What is a grease trap?
A grease trap (also known as grease interceptor) is a plumbing device (a type of trap) designed to intercept most greases and solids before they enter a wastewater disposal system.
In the most basic terms, a grease trap works by slowing down the flow of warm/hot greasy water and allowing it to cool. As the water cools, the grease and oil in the water separate out and float to the top of the trap.
Grease trap usually made from stainless steel or plastic in a rectangular box or cylindrical round with water inlet and outlet, and some other technical walls inside.

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The importance of grease trap
It is perfectly normal for a small amount of waste oil to enter into septic tanks or other treatment facilities. However, when there is a large amount of grease such as in restaurant kitchen, the waste oils can quickly overwhelm either a septic tank or treatment facilities. This makes it critically important for restaurant owners who want to make sure they do not have to pay for the high cost of replacing a septic system or fixing the drainage pipe system … or fines imposed by the government if they are on public sewage.
In addition, grease traps contribute to the cleaning of kitchen waste water into the public water treatment system. This is an environmental protection equipment.
So, installing grease traps in restaurant kitchen is the best and “must” solution.
How does grease trap works?
When waste water flows into a water disposal system, such as a septic tank, the oil in this waste water forms scum at the top of the septic tank. This floating layer of scum has to be cleaned out on a periodic basis if it gets too dense for the water to continue flowing.
Most of the time this oily layer of scum is slowly digested and broken down by microorganisms that are part of the anaerobic digestion process.
However, when there are large amounts of oil, such as from sinks or dish washing machine in restaurant kitchen, the result is often for it to overwhelm the abilities of the septic tank and this will result in blockage of pipes, and the inability to dispose of solids properly in the septic tank system.
Additionally, some types of high viscosity fats such as cooking grease or lard tend to solidify when they get cold. These can then combine with other disposed solids which can create a significant blockage problem.
People typically install traps between the sinks in the kitchen and the sewer system. The traps reduce the number of fats, oil, and greases that enter the sewer system.
Source: https://fupur.com/grease-trap/