A Guide to Grease Traps

A grease interceptor or grease trap is a device designed to collect and prevent fats, oils, and grease (FOG) from entering plumbing systems, septic tanks, and waste water treatment facilities.

Disposing of FOG

In a home, safely disposing of FOG is simple enough; pour it into an empty can or container and throw it in the trash rather than down the drain. However, commercial kitchens in restaurants, bars, or hotels produce a much greater volume of FOG making this method of disposal impractical and their need for a grease interceptor, essential.

Grease interceptors: Design & function

Grease interceptors are designed to separate FOG from wastewater. The interceptor works by slowing down the flow of hot, greasy water coming from commercial pot washing sinks or dishwashing machines. The wastewater then cools and separates from FOG, which floats to the top of the trap allowing only water to continue flowing through the pipes.

Any solids that make their way into the grease interceptor, sink to the bottom and don’t inhibit the flow of wastewater.

Just as many municipal governments require any industrial operation to use a grease interceptor, organizations must also regularly pump out the grease that accumulates inside. The City of Toronto, for example, requires food service establishments to clean out their interceptors at a minimum of once a month as per the Food Service Establishment Environmental Code of Practice.

Hydromechanical vs gravity grease interceptor

There are two different types of grease interceptors, each one dependant on the amount of wastewater flowing down the drain.

Hydromechanical grease interceptors are smaller and often located indoors, even right under the sink. They use a system of baffles or barriers to slow down the flow of wastewater allowing it more time to cool and separate from FOG. Air ventilation also aids in this separation by helping cool the water inside the tank.

Gravity grease interceptors require a minimum volume of 300 gallons. With two or more compartments to slow the rate of flow, these interceptors rely on gravity and the buoyancy of FOG to cause separation.

Find out the capacity of your sink in cubic inches:

Length x Width x Depth = Sink capacity in cubic inches*

e.g. 24 x 24 x 14 = 8,064 cu. in.

*If there are multiple sinks connected to the same interceptor, calculate each of their capacities in cubic inches and add them together before moving on to the next step.

Convert the capacity into gallons per minute (GPM):

Capacity in cu. in. / 231 = Flow rate, GPM

e.g. 8,064 cu. in. / 231 = 35 GPM

Adjust for displacement (this takes into account the actual usable space in your sink since it likely won’t ever be filled to the brim)

Calculated flow rate, GPM x 0.75 = Final flow rate, GPM

e.g. 35 GPM x 0.75 = 26 GPM

The result represents the recommended flow rate of the grease interceptor needed in order for it to function as intended. The Plumbing & Drainage Institute (PDI) recommends selecting the larger size when your sink’s flow rate falls between two interceptor sizes.

Correct sizing is very important. Too small of a grease trap can cause FOG to slip through or require cleaning far more often. If your trap is too large, sulphuric acid could build up inside the tank damaging it and the drainage system. Consulting with a professional is always a good idea before purchasing and installing a grease interceptor for your establishment.