Interceptor / Separator Sizing Calculator — BS EN 1825 & BS EN 858

Not strictly. In its classic sense an interceptor (or intercepting / disconnecting / Buchan) trap is a water-seal U-trap fitted at the last inspection chamber before the public sewer to block foul gas — it has no sized chamber and no volume calculation. Trade usage applies "interceptor" loosely to grease traps and oil separators too. This calculator sizes those latter two: grease separators to BS EN 1825 and oil/light-liquid separators to BS EN 858.

A grease separator is needed where drainage serves a commercial hot-food kitchen — Approved Document H 2.21 calls for a grease separator complying with BS EN 1825-1 and designed to BS EN 1825-2. An oil/light-liquid separator (BS EN 858) is needed where fuel or oil could enter the drainage system: forecourts, car washes, vehicle workshops, and most areas draining to surface water. Water-company trade-effluent consent or an environmental permit usually applies. Use our Sewer Connection Calculator for the connection design to the public sewer.

BS EN 1825-2 sizes a grease separator by nominal size: NS = Qs × ft × fd × fr, where Qs is the peak wastewater flow (L/s), ft is the temperature factor (1.0 up to 60°C, 1.3 above), fd is the density factor (1.0 up to 0.94 g/cm³, 1.5 above) and fr is the cleaning-agent factor (1.0 with no detergents, 1.3 where detergents are used). The result is rounded up to a standard nominal size — the BS EN 1825-1 preferred series is NS 1, 2, 4, 7, 10, 15, 20 and 25 (larger sizes are available on request). The sludge-trap volume is then at least 100 × NS litres (200 × NS for abattoirs and similar). There is no "4-minute retention time" in BS EN 1825 — sizing is by nominal size, not flow × minutes.

BS EN 858-2 sizes by nominal size too. A full-retention separator is sized to the system design flow — normally the flow from a 65 mm/hr rainfall over the catchment — and treats everything. A bypass separator fully treats up to a 6.5 mm/hr rainfall (over 99% of UK rainfall events) and lets larger storm flows bypass; the Environment Agency formula is NSB = 0.0018 × A, where A is the impermeable catchment area in m². The minimum working capacity is 1,000 litres (excluding silt), or 7,600 litres for petrol forecourts.

Grease traps in commercial kitchens typically need emptying every 4–8 weeks, or when the grease and solids layer reaches about 25% of the working depth (the "25% rule"). Oil separators should be serviced at least every six months. All collected waste must be removed by a licensed waste carrier and disposed of properly.

Class 1 separators (BS EN 858-1) discharge effluent with less than 5 mg/L residual oil — required where discharge goes to surface water or a sensitive watercourse, and they use a coalescing plate pack. Class 2 separators allow up to 100 mg/L and are suitable only where discharge goes to the foul sewer (with the water company’s consent). This tool sizes the nominal size; the class you need is set by the discharge route, so confirm it before selecting a unit.

Generally no planning permission is needed for underground separators, as they are 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 (or to register an exemption) if discharging to a watercourse or the ground.