Biosolids treatment and managementDocument Transcript
B SD O OS l l TREATMENT MANAGEMENT Processes for Beneficial Use edited by Mark J. Girovich Wheelabrator Clean Water Systems lnc. Annapolis, MarylandMarcel Dekker, Inc. New York. Basel Hong Kong
Library of Congress Cataloging-in-Publication DataBiosolids treatment and management : processes for beneficial use / edited by Mark J. Girovich. p. cm. - (Environmental science and pollution control series ; 18) Includes bibliographical references and index (p. ). ISBN 0-8247-9706-X (alk. paper) 1. Sewage sludge-Management. I. Girovich, Mark J. 11. Series: Environmental science and pollution control ; 18. TD767.B55 1996 628.3644~20 95-5 1804 CIPThe publisher offers discounts on this book when ordered in bulk quantities.For more information, write to Special Sales/Professional Marketing at theaddress below.This book is printed on acid-free paper.Copyright 0 1996 by MARCEL DEKKER, INC. All Rights Reserved.Neither this book nor any part may be reproduced or transmitted in any formor by any means, electronic or mechanical, including photocopying, micro-filming, and recording, or by any information storage and retrieval system,without permission in writing from the publisher.MARCEL DEKKER, INC.270 Madison Avenue, New York, New York 10016Current printing (last digit):10 9 8 7 6 5 4 3 2 1PRINTED IN THE TJNITED STATES OF AMERICA
PrefaceTreated municipal wastewater solids (biosolids) represent a sipficant and valuableresource which can be recycled for various beneficial uses. The United StatesEnvironmental Protection Agency regulations (40 CFR Part 503 "Standardsfor theUse of Disposal of Sewage Sludge") promulgated on February 19, 1993, haveprovided new momentum and defined regulatory conditions for implementingbeneficial uses of biosolids. Treatment and management of biosolids is one of the most challengingproblems in the wastewater treatment industry. It is complicated by a variety oftreatment and end-use options. New federal and state regulations conerningenvironmental safety and public health have imposed sigdcantly more complexrequirementsfor bimlids treatment,use and disposal. Selectingfeasible options thatc nom to regulations, while minimizing cost, has become very challenging. With ofrOcean disposal no longer allowed, landfills filling up and incineration alreadyee - v and often socially unacceptable,biosolids recycling through beneficial useis gaining in popularity. Such recycling has been practiced by many communitiesforyears in the form of land application of stabilized biosolids. The more advancedtreatment options of digestion, composting,heat drymg and alkaline stabilization havebeen developed in recent years to provide marketable products which, because of theadditional treatment, are usually subject to fewer regulatory controls. This book was conceived late in 1993 after the new environmental, safetyand public health regulations concerning municipal sludge (biosolids) treatment,management and disposal were introduced. It is written by a group of authors withmany years of practice in the field and reflects their unique experience. The text emphasizest euse of biosolids, reflecting the authors strong belief hthat biosolids are a valuable resource and should be beneficially employed in theamtext of envinmnental, safety and public health regulations. By providing valuabletechnical and economic data, this book should prove to be an invaluable resource for . . tors,engineers, consllltants,students and practitioners in the fieldmunicipalwho must know how to evaluate and select the best municipal and industrialwastewater solids (bimlids) treatment, management and disposal options. This bookcontains descriptions of the processing equipment, economics and regulatory andenvironmentalprotection issues related to biosolids management and use. I would like to expms my gratitude to all the people who have been helpful in the creation of this book. Special thanks are extended to Sue Gregory, JamieKaiser, andKathleen Wooldridge (Wheelabrator Clean Water Systems Inc., Bio GroDivision) for their assistance in preparing the manuscript. Mark J. Girovich iii
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ContentsPreface iiiAbout the Contributors xiChapter 1 Biosolidr Characterization,Treatment and U e An s: Overview 1 Mark J. Girovich I. Biosolids Generation and Beneficial U e s II. Biosolids Characterization A. Composition and Beneficial Properties B. Microbiology of Biosolids C. Odors and O h r Nuisances te D. O h r Characteristics te I I Biosolids Treatment for Beneficial Use: An I. Overview A. Beneficial Uses B. Requirementsfor Beneficial U e s C. Treatment Processes: An Overview ReferencesChapter 2 Federal Regulatory Requirements 47 Jane B. Forste I. Historical Backpound and Risk Assessment A. Inimxluction B. Basis for the 503 Regulations C. DataGathering D. Risk Assessment Methodology II. Final Part 503 Regulations A. Introduction B. Exposure Assessment Pathways C. Final Part 503 Standards III. Pathogen and Vector Attraction Reduction References V
vi ContentsChapter 3 Conditioning and Dewatering 131 Robert J. Kukenberger I. Sntroduction II. Conditioning A. Organic Polyelectrolytes B. Polymer Feed and Control Systems C. Inorganic Chemical Conditioning D. Thermal Conditioning III. Dewatering A. Process Description B. Thickening C. M c a i a Dewatering ehncl D. Passive Dewatering IV. Odor Control V. Casestudies ReferencesChapter 4 Digestion 165 Kenneth J. Snow I. Biosolicls Digestion A. Introduction B. Process Fundamentals C. Equipment Review D. Economics of Digestion n. casestudies References Chapter 5 Cornposting 193 Lewis M.Naylor I. Introduction A. Growth ofCornposting in the United States II. Goals of cornposting A. ChemicalQuality B. Biological Quality C. Customer Requirements
Contents vii H . Process Fundamentals I A. Microbial Community B. Environmental Conditions C. Nutritional Considerations IV. Solids and the Cornposting Process A. Types of Solids B. Particlesize V. Process Energetics A. The Biological Fire B. Heat and Temperature C. Temperature Control D. Aeration VI. Preparing a Blended Feedstock A. Dry Solids and Porosity B. Chemical Composition C. Ingredient Selection D. Developing a Blended Feedstock Recipe W. OdorRemoval A. OriginsofOdors B. Odor Control Technologies C. Biofilter Fundamentals and Operations D. Biofilter Challenges WI. Pre- and Post-Processing IX. Marketing A. Marketing Issues X. OutlookandSUmmq ReferencesChapter 6 Heat D y n and Other Thermal Processes rig 271 Mark J. Girovich I. BiosolidsDqmg A. Heat Drying and Production of Fertilizer B. Partially Dried Biosolids II. Heat Drymg Processes
viii Contents III. DryerDesigos A. DirectDryers B. IndirectDryers IV. Major Dryer Parametem A. Evaporation Capacity B. Energy and Dqmg Ar Requirements i V. HeatDrylngSystems A. System Components B. Handling and Treatment of Drying and Heating MeCllUftl C. EnvLonmental Control and Regulatory Issues D. Ekonomics of Heat Drying VI. Production of Fertilizer: Case Studies A. Milwaukee Biosolids Dqmg and Pelletizing Plant B. New York City Biosolids FertilizerFacility C. Baltimore City Fertilizer Facility W. O h r Thermal Processes te A. C a r v e r - M e l d (C-G) Process B. Wet Oxidation (Zimpro Process) ReferencesChapter 7 Alkaline Stabilization 343 Mark J. Girovich I. Introduction II. Alkaline Stabilization A. Pre-Lime and Post-Lime Stabilization B. Process Fundamentals C. Alkaline Materials III. Proprietary Alkaline StabilizationProcesses A. BIO*FIX Process B. N-ViroS i Process ol C. RDP En-Vessel Pasteurization D. ChemfixProcess E. Other Alkaline StabilizationProcesses IV. Economics of Alkaline Stabilization References
contents ixChapter 8 Land Application 389 Jane B. Forste I. Introduction A. Historical Background II. Beneficial Properhes of Biosolids A. Nitrogen Considerations €3. Effects of Organic Matter from Biosolids on Soil Properties C. Effects of O h r Biosolids Constituents te III. Site Selection, Design and Management A. Site Selection B. Nitrogen-Based Agronomic Rates C. Design for Non-Agricultural Sites D. Pathogen Considerations i Land Application n Projects E. Agronomic Considerations IV. Methods and Equipment A. Transportation V. Economics of Land Application V . Monitoring and Recordkeeping I A. GeneralRequirements of 40 CFR 503.12 B. Pathogen Reduction C. Vector Attraction Reduction D. Management Practices E. Monitoring W. Public Outreach A. Communication Channels ReferencesIndex 449
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About the Contributors Mark J. Girovieh, Ph.D., book editor and author of Chapters 1, 6, and 7 has years of experience in mechanical and environmental engineering and project j management and is currently Director of Engineering j for the Bio Gro Division of Wheelabrator Clean Water Systems Inc.Dr. Girovich a earned Ph.D. in mechanical engineering from Moscow Polytechnic Institute (Russia), an M.S. in mechanical engineering 8,~ Kharkov from Polytechnic Institute (Ukraine) and a B.S. in appliedphysics from Kharkov State University. , . L .., i "" 1 He is amemberof the AmericanSocietyofMechanical j Engineers (ASME). The author of numerous papersand several patents, he has specialized in biosolids heat drying and alkalinestabilization over the past 10 years.Jane B. Forste, author of Chapters 2 and 8, is currently Vice President ofTechnical Services the Bio Gro Division of Wheelabrator Clean Water Systems forInc. Ms. Forste holds B.S. and MS. degrees in agronomy from the University ofVermont. She has over 20 years of experience in the field and has frequentlypublished and presented papers at professional conferences. She is a CertifiedProfessional Agronomist Registered Environmental Professional. Ms. and Forsteserves on committees for the Water Environment Federation (WEF), AmericanSociety for Testing and Materials (AS"), Association of Metropolitan SewerageAgencies (AMSA) and the National Association of Professional EnvironmentalCommunicators(NAPEC). Ms. Forste develops technical input federal and state toregulatory agencies and has worked extensively with civic and politicalorganizations.Robert J. Kukenberger, P.E., author of Chapter 3, is currently Executive VicePresident of Blasland, Bouck & Lee, Inc. and is responsible the firms water and forwastewater engineering programs nationwide. Mr. Kukenberger received aB.S.in Civil Engineering from Tri State University and from the University of an M.S.Rhode Island. The author of numerous technical publications, Kukenberger Mr.has over 8 years of experience in the field biosolids treatment and management.
xii About the ContributorsLewis M. Naylor, Ph.D., author of Chapter 5, is Environmental Scientist for Blackand Veatch. Dr. Naylor received his Ph.D. in Civil Engineering from Iowa StateUniversity, M.S. degrees in Chemistry from the University of Northern Iowa andEnvironmental Engineering from the University of Iowa, and B.S. degree inChemistry from Bluffon College (Ohio). He has published numerous papers oncomposting, recycling and beneficial use of biosolids. Dr. Naylor is a member ofthe American Chemical Society (ACS), American Water Works Association(AWWA), and Water Environment Federation (WEF). For more than two decades,he has assisted industries and communities in developing beneficial use andrecycling projects. Dr. Naylor has been associated with Cornell University’sDepartment of Agricultural and Biological Engineering and Wheelabrator CleanWater Systems Inc., IPS Division.Kenneth J. Snow, P.E., D.E.E., author of Chapter 4, is a professional engineerwith over 20 years of experience with industrial and municipal wastewatertreatment design, construction and operation. He is currently President ofCorporate EnvironmentalEngineeringLnc. in Worcester,Massachusetts. Mr. Snowholds a B.S. degree from Lowell Technological Institute and an M.S. degree fiomNortheastern University.
Biosolids Characterization, Treatment andUse: An OverviewMark J. GirovichWheelabrator Clean Water Systems Inc.Annapolis, Maryland1 BIOSOLIDS GENERATIONAND BENEFICIAL USEMunicipal wastewater treatment produces two products: clean water and waterslurries which are usually referred to as sludges. While clean water is dlsposed of directly into the environment, it is not feasible,environmentallyor economically, to do so with the solids generated by wastewatertreatment processes. They must be treated prior to disposal or beneficial use tocomply with public health, safety, environmental and economic considerations. Municipal wastewater treatment solids contain si@cant amounts of organicmatter as well as inorganic elements which represent a valuable resource. Thesecomponents of the solids, &er appropriate treatment, can be beneficially used(recycled) as a fertilizer, soil amendment or other beneficial use products. Energycontained in the solids can be recovered. Ash resulting fiom the solids incinerationcan also be used beneficially. Recognizing potential value of wastewater solids,the term biosolids is used int i book to mean a product of the wastewater solids treatment that can be beneficially hsused. In 1989,5.4milliondry metric tons of municipal solids were produced per yearby approximately 12,750 publicly owned treatment works (POTWs) [l]. Current(1995) municipal solids production is approximately 8.0 million dry metric tons peryear and is expected to increase substantially by the year 2000 due to populationgrowth, improvements in POTW operation and stricter treatment standards. 1
2 GirovichTreatment and disposal of municipal solids in the USA is a growing businessestimated at several billion dollars mually. This business, similar to the municipalwastewater treatment industry, is financed by taxation (federal grants, state taxes,sewer fees, etc.) Quantities and quality of biosolids produced by the POTWs vary widely anddepend upon the origin of wastewater, type of treatment and plant operationalpractices. Biosolids in the U.S. have beenmanaged as follows (1992, dry metric tons): rmm 1-1 DISPOSALOF MUNICIPAL SOLIDS IN THE USA [13 in Co-dispo~al MSW landfills I 1,818,700 34.0% Land application 1,785,300 33.3% Incineration 864,700 16.1% Surface dimsal 553,700 10.3% ocean disposal* 335,500 6.3% TOTAL 5,357,900 100%* Ended in June 1992 In the land applicaiim category, te following beneficial use options are growing hin popularity (design capacity, dry metric tons per year, 1995 estimates): * Heat Drylng and Pelletizing 375,000 - Composting 550,Ooo * Alkaline Stabilization 650,000 The European U i n (Ev> (12 countries) generated over 6.5 million dry metric notonsof municipal solids i 1992. By theyear 2000, the amount will increase up to 9.0 nmillion dm@ear. The EU directives of 1986 and 1991 generally promote beneficialuse of sewage sludge solids . The future of beneficial use in the EU will depend onthe legislation regarding standards for beneficial use and incineration as well as theacceptance of beneficial use versus incineration by the population. At present, thebeneficial use in the EU varies from 10 to 60 percent (Table 1-2). Japan generates over 1.4 million dm@ear w t approximately60% incinerated. ihAccurate data on solids disposal in developing countries of Eastern Europe, Asia,
Biosolids Characterization,Treatment and Use 3Latin America and Africa are not available at present. Detailed discussion onbiosolids disposal worldwide is provided in References  and [111. The US Environmental Protection Agency (EPA) actively promotes beneficialuse of municipal solids because it decreases dependence on chemical fertilizers andprovides signtficant economic advantages. Over 20 years of research have beendevoted to the use of biosolids on agriculture and similar beneficial applications.Beneficial use includes:* application to agricultural and nonagricultural lands alone or as a supplement to chemical fertilizers* application in silviculture to increase forest productivity
4 Girovich* use in home lawns and gardens useongolfcomes* application to reclaim and revegetate disturbed sites such as surface-mined areas* use as a daily, maintenance and final cover for municipal solid waste (MSW) landfills. Land application is essentially the placement of appropriatelytreated biosolidsin or on the soil in a manner that utilizes their fertilizing and soil conditioningproperties. It includes agricultural, forest and site reclamation applications, and anumber of biosolids-derived products, such as dlgested, dried, chemically or heathreated biosolids, and compost. Biosolids-derived products are distributed in variousforms, such as in bulk, packaged, further processed, enriched, sold to public, etc.They are applied to agricultural and nonagricultural lands, soil reclamation andrevegetation sites, forests, lawns, gardens, golfcourses, parks, and so forth. Surface disposal includes disposal in monofills and dedicated sites. @Iisposalon dedicated sites is a beneficial use method of applying biosolids at greater thanagnmomic rates a sites speczdy set aside for this purpose to restore disturbed soils t(e.g., strip mines).] Incineration is generally regarded as a nonbeneficial disposal method of solidsmanagement unless heat recovery for steam and electricity generation is included inthe process. One o more levels of treatment (i.e., primary, s e c o n w and tertiary) are used rto clean wastewater. Each level of treatment provides both greater wastewater clean-up and greater amounts of municipal solids. primar?r solids are removed by gravity settling at the beginning of the wastewater treatment process. They usually contain 3.0 to 7.0 percent total solids (%TS), 60 to 80 percent of which is organic matter (dry basis). The primary solids are generated at the rate of approximately 2,500 to 3,500 liters (660to 925 gallons) per each million liters of wastewater treated.* Secondarv solids are generated by biological treatment processes called secondary treatment (e.g., activated sludge systems, trickling filters and other attached growth systems which utilize microbes to remove organic substances from wastewater). They usually contain fiom 0.5 to 2.O%TS. The organic content of the secondary solids ranges from 50 to 60%. About 15,000 to 20,000 liters (3,965 to 5,285 gallons) of these solids are generated per each million liters of wastewater treated.- Advanced (tertiary) solids are generated by processes such as chemical precipitation and filtration. The solids content varies fi-om 0.2 to 1.5%TSwith the organic content of the solids in the 35% to 50% range. About 10,000 liters of solids (2,642 gallons) are generated additionalIy per each million liters of wastewater treated.
Biosoiids Characterization, Treatment and Use 5-ABLE 1-3 SOLIDS GENERATION r i i Item Unit Primary Amount liters 2,500- 15,000- 10,000 generated 3,500 Percenttotal % 3.0-7.0 0.5-2.0 0.2-1.5 solids Dry biosolids metric tons per 0.1-0.15 -.-.1 02 03 0.02-0.15 tons per million I 0.42-0.55 0.8-1.2 ~ 1 0.08-0.6 I Table 1-3 provides for quick, approximate estimates of the total biosolidsgenerated as a function of a POTW wastewater influent flow. For example, a 30milliongallon per day P O W (30 mgd) with primary and secondary treatment (typicalfor a large number of the POTWs in the USA) will generate 30 mgd x (.485+ 1.O)= 44.5 dry tuns of biosolids per day (dtd) approximately. (0.485and 1.O are averagebiosolids generation for primary and secondary treatment, tons per million gallonsrespectively.)IL BIOSOLIDS CHARACTERIZATIONC a a t r z t o of biosolids by their source (primary, secondary, etc.) provides only hrceiainlimited information about their properhes. There are numerous other physical,chernical and microbiological parameters which are important for biosolids treatmentand management.k Composition and Beneficial Properties1. Solids ConcentrationSolids concentrations are measuredand expresseL either as mg (milligrams per liter)o as percent (%) of solids. In all cases in this book it is assumed that: 10,000mgA r= 1% total solids (TS). Note that percentage of total solids is weighdweight ratio while solidsconcentration(mgA) is weightholumeratio. The equation above is valid only with theassumptionthat specific gavity of biosolids is equal to that of water (1 .O) which, in
6 Girovichmany wastewater solids,especially thoseof industrial origin, is not true. The standardprocedurefor determhhg solids concentration employs drying a measured volume ofbiosolids to a cunstant weight at 103 - 105 C. The solids concentration is the weight O Oof dry solids divided by the volume of the sample expressed in mgA. In order todetermine percent of total solids (%TS) as a weightlweight ratio, the identicalprocedure is applied with a measured weight of the biosolids sample. Total volatile solids f%TVS) are determined by igniting the dry solids at 550"+- 50°C in a furnace with excess of oxygen. The residue is referred to as non-volatileor as fixed solids (ash) and the loss of weight on ignition determines total volatilesolids, Both %TS and %TVS are widely used in the biosolids treatment andmanagement practices as measures of dry matter (or moisture) and organic(combustible) matter in the biosolids. Total sumended solids (TSS) refers to the nonfilterable residue retained after h O -fdtration of a sample of t e liquid biosolids and then dried at 103 105O C to removewater. The concentration of TSS is the weight of dry solids divided by the volume ofthe sample usually expressed in mgA. Determination of volatile suspended solids is identical to that of total volatilesolids using loss on ignition methods. Total solids are the sum of the dissolved andsuspended solids.  Water in biosolids is usually categorized as follows:* Free water is not attached to the biosolids particles and it can be separated by gravitational settling.- Floc water is trapped within the flock and can be removed only by mechanical forces, which are usually much greater than gravitational force... Carillarv water adheres to individual particles and can be also separated by mechanical forces.- Intracellular and chemicallv bound water is part of cell material and is biologically and chemically bound to the biosolids organic and inorganic matter. Approximate amounts of energy required to remove water h m biosolids byvarious methods are as follows (per cubic meter of water) :- Gravity (thickening): 10 KW to achieve 2-6%TS Mechanical dewatering: 1-10 KW to achieve 1530% TS* Thermal drying: lo3KW to achieve 85-95% TS In other words, water removal requires approximatelyone million times moreenergy to achieve 95% TS than to achieve 2-6%TS in the biosolids.
Biosoli& Characterization,Treatment and Use 72. Chemical CompositionChemical c a n p i t i o n of municipal solids varies greatly depending upon their originand methods of treatment. Biosolidsc n a n organic matter, macro and micronutrients and water important utifor plant growth. Sixteen (16) elements out of ninety (90) found in plants are knownto be essential for plant growth and most of these elements are present in biosolids.The elements are carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, s l u ,ufrcalcium, magnesium, iron, boron,manganese,copper, zinc, molybdenum and chlorine.Except for boron, animals require all of these elements and, in addition, sodium,iodine, selenium and cobalt. Some of these elements, however, can be detrimental tohuman, plant o animal life if they are present above certain limits (e.g., copper, Zinc, rmolybdenum, and chlorine) . Certain metals and synthetic organics have been proven to be detrimental andeven toxic to human, animal and plant life at certain levels and, as such, are regulatedby respectivestatutes. Extensive studies conducted in the 1980s by U.S. EPA  andsubsequent analysis of the potential pollutants concentration, fate, toxicity anddetrimental effects on humans and environment resulted in new federal regulationswhich limit concentrations of ten "heavy" metals (arsenic, cadmium, chromium,copper, lead, mercury, molybdenum, nickel, selenium and zinc) in biosolids appliedto land or disposed of by various means. Synthetic organics (organic pollutants) arenot regulated at present (1 995) unless they are at the levels that make biosolidshazardous and subject of other federal regulations (e.g., RCRA, TOSCA). Major solids characteristics are provided in Table 1-4 [S]. Nutrients present in biosolids are absorbed by plants as water soluble ions, mostly throughthe mts. Dissolved mineral matter in biosolids (and in soil) is present Ca",IMe, as cations @ + K Na and low levels of Fe", Mn*+,Cu", A13, Zn") and ,, anions (HCO, COP, HSO;, SO:, Cl-,F-, HPO;, &PO;). Table 1-5 illustrates the nutrients ionic forms generally present in biosolids and available for plants. Note t a carlxm is absorbed by plants mostly through leaves as COz;hydrogen ht fiom water as E, HOH, and oxygen through leaves as 0,, OH- and CO,.3 . MacronutrientsThe elements generally recognized as essential macronutrientsfor plants are carbon,hydrogen, nitrogen, phosphorus, potassium, calcium, magnesium and s l u . ufrNitrogen, phosphorus and potassium are the most likely to be lacking and arecommonly added to soil as fertilizers. Although biosolids contain relatively low levels of macro and micronutrientswhen applied to soil at recommended rates they can supply all the needed nitrogen,phosphorus as well as calcium, magnesium and many of the essential micronutrients.
8 GirovichNitrogen, phosphorus and potassium (usually referred to as N-P-K) play significantroles in biosolids beneficial use. E 1-4 MUNICIPAL. SOLIDS C MCTERISTICS :5] E Ie tm Primary Secondary Total dry solids (TS), % - 3.0 7.0 I 0.5 - 2.0 Volatile solids (% of TS) 60 - 80 150-60 Nitrogen (N, % of TS) 1.5 - 4.0 I 2.4 - 5.0 Phosphorus (P,O,, % of TS) 0.8 - 2.8 0.5 - 0.7 Potash K O , % of TS) ~ Energy content (BTUAb, dry basis) PH 10,000 - 12,500 5.0 - 8.0 I 8,000 - 10,000 6.5 - 8.0 Alkalinity (m@ as CaCO,) Range Median a. Arsenic 1.1 -230 10 b. Cadmium 1- 3,410 10 c. chromim 10 - 99,000 500 d. Copper 84 - 17.000 800 e. Lead 13 - 26,OOO 500 f Mruy ecr 0.6 - 56 6 g. Molybdenum 0.1 - 214 4 h. Nickel 2 - 5,300 80 i. Selenium - 1.7 17.2 5 j. Zinc 101 - 49,000 1,700 k. Iron 1,OOO- 154,000 17,000 1. Cobalt 11.3 - 2,490 30 m. Tin 2.6 - 329 14 n. Manganese 32 - 9,870 260
Biosolids Characterization, Treatment and Use 9 Nitrogen is the most critical part in the plant growth. Poor plant yields aremost often due to a deficiency of nitrogen. It is a constituent of plant proteins,chlorophyll and other plant substances. Biosolids nitrogen exists as organic andinorganic compounds. Organic nitrogen is usually the predominant form of nitrogenin soils (90 percent) and in biosolids and it is not available to plants. It m s be utbacterially converted to ammonium (NH,+) and eventually oxidized to nitrate (NO;)to become biologically available. copper cu cu2+ Zinc zn Zn2+ Molybdenum Mo MOO; (molybdate) Boron B H,Bo3, W O i rB(OH)i Nitrogen is a unique plant nutrient because, unlike the other elements, plants canabsorb it in either cation ( H+ or anion (NO;) form. Nitrogen forms not absorbed N ,)by plants volatilize o oxidize and are lost to atmosphere as N, or N20. Volatilization rof ammofiium ion dependsupon pH. The higher the pH, the more nitrogen is releasedas gaseous ammonia(NH3). The nitrogen requirements of different plants range fiom 50 to 350 kg perhectare (45-3 12 pounds per acre). [ 11 Heavy application of nitrogen-containingbimlids or chemical fertilizermay result in unused nitrate migrating into surface orunderground water with adverse health and environmental effects.
10 Girovich -The nitrogen content of primary biwlids is in the range of 2% 4%; in secondaryandanaerobicallydigested biosolids it is in the 2% - 6% range (dry basis) . Nitrogen in biosolids is usuallydetermined as organic nitrogen (Org-N), solubleammonia nitrogen (NH,-N), soluble nitrate-nitrogen (NC) -N) and total (Kjeldahl)nitrogen (TKN). Numerous studies have been conducted on the rate of organic nitrogenamversion into the biologicallyavailable forms ( H+ NO,?, called mineralization. N ,,One method of determinkg nitrogen mineralization and availability is based onassumptionthat 15% of organic nitrogen becomes available to plants during the firstyear and 6% of the remaining, or residual organic nitrogen, is released during thesecond, 4% during the third, and 2% during the fourth growing season afterapplication. B s d on these assumptions, the available nitrogen per ton of dry aebiosolids and application rate can be calculated . Nitrogen is usually the limitingfactor in biosolids land application, unless they have been stabilizedby an alkalinematerial (he) or contain excessive amounts of heavy metals. In this case, calciumo heavy metal@)becomes the limiting component. A detailed nitrogen discussion is rprovided in Chapter 8. Nitrogen is available fiom chemical fertilizer in a relatively concentrated form,typically 8 to 40percent The chemical fertilizers nitrogen is valued, however, at $2to $4 pea percent point while the same for organic nitrogen in biosolids is $12 to $17per percent (1994). Phosphorus (P) is the second most critical plant nutrient. The nucleus of eachplant c l containsphosphaus. Cell division and growth are dependentupon adequate elsupply of phosphorus. The most common forms of phosphorus are organicphosphorus and various forms of orthophosphates (H$0 ; HPO - , PO: -) and ,polyphosphates,such as Na@O,),, Na,P,O,,, Na,P,07. Phosphorus is not readilyavailable in most unfertilized soils and is derived primarily fi-omphosphates releasedby organic matter decomposition. Organic phosphorus is decomposed by bacterialactioninto orthophosphatePO;. Polyphosphates also decompose (hydrolyze) in waterinto orthophosphates. Sigdicant portion of phosphorus compounds are watersoluble. Stable orthophosphate (PO,) is predominate form which is absorbed byplants. Primary wastewater solids contain relatively small amounts of phosphorus.Secondary biosolids contain greater amounts of phosphorus which is generallyremoved from wastewater by biological means. Amounts of phosphorus in biosolids depends on phosphorus concentrationin theinfluent and type of phosphorus removal used by wastewater treatment plants. Conventional primary and waste activated processes remove only 20-30% of an influent phosphorus and, therefore, biosolids resulting fiom these processes contain a small amount of phosphorus (0.1 to 2%).
Biosolids Characterization, Treatment and Use 11 This amount is Sutticient for plant growth when biosolids are applied at theniirogen requirement rate. Excessive amounts of phosphorus can eventually be builtup in the soil and result in leaching. The phosphorus requirementsof different plants range from 0 to 95 kilogramsper hectare (0 to 85 pounds per ar) Phosphorus is not removed fiom wastewater ce.by aerobic o anaerobic digestion. Only chemical precipitationusing aluminum and rironcoagulants or lime is effectivein phosphorus removal. Biosolids resulting h mthese processes may contain greater amounts of phosphorus. The following reactions convert phosphorus b m wastewater into the formsfound in biosolids. 1. Chemical coagulation by alum, Al,(SO,),, and ferric chloride, FeCl,, respectively: Al,(SO,), 14.3 KO + 2 P : O = 2 AlPO, + 3 SO: + 14.3 H20 FeC1, +PO;- = FePO, + 3 C1 2. Phosphorus removal by addition of slaked lime: Ca(HCO,), + Ca(OH), = 2 CaCO, + 2 &O (removal of water hardness) 5 Ca2++ 4 OW + 3 HPO,2- = Ca, (OH)(PO,), + 3 KO Calcium ion in the second reaction forms precipitates containing orthophosphate. Potassium (K) is needed by plants for various functions, including maintainingcell permeability, increasing resistance of plants to c r a n diseases, and aiding in etitranslocation of carbohydrates. Potassium in most soils is found in more thansufficient amounts, however, it is not bioavailable. As a result, use of potassiumfertilizers is required. About 150 pounds per acre of potassium is needed for plantgrowth. Biosolids usually Contain small amount of potassium (0.02 to 2.5% drybasis). 4. Other Inorganic Nutrients Calcium (Ca) is rarely deficient in plants. It is needed for cell division, it makes cells more selective in their absorption and it is a constituent of a cell wall. M s soils ot (except sandy and strongly acid ones) contain suilicient calcium supply. Calcium is supplied to the plants by soluble calcium ions fiom calcium containing minerals such as CaCO, Addition of CaO, Ca(OH), or CaCO, to soils is done to correct the pH of strongly acid soils to improve nutrient availability rather than to supply calcium for plant growth. Biosolids contain calcium in small amounts unless they are the result of processes where lime is used (e.g., lime stabilization). Mamesium (Mg) is important in chlorophyll formation. There is one atom of magnesium in each chlorophyll molecule. There would be no green plants without
12 Girovichmagnesium. Most of the plant magnesium, however, is found in plant sap and thecytoplasm of cells. Magnesium is taken up by the plant roots as cation M e . Magnesiumbioavailability is affected by other ions such as potassium, calcium and nitrogen.Magnesium requirements of different plants range fiom 9 to 36 pounds per acre.Biosolids usually contain small amounts of magnesium (0.3% - 2%,dry basis). sulfirr ranks in importance w t nitrogen and phosphorus as an essential nutrient ihin the formation of plant proteins; it is also required for the synthesis of certainvitamins in plants. Sulfkantaining organic compounds are present in biosolids andbioavailable sulfate (SO;2) is produced as a result of microbial decomposition. Sulfur requirements of different plants range fiom 10 to 40 pounds per acre.Biosolids contain fiom 0.6% to 1.5% of sulfur (dry basis).5. OrganicMatterOrganic matter, dead or alive, is largely carbon (approximately 58% by weight), withlesser amounts of hydrogen, oxygen and other elements such as nitrogen, sulfur andphosphorus. Organic matter in biosolids contains proteins, carbohydrates,fats--compoundscomposed of long chains of molecules with molecular weights ranging from severalhundred to several million. Organic matter is a nutrient source for plants and microorganisms, in sods itimproves water infiltration, aeration and aggregation of soil particles. Microorganisms (bacteria, protozoa, fungi, and others) decompose biosolidsorganic matter and use some decomposition products (carbon, nitrogen and otherelements) for reproduction and, as a result, change the biosolids organic matter andrelease c r a n products of decomposition (e.g., carbon dioxide, methane, volatile etiorganics including odor pollutants, ammonia, nitrogen, water) into the environment.Bacteria, actinomyceteSand fungi are the most active decomposersof organic matter,but some algae, protam, rotifers, nematodes and others participate in organic matterdewnposition These organisms also interact in a complex m n e at various stages anrof wastewater and biosolids treatment processes. Most soils Containrelatively snall amount of organic matter (1 -5% in the top 10inches of soil) but it largely determines soil productivity. The biosolids organic matter can have a profound effect on the soil physicalpropxt~es as soil fertility, humus formation, bulk density, aggregation,porosity, such and water retention. A decrease in bulk density, for example, provides for a better environment for plant root growth. The high organic carbon content of biosolids provides an immediate energy source for soil microbes. The nitrogen in biosolids is in a slowly available organic form which provides for a reliable nitrogen supply. Increasedaggregation results in better tilth and less potential for erosion and reduction
Biosolids Characterization, Treatment and Use 13of runoff. Water retention and increased hydraulic conductivity provide necessarywater for plants, especially during drought. Biologically active organic components of biosolids include polysaccharides,such as cellulose, fats, resins, organic nitrogen, slu and phosphorus compounds, ufretc.; they contribute to the formation of soil humus, a water-insoluble material thatbiodegrades very slowly and is the product of bacterial decomposition of plantmaterial.6. MicronutrientsMicronutrients such as iron, zinc, copper,manganese, boron, molybdenum (for N-htion), sodium,vanadium, and chlorine, are needed by plants in small quantitiesbutthey are quite important as catalysts in numerous biological processes. The role ofchlorine, except for its part in root growth, is not well known. Excessive quantitiesof some of the micronutrients can render biosolidshazardous to human health, plantand animal life. Soil and biosolids pH influences micronutrient availability. All metals exceptmolybdenum are more bioavailable at low pH (acidic environment). In near neutraland alkaline environments,metals form insoluble oxides or hydroxides and becomenonbioavailable. Also, toxicity caused by excess level of micronutrientsmay O C C U T .A detailed discussion concerningmicronutrients is provided in Chapter 8.7 . PollutantsBiosolids usually contain organic and inorganic components which can adverselyaffect plant and animal life as well as human health if present at excessive levels.Inorganic pollutants include ten "heavy" or trace metals presently regulated by theU.S. EPA arsenic, cadmium, chromium,copper, lead, mercury, molybdenum, nickel,selenium and zinc. Ranges and mediancOncentrafioIlS of heavy metals in biosolids areprovided in Table 1 4 . Arsenic, a toxic metalloid, has been the chemical villain of more than a fewmurder plots. The combustionof coal introduces large quantities of arsenic into thea w t S o m e f m l y used pesticides contain highly toxic arsenic compoundsas well as some mine tailings. The U.S. EPA has classified arsenic as a humancarcinogen. Arsenic content in biosolids varies widely (Table 1-4). Cadmium comesh r nmetal plating and mining wastes. Cadmium is chemicallysimilar to zinc (both are divalent cations) and replaces zinc causing acute cadmiumpoisoning (ludney damage, high blood pressure and destruction of red blood cells).According to the U.S. EPA, cadmium is a probable human carcinogen. Cadmiumcontent in biosolidsvaries fiom few to over 3,500 mg/kg, dry basis. Chromium comes fkom metal plating and mine tailing. It is an essential traceelement. Hexavalent chromium is classified by the U.S. EPA as a human carcinogen,
14 Girovich Cmuer unnes fiom industrial discharges, mining and mineral leaching; it is anessential trace element, not very toxic to animals, toxic to plants and algae at moderatelevels. Copper is not classified as a carcinogen. Lead comes from a number of industrial and mining sources, leaded gasoline,plumbing, lead bearing minerals, etc. It causes kidney, reproductive system, liver,brain and central nervous system disfunctions. Lead is classified by the U.S. EPA asa probable human carcinogen. Mercury generates the most concern of the heavy metal pollutants. It comesfrom minerals, coal combustion, pesticides and fungicides, batteries, andpharmaceutical products. Toxicological effects include neurological damage, birthdefects, depression, etc. Mercury tends to accumulate in seafood due to formation ofsoluble organic mercury compounds. The U.S. EPA has not classified thecarcinogenicity of mercury because of inadequate evidence. Molybdenum comes fiom natural sources and industrial discharges; possiblytoxic to animals, essential for plants. The US. EPA has not classified the carcinogenicityof molybdenum because of inadequate evidence. Nickel sources are mineralsand industrial discharges. Nickel deficiency has not been demonstrated to be essential for the proper growth and development of plants or animals. However, excessive levels of this element produce toxic effects in plants, animals and humans. The U S . EPA has classified nickel as a probable human carcinogen. Selenium sources are minerals, coal and industrial discharges. It is essential at low levek, toxic at higher levels. The U.S. EPA has not classified the Carcinogenicity of selenium because of inadequate evidence. comesh indushial waste, metal plating and plumbing. It is an essential element in many metall- and aids wound healing; it is toxic to plants at higher levels. Zinc occurs in biosolids in relatively large quantities and could limit land application. Zinc requirements vary considerably from crop to crop (e.g., alfalfa requirements are 0.5 pounds per acre, approximately). Organic Dollutants, among others, include hazardous organic substances - regulated under RCRA or TOSCA standards. Moredetailedstudyofbiosolidspollutantscanbefoundin, [lo], [ll], , and .B Microbiology of Biorolida .Biosolids contain diverse life forms. These microscopic living organisms have bothbeneficial and detrimental roles in the biosolids treatment and use practices. Microorganisms in biosolids can be categorized into bacteria, includingactinomycetes, Viruses, helminths (parasitic worms), protozoa, rotifers and fungi. Al m t d number of these organisms are pathogenic (i.e., they may cause various human iieand animal diseases). One of the major objectives of biosolids treatment is pathogen
Biosolids Characterization, Treatment and Use 15elimination or reduction to acceptable levels. Current regulations do not addressprotozoa, rotifem and fungi as organisms subject to pathogen requirements becauseof the lack of analytical methods and because EPA concluded that these organisms areunlikely to survivewastewater and biosolids treatment processes and, thus, should notcause an adverse effect in biosolids use [I]. Bacteria are the smallest living organisms. Most bacteria reproduce by divisioninto two identical cells. Bacteria are very diacult to class@. They are composed ofwater (80%) and dry matter (20%), about 90% of which is organic. Bacterial drymatter contains carbon (48%), nitrogen (7- 12%),phosphorus (1 .O-3.0%), potassium(1.0 - 4.5%), sulfur (0.2-1.0%)and trace elements, such as magnesium, sodium,calcium, iron, copper, manganese, and molybdenum. Bacterial organic mattercontains proteins (50-60%), carbohydrates (6-15%), lipids and organic acids . Actinomvcetes are a large group of organisms which look like elongated cells orflaments. Same actinomycetes have characteristics common to fungi. Actinomycetesare commonin biosolids and soils. They are saprophytes and decompose a wide rangeof organic compounds such as difficult to decompose long chain hydrocarbons,complex aromatic compounds, pesticides and dead microbial biomass. They alsodecompose readily degradable compounds such as amino acids, sugars and organicacids. They u u l y grow more slowly than most other bacteria. Some actinomycetes salare aerobes, some are anaerobes,and they thrive at a pH of 6.0 to 8.5. Actinomycetesare o h found in activated biosolids and scum developed over the aeration tanks andsecondary clarifiers. The most fiequently reported genera in activated sludgeprocesses are Arthrobacter, Corynebacterium, Mycobacterium, Nocardia andRhodococcus [1 21. Coliform bacteria, especially fecal coliforms,are natural, normally harmlessmicroscopic inhabitants of the intestines of all warm-blooded animals including humans. Coliform bacteria coexist in fecal material with pathogenic or disease- Causing organisms such as Certain bacteria, viruses and protozoa. Coliform bacteria are also found in soil and on vegetable matter. They are highly concentrated in wastewater and biosolids and generally sparse or not present in other habitats. The presence of coliform bacteria in biosolids and water is considered an indication of Contamination. Typical average coliform bacteria populations in biosolids and other materials are (organisms per gram, dry weight) [l 11:- Unstabilized biosolids: l,ooo,000,000- Aerobically digested biosolids: 30,000-6,OOO,000* Humanfeces: 50,000,000,0008 Disinfected effluent: 100- Wastewater: 8,ooo,000 One of the coliform bacteria, Escberichiu coli ( . coli), is a natural inhabitant Eonly in the intestines of warm-blooded animals and, therefore, its presence indicates
16 Glrovichfecal contamination and possible presence of pathogens. Fecal coliforms are theprincipal indicator organisms (along with Salmonella sp. bacteria) for evaluatingthemicrobiologicalcontamination of biosolids. Fecal stre~tomcci include the entexwmxs group and several species associatedwith nonhuman warm-blooded animal wastes. Enterococci are enteric bacteria fiomhumans and their presence indicates contamination of human origin. Fecal coliformt fecal streptococcusratio can help idenw sources of contamination. Ratios greater othan 4.4 indicate fecal pollution of human origin; ratios less than 0.7 indicate fecalpollution b m nonhuman sources. Salmonella SD. routinely found in unstabilized biosolids, compost handlingfacilities and 1andfYls cause various diseases such as acute gastroenteritis (foodpoisoning), typhoid fever, and salmonellosis. n i a o s very useful in assessing microbiological contamination, no While i d c t ri d c t rspecies is perfect. Coliforms, for example, die off very quickly in water (half niaolife is about 15 hours and only few coliforms survive more than 3 days); however,coliforms may live significantly longer in biosolids. V r s s are acellular particles which carry genetic reproductive information but iueare incapable of living outside a host cell. They are extremely small (0.01-0.25micmmter cr micron, pn)and extremely host-cell specific. More than 100 differentviruses may be present in untreated biosolids. The major concerns with viruses aret e rpotential for disease transmission and conditions necessary for their destruction hiin biosolids. Diseases associated with specSc pathogenic bacteria and viruses and aresummarizedin Table 1-6. Protozoa ( f r tanimals") are very small (5 to lo00 pm in size), single celled "isanimals comprising a diverse group. Protozoa need water; they are present in allaerobic wastewater treatment plants. Their role is important in the activated solidsprocesses (up to 5 , O o r g a n i d d or about 5% of the dry weight of suspended 0O O solids in the mixed liquor); they are present in trickling filters; rotating biological contactors (RBC); oxidation ponds, and natural and manmade wetlands. Protozoa are parasitic (live on or in other life forms) or fi-ee living. Many areparasites of animals. Protozoa are of four different nutritional types: autotrophs (plant-like forms capable of absorbing sunlight and using carbon dioxide to produce organic compounds); saprobic organisms (animal-like forms that do not contain chlqhyll IWT require light but rely on the organic soluble compounds); phagotrophs (forms t a feed on bacteria), and carnivorousprotozoa which feed on other protozoa. ht Some protozoa require oxygen and some do not use it and are often unable to g o in its presence. rw Optimum pH for protozoa survival is in the 6.0 to 8.0 range. At pH below 5.0 and higher than 8.0 their population is adversely affected. Light is important for autotrophs. Many protozoa compete with one another for bacteria as a food supply, while others compete with bacteria and other saprobic protozoa for soluble organics.
Biosolids Characterization,Treatment and Use 17 Fmtcma play a sigmficantrole in bacterial removal from wastewater includingpathogenic bacteria that cause diseases such as diphtheria, cholera, typhus andstreptococcal infiions, as well as removal of fecal bacteria such as Escherichia coli.Protozoa are helpful in flocculating suspended particulate matter and bacteria thusaiding both clarification of the effluent and formation of biosolids. Such protozoa as Entamoeba histoktica and Giardia lamblia are common inbiosolids, especially in warm climates. Their cysts are quite resistant to disinfection. Rotifers ("wheel bearers") are the simplest and smallest of themacroinvertebrates found in wastewater and biosolids. They are fiee swimmingorganisms and range in size from 40 to 500 um (microns) and have an average lifespan of 6 to 45 days. Rotifers perform many beneficial roles in stabilizing the organicwastes of lagoons and fixed-filmand activated biosolids processes. In lagoons rotifersfeed on phytoplankton or algae. In the activated sludge process, rotifers consumelarge quantities of bacteria and enhancefc f d o n . Generally in aerobic processes lcthe rotifers large consumption of bacteria and solids contributes to BOD reduction. Helminths (parasitic intestid worms and flukes) and Nematodes (roundworms)are tire living, microscopic (0.5-3.0millimeters long and 0.02 to 0.05 mm wide) andnacn>scopic organisms (Rrcaris lumbricoides (roundwonn), sometimes reaching 20-40cm in the intestine) whch include various worms such as hookworms, pinworms,whipworms, eelworms, roundworms, and many others. They are present in aerobicprocesses with abundance of oxygen and microbial food (e.g., trickling filters,activated sludge processes). Some nematodes survive temperatures as high as 117O F(47°C)and are most active at pH between 3.5 and 9.0. Ascaris lumbricoides are a serious potential danger to humans from biosolids.Ms helminths and nenato$eeggs and cysts tend to accumulate in primary biosolids. otTable 1-6 lists major pathogenic h e h t h s and nematodes and potential diseases. m. Over 80,OOO fungal species have been identified, most of whichdecompose organic matter. They can be broadly categorized as yeasts or molds.Fungi consist of tubular, filamentous branches 10 to 50 um in diameter. Theyreproduce by forming spores which are quite hardy. About 50 fungal species cancause human infections afFecting primarily lungs, skin, hair and nails. Fungi are less dependent on moisture than bacteria and can grow on drybiwlids absorbing moisture fiom atmosphere. They can withstand broad range of pH and temperature. Fungi are important industrial microbes. Yeasts are used in production of alcohols and to synthesize antibiotics, organic acids and enzymes. Most wastewater treatment processes remove pathogens from the wastewater and trader them i t biosolids. While wastewater is being cleaned, biosolids that are no generated by the treatment processes contain a number of pathogenic organisms. Primary biosolids have large numbers of alive and dead protozoa cysts and helminth and nematode eggs. Secondarybiosolids contain sigdicant numbers of bacteria and viruses. For example, Salmonella are removed at 90% to 99% efficiencies fiom the water and end up in the biosolids.
18 Girovich orb- Dlsepee Mode of Trnnsmlssion comment24L Bacteria and Actinomywtca1.1 Colifonn species1.2 Mbrio cholera1.3 Salmonella species Chmmon in biosolids1.4 Salmonella @phi I Typhoidfever I$ : in biosolids1.5 Shinella Shigellosis (bacillary djwnbry) Polluted water1.6 Bacillus Anrhracis Anthrax Diseascofanimals,rartin humans I I ~~~~~ ~1.7 Brucella Brucellosis Infectedmilkormeat Found in biosolids1.8 Mycyobacterium Tubennrlosis Found in biosolids tllhlOSis1.9 Leptospira Leptospkis cordaminaiadfoodanddrink inrerohaemorrhagiae Found i biosolids n1.10 Yersinia entercolitica Gdmenkh .. contaminatedfoodanddrink1.11 Esherichia coli Gastroententi8 contaminatedwaterandfood (usually Common i biosolids n nonpathoe~c)1.12 CbsMdium rerani Tetanus Woundcontact Found i biosolids n 1.13 Nocardia spp. Lung disease (nocardiosis) Inhalation and contact with skin Found i biosolids n
Biosolids Characterization, Treatment and Use 19TABLE 1-6 (COW.) I Mode of TrPRmtlsston Commenb1.14 Actinomycetes israelii Actinomycosis (meninsitiS. Inhalation and contact with mdocarditis,genitalinfdons) skin Found i biosolids n1.15 Camphlobacterspp. Acute enteritis contaminatedfoodanddrink Found i biosolids n2.1 Polio virus Poliomyelitis F d in biosolids Polio vaccine eliminates disease2.2 v i HeDatitis A Found in biosolids2.3 Cowadcievirua, e i g t s diarrhea Inhalation,water Mild infdons, m n n i i , echovirus ininf~heartdisease, Found in biosolids conjunctivitis2.4 Adenovirus,reovirus Respkatoryinfections,influenza, Inhalation,water cold&bronchitis. diarrhea Found in biosolids2.5 Rtvrsclcvrs oaiu,aiiiu Viralgssh’oenteritis Inhalation,water3.1 Entamoeba histolitica Amoebic dysentery In untreated biosolids used aa a fertilizer,resistant to disinfeaim 3.2 Giardia lamblia Giardicrsis Cysts are not destroyed by disinfection; found in biosolids ~~ 3.3 Criptospridium Found i biosolids n 3.4 Balantidiurn coli Dysentcly Found in biosolids 4.1 Ascaris lumbricoides; Ascariasis (large inlestinal Ingestion ofeggs in food or ascaris suum roundworm) drink Awominal pain, digestive F d i biosolids wet and n dkhhms, fever, chest pain dry;mostcommonof hellninth
20 Girovich TABLE 1-6 (COW.)I I I Disease I ModeofTrnnsmission Comments 4.2 Ancylostoma Ingestionof eggs duodenale, Necator Found in biosolids americanus 4.3 Enterobius Pinworm (enterobiasis) Ingestion of eggs vennicularis Easily curable with drugs 4.4 Trichuris trichiura Whipworm (irichuriasis) Ingestion of eggs Abdominalpain,diarrhea Easily curable with drugs Found in biosolids14.5 Taenfasaginato IAbdominal pain, disturbances IFound in biosolids 4.6 Cat, dog,beef, pork ~~ W r infections i humans om n I Ingestion of eggs Ingestion of eggs 4.7 Variouskernatodes Intestid, lung and liver flukes (flukes) Found i biosolids n 5.1 Aspergillusficmigatus Aspergillosk, lung infection Inhalation of spores Found i biosolids and n compost,mostcommonand serious offungal infections ~~I 5.2 Candida albicans ~~ ~~ I intestidtract) Lung infection I Candidiasis (iection of lungs, skin, Inhalation of spores Inhalation of spores 5.3 Coccidioides immitis and Histo-plasma Fungus grows on biosolids in capsulatum w r and moist conditions am 5.4 Blastomyces Blastomycosis(lung infection) Inhalation of spates dennatitides 5.5 cryptococcus Cryptoauwxis (lung infection) Inhalation of spores neoformans 5.6 Sporothrix schenkii Sporotrichosis Brokenskincontact When biosolids are applied to land, humans and animals can be exposed to pathogens directly by coming into contact or indirectly by Consuming water or food Contaminated by pathogens. Insects, birds, rodents and people involved in biosolids processing can also contributeto the exposure.
Biosolids Characterization, Treatment and Use 21 Some pathogenic organisms have limited survival time in the environmentTable 1-7 provides the survival times for major pathogens in soil and on plants. Pathogens can be destroyed by various treatments, such as heat (hightemperature) ‘generated by physical, chemical or biological processes. Sufficienttemperatures maintained for sufficiently long t i e periods can reduce bacteria, viruses,protozoan cysts and helminth ova to below detectable levels (helminth ova are themost resistant to heat treatment). Chemical processing with disinfectants (e.g.,chlorine, ozone, lime, etc.) can also reduce bacteria, viruses and vector attraction.High pH conditions, for example, may completely destroy viruses and bacteria buthave little or no effect on helminth eggs. Gamma and high energy electron beamradiation treatment depends on dosage with viiuses being most resistant. Reductionof pathogenic organisms can be measured using microbiological analysis directly orby monitoring non-pathogenic organisms-indicators.TABLE 1-7 PATHOGEN SURVIVAL TIME IN THE ENVIRONMENT Existing regulations require monitoring for representative pathogens and non-pathogenic indicator organisms (fecal colifom bacteria). Direct monitoring isrequired for the three common types of pathogens: bacteria, viruses and viablehelminth ova. [ I ] There are test procedures available to determine viability of helminth ova andcertain types of enterovirus species. No test procedure is available for bacteria. Whendirect monitoring of bacteria is important, Salnionellu sp. are used. They are goodindicators of reduction of other bacterial pathogens. Microorganism density is defined by cui-rent regulations as nuitrber o fmio-oorganisnw per zinir mass of rota1 solids (dy weight) [IS]. For liquid biosolids, density is determined as number of microorganisms per 100milliliters. The microorganisms in biosolids are associated with the solid phase.When biosolids are dewatered, the number of microorganisms per unit volumechanges significantly whereas the number per unit mass of solids remains almostconstant. Units and methods used to count microorganisms vaiy (e.g., viable ova are
22 Girovichobserved and individually counted under a microscope). Viruses are counted inplaque-forming units (PFU). Each PFU is an infection zone were a single virusinfcctcd a layer of animal cells. For bacteria the count is in colony-foiming units (CFU) or most probablenumber (MI"). CFU is a count of colonies on an agar plate and it is not necessarilya count of individual organisms. MPN is a statistical estimate of numbers in anoriginal sample. The sample is diluted at least once into tubes containing nutrientmedium; there are several duplicates at each dilution. The original bacterial densityin the sample is estimated bascd on the numbcr of tubes that show growth. Under existing regulations, the pathogen density limits are expressed as numbersof PFUs, CFUs, or MI" per four (4.0) grams total solids. (This approach wasdeveloped because most ofthe tests start with 100 mL of biosolids which typicallycontain 4 grams of solids.) Densities of total and fecal colifoim, however, are givenon a "per gram" basis. , [ 151C. Odors and Other NuisancesBiosolids treatment and managcmcnt practices are greatly influenced by odor andother pollutant and nuisance factors such as dusts (particulate matter), gaseous andliquid discharges, and vectors (insects, birds, rodents, etc.). Biosolids are mherently odorous and if handled improperly can create offensiveodors and other pollutants and nuisances which may cost a lot of time and money tocon-ect. Control of odors is one of the most difficult problems in biosolids treatment andmanagement. It is especially true if biosolids are beneficially used. Dealing with odors and nuisances is often not a technical issue but rather amatter of public acceptance, public education and human perception. Regulatoryrequirements concerning odors are often poorly defined leaving much room forinterpretation and arbitrary action. Odor is an increasingly sensitive subject, and it isone of the first (and frequently most important) issues concerning the public. 1 . Odor co"l~olilld~9Although the exact origin, chemical composition and fate of many odors frombiosolids are difficult to classify and not well studied, it is also true that theoverwhelming majority of odor compounds are by-products of decomposition anddecay of organic matter. Therefore, the most effective control measures are based onpreventing or managing this decomposition cycle. Raw organic material brought by the influent settles into primary solids largelywithout microbiological decomposition. Primary solids are untreated solids fromresidential and industrial sources which makes them potentially the most odorous.Secondaiy solids are primarily products of aerobic microbiological activity.
Biosolids Characterization,Treatment and Use 23Microorganisms decompose and metabolize organic matter, decrease the amounts ofraw organics and increase organic matter attributable to live and deadmicroorganisms. Microbiological decomposition of raw organic materials results in the use ofsome elements (carbon, nitrogen and others) by microbes, formation of new andmodified organic compounds and the release of products such as carbon dioxide,water, hydrogen sulfide, ammonia, methane and considerable amounts of otherpartially decomposed organic compounds. A sigmficant number of these organiccompounds are strong odor pollutants. Fatty acids (e.g. acetic acid, a.k.a. vinegar), ammonia and amines (organicderivatives of ammonia), aromatic organics based on the benzene ring with nitrogena o (e.g. indole and skatole), hydrogen sulfide, organic sulfides (e.g. mercaptans) and tmnumerous other organic compounds have been identified as malodorous pollutants. Classifidon of t e e odor pollutants is difficult due to their low concentration, hscomplexity of molecular sh-ucture, often short life span in the air, variety of sourcesand conditions, etc. However, two large groups can be identified: (1) sulfiu-containing organic compounds and (2) nitrogen-containingcompounds. Mercaptans(general formula C&SH) comprise a large group of strong odor pollutants in thesulfur containing group along with various organic sulfides ( C W ) and hydrogensulfide N S ) . In the nitrogen containing group, various complex amines (CJ-IJW) are strong odor pollutants along with ammonia (NH3)and some others containing N,NH, radicals. With few exceptions (notably ammonia and some amines), theirthreshold odor concentration is very low (fiaction of ppb). Table 1-8 lists these odorpollutants, their chemical formula and their recognition threshold.2. Odor MeasurementInstrument-based odor detection has been advancingrapidly, able now to detect odorsin parts per billion concentrationfor a sigdicant number of compounds. Instrumentalodor detection preferred by the engineering Community has a limited use due to thecomplexity and variety of odors,considerable amount of time and money required and,more importantly, the availability of the human nose, still the best receptor that sensesodors better (in some cases three orders of magnitude better) than any instrument hasyet been able to do. Therefore, the human nose is still the m s common means for otdetecting and measuring odors. The human nose provides a subjective odor evaluation and a number oforganoleptic methods (i.e., using the human olfactory system) have been developedto improve reliability and to quant.@ odor compounds. M s common is the "odor otpanel technique" (ASTM Method E-679). The odor panel technique involves a panel of four to sixteen members who are exposed to odor samplesdiluted with odor-fiee air. The number of dilutions requiredto achieve a f%y percent positive response fiom the members is called the threshold
24 Glrovichodor concentration (TOC) which is considered to be the minimum concentrationdetectable by the average person. If, for example, nine volumes of diluting air addedto cme volume of odor sample generate a SffY percent positive response, this pseudo-concentrationis reported as ten dilutions to TOC. In this case, the above sample is&lined as having an "odor concentration" of ten odor u i s (100~). An odorous ntcOmpOund diluted to its TOC has a concentration of 1.Oou. The stronger the odor, theh is its odor Unit number. Several Merent nomenclatures are used to determine imthe number of required dilutions: a) the odor unit (ou); b) the effective dose at 50%positive panel response (ED&; c) dilutions to threshold OR);d) dilution ratio 2, etc.AU of t e eam essentially the same. The odor unit and ED, are most common units hsin odor panel technique. The TOC is the minimum concentrationof an odorant thatwill arouse a sensation. A numbex of diffaent TOCs have been determined. The mostcommon is odor recognition threshold (Table 1-8), i.e. the odorant concentrationatwhich 50% of the odor panel detected the odor.[161 odw cuncentratimin excess O 50U are easily recogrued by most people and, fregardless of odor type, represent the threshold level of complaint (distractionthreshold) [l 11. At lOou complaints are assured. An ambient air odor concentrationof 2ou maximum is often adopted at the plant boundary to avoid public complaint. Anumber of inherent difficulties are associated with organoleptic odor measurement.Collection and storage of samples is often an unreliable and expensive process;comparing results of Merent methods is quite confusing. A detailed discussion ofodor science, measurements and odor treatment is provided in [ll], , , , . Complete elimination of odors may be the goal; however, the best way to avoidcomplaints is through odor prevention, management and control to acceptably lowlevels. Portable Instrumental Methods: Currently there are several portable instrumental methods that can measure air concentrations of a number of gaseous compwnds(ammonia, hydrogen sulfide, etc.) quite accurately even to very low concentrations. For example, to measure H.$, there are two types of portable instruments: a gel-cell detector which detects &S in the 0- 100 ppm range and a more sensitive gold fl conductivity type able to detect as little as im one ppb (e.g., marketed by Arizona Instrument Co.). Draeaer Detector Tubes: Draeger detector tubes measure one odor compound at a t m . A sealed graduated tube contains a special packing which changes ie color proportionately to the odor compound concentration. Ammonia, hydmgensulfide,dimethyl &ide,and some other odors can bedetected by
26 Girovich Approximate Odor Odor ChnrPetertstlC PoUutnnt Formula Recognition Odor Threshold @PWEthyl& GWHi 800 AmmdacalMethylamine CH,NH, 200 Putrid, fishyTriethylamine (GH,XN 80 Ammoniacalothers:Cadaverine NH,*(CH,),*NH, c1.0 Decayingflesh Indole WJYH <1.0 Fecalpyridine C6HP 3-5 Irritating Skatnle w 1-2 Fecal Acetaldehyde CH,CHO 4.0 Pungent, h i t y Chlorophenol CI*C,H,O 0.2 Mdcnl eiia color proportionately to the odor compound concentration. Ammonia, hydrogen sulfide, dimethyl sulfide, and some other odors can be detected by Draeger tubes. Certain compounds present in the air during testing may cause interference and affect the accuracy of the odor measurement by Draeger detector tubes. Portable G s Chrmnatomhs Portablegas chromatographs are able to analyze a many different compounds. Flame ionization BID) and flame photometric detection (FPD) are used to detect hydrocarbon, nitrogen-containing(by FID) and s u h r a t a i n i n g organics (by FPD). Detection hmits are usually in the 10- 200 ppb range [I 13. Mass Smctrometrv/Gas ChromatomaDhv: M s SpectrometrylGas as Chromatography (MSIGS) is based on matching a unique mass spectrum of a sample cOmpOund with the spectra of different known compounds stored in the library. It is very accurate and sensitive, although a time-consuming and quite expensive methodology.
Biosolids Characterization, Treatment and Use 273. Odor CoiilrolNumerous odor control methods are employed in biosolids treatment and disposal.Generally, two fundamental approaches are used:a. Source control: Elimination, minimization and containing odors at the source or point of use by selecting an appropriate treatment, equipment or handling process; andb. Removal of odors generated by a process by treating the process exhaust prior to its release into the atmosphere. Source controls may include the addition of chemicals to suppress generation ofodors (e.g., pH adjustment by lime, injection of odor suppressing chemicals, etc.),aeration, and enclosing of the process and handling equipment.. Odor removal andreduction techniques include absorption (e.g., scrubbing by water solutions),adsorption (e.g., activated carbon systems), theimal destruction (incineration,afterburning), atmospheric dilution, masking, etc. A more detailed discussion onspecific odor conti-ols is provided in Chapters 3,4,5,6 and 7.D. Other CharacteristicsOther biosolids characteristics which have considerable effect on selection andperfoimance of a treatment process and type of equipment are: density particle size, granularity abrasiveness angles of repose, fall and slide flowability and transportability (rheological properties) con-osivity adhesion compressibility hygroscopicity (for dried biosolids) presence of foreign matter, grit and fibrous material 1. TraiisporIabilityLiquid biosolids, whch are physically quite similar to water, can be pumped relativelyeasily using cenmfugal, plunger, progressive cavity, diaplu-ap, piston or other typesof pumps. Liquid biosolids (up to approsimately 6% TS) are usually Newtonian fluids, i.e.,pressure loss is yopoitional to the velocity and viscosity under laminar flow
28 Girovichconditions. The flow becomes turbulent at certain critical velocity (usually 1.2-2.0d s e c . or 4-6 Wsec.). The pressure drop in pumping liquid biosolids may be up to 25 percent greaterthan for water. In turbulent regimes, the losses may be two to four times the losses ofwater. The transition from laminar to turbulent flow regime depends on the Reynoldsnumber which is inversely proportional to the fluid viscosity. The precise Reynoldsnumber at which the change of regimes occurs is generally uncertain for biosolids. Thickened (and especially dewatered) biosolids are non-Newtonian fluids whichbecome plastic as dryness increases. This means, among other things, that thepressure losses are not proportional to flow, so the viscosity is not a constant. Specialprocedures are required to deteimine losses for these biosolids under laminar andturbulent regimes. Prior to treatment, liquid biosolids are usually ground. Grinding is essentialwhen pumping wt progressive cavity pumps, prior to dewatering, heat treatment, or ihother processes. Newer designs of grinders are significantly more reliable and durablethan types used in the past. In certain instances in which quality of the end product is very important (e.g..production of pelletized fertilizer), biosolids degritting and foreign matter removal is skongly advisable. For liquid biosolids, degritting and foreign matter removal can be achieved by using cyclone degritters which employ a centrifugal force to remove heavier grit and foreign particles or by using a microscreen separation. In the case of dried biosolids, the foreign matter is usually removed by screening. fickened and especially dewatered biosolids are significantly more difficult to transport, meter and store and carehl attention should be paid to the selection of pumps, conveyance and storage amangements. Piping design is also an important factor. Biosolids piping is rarely less than 150mm (6 inches) in diameter. Deposits quite often develop inside pipes, resulting in an increase of the losses and, unless cleaning measures are implemented, ultimately in failure. Dewatered biosolids have rather ill-defined characteristics in terms of their flow properties, or rheology. Direct measurement of biosolids rheological properties (viscosity, Reynolds number, shear stress) is time-consuming, not veiy informative and, in most cases, does not provide usehl information. Many biosolids properties are extremely dfiicult to predict due to numerous variables involved. For example, it has been observed that dewatered biosolids (in the 35 to 45% TS range) exhibit unique properties which are often referred to as a "glue-like or "plastic" phase. In that condition, biosolids tend to foim a sticky inass, often shaped as large balls. This glue- like material is veiy difficult to handle, and this condition should be recognized and anticipated when designing biosolids treatment or conveyance. Mechanical conveyors (predominantly screw and belt type) and, more recently, shaftless screw conveyors and sFecial positive displacement, single and double piston
Biosolids Characterization, Treatment and Use 29pumps are used to transport dewatered biosolids. Enclosed conveyors and pumps inparticular have the important advantage of containing odors.2. SrorabilitjStorage of biosolids is often required to accommodate variations in the rate ofbiosolids production and to allow accumulation during periods when subsequentprocessing facilities are not operational (weekends, downtime, etc.). Biosolids storageis also important to ensure uniform feed to various treatment processes such asdewatering, drying, incineration, chemical stabilization, etc. Short-term liquid biosolids storage may be accomplished in settling andthickening tanks. Long-term liquid biosolids storage can be provided in aerobic andanaerobic digesters, specially designed separate storage tanks or in-ground lagoons.Tanks usually have storage capacity ranging from several hours to a few days whilelagoons may provide up to several years of liquid biosolids storage. If storage is forlonger periods, biosolids will deteriorate (e.g., turn septic) unless kept under aerobicconditions. Special prewvative chemicals, such as chlorine, lime, sodium hydroxideand hydrogen peroxide, have been employed to prevent deterioration but these havemet with only limited success. Entirely different problems arise when dewateredbiosolids are stored in silos (tanks), lagoons and stockpiles. The most challengingproblem in this situation is to select a type of conveyance and a device to removebiosolids from the storage silos without material bridging and ratholing, unevendischarge, etc. Usually screw feeders and various types of "live bottom" storage binsare used with varying degrees of success.3. Fuel ValueBiosolids contain organic matcrial and thus have a fuel value that can potentially berealized. Biosolids heat content is approximately 5,500 kcalkg ( 10,000 BTU perpound) of dry volatile solids or 2,500 to 3,000 kcalkg (4,500 to 5,500 BTUAb.) oftotal d ~ matter. By way of comparison, coal has a fuel value of 7,750 kcalkg (1 4,000 yBTUhb.).4. Fertilizer ValueCommercial chemical fertilizers commonly contain nitrogen, phosphorus andpotassium (N-P-K) simultaneously or separately. The N-P-K value of very commonfertilizer is 8-8-8 (i.e., 8% nitrogen, 8% phosphorus [as P,O,] and 8% potassium [as&O]). Biosolids on average (on dry weight basis) contain 4% nitrogen, 0.1YOto 1 .O%phosphorus and an even smaller amount of potassium (an N-P-K 4-1-0).
