Unit1, Building 3, Port link Business Park, Port Road, Letterkenny, Co.Donegal

What is a septic tank?

A septic tank is common in areas with no connection to main sewage pipes provided by local governments.

It generally consists of a tank connected to an inlet waste water pipe at one end and a drain at the other. Modern design of tanks usually incorporates two chambers (each of which is equipped with a manhole cover), which are separated by means of a dividing wall that has openings located about midway between the floor and roof of the tank. Waste from toilets, sinks and washing machines enter the tank via the inlet pipe.

How does a septic tank work?

Household waste enters the septic tank, which is a holding tank generally made of pre-cast concrete or fiberglass and is sized according to the estimated wastewater flow from a given-sized residence.

The septic tank separates the wastewater into three general components — solids or “sludge”, floatables or the “scum layer”, and a zone of relatively clear water.

Anaerobic bacteria (able to live in an oxygen-free environment) perform the first treatment of the wastewater, generating gas that is vented through the vent stack of the building’s plumbing, and breaking the solids into a liquid form.

From the septic tank, the liquid portion of the wastewater flows into the drain field,which provides secondary treatment of the sewage by allowing aerobic (oxygen-using) bacteria to continue deactivating the disease germs that remain in the wastewater.

The drain field also provides filtration of the wastewater as gravity draws the water downwards through the soil layers. In addition, evaporation of water occurs through the layer of soil covering the drain field.

Does my septic tank need to be emptied?

Septic tanks are designed to work automatically; however, most households use cleaning products and washing powder which kill the bacteria which breaks down the waste solids.

It is generally accepted that tanks should be regularly maintained and recently Ireland has introduced new septic tank regulations to ensure your tank meets European rules. It is also recommended that tanks should be emptied every 2 to 3 years.


If you have a septic tank installed in your new home, you must register it with the local county council within 90 days of it being installed


Ensure that the system cannot pollute the environment by discharging or leaking waste anywhere it is not supposed to, that is, into the ground or any water source


Ensure that roof water or surface water run-off cannot enter a domestic waste water treatment system


What does the term percolation mean?

Percolation is simply the process of liquid slowly passing through a filter, for example it is how coffee is made. For a septic tank, the percolation area is an area of soil where treated wastewater is discharged into the ground.

What are the common problems with percolation areas?

There is a number of problems that may occur with your Percolation Area or Septic Tank. One of the most obvious signs that there is something wrong is that there will be a foul smell. Cracks on the lid or on the walls of your tank is also a cause for concern.

It is also important that you de-sludge your tanks regularly otherwise solid untreated materials will overflow into the outlet of your tank, which will then accumulate in the percolation pipework. When septic tanks are not serviced and de-sludged in the recommended time the pumps and blowers will burn out due to the added pressure.

What is a percolation test?

In order to ensure your building site is suitable for a septic tank, a percolation test or “perc test” is required. The perc test will determine the soils absorption rate for a “leach field” or a septic tank drain field. The results of the perc test are required to design your septic tank accordingly.

How to carry out a percolation test?

A percolation test can be carried out by digging a hole at 300mm square to a depth of around 250mm at the site where a septic tank is proposed to be installed. The soil is then saturated fully with water and leave overnight. Refill the hole with water and record the time taken for the water to seep away. Once the water has completely seeped away from the hole, divide the time taken for the water to disappear by the depth of the water in the hole.

Try this same test in 3-4 other trial holes at your site and compare results in each to determine the best suitable place for your septic tank at your site.

How far does my septic tank need to be from my house?

To meet the septic tank regulations, the distance required is 7 meters

Alternative Percolation Types

also known as a Low-Pressure Pipe System (LPP) is a very common method of distributing treated effluent from a sewage treatment system into the soil. The effluent is pumped from the Streamline BAF to a either a splitter valve or a manifold which in turn distributes the effluent trough a number of 32mm laterals to disperse the effluent evenly over the bed.

is a more traditional method of distributing effluent from a septic tank or sewage treatment system. This is option is more ideal for sites with a topography that favours a gravity outlet from the septic tank or sewage treatment system.

Drip Irrigation Systems can be used as an alternative method of distributing effluent into poorly draining soil. A drip system has the advantage of working well on sites with limited soakage as the effluent is evenly distributed across a large network of high pressure pipes with small emitters that let the effluent out at a controlled pace.

Tertiary Sand Polishing Filters treat the waste water from the Streamline BAF system to a higher standard prior to final discharge into the soil and are ideal for restricted sites as they have a small footprint. A sand polishing filter is generally the most viable option on sites with limited available area and/or sites with poor soakage as the effluent discharged from a sand polishing filter does not form a biomat or sludge layer which can impede the rate of soakage into the ground.

