sewage treatment in cork and munster




This following Q & A’s have been prepared to provide answers to the frequently asked questions about on-site wastewater treatment systems for single houses.

If your query is not answered by these Q & A’s and it relates to a site-specific issue then our advice is that you should contact us directly to arrange an on-site consultation where we can try and find a site specific solution.


Q.        What guidance is available from the EPA on installing a treatment system for a single house?
A.         The Wastewater Treatment Manual for Single Houses (2000) published by the EPA provides guidance and is designed to help planning authorities, builders architects/engineers and others to deal with this issue.

Q.        Where can I purchase a copy of the Wastewater Treatment Manual for Single Houses?
A.         There are a variety of ways to obtain a copy of the EPA’s Wastewater Treatment Manuals.

The Wastewater Treatment Manual for Single houses can be downloaded from

Q.        Where can I find a qualified person to carry out a Site Suitability Assessment as part of a planning application for a single house?

A.         Local authorities are responsible for determining who is qualified to carry out site suitability assessments within their functional area.  In some cases local authorities have a list of designated persons who may carry out the assessment; these can be found on their websites or by contacting the Planning or Environment Section of the local authority.  Carhoo Environmental are approved in carrying out Site Suitability Assessments on behalf of Local Authorities and County Councils 

Q.        What are the legal standards that I must comply with in relation to the installation of a wastewater system for a single house?
A.         Septic tanks installed on or after 1 June 1992 must comply with Part H of the National Building Regulations.  The relevant Technical Guidance Document (TGD) - H (Drainage and Waste Water Disposal) calls up the following standards:

Septic tanks serving single houses: Irish Standard Recommendations SR6 of 1991 for Domestic Effluent Treatment and Disposal from Single Dwellings, issued by the National Standards Authority of Ireland (NSAI); and

Septic tanks serving groups of houses: British Standard B.S. 6297: 1983 (incorporating amendment No. 1 of 1990), a Code of Practice for the Design and Installation of Small Sewage Treatment Works, issue by the British Standards Institution (BSI).

The Department of the Environment, Heritage and Local Government issued a Circular Letter to planning authorities on the 31 July 2003 that dealt with groundwater quality in the context of planning decisions. This circular reminded planning authorities that in assessing planning applications they should consider, as well as the proper planning and sustainable development of the area, other relevant Government and Ministerial policies. In this regard, the attention of planning authorities was drawn to the vital importance of good siting and design of necessary development in rural areas; they were also reminded that the current standard for domestic effluent treatment and disposal from single dwelling houses is set out in Recommendation SR6: 1991. Reference was also made to the Environmental Protection Agency’s Manual on Treatment Systems for Single Houses (2000), designed to help planning authorities, builders and others to deal with the complexities of on-site systems, including newer packaged systems.  This manual is currently being revised and the EPA understands that it is the intention of the Department that it will supersede SR6: 1991, which will then be withdrawn by the NSAI. The Department plans to amend TGD-H, to call up the EPA Manual, as soon as it is published.


Q.        How can I go about selecting a wastewater treatment system?
A.         Details of the different wastewater treatment systems, both conventional septic tank systems and package treatment systems, can be found in the Agency’s Wastewater Treatment Manual for Single Houses (EPA 2000).  This manual outlines each of the requirements for wastewater treatment systems. 

Q.        What is included in the term domestic wastewater?
A.         Domestic wastewater includes grey water and sewage from domestic dwellings. Grey water is defined as wastewater that comes from sinks and washing machines, i.e. the wastewater that contains some bleach and detergents.  The grey water from your household is treated in the same way as sewage whereby it is collected in a septic tank and undergoes treatment either in a percolation area or in a secondary treatment unit prior to being discharged via a polishing filter to ground. Rainwater is not classified as grey water and therefore should not be discharged into your wastewater treatment unit.

