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 126.96.36.199 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.