PREFACE iiiPREFACEThe Environmental Protection Agency was established in 1993 to license, regulate and control activities forthe purposes of environmental protection. In Section 60 of the Environmental Protection Agency Act, 1992, itis stated that "the Agency may, and shall if so directed by the Minister, specify and publish criteria andprocedures, which in the opinion of the Agency are reasonable and desirable for the purposes ofenvironmental protection, in relation to the management, maintenance, supervision, operation or use of all orspecified classes of plant, sewers or drainage pipes vested in or controlled or used by a sanitary authority forthe.....treatment or disposal of any sewage or other effluent to any waters". The following is a list of themanuals published to-date: • Wastewater Treatment Manuals - Preliminary Treatment; • Wastewater Treatment Manuals - Primary, Secondary & Tertiary Treatment; • Wastewater Treatment Manuals - Characterisation of Industrial Wastewaters; and • Wastewater Treatment Manuals - Treatment Systems for Small Communities, Business, Leisure Centres and Hotels.This manual has been prepared to provide guidance on the design, operation and maintenance of on-sitewastewater treatment systems for a single house. The National Standards Authority of Ireland publishedstandard recommendations in 1975 (revised in 1991) with the aim of achieving satisfactory practice in thedesign, construction and maintenance of septic tank drainage systems. This manual has been prepared havingregard to the above and will inter alia assist planning authorities, developers, system manufacturers, systemdesigners, system installers, system operators to deal with the complexities of on-site systems. Wherereference in the document is made to proprietary equipment, this is intended as indicating equipment type andis not to be interpreted as endorsing or excluding any particular manufacturer or system.Chapter 1 of this manual contains an introduction to wastewater treatment and the types of on-site treatmentsystems available for a single house.Chapter 2 outlines the steps which should be taken to characterise a site. Characterisation of a site is dividedinto a desk study followed by an on-site assessment. The on-site assessment is subdivided into a visualassessment, a trial hole and a percolation test. The significance of the information collected during the deskstudy and the on-site assessment is summarised at the end of this chapter.Chapter 3 outlines a methodology for choosing the on-site treatment system and the optimum dischargeroute.Chapter 4 includes information on the design, construction and maintenance of a septic tank,soil percolationarea, intermittent filters, constructed wetlands and polishing filters.Chapter 5 includes information on mechanical aeration systems and polishing filters.A site characterisation form for use with this guidance manual is included in Appendix A.This manual was prepared following completion of a research study carried out under the direction of the EPAin the period 1995 to 1997. A seminar on the conclusions of the study was held on the 12th February, 1998.The Geological Survey of Ireland (GSI) in conjunction with the Department of Environment and LocalGovernment (DELG) and the EPA have developed a methodology for the preparation of groundwaterprotection schemes to assist the statutory authorities and others to meet their responsibility to protectgroundwater. Groundwater protection responses have been developed for on-site systems for single houses(DELG/EPA/GSI, 2000). These responses should be consulted when reading this document.The Agency welcomes any suggestions which users of the manual wish to make. These should be returned tothe Environmental Management and Planning Division at the Agency headquarters on the enclosed UserComment Form.
iv WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES ACKNOWLEDGEMENTS In order to examine the current position in relation to on-site systems (in Ireland and internationally) and to produce draft guidelines for their future use, a research project apropos on-site systems was part-financed by the European Union through the European Regional Development Fund as part of the Environmental Monitoring, R&D sub-pr ogramme of the Operational Programme for Environmental Services, 1994 -1999. The sub-programme is administered on behalf of the Department of the Environment and Local Government by the Environmental Protection Agency, which has the statutory function of co-ordinating and promoting environmental research. The consortium awarded the project was led by the Civil Engineering Department, National University of Ireland, Galway. The project leader was Dr. Michael Rodgers, assisted by Mr. John Mulqueen and Mr. Brian Gallagher. Other members of the project team were: Ms. Angela Casey, Mr. John Kenny, Mr. Padraic Ballantyne, Mr. Eamonn Waldron (P.J. Tobin & Co.), Mr. Brendan Fehily (Fehily Timoney & Co.), Ms. Mary Hensey (Hensey Glan Uisce Teo.), Ms. Sheila Davey (Neptune Labs) and Ms. Patricia Brannick (Central Marine Services Labs, NUI Galway). The project was monitored by a Technical Steering Group established by the EPA and included representatives of the EPA, the Department of the Environment and Local Government, the County and City Engineers’ Association and the project consortium. Members of the Technical Steering Group were (in alphabetical order): • Mr. Gerry Carty, EPA • Mr. Tony Cawley, Department of Environment and Local Government • Ms. Lorraine Fegan, EPA • Mr. Frank Gleeson, Sligo Co.Co., representing the City and Co. Engineers’Association • Mr. John Mulqueen, Teagasc • Mr. John O’Flynn, Waterford Co.Co., representing the City and Co. Engineers’Association • Mr. Gerard O’Leary, EPA • Dr. Michael Rodgers, NUI, Galway, Project leader As part of this research study a detailed questionnaire was issued to local authority and health board personnel. The co-operation of those who returned a completed questionnaire is gratefully acknowledged. The output from the research study formed the basis for the development of this manual. The Agency wishes to acknowledge the assistance of Mr. Donal Daly, Geological Survey of Ireland and Mr. Billy Moore, Monaghan County Council in reviewing early drafts of the manual. The Agency would like to acknowledge the National Standards Authority of Ireland for the use of material and diagrams from SR6. The Agency wishes to acknowledge the contribution of those persons listed below, who took the time to offer valuable information, advice, comments and constructive criticism on the draft manual. • Mr. Martin Beirne, Environmental Officers’ Association. • Mr. Dan O’Regan, National Standards Authority of Ireland. • Ms. Louise Mulcair, National Standards Authority of Ireland. • Ms. Yvonne Wylde, National Standards Authority of Ireland. • Mr. Bruce Misstear, Trinity College Dublin. • Mr. Paul O’Connor, Environmental Assessments.
ACKNOWLEDGMENTS v • Mr. Garvan Ward, Biocycle. • Dr. Eugene Bolton, Bord na Mona. • Dr. Hubert Henry, Bord na Mona. • Mr. Jer Keohane, Geotechnical and Environmental Services. • Mr. John Molloy, John Molloy Engineering. • Mr. Seamus Butler, Butler Manufacturing Services. • Mr. Albert Sneider, Aswatec. • Mr. Terry O’Flynn, Banks Douglas Environmental Science.The Agency also wishes to acknowledge the contribution of the Engineering Inspectors of the Department ofthe Environment and Local Government, and the Sanitary Services sub-committee of the Regional Laboratory,Kilkenny, who commented on the draft manual. The authors would also like to acknowledge the assistanceof Ms. Margaret Keegan, Mr. Donal Howley and Ms. Jane Brogan.
LIST OF ABBREVIATIONS viiList of AbbreviationsC Capacity°C Degrees CelsiusAgency Environmental Protection AgencyBAF Biofilm aerated filtersBOD Biochemical oxygen demandBOD5 Five-day biochemical oxygen demandCOD Chemical oxygen demandDELG Department of the Environment and Local Governmentd DayDO Dissolved oxygenDWF Dry weather flowEPA Environmental Protection AgencyFOG Fats, oils and greaseFWS Free-water surfaceg GramGSI Geological Survey Of Irelandh Hourkg KilogramISO International Organisation for Standardisationl Litrem Metrem3 Cubic metresm/s Metres per secondmg Milligrammm MillimetreNHAs National Heritage AreasNUI National University of Irelandp.e. Population equivalentPFP Preferential flow pathsRBC Rotating biological contactorss SecondSACs Special Areas of ConservationS.I. Statutory instrumentSBR Sequencing batch reactorSFS Sub-surface flow systemSS Suspended solidsTSS Total suspended solidsTWL Top water level
viii WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES
1I NTRODUCTION 91. INTRODUCTION1.1 GENERAL • BS 6297: 1983, Design and installation ofIn Ireland, the wastewater from over one third of the small sewage treatment works and cesspoolspopulation - principally those living in dwellings not (British Standards Institution) deals mainlyconnected to municipal sewers - rely on systems with the design of small sewage treatmentdesigned to treat the wastewater at or near the works serving small communities, notlocation where it is produced. These wastewater primarily concerned with septic tank systems;treatment systems are called on-site systems. andMany on-site systems are available for the treatment • US EPA/625/R-92/005 Manual: Wastewaterof wastewater from single houses and are designed Treatment/Disposal for Small Communities.to: • treat the wastewater to minimise In order to examine the current position relating to contamination of soils and water bodies; on-site systems (in Ireland and internationally) and to establish guidelines for their future use, so as to • protect humans from contact with ensure sustainable development, a research study wastewater; was carried out between 1995 and 1997 (as part of the Department of the Environment Operational • keep animals, insects, and vermin from Programme for Environmental Services, 1994- contact with wastewater; 1999). This study was co-ordinated by the D ep a rtment of Civil Engineering, The National • prevent direct discharge of untreated University of Ireland, Galway under the direction of wastewater to the groundwater; the Environmental Protection Agency (EPA) and was funded through the E nv i ronmental Monitoring, • minimise the generation of foul odours; and Research and Development Sub-programme of the Operational Programme. • prevent direct discharge of untreated wastewater to surface water. Some of findings of the research regarding single house treatment systems were:The biological treatment of the wastewater in on-sitetreatment systems occurs, in the main, under aerobic • conventional septic tank systems (septic tankconditions. For example, in a soil percolation area, and percolation area), properly installed andaerobic conditions are present due to the unsaturated maintained, are satisfactory where suitablenature of the soil. subsoil conditions exist;Public health is threatened when on-site systems fail • where suitable subsoil conditions do notto operate satisfactorily. System failures can result initially exist for treatment by means of ain wastewater ponding or forming stagnant pools on conventional septic tank system, sitethe ground surface when the wastewater is not development works may improve the subsoilabsorbed by the soil. In such circumstances of conditions and make the subsoil suitable insystem failure, humans can come in contact with the certain circumstances;ponded wastewater and be exposed to pathogens andfoul odours can be generated. • in certain situations such as when unsuitable subsoil conditions exist, other systems, whichThe three documents commonly used in relation to include mechanical aeration or intermittentthe design of on-site systems in Ireland are: filters for secondary treatment and followed by a polishing filter can be used; • SR6: 1991, Septic tank systems: Recommendations for domestic effluent • all treatment systems including wastewater treatment and disposal from a single dwelling collection systems must be designed, house (National Standards Authority of constructed, commissioned and maintained in Ireland); accordance with recognised standards; and
10 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES • all surface water and groundwater should be 1.3 CRITERIA FOR SELECTION excluded from entering any treatment system. When selecting a treatment system to treat 1.2 CHARACTERISTICS OF WASTEWATER wastewater from single houses, the system chosen: FROM A SINGLE HOUSE SYSTEM • should protect public health; For the purposes of this manual, a single house system refers to a dwelling house of up to ten people • should protect the environment; with toilet, living, sleeping, bathing, cooking and eating facilities. Under no circumstances should • should be economical; rainwater, surface water or run-off from paved areas be discharged to on-site single house systems. To • should operate with minimal maintenance prevent the quantity of wastewater generated in a from the owner; and household, water reducing measures should be adopted. Such measures include: minimising the use • should have a long (> 20 years) lifespan. of high water using equipment such as automatic washing machines and dishwashers, the use of On-site systems for single houses can be divided into showers instead of baths, the use of dual flush two main categories: cisterns in WCs, and the prompt fixing of leaks in household plumbing system. • septic tank systems; and The s t re n g t h of the inflow in terms of BOD • mechanical aeration systems. (Biochemical Oxygen Demand) into an on-site system will largely depend on the water usage in the 1.4 SEPTIC TANK SYSTEMS house; for example, houses with dishwashers may have a wastewater strength reduced by up to 35% 1.4.1 CONVENTIONAL SEPTIC TANK due to dilution even though the total organic load to SYSTEM the treatment system (kg/day) remains the same. Household garbage grinders can increase the BOD A conventional septic tank system comprises a septic loading rate by up to 30% and because these tank followed by a soil percolation area. The septic appliances are becoming more popular their use is an tank functions as a primary sedimentation tank, important consideration. removing most of the suspended solids from the wastewater; this removal is accompanied by a Other important constituents in domestic wastewater limited amount of anaerobic digestion. It is in the include nitrogen, phosphorus and microorganisms p e rc o l ation area that the wastewater undergoes such as coliforms. Table 1 gives typical secondary treatment and is purified. The wastewater concentration values for a number of parameters in from the septic tank is distributed to a suitable soil domestic wastewater. percolation area, which acts as a bio-filter. As the wastewater flows into and through the subsoil, it Typical daily hydraulic loading to an on-site system undergoes surface filtration, s t ra i n i n g, p hy s i c o - for single houses is 180 litres per person. chemical interactions and microbial breakdown. TABLE 1: CHARACTERISTICS OF DOMESTIC WASTEWATER FROM A SINGLE HOUSE Parameter Typical concentration (mg/l unless otherwise stated) Chemical Oxygen Demand COD (as O2) 400 Biochemical Oxygen Demand BOD5 (as O 2) 300 Total solids 200 Total Nitrogen (as N) 50 Total Phosphorus (as P) 10 Total coliforms (MPN/ 100 ml)* 107 - 108 * MPN Most Probable Number
1I NTRODUCTION 11After flowing through a suitable percolation area the loaded with wastewater from a septic tank, a biomatwastewater is suitable for discharge. layer quickly develops along the base and wetted sides of these trenches (Figure 2). The biomat layerA typical septic tank is illustrated in Figure 1 and the consists of a deposit of microorganisms, slimes andattributes of a septic tank are given in Table 2. The sludge which coats the floor and walls of the trenchtank, which should be two-chambered, allows the and enters the subsoil for a short distance inside thewastewater from the dwelling house time to settle infiltrative surface. The biomat drastically lowers theout into three layers viz. the sludge layer, the liquid infiltration through the base and sides, causinglayer and the scum layer (Figure 1). The sludge layer ponding in the trenches. The ponding causesis a blanket of heavy solids and some coagulated wastewater to flow over the entire trench base and inmaterials, which settle out on the tank floor. The a short time leads to a uniform distribution of theliquid layer, while re l at ively free of coarse wastewater over the total length of the trenches.suspended solids, is high in decomposable dissolved Ponded wastewater gradually rises in the trenchesand colloidal organic matter and contains bacteria, accompanied by the development of a biomat alongviruses, worm eggs, larvae etc.; it is allowed to flow the wetted walls of the trench until an equilibrium isto the percolation area through a tee-pipe for reached, causing flow through the sides and base. Andistribution and secondary treatment. The scum adequate depth of gravel aggregate in the trench islayer consists of greases, oils and gas-buoyed solids important for hydraulic function. The biomat layerwhich accumulate as a layer on the surface. then determines the hydraulic loading. Therefore forDetention times should be in excess of 24 hours. long-term successful operation of a perc o l at i o n system, the system should be designed to cope withThe subsoil through which the wastewater percolates the impedance caused by the development of theacts as an attached growth medium for biomat layer along the base and wetted walls of themicroorganisms. As the percolation trenches are percolation trench. Manhole cover with ventilation Manhole cover with ventilation Inlet TWL Outlet Scum Layer Sludge layer Liquid layer CHAMBER NO. 1 CHAMBER NO. 2 SECTION A - A FIGURE 1: A TYPICAL SEPTIC TANK
