Grease trap care: signs it is full and how to keep it working

A grease trap, or grease interceptor, sits between the kitchen sinks and the drain. Inside it the waste water slows down: fat and oil float to the top, food scraps sink to the bottom, and cleaner water flows out to the drain in between.

That means the trap is meant to fill. Every pan, plate and fryer adds a little more fat and food. As the layers grow, the space for water shrinks, the water rushes through too fast to separate, and fat passes into the pipes beyond. There it cools, sets hard on the walls and, in time, blocks the line. The rotting fat and food are also the source of the sour smell that hangs around a neglected kitchen.

The earliest sign is usually the smell: sour and rotten, strongest near the trap cover, the sinks or the floor drains. Next, sinks start to drain slowly, especially when the whole kitchen is busy. You may see a thick layer of fat at the trap cover, or fat floating in the next manhole outside.

The late signs are the expensive ones: water backing up into the sinks, greasy water seeping out around the trap cover, or a blocked line that stops the kitchen. At that point the trap needs emptying straight away, and the pipe after it often needs drain jetting as well.

What goes down the sink decides how fast the trap fills. Scrape plates and pots into the bin before washing. Wipe greasy pans and trays with paper towel first. Collect used frying oil in a sealed container instead of pouring it into the sink. Fit strainers in every sink and empty them into the bin.

Avoid the common shortcuts. Pouring boiling water or strong detergent after the grease doesn't remove it; it carries the fat past the trap, where it sets further down the line. Enzyme and bacteria products may help with smells but they don't empty the trap. And never wash food waste down the drain to save time.

Make it part of the routine: show new kitchen staff the trap and explain why the habits matter, and keep the trap cover clear of crates and equipment so it can be checked.

Skimming the fat off the top can make a trap look better, but it leaves the heavy food solids at the bottom, and they take up room and rot just the same. A proper clean removes the floating fat, the water and the settled solids, then washes down the inside of the trap, its baffles and its inlet and outlet.

Our grease trap cleaning service does this with the Super Combi suction truck. Everything goes into the truck's own tank and leaves with us. Because we work 24/7, including Fridays and nights, we can come before service starts or after the kitchen closes.

It depends on the size of the trap and how much cooking goes through it. A busy kitchen that fries a lot fills a trap far faster than a small one that mostly reheats. The way to find your answer is to keep a simple log: the date of each clean and how full the trap was. If it was very full, clean sooner next time; if it was barely half full, you can stretch it.

Buildings and compounds with several kitchens are easier to manage on a fixed schedule. An annual maintenance contract puts every trap on dates agreed with you, so none of them is forgotten until it overflows.