30 Girovich Organic nitrogen in biosolids is valued commercially higher (typically at $12 to$17 per percent of nitrogen) than chemical fertilizers nitrogen ($2 to $4 perpercentage point). Biosolids can be land applied as a fertilizer substitute, either directly or as anaddtive (filler) in the chemical fertilizer mix or in combination with other beneficialproducts, such as lime, other alkaline materials and sludges, incineration ash, etc.Certain physical requirements (such as density, moisture, amount of dust, particle size,spreadability, uniformity, durability, minimum odors, etc.) must be addressed in orderto use biosolids as a fertilizer or fertilizer additive. Despite considerable amounts of knowledge available for biosolids processingand management, a frustrating and expensive situation may occur if biosolidspi-opei-tiesare not fully evaluated on a case-by-case basis. A smoothly iunning pilotplant, when expanded into a full-scale production process, may unexpectedly fail. Itis even more rislq to assume that a full-scale operation can succeed on the basis ofpreliminary information or laboratory studies. "ExFeiienceis the best teacher" is especially tiue in the sometimes unpredictableworld of biosolids processing and disposal with its many variables and potentiallycostly surprises.IU. BIOSOLLDS TREATMENT FOR BENEFICIAL USE: AN OVERVIEWA. Beneficial UsesThe organic matter and nutrient content of biosolids make them a valuable resourceto use as a fertilizer and soil conditioner for agricultural crops; to improve marginaland reclaim disturbed lands; to increase forest productivity and to stabilize andrevegetate forest land devastated by fires or other natural disasters. Biosolids productsare veiy effective in revegetating areas destroyed by mining and dredge spoil areas.Fertilizing highway median strips and use as a landfill daily, maintenance and finalcover are additional mcthods of beneficially using biosolids. While Section 405(e) ofthe Clean Water Act (CWA) reseivcs the choice of use and disposal practices to localcommunities, the U.S. EPA actively suppoi-ts the beneficial use of biosolids. Whenquality of biosolids with respect to pollutants is a limiting factor for beneficial use,POTWs can implement or increase local pretreatment efforts to control dischargelimits for nondoniestic wastewater discharges in accordance with NationalPretreatment Standards. Over 2,000 POTWs in the United States established theseprograms (1 992). The following beneficial use methods are practiced in the U.S.A. [ I ] :
Biosolids Characterization, Treatment and Use 311. Application on Agricultural LandApproximately 1.17 million dry metric tons, or 21.6% of total biosolids generated inthe U.S.A. (5.4 million dry metric tons in 1989), were used to improve soils foragricultural crops and pastures [l]. Liquid biosolids are usually surface applied orinjected into the surface layer of the soil. Dewatered and dried biosolids are typicallyapplied to the land surface by equipment similar to that used for applying chemicalfermizers,limestone,and animal manures and then incorporated by plowing or discing(except on pastures). The amount of biosolids applied to agricultural land shouldconform to agronomic rates designed to 1) provide the amount of nitrogen needed bythe crop, 2) minimize the amount of nitrogen leached to the groundwater, and 3)minimizeintroduction of pollutants (heavy metals, synthetic organic compounds, etc.)into soil.2. Application on Non-Agricultural LandNon-agricultural land application in 1989 included forest land (32.3 thousand drymetric tons, or 0.6%), public contact sites (166.1 thousand tons or 3.1%) such asparks, golf courses, etc., and reclamation sites (65.8 thousand tons or 1.2%).Disturbed land resulting from both surface and underground mining can beswce&Uy restoredby proper application of biosolids. Approximately 1.48 millionhectares were disturbed in the USA between 1930 and 1971 and only 40% of this hasever been reclaimed. Disturbed land reclamation in Pennsylvania, Ohio, Kentucky,Illinois, West Virginia, Missouri, Alabama and Florida proved to be an excellentoppcntun@for beneficial use of biosolids. Liquid, dewatered and dried biosolids canbe applied at si@icantly larger amounts on non-agricultural and disturbed land (upto 180metric t n per hectare) on a one-time basis. Instead of a nationwide nitrogen osstandard, application rates are established by permits (usually state-issued) on a case-by-case basis.3. Land Application--Sale o r Give-AwayApproximately 12% ofbiosolids generated in the U.S.A. were sold or given away foruse on home gardens, landscaping and similar applications (1989). Presently (1 9 9 3 ,biosolids in this category are either composted (500 thousand dry metric tons peryear), heat dried and pelletized (350 thousand dry metric tons per year) or alkalinestabilized. Compost and pellets, in particular, are easy to transport and store forconsiderable periods of time. They are distributed, marketed and used as a bulkferilizer in agriculture, on lawns, golf courses,ornamentals and home gardens. Thesebiosolids products may also be distributed in various size bags. Significant increase in the heat dried and pelletized biosolids productionoccurred in the late 1980s and early 1990s when a number of municipalities
32 Girovichrenovated (Milwaukee, Wisconsin and Houston, Texas) or started (Boston,Massachusetts; New York, New York; Tampa and Largo, Florida; Cobb and ClaytonCounties, Georgia; Baltimore and Hagerstown, Maryland, etc.) new heat drying andpelletizing facilities. Biosolids pellets produced by these facilities are marketed as abulk organic fertilizer, fertilizer amendment or as a bagged retail product nationwide. Production of heat dried biosolids pellets has gained in popularity and isprojected to increase substantially in the late 1990s to approximately 6% to 7% oftotal U.S. biosolids generation. Production of compost and alkaline stabilized biosolids in the USA has alsoincreased substantially in the 1990s.4. Biosolids as a Laridll Cover MaterialAnother beneficial application of biosolids is as an alternative daily, maintenance andfinal cover material (ACM) for landfills. A municipal solid waste (MSW) landfilltypically uses approximately a six-inch compacted layer to cover the disposed MSWdaily and as maintenance and final cover. The 503 Regulations do not establishstandards for biosolids used as a landfill cover in municipal solid waste landfills(MSWF). Biosolids used as a cover in MSWLF must be suitable for that purpose (40 CFRPart 258). Requirements such as dryness (e.g., passing a paint filter test), granularity,spreadability, compactibility, peimeability, toxicity, odor, pathogen and vectorattraction control, ability to support vegetation (if used as a final cover) must becomplied with for this application. Biosolids chemically stabilized by alkalinematerials such as lime, cement kiln dust, fly ash, etc. and usually mixed with earthenmaterials are used for landfill cover. Biosolids used as a landfill cover provide the following benefits:- reduction of vectors and odors. control of blowing litter reduction of the chance and spread offires reduction of the potential for surface and groundwater pollution* enhancement of aesthetics- cost savings Most state regulations do not require that soil be used as daily cover material atMSW landfills. Regulations generally require that the daily cover should use"compacted layer of at least six (6) inches of suitable material" (e.g., Title 35, SubtitleG, Part 807, Illinois Pollution Control Board Regulations). Biosolids are also used as a final cover material usually mixed with top soil. Inorder to establish a vegetative cover on a closed landfill, noimally about 0.3 to 0.9meters (one to three feet) of biosolids and top soil mixture (1 :1) is applied.
Biosolids Characterization, Treatment and Use 335. Disposal 011Dedicated SitesApproximately 260 thousand dry metric tons per year were applied to these sites in theU.S.A.(1 989). Th~s method is defined as a disposal of biosolids (which may not meetstandards for land application) with minimal attempt to use them beneficially at sitesspecifically set aside for this purpose. Dedicated sites may be owned, operated andmanaged either by a POTW or a private operator. Dedicated sites may beunproductive, disturbed, barren by mining or similar activities with no or minimalorganic content, erodible and acidic. Such sites range fi-om a few to 10,000 acres andmay include a vegetative cover crop. Measures should be taken on these sites toprevent runofi or leaching of the pollutants to waters.6 . Biosolids as an Eiiergv SozirreExcacting energy from biosolids can also be considered as a beneficial use. Energycan be rccovered fiom biosolids in the foim of steadelectricity using biosolids-to-energy processes such as incineration with energy recoveiy, gasification, pyrolysis,and conversion into fuel.7 . Inciiieration Ash UtilizatioriAbout 470 thousand tons per year of biosolids incinerator ash (bioash) is producedfiom 170 municipal biosolids incineration facilities in the USA. Bioash can be usedas a supplement to other beneficial uses of biosolids or as a raw material in productionof building and road cover products. For example, in Osaka, Japan, bioash is used asthe only raw material in manufacture of decorative paving bricks.8. Biosolids in Biological RenrediationMicroorganisms in biosolids can be beneficially used in several ways:* Biosolids disposal in MSW landfills under appropriate conditions enhances decomposition of municipal solid waste and increases production of methane.. Biosolids mixing with hydrocarbon-contamina~ed soils aids in establishing an appropriate microbial population to destroy hydrocarbon pollutants and to remediate sites.. Biosolids addition to the soil structure to establish vegetation can improve the success of reclamation of disturbed and contaminated sites.
34 Girovich9. Nan-Beneflcial UsesNon-beneficial uses of biosolids in the USA (1 989) include biosolids disposal intomonotills (0.15 millionmetric t n of less t a 3.0%), incineration (0.86 million metric o hntonso 16.1%),codisposal in the MSW landfills (1.82 million metric tons or 33.9%) rand surface clsposd (0.55 millionmetxic tonsor 10.3%)[13. The disposal of biosolidsin MSW landfills i regulated under 40 CFR Part 258. In order to be co-disposed with sMSW, biosolids must: (1) be nonhazardous and (2) pass the Paint Filter Liquid Test.If biosolids (sludges) are determined to be hazardous as defined by the ToxicityCharacteristic Leachate Procedure (TCLP), they must be disposed of in compliancewith the hazardous waste regulations (40 CFR Part 261-268). Also, if biosolidscontain more than 50 ppm of PCBs, they must be disposed of in compliance withtoxic waste regulations (40 CFR Part 761). Ocean dumping was banned by an Act ofCongress after 1991. The selection of a technically and economically optimal method of convertingwastewater solids into a beneficial use product is a complex process involvingevaluation of potential markets, economics, technical and environmental protectionissues, public response, regulatory and site specific factors. Figure 1-1 summarizes biosolids generation, treatment and beneficial useoptions.B Requirementsfor Beneficial Use .Prior to beneficial use, municipal solids should be treated to satisfy the followinggeneral requirements relating t human health and environmentalprotection: o1. Pathoven duction: elimination or reduction to an acceptable level of potential diseasecausingorganisms in biosolids (bacteria, viruses, protozoa, helminths).2. Vector attraction reduction: elimination or reduction to an acceptable level of attraction to vectors (e.g., rats, insects,flies).3. Pollutant h t s : pollutants should not exceed lirmts established by federal, state and local regulations. These regulations apply to the trace metals content of biosolids, as well as emissions f?om processing facilities.4. Auulication rates and other specific reauirements (such as monitoring, remrdkeeping, public access, etc.) for biosolids used on agricultural and nonagricultural land, sold to the general public, and used as a landfill cover material must also be met. 1. Pathogen ReductionBiosolids which are beneficially used should satisfy pathogen reduction requirements.Current US. EPA Regulations (Subpart D of 40 CFR Part 503) establish two classes
0 0 0 0 0 0 Biosolids - 1. Land Application 0 0 - 3 7% T.S. Digested Biosolids Dried & Pelletized Chemically Stabilized Biosolids Compost Heat Dried Biosolids Air Dried Biosolids Thermally or Chemically Primary Solids Treated Liquid Biosolids Irradiated BiosolidsFIG.1-1 - 0.5 2.0% T.S. BENEFICIAL USES AND PRODUCTS 2. l L a d m b m 0 3. 0 4. 5. - 2,500 3,500 L 15,000 20,000 L Iacinerahon Q&m Secondary Solids . m e r p v Rccov~y: SteamElectrictiy Conversion to Feel Building 8t Road Material Component BlOSOLlDS GENERATION, TREATMENT AND USAGE. 10,000 L Stabilized Biosolids . 0.2 - 1.5% T.S . Tertiary Solids DISPOSAL Monofilling MSW Co-Landfdlin Incineration without Energy Recovery w * Per million liters of wastewater treated D 3 Q
36 Girovichof pathogen reduction: Class A and Class B. Class A biosolids are essentiallypathogen h e ; Class B biosolids have reduced levels of pathogens, and are subject touse restrictions to prevent immediate and direct public contact [I]. Biosolids that are sold or given away in bags or containers for land application,applied in bulk to a lawn or home garden must be Class A product.a. Class A Pathogen Reduction RequirementsThe implicit objective of all Class A requirements is to reduce pathogen densities tobelow detectable limits which are: Salriioriella sp: Less than 3 MPN per 4 grams of total solids (’I’S), diy weight basis Enteric viruses: Less than 1 PFU per 4 grams of TS, diy weight basis- Viable helminth ova: Less than 1 per 4 grams of TS, dry weight basis The above referenced requirements define microbiological “Cleanliness” of ClassA biosolids. Six alternatives are established for Class A. Any of these alternativesmaybe met for biosolids to be Class A with respect to pathogens. These alternativerequirements are described in detail in Chapter 2.b. Class B Pathogen Reduction ReouirementsClass B requirements can be met by any of the t h e e (3) altematives. The implicitobjective of all three altematives is to ensure that pathogenic bacteria and entericvlluses are adequately reduced in density (as demonstrated by fecal coliforni density)in Class B biosolids to less than 2 MPN or CFU per gram of total solids on diy weightbasis. Viable helminth ova are not necessarily reduced in Class B biosolids.Additionally, site restrictions are applied which restrict crop harvesting, animalgrazing and public access for a ceitain period of time until environmental factors havefurther reduced pathogens. See Chapter 2 for a detailed discussion. Class B requirements apply to bulk biosolids that are applied to agricultural land,a forest, a public contact site, a reclamation site or a suiface disposal site, unless, inthe latter case, biosolids are covered at the end of each operating day.c. Domestic Sentage Pathogen Reduction [503.32(c)lClass A or Class B biosolids requirements apply to domestic septage used on all typesof land. No pathogen-related requirements apply to domestic septage placed on asuiface disposal site. Pathogen reduction in domestic septage applied to agricultural land, a forest ora reclamation site can be met by means of one of two options:
Biosolids Characterization, Treatment and Use 37- all class B site restrictionsunder 503.32(b)(S) must be met, or- the pH of the septage must be raised to no less than 12 by alkali addition and maintained at this level for 30 minutes without adding more alkali,and the site restrictions on crop harvesting m s be met. ut2. VectorAttraction ReductionA significant route for pathogen transport is vector transmission. Vectors are livingorganisms capable of transmitting a pathogen from one organism to anothermechanically (simply transportingthe pathogen), or biologically (by having a role inthe paghogens life cycle). Insects,birds, flies, rodents are the most likely vectors. Current U.S. EPARegulationsestablish 12 options for vector attraction reduction. Compliance With thevector attraction reduction requirements must be demonstrated separately fromcompliancewith the pathogen reduction requirements. Vector attraction reduction options are discussed in Chapter 2.3. Pollutant Limits Metal Concentrations)Current regulations establish numerid limits (ceiling concentrations) for trace metalsthat must be met in order to land apply biosolids. If these limits are not met, biosolidsmust be managed under the surface disposal category. Ifthe biosolids @ l y meets the ceiling limits, additional numerical limits (e.g.,cumulative o annual loading ratas) are also provided for the metals. These numerical rlmt are discussed in detail in Chapter 2. iis Biosolids t a meet "exceptional quality" lmt with respect to the trace metals, ht iisin addition to Class A pathogen and vector attraction reduction requirements, arereferred to as "Exceptional Quality" (EQ) biosolids. The EQ biosolids have fewerrecordkeeping and monitoring requirements, which is particularly beneficial fordistribution and management programs. Biosolids meeting these pollutantconcentrations also do not have to meet annual or cumulative loading limits."Exceptional Quality" metal concentrations are shown in Table 1-9 (Table 3 of$503.13). The requirements for use of biosolids on non-agricultural lands are the same asthosefor use on agriculturallands. The U.S. EPA requires that biosolids be appliedto forest land or a public contact site at a rate that is equal to, or less than, anagronomic rate (i.e., a rate that protects the groundwater and minimizes the amountof nitrogen t a passes below te root zone of the crop or vegetation grown on the site ht hby matching the amount of nitrogen taka up the the crop). This management practicealso applies to a reclamation site, unless otherwise specified by the permittingauthority. In t i case,the permitting authority may allow larger amounts of biosolids hs
38 Girovichto be applied in order to provide nutrients and organic matter to revegetate disturbedsites. Ifthe biwlids are sold or given away in a bag or other container (less than onemetric tn and the metal concentrationsexceed the amount in Table 3 of 5503.13, a o)label must be attached to ensure that the annual pollutant loading limits (Table 4 of5503.13) are not exceeded when the product is used. Pollutant Pollutant Concentration2 (mg/kg) Arsenic 41 Cadmium 39I chromium I 1,200 1I copper I 1,500 I Lead 300 Mercury 17 Nickel 420 Selenium 36 zinc 2,800 Both bulk and bagged products must be treated by a Class A pathogen reductionprocess and one of the vector attraction reduction options to be applied in bulk to alawn or home garden or sold or given away in a bag or other container. Detailed discussion of the technical basis and requirements of the 503Regulations is provided in Chapter 2.C. Treatment Processes: An OverviewMunicipal solids are processed to achieve one or more of the following goals:
Biosolids Characterization, Treatment and Use 39- volume (weight) reduction* pathogen and vector attraction reduction removal of pollutants and odors-- energy generation or recovery production of beneficial use products1. Volume ReductionVolume (weight) reduction is achieved by various types of mechanical water-solidsseparation techniques (dewatering), drying and incineration. Figure 1-2 illustrates volume (weight) reduction achieved by some commonlyemployed processes. Reduction of volume (weight) by water-solids separation(dewatering) provides considerable economic advantage by reducing costs of storageand transportation. A sigdcant number of treatment processes use dewateredbiosolidsonly. High dryness dewatering is importantfor heat drymg and incinerationbecause it reduces dramatically the amount of energy required. 1 OO?! B l i g or Alkaline Additives ukn 80% OrganicMatter InorganicMatter 60% 40% 20% o?! Liq.Bimlids Dew. Bimlids Compost Ahlinc Stab. Drying Inckatim FIG. 1-2 VOLUME REDUCTION
40 GIrovfch2. Pathogen and VectorAttraction ReductionAs described previousl in t i Chapter, a treatment is required to eliminate or reduce hsto acceptable levels the pathogenic organisms in biosolids and the vector attractionproperties prior to beneficial use or disposal. These goals can be achieved byphysical, chemical and biological means as Table 1- 10 illustrates.3, Control of Odors and PollutantsReducing or eliminating pollutants from biosolids, air emission and odor controlrepresent one of the most diacult t s s in biosolids treatment. ak Dilution (addition of certain materials such as lime) reduces pollutantconcentation in biosolids. Special lreatments are used to reduce amount of pollutantsin liquid, solid and gaseous discharges. Reduction of odors is achieved by variousmeans, such as chemical treatment (addition of oxidizing agents such as potassiumpermanganate, lime, etc.) scrubbing, absorption, adsorption, masking, andincineration.Detailed discussion on specific air emission and odor control is providedin other chapters.4. Environmental ImpactBmi s treatment impacts upon environmental, safety, and public health conditions i ldhcludmg (butnot l m t dto) air pollution, odors,liquid sidestreams,noise, siting, and iiepublic acceptance. C r a n treatment processes generate more gaseous, liquid, etiparticulate matter and other emissions than others (e.g., incineration) and, therefore,have a greater potential social, environmental and health impact. Processes withgreater impact usually require higher capital (and often higher operating) costs andmay experience difficulty in achieving public and regulatory support and approval. It is impossible to quant@ social, environmental and public health impact ofdifferentprocesses,but Table 1- 10 attempts to provide a qualitative evaluation of suchpotential impacts.5 . Economic ImpactThe bimlids treatment and beneficial use market in the USA is estimated to be in the$1.5 to 2.0 billion range annually (1995). About 9.0 million dry metric tons are to betreated,berdicially used or disposed. Costs of treating and beneficial use of biosolidsvaries widely h m as low as $90 for land application of stabilized biosolids to as highas $400 per dry ton for heat drymg, including operational costs and cost of capital(Fig. 1-3). Potential revenue sources are: 1) sale of final products, and 2) sale ofenergy. Sale of bimlids derived products such as pellets and compost generally havehigher revenue potential than other end products. Sales of digester gas, production of
TABLE 1-10 SUMMARY Treatment Volume Pathogen Potential for Reduction of F i Revenue Reduction Reduction Energy Pollutants Product Potential RecoveryDewatering -1 signifcant No I I Dewatered Medium to I1 Medium No Ti& No Biosolids ~ High IAr Drying i(1O0C/2-3mos.) I C1assB I No 1”. LOW VeryLOW very LOW MediumMesophilicAerobicDigestion I No Medium Medium No LOWAnaerobicDigestion I No Class B I Significant’ I No I Liquid, ClassB LOW No LOW LOWAerobic Medium MediumMesophilic Small Medium 1 1 1 I LOWHeat Treatment No ClassA Medium No Liquid, !$urnto Medium No LOWof Liquid Class A to HighBiosolids
TABLE 1-10 (CONT.) Treatment Pmcess Volume Reduction Pathogen Reduction Potentialfor Energy Recovery I ~~ Reductionof Pollutants I 1 Final Product Capital cost O&M cost Potential Pasteurization No Class A No 1 I Dewatered, ClassA I Low Medium No I Low 1 No Linie/Alkaline Small Class B No Yes’ Alkaline Low Medium Low Medium Increase Stabilized ~ ~~ Irradiation No Class A No No Dewatered No Data No Data No No Data I Biosolids, Class A I Heat Drying for Significant Class A Significant No Dried High Medium No Low to Autogenous Biosolids. to High Medium Incineration or Fuel Land Application (40-80% TS) Heat Drying & Pelletizing (90% TS) High Class A Significant 1 No I 2;”. I Fertilizer High Medium to High No I Medium Incineration Highest NIA High NIA I High to Very High High No I MediumI Wet Air Oxidation No Class A Medium I Possible I Liquid Medium to High No I Medium
TABLE 1-10 (CON".)1 Treatment Volume Redudion of Capital Process Reduction Reduction Energy Pollutants Product Impact Multi-effea Possible Very High Evaporation (Carver Greenfield Process) (Biosolids to Oil) I Low Nf A I High Melting VeryHigh I NIA I Medium NIA Building Materials I High1. Produclioii of niethaiie gas.2. Low to high decrease in pollutant concentralion (on dry basis) in thejnal produci due to addition ofprocessing material. N/A - Not Applicable
44 GirovichFIG.steam and electricity are potential sources of revenue fiom anaerobic digestion andincineration. C r a n processes provide potential for energy recovery and resultant etireduction in energy cost (e.g., heat treatment of liquid biosolids, heat drymg,incineration,multi-effect evaporation). Selecting an optimum treatment process for biosolids is a complicatedprocesswhich should take into account technical, economic and site specific issues. Detaileddiscussion on econOmics of various treatment and beneficial use methods is providedin other chapters.
Biosolids Characterization, Treatment and Use 45REFERENCES1. U.S. EPA, Subpait D, 40 CFR Part 503. "Standards for the Use or Disposal of Sewage Sludge", Federal Regisfer, Vol. 58, No. 32, February 19, 1993.2. J.E. Hall, F. Dalimer, Eur*opeatiSewage Sludge Sutvey, April 1994.3. StandardMethds for the Examination of Water and Wastewater. 17th Edition, Washington, D.C. Anierican Public Health Associafioti. 19804. M.J. Girovich, Simultaneous Sludge Diying and Pelletizing, Water Engineering atidManagenienf, March 1990.5. Metcalf & Eddy, Inc. Wastewater Engineering. McGraw-Hill Publishbig Co. 1991.6. R.L. Donahue, R.W. Miller & J.C. Shickluna. Soils: An Introduction to Soils and Plant Growth, Prenfice Hall, Itic. 1977.7. Municipal Sewage Sludge Management, Vol. 4, Water Qiialip Mariagenietit Library, Teclinortric Pirbliskitig Co. 1 992.8. Beneficial Use of Waste Solids, hlat~iial f Pvacfice FD-15. WPCF. 1988. o9. S.E. Manahan. Environmental Chemistiy. 5 t h Edition. Lewis Pub/islier*s,]tic. 1991.10. Design of Municipal Wastewater Treatment Plants, Volume 11, Chapters 13-20, WEF Manual of Practice No. 8, ASCE Manual and Report on Engineering Practice No. 76. Book Press, Inc., Veimont. 1991.11. C . Lue-Hing, D.R. Z e n , & R. Kuchenrither, Municipal Sewage Sludge Management: Processing, Utilization and Disposal, Vol 4, Wafer Quality Matiagenietit Libraty, Tecktioniic Publishing Co. 1992.12. Wastewater Biology: The Microlife. Water Polliifion Control Federation, 1990.13. U.S. EPA, Fate of Priority Pollutants in POTWs. EPA 44011-82-303, Washington, DC. 1982.14. U.S. EPA National Sewage Sludge Suwey, Washington, DC, 1988.15. U.S. EPA, Control of Pathogens and Vector Attraction in Sewage Sludge: Environmental Regulations and Technology, EPA 625/R-92/0 13, December 1992.16. R.T. Haug, The Practical I-Iandbook of Compost Engineering: Principles and Practice, Lewis Publisliers, 1993.17. J.Y. Huang, G.E. Wilson & T.W. Schroepher. Evaluation of Sludge Odor Control Alternatives, Journal of Etivirotittieri~alEtigineetitig, Div. ASCE 6, 1978.18. Threshold Limit Values for Chemical Substances in Workroom Air, Nafional Sa&y News, 1976.19. C. Wilber & C. Munay, Odor Source Evaluation, BioCycle, Vol. 31, No. 3, March 1990.20. Odor Control for Wastewater Facilities, illanual of Practice No. 22, Washington, DC, WPCF, 1979.
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Fed era1 Reg u 1atory RequirementsJane B. ForsteWheelabrator Clean Wafer Systems Inc.Annapolis, MarylandI. HISTORICAL BACKGROUND AND RISK ASSESSMENTA. IntroductionThe Clean Water Act (CWA), enacted in 1972, was adopted to "restore and maintainthe chemical, physical and biological integrity of the Nations water". To that end, theAct prohibits discharging pollutants into navigable waters except in compliance withthe statute. The CWA led to the establishment of the National Pollutant DischargeElimination System (NPDES), a pennitting system which establishes specificdischarge limits for each pcnnitted wastewater treatment facility, including industrial,commercial and public sourccs of wastewatcr. In addition to regulating those sources discharging emucnt dircctly into theNations waters, the CWA rcquircs the U.S. EPA to promulgate nationally applicablepretreahncnt standxds to reshict dischxge of pollutants into publicly owned treatmentworks (POTWs). These indirect industrial dischargers, as well as direct dischargcrs, are subject to generally similar standards regarding levels of treatment. In addition,POTWs are required to implement local limits applicable to the specific industrial dischargers within their system to meet local requirements. Thus, direct dischaip-s must comply with NPDES permit emuent limitations and indirect dischargers must comply with pretreatment standards. Prctreatment standards must also be set for pollutants which cannot be treated readily by POTWs or which would inteifere with POTW opcrations [l]. 47
48 Forste The pretreatment program implemented by EPA through these limits is theprimary means for ensuring that the wastewater solids resulting from treatment atPOTWs mean applicable quality standards for ultimate use or disposal. The wastewater from industry, domestic sources from private residences andrunoff from various other sources, must all be treated at POTWs before they can bedischarged to a receiving water. This treatment results in an emuent to be dischargedand the solids known as biosolids. Because implementation of the CWA has rcsultedin greater levels of treatment and pollutant removal from wastewater, the quantities ofsolids resulting from treatment increased steadily during the 1970s. Appropriatemanagement of these increasing amounts of solids has become more critical as theefforts to remove pollutants from wastewater have become more effective. In theUnited States, the quantity of solids generated from municipal wastewater treatmenthas almost doubled since the enactment of the Clean Water Act in 1972. EPAestimates that municipalities cuirently generate more than 6.5 million diy metric tonsof wastewater solids per year. An important means for managing these municipal wastewater solids is throughbeneficial use and recycling. The nutrients and other properties found in thcse othcrsolids make them usehl as a fertilizer and soil amendment. At the same time, and insome situations, the use of these solids presents environmental conceins because ofpotential for contamination by haiiifil pollutants. Because coiitaminatcd orimproperly managed wastewater solids can result in pollutants in those solidsreentering the environnicnt and possibly contaminating various media through avariety of exposure routes, it is important to regulate the management of thesematerials. Failure to do so could have adverse effects on surface and/or groundwater,wetlands and possibly on human health as well. Concern for air quality requiresproper controls over incineration of municipal wastewater solids. The interrelationship among these media means that a coordinated, comprchensive approach which encourages the beneficial use of biosolids and avoids creatingenvironmental loopholes will assure that solving problems in one medium will notcreate problems for another. The 1972 CWA addressed biosolids use and disposal in only a veiy limited circumstance: when use or disposal posed a threat to navigablc waters (i.e., the Act prohibited the disposal if it would result in any pollutant entering navigablc waters unless done in accordance with a peinut issued by EPA). In 1977, Congress amended Section 405 to add Section 405(d) requiring EPA to develop regulations with guidelines for the use and disposal of what was then called "sewage sludge." These guidelines would identify uses, including disposal; spec@ factors to be accounted for in determining the methods and practices for each identified use, and identify concentrations of pollutants that would interfere with each use. The 1979 Interim Final Rule (257 Rule) was issued under the authority of the Solid Waste Disposal Act as amended by the Resource Conservation and Recovery Act (RCRA) of 1976 as wcll as Scction 405(d) of CWA. This 257 Rule was an
Regula tory Requirements 49interim final regulation whosc ciiteiia partially fulfilled the requirement of Section 405of the CWA to provide guidelines for disposal and utilization of biosolids. Becauseof its issuance as a solid waste regulation, the 257 Rule resulted in a trend by manystates to regulate biosolids under their solid waste programs. In fact, the federalrulemaking stated that facilities which did not meet the criteria of the 257 Rule wouldbe classified as "open dumps." As specified by Section 405(d) of the CWA, thecriteria contained in the 1979 regulation were designed "to avoid a reasonableprobability of advcrse effcts on health or the environment from disposal of sludge onland." They were less comprchcnsive than later regulations and addressed only theplacement of biosolids on the land. All such practices were required to addressenvironmentally sensitive areas and diseases (pathogen control) in addition toprotecting food chain crops from the effects of cadmium and PCBs. The evaluationof lead, pcsticides and pcrsistcnt organics was deferred to future rulemaking because information available to the agency at the time was believed to be inadequate to support specific standards.The 257 RuleThe 257 Rule allowed for the beneficial utilization of biosolids as an agricultural soilconditioner or fertilizer through land application by providing criteria that peimittcdthcse activities in such a way as to prevent restricting the flow of the base flood,reducing the teniporaiy watcr storage capacity of the floodplain, or resulting ofwashout of the solid wastc so as to pose a hazard to human life, wildlifc or land orwater rcsourccs. Facilities and practices were also required not to: "cause orcontribute to the taking of any endangered or threatened species of plants, fish orwildlife . . . or result in the dcstruction or adverse modification of the critical habitatof endangered or threatencd species"; cause discharge of pollutants into waters of theU.S. in violation of the National Pollutant Discharge Elimination System (NPDES)under Section 402 of the CWA; contaminate underground drinking water sourcesbeyond the boundary of the solid waste site except as proved by a state solid wastemanagement plan. Many of the foregoing requirements were developed from theperspective of a "facility" site (e.g., a landfill) which bears little or no relation to thebencficial use practices employed for land application. This solid wastc-basedapproach has resulted in somc misinterpretation and misunderstanding of therequircments and the applicability of the 1979 regulation. The 257 Rule is also far less comprehensive than the 503 Regulations, particularly with respect to the pollutants regulated. Cadmium and PCBs were the only pollutants regulated by the 257 Rule. Cadmium was selected as the metal being of greatest concern because of its ability to accumulate in vegetative tissues of crops to levels which could potentially cause adverse effects on human health. To that end, the 1979 regulation was designed to protect land use for the production of food chain crops by imposing both annual and cumulative limits on the amounts of cadmium which could be applied to soils. In addition, the regulation required that the pH of the
50 Forstesolid wastekoil mixture be adjusted to 6.5 or greater at the time of each applicationof any biosolids containing concentrations of 2 m g k g cadmium or more.Cadmium LimitsA phased-in annual cadmium application rate was developed to provide anopportunityfor POTWs to impose pretreatment requirements and reduce the levels ofthe metal in their influent and, therefore, in the solids generated by these facilities. The maximum cumulative application of cadmium was based on two factors:background soil pH and soil cation exchange capacity. Higher cumulative applicationrates were allowed for sites with higher cation exchange capacities that also hadbackground soil pHs equal to or greater than 6.5. These higher cumulative applicationrates were allowed for soils with background pH of less than 6.5 if: (1) only animalfeed crops were grown, ( 2 ) pH was adjusted to 6.5 or greater each time an applicationoccurred; (3) a facility operating plan was developed to demonstrate to whom theanimal feed would be distributed to preclude ingestion by humans; and (4)fbtureproperty owners were notified that high cadmium application rates had been used onthe site and that food chain crops should therefore not be grown. The net effect ofthese requirements was to establish controls based on the best available scientificestimates of cadmium bioavailability at the time. As will be discussed later in thischapter, many of the assumptions and the data used to develop these limits wereunnecessarily conseivative and/or based on an incomplete understanding of themechanisms of cadmium uptake and accumulation in crops. Nevertheless, this Rule (along with recommendations for cumulative loading limits developed by the U.S.Department of Agriculture) provided the federal basis for land application programs in the United States for considerably more than a decade after becoming effectivc October 15, 1979.PCB LimitsUnder the 257 Rule, solid waste containing concentrations of PCBs equal to or greaterthan 10 m g k g had to be incorporated into the soil when applied to land used forproducing animal feed, including pasture crops for animals raised for milk. Suchincorporation was not required if it was assured that the PCB content was less than 0.2m g k g (actual weight) in animal feed or less than 1.5 mgkg (fat basis) in milk.Pathogen and Vector Attraction ReductionPathogen reduction and vector attraction reduction requirements of the 1979rulemakingwere also included in an appendix to the 503 Rule (with some additionalmonitoring requirements imposed for the Processes to Further Reduce Pathogens(PFRP) options). The processes to reduce pathogens in the 257 Rule weretechnology-based with no requirements for microbiological monitoring, as have beenimposed in the 503 Regulations discussed later in this chapter.
Regulatory Requirements 51 In addition to cadmium, PCBs, pathogen and vector requirements, the 257 Rulecontained provisions based on requirements for a solid waste facility, i.e. prohibitingopen burning, generation of explosive gases, fires, bird hazards and access. Even as it was published, EPA and the scientific community identified gaps andlimitations in the 257 Rule and began efforts to develop a more comprehensive risk-based rule encompassingmore of the pollutants of concern than those contained in the257 Rule, which remained an "interim final" rather than "final" rulemaking. During the next decade, various study groups and task groups within EPA wereformed to develop a final rule as mandated under the CWA. It was not until 1989,however, that such a rule was proposed to meet the mandate of Section 405(d) of theCWA. Changes in the Act enacted in 1987 for the first time set foi-th a comprehensiveprogram for reducing the potential environmental risks and maximizing the beneficialuse ofbiosolids. At that time, the Act established a time table for the development ofuse and disposal guidelines. Under amcnded Section 405(d), EPA was charged to firstidentify, based on available infoimation, toxic pollutants which may be present in [biosolids] in concentrations which may affect public health and the environment. Then, for each identified use or disposal method, EPA was required to promulgate regulations that specify acceptable management practices and numerical limitations for biosolids that contain these pollutants. These regulations must be "adequate to protect human health and the environment from any reasonably anticipated adverse affect of each pollutant" (Section 405(d)(2)@)). The statute also required EPA to promulgate regulations i two stages and periodically review these regulations for the n purpose of identifLing additional pollutants for regulation. Since Congress provided little guidance for the Agency in carrying out the broad mandate to protect public health and the environment, EPA needed to address a number of issues including: scope of the regulation, what pollutants should be covered, what pathways of exposure should be evaluated, what individuals or groups of individuals, plants or animals were most likely tobe affected, as well as what type of modeling and risk assessment would be addressed in the final Rule. The Agency also needed to determine at what concentrations a pollutant can adversely affect human health and the environment, and what constitutes an adverse effect. The question of an acceptable level of risk had to be addressed in meeting the "adequate protection" mandate of the Act, taking into account background pollutant levels and the uncertainties inherent in all risk analyses. The Agency conducted an extensive infoimation gathering and analytical program beginning in 1984 to support the development of the final regulation. M e r the 1987 amendments to the CWA were adopted, the Agencys efforts increased and resulted in the development of important documentation which formed the basis for the 503 Regulations.
52 ForsteB. Basis for the 503 RegulationsWith the promulgation of the 503 Regulations on February 19, 1993, EPA met itslongstanding obligation under the CWA to establish standards for the use and disposalof "sewage sludge." This undertaking represented an unprecedented effort on the partof the Agency to assess the potential for pollutants in wastewater solids to affect publichealth and the environment through various routes of exposure. This enormouslycomplex effort required the Agency to address issues that affect many other EPAregulatory programs. For example, the risk evaluated in the 503 Regulations forbiosolids applied to land required EPA to evaluate human exposure throughinhalation, direct ingestion of soil fertilized with biosolids and through consumptionof crops grown on such soil, as well as many others. The agency assesscd potentialrisk of contamination of drinking water or surface water sources when biosolids aremanaged on the land, and evaluated potential direct effects on crops, cattle, aquaticspecies and wildlife. In addition, EPA evaluated the effect of emissions frombiosolids incinerators on human health. Thus, the development of the 503 Regulationshad an impact on Agency activities under the Clean Air Act (CAA),the Resourcc Conservation and Recovery Act (RCRA), thc Toxic Substances Control Act (TSCA), and the Comprehensive Environmental Response Compensation and Liability Act (CERCLA), as well as the Safe Drinking Water Act (SDWA). The coordination of this rulemaking with standards of other Agency programs presented a significant challenge to the development of the 503 Regulations. The challenge was particularly great with respect to non-human health effects, given the more limited infoimation available in that area and the less well-developed methods available for cvaluating such effects as comparcd to human health effects. In promulgating the 503 Regulations, EPA expressed confidence that they do in fact adequately protect public health and the environment from all reasonably anticipated adverse affects, as required by CWA Section 405(d) based on the following: EPA evaluated the regulations for aggregate national health impact and found, based on very conservative assumptions that probably overstate exposure, that there are virtually no effects when biosolids are disposed of on land or used as a soil conditioner or fertilizer in compliance with the Rule. Even when biosolids are incinerated, the potentially greater human population exposed to emissions is still subject to extremely small risk. Beneficial use of biosolids is not new in the United States. In the development of the 503 Regulations, EPA and the Technical Peer Review scientists reviewed available scientific and technical literature and found no evidence that the use of biosolids is causing significant or widespread adverse effects.
Regulatory Requirements 53 The promulgation of the 503 Regulations represents the Agencys first step (Round I> of this rulemalung. The CWA statutory requirements mandate that the Agency develop regulations in two steps and revise such regulations periodically if additional information suggests such a need. EPA has expressed the view that the Regulations as promulgated in 1993 areadequately protective because most of the effects that the Regulations are designed toprevent are chronic, not acute. Even in the unlikely event that new information woulddictate reconsideration of some of the determinations of the SO3 Regulations, noadverse short-term human health consequences would be experienced since thestandards already protect against chronic effects and thus are well below acute effectlevels. The Agency is also committed to investigate many of the assumptions used indeteimining the regulatory levels contained in the SO3 to insure consistency with broadprotection of public health and the environment. Under Section 40S(d) of the CWA, EPA identified, based on availableinformation, pollutants which may be present in biosolids in concentrations whichmight affect public health andor the environment. Then, for each identified use ordisposal method, EPA specified acceptable numerical limitations and managementpractices for biosolids that contain these pollutants. In the absence of specificgudance fiom Congress in carrying out the broad mandate to protect public health andthe environment from reasonably anticipated adverse affects of each pollutant, theAgency was required to address and clarify a number of significant issues.Remlatorv IssuesTo determine standards that would adequate protect public health and thcenvironment, EPA had to detemiine the following: Scope of the Remulation Since different types of biosolids are generated and are used or disposed in different ways, the question of which types to regulate and which use/disposal methods to regulate needed to be clearly defined. Pollutants Covered The basis for the Agencys selection of pollutants for regulation had to be defined. Exposure Pathways Air, water and soil as media of transport of pollutants had to evaluated in establishing risk-based limits for pollutants. Tarpet Organisms Individuals or groups of individuals, plants or animals which are most likely to be affected by the pollutants in biosolids had to be identified. Models The Agency had to simulate the movement of the pollutants in biosolids into and through various environmental media to the target organisms.
54 Forste Tvpes of Risks Potential human health and environmental effects from the use or disposal of biosolids (e.g., eating foods grown on soil to which biosolids have been applied) had to be identified.0 Effect Levels The "response"to the presence of a pollutant by plant, animal and human systems had to be evaluated to determine at what point variation constitutes an adverse effect. Acceptable Level of Risk The Agency had to decide what level of risk adequately protects human health and the environment; this was unclear in the congressional mandate and varies within other EPA programs.0 Backmound Pollutant Levels The Agcncy had to evaluatc the impact of sources of pollutants other than biosolids (e.g., lead from gasoline). Uncertainties The Agcncy had to measure and account for the unavoidable uncertainties in its analyses by using appropriately conservative assumptions and margins of safety. Types of Effects Evaluated EPA had to determine whether to evaluate health and environment effects on the most exposed target organisms and/or based on the incidence of adverse effects on the total population associated with use or disposal.0 Pollutant Limits The Agency had to determine whether to establish a single pollutant limit for all. use or disposal practices or separate pollutant limits for each of these. Form of the Pollutant Limits Pollutant limits could be expressed as limitations on concentrations in biosolids, on pollutant loading rates, pollutant emission rates, or various other limitations.0 Regulatorv Responsibility The Agency had to determine who would be responsible for meeting the requirements in the Rule (e.g., end user, treatment works, service contractor). Impacts A cost benefits analysis of the Rule had to be developed in order to determine its effects. The final 503 Regulations contained a number of important, fundamental assumptions based on the Agencys interpretation of the requirements of Section 405(d) of the CWA and the various regulatory issues described above. These assumptions include the following:
Regulatory Requirements 551. Control QualityBy cstablishing limits on biosolids quality, the 503 Regulations create incentives fortreatment works to generate less contaminated biosolids. Those facilities that do notmeet the quality conditionsunder the 503 must improve the influent quality (e.g., morerigorous pretreatment), provide better treatment of the solids (e.g., improved pathogenreduction) or select another use or disposal method.2. Reduce Waste and Use BeneJiciallyEPAs 1984 Beneficial Use Policy and the 1991 Interagency Policy on Beneficial Usestrongly supports beneficial use practices for biosolids--the term the Agency notes isused to distinguish solids which can be beneficially recycled. The Agencybelieves that the improved productivity of land using the soil conditioning propertiesand nutrient content of biosolids has human health and environmental advantagesbeyond those that are directly associated with applying this material to the land. TheAgcncy sees secondary or related bcnefits of using biosolids resulting from a reductioni adverse human health effects of incineration, a decreased dependcnce on chemical nfertilizers and a reduction in he1 or energy costs that may be associated withincineration. In finalizing the Rule, the Agency considered and placed emphasis onapproaches that supported its policy of beneficial use.3. Mininiize Risks to Iiidividuals and the General PopulationThe Agency evaluated the effects of each pollutant on a highly exposed individual,plant or animal (HEI) and on populations at higher risk. It also examined regulatoryoptions that would base the Rule on: aggregate incident analyses only (the effect onthe whole population); the ME1 analyses only; and a combination of these twoapproaches. The final 503 Regulations use an HE1 analysis supported by aggregaterisk assessment on higher risk populations or special sub-populations (e.g., children)to insure protection of public health and the environment.4. Proniulgate Reasonable Standards In establishing standards under 405d of the CWA, the Agency evaluated long-term pollutant exposure and circumstances leading to: 1) increased toxicity and potency of a pollutant in the environment, and 2) increased intensity of an adverse effect that the pollutant might have on human health or the environment. While this approach is used throughout the Rule, it does not include every conceivable combination of adverse conditions; however, based on the conservative assumptions built into EPAs modelling effort, the Agency expccts that few, if any, individuals will reccive greater doses of a pollutant than the doses used to establish the standards. Therefore, EPA has
56 Forstedetermined that the 503 Rule meets the statutory directive contained in 405d. EPAconcludes that adequate protection does not require the adoption of standards forexposure conditions that are unlikely and from which effects are not significant orwidespread.5. Proniulgate an Implenientable RuleThe flexibility associated with site-specific analyses was weighed by the Agencyagainst the simplicity of national numerical limits and a self-implementing rule.Regulations that allow exceptions for every conceivable contingency would becumbersome and difficult to implement. Therefore, the limited exceptions in 503 tonational pollutant limits are restricted to circumstances in which site specificconditions may make a significant difference in the pollutant limits withoutcompromising public health and environmental protection. In such cases where site-specific conditions are appropriate, recalculated numerical pollutant limits for that sitecan be imposed using EPA approved models by providing supporting information andrecalculated numerical limits to the permitting authority for approval. The Agencyplaces primary responsibility on the treatment works for insuring that biosolids aremanaged as required. Because the 503 Regulations are designed to be self-implementing, they spell out the requirements which apply to persons using ordisposing of this material.6. Provide f o r Future ExpansionSince the 503 Regulations are necessarily limited by the adequacy of idolmationavailable at the time, they contain standards for pollutants and use or disposal practicesfor which sufficient information exists. EPA may expand and refine these standardsin the future. The National Sewage Sludge Survey (NSSS) was conducted by the agency to fillsome ofthe information gaps. Among other things, it gathered additional infotmationon the pollutants in biosolids, as well as how they are used and disposed. Thosepollutants and methods of use or disposal not covered by the February 19, 1993 FinalRule will be evaluated for coverage under subsequent phases of503 rulemaking andas adequate data are developed. The 503 rulemaking was unique not only with respect to the complexity of thetask at hand, but the extensive review and input which was received by the Agency onthe technical standards. Experts from both inside and outside EPA reviewed thescientific literature and provided additional data and scientific and technical inputwhich enabled the Agency to expand and refine the standards during the timefollowing the comment period and before promulgation of the final standard.Reviewers included the EPA Science Advisoly Board, the Cooperative State Research Service, the Regional Research Technical Committee (the W- 170 Committee),
Regulatory Requirements 57representatives of academia and other scienticltechcal entities with specificexperience in the areas covered by the Rule. A complete listing of public commentsand documents used in developing the final Part 503 can be found i .n The phased-in approach for this rulemakmg specified under Section 405d of theCWA provides for review of the Regulations at least every two years to identifyadditionalpollutants and promulgating regulations for such pollutants. The EPA willuse data from the NSSS to identify additional pollutants in the second round ofrulemaking. The 503 Regulations establish standards for final use or disposal when biosolidsare applied to agricultural and non-agricultural land (including products sold or givenaway), placed in or on surface disposal sites or incinerated. The standards do notapply to processing of biosolids before their ultimate use or disposal. In addition, the503 Regulations do not specify operating methods or requirements for biosolidsentering or leaving any particular treatment process. They also do not establishstandards for biosolids disposed with municipal solid waste in Municipal Solid WasteLandfills (MSWLF) or used as a cover material at MSWLF sites. The joint authorityof Sections 4004 and 4010 of RCRA and Section 405d of the CWA contain therequirements adopted by the Agency for MSWLFs that apply to biosolids placed in such landfills. Disposal of biosolids in MSWLFs is regulated under 40 CFR Part 258 . Treatment works which use an MSWLF to disposal of their biosolids must insure that the material is non-hazardous and passes the Paint Filter Liquid Test. By meeting these requirements, the treatment works will be in compliance with Section 405(e) of the CWA. The standards also are not applicable to biosolids co-incinerated with large amounts of solid wastes . However, the standards established in the 503 Regulations do apply to biosolids incinerated with incidental amounts of solid waste used as an au>cilimyhe1 (i.e., 30% or less solid waste by weight). Biosolids generated by all publicly and privately owned treatment works treating domestic sewage and municipal wastewater are subject to the provisions of the 503 Regulations. They do not apply to domestic sewage that is treated along with industrial water by privately owned industrial facilities. Since EPA does have the authority under Section 405d of the CWA to regulate industrial sludges with a domestic sewage component, it plans to consider regulating these materials in the future Part 503 rulemaking, In the meantime, until the agency develops such regulations, those sludges (as well as non- hazardous industrial sludges without a domestic sewage component) will be regulated under 40 CFR Part 257. The 503 Regulations do not establish disposal standards for wastewater solids that are determined to be hazardous under procedures contained in Appendix I1 of 40 CFR Part 261. Such hazardous materials must be disposed of in compliance with the Regulations in 40 CFR Parts 261-268, and compliance with these requirements will constitute compliance for purposes of Section 405 of the CWA. Also, wastewater solids containing 50 ppm or more of PCBs are excluded from the 503 Regulations and
58 Forstemust be disposed of according to the requirements established in 40 CFR Part 76 1.Therefore, since EPA has not established specitic standards for the disposal of PCBcontaminated wastewater solids, a disposer complying with 40 CFR Part 76 1 will bein compliance with Section 405d of the CWA. The Ocean dumping ban act of 1988 prohibits any person from dumpingwastewater solids into ocean waters after December 31, 1991. A phased-inimplementation under the Marine Protection Research and Sanctuaries Act (MPRSA)resulted in the cessation of such disposal at the end of June 1992.C. Data GatheringAs required by Section 405(d), EPA relied on available information to develop the503 Regulations. The primary source of this information regarding the occurrence andconcentration ofpollutants in biosolids came from data on 40 pollutants from POTWsin 40 cities (40 City Study). [ 101 This database was selected as the primary source ofinformation on pollutant concentrations because it provided the most comprehensiveand best documented nationwide database available at the time.40 Citv StudyHowever, EPA recognized scveral deficiencies in using the "40 City Study" data. Thekey deficiency was the fact that this study did not generally provide data on finalprocessed biosolids. In addition, POTWs selected for the 40 City Study were notselected following statistical methods which would provide unbiased nationalestimates of pollutant concentrations in biosolids. EPA also recognized that biosolids quality had changed since 1978 due to theinitiation of many pretreatment programs, development of new industrial facilitiesdischarging to the POTWs, and changes in wastewater treatment processes. TheAgency therefore concluded that pollutant concentrations from the 40-city study didnot accurately reflect the current quality of biosolids. In addition, advancements inanalytical methodology since the 40-city study provide more accurate pollutantanalysis. Other data sources on biosolids quality also suffered from deficiencies whichmade them inappropriate for establishing regulations.National Sewage Sludge Survev (NSSS)To develop more appropriate data to replace or at least supplement the data used todevelop Part 503, EPA conducted the National Sewage Sludge Survey to obtain acurrent and reliable database. This information will also be used to develop a list ofpollutants fi-om which the Agency will select additional pollutants for further analysisand potentially for regulation under Section 405(d) of the CWA. The data collection effort for the NSSS began in August 1988 and was completedin September 1989. EPA collected and analyzed biosolids samples at 180 POTWsand analyzed them for more than 400 pollutants. Using detailed questionnaires, the
RegulatoryRequirements 59survey also collected information on the use and disposal practices from 475 publictreatment plants with at least secondaxy treatment. The NSSS results provided EPAwith information necessary to establish numerical pollutant limits in the final Part 503Regulations to encourage the beneficial use of biosolids and provide a greater degreeof public health and environmental protection than was contained in the February 6, 1989 proposal. To establish numerical limits, data from the NSSS regarding pollutantconcentrations were required to estimate the level of risk which current quality andcurrent use or disposal practices pose to the environment. The survey informationenabled the Agency to fUrther evaluate its regulatory approach and "cap" pollutants atthe 99th percentile concentration in instances where the Agency believes that thestrictly risk based numerical limitations do not provide an adequate margin of safetyto protect public health and the environment. EPA also used the results of the NSSS to attempt to assess potential shifts amongvarious use or disposal practices as a result of the final regulations, since effect of a rule is important in determining how rapidly it should be implemented. For example, a slight impact from a particular numerical limitation may make immediate implementation of the regulations appropriate; however, if wide shifts in current methods of use or disposal are anticipated from the numerical limits, POTWs may need assistance in developing more stringent pretreatment limits or in adopting alteinative use or disposal practices. EPA will also study the analpcal results of NSSS to identify a preliminary list of pollutants for second round rulemaking, based to some degree on pollutants that have elevated concentrations in biosolids. Final decision on which pollutants to regulate in the second round will depend on availabilityof data regarding a pollutants toxicity and environmental fate, effect and transport properties. EPA will follow a similar process in identifying these pollutants as they did for the 1989 rulemaking. Participants in the NSSS were selected from 11,407 POTWs in the U.S., including Puerto Rico and the District of Columbia. These facilities have been identified in the EPA 1986 Needs Survey as having at least secondaiy wastewater treatment (primary clarification followed by biological treatment and secondary clarification). The effort consisted of both a questionnaire and an analytical survey. The POTWs included in the two samples were selected according to stratified probability design. All POTWs in the analytical survey were selected from among those that had already been selected to receive the questionnaire. The questionnairewas designed to allow results to be analyzed separately by flow rate group and by biosolids use and disposal practice. Four hundred seventy-nine POTWs provided idormation concerning service area, operating parameters, general biosolids use and disposal practices, pretreatment programs, wastewater and biosolids testing frequencies and financial information. The POTWs also provided use and disposal practice specific information and indicated which practice(s) would be likely alternatives to their current ones.
60 Forste A total of 208 POTWs fiom four flow rate categories were selected for samplingand analysis. EPA contract personnel collected the samples which were analyzed fora total of 412 analytes. These analytes include every organic, pesticide, dibenzohran,dioxin and PCB for which EPA has gas chromatography and mass spectrometry( G C M S ) standards. The remaining pollutants are inorganic. Pollutants were alsoselected based on: 1) the CWA Section 307(a) priority pollutants, 2) toxic compoundshighhghted i the domestic sewage study, and 3) Resource Conservation and Recovery nAct (RCRA PL94-580) Appendix VIII pollutants. Sampling preservation and analytical protocols were developed specifically forthe NSSS. Analytical methods 1624 and 1625 were adapted from methods to dealwith the biosolids matrix for volatile and semi-volatile organics respectively. Pesticides and PCBs and dibenzohans and dioxins were analyzed using methods 16 18 and 1613, respectively. Metals and other inorganics and classicals wereanalyzed by standard EPA methods. The analytical methods were either developed,chosen or specifically adapted for the biosolids matrix to give the most reliable,accurate and precise measurement of the 4 12 analytes undertaken.Incinerator Field StudiesA series of field studies was initiated by EPA in 1987 to support the 503 incineratorrulemaking effort. These on-site tests were designed to obtain information about: 1)percentage of hexavalent chromium in the total chromium in the exit gas, 2)percentage of nickel subsulfide in the total nickel in the exit gas, 3) total hydrocarbon(THC) emissions data, and 4) organic compounds in the exit gas. Information was collected at 10 biosolids incinerators, eight of which weremultiple hearth and one was a fluidized bed; these incinerators had variouscombinations of air pollution control devises, including wet scrubbers and wetelectrostatic precipitators. For the final Rule, EPA developed allowable pollutant concentrations for metalsin biosolids based on risk-specific concentrations. The risk-specific concentration forchromium depends on the percentage of hexavalent chromium in the total chromiumin the exit gas. EPA determined, based on tests at several incinerators, that theconversion to hexavalent chromium will vary with the type of biosolids incinerator andair pollutant controls. Based on those results, EPA derived different risk-specificconcentration values based on four combinations of biosolids incinerators and airpollution control technologies (see Table 10 of the final Part 503 Regulations). The nickel speciation tests revealed that nickel subsulfide is not emitted frombiosolids incinerators above the detection level for the analytical methods used in thetests. EPA therefore decided to be protective and base the standard risk-specificconcentration for nickel on the higher of the two detection limit values for nickel subsulfide. The risk specific concentration for nickel in Table 9 of the final Rule is based on the occurrence of ten percent nickel subsulfide in total nickel emitted from a biosolids incinerator.
Regulatory Requirements 61 EPA used data from total hydrocarbon concentration in exit gas along with theaggregate risk analysis as the basis for the THC operational standards in the final part503 regulations. This standard is technology-based since it is based on perfoimancedata from biosolids incinerators. Information on total organic pollutants and THC inthe exit gas provided the basis for THC being used as a surrogate for measuringorganic compounds in the exit gas. Tests show that there is a significant correlationbetween THC and organic compounds and THC is easier and less expensive tomonitor than are total organics. THC can also be measured on a continuous basiswhich enhances operating and management practices, Knowing which organicpollutants are in the exit gas (or potentially in the exit gas) allowed EPA to developan ambient risk-specific concentration for the organic compounds. This value wasthen used to estimate the risk level for the technology-based THC limits which is anexit gas concentration. Details on the biosolids incinerator field studies can be foundin the technical support document for incineration.Domestic Septage StudyTo characterize domestic septage, EPA initiated a 1981 sampling and analysis studybecause data on organic pollutants and domestic septage were not available. As partof h s study, nine samples of domestic septage were collected and analyzed for over400 pollutants. Analytical results from the study were used to calculate the annualapplication rate equation for domestic septage for the final rule (based on totalKjeldahl nitrogen and ammonia concentrations), and to justify the domestic septageannual application rate. Details on the domestic septage study are found in thetechnical support documents for land application.D. Risk Assessment Methodology1. IntioducrionIn developing the 503 Regulations, the U.S. EPA had to set up and justify numericallimits for certain pollutants. EPA developed the numerical limits using the exposureassessment models as described in this Chapter, and in the final 503 Regulations alsoestablished an operational standard for total hydrocarbons emitted by biosolidsincinerators. The models incorporated well established measures of human health orenvironmental protection as the design endpoint. Thus EPA based its environmentalassessment on human health or environmental criteria already published orpromulgated by the Agency, on human health criteria developed by the Agency, or onplant or animal toxicity values published in scientific literature. In its exposure assessment, if the Agency had not published or promulgatedcriteria for specific pollutants, EPA evaluated non-cancer human health risks frompollutant exposure using reference doses. EPA evaluated cancer risk using cancerpotency factors listed in the Agencys Integrated Risk Information System (IRIS). In
62 Forsteall cases, the Agency used cancer potency values corresponding to an incremental,carcinogenic risk level of 1 x to evaluate the risk from pollutants found inbiosolids (a 1 x lo4cancer risk implies that one additional cancer case will occur ina population of 10,000 exposed at that level for 70 years). The final limits forpollutants ensure that the use and disposal of biosolids does not result in ambientconcentrationsof the regulated pollutants that exceed an incremental carcinogenic risklevel of 1 x For biosolids disposed of in or on surface disposal sites (including monofills) orincinerated, the treatment facility may submit modelling and data analysis for certainphysical parameters relating to the site. The permitting authority will review andapprove the facility’s site specific modelling and data analyses used to recalculatenumerical limits used in EPA’s approved exposure assessment methods. Therecalculated numerical luruts will therefore be based on EPA approved models and thesame health and environmental criteria as the national numerical limits and, therefore,the recalculated limits will also adequately protect human health and the environment. The 503 Regulations establish requirements for pathogenic organisms orpathogenic indicator organisms such as fecal coliforms in biosolids applied to the land.It also includes requirements for destroying or reducing the characteristics of biosolidsthat might attract birds, insects, rats and other animals (so-called “vectors”). Basedon “vector”exposure to the pathogenic organisms potentially present in biosolids andpotential spread of disease from these disease vectors to humans, the Rule requiresmeasures for reducing the attraction of vectors to biosolids. These measures includereduction or destruction of the odor-causing properties of biosolids that lure insects and animals by a variety of means. Management practices and general requirements to protect human health and prevent environmental abuse supplement the numerical pollutant limits contained in the 503. In addition, small quantities of biosolids sold or given away in a bag or other container may require labelling with instructions on proper use of the product if such a product does not meet the most stringent requirements of the 503 with respect to pollutant concentrations. Monitoring, recordkeeping and reporting requirements are also contained in 503 at specified frequencies depending on the quantity of biosolids used or disposed by a treatment facility. The pollutants for which a treatment facility must monitor their biosolids depend on the use or disposal method employed, and recordkceping and reporting requirements are also specific to each use or disposal method. The 503 Regulations cover nearly 35,000 treatment facilities, including priniaiy treatment POTWs, secondary and advanced POTWs, privately owned treatment works, federally owned treatment works and domestic septage haulers. Based on the NSSS, EPA estimates that the 503 will affect approximately 6,300 of the 12,750 secondary, advanced and primary POTWs that use one or more of the disposal practices included in the 503. These 6,300 facilities generator or treat approximately 60% of the biosolids produced in the US. Of the remaining POTWs, EPA estimates
Regulatory Requirements 63that 2,700 dispose of their biosolids (34% of the total generated) in MSWLFs that areregulated under 40 CFR Part 258. The remaining 3,750 POTWs use other disposalpractices not covered in the 503 or the MSWLF rule. As noted above, in some casescompliance with the requirements of the other practices also constitutes compliancewith 405(d) of the CWA. EPA’s aggregate risk assessment estimated that the pre-503 use and disposalpractices contributed from less than one up to five cancer cases total for the entire USpopulation. The lifetime cancer risk to a highly exposed individual fi-om existingpractice ranged from 6 to 10-4for land application and surface disposal and from 6 x lo4 to 7 x 10” for incineration. Other health effects associated with usc and disposalprior to 503 wcre evaluated by the Agency as primarily relating to lead exposure. Thebaseline risks for the 503 are extremely low as compared to aggregate risks identificdby the Agency in other environmental rule making efforts. The aggregate risk assessment estimates are discussed later in this chapter. EPA’s regulatory impact analysis estimates that approximately 130 of the 6,300 aEected POTWs may produce biosolids which do not meet the numerical limits of the 503 Regulations (however, some POTWs may come into compliance by using site specific data to calculate new numerical limits and imposing stricter pretreatment requirements on industrial discharges). T e c h c a l support documents, aggregate human health risk analyses, regulatoiy impact analyses and the preamble to the 503 discuss the factors that EPA considered, the data and comments that were evaluated, and the determinations that it made in developing the Regulations. This information is containcd in 15 parts: Part I contains generation, volume and constituents of biosolids and ways in which communities commonly use or dispose of their biosolids, the bcnefits of using biosolids and the risks associated with their disposal P r I1 lists existing federal and state requirements for use and disposal and their at relationship to 503 standards. Part I11 describes the Agency‘smethods of selecting pollutants and developing the final Rule through a series of exposure assessment models. Part IV briefly describes the February 6, 1989 proposed rule. Part V describes the data collection and evaluation of the NSSS. Parts VI and VII discuss alteinativc regulatory approaches and commcnts thc Agency considered. P r VIII discusses proposed exposure assessment methods and public comments at considered by the Agency in developing exposure assessment methodology. Part IX describes criteria used to select pollutants for regulation in the final Part 503 Rule. Part X describes the methods used to analyze the aggregate human health effects on highly exposed individuals and the nation from existing practice.
64 Forstea Part XI describes, in several subparts, the requirements that apply to the use and disposal of biosolids. Part XI also contains the requirements for septage use and disposal, pathogen and vector attraction requirements and monitoring, recordkeeping and reporting requirements.0 Part XI1 discusses implementation of the Rule through federal and state permit programs and the self-implementing nature of the 503 Regulation. A separate rulemaking by the Agency promulgated state program management requirements and changes in the NPDES permitting requirements. [ 101 Part XI11 describes benefits, costs and regulatory impact of the Rule along with discussion of the data limitations and assumptions. Part XIV provides information on obtaining copies of the 503 Rule, the technical support documents, risk assessment and regulatoq impact analysis.0 Part XV describes changes in 40 CFR Parts 257 and 403 (i.e., revisions to part 403 and removal from coverage in Part 257 of biosolids use and disposal methods which x e now subject to the standards established in 40 CFR Pait 503. Part XV lists the subjects in 40 CFR Parts 257,403 and 503.2 . Selection oJPollutants for RegulationIn 1984 EPA began to assess which pollutants likely to be found in biosolids shouldbe examined closely as candidates for numerical limits. A list of approximately 200pollutants in biosolids that if disposed of improperly could cause adverse human healthor environment al€ects was developed. The list was then revised by adding or deletingpollutants using the potential risks to human health and the environment when aparticular pollutant was applied to the land, placed in a landfill or incinerated as thetest for inclusion or exclusion on the revised list. EPA also requested that the mostlikely routes which a pollutant could travel to reach target organisms (human, plant orwild or domestic animals) be identified. This process resulted in a recommendationby the experts that the agency gather additional environmental information onapproximately 50 pollutants listed in Table 2-1. In 1984 and 1985, EPA gathered data on the toxicity, persistence, transport andenvironmental fate of the 50 pollutants being evaluated. The Agency also developedpreliminay information on the relative frequency of concentration of these pollutantsin biosolids by analyzing the product of 43-45 POTWs (depending on the pollutant)in 40 cities. This 40-city study provided data on concentration of 40 pollutants (1 2metals, 6 base neutral organic compounds, 6 volatile organic compounds, 9 pesticides, and 7 PCBs). Using this preliminay infoimation, EPA assessed the likelihood that eachpollutant would adversely affect human health or the environment. For this effort, EPA used screening models and calculations to predict the concentration of a pollutant that would occur in surface or groundwater, soil, air or food, then compare that prdcted concentration with an Agency human health criterion, such as a drinking
Regulatory Requirements 65TABLE 2-1 POLLUTANTS SELECTED FOR ENVIRONMENTAL PROFILES/I-IAZARDS INDICES Pollutants Landfill Incineration AldridDieldrin X ~1 Arsenic I x X X Benzene X X Benzo(a)anthracene X Benzo(a)pyrene x XI I X X X Cadmium X X X Carbon tetrachloride XI Chlordane* X X Chlorinated dibenzodioxins X Chlorinated dibenzofurans X Chloroform X Chromium* X ~ X X Cobalt X X Copper* X X X Cyanide* X X DDTDDDDDE X X X 2,4-Dichlorophenoxy-acetic acid* I -x Dimethylnitrosamine* X
66 Forste TABLE 2-1 (CONT.) Nickel* X X X PCBs X X X Pentachlorophenol* X X Phenathrene X X Phenol X Selenium* X Tetrachloroethylene* X Toxaphene X X X Trichloroethylene X X
Regulatory Requirements 67 TABLE 2-1 (CONT.) Trichlorophenol X Tricresyl phosphate X Vinyl chloride X Zinc* X X X* Pollutant evaluated found not to interfere with one or more use/disposal option(s).water standard, to determine whether the pollutant could be expected to have anadverse effect on human heallh. For this initial screening, EPA assumed conditionsthat would maximize the pollutant exposure of an individual animal or plant, as wellas the worst possible pollutant related effects. Based on concentration, toxicity, persistence and other factors, EPA scored eachpollutant and ranked them for more rigorous analysis, excluding two categories ofpollutants for M e r evaluation. Excluded were those which, when compared to asimple index, presented no risk to human health or the environment at the highestconcentration found in the 40-city study or other databases, or, secondly, lacked humanhealth criteria or sufficient data to be evaluated at the time. [ 1 11 Table 2-1 notes the pollutants EPA did not analyze hrther because of theirdemonstrated lack of negative eflect on human health or the environment. Thesepollutants are also included in the list of pollutants for which eligible POTWs, by complyingwith the requirements in Part 503, may under 40 CFR Part 403, apply for authorization to grant removal credits to their industrial dischargers, shown in Table 2-2. TheEPA also proposed to include septage from septic tanks in the definition of "scwage sludge" and thus within the scope of the proposed requirements. EPA did not propose separate standards for septage from septic tanks but rather regulated septage under the various use or disposal practices contained in the proposed Rule.3. Proposed Standards - 1989The pollutant limits, management practices and other requirements were specific tothe use or disposal method employed. The use or disposal methods included in theproposal were 1) application to agricultural or non-agricultural land, 2) distributionand marketing (referred to in the final SO3 Regulations as sale or giveaway ofbiosolids), 3) disposal in monofills, 4) disposal in on surface disposal sites, and 5)incineration.