12 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES TABLE 2: ATTRIBUTES OF A SEPTIC TANK A properly constructed septic tank will: Retain and remove 50% or more solids; outflow from tank contains about 80 mg/l solids Allow some microbial decomposition Accept sullage (i.e. water from baths, wash hand basins etc.) Accept water containing detergents Reduce clogging in the percolation area Not fully treat domestic wastewater Not work properly if inadequately maintained Not significantly remove microorganisms Not remove more than 15 - 30 % of the BOD Not operate properly if pesticides, paints, thinners, solvents, disinfectants or household hazardous substances are discharged to it Not accommodate sludge indefinitely Not operate properly if surface waters (i.e. roofs etc.) are discharged to it Distribution box initial operation - no biomat Biomat formation (Order of weeks) Biomat formation Biomat formation with extension of clogging of the base and adjoining walls of trench FIGURE 2: ILLUSTRATION OF BIOMAT FORMATION ON THE BASE OF A PERCOLATION TRENCH
1I NTRODUCTION 13 Soil Distribution laterals Geotextile cap Distribution gravel Washed gravel Top soil Filter sand filter gravel or permeable soil Gravel, fractured bedrock, high water table or impervious soil FIGURE 3: SCHEMATIC DIAGRAM OF A SOIL COVERED MOUND SAND FILTERA percolation area is considered "failing" when (i) it for the other filters are usually installed incauses a backing up of wastewater in the distribution prefabricated containers (prefabricated intermittentbox or (ii) it does not keep untreated wastewater filters).below the surface of the land or (iii) it does not treatthe wastewater before it reaches groundwater or 1.4.3 CONSTRUCTED WETLANDSsurface water. Constructed wetlands can also be used for theIn Ireland, a significant number of septic tank t re atment of wastewater from single houses.systems do not function properly, mainly because Wetlands are areas with high water tables whichthey have been poorly constructed, installed, promote aquatic vegetation or water tolerant plantsoperated, maintained or, are located in areas with such as reeds.unsuitable subsoils, or percolation of the septic tankeffluent is through a soakaway. It is important to Primary treatment by a septic tank is used prior tonote, however, that in the absence of a connection to discharge to a constructed wetland. In the wetland,a sewer system, one of the most appropriate and cost the wastewater from a septic tank is treated by aeffective means of treating wastewater in a suitable combination of physical, chemical and biologicalsite is a properly constructed and maintained processes that develop through the interaction of theconventional septic tank system. plants (reeds), the growing media (gravel) and microorganisms. These processes include settlement1.4.2 FILTER SYSTEMS and filtering of suspended solids, biodegradation, plant uptake and chemical interactions.Where the subsoil is unsuitable for treating thewastewater from a septic tank, filter systems may be There are two different types of constructed wetlandsused. These include intermittent soil filters, sand and they are characterised by the flow path of thefilters, peat filters and other filters using materials water through the system (Figure 4). In horizontalsuch as plastic foam filters and geosynthetic strips. flow constructed wetlands, wastewater is introducedIntermittent soil filters comprise suitable soils placed at one end of a flat to gently sloping bed of reeds andoften in the form of a mound, through which septic flows horizontally across the bed to the outfall end.tank effluent is filtered and purified. Intermittent In the second type, called the vertical-flow wetland,sand filters consist of one or more beds of graded the wastewater is dosed unifo rm ly over, andsand underlain at the base by a filter gravel or intermittently onto the media, and gradually drainspermeable soil layer to prevent outwash or piping of vertically to a drainage network at the base of thethe sand. Soil covered intermittent sand filters may media. Constructed wetlands should be securelybe underground, part underground and part fenced off to prevent access by unauthorised persons,overground, or overground. The latter two especially children.constructions are commonly referred to as moundsystems (Figure 3). Fibrous peat and plastic media
14 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES Horizontal flow wetland Septic tank Vertical flow wetland Septic tank FIGURE 4: TYPES OF CONSTRUCTED WETLANDS (SECTION) 1.5 MECHANICAL AERATION SYSTEMS soil or imported soil, whereas sand polishing filters comprise stratified layers of sand. In recent years, many mechanical aeration systems have come on the market; these offer a solution in 1.7 SITE DEVELOPMENT some cases where a site may be unsuitable for treating the septic tank wastewater, or an alternative Where a site is initially unsuitable for a septic tank to the conventional septic tank system. These system, site development works may improve the systems include the following: site and make it suitable for the development of an on-site system. • biofilm aerated (BAF) systems; Site development works could include lowering the • rotating biological contactor (RBC) systems; water table, raising the ground surface by filling with and suitable soil, part replacement of the subsoil by suitable soil or subsoil loosening. After carrying out • sequencing batch reactors (SBR) systems. the necessary improvements, the site should be reassessed to establish whether the improved soil is BAF systems may consist of a primary settlement satisfactory. tank, aerated filter media and a secondary settlement tank. RBC systems consist of a primary settlement 1.8 SITE CHARACTERISATION tank, a biological treatment compartment and a secondary settlement tank. These systems are The objective of a site characterisation is to obtain similar to conventional trickling filter systems in that sufficient information to determine if the site can be the microorganisms carrying out the secondary developed for an on-site system. Characterising the treatment are attached to an inert media surface. site involves a number of stage s . These should Sequencing batch reactors (SBR) consist of a include: primary settlement tank and a reactor in which • a desk study, which collects any information biological treatment and clarification occur. that may be available on maps etc. about the site; 1.6 POLISHING FILTERS • a visual assessment of the site, which defines Polishing filters should be used to treat wastewater the site in relation to surface features; from intermittent filters, constructed wetlands and mechanical aeration systems. These filters consist of • a trial hole to evaluate the soil structure, depth either soil or sand and are employed to reduce to rock and water table; and microorganisms, phosphorus, and nitrate nitrogen. Soil polishing filters may comprise in-situ, improved • percolation tests.
1I NTRODUCTION 15Figure 5 below summarises the protocol to be occurring and its consequences that is the basis offollowed to select and design an on-site system. risk assessment. Risk management involves site assessment, selection of options and implementationThe concepts of ‘risk’, ‘risk assessment’ and ‘risk of measures to prevent or minimise the consequencesmanagement’ have recently become important tools and probability of a contamination event (e.g. odourin environmental protection. Risk can be defined as nuisance or water pollution). The methodology forthe likelihood or expected frequency of a specified selection and design of an on-site system in thisadverse consequence. Applied for example to manual embraces the concepts of risk assessmentgroundwater, a risk expresses the likelihood of and risk management.contamination arising from a proposed on-sitetreatment system (called the hazard). A hazard The remainder of this manual sets out how a sitepresents a risk when it is likely to affect something characterisation should be completed and a choice ofof value (the target, e.g. surface water). It is the on-site system made.combination of the probability of the hazard SITE DESK RESTRICTIONS STUDY NO SITE RESTRICTIONS NOT SUITABLE ON-SITE INVESTIGATION VISUAL ASSESSMENT TRIAL HOLE SITE PERCOLATION IMPROVEMENT TESTS FILTER SYSTEM AND POLISHING FILTER Or SITE UNSUITABLE** UNSUITABLE * CHARACTERISATION MECHANICAL AERATION SYSTEM * This option may not always be available AND POLISHING FILTER SUITABLE ** Site may not always be suitable for an on-site system FILTER SYSTEM /MECHANICAL AERATION SYSTEM CONVENTIONAL SEPTIC TANK SYSTEM AND POLISHING FILTER Or FIGURE 5: SELECTING AN ON-SITE TREATMENT SYSTEM FOR A SINGLE HOUSE
16 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES
2S ITE CHARACTERISATION 172. SITE CHARACTERISATIONThe purpose of a site assessment is to determine the A desk study involves the assessment of availablesuitability of the site for an on-site treatment system. data pertaining to the site and adjoining area thatThe assessment will also help to predict the may determine whether the site has any restrictions.wastewater flow through the subsoil and into the Information collected from the desk study shouldsubsurface materials. include material related to the hy d ro l ogical, hydrogeological and planning aspects of the site,The key to installing a reliable on-site system that which may be available in maps or rep o rt s .minimises the potential for pollution is to select and Hydrological aspects include locating the presencedesign a suitable treatment system following a (if any) of streams, rivers, lakes, beaches, shellfishthorough site assessment. For a subsoil to be areas and/or wetlands while hydrogeological aspectseffective as a medium for treating wastewater, it include:must retain the wastewater for a sufficient length oftime, and it must be well aerated. • soil type;Only after a site evaluation has been completed can • subsoil type;an on-site system be chosen. The info rm at i o ncollected in the evaluation will be used to select the • bedrock type;on-site system. • aquifer type;In designing a soil percolation area to treatwastewater, three factors must be considered: • vulnerability class; and • the suitability of the site; • groundwater protection response (refer to the DELG/EPA/GSI groundwater protection • the suitability of subsoil and groundwater scheme and groundwater protection conditions, and responses for on-site systems for single houses). • the permissible hydraulic load on the subsoil. The Groundwater Protection Schemes provideTo determine these considerations a site guidelines for developers in assessing groundwatercharacterisation is undertaken. This includes: vulnerability and for the planning authorities in carrying out their functions, and a framework to1) a desk study; and assist in decision-making on the location, nature and control of developments and activities (including2) an on-site evaluation, consisting of : single house treatment systems) in order to protect groundwater. The density of on-site systems should • a visual assessment; be considered also at this stage. The protection responses required to protect groundwater from on- • a trial hole; and site systems should be satisfied. Where no scheme exists, interim measures as set out in the • percolation tests. Groundwater Protection Schemes should be adopted. Each site is specific and local factors should be taken2.1 DESK STUDY into account in using this guideline information.The purposes of a desk study are to: Planning aspects include: • obtain information relevant to the site, which • zoning in the development plan; will assist in assessing its suitability; • presence of significant sites (archaeological, • identify targets at risk; and natural heritage, historical etc.); and • establish if there are site restrictions. • past experience of the area.
18 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES 2.1.1 INTERPRETING THE RESULTS OF THE drained areas, or on convex slopes are most DESK STUDY desirable. Sites which are in depressions, or on the bottom of slopes or on concave slopes are less The information collected from the desk study desirable. should be examined and the following should be considered for all treatment options: The principal factors which should be considered are, relief, shape and form, rock outcrops, wells and • zoning (including groundwater protection wat e rc o u rs e s , land use, vege t at i o n , t ra m p l i n g schemes): Zoning for groundwater protection damage to the soil by livestock, seepage, boundary schemes outlines the aquifer classification in of property, and old building foundations. the general area and the vulnerability of the groundwater. The groundwater protection Slope: It is more difficult to install pipework and responses will provide an early indication of ensure that the wastewater will stay in the soil if the the probable suitability of a site for an on-site land has an extreme slope. Where there is surface system. The on-site assessment will later water run-off and interflow, low-lying areas and flat confirm or modify such responses; areas generally receive more water. This accounts to some extent for the occurrence of poorly drained • presence of significant sites: Determine soils in low-lying areas. Soils with poor drainage, whether there are significant archaeological, however, may also be found on good slopes where natural heritage and/or historical features the parent material or the subsoil is of low within the proposed site. To avoid any p e rm e ab i l i t y. Provision must be made for the accidental damage, a trial hole assessment or interception of all surface run-off and seepage, and percolation tests should not be undertaken in its diversion away from the proposed percolation areas, which are at or adjacent to significant area. sites (e.g. SACs, NHAs etc.), without prior advice from Duchas, the Heritage Service; Proximity to surface fe at u re s : M i n i mu m separation distances as set out in the following • nature of drainage: A high frequency of ch ap t e rs should be maintained from specified watercourses on maps indicates high or features. The presence/location of surface features perched watertables; and such as wells/springs, watercourses, dwelling houses on adjacent sites, site boundaries, roads, steep • past experience: Is there evidence of slopes, etc. should be noted. satisfactory or unsatisfactory local experience with on-site treatment systems? Wells: Wells should be considered as targets at risk. The groundwater flow direction, where it can be 2.2 ON-SITE ASSESSMENT inferred; the number of wells; the presence of any wetlands, and presence of any karst features should 2.2.1 VISUAL ASSESSMENT be noted. The purposes of the visual assessment are to: Drainage: A high density of streams or ditches tends to indicate a high water table and potential risk • assess the potential suitability of the site; to surface water. Low density of streams indicates a free draining subsoil and or/bedrock. • assess potential targets at risk (adjacent wells); and Type of vegetation: Rushes, yellow flags (irises) and alders indicate poor percolation characteristics • provide sufficient information to enable a or high water table levels. Grasses, trees and ferns decision to be made on the suitability of the may indicate suitable percolation characteristics. site for the wastewater to be treated and the Plants and trees indicating good drainage and poor location of the proposed system within the drainage are illustrated in Appendix E. site. The principal factors which should be considered are listed below. Ground condition: The ground conditions during the on-site investigation should be noted. Trampling Topography and landscape: Topography reflects damage by livestock can indicate impeded drainage the relief of the site. Landscape position reflects the or intermittent high water tables, especially where location of the site in the landscape e.g. crest of hill, accompanied by widespread ponding in hoof prints. valley, slope of hill. Sites which are on level, well The factors examined during a visual assessment and
2SITE CHARACTERISATION 19 TABLE 3: FACTORS TO BE CONSIDERED DURING A VISUAL ASSESSMENT Factor Significance Water level in ditches and wells Indicates depth of unsaturated subsoil Shape, slope and form of site May indicate whether water will collect at a site or flow away from the site Presence of watercourses May indicate low permeability or a high water table Presence and types of rock outcrops Insufficient depth of subsoil to treat wastewater allowing it to enter the groundwater too fast Proximity to adjacent percolation areas and/or houses May indicate too high a loading rate for the locality and/or potential nuisance problems Land use and type of grassland surface (if applicable) Indicator of rate of percolation or groundwater levels Vegetation type Indicator of the rate of percolation or groundwater levels Proximity to wells on-site and off-site, water supply Indicates targets at risk sources, groundwater, streams, ditches, lakes, surface water ponding, beaches, shellfish areas, and wetlands TABLE 4: MINIMUM SEPARATION DISTANCES IN METRES Type of system Watercourse/ Wells/ Lake Any Site Road Slope stream springs* Dwelling boundary breaks/ cuts Septic tank; prefabricated intermittent filters; 10 10 50 7 3 4 4 mechanical aeration systems In situ intermittent filters; percolation 10 30 50 10 3 4 4 area; polishing filters* This applies to wells down-gradient or where flow direction is unknown. For more information on wells alongside orup-gradient consult DELG/EPA/GSI ground water protection scheme 1.their significance are summarised in Table 3 above. requirements cannot be met, on-site systems cannot be developed on the site. The recommended2.2.2 INTERPRETING THE RESULTS OF THE minimum distances from wells should satisfy theVISUAL ASSESSMENT requirements of the groundwater protection response, which should have been reviewed duringThe minimum separation distances that should be the desk study. In some cases, the requirements ofused in the visual assessment are set out in Table 4. the groundwater protection scheme and responsesThese apply to any on-site system. If any of these may be greater than the distances set out in Table 4.1Department of Environment and Local Government, Environmental Protection Agency, Geological Survey of Ireland2000. Groundwater Protection Responses for On-site Systems for Single Houses.
