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  • 1. SLUDGE TREATMENT IN WASTEWATER: AEROBIC AND ANAEROBIC DIGESTION Ryerson University Wastewater Engineering ES8907 Winter 2010 ZAKI ABEDEEN 8/4/2010 8/4/2010 Wastewater Engineering
  • 2. OUTLINE
    • Introduction to Sludge Treatment
    • Anaerobic Digestion
      • Fundamentals
      • Chemical Pathways
      • Types of Anaerobic Digesters
      • Anaerobic Digester Design
        • Digester Volume
        • Mean Cell Residence Time
      • Digester Operation
      • Moisture/Weight Relationships
      • Digester Heat Requirements
      • Fluid Properties of Sludge
      • Sludge Thickening / Dewatering
    • Aerobic Digestion
      • Fundamentals
      • Types of Aerobic Digesters
      • Kinetics of Aerobic Digestion
      • Design: Mean Cell Residence Time
    • References
    Outline Wastewater Engineering
  • 3. Introduction
    • What is “ Sludge ” ?
      • The term “Sludge” refers to the solids that are separated from a liquid containing suspended solids, when passed through a settling tank.
    Sludge Treatment Wastewater Engineering
  • 4. Introduction: Types of Sludge
    • In a wastewater treatment plant, organic sludge may originate from several sources :
      • 1. Primary (“raw”) sludge – sludge from primary settling of untreated wastewater.
      • Waste activated sludge (WAS) – excess sludge produced by the activated sludge process.
      • Trickling filter secondary sludge (humus) – sludge from secondary settling of trickling filter effluent.
      • Secondary sludge – sludge from secondary clarifier of activated sludge / trickling filter process.
      • Digested sludge – sludge that has undergone biological oxidation.
      • Dewatered sludge – sludge that has had most of the water removed from it.
    Sludge Treatment Wastewater Engineering
  • 5. Sludge in Wastewater Treatment Facility Sludge Treatment Wastewater Engineering Sources of Sludge
  • 6. Anaerobic Digestion Anaerobic Digestion Wastewater Engineering
  • 7. Introduction: Anaerobic Digestion
    • Anaerobic Digestion may be defined as being the biological oxidation of degradable organic sludge by microbes under anaerobic conditions.
    • This process is employed for the treatment of organic sludges and concentrated organic industrial wastes.
    • Most microbes used in this process are facultative anaerobes.
    Anaerobic Digestion Wastewater Engineering
  • 8. Fundamentals of Anaerobic Digestion
    • Anaerobic digestion occurs in an environment with no molecular oxygen and substantial organic matter.
    • The organic material is a food source for the microbes.
    • The microbes convert the food into oxidized materials, new cells, energy, and some gaseous end products (such as CH 4 and CO 2 ).
    • Generalized Equation
    • Organic matter + combined O 2 new cells + energy + CH 4 + CO 2 + anaerobic microbes end products
    • Sources of combined O 2 : CO 3 2- , SO 4 2- , NO 3 - , PO 4 3-
    • Some of the end products: H 2 S , H 2 , N 2
    Anaerobic Digestion Wastewater Engineering
  • 9. Fundamentals of Anaerobic Digestion
    • Microbial action by the anaerobic bacteria consists of 3 stages:
      • Liquefaction of solids ( hydrolysis )
      • Digestion of soluble solids ( by acid-forming bacteria )
      • Gas production ( methanogenesis )
    • The breakdown of the three major food groups are as follows:
      • Carbohydrates Simple sugars Alcohols Organic acids
    • Aldehydes
      • Proteins Amino acids Organic acids + NH3
      • Fats and Oils Organic acids
    Anaerobic Digestion Wastewater Engineering “ volatile acids”
  • 10. Fundamentals of Anaerobic Digestion
    • Most of the bacteria forming these organic acids are facultative anaerobes and are found in soil. There are (i) organic acid forming heterotrophs and (ii) methane producing heterotrophs .
    • The genera of microbes responsible for anaerobic digestion are:
      • Pseudomas
      • Flavobacterium
      • Alcaligenes
      • Escherichia
      • Aerobacter
    • These microbes thrive in a wide pH range.
    Anaerobic Digestion Wastewater Engineering Pseudomonas Escherichia (www.wikipedia.org)
  • 11. Fundamentals of Anaerobic Digestion
    • The methane producing bacteria are all anaerobes .
    • The methane-producing heterotrophs use the organic acids as substrates to form methane and carbon dioxide.
