Two Tools, Very Different Purposes
Oil interceptors and drain covers are both used to prevent oil and fuel from entering surface water drainage systems — but they operate on entirely different principles, are suited to different scenarios, and carry different legal obligations. Confusion between the two is common and can result in either unnecessary expenditure or, more dangerously, inadequate pollution prevention that leaves a site exposed to enforcement under the Environmental Permitting (England and Wales) Regulations 2016 (EPR 2016) and potential prosecution under the Water Resources Act 1991, Section 85 (causing or knowingly permitting a water discharge activity without a permit).
What Is an Oil Interceptor?
An oil interceptor (also called a petrol interceptor or separator) is a permanently installed drainage structure that uses physical separation principles to remove free-phase hydrocarbons from surface water runoff before it enters the sewer or watercourse. They are designed to treat a continuous or regular flow of hydrocarbon-contaminated runoff — not a catastrophic spill event.
UK interceptors are designed and installed to BS EN 858-1:2002 and BS EN 858-2:2003, which classify them as:
- Class I: Hydrocarbon content in treated effluent less than 5 mg/l — required where there is a risk of emulsified oils and where the effluent discharges to a sensitive receiving water
- Class II: Hydrocarbon content less than 100 mg/l — standard specification for most commercial and industrial applications
Under Approved Document H of the Building Regulations 2010, petrol interceptors are required for:
- Car parks of more than 800 m² or more than 30 spaces (discharging to any sewer other than foul)
- Vehicle maintenance workshops
- Any site where it is reasonably foreseeable that significant quantities of oil will be present in surface water runoff
What Is a Drain Cover (Drain Blocker)?
A drain cover or drain blocker is an emergency response tool — not a continuous treatment device. It is used to physically seal a drain opening to prevent a spill (already occurring) from entering the drainage system. Drain covers come in several forms:
- Magnetic drain covers: Attach magnetically to cast iron grates — fast deployment but only suitable for ferrous drain frames
- Folded drain covers: Fabric or polymer covers weighted by filling with water — suitable for non-ferrous drains
- Expanding bungs: Foam or rubber plugs inserted into the drain pipe — suitable for round inlet pipes
- Drain guard sacks: Fabric sacks filled with sand or absorbent material placed over the grate
Drain covers do not treat the water — they prevent it from entering the drain. They must be used in conjunction with a spill kit to recover the spilled liquid before the cover is removed.
Decision Framework: Interceptor vs Drain Cover
| Scenario | Appropriate Control | Regulatory Driver |
|---|---|---|
| Car park >30 spaces with surface water drain | Permanent Class II interceptor | Building Regulations H; EA PPG3 |
| Vehicle workshop with floor drains | Class I interceptor on workshop drain | Building Regulations H; EA PPG13 |
| Fuel delivery/tanker offloading area | Drain covers + spill kit (immediate); interceptor where regular operations | EA PPG2; EPR 2016 |
| Chemical storage area (bunded) | Drain covers outside bund perimeter; bund intercepts spills within | COSHH Reg. 7; EPR 2016 |
| Emergency spill response on any hard standing | Drain covers to seal nearby drains immediately | PPG22; duty of care — WRA 1991 s85 |
| Road or yard runoff (regular hydrocarbon contamination) | Interceptor on surface water outfall | EA PPG3; water company discharge consent |
Interceptor Maintenance — The Forgotten Legal Obligation
An interceptor that is not maintained fails both as a treatment device and as a legal compliance measure. EA guidance requires interceptors to be desludged and inspected regularly — the frequency depends on usage, but quarterly inspection and annual desludging is standard for commercial sites. Key failure modes:
- Full sludge chamber: Once the sludge level reaches the overflow level, hydrocarbons bypass treatment. A full interceptor provides no protection.
- Coalescing element damage: Class I interceptors rely on coalescing elements to capture emulsified oils. If the element is damaged or absent, the unit operates as Class II at best.
- Automatic closure device failure: Most interceptors include an automatic closure device (ACD) that seals the outlet in the event of a large spill. ACDs must be tested annually; a seized ACD will not activate during an emergency.
Where Drain Covers Are Required by Environmental Permit
Many EA environmental permits for activities with pollution potential explicitly require drain covers to be immediately available at specified locations. This will be listed in the site's permit conditions or the associated management system document. Failure to have drain covers at the specified location during an EA inspection constitutes a permit breach — irrespective of whether a spill has occurred.
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