5. PREFACE iii
PREFACE
The Environmental Protection Agency was established in 1993 to license, regulate and control activities for
the purposes of environmental protection. In Section 60 of the Environmental Protection Agency Act, 1992, it
is stated that "the Agency may, and shall if so directed by the Minister, specify and publish criteria and
procedures, which in the opinion of the Agency are reasonable and desirable for the purposes of
environmental protection, in relation to the management, maintenance, supervision, operation or use of all or
specified classes of plant, sewers or drainage pipes vested in or controlled or used by a sanitary authority for
the.....treatment or disposal of any sewage or other effluent to any waters". The following is a list of the
manuals 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-site
wastewater treatment systems for a single house. The National Standards Authority of Ireland published
standard recommendations in 1975 (revised in 1991) with the aim of achieving satisfactory practice in the
design, construction and maintenance of septic tank drainage systems. This manual has been prepared having
regard to the above and will inter alia assist planning authorities, developers, system manufacturers, system
designers, system installers, system operators to deal with the complexities of on-site systems. Where
reference in the document is made to proprietary equipment, this is intended as indicating equipment type and
is 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 treatment
systems available for a single house.
Chapter 2 outlines the steps which should be taken to characterise a site. Characterisation of a site is divided
into a desk study followed by an on-site assessment. The on-site assessment is subdivided into a visual
assessment, a trial hole and a percolation test. The significance of the information collected during the desk
study 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 discharge
route.
Chapter 4 includes information on the design, construction and maintenance of a septic tank,soil percolation
area, 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 EPA
in 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 Local
Government (DELG) and the EPA have developed a methodology for the preparation of groundwater
protection schemes to assist the statutory authorities and others to meet their responsibility to protect
groundwater. 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 to
the Environmental Management and Planning Division at the Agency headquarters on the enclosed User
Comment Form.
6. 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.
7. 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 of
the 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 assistance
of Ms. Margaret Keegan, Mr. Donal Howley and Ms. Jane Brogan.
9. LIST OF ABBREVIATIONS vii
List of Abbreviations
C Capacity
°C Degrees Celsius
Agency Environmental Protection Agency
BAF Biofilm aerated filters
BOD Biochemical oxygen demand
BOD5 Five-day biochemical oxygen demand
COD Chemical oxygen demand
DELG Department of the Environment and Local Government
d Day
DO Dissolved oxygen
DWF Dry weather flow
EPA Environmental Protection Agency
FOG Fats, oils and grease
FWS Free-water surface
g Gram
GSI Geological Survey Of Ireland
h Hour
kg Kilogram
ISO International Organisation for Standardisation
l Litre
m Metre
m3 Cubic metres
m/s Metres per second
mg Milligram
mm Millimetre
NHAs National Heritage Areas
NUI National University of Ireland
p.e. Population equivalent
PFP Preferential flow paths
RBC Rotating biological contactors
s Second
SACs Special Areas of Conservation
S.I. Statutory instrument
SBR Sequencing batch reactor
SFS Sub-surface flow system
SS Suspended solids
TSS Total suspended solids
TWL Top water level
11. 1I NTRODUCTION 9
1. INTRODUCTION
1.1 GENERAL
• BS 6297: 1983, Design and installation of
In Ireland, the wastewater from over one third of the small sewage treatment works and cesspools
population - principally those living in dwellings not (British Standards Institution) deals mainly
connected to municipal sewers - rely on systems with the design of small sewage treatment
designed to treat the wastewater at or near the works serving small communities, not
location where it is produced. These wastewater primarily concerned with septic tank systems;
treatment systems are called on-site systems. and
Many on-site systems are available for the treatment • US EPA/625/R-92/005 Manual: Wastewater
of 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-site
treatment systems occurs, in the main, under aerobic • conventional septic tank systems (septic tank
conditions. For example, in a soil percolation area, and percolation area), properly installed and
aerobic conditions are present due to the unsaturated maintained, are satisfactory where suitable
nature of the soil. subsoil conditions exist;
Public health is threatened when on-site systems fail • where suitable subsoil conditions do not
to operate satisfactorily. System failures can result initially exist for treatment by means of a
in wastewater ponding or forming stagnant pools on conventional septic tank system, site
the ground surface when the wastewater is not development works may improve the subsoil