    • The principle genera are:
      • Methanococcus
      • Methanobacterium
      • Methanoscarcina
    Anaerobic Digestion Wastewater Engineering
      • Methanococcus
      • (Source: microbewiki.kenyon.edu)
  • 12. Chemical Pathways of Methane Production
    • The diagram below shows the chemical pathway of methane production from organic solids and the intermediates formed.
    Anaerobic Digestion Wastewater Engineering Complex Waste Propionic Acid Other Intermediates Acetic Acid CH 4 (McCarthy, 1964) %COD 100% 15% 20% 65% 15% 17% 13% 15% 35% 72%
  • 13. Advantages of Anaerobic Digesters
    • Advantages of anaerobic digesters
      • Methane recovery by most of the microbial biomass produced in aerobic growth (biogas), can be used as alternate fuel source ( green solution ).
      • Reduces production of landfill gas, which when broken down aerobically releases methane into atmosphere (a powerful greenhouse gas).
      • Sludge occupies less volume, easier to dry.
      • Lower operating costs.
      • Odours/flies typically removed from system.
    • Disadvantages of anaerobic digesters:
      • Accumulation of heavy metals and contaminants in sludge.
      • Narrow temperature control range.
      • Installing and managing an interrelated group of systems to safely handle heating of the tank, hydrogen sulphide reduction, methane transfer, heat production, electrical production, inter connection with the electrical grid and surplus heat management.
    Anaerobic Digestion Wastewater Engineering
  • 14. Anaerobic Lagoons
    • There are several types of low rate anaerobic processes for several types of applications.
    • Anaerobic lagoons (Very Low Rate)
    • Main advantage: very low capital cost
    • Low biomass concentration; Very low rate of conversion; Gas production is seasonal; High F/M ratio; Poor growth rates; Little or no mixing occurs; Periodical cleaning at high cost.
    • Usually used for sludge dewatering .
    • Nuisance odours are generated.
    Anaerobic Digestion Wastewater Engineering (Burke, 2001)
  • 15. Types of Anaerobic Digesters
    • The two main types of anaerobic digesters are:
      • Low rate (Conventional) Digesters
          • Intermittent mixing, sludge feeding and sludge withdrawal.
          • When sludge is not being mixed, sludge contents undergo thermal stratification.
          • Detention time = 30-60 days
      • High rate Digesters
          • Continuous mixing (homogeneous)
          • Continuous or intermittent sludge feeding and sludge withdrawal.
          • Detention time = < 15 days.
    • Most digesters are heated and operate in the mesophilic range.
    • Usually made of concrete or steel.
    Anaerobic Digestion Wastewater Engineering (Reynolds, 1996)
  • 16. Features of Anaerobic Digesters
    • Covers
      • Completely mixed reactors can have fixed covers, floating covers, or gas holding covers.
      • Most municipal digesters have floating covers. Floating covers are more expensive than fixed covers and standard diameters range from 15 to 125 ft.
    Anaerobic Digestion Wastewater Engineering (Reynolds, 1996)
  • 17. Features of Anaerobic Digesters
    • Mixing
      • Mixing can be accomplished with a variety of gas mixers, mechanical mixers, and draft tubes with mechanical mixers or simply recirculation pumps. Most municipal digesters are intensely mixed to reduce the natural stratification that occurs in a low profile tank.
      • Figure 19.9 shows the various types of mixing flow regimes used in anaerobic digesters: (a) gas recycle and draft tube (b) gas recycle and injection (c) impeller and draft tube and (d) impeller.
      • The most efficient mixing device in terms of power consumed per gallon mixed is the mechanical mixer.
    (Reynolds, 1996) Anaerobic Digestion Wastewater Engineering
  • 18. Anaerobic Digestion Wastewater Engineering (supernatant) (fresh sludge) (biogas) (digested sludge / sludge withdrawal) (scum) Anaerobic Digester : Diagram (source: HTI Tanks, LLC)
  • 19. Types of Anaerobic Digesters
    • Some municipal digesters in the U.S., and most in Europe have an egg shape with a height much greater than the diameter.
    • Figure 19.12 shows the schematic of a typical egg-shaped anaerobic digester.
    • Advantages:
      • The egg shape enhances mixing while eliminating much of the stratification.
      • Virtually no grit accumulates at bottom of tank.
      • Better control of scum at top of digester.
      • Smaller land requirements.
    • Disadvantages:
      • Expensive.
      • Height can restrict usage around residential areas.
    Anaerobic Digestion Wastewater Engineering (Reynolds, 1996)
  • 20. Two Stage Anaerobic Digesters
    • Common when design population is over 30,000 – 50,000 persons.
