You should have a good working knowledge of your drainage systems. There are two types:

  • Separate drainage systems have two drains, one for foul water (usually painted red) and another for surface water (usually painted blue). The foul water drain carries polluted water, such as sewage and trade effluent, to a sewage treatment works. The surface water drain should only carry uncontaminated rainwater as it goes directly into a watercourse.
  • Combined drainage systems have a single drain that carries both foul and surface water to a sewage treatment works.

What you must do

In Northern Ireland if you want to discharge anything other than clean, uncontaminated water into a surface water drain, or directly into surface water or groundwater, you must have a discharge consent from the Northern Ireland Environment Agency (NIEA).


In Scotland, if you discharge anything to the water environment you may require an authorisation from SEPA - See SEPA: Water - Controlled Activities Regulations (CAR). Some discharges are authorised if you comply with the general binding rules (GBR). In such cases, you do not need to apply for authorisation from SEPA.

You must comply with the conditions of your authorisation.

If your business discharges trade effluent to a public sewer, you must have a valid trade effluent consent from your water and sewerage company. See our guideline: Trade effluent – managing liquid wastes

Use the correct drainage system

You can take simple measures to use the right drainage system:

  • Draw up a drainage plan to identify all drains and update it after building work. If you do not have the expertise in-house, use a reputable drainage business to do the work for you.
  • Colour code drain covers, drainage grills and gullies. Foul water drains should be painted red, surface water drains blue and combined drainage systems can be marked with a red letter C.
  • Check that facilities such as sinks, washing machines, showers and toilets are connected to the public foul sewer or combined drainage system.

If no foul sewer is available, you may be able to use a septic tank or connect to a package sewage treatment plant.

GPP4: Treatment and disposal of sewage where no foul sewer is available

Prevent pollution from high-risk areas

Isolate run-off from refuelling areas from general yard drainage.

Cover areas at high risk of contamination, such as refuelling areas, to prevent run-off from rainfall. Where this is not possible you may need to channel run-off to a collection tank.

If your drainage goes to the foul sewer, you may be required to install an oil interceptor. You may need permission from your environmental regulator or water company to discharge wastewater from your oil interceptor or other treatment system.

PPG3: Use and design of oil separators in surface water systems

You can only discharge roof water run-off directly to the surface water system if it is clean and uncontaminated. You must ensure that it does not pass through anything that could contaminate it, such as an oil interceptor.

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