Environmental guidance for your business in Northern Ireland & Scotland

Septic tanks

If a your domestic property has no connection to a foul sewer, and no connection is possible, you will most likely have a septic tank to deal with sewage and waste water from your premises.

This might be a septic tank that deals only with the waste water from your house, or it might be a septic tank that deals with waste water from a number of neighbouring properties.

Septic tanks use natural biological processes to break down sewage and waste water and produce a discharge that can be made safe and harmless when slowly added to well aerated soil, or by another final treatment.

If the natural processes are stopped, then problems can occur. Non-mains sewerage systems can pollute groundwater (all water lying below the water table or in aquifers) if they are poorly located, built or operated. This can affect water supplies, such as drinking water or water for livestock. It can also pollute surface water in rivers, streams and lochs/loughs.

This guide explains what you should do to look after a septic tank and how to prevent problems arising. It also tells you how to register existing or new septic tanks.

If you are a plumber or builder installing a septic tank there are certain things you must do before installing a tank. This guideline outlines these responsibilities and the authorisation you need before you start work.

Additional resources

Your septic tank will treat domestic sewage and waste water by allowing bacteria to naturally break down potentially harmful substances.

In order to work effectively the sewage and waste water should stay in the tank long enough for the bacteria to do their work.

The solids will settle to the bottom of the tank and what comes out of the tank should be a clear liquid, with no offensive smell. Factors such as temperature, the amount of liquid entering the tank, and chemicals in the waste water can affect how well the tank performs. This liquid does contain the breakdown products of the bacterial action, so contains chemicals such as nitrates and phosphates, as well as any other chemicals that can’t be broken down.

The solids that settle will eventually need to be removed, so to keep a septic tank working effectively regular desludging should be carried out.

Usually the liquid discharge from the tank will be directed into a drainage field, often referred to as a “soakaway”. This is a network of pipes buried under the soil where the liquid discharge soaks into the soil. Here it is further broken down by soil bacteria and is taken up by plants as nutrients. It is important that the soil conditions are right for a soakaway, or problems can arise.

Figure 1 Typical septic tank

In some cases, where the soil is too shallow, or where the water table is too close to the surface, a raised mound of soil can be used. This can be expensive; it may need significant amounts of soil to be imported, and could need a pump to be installed to get the waste water into the mound. Well drained soil with more than 2 metres of soil above the water table is the best option.

If no ground is suitable for a soakaway then the outflow from the septic tank will need further treatment before entering a watercourse. This could be done using a package treatment plant, or biological filters. Discharges can also be further treated using a properly constructed reed bed, or by using a gravel filter. Both these options will require a large area of ground.

A properly constructed soakaway is the best option for dealing with the discharge from your septic tank.

Your septic tank is designed to treat domestic sewage and waste water. These pipes from your house should be connected to the inlet pipe to the tank.

Because the tank works by allowing bacteria to break down the contents, it is important to make sure that nothing enters the septic tank that could prevent that process working.

Too much water

Make sure that your downpipes and drains that carry rainwater are not connected to your septic tank. This could wash out the contents of the tank before it is broken down, and could cause pollution and health hazards.

When you use a washing machine, wait until the cycle is finished before you take a shower. This prevents too much water entering the tank at the same time and flushing out untreated wastes. Think about how much water is leaving your house at any one time, and keep it to a manageable flow.

Too little water

To maintain a healthy population of bacteria your septic tank should have a regular inflow of sewage or waste water. This adds air to the liquid and encourages bacteria to grow. Septic tanks in holiday homes for example can become less effective if left for a long time unused. If this happens then try to let frequent small amounts of waste water enter the tank at first to revive the system.

Chemicals that stop the bacteria working

There are many household chemicals that are designed to kill all bacteria. You should never allow these to enter a septic tank. Killing the bacteria in the tank will result in untreated sewage and waste water leaving the tank, clogging your pipes and creating a nuisance and potential health hazard. Things to avoid include bleach, paint, disinfectants, garden pesticides, medicines, solvents like white spirit and drain cleaning and unblocking liquids that contain caustic soda. There are cleaning products that have been specially designed for use with septic tanks.

Oil grease and fat

These will solidify and clog your pipes and drains. They do not break down in the septic tank so float at the top of the tank creating a thick scum. Fats and oils that enter the soakaway can block the outlets and prevent soil bacteria working.

Items that won’t break down

Make sure that nothing goes into the tank that won’t break down, such as paper (other than toilet paper which is designed to disintegrate) sanitary waste, cotton buds, nappies etc.