    • First stage:
      • Main biochemical reaction is liquefaction of organic solids, digestion of soluble organic materials and gasification.
      • Usually a high rate digester with fixed cover and continuous mixing.
    • Second stage:
      • Some gasification occurs.
      • Main purpose is supernatant separation, gas storage and digested sludge storage.
      • Conventional digester (low rate) with a floating cover and intermittent mixing.
    Anaerobic Digestion Wastewater Engineering (source: www.nptel.iitm.ac.in)
  • 21. Digester Operation
    • Most digesters operate in the temperature range of 30 -38 °C during cold weather to optimize digestion time.
    • The digester gas produced from the process may be used for heating purposes.
    • Optimum pH range is 7.0 - 7.2
      • Maintained by properly seeding fresh added sludge and not excessively withdrawing sludge (should not exceed 3-5% of dry solids weight in digester).
      • Temporary solution for acidification: lime addition
    • Heavy metals may inhibit digestion process. Must be eliminated at the source.
    • The supernatant liquor is the water released during digestion.
      • May have BOD 5 as high as 2000 mg/l.
      • SS concentration as high as 1000 mg/l.
      • Fed back to the influent to primary clarifiers.
    • Table 19.1 shows typical design and operation parameters for anaerobic digesters
    • Table 19.2 shows required conditions for mesophilic anaerobic digestion.
    Anaerobic Digestion Wastewater Engineering
  • 22. Digester Operation (Reynolds, 1996) Anaerobic Digestion Wastewater Engineering
  • 23. Digester Operation (Reynolds, 1996) Anaerobic Digestion Wastewater Engineering
  • 24. Anaerobic Digestion Temperature
    • The digestion time required to digest 90% of degradable solids in primary sludge as a function of digestion temperature is shown in Figure 19.5.
    • Mesophilic range extends to about 42.2°C
    • Thermophilic range is above 42.2°C
    • Optimal temperature is around 35°C
    Anaerobic Digestion Wastewater Engineering (Reynolds, 1996)
  • 25. Digester Design: Volume
    • The volume of digesting sludge in a digester is a function of the volume of fresh sludge added daily, the volume of digested sludge produced daily and required digestion time (days).
    • Empirical evidence (batch experiments) has shown that if supernatant is removed from a batch of digesting sludge as it is produced, volume of remaining digested sludge vs. digestion time is a parabolic function.
    • For a parabolic function : average volume = initial volume – (2/3)(final volume – initial volume) therefore,
    • where: V avg = average volume of digesting sludge (m 3 / day)
    • V 1 = volume of fresh sludge added daily (m 3 / day)
    • V 2 = volume of digested sludge produced daily (m 3 / day)
    Anaerobic Digestion Wastewater Engineering (Fair et al) (19.6)
  • 26. Digester Design: Volume
    • The volume of total sludge in the reactor is given by adding both the digesting and the digested sludge:
    • where: V s = total sludge volume (m 3 )
    • V avg = average volume of digesting sludge (m 3 / day)
    • V 2 = volume of digested sludge produced daily (m 3 / day)
    • t d = time required for digestion (days)
    • t s = time provided for sludge storage (days)
    • The sludge volume normally occupies the bottom half of the digester, and the supernatant liquor occupies the upper half therefore the total sludge volume,
    • V t (m 3 ):
    • where: V t = total digester volume (m 3 ).
    Anaerobic Digestion Wastewater Engineering (19.7) (19.8)
  • 27. Digester Design: Residence Time
    • Digesters may also be designed based on organic loading (kg/m 3 day), or mean cell residence time , θ c ,(days)
    • The mean cell residence time is often referred to as ‘ solids retention time ’ ( θ s ).
    • where: θ c = mean cell residence time (days)
    • X = kilograms of dry solids in digester
    • Δ X = kilograms of dry solids produced per day in digester
    • Since no. of cells in feed is negligible compared to no. of cells in digester, the mean cell residence time is equal to the hydraulic retention time, and we assume the biological reactor is completely mixed therefore: θ c = θ h = θ s
    Anaerobic Digestion Wastewater Engineering (19.9)
  • 28. Digester Design: Residence Time
    • The design of high-rate digesters are usually done with mean cell residence time
    • θ c .
    • As the mean cell residence time decreases, a minimum condition, θ c min ,will be reached when cells are withdrawn from the system faster than they can multiply.
    • For design considerations, θ c design , is much longer than θ c min . Usually, it is 2.5 times the minimum as a rule of thumb.
    • The volume of a high-rate digester is given by:
    • where V = total digester volume (m 3 ).