20 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES 2.2.3 TRIAL HOLE hours to establish the depth to the water table (if present) and should be securely fenced. The soil The purposes of the trial hole are to determine: ch a ra c t e ristics assessed are: t ex t u re, s t ru c t u re, presence of pre fe rential flow paths, density, • the depth of the water table; compactness, colour, layering, depth to bedrock and depth to the watertable. If items of suspected • the depth to bedrock; and archaeological interest are discove re d, contact should be made with the relevant authorities. • the soil and subsoil characteristics. Depth to bedrock and depth to water table: A The trial hole will also help to predict the wastewater depth of 1.2m of suitable free draining unsaturated flow through the subsoil. subsoil, to the bedrock and to the water table below the base of the percolation trenches, must exist at all The trial hole should be as small as practicable, e.g. times to ensure sat i s fa c t o ry treatment of the 1.0 metre x 0.75 metre in plan, and should be wastewater. Sites assessed in summer when the excavated to a depth of at least 1.2 m below the invert water table is low, should be examined below the of the lowest percolation trench. In the case of a level proposed invert of the percolation pipe for soil site the depth of the trial hole should be a minimum mottling - an indicator of seasonally high water of 2.1 m below ground surface. In the case of a tables. sloping site it is essential that an estimate of the depth of the invert of the percolation trench be made Soil texture: Texture is the relative proportions of beforehand. The hole should remain open for 48 sand, silt and clay particles in a soil after screening through a 2 mm size sieve. The rate and extent of many important physical processes and chemical reactions in soils are governed by texture. Physical processes influenced by texture include drainage and moisture retention, diffusion of gases and the rate of transport of contaminants. Texture influences the biofilm surface area in which biochemical and x chemical reactions occur. The soil texture may be characterised using the chart in Figure 6 . To classify a soil/subsoil, it should be wetted and squeezed between the fingers. Soils/subsoils high in sand feel sandy, soils/subsoils high in silt are silky to feel and soils/subsoils high in clay are sticky and Example x: 50% Clay 30% Sand have tensile strength. A guide to assist the 20% Silt cl a s s i fi c ation of soil/subsoils is included in Appendix D. Various soil/subsoil texture classifications schemes exist; Table 5 compares three FIGURE 6: SOIL CLASSIFICATION CHART such classifications and indicates typical percolation rates. TABLE 5: SOIL/SUBSOIL TEXTURES AND TYPICAL PERCOLATION RATES Soil Class Subsoil Unified Class Typical Percolation Classification Classification Classification Rate * (min/25mm) sand medium fine SAND sand; silty sand; clayey sand 1-5 loamy sand silty, clayey SAND sand; silty sand; clayey sand 6 - 10 sandy loam silty SAND silty sand; clayey sand 6 - 30 loam / silt loam sandy SILT silty fine sands - low plasticity 31 - 50** * typical for soil in an uncompacted state and not indurated or hard. ** upper limit of 50 may need to be reviewed in the light of on-going research findings.
2S ITE CHARACTERISATION 21Structure: Soil structure refers to the arrangement illustrated in Figure 8. Where water is supplied to aof the soil particles into larger units or compound soil at a rate less than its permeability, as in the caseparticles in the soil. The soil particles,sand, silt, clay of septic tank effluent, the r ate of flow through theand organic matter, are generally clumped together soil equals the rate of supply in soils of adequateto form larger units called peds. The shape and size permeability.of the peds have a large effect on the behaviour ofsoils. A ped is a unit of soil structure such as an The preferred structures from a wastewater treatmentaggregate, a crumb, a prism, a block or granules perspective are granular (as fine sand), blocky andformed by natural processes. Soil texture plays a structureless-single grain sandy loams, loams andmajor part in determining soil structure. The silt loams. Unstructured massive plastic soilsstructure of the soil influences the pore space, indicate seasonal or continuous saturation and areaeration and drainage conditions. Types of soil unsuitable. Likewise soils with extensive, large andstructure (shape of the ped) are illustrated in Figure continuous fissures and thick lenses of gravel and7 and are: coarse sand may be unsuitable; this suitability will be assessed in the percolation test. • Crumb - peds have curved surfaces. The faces of peds do not fit into the faces of Preferential flow paths: Preferential flow paths neighbouring peds. Commonly found in top (PFPs) are formed in soils by biological, chemical soils. and physical processes and their interactions. They may be randomly distributed or their formation may • Granular - peds composed of single grains be systematic, reflecting the influence of agricultural with curved surface e.g. sands. practices. Research in recent years indicates that PFPs can have a significant influence on the • Blocky - the faces of each ped are nearly movement of ponded or perched water in equal and fit into the faces of neighbouring soil/subsoils where free (non capillary) water is in peds. They are common in loamy soils. direct contact with PFPs. The presence of PFPs should be noted during the trial hole assessment • Prismatic - the soil particles are arranged because their presence may influence the percolation about a vertical axis and are bounded by rate of the subsoil. relatively smooth vertical faces; the vertical faces are longer than the horizontal faces and Soil density: this refers to how tightly the soil grains fit into neighbouring peds; commonly found are packed to gether. Dry bulk density is commonly in clayey soils. classified as low, medium or high. • Platy - peds consist of thin flat plates and are • Low - loose and easily disintegrated into formed where soils dry out rapidly (rare in structural peds when dry to moist; typical of Ireland). many topsoils; • Structureless - massive - soil is not separated • Medium - dry bulk density of intermediate into structural units but occurs as one large magnitude (e.g. 1.3 tonne/m 3); typical of (often plastic) mass; typical of clays and silts. many permeable soils; and • Structureless - single grain - soil has no • High - compact and strong and resistant to visible aggregation; on immersion in water, penetration; typical of some deep permeable soil readily disintegrates into its single grains soils. of gravel, sand, silt and clay. Soils of low and medium dry bulk density are best asThe rate of flow of water through soils of the various percolation soils.structures is in the following order: Colour: This is a good indicator of the state of crumb faster than blocky; blocky faster than aeration of the soil/subsoil. Free draining structureless-single grain. unsaturated soils/subsoils are in the oxidised state at all times and exhibit brown, reddish brown andStructureless massive structure have very low flow yellowish brown colours. Many free draining soils ofrates and can often be regarded as impervious ( e.g. limestone origin with deep water tables are grey atwith a permeability < 10 mm/day). The relationship depth. Saturated soils/subsoils are in a reduced statebetween structure type and water movement is and exhibit dull grey or mottled colours. Mottling
22 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES FIGURE 7: TYPES OF SOIL STRUCTURE ILLUSTRATED FIGURE 8: RELATIONSHIP BETWEEN STRUCTURE TYPE AND WATER MOVEMENT
2S ITE CHARACTERISATION 23(comprising a reddish brown or rusty staining) of the with clay. In some areas a thin, hard, rust colouredsoil layers can indicate the height to which the water impervious layer can develop (iron pans) as a resulttable rises in periods of high rainfall; mottling in a of the downward leaching of iron and manganesegrey matrix (grey with reddish brown mottles) compounds and deposition at shallow depth (lessindicates aeration along old root channels and cracks t h a n 1 m ) . The underlying subsoil often has awhile the matrix remains reduced; this soil layer is satisfactory percolation rate. Enrichment with claysaturated during part of the year. particles and precipitation of iron and calcium and magnesium compounds can lead to very lowLayering: This is common in soils, arising during percolation rates. Such soils can often be improveddeposition and/or subsequent weathering. In soils, by loosening or by breaking the impervious layer.that are free draining in the virgin state, weatheringcan result in downward movement of some of the The factors that are evaluated from the trial hole andclay fraction leading to enrichment of a sub-layer their significance are summarised in Table 6 below. TABLE 6: FACTORS TO BE CONSIDERED DURING A TRIAL HOLE EXAMINATION Factors Significance Soil structure and texture Both influence the capacity of soil to treat and dispose of the wastewater; silts and clays are generally unsuitable Mottling Indicates seasonal high water tables Depth to bedrock Subsoil must have sufficient depth to treat wastewater Depth to water table Wet subsoils do not allow adequate treatment of wastewater Water ingress along walls Indicates high water table Season Water table varies between seasons
24 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES 2.2.4 INTERPRETING THE RESULTS OF THE ch a ra c t e ristics necessary for the treatment of TRIAL HOLE TEST wastewater. The percolation characteristics will be confirmed later by examining the percolation test Table 7 sets out the subsoil characteristics which results. indicate sat i s fa c t o ry percolation and other TABLE 7: TRIAL HOLE - SITE REQUIREMENTS WHICH INDICATE ADEQUATE PERCOLATION CHARACTERISTICS Subsoil Characteristics Requirements Minimum depth of unsaturated permeable subsoil 1.2 m below base of all percolation trenches Percolation trench cross section for a level site Ground level Topsoil 300 mm Gravel 150 mm Distribution pipe & Gravel 100 mm Gravel 250 mm 2000 mm 450 mm Unsaturated 1200 mm Subsoil Minimum depth of unsaturated subsoil to bedrock 1.2 m below invert level of all percolation trenches Minimum depth to water table below invert of all 1.2 m percolation trenches* Texture of unsaturated soil/subsoil Sand (medium fine SAND), Loamy sand (silty, clayey SAND), Sandy loam (silty SAND), Loam and silt loam (sandy SILT); Structure of unsaturated soil/subsoil Granular, blocky; and structureless single grain Colour of unsaturated soil/subsoil Greyish brown, reddish brown, and yellowish brown; grey in the case of many free draining limestone soils Layering in the walls of a percolation trench or No gravel or clay layer should be present below its invert Bulk density of unsaturated soil/subsoil Low to medium *where the dimensions of the percolation trench and unsaturated soil are as shown, the minimum depth to the water table is 2 m below ground surface.
2S ITE CHARACTERISATION 252.2.5 PERCOLATION TESTS the site is not suitable for the treatment of septic tank wastewater by soil percolation. Other options shouldA percolation (permeability) test assesses the be considered such as a constructed percolation area,hydraulic assimilation capacity of the subsoil i.e. the mechanical aeration systems, intermittent filters orlength of time for the water level in the percolation constructed wetlands. Where mechanical aerationhole to fall from a height of 300 mm to 200 mm systems, intermittent filters or constructed wetlandsabove the base of the test hole in a percolation area. are used, the treated wastewater from such systemsThe permeability of each soil class may vary within should discharge to receiving waters (surface oran order of 10 - 100 fold depending primarily on the groundwater) through a polishing filter.particle size grading which reflects the va ry i n gamounts of fine particles, the structure and void ratio Where shallow or impervious soils exist, a soilin each soil class. The procedure for carrying out a percolation area may still be possible by importingpercolation test is set out in Appendix B. suitable soil and placing it in lifts in the proposed percolation area such that there is a minimumThe results of percolation tests are expressed as a "T" thickness of 2.0 m of unsaturated soil with drainagevalue. This is the average time in minutes for the over the bedrock or impervious soil. A trial hole andwater level to fall 25 mm in each of two percolation percolation tests (T - tests) should then be carried outtest holes over the water depth range of 300 mm to (see Appendix B - Percolation Tests for further200 mm in the proposed percolation area. details) in the same way as for in situ soils. Where such soil filling is not feasible, alternative systemsTo carry out a percolation test (which should be followed by a polishing filter may be suitable.within the proposed percolation area), a 300 mmsquare percolation test hole is excavated to a depth of Where an alternative system and a polishing filter are400 mm below the invert of the proposed distribution employed, the nature of the soil or bedrockpipe. underlying the polishing filter determines the disposal route of the treated wastewater. For aTo establish the percolation value for shallow soils polishing filter overlying impervious soils or rocks,that may be used for polishing filters (discussed the treated wastewater is collected in a suitablelater) a modification of the T test is required. For drainage system and discharged to surface waters.this, the test hole is 400 mm below the ground Polishing filters overlying permeable soils, gravelssurface as opposed to 400 mm below the invert of the or bedrock with a T/P value less than 50 maydistribution pipe for the T test. To avoid confusion d i s ch a rge the treated wastewaters to thewith the previous test, this test is called a P test, and groundwater. A flow diagram to assist in the choicethe values are referred to as P values. of an on-site system is shown in Figure 188.8.131.52 INTERPRETING THE RESULTS OF THEPERCOLATION TEST 2.3 INTEGRATION OF THE DESK STUDY AND ON-SITE ASSESSMENT INFORMATIONA "T" value greater than 50 suggests that wastewaterentering such subsoils would cause ponding on-site. Table 8 summarises the information that can beA "T" value less than 1 suggests that the retention obtained from the data collected from the desk studytime for the wastewater would not be long enough to and the on-site assessment. This information is usedprovide satisfactory treatment. If the percolation T to characterise the site and used later to choose andvalue is within the range 1-50* then the site should design an on-site system. An integrated approachbe suitable for the development of a conventional will ensure inter alia that the targets at risk areseptic tank system. identified and protected.Where the "T" value is less than 1 or greater than 50* upper limit of 50 may be reviewed depending on experience.
26 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES TABLE 8: INFORMATION OBTAINED FROM THE DESK STUDY AND ON-SITE ASSESSMENT Information Collected Relevance Factor Determined Zoning (County development plan, groundwater protection scheme, groundwater protection response etc.); Hydrological features; Density of houses; Identifies planning controls Site restrictions and targets at risk Proximity to significant sites; Experience of the area; Proximity to surface features; Depth to bedrock Sufficient subsoil to allow Depth to bedrock treatment of wastewater Texture; Structure; Indicators of the suitability of Unsuitability if prismatic, the subsoil for percolation structureless-massive Bulk density; and of its percolation rate silt or clay. Layering; Colour; A minimum thickness of 1.2 m of unsaturated soil is required Mottling; Depth of the water table to successfully treat septic tank effluent Depth to water table; Drainage (permeability); Identifies suitable soils that have adequate but not excessive (T or P value) Percolation test; percolation rates To assist in the selection of the on-site system and to planning applications for on-site systems for a single standardise the assessment process, a site house. A verification section is included at the end characterisation form has been prepared (Appendix of the form and this should be completed by the A ) . The completed form should accompany all planning authority.
