Grease traps are sized on the flow of water reaching them, not on how big the restaurant is. Get that wrong and the unit will carry grease into the sewer no matter how often it is emptied — which is why undersizing is the fault we are called out to correct most often.
Every credible sizing method comes down to the same pair: how fast water can arrive, and how long the chamber holds it. Everything else is detail.
List every fixture that drains into the trap — pot-wash sink, prep sinks, dishwasher, combi oven, floor gullies — and work out what they can discharge at once during the worst ten minutes of service, not on average. That peak figure is the floor for the unit's rating.
Separation is physics with a clock on it. Fat needs time in still water to rise; solids need time to settle. A chamber that meets the flow number but empties almost as fast as it fills gives grease no chance to separate, so it leaves with the water.
Two kitchens with identical sinks can produce very different FOG loads. Deep-frying, grilling and bulk cooking push far more fat into the drain than a salad-and-sandwich operation, and that pushes the service interval — and sometimes the unit choice — up.
The theoretical answer has to fit the building. Shallow drainage runs, low ceilings in the plant room and tight under-counter voids often rule out the obvious unit, and the practical alternative may be several smaller traps on the heaviest fixtures.
A trap that a hose or tanker cannot reach will not be cleaned properly, whatever the specification says. Access has to be part of the sizing decision, not discovered afterwards.
Kitchens rarely get quieter. If you are likely to add fryers, extend hours or raise covers, specifying with margin is cheaper than replacing the unit two years in.
When a trap is too small, the symptom that shows up is not an overflowing unit — it is grease appearing downstream, drains that slow between services, and odour returning within a day or two of a clean. Operators reasonably read that as "we need to clean more often", increase the frequency, and spend more every month without fixing anything.
The reason is that the failure is hydraulic. The trap separates by slowing water down. Under-rate it and water crosses the chamber too fast for fat to float out, so grease leaves with the flow even on the day it was emptied. More frequent cleaning removes what did separate. It cannot slow the water down.
A dated cleaning record proves you maintain the trap. It does not prove the trap works. Where FOG is reaching the public sewer, an undersized unit will show up regardless of how complete the paperwork is — and in Dubai, drainage and public-health requirements for food establishments sit under Local Order No. 8 of 2002. Correct rating and a real service record are two halves of the same thing. See our Dubai Municipality grease trap regulations guide for what is required.
The standard approach works from flow, not from floor area or cover count. You identify every fixture draining into the trap — pot-wash and prep sinks, dishwasher, combi oven, floor gullies — establish the peak rate at which they can discharge together, and then select a unit rated at or above that flow. The second half of the calculation is retention: the chamber has to hold the water long enough for fat to rise and solids to settle before it moves on. A unit that meets the flow figure but gives almost no retention margin will still underperform at peak service.
Because the failure is hydraulic, not a matter of fullness. A grease trap separates by slowing water down. If the unit is rated below your peak flow, water moves through the chamber too fast for fat to float out, so grease carries over into the sewer even when the trap was emptied that morning. Cleaning more frequently removes what did separate, but it cannot slow the water down. The only fix is a correctly rated unit.
Oversizing is a much smaller problem than undersizing, but it is not free. A very large unit on a light load can hold waste long enough for it to turn septic between visits, which produces odour, and you are paying for capacity and pump-out volume you do not use. It also takes space that a tight kitchen rarely has. The aim is a unit matched to the load with sensible margin, not the biggest one that fits.
It is a widely used maintenance benchmark rather than a sizing rule: service the trap once accumulated grease and settled solids reach roughly a quarter of its working depth. Beyond that point the effective volume left for separation drops sharply and carry-over starts. It is useful for setting a cleaning interval once the unit is installed — it tells you nothing about whether the unit was the right size in the first place.
It is worth rechecking. Adding fryers, extending hours, raising cover numbers or changing to a heavier menu all increase the fat, oil and grease load and the peak drainage flow. A trap that was correctly specified for the original fit-out can end up undersized without anything visibly changing — the first sign is usually that it fills well before the next scheduled visit.
Tell us the fixtures draining into it and we'll tell you whether the unit is rated for them. We supply and install in Dubai and across Sharjah, and service what we fit.