    • Q = fresh sludge flow (m 3 /day).
    • θ c = mean cell residence time (days).
    Anaerobic Digestion Wastewater Engineering (19.10)
  • 29. Mean Cell Residence Time
    • Table 19.3 Suggested Values of Minimum Mean Cell Residence Time.
    Anaerobic Digestion Wastewater Engineering (McCarthy, 1964) Temperature ( °C) Θ c min (days) 18 11 24 8 30 6 35 4 40 4
  • 30. Example 1
    • Low-Rate Digester Problem
    • A heated, low-rate anaerobic digester is to be designed for an activated sludge plant treating the wastewater from 25,000 persons. The fresh sludge has 0.11 kg dry solids/cap-day, volatile solids are 70% of the dry solids, the dry solids are 5% of the sludge, and the wet specific gravity is 1.01. Sixty-five percent of the volatile solids are destroyed by digestion, and the fixed solids remain unchanged. The digested sludge has 7% dry solids and a wet specific gravity of 1.03. The operating temperature is 35 °C and the sludge storage time is 45 days. The sludge occupies the lower half of the tank depth, and the supernatant liquor and the gas occupy the upper half of the tank depth. Determine the digester volume.
    • Solution
    • Input = fresh sludge solids = (25,000 persons)(0.11 kg/cap-day) = 2750 kg/day.
    • VSS = (2750 kg/day)(0.70) = 1925 kg/day.
    • FSS = (2750 kg/day)(0.30) = 825 kg/day.
    • VSS destroyed = (1925 kg/day)(0.65) = 1251 kg/day.
    • Output = Input – Decrease (due to reaction) overall mass balance @ steady-state
    • VSS (in digested sludge) = 1925 – 1251 = 674 kg/day
    • FSS (in digested sludge) = 825 – 0 = 825 kg/day
    Anaerobic Digestion Wastewater Engineering
  • 31. Example 1 (continued)
    • Total solids in digested sludge, TSS = FSS + VSS = 674 + 825 = 1499 kg/day.
    • Fresh sludge volume = (2750 kg/days) (m 3 /1000 kg) (100 kg/5 kg) (1/1.01) = 54.46 m 3 /day.
    • The digested sludge volume = (1499 kg/days) (m 3 /1000 kg) (100kg/7 kg) (1/1.03) = 20.79 m 3 /day.
    • From Figure 19.5, the digestion time at 35 °C (~95 °F) is 23 days.
    • Total sludge volume = (32.01 m 3 /day)(23 days) + (20.79 m 3 /day)(45 days)
    • = 1670 m 3
    • Therefore, total digester volume = (1670 m 3 )(2) = 3340 m 3
    Anaerobic Digestion Wastewater Engineering
  • 32. Example 2
    • High-Rate Digester Problem
    • A heated high-rate anaerobic digester is to be designed for an activated sludge plant treating the wastewater from 25,000 persons. The feed to the digester (primary and secondary sludge) is 56.1 m 3 /day and the operating temperature is 35 °C. Determine the digester volume.
    • Solution
    • From Table 19.3, minimum cell residence time, θ c min , for 35°C is 4 days.
    • Therefore the design cell residence time, θ c = (2.5) θ c min = 2.5(4) = 10 days.
    • Assume: completely mixed biological reactor without recycle, the θ c = θ h .
    • As a result we can use, the volume of a high rate digester formula,
    • V = Q • θ h = (56.1 m 3 /day)(10 days) = 561 m 3 .
    Anaerobic Digestion Wastewater Engineering
  • 33. Moisture-Weight Relationships
    • The specific gravity of a wet or dried sludge, s, depends on: water content, solids content, specific gravity of dried solids, S s .
    • The moisture content, p w , is given by:
    • The percent solids (p s ) is then calculated by (100 – p w ) =
    Anaerobic Digestion Wastewater Engineering where: p w = percent water (%) W w = weight of the water W s = weight of the dry solids (19.11) (19.12)
  • 34. Moisture-Weight Relationships
    • The specific gravity of dried sludge solids, s s , is a function of specific gravities of the volatile and fixed fractions, s v and s f , respectively.
    • Let p v and p f represent percent volatile material and percent fixed material respectively.