3T REATMENT OPTIONS 273. TREATMENT OPTIONS3.1 INTRODUCTION ch a ra c t e ristics suitable for the treatment of wastewater. The percolation characteristics will beThe information collected from the desk study and confirmed later by examining the percolation teston-site assessment should be used in an integrated results. It is important to remember that the subsoilway to determine whether an on-site system can be characteristics (i.e. depth and type of subsoil) as setdeveloped, and if so, the type of system that is out in Table 7, and the control measures outlined inappropriate and the optimum final disposal route of the groundwater protection responses should boththe treated wastewater. Depending on the be satisfied. In some cases the requirements of thecharacteristics of the site, more than one option may groundwater protection responses will be greaterbe available. than 1.2 m of subsoil below the invert of the percolation trench.3.2 CHOOSING AN ON-SITE SYSTEM 3.2.1 SYSTEMS USED WHERE ON-SITEFigure 9 sets out how the information from the desk ASSESSMENT IS SUCCESSFULstudy and on-site assessment is used to choose an on-site system. The procedure for deciding how to If the site satisfies all the specified requirements anddispose of the treated wastewater, i.e., whether it can has a T value between 1 and 50, the site is suitablebe disposed of by soil percolation to groundwater or for the development of a septic tank with a soilbe discharged directly to surface water, is set out in percolation area (conventional septic tank system).the lower half of the diagram. The wastewater froman on-site system cannot be disposed of by soil A septic tank followed by either an intermittent filterp e rc o l ation to groundwater unless the subsoil or a constructed wetland, or a mechanical aerationcharacteristics are suitable for this purpose. unit can also be developed on sites that have suitable percolation characteristics.The desk study information is first examined. Areaswith significant sites e.g. archaeological, natural 3.2.2 SYSTEMS USED IN THE EVENT OF ON-heritage or historical features should be ruled out of SITE ASSESSMENT FAILUREfurther consideration. If past experience indicatesthat there have been problems with the proposed For sites which fail the on-site assessment (refer tosystem in the locality, further investigation may be Figure 9), site improvement works may allow thewarranted before proceeding to the next step. If the development of an on-site system. TheseDesk Study conclusion is that the site is potentially improvement works may include lowering the watersuitable for an on-site system, an on-site assessment table by drainage, increasing the soil depth byshould be carried out. importing suitable soil or replacing existing unsuitable soil. The conditions that give rise to aAs previously mentioned, the on-site assessment high water table are site specific; these includeconsists of a visual assessment, a trial hole and topography, nature of soils, bedrocks and outfalls.percolation tests. The minimum separation distances Some of the problems resulting from theseto pass the visual assessment are set out in Table 4 . conditions are readily solved. Detailed designThese apply to any treatment system. If any of these procedures are available in drainage manuals2 .requirements cannot be met, on-site systems cannotbe developed on the site. Where a site is marginal, Imported soil may be placed in mounds - asfurther investigation may be warranted to identify illustrated in Figure 3 - or level with the groundwhether or not site improvements will suffice. If the surface. The mounds may be constructed partially orvisual assessment indicates that the site is potentially totally overground. Free draining unsaturated soilssuitable, proceed to the trial hole stage of the on-site as detailed in Table 5 should be used. The fill shouldassessment. be placed in layers not exceeding 300 mm thick and lightly compacted. Great care should be taken not toTable 7 sets out the subsoil characteristics which overcompact the soil as this will lead to ponding.indicate satisfactory percolation and other subsoil After each lift is placed, percolation tests should be2 Mulqueen, Rodgers, Hendrick, Keane, McCarthy (1999). Forest Drainage Engineering. COFORD Dublin.
28 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES carried out. A 150 mm square hole is excavated to a • a mechanical aeration system followed by a depth of 150 mm in the placed soil. After presoaking polishing filter; or to completely wet up the soil, 0.5 litres of water is poured into the hole and the time in minutes for the • a septic tank and constructed wetland system water to soak away is recorded. This time should be followed by a polishing filter may be installed between 10 minutes and 2 hours. After these works on the site. have been completed, a second test hole must be excavated in an appropriate location in the improved The treated wastewater from systems other than a soil and a percolation test carried out. conventional septic tank system should be percolated through a polishing filter to reduce microorganisms. Where a minimum of 0.6 m of permeable soil is present or can be placed over the rock and/or the The polishing filter may comprise the in situ soil, water table (i.e. shallow soils) and all other where there is adequate depth of suitable soil, requirements of the on-site assessment are satisfied, imported suitable soil or a combination of the in situ an on-site system can be developed using this soil as and imported soil. Polishing filters may also be a polishing filter in anyone of the following: constructed from medium fine sands placed in layers alternating with layers of 10-20 mm clean washed • a septic tank and an intermittent filter gravels. Typical designs for polishing filters are followed by a polishing filter; given in Chapter 4.
3T REATMENT OPTIONS 29 Desk Study Desk study Fail Archaeological Archaeological Desk study Unsuitable site, NHA Pass This option may not Site always be available improvement Visual Assessment On-site Assessment On-site Assessment Pass Fail Fail Trial Hole Pass Percolation Test Site may not be (T- Test) suitable for any on- site system Pass Fail Conventional septic Wetland Intermittent Mechanical tank system system filter system aeration system Percolation Test (P- Test)* soil sand polishing polishing filter filter Discharge to Discharge** groundwater* A P (or T) test is required to design a soil polishing filter. The hydraulic loading r ate depends on the soil or bedrockand recommended loading rates are as follows: up to 20 l/m2.d for P/T values of 20 or less; up to 10 l/m2.d for P/Tvalues from 21 to 40 and up to 5 l/m2.d for P/T values 41 -50.** The treated wastewaters from the polishing filter may discharge to g roundwater or surface water depending on thenature of the strata underlying the filter. The discharge of treated wastewater from the polishing filters overlyingpermeable soils or bedrocks may go to groundwater. A soil or bedrock with a P/T value of 50 or less is suitable topercolate effluent from a polishing filter to groundwater. The treated wastewater from the polishing filters overlyingsoils or bedrocks with P/T values greater than 50 is collected in a suitable drainage system and discharged to surfacewaters. FIGURE 9: FLOW DIAGRAM FOR CHOOSING AN ON-SITE SYSTEM
30 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES 3.3 CHOOSING THE OPTIMUM DISCHARGE Where it is proposed to discharge wastewater to ROUTE "waters", local authorities should assess the impact of the discharge from the on-site system on the Once the on-site treatment system has been decided receiving water. The parameters to be examined upon, the disposal of the treated wastewater needs to should include: be considered. For septic tank systems with a soil percolation area the treated wastewater will normally • Flow; be discharged to the groundwater. In the case of filters, mechanical aeration systems and wetland • BOD; systems, treated wastewater from the polishing filter may be discharged to the ground or to surface water • Nitrates; (Figure 9). • Ammonia; In the case of a discharge to surface water a licence to discharge is required from the local authority • Phosphates; and under the Water Pollution Acts 1977-1990. Where such a licence is required, the final wastewater • Microorganisms. quality from the on-site system should comply with the requirements set out in the licence. When assessing the impact of an on-site system, local authorities should consider the beneficial uses 3.4 LICENCE REQUIREMENTS of the receiving water. The principal beneficial uses of surface water are, water intended for human The discharge of any sewage effluent to "waters3" consumption after treatment, agriculture, bathing, requires a licence under the Water Pollution Acts b o at i n g, coarse fishery, cooling, game fishery, 1977-1990. Licence applications are processed by general amenity, or industry. Principal beneficial the local authorities. Domestic sewage, however, not uses of groundwater are agriculture, d ri n k i n g, exceeding 5 m3/day, which is discharge d to an industry and raw water intended for human aquifer from a septic tank or other disposal unit, by consumption after treatment. means of a percolation area, soakage pit or other method is not subject to the licensing provisions of Once the beneficial use of the water has been the 1977-1990 Acts. If an on-site system does not established, local authorities should consult relevant comply with all the conditions above, a discharge Regulations, water quality management plans and licence is required for the on-site system. However, any published standards to obtain the relevant it should be noted that a "soakage pit" or similar discharge standard. The treated wastewater from the method is not an acceptable means for treating septic on-site system should comply with the water quality tank effluent and does not comply with the standard set for the receiving waters. requirements set out in this document. 3includes any (or any part of any) river, stream, lake, canal, reservoir, aquifer, pond, watercourse or other inland waters, whether natural or artifical.
4S EPTIC TANKS, PERCOLATION AREAS AND OTHER FILTER SYSTEMS 314. SEPTIC TANKS, PERCOLATION AREAS AND OTHER FILTER SYSTEMS4.1 SEPTIC TANKS 4.1.1 SEPTIC TANK CAPACITYSeptic tanks are primary settlement tanks providing The septic tank should be of sufficient volume toa limited amount of anaerobic digestion. Septic tanks provide a retention time for settlement of theshould comprise two chambers. For best suspended solids while reserving an adequateperformance, septic tanks should have the following volume for sludge storage. The volume required forcharacteristics: sludge storage is the determining factor in sizing the septic tank and this sizing depends on the potential • septic tanks should be much longer than they occupancy of the dwelling which can be estimated are wide to promote settlement of suspended from the maximum number of people that the house solids; can accommodate taking into account the number and types of bedrooms. The tank capacity may be • larger septic tanks are better than smaller calculated from the following formula: tanks because of greater settlement of solids and larger storage volume for liquid and solids; C = 180 . P + 2000 • properly designed baffles provide quiescent conditions and minimise the discharge of where solids to the percolation area; and C = the capacity of the tank (litres) • the inlet and outlet of the septic tank should be separated by a long flow path for the P = the design population with a minimum of 4 wastewater; if the outlet is too close to the persons inlet, solids settlement and grease separation may be inadequate. A minimum capacity of 2720 litres (2.72 m 3) should be provided. This assumes that desludging of theSeptic tanks must be a ble to (i) withstand corrosion septic tank is carried out at least once in every 12-(ii) carry safely all lateral and vertical soil pressures month period. When kitchen grinders are installed,and (iii) accommodate water pressure from inside additional sludge solids are discharged with theand outside the tank without leakage occurring. wastewater and the capacity of the septic tank shouldSeptic tanks must be watertight to prevent (i) be increased by 70 litres for each additional person.wastewater escaping to the soil outside, and (ii) Typical septic tank capacities and dimensions forsurface water and groundwater entering the tank. A rectangular tanks are shown in Table 9, Table 10,leaking tank can cause pollution problems. Figure 10 and Figure 11.
32 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES TABLE 9: TYPICAL CAPACITIES OF SEPTIC TANKS No of Required Dimensions (m) Persons Storage Capacity (litres) length width depth C = 180 . P + 2000 a* d* b* c* 3 2720 2.2 1.0 1.0 1.2 4 2720 2.2 1.0 1.0 1.2 5 2900 2.4 1.0 1.0 1.2 6 3080 2.5 1.0 1.0 1.2 * refer to Figure 11 Cover Ventilation cowl Inlet T-piece Outlet T-Piece Scum layer 300 Freeboard 75 TWL INLE INTLET OUTLET T 350 350 C Sedimentation 200 Gas Buoyed 2200 Flotation 1000 550 liquid layer Sludge Layer SECTION A-A FIGURE 10: LONGITUDINAL SECTION OF A TYPICAL SEPTIC TANK (ALL DIMENSIONS IN MM)
4S EPTIC TANKS, PERCOLATION AREAS AND OTHER FILTER SYSTEMS 33 TWL c 200 450 Section B-B BA A 1000 b 2200 1000 a d B Floor Plan FIGURE 11: PLAN AND SECTION OF A SEPTIC TANK (ALL DIMENSIONS IN MM)
34 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES TABLE 10: TYPICAL DESIGN FEATURES OF A SEPTIC TANK Tank Characteristics Recommended Requirements Tank capacity 2720 litres for 4 persons Tank length to width ratio 2 to 3 : 1 Number of compartments 2 Volume of inlet compartment 2/3 to 3/4 of the total tank capacity Concrete compressive strength 35 N/mm2 at 28 days, minimum Wall thickness 100 mm minimum reinforced concrete or equivalent Roof thickness 125 mm minimum Interior height 1.2 m minimum Liquid depth 0.9 m minimum Freeboard (roof height above liquid) 300 mm Baffle wall liquid opening 450 mm to centre of opening from floor of tank (Figure 10 and Figure 11) Inlet and outlet pipes Minimum internal diameter of 100 mm Bottom end of T-piece 550 mm above floor of tank Difference in elevation of inlet and outlet 75 mm Joints Watertight joints required Ventilation 100 mm diameter pipe in roof with a cowl in each chamber Access covers 600 mm x 600 mm (2 no.) 4.1.2 CONSTRUCTION OF A SEPTIC TANK Standard EN 12566 watertightness test method 4. The design features of a septic tank system are For concrete septic tanks the loss of water measured outlined in Table 10. after 30 min. should be ≤ 0.1 litre per m2 of the internal wet surface area of external walls. For Septic tanks may be cast in situ or may be polyethylene and glass reinforced plastic (GRP) prefabricated from steel, reinforced concrete, glass septic tanks, no leakage is permitted. fibre reinforced concrete or plastic. Two-chambered tanks may be accommodated by having two separate 4.1.4 IN SITU TANKS tanks connected together using a tee-piece baffle in each tank. The principles given for rectangular tanks The following construction standards are should be followed for cylindrical tanks where recommended for in situ tanks: reasonably practical. Some upward adjustment to volumes may be necessary. The roof, outer walls • the floors should be of concrete with a and floors of the tank and all joints should be minimum thickness of 225 mm; watertight. • the walls should be a minimum of 100 mm 4.1.3 WATERTIGHTNESS OF SEPTIC TANKS thick reinforced concrete or equivalent such as 225 mm solid block rendered wall; A septic tank should be watertight up to the top of the tank. Methods employed to test such tanks • the roof should be either cast in situ or should be in accordance with CEN E u ro p e a n precast reinforced concrete; and 4 CEN/TC 165 "Wastewater Engineering" is preparing a series of European standards on small wastewater treatment systems.
