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Site suitability assessment

It is essential in the interests of public health to ensure the safe disposal of wastewater from a development. A site assessment will determine whether or not a particular site can achieve this. In all cases where an application involves a proposal to use a septic tank or other on-site treatment system, a site assessment is required. Applicants must carry out a site assessment in advance of making a planning application. 

Site assessments

There are five key elements in the assessment:

  • A desk study to provide information on soils, geology and groundwater vulnerability
  • A field visit to look at site drainage, vegetation, levels, housing density, water uses in the area.
  • Trial hole to check the depth and type of sub-soils and depth to water table. If the trial hole indicates poor subsoil permeability- there may be little point in proceeding to the percolation test and/or application for planning permission
  • Percolation testing (P/T tests). These tests determine the soil’s ability to filter and move the wastewater. A "T" test is normally done at 800mm below ground level, which is about the level where the pipe from the wastewater system enters the ground. A "P" test is undertaken on sites with shallow soils, at 400mm below ground level. The same test hole dimensions are used, and the same test procedure.
  • Recommendation on a wastewater treatment option. A good assessor will include a full wastewater treatment design with a cross section showing where the pipework enters the soil polishing filter.

The cost of the assessment should be agreed with the site assessor. Ideally, a cost could be negotiated for each part of the assessment described. This allows the applicant to decide whether or not to proceed to a full site assessment, and a full planning application. This approach should be agreed before starting any part of a site assessment. The fee does not include digging the holes. A desk study, together with the experience of the assessor, will indicate a site that is unlikely to pass the assessment. It is more appropriate to spend a smaller fee on this advice, than a larger fee on a full report for a failed site.

Some sites cannot be made suitable to ensure the safe disposal of wastewater from a development. Both treatment and disposal of wastewater is required. One or other, (or both), may be a problem. This may be due to a number of reasons, including poor draining soils or groundwater use for drinking. The site assessment is a risk assessment. The assessor must identify the potential targets at risk, when wastewater is produced on site. This includes the risk of ponding (with impeded drainage), the risk of contamination of groundwater, and the risk of run off to surface water drains and rivers. Ultimately, the decision on the site suitability rests with the planning authority. The site must be capable of providing adequate treatment within the site and dispersion of the total water volume arising in the development. Outfall to surface waters (drains and rivers) is not permitted.

Wastewater treatment for single houses

What is wastewater and where does it go?

Wastewater comes from the toilets, bath, shower, sinks (including garbage grinders), dishwasher and washing machine.It eventually makes its way back to the water table. This is why it’s so important to treat wastewater coming from dwelling houses. In Ireland, almost 500,000 houses are not linked to public sewerage systems, but have their own WWT systems on site. Many of these houses also have private wells. The importance of treating wastewater becomes very clear when you think about the potential of drinking your own wastewater!

What is in wastewater?

A small glass of wastewater contains over a million faecal bacteria, as well as viruses and other disease causing pathogens. Wastewater also contains chemicals like urea, ammonium,metal salts, hormones, vitamins and soluble phosphorus. All of these components will cause serious problems if they accumulate in ground water or surface water.

Who should install the treatment system?

A person who is suitably qualified should supervise the installation. This will include supervision of construction of the percolation area, or soil polishing filter. A suitable distribution box between the treatment unit and the percolation area / soil polishing filter is critical. This will provide a balanced flow throughout the soil filtration area. The wastewater system should be leak tested and commissioned after installation. A certification of supervision of installation will be required under the planning permission.

Can the wastewater treatment be under the drive-way?

No part of the wastewater treatment system should be located under roads, paved areas or driveways. This includes the percolation / soil polishing filter area. Driving on this area can cause damage to pipework and compaction of soils. It is also important to be able to get access to every element of the wastewater treatment system for long term maintenance.

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How often do I have to de-sludge the system?

This depends on the wastewater treatment system and the occupancy of the dwelling. The system supplier will advise on frequency of emptying of sludge from the unit. A septic tank should be desludged at least once per year. An access route for a de-sludging vehicle to the system must be available. Sludge removal and disposal should be undertaken by a contractor, permitted under the Waste Management (Collection Permit) Regulations, 2001.

What about rainwater and roof water?

A separate soakaway is required for storm water disposal. This should be located more than 5m away from a dwelling, and should be as far away as possible from the wastewater treatment system and percolation area / soil polishing filter area (at least 5m).

Which treatment system should I use?

Selection of a wastewater treatment system depends on the outcome of the site assessment. A range of systems exist, of increasing complexity.

The simplest solution is a septic tank and percolation area. This is only suitable for sites with very good drainage and subsoil depth. For sites with less advantageous conditions, a variety of advanced wastewater treatment systems exist, including, intermittent filter systems and mechanical aeration systems.

It is important to check the track record of the proposed system, to ensure its performance is adequate. Do not select a unit on the basis of cost only, or on the basis of the salesperson’s promises. Ask the wastewater treatment supplier where other similar systems are located. Treatment systems vary in terms of their operating costs, maintenance requirements, pumping requirements, emptying frequency etc.

Do I need a maintenance agreement?

For advanced treatment systems - the answer is yes. These systems have moving parts with limited lifetimes. They should be checked for signs of wear and tear, blockages, pump status and general operational status- at the frequency recommended by the supplier. Maintenance should be undertaken by a qualified person, observing health and safety precautions.

Where can I get more information?

Go to the the wastewater advice and guidance section of the Environmental Protection Agency website. The content and documents include an extensive list of frequently asked questions, with answers. The Code of Practice for Wastewater Treatment Systems for Single Houses can be downloaded from the EPA website. 


The Environmental Protection Agency (EPA) have published a new Code of Practice for Domestic Waste Water Treatment System  2021 (Population Equivalent ≤10) . This document provides guidance on the site characterization, design, operation and maintenance of domestic waste water treatment systems.

Links to the revised EPA Code of Practice , explanatory letter and electronic Site Characterisation Form are provided below;–enforcement/waste-water/2021_CodeofPractice_Web.pdf–enforcement/waste-water/2021-Code-of-Practice—Explanatory-Letter.pdf

2021-Code-of-Practice---Electronic-Site-Characterisation-Form-with-note.pdf (

This Code of Practice (2021)  applies to site assessments and associated installations carried out on or after 7th June 2021. The 2009 Code of Practice for Waste Water Treatment and Disposal System Serving Single Houses (i.e. ≤ 10) may be used for site assessments and associated installations commenced before 7th June 2021 or where planning permission has been applied for before that date.

Page last reviewed: 28/11/19

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