Oil Separator (oil-water separator or interceptor) is a below-ground treatment chamber that removes free oils, fuels and associated silt from surface water run-off before it discharges to a sewer, soakaway or watercourse. Separators are specified in the UK to BS EN 858 and are standard on car parks, roads, fuel forecourts and industrial yards where hydrocarbon spillage is a credible risk.
How an oil separator works
Contaminated run-off enters the chamber, where velocity drops and gravity separation takes place: oil droplets, being lighter than water, rise to the surface and are retained, while silt settles into a sump. Class 1 units add a coalescing element — a filter surface on which fine droplets merge into larger ones that rise more readily. An automatic closure device shuts the outlet if the stored oil volume reaches capacity, preventing breakthrough during a major spill.
Classes and configurations under BS EN 858
- Class 1 — discharge concentration below 5 mg/l of oil; required where run-off discharges to surface water or infiltration. Achieved with coalescing separators.
- Class 2 — below 100 mg/l; acceptable for discharge to foul sewer with the water company’s consent.
- Full retention — treats the full design flow (up to 65 mm/hr rainfall); required for forecourts and high-risk sites.
- Bypass — treats the first 6.5 mm/hr of rainfall (which carries most pollution) and bypasses extreme flows; suitable for low-risk areas such as car parks.
Where separators are required
The Environment Agency’s guidance (GPP 3, formerly PPG 3) identifies where separators are needed: fuel delivery and vehicle refuelling areas, vehicle maintenance yards, roads and car parks draining to sensitive receptors, and industrial sites handling oils. Within a SuDS strategy, source-control components can sometimes reduce or replace separators — but forecourts and high-risk areas always require full retention Class 1 units.
Sizing and maintenance
Separators are sized by Nominal Size (NS/NSB) from the contributing impermeable area and rainfall intensity, with silt storage capacity added. BS EN 858-2 and manufacturer software give the selection. All separators require an automatic oil-level alarm to BS EN 858-1, six-monthly inspection and emptying by a licensed waste carrier when oil or silt capacity is reached.
Related ViaCon solutions
ViaCon Treat Solutions include corrugated steel oil separators and precipitators engineered for infrastructure-scale flows. See ViaCon Treat Solutions and our stormwater solutions range. Related glossary entries: vortex separator, silt trap and surface water drainage.
Frequently asked questions about oil separator
What is an oil separator?
An oil separator is a below-ground chamber that removes free oils, fuels and silt from surface water run-off by gravity separation before discharge to a sewer, soakaway or watercourse. UK separators are specified to BS EN 858 and sited on car parks, roads, forecourts and industrial yards.
What is the difference between Class 1 and Class 2 separators?
Class 1 separators use a coalescing element to achieve less than 5 mg/l of oil in the discharge and are required where water goes to a watercourse or infiltration. Class 2 achieves less than 100 mg/l and is acceptable only for discharge to foul sewer with consent.
What is the difference between full retention and bypass separators?
A full retention separator treats the entire design flow, up to a rainfall intensity of 65 mm/hr, and is required for fuel forecourts and high-risk sites. A bypass separator treats the polluted first flush (6.5 mm/hr) and lets extreme flows bypass, suiting lower-risk areas like car parks.
How often should an oil separator be serviced?
BS EN 858 and GPP 3 recommend inspection at least every six months, checking oil and silt levels and the automatic alarm. Contents must be removed by a licensed waste carrier when storage capacity is reached, and after any significant spill.