Grease traps are used as an additional treatment component and are placed before septic tanks. They are usually required for septic systems that serve a business that produces a large amount of fats, oils and grease (FOG), such as restaurants.

A grease trap works by slowing down the flow of warm/hot greasy water and allowing it to cool. As the water cools, the fat, grease, and oil separate and float to the top of the grease trap.

The grease is actually trapped by baffles, which cover the inlet and outlet of the tank, preventing grease from flowing out of the trap. Grease traps must be pumped out on a regular basis. If high levels of FOG enter a septic system, they will clog pipes and other components downstream of the system.