Forstea. Land AmhcationThe original proposed standards for spreading of liquid, dewatered, dried orcomposted biosolids on or just below the surface of agricultural and non-agriculturalland contained difZaent numerical pollutant limits for agricultural and non-agriculturallands. The limits proposed for agricultural land were based on an EPA modeledassessment of potential risk to public health and the environment through 14 pathwaysof exposure. Numerical limits, when applied to agricultural land, were expressed ascumulative loading limits of 10 metals and annuals pollutant loadings of 12 organicpollutants. A cumulative loading rate for each metal defined the limit of how much ofa given metal in biosolids could be added to the soil. This load could be applied allat once or over a period of years from repeated applications. No fbrther applicationwould be allowed once the cumulative loading was reached. Also, the proposedRegulations provided an annual limitation on the quantity of the 12 organic pollutantsthat could be applied to land. The proposed Regulations required owners and operators of treatmcnt works tokeep records on the amount of organic and inorganic pollutants applied to each land application site in order to insure that cumulative and annual loading rates would not be exceeded. In addition, before biosolids could be applied to the land by anyone other than the treatment works, the treatment works would have to entcr into an agreement with the distributor or applier to provide that they must comply with the standards. For non-agricultural land, EPA developed pollutant ceilings for the concentrations in biosolids of these 22 organic and inorganic pollutants. These standards were based on the assumption that pollutants in biosolids applied to non-agricultural land would not reach individuals through the food chain. The ceiling concentration (above which biosolids could not be land applied) were based on the 98th perccntilc values for pollutant concentrations in biosolids based on data from a 198 1-1982 study.b. Distribution and MarketingDifferent requirements were originally proposed for biosolids to be distributed andmarketed (designated in the Final Rule as sold or given away) for use as a fcrtilizerand soil conditioner. EPA proposed to limit the quantity of biosolids (or productderived therefiom) of a given concentration that could be applied to land in one year.The major difference between proposed land application requirements and proposeddistribution and marketing requirements was in the numerical limits for somc of thcorganic pollutants and some metals. For both instances, it was assumed that biosolidswould be used in the production of crops intended for human consumption. Foragricultural land, numerical limits were based on crops intended for direct humanconsumption or fed to animals intended for direct human consumption, whichevcr wasthe more stringent. For the organic pollutants which can accumulate through the food
Regulatory Requirements 69chain, the numerical limit was based on crops fed to animals intended for humanconsumption. In contrast, the distribution and marketing scenario centered on a fruitand vegetable home garden, not a garden in which feed for animals is raised.Therefore, the numerical limits for pollutants in distribution and marketing weresomewhat higher than those for agricultural land application.c. MonofillsEPAs proposed requirements applying to landfills receiving only biosolids containednumerical limits on the concentration of 16 pollutants that could not be exceeded formonofill disposal. These limits were derived from a modelled exposure pathwayanalysis and varied depending upon the type of groundwater under the monofill. Theproposal also provided for site specific limits for monofills in defined circumstances.d. Surface DisposalIn addition to disposal in monofills, EPA developed standards for another widelypracticed land disposal method which EPA called "surface disposal," typically pilesof material placed on the land and defined as areas of land where such material isplaced for a year or longer. EPA concluded that such sites are generally small and inrural areas and therefore did not expose individuals to significant concentrations ofpollutants. The Agency therefore proposed pollutant concentration limits for surfacedisposalbased on the 98th percentile value derived from the data on biosolids quality.Using a 98th percentile data cap would essentially freeze surface disposed biosolidsat the level of quality represented by the database. Based on low aggregate effects ofexisting use and disposal practices, EPA then concluded that this approach wouldadequately protect public health and the environment. In the proposed Regulations,EPA did however commit to revisiting for the final Regulations the issue of whetherto distinguish between different use and disposal methods and to develop exposureassessment models to evaluate potential risks from surface disposal units for the finalrulemaking.e. Pathopen and Vector Attraction Reduction RequirementsSince the solids resulting from wastewater treatment typically include bacteria,viruses, protozoa and helminth ova which can cause diseases (usually enteric diseasesthrough direct human contact wt the organism or through the ingestion of an affected ihanimal), the proposal included requirements for the control of pathogens in biosolids.It also required measures for reducing contact of the disease "vectors" with suchpathogens. The pathogen and vector attraction reduction requirements were requiredfor biosolids applied to agricultural and non-agricultural land, distributed andmarketed or disposed of on a monofill or surface disposal site.
70 Forste Three levels of pathogen reduction were contained in the proposed Regulationswith applicable restrictions on public access, crops and animal grazing which variedwith the particular level of treatment selected. Two sets of numerical limits wereincluded and their applicability depended on whether biosolids were to be used in theproduction of crops intended for human consumption or for animals raised for humanconsumption. The level of pathogen reduction Mered in the proposed Regulations for biosolidsintended for land application and those intended for distribution and marketing. Thehighest level of pathogen reduction was required for distribution and marketing to thegeneral public. On the other hand, land application allowed alternative pathogenreduction standards as long as they were combined with imposed public access andanimal grazing controls and restricted to growing and harvesting of crops to conformto the standards of the pathogen reduction method selected. In developing thepathogen reduction requirements, EPA assumed that except for the applier, therewould be little public contact with the biosolids or with the land to which they wereapplied and that public access would be restricted for a period of time. The premiseundcrlying the distribution and marketing requirements was that biosolids would beused in a home garden where there would be immediate and continuous human contact with the biosolids or with the land to which it was applied and that under such circumstances no restricted access could be assumed.f. Incineration RequirementsThe proposed 503 Regulations contained requirements for biosolids that areincinerated in an incinerator firing only biosolids. Such incinerators were required tocomply with the National Emissions Standards for Hazardous Air Pollutants(NESHAPS) for mercury and beryllium. For lead, arsenic, cadmium, chromium andnickel, the proposed Regulations set limits on the concentration of these mctals in thebiosolids that would be incinerated based on two factors: 1) the control cficiency ofthe incinerator and 2) the dispersion factor (i.e., the relationship between ground levelconcentrations and pollutant emissions). Limits were designed to result in groundlevel concentrations for a given pollutant that would fall below the value required toprotect human health at a cancer risk level of 10.. For lead, the standard NationalAmbient Ar Quality Standard for lead; for h s calculation, biosolids incinerators were iassigned 25% of the air shed loading for lead. The 1989 proposed rulemaking also contained a limit for maximum allowabletotal hydrocarbon concentration in biosolids. Like the metal limits, this limitationwould vary with dispersion factors and control efficiency. It was also designed toprevent ground level concentrations of total hydrocarbons above a level associatedwith a cancer risk of lo, To determine this risk-specific concentration for totalhydrocarbons, EPA used a number of assumptions about which organic pollutants
Regulatory Requirements 71comprised the total hydrocarbon mixture and the levels at which these organics werepresent.g. Monitoring. Recordkeepinp and ReportingThe proposed Regulations required owners and operators of treatment works tosample and analyze their biosolids and keep certain records. The specific pollutantsfor which monitoring was required was determined by the method of biosolids use ordisposal. Frequency of monitoring would vary with the design capacity of thetreatment works. Treatment works were also required to monitor biosolids forcompliance with pathogen reduction requirements when they were to be used ordisposed of other than by incineration. For incineration, the proposal required ownersor operators to monitor continuously for stack hydrocarbon concentrations, feed rate,combustion temperature and oxygen content of the exit gas. Another provision of the proposed 503 Regulations required an agreementbetween the treatment works and the distributor or land applier which would containthe mformation needed for proposed reporting requirements. EPA also proposed thattreatment works applymg biosolids to agricultural land keep records for the life of the treatment works to insure that no cumulative pollutant loading rate would be exceeded for a paiticular parcel of agricultural land to which biosolids were applied. Similar reporting requirements were proposed for non-agricultural lands, with the cxception that treatment works did not have to keep track of annual and cumulative pollutant loading rates and therefore, need only retain records for five years. For surface disposal, five years retention of the analytical data on pollutant concentrations and pathogen reduction were required and ten years of such recordkeeping for monofills. Under the proposal, incinerator records were also required to be kept for five years.4. Risk AssessnientEPAs risk assessment processes and tools have been developed to identily thepotential for adverse aRects associated with a pollutant in order to deteimine what, ifany, measures are needed to protect public health and the environment. In developingthe 503 Regulations, EPA evaluated such risk from individual pollutants present inbiosolids. This risk assessment can be broken down into four stages: hazard identification 5 dose-response evaluation * exposure evaluation * characterization of risks
72 Forste Hazard Identification The first element in risk assessment--hazard identification--determines the nature of the effects that may be experienced by an exposed human or ecosystem from an identified pollutant. Thus, hazard identification helps to determine whether a pollutant poses a hazard and whether sufficient information exists to perform a quantitative risk assessment. By gathering and evaluating all relevant data that help determine whether a pollutant poses a specific hazard and quantitatively evaluating those data on the bases of the type of effect produced, the conditions of exposure and the metabolic processes that govern pollutant behavior within the organism, potential hazards can be identified. Hazard identification may also characterize the behavior of a pollutant in the environment (or within an organism) as well interactions within the environment or within an organism. Hazard identification therefore helps to determine whether it is scientifically appropriate to infer that observed effects under one set of conditions (e.g., in experimental animals) are likely to occur in other settings (e.g.,in human beings) and whether a quantitative risk assessment can be developed from available data. The first step in hazard identification is to gather mformation on the toxic properties of pollutants through animal studies and controlled epidemiological investigations of exposed human populations. Animal toxicity studies are based on assumptions that effects in human beings can be inferred from effects in animals. Animals bioassays include acute exposure tests, subchronic tests, and chronic tests. Acute exposure to high doses for short periods of time (usually 24 hours or less) is most commonly measured as m e d m lethal dose (Ld&-(the dose level that is lethal to 50 percent of the test animals). Ld, also used for aquatic toxicity tests (i.e,, concentrations of the test is substance in water that will result in 50 percent mortality in test species). Substanceswith a low LD, are more acutely toxic than those with higher values; for example, sodium cyanide has an M,, of 6.4 m g k g while that of sodium chloride is 3000 mgkg. Subchronic tests utilize repeated exposure of test animals for periods ranging from 5 to 90 days depending on the animals by exposure routes which comespond to human exposures. Such tests are used to determine the no observed adverse effect level (NOAEL), the lowest observed adverse effect level (LOAEL) and the maximum tolerated dose (MTD). This latter is the largest dose a test animal can receive for most of its lifetime without demonstrating adverse effects (not including cancer). For chronic effects, test animals receive h l y doses of the test agent for approximately two to three years at doses lower than those used in acute and subchronic studies. The number of animals for these tests is also larger since the tests are trying to determine effects that will be observed in only a small percentage of the animals tests. Health effects can also be evaluated using epidemiology, the study of patterns of disease in human populations and the factors the influence those patterns. Well-conducted epidemiological studies are generally viewed by scientists as the
Regulatory Requirements 73 most valuable information from which to draw inferences about human health risks. Unlike the other approaches described above, epidemiological methods evaluate the effects of hazardous substances directly on human beings. They help to identify human health hazards without requiring knowledge of the causative factor and they complement information gained from animal studies. Epidemiological studies compare the health status of a group of people who have been exposed to a suspected causal agent with that of a comparable non- exposed group. In case control studies, individuals with a specific disease are identified (cases) and compared with individuals not having the disease (control) in order to attempt to find common exposures. Cohort studies begin with a group of people (a cohort) considered fi-eeof the specific disease; the health status of the cohort known to have a common exposure is examined over time to determine whether specific conditions or causes of death occur more frequently than might be expected from other causes. The next step in hazard identification is to combine the data to ascertain the degree of hazard associated with each pollutant. EPA generally uses different approaches to assess hazard associated with carcinogenic versus non- carcinogenic effects. For non-carcinogenic health effects (e.g., mutagenic effects, systemic toxicity), EPAs hazard identificatiodweight of evidence determination has not been formalized and is based on qualitative assessment. For carcinogenic risk assessment, EPA groups all human and animal data reviewed into the following categories of degree of evidence of carcinogenicity: - sufficient limited (e.g., in animals increased incidents of benign tumors only) inadequate no data available 0 no evidence of carcinogenicity Human and animal evidence of carcinogenicity in these categories is combined into the following classifications: Group A - human carcinogen Group B - probable human carcinogen B 1 - higher degree of evidence B2 - lower degree of evidence Group C - possible human carcinogen Group D - not classifiable as to human carcinogenicity Group E - evidence of non-carcinogenicity The following factors are evaluated by judging the relevance of the data for a particular pollutant: - quality of data resolving power of the studies relevance of route and timing of exposure appropriateness of dose selection
74 Forste replication of effects number of species examined availability of human epidemiologic study data Hazard identification enables researchers to characterize the body of scientific data in such a way that the following questions can be answered: 1) Is a pollutant a hazard? 2) Is a quantitative assessment appropriate? Conducting such quantitative assessments involves dose-response evaluations and exposure evaluation. Dose ResDonse Evaluation Estimating what dose of a chemical produces a given response for a particular pollutant is the second step in the risk assessment methodology. Evaluating such data characterizesthe connection between exposure to a pollutant and the extent of injury or disease. Most dose-response relationships are based on animals studies, since even good epidemiological studies rarely will have good information on exposure. In the current context, "threshold" characteristics of a pollutant refer to exposure levels below which no adverse health effects are assumed to occur. For effects involving genetic alterations (including carcinogenicity and mutagenicity) EPA postulates that effects may take place at vary low doses and therefore these compounds are modeled with no thresholds. For most other biological effects, the usual assumption is that threshold levels exist. For non-threshold effects, the key assumption is that the dose response cuve for each pollutant exlubitiig these effects in the human population achieves zero risk only at zero dose. Mathematical modeling extrapolates response data fiom doses in the obseived (experimental) range to response estimates in the low dose ranges. EPAs cancer assessment guidelines recommend the use of multi- stage model which yields estimates of risk that are conservative, representing a plausible upper limit of risk. Systemic toxicants or other compounds which exhibit non-carcinogenic and non-mutagenic health effects are assumed to exhibit threshold effects. Dose response evaluations involve calculating what is known as the reference dose (oral exposure) or reference concentration (inhalation exposure) are RfD and RfC respectively. RfDs and RfCs are estimates of a daily exposure to the human population that is likely to be without appreciable risks of a negative effect during a lifetime. The RfD and RfC valucs are calculated from data establishing a no observed effect level (NOEL), no obmved adverse effect level (NOAEL), lowest observed effect level (LOEL), or lowest observed adverse effect level (LOAEL). These values are stated in mgkg per day and values are derived from laboratoiy animal and human epidemiology data. Uncertainty factors are applied to the IUD and RfC values depending upon the level of confidence EPA has in the data used to derive them. Uncertainty factors vary with the nature and quality of the data from which the NOAEL or LOAEL is derived and range from 10 to 10,000.
Regulatory Requirements 75 These uncertainty factors are used to extrapolate from acute to chronic effects, to accounf for differences in species sensitivity or variation in sensitivity in human populations and, if appropriate, to extrapolate from an LOAEL to a NOAEL. If information is available for only one route of exposure, this infomation is used to extrapolate to other routes in the absence of I s and RfCs for those routes. Once an RfD or R E is derived, the next step in risk assessment is to estimate exposure. Exposure Evaluation EPA relies on two methods to determine pollutant concentration: 1) direct monitoring of pollutant levels and 2) using mathematical models to predict pollutant concentrations. Atter environmental pollutant concentrations are determined by one of the above methods, EPA then determines the severity of the exposure by evaluating data on the nature and size of the population exposed, exposure route (i.e., oral, inhalation, dermal), the extent of exposure (concentration x time) and the circumstances of exposure. Collecting monitoring data provides the most accurate information about pollutant concentrations. Such monitoring includes personal and ambient (or site and location) monitoring. To reduce the variability of exposure assessments to individual persons, the technique of sampling air and water and then combining that information with the amount of time spent in various microenvironments (i.e., home, car or office) provides an estimate of exposure. Personal monitoring may also include sampling of human body fluids (biological monitoring or biomonitoring). Biological markers (biomarkers) can be classified as markers of exposure of effect and susceptibility. An example of a biomarker of exposure is lead concentration in blood. A biomarker of effect measures some biochemical, physiologic or other alteration within the organism that points to impaired health. Biomonitoring may also refer to the regular sampling of animals, plants or microorganisms in an ecosystem. Ambient monitoring (or site or location monitoring) involves collecting samples from the air, water, soil or sediment at predetermined locations and analyzing them to determine environmental concentrations at those locations. Exposures can be further evaluated by modeling the fate and transport of the pollutants. Measurements are a direct and preferred source of information for exposure analysis, but such measurements are expensive and are oAen limited geographically. Such data can be best used to calibrate mathematical models that simulate the movement of pollutants into and through the environment with mathematical equations or algorithms that can be more widely applied. Mathematical models must account for both physical and chemical properties related to fate and transport and must document mathematical properties, spacial properties and time properties. Of the hundreds of models for fate, transport and dispersion available for all media, there are five general types: atmospheric,
76 Forste surfacewater, groundwater, and unsaturatedzone, multi-mediamodels and food chain models. These types of models are primarily applicable to pollutants associated with dust and other particles.-. PoDulation Analvsis Population analysis describes the size, characteristicsand habits of potentially exposed human and non-human populations. Census and other survey data may be u e in iden* and describing the populations exposed to a pollutant. sm Integratedexposme a d y s s calculatesexpowe levels and describes the exposed population and uses this information to quantify the contact of an exposed population to each pollutant under investigationvia all routes of exposure and all pathways h m the sources to the exposed individuals. In addition,uncertainty must be described and quantified to the extent possible to achieve a valid exposure analysis. EpAs policy with respect to risk characterizationis designed to convey the extent of the Agencys confidence in risk estimates. It is also important that risk assessment information be clearly presented and separate fiom any risk management considerations. Evaluating how exposure assessments were conducted provides one method for i d e n e g the uncertainties in risks. For example, in the human health risk assessment for the 503 Regulations, the technical support documents define several exposure pathways for the three biosolids management practices. EPA used point estimates for each exposure pathway and did not consider variability of the parameters describingexposure among individuals. B s d on the amount of data and the lack of accepted risk ae assessment methodologies in certain areas, it is difllcult to judge whether the point estimates in the human health risks assessment and assumptionsmade in the ecological effects assessment are likely to underestimateor overestimate actual risks. Some aspects of the risk analysis probably contain coI1sefv(Ltiveor protective assumptions while other factors may bias results in the opposite direction. Also,some assumptions are based on EPA policy and reflect risk management choices; some of which are conservativewhile others are less s . o% Human Health Assessment Based on staradard Agency practice, human-health dose-response assessmentsfor te 503 Regulations are based on RfDs for non-carcinogensand cancer potency h factors for carcinogens. Both of these measures are generally considered conservative, ta is, they predict a greater impact on human health than is likely ht to actually occur. The referencedose i defied as an estimate of a daily exposure s to the human population (including sensitive subgroups) that is likely to be without appreciablerisks of deleterious affects during a lifetime. It is based on themost sensitive adverse dect found by toxicologicaltesting and then applying
Regulatory Requirements 77a series of unmtainly factors so that higher exposures may also not present anyappmiablerisk For example, it assumesthat humans may be an order of magnitudemore Sensitivethanthe animals tested. But in fact humans may also be less sensitive.This Bssesgnenf method also assumes,except as noted, that the risk assessmentsreliedan far theseregulationsare based on exposureslasting a lifetime, whereas in fact theymay be much shorter. Similarly, cancer risks are calculated in the risk assessmentguidelines as "plausibleupper bounds" to the actual risk. Conservativeassumptionsused in these calculations, such as use of the m s sensitiveanimal data in bioassays, otlinear extrapolaticnto low doses, species-to-species conversionbased on surface areaand use of an upper confidence limit for the dose response slope make it unlikely thatcancer risk would be greater than what is calculated; in fact, it could be orders ofmagnitude less or even zero. The application of uncertainty factors in the riskassessment process provides a conservative level of protection even in the face oflimited data on the factors described above.* Home Garden Scenario A primarily focus of concern in developingthe 503 Regulations was the human food chain pathway, that is the production of crops by gardeners and farmers. The population assessed for this pathway were people who used biosolids to produce crops for ter own consumption. EPA made specific assumption about hi a number of variables addressinghuman behavior and properties of biosolids as describedbelow.* Plant Uptake of Metals The risk assessment estimated metal concentrations i plants using a linear n uptake line whose slope was based on the assumption that metal uptake was proportional to cumulative applicationrates of biosolids. The uncertainty as to whether this is appropriate arises fiom data on plant concentration versus application rate which suggests a nonlinear relationship. EPAs assessment of linearity is therefore conwative because application rates allowed under 503 are in g d well in excess of test plot application rates and data ffom long-term (>20 years) studies support the conclusion that actual metal concentrations in plants plateau at these higher application rates (seeFigure 2-1). Anadditid- t in the plant uptake calculation arises fiom the uses y of gemnet& means for all slopes calculated fiom individual studies. Assuming a log normal distribution,the geometric mean provides an estimate of the median (50th percentile) slope. Such a value is useful in estimating uptake for "typical biosolids." Individual studies used by EPA to calculate plant uptake used bimlids with higher metals Concentrations than the "typicalbiosolids" on the
Metal level in biosolids-amended soilFIG. 2-1 ASSUMED VS. ACTUAL UPTAKE OF METALS RESULTING FROM INC R EASlNG CU MULATIVE LOAD1NGS
Regulatory Requirements 79 market today; therefore, these biosolids with higher metal concentrations most likely produced higher plant concentrations than would be likely using current values. A further uncertainty results from the way the geometric mean calculations were performed. A value of 0.001 was used as the uptake slope from studies which showed no significant increase in metal uptake by the crops. Since the geometric mean calculation is very sensitive to the inclusion of low values and it appears that 0.001 is smaller than the upperbound on uptake that would be obtained from "no significant increase in metal uptake" studies, the use of this default slope of 0.001 may therefore underestimate the typical slope for crop uptake. Dietary Consumption Pollutant limits for the 503 Regulations were calculated based on population average food consumption estimates. Survey data from short-term food consumption reports of large surveyed populations provided the basis for these estimates--an accepted basis for estimating population average food consumption rates. Such estimates do not reflect the higher food consumption rates per unit of body weight of young children compared with adults. Another limitation arises from the fact that it is possible that individuals who raise a particular crop may have a highcr consumption rate than do individuals who only buy such a crop. On the other hand, homc gardens do not produce a specific crop year round, which may offset this bias. EPA estimated the amount of food coming from biosolids treated land using USDA survey data on average percentage food consumption from home-grown crops. EPA estimated that large garden plots were required to produce the amount of home grown crops consumed although EPA believes that a relatively small percentage of gardens are that large. Also, because of seasonal factors it may be difficult for most gardeners to produce the quantities of leaf) vegctables that are assumed; leafy vegetables are an important factor in the risk assessment since they tend to have high mctal uptake slopes. Plant Toxicity and Uptake The plant toxicity (pliytotoxicity) assessment was based on the relationships between biosolids application and tissue residue, between tissue residues and growth reduction, and between growth reduction and yield reduction. The relationship between growth reduction and yield reduction is particularly uncertain and will vary with chemicals, crop species and toxic endpoints. Some crops (e.g., beans) and endpoints (e.g., reproduction) may be more sensitive to the effects of biosolids, although other crops (e.g., sudan grass) and endpoints (e.g., mortality) may be less sensitive. There are limited data about non-
80 Forste cultivated forest species and perennials and these may differ in their response to biosolids-borne contaminants. Metal phytotoxicity depends particularly on soil pH and the degree of binding to the biosolids matrix. Since most metals are more available in acidic soil, EPA used the assumption of a soil pH of 5.5 as representing a reasonable worst case condition for agricultural soils. Based upon results from several field studies, EPA believes that metals are bound to the biosolids matrix and remain relatively unavailable biologically.0. Wildlife In the absence of standard methodology at EPA for assessing iisks to wildlife, there are uncertainties about how biosolids application affects terrestrial wildlife and soil biota. Available data and methodologies only describe direct toxicity to a few species. With uncertainties as to how to extrapolate this information to other birds, mammals, amphibians and soil invertebrates whose relative sensitivity to the compounds of concern is unknown. The ecotoxicological analysis focused on cadmium and lead which had the most data available. EPA used a simple linear model of bioaccumulation or bioconcentration from soil to earthworms to shrews and did not model the more complex effects of biosolids contaminants on the terrestrial food chain. In the absence of standard methodologies, EPA did not consider how amendment of forest soils or edges of agncultural fields with biosolids might change the composition of species in the plant community, either through nutrient enhancement or phytotoxicity, and any subsequent ramifications such changes might have throughout the food web. Uncertainty about the impact of biosolids on soil biota exist because the criteria are based solely on a N O E L for the earthworm (Eiseniafoetidu) which may not be the most sensitive or appropiiate species for evaluating many of the chemicals. Possible additional factors influencing soil flora and fauna include adding nutrients to the soil and possible increased exposure to organisms that feed in the litter layer due to the organic matter in biosolids. While these considerations need to be explored further, it should be noted that the same lack of data and uncei-hinties exist with respcct to the current agricultural practices on sites used to produce food and feed crops throughout the US. Section 405 of the CWA requires EPA to develop standards for biosolids use or disposal, to protcct from reasonably anticipated adverse affects, and to promulgate these standards in two stages and to revise the standards periodically. EPA concluded that the standards adopted on February 19, 1993 are adequately protective based on its assessment of available data. To verify the conclusions about the adequacy of these standards, EPA has committed to developing a comprehensive,environmental evaluation and monitoring study. The results will provide a database for the Round 2 biosolids standards and will also help the Agency in its efforts to develop a comprehensive, ecological risk assessment
Regulatory Requirements 81 methodology and to conect any u c r a n i s in future P r 503 rulemakings. The netite at final plan, including study design, will be available for public comment at the time that Round 2 regulation is proposed. Risk Management Auuroach Using risk characterizationinformation, EPA can determine if a "SigIllficant"or "unreasonable"risk exjsts, what controls are needed and how to communicatethis risk to the public and regulated community. The mere identification of risk is not necessarily enough to just@ action. Also,non-risk factors, such as availability and effectivenessof controls, whether alternativesexist and any benefits lost or gained as a result of umtmls must be evaluated by the Agency to reach a decision. Under each exposure scenario, EPA identifies a range of control strategies and regulatory requirements that usually reduce exposure so that the risk or identified effect is put back into balance with the benefits. Using this information provided in t erisk management step, EPA can then select the appropriate control strategy h and means for communicating it. For the 503 regulations, EPAs approach was to establish management practices and numerical limits (standards) to safeguard public health and the environment by examining use or disposal practices and the probability that individuals would be exposed to pollutants Itom these practices. The Agency identified the types of risks involved (e.g., drinking water with pollutant levels exceeding the MCLs for drinking water) and examined the possibility of special populations at greater risks (e.g., small children playing in gardens where biosolids have been applied), and examined whether individuals voluntarily incurred the risks. Finally, EPA used exposure assessment models to project the effecton an individual receiving maximum dose throughout an average life span of 70 years. Aggregate effects analyses were employed to project the incidents of adverse health effects on t e population as a whole (e.g., the number of people h exposed to lead at levels producing adverse health effects). For its regulatoy approach in the proposed Regulations, EPA primarily focused on two types of risks: that to a most exposed individual (MEI) receiving a maximum dose and risk to the population as a whole (aggregate risk). The Agency examined both the individual and aggregate effect of each alternative to balance the uncertainties in the analyses. Available data resulted in the greater emphasis being placed on public health than on environmentaleffects, although environmentaleffects were considered in the determinationof what constituted adequate protection as long as they could be identified even qualitatively. Individual vs. aggregate risk results in divided opinion. Some argue that no individual should be at high risk and t a considering the number of people at risk ht leads to acceptance of higher individual risk when two people are exposed; using maximumindividual risk alone h u t s the effectivenessof the Regulations by not indicating how many people may be affected since they only relate to carcinogenic
82 Forste risk to the MET Aggregate risk can also be seen as an appropriate measure of total public health impact and therefore a good indicator of whether the goal of adequately protecting public health has been achieved. A combination of approaches was selected by EPA for the purposes of the proposed 503; this was revised for the h l Regulations based on current Agency policy, public comment a and scientific peer review. For the proposed 503 Regulations, using the ME1 approach, the Agency identified a most exposed individual plant or animal that remained for an extended period of time at or adjacent to the site where the maximum exposure occurs. EPA used models of 14 exposure pathways to determine the concentration of biosolids-borne pollutants that may be utilized or disposed of in each use and disposal practice without exceeding human health or environmental criterions. These criteria were taken from those already published or promulgated by the Agency, fiom human health criteria developed by the Agency or from plant and animal toxicity values published in scientific literature. For example, to protect sources of drinking water, pollutant limits were developed which would ensure that the Agency’s maximum contaminant levels were not violated. For surface water protection, Water Quality Criteria were used. For carcinogens, the risk specific doses corresponding to an incremental carcinogenic risk level of 1 x 10” were used for all use and disposal practices except when biosolids were distributed and marketed. For distribution and marketing, numerical limits were established so as not to exceed an incremental carcinogenic risk level of 1 x lo6. For all pathways, the human MEIs assumed to be the most sensitive individuals were continuously exposed over a 70 year lifetime. Endpoints for the ecologicals MEIs were conservatively developed using the most sensitive species with steady state duration and concentration of exposure over a critical life period. For all use or disposal practices, carcinogenic risk targets were applied pollutant by pollutant, except for the organic pollutants in the emissions of biosolids incinerators. In this case, EPA set a limit on total hydrocarbon emissions, rather than on each individual pollutant as described previously. For the proposed 503 Regulations, the Agency’s approach to situations where individuals were unlikely to be exposed to biosolids pollutants (i.e., forests, reclaimed lands, and others) a proposed 98th-percentile pollutant concentration was developed based on the assumption that such practices would have negligible impacts on human diets. These pollutant concentration limits also applied to surface disposal since the Agency believed such sites are generally small, located away from population centers and presented little likelihood of exposure.
Regulatory Requirements 835 . Conments/Review oJProposed RuleEPA solicited comments on a wide range of issues, including the hndamentalprinciples of the 503 Regulations, the carcinogenic risk levels, other human health andenvironmentalcriteria, changes resulting from other Agency actions, the models used,die ME1 and aggregate risk analyses, the cost benefits estimates of the Rule and datadeficiencies. The EPA committed to seek and support scientific peer review of thetechnical bases of the rulemaking during the public comment period (1 8 3 days). Themajor peer review groups with which EPA worked during the public comment periodwere: Land Practices Peer Review Committee -- This specially convened group of biosolids experts reviewed in depth the land application, distribution and marketing, monofilling and suiface disposal provisions. The group was comprised of nationally known experts on biosolids use and disposal, including members of the U.S. Department of Agriculture W-170 Committee along with representatives from a broad diversity of views. The Peer Review Committees final report was officially submitted to EPA on July 24, 1989. [ 121 EPA Science Advisory Board -- The SAE3 reviewed the technical bases of the incineration portion of the incineration portion of the regulation. As various SAB committees have reviewed similar EPA incineration regulations in the past, most notably for municipal solid waste combustion and hazardous waste incineration. Their final report of August 7, 1989 was also submitted to EPA. [ 131 During the comment period, EPA received more than 5,500 pages of commentsfrom 656 commenters during the 183 day public comment period. Comments weresubmitted from municipalities, industries, federal and state agencies, professionalassociations, academia and the general public. These comments were provided inaddition to the foimal peer review described above. These public and scientific review groups provided EPA a comprehensive rangcof opinions, comments, and recommendations. Many commenters criticized theAgencys risk assessment as being overconservative for some use and disposalpractices and underconseivative for others. Other significant areas of criticismincluded the risk levels used by EPA ( i c , which risk levels are most appropriate);data selection and the parameters used in the analysis of exposure assessment and thcimpacts that the proposed nile would have on beneficial use of biosolids. Followingthe public comment period, EPA provided public notice of the availability of NationalSewage Sludge Sluvey data along with information and data from the Sewage SludgeIncinerator Study and the Domestic Septage Study. The notice described some of thercsults of the survey and changes the Agency considered making to the proposed 503Regulations as a result of these studies. The notice also requested comments on a
a4 Forstenumber of changes to the use and disposal standards that were being considered forthe 503 Regulations in light of the comments submitted earlier the peer review andnew idormation developed since the February 8, 1989 proposal. During the 60-daypublic comment period for the NSSS notice, the Agency received more than 1000pages of comments from 153 commenters. Many of these comments supported thechanges identified in the notice as potential revisions to Part 503.11. FINAL PART 503 REGULATIONSA. IntroductionThc Agencys proposed regulatory approaches received extensive peer review andpublic comment, focusing especially on the ME1 exposure scenario used and the useof the 98th percentile technique. The Agency agreed with many of the comments provided by the public and thescientific peer review committees and noted that there is no clear guidance in Section405, which contains only limited discussion of how to establish pollutant limits andconcluded that its statutory duty was to protect against reasonable risks to exposedpopulations and not to risk associated with highly unlikely or unusual circumstances.Therefore, the Agency decided to evaluate the risk to a highly exposed individual(I-IEI) instead of the ME1 for the final 503 risk assessment. This approach morerealistically protects the health of individuals or populations which are at higher risksthan the population as a whole. EPA retained a 70-year exposure for the HE1 in the final risk assessmcnt as aconservative assumption in the context of a highly mobile society. Furthermore, this70-year exposure duration represents a steady state assumption that is consistent withthe measure of carcinogenic risk (i.e., the probability of contracting cancer based upona lXetime--70 year-exposure). Tlus esposure assessment assumption, which is in parta policy judgment by EPA, was considered by the Agency to be preferable to the lessconservative alternatives suggested and, in the Agencys opinion, represents anappropriate response to their obligation to protect public health. Furthermore, EPAbelieves that retaining the 70-year assumption insures that the population of highlyesposed individuals will remain extremely small. In preparing the proposed 503 Regulations, EPA used what it believed were "reasonableworst-case assumptions." Each of these has a margin of safety associated with it, depending on the accuracy ofdata and information supporting it. For example, if EPA had insufficient data from biosolids/field studies 011 metals uptake in crops, data from biosolids/pot studies or saldpot studies were used in the original ME1 risk assessment analysis. The margin of safety associated with the data from saltlpot studies is much geater than the margin of safety associated with data from biosolids/pot studies and far greater than the margin of safety associated with data from biosolids/field studies.
Regulatory Requirements 85 The proposed Regulations were developed using each of the above types of datadescribed above and in addition, selected data using only studies which showed aneffect, thus eliminating valid field studies in favor of pot studies or salt studies whichshowed an effect which did not occur at similar levels in the field. Individualresearchers and members of the peer review committee emphasized that data from potand metal salt experiments should not be extrapolated to results found in the field.Thus the peer review concluded that the approach used to select data for individualcontaminants and pathways in the proposed rule was scientifically unsound and neededto be revised. The peer review committee also criticized E P A s identification of an ME1 in theproposed rulemaking. As noted by the committee: "The exposures implied by the MEIs are grossly exaggerated and it is impossible to know the probability that such an ME1 exists. Because the assumptions underlying selection of the various MEIs differ, the extent of conservatism embodied in the various ME1 exposure models also differs. Further, insufficient information is available to compare the relative degree of conservatism of one ME1 with another. Consequently, it can be misleading to compare risks derived from one ME1 with another because different premises were used within and among disposal options. The reliance on "worst-case" scenarios pervades these [proposed 5031 documents and needs to be reconsidered. The ME1 is so rigidly defined that it completely overshadows other components of the risk assessment pathways and even to the extent that it makes the issue of inappropriate technical material almost irrelevant."[12, p. xi] The problem occurred when a series of parameters and assumptions, each havinglarge margins of safety, were used in the same exposure pathway assessment. Thisresults in an unrealistically conservative analysis which the Agency acknowledgedcould conceivably over-regulate a useldisposal practice. Information provided by the NSSS, the incinerator study, the scientific peerreview committees and the public was incorporated into the aggregate risk assessmentfor the final Rule and showed minimal risk from current biosolids managementmethods (pre-Part 503 or base line risk). EPA agreed with the public and thescientific peer review committees that the 98th percentile approach is inconsistentwith the ME1 approach and that numerical limitations derived ftom the 98th percentileapproach do not insure protection of public health and the environment because theylack a formal pathway risk assessment. In developing the proposed 503 standards, theAgency had relied on this approach because it did not have reliable exposureassessment models nor input data and information to conduct the pathway riskassessment for certain practices. The Agency also believed that this 98th percentileapproach was supported by the aggregate risk assessment which showed low exposure
86 Forsteand minimal human health impacts on the population as a whole from the use anddisposal practices which would be governed by this approach. Following theproposal, the Agency workcd with experts within and outside EPA to devclop andrefine modeling techniques and supporting data to conduct a formal pathway riskasscssmcnt for these practices. Public comments and scientific peer review providedEPA with better data and lnfoimation so that numerical limitations for all biosolids useand disposal practices could be derived using risk assessment. For the final Regulations, EPA selected an approach based on a risk to highlyexposed individuals (HEIs) and consideration for higher risk populations (aggregaterisk assessment), not an unrealistic worst-case ME1 approach. EPA believes thisapproach is consistent with the congressional intent to establish standards adequateto protect public health and the environment from reasonably anticipated adverseaffccts of each pollutant. In the final Regulations, EPA evaluated the risk to highly exposed individuals andpopulations from pollutants found in biosolids using differcnt exposure assessmentpathways. In evaluating the standards for the final Rule, the Agency established criteria based not only on cancer risk but on a series of other health and environmental effects, including the overall incidence of othcr serious health effects within thc esposed population as a whole (including average exposed and highly exposed individuals and within special subpopulations such as children). EPA also evaluated effects on plants and animals and considered policy assumptions, estimation of uncertainties and margin of safety, weight of the scientific evidence for human health and environmental effects, other quantified or unquantified health and environmental effects and other impacts associated with use and disposal of biosolids before selecting the final standards. The Agency also determined that in order to insure adequate protection of public hcalth and the environment, they needed to add safety factors to the numerical criteria derived from the exposure assessments. This dccision also served a sccond critical objcctivc in the rulcmaking, that is, to promotc the use of biosolids for their beneficial properties. EPA believes that an inportant component to promote beneficial usc of biosolids involves building public confidence that using biosolids to @-OW food that the public eats is safe, and that adding a margin of safety to the model-derived criteria should help to encourage this practice. In addition, adding safety factors to the model-derived numerical criteria enabled EPA to overcome some of the unquantifiable variables in the real world movement of pollutants fi-ombiosolids to environmental end points. EPA therefore made a number of assumptions to reduce the complexity of actual experience. The Agency believcd that these exposure assessments generated numerical criteria consistent with the stated goals of the rulemaking. Also, through its exposure assessments, EPA derived numerical limitations from metals that represented the total quantity that could be added to the soil. So long as that total quantity (loading) for the metal is not exceeded, the eqosure assessment models predict that there will be no haim to the HEI. Sincc
Regulatory Requirements a7the model does not concern itselfwith whether the total quantity is received in a singleapplication or over time, adopting a purely cumulative loading approach would meanthat biosolids with extremely high metals concentration could be applied to the landso long as the cumulative load is not exceeded. Such an approach, which is strictlyrisk based, could therefore allow for degradation of current biosolids quality. EPAsaggregate risk assessment shows only small effects associated with current use anddisposal--that is, biosolids used at pre-503 pollutant concentration levels presented alow risk and therefore such levels already had an inherent level of protection. The models do not look exclusively at data from the most heavily contaminated biosolids. Therefore, in order to ensure continued protection of public health and the environment EPA determined that existing quality of biosolids applied to land should be "protected" and not allowed to deteriorate above current concentration levels. EPA has stated that implicit in its numerical pollutant limits for land application is the assumption that biosolids with low concentrations of pollutants are safe and to downgrade the quality of biosolids reduces the protective levels inherent in such limits. EPA concluded that its uncertainty about the protectiveness of the numerical criteria derived from the exposure assessment models for land application is increased by adding margins of safety to the numerical criteria. Accordingly, the Agency placed a "ceiling" on the concentration of pollutants in biosolids that may be applied to land at the 99th perccntile pollutant concentration from the NSSS survey. This ceiling concentration is the &r of the 99th percentile pollutant concentration or risk based pollutant limitation and acts as a trigger, dictating when biosolids quality is no longer suitable for beneficial use, regardless of how it is applied to the land. One important purpose of the ceiling limits is to direct the "cleanest biosolids" into beneficial usc practices. The agency has also "capped" the numerical pollutant limits for land application at the 99th percentile pollutant concentration found in the NSSS. If that concentration is lower than the risk-based numerical pollutant limit, this cap dctermines when biosolids quality is suitable for beneficial use under the alternative pollutant limit concept or must be applied using cumulative pollutant loading rates as discussed below. EPA made these risk policy decisions (i.e., the capping and ceiling) to provide an additional margin of safety to protect public health and the environment beyond the risk-based standards developed for the final rule while maintaining quality to encourage utilization consistent with the Agencys beneficial use policy. A complete description of the exposure assessment methodology and risk management issues for the proposcd 503 rulemaking is found at 54 Federal Register 5764 - 5791. The following section describes the exposure assessment pathways modeled in the final Part 503 Rule and the basis for the decisions made in developing the approach for each use and disposal practice. A detailed discussion of the exposure assessment methodology (i.e., models, pathways, parameter values, assumptions and others) and the risk management decisions used by EPA to develop the final Part 503 numerical critcria are contained in the Technical Support Documents.
88 ForsteB. Exposure Assessment PathwaysEPA evaluated 14 pathways of potential exposure to pollutants in biosolids of the finalPart 503 Regulations. The Regulations distinguish between biosolids applied to theland for a beneficial purpose and biosolids disposed of on the land. EPA evaluated potential exposure when biosolids are used as a fertilizer or soilconditioner in one of two ways: agricultural and non-agricultural land application.Agricultural land application includes use to produce food or feed crops commerciallyby agricultural producers on pasture and rangeland and also by a home gardener(formerly described as distribution and marketing in the proposed rule; in the final rulecharacterized as biosolids "sold or given away in a bag or container"). Non-agricultural land includes forest, reclamation and public contact sites. The descriptiveterm "surface disposal"in the final rule applies to biosolids disposed on land either inpiles or i biosolids-only landfills which are also referred to as monofills. For surface ndisposal, EPA evaluated two pathways of exposure. Incineration was evaluated by asingle pathway of exposure--inhalation. The final exposure assessment pathwaysevaluated in the final Part 503 Regulations are: Land Amhxtion (Beneficial Use) Biosolids-soil-plant-human(Pathway 1) Biosolids-soil-plant-home gardener (Pathway 2) Biosolids-soil-child (Pathway 3)* Biosolids-soil-plant-animal-human (Pathway 4) Biosolids-soil-animal-human (Pathway 5) Biosolids-soil-plant-animal toxicity (Pathway 6) Biosolids-soil-animal toxicity (Pathway 7) Biosolids-soil-plant toxicity (Pathway 8)** Biosolids-soil-soil biota toxicity (Pathway 9) Biosolids-soil-soil biota-predator of soil biota toxicity (Pathway 10) Biosolids-soil-airborne dust-human (Pathway 11) Biosolids-soil-surface water-contaminatedwater-fish toxicity-human toxicity (Pathway 12) Biosolids-soil-air-human (Pathway 1 3) Biosolids-soil-groundwater-human (Pathway 14)9 Surface Disposal Biosolids-soil-air-human (Pathway 13) Biosolids-soil-groundwater-human (Pathway 14) Incineration Biosolids-incineration particulate-air-human (Pathway 13)
Regulatory Requirements 89 * Most limiting pathway (regulatory limit) for arsenic, cadmium, lead, mercury and selenium. ** Most limiting pathway (regulatory limit) for copper, lead, nickel and zinc. For situations where EPA determined that these exposure assessment pathwaysfor a particular use or disposal option did not yield adequately protective results,additional management practices were imposed under the Regulations to preventenvironmental abuses and to protect public health. All pathways, except 3 , s and 7, assume the mixing of biosolids with 15 cm (i.c.,six inch plow layer) of the suiface soil, either by incorporation or by naturalweatheringprocesses. Using this assumption, which entails the affected surface layerhaving a mass of two million kghectare, pollutant concentrations in soil (per unit massof soil) could be converted to cumulative pollutant loading rates for metals (mass ofpollutant/hectare of land, In addition, annual pollutant loading rates for organics (massof pollutant per hectare of land per 365-days) could also be converted to pollutantconcentrations in soil. EPA first determined the pollutant concentration in soil that would be allowed(i.e., the maximum pollutant concentration in soil that when taken up by a plant andconsumed by a target organisms does not produce undue risk for a particular pathway).Having made that determination, the model then can be used to determine the allowable pollutant loading rate in two different ways.I. Metals are determined based on a cumulative pollutant loading rate. That is, the total quantity of metals consistent with no undue risk. This number is derived from the allowable pollutant concentration in the soil multiplied by the mass of soil in the top 15 cm of a hectare of land.2. The annual pollutant loading rate for organic pollutants (kg/ha/365 days) takes into account the rate of pollutant loss or decay. A first order decay of organic pollutants is assumed by the model--that is, that the quantity lost per year is directly proportional to the quantity present. Assuming continued annual applications, pollutant concentrations for organics gradually approach a plateau at which the quantity lost each year equals the quantity applied. Therefore, the annual pollutant loading rate is dcteimined such that the concentration levels off at that allowable soil concentration when biosolids are applied for n long period of time. For human exposure pathways, maximum pollutant intake allowed was based onseveral different EPA health effects criteria: reference dose (RtD), recommendeddaily allowance (RDA), or concentration (RE) for noncarcinogens, a risk specific
90 Forstedose for carcinogens based on a risk level for all use and disposal practices. Adaily dietaiy intake derived from a drinking water standard or a drinking waterstandard (MCL). The only exception to this approach is Pathway 11 (inhalation)which limits the pollutant concentration in the soil to an amount which will not exceedthe National Institute of Occupational Safety and Health N O S H ) work place airquality criteria ifsigmficant quantities of soil become airborne. This pathway was nota limiting pathway for any pollutant in the final Regulations Brief summaries of each of the pathways analyzed for the 503 Regulations are asfollows: Pathway 1 - Evaluates liuniati exposure to crops grown with biosolids. This pathway is designed to protect consumers who eat produce grown in a soil to which biosolids have been applied. The environmental endpoint is an HE1 (highly exposed individual) assumed to live where a relatively high percentage of the available cropland receives biosolids. It is assumed that the HE1 eats a mis of crops from land where biosolids were applied and from land where this did not occur. For Pathway 1,2.5 percent of a consumers intake of grains, vegetables, potatoes, legumes and garden fruits is assumed to come from biosolids-amended soil. Pathway 1 assumes uptake of biosolids pollutants through plant roots, not through direct adherence to crop surfaces as crops are assumed to be washed before consumption. This pathway assumes agricultural use in commercial enterprises where crops for human consumption are grown. Pathway 1 also includes the exposure of a person in a non-agricultural setting who ingests wild berries and mushrooms grown in biosolids amended soils. Exposure is based on the uptake of a pollutant by each type of beny or mushroom, a daily consumption of wild bemes and mushrooms, and a fraction of diaerent wild berries and mushrooms grown in biosolids amended soil. Thc H 1 for the non-agricultural Pathway 1 is an individual living where biosolids are E applied to a forest, a public contact site or reclamation site. The dose for this pathway is the RfD for an inorganic pollutant; organics were not evaluated for this pathway because they do not concentrate in wild berries and mushrooms. Pathway 2 - Evaluates the situation it1 which biosolids are added to tlie soil in a home garden. Pathway 2 differs from Pathway 1 primarily in the fraction of food g-oups produced on biosolids amended soil. For this pathway, as much as 60 percent of the EIEIs diet of certain food groups is grown in the home garden where biosolids are used as a fertilizer. Pathway 2s home gardener produces and consumes potatoes, l e a vegetables, legume vegetables, root vegetables and garden fi-uits; grains, cereals and peanuts are not included in these crops because home gardeners do not usually consume such crops when they grow them on biosolids- amended soils.
Regulatory Requirements 91 Pathway 3 -Assesses the hazard to a child ingesting undiluted biosolids. This HE1 is the child who ingests biosolids from storage piles or from the soil surface. Pathway 3 assumes that the biosolids are not diluted with soil when exposure occurs. The ingestion rate of 0.2 grams (dry weight) per day for five years was based on the 1989 EPA soil ingestion directive suggesting this value for higher risk chldren. Pathway 3 uses the integrated uptake biokenetic model (IUBK) rather than extrapolating from cattle data as had originally been proposed. The IUBK model used a lead blood level not to exceed 10 micrograms per deciliter, a 30 percent absorption value and a 95th-percentile population distribution. Using these values in the IUBK model, results in an allowable biosolids concentration of 500 parts per million (ppm). The lead pollutant limit calculated by the peer review committee resulted from the observation that body burdens (absorption) of animals fed up to 10 percent of their diet as biosolids did not change until the lead concentration in the biosolids exceeded 300 ppm. EPA therefore decided to select the more conservative numerical limit for the final rule to minimize lead exposure to children and set the allowable lead concentration at 300 ppm for Pathway 3. Pathway 4 - Evaluates huinan exposure from the consuinption o anitnal f products. The HE1 for this pathway consumes the tissue of foraging animals that have in turn consumed feed crops or vegetation grown on biosolids amended soils. Pathway 4 depends on plant uptake of a contaminant being proportional to soil concentrationsof the contaminant with uptake occurring through the roots to the edible portion or by volatilization from soil to above ground plant parts. In the non-agricultural setting for Pathway 4, an individual consumes meat or products from wild animals that consume plants grown in biosolids amended soils (meat obtained from hunting herbivorous wild animals). For both agricultural and non-agricultural Pathway 4 HEIs, a background pollutant intake is also assumed. Pathway 5 - Evaluates the consumption of anitnal tissue which has been containitiated by direct ingestion of biosolids by the animal. The HE1 consumes the tissue of foraging animals that have incidentally ingested biosolids. As with Pathway 4, the HE1 is assumed to consume daily quantities of various animal-tissue food groups and is also assumed to be exposed to a background intake of each pollutant. Pathway 6 - Establishes level o pollutants in biosolids to protect aiiinials f ingesting plants grown on biosolids amended soils. This pathway assumes the livestock diet is 100 percent forage grown on biosolids amended soil and that the animal is exposed to a background pollutant
92 Forste intake. For Pathway 6, when a sensitive species has been identified for a specific pollutant, that species is used in the exposure assessment (e.g., livestock, domestic grazing animals, birds and rodents). Pathway 7 - Designed to protect the highly sensitivelhighly exposed herbivorous livestock animal which incidentally consumes biosolids adhering to forage crops and/or biosolids on the soil surface. Pathway 7 assumes a 1.5 percent biosolids in the livestock diet and a background pollutant intake for the animal, as well as the most sensitive species for which data are available for each pollutant. Pathway 8 - Evaluates the risk of plant toxiciyfiom pollutants i n biosolids. EPA determined an allowable pollutant concentration in the soil that would be associated with a low probability (I x lo‘) of a 50 percent reduction in young plant growth (not necessarily a mature plant yield reduction). Since phytotoxicity resulting from metals is sensitive to changes in pH, plant species and the degree of binding capacity in the biosolids matrix, EPA elected to “cap“ at the 99th percentile pollutant concentration from the NSSS for some metals. Pathway 9 - Designed to assist in establishingpollutant loading litnits to protect highly exposeahighly sensitive soil biota. Since only limited field data exist which indicate levels at which inorganic pollutants become toxic to soil biota, the database available for earthworms which were raised in biosolids were used to set the criteria for this pathway. These criteria are based on a No Observed Adverse Effect Level (NOAEL) for the earthworm Eisenia foetida. Pathway 10 - Designed to assist in detemiining pollutant loading liriiits to protect highly sensitive/highly exposed soil biota predators (i.e., sensitive wildlye that consuiiies soil biota that have beenfeeding on biosolids-amended soil). A literature review identified what the Agency determined is a pollutant intake level protective of sensitive species in general. Chronic exposure assumes the sensitive species diet to consist of 33 percent of such soil biota. - Pathway 11 Evaluates liuiiian exposure to biosolids pollutants through inhalation. A tractor driver tilling the field is the HE1 for Pathway 1 1 which evaluates the impact of suspended particles of dewatered biosolids. Pathway 1 1 assumes the incorporation of biosolids to a depth of 15 cm and a distance from the driver to the soil surface of 1 m with no more than 10 mglcubic meter (mglm’) of total dust.
Regulatory Requirements 93 Pathway 12 - Designed to protect surface waters for benejkial use in order to protect both human health and aquatic lfe. The runoff of pollutants from soil on w h c h biosolids have been applied is calculated so that the soil pollutant concentrations would not result in exceeding a Water Quality Criterion for a pollutant if the soil enters a relatively small stream. Rate of soil loss was based on the Universal Soil Loss Equation and a sediment delivery ratio. Water Quality Criteria designed to protect human health assume exposure through consumption of drinking water and resident fish and are also designed to protect aquatic life. Pathway 13 (Land Application) - Evaluates the exposure of a fann fami4 inhaling vapors of volatile pollutants that niay be in biosolids applied to the land. Six pollutants were included in this pathway: benzo(a)pyrene, dis(2- ethylhexyl)phthalate, chlordane, DDT, dimethylnitrosamine, polychlorinated biphenyls. These pollutants were selected from EPAs hazard indices screen because they are semi-volatile. Organic pollutants which are highly volatile were not evaluated for Pathway 13 because such compounds would volatilize in the wastewater treatment processes and were therefore not considered to be of concern. Similarly, non-volatile metals were not evaluated in the vapor pathway. The vapor pathway assumes that the total amount of the pollutant spread each year would vaporize during that year. The allowable annual pollutant loading rate is thus equal to the amount that may be allowed to enter the atmosphere per unit area per unit time without exceeding the allowable pollutant concentration in the air. This concentration corresponds to the R E , risk specific dose or an acceptable daily dose derived from an MCL. A plume model was used to determine the resultant pollutant concentration in the air with a never changing wind direction so that the HE1 always remains in the center line of the plume. Pathway 13 (Surface Disposal) - Evaluates the exposure of an individual inhaling vapors o any volatile pollutants for a 70-year period. f The HE1 is assumed to live at the downwind edge of the site and to inhale air, at 20 cubic meters per day, for 70 years, contaminated with volatile organic compounds from a surface disposal site. Volatilization rates were calculated with equations that consider constituent parameters. Allowable lifetime exposure (at a risk level of lo4) is the basis for back-calculating the allowable loss rate to the vapor pathway. This value, divided by the fraction that vaporizes, provides the allowable pollutant concentration at the site.