absorbed by the soil. In such circumstances of conditions and make the subsoil suitable in
system failure, humans can come in contact with the certain circumstances;
ponded wastewater and be exposed to pathogens and
foul odours can be generated. • in certain situations such as when unsuitable
subsoil conditions exist, other systems, which
The three documents commonly used in relation to include mechanical aeration or intermittent
the 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
12. 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
13. 1I NTRODUCTION 11
After flowing through a suitable percolation area the loaded with wastewater from a septic tank, a biomat
wastewater is suitable for discharge. layer quickly develops along the base and wetted
sides of these trenches (Figure 2). The biomat layer
A typical septic tank is illustrated in Figure 1 and the consists of a deposit of microorganisms, slimes and
attributes of a septic tank are given in Table 2. The sludge which coats the floor and walls of the trench
tank, which should be two-chambered, allows the and enters the subsoil for a short distance inside the
wastewater from the dwelling house time to settle infiltrative surface. The biomat drastically lowers the
out into three layers viz. the sludge layer, the liquid infiltration through the base and sides, causing
layer and the scum layer (Figure 1). The sludge layer ponding in the trenches. The ponding causes
is a blanket of heavy solids and some coagulated wastewater to flow over the entire trench base and in
materials, which settle out on the tank floor. The a short time leads to a uniform distribution of the
liquid 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 trenches
and colloidal organic matter and contains bacteria, accompanied by the development of a biomat along
viruses, worm eggs, larvae etc.; it is allowed to flow the wetted walls of the trench until an equilibrium is
to the percolation area through a tee-pipe for reached, causing flow through the sides and base. An
distribution and secondary treatment. The scum adequate depth of gravel aggregate in the trench is
layer consists of greases, oils and gas-buoyed solids important for hydraulic function. The biomat layer
which accumulate as a layer on the surface. then determines the hydraulic loading. Therefore for
Detention 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 with
The subsoil through which the wastewater percolates the impedance caused by the development of the
acts as an attached growth medium for biomat layer along the base and wetted walls of the
microorganisms. 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
14. 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
15. 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 FILTER
A percolation area is considered "failing" when (i) it for the other filters are usually installed in
causes a backing up of wastewater in the distribution prefabricated containers (prefabricated intermittent
box or (ii) it does not keep untreated wastewater filters).
below the surface of the land or (iii) it does not treat
the wastewater before it reaches groundwater or 1.4.3 CONSTRUCTED WETLANDS
surface water.
Constructed wetlands can also be used for the
In 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 which
they have been poorly constructed, installed, promote aquatic vegetation or water tolerant plants
operated, maintained or, are located in areas with such as reeds.
unsuitable subsoils, or percolation of the septic tank
effluent is through a soakaway. It is important to Primary treatment by a septic tank is used prior to
note, 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 a
effective means of treating wastewater in a suitable combination of physical, chemical and biological
site is a properly constructed and maintained processes that develop through the interaction of the
conventional septic tank system. plants (reeds), the growing media (gravel) and
microorganisms. These processes include settlement
1.4.2 FILTER SYSTEMS and filtering of suspended solids, biodegradation,
plant uptake and chemical interactions.
Where the subsoil is unsuitable for treating the
wastewater from a septic tank, filter systems may be There are two different types of constructed wetlands
used. These include intermittent soil filters, sand and they are characterised by the flow path of the
filters, peat filters and other filters using materials water through the system (Figure 4). In horizontal
such as plastic foam filters and geosynthetic strips. flow constructed wetlands, wastewater is introduced
Intermittent soil filters comprise suitable soils placed at one end of a flat to gently sloping bed of reeds and
often 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, and
sand underlain at the base by a filter gravel or intermittently onto the media, and gradually drains
permeable soil layer to prevent outwash or piping of vertically to a drainage network at the base of the
the sand. Soil covered intermittent sand filters may media. Constructed wetlands should be securely
be 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 mound
systems (Figure 3). Fibrous peat and plastic media
16. 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.