Puraflo Tertiary Modules use peat fibre to treat the waste water from the Streamline BAF system to a high standard prior to final discharge into the soil and are ideal for restricted sites as they have a small footprint. Puraflo modules are generally a suitable option on sites with limited available area and/or sites with poor soakage as the effluent discharged from a sand polishing filter does not form a biomat or sludge layer which can impede the rate of soakage into the ground.

SteamPod Tertiary Modules are highly effective way to treat the waste water from the Streamline BAF system to a very high standard prior to final discharge into the soil and are ideal for restricted sites as they have a small footprint. StreamPod modules are generally a suitable option on sites with limited available area and/or sites with poor soakage. The StreamPods are delivered and placed by truck mounted crane.

Selecting What System to Use:

Firstly, a suitably qualified person must carry out a site assessment in accordance with the guidance set out in the Code of Practice Wastewater Treatment and Disposal Systems serving Single Houses (p.e. £10) (2009).

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 on-site wastewater treatment and disposal systems are;
  • Septic tank systems – septic tank and a properly constructed percolation area
  • Secondary Treatment System: Constructed On-site – septic tank, filter system (including constructed wetlands) followed by a polishing filter for discharge to ground.
  • Secondary Treatment: Packaged Wastewater Systems – package system followed by a polishing filter for discharge to ground.
  • Tertiary Treatment Systems – polishing filters, constructed wetlands or package tertiary systems.

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

Who to register with:

Owners of domestic waste water treatment systems are required to register their systems with their water services authority. They can register online, by post or at your local authority office.

  • Online – www.protectourwater.ie – by credit card/debit card.
  • By Post: Registration forms are available online; from City/County Councils; Libraries; Citizen Information Centres or Lo Call 1890 800 800.
  • Payment can be made by cheque, postal order or bank draft and made payable to “Protect Our Water”. Send your form with payment to: Protect Our Water, P.O Box 12204, Dublin 7.
  • Local Authority Offices: Payment by credit or debit card or by cheque, postal order, bank draft or cash can be made in your City or County Council office (no administration fee will be charged by the Council for this).
Soakaway Explained

In its simplest form a Soakaway is a large hole dug in the soil and filled with stone. Rainwater runoff from the impervious surfaces (driveways, roofs etc.) is collected by a pipe and allowed to flow into this large hole.

The stone filled hole is then covered with soil and a covering such as grass/driveway etc. The Soakaway prevents flooding of the surrounding areas by allowing the large pit fill with water during times of heavy rain fall and then infiltrate into the ground slowly.

Why have the planners asked me for a Soakaway Design?

Request from the councils for Soakaway Designs have become more and more common in recent years. This is due to the increased amount of flooding in built up areas.

By implenting Soakaway designs the rainwater runoff from the proposed development is curtailed within the site in question and does not put any extra pressure on the mains runoff water system thus reducing the risks of flooding.

Soakaways should not be built:

(a) within 5 m of a building or road or in areas of unstable land;
(b) in ground where the water table reaches the bottom of the soakaway at any time of the year;
(c) within 5 m of any drainage fields (percolation area), drainage mounds or other soakaways and should be sited down gradient to ensure that the overall soakage capacity of the ground is not exceeded, and the effectiveness of any drainage fields is not impaired;
(d) where the presence of any contamination in the runoff could result in pollution of a groundwater source or resource.

Gutter Sizes & Outlet Sizes
Table 17 Gutter Sizes and outlet Sizes Flow Capacity
Max effective roof area(m2) Gutter size(mm dia) Outlet size(mm dia) Flow Capacity
18.0 75 50 0.38
37.0 100 63 0.78
53.0 115 63 1.11
65.0 125 75 1.37
103.0 150 89 1.37
*Note:Refers to normal half round eaves gutters laid level with outlets at one end sharp edged.Round edged outlets allow smaller downpipe sizes


The layout of the drainage system should be as simple as possible. Changes of direction and gradient should be minimised and as easy as practicable. Access points should be provided only if blockages could not be cleared without them. Connections of drains to other drains or to sewers should be made obliquely, and in the direction of flow. They should be formed using a standard saddle connection.

The system should be ventilated by a flow of air. A ventilating pipe (without an air admittance valve) should be provided, at or near the head of each main drain, to any branch longer than 6 m and on a drain fitted with an intercepting trap (particularly on a sealed system). Ventilated discharge stacks may be used

Pipes should be laid to even gradients and any change of gradient should be combined with an access point

Pipes should be laid in straight lines where practicable but may be laid to slight curves if these can still be cleared of blockages. Any bends should be limited to positions in or close to inspection chambers or manholes and to the foot of discharge and ventilating stacks. Bends should have as large a radius as practicable.