Detergents that contain phosphates

You can buy cleaning products that don’t contain phosphates, these are preferable to ones that do. Phosphates are not broken down in a tank so they pass through and end up in the soil and eventually in watercourses. Phosphate pollution is a significant problem in watercourses, ponds and lochs, so reducing the amount in your septic tank discharge will help to reduce this.

To keep your tank working effectively you should carry out regular maintenance. You should look out for the signs that problems might be developing by regularly checking:

  • Do sinks or toilets in your house drain slowly or back up? The most likely cause of this is a tank that needs emptying, or desludged. Have the sludge removed regularly to prevent problems. This problem could also be caused by blocked pipes. If your tank does not need desludging but the problem continues you should contact a specialist or plumber who can clear the pipes without using chemicals.
  • Is there a detectable odour where the soakaway is located, with damp ground visible and a flush of green grass? – this could be a problem with the soakaway being clogged, discharging to an area where the water table is too high or the soil has poor natural drainage. A specialist will identify the problem and could provide a solution.
  • Is the discharge from the tank clear and odourless? – if it is dark, contains solids and has an odour then something is preventing the septic tanks from working. This could be a build-up of sludge, blocked pipes, or harmful chemicals have been added to the tank which have killed the bacteria. Have it checked by a specialist.
  • If your tank never needs to be desludged then there is a chance that it is leaking, or you have a burst pipe. You should have this investigated by a specialist or plumber as you could be causing water pollution, or even a health hazard.
  • If there is no outflow from the tank then the drains to your tank could be blocked, or there could be a leak from the tank. Again you must have this investigated to find the cause.
  • Is there a noticeable fungal growth, or an odour coming from a nearby stream or watercourse? This indicates that untreated sewage is finding its way to the water. This could be the result of a damaged or leaking tank or pipes, which are releasing raw sewage. Contact a plumber or drainage specialist to fix the problem.


Over time the solids that settle out build up, effectively reducing the volume of the tank and making it less efficient. You must have your tank regularly desludged.

Check the outflow from the tank, if it is clear and odourless then your tank is working well. If it has an unpleasant odour, is discoloured or has any solids in it then it is likely that your tank needs to be have the sludge removed.

Usually a tank should be desludged once a year and you can arrange a regular service from contractors. If you set up regular visits from a business that carries out desludging, it will help prevent problems, and will usually cost significantly less than an unscheduled visit.

Then frequency of desludging will depend on the size of the tank, the number of people using it, and how well you look after it.

Keep access to your tank clear

Locate the access lid for your tank and make sure it is clear of vegetation and soil. Don’t operate heavy machinery or drive vehicles over the ground where the pipes have been laid. This includes the area of ground used for the soakaway.

Don’t allow trees or deep rooted shrubs to become established close to your tank or the soakaway, the roots can damage the pipework resulting in blockages or leaks. In either case the repairs could be costly.

If you suspect that there is a problem, get a plumber or drainage specialist to check out the tank as soon as possible.

Regular checks

It is important that you regularly check the operation of the system.

  • Carry out a fortnightly or monthly visual check on the discharge from system to the drainage field. Your septic tank should have an inspection chamber fitted between the tank and the soakaway. The discharge should be clear and shouldn’t contain any solids.
  • At the same time check the level of the liquid in the tank. If it is very low then your tank could have a leak. If it is very high then you may have a blocked outlet pipe.
  • Check the area of ground where the soakaway is located. Make sure the ground is not saturated, and that there is no odour. Check nearby watercourses, such as ditches and streams to make sure that there is no fungus or odour associated with the tank.

Figure 2: Sewage fungus in a stream

If you find any problems, have a professional look at the tank and the drainage. Problems that are not fixed can lead to pollution and could cause a nuisance to neighbours.

Why does it need to be registered?

An individual septic tank serving a household, if it is properly maintained doesn’t pose a great threat to the environment. Where there are a lot of septic tanks, the combined discharges can have an effect on the quality of groundwater and of watercourses that pass through the area.

Your environmental regulator needs to know where they are located and how many there are. This is in order to make decisions about whether allowing other activities, or discharges, will cause pollution problems for groundwater, streams or rivers or lochs/loughs.

Check if your septic tank is already registered

You can check if your tank has already been registered by contacting your environmental regulator. If you are unsure then it is best to check and avoid making an unnecessary payment. The NIEA and SEPA have records of all registered septic tanks.

Contact your environmental regulator

Register your septic tank

If your tank is not registered then you must register it with your environmental regulator.

