Environmental guidance for your business in Northern Ireland & Scotland

Oil separators for use at construction sites

Oil separators for use at construction sites

A well-maintained oil separator is very effective at removing petrol, diesel and oil from water.

Before you set up your site, decide how you will deal with run-off from car parks, storage areas and vehicle wash down areas. These may contain oil in water runoff.

You may need to channel run-off from your site to more than one method of treatment.

For example, if you use detergent to wash vehicles in a car park or vehicle wash down area the detergent will emulsify any oil in the water and make an oil separator ineffective.

Yard water may not require a separator if suitable sustainable drainage techniques (SUDS) can be used. See PPG 3 for further information.

GPP 3 Use and design of oil separators in surface water drainage systems (Adobe PDF – 78.6KB)

Discharges to water or sewer

Wheel washing at construction sites

What you must do

Sizing: consider separator design

Make sure that the oil separator is big enough to cope with the volume of water that will pass through it. If the flow of water is too fast, the water will be churned up and the separator may not work properly.

You may need to install a storage tank to regulate the flow of water through your separator.


Oil separators (interceptors) that are not maintained do not work properly. They require regular maintenance to ensure that they work correctly and efficiently.

The separator should be filled to its operating level with clean water when it is first installed and every time it is cleaned out.

Any surface scum or debris that has gathered behind the skimmer should be regularly removed and disposed of.

Oil absorbent pads can be used periodically to remove oil and fuel from the surface of the water within the separator (interceptor).

Do not leave oil absorbent pads on the water surface within the separator during periods of high water flow or for longer than the manufacturer's instructions suggest.

These should be disposed of as hazardous/special waste.

Hazardous / special waste

If you are hiring oil separators, contact the plant hire business to establish whether they can undertake their maintenance as part of your hiring agreement.


When planning the layout of your site, consider where the discharge from the oil separator (interceptor) will go and what authorisation you will need for the discharge.

Discharges to water or sewer

Duty of care - your waste responsibilities

Water use and abstraction

SEE ALSO: Oil storage, Controlling surface water runoffDealing with silty water, Dewatering during construction, Drip trays,

Whats new on NetRegs

  • Waste – Duty of Care Roles and Responsibilities

    The Northern Ireland Environment Agency has published a short guide to the duty of care responsibilities including advice and information for waste producers, carriers and those accepting, storing and treating waste.



  • EIA (Agriculture) Regulations for Northern Ireland

    Any person intending to alter the use or management of areas of uncultivated or semi-natural land must obtain prior approval from the Department of Agriculture, Environment and Rural Affairs (DAERA).

    Read more on the DAERA website

  • Guidance from your environmental regulator

    Regulator logos

  • 9 NEW GPPs (Guidance for Pollution Prevention) available now

    The NetRegs team at SEPA, in partnership with The Northern Ireland Environment Agency, Natural Resources Wales and a number of industry bodies have produced 9 new GPPs to replace out of date PPGs. More are coming! Check the available topics

  • New guidance

    New guidance for Start-ups, charities and community projects


NetRegs on NetRegs on youTube

View our latest videos & subscribe to our channel.

NetRegs Update Newsletter

Free monthly email newsletter with environmental updates for Northern Ireland and Scotland

Sign up for free today!


NIEA - Apply online

SEPA - Application forms