94 Forste Pathway 13 (Incineration) - Evaluated exposure of an individual inhaling particulates and gasesfi.oni an incinerator 24 hours per day, 365 days per year f o r 70years. This HE1 is located when the highest annual ground concentration of incinerator emissions occurs. This pathway evaluated five metals (arsenic, cadmium, chromium, lead and nickel) and approximately 70 organic compounds which are represented by a surrogate measure of total hydrocarbons. Allowable metal concentrations are calculated by determining the removal efficiency across the incinerator and air pollution control devise site-specifically and considering the feed rate to the system. Pathway 14 (LandApplication) - Evaluated exposure of individuals obtaining drinking water fioni groundwater directly below a j e l d where biosolids were applied. The allowable pollutant rate was determined from the MCL that must be met at the groundwater interface with no allowance for dilution, the decay rate of a pollutant and other factors that affect either the time period for decay or the dispersion of the peak conccntration. Pathway 14 (Surface Disposal) - Evaluated exposure o individuals who f obtained their drinking water from wells located 150 iiielers or less downgradient, using eitherMCLs or iisk level of I x IQfor an HEI consunling two liters of water per day over a 70year lifetime. Numerical limits were derived for both covered and uncovered suiface disposalsites, considering pollutant losses through three processes: volatilization, on-sitedegradation, and leaching. Public comments and the scientific peer review of 503 resulted in changes inmany of the assumptions and data originally proposed for the exposure assessmentmethodology. These changes resulted in more realistic exposure scenarios re1ative tothe use of biosolids which are land applied for agricultural purposes. A discussion ofthe major comments, responses and actions taken by EPA are contained in the finalPart 503 Regulations and the Techrucal Support Document for Land Application. [ 141These sources provide detailed information on the Agencys response to the significantcomments on the exposure pathways and the final action in the 503 Rule resultingfrom the Agencys evaluation of such comments.Domestic SeDtaneMany of the 130 commenters who provided input to the Agency on the proposedregulations for septage, opposed the 1989 proposed limits as overly stringent,burdensome and with little or no environmental benefit. As a result of thesecomments, the Agency agreed that the regulation of septage in a manner similar to
Regulatory Requirements 95biosolids would have been difficult to implement and therefore unlikely to achieve theAgencys public health and environmental statutory objectives. Based on thisassessment, EPA developed an alternative regulatory strategy for septage whichreplaced the 503 pollutant monitoring and cumulative pollutant loading limits used forbiosolids with a single hydraulic loading rate (30,000 gallons per acre per year). Thcrevised approach also requires short-term lime stabilization to control pathogcn andvector attraction or site use and access restrictions in the absence of pH control. EPAalso simplified many of the monitoring, recordkeeping and reporting requirements fordomestic septage. The h a l rule on domestic septage contains limits on hydraulic loading based onan average septage analysis to provide an estimated crop nitrogen rate. Short-teiialkaline stabilization for septage may be achieved by raising the pH to 12 or greaterfor 30 minutes. If alkaline stabilization is not possible and domestic septage is appliedto agricultural land, crops whose edible portions may contact the surrace soil and rootcrops grown in the soil are prohibited for 14 and 38 months rcspectively following septage application. In addition, for sites where potential for public exposure is high (e.g., parks and recreational areas) public access must be restricted for 12 months following application of domestic septage; whcre such potential esposure is low, public access must be restricted for 30 days. Similarly, animals must not be allowed to graze or feed crops harvested for 30 days. [ 151C. Final Part 503 StandardsThe standards contained in the final Part 503 Regulations consist of generalrequirements, pollutant limits, management practices, operational standardsand requirements for frequency of monitoring, recordkeeping and reporting.In the final regulation, EPA uses the term "land application" in a restrictive sense todelineate clearly between different regulatory requirements. Since biosolids arenot only disposed on land, but in many cases also used to condition the soil orprovide nutrients, the Part 503 uses the phrase land application only when referringto biosolids used for their beneficial properties. When biosolids are disposed of byplacing them on the land, Part 503 rcfers to this practice as "surface disposal." Requirements for biosolids applied to the land differ for bulk material or thatsoldgiven away in a bag or other container. The latter is used to distinguish situationsin which biosolids are typically applied in small amounts in a single application (e.g.,home gardens) from those in which biosolids may be applied in large quantities ovcrwide areas (e.g., agricultural and reclamation use) or bulk material. Many of therequirements in the regulation apply to the "person who prepares" biosolids--referringto the person or entity that effectively controls the quality of the biosolids or thematerial derived therefrom that is ultimately either used or disposed. For esamplc, insituations where a treatment works generates biosolids that are blended with othcr
96 Forstesubstences, the person blending is the one who prepares the biosolids since he controlsthe quality of the material that is ultimately u e or disposed. sd The final rule requires that any person who generates biosolids or derives amaterial from biosolids, any person who applies biosolids to the land, anyownedoperator of a surface disposal site, and any person who fires biosolids in anincherator must maintain certain records. It also establishes reporting requirementsfor Class I biosolids management facilities, publicly owned treatment works (POTWs)with a design flow rate 2 1 million gallons per day, and POTWs that Serve 10,000people or more. The "surface dqmd"subpart (C) in tefinal regulations combines the originally hproposed subparts on monofills and surface disposal because differences betweentheae two did not merit separate treatment, since in either case the material is placedon the land for final disposal and may present essentially similar potential threats topublic health and the environment. EPA also moved the subpart on removal credits fiom the final Part 503Regulations to the general pretreatment regulations (40 CFR Part 403) since theAgency believed ta these lists of pollutants eligible for removal credit more logically h!belonged with the pretreatment requirements.TABLE2-2 POLLUTANTS IN PART 503 ELIGIBLE FOR A REMOVAL CREDIT Uae or Diapoepl Prnctke Unlined Linedb Arsenic - - 100 - AldrinlDie1cb-h(Total) 2.7 - - - Benzane 16 140 3400 - -oopyrene 15 ~ 100 looc - Bis(2-ethylhexyl)ate - looo 100 I cadmium - 100 l00C - Chmmium - - 100 - copper - 4 6 looC 1400 DDD, DDE, DDT (Total) 1.2 2000 2000 -
Regulatory Requirements 97TABLE 2-2 (COW.)Mermry - 100 lOOC I Molybdenum - 40 40 - Nickel - - 100s -N-N- ylamine 2.1 0.088 0.088 - Pentachlorophenol 30 - - - Phenol I 82 82 - Polychlorinated biphenyls 4.6 <50 <50 - Selenium - 4.8 4.8 4.8 Toxaphene 10 26 26 - Trichloroethylene 1O C 9500 100 - zinc - 4500 4500 4500 - -Key: LA land application, SD surface disposal I - incinerationSewage sludge unit without a liner and leachate collection system Sewage sludge unit with a liner and leachate collectionsystem Value expresd in grams per kilogram - dry weight basis. Table 2-2 identifies pollutants by use or dqosal practice and a concentrationforeach pollutant which was determined by the EPA not to pose an unreasonable risk topublic health and the environment if the concentration for those pollutants in thebimlids are below the concentrationsgiven.[161 Pollutants were placed on this listfor one of two reasons. First, available data at the time the original list of pollutants
98 Forsteof concern for Part 503 was developed indicated that the existing concentrations ofpollutants in biosolids did not exceed those concentrations contained on the list inTable 2-2. EPA also determined that at those concentrations the pollutants do notpose a threat to public health and the environment at the highest levels detected.Secondly, the pollutant was placed on the GI1 list shown in Table 2-2 if, afterdetermining a risk level, EPA decided not to regulate this pollutant in the final Part503 Regulations. The concentration specified for those pollutants to be eligible forremoval credits are the concentrations developed during the risk assessment. Thesepollutants are designated with an asterisk on the list in Table 2-2. Based upon either low concentration or low risk as described above, a removalcredit is available for those pollutants with respect to the use/disposal of biosolids ifthe concentration of the pollutant in the biosolids is less than or equal to theconcentration shown in Table 2-2 and if the treatment works complies the applicablerequirements in 40 CFR 403.7. A POTW applying for removal credits must provide proof that the pollutantconcentrations in its biosolids do not exceed the pollutant concentrations shown inTable 2-2. If subsequent monitoring reveals pollutant concentrations above thoselevels or any more stringent level in the biosolids permit, the POTW is no longereligible for removal credit authority for that pollutant. Removal credit eligibility ish t e d to those pollutants regulated specifically in Part 503 and to pollutants that theAgency determines do not threaten public health and the environment as specifiedconcentrations. The general provisions of Part SO3 establish standards for biosolids applied to theland or disposed by surface disposal or incineration. The requirements for reducingorganisms in biosolids that cause disease (pathogens) are contained in Subpart D ofPart 503. Either Class A or Class B pathogen reduction requirements must be metwhen biosolids are applied to the land or placed on a surface disposal site. In addition,the regulations require reduction of vector attraction, that is, control of those characteristics of biosolids that attract disease-spreading agents (e.g., flies or rats)when applied to the land or placed on a surface disposal site. There are no pathogen or vector atlraction reduction requirements for biosolids fired in an incinerator which achieves such reduction during the incineration process. For land application, the pollutant lmt and management practices protect public iis health and the environment from reasonably anticipated adverse affects of arsenic, cadmum, chromium, copper, lead, mercuiy, molybdenum, nickel, selenium and zinc in the biosolids. EPAevaluated the risks associated with organic pollutants and other inorganic pollutants in biosolids and concluded that numerical pollutant limits for these are not required for the reasons described in this chapter. For surface disposal, the pollutant limits and management practices are established to protect public health and the environment from the reasonably anticipated adverse affects of arsenic, chromium and nickel in the biosolids. For incineration, these limits and practices are based on arsenic, beryllium, cadmium,
Regulatory Requirements 99chromium, lead, mercury and nickel in the biosolids. In addition, EPA established anoperational standard for biosolids incinerators that limit the total hydrocarbons (THC)in the exit gas from a biosolids incinerator stack. The recordkeeping requirements of the 503 Regulations specify who mustdevelop and retain information, what information must be developed and the lengthof time such information must be kept. Section 405(f) of the CWA provides thatpermits issued to a publicly owned treatment works or any treatment works treatingdomestic sewage shall include conditions to implement the Part 503 Regulationsunless such are included in permits issued under other federal or approved stateprograms. Thus,Part 503 requirements may be implemented through a CWA permit,a Subtitle C Solid Waste Disposal Act Permit, a Part C Safe Drinking Water ActPermit, a Marine Protection Research and Sanctuaiy Act Permit, a Clean Air ActPermit, a permit under an approved state program or an EPA-issued "Sludge Only"permit. However, it should be noted that the requirements in the Part 503 must be meteven in the absence of a permit, i.e., the Part 503 is self-implementing. Thus aresponsible person must become aware of the Part 503 standards, comply with them,perform appropriate monitoring and recordkeeping and, if applicable, report information to the permitting authority even when a permit is not issued. These standards are also directly enforceable against any person who uses or disposes of biosolids through any of the practices addressed in the final regulations. An enforcement action can be taken against a person who does not meet those requirements even in the absence of a peimit. Relevant dates for the biosolids program are shown in Table 2-3.Pollutants Selected for RegulationNumerical limits in the 503 are imposed for 10 metals rather than the 28 inorganic andorganic pollutants contained in the proposed regulation. EPAs decision to select thefinal 10 metals for regulation was based on analysis of information provided on theproposed Rule and the data from the National Sewage Sludge Survey. The Clean Water Act language requires EPA to develop standards on the basisof available information on pollutants toxicity, persistence, concentration, mobility orpotential for exposure. These criteria led to the original proposal of 28 pollutants,however, the Agency deteimined for the final rule that some of these should not beregulated, either because they are not present in biosolids, or, if present, the potentialfor exposure and therefoi-ehealth or environmental risk is small. EPA concluded that the concentrations of a pollutant do not pose a public healthor environmental risk if: 1. the pollutant is banned or restricted or no longer manufactured or used in manufacturing a product 2. based on data from the NSSS the pollutant was not found in biosolids at significant frequencies of detection
100 Forste ABLE 2-3 RELEVANT DATES FOR THE BIOSOLIDS PROGRAM Publication of Part 503 at 58 FR 9248 February 19,1993 ~~~ Publication of amendments to Sewage Sludge Permit Program February 19, 1993 regulations at 58 FR 9404 Effective date of Part 503 March 22,1993 Requirements for monitoring and recordkeeping under Part July 20, 1993 503 become effective (except for THC) Compliance date for Part 503 requirements other than February 19,1994 monitoring, recordkeeping and reporting (where construction is not required) Rcquireinents for reporting under Part 503 become effective February 19,1994 (except for THC) Limited permit application information due from sludge-only February 19,1994 facilities (not needing site-specific limits) Date for closure of active sewage sludge units 1) located within March 22, 1994 GO meters of a fault that have displacement in Holocene time (unless authorized by the permitting authority); 2) located in a wetland (unless authorized under an NPDES permit); or 3) located in an unstable area Compliance date for Part 503 requirements other than February 19,1994 monitoring, recordkeeping and reporting (where construction is required) Date when active sewage sludge unit owners/operators must 180 days prior to the submit closure plans date the unit closes Permit application information duc from facilities with NPDES At the time of the pcrniits (not needing site-specific limits) next NPDES permit renewal Permit application information due from facilities who 180 days prior to the commence operation after 2/19/93 date proposed for commencing operation
Regulatory Requirements 1013. the risk assessment for the pollutant shows no reasonably anticipated adverse affects at the 99th percentile concentration found in biosolids as per the NSSS. The NSSS was specifically designed to resolve data deficiencies which EPAacknowledged existed in the information used to develop the proposed rule. Based on the results of the survey and the status with respect to the production ofa pollutant, the above three criteria provide a reasonable certainty that numericalpollutant limits are unnecessary. For example, if the pollutant is banned fromproduction, it is highly unlikely that it will be present in biosolids and there is noconsequent need to establish numerical limits. For a number of the banned and nolonger manufactured pesticides that EPA had proposed to regulate, examination of theNSSS data confirmed that these pesticides were not present in todays biosolids.Fourteen pollutants meet the criteria detailed above and are no longer regulated in thePart 503 Rule: aldriddieldnn benzene benzo(a)pyrene bis(2-ethylhexy1)phthalate chlordane DDT (and its derivatives DDD and DDE) dimethyl nitrosamine heptachlor hexachlorobenzene hexachlorobutadiene lindane polychlorinated biphenyls toxaphene trichloroethylene Details on the analysis used in deciding not to regulate are contained in AppendixA of the Technical Support Document for Land Application.m e g a t e Risk AssessmentEPAs aggregate risk assessment evaluated the "baseline" public health impacts ofbiosolids use and disposal without the 503 Regulations, assessed the impacts afterimplementation and established the difference between these two estimates as thepublic health "benefit"of the final rule. For the baseline estimates, EPA used a sample of plants from the NSSS and usedadditional information on the plants from the questionnaire portion of the NSSS todevelop a profile of use and disposal practices across the country. The Agency also
102 Forsteused the data to assign pollutant concentrations under baseline conditions to treatmentplants in different categories. Data describing cancer potency or other effects were used to develop measuresof individual risk. The results provided an aggregate determination of the likelynumber of individuals experiencing each effect expected per year in the affectedpopulations as a result of biosolids. Details on the aggregate risk assessment arecontained in the Technical Support Document, "Human Health k s k Assessment forthe Use and Disposal of Sewage Sludge: Benefits of Regulation"[l7]. For landapplication, EPA used average values for the pollution concentrations from facilitiespracticing land application as deteimined by the NSSS. The mathematical modelscontained in 503 were used to predict the transport of pollutants to groundwater andambient air for the baseline assessments. Additional modelling was used to predictthe uptake of pollutants from treated soil to agricultural crops and animal tissues andto estimate contamination of surface water. Using average population densities, theAgency established current human exposure to and risk from each pollutant inbiosolids through each relevant exposure pathway. The final step in the assessment of current risks was to extrapolate results to thenational level based on the estimated number of facilities practicing land applicationand the quantity of biosolids applied. The baseline assessment showcd a pre-503 riskof less than one possible cancer case, 1000 possible cases of individuals who wouldexceed a threshold lead concentration and 500 possible individuals experiencing leadrelated effects. In no CRSC did the average exposure to the other inorganicpollutants for which limits are included in the land application exceed the reference dose for the pollutant during the baseline assessment. The aggregate risk assessment & implementation of the final part 503 land application regulations are: <1 possible cancer case, <I possible individual who exceeds a blood lead level and 4 lead case. The results indicate that pre-503 use and disposal practices for biosolids posed little risk to public health. Sufficient data were not available for deteimining how the regulation would reduce human exposure and risk from land application; therefore, benefits from regulating this practice range from 0 to 100 percent of the estimated baseline risks. Even though the estimated baseline risks from all biosolids use and disposal practices combined were very low, the aggregate risk analysis shows that the adoption of the final standards will further ensure that these use and disposal practices will not jeopardize public health in the future. Relationship to Other Requirements As noted above, biosolids disposed in a municipal solid waste unit must meet the Part 258 requirements, and if uscd as a landfill cover, must be suitable for that purpose. Even though thc final 503 Regulations do not have a stormwater requirement, the CWA required that EPA, under its NPDES permit application requirements, control stormwater associated with industrial activity. Thus, treatment works treating
Regulatory Requirements 103domestic sewage or any device or system used in the storage, treatment, recycling andreclamation of municipal or domestic sewage, including land dedicated to the disposalof biosolids that are located within the confines of the facility with a design flow of 1.OMGD or more or required to have an approved pretreatment program under 40 CFRP r 503, must control stormwater. Specifically not included are farm lands, domestic atgardens or land used for a beneficial use of biosolids which are not physically locatedwithin the facility. For this category, EPA viewed fac es such as large treatmentworks that may experience spills and bubbleovers (e.g., on-site composting andchemical storage) as suitable candidates for stormwater permits. Such activities areconsidered to be more akin to industrial activity in scope and size and hence shouldbe required to obtain an NPDES permit for stormwater discharges. The 503 Regulations do not establish requirements for the specific treatment of biosolids except for properties (other than chemical composition) that may pose a threat to public health and the environment. Therefore, requirements to reduce pathogens and vector attraction are provided while processes used to prepare biosolids for final use or disposal (e.g., composting) are not subject to the 503 standards. The pathogen requirements of Part 503 are not risk-based but are pei-fomiance standards based on the demonstrated ability of treatment processes to reduce pathogens. Such requirements include raising the temperature of the biosolids and maintaining that temperature for a specific period of time. While EPA encourages the beneficial use of biosolids through such practices as land application, the specific use or disposal practice selected by a municipality or authority responsible for biosolids remains a local determination and responsibility. Therefore, the Part 503 does not establish requirements for the selection of a use or disposal practice. The Part 503 Regulations do not establish requirements for use or disposal of biosolids from an industrial facility which treats industrial wastewater because those materials are not biosolids which are generated during the treatment of donicslic sewage in a treatment works. If industrial wastewater solids are disposed on the land, the requirements of 40 CFR Pait 257 must be met. The 1987 Water Quality Act included under Section 405(d) industrial manufacturing and private processing es that treat domestic sewage combined with industrial wastewater. During the development of the P r 503, insufficient time was available to develop standards for at use or disposal ofwastewater solids generated at such facilities. In addition, EPA did not have sufficient information on the number of such facilities, the amount of solids generated at them and the practices through which the solids are used or disposed in order to evaluate the impact of the Part 503 numerical standards. The Agency also questioned whether the models and data used to develop the numerical limits in 503 are appropriate for industrial wastewater solids which also have a domestic scwage component. For these reasons, the Part 503 does not establish requirements for the use or disposal of biosolids generated at an industrial facility during the treatment of industrial wastewater combined with domestic sewage. Such materials may be
104 Forsteconsidered in future revisions to the Part 503. It should be noted however that the Part503 Regulations do apply to biosolids generated at an industrial facility during thetreatment of only domestic sewage, that is, without combining the domestic sewagewith industrial wastewater. Thus all industrial wastewater treatment facilities that treatdomestic sewage, whether generated on site or off site, are considered TreatmentWorks Treating Domestic Sewage and may be required to apply for a permit under 40CFR 122.21. The Part 503 Regulations do not establish requirements for wastewater solidsdetermined to be hazardous; these materials must be used or disposed under the,applicable requirements of 40 CFR Parts 260 through 268. Biosolids having aconcentration of polychlorinated biphenols 3 50 mgkg of total solids (dry weight)must be used or disposed in accordance with the requirements of 40 CFR Part 76 I ,not the P r 503 requirements. Ash from incinerated biosolids also are not subject to atthe Part 503 but must be used or disposed in accordance with the appropriate requirement (e.g.,40 CFR Part 257 when disposed on the land). Wastewater grit and screenings are not subject to the Part 503 Regulations as they have completely different characteristics than biosolids and must be disposed in accordance with appropriaterequirements (e.g., 40 CFR Part 257 when disposed on the land). Solids generated from the processes which produce drinking water also are not subject to Part 503 since they are not generated during the treatment of domestic sewage in a treatment works. Since the characteristics of domestic septage and the characteristics ofcommercial septage (e.g., grease from a grease trap at a restaurant) and industrial septage (e.g., from a septic tank or similar treatment works that receives industrial wastewater) are different, the Part 503 requirements for domestic septage do not apply to commercial or industrial septage and these are excluded from the Part 503 Regulations.Requirements for a Person Who Premres BiosolidsWhen biosolids are prepared to be applied to the land, placed in a surface disposal siteor incinerated, the person who performs such preparation must meet the applicablerequirements in the Part 503. This preparer could be the person who generatesbiosolids during the treatment of domestic wastewater or a person who derives amaterial derived fiom biosolids. The latter would include, for example, a person thatblends biosolids with some other material or a private contractor who receivesbiosolids from a treatment works and then blends the biosolids with some othermaterial (e.g., a bulking agent). When biosolids are part of any material, the person"derived" the material fiom biosolids and similarly any time the quality of the biosolidsis changed, a "material" is derived from those biosolids. Although not explicitly defincd in the 503 Regulations, biosolids which meet aClass A pathogen reduction requirement, an appropriate vector attraction reductionrequirement and contain metals concentrations below those in 503.13(b)(3) (pollutantconcentration limits) are not subject to any further regulatory requirements of 503
Regulatory Requirements 105beyond monitoring and reporting to establish conformance with the three requirementsspecified above. Thus, a person who blends such biosolids is not subject to anyfurther regulatoly requirements, including monitoring and reporting. This distinctionis especially important for products (e.g., heat-dried biosolids) which are often soldto a dealer who then blends the biosolids with some other material for sale to theconsumer.Pollutant LimitsBased on numerous comments on both the 1989 proposed and the 1990 NSSS, EPAconcurred with the view that protection of public health and the environment allowedfor the development of standards for biosolids that receives minimal regulation as aresult of meeting certain quality requirements. Such materials may either meet ClassA pathogen reduction requirements and be subject to no fi.uther federal regulation, ormeet Class B pathogen reduction requirements and be required to conform to thegeneral requirements and management practices for land application. The numerical s t a n h d s for biosolids not subject to cumulative pollutant loadinglimits result from the loading rates for inorganic pollutants (i.e. trace or "heavy"metals) through the various exposure assessment pathways described above. EIAderived the pollutant concentrations in such materials from calculations using thealready established cumulative pollutant loadings and applying certain conservativeassumptions to back calculate to a pollutant concentration. Maximum metalconcentrationsfor such biosolids are shown in Table 2-4 (Table 3 of 6 503.13). Sincepollutant concentrations are based on the cumulative pollutant loading rates andinclude the same conservative safety factors, they provide the same degree ofprotection to human health and the environment as is provided by the cumulativepollutant loading rates. For the final rule, the cumulative pollutant loading rate was converted to anannual pollutant loading rate by assuming that the entire load for the pollutant was applied in one year, which EPA believes is a conservative assumption and unlikely tooccur. Annual pollutant loading rates were then converted to pollutant concentrationsusing the following equation: C APLR xo.001 ....................... AWSARWhere: C = pollutant concentration in m g k g (dry weight) APLR = annual pollutant loading rate in kg/hectare/365 day period (diy weight) AWSAR = annual whole sludge application rate in metric tonshectare/ 365 day period (dry weight) 0.00 1 = conversion factor
106 Forste EPA assumed that the AWSAR used in this equation is 10 dry metrictonshectare/year for 100 consecutive years. EPA believes that a pollutantconcentration derived fiom h s equation is conservative because it is unlikely that anyone site will receive 10 metric tons of biosoliddhectarehjear for 100 consecutiveyears. It is also unlikely that nutrient-based (agronomic) application rates foragricultural land will require such annual application rates for 100 consecutive years.TABLE 2-4 POLLUTANT CONCENTRATION (FC) BIOSOLIDS* %Biosolidsmeeting these pollutant concentration lintits are not required to be tracked with respect to cuinirlative loadings on sites where they are applied to land. If biosolids meet the pollutant concentrations calculated using the above equation, the more stringent Class A pathogen reduction requirements and one of eight vector attraction requirements in the final 503 Rule, the general requirements and management practices for land application do not apply if such biosolids are applied to the land. However, minimum frequency for monitoring, recordkeeping and reporting however do apply since the EPA needs to have information to determine that
Regulatory Requirements 107 the 503 quality requirements are being met. This reduction of general requirements and management practices applies both to bulk biosolids applied to the land and to biwlids sold or given away in bag or similar container. It also applies to a material derived ffom biosolids if that material is derived fiom a biosolids that does not meet the three q d t y requirements. Land AvDlication The 503 Regulations recognize two broad categories of biosolids applied to the land and establish requirementsfor each. For both categories,biosolids must meet metal ceiling concentrations, otherwisethey cannot be applied to the land. The first broad categq is bulk biosolids applied to the land, that is, biosolids that are not sold or given away in a bag or other container. These biosolids must meet either pollutant concentration limits or the amount of the pollutant applied to the land in the bulk biosolids must not exceed a cumulative pollutant loading rate. The ceiling limits, cumulative pollutant loading rates and annual pollutant loading rates are shown in Table 2-5 (Metal Concentrations and Allowable Loadmg Rates). In addition, pathogen and vector attraction reduction requirements must be met for bulk biosolids applied to the land and general requirements and manageme$ practices may have to be met depending on the degree of treatment for pathogen reduction of the bulk biosolids. For t e second broad categq--biilida sold or given away in a bag or other h container for application to the land--oneof two pollutant limits also must be met. The biosolids must meet the pollutant concentrations in Table 2-4 for bulk biosolids or the amount of a pollutant applied to the land annually must not be exceeded in an annualpollutant loading rate (Table 2-5). Annual pollutant loading rates are used to calculate the application rate specified on a label on the bag or other container. This rate cannot be exceeded and is based on EPAs estimation of 20 annual applications for biwlids combined wt the cumulative pollutant loading limits (the CPLR + 20). ih The annual rate i for the whole biosolids, not just for a single pollutant, although one s such pollutant will govern the annual rate. In addition, these biosolids must meet the highest quality (Class A) pathogen reduction requirements and a vector attraction reduction requirement. Depending on quality, such biosolids may also be subject to general requirements and a management practice. The general requiranentSand management practices for biosolids are contained in Figures 2-2 and 2-3. The land application requirements apply to any person who prepares biwlids t a are applied to the land, any person who applies biosolids to the ht land, to the biosolids applied to the land, and to the land on which biosolids are applied.
Pollutant Ceiling Cumulative Annual Pollutant Concentretion* Pollutant Loading Rates Pollutant (ww Loading Rates WLR) (Pc) (CpLR) @@la) Wac) (kglhalyr) (Ib/ar/yr)Nickel 420 420 375 21 19Selenium 100 100 89 5 45 .Zinc 7,500 2,800 2,500 140 125 ~ ~Asper§ 503.13, Table I , 2 and 4. Wastewaterbiosolicis or biosolids-derivedproductsthat are to be land-applied, usedfor reclamation purposes, or distributed and marketed must meet these criteria at a minimum. Molybdenum limits (other than ceiling concenb-ations)have been remmedfiom the 503 Regulationspending re-evaluation of scientific data and re-proposal.
Regulatory Requirements 109For Bioaolidn Meeting 503.13@)3,50332(n) Additional Requirement. for CPLR Biosolidrand one of 5 3 3 ) 1 through @)@) 03@() The applier m s not@ the permittingauthority utNone (unless set by EPA or State permittmg in the State where bulk biosolids are to be appliedauthority on a CBsebyCBsebasis for bulk prior to the initial applicationof the biosolids.biosolids to p t e c t public health and the This is a onetime notice requirementfor eachenvironment). land applicationsite each time there is a n w e applier. The notice m s include: utFor PC and CPLR B l m U + the location (either street address or latitudeTheprepma. must not@ and provide and longitude)of the land applicationsite;informationnecessBty to comply with the Part and503 land applicationrequirementstotheperson 0 the name, addreaq telephone number, andwho applies bulk biosolids to the land NPDES permit number ( ifapproPriate)of The prepare who provides biosolids to awther the person who will apply the bulk biosolids.persoa who further prepares the biosolids for The applier m s obtain records (if available) utapplicationto the land m s provide this person ut h m the previous applier, landowner, orwith notification and informationnecessary to pennittingauthoritythatindicatatheamountofcomply with the Part SO3 land application each CPLR pollutant in biosolids thathave beenreqUirlXllentS. applied to the site since July 20, 1993. In Theprepara m s provide written notification ut addition:of the total nitrogenCOncenMion (as N on a dry- + when these records are available,the upplier mustusethisinfmationtodeterminetheweight basis) in bulk biosolids to the applier ofthe biosolids to agricultural land, forests, public additionalamount of each pollutant that canoontact sites, or reclamation sites. be applied to the site in Bcoordatlcewith the The upplier of biosolids m s obtain ut CPLRS in Table 2-5;infomtion 7 comply with the Part to + the upplier m s keep the previous records utSO3 land applicatronrequirementS, apply and also record the additionalamount ofbiosolids to the land in BEcordance with the Part each pollutanthe or she b applying t t e o h503 land application requirements, and provide dte; andnotice and necess~ly informationto the owner a when records ofpast known CPLRleaseholder of the land on ~ c biosolids are h applicationssince July 20,1993,arenotapplied. available,biosolids subjectto CPLRs cannot be applied to that site. out of state u e s Ifbiosolids meeting CPLRs have not been Thepreparermustprovidewittennotification applied to the site in excess of the l i t since July (pior to the initial applicationof the bulk 20,1993,the CPLR limit for each pollutant in biosolids by the applier) to the permitling Table 2-5 will determine the maximum amount authority in the State where biosolids are of each pollutant that can be applied in biosolids pFoposedto be land applied when bulk biosolids E aro gonerated inoae state andtranaferredto + all applicablemanagement practices are ~ 0 t State ~ applicationto the land The h for followed, and notificationm s include: ut + the applier keeps a record of the amount of+ the location (either street address or latitude and longitude) of each land applicationsite; each pollutant in biosolids applied to any given site.~+ theapproltimatetimeperiodthebulk The applier m s not apply additional biosolid ut biosolids will be applied to the site; under the cumulativepollutant loading c4mcept tc + the name, addaess,telephone number, and a site whem any of the CPLRs have beenreached National Pollutant Discharge Elimination System (NPDES) number for both the * The preparer is either the person who g p preparer and the applier of the bulk the biosolids o the person who derivesa matend r biosolids; and &ombiosolids. 0 a&iitionalinformationorpermitsinboth Statea,.ifrequired by the permitting -ty. FIG. 2-2 PART 503 LAND APPLICATION GENERAL REQUIREMENTS, Source: U.S. €PA. A Plain English Guide to The €PA Part 503 Biosolids Rule. EPA/8324?-93/003. September 1994
110 Forst eFor Biosolids Meeting 503.13(b)3,503.32(a) and one of 503.33(b)(l) through (b)(8)None (unless established by EPA or the State perniitting authority on a ease-by-case basisfor bulk biosolids to protect public health and the environment). For PC (Pollutant Concentration) and CPLR (Cumulative Pollutant Loading Rate) Biosolids These types of biosolids cannot be applied to flooded, frozen, or snow-covered agricultural land, forests, public contact sites, or reclamation sites in such a way that tlic biosolids enter a wetland or other waters of the United States (as defined in 40 CFR Part 122.2, which generally includes tidal waters, interstate and intrastate watcrs, tributaries, tlie territorial sea, and wetlands adjaccnt to tlicse waters), cxccpt as providcd in a periiiit issued pursuant to Section 402 (NPDES perinit) or Scction 404 (Dredge and Fill Pcrniit) of the Clean Water Act, as anieiidcd. These types of biosolids cannot bc applicd to agricultural land, forests, or reclaination sitcs that arc 10 nicters or less froin U S . waters, unless otherwise specificd by tlie permitting authority. If applied to agricultural lands, forests, or public contact sites, these typcs of biosolids must be applied at a rate that is equal to or less than the agronomic rate for nitrogen for the crop to be grown. Biosolids applied to reclamation sites may exceed the agronomic rate for nitrogen as specified by tlie permitting authority. These types of biosolids must not harm or contribute to the harm of a threatened or ciidangered species or rcsult in the dcstruction or adverse modification of the spccics critical habitat when applied to the lant. Thrcatencd or endangered species and their critical habitats are listed in Section 4 of the Endangered Species Act. Critical habitat is defined as any placc where a Ilircatcncd or ciidangered species lives and grows during any stage of its life cycle. Aiiy dircct or indirect action (or the rcsult of any dircct or indirect action) in a critical habitat that diminishes the likelihood of survival and recovery of a listed species is considered destructionor adverse modification of a critical habitat. For APLR (Annual Pollutant Loading Rate) Biosolids A label must be affixed to the bag or other container, or an infonnation sheet must be provided to tlie person who receives APLR biosolids in other containers. At a minimum, the label or information shcct must contain tlie following information: + the name and address of the person who prcpared the biosolids for sale or giveaway in a bag or othcr contaiiicr; + a statement that prohibits application of the biosolids to the land except in accordance with tlie iiistructions on the label or information sheet; + an AWSAR (see Figure 2-6) for tlic biosolids that do not cause tlic APLRs to be exceeded; and + the nitrogen content. FIG. 2-3 PART 503 LAND APPLICATION MANAGEMENT PRACTICE REQUIREMENTS Source: US.EPA. A Plairi English Guide to The EPA Part 503 BiosolidsRule. EPA/832/R-93/003.Septenr ber I994
Regula tory Requirements 111 For land application, there are instances in which not all the requirements must bemet to comply with the standards: If the biosolids or material derived therefi-om meets certain pollutant limits (see Table 2-4) and operational standards for pathogen and vector attraction reduction as well as fiequency of monitoring, recordkeeping and reporting for land application, the general requirements and management practices do not apply if these three quality requirements are met. For a bulk material derived from biosolids, the land application subpart requirements do not apply when the derived material meets the pollutant concentrations in 503.13(b)(3), the Class A pathogen reduction requirements in 503.32(a) and one of the vector attraction reduction requirements in 503.33(b)( 1) through 503.33(b)(8). In this case, the biosolids used to produce the bulk matcrial already meets the three quality requirements. In the fust instances cited above, an EPA Regional Administrator or a state dircctorin a state with an approved management program may require that any or all of thegeneral requirements or management practices be met on a case-by-case basis, evenwhen the biosolids or bulk material derived therefrom meets the three qualityrequirements, if found that these are needed to protect public health and theenvironment from any reasonably anticipated adverse effect of a pollutant in thebiosolids. The final Part 503 Regulations do not authorize the EPA RegionalAdministrator or the state dircctor to impose the general requirements andmanagement practices on a bulk material derived frJm a biosolids that already meetsthe three quality requirements. Because the 503 requirements do not apply tobiosolids used to make that material once it has met the identified qualityrequirements, no records have to be kept on who receives those biosolids or whathappens to them after the thrce quality requirements are met. For biosolids sold or given away in a bag or other container, the generalrequirements and management practices do not apply when the biosolids or material derived therefrom meet the three criteria dcscribed above. However, monitoring, recordkeeping and reporting requirements do apply to such biosolids derived material. Ceiling concentrations are included in the Part 503 to address EPAs concein about the potential impact of a "dirty" biosolids on public health and the environment. They are the less stringent of two values: the cumulative pollutant loading rate from the land application exposure assessment and assumed 100 year site life at an assumed annual application rate of 10 metric tonshectare the 99th percentile concentration fiom the National Sewage Sludge Suvey, whichever is less stringent. EPA concluded that when pollutant concentrations in biosolids do not exceed the ceiling concentrations, the potential for short term impacts on the environment from land application are greatly reduced. Two pollutant limits for bulk biosolids applied to the land are contained in the Part 503. The first limit consists of cumulative pollutant loading rates in 503.13(b)(2) (see
112 ForsteTable 2-5). The cumulative loading rate is the total amount of an inorganic pollutantthat can be applied to an area of land. Cumulative pollutant loading rates must be metwhen bulk biosolids that do not meet the pollutant concentrations contained in503.13@)(3). To comply with this requirement, the amount of each pollutant in thebulk biosolids must be known, records must be kept of the amount of each pollutantapplied to each site. When the cumulative pollutant loading limit for any of thepollutants i Table 2-5 is reached for a site, no more bulk biosolids may be applied to nthat site. The ceiling concentrations and cumulative pollutant loading rates must bemet when bulk biosolids are applied to agricultural land, forests, a public contact siteor a reclamation site. Equation (2-1) can be used to estimate site life for bulk biosolids with a particular quality and for a c r a n Annual Whole Sludge Application Rate (AWSAR) when the eticumulative pollutant loading rates are met. When either the quality of the bulk biosolids or the AWSAR changes, the site life for the land also changes. In order to use this equation, the concentrations of the regulated metals must be determined on a dry weight basis and the AWSAR determined-- usually based on a selected agronomic rate. The Annual Pollutant Loading Rate (APLR) may be calculated as follows: APLR = C x AWSAR x 0.001 .................... (2-2)Where:APLR = Annual Pollutant Loading Rate for an inorganic pollutant in kilograms of pollutant per hectare per 365 day periodC= measured pollutant concentration in milligrams of pollutant per kilogram of total solidsAWSAR = Annual Whole Sludge Application Rate in metric tons per hectare per 365 day period0.001 = a conversion factor The years an inorganic pollutant can be applied to the land is calculated by dividingthe cumulative pollutant loading rate (CPLR) by the APLR calculated in accordancewith equation (2-1). The lowest number of years calculated in this manner is theperiod that this bulk biosolids can be applied to the land without causing any of thecumulative pollutant loading rates to be exceeded, This calculation is used only toestimate the number of years biosolids can be applied to a site; when the pollutantconcentrations change or when the AWSAR changes, this estimate will also change.
Regulatory Requirements 113EXAMPLE: Given: Step 1 - Pollutant concentrations in bulk biosolids (dry weight basis): oncentration - Step 2 - Assume thc annual whole sludge application rate for the bulk sewage sludge is 10 MThd36.5 day period base on agronomic requirements. Step 3 - Calculate the annual pollutant loading rates for the pollutants using equation (2-2): ~~ Zinc 1201 12.00
114 Forste CPLR APLR Years Pollutant (kgW OCghdyr) (CPLWAPLR) Arsenic 41 0.10 410 Cadmium 39 0.07 557 chromium 3000 8.5 353 Copper 1500 7.4 202 Lead 300 3.50 85 Mercuiy 17 0.05 340 Molybdenum 18 0.09 200 Nickel 420 0.42 1000 Selenium I 100 I 0.05 I 2000 I Zinc I 2800 I 12.0 I 233 I Step 5 - Determine the lowest number of years calculated in Step 4 For this example, the lowest number of years is 85 for lead. Bulk sewage sludgewith the inorganic pollutant concentrations given in Step 1 of this procedure can beapplied to the land at an AWSAR of 10 M T h a for 85 years. After that period, thecumulative pollutant loading rate for lead is exceeded. To develop the pollutant concentrations in Tabre 2-5, the cumulative loading rateswere converted to annual pollutant loading rates and the calculated annual pollutantloading rates and an assumed AWSAR were used by EPA in Equation (2-1) tocalculate a pollutant concentration (C = APLR f AWSAR x 0.001). The pollutant concentrations calculated using the equation (2- 1) were compared tothe 99th percentile concentration values for these pollutants from the NSSS. Thus thepollutant concentration limits in Table 2-4 are either the concentration calculatedusing this equation or the 99th percentile concentration, whichever is more stringent.To convert the cumulative pollutant loading rates to annual pollutant loading rates,EPA assumed that the life of the sitc where biosolids are applied is 100 years and thatthe AWSAR is 10 metric tons/hectare/year based on an estimated agronomic rate.
Regulatory Requirements 115 Because the pollutant concentrations shown in Table 2-4 are based on conservative(100 year) site lives and typical AWSARs, the Agency concluded that records do nothave to be kept of the amount of each inorganic pollutant in biosolids applied to a site.The pollutant concentrations are monthly average concentrations which may not beexceeded in bulk biosolids in order to be exempt from the recordkeeping requirement. EPAs use of the 99th percentile "cap" has been the subject of litigation. Aunanimous three-judge panel of the U.S. Court of Appeals for the D.C. Circuit onNovember 15,1994 remanded to EPA the limits or caps on chromium and selenium.The Court rejected EPAs reliance on the National Sewage Sludge Survey forestablishing the parameters by determining what limits could be met by 99% of thePOTSs responding to the survey, stating that the 99th percentile caps are not relatedto risk. The 3,000 mgkg maximum loading for chromim and the 100 m a g cap for selenium were rejected because EPA had failed to meet the statutory burden of assessing the risk to public health and the environment. EPA will have to redetermine those caps as mandated by the Court. The Court also found EPA had failed to establish a rational relationship between the assumed application of all the covered pollutants, including those that are risk-based, in liquid biosolids and the actual usage of heat-dried biosolids. This part of the regulation was also remanded to EPA either to just@ its assumptions or to provide more tailored caps. Since no risk-based chromium limit can be established based on available field data (which show no adverse effects from chromium) it is likely that chromium will be deleted fi-om the 503 Regulations. It is also expected that the Agency will reevaluate and impose risk-based limits on selenium. For biosolids sold or given away in a bag or other container which do not meet the pollutant limits but are below the ceiling concentrations, annual pollutant loading rates must be met as shown in Table 2-5. These annual pollutant loading rates assume a cumulative pollutant loading rate to be reached in 20 years. That is, an assumed site life of 20 years. EPA concluded that 20 years is a conservative assumption because most biosolids sold or given away in a bag or other container will be applied to a lawn, a home garden or public contact site which EPA believes are unlikely to receive biosolids for longer than 20 years, particularly 20 consecutive years. These annual pollutant loading rates for "bag or other container" biosolids are also being challenged and may be deleted from Part 503 if that challenge is upheld. Domestic Septage A separate pollutant limit is contained in the 503 Regulations for domestic septage applied to agricultural land, forest or a reclamation site. This annual application rate is based on an estimate of the amount of nitrogen needed by the crop grown on the application site by the following equation:
116 Forste N ANC x AAR x 8.34 ..................... .(2-3)Where:N= pounds of nitrogen needed by the crop per acre per 365 day periodANC = available nitrogen concentration in milligrams per literAAR = annual application rate in million gallons per acre per year8.34 = a conversion factorSolving for the annual application rate as follows: N AAR ................................ (2-4) ANC x 8.34 The 503 Regulations require that the allowable annual application rate becalculated using the following equation, which is equation (2-4) with a valuesubstituted for ANC x 8.34: N AAR ................................ (2-5) 0.0026 In tlus equation, 0.0026 is the product of an available nitrogen concentration andthe conversion factor 8.34/1 million. This concentration was calculated using valuesfor Total Kjeldahl Nitrogen (TKN) and ammonia-N in domestic septage from ninesamples collected in Madison, Wisconsin. Tk available nitrogen concentrations werecalculated assuming that:5 domestic septage is applied to the site every year0. domestic septage is injected below the land surface*a none of the ammonia-N in the domestic septage is lost through volatilization00 most of the organic nitrogen (TKN minus ammonia) in domestic septage becomes available over a three year period (50,20 and 10 percent in years 1 , 2 and 3 respectively)00 "steady state" conditions with respect to available nitrogen are achieved in the third year after application of domestic septage
Regulatory Requirements 117 EPA allowed the annual application rate for domestic septage to be used onlywhen it is applied to agricultural land, forests or a reclamation site because siterestrictions are imposed on such sites. EPA assumed that the applier has control overthe application site. Because of the difficulty of imposing site restrictions on a publiccontact site, lawn or home garden application of domestic septage to such site anannual application rate is prohibited under 503.III. PATHOGEN AND VECTOR ATTRACTION REDUCTIONBiosolids applied to agricultural land, forest, public contact sites or reclamation sitesmust meet either Class A or Class B pathogen reduction requirements. Class A biosolids are essentially pathogen free; Class B biosolids have reducedlevels of pathogens, and are subject to use restrictions to prevent immediate and directpublic contact.Class A Pathopen Reduction RequirementsThe implicit objective of all Class A requirements is to reduce pathogen densities tobelow detectable limits which are:*a Salmonella sp. : Less than 3 MI" per 4 grams of total solids (TS), dry weight basis.0- Enteric viruses: Less than 1 PFU per 4 grams of TS, dry weight basis.0. Viable helminth ova: Less than 1 per 4 grams of TS, dry weight basis.The above-referenced requirements define microbiological "cleanliness" of Class Abiosolids. Six alternatives are established for Class A. Any of these alternatives maybe met for biosolids to be Class A with respect to pathogens. These alteinativerequirements are described below and summarized in Table 2-6.Alternative 1: Thermallv Treated Biosolids [503.32(a)(3)]For pathogen reduction, microbiological requirements time-temperaturerequirements must be met. Four time-temperature regimes are specified based onpercent of total solids and operating parameters.Alternative 2: Biosolids Treated bv Alkaline Materials [503.32(a)(4)]Elevating and maintaining a certain pH level for certain time (pH-time requirements)-air drying to certain level of dryness. No microbiological limits are established.and
118 ForsteABLE 2-6 CLASS A PATHOGEN REDUCTION ALTERNATIVES Process Requirements Either F. colifomi or No option to substituteUilknown Either F. colifonn or Yes2 yes3 operational monitoring forProcesses Salmonelia microbiological monitoring.PFKP Either F. colifomi or Seven PFRP processes listed inProcesses Salmonella No No /Appendix B, Part 503.Process Either F. colifonn orequivalent No No Process equivalent to PFRP. ~aliiioiie~ato PFRPNotes: -the density of fecal colifonn must be less than 1,000 MPN per gram of total solids (dry weidit basis) oLthe density of Salmonella sp. bacteria must be less than 3 MPN per 4 grams of total solids (dry weight basis). hi addition to requiretnaitsin 1. above, the density of enteric viruses must be less than 1 PFU per 4 grams oftotal solids (dry weight basis). In addition to rquiremcnts in I. and 2. above, the density of viable helminth ova must be less than 1 per 4 grams of total solids (dry weight basis).Alternative 3: Biosolids Treated in Other Processes [503.32(a)(5)]This alternative requires comprehensive monitoring of bacteria, enteric viruses andviable helminth ova. If a process has been proven to meet microbiological limits,monitoring operating paranicters can be used instead of microbiological monitoring.Alternative 4: Biosolids Treated in Unknown Processes [503.32(a)(6)]This Alternative is similar to Alternative 3, but with no option to substituteoperational monitoring for microbiological monitoring.
Regulatory Requirements 119Alternative 5: Use of PFRP [503.32(a)(7)]Biosolids must be treated in one of the seven (7) specified Processes to FurtherReduce Pathogens (PFRP) listed in Appendix €3 of the 503 Regulations (see Table2-7) & comply with microbiological limits.Alternative 6: Use of a Process Eauivalent to PFRP [503.32(a)(8)]Any process determined by the EPAs Pathogen Equivalency Committee (PEC) to beequivalent to a PFRP compliance with microbiological limits. Several processeshave already (1994) been recommended as equivalent to Class A.In addition, it is required that:( I ) Pathogen reduction be accomplished before or at the same time as vector attraction reduction except for alkaline treatment and drying.(2) Certain microbiological limits be monitored to detect regrowth at the time of use or disposal, or when biosolids are prepared for sale/give-away [503,32(a)(3)- (811.Class B Pathogen Reduction RequirementsClass B requirements can be met by any of the three alternatives shown below. Theimplicit objective of all three altematives is to ensure that pathogenic bacteria andenteric v i ~ are adequately reduced in density (as demonstrated by fecal colifoim wdensity in Class B biosolids of less than 2 million MPN or CFU per gram of totalsolids on dry weight basis). Viable h e h t h ova are not necessarily reduced in ClassB biosolids. Additionally, site restrictions are applied which restrict crop harvesting,animal grazing and public access for a certain period of time until environmentalfactors have further reduced pathogens. The Class B requirements apply to bulk biosolids that are applied to agriculturalland, a forest, a public contact site, a reclamation site or a surface disposal site,unless, in the latter case, biosolids are covered at the end of each operating day. Alternative 1 : Monitoring of Fecal Colifonn [503.32@)(2)] This alternative requires that seven samples of treated biosolids be collected at the time of use or disposal and the geometric mean fecal colifom density of these samples be less than 2 million CFU or ME" per gram of total solids (dry weight basis). Alternative 2: Use of PSRP [503.32@)(3)] Biosolids must be treated in one of the five (5) Processes to Significantly Reduce Pathogens (PSRP) listed in Appendix B of the 503 Regulations (see Table 2-7). No microbiological monitoring is required.
120 ForsteAlternative 3: Use of Processes Eauivalent to PSRP [503.32@)(4)]Any process detennined by the EPAs Pathogen Equivalency Committee (PEC) to beequivalent to PSRP. Several processes have already (1 994) been recommended asequivalent.TABLE 2-7 CLASS A (PFRPI AND CLASS B (PSRP) OPTIONS  Processes to Further Reduce Pathogens Processes to Signiflcnntly Reduce (PFRPs) Pathogens (PSRPs) 1. Composting 1. Aerobic Digestion Using either the within-vessel coniposting Biosolids are agitated with air or oxygen method or the static aerated pile coniposting to maintain aerobic conditions for a method, the temperature of the biosolids is specific mean cell residence time (i.e., maintained at SSEC (131EF) or higher for 3 solids retention time) at a specific day;. temperature. Values for the mean cell residence time and temperature shall be Using the windrow composting method, the between 40 days at 20EC (68EF)and 60 temperature of the biosolids is maintained at days at l5EC (59EF). 55EC (131EF) or higher for I5 days or longer. During the period when the compost is maintained at 55EC (131EF) or higher, there shall be a minimum of five turnings of the windrow. 2. HeatDryIng 2. AlrDrying Biosolids are dried by direct or indirect contact Biosolids are dried on sand beds or on withhot gases to reduce the moisture content of paved or unpaved basins. The biosolids the biosolids to 10% or lower. Either the dry for a minimum of 3 months. During temperature of the biosolids particles exceeds 2 of the 3 months, the ambient average 80EC (176EF) or the wet bulb teniperature of daily temperature is above OEC (32EF). the gas in contact with the biosolids as the biosolids leave the dryer exceeds 80EC (1 76EF). 3. Heat Treatment 3. Anaerobic Digestion Liquid biosolids are heated to a temperature of Biosolids are treated in the absence of air 180EC (356EF) or higher for 30 minutes. for a specific niean cell residence time (i.e., solids retention time) at a specific temperature. Values for the mean cell residence time and temperature shall be between 15 days at 35EC to SSEC (131EF) and 60 days at 20EC (68EF).
Regulatory Requirements 121 TABLE 2-7 (CONT.) 4. Therniophllic Aerobic Digestion 4. Cornposting Liquid biosolids are agitated with air or oxygen Using either the within-vessel, static to maintainaerobic conditions and the mean cell aerated pile, or windrow composting residence time (i.e.., the solids retention time) of methods, the temperature of the biosolids the biosolids is 10 days at 55EC (131EF) to is r a i d to 40EC (104EF) or higher and 60EC (140EF). remains at 40EC (104EF) or higher for 5 days. For 4 hours during the 5-day period, the temperature in the compost pile exceeds 55EC (13 1EF). 5. Beta Ray Irradiation 5. Linie Stabllizatioii Biosolids are irradiated with beta rays from an S d c i e n t lime is added to the biosolids to electron accelerator at dosages of at least 1.0 raise the pH ofthe biosolids to 12 afler 2 megarad at rmm temperature (ca. 2OEC hours of contact. [68EF]). 6. G m i a Ray Irradlntlon Biosolids are irradiated with ganuna rays from certain isotopes, such as Cobalt 60 and Cesium 137, at dosages of at least 1.0 megarad at room temperature (ca. 20EC [68EF]). 7. Pasteurization The tempaature ofthe biosolids is maintained at 70EC (15PEF) or higher for 30 minutes or longer.Site restrictions with respect to types of crops, animal grazing, turf growing andpublic access are also required for Class B biosolids [503.32(b)(5)], as shown inFigure 2-4. Class A pathogen reduction requirement for bulk biosolids applied to a lawn orhome garden is based on the infeasibility of imposing site restrictions on such areas.Rather than imposing site restrictions which would be needed if the bulk biosolidsmet only Class B pathogen requirements, EPA has also imposed the Class Arequirements for biosolids sold or given away in a bag or other container for landapplication.
122 Forste ~~ Food crops with harvested parts that touch the biosolids/soil mixture and are totally above the land surface shall not be harvested for 14 months after application of biosolids. Food crops with harvested parts below the surface of the land shall not be harvested for 20 months after application of biosolids when the biosolids remain on the land surface for four months or longer prior to incorporation into the soil. Food crops with harvested parts below the surface of the land shall not be harvested for 38 months after application of biosolids when the biosolids remain on the land surface for less than four months prior to incorporation into the soil. Food crops, feed crops and fiber crops shall not be harvested for 30 days after application of biosolids. Animals shall not be allowed to graze on the land for 30 days after application of biosolids. Turf grown on land where biosolids are applied shall not be harvested for one year after application of the biosolids when the harvested twf is placed on either land with a high potential for public exposure or a lawn, unless otherwise specified by the permitting authority. Public access to land with a high potential for public exposure shall be restricted for one year after application of biosolids. Public access to land with a low potential for public exposure shall be restricted for 30 days after application of biosolids.FIG. 2-4 CLASS B/SEPTAGE SITE RESTRICTIONS Domestic Septage Pathogen Reduction r503.32(c)]Class A or Class B biosolids requirements apply to domestic septage used on alltypes of land. No pathogen-related requirements apply to domestic septage placedon a surface disposal site. Pathogen reduction in domestic septage applied to agncultural land, a forest ora reclamation site can be met in one of two options:
Regulatory Requirements 123 all Class B site restrictions under 503.32(b)(5) must be met; orc the pH of the septage must be raised to no less than 12 by alkali addition and maintained at this level for 30 minutes without adding more alkali, the site restrictions on crop harvesting must be met. The same site restrictions that apply to Class B biosolids applied to the land arealso imposed when domestic septage is applied, unless a pH requirement fordomestic septage is met along with site restrictions concerning the harvesting ofcrops. These restrictions prohibit harvesting of crops, grazing of animals and publicaccess to the site for a period of time. Restrictions on crop harvesting are part of thissecond alternative because EPA does not believe adequate pathogen reduction isachieved by pH adjustment to allow crops to be harvested immediately after applyingdomestic septage.Vector Attraction ReductionA sigrdicant route for pathogen transport is vector transmission. Vectors are livingorganisms capable of transmitting a pathogen from one organism to anothermechanically (simply transporting the pathogen), or biologically (by having a role inthe pathogens life cycle). Insects, birds, flies, rodents are the most likely vectors. Current USEPARegulations establish twelve options for vector attraction reduction (VAR).Compliance with the vector attraction reduction requirements must be demonstratedseparately from compliance with the pathogen reduction requirements. Vectorattraction reduction options are summarized in Table 2-8. When bulk biosolids are applied to agicultural land, forests, public contact sitesor reclamation sites, one of ten VAR requirements must also be met. This vectorattraction requirement is in addition to the pathogen requirements and both must bemet. One of the first eight VAR options must be met when bulk biosolids are appliedto a lawn or home garden. Injection below the land surface and incorporation intothe soil cannot be used to achieve VAR in these cases since implementation wouldbe dflicult ifnot impossible. Similarly, one of the first eight VAR requirements mustbe met for biosolids sold or given away in a bag or other container for the reasonsjust cited. For domestic septage, the pH elevation to 12 or higher for 30 minutes, injectionbelow the surface of the land or incorporation into the soil after application will allmeet the VAR requirements of 503.Management PracticesWhen bulk biosolids are applied to agricultural land, forests, a public contact site ora reclamation site, the management practices set forth in Figures 2-2 and 2-3 areimposed. These management practices do not apply for bulk biosolids applied to a
124 Forstelawn or home garden based on EPAs determination that large amounts of bulkbiosolids will not be applied to lawns or home gardens for several applications andthe pollutant limits already insure a high degree of protection. In addition, themanagement practices in many cases have no relevance in the lawn or home gardensetting.MonitoringMonitoring frequency requirements in the 503 Regulations apply to both pollutantconcentrations in biosolids and for compliance with pathogen reduction and vectorattraction reduction requirements. Frequency of monitoring requirements also areincluded in the subparts on surface disposal and incineration in order to make thePart 503 self-implementing.TABLE2-8 VEC )R ATTRACTION REDUCl IN OPTIONS  Requirement What is Required? Most Appropriate For: Option 1 At least 38% reduction in Biosolids processed by: 503.33 volatile solids during biosolids Anaerobic biological @XI) treatment treatment Aerobic biological treatinelit Chemical oxidation Option 2 Less than 17% additional Only for anaerobically 503.33 volatile solids loss during digested biosolids that cannot (bX2) bench-scale anaerobic batch meet the requirements of digestion for 40 additional days Option 1 at 30EC to 37EC (86EF to 99EF) Option 3 Less than 15% additional Only for aerobically digested 503.33 volatile solids reduction during biosolids with 2% or less (bx3 1 bench-scale aerobic batch solids that cannot meet the digestion for 30 additional days requirements of Option 1 -- at 20EC (68EF) e.g., biosolids treated in extended aeration plants Option 4 SOUR* at 2OEC (68EF) is Biosolids froni aerobic 503.33 1.5 nig oxygenlhrlg total processes (should not be used (bX4) solids for composted biosolids) Option 5 Aerobic treatinent of the Composted biosolids 503.33 biosolids for at least 14 days at (Options 3 and 4 are likely to (bX5) over 40EC (104EF) with an be easier to meet for biosolids average temperature of over froni other aerobic processes) 45EC (1 13EF)
Regulatory Requirements 125TABLE 2-8 (cow.)Option6 Addition of sufficient alkali to A c l - r a e biosolids Uaitetd503.33 raisethepHtoatleast12at (alkalies include lime,fly as4@X6) 25°C(770F)andmaht&a kilUdUStandWo0ddl) pH 2 12 for2 hours and apH 2 11.5 for22 m r hours oeoption7 Biosolids t e t d by an rae503.33 aerobic or anaerobic process@X7) (i.e., biosolids that do not contain unstabilized solids generatedinprimary wastewatertreatment)Option 8 Percent solids 290% prior to Biosolids that Oontain503.33 mixhgwith dher malerials unstabilii solids generated@X8) inprimaryWastewater mdment (e.g., any heatdriedOption9 Biosolids are injected into soil Biosolids applied to the land503.33 so t a no significant amount of ht or placed on a surface disposalex91 biosolids are present on the site. Domesticseptage landsurface 1 hour after applied to agricultural land, a hj4m except Class A forest, or a reclamation site, or biosolids which m s be ut placedona surface disposal injectedwithin8hoursafter site option 10 Biosolids inoapotatedintothe Biosolids applied to the land 503.33@X10) soil withiu 6 hours after application to land or sitc. D o m e s t i c w e placementonaBurfacedisposal applialto agriarltural l n , ad site, except Class A biosolids forest, or a reclamation site, or which m s be applied to or ut placedona d i d i s p o d placedonthelandsurfa Site within8hoursafterthe DathORCXI radudOn DlWXSS option 11 Biosolids placed on a surface Biosolids a domestic septage 503.33 disposal sitemustbe m v d placed on a surface disposal @X11) with soil or other material at site the end of each opffatingday option 12 pH of domestic septage m sut Domestic septage appliedto 503.3 be d t o 2 12 at 25°C ( 7 7 3 by alkali addition and reC~amation or p l d on a site @XI21 . . mmtmedat 212 for30 surface dispapal site minutea without adding mom alkali I SOUR:S@c Oxygen Uptake Rate
126 Forste The frequency of monitoring is based on the amount of biosolids used ordisposed annually. For biosolids sold or given away in a bag or other container, theland application monitoring frequency is based on the amount of biosolids receivedby the preparer. The frequency of monitoring specified for the lowest range (i.e., > 0 but < 290metric tons per year) indicates that when biosolids are not used or disposed duringa 365 day period no monitoring for the requirements of Part 503 is required.Biosolids must be monitored only when an amount is used or disposed in a 365 dayperiod (Table 2-9).TABLE 2-9 MONITORING FRE 2UENCY Amount of Biosolids* Estimated Size of Frequcncy (Metric Tons pcr 365 Day Facility (MGD)*** Period - Dry Wcight Basis) >O to ~ 2 9 (320)** 0 ,290 to <1,500 (1,653)** B0.9 to 4 . 5 once per quarter I once per 60 days I ~~ ~ 31,500 to <15,000 (16,530)** 24.5 to <45I 215,000 (>16,530)** L45 I oncepermonth I * Either the aitioutit of bulk biosolids applied to flie latid or the amourit of biosolids received by a persoti who prepares biosolids that is sold or given away in a bag or other containerfor application to the latid. ** Values in parentheses are dry tons. *** These valrres are not listed in the 503 Regulatiotis but are rule-ofthurrrb values based oti one diy loti of biosolids beiiig produced for eveiy iirillioti galloiis per day &fGD) o wastewaterprocessed. f Under the 503 Regulations, the peimitting authority can reduce the fiequencyof monitoring for pollutant concentrations and pathogen requirements after two yearsof monitoring at the fiequency specified in P r 503. However, monitoring fiequency atmust be at least once per year when biosolids are applied to the land. Reducing thefiequency of monitoring for pathogen densities addresses enteric viruses and viablehelminth ova in biosolids. As part of those requirements, the biosolids must beanalyzed for viruses and ova each time the biosolids are monitored. After those twoorganisms are found in the influent to the pathogen treatment process and after tlie
Regulatory Requirements 127required reduction for those organisms is demomtrated through the treatmentprocess, the biosolids do not have to be monitored for enteric viruses and viablehelminth ova when values for the process operating parameters are consistent withthe documented values for those parameters. The potential reduction in frequencyof monitoring applies only to pathogen density reduction requirements for entericviruSes and viable h e h t h ova and cannot be reduced for the other pathogen densityrequirements. Every container of domestic septage applied to agricultural land, forests orreclamation sites must be monitored to establish compliance with the pH adjustmentrequirement when that requirement is met. Each container (e.g., each tanldtiuckload) must be monitored because there is no way to ensure that the pH requirementwill be met if only a certain number of containers are monitored.RecordkeeringThe final Part 503 Regulations contain recordkeeping requirements for bulk biosolidsand bulk material or biosolids sold or given away in a bag or other containerregardless of whether such biosolids meet the requirements which would exemptthem from the cumulative pollutant loading limits contained in Table 2-5.Recordkeeping requirements also apply to domestic septage applicd to agriculturalland, forests or reclamation sites. Recordkeeping requirements specify the information that must be developed, theperson who must develop and retain the infoimation and the period h a t thisinformation must be retained. The infoimation that must be developed depcnds onwhich pollutant limits are met and which pathogen and vector attraction reductionrequirements are met; the infomation is needed to show that the requirements i nSubpart B are met. For biosolids that do not meet the pollutant concentrations in503.13(b)(3), the more stringent Class A pathogen requirements and one of thevcctor attractionrequucments in 503.33(b)(I ) through (8), the person who preparesthe biosolids must develop certain information (e.g., pollutant concentrations in thebiosolids) and the applier must develop other information (e.g., the record of theamount of each pollutant applied to the land). These persons may be the treatnicntworks, an agent of the treatment works, a private contractor or some other personFor example, when the bulk biosolids meet the pollutant concentrations and the lessstringent Class B pathogen requirements, the person who applies the biosolids to thcland must develop infoimation concerning how the restrictions on the application siteare met and keep those records. Depending on which pollutant limits are met,infoimation must usually be retained for five years. However, when cumulativepollutant loading rates are met, records for certain information have to be retained indcfinitely in order to know how much of an inorganic pollutant has bccn applied tcj the land.
128 ForsteFuture EPA Rewlatorv Actions on Part 503 ProgramOn April 3, 1995, EPAs Office of Water published a preliminaxy report onregulatory streamlining measures within the Agencys water program regulations.Targeting the Part 503 biosolids management program, EPA plans to revise therequirements to delete chromium pollutant limits, increase the ceiling limits forpollutants in areas with high background levels of the pollutant, and eliminate the useof the 99th percentile to set pollutant concentrations. Revisions may also includenew pollutant lmt for molybdenum, revised requirements for incinerator emissions, iisand revisions to reporting and recordkeeping requirements. EPAs Dioxin Reassessment is currently under review, and the agenda itemsinclude: an EPA overview of the Agencys proposed dioxin research program an overview of EPAs regulatory programs that may be impacted by the Dioxin Reassessment updates from EPA and others on the Draft Dioxin Reassessment public comment responses and the Dioxin Call-in fiuther discussions of known and unknown sources EPAs Science Advisory Board (SAB) will review the Agencys dioxin riskassessment. Two separate charges--an exposure document charge consisting of 20questions and a health document charge consisting of 23--will be addressed. Theissue of toxicity equivalence factors (TEFs) is likely among the most controversialissues to be examined. TEFs are used to calculate a toxic potency, measured in termsof the potency of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), for various dioxin-like chemicals. Data h i s h e d to EPA concluded that POTWs are nominal sourcesof dioxin releases to the environment. EPA will propose and promulgate additional standards for the use or disposalof biosolids (Round 11) following a review of contaminants from a list of 31pollutants listed in a Consent Decree dated May 25, 1993. The Agency will thendevelop regulations that will establish requirements for biosolids under the followingconditions: when applied to the land for a beneficial purpose, when disposed on landby placing on surface disposal sites, and incinerated. The court-ordered schedulecalls for proposed Round I1 regulations by December 1999 and final promulgationby December 2001. EPA may significantly reduce the number of pollutants forM e r regulatory action and focus on only those with a human health impactpotential. EPA is scheduled to propose claracations to ease implementation of the Part 503 rules and to address inconsistenciesin the regulations in 1995. According to an EPA internal memo a Federal Register notice will propose the Agency: (1) clarify the land application rules to indicate that pollutant ceiling concentration limits pertnin to any land-applied biosolids; (2) delete the requirement to record the time of day
Regulatory Requirements 129biosolids or septage is applied to land; (3) spec@ biosolids pH must be reported atthe standard temperature of 25 degrees Celsius; (4) require seven samples of ClassA sludges by analyzed for fecal colifom, (5) allow permitting authorities todetermine equivalency for options to reduce vector attractions reduction options otherthan those specified in Part 503; (6) change site-specific pollutant limits for biosolidsincinerators fi-om daily conmtxations to an average monthly concentration; (7) allowpermitting authorities to delete site-specific limits for incineration if the pollutant areextremely high; and (8) modify the certification statement used throughout Part 503to allow biosolids authorities to certitjl that compliance information was collected,whether or not requirements were met.REFERENCES1. 33 U.S.C.A. 1251 et. seq.2. U.S. Environmental Protection Agency. Standards for the Use or Disposal of Sewage Sludge. 40 CFR Parts 257,403 and 503. February 19,1993.3. U.S. Environmental Protection Agency. 40 CFR Part 257, Criteria for Classification of Solid Waste Disposal Facilities and Practices. Federal Register 44( 179):53438-53464. 1979.4. US. Environmental Protection Agency. Municipal Sludge Management Policy. Federal Register. June 12, 1984.5. U S . Environmental Protection Agency. Interagency Policy on Beneficial Use of Municipal Sewage Sludge on Federal Land. Federal Register. July 18, 199 1.6 . National Sewage Sludge Survey. 55FR472 10. November 1990.7. U S . Environmental Protection Agency. Response to Public Comments on the Proposed Part 503 Sewage Sludge Use and Disposal Regulations and the National Sewage Sludge Notice of Availability. Washington, D.C. 1992.8. 56FR50978. October 9, 1991.9. 56FR5507. February 1 1, 1991.10 US. Environmental Protection Agency. Fate of Priority Pollutants in Publicly- Owned Treatment Works. Vol. I. Efluent Guidelines Division, Washington, D.C. EPA 44011-82-303. 1982. 11 U.S. Environmental Protection Agency. Ground-Water Protection Strategy. Washington, D.C. 1984. 12. U.S. Department of Agriculture. Peer Review - Standards for the Disposal of Sewage Sludge US. Environmental Protection Agency Proposed Rule 40 CFR Parts 157 and 503. 1989. 13. U.S. Environmental Protection Agency. Review of Proposed Sewage Sludge Incineration Rules (40 CFR Part 257 and 503). EPA-SAB-EEC-89-035. 1989.
130 Forste14. U.S. Environmental Protection Agency. Technical Support Document for Land Application of Sewage Sludge. EPA 822R-93-001a. November 1992.15. U.S. Environmental Protection Agency. Domestic Septage Regulatory Guidance: A Guide to the EPA 503 Rule. EPA 832-B-92-005. September 1993.16. U S . Environmental Protection Agency. Standards for the Use or Disposal of Sewage Sludge, Appendix G.11. February 19, 1993.17. US. Environmental Protection Agency. Regulatory Impact Analysis of the Part 503 Sewage Sludge Regulation. EPA 821-R-93-006. 1993.
3Conditioning and DewateringRobert J. KukenbergerBlasland, Bouck 8 Lee, Inc.Syracuse, New Yo&L INTRODUCTIONConditioning and dewatering is an integral and often necessary process associated withreuse or disposal of wastewater solids. Biosolids conditioning, the first step i the nprocess, prepares liquid biosolids by changing chemical and/or physical properties toallow separationof the solid and liquid fractions. Dewatering, the separation of solidand liquid portions of biosolids slurries, is generally accomplished by mechanicalmeans,although passive systems, such as drying beds and natural fieeze/thaw lagoons,are used where weather permits. Thickening can be the first phase of dewatering, orcan function as a stand-alone process to remove a portion of the liquid fiaction,creating a thick slurry. Conditioning and dewatering prepares the biosolids for subsequent processes,such a dqmg, incineration, composting, alkaline stabilizationor for beneficial use as sa soil amendment and fertdizer, or for disposal in landfills. The volume and weightof liquid or slurry biosolids can be reduced by as much as 20-fold, although typicalprocesses wl achieve a 10-fold reduction in volume and weight. Handling ilcharacteristicsare also improved, as dewatered biosolids can be handled as a solid orsemi-solid rather than as a liquid slurry. This chapter focuses primarily on chemical conditioning wt organic ihpolyelectrolytes (polymers) and mechanical dewatering. Inorganic chemicalconditioning (e.g., lime and ferric chloride) is also covered; however, the use ofinorganic chemicals is becoming less common as polymer technology improves. 131
132 KukenbergerHigh-pressure thermal treatment obviates the need for chemical conditioning.Thermal conditioning at atmospheric pressure in conjunction with polymers is a newtechnology and holds promise in plants that have low-cost heat or waste heat availablefor heating biosolids slunies prior to dewatering. Mechanical dewatering is generally favored over passive dewatering (dryingbeds and fi-eezekhylagoons) as a primary means of dewatering, except in parts of theworld that have climates favorable to such technologies or for small treatment plants.Mechanical dewatering machmes are continuously being developed and improved, andnew and improved equipment combined with improved conditioning chemicals hasresulted in production of diyer biosolids cakes. New technologies are also beingdeveloped, requiring planners, designers, and operators to stay current with the latestconditioning and dewatering technologies. Dewatering processes discussed in this chapter include: Belt filter presses Centrifuges Pressure filters (plate and frame presses) Vacuum filters Screw presses Rotaiy presses Diying beds and fi-eezelthaw lagoons Bag dewatering Biosolids thickening is an important process that can be accomplishedindependently from dewatering or as an integral part of the dewatering process.Independent thickening processes include: Gravity belt thickening Centrifuges Flotation thickening 0 Gravity thickening 11. CONDITIONING Dewatering of most biological solids is not practical without conditioning by either chemical or physical means, including: - Organic and inorganic chemical addition Theimal treatment (with and without increased pressure) Freezelthaw Addition of bulking material
and Conditioning 133 Conditioning represents a signifcant cost component of biosolidsandor reuse disposal; therefore, particular attention needs to be given to this process during planning, design, and operations. A. Organic Polyelectrolytes The use of polymers in the wastewater industry began in the1960s [l]. Biosolids dewatering equipment technology has developed substantially since the introduction to of polymers, leading the extensive use polymers for conditioning. Polymers of are manmade, long-chain molecules which function in several ways coagulate and flocculate solids particles. The most important bonding mechanisms associated with polymer conditioning are: Electrostatic attraction van Waals der attraction Chemical bonding Electrostaticattraction is similartomagneticattractionandreliesonthe attraction of positively and negatively charged particles. Van Waals attraction der is a weak attraction caused polarization. van der Waals attraction can be explained by qualitatively by considering that even neutral atoms molecules have systems of and oscillating charges that attract each other. Chemical bondingattraction of highly is an polar molecules. Chemical forces extend only over very short distances . + W Daa1abmz.d Coagulation rmlabnm Catlonio Flocculation Figure 3-1 POLYMER COAGULATION AND FLOCCULATION
134 Kukenb erger Polymers used for conditioning of biosolids can be categorized by charge,molecular weight, and foim. Polymers are available as cationic (positive), anionic(negative), or nonionic (neutral). Cationic polymers are most often used forconditioning of wastewater biosolids; however, anionic and nonionic polymers haveapplications for some biosolids either independently or in conjunction with cationicpolymers. For example, very old biosolids can lose their charge and may require acombination of anionic and cationic polymers for eEective conditioning . Figure3- 1 depicts negatively charged biosolids particles conditioned with cationic polymers.Polymers for wastewater treatment and biosolids conditioning are usually purchasedin dty, liquid, or emulsion form.TABLE 3-1 MAJOR POLYMER MANUFACTURERS Allied Colloids, Inc. Calgon 2301 Wilroy Road Water Management Division P.O. Box 820 P.O. Box 1346 Suffolk, VA 23439-0820 Pittsburgh, PA 15230-1346 Callaway Clieniical Cytec Industries Division of Exxon Chemical C o 5 Garret Mountain Plaza P.0.Box 2335 W. Patterson, NJ Columbus, GA 3 1993 (201) 357-3100 (800) 241-7543 Diatec Enviroimiental Nalco Chemical Co. 1245 Paramount Drive One Nalco Center Butavia, IL 60510 Naperville, IL 60566 (800) 541-8041 (708) 305-1000 Rhoiie-Poulenc, Inc. Secodyne, Inc. (Fomierly Polypure, Allied Signal) P.O. Box 10717 One Catehall Drive Detroit, M1 48210 Parsippany, NJ 07054 (800) 824-0933 (201) 232-2900 Stoc~lausen, lnc. 2408 Doyle Street Greensboro, NC 27406 (919) 333-35611. ClassiJicatioii of PolyiireisCationic polymers ai-e foimed by joining aciylarnide monomers together or bymodification of acrylamide monomers. Manufacture of various polymer types and
Conditioning and Dewatering 135specific polymers are beyond the scope of this book; however, the reader is directedto the "Guidance Manual for Polymer Selection in Wastewater Treatment Plants" forspecific polymer manufacturing and development information . Major polymer manufacturers in the USA are shown in Table 3-1.2. Foriiis o Po!ytner.s fDrv PolymersDry polymers are manufactured as powder, granules, beads, or flakes. Dry polymershave a high active content, typically 90-95% . Dry polymers have proven to bevery effective for biosolids condilioning; however, handling of diy polymers requiresmore manual labor than handling of liquids and emulsions.EmulsionsEmulsions are dispersions of polymer particles in hydrocarbon oil. Emulsions arehigh molecular weight polymcrs with high solids. Active polymer levels of emulsionsusually range from 25 to 50%. Emulsions are pourable, clear to white milky liquidswt viscosities ranging fi-om 300 to 5,000 centipoise (cps) . Emulsions are stored ihin tanks and transported with pumps; therefore, many operators prefer emulsions overdry polymers.MannichsMannich polymers have high molecular weight and very high viscosity, typicallyrangng kom 45,000 to 60,000 cps. As a cornpaison, room temperature molasses hasa viscosity of 24,000 cps, and water has a viscosity of 1 cps. Mannich polymers havea active solids level of 2-10%. The pH of Mannich polymers can be as high as 12. nMannich polymers have a I-elativelyshort shelf life, which is a factor in purchase,delivery, and storage. Maiuiichs are manufactured using formaldehyde, which hasraised concerns regarding the health and safety of workers using mannichs and theenvironmental impacts associated with beneficial reuse of mannich-polymer-treatedbiosolids .Liquid PolvmersThere is a wide range of liquid polymer types available. Liquid polymers have low tomedium molecular weight, viscosity ranging fiom 1 to 6,000 cps, and active solidsranging from 10 to 60%. The pIH of liquid polymers is generally neutral to acidic.Shelf life can be from two months to one year depending on the manufacturer .3 . Polynier. SelecliotiDesign of new or upgraded facilities should include the capability of handling both dryand liquid or emulsion polymers to give the operator flexibility. Selection of the mosteffective polymer for each application is based on the following criteria:
136 Kukenberger Biosolids characteristics - Performance objectives Type of dewatering process employed Downstream impacts of filtrate or centrate Cost Jar testing is used for screening to determine which polymers have the mostpotential to be effective in full-scale dewatering applications. Polymer testing isusually conducted as an annual event; therefore, the operator should attempt to obtainbiosolids samples for testing that represent, as closely as possible, conditions for theupcoming year. If a side-by-side comparison of various polymers is being made, thetests should be done simultaneously to be sure the biosolids are of similarcharacteiistics. Ifcost is to be a major selection factor, preliminaiy cost estimates canbe made prior to jar testing based on dosage and cost estimates from the polymervendors. Cost is typically expressed as cost per unit weight of biosolids treated(dollars per dry ton of biosolids). Polymer vendors and manufacturers will generallyprovide jar testing to screen various polymers and will assist the operator on full-scalefield testing. Biosolids should be characterized to determine solids concentration, inorganic(ash) content, biological solids content, and biosolids chemishy. Concentrations ofsolids can change due to changes in the wastewater and biosolids treatment practices.Lower solids concentrations will generally require more polymers (on a pounds perdry ton basis). Expected changes in solids concentrations should be assessed, andappi-opiate adjustments made, during jar testing and field testing of conditioning anddewatering processes. Biosolids with low inorganic (high ash) content usually have cationic chargeneutxalization requirements. Fresh or digestcd biosolids generally have an ash contentof 1 5 5 0 % and eshibit cationic charge neutralization demands. Higher ash contents(>50%) may be encountered from lime-stabilized or chemically treated biosolids.Extremely old biosolids, such as lagoon-stored biosolids, may also have high inorganiccontent. Biosolids with greater than 50% inorganic content may exhibit either ananionic or nonionic charge demand. Bench testing with various polymers should bedone to verify the ionic charge demand, and subsequently the most appropriatepolymer. In some cases, a combination of anionic, nonionic, and cationic polymerswill be more effective than any single form of polymer due to the charge neutralizationrequirements. The percentage of solids resulting fi-om biological treatment processes can significantly impact conditioning. Solids produced by biological (secondary) treatment processes are more difficult to dewater than raw pl-imaiy or chemical solids [ 5 ] . Unconditioned biological solids have poor characteristics for mechanical dewatering. The trend in larger wastewater treatment fac es is to condition secondary solids separately from raw or primaiy solids. It is generally less costly to
Conditioning and Dewatering 137treat these solids separately; however, the cost and complexity of two separate systemsmay offset the advantages, particularly in smaller facilities. Extended storage of biosolids usually increases conditioning requirements. Onlywell stabilized biosolids should be stored prior to conditioning and dewatering toprevent anaerobic conditions. Biosolids should be processed within a few hours ofproduction; however, aeration of raw biosolids for up to 4 or 5 days can improvedewatering characteristics. Dewaterability deteriorates significantly if biosolids arestored for longer than 5 days. Anaerobically digested biosolids generally have higher polymer dosagerequirements than raw or unprocessed primary biosolids, and are generally moredifficult to dewater. Polymers with moderate cationic charge and moderate to highmolecular weight are usually most effective on anaerobically digested biosolids.Polymer dosage requirements for aerobically digested biosolids are similar to dosagerequirements for anaerobically digested biosolids, however, more highly chargedpolymers are typically required.B. Polymer Feed and Control SystemsPolymer feed systems are selected based on the form of polymer to be used. In largerwastewater eeatment plants, designers will often select multiple feed systems to allowthe operator to select the most appropriate and cost-effective polymer form.1 , Polyirier Feed Systerri RequisenrentsThe key requirements of a polymer feed system are: Proper polymer storage Complete dissolution to prevent polymer waste Lack of undissolved particles to prevent clogging of equipment and pipes Proper mixing Proper equipment sizing Optimum dosage - underfeeding does not accomplish results and overfeeding can upset treatment and waste polymers Polymer feed systems should allow for dosage variations of 20 to 1 to give theoperator adequate flexibility. Multiple polymer application points into the biosolidsstream should be provided. It is necessaiy to provide diy, temperature-controlledstorage areas to prolong the life of the polymer. Make-up water should have a pH of6-8, be fkee of suspended solids, have a specific conductivity of < I ,200 micromho percentimeter (pmhohm), total dissolved solids of 4 0 0 mg/L, and temperature of 10"-50°C (50" -120°F). Residual chlorine in the make-up water should be less than 0.5mg/L [ 11. Typical polymer feed systems are depicted in Figure 3-2. Polymer feed
138 Kukenbergersystems can be assembled from components or purchased as skid-mounted packagesystems. Compact polymer blending systems have been shown to be successhl in manyapplications. Polymer blending systems can eliminate the need for mixing and agingtanks, which saves considerable space and operator attention . Dry Feeder Llquld or Emulrlon Storage TMk Feed Transln UetMblg Pumps Pumps Pump.Figure 3-2 SCHEMATIC OF A TYPICAL POLYMER FEED SYSTEM2. Polynier COWO~ SysteimThe control of polymer feed for biosolids conditioning affects the thickening anddewatering perfoimance, quality offiltrate or centrate, and the cost of chemicals. Withpolymer conditioning, “more is not better” because overdose as well as underdose canhave deleterious effects on dewatering efficiency. Manual control is typicallyaccomplished by measuring cake concentration, filtratelcentrate quality, and, in beltfilter press installations, by visual obseivation of the conditioned biosolids on thegravity deck. Several technologies have recently become available for automatic or semi-automatic control of polymer feed, including streaming current detectors, sludgeflocculation controllers, and reflective (infrared) scanning of gravity drained biosolids. The streaming current detector measures colloidal charge (streaming current) inthe filtrate or centrate fi-om the biosolids dewatering process. Research has shown thata near zero charge indicates optimum dewaterability and that the centratekltrateexhibits nearly the same charge as conditional biosolids . Therefore, streamingcurrent measurements of filtrate or centrate can be used to control polymer feed. Biosolids flocculation controllers measure the rheological characteristics of thebiosolids before and after conditioning. Relationships have been developed betweenpolymer addition and rheology, which can be used to optimize polymer addition and
Conditioning and Dewatering 139dewaterability. Flocculation controllers control polymer feed by alternately measuringraw and conditioned biosolids rheological characteristics to optimize polymer feed.Flocculation controllers appear to have difliculties in controlling polymer dose overa wide range of biosolids conditions [ 7 ] . Automatic scanning of biosolids reflective characteristics on the gravity drainagesection of belt filter presses can be accomplished with infrared sensors . Infraredsensors are placed along the length of the gravity deck to evaluate drainageeffectiveness. Signals from the sensors can be used to control polymer feed, beltspeed, and biosolids feed rate. The principle is essentially the same as visualobservationby the operator, except that the sensing and control is continuous and canpotentially save up to 20-30% of the polymer dose [S], .3. Polynier ConsuiiiptioiiOrganic polymer consumption depends on a wide range of variables. Belt filterpresses (BFPs) typically use polymers to condition biosolids to achieve separation ofthe water from the biosolids sluiiy. Use of a wide range of dosages, ranging from 1to 25 g/kg dry solids (2-50 lbdton), has been practiced, although typical dosagerequirements for BFPs are between 3 to 6 g k g dry solids . Belt filter presses relyon significant removal of water in the gravity drainage section, which is highlydependent on good conditioning. Underconditioning causes poor dewatering in thegravity section, and overconditioning causes blinding of the filter press cloth (media). Organic polymers are also used with solid bowl centrifuges for wastewaterbiosolids dewatering. The recently developed "high solids centrifuges" rely heavilyon good polymer conditioning. Typical dosages range from 2-4 g k g (4-8 lbs/ton) fortraditional solid bowl centrifuges and up to 10 g/kg (20 Ibs/ton) for "high solids"centrifuges . In the past, organic polymers have not been used extensively for plate and frame(pressure) filter presses. Poor and inconsistent dewatering and media blinding havebeen historical problems associated with polymers used with plate and frame filters.As polymer technology continues to improve, however, conditioning for plate andframe filtration has become more effective. Also, the use of precoat agents, such asdiatomaceous earth or fly ash, prevents cloth blinding. As a result, polymerconditioning prior to plate and frame filtration is becoming more popular. Historically, gravity drying bed dewatering was accomplished withoutconditioning. Over the past several years, however, improved polymer technology has led to conditioning prior to applying biosolids to drying beds, which has improveddying bed performance. Vacuum-assisted diying beds rely on polymer conditioningfor adequate dewatering, but care must be taken to prevent floc break-up, which typically occurs with pumping or transporting over distances. Floc break-up can beprevented by conditioning near the discharge to the diying beds. Typical dosage forvacuum-assisted diying beds is 15 to 25 g k g (30-50 lbs/ton) [I].