    • Rearranging,
    • For engineering purposes, we may assume specific gravity of s v = 1, and
    • fixed s f = 2.5, therefore,
    Anaerobic Digestion Wastewater Engineering (19.13) (19.14) (19.15)
  • 35. Moisture-Weight Relationships
    • The specific gravity of wet sludge, s, may be determined from the equation:
    Anaerobic Digestion Wastewater Engineering (19.16)
  • 36. Sludge Quantities/Solids Concentrations
    • Sludge pumping records in various plants may be used to determine amount of primary and secondary sludge required in plant design, however if this data is not available we may use the following equation, developed by (Eckenfelder and Weston, 1956),
    • kg solids /day = YS r - k e Х
    • where: Y = yield coefficient, kg of TSS or VSS per kg of BOD 5 or COD removed per day (kg/kg)
    • S r = kg of BOD 5 or COD removed per day (kg/day)
    • k e = endogenous coefficient (day -1 )
    • X = mass of TSS or VSS in the aeration tank (kg)
    • Table 19.4 shows some typical values of sludge quantities and solids concentrations.
    Anaerobic Digestion Wastewater Engineering (19.17)
  • 37. Sludge Quantities/Solids Concentrations (Reynolds, 1996) Anaerobic Digestion Wastewater Engineering
  • 38. Example 3
    • Sludge Quantities
    • A low-rate trickling filter plant treats the wastewater from 5000 persons. Short-term analyses using composite samples of the influent wastewater show the BOD 5 and suspended solids concentrations to be those expected from average values for municipal wastewaters in the US. The plant gets 95% BOD 5 removal, 33% BOD 5 removal by the primary clarifier, and 65% suspended solids removal by the primary clarifier. Assume that the primary sludge has 4% dry solids, primary sludge specific gravity = 1.01, secondary sludge has 5% dry solids, the specific gravity = 1.02, and the biological solids produced is 0.35kg/kg BOD 5 removed. Determine the primary and secondary sludge produced per day.
    • Solution
    • The primary solids = (5000) × (0.091)(0.65) = 295.8 kg/day.
    • The primary sludge flow rate = (295.8 kg/day) × (100 kg/4.0 kg)(L/kg)(1/1.01) = 7320 L/day.
    • The BOD 5 to the secondary units = (0.077)(5000)(1 - 0.33) = 258.0 kg/day.
    • The BOD 5 from the plant = (0.077)(5000)(1 – 0.95) = 19.3 kg/day. The BOD 5 removed by secondary units = 258.0 – 19.3 = 238.7 kg/day.
    Anaerobic Digestion Wastewater Engineering
  • 39. Example 3 (continued)
    • The biological solids produced per day = (238.7 kg/day)(0.35 kg/kg) = 83.55 kg/day.
    • The secondary sludge flow rate = (83.55 kg/day)(100 kg/ 5 kg)(L/kg)(1/1.02) = 1640 L/day
    Anaerobic Digestion Wastewater Engineering
  • 40. Digester Heat Requirements
    • It is necessary to heat the digester in cold climates (such as Canada) to maintain the digester temperature within the desired mesophilic temperature range.
    • The heating system must:
      • Heat the influent fresh sludge entering the digester.
      • Make up for heat losses through walls, bottom and cover of digester.
    Anaerobic Digestion Wastewater Engineering (Reynolds, 1996)
  • 41. Digester Heat Requirements
    • The heat required to raise the temperature of incoming sludge is given by:
    • where: Q s = heat energy required (J/h)
    • P = weight of fresh dry sludge solids added per day (kg)
    • p s = percent dry solids in fresh sludge (%)
    • T d = temperature in the digester (ºC)
    • T s = temperature of the fresh sludge (ºC)
    • c p = specific heat constant = 4200 (J/kgºC)
    Anaerobic Digestion Wastewater Engineering (19.18)
  • 42. Digester Heat Requirements
      • The heat required to make up for the losses through top, walls and bottom is given by:
    • where: Q d = heat energy required (J/h)
    • C = coefficient of heat flow (J/m 2 -h-ºC)
    • A = percent dry solids in fresh sludge (m 2 )
    • Δ T = temperature in the digester (ºC)
    • The C value is dependent on the properties of the material
      • C(slab) = 1634; C(air) = 6128; C(floating cover) = 3268 (Imhoff et al, 1971)
    • The heating skid is placed externally since placing it internally in previous designs had caused sludge to cake and reduce thermal efficiency.
    Anaerobic Digestion Wastewater Engineering (19.19)
  • 43. Example 4
    • Sludge Heat Requirements
    • An activated sludge treatment plant has an anaerobic digester and serves a population of 25,000 people. The mixture of primary and secondary sludge amounts to 0.11 kg/cap-day, the fresh sludge has 4.5% solids on a dry basis, and the digester operating temperature is 35ºC. During the coldest month, January, the sludge temperature is 12.8ºC. The specific heat constant is 4200 J/kgºC. Determine the heat required to raise fresh sludge temperature to that of the digester.
