Summary

Petrol and oil interceptors are required on any site where hydrocarbons from vehicles or plant could reach surface water drains — car parks, forecourts, garage workshops, bus depots, commercial vehicle parks, and industrial yards. The purpose is to separate floating hydrocarbons (petrol, diesel, oil) from surface water runoff before it reaches a watercourse, soakaway, or surface water sewer.

The regulatory framework sits across two regimes: Building Regulations Approved Document H sets out drainage design requirements, while the Environment Agency (EA) in England (Natural Resources Wales, SEPA in Scotland) controls what can be discharged to controlled waters under the Environmental Permitting Regulations 2010. Getting the interceptor sizing, class, and maintenance wrong can result in EA enforcement action, prosecution, and significant fines under the Water Resources Act 1991.

The most common mistake is installing a Class II interceptor on a surface water outfall where Class I is required — or installing no interceptor at all on a car park drainage system connected to a watercourse. Local drainage design professionals and architects regularly overlook this requirement when specifying car park drainage.

Key Facts

  • BS EN 858-1 — design principles, performance, testing, marking and quality control for oil separators
  • BS EN 858-2 — selection of nominal sizes, installation, operation and maintenance
  • Class I separator: residual hydrocarbon content ≤5 mg/l (required for discharge to surface water/controlled waters)
  • Class II separator: residual hydrocarbon content ≤100 mg/l (acceptable for discharge to foul sewer)
  • Bypass separator: passes peak storm flows around the interceptor; only interceptor flow treated
  • Full-retention separator: all flow passes through interceptor; no bypass; required for high-risk sites
  • Nominal size (NS): based on peak flow rate plus 5× peak flow for bypass calculation
  • Coalescence filter: Class I units use a coalescing filter to achieve ≤5 mg/l — requires regular replacement
  • Silt trap: required upstream of interceptor; minimum 2× interceptor volume; separate chamber
  • Automatic closure device (ACD): required on Class I units; closes the outlet if a large oil spill is detected
  • EA registration/exemption: most surface water discharges from commercial sites require EA permit or exemption
  • Maintenance frequency: minimum annual inspection; Class I units typically quarterly coalescing filter check
  • Disposal of separated oil: must be collected by a licensed waste contractor (EA registered waste carrier)

Quick Reference Table

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Site Type Minimum Class Bypass? ACD Required? Notes
Domestic driveway (≤50m²) Not required — — Small area, low risk
Private car park (>50 vehicles) Class II to foul sewer Optional No Check if adjacent to watercourse
Car park (surface water outfall) Class I Full retention or bypass Yes EA permit likely required
Petrol filling station Class I full retention No Yes EA permit required
Garage workshop Class I or II Depends on outfall Yes if Class I Covered areas may drain to foul
Bus/lorry depot Class I (surface water) Full retention Yes High oil risk from heavy vehicles
Industrial yard Depends on process Depends Depends EA risk assessment needed
Vehicle washdown area Class I Full retention Yes Discharge to foul sewer preferable

Detailed Guidance

Sizing to BS EN 858

The nominal size (NS) of an interceptor is calculated from:

NS = Qr + ff × Qs

Where:

  • Qr = maximum rainfall runoff rate (l/s) from the catchment area
  • ff = factor for firefighting water (typically 0 unless fire risk is high)
  • Qs = maximum flow from industrial processes (l/s)

Rainfall runoff is calculated using the standard formula: Q = C × i × A, where:

  • C = runoff coefficient (0.9–0.95 for impermeable surfaces)
  • i = rainfall intensity (mm/hr) for the 1-in-1-year storm (typically 50 l/s/ha in the UK)
  • A = catchment area (ha)

For a 2,000m² car park (0.2 ha) with C = 0.90 and i = 50 l/s/ha: Q = 0.90 × 50 × 0.20 = 9 l/s → NS9 separator

Standard nominal sizes: NS3, NS6, NS10, NS15, NS20, NS30, NS50, NS100, NS200+

For bypass separators, the bypass is sized at 5× Qr; only the first-flush (Qr) passes through the interceptor chamber. This is acceptable for car parks where first-flush runoff carries the highest hydrocarbon concentration.

For petrol stations and high-risk sites, a full-retention separator is required — all flow passes through the separator chamber regardless of storm intensity.

Class I vs Class II: When Each Applies

Class I is required whenever the separated effluent discharges to:

  • A surface water sewer
  • A watercourse (river, stream, ditch)
  • A soakaway
  • Any controlled water under the Water Framework Directive

Class I separators achieve ≤5 mg/l through a coalescing filter — a cartridge of fine filament material that causes small oil droplets to agglomerate and rise to the surface. The filter must be inspected quarterly and replaced when saturated (typically 6–18 months depending on traffic).