Q.        How do you determine what type of wastewater treatment system is required for a single house?
A.         Firstly, a suitably qualified person must carry out a site assessment in accordance with the guidance set out in the EPA Wastewater Treatment Manual: Wastewater Treatment Systems for Single Houses.  This is then followed by the selection of a suitable wastewater treatment system, which is dependent on the site conditions determined during the site assessment.

The choices of systems are

  • Conventional Septic tank system – septic tank and a properly constructed percolation area
  • Intermittent Filter System – septic tank, filter system (including wetlands) and polishing filter for discharge to ground.
  • Mechanical Aeration system – package system and a polishing filter for discharge to ground. 

It may be required to discharge effluent to surface waters if the percolation tests fail, in which case a discharge from your local authority is required. 

Q.        What is an acceptable housing density?

A.         The EPA manual sets out minimum separation distances between wastewater treatment systems and vulnerable features including houses, wells and watercourses. However, the planning authority may increase these distances where it is deemed appropriate. One of the limiting factors for the siting of a wastewater treatment system is the existing level of nitrates in the groundwater.  If the nitrate levels in the groundwater in a particular area are elevated due to the high density of wastewater treatment systems in that area, then the Local Authority may seek additional information (including dilution calculations) to assess any potential impact on the groundwater quality from any proposed development.


Q.        Do I need a licence to discharge directly from a wastewater treatment system to surface water or groundwater?
A.         A water pollution discharge licence is required from the local authority

  • If the treated effluent is discharged to surface waters


  • If the quantity of treated wastewater is greater than 5m3/day and is being discharged to groundwater.

Contact your Local Authority’s Environment Section for more information or to apply for a water pollution licence. Application forms may be downloaded from the local authority’s website. Some Local Authorities do not allow discharges from a single house to surface waters.


Q.        Who installs Wastewater Treatment systems? What are the requirements?
A.         It is recommended that a suitably qualified installer install wastewater treatment systems; this includes conventional septic tank systems, filter systems and mechanical aeration systems, and the latter two require that polishing filters are also installed.  Usually, the manufacturer will recommend a person who is suitable to install the system or may install it themselves.  This person is then responsible for the testing of the system after installation, to ensure that it is working effectively. In some cases planning authorities’ condition that the installation be certified and in this case contact the local authority for more details as to their requirements.

Q.        Who certifies the design of Wastewater Treatment systems?
A.         The Circular issued by the Department of the Environment, Heritage and Local Government to planning authorities on the 31 July 2003 stated that innovative effluent treatment systems should be certified by the NSAI Irish Agrement Board (IAB) or by the Agrement Board (or equivalent) of an EEA Member State - where the latter certificate ensures in use an equivalent level of safety and suitability.



Q.       What is involved in carrying out a Site Assessment?

A.         A Site assessment deals with the characterisation of the ground conditions of the site and then the selection of an appropriate wastewater treatment system. The objective of site characterisation is

  • to determine if the site can adequately treat the wastewater;
  • to check that  the treated wastewater can get away; and
  • to check that the minimum site separation distance can be achieved.

Q.        What is a percolation test?
A.         A percolation test is a method of assessing the ability of the subsoil to allow water to percolate to the water table (i.e. how water can pass through the soil). In the test a small hole is excavated and the time taken for the water to drop in minutes is recorded.  It is recommended that a suitably qualified person carry out percolation tests.

Q.        What is the minimum recommended distance between two percolation test holes?
A.         Test holes should be located at either side of the proposed percolation area (but not within it), to ensure that the percolation characteristics are assessed across the percolation area.

Q.        What is the difference between “T” and “P” tests?
A.         Both of these are percolation tests that assess the ability of the subsoil to allow water to percolate to the water table. The main difference is that they are carried out at different levels. A “T” test is used to test the suitability of the subsoil at depths greater than 400mm below the ground level. A “P” test is carried out at ground level where there are limiting factors such as high water table or shallow rock.

