When do I need an interceptor trap?

An interceptor trap is required where trade effluent containing oil, grease or fuel could enter the drainage system. Common applications include commercial kitchens (grease traps), car washes, petrol stations, workshops, and any premises where oils could enter surface water drains. Building Regulations Part H and water company requirements apply.

How is interceptor size calculated?

Interceptor volume is based on the peak flow rate multiplied by the retention time. The retention time allows fats, oils and grease (FOG) to float to the surface for separation. Kitchen grease traps typically need 4 minutes retention; oil interceptors for car washes need 6 minutes. BS EN 858 and BS EN 1825 give design guidance.

How often should interceptors be serviced?

Grease traps in commercial kitchens typically need emptying every 4-8 weeks, or when the grease layer reaches 25% of the tank depth. Oil interceptors should be emptied at least quarterly. All waste must be collected by a licensed waste carrier and disposed of properly.

What is the difference between a Class 1 and Class 2 interceptor?

Class 1 interceptors (BS EN 858) discharge effluent with less than 5mg/L oil content — used where discharge goes to surface water or sensitive watercourses. Class 2 allows up to 100mg/L and is suitable where discharge goes to the foul sewer. Class 1 typically requires a coalescing plate pack.

Do I need planning permission for an interceptor?

Generally no planning permission is needed for underground interceptors, as they are classed as drainage works under permitted development. However, you need a trade effluent consent from the water company if discharging to the public sewer, and an environmental permit if discharging to a watercourse.

TradeCalculator

Interceptor Trap Calculator — Chamber Volume & Service Schedule

Calculate grease trap or oil interceptor chamber volume from flow rate and retention time, plus pipe sizing and annual service schedule.

Use Type

Base flow: 2 L/s per outlet

Number of Outlets

Drain outlets serving the interceptor

Peak Flow Rate (optional)

Leave 0 to auto-calculate from use type

Unit Price (optional)

Interceptor installed cost

How We Calculate This

This calculator sizes an interceptor trap based on the design flow rate and required retention time for the specific use type.

The formula

Chamber Volume = Flow Rate × Retention Time / 1000 × 60

Design flow rates by use

Commercial kitchen: 2.0 L/s per outlet
Car wash: 5.0 L/s per bay
Workshop: 3.0 L/s per drain
Restaurant: 1.5 L/s per outlet

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.