In Northern Ireland you should visit the Northern Ireland Environment Agency website for an application form and guidance documents.

NIEA: Septic tanks and domestic discharges

In Scotland you should visit the SEPA website. You can register online or by post:

SEPA: Registering your existing septic tank

If you are the owner of the septic tank then you are responsible for its safe and effective operation.  Problems can occur with septic tanks and they can be described as:

  1. Problems with the operation of the septic tank
  2. Problems that affect nearby watercourses and groundwater
  3. Problems that affect your neighbours

1. Problems with the operation of the septic tank.

Make sure you know how best to look after your septic tank, including What shouldn’t go into a septic tank and Maintaining your septic tank (Links to the above pages)

Your tank should be able to deal with the quantity of sewage and waste water you produce. It should treat it, and produce a discharge that doesn’t smell or flood the ground.  The discharge should not cause problems by polluting watercourses. If it fails to operate correctly, then you should contact a drainage specialist or plumber who can remedy the problems.

2. Problems that affect nearby watercourses and groundwater

If your septic tank is discharging effluent that is pollution nearby streams, or is polluting groundwater then this is a matter for your environmental regulator, either the NIEA or SEPA. They may require you to carry out work that will prevent pollution of the water environment.

3. Problems that affect your neighbours

If your septic tank:

  • produces a foul smell that causes offence to your neighbours
  • discharges to a soakaway that is ineffective and creates damp ground with an offensive odour that affects your neighbour’s property
  • affects a water supply on your neighbour’s property

then your neighbours can contact the environmental health department of your local council. Your council can use laws designed to deal with nuisances, and can require you to carry out works to stop the nuisance and to prevent it happening again.

The correct installation and correct management of the tank should prevent these types of issues arising.

The first step is to confirm that no connection to the foul sewer is possible. Your local council will check this first when deciding the acceptability of your plan. If a foul sewer is available you should connect to it.

You should then inspect your site to check:

  • Is there anything that would prevent the use of a septic tank, for example if there is a well or borehole supplying drinking water close to your site?
  • Is there enough space to construct a soakaway, or drainage field that will be large enough for your needs? See the section on Calculating the area of a soakaway.
  • Are the ground conditions suitable for a soakaway? Is there evidence of waterlogging or a water table close to the surface, very shallow soil or soil with very heavy clay content that prevents percolation.
  • Is there a suitable slope, since the drainage will rely on gravity? If there is a low point where a soakaway could be constructed, is it prone to flooding or is the water table close to the surface?

If there are any problems with the site then you might need to consider alternative locations, or alternative sewage treatment systems.

Choosing a tank

You should choose a tank that has enough capacity to deal with the size of your house. This should not be the number of people that actually live there, but the maximum number who could live there. The tank must be capable of dealing with the waste water and sewage from the maximum number of people who could live in the building at some point in the future.

Check with the supplier that the tank conforms to the Construction Products Regulations.

Drainage design

Good drainage design is key to ensuring that sewage does not end up in groundwater. The key factor you need to consider is the availability of a suitable area of ground where you can construct a soakaway.

You should make sure that your soakaway, also known as a drainage field, conforms to the British Standard:

BS 6297 Code of practice for the design and installation of drainage fields for use in wastewater treatment

To protect the water environment and human health, you will need to make sure:

  • Your drainage field is at least 50m from any well of borehole supplying drinking water.
  • The soil should be of a type that will allow the further treatment of the effluent from the septic tank.
  • If soil conditions are unsuitable, you may want to consider a drainage mound. This can increase the distance between the disposal pipes and the water table to allow further treatment to take place. They do not overcome problems with soil that does not allow waste water to percolate through. Drainage mounds can be expensive to build and need a large area of land. They can’t go on land that has a slope greater than about 7 degrees and may need pumping equipment.

Calculate the area for a soakaway

The area of ground needed for a soakaway depends on the number of people the septic tank is designed for (the person equivalent) and the type of soil in the drainage field. In particular it is a measure of how fast water can soak through the soil, its percolation value, which is important.

The “person equivalent” is the number of people who could live in the building (or buildings) that the septic tank serves. It is not the number who actually live there, but is based on the size of the house.

The “percolation value” is calculated by measuring how quickly water soaks into the soil under controlled conditions. This value must be calculated using a method that conforms to the standard BS926 1983. Your local council building control may want to witness the test being carried out.