    • Solution
    • The number of kilograms of fresh dry sludge solids added per day is (25,000 capita)(0.11 kg/cap-day) = 2750 kg/day.
    • The heat required will be,
    • Q s = (2750 kg/day)(100 kg/4.5 kg) (35 ºC – 12.8ºC)(day / 24 hours)(4200 J/kgºC) =
    • = 237,000,000 J/hour
    • Q s = 237,000,000 J/hour
    Anaerobic Digestion Wastewater Engineering
  • 44. Example 5
    • Digester Heat Losses
    • The anaerobic digester in Example 4 is 22.9 m in diameter, wall height of 9.14 m, and has a floating cover. The digester is recessed 0.61 m in the earth and has a freeboard of 0.91 m. For insulation, the earth is mounded around the digester up to a height of 4.57 m. A wall height of 9.14 m – (0.61 m + 0.91 m + 4.57 m) = 3.05 m is exposed to air on the outside and to digester contents on the inside. The average temperature in January is 7.2ºC. The digester operating temperature is 35ºC, and the dry earth mounded around digester has temperature = avg. monthly temperature = digester operating temperature. Below the digester, earth temperature is 35ºC and temperature of earth around wall is 35ºC. Determine heat required to make up losses from digester.
    • Solution
    • The area of the floating cover is ( π /4)(22.9 m) 2 = 411.9 m 2 .
    • The area of the wall where air is on the outside and digester contents are on the inside is ( π )(22.9 m)(4.57 m) = 328.8 m 2 .
    • The temperature of the dry earth mounded around the digester is ( 35ºC + 7.2ºC)/2 = 21.1ºC
    Anaerobic Digestion Wastewater Engineering
  • 45. Example 5 (continued)
    • The heat required, Q d , to make up digester heat losses is:
    • (411.9 m2)(3268 J/m 2 -h-ºC)(35ºC – 7.2ºC) + (219.4 m 2 )(6128 J/m 2 -h-ºC)(35ºC – 7.2ºC) + (328.8 m 2 )(1634 J/m 2 -h-ºC) × (35 ºC – 21.1ºC) = 82,300,000 J/h.
    • Q d = 82,300 kJ/h
    Anaerobic Digestion Wastewater Engineering
  • 46. Sludge Gas Utilization (Biogas)
    • Digester gas (biogas) consists approximately of (55-75)% methane and (25-45)% carbon dioxide, water vapour, trace amounts of hydrogen sulphide (H 2 S), hydrogen and nitrogen.
    • Explosive! Hydrogen sulphide is a corrosive and toxic gas.
    • To be utilized as fuel, the gas must be scrubbed to be used in turbines or engines (if H 2 S is > 0.015% by volume).
    • Used in many Asian countries for household uses, such as cooking. Countries such as Denmark and Germany effectively use biogas emitted from farms and central locations to produce electricity and heat.
    Anaerobic Digestion Wastewater Engineering
  • 47. Fluid Properties of Sludge
    • Frictional head losses for wastewater sludge may be calculated using hydraulic formulas, such as the Hazen-Williams equation .
    • where: V = velocity (m/s)
    • C HW = Hazen-Williams friction coefficient
    • R = hydraulic radius (m)
    • S = slope of energy gradient
    • Sludge exhibits laminar, transitional and turbulent flow at higher Reynolds number than water.
    (SI units) Anaerobic Digestion Wastewater Engineering (19.20)
  • 48. Example 6 Anaerobic Digestion Wastewater Engineering (Reynolds, 1996)
  • 49. Example 6 (continued) Anaerobic Digestion Wastewater Engineering
  • 50. Example 6 (continued) Anaerobic Digestion Wastewater Engineering
  • 51. Sludge Thickening and Dewatering
    • Fresh sludge is often thickened , to increase solids content prior to the digestion process (for larger wastewater plants).
      • Reduces daily fresh sludge volume.
      • Decreases size of digester.
      • Reduces amount of supernatant liquor.
      • Types: Gravity thickeners, Centrifuges
    • Anaerobic sludge is dewatered by a variety of processes:
      • Air drying
      • Vacuum filters
      • Filter presses
      • Lagoons
      • Centrifuges
    Anaerobic Digestion Wastewater Engineering
  • 52. Aerobic Digestion Aerobic Digestion Wastewater Engineering
  • 53. Introduction: Aerobic Digestion
    • Aerobic digestion is defined as the biological oxidation of organic sludges under aerobic conditions (in the presence of O 2 ).