140 Kukenberger Typical Polymer Dosage Application gram dry polymer/kilogram dry nolidn Vacuum AssistedC. Inorganic Chemical ConditioningInorganic chemicals used for biosolids conditioning prior to dewatering typicallyinclude iron salts (fenic chloride, ferrous chloride, and ferrous sulfate) and lime. Lime (CaO and Ca(0H)J and ferric chloride (FeCl,) are the most widely usedinorganic chemicals for biosolids conditioning. U e of inorganic chemicals has sdecreased in recent years because of improved polymers. As compared to polymers, inorganic chemicals may be more effective wt ihcertain biosolids, particularly when employing filter presses and vacuum filters,however,the use of inorganic chemicalsmay: require a higher dosage rate; increaseweight and volume; reduce the heat value of the biosolids; and add metals to the finalproduct.
Conditioning and Dewatering 1411. Ferric ChlorideFenic chloride is available commercially in liquid foim as a 30-35% solution. In coldclimates, ferric chloride may be shipped at lower concentrations to avoidcrystallization. Commercial ferric chloride is an orange-brown corrosive and acidicsolution. Fenic chloride is used as a conditioner to improve biosolids dewaterability bydestabilizing negatively charged particles. Flocculation with ferric chloride requiresa pH above 6; therefore, lime is usually added to offset the pH reduction caused byferric chloride. Further, rapid mixing should be provided to cause particle collisionand promote flocculation. All materials handling equipment for ferric chloride must be coirosion resistant(i.e,, non-metallic). Pumps, valves, piping, tanks, etc., are provided in plastic-,fiberglass-, or i-ubber-lined steel. FeiTic chloride can cause severe burns to eyes andskin, and fiunes can cause uiteinal buns if inhaled. Protective gear and specific safetyprecautions must be adhered to when handling ferric chloride.2 . LimeLime is commercially available as quicklime (CaO) and hydrated lime (Ca(OH),). Quicklime is manufactured by burning crushed limestone in high temperaturekilns. During the buning process, carbon dioxide is driven off, leaving calcium oxide(CaO). Quicklime is usually sold in pebble form for ease in handling. To usequicklime in wastewater processes, it is sluii-ied with water to form calcium hydroxide(Ca(OH),) prior to use. Hydrated lime is a powdered foim of Ca(OH),. Commercially available hydratedlime is prefeired for small and inteimediate applications because it does not requireslaking. For larger applications (greater than 2 tons per day), quicklime is usuallymore economical, even though slaking is required. Lime is used as a biosolids conditioning agent to raise the pH, reduce odors, andadd bulk via formation of calcium carbonate and calcium hydroxide complexes. Lime dust is an irritant if inhaled and can cause buins to skin and eyes due toheat generated when CaO comes in contact with water.3 . Applicalioii of Litire and Ferric ClilorideLime and feiric chloride are often used together to condition biosolids for dewateringon filter presses and vacuum filters. With filter presses, the dewatering process isaccomplished in batches, therefore, biosolids are conditioned in a batch (or feed) tankprior to filling the filter press. Vacuum filtcrs are operated as a continuous process;therefore, lime and feiiic chloride are added just upstream of the dewatering machine.
142 Kukenb erger Lime and ferric chloride are not typically used for belt filter presses orcentrifuges, as organic polymers are better suited for these dewatering machines. F a n c chloride reacts with biosolids by neutralizing negatively charged particlesand forming fmic hydroxide complexes. Because alkalinity is utilized in the reaction,pH is lowered. Lime is used to raise the pH, provide additional alkalinity, and therebyallow the reactions involving feiiic chloride to be more eEcient. Lime also formscalcium carbonate and calcium hydroxide complexes, which provides a matrix in thebiosolids, improving dewaterability . Feiiic chloride is typically dosed at 2- 10% of solids on a dry weight basis. Limeis usually dosed at 0-40% on a diy weight basis.4. Other Inorganic Cheniicols and AdditivesFerrous sulfate (Fe(S0.J) and ferrous chloride (FeC4 ) (waste pickle liquor - a by-product of the steel industiy) may be used in lieu of ferric chloride. Evaluation ofalternatives to feiric chloride should be based on performance, cost, and availability.Also, waste pickle liquor may contain by-products of the steel manufacturingprocesses, which could cause quality problems associated with reuse or disposal ofbiosolids. Ash, pulverized coal, saw dust, and diatomaceous earth have also been used asadditives for filter presses and vacuum filter dewatering. These additives have limiteduse and depend on biosolids characteristics, type of dewatering equipment, andreuse/disposal options. These additives are noimally used in conjunction with organicor inorganic chemicals and serve as a filter aid or "precoat" to allow release of the cakefiom the filter cloth. The addition of diatomaceous earth, fly ash, and cement kiln dustwese found to be successful when using polymers with plate and frame filter pressesPI.5. Selection CriteriaThe decision to use inorganic chcmicals, particularly lime and fen-ic chloride, shouldbe based on the following criteria: 00 Perfoiinance of conditioning chemicals, as detelmined by testing or full- scale operations -0 Type of dewatering equipment proposed or in use 0. Life cycle costs 0. Materials handling and storage requirements 0- End use or disposal options for the dewatered biosolids
Conditioning and Dewatering 143 D. Thermal Conditioning 1 . High-Pressure Tliermal Conditioning Thermal conditioning of biosolids at elevated pressures breaks down (lyses) the cell wall of the microorganisms in biological solids, which allows the release of bound water. High-pressure theimal conditioning alters the physical properties of biosolids, which results in a drier cake a s compared to that produced by chemical conditioning. High-pressure thermal conditioning is described in Chapter 6. High-pressure themially conditioned biosolids will thicken by gravity to a range of 6 to 15% solids and can be mechanically dewatered to a range of 35 to 75% solids [ 101. Vacuum filters are typically used for dewatering biosolids that have been conditioned by high-pressure thermal treatment. 2. Tliervially Enhanced Dewatering Mixed primary and biological solids dewatering can be enhanced by preheating the solids to 60°C (140°F) at atmospheric pressure prior to dewatering. Cake solids concentration may be inci-eased by up to 6%, and polymer usage may be decreased by as much as 25% with this process [ I I]. This process may be cost effective when waste heat is available fiom biosolids incinerators or fiom other sources. Digested or thickened biosolids are heated in heat exchangers prior to polymer addition and dewatering. Centrate can be returned to the head of the plant or used as a heat source in the biosolids heat exchanger. The potential for odors is greater if heated biosolids or centrate/filtrate is exposed to the atmosphere prior to cooling; therefore, odor conwool facilities need to be designed and sized to address this potential. Figure 3-3 depicts a thermally enhanced mechanical dewatering system. - PoIynmr Centrate 180 C Water Polymer I Dewatared BlOSOlld8 Spiral Heat Dewaterho Exchanoer YachheFigure 3-3 SCHEMATIC OF THERMALLY ENHANCED DEWATERING SYSTEM
144 Kukenberger1 1 DEWATERING 1.A. Process DescriptionBiosolids are produced as a by-product in wastewater treatment facilities as a liquidor sluny, generally from less than 1% solids concentration up to 10% solidsconcentration. Further handling and processing of biosolids is enhanced by thickeningand/or dewatering. Most thickening devices can achieve a solids concentration of 5-lo%, and dewatering devices can achieve 20-50% solids concentration. Factorsaffecting the thickened and dewalered solids Concentration are: Characteristics of biosolids Type of conditioning employed Type of thickening or dewatering device Thickening processes include gravity and flotation thickening, centnfUgethickening, gravity belt thickeners and sotaiy drum thickeners. Dewatering processes include mechanical devices and passive systems (e.g.,drying beds). Mechanical devices discussed herein include belt filter presses,cenhlfuges, pressure filters, vacuum filters, screw presses, and rotary presses. Passivesystems include conventional and vacuum assisted drying beds, freezekhaw lagoons,and bag dewatering systems.B. ThickeningBiosolids thickening reduces the volume of material to be handled in subsequentprocessing steps. Thickening prior to digestion reduces the size of digesters, reducesenergy requirements for heating anaerobic digesters, and increases digestionefficiency. Thickening reduces storage and hauling requirements when biosolids aserecycled in liquid fosm. The Madison Wisconsin Metropolitan Sewerage District(MMSD) thickens digested biosolids with gravity belt thickeners (GBTs) prior toliquid land application. MMSD achieves an average thickened solids concentrationof over 6% with GBTs on a feed solids concentration of 2-3% [ 121. Thickening priorto dewateiing may be beneficial with regard to sizing of dewatering equipment, higherthsoughput, and higher dewatered solids concentration. Most mechanical dewateringsystems provide thickening integral to the dewatering process.1. Gravity ThickeningIn gravity thickening, biosolids are concentrated by gravity settling and compaction.Gravity thickeners are typically used for piimaiy biosolids, lime sludges, and
Conditioning and Dewatering 145combined primary and secondaiy (waste activated) biosolids. Gravity thickeners areusually circular tanks with steep cone-shaped bottoms to promote bottom withdrawalof thickened biosolids. Gravity thickeners can achieve undeiflow solidsconcentrations of 5- 10% on primaiy biosolids and 4-6%on combined primary andwaste activated biosolids [ 131. Gravity thickeners are sized based on solids massloading per unit area. Overflow rates are also an important criteria. Too high of anoverflow rate can cause solids carry-over, and too low of an overflow rate may causeseptic conditions. Dilution water, such as plant effluent, is typically added to maintainsufficient overflow rates. Typical design mass solids loading and oveiflow rate ispresented in Water Environment Federation Manual of Practice No. 8 [ 131.2. Dissolved Air. Flolariori (OAF) ThicketiitigDissolved air flotation thickening is typically used for secondary waste-activatedbiosolids, which do not settle as well as primary biosolids. DAF thickeners rely on theaddition of air in a pressurized system. Microscopic air bubbles attach to the solidsparticle to promote flotation. DAF thickeners are either circular or rectangular withsurface skimmers and bottom solids removal mechanisms. Design criteria includesolids loading rate, hydraulic loading rate, and air to solids ratio [ 131. DAF thickenerscan produce a solids concentration of3-5% without conditioning and 3 . 5 4 % with theaddition of polymers [ 131.3. Ceiilvi~ugal icketI it iy TI1Centnhgal thickening is commonly applied to waste-activated biosolids. Solids bowlc e n h ~ g e for thickening are essentially the same as those used for dewatering. The smajor differences between thickening centrifuges and dewatering centrifuges are thelocation of the discharge ports and the configuration of the scroll (conveyor). Typicalthickened solids concentrations with solid bowl centrifuges range from 3-10% .Solid bowl centrifuges are discussed in more detail in Section C.2.4. Graviv Bell Tliicketiers (GBT) and Roiary Drvttr TliicketiersGravity belt tluckenas and rotaiy drum thickeners are similar to the first stage of beltfilter presses. Polymer conditioning is essential with municipal biosolids, as theseunits rely on liquidsolid separation and drainage of the free liquid through a porousbelt or wedge-wire drum. Gravity belt thickeners typically achieve 4-10% solidsconcentration [ 131.
146 Kukenberger C. Mechanical Dewatering 1. Belt Filter. Presses Belt filter presses (BFPs) are mechanical dewatering machines that utilize two or more porous belts to dewater biosolids. There are three stages of dewatering on a BFP: * . Gravity drainage 0. Compression 0. Shear The BFP is a continuous process and relies heavily on polymer conditioning. Polymer conditioning agglomerates suspended solids into flocs for initial separation of the liquid fi-om the solids. Conditioning also builds a structural matnx in the biosolids that can withstand gradual increase in pressure and shearing action. The gravity drainage section of a BFP is a porous belt (or rotaiy dlum) that allows f e liquid to drain through the belt pores (or drum slots) leaving behind a layer ie of partially dewatered (or thickened) biosolids. Gravity drainage is a necessary first step of dewatering to allow the compression zone to hnction without losing biosolids out the edges of the belts. The compression section of the BFP contains two belts which sandwich the biosolids. As the two belts travel over decreasing diameter rollers under pressure, compression and shear is created. Liquid is removed to create a biosolids cake of typically 16-30% solids concentration. Dewatered biosolids are discharged with the assistance of a "doctor" blade, and the belts are washed with a water spray. Washwater can be potable or plant effluent water; however, if plant water is used, it must be fiee of suspended solids to prevent clogging of spray nozzles. Condiuonlng Lone Dhcharge TO Pressure Zone t Eiosolkls Feed Gravlly Drainage Zone- Figure 3-4 BELT FILTER PRESS
Conditioning and Dewatering 147 Automatic belt tracking systems are provided by means of a roller tensioningsystem. The rollers are driven by variable speed motors or hydraulic drive to allowinfinite variation of belt speed. Belts are typically spliced for easy removal andreplacement by the operator. Figure 3-4 depicts a typical belt filter press mechanicallayout. Performance of the belt filter press is controlled by biosolids feed rate, polymerselection, polymer dosage, point of feed, and belt speed. An experienced operator canvisually observe dewatering characteristics on the first stage (gravity) section of thebelt filter press to determine whether the biosolids are properly conditioned. Inaddition, solids concentration of the dewatered biosolids and solids capture (qualityof the filtrate) are monitored to determine oierall pelfoimance.Centrifuges for thickoning and dewatering of biosolids are typically solid bowlmachines. Solid bowl centrifuges are operated as a continuous process. Thesemachmes rely on centrifugal force that separates the solid and liquid fractions. Solidsare forced away fiom rotating axes of centrifuges, due to the differences in densitiesof the solid and liquid phases, and the liquid moves toward the center of the machine. Both inorganic chemicals and organic chemicals (polymers) have been usedsuccessfully with centrifuges. Due to advances in polymer technology and centrifugemachine design, polymers are now used for most centrifuge dewatering systems onmunicipal wastewater biosolids. Modem centrifuges for dew atering biosolids consist of a solid bowl, usuallytapered on one end, a scroll conveyer, a case to cover the bowl and conveyor, a heavycast iron base, a main drive, and a back drive. The main drive rotates the bowl, andthe back dnve assembly controls the conveyor speed. The range of differential speedsbetween the bowl and conveyor vary with different manufacturers. There are twoconfigurations of solid bowl machines: co-cuirent and counter-current. In the co-current design, the liquid and solid phases travel in the same direction, and liquid isremoved by an internal skimming device or ports located in the bowl. In the counter-current design, liquid travels in the opposite direction from solids, and liquidoverflows weir plates. Figure 3-5 shows the main components of a counter-currentsolid bowl centrifuge. Over the last 5 to 10 years there have been significant improvements incentrifuge design. These new machines, referred to as "high solids" or "compression"centrifuges, provide improved performance and less maintenance than oldercentrifuges. Recent improvements include: Higher operating speeds . 0. Less differentid in speed between conveyer and bowl, resulting in less wear
148 Kukenberger Higher solids retention and more compression Better materials of constiuction The current generation of cenh-tfbges can achieve solids concentrations from 25-35%, 510% higher than earlier generation centrifuges and standard BFPs [ 141. The operator controls the centrihge perfoimance by vruying feed rate, type ofconditioning chemicals, dosage rate, and the bowl/conveyor speed differential. Withsome manufacturers, the pool depth in the centrihge can be varied, thereby affectingdewatering peifomance. The operator monitors solids concentration and solidsrecovery (centrate quality) as primaiy perfoimance parameters. Damtarlno Barnoh Figure 3-5 SOLID BOWL COUNTER-CURRENT CENTRIFUGE3 . Pressure FiltersPressure filters (commonly refemed to as plate and frame presses) used for biosolidsand industrial sludge dewateiing are recessed plate presses, either fixed volume orvaiable volume (diaphragm) machines. The teim "plate and frame press" is derivedfrom the food industry, which developed the forerunners of todays pressure filters.Pressure filters are operated as batch processes and require storage and batch tanksto contain feed biosolids in enough volume to fill the press. In large systems, multiplepresses can be used to reduce storage volume. Modein filter presses can producedewatered biosolids with a solids content in escess of 40% [ 131. Recessed plate pressure filters are horizontally aligned to allow discharge ofdewatered cake by gravity into a receiving bui or conveyor (see Figure 3-6). Recessedplates are placed on a fixed fi-ame which has a closure mechanism that presses theplates together and holds them in position during the filling and dewatering cycle.
Conditioning and Dewatering 149 For a fixed volume press, the biosolids are pumped into the recesses over aperiod of several hours to continually increase pressure and force the liquid throughthe filter cloth. JAW pressure units achieve a pressure of up to 100 pounds per squareinch (psi) (690 kilopascal [@a]), and high pressure units achieve a pressure of up to225 psi (1550 Wa). The maxirnurn pressure is maintained until the filtrate essentiallystops, signifying the end of the dewatering cycle. The variable volume press has a diaphragm behind the cloth media. Biosolidsare pumped into the press until the recessed chambers are filled. Pumps are then shutoff, and air or water is pumped into the diaphragm to create pressure in the recessedchamber. With variable volume presses, higher pressures can be achieved, cycletimes are reduced, and generally more consistent dewatering results can be achieved.However, diaphragm presses are more costly than fixed volume presses and requiremore auxiliary equipment. Biosolids to be dewatered in pressure filters are typically conditioned with limeand feinc chloride. Recently, some municipal biosolids dewatering installations havehad success with polymers alone; however, cloth blinding and difficulty with cakerelease have been generally observed with polymer-conditioned biosolids. However,the small decrease in performance with polymers may be offset by lower chemicalcosts, reduced ammonia odors, and lower metals content in dewatered biosolids. Operator control of performance is limited to conditioning; therefore, chemicalselection and dosages are critical control parameters. rFilter Plates Figure 3-6 PRESSURE FILTER
150 Kukenberger4. Jfacu14111 FilteisVacuum filters have been in existence since the late 1800s and have been used in theUnited States for dewatering municipal sludge since the 1920s. The use of vacuumfilters for biosolids dewatering has sharply decreased in recent years due todevelopments of more efficient dewatering equipment. Vacuum filters are still usedwith high-pressure, thennal-conditioned biosolids, since high solids concentrations(30-50%) can be achieved. Vacuum filters are also used for dewatering lime sludgefrom lime softening water treatment plants, particularly in the southeastern UnitedStates. The most common vacuum filter consists of a large, horizontally mountedrotating drum, whch is covered by a porous cloth or metal coils (see Figure 3-7). Thebottom portion of the drum is submerged in a vat of biosolids. As the drum rotates,biosolids are picked up on the porous medium under a vacuum. The divm is dividedinto sections, and vacuum is applied by a rotary valve. The filter operates in threezones: Cake foi-niation Cake dewatering Cake discharge Vacuum filters can be successhlly used with organic (polyelectrolyte)conditioning, inorganic (lime and fen-ic chloride) conditioning, and thermalconditioning. Operating variables include drum speed, liquid level in the vat, vacuum level,and conditioning agent dosages. Cake Feed Figure 3-7 VACUUM FILTER
Conditioning and Dewatering 1515. Rotaiy PressThe rotary press is a recent development in biosolids dewatering. Biosolids arepumped into a peripheral channel that has walls made up of rotating filter elements.As the mechanism rotates, compression is created and liquid is forced through thefilter elements. Cake is formed in the interior channel and extruded. The rotary pressis depicted in Figure 3-8 [ 161. The rotary press is a continuous dewatering process; therefore, ancillaryequipment would be similar to that required for belt filter presses and centrifuges.Polymer conditioning is a necessaiy step for proper dewatering via rotary press. Themachine requires relatively small floor space and is available in several sizes and insingle- or multiple-channel arrangements. Static Mechanlcal SECTION A-A Figure 3-8 ROTARY PRESS6. Screw PressesScrew presses are manufactured as low-pressure, inexpensive machines for smallerinstallations when high solids concentrations are not required, and as large, high-pressure machines that produce high solids concentrations. The low-pressure unit is typically mounted vertically, either trailer mounted oras a permanent installation. Typical solids concentrations on municipal biosolids fromlow-pressure screw presses are 8- 15%. Horizontally mounted, high-pressure screw presses are typically used fordewatering fibrous materials such as pulp and paper sludge . High-pressurescrew presses are not widely used in the wastewater industry.
152 KukenbergerD. Passive Dewatering1. Conventional Dtying BedsD y n g beds for municipal biosolids dewatering have been designed and constructedwith many variations, including: 0 Open sand beds Open paved beds with and without drainage channels * Covered sand and paved beds 0 Vacuum-assisted drying beds Freeze/thaw lagoons and diying bedsA typical covered, paved drying bed is depicted in Figure 3-9 Open For Ins Ven tlletlon onall SandJ LPerforated Drain To’ LLiner Trench Head Of Plant Figure 3-9 CROSS SECTION OF COVERED/PAVED DRYING BEDS Diymg beds require larger land area than mechanical dewatering, are generallymore labor intensive, and are subject to weather conditions. Because they are typicallyopen to the atmosphere, odor control is difficult, and diying beds used in highlydeveloped areas may result in odor complaints. However, drying beds remain in usefor many municipal biosolids dewatering operations. Diying beds are used in smallcommunities across the United States and at some large plants in areas where theweather promotes rapid diying. In recent years, polymer conditioning has been usedsuccesshlly with diying beds to promote liquidsolids separation and fasterdewatering. Mechanical mixing of the biosolids following npplication to the bed isalso being used, especially on larger installations. During the first several hours of
Conditioning and Dewatering 153drying, the primary mechanism of liquid removal is by gravity drainage. Sand bedsand paved beds with drainage channels are equipped with an underdrain system.Filtrate is typically directed to the head of the treatment plant for treatment with theplants influent. After initial gravity drainage is essentially complete, evaporationbecomes the primary dewatering mechanism. Final solids concentration depends onthe weather conditions and the length of time the biosolids remain on the bed. Finalsolids concentration from drying beds can be as high as 80%; however, dust canbecome a problem if the solids become too dry.2. Vacuirin-AssistedDtyiiig BedsVacuum-assisted drying beds are constructed of a wedge-wire support system whichallows fieewater to be removed much more rapidly than by gravity drainage. Liquidbiosolids are pumped over the slotted surface, and vacuum is applied to the undersideof the support system to enhance gravity drainage. If consbucted outside, the biosolidsare allowed to continue dewatering by evaporation after free water is removed byvacuum. Biosolids are typically conditioned with polymers prior to placement onvacuum-assisted drying beds, allowing rapid removal of the liquid fraction from thesolids.3 , FreezeRiraw LagoonsFreezehaw lagoons and diying beds have been shown to be effective in dewateringbiosolids and alum sludge generated in water treatment plants [ 181. The freezing andthawing process separates the liquid and solid fraction, and it is believed that freezingbreaks all walls, allowing removal of bound water after thawing occurs. Mechanicalfreezelthaw devices have been developed; however, energy requirements haveprevented development of h s technology; therefore, freezelthaw systems are generallylimited to climates that can support natural freezehhaw cycles.4. Bag DewateringSmall treatment plants with wastewater flow rates of less than 200,000 gallons per day(gpd) can dewater biosolids cost effectively with a bag dewatering system. The bagsystem is a gravity dewatering system with no moving parts. Polymer-conditionedbiosolids are pumped into a gravity drainage plenum, which provides initialdewatering and distribution to the bag chambers. Woven polypropylene bags arefilled with biosolids and allowed to drain by gravity. Filtrate is collected in a panplaced underneath the bags and returned to the head of the plant. Cycle time is 4 to6 hours - 2 hours for filling and 2 to 4 hours for drainage. A standard 22-gallon bagyields approximately 20 pounds of d y solids per bag at about 10-1 2% dry solids [ 191,
154 KukenbergerBags can be stacked outdoors for further diying by evaporation to as high as 60%solids concentration. Figure 3-10 depicts a bag dewatering system. ,- Levd Probe Emergency Overllow - pipe Figure 3-10 BAG DEWATERING SYSTEMIV. ODOR CONTROLOdor-producing compounds released from municipal wastewater biosolids includeinorganic gases, hydrogen sulfide and ammonia, and organic gases, such as indoles,skatoles, mercaptans, and nitrogen-bearing organics. A list of odor-producingcompounds in biosolids is included in Table 1-8. Hydrogen sulfide (H,S) is the mostprevalent odorous gas associated with domestic wastewater. Hydrogen sulfide isproduced during anaerobic decomposition of the organic matter in wastewater;therefore, keeping wastewater and biosolids fiom becoming septic preventsproduction of hydrogen sulfide. Organic vapors, particularly mercaptans, are oftenproduced during conditioning and dewatering. Mercaptans have an extremely lowodor threshold, that is, they are detectable by the human nose at very lowconcentrations. Odor thresholds for mercaptans range from 2.9 x I0-j to 1 . 1 xThe odor threshold for hydrogen sultide is 4.7 s 10-4. Mercaptans can create odorproblems at concentrations as low as 20 times less than hydrogen sulfide. Odor control can generally be classified in three categories: 1) prevention of theformation of odorous compounds, 2) treatment of odorous gases prior to release to theatmosphere, and 3) odor modification and masking. Prevention of the formation ofodorous compounds is accomplished by chemical addition to the liquid stream,maintaining aerobic conditions and good housekeeping. Chemicals added to liquidbiosolids to reduce odors include hydrogen peroxide (H,O,) and potassiumpermanganate (KMn04) . These chemicals are strong oxidizing agents that reactwith H,S to form non-odorous compounds.
Conditioning and Dewatering 155 Masking of odors is accomplished by replacing or overwhelming anobjectionable odor with a more pleasant one. This approach of adding "perfume" doesnot solve the problem and should not be considered as a permanent odor controlalternative. Collection and treatment of odorous gases from biosolids processes is generallyrequired at treatment plants located in populated areas. The production of H,S,mercaptans, and ammonia is unavoidable, and the low odor threshold associated withmany odorous compounds leads to odor problems and complaints from nearbyresidents. Collection of odorous gases requires enclosure of biosolids handling andtreatment units, or containment within a building. To reduce air flow rates and odortreatment unit sizing, and for worker comfoi-t, it is beneficial to contain odorous gaseswhen possible. Containment can be accomplished by enclosing handling andtreatment units and utilizing systems that are self-contained. For example, odor containment and control was an important selection criteriafor dewatering and conveying equipment at the Yonkers Joint Treatment Plant(YJTP), Westchester County, New York. Centrifuges were favored over opendewatering units, such as BFPs and filter presses, and dewatered biosolids pumpingunits were selected in lieu of open conveyors. The biosolids dewatering system isentirely enclosed in buildings, including truck loading bays. Figure 3-1 1 is aschematic of the odor containment and control system at the YJTP. Odor treatment systems for biosolids dewatering and handling include: Wet scrubbers (pack tower and mist type) Activated carbon adsorption Biofilters Figure 3-11 BlOSOLlDS DEWATERING ODOR CONTROL SCHEMATIC. YONKERS JOINT TREATMENT PLANT, WESTCHESTER COUNTY N.Y.
156 Kukenberger r Alr DlstrlbutlonZone Alr Discharge Polyethylene Liner J LDrsln Pipe (Vpical) Clay Cut Off Wall pipa Figure 3-12 CROSS SECTION OF BlOFlLTER Wet scrubbers use a chemical solution to remove odors from the odorous airstream. Selection of chemicals for wet scrubbers should be based on the odorouscompounds to be removed. The most common scrubbing solutions are sodiumhypochlorite and potassium permanganate with water. Hydrogen peroxide, ozone, andproprietary chemical solutions are also employed. Adjustment to pH may bebeneficial with some chemicals; therefore, acid and alkaline solutions may be providedwith wet scrubbers. Activated carbon adsorption can be employed as the primary odor treatmentsystem, or as a polishing step. When used as a polishing step, a bypass may beprovided so that the carbon is only used when necessary, thereby extending the life ofthe carbon bed. Carbon adsorbs a wide range of odorous compounds; however, dueto the non-selectivity of carbon, the capacity of the carbon can be prematurelyexhausted due to the adsorption of non-odorous hydrocarbons . Biofilters use bacteria naturally present in soil and compost to biodegradeodorous compounds. Biofilten can be custom designed (in ground) filters, as depictedin Figure 3-12, or manufactured units [2 I]. Custom designed biofilters should meetthe following criteria: Collect leachate by sloping the bed floor and using a liner and collection system with n drainage pipe fitted with a valve or water trap to prevent leakage of untreated air through this pipe Prevent leakage around the perimeter of the bed and along the air supply pipe by estending the active media past the edge of the air distribution zone and using a flange or other device around the air supply pipe as it enters the biofilter Specify corrosion resistant air supply duct work and piping Evaluate the method and frequency of media replacement
Conditioning and Dewatering 157 Provide a water supply for surface sprinklers and an inlet air humidifier Use geotextile fabric between the active media and the air distribution zone to prevent migration of fines Provide condensation drainage holes, as necessaiy, in the air supply piping Avoid surface compaction by keeping equipment and vehicles off the bed - during construction and operation Maintain perfoimance through regular operator attention to bed conditions and maintenance Combustion oxidizes odorous compounds with direct flame oxidation or catalyticoxidation. In direct flame oxidation, the odorous air is mixed with combustion air attemperatures of 480 to 815°C (900-1 500°F) [EPA]. In catalytic oxidation, a catalystcauses oxidation to occur at lower temperatures and in the absence of a direct flame.The odorous air is preheated to 3 15 to 480°C (600 to 900°F). Catalytic combustionis not well understood, but generally occurs in three steps: adsorption on the activesurface, oxidation, and desorption of the reactant products. Catalytic combustion isonly recommended when the concentration of odorous compounds is greater than 1,000 ppm. Catalytic combustion is more widely used for removal of organics fromindustrial processes [EPA]. Other odor treatment systems include adsorption on activated aluminaimpregnated with potassium peimanganate and woodkhips with iron oxide, andoxidation with ozone. These systems can be manufactured package systems or customdesigned systems for specific applications. With less common systems, pilot testingshould be performed to assure that the odorous compounds can be removed and todevelop design and sizing criteria.V. CASE STUDIESTwo case studies are presented in this chapter:1. Westchester County, New York High Solids Centrifuge Dewatering Facility - Yonkers Joint Treatment Plant2. Onondaga County, New York Belt Filter Press Dewatering Facility - Syracuse Metropolitan Treatment Plant The Yonkers Joint Treatment Plant (YJTP) dewatering facility in WestchesterCounty is a 60 d ~ y per day system constructed in 199 1 to replace ocean disposal. tonThis facility processes biosolids produced by the 120 mgd Y JTP and several smallerCounty treatment plants. Figure 3- 13 is a schematic of the biosolids processingsystem at YJTP. Primary solids are gravity thickened, and secondary solids arehckened by flotation. Both solids streams are anaerobically digested and blended in
Conditioning and Dewatering 159 Lime ind Kln Dlul Belt Filter Gravity Anieroblc Stormgo Thickeners Digesters Ylxeri Pump. Thkksned Digested DrVho and Elosolids Blosollds PurnDs Pump8 Figure 3-15 BlOSOLlDS TREATMENT SCHEMATIC SYRACUSE METROPOLITAN TREATMENT PLANT ONONDAGA COUNTY N.Y.TABLE 3-3 SYRACUSE METROPOLITANTREATMENT PLANT DEWATERING FACILITY 0 :RATING PARAMETERS  Cationic Polymer Six Belt Filter Presses Feed Solids Concentration 2.37 (1994 Average) 40 Waste Activated Secondary Solids 60 (% of Total) Emulsion (1690 Secodyne) I Dewatered Solids Concentration 35.5 lbdton D y Solids, as r delivered 35% active 20.8 (1994 Average) II ($/G ~~ ~~ ~ conditioning Cost ton) ~ $29 (1 995) Solids Concentration 0.35% BOD 640 mg/L Nitrogen 316mglL Quantity 1.5 mgd
Conditioning and Dewatering 163storage tanks. Stored biosolids are pumped to a battery of four Sharples DS 706 solidbowl, continuous feed, scroll-type high solids centrifuges, shown in Figure 3- 14.Dewatered biosolids drop by gravity into a battery of four Schwing KSP-25V twincylinder, hydraulically driven, reciprocating dewatered solids piston pumps. Thed e w a t a d solids pumps discharge to storage hoppers. Storage hoppers discharge bygravity to a truck loading bay. Dewatered biosolids are currently trucked to a landfillstabilization facility, followed by beneficial use. The long-term plan will incorporatetrucking to an N-Vir& facility for additional treatment and ultimate beneficial reuse.Table 3-3 lists the key parameters of the Yonkers Dewatenng Facility. The average feed rate to each press is 85 gallons per minute at 2% solidsconcentration. The BFPs are fed with Robbins & Myers I60 HSI Moyno progressivecavity pumps (Figure 3-17) from a holding tank. Dewatered biosolids are conveyedby a serpentine conveyor to an N-Viro@ stabilization process (Figure 3-18). Stabilized biosolids are applied to agricultural land surrounding the Syracuse, New York area.REFERENCES1. Sludge Conditioning,Manual of Practice No. FD- 14, Water Pollution Control Federation, 1988.2. W. Strumm and J. J. Morgan, Aquatic Cheniistw, John Wiley & Sons, Inc., 1970, pp. 447,503-507.3. J. J. Cunnkigham, President, Cunningham Environmental Support, Inc., personal communication with R. 1. Kukenberger, Blasland, Bouck & Lee, Inc., April 1995.4. S. K. Dentel, M. M. Abu-Orf, and N. J. Griskowitz, Guidaance Manual for Polynier Selectiori in Wastewater Treattiient Platits, Project 9 I -1SP-5, Water Environment Research Foundation, 1993.5. Operationatidhfaitiretiance o Sludge Dewatevitig System, Manual of Practice f No. OM-8, Water Pollution Control Federation, 1987.6. J. J. Saraceni, Onondaga County Department of Drainage and Sanitation, Memorandum to Blasland, Bouck & Lee,Inc., Subject: metro sludge dewatering facility operating data, and personal communication with R. J. Kukenberger, April 1995.7. S. K. Dentel and M. M. Abu-Orf, WEW: Full-scale evaluation of available sludge conditioning control technologies, #AC943805, presented at the Water Environment Federation 67th Annual Conference & Exposition, Chicago, Illinois, October 14- 19, 1994. 8. Andritz PolyScan, Automated control system for sludge dewatering operations, manufacturers literature, Andritz Ruthner, Inc., Arlington, Tesas, 1994.
164 Kukenberger9. S. Benitez, A. Rodnguez, and A. Suarez, Optimization technique for sewage sludge conditioning with polymer and skeleton builders, Water Resources, 28:10, pp. 2067-2073 (1994).10. Characteristics of thermally treated sewage sludge, Technical Bulletin 2302-T, Zimpro Inc., Rothschild, Wisconsin, December 1978.11. Alfa Laval Thermal, Spiral heat exchangers save up to 25% on sludge disposal costs, manufacturers literature, Alfa Laval Thermal Inc., Richmond, Virginia.12. Blasland & Bouck Engineers, P.C., 201 Facilities Plan Anienclrnent, Madison Metropolitan Sewerage District, Volume I1 - Technical Memoranda, February 1991.13. Design ofhlunicipal Wastewater Treatment Plants, Volume 11: Chapters 13-20, Manual of Practice No. 8, Water Environment Federation and Manual and Report on Engineering Practice No. 76, American Society of Civil Engineers, 1992, pp. 1 148-1250.14. E. A. Retter and R. Schilp, Solid-bowl centrifuges for wastewater sludge treatment, presented at the Filtech Europa 93 Conference, Karlsruhe, Germany, October 19, 1993, reprinted in Filtvafionci! Separalion, June 1994.15. A. Davarinejad and N. Voutchkov, Comparison of high solids centrifuge performancefor digested and undigested solids dewatering, Proceedings o WEF f Specialty Cotlference, June 1994, Chapter 7, pp. 47-56.16. The Rotary Press, A new solution to all your dewatering problems, manufacturers literature, Les Industries Fournier Inc., Black Lake, Quebec, Canada.17. Andritz, A Dewatering Profile, manufacturers literature, Andritz Ruthner, Inc., Arlington, Texas.18. C. S. Martel, Fundamentals of sludge dewatering in freezing beds, Water-Science Technology, 28: 1, p. 29 (1 993).19. L. Craig and B. Sheker, In the bag: a small flows option, Operations Forum, March 1995, pp. 22-25.20. Odor and Corrosion Control in Sanitary Sewerage System and Treatment Plants, Design Manual, 625/1-85/018, USEPA, October 1985.21. R. W. Frachetti and R. J. Kukenberger, Biofilters for odor control, Operations Forum, 1 1 :4 (1 994).22. T. Lauro, Deputy Director, Wastewater Treatment, Westchester County Department of Environmental Facilities, personal communication with R. J. Kukenberger, Blasland, Bouck & Lee, Inc., April 1995.
DigestionKenneth J. SnowCorporate Environmental Engineering Inc.Worcester, MassachusettsI. BIOSOLIDS DIGESTIONA. IntroductionDigestion typically refers to biological digestion of biosolids, either aerobic oranaerobic. Digestion of solids generated from wastewater treatment processes hasbeen employed as a means of stabilization, volume reduction and pathogen reductionfor many years. The primary objectives of stabilizing biosolids in the recent past wasdependent on the regulations which governed the method of ultimate disposal such asIandfX, ocean dumping, incineration, or land application. Of these disposal options,land application has typically been considered more of a beneficial use than the otheroptionsbut the old regulations, 40 CFR Part 257, did not provide a clear definition ofbeneficial use. Options for the method of disposal are decreasing. Landfills are filling up andpermitting of new landfills has become very expensive. Several states have bannedlandfilling of sludges and Congress has banned ocean dumping. In addition,implementation of advanced treatment of wastewater over recent years has generatedeven more biosolids. As a result, the current objectives for many municipalities, for digestion andother methods of biosolids treatment, is to accomplish beneficial use as defined by the40 CFR 503 regulations, published February 1993. 165
166 Snow Biosolids generated fi-om municipal wastewater treatment plants typicallyoriginate fiom primary clarification and " s e c o n w biological processes such astrickling filters, rotating biological contactors, and activated sludge. Grease,skimmjng and grit are usually disposed of separately and are not processed with theother biosolids. In addition, many municipal wastewater treatment plants acceptresidential septage via tanker trucks. Septage is introduced at a variety of locationsin the treatment process train such as influent headworks, gravity thickeners, ordigesters. Chemical addition to the wastewater process and methods of tertiarytreatment contributes to the volume and varying characteristics of biosolids. Alumaddition for phosphorus removal, polymer addition for e n h a n d solids separation andactivated carbon addition for removal of a variety of substances are examples. Thebiosolids are often thickened prior to digestion. Digester configurations have been fairly standard over the past fifty years.Anaerobic digesters are typically two-stage concrete cylindrical covered tanks with conical bottoms. The biosolids are normally pumped into the first stage tank, where the contents are mixed and most of the digestion occurs. The displaced solids overflow into the second stage where the solids are thickened via settling and are then removed for disposal. The supernatant is generally directed back to the wastewater process for furthertreatment Aerobic digesters are generally open top tanks equipped with diffused o surface mechanical aerators. The shape of the tank is usually dictated r by the selected method of mixing andor aeration. They are often equipped with some means of decanting to provide some degree of thickening. Again, the supernatant is usually directed back to the wastewater treatment process. Fairly recent improvements (in t i country) to digestion processes are generally hs directed toward increasing efficiency, reducing maintenance, and meeting the new 503 Regulation pathogen and vector attraction reduction criteria. The improvements include: Egg-shaped anaerobic digesters which have improved efficiency and sigtufhntly reduced maintenance primarily by modifying the reactor codiguration. Autothermal Thermophlic Aerobic Digestion (ATAD) which is more efficient than conventional aerobic digestion and increases the degree of pathogen destruction. Pre-stage ATAD, which is used upstream of anaerobic digesters to accomplish pathogen destruction to meet Class A 503 regulation criteria, is typically retrofitted into existing plants employing anaerobic hgesters. In addition, a pasteurization step can be added to the above digestion processeswhich can further enhance pathogen reduction and, in some cases, produce a higherclass bimlids end product. Pasteurization is not considered biological digestion anddiscussion will be limited on this process.
Digestion 167 For new digester installations,process selection depends on ultimate use (Class)of the biosolids (dependent on marketability), volume of biosolids to be processed andeconomics. When reb-ofitting the digestion process of an existing treatment plant, theexisting equipment, structures and process configuration must be taken intoconsideration to take economic advantage of previous investments.B. Process Fundamentals1. Aerobic DigestionBacteria growth in a batch reactor can be represented by Figure 4-1. The aerobicdigestion of biosolids originating from biological processes such as activated sludgeis primarily endogenous, i.e., microorganisms consuming their own protoplasm. Thisis represented by the "death" phase portion of the growth curve. When biosolids fromprimary settling are introduced, the organic material is first oxidized resulting in thegrowth of new cells represented by the "growth" phase of Figure 4-1. Themicroorganisms are oxidized to water, carbon dioxide, and ammonia in accordancewith: C,H,NO,(Cells)+SO, = 5C0,+2H20+NH,. I Stationary phase A I Time FIG. 4-1 BACTERIA GROWTH CURVE.
Snowa. Mesophilic Aerobic Digestion In conventional (mesophilic) aerobic digestion, the ammonia is further oxidizedto nitrate. A maximum of about 75 percent of the microorganism cell can be oxidizedas the remainder of the cell is inert material or organics which cannot be biologicallydegraded. The operation of a conventional aerobic digester is fairly simple providedequipment design and layout is not a hindrance. The operator needs to maintainsufficientdissolved oxygen (typically no less than 1 - 2 mgA throughout the reactor),mixing, and residence time. In addition, decanting or drawing off the supernatant after30 to 60 minutes of settling will provide a thicker end product which will reducepower andor chemical requirements for subsequent treatment such as dewatering.Scum removal, foam control and odors or treatment of off gas may also need to beaddressed depending on equipment configuration. Dissolved oxygen can be monitored using a portable meter with a probe whichcan be lowered into the reactor. Adjusting blower or surface aerator speed can beused to minimize power consumption. Adequate mixing can be penodically checked by simply probing the tank bottomto feel for solids which have been deposited or use of a velocity meter which can be lowered into the reactor. A velocity of 2 to 3 feet per second is usually adequate. Often the aeration equipment is also used to provide mixing. Depending on reactorconfiguration, either mixing or dissolved oxygen concentration will govern equipment operation. Laboratory tests such as the percent of volatile reduction, specific oxygen uptake rate, and biological testing, may be used to determine the required residence time. The actual monitoring and testing requirements will be based on compliance with the 503 regulations and the ultimate disposal method. Decanting, if the process is so equipped, to thicken the biosolids is usually conducted prior to andor after the digestion period. Decanting is usually used when a batch process scheme is employed. The method of decanting is dictated by equipment configuration. Thickening following digestion, as a separate process, is also common.b. Theimophilic Aerobic DipestionAutothermal thermophilic aerobic digestion (ATAD) is aerobic digestion underthermophlic conditions (4OoCto SOOC). The term autothermal is used to describe thistechnology because no outside heat source is required. The organic decompositionduing digestion releases heat and maintains the thermophilic operating temperatures(typically 55C), provided the reactor has adequate insulation. The ATAD process hasseveral benefits over conventional aerobic digestion [I]:
Digestion 169 h h biosolids treatment rate. g Decreased reactor volume. More effective pathogen reduction. More effective vector attraction reduction. Higher volatile solidsreduction resulting in reduced downstream equipment sizes. The ATAD process can effectively produce biosolids that exceed EPA Class Apathogen reduction criteria. It can also consistently meet the specific oxygen uptakerate and volatile solids reduction U.S. EPA 503 vector attraction reductionrequirements. Thermophilic Aerobic Digestion is a Process to Further ReducePathogens (PFRP), as defined in the U.S. EPA 503 regulations. The ATAD processcan meet the PFRP definition [ 11. ATAD digestion systems are typically two-stage processes with mixing, aerationand foam control equipment. Single stage systems can provide similar volatile solidsdestruction but cannot destroy pathogens to the same degree. Figure 4-2 illustrates atypical ATAD system configuration. Pre- and post- thickening is usually employed.The Fuchs system (Germany) is the most common system and design and operationalparameters discussed below are generally based on this process configuration. Typicaldesign parameters are listed in Table 4-1 . EXHAUST AIR BlOSOLlDS t1 1 / TO THICKENER/ DEWATERING THICKENER/STORACE ATAD RUCTORS SOUDS STORACEFIG. 4-2 TYPICAL ATAD CONFIGURATION. Key design and process requirements include biosolids feed at 4% to 6%solids with a minimum of 2.5% volatile suspended solids. Biosolids at less than 4%solids may not allow sufficient heat generation to achieve thermophilic conditions.Biosolids at greater than 6% may prevent adequate mixing or oxygen transfer. Heat
170 Snowp h t i o n i typically between 14,200 and 14,600kjkg 0, and oxygen requirements sare 1.42 kg O@g volatile suspended solids. The thermophilic temperature rangeinhibits nitrificationand the associated oxygen demand. Temperatures should be keptbelow 65OC to prevent resolublizationof organics [11. Parameter Range Feed Total Solids 4%-6% Feed Volatile Solids 2.5% min. Detention Time 5 6 days Temperature 35-50°C Reactor 1 50-65°C Reactor 2 Air Input/m’ Reactor Volume 4 m’h The reactors typically are conical with flat bottoms.If grit is not removed in anupstream process, it may accumulate in the reactor and it may be necessaq to providea conical b t o or other means to facilitate cleaning or prevent deposition. otm The method of operation of the ATAD system dictates performance, especiallywith respect to pathogen destruction, and establishes sizing criteria for biosolids feedpumps and gravity flow lines. Biosolids are fed in batch once per day. The solids feedvolume, typically ane tid of a single reactor volume, should be delivered in less than hr1 hour.This allows about 23 hours per day of undisturbed digestion needed to attainhigh pathogen destruction. The sequence of batch feeding is as follows:s Stop aerationhmag and transfer digested solids fiom the second reactor to the storage/thickenertank.r Transfer solids f o the first to the second reactor. imr Transfer feed solids into first reactor and start aeratiodrnixing. The above sequence prevents parually treated biosolids fiom reinoculating thefinal stabilized product. The ATAD process generates foam during digestion. It has been speculated thatfoam provides some insulating value, improves oxygen utilization, and enhances
Digestion 171biological activity. Cuyent designs provide for foam control as opposed to elimination.The foam bubbles are reduced in size, using foam cutters, which provides a denserfoam. A freeboard of 0.5 to 1.0 m in the reactors should be allowed for foam. Thickening of the digested biosolids is generally desirable prior to transport forultimate reuse. It should be noted that the sludge must be cooled (possibly in a storagetank) prior to using gravity thickening since thermal convection currents result in poorperformance. Normally, some storage of the digested biosolids will be needed to accommodateremoval and disposal fi-equencies. Uncovered tanks equipped with mixers but with noaeration will not result in objectionable odors if mixing occurs daily for at least anhour and ifthe biosolids are fully stabilized. Reactivation of pathogens or reinfectionhas not been a problem using the above equipment and procedures. Experience has shown that some odors are generated from the ATAD processand may, depending on receptors, require treatment. Odors will be more objectionableshould the process be overloaded resulting in incomplete stabilization or if storagetanks are not mixed daily as previously mentioned. In general, aerobic digestion can, depending on design and equipment selection,have the following advantages over anaerobic: Impact to the wastewater treatment process is reduced due to lower strength @OD) supernatant recycle. Resulting biosolids dewater better. Less potential for odors and safety hazard due to explosive gases.Disadvantages: Does not produce methane which is a useful by-product. Supplying oxygen generally results in higher power cost.2. Anaerobic DigestionAnaerobic digestion can be described as a multistage process where microorganismsbreak down various types of organics into simpler organics which are furtherconverted by other types of microorganisms into even simpler organic compounds andCO,, KO, Q , and Q S . In the first stage, the biosolids, which are made up of Ccomplex organics, lipids, l i p n s , proteins, and cellulose, are broken down into organicacids, alcohols, ammonia and carbon dioxide. The first stage products are brokendown into hydrogen, carbon dioxide, and low molecular weight organic acids duringthe second stage. The microorganisms responsible for this conversion are referred toas “acid formers.” During the third stage, acetate, carbon dioxide and hydrogen areconverted to methane and carbon dioxide and water. Two types of “methane-forming”bacteria are responsible for this third stage; the first converts carbon dioxide and
172 Snowhydrogen to methane (CH,) while the second type converts acetate to methane andcarbon dioxide. Refer to Figure 4-3 for a schematic description. The thrd stage is generally considered to be rate limiting since methane-formingbacteria grow more slowly than the acid formers. In addition, the methane formers arealso more sensitive to pH. The pH must range between 6 and 8, otherwise, un-ionizedvolatile acids (below pH 6 ) or un-ionized ammonia (above pH 8) is toxic to methaneformers. The operation of a typical two stage anaerobic digester is somewhat morecomplicated than an aerobic digester. A number of parameters must be monitored: COMPLEX ORGANICS LIPIDS LIGNINS PROTElNS CELLULOSE ORGANlC ACIDS ALCOHOLS AMMONIA CARBON DIOXIDE ~ 1 HYDROGEN CARBON DIOXIDE ORGANIC ACIDS ACETATE I CH,COOH acetate HO FIG. 4-3 BlOSOLlDS CONVERSION IN ANAEROBIC DIGESTION.
Digestion 173 - Alkalinity PH Temperature Volatile solids loading Gas production Volatile acid concentration The methane-foiming bacteria are very sensitive to pH and will only h v ebetween pH 6.5 and pH 8.5. Therefore, operational monitoring focuses on providingthe appropriate environment to keep these bacteria viable. Overloading with feedbiosolids is a typical problem with anaerobic digesters. When the feed rate of volatilesolids in the incoming sludge increases, the acid forming bacteria grow rapidly andproduce more acids than the methane forming bacteria can digest. As a result, theexcess acids lower the pH to the point where the methane formers can not function andthe process stops. To help ensure the pH will remain in the proper range, alkalinity isalso monitored. Generally, a range of between 2,000 to 3,500 mg/l CaCO, isacceptable. In conjunction with pH and alkalinity, volatile acid concentration should bemonitored. Typically, the range of volatile acids is from 50 to 300 mg/l for welldigested sludge. The optimum value of the above parameters which provide for the most efficientdigestion will vary with different types of sludges. Therefore, the operator mustmonitor and track the results to determine the optimum range for each parameter.C. Equipment Review1. Aerobic DigestionEquipment used for aerobic digestion of biosolids must be able to provide thoroughmixing and sufficient dissolved oxygen. Tank geomehy, aeratodmixer location, andsuspended solids concentration are factors which affect oxygen transfer and mixingefficiencies. Scum removal, foam control, decanting of supernatant and off gas controlmust also be considered.Conventional Aerobic DigestionConventional aerobic digestion equipment designs have included a variety of tankgeometries. It is usually dependent on the type of mixing and aeration equipmentselected. Conventional aerobic digesters are often open top rectangular concrete tanks,sometimes with sloped bottoms to facilitate solids removal and cleaning, into whichbiosolids are pumped. The liquid level in the digester varies as biosolids areintroduced or removed.
174 Snow Mixing and aeration equipment can be designed as independent devices toaccomplish mixing or aeration, as a single piece of equipment to accomplish bothmixing and aeration or various combinations of both. All mixing and aerationequipment both mix and aerate but may be designed with a primary function as oneor the other. There are many types of aeration/ mixing devices available. Equipment designed primarily for mixing would include Turbine or impeller type mounted on a vertical shaft driven by a surface mounted motor. Pumps which draw liquid from one part of the reactor and discharge to another part in a manner to promote circulation. Equipment designed primarily for aeration in conventional aerobic digestersinclude mechanical surface aerators and diffused aerators. For diffised aeration, an external blower delivers air to a diffiser located nearthe bottom of the digester. Positive displacement blowers are often used when theliquid level i the digester is designed to rise and fall. Diffusers are categorized by the norifice size which detemiines the size of bubble produced; fine bubble or coursebubble. Course bubble d i h s e r s are nomially used since the high concentrations ofsolids tends to clog the fine bubble orifices more rapidly. Diffisers can be mountedon a "swing" a m pipe header to allow lifting out of the digester or can be mounted to rthe digester floor. As air is blown through the diffiser, small bubbles form providinga large aidliquid interface to promote oxygen diffusion and transfer. The action of theair rising also induces mixing. Mixing can be enhanced by strategic location ofdiffusers to develop circulation currents. The blowers can have variable speed dnvesor two speed motors to optimize energy consumption. Mechanical surface aerators are usually constructed of a partially submergedimpeller driven by a vertical shaft connected to an electric motor. The degree ofimpeller submergence is usually critical so the liquid level in the digester must becontrolled or fixed when the aerator is rigidly mounted. As an option, the surfaceaerator can be mounted on floats allowing it to rise and fall with the liquid level. Asthe impeller rotates, the liquid is "sprayed" radially outward. The turbulence andformation of small liquid droplets cause a large airfliquid interface which promotesoxygen transfer. Depth of submergence, often controlled by adjustable weirs on thedigester outlet, and variable or dual speed motors allows control of power usage. Equipment designed to provide both mixing and aeration include: Static aerator Mechanical aspirator Venturi aspirator
Digestion 175 Static aerators are rigidly mounted, usually in a grid pattern, across the floor ofthe digester. Static aerators are usually cylindrical tube-shaped, vertically mounted andair is introduced into the bottom to the tube. A series of baffles and the current inducedby the rising air draws the liquid into the base of the tube, mixes it, and dischargesfrom the top. U x i n g and aeration are controlled by adjusting air flow to the staticmixers. Mechanical aspirator type mixeiherators are manufactured in a variety ofconfigurations and are selected bascd on digester geometry, location within thedigester and the size and shape of the effective mixing zone. Generally, the equipmentconsists of an impeller mounted on a hollow shalt which rotates at high speed. Therotation ofthe impeller creates low pressure which draws in (aspirates) air. The venturi type mixerherator employs a pump which directs the liquid through a venturi device. Air is drawn in to localized low pressure, and the discharge stream is directed such that the force of the stream promotes mixing. The discharge can be submerged or discharge above the liquid level, depending on the application. Conventional aerobic digesters typically use plant water sprayed through a series of nozzles across the top of the liquid. The spray breaks up the foam but has the disadvantage of adding "dilution" water to the biosolids. Decanting is typically conducted by shutting off the mixing and aeration equipment and allowing the solids to settle. The relatively clear supernatant is then drawn oE. Telescopic valves, submersible pumps that can be raised and lowered into the liquid, and multiple draw off lines at various elevations along the tank wall are some of the more common configurations used to decant and thus thicken biosolids before or after digestion. In some cases the digesters are covered and the off gas is directed to some type of treatment such as intake to blowers providing air to aeration basins, scrubbers, or activated carbon canisters. Off gas treatment is usually to control odors.Autothermal Thermonhilic Aerobic DigestionATAD digesters are relatively new and equipment types in use are limited. The ATADreactor installations are usually insulated cylindrical aboveground steel tanks whichare fully enclosed. Steel is cumently less expensive than concrete but retrofittingexisting concrete tanks may be desirable. The efficiency of aeration and mixing equipment is of particular concein in thethermophilic process in order to minimize the heat loss from the reactor through airexhaust. The Fuchs ATAD system, which is one of the more common, uses theaspirator type mixer/aerator described above. The CBI Walker, Inc. system employsthe ventun type mixedaerator with exteinal liquid recirculation. The biosolids arepumped from the top third of the reactor through a venturi where air is drawn in anddischarged back into the reactor near the bottom. This system also has the ability to
176 Snowrecirculate air from within the reactor head space to the venturi aspirator which canbe used to slow down the biological oxidation should temperatures become too great. Foam is generated in ATAD reactors. As previously discussed, foam control, notelimination, is desirable. Foam cutters are typically set at a specific height above theliquid level. An impeller, which can be mounted horizontally or vertically, rotates andthe blades “cut”the foam, reducing the bubble size, making it more dense and lessvoluminous. Various foam control, mixing and aeration equipment are available from severalmanufacturers, some of which are proprietary. Babcock, CBI Walker, Fuchs, Limus,and Thieme are some of the manufacturers. Odor control equipment to treat the offgas from the reactors will, in many situations, be needed. Depending on stateregulations, air peimits may be required.2. Anaerobic DigestionAnaerobic digestion must take place i reactors which are sealed from the atmosphere. nCovered cylindrical concrete tanks have been the most frequently designed. Morerecently, egg-shaped digesters are being designed and installed and offer improvedefficiency and reduced maintenance. The digester equipment must provide for mixing,scum and grit removal, foam control, off gas handling and heating. Conventional cvlindrical dipesters employ a variety of covers to seal the contentsfrom the atmosphere which maintains anaerobic conditions, collects useful gas,minimizes odors,provides insulation, and prevents explosive mixtures of air and gasfrom forming. Covers are either floating or fixed. Floating covers offer severaladvantages; prevention of the formation of excess pressure or vacuum or drawing ofair into the digester during transfer of digester contents and, in some instances,minimizes scum/foam accumulation because the cover rests directly on the liquidsurface. There are a variety of cover types available. Conventional anaerobic digesters are equipped with a variety of mixing devicesincluding gas recirculation and mechanical mixing methods . The major gasrecirculation mixing systems include: sequentially discharged lances floor mounted diffusers draft tubes bubble guns Each requires some type of compressor. Gas recirculation usually promotesfoaming problems. Mechanical mixing typically consists of a motor, drive shaft andpropeller combination or a recirculating pump. Propeller mixers can be mounted atvarious locations throughout the digester but most are used with draft tubes to directflow. The recirculating pump mixer withdraws biosolids from one or more locations,
Digestion 177sometimes through a draft tube, and discharges through nozzles at the digester wall.The nozzles direct the flow to induce a circulation pattern. Convention digesteis are not typically equipped with grit, scum or foam removaldevices. Mixing is usually relied upon to prevent accumulation. Many digestersexperience problems as a result and must be cleaned every 3 to 8 years . Anaerobic digesters generate methane which can be used as fuel to provide heatto maintain proper operating temperatures of the digester contents andor buildingheat. External hot water heat exchangers are frequently used to heat the digestercontents. The gas is commonly used to fire hot water boilers or as a primary fuel forreciprocating internal combustion engines. Gas collection equipment includes flametraps, pressure relief valves, check valves, waste gas burners, accumulators andcompressors. k - s h a D e d digesters (ESD) have eliminated the shaip transition along thedigester wall and reduced the liquid surface area at the top. The ESD offers a numberof advantages over conventional digester configurations which are primarily a resultof the modified shape and combination of mixing methods: Mixing, provided by gas injection, propeller or pump in combination with a central draft tube, is very effective in concert with the shape of the EDS. The mixing energy is lower which results in operating cost reduction. Combinations of these mixing methods can be employed. The thorough mixing improves digestion efficiency by better substrate/microorganism contact and elimination of "dead" zones. Grit accumulation is eliminated by the shape of the ESD and thorough mix systems. Eliminating the cleaning of grit, the lost volume due to accumulated grit, and the associated cost and down time improves efficiency and economics. Scum/foam accumulation can be effectively controlled as a result of the limited liquid suface and operation of the central draft tube mixer in the down mode. In addition, a scum chute at the top of the digester allows removal of accumulated floatables if needed. Eficiency and economics are improved for the same reasons as stated for grit accumulation. Generally, the footprint is smaller and there is usable space below the digester. Figure 4-4 illustrates a ESD in schematic form which is based on the CBIWalker, Inc. design. The mixing system shown utilizes a jet pump with a.draft tubeassembly. The valves and heat exchanger are not shown for simplicity. The jet pumpcan discharge into either the top or the bottom of the draft tube. This promotesblending of either floating or settled materials with the remainder of the digestercontents. The jet pump directs the motive fluid into one end of the draft tube whichdraws in additional liquid. This mixes liquid which is taken from different locationswithin the digester, resulting in a more homogeneous mixture. This pump systemeliminates moving parts from within the digester. In place of the jet pump, a
178 Snow DIGESTER W 1 I I COLLECTION SCUM FUNNEL DIGESTER VESSEL -D M TUBE DIGESTEDBlOSOLlDS 1 1FEEDBlOSOLlDSFIG. 4-4 EGG SHAPED DIGESTER.