17. 1I NTRODUCTION 15
Figure 5 below summarises the protocol to be occurring and its consequences that is the basis of
followed to select and design an on-site system. risk assessment. Risk management involves site
assessment, selection of options and implementation
The concepts of ‘risk’, ‘risk assessment’ and ‘risk of measures to prevent or minimise the consequences
management’ have recently become important tools and probability of a contamination event (e.g. odour
in environmental protection. Risk can be defined as nuisance or water pollution). The methodology for
the likelihood or expected frequency of a specified selection and design of an on-site system in this
adverse consequence. Applied for example to manual embraces the concepts of risk assessment
groundwater, a risk expresses the likelihood of and risk management.
contamination arising from a proposed on-site
treatment system (called the hazard). A hazard The remainder of this manual sets out how a site
presents a risk when it is likely to affect something characterisation should be completed and a choice of
of 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
18. 16 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES
19. 2S ITE CHARACTERISATION 17
2. SITE CHARACTERISATION
The purpose of a site assessment is to determine the A desk study involves the assessment of available
suitability of the site for an on-site treatment system. data pertaining to the site and adjoining area that
The 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 should
subsurface 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 presence
design a suitable treatment system following a (if any) of streams, rivers, lakes, beaches, shellfish
thorough site assessment. For a subsoil to be areas and/or wetlands while hydrogeological aspects
effective as a medium for treating wastewater, it include:
must retain the wastewater for a sufficient length of
time, 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 n
collected in the evaluation will be used to select the • bedrock type;
on-site system.
• aquifer type;
In designing a soil percolation area to treat
wastewater, 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 provide
To determine these considerations a site guidelines for developers in assessing groundwater
characterisation is undertaken. This includes: vulnerability and for the planning authorities in
carrying out their functions, and a framework to
1) a desk study; and assist in decision-making on the location, nature and
control of developments and activities (including
2) 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 taken
2.1 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.
20. 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
21. 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 or
up-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 recommended
2.2.2 INTERPRETING THE RESULTS OF THE minimum distances from wells should satisfy the
VISUAL ASSESSMENT requirements of the groundwater protection
response, which should have been reviewed during
The minimum separation distances that should be the desk study. In some cases, the requirements of
used in the visual assessment are set out in Table 4. the groundwater protection scheme and responses
These 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 Ireland
2000. Groundwater Protection Responses for On-site Systems for Single Houses.
22. 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.
23. 2S ITE CHARACTERISATION 21
Structure: Soil structure refers to the arrangement illustrated in Figure 8. Where water is supplied to a
of the soil particles into larger units or compound soil at a rate less than its permeability, as in the case
particles in the soil. The soil particles,sand, silt, clay of septic tank effluent, the r ate of flow through the
and organic matter, are generally clumped together soil equals the rate of supply in soils of adequate
to form larger units called peds. The shape and size permeability.