    • Most microbes used in the process are facultative ; however some are obligate aerobes such as nitrifying bacteria , such as Nitrosomanas and Nitrobacter.
    Aerobic Digestion Wastewater Engineering
  • 54. Fundamentals of Aerobic Digestion
    • Primary sludge is mixed with waste activated sludge (WAS) or trickling filter (humus) and is aerobically digested:
    • Generalized Equation
    • Organic matter + O 2 new cells + energy + H 2 O + CO 2 + aerobic microbes other end products
    • Some of the end products: NH 4 + , NO 2 - , NO 3 - , PO 4 +
    • Considerable nitrogen converted into nitrate ion.
    • Living cell mass reduced by endogenous decay by following equation:
    • C 5 H 7 NO 2 + 5O 2 5CO 2 + 2H 2 0 + NH 3
    • The ammonia produced from this auto-oxidation is further oxidized into nitrate ion, represented by the following biochemical equation:
    • C 5 H 7 NO 2 + 7O 2 5CO 2 + 3H 2 0 + H + + NO 3 -
    Aerobic Digestion Wastewater Engineering
  • 55. Types of Aerobic Digesters
    • Batch-Operated Aerobic Digester
      • Operated on a fill and draw basis.
      • More expensive.
      • Not useful for large sludge quantities.
    Aerobic Digestion Wastewater Engineering (Reynolds, 1996)
  • 56. Types of Aerobic Digesters
    • Continuous-Flow Aerobic Digester System
      • More commonly used.
      • Lower operational costs.
      • Constant environmental conditions, aids in rapid digestion.
      • Thickener usually included, either upstream or downstream from the digester (Figure 20.3)
      • Thickened sludge has solids content 2 to 5 times that of feed sludge, which has the desirable result of producing a longer mean cell residence time .
    Aerobic Digestion Wastewater Engineering (Reynolds, 1996)
  • 57. Advantages of Aerobic Digesters
    • Advantages of aerobic digesters:
      • Fewer operational problems.
      • Less daily maintenance.
      • Lower BOD concentrations in supernatant liquor.
      • Lower capital costs.
    • Disadvantages of aerobic digesters:
      • Higher energy requirements – lot of aeration and mixing required.
      • No methane produced (i.e. no useful by-product).
      • Digested sludge has lower solids content, thus volume of sludge to be dewatered is much larger.
    Aerobic Digestion Wastewater Engineering
  • 58. Kinetics of Aerobic Oxidation
    • The rate of aerobic oxidation of solid organic materials is represented by a pseudo-first-order biochemical equation:
    • where: dX = change in biodegradable organic matter
    • dt = time interval
    • K d = reaction rate (degradation constant)
    • X = concentration of biodegradable matter at time t
    Aerobic Digestion Wastewater Engineering (20.4)
  • 59. Kinetics of Aerobic Oxidation
    • Equation (20.4) may be rearranged for integration between definite limits:
    • Where X t and X o represent biodegradable matter at times t = t and t = 0.
    • Integration gives:
    • Rearranging gives:
    • This equation (20.6) will plot a straight line on logarithmic scale, with X t / X o on the y-axis and t on the x-axis. Slope = -K d / 2.303
    Aerobic Digestion Wastewater Engineering (20.5) (20.6) (20.7)
  • 60. Mean Cell Residence Time
    • The hydraulic detention time is dependent on nature of sludge and operational temperature.
    • At 20 °C, usual detention times are tabulated, see Table 20.1
    • The required hydraulic detention time at temperatures other than 20 °C is given by the equation:
    • where: θ h2 = hydraulic detention time (days) at temperature T 2 ( °C).
    • θ h20 = hydraulic detention time at 20 °C
    • T 2 = temperature ( °C)
    • Note: θ ranges between 1.02 and 1.11.
    Aerobic Digestion Wastewater Engineering (20.8)
  • 61. Aerobic Digester Design Parameters (Reynolds, 1996) Aerobic Digestion Wastewater Engineering
  • 62. Mean Cell Residence Time
    • The mean cell residence time ( θ c ) or solids retention time, SRT ( θ s ) is given by:
    • where: X = mass of solids in digester (kg)
    • Δ X = solids produced per day in digested sludge (kg)
    Aerobic Digestion Wastewater Engineering (20.9)
  • 63. Example 7
    • Aerobic Digester
    • An aerobic digester is to be designed for an activated sludge plant treating WW from 10,000 persons. The flowsheet is shown. The primary and secondary sludge are to be blended in a blended tank and then thickened and then sent to digester. Pertinent data: BOD 5 = 200 mg/L, influent SS = 250 mg/L, average flow = 380 L/cap-day, 33% BOD 5 removal and 62% SS removal by primary clarifier, primary sludge has 5% solids, sludge density index = 6000 mg/L, secondary sludge = 0.031 kg/cap-day, volatile solids for primary and secondary sludge = 70%, and the specific gravity of primary, secondary, blended and thickened sludge = 1.01. Determine:
    • 1. Primary, secondary and blended sludge flow in m 3 /day.