4S EPTIC TANKS, PERCOLATION AREAS AND OTHER FILTER SYSTEMS 35 • for safety, the roof should be strong and stable It should be vented by means of a vent pipe above enough to prevent interference from children the eaves of the house (Figure 13). A manhole and others. should be provided for rodding the drain and should be located within one metre of the septic tank. The4.1.5 PREFRABICATED TANKS drain should include, at an appropriate location an access junction, to facilitate a future connection to aPrefabricated tanks should be manufactured from sewer network.suitable materials (e.g. pre-cast concrete, glassreinforced plastic, glass reinforced concrete) and the 4.2 PERCOLATION AREASrequirements stated above for capacity, hydraulics,strength and water-tightness should be observed. The most important component of a conventionalPrefabricated septic tanks are to be preferred to those septic tank system is the percolation area. Septiccast in situ because quality control and testing can be tanks remove most of the suspended solids andundertaken at the factory. In the case of light grease from the wastewater, but it is in theprefabricated tanks, attention should be paid to the percolation area that the wastewater is treated. In therisk of flotation of the tanks as a result of conventional percolation trench method, thegroundwater pressure or surface run-off gaining wastewater is allowed to flow by gravity into aaccess to the excavation. distribution box which distributes the flow evenly into the several distribution pipes in the percolation4.1.6 LOCATION OF SEPTIC TANKS trenches. Wastewater flows out through orifices in the distribution pipes into a gravel underlay whichRecommended minimum distances of separation of then distributes it on to the soil, where it undergoesseptic tanks and percolation areas and filters from a biological, physical and chemical interactions thatvariety of features are shown in Table 4. Provision eliminate or reduce the contaminants. For effectiveshould be made for access for a sludge tanker and treatment, the wastewater must enter the soil; if themaintenance equipment to desludge the tank. base or walls of the percolation trench are compacted or glazed or otherwise damaged during excavation,4.1.7 ANCILLARY CONSTRUCTION they should be scratched with a steel tool such as aMATERIALS rake to expose the natural soil surface. It is equally important that the wastewater remains long enoughAll materials used in the construction of the works in the soil; the residence time is controlled by theshould comply with the requirements of the Building hydraulic loading and the rate of flow into the sidesRegulations and the releva n t Technical Guidance and base of the trench.Document. 4.2.1 HYDRAULIC LOADING RATES4.1.8 DRAIN FROM HOUSE TO SEPTIC TANK The percolation rate through the trench base andThe drain to the septic tank should be at least 100 sidewalls is controlled by the biomat on the floor andmm in diameter. It may be of earthenware, concrete, sides of the trench rather than by the subsoil itself inor uPVC or similar materials. It should be jointed to the case of all suitable subsoils. The percolationgive a watertight drain and should be laid to the rates, measured as they are on virgin subsoil usingminimum gradients listed in Table 11. clean water, cannot be used for the design of the hydraulic distribution system and length of percolation trench (Figure 2). A loading rate of 20 l/m2.d is recommended to take into account the effect TABLE 11: MINIMUM GRADIENTS FOR DRAIN TO of the biomat. The minimum length of percolation SEPTIC TANK trench required is given in Table 12. Drainpipe Material Minimum e.g. at a loading rate of 20 l/m 2.d and a wastewater use of 180 l/person.d, the invert area of percolation Earthenware 1 in 40 trench required for a 4-person household is 36 m2 . Concrete 1 in 40 If the width of the invert of the percolation trench is uPVC 1 in 60 450 mm, then the length of percolation trench required is 80 m.
36 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES 4.2.2 GENERAL TABLE 12: MINIMUM PERCOLATION TRENCH LENGTH No wat e rm a i n s , service pipes, access roads, Number of People Required Length driveways or paved areas should be located within in the House of Trench (m) the percolation area. 3 60 The layout of the distribution pipes should make 4 80 optimum use of the available site and be consistent 5 100 with the recommendations in Table 4 and Table 13. 6 120 Land drainage pipes have narrow slots and are likely to clog; hence they are unsuitable as percolation 7 140 pipes. A plan and section of a conventional septic 8 160 tank system layout is given in and a distribution box 9 180 is detailed in Figure 13 and Figure 14. 10 200 TABLE 13: DETAILS OF A TYPICAL PERCOLATION TRENCH Percolation Trench Characteristics Recommendations Length of distribution pipe 20 m maximum in each trench Minimum separation distance 2 m (2.45 m centre to centre) between percolation trenches (Figure 13) Diameter of pipe from septic tank 100 mm Slope of pipe from tank to 1 in 40 for earthenware or concrete, 1 in 60 for uPVC distribution box Slope of percolation trench from 1 in 200 distribution box Distribution (percolation) pipes • 100 mm bore, perforated (typically at 4,6 and 8 o’clock) smooth wall PVC drainage pipes with perforations of 8 mm diameter at about 75 mm centres along the pipe; or • pipes with similar hydraulic properties. Width of percolation trench* 450 mm Depth of percolation trench About 800 mm below ground surface depending on site (Figure 12) Backfilling of percolation trench 250 mm of 20-30 mm washed gravel or broken stone aggregate (Figure 12) on invert; pipe laid at a 1 in 200 slope surrounded by 20-30mm clean washed gravel or broken stone aggregate and with 150 mm of similar aggregate over pipe; geotextile layer followed by topsoil to ground surface. * any compaction or glazing of the soil surfaces on the invert and sidewalls on the trench should be undone by scratching to expose a natural soil surface with a steel tool such as a steel rake or trowel.
4S EPTIC TANKS, PERCOLATION AREAS AND OTHER FILTER SYSTEMS 37 Ground level Topsoil 300 mm Geotextile Gravel 150 mm 800 mm Distribution Pipe Gravel 100 mm Base of the Gravel 250 mm percolation trench 450 mm 1200 mm Unsaturated Suitable Soil FIGURE 12: SECTION OF A PERCOLATION TRENCH Access chamber(for rodding and possible later connection to public sewer) Percolation area 2m 2.45m 100 mm Ø 100 mm Ø Vents 2.45m 7m (min) to Distribution box 20 m (max.) length septic tank House 10 m (min) to percolation area PLAN Eaves vent Percolation pipework Septic tank Distribution box vents 1 in 40 slope for earthware or 1 in 200 slope concrete: 1 in 60 for uPVC SECTION FIGURE 13: PLAN AND SECTION OF A CONVENTIONAL SEPTIC TANK SYSTEM
38 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES 100 mm 600 mm 100 mm 100 mm Varies to Effluent suit site 600 mm from tank effluent A A from tank 50 mm 100 mm 100 mm 100 mm SECTION PLAN FIGURE 14: PLAN AND SECTION OF A TYPICAL DISTRIBUTION BOX 4.3 CONSTRUCTION REQUIREMENTS FOR accessible, and of suitable size (minimum 600 x 600 PERCOLATION PIPES mm) for maintenance and inspection. 4.3.1 PERFORMANCE OF WORK AND 4.3.5 VEGETATION INSTALLATION OF THE SYSTEMS Th e growth of any type of tree or plant which Earth moving machinery should not circulate over develops an extensive root systems should be limited the percolation area after pipework and backfilling to a minimum distance of 3 m from the percolation of trenches have been completed. Access manholes area. This restriction also applies to the cultivation should be located at ground surface so that they are of crops necessitating the use of machinery, likely to a c c e s s i bl e. The distribution box (Figure 14) disturb the percolation trenches. comprises a chamber which divides the effluent from the septic tank equally between the distribution pipes 4.4 MAINTENANCE OF SEPTIC TANKS AND supplying the percolation area. The distribution box PERCOLATION AREAS must be designed and constructed to ensure equal distribution among the various distribution pipes. If Regular maintenance of the septic tank and necessary, special fittings may be used to facilitate percolation area is very important for the satisfactory this. performance of the system. The septic tank should be desludged a minimum of once per year or when: 4.3.2 GENERAL PRECAUTIONARY MEASURES FOR EXCAVATION OF • scum is noticeable in the second chamber of TRENCHES the tank; and/or Earthworks should normally be carried out on dry • the depth of sludge in the second ground. Trenches should be backfilled as soon as compartment is greater than 400 mm. possible after excavation. The depth of sludge can be checked using the 4.3.3 INSPECTION OF PERCOLATION PIPES following technique: Cutting and drilling of pipes should be clean and (i) use a 2 m pole and wrap the bottom 1.2 m smooth. Before installation, the holes in the with a white rag; percolation pipes should be inspected to ensure that they are not blocked. (ii) lower the pole to the bottom of the tank and hold there for several minutes to allow the 4.3.4 ACCESS AND INSPECTION sludge layer to penetrate the rag; and Access and inspection covers should be visible and (iii) remove the pole and note the sludge line, flush with the ground surface without allowing the which will be darker than the coloration entry of surface water. All covers should be caused by the liquid waste.
4S EPTIC TANKS, PERCOLATION AREAS AND OTHER FILTER SYSTEMS 39The percolation area should be inspected regularly. 4.5 FILTER SYSTEMSSigns of ponding indicate blockage or insufficientpermeability. As discussed in Chapter 3, apart from a soil percolation area (conventional septic tank system) as4.4.1 ADVANTAGES AND DISADVANTAGES already outlined, the wastewater from a septic tankOF CONVENTIONAL SEPTIC TANK can be treated by the following filter systems:SYSTEMS • a soil percolation system such as a placed soilThe advantages of a septic tank and soil percolation often in the form of a mound (intermittent soilsystem include: filter system) as already outlined; • easy management; • an intermittent sand filter followed by a polishing filter (intermittent sand filter • no external power requirements; system); • no noise emissions; • an intermittent peat filter followed by a polishing filter (intermittent peat filter • natural treatment process yielding a high system); quality effluent; • an intermittent plastic and other media filter • cost effective treatment system; and followed by a polishing filter (other intermittent media filter systems); or • no need for polishing filter. • a constructed wetland followed by a polishingThe disadvantages of a septic tank and soil filter.percolation system include: The typical layout for the treatment of wastewater • size of area required; using an intermittent filter or a constructed wetland is illustrated in Figure 15. The site conditions will • greater depths of subsoil to treat the influence the requirement for pumping the wastewater relative to other systems; and wastewater through the different treatment units; however, intermittent filters and most polishing • unsuitable for some subsoil types. filters require pumping. INTERMITTENT FILTER SYSTEM POLISHING SEPTIC TANK or FILTER** CONSTRUCTED WETLAND PUMPING CHAMBER* PUMPING CHAMBER* * Pumping is always required for intermittent filters. If the topography or the design permits, gravity systems may be possible for constructed wetlands and polishing filters ** The intermittent filter and the polishing filter may be combined into one unit in certain cases FIGURE 15: ILLUSTRATION OF AN INTERMITTENT FILTER OR CONSTRUCTED WETLAND SYSTEM
40 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES 4.6 SOIL FILTER SYSTEMS 4.7 SAND FILTER SYSTEMS 4.6.1 INTERMITTENT SOIL FILTER 4.7.1 INTERMITTENT SAND FILTERS An intermittent soil filter system is principally Intermittent sand filter systems can be used to treat applicable to a placed or improved soil with a wastewater from a septic tank. They are normally 1 ≤ T/P ≥ 50 but can also be used on in situ soil with used where the soil is unsuitable for a soil a top layer removed to accommodate gravel. p e rc o l ation system. Intermittent sand filters are Wastewater from the septic tank is allowed to flow effective and easy to operate. They require only into a pumping chamber, in which pump and flow small areas and are relatively cheap to construct. controls are housed. The effluent is then pumped intermittently, 3-4 times per day onto a manifold Two types of intermittent sand filters are commonly with lateral distribution pipes provided with orifices. used, namely, soil covered and open. Soil covered These laterals are arranged in parallel and intermittent sand filters may be underground (Figure surrounded by a gravel layer, resting on top of the 16), part underground and part overground, or soil that often is a mound but can also be level with overground (Figure 3). The latter two constructions ground surface; the gravel distributes the wastewater are commonly referred to as mound systems. Open evenly onto the soil surface. The wastewater dose intermittent sand filters are constructed similar to the enters the soil and percolates slowly downwards. If covered sand filters, but without the soil cover i.e. the the native in situ soil surrounding the placed soil is a gravel distribution layer is exposed at the surface to coarse sand or gravel, the percolation soil and gravel allow for inspection and periodic maintenance. overlay should be lined at the sides; if the native in They are preferably underground with the top of the situ soil is impervious, an underlay of gravel with a gravel at ground surface. pumping sump is required to remove the purified effluent to a surface water body e.g. watercourse, for Intermittent sand filters are single-pass slow sand which a discharge licence may be re q u i re d. filters, which support biofilms. They consist of one Hydraulic loading is 4 l/m2.d on the plan area of the or more beds of graded sand commonly 600 - 900 soil. The layout and arrangement of the pipework mm deep, underlain normally by a layer of filter are described in greater detail in Section 4.7 under gravel about 200 mm thick to prevent outwash or Sand Filter Systems. piping of the sand. Septic tank wastewater is pumped intermittently 3-4 times per day onto the surface of 4.6.2 ADVANTAGES/DISADVANTAGES OF the sand bed through 25 mm diameter lateral pipes SOIL FILTER SYSTEMS with orifices, embedded in a 200 mm thick layer of d i s t ri bution gravel. In soil covered fi l t e rs , a The advantages of soil filter systems include: geotextile is used to separate the soil cover from the distribution gravel. The wastewater from the septic • easy management; tank flows through the sand bed where it receives treatment. The wastewater treatment takes place • high quality effluent; under predominantly unsaturated and aerobic conditions. In a soil covered filter, both the • high operational flexibility that can be used distribution gravel over the sand and the drain filter for nitrification, denitrification and gravel under the sand are vented; the vents are phosphorus removal; extended vertically above ground or mound level and capped with a cowl or grid. In an open filter only the • stable treatment process; and drain filter gravel is vented. • no need for a polishing filter. In impervious soils, all surface run-off and seepage from the surrounding soil should be cut off by The disadvantages of soil filter systems include: shallow interceptor drains, the depth of which depends on the depth to the impervious layer. The • pumping is required for influent distribution; interceptor drain should be 2 m distant from the edge of the sand filter. These drains comprise land • odours may occur from open filters; and drainage pipes overlain to ground surface with permeable gravel or broken stone aggregate. These • filter may clog giving rise to waterlogging. i n t e rc eptor drains are brought to the nearest watercourse or stream into which they outfall. In the case of overground sand filters, the collector drains