Digestion 179mechanical mixer (propeller) can be located near the top portion of the draft tube. Thismixer also mixes the contents fiom top to bottom. Compressed gas is not used ineither configuration which minimizes foaming problems.D. Economics of DigestionOne of the primary factors in selecting a digestion process is the ultimate disposal oruse of the biosolids, i.e., production of Class A or Class B end products. This mustbe determined first so the appropriate methods of digestion can be compared on aneconomic basis. Conventional aerobic digestimcannot reliably meet Class B pathogen and vectorattraction reduction criteria when digesting primary and secondafybiosolids at typicalretention times . In some cases, achieving Class A or B end products is notrequued, yet it still may be economical to do so as illustratedby the study conductedby the Grand Chute-MenashaWs Sewerage Commission . et In 1993, the Grand Chute-MenashaWest Sewerage Commission conducted acost comparison between conventional anaerobic digestion with gas mixing andATAD. The upstream wastewater process included primary cl&ers followed byactivated sludge at an average design flow of 5.2 MGD. The schematic arrangementfop the two digestion process is shown in Figures 4-5 and 4-6 located in the followingsection The cost for the digestion process, excluding thickening, dewatering, and odorcontrol, are as shown below. CONVENTIONAL ANAEROBIC ATAD CAPITAL $2,455,000 $1,270,000 AnnualO&M Power $ 5,000 $ 60,Ooo Labor $ 52,500 $ 35,000 Parts & Supplies $ 15,000 $ 15,000 Digester Heat $ 15,000 $ TOTAL 0&M $ 87,500 $ 110,Ooo To meet Class A standards, a comparison between ATAD and ATAD followedby anaerobic ESD can be made. The cost has been found to be within 10% forfacilities t e t n five to eight MGD. The ATAD followed by anaerobicESD will be raigmore economical at larger facilities.
180 Snow For Class B product, the cost to digest biosolids from wastewater facilitiestreating between three to five million gallons per day (MGD) is about the same forATAD and anaerobic ESD. Digestion costs at smaller facilities will generally belower using the ATAD process. This is primarily due to capital cost of someequipment, such as gas monitoring and safety, in anaerobic systems which change verylittle with changes in digester size. Life cycle costs are very important when conducting a comparison betweenprocesses. Labor cost associated with operation are about the same for all threeprocesses. However, energy costs are about half the cost for systems which utilizeanaerobic digestion. The reduced energy cost combined with the energy productionpotential makes the anaerobic digestion process more attractive for the larger facilities.Utilization of the ATAD system with anaerobic digestion hrther enhances the energyproducing characteristics of the anaerobic system. Capital cost comparison between methods of digestion presented below will belimited to the more advanced processes, i.e., autothermal thermophilic aerobicdigestion (ATAD), anaerobic digestion utilizing Egg-Shaped Digesters (ESD), andaerobic thermophilic followed by anaerobic ESD. Biosolids characteristics andvolume generated from a typical municipal secondary treatment facility will be acommon basis of comparison. The capital cost information on the following pages has been provided by CBIWalker for their AutoThermm (ATAD), ESD, and AeroThennTM(ATAD followedby ESD) systems [S]. In developing the three different designs and associated capital cost, a newfacility is assumed. Each of the costs are based on the same design information and assume a site i the Midwest with open shop construction forces for the field erection. n The cost estimates provided for the three designs were developed by comparing information from recent (1995) projects with an accuracy of about 10% to 15%. Other factors which have an impact on the cost estimates are the site location, time ofyear in the field, final design of the system and its components, system layout and theexisting facility. Regardless of the system preference, one should consider life cycle costs whenmalung a final selection of a system. The ATAD ( AutoThermTM) system is the leastcapital cost for the design example. However, as an aerobic digestion process, the ATAD system is an energy consuming process. The ESD system is an anaerobic process and therefore is an energy producing process. While the ATADESD (AeroTheimm) system includes an aerobic process, it is utilized with anaerobic digestion to produce a two stage digestion process. This results in a net energy production similar to the anaerobic digestion process.
Digestion 181 ATAD DESIGN AND CAPITAL COST MIDWEST WWTP APRIL 1995 PROCESS DESIGN INFORMATIONA. PLANT DESIGN SUMMARY: Plant Design Flow Rate - 2.0 MGD (Current) 4.5MGD (Future) Sludge Design Flow Rate - 8,100 GPD (Current) 18,300 GPD (Future) Design Loading - 3400 #/Day Solids Loading (Current) 7600 #/Day Solids Loading (Future) 5% Feed (Assumed Design) 75% Volatile Solids Present Facility - Activated Sludge Facility 45% Primary 55% Secondary No Existing Sludge Treatment SystemB. ATAD DESIGN SUMMARY: Reactor Design - 3 @ 39,600 Gals. Capacity, Each Reactor Detention - 6.5 Days @ Future Design Flow Air Injection - Aspiration Through Venturi Effect Mxing Requirements - External Pump Recirculation Heating Requirements - External Heat Not Required Energy Requirements - 150 k W l 0 0 0 Gals. (Raw Feed Sludge) Sludge Storage - 1 Vessel @ 370,000 Gals. Detention Time - 20 Days @ Future Design Flow Energy Requirements - 5 k W l 0 0 0 Gals. (Raw Sludge Feed)
182 Snow ATAD CAPITAL COST ESTIMATE MIDWEST WWTP APRIL 19951. System Effective Footprint FT2 -x 1 0 50 2 2. Site Work: $ 475,000.00 - Control Building Excavatiodl3ackfYl Concrete Foundation3. Tankage: $ 880,000.00 - Vessels Insulation System Stair Tower Access Walkway & Platforms4. MechanicalElectrical Process Equipment (Installed): $1,150,000.00 Mixing System - Air Injection Pumps & Valves Process Piping Instrumentation & Control Process Engineering Start-up Service5. Vent Air Odor Control System (Installed) $ 50,000.006. Miscellaneous $ 245,000.00 TOTAL COSTS $2,800,000.00
Snow ESD CAPlTAL COST ESTIMATE MIDWESTWWTP APRIL 19951. System Effective Footprint 4950 FTZ 45X1102. Site Work: $ 580,000.00 ControlBuilding Excavation5acldill Concrete Foundation3. Tankage: $1,540,000.00 Vessels Insulationsystem StairTower 0 Access Walkway & Platforms4. MechanicaVElectrical Process Equipment (Installed): $ 720,000.00 Mixing system Heating System pumps & valves Process Piping Gas Safely Equipment Instrumentation & Control Process Engineering start-up service5. G s Storage (Installed) a $ 40,000.00 500 FT Capacity6. Miscellaneous $ 280,000.00 TOTAL COSTS $3,160,000.00
Digestion 185 ATAD/ESD DESIGN AND CAPITAL COST MIDWEST WWTP APRIL 1995 PROCESS DESIGN INFORMATIONA. PLANT DESIGN SUMMARY: Plant Design Flow Rate - 2.0 MGD (Current) 4.5 MGD (Future) Sludge Design Flow Rate - 8,100 GPD (Current) 18,300 GPD (Future) Design Loading - 3400 #/Day Solids Loading (Current) 7600 #/Day Solids Loading (Future) 5% Feed (Assumed Design) 75% Volatile Solids Present Facility - Activated Sludge Facility 45% Primary 55% Secondary No Existing Sludge Treatment SystemB. ATAD DESIGN SUMMARY: Reactor Design - 1 @ 19,800 Gals. Capacity Reactor Detention - 26 Hours @ Future Design Flow Air Injection - Aspiration Through Venturi Effect Mixing Requirements - External Pump Recirculation Heating Requirements - 360,000 BTU/HR Energy Requirements - 20 k W 1 0 0 0 Gals. (Raw Sludge Feed)C. ESD DESIGN SUMMARY: Digester Design - 1 @ 278,000 Gals. Capacity Vessel Digester Detention - 15 Days @ Future Design Flow Mixing Design - External Pump Recirculation Heating Requirements - Included in the ATAD Design Summary Energy Requirements - 10 k W l 0 0 0 Gals. (Raw Sludge Feed) Gas Production - 15 FT31#VSDetroyed Methane Content - 65% Methane Gasholder - 500 FT’ Sludge Storage - 1 Vessel @ 370,000 Gals. Detention Time - 20 Days @ Future Design Flow Energy Requirements - 5 k W 1 0 0 0 Gals. (Raw Feed Sludge)
186 Snow ATAD/ESD CAPITAL COST ESTIMATE MIDWEST WWTP APRIL 19951. System Effective Footprint 6ooo FT - 5UX1202. Site Work: $ 530,000.00 4 Control Building 4 ExcavationBacMill 4 Concrete Foundation3. Tankage: $1,460,000.00 ATAD Vessels Anaerobic DigesterIStorage Vessels Insulation System Stair Tower Access Walkway & Platforms4. MechanicalElectrical Process Equipment (Installed): $ 950,000.00 Mixing System 4 Air Injection System 4 Heating System Pumps & Valves 4 Process Piping 4 Gas Safety Equipment Instrumentation & Control 4 Process Engineering 4 Start-up Service5. Vent Air Odor Control System $ 20,000.006. Gas Storage (Installed) $ 40,000.00 500 FT3 Capacity7. Miscellaneous $ 300,000.00 TOTAL COSTS %3,300,000.00
Digestion 187 When developing the life cycle costs, the following should be addressed:1) Manpower Both systems will require about the same manpower for operation of the process.2) Energy The ATAD (AutoTherm) system utilizes about 150 k W 1 0 0 0 gals. of raw sludge feed per day. The ESD system utilizes about 15 k W 1 0 0 0 gals. of raw sludge feed per day. The ATADESD system utilizes about 20 k W 1 0 0 0 gals. of raw sludge per day. Both the ESD and the ATADESD system will produce about 15 fi’ of gasAb volatile solids destroyed.3) Solids The volatile solids reduction for an aerobic digestion system Reduction has been characterized to be within a 40%-50% range. Anaerobic digestion is typically expected to be within a 45%-55% range. Many times there will be other factors to consider which may be more or lesstangible for a given project. Some things worth considering include the type ofwastewater facility and location. Northern climates with low volatile waste sludgemay require additional heat for the ATAD system. This will drive up the capital costand make the process less attractive for a given project. Warm climates may also bemore favorable to the ATAD system due to little need for heat in the winter and nodesire to use cogen as a means for reducing electrical costs.11. CASE STUDIESSeveral studies have been conducted which compare conventional anaerobic digestionto the more recent aerobic and anaerobic thermophilic processes. More specifically,the ATAD and thermophilic anaerobic processes were evaluated during two studiessummarized below .1. ATAD vs. Convenlional Anaerobic DigestionThe Grand Chute Menasha West Wastewater Treatment Facility, Wisconsin, amedium sized facility with projected flows of 5.2 MGD for the year 2010, wasconstructed in 1983 and was in need of expansion and upgrade. Specific biosolidsprocess for pathogen and volatile solids reduction were the focus of concern and couldnot be met using the existing gravity thickenindaerobic digestion process. The planned liquid process train includes fine screens (0.635 cm opening),vortex type grit removal, primary clarification, activated sludge, ferric chlorideaddition to the secondary process for phosphorous removal and UV disinfection. The
ia8 Snowprojected biosolids from the primary clarifiers and waste activated sludge processassume 50% suspended solids removal as the primary clarifier, 0.6 kgkg BODremoved in the activated sludge process and a 30 mg/l dosage of fenic chloride. Primary Secondary Ferric Solids Solids Chloride Total Annual Avg. (kdday) 1,954 1,549 1,077 4,580 Max. Month (kg/day1 2,344 1,796 1,322 5,462 - To OWATERING PRIMARY & THICKENED SUPERNATANT WASTE ACT. SLUDGE L METHANE (MIXING CAS) + FIG. 4-5 PROCESS FLOW DIAGRAM, ANAEROBIC DIGESTION.
Digestion 189 DIGEST€ rn iuir I 1 AERATION SLUDGE / & MIXING WITHDRAWALFig.4-6 PROCESS FLOW DIAGRAM, AUTO-THERMAL AEROBIC DIGESTION. Parameter ATAD Anaerobic Present wrh cost ot 23% less -- Safety No explosive gases GeneratesMethane Mechanical Simplicity -- H s floating covers, and a gas handlinglsafety equipment Odors None if well H s odors a operated . Tank Size 6 day hydraulic 25 day hydraulic retention time retention time Quallfy for PFRP I Yes I No I The process flow schematicswhich are the basis for the comparison betweenanaerobic digestion and ATAD are shown in Figures 4-5 and 4-6.
190 Snow In this study, the ATAD process for solids treatment was selected overconventional anaerobic digestion. The ATAD process was evaluated to be lessexpensive for both capital and operation and maintenance, safer, mechanically simpler,less odorous, smaller with respect to tank sizes and able to qualify for PFRP. Table4-4 presents the advantages and disadvantages.2 . Thennophilic Anaerobic vs. Conventional Anaerobic DigestionThe Greater Vancouver Regional District conducted full-scale thermophilic anaerobicdigestion testing to determine if a Class A sludge, based on pathogen density criteriacould be achieved. This experiment was conducted at the Lions Gate Wastewater Treatment Plantwhich is a 20.5 MGD average diy weather flow with primary treatment only. Theprimtxy sludge feed into the digesters during the experiment ranged between 130 and160 m3/day at a estimated 5% solids. Two of the existing four digesters, operated in nseries in contmuous flow mode, were used for digestion. These two digesters providea retention time of 20 days each. The remaining two digesters were used for storageof digested sludge prior to dewatering. The experiment intended to operate only the first digester at the thermophilictemperature of 55" C at a retention time of 20 days. This far exceeded the Part 503process requirement for Class A combination of time and temperature: D = 50,070,000/10 0 ~ 1 4 L where: D = Retention Time in Days t = Temperature in Degrees CUsing the above formula, only 1 .O days retention time is required at 55°C. No provisions were made to cool the biosolids between the first and seconddigester. Consequently, the temperature of the second digester rose to 47°C whichresulted in poor pei-foimance. To correct this problem, the temperature of the seconddigester was raised to 55°C. The process then stabilized and concentrations of fecalcolifoim in the second stage digester were sustained below Class A levels of 1,000MPN per gram total solids for several months. The fecal coliform concentrations inthe first stage digester were not below Class A levels. Several important conclusions andor observations can be identified based onthis study:
Digestion 1910. Single stage continuous feed thermophilic (55°C) anaerobic digestion of primary sludge with a retention time of 20 days was unable to meet Class A pathogen levels.*. Two stage continuous feed thermophilic (55°C) anaerobic digestion of primary sludge with a retention time of 20 days in each stage is able to meet Class A pathogen levels.0. Modification of existing mesophilic continuous feed digesters to achieve thennophilic operation may be economically attractive depending on marketing options for biosolids.0. Si@icant odors from subsequent dewatering process are likely. Odor control should be planned.REFERENCES 1. M. Poeckes, D. Oerke, M. Maxwell, S. Rogowski, and I-I. Kelly, Evaluation of the Autotheimal Thermophilic Aerobic Digestion (ATAD) Biosolids Stabilization Process to Meet the New EPA 503 Requirements, Proceedings of the WEF Specially Coilference, Washington D.C., June, 1994.2. U. S. EPA, Autotheimal Thermophilic Aerobic Digestion of Municipal Wastewater Sludge: Environmental Regulations and Technology, EPA 625110- 90/007, September 1990.3. Anaerobic Sludge Digestion, IYEFMmiral of Practice No. 16.4. G. Balog, K. L. Nelson, and M. A. Patel, Back River Egg-Shaped Digesters: Technology-Transfer Adapts German Approach to Wastewater Solids Digestion, Proceedings of the WEF Specially Conj&wice, Washington D.C., June, 1994. 5. T.E. Vik & J.R. G k , Evaluation of the Cost Effectiveness of the Auto Thermal Aerobic Digestion Process, Proceedings of WEF 66111 ilriti~al Corlferetice, 1993.6. G. Volpe, B. Rabinowitz, C. Peddie & S. Krugel, Class A (High Grade) Sludge Process Design for the Greater Vancouver Kegional District Annacis Island Wastewater Treatment Plant, Proceedings of IVEF 66th Atitiiral Corference, 1993. 7. G. Shmp, J. Sandino and R. ShamsKhoizani, Peifoimance of Aerobic Digestion in Meeting the 503 Requirements for Pathogen and Vector Attraction Reduction, Proceedirigs of die WEF Specialty Cotlferetice, Washington D.C., June, 1994. 8. J. Currie, P.E., CBI Walker, Inc., Aurora, Illinois, Personal conversation (Unpublished).
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CornpostingLewis M. NaylorBlack and VeatchGaithersburg, MarylandToo often scientists r e b on their sophisticated instrumentation and computers andnot on their unique abiliv to reason. Linus PaulingL INTRODUCTIONBiological decompontionis as ancient as the existence of organic matter on the earth.With division of the frtcell and germination of the first seed, amino acids making up isproteins and glucose links in cellulosic chains initiated the first step toward chemicaland biological breakdown, returning to the earth nutrients and energy for other lifeforms. This natural process of cleansing the surface of the earth enabled life as weh o w it to exist today. It is the first step in the process of composting. Without the natural decomposition of dead organic materials, recycling of thenutrients and the biochemical energy in the carbon contained in organic matter wouldbe slowed dramatically. While periodic fires could rapidly release such energy andnutrients, organic matter production vastly exceeds the scale of the land area fordecomposltimof organic matter involved in fires. Without this natural decompositionprocess, dead organic matter would accumulate at the rate of about 15 feet of deptheach millennium. Human beings have walked the earth for a million years or so,andplant life has existed much longer. Thus,just since humans began to walk the surfaceof the earth, dead organic matter in the absence of decomposition would haveaccumulated to a depth of nearly 3 miles. Cornposting is a natural process of aerobic, thermophilic microbiologicald@e m of organic wastes into a stabilized, useful product that is free of odors andpathogens, w l not attract rodents and insects, and can be used beneficially for ilhorticultural and agricultural purposes. During this process the waste is stabilizedbiologically. This means that the readily biodegradable components of a mixture 193
194 Naylorof organic materials being composted are stable or resistant to change biologically.Wastes are broken down sufficiently that further decomposition is very slow anddoes not cause problems during use of the material. Such problems could includeodors, attraction of flies, or regrowth of pathogens. Virtually all carbonaceous, biodegradable materials are compostable undersuitable environmental conditions. Such conditions are essentially those favorablefor microbial growth: appropriate moisture content, an aerobic environment, andbiologically available carbon for energy and nitrogen for growth and reproductionof the microbial population. The blended feedstock to be composted shouldcontain about 40% dry solids (60% moisture), a balanced nutrient (nitrogen) andenergy (biodegradable carbon) supply. The aerobic process should supply periodicmixing to improve the contact between food and microorganisms and increase therate of stabilization.A. Growth of Composting in the United StatesBeginning in the 1950’s and continuing through the 1960’s enthusiasm exceededthe reality of operations and the demand for mediocre quality compost in theUnited States. The chief problems were unrealistic economic expectations, poorcompost quality, and process failure. Facilities started up with a notion that thevalue of the finished compost would pay for operations and capital cost of thesystem. Alas, neither the demand nor the price of the compost met expectationsand nearly all facilities from that era failed, or at least hesitated severely. Thisrough start placed a gloomy shadow on composting as a viable, economicalternative to landfilling and combustion. However, in the last 10 years or so theindustry has matured and composting is perceived as a legitimate, engineeredtechnology with wide scale opportunities for recycling organic residuals. We can get a notion of the growth of the composting industry, chronicledannually since 1985 by BioCycle, Journal of Composting and Recycling. InDecember of each year BioCycle publishes a summary of composting activities inthe United States. Data show that in the decade since 1985, the number of compostfacilities in all stages of development has grown from 173 to 3 18 . Of thesefacilities, the number in construction, desigdpermitting, planning/consideration,pilot operation, and those shut down has remained relatively constant. Theremarkable growth has been in the number of operational facilities, their numberhaving grown from 79 to 20 1. Of the operational facilities in the United States, the largest growth in terms of number of facilities is the aerated static pile, growing from 49 to 99. In terms of the percentage increase, the in-vessel facilities have grown by a factor of 15. In- vessel type systems can be divided into vertical silo type reactors, agitated bed systems, tunnel reactors, and rotary drum units. In 1985, there were 3 operating in- vessel composting systems, all of the silo type, with an additional 11 silos or
Composfing 195agitated bed facilities in some phase of construction or start-up. By 1994,45 in-vessel systems were operating or in construction. Of these were 26 agitated bedfacilities, 15 silo type reactors, 3 rotary drums, and 5 tunnel reactors. Compost quality and marketing strategies have likewise improved markedly.The horticultural industry has shown a profound recognition of the value ofcompost as a soil amendment. Increasingly compost is specified by landscapersand architects. University research has more fully defined the value of compost toimpart disease resistance for container crops, field plantings, and turf grass. Market surveys for compost suggest two quite different markets: a highvolume market (Figure 5-1) and a high dollar market (Figure 5-2) [ 151. The highvolume market, estimated at nearly 900 million cu. yd. by The CompostingCouncil, is based almost entirely on marketing compost to agricultural crops. Incontrast, the high dollar value is based on marketing a high quality productsupported by technical staff. Customers use directly or resell the compost in bulk,bags, or with a horticultural product such as turf grass sod or a shrub. Obviously,both markets are important. The market targeted for a specific compostingprogram will dictate to a large extent the range of ingredients selected for thecompostable blended feedstock and the post-processing required.11. GOALS OF COMPOSTINGThe overall goal of composting is to produce a high quality compost using atechnology that is protective of the environment. When the focus is on theproduction of a quality product, the facility will remain open, the process willsucceed and the compost will be readily marketable. This goal is so important thatit precedes all others as the foundation of a successful project. Basically, there arethree requirements of quality compost.A. Chemical QualityCompost produced must meet the proscribed standards of the environmentalregulatory community. The primary standards are those of state and federalregulators. For compost containing biosolids, these would include standardspromulgated by the U.S. EnvironmentalProtection Agency as part of the U.S. EPAPart 503 Rule, discussed at length in Chapter 2 . Those standards which weredeveloped through a risk-based approach assure that the compost would be safeenough to eat, though few would look forward to such an opportunity. Thestandard for lead content of biosolids products, for example, is based on a childconsuming minute quantities of biosolids daily for several years. Thus, achievingthe regulated chemical standards assures not only the manufacturer of compliance,but also the ultimate compost user of a virtually risk-free product.
196 Naylor Landfdl cover Reclamation Sod production Silviculture Agriculture Fig. 5-1 The potential high volume market for composted materials.  lo 1v;x Landscapen Topsoil Bagsed Container Field Fig. 5-2 The potcntial high value market for compost. [ 151
Composfing 197B. Biological QualityWhile the presence of certain metals is of concern in compost used withoutrestriction by the general public, the presence of pathogens may be of morepractical significance. Toxic effects of most metals become apparent only aftermany years of exposure through ingestion. In contrast, the presence of biologicalpathogens such as bacteria, viruses, parasites and protozoa can be immediate anduncomfortable. Achieving conditions which assure complete pathogen destructionduring composting is essential to meeting not only the regulatory requirements, butalso to protecting the most sensitive of compost users. An infective dose of pathogenic organisms over time is generally required forhealthy persons, but certain sensitive individuals may respond differently. As anexample, a mother and son bring home several bags of a composted biosolidsproduct. The mother begins to work the compost into her flower bed, while the sonusing his toy dump truck faithfully delivers the compost to each appointed plant.All the while he is sucking on a piece of hard candy, which he allows absentmindedly to fall out of his mouth into the toy truck load of compost. The young lad glances surreptitiously at his mother and quickly picks up the piece of candy and pops it back into his mouth. Compost manufacturers need to consider this young lad as the person they are protecting as they monitor the conditions required to meet highest standard of pathogen reduction.C. Customer RequirementsThe final standard the compost manufacturer must meet is the one established bythe customer. For the most part, such standards are not reviewed by regulatorybodies, but they are equally important to marketing the finished compost.Customers have requirements that relate both to their perception of what compostought to be, such as color, texture, and aroma, as well as how the compost performsin a growing medium, e.g. pH, salinity, and nutrient levels. The compost mustmeet such requirements, which will vary enormously between customers, or thecompost will not be readily marketable. Thus, the three goals of compost quality are to meet chemical quality,biological quality, and client requirements. When these goals are achieved, the composting process will be effective and the compost will be readily marketable. In. PROCESS FUNDAMENTALS Composting is a living process and proper care and feeding of the active microbial community is essential to good composting.
198 NaylorA. Microbial CommunityThe microbial community does not consist of a single type or even a single groupof organisms. Rather, many different types of bacteria and fungi play importantroles in the decomposition of the organic matter. The conditions produced by onepopulation of organisms establish the character of the food and the workingenvironment of a subsequent group. The formation of compost is a natural, but extremely complex biochemicalprocess in which cellulosic materials, proteins, fats, and carbohydrates aredecomposed and transformed into a humus-like material. The vast and immenselydiverse population of fungi, protozoa, and bacteria initiating this process grow,reproduce, and die as environmental conditions in the composting materialschange. Cell contents and cell walls of these organisms are decomposed bysuccessive generations of microbes, contributing to the humus content of finishedcompost, along with the non-degradable or slowly degradable fraction of the inputfeed-stock. This is the final stage in the production of the finished compost, aliving fertilizer and a marketable product. Complex carbohydrates and proteins are degraded into simple sugars andamino acids by bacteria and fungi. Temperatures rise into the thermophilic rangein the moist composting environment, cooking and softening organic matterparticles and enabling physical break-down of the material. With physicaldegradation, new surfaces are exposed expanding the available food and supportinga larger microbial population. As the most easily degradable materials decompose,woody materials remain in predominance. These materials which consist of layers of cellulose bound by glue-like lignin are degraded by fungi. As the intensebiological activity diminishes, temperatures gradually decrease and the composting materials begin to dry, forming conditions suitable for increasingly complex groups of fungi. While the composting process never grinds completely to a halt, the process is so slow that physical and chemical changes in the materials become noticeable only over months. The compost at this stage is ready to use, and with plant growth the recycling of nutrients and energy continues. Four general physiological groups of microorganisms are active within the temperature range found commonly in well-managed composting processes. As illustrated in Figure 5-3, these groups are psychrophiles, mesophiles, facultative thermophiles, and thermophiles. Psychrophiles grow or tolerate in temperatures found in a northern climate, -5 to 35°C. Mesophiles thrive on temperatures found in the tropics, about 15 to 45°C. Facultative thermophiles tolerate temperatures from room temperature, 25"C, to that of scalding water, 60°C. Finally, thermophilic organisms tolerate very high temperatures ranging from 45 to 75 "C, well into the range at which food can be cooked and milk pasteurized. In general, growth rate increases with temperature, as illustrated in Figure 5-4. However, the relationship is not linear within any one physiological group over its entire
Cornposting 199temperature tolerance range. Each group can survive throughout its temperaturetolerance range, but thrives within its preferred range. 80 70 60 50 z - $ E 0) 40 Thermophiles 30 s 0. I- 20 Facultative thermophiles 10 Mesophiles 0 Psychrophiles -1 0 Physiological groups FIG. 5-3 TEMPERATURE RANGE FOR GROWTH O F SEVERAL PHYSIOLOGICAL GROUPS OF ORGANISMS [Zl]. While growth rate of individual organisms increases with temperature, fewerindividuals of the population thrive. As noted in Figure 5-5, the total number ofcells produced by a physiological group diminishes as temperature increases.Within the mixed culture of the compost heap, all physicological groups arepresent. As each groups activity diminishes because of temperature sensitivity,the next most heat tolerant takes over the decomposition labor. At some point,however, even the thermophiles struggle with the heat and this heat stress beginsto reduce the rate of the composting process. The overall objective of temperaturecontrol is to maintain a temperature that accommodates the maximum number oforganisms with the greatest average biological activity. Similar to other living organisms, the composting microbial population hascertain requirements for growth and reproduction. Metabolic activities of theorganisms reflect environmental conditions as they vary from optimum to tolerableto intolerable.
200 Naylor Regremion line: Rate, Ill,hr = 0.0438 + 0.016 x T.C 0.4 -2x 0 5 10 15 - 20 25 Temperature, .CFIG. 5-4 INCREASE IN GROWTH RATE OF A PSYCHROPHILIC BAClLLUS SP. WITH TEMPERATURE . psychrophllla Baclllus sp.
Cornposting 201B. Environmental ConditionsConditions which a compost facility operator must pay close attention to aretemperature, oxygen levels, and moisture of the cornposting materials. The optimum temperatures for composting are in the range of 35 to 65°C.Microorganisms that efficiently drive the composting process will be most activewithin this temperature range. As noted previously, mesophilic and thermophilicbacteria and fungi predominate at various stages within the composting process.The growth dynamics of celluloyticbacteria and fungi (those microbes which breakdown cellulose) during composting of organic waste is shown Figure 5-6. As thecomposting and curing process continues, more and more microbial activity mustbe directed at decomposition of the lignin and cellulose in the relatively non-biodegradable woody products. Since fungi tend to be more proficient at lignindecomposition their population continues to increase, whereas the bacterialnumbers decline. V I I I . ~~ I I I 0 10 20 30 40 50 Time, days FIG. 5-6 CHANGE IN THE NUMBERS OF CELLULOYTIC BACTERIAL AND FUNGI DURING THE COMPOSTING AND CURING PROCESS.
202 Naylor Sufficient oxygen must be present in the composting mixture to sustain theneeds of the aerobic bacterial population. Five to 10 percent oxygen will generallysuffice for aerobes. However, measurement of oxygen concentrations within thebulk compost pile can give misleading results. Oxygen levels within the center oflarge, non-porous clumps may drop to zero even while reading 10% oxygen withinthe bulk pile. Simple forced air or suction aeration of a compost pile can beinadequate when the composting material tends to stick or clump together.Physical turning and abrasion to break up the clumps is generally beneficial toassure good oxygen penetration throughout the entire composting mass. Moisture levels in the composting materials must be maintained at about 40to 50% to support the physiological systems of the compost microbes. Cell wallsmust be permeable to the flow of soluble nutrients into the cell by osmosis.Enzyme systems must be able to function properly. While complete desiccationcan cause gradual decline and complete cessation of biological activity, reductionof moisture content in the compost will reduce the composting efficiency andrestrict the active organisms to those more tolerant of low moisture. As with thecase for temperature, excessively low or high moisture contents of a compostingmixture can impair the composting process by forming conditions that favor thegrowth of organisms less efficient in the degradation process. For example, if the mix is excessively wet, poor oxygen penetration is likelybecause of voids being plugged with water. Formation of very low oxygenconditions can enhance the survival of acid-forming bacteria that can decrease thepH of the composting materials. The acids formed, termed volatile acids becausethey readily vaporize, possess an odor some find unpleasant. Such is the case forthe formation of vinegar (acetic acid) and other volatile longer-chain,more odorous acids formed in a pile of fermenting apples.C. Nutritional ConsiderationsAll living organisms have fundamental nutrient needs: carbon (C), nitrogen (N),phosphorus (P), sulfur ( S ) , trace nutrients, and minor amounts of vitamins. Waterdiscussed above is likewise a nutrient solvent and carrier as well as anenvironmental requirement. The basic raw materials or feed-stock used and metabolized during compostingare composed of cellulosic materials (polysaccharides), proteins (sources ofnitrogen and sulfur), and sugars, fats and carbohydrates (energy sources).Polysaccharides are essentially polymers or long chains of glucose sub-units withthe formula (-C6H,205-). Proteins are composed of various amino acids, alsolinked to each other in long chains. A common chemical form of an amino acid is(R-CH(NH,)-CO-NH-). Some of the amino acids also contain sulfur. Thesematerials make up the diet of the composting organisms which must be balancednutritionally for optimum composting.
Cornposting 203 Appropriate and balanced amounts of such nutrients are generally present ina heterogeneous composting mix containing material such as food waste, biosolids,source separated organic waste, leaves and yard waste. However, the ratio ofcertain nutrients, notably carbon and nitrogen, can be out of balance for certaintypes of mixes. Carbonaceous and nitrogenous compounds are important to living organismsas a source of energy (biodegradable carbonaceous compounds) and protein(biodegradable nitrogenous compounds) for growth and reproduction. Duringcomposting, these microbes metabolize 15 to 30 parts of carbon for each part ofnitrogen. This is true for organisms as large as a dairy cow to those as small as thedecomposer bacterial cell. While balancing a feed ratio for a Holstein may needto be more precise, the blended feedstock for composting facility must be adjustedfor the relative amounts of carbon and nitrogen. Thus, a compost facility operatormust consider himself to be a microbial nutritionist as well as manager ofenvironmental conditions within the cornposting materials. The compost process manager should have access to a variety of materials tobalance the nutrient and moisture content of the blended feed stock. In general,the ratio of biodegradable carbon to biodegradable nitrogen (the C:N ratio) in ablended compost feedstock is suggested to be about 20 to 40 for good composting.Organic wastes such as biosolids tend to be moist and nitrogen rich, relative to their content of carbon, with C:N ratios of typically 5 to 10. Materials such as sawdustused as amendments, in contrast, tend to be dry and carbon rich. Sawdust has atypical carbon to nitrogen ratio of about 200 or more. Since the blended feedstock should have a C:N ratio of about 20 to 40, mixing these two materials together balances the moisture content as well as the carbon to nitrogen ratio. The relatively wide, acceptable C:N ratio range is indicative of two things: 1, the enormous dietary flexibility within the heterogeneous microbial community (How many of us weigh out our protein, fiber, and energy components each day?), and 2, the dearth of research to enable understanding of the biodegradability of various feedstocks and the consequences of improving the precision of the optimum C:N range. Carbon is combined with organic matter, and can be estimated from the organic matter content because of the relative uniformity of organic matter composition. Organic matter is the principal component of the volatile solids of a material. An ingredient in a compostable mix consists of a liquid fraction, i.e. water, and a solid fraction or dry matter, as depicted in Figure 5-7. Most analyses express the composition of compostable materials as a percentage of dry matter. A proportion of the dry matter consists of minerals with the remainder making up organic matter which is combustible. This combustible fraction is considered to be volatile since it can be burned off, and is named volatile solids. The mineral residue, or ash, is often referred to as futed solids. Since carbon is a portion of the organic matter, percent carbon in a material can be estimated indirectly from its
204organic mattercontent. A varietyofratioshavebeenused to estimate theproportion of carbon in organic matter.The values for percentage carbon insoilorganic matter vary h m approximately to 58% estimate commonlyused is about 55.6% of organic matter carbon, based on data nearly a century is halfold Evaluating the nitrogen content of materialsis generally straightforward andaccurate. Most feeds and forage laboratories, as well as environmental testinglaboratories have provisions to test materials for total kjeldahl nitrogenammonia, nitrate, and nitrite. The total kjeldahl nitrogen content includes organicnitrogen, that bond with organic matter, and ammonia. In general, the TKN doesnot include all of the nitrate and nitrite in a sample, and these anions must beanalyzed separately, usually the liquid phase in fiom leaching a sample. However,in terms of composting process and feed stock control, nitrate and nitrite contentis not very important. TKN concentrations in individual components range from5,000 to 60,000 mg/Kg, whereas the concentration nitratehitrite combined isgenerally less than100 mg/Kg. FIG. S-7 COMPOSITION OF THE SOLIDS IN A COMPOSTABLE
composting 205 The operator can evaluate the C:N ratio of his compostingmaterials reasonablywell h weight and percentage composition and once the blended feedstock has thebegun to compost. Unforhmately, at this point adjustingthe mix is nearly impossible.As mixtumlow in carbon, e.g. C:N < 20,begin to compost, the compostingprocessis likely to be accompanied by the loss of excess nitrogen as ammonia because of theproportionally large amount of nitrogen. Conversely, mixtures deficient in nitrogen,e.g. C:N > 40, tend to compost slowly, and have poor heat generation. From anoperator’s perspective, some ammonia aroma should be evident during compostingand the mix should heat up well. However, if very high concentrationsof ammoniaare released, causing eye irritation or breathing difliculty, then the operator shouldknow that the C:N ratio is too low and adjust the proportion of biodegradablecarbonaceous materials in subsequent batches. Experiencx is a pow& teacher ofcompost process managers.N. SOLIDS AND THE COMPOSTINGPROCESSC c n n w g as a non-biological process is essentially a materials handling exercise.This includes managing moisture, particle size, biodegradability, and porosity. Theseproperties control to a large extent the efficiency of gaseous exchange within thecomposting materials, heat output and temperature achieved.A. Types of SolidsMaterials to be composted consist of dry matter and water. The dry matter, or drysolids @S), consists of organic matter and minerals. The organic matter iscombustible and can be burned off or volatilized, leaving the mineral fraction or ash,Figure 5-7. The ash o fixed solids (FS) consist of minerals including calcium (Ca), rmagnesium (Mg), sodium (Na), iron (Fe), manganese (Mu), and trace metalcompounds. The anion moiety of the metal compounds include carbonates (CO?),bicarbonates (HCO;), sulfates (SO:-1 phosphates (PO:- ), nitrates (NO; ), and other ,auions. The carbonates and bicarbonates help buffer the acidity and alkalinity of thecOmpOShng materialsby reacting reversibly with the carbon dioxide produced duringthe composting process. The combustible fraction or volatile solids (VS) is correlated highly with theorganic matter cunteslf and reliably indicates the organic matter content of a material.A portion of the volatile solids is biodegradable (BVS). This ffaction can be furtherbroken down into thosematerials that are readily biodegraded, e.g. biosolids and foodwaste, and those that are slowly biodegraded such as woody materials. For many operators, little clarity exists regarding biodegradability of
206 Naylorcarbonaceous materials and fibrous materials. In fact, even at the research levelthere is much to be learned and taught regarding biodegradability and factorsinfluencing biodegradability of organic wastes. As a consequence, operators usetheir best judgment until corrected by experience. In general we can judge severalhigh nitrogen, fairly biodegradablematerials, Table 5-1, to include biosolids, springand early summer grass clippings, and certain fresh animal manures withoutbedding. Highly biodegradable, carbonaceous materials include shredded leaves,fruit and vegetable waste, and apple pomace. Two practical approaches toassessing the biodegradabilityof organic materials are to consider their digestibilityby animals and humans and to recall their disappearance in garden compost pilesor when worked into the soil and broken down by soil microorganismsand macro-invertebrates. Respirometry has been used to experimentally judgebiodegradability. In this experimental method, a substance is allowed tobiodegrade under known conditions of temperature and pressure, and either oxygen absorbed or carbon dioxide evolved is measured during a specific time period. The oxygen absorbed or carbon dioxide evolved is a function of the conversion of bound carbon to carbon dioxide.B. Particle SizeParticle size is one of the most important physical properties of the compostingmaterials. Both the physical size and distribution of the particle sizes are importantto porosity of a compostablemix. The more uniform in size the materials in a mix,the greater the porosity of the mix. Conversely, as the heterogeneity of particlesizes widens, the porosity decreases. Small particles fill the voids between largerparticles, Figure 5-8, restricting the flow of water and gaseous exchange. Most composting technologies now include mixing of the composting massbecause developing a uniform particle size is difficult to achieve under even thebest of conditions for most composting facilities. Grinders and shredders, unlessfollowed by screens to grade the product, produce a very heterogeneous materialwith coarse particles and chips to fine dust and grit. Regular mixing of a blendedfeedstock with a broad particle size range ameliorates some of the negative effectsof low porosity by exposing the entire pile of material to the atmosphere. 1. Porosity Reduced porosity can have a number of undesirable effects during the composting process. To maintain an aerobic composting environment, thorough exchange of oxygen and carbon dioxide must take place. As the porosity increases, exchange becomes more efficient. Heating and cooling are likewise important control parameters. In active compost piles, heat generated can produce temperatures too high for efficient composting. Air flowing through the composting materials
Cornposting 207 Uniform particle size Heterogeneous particle sizes FIG. 5-8 UNIFORM PARTICLE SIZE PRODUCES GOOD POROSITY WITH LOTS OF VOIDS FOR EXCHANGE OF GASES AND WATER. HETEROGENEOUS PARTICLES FILL IN VOIDS, REDUCING POROSITY
NaylorTABLE51 DRY SOLIDS AND C:N RATIO IN BIODEGRADABLE ORGANIC MATERIALSDrganic Material Resources DS, %I I I C:N ratio Biodegradability.Apple pomace  20 - 35 48 H >75%Fruit waste 10 - 25 43 H 775%Vegetable waste 10 - 25 28 H > 80%Mixed food waste & paper 25 - 35 14 H 55 %Fruit and vegetable materials 54 22 H 75 %(a%. 1Mixed paper 80 - 95 177 L 20 %Newsprint 95 924 L 20%Corrugated paper 90 - 95 427 L 20 wWaxed m l cartons ik 90 - 95 560 L 20 w
Cornposting 209 Organic Material Resources DS, % C:N ratio BiodegradabilityPaper food cartons 90 - 95 300 L 20 %Junk mail 95 226 L 20 %Paper and paper products 95 324 L 20 %(avg.1IBacteria IC5H702N I 4.29 I Fungi C 1OH 1706N 8.57 Wood C295H4200186N 253.0 Grass C23H380 17N 19.7 Garbage C 16H2708N 13.7 Food wastes C 18H26010N 15.4 Mixed paper C266H4340210N 228.0 Yard wastes C27H380 16N 23.1 Biosolids ClOH1903N 8.57IRefuse 1 IC64H104037N I 54.9 IIRefuse 2 IC99H148059N I 84.9 I II Biodegradability, expressed as % of volatile solids, estimated. I
210 Naylorwill absorb moisture and heat energy, cooling and drying the material. If the airflow is restricted, cooling may be non-uniform leaving hot spots in the compostingmaterials which could also become deficient in oxygen. Such hot, poorly aeratedpockets are ripe for odor production when the compost is prepared for use.2. BiodegradabilityParticle size can influence gross biodegradability of a material. Composting is asurface phenomenon since micro-organismsare active only on surfaces. They haveno way to access, for example, the interior of a wood chip. As the overall surfacearea of a given mass of waste increases, the quantity of food available for themicrobes will increase. Hence, the overall rate of conversion of biodegradablematerial into biologically stable compost will increase. While the biodegradabilityof a material at the molecular level may not vary appreciably with particle sizebecause the rate of biochemical transformations and enzymatic reactions at aspecific temperature are fKed, a smaller particle size does expand the surface areafor the microbial attack. An expanded surface area enables the support of a largernumber of organisms each of which contributesto the overall biodegradation of thematerial. As an example, shredded leaves have a larger surface area and breakdown more rapidly than unshredded leaves because a greater surface area isexposed for degradation. Conceptually, we may think of the composting materials as spheres. It can beshown that the surface ( S ) to volume (V) ratio is equivalent to 3/r, where r is theradius of the sphere. As r decreases the S to V ratio increases. For example, asphere with a radius of one cm. has a volume of about 4.2 cu cm and a surface areaof about 12.6 sq cm. The surface to volume ratio will be 3 : 1. If this small sphereis divided into four equal spheres containing the same total volume as the initialsphere (4.2 cu cm) ,each will have a radius of about 0.63 cm, a total volume of 4.2cu cm, but the total surface area will be 20 sq cm. The resulting surface to volumeratio increasesto 4.7. Now if that one cm. sphere is ground into a 100 tiny spheres,the surface area to volume ratio will be about 14, with the total surface area exposed for biological degradation also 14 times as great. This example also illustrates the importance of regular agitation which physically degrades lumps that may be present in the raw materials, reducing their size and increasing their total surface. Increasing the surface area cannot increase the metabolic rate of the microorganisms in terms of unit weight of material metabolized per organism per unit of time. The basic biochemistry does not change. However, the greater surface area exposed does increase the total amount of food available to the organisms so that greater numbers can be supported. Thus, the practical impact and advantage of reduced particle size is that the rate of composting in terms of tons per hour will be increased.
Composfing 211V. PROCESS ENERGETICSLiving organisms convert food into energy for growth and synthesis of new cells.This process is not 100% efficient. As a consequence waste energy is produced.Within the composting process the waste energy appears as sensible heat and is thebasis for warming the composting materials.A. The Biological FireThe source of the heat energy within the composting process is the metabolicoxidation of organic matter into carbon dioxide and water. The heat generated bythe intense biological activity of the organisms is the driving force of the compostprocesses. Conceptually,the composting process could be considered a bioLogical$re. Biodegradable volatile solids are the fuel for this biological fire. Oxygen isconsumed, and metabolic water, carbon dioxide, heat energy, and compost areproduced. The compost produced represents the materials that do not biodegradewithin the time frame of the managed compost process.Microbes Fuel (biodegradables) + 0, -> Wafer + CO, + Energy + Compost (slowly biodegradables) . . . . . . . . . . . . . . . (5-1) The heat energy may be lost by ventilation or radiation to the atmosphere, orit may be conserved within the composting materials. Whether lost or conserved,the heat energy warms the environment and control over the segment of theenvironment warmed is the basis for achieving temperature increases within thecomposting mass.B. Heat and TemperatureHeat energy is the capacity to do work, such as warm the composting mass orevaporate water. Temperature is a sensory measurement. On a personal level, hotand cold are relative and judged differently from one person to the next. Toincrease our objectivity and eliminate personal bias, temperature is measuredagainst a standard using a thermometer or other recording instrument. By way of illustration, temperature is comparable to the speed of an object.Following the same analogy, heat is similar to the momentum of the object: thespeed times the weight of the object. A bird in flight and a moving truck may havethe same speed, but the momentum of the two objects and their relative effectswhen they bump into a window of a house is enormously different. Thus, temperature is not a good basis on which to judge the heat energypossessed by an object. Heat energy is a function of not only the temperature of
212 Nayloran object, but also its mass. For example, a bathtub full of warm water containsmore heat energy than a cup of hot tea even though the temperature of the tea ismuch higher. Heat energy is quantified by: Heat energy = mass of an object x heat capacity x temperature . . . . . . . . . . . . . . . (5-2) In this equation, temperature is measured in degrees Celsius or Fahrenheit.Heat energy is measured in joules, calories or British Thermal Units. (The ironyis that Americans are the last major country to continue to use the British ThermalUnit or BTU on a regular basis.) Heat capacity, or specific heat, of an object is thecapability of that object to absorb or release heat as related to its temperaturechange. Specific heat is measured in units of calories per gram per degree Celsius. Water has a specific heat of one calorie per gram per degree Celsius, or oneBTU per pound per degree Fahrenheit. That means that one calorie of heat energywill increase the temperature of one gram of water by one degree Celsius.Conversely, one gram of water cooling by one degree Celsius will release onecalorie of heat energy. Not all materials have the same heat capacity. While water has a heatcapacity of 1.00, wood has a heat capacity of about 0.4, and air 0.25. The drymatter of compost probably has a heat capacity of about 0.4. Based on the heatcapacities of these three materials, a given amount of heat produced during thecomposting process will increase the temperature of the same weight of air themost, of the compost dry matter the next greatest, and of water the least. Onecalorie of heat would increase the temperature of one gram of air by four degreesCelsius: heat energy, cal Temperature change, A " C = wt. of object, g x heat capacity, ca1lgl"C T,h 0 C = 1 cal = . . . . . . . . . . . . . (5-3) 1 g x 0.25 cal1gl"C As the proportion of water in the cornposting materials increases, i.e., thewetness of the compost increases, more and more energy will need to be generatedto increase the temperature of the composting materials because of the greater
Cornposting 213capacity of water to absorb heat energy. For a given amount of heat generated, asmaller temperature increase will be observed. As will be noted later in thisChapter, this has important implications for the compost process manager since heis interested in achieving specific minimum temperatures during the compostingprocess.C. Temperature ControlTemperatures in the thermophilic range are beneficial to the composting process.They increase the rate of conversion of biodegradable organic matter intobiologically stable compost. Such temperatures assure rapid stabilization of theorganic matter, and destruction of plant and animal pathogens and weed seeds.However, the temperature of the composting materials must be managed carefullyto assure optimum environmental conditions for the microbial population.1. Feedstock AdjustmentsIt is evident that for a given weight of material, a larger amount of heat producedwill result in a greater temperature rise. One question the compost processmanager must consider is how can the character and composition of that weightof material be managed to result in the greatest temperature increase. Conditionsinfluencing temperature change of composting materials include: a. the heat production capability of the material, b. the ambient temperature surrounding the composting materials, c. the moisture content of the composting materials, and d. the heat losses from the composting materials to the external environment. Heat production of the composting materials is, as we learned earlier, afunction primarily of the biodegradability, and secondarily, of the particle size ofthe composting materials. Any change the process manager can make to improvethe biodegradability of a blended compost feedstock, or of the compostingmaterials, will enhance the heat output. For a specific material, a smaller particlesize will generally improve the rate of conversion (composting rate) of the bulkmaterial to carbon dioxide, water, and heat because of the larger surface areaexposed for microbial attack. Changing the biodegradable character of the input ingredientsto the feedstockcan also enhance heat output. Compost mixes rich in highly carbonaceous, slowlybiodegradable sawdust tend to reduce heat output. In contrast, increasing theproportion of a highly biodegradable material such as food waste will increase theheat output. Using the illustration of diet, most of us know what contributes to our
rABLE 5-2 CHEMICAL PROPERTIES OF COMMON FOODS 
s 3 D g.TABLE 5-2 (CONT.) (Q1 Element Bibb Fried Liver 2.0% Milk Salad Oil Frozen Peas Boiled Raw Spinach White Sugar I Lettuce Potato g/100 g dry weight of food, or YO basis dry Y cn
216 Naylorcaloric intake and potential weight gain most. Certainly it is not high fibervegetables such as celery or lettuce, Table 5-2. Rather it includes sugars, meats,fats and oils. These have a high digestibility and fats and oils have a high caloricdensity. This is not to imply that the process manager should add disproportionateamounts of fats and oils to the blended feedstock. It does suggest that one can usecommon sense in developing a blend of materials of diverse nature so as to enhancethe overall biodegradability and heat output of a compostable mix. Temperature increases within a composting mass are also a function how theheat produced is absorbed by the individual components of the feedstock, andconserved or lost from the composting mass. Heat energy produced by the biological fire of the composting process isabsorbed by the materials that make up the blended feedstock. Since temperatureswithin a well-managed composting mass will be in the range of 40 to 60"C,everything mixed into the pile will have to be brought to desired compostingtemperature, i.e. 55 to 60°C. This includes not just the solid ingredients, but alsoall of the air that is added to the mix to maintain aerobic conditions. During hotweather when ambient temperatures approach 30°C. However, as ambienttemperatures drop, additional energy will be absorbed just to warm up the composting mass to the operating temperature. For cold weather composting, particularly in the northern climates where temperatures can fall to - 10 to -2O"C, considerably more care and thought must be given to blending the compost ingredients to maximize biodegradability and minimize extraneous heat losses. Thus, under these cold weather conditions, air used to ventilate the compost may have to be warmed by 50 to 80°C. The input ingredients will need to be warmed by 30 to 50°C. If the energy output from the composting process is unable to match these heat demands, temperatures mandated for pathogen destruction will not be achieved. The blend of materials is also important from the perspective of the proportion of water in the composting materials. As the proportion of water in the mix increases, the potential temperature rise will decrease because of the greater heat capacity of water relative to the compost dry matter. With a larger proportion of water, the ratio between the heat producing fraction (the biodegradable material) and the heat absorbing fraction (the water) decreases. As a result, during cold weather the compost process manager should develop mixes that are drier and more biodegradable than would be needed during warm weather. 2. Facility Design ConsiderationsHeat loss and conservation is also related to the size of the composting mass andto the compost technology. In contrast to the desirability of a larger surface tovolume ratio to enhance biodegradability,best heat conservation is obtained withthe lowest surface to volume ratio. For example, i a small compost pile a portion n
Composfing 217of the heat energy will be radiated to the atmosphere and lost, reducing potentialtemperature increases in the composting materials. Such would be observed inmany backyard compost piles which tend to be fairly small and not well-insulatedfiom the outside temperature. Larger compost piles a have smaller, heat radiatingsurface area per unit of volume than do small piles. In a well-managed compostingfacility, heat energy will be conserved within larger compost piles. Compostwithin an enclosed composting container or vessel is engineered to minimize heatloss. Pile depth is managed and container walls can be insulated to optimizetemperature increases.D. AerationAir circulation through the composting materials is an important process controlfeature. Aeration cools the vigorously composting material, removes excessivemoisture to produce a product that is readily screened and transported, andexchanges oxygen and carbon dioxide to maintain aerobic conditions within thecomposting materials.1. Temperature Control Air is circulated through the composting material to maintain the optimumtemperature environment for the microbial composting community. Whilesustaining temperatures of 55 to 70°C for several days is needed for assuredpathogen control, such high temperatures are not necessarily advantageous to theoverall composting process efficiency. Extended high temperatures can destroybeneficial organisms that are essential to the cornposting processes. Air can be circulated through the composting materials naturally, bymechanical turning, or by using mechanical forced air or suction. As the air flowsthrough the composting materials, heat is absorbed by the cooler air and carriedfrom the composting materials decreasing the temperature. Best temperature control is effected using a temperature feedback mechanism controlling a mechanical aeration system. In this approach the temperature of the composting materials is sensed electronically. The observed temperature is compared with a set-point using a microprocessor,and a blower is turned on if the observed temperature exceeds the set-point. The sensor monitors the compost temperature periodically and when the temperature drops below the set-point, the blower is turned off. The cycling of blowers off and on can produce a very uniform temperature within the vicinity of the temperature sensor. However, if the porosity of the composting material is poor, air flow through the mix and, hence, temperature uniformity may be non-uniform. Non-uniform air flow can result in hot spots that are not cooled, anaerobic conditions, or overly cool locations. Regular agitation or turning along with mechanical aeration will improve the
218 Naylorhomogeneity of the material and keep the compost mix open and porous to goodair movement.2. Moisture RemovalAeration not only sweeps heat from the composting materials, but also removeswater vapor. Removal of water vapor has two effects. First, the removal of thewater vapor begins the compost drying process. Second, the evaporation of waterand removal of the vapor cools the compost. The heat that develops during composting warms the composting materials,but it also evaporates water. This water is derived from two sources: 1, moisturein the feed-stock (biosolids, wood chips, sawdust, etc.) and 2, metabolic water frommicrobial oxidation of the feed-stock carbon or volatile solids. The incoming feed-stock generally contains about 40 to 45% dry matter, orabout 55 to 60% moisture. During composting, aeration sweeps out a portion ofthis moisture which has vaporized, leaving the drier product with only 40 to 50%moisture. This amounts to removal of a substantial quantity of water in vaporform. For example, for each 100 tons of raw material composted daily, its weightafter composting will have diminished to about 67 tons, assuming no changes inthe weight of the dry matter. Thus, 33 tons of water will be evaporated andremoved from the composting materials each day. On an annual basis, this 33 tonssurges to nearly 12,000 tons of water, equivalent to approximately 3,000,000gallons of liquid water. Ignoring the management of this water vapor, andpotentially corrosive liquid water condensate, has caused untold pain and sufferingamong compost process managers and engineers. The second source of water removed by aeration is metabolic water which isderived from the composting materials. If we consider, as an example, that the fuelfor the biological fire is the cellulose monomer glucose, we may estimate the amount of water that will be produced by the metabolic processes of the composting organisms by degradation of volatile solids. From the following equation, and the associated formula weights of the materials:C,H,,O, + 6 O2 --> 6 C02 + 6 H 2 0 + Energy . . . . . . . . (5-4) 180 192 264 108 From this equation, we can show that for every 180 pounds of cellulose orglucose monomer (volatile solids) lost, approximately 108 pounds of water isproduced. Scientistshave suggested that about 15 to 25% of the dry weight of thecomposting materials is lost or biologically burned during the composting process.Each day the volatile solids loss from a 100 wet ton load at 40% solids couldamount to 4.7 tons, with the consequent release of about 3.3 tons of metabolic
Cornposting 219water. On an annual basis, the formation of this water amounts to about 1,200tons, or approximately 292,000 gallons. A portion of this water will also beevaporated by the aeration process. While aeration cools the composting materials as the air absorbs heat,evaporation of water is the major cooling factor. As noted earlier, air has a heatcapacity of 0.25 caVg/"C, as contrasted with water which is 1.00 caVg/"C. Coolair moving through the hot compost will increase in temperature by 4°C for everycalorie of heat energy absorbed. However, evaporation of water absorbssubstantially larger quantities of heat energy. To evaporate one gram of water at 100°C will require 540 calories of energy. At a temperature of 70", about 560cal/g is required to evaporate a gram of water, over ten times more than the 50calories that would be required, for example, to increase the temperature from 20°C to 70°C. As is evident, evaporation of water is the greatest heat absorbingkooling force within the composting material. While adequate aeration is required to control temperature and remove water vapor, over-aeration of the composting materials can prove detrimental to the composting process. Over-aeration can remove so much heat and moisture that the composting materials cool excessively,reducing the metabolic rate of the microbes and, as a consequence, the composting rate. Over-aeration can also dry the composting materials excessively so that metabolic activities of the microbes is reduced, or in some cases, stopped altogether due to lack of moisture in the materials. Thus, careful control of aeration, and process monitoring is important to good composting.3 . OxygenationThe third hnction of aeration is to maintain adequate oxygen and, hence, anaerobic environment in the composting materials. Compost aeration (the processair) is provided through the aeration piping located typically under the compostingbeds. The composting materials are also aerated during agitation and turning,exposing each particle of the composting materials to the atmosphere. Maintainingan aerobic environment for the compost organisms assures that these organismsoperate at peak efficiency and prevents most odors associated with anaerobicconditions from forming.VI. PREPARING A BLENDED FEEDSTOCK As the compost process manager considers ingredient options for a blended feedstock, the physical and chemical characteristicsof each potential material must be known. There must also be some notion of how each material will influence the overall character of the compostable mix. Of the most important characteristics,
220 Naylorpercent dry solids (%DS) is probably the number one consideration, followedclosely by the carbon to nitrogen ratio (C:N). Then would follow biodegradability,porosity, and finally a host of characteristics related more to marketing a well-composted material than operating the compost process successfully. Sincematerials used to adjust such properties amend the physical and chemical characterof the blended feedstock, they are often called amendments. Thus, the processmanager must know his ingredients,how they impact the composting process, and ABLE 5-3 AMENDMENT SELECTION Selection of an amendment to modify and enhance physical and chemical properties of blended feedstock for optimum composting is a key consideration in the production of high quality, readily marketable compost. Factors to consider during selection of an amendment include: 1. The compost customers requirements 2. Dryness of the amendment 3. Organic matter content 4. Bulkdensity 5. Porosity and particle size distribution 6. pH, nitrogen concentration and other chemical considerations 7. Color and texture Guidelines for Optimum Performance Parameters I Minimum I Maximum I Typical Dry matter, YO 50 95 55 to 75 Organic matter, 50 95 60 to 80 Y O Bulk density, 8 30 15 to 30 lb/cu ft PH 4 7.5 (except ash) 6 to 7 Nitrogen, % 0.01 2 0.5 to 1.0 Mulch Topdressing General Use Particle size, in 112 to 2 1/16 to 1/4 114 to 1 Color dark brown brown to black brown to black Texture coarse fine medium
Cornposting 221how they will ultimately influence the marketability of the finished compost.Guidelines for selection of amendments are suggested in Table 5-3. A second, though no less important basis for developing a blended feedstockis that the compost process manager must be guided by the initial purpose fordeveloping and operating the compost facility. Operational objectives for acomposting facility range from being a solution to a waste management problemto manufacturing compost for an identified compost market. Such objectives mayset fairly narrow constraints on the ingredient options by the process manager. Forexample, many compost facilities are developed to help solve a problem withbiosolids, leaves, and yard wastes. For such facilities, the goal is to develop ablend that 1) composts all of the targeted wastes within a timely fashion, and 2) produces a marketable product. So long as the identified blend of wastesproduces a readily compostable mixture the process manager’s daily activities arestraightforward. However, when that blend of materials no longer produces areadily compostable mix under the prevailing conditions, then the operator willneed to seek out and evaluate other ingredient options.A. Dry Solids and PorosityObtaining appropriate dry solids content of a blended feedstock with sufficientporosity is important biologically to give the composting microbes adequatemoisture, and physically to enable gaseous exchange. Important physicalproperties of individual materials that influence dry matter content and porosity ofa blended feedstock include dry matter content, moisture absorptivity, particle size,distribution of particle sizes, and bulk density. For the most part, a good rule is to develop a blended feedstock containing 40to 50% dry solids. Agitated bed composting facilities will be able to handlematerials at the lower end of the range while vertical silo-type systems andwindrows will need to be at the higher end. Assuring adequate porosity is the mainreason for the dry solids adjustment. Excessive porosity is not normally a problemso long as the correct moisture content and good biodegradability exists in thefeedstock. Porosity is assured when voids between bulk particles such as woodchips are filled with air and not with water or small particles. Windrows rely onporosity within the blended feedstock to enable gaseous exchange, whereasporosity in agitated beds is derived in part from regular turning.1, Increasing Dry SolidsIngredient options for increasing the dry solids content are listed in Table 5-4,along with some comments on their use. The materials include both organic andmineral products, not all of which will be appropriate at each composting location.For example, use of shredded pallets as a source of dry matter and bulkiness may
222 Naylorbe economical in an area with a pallet manufacturing or recycling facility. In themid-west where agriculture predominates,use of various agriculturalwastes or by-products may be more economical or reliable. Wood fred power plants notuncommon in the northeast may make wood ash an important dry mattercomponent in a blended feedstock in that area. Each locale must evaluate materialsdiscarded within a reasonable hauling distance and consider opportunities to worksuch materials into a blended feedstock as beneficial ingredients to the process orthe marketing and use of the finished compost. If the purpose of an amendment is to increase the dry matter content of ablended feedstock, usually the drier the material, the greater its value. As will benoted later, particle size and moisture absorptivity are also important because oftheir effect on porosity and absorption of free water. Material Comments on the Product Wood chips, shavings, can be dusty, avoid nails in all woody shredded woody construction waste materials, very dryI Shredded pallets I very dry, course finished product, coarseness will improve porosity I Shredded brush & leaves, dryness will vary, coarseness will improve porosity, bark, plant fibers such as some limitation on availability, leaves & grass are very shredded hay biodegradableI Kiln dried sawdust & green sawdust I very dry & dusty, smaller quantities, moisture will vary widely, competition for farm use I Shredded paper shred finely, very dry, moderately biodegradable ~~ dry, paper covering is an organic matter source, gypsum may help hold ammonia, presence of gypsum in compost a plus Wood ash very fine material, may reduce porosity, very dry & dusty, carbon particles may absorb odors Coal fly ash not a food grade material, will require regulatory review, some restrictions on use, must pass TCLP test
Cornposting 223 TABLE 5-4 (CONT.)I Material ~ -1 Comments on the Product I Oat hulls, rick hulls hulls are very stiff, low biodegradability & porosity Grain milling & cereal waste corn, wheat, oats fines, very dry & biodegradable Wet grains wet, very biodegradable Poultry broiler bedding nitrogen source, can be dry & coarse, good porosity High sugar waste very biodegradable, good energy, high C:N~ - Organic Wastes and Slurries Dewatered biosolids waste activated, anaerobically & aerobically digested, air dried material, raw primary, dewatered septage, low C:N, biodegradable Dewatered water treatment low volatile solids, may be wet, low biodegradability, plant sludge presence of Al, Ca, or Fe Dewatered milk wastes, potentially very odorous, may contain diatomaceous processing wastes, brewery earth, high biodegradability, good energy source wastes, beverage filter cake residues Paper mill fibrous residuals biodegradability varies, variable organic matter & biosolids content, low nitrogen, check molybdenum Coffee & tea wastes, fruit highly biodegradable, acidic, low nitrogen content processing wastes Meat packing wastes, high nitrogen, highly biodegradable, high odor hatchery wastes, fish potential processing wastes, egg breaking wastes From time to time, porosity needs may be less important than increasing the dry solids content of a mix. In such cases, additional options are available. Several very dry materials that happen to be fairly fine are available and sometimes may be obtained at little or no cost. These would include sawdust, either kiln-dry or green, wood ash, ground dry wall (gypsum), and perhaps coal fly ash, if allowed by regulation. Some materials that may be available locally in selected areas include ground corn cobs, dust from grain storages, animal bedding such as wood shavings from horse stables, and shredded papers. Each of these has specific benefits, but also handling problems. For most of the very fine materials such as kiln dry sawdust, dustiness is a problem the operator must be prepared to contend
Naylorwith. For a few, such as grain dust or animal bedding, rapid biodegradability orodors may need to be considered. However, these problems are not insurmountableand the process manager should consider broadly all ingredient options availableas tools to manage the composting process.2 . PorosityIn general, bulky, woody materials, one-inch and smaller in diameter, make goodamendmentsto increase dry solids content as well as porosity. Such materials arealso called bulking agents for obvious reasons: they increase the bulkiness orporosity of a mix. Post-process screening can be used to remove and reuse thelarger pieces, enabling the operator to get multiple-duty from the chips. This canbe particularly important if the chips must be purchased, or their procurement cost,e.g. chipping, grading, and transport, is higher than their cost of simple disposal.However, in some cases the porosity lending materials were considered in theoriginal design to be included in the fmished product as a means of their dispositionand beneficial use. In such instances, the process manager must give additionalthought on how the materials will impact the marketability of the fmished product.3 . Particle SizeThe particle size distribution of a dry amendment will influence the porosity of amix. A uniform particle size will result in many voids between the particles, justlike a stack-up pile of oranges. However, if there is a wide range of particle sizes,the smaller particles will plug the voids between the larger particles. While the mixmay be drier, the porosity may not be appreciably improved. See Figure 5-8.4. Bulk DensityBulk density of a material can be important in terms of the finished weight andtransportation economics of the marketable compost, transportation cost of theamendment, ease of turning the composting mixture. Materials can vary f o rmextremely light for Styrofoam beads sometimes added to the finished product formarketing purposes to relatively dense in the case of wet wood chips.5 . Moisture AbsorptivityMoisture absorptivity of a dry amendment may be an important consideration ifone or more of the other ingredients contains some fiee water. Free water is waterthat will flow from a material by gravity. For example, a dropped, fresh, ripewatermelon which contains 90% water will manifest free water or juice, whereasa crushed potato that contains 80% water, most of which is inside cell walls (bound
Composting 225water) has almost none. If the objective of adding a dry amendment is to absorb thefree water within a mix, then the absorptivity of that material is important.Shredded rubber tires have from time to time been used as an amendment. Suchmaterial would surely provide porosity, but would have no impact on free water.In contrast, shredded paper would absorb a great deal of water, but would notincrease porosity appreciably.6 . StickinessStickiness is a troublesome property than can be adjusted by increasing the drysolids content of a mix. This problem tends to assert itself with fluid wastes thathave been dewatered using polymers. Under such conditions the overly moist mixwill stick to equipment such as conveyors and loaders, increasing the weight loadon the equipment and bearings. In this case a drier mix tends to be less sticky,turns easier, and keeps the mix reasonably porous to air. A good test of whethera mix is too wet or too sticky is to make a ball about the size of an apple with themix. If the ball barely holds together, the dry solids content is about right. If theball sticks together firmly, even if dropped on the composting floor, more drymatter is needed.7 . Adding MoistureGenerally we consider options for increasing the dry solids content of acompostable mix, but there are circumstances in which a reduction in dry solids,i.e., more moisture, is warranted. For example, biosolids are often used as a co-composting ingredient with leaves or other dry organic residues. Where leaves andyard trimmings are the main focus, dewatered or even fluid biosolids could beconsidered the amendment. The water in the biosolids is an essential componentin the blended feedstock. One could consider that in this case, the biosolids areadded as a source of moisture and nitrogen. Dry solids adjustment may involve either increasing or decreasing the solidscontent of a blended feedstock, depending on the ingredients. However, thecompost process manager must focus on producing quality compost and all that isimplied in that objective. Initially the emphasis is on developing a compostablemix and managing the process successfully, but ultimately the customer’srequirements are primary. It is clear that these two requirements are intertwinedand both the process and the customer must be satisfied.B. Chemical Composition Chemical composition is of interest in a blended feedstock in order to judge the carbon to nitrogen ratio of the mix and to assure that the mix meets regulatory
Naylorstandards and compost user requirements.1. Carbon to Nitrogen RatioThe main purpose of adjusting the carbon to nitrogen (C:N) ratio is to assure thecompost microorganismsa balanced diet. As discussed earlier, the carbon providesthe energy and the nitrogen furnishes the protein for growth and reproduction.Materials that are rich in carbon include woody materials, paper, sugar, and certaintypes of wood ash. However, these materials do not all have the same proportionof carbon that would be available to a microbe during the time frame of a typicaltwo to four week composting period. A common notion is that the amendment is added to adjust the C:N ratio aswell as the dry solids content of a blended feedstock. While this is not an incorrectstrategy, it is certainly not an adequate picture of the true situation. The dry solidsadjustment can be made with all sorts of dry materials that may or may not haveany impact whatsoever on the C:N ratio. Obviously, an amendment such as kilndry sawdust will adjust the dry solids content of a mix, and will have some effecton increasing the C:N ratio since the sawdust is very fine, with a high surface tovolume ratio. In contrast, wood chips that are one-inch diameter by a quarter inchthick will have almost no impact on the C:N ratio. Microbes have relatively littleaccess to the bulk of the carbonaceouscomponent buried within the interior of thechip. Likewise having no effect on C:N are materials such as wood ash withchunks of carbon which are virtually non-biodegradable and ground up gypsum(calcium sulfate) board which is very dry,but contains no carbon at all. Sometimesnon-woody materials make the best carbon sources. These may or may not be dry.Examples include sugary wastes, fruit wastes, and shredded leaves which have highC:Nratios and are very biodegradable. Thus, as the operator evaluates the overallblend, consideration must be given to the biodegradabilityof the materials as wellas their C:N ratio to effectivelyjudge their ability to adjust the C:N of the blendedfeedstock.2 . Regulated ChemicalsAll ingredients incorporated into the blended feedstock contain all ofthe chemicalsregulated by environmental agencies, although for many their concentrations areof no significance. There is a notion on the part of some that certain materials areclean whereas others represent potential risk as a consequence of the presence ofsuch chemicals. In reality, presence equals neither toxicity nor hazard. Allmaterials represent a continua of chemical compositions and concentrations.Beneficial use of a material is possible because the composition is known, theappropriate rate of use to achieve some beneficial end is understood, and a use rateis identified that makes negligible any potentially harmful effects due of the
Cornposting 227presence of chemicals. For example, application rates of chemical fertilizers areestablished to assure nitrogen is not lost to ground water. The proportion ofbiosolids compost blended into a container mix is adjusted to avoid salinityproblems. Nonetheless, strict attention must be paid to the chemical compositionof individual components of the blended feedstock. In general, it is wise to have a chemical analysis of a material beforeincorporating it into the compostable mix. Once a material is mixed into thefeedstock, separation is virtually impossible. If a material is known to containconcentrations of a regulated chemical in excess, blending with other materials soas to produce a finished product that meets the standard is acceptable under EPAPart 503, but not necessarily under all state environmental regulations . Insome cases, blending into a mix a material containing excessive concentrations ofa regulated chemical will cause the entire mix to enter the realm of questionablematerial. It is best to know the regulatory situation and to know the compositionof the materials used in the blended feedstock prior to mixing.3 . Non-regulated ChemicalsWhile the regulatory community has a set of chemicals that it is concerned with,this set does not overlap very much with chemicals of concern to the users of thefinished compost. Where such chemicals do overlap the concern tends to besufficiency rather than excess. Users are interested primarily in macro-nutrientsand trace minerals, pH, organic matter, salinity, along with physical characteristicssuch as porosity, water holding capacity, and bulk density. The macro-nutrients consist of nitrogen, phosphorus, and potassium. Thesenutrients are of interest in the blended feedstock for several reasons. Nitrogen inthe materials will influence the C:N ratio, and have some effect on the nitrogencontent of the fmished product. Phosphorus and potassium content will influencethe finished product quality. Phosphorus is often listed as soluble phosphoric acidon description sheets of the nutrient content of materials with a guaranteed orapproximate fertilizer value. What is meant is not that the material containsphosphoric acid, but that the water soluble phosphorus concentration is expressed as phosphoric acid, usually as P,O,. This formula gives a P205concentration about2.29 times higher than the concentration listed as P. The reason is that the formula contains oxygen. A similar situation exists for potassium which is usually listed as soluble potash, K,O. In this case the value as K 2 0 is about 1.21 times higher than for K. The following two formulas show how the proportion of K (eq. 5-5) and of P (eq. 5-6) in the respective oxide forms is calculated: 2xK- -- 2x39 - - 1 - K,O (39x2+16) 1.21 . . . . . . . . . . . . . (5-5)
228 Naylor -- - 2xP 2x31 - 1-- . . . . . . . . . . . . . . . (5-6) P,O, (2x31 +5x16) 2.29Other elements that may be present at appreciable concentrations include calcium,magnesium, sodium, aluminum, and iron. Calcium, magnesium, potassium, andsodium (Ca, Mg, Na, and K) contribute to the salinity of a material, and correlateclosely with pH. Salinity and pH are of interest to the compost process managersince excess salinity can impact the finished product quality and pH can influencenitrogen loss as ammonia. Aluminum and iron (A1 and Fe) can react with solublephosphorus to form an insoluble compound, and reduce plant available phosphorusin the finished product. Calcium, magnesium, potassium, and sodium are alkaline earth elements thatwill contribute to the salinity of a material and ultimately to the finished productsince leaching losses of salts will be negligible. Excessive salinity in the finishedproduct will reduce the range of uses to those which are less salt sensitive, e.g.mulch around trees and shrubs or field plantings in humid regions. Salinity tolerantplants include most grasses, cotton, and the beet family. Of the four elements,sodium is the least essential to most plants. Thus, the operator needs to reviewanalyses of individual ingredients for excessive salinity or, as a general indicator,sodium. Concentrationsof sodium high enough to be a problem obviously dependon the proportion of a particular ingredient in the blended feedstock. However, asthe concentration of sodium rises above one percent in a material, the managerneeds to consider potential impacts on the fmished product. At high enough concentrations lime or other alkaline materials can contributeto the initial of the mix. High pH materials are commonly derived from limetreatment of a material such as biosolids or potable water treatment residuals. pH is generally only of transient consequenceto the composting process because of thebuffering action of carbon dioxide, a weak acid, and ammonia, a weak base. Ammonia is released as a natural consequenceof the composting process. Ahigh pH in a blended feedstock will tend to encourage volatilization of nitrogen as ammonia during aeration or turning of the composting materials. At pHs greater than 8.0 to 8.5, loss of the volatile ammonia can be uncomfortable to operators in confined composting spaces, whereas at pHs of 7.5 or lower, ammonia loss may not be particularly noticeable. neutral alkaline NH,+ C-> NH, + W . ............................ (5-7) non-volatile volatile
TABLE 5-5 COMPOSITION OF FOOD PROCESSING RESIDUALS [171 quor). 3. Dewaiere N 5:
230 NayIor This chemical reaction is reversible and ammonia, along with carbon dioxide,(eq. 5-8) buffer the pH of the composting materials. Ammonia will react withvolatile acids and other acidic compounds, such as acetic acid, formed duringcomposting, neutralizing the acids. acetic acid ammonia ammonium acetate HC2H301+ NH3 &> NU,+ + C2HsOf.. . . . . . . . . . . . . . . . ( 5 - 8 ) volatile non-volatile Since the reaction tends to reduce the content of the volatile, odorouscomponents, this reaction may reduce or at least change the odors evident at acomposting facility. However, balancing the pH of the composting materials so asto neutralize all odors by this strategy is not practical. Within the composting process, a pH increase is generally temporary. Theenormous quantity of carbon dioxide, a weak acid, produced during the compostingprocess buffers changes in pH. The buffering reaction is reversible, but tends todepress the pH of the composting materials by reaction of carbon dioxide withalkalinity: C02+OH-c->HC03- o C O , ” + H + . . . . . . . . . . . . . . . . . . . (5-9) As a consequence of the buffering action of ammonia and carbon dioxide,well made compost tends to have a pH in the range of 7.5, even though the initialpH of the blended feedstock may vary from pH 5 to pH 10. Aluminum and iron are frequently added to slurries to aid in dewatering, orin some cases to water or wastewater to react with soluble phosphate(orthophosphate). Aluminum forms a very insoluble precipitate which can besettled out with biosolids or other sludge to produce a very low phosphoruseffluent. When unreacted aluminum is present in a waste it will react withphosphorus from other components in the composting feedstock, and can reducethe water soluble or plant available proportion of the phosphorus in the finishedproduct. This will not be evident, however, from the analysis of the totalphosphorus (strong acid soluble) present in the finished product but will be shownin certain tests for plant available phosphorus. Aluminum is not an essential plantnutrient, but is phytotoxic at low pH. Iron has a similar effect on availablephosphorus, although not so strong as aluminum, plus the advantage that the iron is an essential nutrient. Reaction of soluble aluminum with soluble phosphorus in materials to be composted is not usually desirable. One approach to judge the extent to which the aluminum has reacted with the phosphorus is to compare the percentages of each in a material. The formula for aluminum phosphate is AlP04. The molar ratio of
Composfing 231A1:P in the formula is 1:1. Since the atomic weight of A1 is 27 and that of P is 32,an A1:P percentage ratio of 27:32 could theoretically precipitate all the phosphorus.However, soluble aluminum can react with other cations. As a practical matter amolar ratio of I .4:1 was observed experimentally to precipitate only 75% of thesoluble phosphorus from wastewater . Thus, it can be shown that 1.2% A1 ina material will precipitate 1.O% phosphorus. From the perspective of the compostprocess manager reviewing an analysis report, we can say that the percentagephosphorus in excess of the percentage aluminum will approximate the unreactedand potentially plant available phosphorus. So if a waste contains 5% A1 and 2%P, then the manager could expect the waste to react with other sources of P in othermaterials in a blended feedstock. If the %P exceeds the %Al, then most of the A1will have reacted and should not be a problem due to reactivity with solublephosphorus.C. Ingredient Selection1. Chemical Properties o Common Materials fOne of the problems compost facility managers face is lack of ready informationon the composition of common materials. Balancing ingredients from materialsreceived at a composting facility is always a challenge because analyses of suchitems are not always available prior to receipt or in some cases at the time ofdelivery. Dry solids can be tested rapidly using a microwave drier, but otherelements are not so straightforward. Data are presented for various foods and foodprocessing residuals (Tables 5-2 and 5-5 ), wood processing by-products and fiberresiduals (Table 5-6), and various types of animal wastes (Table 5-7). Analysesfurnish data that would help answer questions related to dry solids content, C:Nratio, concentrations of regulated or non-regulated elements. While not as good asan analysis of the actual material, these data are useful as a guide for preliminaryevaluation and selection of amendments and other ingredients in a blendedfeedstock.2. Food Processing WastesSource separated organic wastes and various residuals fiom food processing (Table5-8) are ideal materials to blend with biosolids in a blended feedstock. The wastesderived from the processing of fruits and vegetables are highly biodegradable,lower in C:N than biosolids, and as a result can supply a readily biodegradablesource of carbon. Pallets, corrugated boxes, and slip sheets can be shredded andused as a source of dry matter in the blend. In a highly controlled compostingfacility, a compostable blend of certain meat packing wastes can be speciallyformulated. Not all wastes fiom the food processing facility are appropriate. To
Cornposting 235be avoided are salty residues, metal and glass contamination, plastic strapping andsheeting, and in general slurries from treatment of cooling water or boilerblowdown.D. Developing a Blended Feedstock RecipeCornposting is an art and a science. To learn the art takes a good deal ofexperience, whereas working with blended feedstock recipes based on science canfurnish even the novice with the correct starting point. For the experienced, thescientific approach can at least help the process manager know why the mix did ordid not compost well. This section discusses the science of developing a good mix.The process manager is a chef, and for the preparation of a good product he mustknow his ingredients and understand how these ingredients interact during thecomposting process, their effect on the finished product, the process biology, andthe specific composting technology used. Understanding the art will come withtime.1. Facility Design ConsiderationsEach composting facility is sized based on a specified quantity of biosolids at aspecified minimum per cent dry solids, and a minimum per cent dry solidsamendment. A typical composting period is three to five weeks during weatherconditions when composting temperature levels can be maintained and reasonabledrying of the composting materials can be expected. Under these conditions,marketable finished compost containing 50 to 60% solids would be projected. Impact of Drv Solids. As properties of the input materials vary from theseexpected values, capacity of a facility and the dryness of the finished product willalso depart fiom projected values. As an example, the capacity of a facility will bediminished as the biosolids dry solids content drops below 20%. With a wetterbiosolids material, a greater weight and volume of amendment at, for example,55% solids must be added to bring the compostable mixture to 40% solids. Sincea larger proportion of a facility is filled with amendment, the capacity of theequipment and the facility to handle biosolids is diminished. Likewise, as theamendment dry solids dips below 55%, a greater volume will be required to h i s hdry matter as an amendment for the 20% biosolids to produce the desired dry solidscontent in the input mix. Conversely, as the solids content of biosolids oramendment increases, larger quantities of biosolids or smaller quantities ofamendment can be used. Thus, the dry solids contents of the raw materials to be composted have great importance in the capacity of a composting facility as well as the quantity and cost of procurement of amendment. Nutrient Considerations. A second factor is that nutrient deficiencies can impair the compost process efficiency. Such deficiencies can also limit drying of
TABLE 5-8 COMPOSTABLE WASTE FROM THE FOOD PROCESSING INDUSTRY Raw vegetables and fruit spoiled, bruised, spilled shole or partial items, e.g. apples, corn cobs, mushroom stems and butts, skins, peel and cores, pea and bean waste, cherries and grapes Process residues peeling waste, cooked product, sugar solutions or other concentrated high BOD liquid or semi-solid wastes, grains or whole fiuitlvegetables, spoiled processed product, pie fill, edible fats and oils, sugar beet pulp Juice Operations juice pressing residuals (pomace) with or without press aids such as shredded paper or rice hulls, filter aid with retained materials, apple or grape pomace, juice and beverage filtration residues from wineries, breweries, fruit juice operations, fermentation residues fiom wine tanks, spent grains from breweries Waste treatment waste screenings, wastewater treatment primary (settleable) and secondary (biological) residuals or sludges Solid waste shredded pallets, cardboard boxes and slip sheets, labels Bread. cakes. and cereals whole bread, spilled flour or grain dust fiom silos, chocolate chips, cereal processing residues Fish and chicken processing and packing (depending on locale and regulations) whole or partial birds, hatchery wastes, fish waste, blood if absorbed into sawdust or other dry material, paunch manure, high strength egg breaking wastes and shells Avoid: high salt residues (pickling operations), dilute liquid wastes, cooling water and boiler blow-down water, metal glass or hydrocarbon compound contaminated wastes, waste containing non-compostable solid waste such as large seeds or pits.Note: Shredding and screening with recycle of overs will be required for most food processing waste.
