Hydromechanical grease interceptors (HGIs) (formerly named grease traps) treat kitchen wastewater from food service establishments (FSEs) using gravity separation aided by vented flow control. They are typically installed indoors and connected to one to four sinks in the kitchen and can be either above floor or below. They accumulate fats, oil and grease (FOG) and solids over time in a relatively small separator tank allowing the treated wastewater to discharge to the sanitary sewer (see the figure below).
HGIs may be made of steel, fiberglass or polyethylene, typically consisting of a single compartment with baffle arrangements. HGIs are sized based on flow rate and the pounds of FOG that they can store. Typically, they treat 20 to 50 gallons per minute (GPM), store 40 to 100 pounds of FOG, and are 15 to 60 gallons in volume. Vented flow control devices must be installed upstream of HGIs to control the wastewater flow to match the certified flow rate of the HGI. If this flow control device is not installed, the HGI may not perform properly when the flow exceeds the certified flow rate.