Q.        I have got two very different percolation test values from my site – what do I do?
A.         It is then recommended that a third test be carried out.  The results of this test should be examined in light of the other two test results.  A detailed examination of the subsoil types within the trial hole and the individual test holes should be undertaken to determine the dominant site conditions.  The dominant subsoil type in the test holes and trial hole should be taken as representative of the site conditions and therefore the percolation test results that equate to that should be used.

Q.        What can I do if the T test fails?
A.         If a T test fails; a P test should be carried out.  This will establish if it is possible to install a constructed percolation area or a polishing filter. It determines whether the upper layers of the subsoil are permeable enough to allow percolation of the treated wastewater into the ground. 
If the P test passes it may be possible to install a secondary treatment unit with a polishing filter – see Chapter 4 and 5 of the EPA Manual for Single Houses for more details.
If both the P and T test fail, it is only permitted to discharge the effluent to surface waters.  A water pollution licence must be obtained from your Local Authority.  See Section 2.2, “On-Site Assessment”, of the Wastewater Treatment Manual for Single Houses, for more information on P and T tests.

Q.        What information can a local authority request in relation to the suitability of a site?
A.         A planning authority is currently entitled to request any information on site suitability for an on-site wastewater disposal system from the applicant that it considers necessary to make a decision on an application for planning permission.  The Department of the Environment, Heritage and Local Government has indicated that they intend to make revised planning regulations shortly which will require applicants, in cases where it is proposed to dispose of wastewater from the proposed development other than to a public sewer, to submit information on the on-site treatment system proposed and evidence as to the suitability of the site for the system proposed with the planning application.

Q.        How can I tell if I have a high water table?
A.         A high water table can be determined by constructing a trial hole and leaving it open for 48 hours during the site assessment.  The water level should be measured from ground level.    

Q.        What can I do if I have a high water table?
A.         A depth of 1.2m of free draining subsoil to the bedrock must exist at all times for a conventional septic tank system.  If this is not the case, the use of a conventional septic tank system is not recommended but a suitable secondary treatment system may be installed if the subsoil has a percolation rate (T/P test result) between 1-50 and there is a minimum of 600mm unsaturated subsoil (i.e. water table is at least 600mm below the ground surface) so that a polishing filter may be constructed. 
Q.         Is it permissible to have more than one house sharing a septic tank and well?
A.         This is a matter for the planning authority.

Q.        Are all sites suitable for the installation of a wastewater treatment system of one kind or another?
A.         In some cases the site conditions could mean that the treatment of wastewater may not be possible on site without the risk of causing water pollution. The site suitability assessment process will determine this but the ultimate decision on whether a site is suitable will rest with the local authority or planning authority.


Q.        How far away from a wastewater treatment system is it safe to locate a well so as to prevent contamination?
A.         The minimum recommended distance between a water well and a wastewater treatment system (including percolation area or polishing filter) septic tank is set out in the Groundwater Protection Scheme & Responses, where zoning for groundwater protection schemes outlines the aquifer classification in the general area and the vulnerability of the groundwater. The minimum distance of wells from wastewater treatment systems and percolation areas/polishing filters are set out in the following table:

Recommended Minimum Distance between a Receptor and a Percolation Area or Polishing Filter

T or P Value 1

Type of soil/subsoil *

Depth of soil/subsoil

Minimum distance (m) from receptor to percolation area or polishing filter ****




(m) above bedrock
(see note 1,2,3,6)

Public Water Supply

Karst feature

down-gradient domestic well or
 flow direction is unknown

Domestic well
(no gradient)

up-gradient domestic well



CLAY; silty, sandy CLAY (e.g. clayey till); CLAY/SILT.












10 -30

Sandy SILT; clayey, silty SAND; clayey, silty GRAVEL (e.g. sandy till).

























* The distance from the percolation area or polishing filter means the distance from the periphery of the percolation area or polishing filter and not the centre.