4S EPTIC TANKS, PERCOLATION AREAS AND OTHER FILTER SYSTEMS 41to remove the filtrate are excavated in the soil top with uniformity coefficients (D60/D10) less than 4layer at appropriate spacings into the top of the (Table 14). The smaller the effective grain size, theimpervious layer, piped and backfilled with filter higher the level of tre at m e n t , the lower thegravel to original ground surface. The wastewater permissible hydraulic loading and the more frequentfrom the intermittent filter is normally collected in a the need for maintenance. The lower the uniformitychamber, from where it is discharged to a polishing coefficient, the longer the filters life-span and thefilter. In some cases, the in situ topsoil underneath less the potential for elutriation downwards of thethe intermittent filter may have sufficient depth (see finer particles which could result in clogging. Allsection 4.11) on its own or with placed imported soil filter gravels must be designed on filter principles(i.e. site improvement works) to act as a polishing after for example, viz.:filter. D15 Filter gravel/D85 Sand < 5;In very permeable (gravelly) sites, the filtrate fromthe intermittent sand filter, after passing through a 4 < D 15 Filter gravel/D15 Sand < 20;polishing filter, may percolate to the groundwater;filter gravel at the base of the sand filter is not D50 Filter gravel/D50 Sand < 25,required where the polishing filter is directlyunderneath the intermittent filter. An impermeable where D15, D50 and D 85 are the particle sizes from aliner is used to seal off the sides of the intermittentsand filter to prevent possible bypass into the grading curve at 15, 50 and 85% finer by weightgravelly soil when the filter is underground; this ordinates, respectively.bypass could occur when a flooding dose is appliedto the distribution gravel. Where the polishing filter 184.108.40.206 LOADING RATESis offset, the entire intermittent filter must beenclosed in a leak proof liner. Loading rates vary with the characteristics of the filter sand and the type of design. Rates typically220.127.116.11 DESIGN CRITERIA vary from 40 - 100 l/m2.d* . Soil covered filters should have the lowest hydraulic loadings. HigherSand selection is decided first and is based on rates than those above result in impaired wastewatergrading curve characteristics. Effective grain sizes quality and an increase in the frequency of(D10) for soil covered and open sand filters are in the maintenance. Design criteria are shown in Table 14.range 0.7 - 1.0 mm and 0.4 - 1.0 mm respectively* USEPA (1992). Wastewater Treatment/Disposal for Small Communities. Manual No. EPA/625/R-92/005
42 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES Effluent from septic tank Threaded 100 mm perforated underdrain with 5 mm holes Inspection lid 3 mm holes facing up 25 mm PVC lateral PLAN Geotextile Inspection/vent Top soil Distribution pipes Existing ground level Grass embankment Sand 900 mm Filter Clean washed gravel gravel or broken stone 75 mm SECTION FIGURE 16: SCHEMATIC DIAGRAM OF A SOIL COVERED INTERMITTENT SAND FILTER FOR AN IMPERVIOUS SOIL
4S EPTIC TANKS, PERCOLATION AREAS AND OTHER FILTER SYSTEMS 43 TABLE 14: DESIGN CRITERIA FOR INTERMITTENT SAND FILTERS Design Factor Soil Covered Open Effective grain size (D10), mm 0.7 - 1.0 0.4 - 1.0 Uniformity coefficient (D60/D10) < 4.0 < 4.0 Depth of sand filter, m 0.6 - 0.9 0.6 - 0.9 Hydraulic loading, l/m2.d 40 - 60 50 - 100 Dosing frequency, no./day 2-4 1-4USEPA, 19918.104.22.168 APPLICATION OF SEPTIC TANK 4.7.2 ADVANTAGES/DISADVANTAGES OFWASTEWATER SAND FILTER SYSTEMSThe wastewater should be applied uniformly to the The advantages of sand filters include:surface of the filter sand at intervals such that thewastewater completely infiltrates the sand. This • easy management;allows ample aeration of the filter sand to effectaerobic treatment. Even distribution may be • high quality effluent;obtained by pumping the wastewater through evenlyspaced lateral pipes with evenly spaced orifices • high operational flexibility that can be usedembedded in a distribution gravel. Dosing for nitrification, denitrification andfrequencies are related to the type of filter sand. A phosphorus removal; anddosing frequency of 3 - 4 times daily isrecommended. Dosing tanks are sized for the • stable treatment process.maximum daily dose to be used. The disadvantages of sand filters include:In a typical design for a 4-person household with ahydraulic loading of 0.72 m3/day using a loading rate • need for a polishing filter;of 100 l/m2.d, a plan area of 7.2 m2 of intermittentfilter is required. This can be accommodated in a • costs are higher than natural percolationtypical layout of 8 number 25 mm diameter lateral areas;pipes each 1.5 m long and spaced 0.6 m apart, andserved by a central manifold. Orifices with a • pumping is required for influent distributiondiameter of 3 mm are drilled in the lateral pipes at and in most cases for the distribution of theequal spacings of 0.3 m to uniformly distribute the filtrate to a polishing filter;wastewater onto the filter media. The wastewater ispumped onto the intermittent filter in 4 doses, each • odours may occur from open filters; andof 180 litres over a duration of about 2 minutes. • filter may clog giving rise to waterlogging.22.214.171.124 OPERATION AND MAINTENANCE 4.8 PEAT FILTER SYSTEMSIntermittent sand filters require little control andmaintenance. The main tasks are servicing of the Fibrous peat filters are used as intermittent opendosing equipment,monitoring of the wastewater, and filters to treat septic tank wastewater. The peat fibrespossible maintenance of the sand surface in open are placed in modules and compressed. Thesand filters. Soil covered sand filters are expected to thickness or depth of the compressed peat is aboutwork without maintenance throughout their working 0.7 m and its dry density is about 200 kg/m3. Thelife. When desludging the septic tank, the pump surface area of one commercial fibrous peat filter issump should also be desludged. sized at 1 m2/person. The hydraulic loading rate on peat filters varies depending on the type of peat employed. Commercial fibrous peat filters are
44 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES presently designed at hydraulic loading rates in 4.9 OTHER INTERMITTENT MEDIA FILTER excess of 100 l/m2.d. A layer of lightweight coarse SYSTEMS grained aggregate such as shells is placed on top of the peat to promote uniform distribution and Other intermittent media filter systems may from minimise disturbance. A pipe distribution network time to time be introduced to treat wastewater. Such fitted with orifices as illustrated in Figure 16 or a filter products could include geotextile strips and spray irrigation system is used to evenly distribute other media that can be used to attach biofilms. the wastewater that is pumped from the second Many, such as geotextile strips will operate in a chamber of the septic tank or from a plastic sump manner similar to fibrous peat filters while others fitted with a baffle filter, downstream of the septic may employ novel ways to attach biofilms. Where tank. Each module of a modular unit should be such products are introduced, i n d ep e n d e n t provided with a cover. evaluation should be carried out to verify the manufacturer’s design loadings. Other intermittent 4.8.1 OPERATION AND MAINTENANCE media filters should be followed by polishing filters. The surface of the peat filter should be examined 4.10 CONSTRUCTED WETLANDS periodically for signs of ponding. When desludging the septic tank, the pump chamber should also be A constructed wetland system is another option used desludged. to treat wastewater from a septic tank ( Figure 17 and Figure 18)5. As mentioned in Chapter 1, constructed 4.8.2 ADVANTAGES/DISADVANTAGES OF wetlands can be characterised by the flow path of PEAT FILTER SYSTEMS the wastewater through the system. In horizontal flow constructed wetlands, wastewater is introduced The advantages of peat filters include: at one end of a flat to gently sloping bed of reeds and flows across the bed to the outfall end. If the surface • easy management; of the wastewater is at or above the surface of the wetland media, the system is called a free-water • high quality effluent; surface (FWS) hori zo n t a l - fl ow wetland. If the surface of the wastewater is below the surface of the • high operational flexibility that can be used wetland media, the system is called a sub-surface for nitrification, denitrification and (SFS) horizontal flow wetland. As it fl ow s , phosphorus removal; and microorganisms attached to the reeds and support media purify the wastewater. The media can consist • stable treatment process. of soil (free-water surface), gravel or other suitable material. In the vertical-flow wetland (Figure 18), The disadvantages of peat filters include: the wastewater is distributed uniformly over, and intermittently onto the media, and gradually drains • need for a polishing filter; vertically to a drainage network at the base of the media; as the wastewater drains vertically, air re- • costs are higher than natural percolation enters the pores in the media. The media used in the areas; vertical-flow wetland can consist of a layer of sand overlying a laye r o f grave l . The sand must be • pumping is required for influent distribution protected from erosion and piping. and in most cases for the distribution of the filtrate to a polishing filter; • odours may occur from open filters; and • filter may clog giving rise to waterlogging. 5 Cooper, P.F., Job, G.D., Green, M.B. and Shutes, R.B.E. (1996). Reed beds and constructed wetlands for wastewater treatment. Water Research Centre, Swindon, U.K.
4S EPTIC TANKS, PERCOLATION AREAS AND OTHER FILTER SYSTEMS 45 Phragmites Roots & Level Rhizomes surface Inlet Typical depth 0.6m Adjustable discharge outlet Gabion Gravel Gravel Sand & inlet zone Impervious Slope 1% liner FIGURE 17: SUB-SURFACE (SFS) HORIZONTAL FLOW WETLAND Legend: 1 ‘Sharp sand’ 2 6mm washed pea-gravel 3 12mm round washed gravel Intermittent dosing pipe Phragmites 4 30-60mm round washed gravel Inlet Solid pipe ~ 80mm 1 Perforated pipe Graded 2 ~ 150mm filter material ~ 100mm 3 ~ 150mm 4 Outlet liner drainage 1% slope pipes FIGURE 18: VERTICAL FLOW WETLAND4.10.1 OPERATION AND MAINTENANCE 4.10.2 DESIGN CRITERIAConstructed wetlands should be inspected weekly. For a horizontal-flow sub-surface wetland, the planFlow distribution should be carefully examined for area (Ap) requirement for BOD removal is about 5channelling and orifice blockage. Sidewalls should m2/person. The depth of the bed is about 0.6 mbe maintained. Rabbits, weeds and plant diseases can placed on a base with a slope of 0.5-2.0%. The widthcause damage to the reeds. Solids from the of the constructed wetland can be calculated fromwastewater will reduce the pore space in the media the following equation:especially at the inlet end of a horizontal-flowwetland and it may be necessary to replace some of Ac = Q/( k.i)the media after a period of time. Vegetation growth,flows, and influent and wastewater quality should be where Ac is the cross sectional area (m2) of themonitored. The wetland should be securely fenced wetland at the inlet, Q is the average wastewater flowoff.
46 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES rat e ( m3/d), k is the permeability (m/d) of the 4.11.1 SOIL POLISHING FILTERS wetland media and i is the hydraulic gradient equal to the slope of the bed. For a four-person household, Soil polishing filters may comprise in situ soil, Ap is 20 m2; for a wetland media, with a depth of 0.6 improved soil or imported soil. Where grass growth m and a permeability of 50 m/d (sand), laid at a is allowed, there can be a large reduction in NO3-N. gradient of 1%, a suitable width is 2.5 m and the These soils should have percolation values (P or T) length would then be 8 m. in the range of 1-50. Dosing may be by gravity or by pumped arrangements. 4.10.3 ADVANTAGES/DISADVANTAGES OF CONSTRUCTED WETLANDS In typical layouts, the polishing filter: The advantages of constructed wetlands include: (i) may underlie an intermittent filter such as a peat filter with the effluent being spread out • low construction and running costs; over a shallow gravel layer (Figure 19); any exposed polishing filter area may be soil • easy management; covered and grassed (Option 1); • excellent reduction of biochemical oxygen (ii) may underlie or may be offset from a demand (BOD5) and suspended solids (SS) secondary treatment unit; loading may be from septic tank effluents; and by a pumped arrangement as in an intermittent sand filter (Figure 20) (Option • secondary benefits in terms of potential 2); and wildlife habitat enhancement. (iii)may be offset from the secondary treatment The disadvantages of constructed wetlands include: system; loading may be by gravity into percolation trenches (Figure 21) (Option 3). • lack of agreed design criteria; Recommended loading rates for polishing filters are: • need for a polishing filter; • up to 20 l/m 2.d for P/T values 1-20; and • systems remain unproven for other than BOD5 and SS removal; • up to 10 l/m 2.d for P/T values 21-40; and • security and safety; • up to 5 l/m 2.d for P/T values 41-50. • concern about disease vectors; and 126.96.36.199 Option 1- Direct Discharge • difficulty in maintaining uniform distribution In the case of spreading the secondary effluent over of flow at inlet. a polishing filter using a distribution gravel and with direct discharge from the polishing filter to 4.11 POLISHING FILTERS groundwater through soil or bedrock the loading rates on the soil or bedrock should not exceed All intermittent filter systems, constructed wetlands (Figure 19); 20 l/m2.d for P/T values 1-20, 10 l/m2.d and mechanical aeration systems (discussed later) for P/T values 21-40 and 5 l/m2.d for P/T values 41- require a polishing filter following the secondary 50. treatment stage (Figure 15). The polishing filter can reduce microorganisms, phosphorus, and nitrate nitrogen in otherwise high quality wastewater effluents. Figure 19, Figure 20, Figure 21 and Figure 22 illustrate three possible loading arrangements for polishing filters. Polishing filters may be of two types: soil or sand.