Cornposting 237 the finished product. Composting is a microbiological process in which nutritionof the organisms must be considered. Under most circumstances, the wastematerials to be composted contain adequate nitrogen and energy. Biosolidsmaterial is very moist and contains plenty of nitrogen. Amendment tends to bemuch drier, and contains much less nitrogen. Balancing these two materials notonly adjusts moisture in the input mix, but also shifts the overall nitrogen balanceof the mix. The ratio of nitrogen to carbon (protein to energy) as discussed earlier,is an important consideration in the composting process, especially as the input mixbecomes very rich in biosolids or very rich in amendment. For example, an inputmix will tend to be rich in amendment or yard waste if the biosolids material isvery wet. As the amendment increases in moisture, even larger proportionsmust be added, M e r widening the carbon to nitrogen ratio, or C:N ratio. At somepoint the composting process will become nitrogen limited. An acceptable upperlimit for the C:N ratio is 40. As the C:N ratio becomes larger and larger, the microorganisms begin tobecome nitrogen limited. In such a situation, while we are not in troublenutritionally, the composting process begins to slow causing a hrther retardationin the rate of energy or heat production. With less energy produced, the dryingcapability will be diminished resulting in a wetter finished compost. Energv Considerations. Finally, not only does the biosolids compostingcapacity diminish as amendment solids content decreases, but the dryness of thefinished product may also decrease. The microbially available energy content ofmost amendment materials tends to be lower than that of the biosolids. Volatile solids derived from biosolids is about 30 to 50% biodegradable, whereas for an amendment such as sawdust the volatile solids are only about 15 to 25% biodegradable. Thus, on a dry weight basis, energy available microbially from sawdust may be comparable to that from biosolids even though the biosolids may contain only 50% VS as compared with sawdust which may contain 90% VS. As the amendment dry solids content decreases, proportionately more is needed to produce an input mixture with the desired solids content, for example 40% in this case. As the proportion of sawdust in the input mixture increases, the available energy decreases. With less energy available to warm the compost and evaporate water, more moisture remains in the compost. Hence, the finished product will tend to be wetter (lower dry solids content) when the amendment is wetter.2. The Two Ingredient MixThe simplest compostable mix will consist of two ingredients: biosolids and a dryamendment. For example, biosolids and sawdust, biosolids and shredded yardwaste, or biosolids and wood chips are compostable mixes. Literally hundreds ofsuch mixes are prepared daily in all sorts of composting facilities throughout the
238 Naylorworld. The single objective of the recipe is generally to develop a mix with thecorrect dry solids content, about 40 to 50% dry solids, depending on thecomposting technology. As it turns out, such mixes have a C:N ratio of about 20to 40, which is appropriate. The mixes usually have adequate porosity. And themixes usually compost well. Thus, for ordinary day-to-day composting balancingthe dry solids content is a good approach, and certainly a good first step, todeveloping a compostable mix. Obtaining the correct dry solids content at the time the blended feedstock isprepared is important to successful composting, not to mention low frustrationlevels by the staff. Once the initial mix has been prepared, wetting is inconvenient,if not very difficult, and drying a too wet mix is nearly impossible withoutcompletely starting over and adding more dry material. Weight basis mix ratios. The proportion of biosolids and amendment in a mixTABLE 5-9 CALCULATION OF WET WEIGHT OF AN AMENDMENT TO ADJUST DRY SOLIDS CONTENT OF A COMPOSTABLE BLENDED FEEDSTOCK drv weight waste + drv weight amendment = %DS mixture or %DS, wet weight waste + wet weight amendment wet weight waste x YoDS, + wet weight amendment x %DS, = %DS, wet weight waste + wet weight amendment wet weight waste x YoDS, + wet weight amendment x %DS, = %DS, (wet weight waste + wet weight amendment) = %DS, x wet weight waste + %DS, x wet weight amendment wet weight amendment x YoDS, - wet weight amendment x YoDS, =wet weight waste x YoDS, - wet weight waste x %DS,v wet weight amendment (%DS, - %DS,) = wet weight waste (%DS, - YoDS,) &DSADS,) = wet weipht waste (DS, - %DS,) wet weight amendment wet weight amendment = wet weight waste WODS,-O/DS,~) (%DS, - YoDS,) Where: w = waste, a = amendment, m = blended feedstock mixture, YoDS = percent dry solids
Cornposting 239 are a function of the dry solids content each of these two ingredients and the target dry solids content of the blended feedstock. The derivation of mathematical equation, shown below, for calculation ofthe amendment quantities (weight basis) is given in Table 5-9. wet weight waste (%DS,-%DS,) . . . .(5-10) Net weight amendment = (YoD S,-YO S,) D Where: w = waste, a = amendment, and m = blended feedstock mixture; %DS = percent total dry solids of the indicated material as subscript Using this equation, the amendment weight varies the greatest when in the range of 50 to 60% as shown in Figure 5-9. For example, the quantity of amendment required, weight basis, at 50% DS to achieve 40% DS in the input mix is double that required at 60%. But the amount of amendment required at 60% is double that required at 80%. Similar differences are noted in the effect of the dry solids in the biosolids on the amendment requirements. Thus, dry solids content c 30i Bio6olids. % dry solids +12% DS +18% DS -24% DS ~ 3 0 DS % U Ic 0.5 .- Weight ratio6 50 55 60 65 70 75 130 85 90 Amendment, %DSFlg. 6-9 INFLUENCE OF DRY SOLIDS CONTENT OF BIOSOLIDS AND AMENDMENT ON THE WEIGHT RATIOS OF THE TWO INGREDIENTS TO ACHIEVE 40% DS IN THE BLENDED FEEDSTOCK.
240 Naylorof the available amendments and of the biosolids are very important considerationsas the compost process manager evaluates the supply of dry materials availableto be used in a biosolids-amendment input mix. This ratio is important totransportation costs, truck traffic, finished product handling and post-processing,quantity of finished compost to be marketed, and capacity of storage facilities. Itis also an important consideration in evaluating the quantity of a dry waste materialsuch as yard waste that can be absorbed by the composting facility. Volume basis mix ratios. Mixing materials on a weight basis provides themost accurate, reproducible, and generally most successful approach. However,materials are mixed from time-to-time on a volumetric basis, especially once thebulk density of the material is well-known fiom practice. To develop a mix basedon the volume of the ingredients their bulk densities must be known. Bulkdensities were calculated from weight of a cubic foot of material for varioussamples of sawdust, a common amendment, and the data are plotted in Figure 5-10.As is clear from the graph, considerable variation exists in the bulk density of sawdust samples, even samples with the same dry solids content. Sawdust is a relatively homogeneous material as contrasted with, for example, yard wastes, leaves, and even wood chips. The latter materials are non-uniform, and pack tightly or loosely depending on the particle size, moisture, and stiffness (green vs. dry). Developing mixes based on volume ratios for such non-homogeneous materials is not very accurate, but experience operators have had reasonable Fig. 6-10 BULK DENSITY OF SAWDUST AS A FUNCTION OF DRY SOLIDS CONTENT.
Cornposting 241success. Nonetheless, mix ratios based on weights of ingredients are always moreaccurate than those based on volumes. As the dry solids of a homogeneous amendment such as sawdust decreases,i.e. the material becomes wetter, the bulk density increases (Figure 5-10). Waterhas filled openings in the material that were previously filled with air. From Figure5-9, a decrease in the dry solids of an amendment will increase the quantityrequired to achieve a desired dry solids content in the blended feedstock. Thisrequirement for a larger weight of an amendment when combined with an increasein bulk density as amendment moisture increases would seem to be self-compensating. But these two factors do not quite compensate for each othervolumetrically, as shown in Figure 5- 1 1. Roughly half as much amendment byvolume is required at 65% DS as at 50% DS. It should be noted that the mix ratios shown in Figure 5-9 (weight basis) andFigure 5- 1 1 (volume basis) are specific to the conditions represented on the graphs.Those conditions are 40% DS in the blended feedstock, amendment bulk densityas represented by the curve in Figure 5-10, and a biosolids bulk density of 59 lb/cuft. Volumetric ratios are particularly sensitive to variations in bulk density of the 2 1 Volume ratios Biosolids, % dry solids 5 +12% DS --A-18% DS -+-24% DS - - 30% DS 0 .. C al E 0 v 0 .- c 0 U Biosolids bulk density = 59 lblcu ft U A v m K 01 I I I I I I I 1 50 55 60 65 70 75 80 85 Amendment, % DS Fig. 5-11 INFLUENCE OF DRY SOLIDS CONTENT OF BlOSOLlDS AND AMENDMENT ON T H E VOLUME RATIOS OF T H E T W O INGREDIENTS TO ACHIEVE 40% DS IN THE BLENDED FEEDSTOCK.
242 Nayloramendment. If the conditions change from these assumptions, the curves becomeless representative and should be redrawn for better accuracy.3. Multiple Ingredient MixesCompostable feedstocks have become more complex as compost process managersbecome increasingly knowledgeable and a wider diversity of ingredients becomeavailable. Public works directors and government officials are becomingincreasingly aware of the ability of compost facilities to handle the sourceseparated organic wastes portion of the solid waste stream. These wastes includegrocery store wastes; certain non-recyclable paper products; food wastes fromhomes, restaurants, fast food establishments, hospitals and schools; and a widevariety of organic wastes and residues from industries. Developing a recipe for such a complex mixture can be a challenge becauseof the diverse physical and chemical properties of the various ingredients.However, all potential ingredients with the exception of mineral residues such aswood ash are biodegradable within the composting process just as they would bein a natural setting such as a woods or field. Blending such materials does notguarantee rapid composting, but the process manager can expect that the materialswill eventually break down. The manager’s objective is to optimize the blend ofcomposting materials and the composting conditionsto allow the fungi and bacteriato function most efficiently. The first objective the process manager must address is to adjust is the drysolids content of the blended feedstock. As with the two ingredient mixture, afeedstock with appropriate dry solids content will generally have adequate porosityand a reasonable C:N ratio. The simplest approach is to convert the diverseingredients to a two component mixture by combining the relatively moistmaterials into one group, and then choosing one dry material or a group of drymaterials as the amendment. Then the recipe for the feedstock mixture thusdivided can be developed using the formula for the two component mix. A blended feedstock recipe calculation example is shown in Table 5- 1Oa. A blank template is provided in Table 5-lob. The calculations directed in Table 5-10 can be readilytransferred to a computerized spreadsheetto make the recipe calculator interactive. The example in Table 5- 1O suggests the availability at a composting facility a of biosolids, paper, yard trimmings, food waste, dry IC&I (industrial, commercial, and institutional wastes), and a dry amendment that is used to balance the solids content of the mixture of the other ingredients. The wet (as received) weights of the waste materials shown are arbitrary for this example. In practice, the materials and their weights will be those actually available or received at the composting facility. No weight or mass units are shown in the example since it is the weight ratio of the various materials that is key. While yard trimmings are shown here as a waste,
Cornposting 243the material could also be used as an amendment, leaving a blank for the yardtrimmings box. I other words, the example shows a weight of 10 for the yard ntrimmings, and a weight of 14 for amendment. We couldjust as easily use a weightof 24 for the yard trimmings. ABLE5-10a MULTIPLE INGREDIENTSCOMPOST RECIPE CALCULATOR We i n r de Q Dry SolIda, Wet Weight Dry Weight % fill in 96DS and w t w i g h t multiply divide to get Biosolids paper YardTrimming 55% Food Waste 32% Dry IcBtI divide dry d u e t wt add add x 100toget%DS wetwts dry wts Total wastes Blendedfeedstock << Fill in desired Yo dry solids i feedstock n Amendment %dry solids inamendment amendmaRYoDS x wetwt= wet weight dry weight wet weight (~alc.below) = H w to eplcnlntethe weight of amendment to nchleve the dealred % dry r o ~ d aa the blended o h feedstock: 1. Add up dry weights of in@enta. Add up weights of ingredients. Keep all Units the same, for example, all weights in tons, Mg, kilograms or Ibs. 2. Divide t t l oa weight by the tdal wet weight to give percent dry solids of the compositemix 3. Calculate amdment guantity t achieve the desired percent solids in the blended feedstock o Amendment wt = - (wet wt wastes x feedstodc DS %I 100 drv wt wastes) - (amendment DS YO feedstock DS %)I 100 wetweieht x feedstockDS.%+ 100 = value = 12.11 a-b - lamend DS. YO feedstock DS. YO) + 100
244 Naylor Depencllng on the actual materials, their weights, and their dry solids content, itis possible for the calculated amendment weight to be negative value. When this is thecase,the dry solids content of the mixture of the wastes will be higher than that desiredin the blended feedstock In such cases, a material wetter than the blended feedstockwl be used to balance the dry solids content. For example, ilTABLE 5-lob MULTIFLE INGREDIENTS COMPOST RECIPE CALCULATOE Ingredients Dry Solids, Yo Wet Weight D y Weight r ill in % DS and wet weight 100 divide dry wt/wt wt add add X loOtoget%DS wetwts dry wts rdal wastes Blendedfeedstock n n 0 << Fill i desired %dry solids i feedstock Amendment << Fill in known% dry solids in amendment ammdment%DSxwetwt= wet weight dry weight wet weight (~alc. below) = 1 1 0 H w to cpleulnte the weight of amendment to achieve the dedred % dry solids in the blended o 1. Add up dry weigMs of ingredients. Add up weights of ingredients. Keep all units the same, for example, all weights i tons, Mg, kilograms or Ibs. n 2. Divide total dry weight by the total wet weight to give percent dry solids of the composite mx i 3. Calculate amendment quantity to achieve the d e s i percent solids i the blended feedstock n -wt= - (wetwtwastes X feedstock DS %I 100 drvwtwastes) - (amendment DS YO feedstock DS YO) 100 I wetweight X n1 - + TI= dryweightwastes =
Composfing 245food waste or biosolids could become the amendment if a wet material is neededto lower the percent dry solids of the waste materials.4. Carbon to Nitrogen AdjustmentsThe C:N ratio is important in developing a compostable mix in order to assureproper nutrition for the microbiological decomposers involved in the compostingprocess. The C:N ratio is calculated by dividing the % carbon (%C) by the % totalnitrogen (%N). As noted previously, the % carbon (%C) can be estimated fiom the% volatile solids (%VS). Research has indicated a very general relationshipbetween these two parameters fiom the study of soils . This study suggests thatthe % volatile solids divided by 1.8 approximates the % carbon in a sample.Although this is only an estimate, it does provide guidance in developingcompostable mixtures.TABLE 5-11 CALCULATION OF C:N RATIOS Total Mix$ 1.2% 78% 43% 35 t % Carbon = % VS + 1.8 tt C:N=%C+%N % Blended Feedstock * Dry weight. These weights may be transferred from the data in Tab. 5-10a.
246 Naylor In general, it is best to develop a blended feedstock recipe initially byadjusting the dry solids content to the desired level because moisture content is socritical to good composting. Then the C:N ratio can be checked to be sure it iswithin the range of 20 to 40. If the ratio is much outside this range, subsequentbatches can be adjusted. Adjusting material that is already prepared is usually notnecessary. Unless the biosolids used in the initial mix is very dry (e.g. > 30% DS),a C:N of about 15 to 18 is the lowest that would ordinarily be obtained with abiosolids -wood chips or sawdust mixture provided the dry solids content of themix is correct. Mixes with this much nitrogen will be expected to generate a goodbit of ammonia when they are turned or aerated. In a static pile situation, thesituation can be corrected if the piled material can be leveled, more carbonaceousmaterial layered on top with a front end loader, and then the pile reformed. Forvessel type systems, adjustment is generally not practical. A procedure to calculate the C:N ratio of a blended feedstock is presented inTable 5-1 1. Weight of dry solids for each of the waste materials, the amendment,and the input mix can be transferred from Table 5-10a and used as input valueswhere appropriate in Table 5-1 1. If spreadsheets are used for the calculations,these can be linked so that the data is automatically transferred and C:N calculationis immediate. This enables working interactively with the quantities of thematerials to adjust not only the dry solids content, but also the C:N ratio. This isnot to say, of course, that a compost process manager must have a computer to dogood composting. A hand calculator can work just as well, but will take somewhatlonger. If a deficiency of carbon or nitrogen is evident to the process manager,corrective steps in the next mixture should be undertaken. A large number ofmaterials with their dry solids content, C:N, and estimated biodegradability are listed in Table 5-1. Review of these materials may be helpful to the processmanager in locating ingredients that can correct the available carbon or available nitrogen in the blended feedstock.5. Energy BalanceAs discussed earlier, the energy output as heat during composting is the drivingforce of the process. Energy generated is used to increase temperatures into thepathogen killing range, to increase the microbial rate of decomposition, to elevatethe temperature of the feedstock and air, and to evaporate water. When a mix hasinadequate energy, one of these uses suffers, usually moisture evaporation. Occasionally it is useful to be able to estimate the energy content of variouscomponents in a compostable mix and the blended feedstock. This is especiallyhelpful if an input mix contains a high proportion of inert (non-biodegradables suchas sand or ash) or slowly biodegradable materials (lots of paper or wood chips).
Composting 247Energy can be estimated from the percent biodegradabilityof the volatile solids andthe amount of energy obtained from degradation of those solids. Oxidation ofvolatile solids may be estimated from the energy output during combustion of amaterial. For organic matter the common value used is 10,000 BTUs per pound.While this value may not be exactly correct for each ingredient, it will furnish anapproximation of the relative heat output from a mixture of ingredients. A protocol for calculatingthe estimated energy from ingredients and for theblended feedstock is presented in Table 5-12. This energy is considered to bereleased during the three to seven week composting period plus curing time.Weights and solids data for this calculation can be obtained from Tables 5-10 and5- 1 1. Estimates for biodegradability are shown in Table 5- 1. Tables 5- 10 to 12developed as computer spreadsheets can be linked to automatically update datatransfers and make the blended feedstock recipe (materials balance), C:N ratio, andenergy output interactive. Data calculated from Table 5- 12 also enables estimationof the dry weight and wet weight of the finished product. Examining the thermodynamicsof the composting process can lead to a better understanding of energy output during the composting process, how that energy is used, and the inter-relationship of biodegradability, volatile solids lost, metabolic water formed, and the weight of the finished compost. To produce a dry, high quality product, the composting process must generate enough energy to not only heat the compost to pathogen-killing temperatures, but also to evaporate moisture in the composting materials. This energy is produced by the microbial oxidation of biodegradablevolatile solids, the biological fire. This discussion assumes an energy yield of 10,000 BTU per pound of volatile solids destroyed, and the energy to remove the moisture (latent heat of evaporation) is considered to be about 1,000 BTU per pound of water evaporated at 140°F. For the purposes of this example, the biosolids dry matter is considered to contain 60%volatile solids, of which 35% is considered to biodegrade within the active composting period. While the amendment contains a greater concentration of volatile solids, 90%, this material is known to be less biodegradable because of the lignin in the wood. The amendment is considered to be 25% biodegradable assuming it to be primarily ground up wood mixed with some leaves and/or grass clippings. The example data in Table 5-13 illustrates a case in which 30 wet tons of biosolids are composted with 40 wet tons of amendment. The blended feedstock contains 70 wet tons or 28 dry tons at 40% DS. Of the dry material in the feedstock, 3.60 tons of VS is derived f o biosolids and 19.8 tons from rm amendment, for a total of 23.4 tons. During the cornposting process, 1.26 tons of biosolids VS plus 4.95 tons of amendment VS for a total of 6.21 tons VS are biologically burned releasing about 124.2 million BTU of energy, eq. 5-1 1 .
248 NaylorTABLE 5-12 ESTIMATION OF ENERGY CONTENT Example DS,% Wetwt. Drywt. VS,% Weight Weight Material % vs Water a b axb C axbxc b-a x b Biosolids 30.0 6.0 60% 3.60 24.0 Paper 5.0 4.5 60% 2.70 0.5 Food Waste Dry IC&I Amendment Totals 10.0 10.0 2.0 14.0 71.0 1 ::: 1 1 1 :I 1 5.5 3.2 28.4 80% 95% 90% 90% 4.40 3.04 1.35 6.93 22.0 4.5 6.8 42.6 Total Mix I 40% a+b d e - II 78% f+e f I x 100% x 100% Biosolids 6.0 3.60 35% 1.26 12,600 4.7 Paper Yard Trimmings Food Waste Dry IC&I Amendment Totals Total Mix k I 32% I I I +k x 100% Y BTUs = Ib VS lost x 10,000. Example assumes weights are in Ibs. Multiple by 2000 for tons, or other factor to convert to other units. t Biodegradability of volatile solids. tt Ib VS lost = Ibs VS x % biodegradability
Composffng 249 6.21 tons VS lost x 2,000 lbshon ......................... .(511) x 10,000 BTUAb VS lost = 124.2 M BTU Assuming the 28 tons of dry solids and 42 tons of moisture increase intemperature by 70°F fiom 70°F to 140°F during composting, approximately 6.86millionBTUs of energy will be absorbed by the campostingmaterials (eq. 5- 12), plusan additional amount for the air moving through the materials and warmed. In thisexample, we will ignore the quantity of heat absorbed by the air. Further details onenergy balances and composting thermodynamics are discussed in exhilarating detailin the text by Haug . The amount of heat required to warm water and solids is,respectively, 1.OOBTUAb-F" for water and 0.25 BTUAb-F" for solids.(28 tons solids x 2,000 lbhon x 0.25 BTUAb-F + 42 tons waterx 2,000 lbhon x 1.00 BTUAb 4) x 70 O F = 6.86 M BTU .......................... (5-12) Most of the energy will be absorbed by evaporation of water. The blendedfeedstock mix is 40% DS initially, and the finished compost is considered to be 55%DS. From Table 5-13, the 70 wet tons of feedstock contains 42 tons of water and 28tons of dry matter. During the composting process 6.21 tons of volatile solids arebiologically b u m 4 leaving only 21.8 tons of dry matter. If this remaining material,the finished compost, is 55% dry solids, the wet weight will be:21.8 dry tons + 0.55 dry tons per wet tons = 39.6 wet tons .......................... (5- 13)39.6 wet tons - 21.8 dry tons = 17.8 tons water in the finished compost42.0 tons water in feedstock - 17.8 tons water in compost = 24.2 tons water lost However, the biodegradation of the volatile solids generates water as well ash a ,ammonia, and carbon dioxide, eq. 5-14. This water is a product of the metabolic etactivities of the microbes and is called metabolic water. The quantity of metabolicwater can be calculated assuming that the biodegradable volatile solids have theempirical formula C,,,H,,O,N, formula weight of 201, which is oxidized by 12.5 molesof O2producing 10moles of CO, 1 mole of NH and 8 moles of 5 0 ,formula weight18 .
250 NaylorTABLE 5-13 ESTIMATION OF ENERGY CONTENT -EXAMPLE Example DS, % Wet,wt. Drywt. VS, % Weight Weight Material vs Water a b axb C axbxc b-axb a+b d e fie f g x 100% x 100% Weight Weight Loss? Wt. VS BTU Final wt. DS vs %ofVS 1osttt outputg DS h i j ixj h-ixj Food Waste Amendment I + k x 100% .Y BTUs = Ib VS lost x 10,000 BTU per Ib. Data shown here assume weight units are in tons. The calculation is: M BTUs = (tons Vs lost x 2000 Ib/ton x 10,000BTUllb) + 1,000,000 t Biodegradability of volatile solids. tt Ib VS lost = Ibs VS x % biodegradability
Composting 2516.2 1 tons 4.4 tonsC,,H,,O3N + 12.5 0 2 --> 10 CO,+ NH3 + 8 HZO . . . . . . . . . . . . . . . . (5-14) 201 8 x 1 8 = 144 Oxidation of 6.21 tons of biodegradable volatile solids as calculated in Eq. 5- 14, produces (6.21 x 144 -+ 201) x 2,000 = 4.4 tons of water. This weight must bealso added to the total evaporated. Adding the weight of the metabolic water (4.4tons) to the amount calculated earlier (24.2 tons) we get a total of 28.6 tons ofwater lost. Evaporation of this water at 140°F will require 57.2 million BTUs forevaporation, over 8 times more energy than for warming the composting materials.28.6 ton x 2000 Ib/ton x 7,000 BTU/lb water lost = 57.2 M BTU ............................. (5-15) Adding the 57.2 M BTU for evaporation of water to the 6.86 M BTU to warmthe compost gives a total energy absorption of 64.1 M BTU. This leaves 60.1million BTU in excess that will be absorbed by the air used to cool the compost orlost to the environment.VII. ODOR REMOVALOdor ranks number one of all the pollutants that may get the publics immediateattention. Complicating the odor problem is the highly subjective nature of odor.What one person finds acceptable, another many find extremely objectionable.Odors are produced on the farm, during rendering and food processing activities,and by a variety of waste management activities, including composting.Agricultural areas have organized into political agricultural districts with legalright-to-farm laws supporting normal farm operations including the spreading ofmanure with any associated odors. Thus, odors are not specific to biosolids orcomposting, but are common in all areas of life from the home to the farm. Thekey to being a good neighbor, whatever the odor source, is to keep the odorsseparate from the noses. For a composting facility to be successful and continueoperations, this objective must not just be a goal, but an accomplishment. In recent years the science and technology of odor control has greatlyimproved. Odor control can be provided using a number of strategies including thefollowing: * Control of composting process c Enclosure of composting areas . I Treatment of odorous pollutants c Locating composting site at safe distance from receptors
252 NaylorA high percentage of composting facilities use biofiltration but a number of verylarge facilities do use chemical scrubbers. No reports of commercialuse of thermaloxidizers can be found in the literature researched. This section reviews causes of odors and their removal by biofiltration, anincreasingly used odor control technology.A. Origins of OdorsOdors are produced during incomplete microbial oxidation of organic materials,principally carbohydrates and proteins. Carbohydrates are chemicals that consistof carbon, hydrogen, and oxygen in the general form - (C,H,,O,) -. Mostcommon of the carbohydrates are cellulose and sugars found in virtually all naturalproducts. In the absence of adequate aeration, carbohydrates break down intoalcohols, esters, aldehydes, and organic acids. Each of these natural compoundshas a characteristic aroma from sweet to sour to putrid. Proteins are compoundsconsisting of carbon, hydrogen, nitrogen, oxygen, and sulfur. Odors producedfrom degradation of proteins in the absence of adequate oxygen consist ofammonia, amines, mercaptans, and others. Some of the nitrogen and sulfurcontaining odors are very intense, tend to be more objectionablethan carbohydrate-based odors, and may be evident at atmospheric concentrations in the ppm and ppbrange. Composting biosolids aerobically will generate some odors as a consequenceof normal biological activity. Intensity and character of such odors will be afunction of the C:N ratio of the blended feedstock, the quantity of materialcomposted, odor dilution potential, operational factors such as agitation,temperature of the composting process, and the odor potential of raw materials.While odors from such sources are not readily controlled, other potential sourcescan be minimized. Prevention is the best initial step for control of odors. Maintenance of aerobic composting conditions will help to a large extent in minimizing formation of odors. But this cannot eliminate all odors. Sources of preventable odors are spills, biodegradable amendment storage piles, and even clean up activities. Non- composted biosolids tracked onto roads, parking areas, or even off site are troublesome due to both odors and appearance. Good housekeeping in the composting area is essential to eliminate unwanted debris or piles of fresh materials from accumulating. Any spill or even clean up activity leading to evaporation of volatile organic compounds will contribute to odors. A bucket on a tractor is an ideal way not only to clean up spills, but to also generate an odor-producing thin layer of biosolids on a hard surface. But biosolids are not the only sources of odors. Other sources would include wet leaves and especially grass clippings used in the composting process. Stockpiles of such materials may turn anaerobic and release odors during turning.
Cornposting 253 Once the preventable odors are eliminated,the focus is on minimizing contactof the residual odors with sensitive noses. This is accomplished through acombination of facility isolation, odor control technologies, and natural dilutionwith air. Facility isolation must be a consideration during the initial planningprocess, and subsequent changes are seldom an option. Thus, odor destruction anddilution of any residual odors are the basis of being a good neighbor.B. Odor Control TechnologiesAttempts at odor control have included masking agents (covering up the odor withanother, more acceptable odor); oxidation of the odorous material with ozone,potassium permanganate, or chlorine containing compounds in commerciallyavailable wet scrubbers; adsorption using activated carbon filters; thermaloxidation using afterburners; and absorptiodbiological oxidation using biofilters.All of these technologies can be successful in removing odors. However the chiefbenefit of the biofilter is the simplicity of this technology matching the simplicityof composting. Although biofiltration is accepted to varying degrees within the compostscience and engineering community, biofilter use is growing. Major benefits ofbiofiltration relate to its simplicity. Odor removal is reasonably effective even under poor management, and very effective under average to best management. A broad spectrum of chemicals and concentrations can be removed. Biofilter media materials are readily available through local landscaping supply yards. Piping and blowers to distribute air to the biofilter are readily available at many equipment supply houses. Virtually no chemicals are required for successful operation. Moisture flowing through the biofilter contains mostly soluble salts (nitrates, carbonates, and sulfates). This liquid can be captured in a sump and may be returned to the composting materials as a source of moisture. The biofilter is resilient to varying environmental conditions such as heat and cold, snow and rain, although much less so to drought. Relatively simple and infrequent maintenance is required for satisfactory odor removal. While there are a number of benefits, the long term biofilter efficiency is dependent on the maintenance of the physical, chemical, and biological condition of the biofilter media.
Nay/orC. Biofilter Fundamentals and OperationsA properly operating biofilter is, at its most fundamental level, a forced air,unmixed, very high C:N ratio compost pile. In most cases heat build-up isnegligible, though fieezing is uncommon in northern climates. Components of themedia must furnish porosity for good air flow and low head loss through the media,physical structure to support the media and minimize collapse during the life of thebiofilter, an active biological population, and moisture for support of this biologicalcommunity. To function efficiently, the biofilter must be maintained to sustainboth the physical structure and the biological population. The success of a well-maintainedbiofilter to control odors is a function of theefficiency of two processes: sorption and regeneration.1. Sorption ProcessesThe biofilters odor removal capability is due initially to interaction with particlesurfaces, the active sites. This takes place by adsorption onto particles of themedia, absorption or dissolution into water, chemisorption, catalytic contact onparticle surfaces, and ion exchange on particle surfaces. Odorous compounds inan air stream flow through the biofilter media matrix and become attached throughadsorption or absorption onto particle surfaces, removing the odors from the airstream. Since removal occurs onto surfaces, the surface area of the media is fairlyimportant. Thus, odor removal efficiency is a function the strength of thephysicaVchemica1 sorption processes and the surface area of the sorptive particlesin the biofilter. For example, hydrogen sulfide can be precipitated onto particles as sulfides ofiron or other metals. Hydrogen sulfide is first dissociated into HS- and Hand thenprecipitated as the metal sulfide. In the case of ammonia, the ammonia is dissolvedin water, dissociating into m andOH with sorption of the ammonium ion, andneutralization of the hydroxyl ion by soil acidity. Volatile, long-chain, lipophilicorganic molecules are sorbed most effectively by moist organic matter. However,the sorption capacity of the particles is limited, and regeneration of the mediumssorption capacity must be achieved by chemical and microbial oxidation of theodorous compounds. When the biofilter is in long-term use, the sorption--regeneration processes reach steady-state conditions.2. Regeneration ProcessesRegeneration of the sorbed chemical is moderated by microbial biodegradation,thermal processes, and chemical reactions. Thermal and chemical removal bybiofilters play a small role in odor removal because of their high energyrequirements. In contrast, microbial processes, which have low energy
Composting 255requirements, are the dominant mechanisms in biofilters. Biofilter media possessan abundance of diverse soil bacteria and other organisms which participate inthese processes. Because these organisms are dependent on the water in the soil fortheir biological processes, biodegradation and oxidation of sorbed odors is mostefficient in media with adequate moisture. As an example, hydrogen sulfide, and H S are oxidized by thiobacillus bacteriato hydrogen ions and sulfate ions, odorless chemical entities. In the case ofammonia, the NH4+is oxidized to nitrite by nitrosomonas bacteria, and then tonitrate by nitrobacter bacteria. Volatile organics such as the odorous butyric acidare oxidized to carbon dioxide and water by a succession of bacterial groups.3 . Steady-state OperationThe capacity of the biofilter media to remove odorous chemicals depends on thesimultaneous operation of both the sorption processes and the regenerationprocesses. As a result, the biofilters odor removal potential can be exceeded byoverloading the system through excessive air flow rates, so that sorption rates arelower than the rate at which the chemicals pass through the filter. Once all thesorption sites are occupied, odor removal is rapidly diminished. A second limitingfactor is the microbial regeneration rate of the sorbed chemical which must equalor exceed the sorption rate. Toxic chemicals can interfere with microbialprocesses until a microbial population develops which can metabolize the toxicchemical. Soil acidity may reduce the removal efficiency due to an inhospitableenvironment for soil bacteria. Soil acidity may also increase removal by reactingwith ammonia to form the non-volatile ammonium ion. In most cases of biofilterfailure, however, the limiting factor is simply overloading of the filter rather thanimpairment of microbiological processes because of the great diversity andnumbers of soil bacteria.4. Design SuggestionsConstruction of biofilters is straightforward with proper planning and adequateattention to design and construction. Key factors in biofilter design are mediacomposition, air retention time, head loss through the media, and pollutant loadingrate. Biofilter media generally consist of a blend of materials to achieve the desiredproperties of structural longevity,high C:N ratio, porosity, biological activity, andabsorptivity. Individual components of the biofilter sorbing media have included wellcomposted leaves or yard trimmings, wood chips, bark, peat, sand, soil, curedbiosolids compost, fibrous peat, volcanic, and ash. The choice of media tends tobe somewhat site specific. After placing, the media the should have a porosity inthe range of 32 to 55%, possess 40 to 60 % moisture, and the pH should be in the
256 Naylorrange of 5.5 to 8.0. Odorous gases must reside in the filter long enough to achieve maximumsorption. While residence time is influenced by the structure, permeability, andporosity of the filter media, the volume of the bed has been treated for designpurposes as an unobstructed volume. Thus, residence time is determined by thevolume of the filter divided by the specific loading rate of the waste gas [l]. Airflow rates have varied from 0.5 cubic feet per minute per square foot of biofiltersurface area (cfm/ft2)tomore than 30 cfndff, with a range of 1 to 6 cfm/fi . Atleast 30 seconds is suggested for gases with low odor concentrations. Retentiontimes of 45 to 60 seconds are commonly used for biofilters in the northeasternUnited States [ 161. To achieve these contact times at high airflow volumes usinga biofilter media of typically three to four feet in depth, the biofilter footprint canbe quite large. With this media depth, a good rule-of-thumb is an air flow rate ofabout 3 cfm/ft2. Surface area requirements can be a significant disadvantage for locations where land is at a premium. Several equipment suppliers are marketing enclosed biofiltration units of varying capacities. Most include some type of air moistening capability and are ready to be attached to a point source of odor.D. Biofilter ChallengesWhile there are a good many positive aspects of biofiltration, there are also manychallenges that must be taken care of during design, construction and operation.Biofilter media are considered to possess uniform porosity and uniform moisture.These properties are important to uniform air flow and efficient odor removal, butare seldom achieved throughout the filter. 1. ConstructionProblems during placement include compaction, vertical and/or horizontalinterfaces between consecutive placements, non-uniform mixing of the componentmaterials, and non-uniform composition of materials. During construction, mediamust be placed without compaction using a back hoe or a crane. After placementthe media should be leveled by hand raking. Traffic must be avoided on the mediaduring or after placement. Compaction decreases porosity, resulting in non-uniform air flow and poor odor removal. 2. Operation Short circuiting of odorous air through the biofilter reduces odor absorption efficiency. Short circuits can be caused by tap roots of plants allowed to grow in the media, by dry zones which possess more porosity than adjacent moist areas, and
Cornposting 257macro-pores caused by earthworms, rodents, or water flow. Leaks can be causedaround the edge of the biofilter by improperly placed or sealed liners. Although there is some concern with overly wet media, drying tends to be amore common problem. Air flowing to the biofilter is seldom saturated withmoisture, and thus has the potential to remove moisture from the saturatedatmosphere within the media voids. Removal of water from the voids in thebiofilter increases porosity. With higher porosity comes preferentially higher airflow and the capacity to firher dry the media. Thus, once a dry channel forms inthe media, the drying problem always gets worse. Rewetting of the media shouldbe initiated as soon as a media dryness is discovered. Biofilter media can be rewetted by moistening the air flowing to the biofilter,using soaker hoses below the surface, and providing surface irrigation. Rain andespecially snow are very effective in maintaining media moisture levels. Whereextensive drying has occurred, the dry media may need to be excavated, rewettedin bulk, and then replaced. Turning off airflow to the section of the biofilter beingwetted can improve penetration of the water. Many biofilters are now designedwith synthetic liners enabling water flowing through the media to be directed to asump. Water from the sump may contain nitrate, carbonate, and sulfate salts and should be tested prior to returning to the biofilter surface or addition to compostingmaterials. The biofilter media tends to degrade physically with time. Even though the media possesses a very high C:N ratio, odors from biosolids composting operation generally contain varying amounts of ammonia. As the ammonia is absorbed by the media, microbes can use this nitrogen source for degradation of the carbonaceous media. The consequences of this physical breakdown are shrinkage, cracks, and structural collapse. Consequences include direct leaks, shorter retention times, increasing pressure drop, and poor odor removal. The life of the media will be affected by its biodegradability, the nature and loading rate of the gases flowing to it, and the maintenance it has received. A three to seven year media life is common. 3. Monitoring Media Condition On a weekly basis, record the pressure drop through the media, check for dry spots on the surface, remoisten as necessary, and remove weeds and grass from the surface. Increases in static head pressure indicate plugging or media collapse, while decreases may indicate leaks. Variation from week to week is normal when induced by rainfall or other temporary changes. What is important are pressure changes that persist, or trends over time. Graphing the data is often helpful to get a visual presentation of changes in head pressure within the biofilter. Each month make an extensive survey of the media surface and correct any problems. These include surface porosity, moisture in the media, rodent holes that could develop
258 Naylorinto short-circuits, and any odor leaks. On an annual basis, chemically test samplesof the biofilter media to check pH, and concentrations of nitrogen compounds.Screen the media at one inch, and measure the C:N ratio on the finer materials.Excavate and examine the media from surface to base to assess its physicalcondition.4. Monitoring Air QualiySamples of dischargedltreatedair can be checked fiom if required using an invertedm e 1 or plastic bucket dischargingto a plastic sampling bag. Samples should beselected randomly at points on a grid superimposed on the media surface.Collection of a number of samples is recommended because of non-uniform airflow due to non-homogeneityof the media. Monitor air flow rates and temperatureat each collection point and at a monitoring point leading to the biofilter to evaluateairflow rates and mass loading accurately. Untreated and treated air samples canbe analyzed for specific chemicals of interest or by odor panelists. The success of the biofilter is derived from a combination of physical,chemical, and biological processes that occur in the media. Odors are sorbed bythe organic matter particles, dissolved in moisture, and broken down and destroyedby microorganisms. The process is self-sustainingand no supplemental chemicalsare needed. Consequently, little maintenance is required and any liquids generatedare often recyclable.VIII. PRE- AND POST-PROCESSINGA successful composting facility will need a receiving and processing train toprepare the ingredients for the composting process and the finished compost for thecustomer. This includes a receiving area, shredding or particle size reductioncapability, moving the finished compost to a screening area, and a curing orstorage area. he-processing prepares the raw materials for the composting process and theblending together of ingredients to be used to develop the finished compost. Theraw materials will affect the quality of the finished compost and its marketing sothe presence of any contaminants must be evaluated at this stage. The receiving area should enable receipt of wet materials such as dewateredbiosolids and dry materials such as wood chips, sawdust, leaves, or yard trimmings.Provisions must be made to store dry materials before and after shredding.Sawdust and wood chips can be readily stored under cover to keep them dry; thepurpose of the dry materials is to use their dryness to adjust the moisture contentof the biosolids. Leaves and yard trimmings can be stored outside before grindingand after grinding, if necessary. These piles are essentially static compost piles and
Cornposting 259will generate heat that tends to keep the materials fairly dry on the inside. Becausethe piles do generate heat, periodically monitoring their temperature is essential toavoid overheating, charring, or worse. If heat builds up beyond 60 to 70"C, thepiles should be turned and aerated to release the heat. The dry materials will be processed to uniform particle size by shredding usinga tub grinder, a slow speed auger shredder, or other type of grinding or shreddingtechnology. The particle size should be guided by the composting technologyselected and the requirements of the customer for the finished compost. Once thedry materials to be used as amendments are received and ground, these materialswill need to be stored until blending into the composting feedstock. The purpose of post-processing is to prepare the finished compost formarketing. Preparation will include conveyance of the finished compost from the active composting area to the curing, screening, and preparation areas. Trommel screens and belt shredders are frequently used. Shredding can precede or follow curing. In some cases, double screening is preferable, especially for the horticultural market where price opportunities and product quality required enable somewhat more time to be spent on product quality control. The compost will need to be cured and stored for 30 days to up to 6 months or more. The length of curing and storage is a function of the composting process, regulatory requirements, marketing strategy, and customer requirements. One benefit of screening before curing is that the quantity of material stored is smaller, reducing the size of the curing area. On the other hand, the presence of coarser materials in the curing compost tends to keep the static pile more open, enabling some exchange of air and heat release. Compost can be cured and stored outside. Heat generated by this product tends to dry the piles on the inside. The piles will become wetted on the surface to a depth of 6 to 12 inches. The site should be laid out to avoid surface water run-on to the storage area. In most cases, site runoff control is required. Surface water from the area can be collected and used as a moisture source in the process in many cases.IX. MARKETINGRecycling composted organic wastes through soil-plant systems provides anenvironmentallybeneficial, ultimate disposition of such materials. Such a programis possible only if the product meets the quality compost standards: chemicallyacceptable to regulatory agencies, biologically free of pathogens and weed seeds,and meets the needs and expectations of prospective customers. The composting process transforms biosolids and other organic residues intoa material with a physically appealing character, in contrast with the generalacceptability of raw materials. Finished compost contains carbon, oxygen,hydrogen, nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, and a
260 Naylordiverse group of trace elements such as copper, zinc, iron, and boron. Theseelements as well as the bulk provided by the organic matter in compost canimprove the humus content of soil and contribute greatly to the chemical andphysical properties of a fertile soil. However, once the finished compost isprepared, the necessary next step is distribution and marketing of the product to theconsumer. Composted biosolids are marketed to a large extent for the establishment andmaintenance of turf-grass, garden and horticultural use, production of nurserycrops, and as a source of organic matter in bedding plant mixes. This practicesaves customers money while yielding excellent horticultural and floriculturalresults. These markets depend on readily available sources of consistent qualityand supply of organic matter for production of high quality plants. Compostderived from an effective composting process guided by a quality assuranceprogram is a good competitor in such markets.A. Marketing IssuesIssues that can influence marketing fall into two general areas: (1) consistentcomposition and assured delivery, (2) quality concerns: physical, chemical, andbiological.1. Consistent Composition & Assured DeliveryOn-time delivery of the required quantity of a soil amendment with a knownquality is essential. While sizable budgets are allocated for the use of growingmedia and soil amendments, they may be small relative to the cost of an entireproject. Composts may be blended with other ingredients such as lime, soilacidifiers, sand, etc. to achieve desired properties in the finished product. Theseblends are used to produce platings that account for the entire year’s sales. If thequality of the compost changes during the blending program, the finished soilproduct may be unsuitable for the planned use. Plant production schedules,fertilization programs, harvest schedules and marketing plans are heavilydependent on the quality of the growing media and its consistent performance. Growers, landscapers, top soil dealers, golf course superintendents, andgreenhouse managers cannot afford to have a project delayed nor the quality oftheir work diminished because of unreliable delivery schedules or compost of aquality different from what they were expecting. Delays cost money and can holdup work that must follow in sequence. Inability of a compost supplier to meet boththe delivery quantity and schedule can mean lost business by a customer. While high quality compost can often deliver better results at a lower cost, theseller of the displaced material will still be competing for the same business. Tosuccessfullymarket compost in a competitive environment, the producerhupplier
Composting 26 1must achieve assured delivery of the quality and quantity of the material orderedat a competitive price.2. Quality ConcernsPhysical, chemical, and increasingly, biological properties of compost are of majorimportance in determining compost use and value. Physical properties influenceporosity to air and water; chemical characteristics control nutrient availability,while biological properties can influence plant disease control. Phvsical DroDerties. Physical properties of interest to users would includetraditional characteristics such as porosity (70 to 90%), bulk density (0.1 to 0.6),water holding capacity (100 to 200%), and organic matter content (60 to 85%). But color and particle size can also be important considerations.Customers tend to compare the color and texture of the new product with whateverthey are currently using. Education by the salesperson,actual use by the customer,and analysis by a reliable soils laboratory can improve acceptability of a newproduct. In general, compost must match physical properties of currently used products, or the salespersonmust be prepared to effectively demonstrate equivalent or better performance. Although woody particles are acceptable and even desirable in compost, the presence of non-organic materials should be evaluated carefully. Unacceptable contaminants would include pieces of sheet plastic larger than about I8 inch, metal objects, and sharp pieces of glass (but probably not rounded pieces the size of a pebble or smaller). Not all non-organic materials are unacceptable, however. For example, shredded dry wall (gypsum board) has been included in compost, and may be acceptable chemically and physically. The gypsum particles add sulfur and calcium, and have a physical appearance similar to perlite, a common ingredient in soil mixes. The principal value of compost, though, is its ability to supply organic matter. Physical characteristics of compost provide important soil conditioning benefits . These benefits include enhanced aggregation (the ability of the soil to form discrete particles or aggregates), increased soil aeration (helps keep the soil biologically healthy), lowered bulk density (lightens the soil which eases weeding and improves root development),and increased water infiltration and water holding capacity (maintains the proper moisture balance in the soil for plant growth). Compost added to soil improves spring warming and enables earlier working of the soil and planting of crops. Compost added to sandy soils increases the water holding capacity and the amount of water available to plants. This reduces the need for irrigation and minimizes the loss of soluble nutrients such as nitrates. In heavy textured clay soils the added organic matter increases the soils permeability to water and air. Such improvements enhance the plant root environment and thus, aid root development and overall plant growth.