  • Depths are measured from the invert level (i.e. the base) of the percolation trench.
  • Depths and distances can be related by interpolation: e.g. where the thickness of silty, sandy CLAY is 1.2 m, the minimum recommended distance from the well to percolation area is 40 m; where the subsoil thickness is 3.0 m or more, the distance is 30 m; distances for intermediate depths can be approximated by interpolation.
  • Where bedrock is shallow (<2 m below invert of the trench), greater distances may be necessary where there is evidence of the presence of preferential flow paths (e.g. cracks, roots) in the subsoil.
  • Where the minimum subsoil thicknesses are less than those given above, site improvements and systems other than conventional systems, as described in EPA (2000), may be used to reduce the likelihood of contamination.
  • If effluent and bacteria enter bedrock rapidly (within 1-2 days), the distances given may not be adequate where the percolation area is in the zone of contribution of a well.  Further site-specific evaluation is necessary.
  • Where it is known that bedrock is karstified or highly fractured, greater depths of subsoil may be advisable to minimise the likelihood of contamination.

Q.        What are the minimum separation distances to rivers, beaches, lakes, etc?
A.         The minimum recommended distances for the above are set out in Table 2 below.  The minimum distances for wells are set out in Table 1 above.

Table 2: Minimum Separation Distances in metres


Type of System




Watercourse or


Heritage Features NHA/SAC






Site Boundary








Septic tank; prefabricated intermittent filters; mechanical aeration


See Table 1


























In situ intermittent filters; percolation area; polishing filters


See Table 1

























* The distances required are dependent on the importance of the feature.  Therefore, advice should be sought from the local authority heritage officer, the Heritage Service or the National Wildlife Service.


Q.        Is a reserve percolation area required?
A.         The Agency’s manual does not require a reserve percolation area but requires a rigorous site assessment, correct installation and proper maintenance of the wastewater treatment system. In addition, minimum separation distances must always be achieved.

Q.        Can I have trench widths greater than 450mm in my percolation area?
A.         The recommended trench width is 450mm.  The percolation trench lengths given in the Agency’s Wastewater Treatment Manual for Single Houses is dependent on this trench width of 450mm.

Q.        What is a polishing filter?
A.         Polishing filters consist of either soil or sand and are employed to reduce micro-organisms, phosphorus and nitrate nitrogen from wastewater. They are used to treat wastewater from intermittent filters, constructed wetlands and mechanical aeration systems and to allow for the discharge of treated wastewater to ground. See the Wastewater Treatment Manual for Single Houses, Section 4.11 for more information.

Q.        What is the difference between a soil polishing filter and a sand polishing filter?
A.         Soil polishing filters comprise in-situ or improved or imported soil, whereas sand polishing filters are comprised of layers of sand. See the Wastewater Treatment Manual for Single Houses, Sections 4.6 and 4.7 for more information. 

Q.        I want to use a constructed wetland as my wastewater treatment system.  What should I do?
A.         A site suitability assessment is required to be completed prior to selecting any system.  If the site is suitable for a constructed wetland then the following should be noted.
Any discharge from a constructed wetland to surface water requires a water pollution discharge licence in accordance with the Water Pollution Acts 1977-1990. Specific information can be obtained from the local authorities.
If discharging from the constructed wetland to ground, a polishing filter will be required.  If the discharge is greater than 5m3/day, then a water pollution discharge licence is a required as well as additional prior investigation
It should also be noted that constructed wetlands should be inspected weekly to ensure that there is no evidence of varying flow distribution or blockage, that the sidewalls are maintained and that the reeds have not been damaged. See the Wastewater Treatment Manual for Single Houses or contact the Environment Section of your local authority for more information.



Q.        Can I place a car park or driveway over the percolation area or polishing filter?
A.         Roads, driveways or paved areas or any underground services must not be located within the disposal area.  This is due to the need to have easy access to the site for maintenance and also to prevent problems with the system due to the potential for damage to the pipework and compaction of the filter materials.