4S EPTIC TANKS, PERCOLATION AREAS AND OTHER FILTER SYSTEMS 47 Secondary treatment unit e.g. intermittent peat filter Ground Level grassed soil overlay distribution gravel soil polishing filter (size dependent on P or T value) to groundwater FIGURE 19: INTERMITTENT FILTER OVERLYING AND LOADING A SOIL POLISHING FILTER 3 mm holes facing up Secondary 25 mm lateral treatment unit pumping chamber Polishing filter FIGURE 20: SECONDARY TREATMENT UNIT FOLLOWED BY A SOIL POLISHING FILTER188.8.131.52 Option 2 - Pumped Discharge 184.108.40.206 Option 3 – Gravity Pipe DischargeIn the case of hydraulic loading by pumping, in a soil In the case of loading through percolation trencheswith a P/T value between 21 and 40 the minimum with a P/T value of 1-50 (Figure 21), a greater areaarea of polishing filter required for a 4 person is required. For trenches 450 mm wide at 2 mhousehold (i.e. 4 x 180 l/person/d) is 0.72m3/0.01m spacing the minimum length required for a 4 person= 72m 2 (9m long x 8m wide). Figure 20 illustrates household is 64m and the land area is 157m2, basedthe loading arrangement for such a soil polishing on a P/T value of 21-50 and a loading rate of 25filter. The treated wastewater from the secondary l/m2.d on the trench base. The loading rate can bet re atment unit is pumped to a manifold and increased to 50 l/m2.d for subsoils with a P/T valuedistribution pipes provided with 3 mm diameter in the range 1-20. The length of percolation trenchorifices. The distribution pipes are embedded in a for secondary treated wastewater for the different100-200 mm thick layer of gravel. percolation values is give n i n Tabl e 1 5 . Treated wastewater from the secondary filter flows by
48 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES gravity to a distribution box which distributes the is impervious, a graded gravel layer with drains flow evenly into the several trenches. should underlie the polishing filter and these drains should outfall to a watercourse or stream. Where a All soil polishing filters must have a minimum polishing filter is constructed in contact with a very thickness of 600 mm of free-draining unsaturated permeable gravel or sand stratum in the soil and is soil between the point of infiltration of the effluent pressure dosed into a surface gravel layer, the sides and the water table and bedrock. They may be at of the filter must be enclosed by an impervious liner ground surface or partially or totally above ground to prevent bypass of flooding doses directly to the surface (Figure 3). Where the native soil at the site groundwater. 2.45 m Secondary Gravity treatment flow Vents unit Percolation trench Distribution box FIGURE 21: SECONDARY TREATMENT UNIT FOLLOWED BY A PERCOLATION TRENCH TABLE 15: MINIMUM TRENCH LENGTHS IN A SOIL POLISHING FILTER Estimated maximum Required length of Required length of number of people in trench* (m) for trench* (m) for the house based on T/P values T/P values number of bedrooms 21-50 1-20 (loading at 25 l/m 2.d) (loading at 50 l/m 2.d) 3 48 24 4 64 32 5 80 40 6 96 48 7 112 56 8 128 64 9 144 72 10 160 80 * 450 mm trench
4S EPTIC TANKS, PERCOLATION AREAS AND OTHER FILTER SYSTEMS 494.11.2 SAND POLISHING FILTERS on a 75 mm layer of pea-sized gravel. The middle sand layer is 100 mm of medium-fine sand 0.2-0.7Sand polishing filters comprise stratified sand filters m m w i t h a D10 of 0.29 mm and a uniformity( Figure 22 and Figure 23)6. In a typical layout, three coefficient of 1.7. This layer rests on a 75 mm la yerlayers of sand decreasing in coarseness with depth of pea-sized gravel. The bottom sand layer is a 200are separated from each other by thin layers of mm layer of fine sand 0.1-0.5 mm with a D10 of 0.18washed pea-sized gravel. The surface layer mm and a uniformity coefficient of 1.7. This layercomprises a pea-sized gravel, e.g. 10-20 mm gravel rests on a 100 mm layer of pea-sized gravel, which isaggregate of 100 mm thickness in which pressure underlain by a laye r o f graded gravel in whichdistribution pipes are placed overlain by a geotextile drainage pipes are placed. A thin (50 mm) layer ofand soil cover. The gravel layer serves to distribute sand below the graded gravel protects the liner fromthe secondary effluent evenly over the underlying damage. The hydraulic loading should not exceed 60sand layer. The top layer of sand is a 200 mm thick 1/m2.d. The filter can be soil covered and sown withlayer of 0.4 - 1.4 mm coarse sand with a D10 of 0.56 grass.mm and a uniformity coefficient of 1.7. This rests 3 mm holes facing up Secondary 25 mm lateral treatment unit pumping chamber Sand polishing filter FIGURE 22: SECONDARY TREATMENT UNIT FOLLOWED BY A SAND POLISHING FILTER 100 mm distribution gravel (10-20 mm) 4.11.3 PAST EXPERIENCE (DESK STUDY) 200 mm sand (0.4-1.4 mm); D10 = 0.56 All intermittent filter systems, constructed wetlands and mechanical aeration systems require a polishing 75 mm pea gravel (10-20 mm) filter following the secondary treatment stage. The 100 mm sand (0.2-0.7 mm); D10 = 0.29 polishing filter produces a high quality effluent. The 75 mm pea gravel (10-20 mm) advice provided above allows effluent from a polishing filter to discharge to ground provided the 200 mm sand (0.1-0.5 mm) D10 = 0.18 subsoil has a P/T value less than 50. Otherwise the discharge must be directed to surface water and a 100 mm pea gravel (5-10 mm) licence obtained from the local authority. However, 150 mm graded gravel (5-30 mm) where previous experience (Desk study) suggests, 50 mm sand (0.2-0.7 mm); D10 = 0.29 that a subsoil with a T/P value greater than 50 will not result in ponding, consideration may be given in FIGURE 23: SCHEMATIC CROSS SECTION OF A SAND such circumstances to permit the effluent to POLISHING FILTER discharge to ground. In any case, where soil is used as the polishing filter (as an alternative to a stratified sand filter), such soil must have a T/P value in the range 1-50.6 Nichols, D. J., Wolf, D. C. Gross,M. A.,and Rutledge, E. M., “Renovation of Septic Effluent in a Stratified Sand Filter".ASTM STP 1324. American Society for Testing and materials, 1997.
50 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES
5M ECHANICAL AERATION SYSTEMS 515. MECHANICAL AERATION SYSTEMS5.1 GENERAL units, with all chambers in one unit. BAF systems are normally constructed in either glass reinforcedMechanical aeration systems may be used to treat plastic (GRP), concrete or steel.wastewater from a dwelling house where a site isunsuitable for a conventional septic tank system or The BAF system is a biofilm system. Thethey may be used as an alternative to septic tank microorganisms are attached to the filter media in thesystems on suitable sites. The effluent from all secondary treatment stage. The media normally havemechanical aeration systems should be treated on a a high specific surface area (m2/m3) and can consistpolishing filter. Many systems are available on the of plastic modules or a granular material. Wheremarket and include the following: granular media are used the system may require backwashing to prevent clogging of pore spaces. The • biofilm aerated filter (BAF) systems; required surface area of the media can be determined using an organic loading rate of 5 g BOD/ m2.d of • rotating biological contactor (RBC) systems; settled sewage. For a single house with 4 persons,the and required area is about 32 m2 based on a per capita loading of 40 g BOD/d of settled sewage. • sequencing batch reactors (SBR). Normally the BAF system provides carbonaceousMechanical aeration systems should be assessed oxidation but can be designed to provideunder the following headings: nitrification. Grease should not be allowed to enter the aerated zone. • Costs: these should include the capital, running and maintenance costs; 5.2.1 OPERATION AND MAINTENANCE • Experience with proposed system: In the case of BAF systems the owner normally takes information sought from other users should out a maintenance contract with the manufacturer. include: This is advisable due to the high level of skill necessary to service and maintain pumps and • durability of the components of the system; compressors. BAF systems require desludging, and a properly designed BAF system should provide for • ease of operation and inspection; and adequate sludge storage capacity in the primary settlement compartment. Sludge levels should never • frequency of maintenance. be allowed to rise, as any entry of sludge into the media compartment will cause problems. AllAppendix C contains an evaluation form which mechanical and electrical components requireshould be completed to compare the available on-site periodic checking. In many cases, manufacturerstreatment systems. install an alarm system to alert the owner of malfunction.5.2 BAF SYSTEMS 5.2.2 ADVANTAGES AND DISADVANTAGESBiofilm aerated filter (BAF) systems can be used tot re at wastewater from single dwellings. A BAF In terms of treating wastewater from singlesystem may consist of a primary settlement tank, an dwellings, the BAF system offers the followingaerated submerged biofilm filter and a secondary advantages:settlement tank. Solids are sometimes returned fromthe secondary settlement chamber to the primary • ease of operation;settlement chamber to facilitate desludging and toavoid sludge rising due to denitrification. There • low noise level;should be adequate sludge storage capacity in thep ri m a ry settlement chamber. Normally BAF • can function under conditions of shocksystems which are used to treat wastewater from loading, which are common in singlesingle dwellings can be purchased as prefabricated dwelling situations;
52 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES • possibility of nitrification and denitrification 5.3.2 ADVANTAGES AND DISADVANTAGES if properly designed; and The advantages of an RBC system for use in single • low fly nuisance. dwelling situations include: Disadvantages include: • ability to function under conditions of shock loading, which is common in single house • need for a polishing filter; situations; • package plants may have small sludge • low noise levels; storage volumes that could lead to overloading of the biofilm; • no fly nuisance; • pumps and compressor, which are usually • low head loss through the system; and required, will need skilled maintenance; and • possibility of nitrification and denitrification, • grease, if allowed to enter the aeration zone if properly designed. may cause problems with media. Disadvantages include: 5.3 RBC SYSTEMS • need for a polishing filter; A rotating biological contactor (RBC) system consists of a primary settlement tank, a secondary • package plants may have small sludge treatment compartment and a secondary settlement storage volume that could lead to overloading tank. In this system the microorganisms are attached of the biofilm; to an inert media surface and the inert media are mounted on a shaft that is turned by an electric • grease, if allowed to enter the contactor zone motor. These media are partially submerged in the may cause problems by coating the media; wastewater. A biofilm develops on the media over and time; it is this biofilm which treats the wastewater. The settled sludge in the secondary settlement tank is • skilled personnel are required to service and sometimes returned to the primary settlement tank. maintain motor and pumps. There should be adequate sludge storage capacity in the primary settlement chamber. RBC units can be 5.4 SEQUENCING BATCH REACTOR purchased as packaged treatment units for single dwellings; these units normally contain all three The sequencing batch reactor (SBR) process is a compartments in one unit. The required surface area form of activated sludge treatment in which aeration, of the media can be determined using an organic settlement, and decanting can occur in a single loading rate of 5 g BOD/ m2.d of settled sewage. For reactor. The process employs a five-stage cycle: fill, a single house with 4 persons, the required media react, settle, empty and rest. Wastewater enters the surface area is about 32 m2 based on a per capita reactor during the fill stage; typically, it is loading of 40 g BOD/d of settled sewage. Grease aerobically treated in the react stage; the biomass should not be allowed to enter the contactor zone. settles in the settle stage; the supernatant is decanted during the empty stage; sludge is withdrawn from 5.3.1 OPERATION AND MAINTENANCE the reactor during the rest stage; and the cycle commences again with a new fill stage. For single In the case of the RBC system, the owner normally house systems the reactor is preceded by a primary takes out a maintenance contract with the settlement tank. Grease should not be allowed to m a nu fa c t u re r. A properly designed RBC system enter the reactor. should provide for adequate sludge storage capacity in the primary settlement compartment. RBC Critical components of an SBR system include the systems require desludging. All mechanical and aeration/mixing process, the decant process, and the electrical components require periodic checking. control process. SBR systems are capable of The structural condition of the RBC unit should be producing a high-quality effluent. They can be checked periodically. Any unusual noise from the modified to remove nitrogen and phosphorus. unit should be investigated. Since the SBR system provides batch treatment of
5M ECHANICAL AERATION SYSTEMS 53 TABLE 16: DESIGN CRITERIA FOR THE SBR PROCESS* Parameter Range Total tank volume 0.5 - 2.0 times average daily flow Number of tanks Typically 2 or more Solids retention time (days) 20 - 40 Aeration system Sized to deliver sufficient oxygen during aerated fill and react stage Cycle times (hr) 4-12 (typical)wastewater, it can accommodate wide variations in reactor;flow rates that are typically associated with singlehouses. The SBR technology is well established in • need for a polishing filter;other countries. • package plants may have small sludgeDesign criteria for the SBR process are summarised storage volume that could lead to overloadingin Table 16. of the system;5.4.1 OPERATION AND MAINTENANCE • pumps and valves are used and these require skilled maintenance; andIn the case of the SBR system, the owner normallytakes out a maintenance contract with the • grease may cause problems if allowed tom a nu fa c t u re r. A properly designed SBR system enter the reactor.should provide for adequate sludge storage capacityin the primary settlement compartment. SBR 5.5 OTHER TREATMENT SYSTEMSsystems require desludging. All mechanical andelectrical components require periodic checking. Other treatment systems may be introduced fromThe structural condition of the SBR unit should be time to time to treat wastewater. Such systemschecked periodically. Any unusual noise from the include other activated sludge systems, membraneunit should be investigated. bioreactors and composting units. Where such products are introduced independent eva l u at i o n5.4.2 ADVANTAGES AND DISADVANTAGES should be carried out to verify the manufacturer’sOF SEQUENCING BATCH REACTORS design loadings. In addition, the evaluation criteria set out in Appendix C should be consulted.SBR systems have the following advantages: Polishing filters should typically follow such systems to reduce micro-organisms to re q u i re d • simple and reliable; levels. • ideally suited for wide flow variations; 5.6 LOCATION OF MECHANICAL AERATION SYSTEMS • high quality wastewater achievable; and Recommended minimum distances of separation of • high operational flexibility, which can be mechanical aeration treatment systems should be as used for nitrification, denitrification and listed in Tabl e 4 . The recommended minimum phosphorus removal. distances from wells should satisfy the requirements of the groundwater protection response, whichSBR systems have the following disadvantages: should have been reviewed during the desk study. In some cases, the requirements of the groundwater • complex control system; protection scheme may be greater than the distances set out in Table 4. All mechanical aeration systems • frequent sludge wasting required in the require desludging, usually, at yearly intervals and* USEPA, 1992
54 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES provision should be made for access for a sludge does not produce an outflow with a low BOD and tanker. suspended solids concentrations, the polishing filter may clog. 5.7 POLISHING FILTERS FOR MECHANICAL AERATION SYSTEMS Polishing filter systems should be designed in accordance with the procedures outlined in 4.11- The treated wastewater from mechanical aeration Polishing Filters. A typical layout for the treatment systems should be treated in a polishing filter of wastewater using a mechanical aeration system is system, the primary purpose of which is to reduce illustrated in Figure 24. microorganisms numbers in the treated wastewater to required levels. If the mechanical aeration system POLISHING MECHANICAL AERATION SYSTEM FILTER PUMPING CHAMBER* * If the topography or the design permits, gravity systems may be possible. FIGURE 24: MECHANICAL AERATION AND POLISHING FILTER SYSTEM
REFERENCES AND FURTHER READING 55REFERENCES AND FURTHER READING • Cooper, P.F., Job, G.D., Green, M.B. and Shutes, R.B.E. (1996). Reed beds and constructed wetlands for wastewater treatment. Water Research Centre, Swindon, U.K. • Department of Environment and Local Government, Environmental Protection Agency, Geological Survey of Ireland (1999). Groundwater Protection Schemes. Geological Survey of Ireland, Dublin. • Department of Environment and Local Government, Environmental Protection Agency, Geological Survey of Ireland (2000). Groundwater Protection Responses for On-site Systems for Single Houses. Geological Survey of Ireland, Dublin. • EPA (1999). Wastewater Treatment Manuals: Treatment Systems for Small Communities, Business, Leisure Centres and Hotels. • Mulqueen J., Rodgers M., Hendrick E., Keane M., McCarthy R., (1999). Forest Drainage Engineering. COFORD Dublin. • Nichols, D. J., Wolf, D. C. Gross, M. A., and Rutledge, E. M., (1997). Renovation of Septic Effluent in a Stratified Sand Filter. ASTM STP 1324. American Society for Testing and materials,. • US EPA, (1992). Wastewater Treatment/Disposal for Small Communities, Manual No. EPA/625/R- 92/005.
