How Does a Grease Trap Work?

Types

Grease traps are used by businesses, schools, hotels, hospitals and other large institutions that use commercial sized kitchens. There are two major varieties of grease traps: point-of-use grease traps are smaller traps that are attached to output lines for specific devices and larger single-line traps (or interceptors) are used for the entire kitchen and may hold up to 500 gallons of waste.

The waste collected by grease traps is known as FOG (fat, oil and grease). Grease traps will also collect non-floating solids. Most municipalities require that large kitchens have at least one large grease trap located on the premises to keep the sewer lines from being polluted with FOG and large food particles from kitchens. If these contaminants are not kept in check, they can cause severe problems city drainage systems as they collect and coat the pipes.

Function

The first thing that you will notice when you install your grease trap is that it is not added directly to the drain of the sink or appliance that it is filtering. There is a flow control valve that keeps the water from reaching the grease trap too quickly before the trap itself. Another thing that may catch your eye is that there is no actual screen used to filter the FOG and debris out of the water.
The distance between the drains and the grease trap is specifically used to cool the water on its way to the trap. As the water cools, there are two principles at work; the first is that food debris sinks, and the second is that FOG floats. The flow control will keep the water from washing the debris from the trap during times of faster flow.
The grease trap is a well that keeps the floating oils at the top of the water from passing through by making the water fill the well. There is a wall that travels part of the way down the well on the input side so that the water needs to pass under it to exit through the drain. The FOG is blocked from traveling to the drain by the wall (because it floats), and the larger food debris sinks to the bottom of the trap and cannot travel up to the drain on the other side of the wall.

Cleaning

There are bacteria that should be introduced to your grease trap. These facilitate the breaking down of the FOG in your grease trap. You should never add any other cleaners, such as bleach, ammonia, or enzymes, as they will interfere with the bacteria. These bacteria will help keep the odors to a minimum and allow the FOGs to be digested, reducing the frequency of cleanings. The City of Boston suggests that a proper cleaning schedule for grease traps is once per month for smaller units and quarterly for the larger in-ground interceptors.
If you plan on cleaning your grease trap yourself, you will need to remove the lid and ladle off the FOG. It is best to deposit into an absorbent material, such as kitty litter, so that it can be thrown away with your solid waste. Do not wash it down the drain. Use a strainer to remove as much of the solid waste as you can from the bottom of the trap and scrape away build-up from the inside of any baffles in the drain line to the grease trap.
If you would prefer that a service would handle your grease trap cleanings, you can call a recycler to pump out your trap. You can easily locate vendors that provide this service, usually the vendor that recycles your waste oil from the deep-frying machine will also pump your grease trap. If you are having difficulty finding a vendor, contact your local health department for suggestions.