262 Naylor Chemical characteristics. While the major use of compost is related to itsorganic matter content, the macro- and micro-nutrient content are also important.The nitrogen (N), phosphorus (P), potassium (K) and calcium (Ca) composition ofcomposts will vary depending on the source of the wastes. However, compost drymatter typically contains about 0.5 to 3.0 % total nitrogen, 0.5 to 3.0 % P,05, 0.2to 2.0 % K20, 0.5 to 5 % calcium, and 0.1 to 3 % iron. Concentrations of thesenutrients substantially beyond this range, suggesting higher nutrient content, canbe produced by adjusting with sources of fertilizer concentrates such as guano,bone meal, blood meal, or rock phosphate. In addition, most composts containvariable amounts of other macro-nutrients such as magnesium and sulfur, andmicro-nutrients such as boron, copper, manganese, molybdenum, and zinc. Total analysis of several biosolids based composts and the amendment usedare listed in Table 5-14. Analyses include macro-nutrients and several micro-nutrients of horticultural interest, pH (5050 mix in water), total solids (dry mattercontent), volatile solids (organic matter content), fixed solids (ash content), carbonto nitrogen (C:N) ratio, and N-P,O,-K,O fertilizer value (on a dry solids basis). Growers are also interested in the concentrations of nutrients that plants canactually access during the growing season. Available or exchangeablenutrients arethe concentration of nutrients present in a material in a form that a plant root canabsorb, sometimes within a specific time period. For this test, sample processing is less vigorous since the laboratory is trying to simulate chemically the same extracting power as fluids emitted by root hairs or the potential for the nutrient to dissolve in soil water. In this case prior to actual chemical analysis, the sample is typically prepared by extraction with a weak acid solution such as acetic acidammonium acetate buffer. Results are commonly expressed as ppm or mg/l in the extracting solution or the sample dry matter, or sometimes as lbs. per acre. In some cases the laboratory reports extractable nutrient concentrations for the sample and also the optimum or typical range of such concentrations for commercial container mixes. This strategy is useful to a potential customer because it enables the economic and cultural comparison with a familiar growing media. The total and available nutrient analysis present a fairly accurate estimate of the nutrients in compost in both the long term (total) and the short term (available). The exception may be nitrogen availability. Forms of nitrogen present in most composts can be divided into inorganic nitrogen and organic nitrogen. Inorganic nitrogen forms include principally nitrate and ammonia. These forms are sometimes described as mineral forms because they can be found in nature or can be purchased commercially as a mineral such as sodium nitrate. Since these two nitrogen forms are soluble and can be extracted by water or extracting solutions, available nitrogen estimated by extraction will report both nitrate and ammonia. However, organic nitrogen is chemically bound to organic matter and the organic matter must be biodegraded or mineralized to release the nitrogen. Estimating the
Cornposting 263amount of nitrogen that may become available as the organic nitrogen mineralizes(biodegrades into the mineral or inorganic form) is not described by either test.While there are no tests to determine precisely the proportion of this organicnitrogen that may become available from a specific compost during a growingseason, the results of many laboratory studies of nitrogen release from compostssuggest that about 10 to 15% of the organic nitrogen will be released during thegrowing season. The residual nitrogen is retained in the soil and continues toprovide nitrogen for plant growth for several years. Compost is a living fertilizer. Because some the nutrients in the compost arecombined with organic matter, these nutrients, especially nitrogen, are released asthis organic matter slowly breaks down through biological activity of living micro-organisms. This biological activity is moderated by the same environmentalconditions that influence plant growth: moisture and temperature. Thus, as withmany organic fertilizers, the plant nutrients tend to be released gradually, and inresponse to the conditions that also stimulate plant growth. Such a release patterntends to result in more efficient use of the nutrients. Because of this slow releasenature of compost nitrogen, a second, but equally important, benefit is that nutrientloss through leaching is greatly reduced or eliminated. Specific parameters. While environmental regulators show a great deal ofinterest in metal concentrations in compost, potential customers, i.e. growers, havemuch simpler analytical interests once the compost has passed the regulatorystandard. Thus, a quality assurance program should focus on the grower’s interests. Analyses of interest are typically pH, soluble salts, organic matter content, C:N ratio, and odor. The pH of a growing media strongly influences the solubility of minerals inthe root zone of the plant and the hospitably of the root zone to plant pathogens. As the growing media becomes more acidic or lower pH, the solubility of aluminum, iron, manganese, zinc, copper, and macro-nutrients such as calcium, potassium, magnesium, and phosphorus increases . For ericaceous plants such as azaleas, gardenias, rhododendron, or laurel a growing media pH of 4.5 - 5.5 is necessary. On the other hand, non-ericaceous plants prefer a media pH of 5.5 to 6.8. Turfgrass recommendations suggest a soil pH of 6 to 7, but can use higher pH. Most composts have a pH of 6.8 to 7.5, although some composts can be lower (Table 5-14). Thus, testing the pH of compost or other material intended for use is important so that the pH can be adjusted, if necessary, for the intended crop. Concentrations of soluble salts tends to be higher in compost than that present in common media such as peat, bark, soil, or sand. For example, biosolids can be rich in calcium, potassium, magnesium and sodium. These elements contribute to the soluble salts or salinity of a compost. While such minerals are essential for plant growth, excess can damage plant roots and inhibit germination, especially in container crops. The carbon to nitrogen ratio of a compost, usually shown as C:N, is important
266 Naylorbecause it indicates generally whether a compost will compete with plants fornitrogen. The C:N ratio should be in the range of 15 to 25, i.e. the carbon contentshould be 15 to 25 times greater than the nitrogen content. Extra soluble nitrogencan be added to a growing media to prevent nitrogen deficiencies, provided theproblem is known ahead of time. Alternatively, if the problem is discovered afterplanting, a soluble nitrogen source can be added during watering. Odor is a very personal sensation and one that can influence compost sales.Compost produced fiom wastes tend to have some odor associatedwith them. Theodor often reflects the material being composted. For example, some ammoniamay be noted in compost produced fiom manures. Biosolids-based composts tendto have a musty aroma. While such odors may be of some concern initially, theyusually dissipate once the material is mixed with the other ingredients orincorporated into field soils. Odor in composts applied to the soil surface as asurface mulch or top-dressing dissipates after watering or exposure to sun, air, andrain. The public and compost customers often have questions regarding thebiological condition of compost, particularly biosolids derived compost. Thesequestions center on pathogens. Compostingtime and temperature very effectivelydestroy pathogens: bacteria, viruses, protozoans, and helminths. State and federalregulation of these conditions have been in effect for many years, and experienceindicates that where these regulations have been followed, pathogens areeffectively eliminated fiom compostedbiosolids. However, human pathogens maynot be the problem grower customers are most interested in. Weed seeds in compost are devastating to a landscaper who has investedhundreds of hours in developing a beautiful garden only to discover after the firstrain a blanket of weeds. The compost facility will effectively destroy weed seeds if the time and temperature requirements are met. However, that is only the firststep. The next step is protecting the compost fiom being infected with wind-blownweed seeds during storage or curing outside. Protection can be achieved by covering the material, or pushing aside the surface layer into a pile allowing it to reheat and destroy the weed seeds. Weed control around the storage site can also be effective in isolating the compost fiom weeds. A property of compost that has long been suspected by organic gardeners, and is now being investigatedand proven scientificallyis the ability of some composts to control plant pathogens. Homick, et al. reported that as early as 1974, composts were thought to control some root rots and plant pathogens, and that in 1983 research suggested that plant nematodes would also be controlled . Kuter, et al. found that specific h g a l populations colonizing compost were responsible for suppression of Rhizoctonia damping-off . Nelson has continued this work focusing on pythium blight and dollar spot control on turf and has found that bacterial populations can also play an important role [ 181[ 191. Much of the current understanding of the widespread mechanisms of disease suppressing effects of
Cornposting 267compost has been reviewed by Hoitink, Boehm, and Hadar [ 1 11.3. Quality CompostAchievement of quality compost is not only indicative of a well-operatingcomposting facility, but essentialto successfulcompost marketing. Marketing suchproducts within a competitive economic environment requires a sound qualityassurance program focused on delivering a material with consistent physical,chemical and biological properties. While meeting environmental regulations isrequired by law for biosolids derived composts, customers have an additional setof chemical parameters of interest. These include organic matter content, total andavailable nutrients, pH, carbon to nitrogen ratio, salinity, and odor. Compostcustomers rely on the supplier to have the capability to deliver not only a consistentproduct, but to deliver that product in the required quantity and on schedule aspromised. Recycling composted biosolids as a soil amendment is not onlyenvironmentally responsible, but sound from a plant production and economicperspective as well.X. OUTLOOK AND SUMMARYIn the past decade composting has developed from a fledgling art into anengineered technology. The science of the composting process is increasingly wellunderstood. Mechanical composting technologies are improving in reliability andease of operation. Process control has become computerized to enhance theenvironmental conditions for the compost microbial community and improveprocess efficiency. Best of all, the quality assurance of the finished product isincreasingly being guided by customer requirements and process knowledge,improving the marketability of the compost. While substantial strides have been made, there is opportunity forimprovement. Continuing efforts must be directed toward economical andeffective odor management strategies. Because of the potential growth within theindustry, effort is needed to expand even further opportunities for beneficial use ofcomposted biosolids. Public acceptance of biosolids products varies from place toplace. While we have supportive regulations guiding biosolids use backed by asound technical understanding of risks and benefits, the societal aspects needW e r work. Perhaps it is time to request the assistance of the social scientists inmoving forward on this front. Technically the priorities for guiding biosolids composting are to know thequalities of the biosolids and the amendments used in developing the blendedfeedstock, and to know the expectationsof the market. Compare the quality of thecompost produced with that likely to be expected by the compost market. Once
268 Naylorthe inputs and the markets are known then decisions can be made regardingselection of the most appropriate composting technology whether in-vessel orwindrow type. Selection of pre- or post-processingtechnologies will be guided bythe characteristics of the ingredients, the quality of the fmished compost, and thecomposting technology. In short, with a sound understanding of the art and the science of compostinga quality product can be produced. A quality product is the foundation of readilymarketable compost.REFERENCES1. E. Allen, H.U. Hartenstein, Biofiltration: An Odor Control Technology for a Wastewater Treatment Facility, Environmental Engineering Sciences Dept., University of Florida, Gainesville, FL. 1986.2. L. Allison, Organic Carbon, p. 1367 In Methods o Soil Analysis, C. A. Black, f Ed., American Society of Agronomy, Madison, WI, 1965.3. G. Arkin, H. Taylor, Modifying the Root Environment to Reduce Crop Stress, ASAE Monograph No. 4, American Society of Agricultural Engineers, St. Joseph, MI. 1981.4. N. Brady, The Nature and Properties of Soils, Macmillan Publishing Co., New York, 1984.5. G. Bugbee, C. Frink, Quality of Potting Soils, Bulletin 812, The Connecticut Agricultural Experiment Station, New Haven, CT, 1983.6. M. de Bertoldi, F. Zucconi, Microbiology of the Transformation of Urban Solid Waste in Compost and its Use in Agriculture, Ingeneria Ambientale 9 (3): 209-216, 1980.7. N. Goldstein, D. Riggle, R. Steuteville, Biosolids Cornposting Strengthens its Base, BioCycle 35 (12): 8-57, 1994.8. N. Goldstein, Sewage Sludge CompostingFacilities on the Rise, BioCycle 26 (8): 19-24, 1985.9. T. Haug, The Practical Handbook of Compost Engineering, Lewis Publishers, Boca Raton, Florida, p. 249, 1993. 10. C. Henry, M. Knoop; K. Cutler-Talbott, Technical Information on the Use of Organic Materials as a Soil Amendment:Literature Review, University of Washington College of Forest Resources, Seattle, WA, 1990. 11 . H. Hoitink, M. Boehm, Y. Hadar, Mechanisms of Suppression of Soilborne Plant Pathogens in Compost-amended Substrates, p. 601. In Science and Engineering o Compost, Eds. H. Hoitink and H. Keener, Renaissance f Publications, Worthington, OH, 1993. 12. S. Hornick, et al., Utilization of Sewage Sludge Compost as a Soil
Cornposting 269 Conditioner and Fertilizer for Plant Growth, Agriculture Information Bulletin Number 464, U.S. Department of Agriculture, Agricultural Research Service, Beltsville, MD, 1984.13. Incineration Application of Combustion, Wheelabrator Technology, Inc., Hampton, NH. Internal Document, J. Sciabba, April 1, 1994.14. G. Kuter, et al., Fungal Populations in Container Media Amended with Composted Hardwood Bark Suppressive and Conducive to Rhizoctonia Damping-off, Phytopathology 73, 1983.15. R. Monk, Digging in the Dirt, Unearthing Potential, World Wastes 37 (4): CS2-CS14, 1994.16. L. Naylor, P. Gormsen, Odor Control by Biofiltration in the Northeast: Inventory and Operations, Presented at the New England Water Pollution Control Association, Boston, MA, January 1992.17. L. Naylor, W. Doty, Food Processing Residuals: Management Strategies for Beneficial Use. pp. 115 - 125 in Proc. Con$ Waste Management and Water Conservation in the Food Processing Industry, P.D. Robillard and H.A. Elliott, Eds, The Pennsylvania State University, University Park, PA, 1989.18. E. Nelson, C. Craft, A Miniaturized and Rapid Bioassay for the Selection of Soil Bacteria Suppressive to Pithium Blight of Turfgrasses, Phytopathology 82,206-2 10, 1992.19. E. Nelson, C. Craft, Suppression of Dollar Spot on Creeping Bentgrass and Annual bluegrass turf with compost-amended topdressings. Plant Disease 76, 954,1992.20. Northeast DHIA Forage Laboratory, 1994 Tables of Feed Composition, Northeast DHIA, Ithaca, NY. 1994.21. M. Pelczar, Jr., R.D. Reid, Microbiology, McGraw-Hill Book Co., New York. p. 115, 1972.22. Second interim report of the Inter-departmental Committee on Utilization of Organic Wastes, New Zealand Eng., November-December, 1951.23. G. Tchobanoglous, F. Burton, Wastewater Engineering: Treatment, Disposal, and Reuse, McGraw-Hill Publishing Co., New York, p. 744, 1991.24. U.S. EPA, Subpart D, 40 CFR Part 503. Standards for the Use of Disposal of Sewage Sludge, Vol. 58, No. 32, February 19, 1993.25. B. Watt, A. Merrill, Composition of Foods, Agricultural Handbook No. 8, U.S. Department of Agriculture, Washington, D.C., 1975.
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Heat Drying and Other ThermalProcessesMark J. GirovichWheelabrator Clean Water Systems Inc.Annapolis, MarylandHeat (thermal) treatment in biosolids processing has been used for severalpurposes, such as a) promoting separation of solids from liquids by releasing bound(cell) water to improve dewaterability; b) meeting pathogen and vector attractionreduction requirements (i.e., sterilization and disinfection), c) enhancing waste-activated biosolids digestibility; d) incineration; and e) drying and production ofbiosolids-derived fertilizer. To meet current U.S. EPA 503 Regulations pathogen and vector attractionreduction requirements, the biosolids heat treatment processes should meet certaintime-temperature requirements depending largely on the solids content(Subparagraphs C & D of §503.32(3)(ii), [l]). These requirements are typically applied for direct thermal treatment of liquid biosolids ( . 7% TS), aerobic 1thermophilic stabilization (ATS) processes and pasteurization. This chapter’s focus, however, is on biosolids heat drying for production of beneficial use products.I. BIOSOLIDS DRYINGProduction of dried beneficial use products from municipal solids offers a viablemanagement solution for many municipalities. Open air solar drying has beenpracticed for many years using fiee, nonpolluting, renewable and abundant energy 271
272 Girovichsource. However, using a solar radiation is not an applicable method for theoverwhelming majority of municipalities in the U.S. where more reliable sourcesof energy must be used. Air (solar) drying is only feasible at favorable climate conditions (significantpercent of sunshine, rate of precipitation, air temperature, wind velocity, etc.).Biosolids are dried to a level of dryness which depends upon largely uncontrollablecircumstances and, therefore, the establishment of a standard operating procedureis impossible. For air drying, biosolids are typically placed on an open surface (lagoons,sand drying beds, etc.) and left there until they have dried to a desired solidscontent. Biosolids should be stabilized prior to air drying to prevent odors and vectorattraction. Air drying can be enhanced by a vacuum or mechanical agitation, withthe latter possible only if biosolids initial dryness is at least 12-15% TS. Anothersolar drying method tested in Florida is to dry biosolids by air heated in solarpanels; however, this technology has not developed beyond a pilot plant. Air drying is a low cost alternative but it requires large parcels of land andcooperating weather. It has been practiced primarily in the Southern United States,however, some installations have been as far north as Chicago, Illinois. Air driedbiosolids are Class B products with respect to the pathogen and vector attractionreduction requirements and as such they can be land applied on the agricultural sites specifically permitted for this purpose.A. Heat Drying and Production of FertilizerThe rapidly growing biosolids treatment option for beneficial use is heat drying.Heat drying and production of biosolids-derived fertilizer has been in use in theU.S. since 1925 when the City of Milwaukee, Wisconsin produced Milorganite, afertilizer made from municipal solids. Biosolids heat drying in general provides the following major advantages: eliminates pathogens and sterilizes the end product, fully satisfying the U.S. EPA "Standards for the Use or Disposal of Sewage Sludge" (40 CFR Part 503) reduces the volume and weight of the biosolids considerably preserves and recycles valuable organic and inorganic nutrients found in biosolids converts biosolids into a valuable end product with revenue potential (fertilizer, soil amendment or fuel) improves transportability, storability and social acceptance of biosolids derived products
Thermal Treatment 273Heat drying can generate the following products:a) dried biosolids pellets with no less than 90% TS for marketing as a fertilizer;b) partially dried biosolids for land application and landfilling (typically 40-60% TS); andc) dried biosolids for incineration, i.e. fuel production (40-90% TS). In order to successfully market biosolids fertilizer, the following majorcriteria should be met:1. Moisture ContentPellet moisture content should not exceed ten percent (10%) since a highermoisture content encourages microbiological regrowth and product overheating.The finished product with minimum dryness of 90% is a Class A product and assuch it can be used as fertilizer with no site restrictions provided its heavy metaland synthetic organic compounds content is in compliance with the limitsestablished by federal and local regulations.2 . Nutrient ContentPellet fertilizer value is based primarily on nitrogen content. Nitrogen is theprimary plant nutrient found in biosolids, typically in the range of 4% - 6% on adry weight basis. As a "stand alone" fertilizer product, pellets should contain aminimum of 5% total nitrogen consistently. If used as a fertilizer ingredient to beblended with other fertilizer materials, pellets should contain at least 3.0%totalnitrogen. Phosphorus (as P,O,) and water soluble potassium (as y 0) are alsoimportant nutrients which are usually present in biosolids at low concentrations. Heat drying generally does not diminish the fertilizer or soil amendingqualities of biosolids as other treatment processes can. However, certain loss(approximately 20% - 60%) of water soluble ammoniacal nitrogen (M-h+) been hasobserved during heat drying, especially if the incoming biosolids pH is aboveneutral. Fortunately, the biosolids ammoniacal nitrogen is typically in the 0.25%- 0.75% (dry weight) range only. Organic nitrogen, the predominant form ofnitrogen in biosolids, on the other hand remains practically unchanged. Organicnitrogen has the advantage of a slow release rate over time and it is much lesssusceptible to run-off loss after rainfall. Micronutrients,for example iron, may alsoenhance the product value. The pellets can also be enriched by adding nutrients(nitrogen, magnesium, iron, etc.) as needed prior to marketing.
274 Girovich3. Physical QualityThe fertilizer industry demands biosolids fertilizers be consistent with otherfertilizer materials they utilize. The U.S. Fertilizer Institutes (Washington, D.C.)Bulk Blend Quality Control Manual (1987) addresses some properties for thefertilizers amendments (fillers). These properties include particle size, bulkdensity, uniformity, chemical compatibility, pH, critical humidity and visualappearance. Physical quality criteria include: Particle Size Range - For most agricultural applications, pellets should fall inthe size range of 1 to 4 millimeters. For example, granular commercial fertilizerparticle size specificationused by the U.S. Agency for International Development(USAID) establishes: 0% wt over 4.75 mm (US #4 mesh) 90% wt minimum in the 1.OO to 3.35 mm (US #6 to #18 mesh) 2% wt maximum below 0.6 mm (US #30 mesh) Some agricultural and other non-agricultural fertilizer outlets may requiredifferent pellet sizing. Pelletized biosolids with narrow particle size distributionand minimum dust command higher prices. Bulk Density - The bulk density of most fertilizer constituents ranges from 40to 80 pounds per cubic foot. Typically, biosolids fertilizer bulk density values fallat the lower end of this range or below. DurabiliR - The pellets should be able to withstand the normal rigors oftransportation, handling and mixing without producing excessive levels of dust.4. Pathogen and Vector Attraction ReductionThe pellets must comply with the Class A standards for pathogen and vectorattraction reduction requirements contained in the 503 Regulations. In order to doso, the biosolids heat drying should meet either the direct microbiological limits(see Chapter 2) or the following process-based requirements: biosolids are dried in direct or indirect contact with hot gasses to reduce the moisture content to 10% or lower (290% TS) either the temperature of the biosolids particles exceeds 80°C (176°F) or the wet bulb temperature of the gas in contact with biosolids as biosolids leave the dryer exceeds 80°C (176°F). Heat drying is an effective pathogen and vector attraction reduction processwhich produces Class A biosolids. If the biosolids also meet applicable quality
Thermal Treatment 275standards with respect to trace metals and synthetic organics, they may be soldwithout restriction for use on all crops. The pellets must also comply with federal, state, and local standardsgoverning marketing and distribution of biosolids as a fertilizer. Depending upon the desired outlet, other qualities will determine pelletmarketing success. A very important characteristic is odor, which is largelydependent upon the type of biosolids processed (e.g., raw, primary, secondary,digested, etc.). Heat drying with pelletization generates so far the most acceptable, safe, oftenrevenue-producing product. Pellet fertilizer qualities which can be assigned a value, depending upon theapplication, include: Slow-release organic nitrogen content Micronutrient content - Organic matter content Blended fertilizer material conditioning properties Reduced likelihood of plant nutrient contamination of groundwater compared to many chemical fertilizers5. MarketsBiosolids fertilizers meeting regulatory standards can be used for a range offertilizer applications from the most basic such as agricultural fertilizer filler to themore sophisticated such as the feeding of sensitive turf and ornamental plants. Assuch, the application opportunities for biosolids exist throughout the U.S. andbeyond. Presently ,there is a substantial demand for heat-dried biosolids fertilizers inthe US. The Florida fertilizer industry consumes biosolids-derived products foragricultural applications including citrus, other fruit, and vegetable fertilization.Fertilizer blenders can replace currently used lime and clay by biosolids pellets asa filler material. Non-agricultural applications include golf courses, fairways,greens and tees; residential and commercial property lawn care; and landscapeplant nursery feeding. Florida is currently the distribution end point for a numberof in- and out-of-state pellet producers. Number of producers and amount ofpellets are expected to increase in upcoming years which may potentially flood the Florida market. Major new markets outside the primary market area of Florida should certainly be developed to accommodate the increasing production. Biosolids product marketing typically involves displacement of existing product rather than creation of a product to satisfy an unmet demand; and, therefore, effective marketing depends on a demonstration that the products benefits exceed those of competing products .
276 Girovich Typically, biwlids fertilizer prices in Florida range from $1 5 to over $100 perton at the point of use and depend largely on nitrogen content (up to $15.00 perpercent point of nitrogen for a good quality product). Products with establishedreputation and guaranteed nutrient composition, such as Milorganite, commandsignilicanehigher prices in the fertilizer market ($90 - $120 per ton). Future pricedynamics will depend upon supply-demand relationship wt a possibility of ihsi@cant price decrease due to flooding of the prime Florida market. New marketsmust be developed, especiallynear production plants to Sustain reasonable price. The production of a fertilizer from the municipal biosolids offers a viablemanagement solution for many municipalities with sufficient biosolids generation,especially those located in densely populated areas. This option is stimulatedthroughs i m c a n t improvement in biosolids quality throughout the U.S. and it grows quiterapidly. At present, large biosolids heat drymg facilities operate in New York, NewYork; Milwaukee, Wisconsin, Boston, Massachusetts; Tampa and Largo, Florida,Baltimore, Maryland; Houston, Texas, and other locations with total annual designcapacity of over 350,000 dry tons. A list of heat drymg and pelletizing facilitiesproducing commercial fertilizer in the U.S. is provided in Table 6-1. By the end of the 1990s, heat dried bimlids production for fertilizer applicationin t e U.S. is projected to increase from the current 0.3 million tons approximatelyto hin excess of 0.5 million tons per year with the majority of this amount being utilized in the South. This production level will still comprise only a small portion of the totalU.S. chemical fertilizer consumption (approximately50 million tons annually).B Partially Dried Biosolids .Bimlids can be dned partially, i.e. to less than 90% TS, (typically i the 40% - 70% nTS range), in order to reduce volume prior to land application or landfilling. Partiallydried biosolids are used also for subsequent autogeneous incineration. Landfilling of partially dried biosolids is typically practiced when they containtrace metals and other pollutants in excess of the limits established by the 503Regulations and could not be used as a fertilizer. Other requirements of landfillingshould also be met. Partial drymg of biosolids makes economic sense mainly because it reduces thevolume and cost of disposal. Heat +g is alsoutilized to reduce or eliminate the amount of externalenergy required for biosolids incineration (i.e., to reach an autogenous
Thermal Treatment 277TABLE 6-1 BIOSOLIDS COMMERCIAL HEAT DRYING AND PELLETIZING FACl ITIES IN THE USA (1994) Location Capacity, dtd (Evaporation Pollution Control Capacity, Wilwaukee, 200.0 Direct rotary drum 1925 Wisconsin (12 x 4.1) drying, cyclones & (renovated in electrostatic 1994) precipitators Zlayton 32.0 Direct rotary drum 1980 County, (2 x 2.0) drying, wood chip combustion Largo, Florida 36.0 Direct rotary drum 1976 (2.5) drying, after-burners (renovated in ~ 1993) Cobb County, 32.0 Direct rotary drum 1980 Georgia (2 x 2.0) drying, chemical scrubbers Tampa, 64.0 Direct rotary drum 1987 Florida (2 x 4.0) drying, after-burners Hagerstown, 14.0 Direct rotary drum 1990 Maryland (1.5) drying, chemical scrubbers Boston, 130.0 Direct rotary drum 1991 Massachusetts (4 x 4.5) drying, after-burners New York, 300.0 Direct rotary drum 1993 New York (6 x 5.0) drying, after-burners Houston, 90.0 Flash drying 1976 (7) Texas (9 x 1.0) 1987 (2) Baltimore, 1 10.0 Indirect multitray 1994 Maryland (3 x 4.5) dryers (3)
278 Girovich Pollution Control* Design capacity available in 1995.incineration). The dried biosolids contain a substantial amount of energy (5,000 -9,000 BTUAb, dry weight) and can be used as a fuel. Two methods of bimlids drylng are used for autogenous or partially autogenousincineration:1. The entire amount of biosolids is dried to an autogenous level (typically 40% TS) and then fed into an incinerator.
Thermal Treatment 2792. A portion of biosolids is dried to no less than 90% TS and mixed with wet biosolids feed to arrive at an autogenous level prior to feeding an incinerator. A portion of the dried biosolids can also be diverted for marketing as a fertilizer. Biosolids drying for autogenous incineration is most popular in Europe andJapan where cost of energy is high and the biosolids use as a fertilizer is limited.A substantial number of incineration systems there employ indirect horizontaldryers to dry biosolids to autogenous level prior to incineration. The second method can be used for a dual purpose system, i.e. forsimultaneous production of pellet fertilizer andor incineration during inclementweather or market downturns. (See Figure 6-16)11. HEAT DRYING PROCESSESDiversity of materials to be dried and relatively high capital cost of drying systemsresulted in the evolution of many dryer and drying process designs. Drying isessentially the process of thermally removing volatile substances (above all,moisture in case of biosolids) to yield a dry product. All dryers are evaporators,i.e., they convert water to vapor which expands the volume about 1500 times thusexpelling the vapors. Moisture in biosolids may be either free (unbound) orbiologically and chemically bound. During the first stage of drying, the drying(evaporation) rate is constant. The biosolids particle surface contains free moisturewhich evaporates at constant rate. At the end of the constant rate period moisturehas to be transported from the inside of the particle to the surface by capillary and diffusion forces and the drying rate begins to fall. As the dry surface develops on the particles and as the free moisture content is lowered by drying, the rate of internal movement of this moisture decreases and the drying rate falls continuously. The bound moisture evaporation proceeds even more slowly for it requires breaking the biological cell structure or chemical structure of compounds having bound water in their composition. As a result, the drying rate falls even more rapidly. This is of course the mechanism of all solids drying processes but it is considerably complicated by many other factors, such as mechanical agitation, mixing, etc. In the direct rotary drum dryers, for example, biosolids cascade through the dryers with the cascading interrupted by the "resting in flights" periods. In indirect drying, the solids are agitated and mixed by paddles, discs, stationary bars, etc. These mechanical motions are design-specific: and, therefore, the process is extremely difficult to describe in mathematical equations of heat and mass transfer.
280 Girovich WATER VAPORDRYING MEDIUM CASCADING OR SUSPENDED PARTICLES DIRECT INSULATED SHELL (CONVECTIVE) DRYING WATER VAPOR BlOSOLIDS PARTICLES INDIRECT (CONDUCTZVE) DRYING Fig.6-1 DIRECT VS. INDIRECT DRYING
Thermal Treatment 281 Extensive work has been done in developing theories of drying. A systematiccomprehensive review and classification of drying processes and dryers is beyondthe scope of this book and the reader is referred to specialized sources , [ ] , 4,[61. The most common classification of drying is based on methods of heattransfer, namely: 1) conduction or indirect drying; 2) convection or direct drying;3) radiant drying and 4) dielectric (microwave) drying. Freeze drying can beclassified as a special case of conduction drying. Dryers can also be classifiedbased on the type of drying hardware (rotary drum, fluidized bed, spray, tray,conveyor, etc.), mode of operation (continuous, batch), residence time, etc. Both indirect and direct drying processes are used in biosolids treatment (Figure 6-1):1. Indirect (conductive) drying uses a heat medium such as steam, thermal oil orhot gas to heat a metal surface over which biosolids are transported. Conductiontransfers heat from the hot metal surface to the cooler particles and then betweenthe particles. It is the principal heat transfer mode for indirect dryers. Indirectdrying is a solid-liquid-vapor system and the process is nonadiabatic, i.e., heat isadded from outside to the dryer. In indirect drying, solids and water can be heated above the waters boilingpoint. Indirect drying is more efficient and uniform when agitation brings cooler,wet particles into contact with the heating surface. Heat transfer fluids in the indirect drying may be of either the condensing type (e.g. steam) or the liquid type (e.g., hot water, oils). Indirect drying has several distinctive features: 1) cross contamination is avoided since the product does not contact the heating medium; 2) since a very small amount of air is introduced, air handling, cleaning and odor control is considerably less expensive; and 3) fire and explosion potential in the dryers is reduced significantly. Examples of indirect dryers are horizontal "en masse" (hollow disc, paddle, screw, etc.), vertical multi-tray, rotary, mechanically fluidized bed dryers, etc. 2. Direct (convection)drying heats the biosolids by contacting them directly with hot gas (air). Direct drying uses relatively large amounts of hot air (usually mixed with combustion products). Convection transfers heat between adjacent volumes of hot air and particles, within porous particles and outside of all particles. It is the principal heat transfer mode for direct drying. Direct drying is a solid-liquid- vapor-gas system and the process is adiabatic, i.e., no heat is added or taken from the dryer, assuming perfect insulation . In direct drying, water and solids are not heated much above the wet-bulb temperature which is well below the waters boiling point. The higher water vapor pressure drives water vapor into the gas which has a low vapor pressure. Drying stops when these two pressures are equal.
282 Girovich Direct dryers may use hot air, combustion products, inert gas, and superheatedvapor as the heating medium. Examples of direct dryers are rotary drum,fluidizedbed, flash, spray and conveyor dryers. Direct rotary drum dryers are theworkhorses of biosolids drying; they are the least expensive of all dryers and havehigh unit capacity. However, all direct dryers have a common disadvantage: the amount ofheating medium (usually hot air mixed with products of combustion) is large whichresults in energy loss in the exhaust and requires expensive air handling andpoilution control equipment to clean the gas prior to its discharge to theatmosphere. This serious disadvantage can be neutralized to a certain degree byusing drying air recirculation as described in the following sections. Both drying processes are complex. Drying involves a number of physicaland chemical processes such as heat and mass transfer, mixing, combustion,conveying, size separation, psychrometry,evaporation, etc. So far, however, thereis very little help and transfer from the theoretical studies to the drying practice.Drying is still more art than science and more so for biosolids drying.111. DRYER DESIGNSNumerous dryer designs have evolved for drying of various products includingbiosolids. Table 6-2 depicts dryer design availability and use for biosoiids dryingin the USA (1995). Dryer Design Direct Indirect Installations in the USA 1. Rotary drum, single pass A A 14 (direct) 2. Rotary drum, triple pass A NA 19 (direct) 3. Horizontal, "en masse" NA A 8 4. Multi-tray A A 3 (indirect) 5. Flash A NA 9 (direct) 6. Fluid bed A A 1 (direct) 7. Conveyor A A 0 8. Spray A NA 0 A = commercially available NA = commercially not available
Thermal Treatment 283 The following dryer designs are currently used in the U.S.A. for commercialproduction of biosolids-derived fertilizer (Table 6-2): Rotary drum, single and triple pass, direct drying (9 operational installations, over 30 dryers) Horizontal "en masse", indirect drying (6 operational installations, 8 dryers, including partial drying for autogenous incineration)rn Flash dryer, direct type (4 operational installations, 9 dryers) Multi-tray, indirect drying (one operational plant, 3 dryers) There are also a few pilot installations in the U.S.A. using radiation heateddryers and directly heated fluidized bed dryers. Dielectric heating has not yetdeveloped beyond the experimental phase.A. Direct Dryers1. Rotary Drum DtyersIn the rotary drum dryers, drying air supplies the heat and carries off the evaporatedmoisture. Volume of drying air is the most important factor which determines thedryer and the drying system cost. The best drying energy efficiency is achieved atthe highest air inlet and the lowest air outlet temperatures. These temperatures arelimited by various factors (fire hazard, type of fuel, design of the heat generator,sensitivity of the end product, water condensation at the downstream equipment,etc.). Direct rotary drum dryers typically use cocurrent air flow which allows highinlet temperature, up to (900°F) for biosolids. Exit temperature varieswithin 85"- (180"-200°F). In cocurrent flow with high inlet temperaturethe moisture does not protect the small particles from over-heating and potential fire; however, the drying is fastest in this case. Most drying takes place as the solids cascade off lifting flights (or get fluidized) and the air passes around them. The dryer shell also heats the solids. In rotary dryers, solids are moved through the shell by the combined effect of air flow, gravity, internal flight design, speed of rotation and the shell slope. Baffles and dam rings are also used to retard the forward motion of the solids and to increase their residence time. Evaporation of free water typically occurs in minutes. The smaller particles dry faster and are carried off more readily and often are overdried. Their removal and return to the process may be troublesome for it requires special air-solids separation and transport techniques. The end product physical characteristicsimportant for biosolids beneficial use are moisture content, particle size distribution, density and amount of dust. Due to rapid evaporation in cocurrent rotary dryers, a particles density often decreases
284 Girovichand particle sue distribution widens. Significant amounts of fine particles (dust)are produced by abrasion and overheating. Typical rotary d u speeds are between 5 and 25 rpm depending upon the rmdrums diameter. The percent of the shell loading is typically 10 to 20 percent andthe solids retention time is 10 to 25 minutes. Drying air retention time is onlyseveral seconds. Single pass and triple pass direct rotary dryers are used to drybiosolids. At low capacities and low gas temperatures, direct drum dryers lose their costeffectiveness. The biosolids fed into the dryer must be a post glue-like phase, granular (i.e.,50 - 60% TS) material, therefore, dry material recycling and backmixing isrequired to avoid sticky biosolids inside the dryers and to generate a pelletizedproduct. D y fine material recycling and backmixing in the upstream mechanical r,mixer is extremely important because it forms wet pellets (50-65%TS) in the mixerwhich are subsequently dried in the dryer resulting in high quality pelletizedproduct with 90-95%TS. Two types of direct rotary drum dryers are used in biosolids drying: single and triple pass. The triple pass dryer (Figure 6-2, courtesy Baker-Rullman, Inc.) technical data is provided in Table 6-3. Fourteen single pass and nineteen triple pass rotary drum dryers are currently (1995) used in the USA for biosolids drying and pelletizing.TABLE 6-3 TRIPLE PASS ROTARY DRUM DRYER (courtesy of Baker-Rullman, Inc.) 1. Biosolidsfeed at 20% TS; product at 2 90% TS; dry recycle; 24 hour day. 2. Dry air inlet = 480°C (900"F), outlet = 85°C (180°F). 3. MBH = million BTU/hr; ACFM = actual cubicfeet per minute; dmtd = dry metric ton per day.
MAS H UT TO APCCOMAIR ----- Fig.6-2 TRIPLE: PASS ROTARY DRUM D R n R (Courtesy of Baker Rullman Co.) l ) B U M R , 2)FURh?ACE, 9)OUTER INSULATED SHELL, 4)INTEMEDU TE DRUU, 5)L"ER DRUM, S)SEALS, ?)A.lR-SGLIDS SEPARATOR, 8)DRNB BASE. 9)FUGHTS. 1O)IDLER BASE
286 Girovich Numerous other single and triple pass dryers are used in Canada, Europe andJapan for biosolids drying. The rotary drum dryer is the most widely used designwith the lowest capital cost. Disadvantages of direct rotary dryers, especially ones without drying airrecirculation (so called open-cycle system), include: heat losses in the dryersexhaust; use of expensive systems to clean large amounts of air from particulateand gaseous pollutants including odors often with application of costlyafterburners; potential for dust fire and explosion, Flash Drver (FD) is a direct type, cocurrent unit that is essentially a longvertical tube with no moving parts (similar to pneumatic conveyance). The FDshave short solids residence time (0.5-3.5 seconds) and high unit evaporation rate.Good mass and heat transfer rates are obtained due to high relative velocitybetween the feed and drying air. Typically, @/wet biosolids mixture is fed in a cage mill where hot gas is alsointroduced. Drying occurs in the mill, ductwork and partially in the cyclones.Dried material is separated from drying air in the cyclones. Vapors are burned inthe heat generator ( h a c e ) . The FD do not produce high quality pellets. The FDsare used in biosolids drying in the USA by the City of Houston, Texas (nine flashdryers by C-E RaymondCombustion Engineering Co.). Houactinite, a biosolids derived product produced in Houston is marketed primarily in Florida. Disadvantages are: complex operation and potential for fire. Fluid bed h e r s fFBD) are also direct, cocurrent units. The FBD involves the suspension of solid particles in upwardly moving hot air, which is introducedthrough a distribution plate. The FBD are usually of direct type, however, in recent years indirect FBD have been introduced. The biosolids feed must be granular and backrnixing and feed grinding before drying is typically required; particles should not be so heavy that they drop out. Retention time is very short (few seconds) because air velocity must be kept above a certain minimum. The FBD do not produce a high quality pelletized product; however, the end product is marketable. Numerous fluid bed dryer designs are available (stationary, vibrated fluid bed, continuous fluid bed, mechanically agitated, etc.) However, only a few experimental FBDs for biosolids fertilizer production are in operation in the USA. Disadvantages include: poorly granulated product; potential for dust fires/explosions and complex operation. Convevor Drvers use steel belt, screw or vibrating trough to carry the biosolids through a directly or indirectly heated chamber. Depending upon the design, hot air may or may not pass through the bed of biosolids on the conveyor. Indirect conveyor dryer may use radiation as a means of heating.
Thermal Treatment 287B. Indirect DryersIndirect dryers also come in a variety of designs. The most popular design ishorizontal, "en-masse" (i.e., moving, mixing and drying biosolids in a plug or semi-plug flow fashion) type. In this design, biosolids are simultaneously heated, driedand transported horizontally from the feed to the discharge end. Other types ofindirect dryers include multi-tray, fluidized bed, rotary drum and conveyor designs. Indirect "en masse" dryers come in a variety of shapes and forms. Some unitshave single rotors while others may have two or more. Flights may be fullycircular (disc), helical (heated screws), blade-like (paddles) or circular segments.Flight cross sections may be tapered, parallel, or wedge shaped. The stator(housing) is horizontal and it may be cylindrical, U-shaped or ox-bow in section. The indirect dryers use very little or no drying air and as a result they aregenerally more energy efficient than direct ones. The energy required to operatedryers is delivered by an external energy source, typically steam boiler or thermaloil heater. The heat is transferred from hot metal surface to biosolids. A significant number of proprietary horizontal "en massel designs have beendeveloped by various manufacturers (Stord, Bepex, BUSS, Blaw-Knox,Tsukishima Kikai, Nara Machines, Komline-Sanderson, and others). Indirect disc dryer (Figure 6-3, courtesy of Stord, Inc.) data is provided in Table 6-4. In the disc (flight) dryer, the external surface of the discs (flights) provides 85 to 100% of the total heat transfer surface of the dryer. The heat transfer medium, usually steam, flows into the discs (flights), and is discharged as condensate after transferring its latent energy to the product. Other thermal mediums, such as hot water or hot oil, may be used as well. The filly circular, hollow discs of the Stord dryer (Figure 6-3) provide a smooth heat transfer surface, which reduces fouling, product accumulation in "dead" spots and wear. The rotor rotates at a constant speed . The discs are fabricated of carbon or stainless steel, or a combination of both. The stator is filled with biosolids so that the rotor is submerged and the heating surface is almost fully utilized. Agitator (scraper) bars are mounted fiom the stator, ending just above the rotor shaft at the base of the discs. Large paddles are mounted on the tips of the discs and each disc has a combination of neutral and adjustable paddles. A portion of the upper part of the stator extends into a plenum to form a vapor dome. The vapor exits from the dome near the product inlet where the evaporation rate is the greatest. Wet biosolids enter the dryer near the top of the unit on one end and the finished product exits the dryer at the opposite end. The dryer is operated as a lclosedl unit. No air is deliberately allowed to enter and the exhaust is nearly pure
h) 00 00 EXHAUST / IFig.6-3 INDIRECT DISC DRYER (Courtesy of Stord, Inc.) 1)EOLLO W STATOR (HOUSIUC), 2)HOLLOW ROTOR. 3)PRODUCT DISCBdRCB. P 4)CONDENSAT.E OUTLET, S)rrouoW DISCS, 8 W E R BARS, 7)VBpoR DOME, 8 BXaaUST OUTLET, 0)DRIVR. i0)SU PORTS, 1l)DISCS PADDLES, i2)KATBAIdt INLET, 13)STYU.M INLET
Thermal Treatment 289TABLE 6- 4 DISC DRYER TECHNICAL DATA Parameter Unit Range Heating Surface ;I 1 Evaporation Capacity 450 - 3,500 I 3 I Weight I tons, metric 12 - 75 I Horsepower kLca1 30 - 200 Number of discs Specific Thermal Rate 650 - 670() I (STR) I kg H2O I I 7 Specific Evaporation Rate kg H,O (SEW m2 hr.1. Feed: 22% - 30% TS; product: 25% - 95% TS; no recycle, constant rate heat transfer.2. Product at 95% TS. SER increases to 16.0 ifproduct i at 35% TS. Lower s values of STR and SER are for higher product dryness. 100% surface utilization.3. SER - Specific evaporation rate, STR - Specific Thermal Rate, see Section IV of this Chapter.vapor. It is intended that biosolids co-rotate with the rotor. However, simultaneousco-rotation is prevented by the agitator bars. The disc tip paddles aid theprogression of the product in both rotational and longitudinal directions. Thebiosolids travel through the dryer in a plug flow fashion, and they must passthrough the annulus formed between the inner wall of the stator and the tips of thediscs. Heat is transferred from the heat transfer surface to the biosolids by contactand by internal distribution of the heat in the product. Disc dryers manufactured by Stord, Inc. are used in the USA primarily forproduction of partially dried biosolids for landfilling, land application orsubsequent incineration. In order to produce biosolids pellets, mechanicalpelletizing equipment is added to the dryer. Twelve Stord disc dryers are currentlyused to dry biosolids in the USA. Five of them produce dried biosolids forsubsequent incineration (Buffalo, NY; Pittsburgh, PA; Los Angeles, CA).
F’ig.6-4 SOLID PADDLE DRYER (Courtesy of Tsukishima Kikai Co.) 1 ) 4 I R T E W INLET, 2)STEAM INLET, 3)EXlUUST OUTLET, 4)SOLID PADDLES, 5)CONDENUTE OUTLET, 6 HOLLOW ROTOR, 7)MTERI.U OUTLET. 8)HOLLOW 0 STATOA. @)SUPPORTS, 1 OLRIVE
Thermal Treatment 291TABLE 6-5 NARA PADDLE DRYER TECHNICAL DATA (Courtesy of Komline Sanderson) Parameter Unit Evaporation Capacity kg/hr 35 - 1,500 1,600 - 3,000 3 4 Weight Horsepower tons, metric kw 1-50 2-95 I I 40 - 70 45 - 95 5 Specific Thermal Rate kcal 650 - 680() (STR) kg H2O Specific Evaporation 10 - 16") Rate (SER) m2 8 hr. Lower values are for higher product dryness. Indirect two or four shaft Nara Daddle drvers manufactured by Komline-Sanderson, Inc. have hollow steam (or oil) heated rotors and hollow heated wedgeshaped, self-cleaning blades. The omega shaped stator (trough) is also heated.Typical rotor speed is 10 - 30 rpm. Major Nara paddle dryer technical data areprovided in Table 6-5. Other types of indirect horizontal dryers, solid paddle and inclined flight areshown in Figures 6-4 and 6-5 respectively (courtesy of Tsukishima Kikai Co.). Solid paddle dryer (Figure 6-4) has multiple solid (not heated) paddlesinstalled onto the hollow steam heated rotor contained in the hollow steam heatedstator (housing). Heat is conducted through stator and rotor to the biosolids. Rotormoves at relatively high circumferential speed (5 - 15 m/sec). The paddlesperform multiple actions: agitate, scrap, transfer and granulate (particle size: 2 -15 mm). Disadvantages are: short retention time (1 - 5 min.); high productmoisture content (40% - 6O%TS); relatively high electric power demand.Currently no paddle dryers of this type are in use in the USA. Paddle dryers areused mainly in Japan to dry biosolids prior to incineration. Specific evaporationrate (SER) is in the 9 to 18 (kg H20/m28 hr.) range. Lower value of the SER is forhigher product dryness. Steam heated inclined flight dryer (Figure 6-5) originates from the jacketeddual-shaft screw conveyors. Both rotor and stator conduct heat. Biosolids aretransported and mixed by solid (not heated) flights in a plug-flow fashioneliminating the need for paddles or scraper bars. The rotor speed is relatively high
N (0 NFig.6-5 INCLINED FLIGHT DRYER (Courtesy of Tsukishima Kikai Co.) 1)AlilTERUL INLET. 2)EXlUUST OUTLET, S)PADDLES, 4)INCUNED FLIGHTS, 5)VdpoR DOME, 6)DAM ADJUSTER, 7)CONDBNSATE OUTUT, 8)MBTERUL OUTLET, 9)HOLLO W ROTOR, 1O)HOLLO W STATOR, 1 l)SUPFORTS, I2)DRWE. f 3)STEAhf I N U T
Thermal Treatment 293(30 - 45 rpm) which ensures good mixing and heat transfer. Typical productdryness is in 45 - 60% TS; SER is 10 - 20 kg H,0/m2 -hr. Granulation is poor (5- 40 mm, with lumps). These dryers are also used in Japan for drying prior toincineration or for volume reduction. Advantages of the en masse horizontal dryers include: compact, horizontalarrangement; effective utilization of heating surface; relatively high energyefficiencyand low cost of environmentalcontrol. Disadvantagesare: end productis poorly granulated, dusty with lumps. Production of high quality pellets is notfeasible without additional processing ( e g , mechanical compaction, milling,screening, etc.). Indirect multi-tray drver (Figure 6-6, courtesy of Wheelabrator Clean WaterSystems, Inc.) is a vertical multi-tray dryer to which biosolids are fed via the topinlet and moved by rotating arms equipped with adjustable scrapers from onehollow, oil heated tray to another in a zigzag motion until they exit at the bottomas a dried, pelletized product with dryness in excess of 90% TS. The rotating armsare installed on a vertical shaft rotated by a variable speed drive. The dryers doughnut-shaped trays are heated by recirculating thermal oil. The dryers scrapers move and tumble biosolids over the trays heated metal surface in small windrows and thin layers (30-60 mm) constantly exposing new surfaces for heating and thus improving heat transfer. Only top surface of the tray is in contact with biosolids. Multi-tray dryers are typically fed by a biosolids mixture with 55-75% TS to avoid glue-like plastic phase inside the dryer and, therefore, they require dried material recycle and backmixing. The biosolids mixture is prepared upstream of the dryer by mixing dry fine recycled material (2 90% TS) with wet biosolids (20-30% TS). In the upstream mixer, dry particles, which form a nucleus of hture pellets, are coated with wet biosolids-forming pellets with a dry core and thin wet coat. In the dryer, only the outside thin (0.1-0.2 mm), wet layer has to be dried. In the drying systems based on the multi-tray dryer this process is repeated several times until the pellets grow to a predetermined size and then removed for beneficial use. As a result of this "pearling"process, water evaporation is more effective and heat transfer rates are higher than in the "en masse" dryers. Pellets produced by this process tend to have higher bulk density and less dust than other dryers. Indirect multi-tray dryers are licensed from Seghers Engineering Company (Belgium) and marketed under a trade-name PelletechO by Wheelabrator Clean Water Systems, Inc., Bio Gro Division [ll], 11141. Major multi-tray PelletechB technical data are provided in Table 6-6. Advantages of the multitray dryer are: low cost of environmental control; high heat transfer rates; low power consumption; high quality of pellets with narrow particle size distribution and minimum dust; small footprint due to vertical arrangement.
294 Girovich Fig.6-6 INDIRECT MuLmRAY (PELLeTECH@) DRYER (Courtesy of Wheelabrator Clean Water System, Inc.) 1)INSLVATED SEE& 2 E O U Y TRdYS, 3 ROTATING ARMS WITa SCRAPERS. 4 ) T d R W L OIL INLET .dlNIFOLD, 6)S€L4FT, 6 ) D i U W . 7)OUTLET OIL MANIFOLD, 8)UPPER BBdRINcr @)LOWER BZWUNG 1O)PRODUCT LNUT AND 0 U m T VXLVZS, 1I)DEFLECTOR. 12)TRdYS SKIRTS, IS)ClXAN-UP SCRMER, 14)SUPPORTS
Thermal Treatment 295 Disadvantages are: significant vertical space required; relatively high weightand cost; only half of the heating surface is fully utilized.TABLE 6-6 MULTI-TRAY PELLETECHO DRYER TECHNICAL DATA (courtesy of WCWS)I. Only top tray surface is in contact with biosolids.2. Feed @ 65% TS; pellets @ 2 90% TS; dry recycle; oil @ (500°F). 3. Lower values are for higherfeed dryness. IV. MAJOR DRYER PARAMETERS A. Evaporation Capacity Biosolids quantity is expressed in either dry or wet weights. Dry and wet weights are related as follows: D =Wed ............................ (6-1) D = dry weight W = wet weight d = biosolids dryness, TOTS
296 Girovich 20 25 30 FEED DRYNESS, X TS Fig.6-7 EVAPORATION CAPACITY AND ENERGY DEMAND VS. FEED DRYNESS A dryers throughput, whether it is direct or indirect, can be measured by itsevaporation capacity and by its dry weight capacity. Traditionally in the U.S.throughput is expressed in dry tonnage (e.g., dry ton per day); however, it is oftenmore useful to express production in terms of evaporation capacity (e.g., tons ofwater evaporated per unit of time). The relationship between the dryersevaporation capacity (E) and dry weight capacity (D) is: E =D (k i)- . . . . . . . . . . . . . . . . . . (6-2)E = evaporation capacity, i.e., weight of water evaporated per unit of time;D = dry weight capacity (throughput), i.e., weight of biosolids dried to 100% TSper unit of time;d , do = biosolids input and output dryness (%TS).
Thermal Treatment 297 For example, if the biosolids input dryness is 22% TS, the end productdryness is 92% TS, and dry biosolids throughput required is 1.O dry tonhour, thenthe evaporation rate needed is E = 1.0(1/.22 - U.92) = 3.46 tons of water.Evaporation rate more than any other process parameter determines the size and,therefore, cost of the dryer and drying system. Biosolids feed dryness is the most significant process parameter whichdetermines the dryer energy requirements. Figure 6-8 illustrates that at thebiosolids feed dryness of 30% TS the evaporation rate and amount of energyrequired to dry one ton (2,000 lbs.) of dry biosolids is almost two times less thanat the feed dryness of 18% TS. Assuming the cost of energy at $4.50 per million BTU, the feed dryness increase from 18% TS to 30% TS will save approximately $34 per dry ton of biosolids. Figure 6-8 also illustrates that the higher feed dryness increases the drying systems evaporation rate or, in other words, the same system can process more biosolids if the feed has higher solids content. For indirect dryers, it is also useful to express the dryers evaporation capacity as amount of water evaporated per unit of heating surface per unit of time, i.e., by using Specific Evaporation Rate (SER). The SER indicates how efficient the indirect dryer heating surface is utilized. . . . . . . . . . . . . . . . . (6-3) Where: SER = specific evaporation rate, kg/m**hror BTU/ft2*hr E = total dryer evaporation capacity, kg (lbs) of water evaporated per hour S = indirect dryer heating surface, mz (ft2). Heat transfer in the indirect dryers occurs typically in several zones: heating, constant rate drying and falling rate drying. For each zone, the SER will be different. The SER can be obtained from a dryers manufacturer. The SER allows one to quickly estimate the amount of heating surface for a required evaporation capacity. B. Energy and Drying Air Requirements All dryers, direct or indirect, require an external source of heat energy. Usually dryers are fired on natural gas or oil. Digester gas or other sources of heat energy can also be utilized.
298 Girovich A dryer can be characterized by the amount of heat energy required toevaporate a weight unit of water expressed as kilocalories per kilogram or BTU perpound of water evaporated (kcaykg or BTU/lb). Total amount of heat energy required by a dryer is: QT= Q l + Q2+ Q,+ Q4 . . . . . . . . . . . . . . (6-4)Q1= energy required to heat dry substance in the feed;Q2= energy required to heat water in the feed up to an evaporation temperature;Q3= energy required to evaporate water and to heat water vapor;Q4= dryers heat losses (for well insulated dryers it is usually less than 1% of totalenergy required). Q, is the largest energy component (approximately 970 BTU per pound ofwater, or 540 kcalkg at standard conditions). The heat losses also occur outside the dryers (in conveyance, feed and producthandling systems, air pollution control, afterburners, etc.) and, as a result, theoverall drying systems energy demand is considerablyhigher than that of the dryerproper, usually by 10% to 20%depending on the drying system design and energyrecovery provisions. It is useful to characterize dryers by their specific thermal rate (STR), i.e.,amount of heat energy required to evaporate a unit of water. The greater the STRthe less efficient is the dryer.STR = Specific Thermal Rate, kcaykg (BTU/lb) of water evaporated;QT= total amount of energy required, kcal (BTU) per hour;E = evaporation capacity, kg (lbs) of water evaporated. The average STR for modem dryers is typically in the 640 - 780 kcal per kgof water range (1,150 - 1,400 BTU/lb) depending on inlet and outlet temperaturesof drying and heating mediums, dryers throughput, design, etc. As with the SERs,the STRs are different for different heating zones. The STRs are provided andmust be guaranteed by a dryers manufacturer. The overall drying systems STR is higher than that of the dryer proper. M s asand energy balance calculations are performed to determine the dryers heat
Thermal Treatment 299requirements, heating surface, heating medium flow, evaporation rate, and otherdryer parameters. The following input data are typically required:0 biosolids feed rate, moisture content and inlet temperature product moisture content and temperature drying air conditions (for direct dryers) heating medium type, temperature and heat transfer rates (for indirect dryers) Major calculating needs are associated with determination of heat needed forevaporation and, for direct dryers, amount of drying air. Calculations alone do notcomplete a design. Experience, manufacturers data and tests are essentialcomponents of dryer and drying process design.a. Indirect DryersHeat transfer in the indirect dryers occurs typically in three zones: heating,constant rate and falling rate drying. In the first zone, wet biosolids are heated by hot metal surface of the dryerand the heat transfer coefficient is initially high due to high temperature differentialand abundance of free water. As biosolids temperature rises, the heat transferbegins to fall. In the second zone, the entire mass is heated almost to a boilingpoint (100°C) and evaporation of ffee water is essentially completed at the end ofthe zone. This zone is characterized by an approximately constant heat transfercoefficient. In the third zone, water bound in the biosolids is evaporated, and as thebiosolids dryness increases it becomes more and more difficult to evaporate theremaining water. The heat transfer coefficient falls sharply. Figure 6-8 illustrates an indirect drying process in the horizontal steam heateddryer. Moisture content of the feed and heat transfer rates are plotted againstdryers length . Typically, about 15 percent of dryer length (heating surface) isused to reach the boiling point, 25 percent of the length is at the constant and 60percent is at the falling heat transfer rates. For each zone, amount of heat energy transferred from the heating surface tothe product is defined as follows (heat losses are not included):Q = heat transferred, kcaVhr or BTU/hr;S = heating surface, m2(ft2);T = log mean temperature difference, "C (OF); ,U = heat transfer coefficient (different for different zones);kcaVm2*"C-hror BTU/ftz*"F*hr
300 Girovich . . . . . .. . . . 05-71ti, to= biosolids temperatures, inlet and outlet, "C (OF);Ti, To= heating medium temperatures, inlet and outlet (steam, oil); "C ( O F ) Heat transfer coefficients, U,are typically in the 40 - 80 kcaVm2*"C=hr 8 or- 16 BTU/ft2-"Phr range with higher values for the constant rate zone. Indirect dryer calculations are based on mass and energy balance using heattransfer coefficients obtained experimentally for various drying zones. Thesecalculations are quite straightforward and center around determining evaporationcapacity, heat energy and amount of heating medium flow needed, taking intoaccount feed and product moisture and temperature, heat medium type (steam, oil)and temperatures, heat sources and losses, etc.Thermal Fluid vs. SteamThermal fluid heating systems provide certain advantages as compared to steam. Thermal fluid heating systems are used typically for process temperaturesbeyond the range of steam. It is often overlooked that thermal fluid systems areeconomical compared to steam heating, even at temperatures where steam can beused easily. As the steam temperatures increase, so does pressure. For a steamtemperature of 187"C, the steam pressure is 10.54 kg/cm2 (150 psig). Thisincreases to 30 kg/cm2(425 psig) for steam temperatures of 230°C. Such highpressure steam systems are expensive to install and operate. This is the mainreason for selecting thermal fluid versus steam. Heat transfer fluids (thermal oil) are manufactured from refmed petroleum,synthetically formulated hydrocarbons or siloxanes (silicone). They are able toprovide high temperature at very low vapor pressures. Thermal oil systems, if usedproperly, offer safety, low maintenance and extended operating lifetimes. For process temperatures of about and above, thermal fluid heating issuperior to steam heating in almost all respects. Thermal fluid heating uses only sensible heat since it is in a liquid state. Hence, there are no "condensate losses." This is one of the reasons why the thermal fluid heating system has better efficiency than steam. Thermal fluids have relatively high boiling points, depending upon their application. The operating pressures of thermal fluid systems are from 5 to 15 kg/cm2.
m U LO4 rI &: 2,000 I _-- -1----1---- - - --I---- r - E I I I I92 1.600 2. 40n s B vi UJ Y 1,200 - E 30 40 & 8 n3 V t; zEF aoo - 20 60 3 80E8 ! I -l a nv 400 - 5 10 80 I v8 1 3 I I 1 I 0 I 20 40 1 60 a0 ___t 100% PERCENT OF DISTANCE THROUGH DRYER HEATING1 CONSTANT RATE L FALUNG RATE ZONE ZONE ZONE FIG.6-8 INDIRECT DRYER HEAT TRANSFER COEFFICIENT U, STR AND SER.
302 Girovich The selection of thermal fluids is essentially dependent on the maximumtemperature required by the process. Each thermal fluid has a maximumtemperature recommended by the manufacturer. The life of the fluid is dependenton the maximum film temperature and not necessarily the bulk temperatures.When the fluid is being heated, the film of fluid adjacent to the tubewall andexposed to the flame is at a higher temperature than the average temperature of thefluid at that point. For a well-designed heater, the film temperature would be about30°C higher than the bulk temperature. The heart of the thermal fluid system is a heater where the fluid is heated.The hot fluid is pumped to the process to give up its heat and then returned to the heater. The fluid expands considerably during heating so an expansion tank is required to prevent system overpressure. The thermal fluid heater is generally a forced circulation unit. Sufficient velocities, about 3 to 5 m/s, are maintained in the coil to prevent overheating of the fluid. The licensed boiler engineer is not required to operate thermal oil heater. The thermal fluid system does have disadvantages: Petroleum-based thermal fluids are flammable, and some of them are toxic, so precautions have to be taken. Accidents have happened, but very few in chemical plants with good maintenance. Care must be taken in using thermal fluid systems because any leakage may have serious consequences. Temperature control is more complex than in a condensing system (like steam) though temperature control of 2 to can be achieved. The heat transfer coefficients on the thermal fluid side are lower than steam condensing coefficients. But the higher temperature of thermal fluid increases the overall heat transfer rate. In practice, the heat transfer rate is always equal to or more than that of steam. When the process needs a cooling cycle, design of heat exchanger for cooling the thermal fluid is quite complex. Measures must be taken to prevent oil leakage, to avoid water in the system and maintain the systems tightness and cleanliness. Only high temperature closed- cell insulation should be used in leakage-prone areas; the systems equipment (pipes, pumps, expansion joints, strainers, gaskets, packing seals, flanges, etc.) must be in accordance with pertinent standards and best engineering practice. The amount of thermal oil or steam flow for indirect drying can be determined using formulae (6-8), (6-9) and (6-10) below. Formulae (6-9) and (6- 10) are used for noncondensing heating medium (thermal oil, hot water) and formula (6-8) for the condensing medium (steam).
Thermal Treatment 303 . . . . . . . . . . . . . . . (6-8) .............. (6-9)Formula (6-9) can be written as follows: F = QT . . . . . . . . . . . . . (6-10) T0)*60 CD*d*(Ti-Where:m = mass flow of heating medium, kg/hr (lbhr)Q T = total amount of energy required, kcaVhr (BTUhr)C, = specific heat, kcaVkg."C or BTU/lb*"FTi, To= heating medium temperature, inlet and outlet, "C ( O F )hi, h, = heating medium enthalpies, inlet and outlet, kcaVkg (BTU/lb)d = density of thermal oil at operating temperature, Ib/galF =thermal oil flow, gpm For example, to evaporate four (4.0) tons of water per hour in an indirect dryer with the STR = 1,260 BTU/lb, the amount of heat energy required is: QT= STR x 4.0 x 2,000 = 1,260 x 4.0 x 2,000 = 10 million BTU/hr, approx. (STR, Specific Thermal Rate, can be obtained kom the dryers manufacturer data). Using formula (6-lo), (thermal oil with C = 0.65, d = 6.2 lb/gal; Ti = 450°F; , To= 410°F) flow of thermal oil through the dryer: F = 10 x lo6 : 0.65 : 6.2: (450 - 410) : 60 = 1,034 gpm b. Direct Drvers Calculations for direct dryers with hot air as a drying medium are quite different from those for indirect dryers. They are also based on mass and energy balance but involve the psychrometric relations of moist air.
304 Girovich In the direct dryers, as soon as wet biosolids particles come into contact withdrying air, evaporation begins from the saturated vapor film which is quicklyestablished at the particles surface. The temperature of the biosolids particlesurface approximates to the wet-bulb temperature of the drying air. At fust, thereis sufficient moisture within the particle to replenish that lost at the surface.Unbound moisture transport from within the particle maintains saturated surfaceconditions and, as long as this is possible, evaporation proceeds at a constant rate.After a dried shell forms at the particles surface, evaporation is dependent largelyupon the moisture diffusion through the dried surface and, as a result, evaporationrates begin to fall. The addition of heat to a wet particle is insufficient by itself to dry a particle.Removal of moisture in the direct drying depends upon humidity (moisturecontent) of the drying air. To maintain high drying rates, cool humid air around aparticle must be moved away and replaced with hot, low humid air. Humidity (amount of water vapor or absolute humidity) defines the moisturecontent of the air and it depends upon the air temperature and is independent of airpressure. Absolute humidity is expressed as weight of water vapor per unit weightof dry air (kgkg DA). Absolute humidities for air-water mixtures (i.e., drying air)are read directly from psychrometric charts (enthalpy/temperature--humidity).Relative humidity is water vapor content expressed relative to water content at saturation at the temperature of the mixture. The mixture relative humidity at saturation is designated 100%. Saturation temperature (dew point) is the temperature to which wet air must be cooled at constant pressure to cause condensation. The dry bulb temperature of air-water mixture can be recorded by an ordinary thermometer. Wet bulb temperature is recorded by a thermometer whose surface is saturated with liquid water and exposed to an air-water vapor mixture. The wet bulb temperature is lower than dry bulb temperature. The wet bulb temperature corresponds to the temperature at which the air would be saturated without any change in its heat content. For the saturated mixture wet and dry bulb and dew point temperatures are the same. Air-water mixtures are unique in that for all practical purposes, wet bulb and adiabatic saturation temperatures can be considered identical and wet bulb and adiabatic cooling lines coincide on psychrometric charts. Psychrometric charts allow determination of important direct drying process parameters such as evaporation rates, energy demand and efficiency, drying air parameters, wet bulb temperature, particle temperature, etc. Psychrometric charts are often used to illustrate or calculate direct drying operations. Several forms of psychrometric charts are used. Mollier charts plot moisture against enthalpy (in metric units) and are widely used in Europe and elsewhere outside the US. (In the US, Mollier diagram refers to an enthalpy-entropy chart). The most common US psychrometric chart plots temperature and enthalpy on the ordinate (Y-axis) against humidity in kgkg of dry
Thermal Treatment 305air (DA) on the abscissa (X-axis). Psychrometric diagrams along withcomputerized calculations are used by direct and indirect dryer manufacturers.These calculations are design specific and the reader is referred to specializedsources and manufacturers data , , . Typically, dryer calculations are based on a computer program developed bya dryer manufacturer and vary from one type of dryer to another. Calculations,however, cannot complete a design. Empirical data, tests, and rules of thumb arerequired to transform data into a successful drying system design. However, a simplified method of determining two major direct dryerparameters, drying air flow and total heat requirements which are essential forsizing and estimating the price of the drying system is described below . The nomograph in Figure 6-9 depicts dryer inlet (heavier curved lines) andoutlet temperatures (lighter curved lines). Drying air flow in cubic feet per minuteper pound of evaporated water (cfm/lb) is plotted on the abscissa against specificthermal rate (STR in BTU per pound of water evaporated) plotted on the ordinate. The nomograph is based on typical conditions: open cycle drying (oncethrough drying air flow); atmospheric pressure; air supply temperature 60°F and0.007 pounds of moisture per pound of dry air; feed at 60°F and 50% TS; heat loss-0.5%. With natural gas or indirect heat the error in the airflow rate is less than 0.5%.To correct the heat load, add 4% for natural gas heating and subtract 7.5% forindirect heating. Both values are roughly 9% higher at 80% TS and 9% lower at20% TS in the feed . As an example, an 800°F inlet and 200°F outlet temperature linesintersection has coordinates of 2.75 and 1500 approximately (Figure 6-9). Thus,the evaporation rate of 8,000 l b s h (4 to*), an airflow of 2.75 x 8,000 = 22,000cfm (at the dryer outlet) and a total heat energy demand of 1,500 x 8,000 = 12million B T U h are required.V. HEAT DRYING SYSTEMS (€IDS)A. System ComponentsDryers comprise an important,but not the only part of a heat drying system (HDS).Each HDS contains the following major components: (1) Dryer (2) Heat Generator (Furnace) (3) Feed Preparation and Handling (4) Dried Product Preparation and Handling (5) Air Pollution and Odor Control
UE0 Drying Air Flow/Evaporation, CFM per poundhour of evaporated waterFig. 6-9 DRYING AIR FLOW AND ENERGY DEMAND IN DIRECT DRYERS
Thermal Treatment 3071. Heat Generator (Furnace)All HDS require an external source of energy to heat the drying air (direct drying)or heating medium (indirect drying) such as thermal oil or steam. Typically,natural gas is used by a majority of the existing HDS; however, digester gas andwaste heat from other sources (e.g., incinerator flue gas, steam, gas turbine exhaust,etc) is utilized also. For direct HDS, the heat generator (furnace) fired on natural gas is usually acylindrical refractory lined chamber with a natural gas or dual fuel burner installedat the front end. Combustion air fan, burner controls and necessary ductwork arealso part of the heat generator. The burner combustion products reach hightemperature of 1700-1800°C (3 100-3250°F). For direct drying this temperatureis reduced to an acceptable level of 37W50"C (700-850°F) prior to introductionin the dryer by addition of ambient air or air vented from other parts of the HDS,other equipment, housing building, etc. In indirect drying, high temperature combustion products produce steam ina boiler or heat thermal oil, water or air indirectly.2. Feed Preparation and HandlingEach HDS requires that the feed (mechanically dewatered biosolids or mixture ofDWB and dry recycled product) be prepared prior to entering the HDS. CommonlyDWB is either conveyed into the dryer without mixing with dry recycle orDWB/dry recycle mixture is prepared in a mixer (e.g., rotary drum, multi-tray,fluid bed, flash dryers, etc.). Formation of wet pellets occurs in the mixer andselection of the mixers design is very important. Single shaft, high speed pinmixers or dual shaft relatively slow speed ( 2 W O RPM) paddle mixers (pug mills)are typically used. Measures must be taken to reduce wear of pins or paddles usingabrasion resistant alloys. Wet pellets should be preserved from deterioration asmuch as possible prior to their introduction into the dryer.3. Dried Product Preparation and HandlingIn the modem HDS which produce pelletized biosolids, dry product (290%TS)exiting the dryer typically undergoes the following treatment: (1) screening to separate fine, oversized and on-spec ( 1 4 mm) size fractions; (2) recycling the fine fraction to mixer; (3) reducing (crushing) the oversized fraction and recycling the fine material to mixer;
308 Girovich (4) cooling the "on-spec", commercial size pellets prior to storage to no less than 30°C (90°F) to avoid problems in storage; (5) storing the commercial pellet fertilizer in silos provided with overheating and fire and explosion prevention devices. In the HDS using heat drying with subsequent mechanical pelletizingadditional treatment of dry product is required (i.e., compaction, screening, milling,etc.).4. Air Pollution and Odor ControlControl of particulate matter (PM) and gaseous pollutants including malodorouscompounds has played an extremely important role in the evolution of the HDS.With air emission and odor control regulations becoming more stringent on federaland state levels and ever growing public concern about environmental impact ofany industrial process the utmost attention should be given to the design of theHDS air pollution and odor control equipment. Historically, removal of particulate matter (dust) was the prime co