What is a grease trap and how often should it be cleaned?

When warm water from the kitchen enters the trap, its speed drops suddenly. In the slower water, grease and droplets of cooking oil rise and form a layer on the surface, while heavy pieces such as rice, breadcrumbs and vegetable scraps settle on the bottom. The baffle plates inside stop the water running in a straight line from inlet to outlet and give it time to separate.

The outlet pipe is positioned so that it takes water from the middle layer of the chamber. That way the water going on to the line carries neither the grease from the surface nor the sludge from the bottom. This arrangement only works while both layers stay below a certain thickness. When the surface grease thickens downwards or the sediment at the bottom rises, the middle layer narrows and grease starts to slip into the outlet pipe.

Every commercial kitchen that fries, grills, cooks doner or pide, or makes desserts and pastry produces greasy water. Restaurants, cafés, hotel kitchens, school and factory canteens, patisseries and catering kitchens are the places where traps are most widely used. In cafés, milk, cream and butter also leave more grease behind than people expect.

Home kitchens usually have no trap. At home, wiping the pan with a paper towel and collecting used cooking oil in a bottle is a good enough habit to keep grease out of the drain. In businesses, the type of trap is chosen according to the size of the kitchen and whether there is room under the floor.

A trap does not fill up overnight. The symptoms usually follow one another over a few days. If staff who notice them report it to the manager straight away, an overflow in the middle of service can be avoided.

There is no single frequency that suits everyone. A grill restaurant that is full every evening and a small café serving only breakfast build up grease in their chambers at very different speeds. Small under-counter traps are often emptied by staff as part of a weekly routine, while large below-floor chambers are cleaned less often but at planned intervals.

The practical way to find the interval for your business is to open the lid regularly during the first few months, measure the thickness of the surface grease and the sediment at the bottom, and write it in a logbook. The date that marks the point just before the layers begin to narrow the middle zone is your cleaning interval. Review it again when the menu changes, when a new fryer is added or when customer numbers rise in the summer season.

Grease that escapes from a full trap travels down the line with the warm water and sticks to the pipe walls as the pipe cools. Over time, crumbs, soap and sediment collect on top of it and the inner diameter of the pipe narrows. In the end the blockage shows up as dirty water coming back through the floor drain in the restaurant's own kitchen. If the business is in the same building, neighbouring shops and flats are also affected through the shared line.

In a chamber that is left uncleaned for a long time, food scraps rot and foul-smelling gases form. These gases corrode the surface of the concrete chamber and the metal lids, and the smell around the lid can reach from the kitchen into the dining room. The built-up waste also attracts cockroaches and rodents, which is why the area around the lid is one of the first places looked at in a hygiene inspection.

Cleaning is done when no water is flowing from the kitchen, preferably after service has closed or before opening. When the lid is opened, the area is ventilated so the gas inside can disperse, and the work is done without leaning into or entering the chamber. The grease and sludge that are collected go into sealed containers. The waste oil is set aside to be handed over to companies authorised to collect it, and is never poured into a sink, a toilet or a street grate.

Once the chamber has been emptied, the pipe from the trap outlet onwards is checked as well. If escaped grease has formed a layer in the line, it is cleaned with water jetting, and if the blockage keeps coming back, the inside of the pipe is viewed with a camera to tell whether the problem is in the trap or further down the line.