56 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES GLOSSARY Biofilm: a thin layer of microorganisms and organic polymers attached to a medium such as soil, sand, peat, and inert plastic material; Biofilm aerated filter a treatment system normally consisting of a primary settlement tank, an aerated (BAF): biofilm and, possibly, a secondary settlement tank. The system is similar to the conventional percolating filter system except that the media are commonly submerged and forced air is applied; Biochemical oxygen BOD is a measure of the rate at which microorganisms use dissolved oxygen in demand (BOD): the biochemical breakdown of organic matter in wastewaters under aerobic conditions. The BOD 5 test indicates the organic strength of a wastewater and is determined by measuring the dissolved oxygen concentration before and after the incubation of a sample at 20°C for five days in the dark. An inhibitor may be added to prevent nitrification from occurring; Biomat: a biologically active layer that covers the bottom and sides of percolation trenches and penetrates a short distance into the percolation soil. It includes complex bacterial polysaccharides and accumulated organic substances as well as microorganisms; Chemical oxygen demand COD is a measure of the amount of oxygen consumed from a chemical oxidising (COD): agent under controlled conditions. The COD is generally greater than the BOD as the chemical oxidising agent will often oxidise more compounds than microorganisms; Constructed wetlands a wastewater treatment system which includes a septic tank, providing mainly (CW): primary treatment, followed by a wetland system supporting vegetation, which provides secondary treatment by physical and biological means. Constructed wetlands are also used for tertiary treatment; Conventional septic tank a wastewater treatment system that includes a septic tank mainly for primary system: treatment, followed by a percolation system in the soil providing secondary and tertiary treatment; Distribution box: a chamber between the septic tank and the percolation area, arranged to distribute the tank wastewater, in approx i m at e ly equal quantities, through all the distribution pipes leading from it; Groundwater protection control measures, conditions or precautions recommended as a response to the response: acceptability of an activity within a groundwater protection zone as set out in the DELG/EPA/GSI document Groundwater Protection Responses for On-site Systems for Single Houses; Groundwater protection a scheme comprising two main components: a land surface zoning map which scheme: encompasses the hydrogeological elements of risk and a groundwater protection response for different activities; Mottling: the occurrence of reddish/brown spots or streaks in a matrix of dark grey soil; the reddish/brown spots or streaks are due to intermittent aeration and the grey colours may be due to anaerobic conditions; Organic Matter: mainly composed of proteins, carbohydrates and fats. Most of the organic matter in domestic wastewater is biodegradable. A measure of the biodegradable organic matter can be obtained using the biochemical oxygen demand (BOD) test;
GLOSSARY 57Pathogenic Organisms: these potential disease producing microorganisms can be found in domestic wastewaters. Organisms, such as E. coli, and Faecal streptococci, with the same enteric origin as the pathogens are used to indicate whether pathogens may be present or not in the wastewater;Peat filter: a filter system consisting of peat used to treat wastewater from a primary settlement tank (usually a septic tank) by biological and physical means;Perched water table: unconfined groundwater separated from an underlying body of groundwater by an impervious or perching layer;Percolating filter system: a wastewater treatment system consisting of primary settlement and biological treatment (effected by distributing the settled liquid onto a suitable inert medium to which a biofilm attaches) followed by secondary settlement;Percolation area a system consisting of trenches with pipes and gravel aggregates, installed for the(soil absorption system) purpose of receiving wastewater from a septic tank or other treatment device and transmitting it into soil for final treatment and disposal. This system is also called a drain field, seepage field or bed, distribution field, subsurface disposal area, or the treatment and disposal field;Preferential flow a generic term used to describe the process whereby water movement follows favoured routes through a porous medium bypassing other parts of the medium. Examples include, pores formed by soil fauna, plant root channels, weathering cracks, fissures and/or fractures;Rotating biological a contactor consisting of inert plastic modules mounted in the form of a cylindercontactor (RBC): on a horizontal rotating shaft. Biological wastewater treatment is effected by biofilms that attach to the modules. The biological contactor is normally preceded by primary settlement and followed by secondary settlement;Sand filter: a filter system consisting of sand used to treat wastewater from a primary settlement tank (usually a septic tank) by biological and physical means;Sludge: the material which settles in the bottom of the primary/secondary settlement tank;Soil structure: the combination or arrangement of individual soil particles into definable aggregates, or peds, which are characterised and classified on the basis of size, shape, and degree of distinctiveness;Soil texture: the relative proportion of various soil components, including sands, silts, and clays, that make up the soil layers at a site;Soil (topsoil): the upper layer of soil in which plants grow;Subsoil: the soil material beneath the topsoil and above rock;Suspended solids (SS): includes all suspended matter, both organic and inorganic. Along with the BOD concentration, SS is commonly used to quantify the quality of a wastewater;Total nitrogen: mass concentration of the sum of Kjeldahl (organic and ammonium nitrogen), nitrate and nitrite nitrogen;Total phosphorus mass concentration of the sum of organic and inorganic phosphorus;
58 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES Trench: also referred to as a percolation trench, means a ditch into which a single percolation pipe is laid, underlain and surrounded by gravel. The top layer of gravel is covered by soil; Unsaturated soil: a soil in which some pores are not filled with water; these contain air; Wastewater: the discharge from sanitary appliances, e.g. toilets, bathroom fittings, kitchen sinks, washing machines, dishwashers, showers etc.; Water table: the level of the surface of the groundwater in a trial hole or other test hole.
APPENDIX A 59APPENDIX A: SITE CHARACTERISATION FORM1.0 GENERAL DETAILS (From planning application)PLANNING APPLICATION Ref. no.:NAME & ADDRESS OF APPLICANT:SITE LOCATION AND TOWNLAND:TELEPHONE NO: FAX NO: E-MAIL:MAXIMUM NO. NO. OF NO. OFOF RESIDENTS: DOUBLE SINGLE BEDROOMS: BEDROOMS:PROPOSED CAPACITY OF SEPTIC NUMBER OF CHAMBERS:TANK (litres) (if applicable): mains private well/borehole group well/boreholePROPOSED WATER SUPPLY:(tick as appropriate)2.0 DESK STUDYSoil type: Bedrock type:Subsoil type: Aquifer type:Vulnerability class: Groundwater protection response:Presence of significant sites (archaeological, natural and historical):Zoning in county development plan:Past experience in the area:Comments:(Integrate the information above in order to comment on: the potential suitability of the site, potentialtargets at risk, and/or any potential site restrictions).
60 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES Sketch of site showing measurement to Trial Hole location and Percolation Test Hole locations, wells and direction of groundwater flow, proposed house (incl. distances from boundaries) adjacent houses, watercourses, significant sites and other features. North point should always be included. [A copy of the site layout drawing should be used if available]
APPENDIX A 613.0 ON-SITE ASSESSMENT 3.1 Visual AssessmentTOPOGRAPHY: SLOPE:LANDSCAPE: STEEP (>1:5) SHALLOW (1:5-1:20)GEOLOGY: RELATIVELY FLAT (<1:20)SURFACE FEATURES:OUTCROPSHOUSESDITCHES*WELLS*SPRINGSKARST FEATURESROADSWATERCOURSE*LAKES/SURFACE WATER PONDING/BEACHES/SHELLFISH AREAS/WETLANDSSITE BOUNDARIESEXISTING LAND USE* note water levelLOCAL DRAINAGE:TYPE OF VEGETATION:GROUND CONDITION:COMMENTS:(Integrate the information above in order to comment on: the potential suitability of the site, potentialtargets at risk, the suitability of the site to treat the wastewater and the location of the proposedsystem within the site).
62 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES 3.2 Trial Hole Hole should be approximately 1m x 0.75m in plan and a minimum of 2.1 m deep Date and time of Date and time of Depth of Trial Hole (m): Excavation: Examination: Depth from ground surface to bedrock (m): Depth from ground surface to water table (m): Soil type: Subsoil type: Additional Soil/Subsoil Information Texture Structure Bulk Colour* Preferential 0.1 m density flowpaths 0.2 m 0.3 m 0.4 m 0.5 m 0.6 m 0.7 m 0.8 m 0.9 m 1.0 m 1.1 m 1.2 m 1.3 m 1.4 m 1.5 m 1.6 m 1.7 m 1.8 m 1.9 m 2.0 m 2.1 m 2.2 m 2.3 m 2.4 m 2.5 m Other information (e.g. depth of water ingress) * All signs of mottling should be recorded
APPENDIX A 63 3.3 Percolation Test Type of test: T-Test or P-TestPercolation Test Hole 1 2Depth from ground surface to top of hole (mm) (A)Depth from ground surface to base of hole (mm) (B)Depth of hole (mm) [B - A]Dimensions of hole [length x breadth (mm)]Each hole must be pre-soaked twice before the test is carried out (from 10.00 am to 5.00 pm andfrom 5.00 pm to next morning)Date of test:Date pre-soaking started:Time filled to 400 mmTime water level at 300 mm Percolation 1 2 Test Hole No. Fill no. Start Time Finish Time ∆t (min) Start Time Finish Time ∆t (min) (at 300 mm) (at 200 mm) (at 300 mm) (at 200 mm) 1 2 3 Average ∆t Average ∆t Average ∆t/4 = [Hole No.1] _____(t1) Average ∆t/4 = [Hole No.2] _____(t2)T value = (t 1 + t2)/2 =_______(min/25 mm)Result of Test : T=Comments:
64 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES 4.0 CONCLUSION: (Integrate the information from the desk study and on-site assessment (i.e. visual assessment, trial hole and percolation tests) above and conclude the type of system that is appropriate. This information is also used to choose the optimum final disposal route of the treated wastewater). Suitable for (delete as appropriate): (a) septic tank and soil percolation system (b) septic tank and intermittent filter system and polishing unit; or septic tank and constructed wetlands and polishing unit (c) mechanical aeration system and polishing unit and SUITABLE for discharge to surface water/groundwater (delete as appropriate) 5.0 RECOMMENDATION: Propose to install:. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . and discharge to surface water/groundwater (delete as appropriate) Signed: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Address . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Qualifications . . . . . . . . . . . . . . . . . . . . . . . . . . . Date of Report: . . . . . . . . . . . . . . . . . . . . . . . . . Phone: . . . . . . . . . . . . . . . . . . . . . Fax:. . . . . . . . . . . . . . . . . . . . . . E-mail: . . . . . . . . . . . . . . . . . . . 6.0 VERIFICATION (by Local Authority): Site visit Ë Date: Inspection of Trial Hole Ë Date: Inspection of Percolation Test Holes Ë Date: Comments SIGNED: Date:
APPENDIX B 65APPENDIX B: PERCOLATION TESTSThe percolation test comprises the measurement of the length of time for the water level in the percolation testhole to fall from a height of 300 mm to 200 mm above the base of the hole.PERCOLATION TEST (T Test) PROCEDUREDay 1: Two percolation test holes should be dug in the proposed percolation area. Each hole should be 0.3 mx 0.3 m and 0.4 m deep below the proposed invert level of the distribution pipe (Figure 25). The bottom andsides of the hole should be scratched with a knife or wire brush to remove any compacted or smeared soilsurfaces and to expose the natural soil surface.Clear water should be carefully poured into the hole at about 10.00 am so as to fill it to the full height of 0.4m. The water should then be allowed to percolate. At about 5.00 pm the hole should once again be filled tothe full height of 0.4 m and allowed to percolate overnight.Day 2: The hole should be filled with clear water at about 10.00 am and the water should be allowed to dropsuch that there is 0.3 m of water in the hole. Thereafter, the time in minutes required for the water to drop 100mm, that is from 0.3 to 0.2 m, in the hole should be recorded. The hole should then be refilled to the 0.3 mlevel again and the water allowed to drain to the 0.2 m level and the time again recorded. The filling andmeasurement of the percolation rate through the hole should be repeated two times – three tests in all.The average value in minutes of the three recordings should then be divided by 4 to give the time required fora fall of 25 mm or the percolation value "t". The same procedure should be repeated in the second hole in thepercolation area.TEST RESULTSThe time for the 25 mm drop ("t") for each of the test results in the percolation area should be averaged togive the value "T". A proposed percolation area whose "T" value is less than 1 or greater than 50 should bedeemed to have failed the test. Ground level proposed invert Invert of of distribution pipe pipe Water level 0.4 m 0.3 m 0.3 m square FIGURE 25: PERCOLATION TEST HOLE
66 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES PERCOLATION TEST (P Test) FOR SOIL POLISHING FILTERS To establish the percolation value for soil polishing filters and to determine the discharge route for secondary treated effluent where shallow subsoils exist, a modification of the percolation test as described above is required. The test procedure is identical to that outlined above but in this situation the trial hole dimensions should be 0.3 m x 0.3 m and 0.4 m deep below the ground surface (Figure 26). To avoid confusion with the previous test, this test is called a P type percolation test and the values are referred to as P values. TEST RESULTS The time for the 25 mm drop ("t") for each of the test results in the percolation area should be averaged to give the value "P". A proposed percolation area whose "P" value is less than 1 or greater than 50 should be deemed to have failed the P test. Ground surface Water level 0.4 m 0.3 m 0.3 m square FIGURE 26: PERCOLATION TEST HOLE FOR SHALLOW SOILS
APPENDIX C 67APPENDIX C: EVALUATION OF TREATMENT SYSTEMS Factor Treatment Option No. 1 Treatment Option No. 2 Capital cost £ Construction costs prior to delivery £ Additional costs prior to commissioning Annual running cost £/annum Installation and commissioning service available Maintenance agreement available Cost of annual maintenance agreement £ Design criteria* Performance - % reduction in BOD, COD, TSS Performance - % reduction Total P and Total N Performance - % reduction Faecal coliforms Beneficial uses of the receiving water Guarantee available AGRÉMENT certification Recommendations from other users Expected life of the system Power requirements kW/d Power requirements – single phase/three phase Ease of operation Daily, weekly and annual maintenance requirements Licence required (Water Pollution Act Licence) Access requirements for sludge removal Sludge storage capacity (m 3)* in the case of biofilm systems the organic and hydraulic loading rates in g/m 2.d and l/m 2.d respectivelyshould be quoted
68 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES APPENDIX D: SOIL/SUBSOIL CLASSIFICATION CHART START HERE: Rub moist Soil/Subsoil between thumb and fingers Soil Classification Subsoil Classification Range of (Soil Scientist or Agricultural) (Geotechnical)2 T values YES Sandy and rasping sound? Sand SAND 1-5 OR NO silty or clayey Loamy Sand 6 - 10 SAND Slighty sandy, faint YES rasping sound? Sandy Loam 1 very sandy SILT 6 - 30 NO Smooth soapy feel, YES Silt Loam clayey, sandy SILT 31 - 50 no grittness? NO Very smooth, slighty YES Clay loam silty, sandy CLAY >50 sticky to sticky? NO Very smooth, sticky YES Clay CLAY >50 to very sticky? NO START again or soil is organic 1 Loam:A soil composed of a mixture of sand, silt and clay such that the properties of no one group dominate its characteristics is called a Loam. 2 Classification system used in BS 5930:1981.
APPENDIX D 69APPENDIX E: INDICATOR PLANTS OF DRAINAGEThe following illustrate plants which: • indicate dry conditions throughout the year (good drainage); and • indicate wet conditions through the year (poor drainage).Some of the plates below illustrate the plants in flower, this aspect should be ignored. Plants in flower, orotherwise, do not change their indicator status. Note that an alder is a tree. DRY CONDITIONS Creeping Thistle Bracken Common Ragwort WET CONDITIONS Alder Soft Rush Big Iris
USER COMMENT FORMNOTE: Completed comments to be forwarded to: The Environmental Management and Planning Division, Environmental Protection Agency, P.O. 3000, Johnstown Castle Estate, Wexford. Document Title: Wastewater Treatment Manuals: Treatment Systems for Single Houses____________________________________________________________________________________CONTENTS:____________________________________________________________________________________STYLE:____________________________________________________________________________________INFORMATION:____________________________________________________________________________________SUGGESTIONS FOR FUTURE EDITIONS:____________________________________________________________________________________NAME................................................................... ORGANISATION.......................................................ADDRESS.....................................................................................................................................................DATE.............................................. PHONE ....................................... FAX...........................................