Class II is acceptable for discharge to foul sewer with water company consent. The water company must be consulted — most require a Trade Effluent Consent for commercial sites. Class II separators rely on gravity separation only; no coalescing filter.

Automatic Closure Device (ACD)

Class I separators must be fitted with an automatic closure device (also called an automatic closure valve or ACV). This sensor-operated valve closes the outlet if the oil layer in the separator exceeds the design capacity — preventing a large spill event from passing through the outlet untreated.

ACDs are typically:

  • Float-operated (mechanical — simple, low maintenance)
  • Electronic sensor (more accurate, requires power and maintenance)

ACDs must be tested at every maintenance visit to confirm they operate correctly.

Maintenance Requirements

BS EN 858-2 requires that separators are maintained to a written maintenance schedule. Minimum requirements:

Activity Frequency
Visual inspection (oil layer depth, outlet check) Quarterly (Class I); 6-monthly (Class II)
Coalescing filter inspection Quarterly (Class I only)
Coalescing filter replacement As required (typically 6–18 months)
ACD test At each inspection
Full desludge and clean Annual minimum; more frequently if silt trap fills quickly
Oil removal by licensed contractor When oil layer >80% of capacity or at annual clean
Maintenance log update After every visit

A maintenance log must be kept on site and available for EA inspection. The log should record oil volume removed, condition of filter, ACD test result, and any defects.

Failure to maintain an interceptor is a criminal offence under the Environmental Permitting Regulations 2010 if it results in a discharge causing water pollution.

Environment Agency Requirements

In England, most discharges from commercial sites to surface water require either:

  1. Permit under EPR 2010 (Environmental Permitting (England and Wales) Regulations 2010) — for higher-risk sites
  2. Exemption registration — for lower-risk discharges meeting standard conditions

EA Regulatory Position Statements (RPS) provide guidance on when an exemption applies. For car parks discharging to surface water, the most relevant is the EA's guidance on urban surface water drainage. The EA can require remediation or shutdown of drainage systems that do not meet standards.

For new developments, the local planning authority will often condition the installation of an oil interceptor as part of the drainage strategy (SUDS design — see suds design).

Frequently Asked Questions

Does a domestic car park or driveway need an oil interceptor?

Generally no — domestic driveways are exempt. However, if a driveway drains directly to a ditch or stream, and the land has more than 50 vehicles regularly present (e.g., a car storage yard operated from a home), EA guidance should be checked. For a private family home with 1–3 cars, standard permeable paving or drainage to a soakaway is acceptable without an interceptor.

Can I discharge interceptor-treated water to a soakaway?

Only if you have Class I treatment. Even then, the Environment Agency's view is that discharging hydrocarbons to ground is a pollution risk, and they may require a permit or condition the soakaway design. In practice, treated Class I discharge to a correctly designed infiltration basin or soakaway is often accepted for lower-risk sites. Always check with the EA or Natural Resources Wales before installing.

What happens if the interceptor fills up with oil?

If the oil layer in the separator reaches the outlet level, untreated oil passes through to the watercourse — a pollution incident and a potential prosecution. The ACD is designed to prevent this, but ACDs can fail. This is why regular inspection and timely oil removal by a licensed contractor is essential. Licensed waste contractors must hold a current EA waste carrier registration.

Who do I notify if I accidentally discharge oil to a drain?

Report oil pollution incidents immediately to the EA Incident Hotline: 0800 807 060 (24 hours). Prompt reporting and evidence of immediate containment action will be taken into account in any enforcement decision.

Regulations & Standards

  • BS EN 858-1:2002 — Oil separators; design principles, performance, testing, marking and quality control

  • BS EN 858-2:2003 — Oil separators; selection of nominal sizes, installation, operation and maintenance

  • Environmental Permitting (England and Wales) Regulations 2010 — framework for EA permits and exemptions

  • Water Resources Act 1991 — makes it an offence to cause or knowingly permit water pollution

  • Building Regulations Approved Document H — drainage design for buildings

  • BS 7361-1 — centrifugal and gravity separators (older standard, largely superseded by BS EN 858)

  • Environment Agency: Oil Separator Guidance — PPG 3 guidance note

  • BS EN 858 via BSI — standards purchase

  • EA Environmental Permitting Guidance — permits and exemptions

  • CIRIA C697: SUDS Design Manual — surface water drainage context for interceptors

  • suds design — sustainable drainage systems and surface water management

  • grease traps — FOG (fats, oils and grease) interceptors for commercial kitchens

  • underground drainage — general drainage installation

  • part h drainage — Building Regulations drainage overview