The area of ground required, in square metres, is then found by the formula:

A = P x Vp x 0.25 (A = area in square metres, P = person equivalent, Vp is the percolation value)

The drainage pipes that make up the soakaway should never be more than one metre below the ground surface. The pipes should always be at least one metre, preferably more, above the water table in winter.

It is important to select a location that is away from watercourses, wells or boreholes, and does not have the potential to cause a nuisance to neighbours.

Technical guidance for constructing septic tank drainage fields can be found in the building regulations.

In Northern Ireland:

The Building Regulations (Northern Ireland) 1990 Technical Booklet N

Also available from The Stationary Office, Belfast on 02890 238 451

In Scotland:

Scottish Government: Technical Handbook – Domestic - section 3.9

Package treatment plants

Package treatment plants are self-contained units that will produce a safe discharge from partially treated waste water and sewage from a septic tank. They are designed to carry out the biological treatment that a well-constructed soakaway would provide. They are made in different sizes to cope with the sewage and waste water from different sized dwellings.

Package treatment plants will deal with waste water and sewage effectively, but have the drawback that they usually require a power source as well as regular maintenance.

Reed Bed Systems

You can use a reed-bed or wetland system to improve the quality of effluent discharges from septic tanks. This enhanced level of treatment might be required before a discharge is allowed into a sensitive or small watercourse, a watercourse that receives many discharges, or a drainage field where groundwater is vulnerable.

Reed beds are built on an impermeable base, usually with a layer of gravel above. This provides suitable growing conditions for species of plant that can absorb organic material in the sewage effluent, and in this way purify the discharge from the septic tank.

Reed beds are effective, and have the advantage over a package treatment plant that they require no power and need less maintenance. They do however require a significant area of land to be used if they are to work effectively. Package treatment plants can be a more practical and effective solution.

Reed bed systems are not the preferred option in Northern Ireland.


If your septic tank can’t be drained to a soakaway, you will need to get an authorisation from the NIEA or SEPA. This will require treatment of the septic tank discharge before it reaches the water environment. You may be required to install a package treatment plant or reed bed in order to get an authorisation.

You must contact the NIEA or SEPA before you construct a reed bed to ensure that your plans will provide satisfactory treatment for the effluent in the local circumstances. A reed bed must be properly designed, constructed and maintained. In Northern Ireland reed beds are not generally considered to provide adequate treatment and are unlikely to be approved. Contact your environmental regulator

Guidance on how to construct a reed bed is available from the Building Research Establishment.

Building Research Establishment Good Building Guide No 42 (GG42) Reed Beds 2001. ISBN 1860814379.

BRE, http://www.brebookshop.com/, Tel: 01344 404 407

Waterless toilets – Chemical and composting toilets

Chemical toilets or composting toilets are an option you might consider if you want temporary or mobile sewage treatment, or if your site is remote and away from sewers, electricity or running water.

Waterless chemical toilets are self-contained systems that rely on chemicals (biocides) to control foul odours. They are often used by caravans, and at locations like campsites, construction sites and at other large events.

You should dispose of the chemical toilet wastes at a central disposal point, common in camping and caravanning sites, or by disposal to the foul sewer. You should contact the sewer provider before emptying to the foul sewer.

You must never dispose of chemical toilet waste into a watercourse, surface drain, the ground or groundwater. Your duty of care for waste means that you must dispose of this waste in a way that doesn’t harm the environment.

Composting toilets use natural processes to convert waste matter into compost. They are useful at remote sites, where there is no public sewer or mains water supply.

They may require maintenance and the addition of materials such as sawdust to aid the composting process. Some may produce concentrated fluid fertiliser or dry compost for use. These should not be discharged to a watercourse.

Northern Ireland

Water (Northern Ireland) Order 1999 SI 662 (including amendments up to 2004)

Revokes and replaces the Water Act (Northern Ireland) 1972 and makes provision for discharge consents. Enables the DoE to set water quality objectives and prevent pollution from anti-pollution works.

Water Environment (Water Framework Directive) Regulations (Northern Ireland) 2017 SR81

Revokes and replaces the Water Environment (Water Framework Directive) Regulations 2003. They transpose the Water Framework Directive.  The Regulations will continue to provide for the monitoring, assessment and improvement of the condition of water bodies in order to meet the objectives of the Water Framework Directive.


Water Environment (Controlled Activities) (Scotland) Regulations 2011 SSI 209

Replaces CAR 2005/348 (as amended). Increases the transparency and efficiency of processing applications for water use licences, and increases flexibility to deal with emergencies with imminent risks of serious harm to people, property or the environment.

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