Special precautions should be taken to accommodate the effects of settlement where pipes run under or near a building, on piles or beams, in common trenches or in unstable ground. Precautions may also be necessary in situations involving surcharging of drains.

What is a Polishing Filter (Emma Toner House)

Polishing filters consist of either soil or sand and are employed to reduce micro-organisms from wastewater. They are used to treat wastewater from intermittent filters, constructed wetlands and packaged treatment systems and to allow for the discharge of treated wastewater to ground. See the Section 10 of the CoP for more information.

Packaged wastewater treatment systems

Packaged wastewater treatment systems may be used to treat wastewater from a building where the site is unsuitable for a septic tank system or they may be used as an alternative to septic tank systems. These systems should conform to I.S. EN 12566-3: 2005/A1: 2009 Packaged and / or site assembled domestic wastewater treatment plants, and its National Annex.

The system should be designed for a minimum hydraulic daily load of 150 l/person/day based on the number and size of bedrooms and a minimum organic daily load of 60g BOD/person/day to ensure adequate treatment is provided.

is required to ensure that the effluent is treated to this standard. In nutrient sensitive areas, more stringent performance standards for nitrogen and phosphorous may be necessary.

All such systems must have a treatment efficiency capable of meeting the minimum performance effluent standards (see below)

The sludge storage capacity should be checked with the manufacturer to establish the necessary frequency of de-sludging. All package wastewater treatment systems should be provided with an alarm to indicate operation failure in line with the requirements of I.S. EN 12566-3: 2005/A1: 2009 Packaged and / or site assembled domestic wastewater treatment plants.

Different Types of Treatment Systems:
  • (a) Biofilm Aerated Filter (BAF) systems;
  • (b) Rotating Biological Contactor (RBC) systems;
  • (c) Sequencing Batch Reactors (SBR) systems;
  • (d) Membrane filtration systems.
Tertiary treatment systems

The term tertiary treatment system includes polishing filters, constructed wet lands and packaged tertiary treatment systems. They provide additional treatment to wastewater from secondary treatment systems. Tertiary treatment systems should comply with the EPA Code of Practice – Section 10 and prEN 12566 Part 7 – Prefabricated tertiary treatment units (when available).

How a Septic Tanks Works

Waste water enters the septic tank through the pipelines leading from the dwelling. While sitting in the tank, the solids and the liquids that make up the waste water begin to separate by natural breakdown. The solids sink to the bottom forming the sludge layer and the lighter particles of waster rising to the top, forming a layer known as scum. The sludge is left behind and eventually begins to build up which is why a septic tank must be emptied every few years.

The waste water is now up to 90% free of solid waste and known as effluent, flows into the second chamber in the tank. Here, due to pressure in the tank caused by the incoming waste water in the first chamber, the effluent flows out into the percolation area through a series of perforated pipes. Once the effluent flows out through the pipes, it then flows through a layer of gravel before it finally reaches the soil.

This is the last stage of the septic tank process where the biological organisms in the soil treat the waste ensuring that it permeates down and eventually out of the soil, joining up with the water on the surface. This stage is probably the most important as it ensures that the waste is no longer a health or environmental hazard which is why extensive soil testing is carried out prior to a septic tank installation to ensure that the soil is suitable.

Irish Septic Tank Design and Construction Guide

Please refer to the recommendations of your Site Assessor but as a general rule of thumb: Slope of pipe from Septic Tank to distribution box 1 in 40 for earthenware or concrete 1 in 60 for uPVC. Slope of percolation trench from distribution box 1 in 200.

Length of percolation pipe in each trench 18m maximum. Minimum separation distance between percolation trenches 2 m (2.5 m centre to centre) Diameter of pipe from septic tank to distribution box 100–110mm. Percolation pipes 100 mm bore, perforated (typically at 4, 6 and 8 o’clock) smooth wall PVC drainage pipes with perforations of 8-mm diameter at about 75-mm centres along the pipe or Pipes with similar hydraulic properties.

Width of percolation trench 500 mm. Depth of percolation trench is about 850-mm depth below ground surface BUT this will be the Site Assessors responsibility to decide.

Backfilling of percolation trench 300mm of 8- to 32-mm washed gravel on invert; pipe laid at a 1 in 200 slope surrounded by 8- to 32-mm clean washed gravel and with 150 mm of similar gravel over pipe;

geotextile layer followed by 300 mm topsoil to ground surface Geotextile Geotextile should be in accordance with EN ISO 10319. Access/Inspection points and vents are recommended for the ends of the percolation pipes; the covers should be visible and installed to prevent entry of water. They may also be used for rodding/scouring purposes.