of the peds have a large effect on the behaviour of
soils. A ped is a unit of soil structure such as an The preferred structures from a wastewater treatment
aggregate, a crumb, a prism, a block or granules perspective are granular (as fine sand), blocky and
formed by natural processes. Soil texture plays a structureless-single grain sandy loams, loams and
major part in determining soil structure. The silt loams. Unstructured massive plastic soils
structure of the soil influences the pore space, indicate seasonal or continuous saturation and are
aeration and drainage conditions. Types of soil unsuitable. Likewise soils with extensive, large and
structure (shape of the ped) are illustrated in Figure continuous fissures and thick lenses of gravel and
7 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 as
The 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 and
Structureless massive structure have very low flow yellowish brown colours. Many free draining soils of
rates and can often be regarded as impervious ( e.g. limestone origin with deep water tables are grey at
with a permeability < 10 mm/day). The relationship depth. Saturated soils/subsoils are in a reduced state
between structure type and water movement is and exhibit dull grey or mottled colours. Mottling
24. 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
25. 2S ITE CHARACTERISATION 23
(comprising a reddish brown or rusty staining) of the with clay. In some areas a thin, hard, rust coloured
soil layers can indicate the height to which the water impervious layer can develop (iron pans) as a result
table rises in periods of high rainfall; mottling in a of the downward leaching of iron and manganese
grey matrix (grey with reddish brown mottles) compounds and deposition at shallow depth (less
indicates aeration along old root channels and cracks t h a n 1 m ) . The underlying subsoil often has a
while the matrix remains reduced; this soil layer is satisfactory percolation rate. Enrichment with clay
saturated during part of the year. particles and precipitation of iron and calcium and
magnesium compounds can lead to very low
Layering: This is common in soils, arising during percolation rates. Such soils can often be improved
deposition and/or subsequent weathering. In soils, by loosening or by breaking the impervious layer.
that are free draining in the virgin state, weathering
can result in downward movement of some of the The factors that are evaluated from the trial hole and
clay 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
26. 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.
27. 2S ITE CHARACTERISATION 25
2.2.5 PERCOLATION TESTS the site is not suitable for the treatment of septic tank
wastewater by soil percolation. Other options should
A 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 or
length of time for the water level in the percolation constructed wetlands. Where mechanical aeration
hole to fall from a height of 300 mm to 200 mm systems, intermittent filters or constructed wetlands
above the base of the test hole in a percolation area. are used, the treated wastewater from such systems
The permeability of each soil class may vary within should discharge to receiving waters (surface or
an order of 10 - 100 fold depending primarily on the groundwater) through a polishing filter.
particle size grading which reflects the va ry i n g
amounts of fine particles, the structure and void ratio Where shallow or impervious soils exist, a soil
in each soil class. The procedure for carrying out a percolation area may still be possible by importing
percolation test is set out in Appendix B. suitable soil and placing it in lifts in the proposed
percolation area such that there is a minimum
The results of percolation tests are expressed as a "T" thickness of 2.0 m of unsaturated soil with drainage
value. This is the average time in minutes for the over the bedrock or impervious soil. A trial hole and
water level to fall 25 mm in each of two percolation percolation tests (T - tests) should then be carried out
test holes over the water depth range of 300 mm to (see Appendix B - Percolation Tests for further
200 mm in the proposed percolation area. details) in the same way as for in situ soils. Where
such soil filling is not feasible, alternative systems
To carry out a percolation test (which should be followed by a polishing filter may be suitable.
within the proposed percolation area), a 300 mm
square percolation test hole is excavated to a depth of Where an alternative system and a polishing filter are
400 mm below the invert of the proposed distribution employed, the nature of the soil or bedrock
pipe. underlying the polishing filter determines the
disposal route of the treated wastewater. For a
To 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 suitable
later) 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, gravels
surface as opposed to 400 mm below the invert of the or bedrock with a T/P value less than 50 may
distribution pipe for the T test. To avoid confusion d i s ch a rge the treated wastewaters to the
with the previous test, this test is called a P test, and groundwater. A flow diagram to assist in the choice
the values are referred to as P values. of an on-site system is shown in Figure 9.
2.2.6 INTERPRETING THE RESULTS OF THE
PERCOLATION TEST 2.3 INTEGRATION OF THE DESK STUDY
AND ON-SITE ASSESSMENT INFORMATION
A "T" value greater than 50 suggests that wastewater
entering such subsoils would cause ponding on-site. Table 8 summarises the information that can be
A "T" value less than 1 suggests that the retention obtained from the data collected from the desk study
time for the wastewater would not be long enough to and the on-site assessment. This information is used
provide satisfactory treatment. If the percolation T to characterise the site and used later to choose and
value is within the range 1-50* then the site should design an on-site system. An integrated approach
be suitable for the development of a conventional will ensure inter alia that the targets at risk are
septic 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.
28. 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.