    • 2. Solids concentration in blended flow.
    • 3. The thickened sludge flow if thickener increases solids by 2.5 times and supernatant liquor has negligible SS.
    • 4. The supernatant liquor flow.
    • 5. Digester volume if minimum operating temperature is 18 °C.
    • 6. Oxygen requirements of digester if 90% of primary BOD 5 is destroyed and 65% of volatile solids in WAS are destroyed.
    • 7. The air required if air has 0.281 kg O 2 /m 3 and field transfer is 4%. Is mixing adequate?
    Aerobic Digestion Wastewater Engineering
  • 64. Example 7 (diagram) Aerobic Digestion Wastewater Engineering
  • 65. Example 7 (continued)
    • Solution
    • The primary sludge SS are:
    • = 589 kg/day
    • The primary sludge flow is:
    • The waste activated sludge (WAS) is = (10,000 cap)(0.031kg/cap-day) = 310 kg/day.
    • The waste activated sludge flow is:
    • The flow leaving the blending tank (“blended flow”) = 11.7 m 3 /day + 51.2 m 3 /day = 62.9 m 3 /day
    Aerobic Digestion Wastewater Engineering
  • 66. Example 7 (continued)
    • The solids content in WAS = 6000 mg/L or 0.6%.
    • A mass balance on solids at the blending tank gives:
    • Q ps • C ps + Q w • C w = (Q ps + Q w )C 1
    • (11.7)(0.05) + (51.2)(0.006) = (11.7 + 51.2) C 1
    • Rearranging, C 1 = 0.014 = 1.4%
    • The blended sludge, Q 1 , is 11.7 m 3 /day + 51.2 m 3 /day = 62.9 m 3 /day.
    • A mass balance on solids at the thickener gives:
    • Q 1 C 1 = Q 2 C 2
    • since solids increase 2.5 times,
    • C 2 = (1.4)(2.5) = 3.5%. The thickened flow, Q 2 , is
    • Q 2 = Q 1 (C 1 /C 2 ) = (62.9)(1.4/3.5) = 25.2 m 3 /day
    • A balance on the flows at the thickener gives Q 1 = Q 2 + Q sl
    • Q sl = Q 1 - Q 2 = 62.9 - 25.2 = 37.7 m 3 /day
    Aerobic Digestion Wastewater Engineering
  • 67. Example 7 (continued)
    • From Table 20.1, we can assume θ h = 20 days at 20°C,
    • The correction for temperature is given by:
    • Assume θ is mean value of 1.02 and 1.11 = 1.065. Thus,
    • θ h2 = 20 • 1.065 (20-18)
    • = 22.7 days
    • The digester volume V = Q • θ h2 = (25.2 m 3 /day) (22.7 days) = 572 m 3
    • The oxygen required is for BOD 5 removal and WAS destruction. The BOD 5 removed is:
    • = 226 kg/day
    Aerobic Digestion Wastewater Engineering
  • 68. Example 7 (continued)
    • the WAS destruction:
    • Assume from Table 20.1 that 1.9 kg O 2 / kg BOD 5 removed and 2.0 kg O 2 / kg VSS destroyed. The oxygen required is:
    • O 2 / day = (226 kg/day)(1.9) + (141)(2.0)
    • = 429 + 282 = 711 kg/day
    • The air required is:
    • The air per 1000 m 3 is
    • air = 43.9 / 0.572 = 77 m 3 /min-1000 m 3
    • since 77 > 60 from Table 20.1, the mixing is adequate.
    Aerobic Digestion Wastewater Engineering
  • 69. References
    • Unit Operations and Processes in Environmental Engineering, Reynolds and Richards, PWS Publishing Company, Boston, MA, 1996.
    • Wastewater Engineering Treatment and Reuse, Metcalf & Eddy, McGraw-Hill, New York, NY, 2003
    • Biological Waste Treatment, Eckenfelder, Pergamon Press, 1966
    • Biology of Wastewater Treatment, Gray N.F., Oxford Science Publications, 1989
    • Waste Anaerobic Digestion Handbook, Burke D.A., June 2001.
    References Wastewater Engineering