Q.        How do I construct a percolation area on a sloping site?
A.         Mound systems can be constructed on a sloping site, if the slope of the site does not exceed 12%. They must be constructed carefully along the contour to ensure that minimum installation thickness is maintained and to assist the even distribution of wastewater.   

Q.        How do I dispose of the rainwater and clean surface runoff from my site?
A.         Uncontaminated water should be disposed of by means of a soak pit/soakaway.  The soakaway should be designed in accordance with the guidance provided in BS8301 and in BR DG365. It should not be located within 5m of any dwelling and as far away as possible from the percolation area (at least a minimum of 5m separation distances should be used).


Q.        How often do I have to de-sludge the septic tank or wastewater treatment system?
A.         It is recommended to de-sludge a septic tank at least once a year but this varies with the system’s capacity and use.  You must de-sludge the septic tank if scum is present in the second chamber or if the sludge comes up to about 400mm from the bottom of the tank. A minimum of 75mm of sludge should remain in the tank to assist in the re-seeding of the new sludge.  Regular maintenance is required to ensure that the septic tank operates effectively and that solids do not enter the percolation area and clog the distribution pipe work.

Q.        Who can I get to de-sludge the tank?  What controls are in place?
A.         It is recommended that a waste contractor that has an appropriate waste collection permit be employed to de-sludge a septic tank.  A list of approved permit holders is available from the Environment Section of your Local Authority.

Q.        What are grease traps and what do they do?
A.         Grease traps capture the oil and grease from the flow of wastewater by slowing down the flow of hot greasy water through the trap and allowing it to cool.  As it cools, the grease and oil separate out of the water and float to the top of the trap.  The cooler water then flows to the septic tank where it is treated.  Grease traps are usually not included in the design of a domestic wastewater system but are mandatory in systems treating water from restaurants, hotels and any other businesses that supply food. 
Due to the absence of these systems in single dwellings it is highly important that people do not allow any fats, grease or oils to enter their septic tank systems.  The inlet pipes can become clogged up by the fats and grease and therefore can reduce the treatment rate of the septic tank system.  To insure that this doesn’t happen to your system, all fats, grease and oils must not be disposed down the sink or drains.

Q.        Do I really need to renew my maintenance agreements?
A.         Yes.  All secondary treatment systems require ongoing maintenance to ensure that the system is providing adequate treatment of the wastewater.  An on-going maintenance agreement should be made and renewed with an appropriately qualified person to ensure that your wastewater treatment system is working effectively at all times.

Q.        Will bleach or disinfectants harm the septic tank system?
A.         Normal amounts of household bleach, disinfectants and detergents will not harm the septic tank system.  However, excessive amounts of bleach will temporarily reduce the treatment capacity, as the microorganisms needed to treat the biological waste will be killed off.  In saying this, the system should return to full performance capacity within a short period of time. It is important to be aware of potential effects that excessive use of these chemicals will have on your wastewater treatment system.



Q.         I am concerned that my neighbours’ treatment system is contaminating my well. What can be done about this?
A.         You are advised to contact the local authority in the first instance.  Both the planning and environment sections of the local authority should be contacted. By consulting the planning files, an assessment of whether or not the installed wastewater treatment system is as per the conditions of the planning permission can be made.  In relation to the concern that the wastewater treatment system is potentially contaminating the well, advice should be sought from the environment section. 

Q.         There is a problem with my neighbours’ treatment system, which is flooding onto my property. What can be done to get this resolved? 
A.         Contact the environment section of the local authority for advice.

Q.         What role does the EPA have in relation to checking that a treatment system is suitable and installed properly?  
A.         The EPA does not have a role in assessing compliance with planning conditions.  It does however, provide advice to local authorities in relation to wastewater treatment through the publication of guidance manuals and advice where requested. It is the responsibility of the local authority through its planning section to enforce the conditions of planning and the environment section enforces water pollution legislation.