Grease Trap / Separator Sizing

EN 1825-2 sizes a separator on the peak wastewater flow it must handle, and it offers two independent ways to estimate that flow. The appliance method works from the equipment: each sink, dishwasher, and kettle has a characteristic discharge flow, and these are summed — but with a twist. A simultaneity factor reduces the total to reflect that the appliances do not all empty at the same instant, and that factor falls as the appliance count grows, just as it does in fixture-unit plumbing. A single sink discharges at nearly half its rated flow on the design basis; a bank of several discharges at a much smaller fraction each.

The meals method comes at it from the other direction, estimating flow from how much food the kitchen produces. It multiplies the meals served per day by a litres-of-wastewater-per-meal figure and a peak factor that captures how concentrated the service is, then spreads that over the operating hours. A speciality restaurant with long sittings has a very different peak profile from a works canteen that serves a thousand lunches in ninety minutes, and the kitchen-type factors encode exactly that.

The standard expects both routes to be considered, and the larger result wins. This matters because the two can disagree sharply: a lightly equipped kitchen with huge throughput is governed by meals, while a heavily equipped one serving few covers is governed by appliances. Running both and taking the maximum, as this calculator does, is the safe and standard-compliant approach.

A separator works by giving grease time to float, and anything that makes the grease less inclined to separate forces a larger unit to recover that time. EN 1825-2 captures the three main culprits as multiplying factors on the design flow. The density factor fd allows for heavier greases — above 0.94 kg per litre the fats are closer to water's density and rise more reluctantly, so the size steps up by half again. It is a reminder that the type of cooking, not just its volume, shapes the duty.

The temperature factor ft addresses hot discharge. Grease that enters above about 60 °C is still liquid and well mixed with the water, and it only begins to separate as it cools inside the unit — so a hotter inlet needs a bigger chamber to allow that cooling and rising to happen before the water leaves. Kitchens with high-temperature wash processes or steam cleaning routinely trip this factor.

The detergent factor fr is often the most punishing, because cleaning agents are designed to do exactly what a separator does not want: emulsify grease into a stable suspension that will not float. Where degreasers and rinse aids are in regular use, the standard raises the size by a third or more, and continuous or aggressive chemical use raises it further still. Ignoring these factors is the classic way a separator that looks adequately sized on raw flow turns out, in practice, to pass grease straight through.

The nominal size NS that this calculator returns is the flow rating — the headline number that selects a product family and goes into the tender. But a separator is more than a flow rating, and EN 1825 sets out the rest. Each NS comes with required volumes for grease and sludge storage, because the unit has to hold what it captures between cleanings without re-releasing it. A separator sized correctly on flow but starved of storage volume fills quickly and starts carrying grease over, defeating its purpose.

Maintenance is the other half of the equation, and it is inseparable from the sizing. The storage volume and the expected grease load together set how often the unit must be emptied — typically on a regular schedule, often monthly. A larger NS or a larger storage volume buys longer intervals; a tight selection demands diligent servicing. Designing the separator without agreeing the maintenance regime is how units end up neglected and overflowing.

Finally, installation and local rules complete the picture. The separator needs accessible covers for cleaning, correct ventilation, protection from freezing where relevant, and compliance with whatever discharge limits the local authority sets on the outlet. The calculation here gives you the engineering size quickly and correctly, which is exactly what early design and pricing need — then the storage volume, maintenance plan, and installation details turn that size into a separator that actually keeps the drains clear.