The purpose of a grease trap (or interceptor) is to prevent grease and food solids from entering the sanitary sewer system (where it can cause expensive clogs). It accomplishes this goal by capturing all of the waste deposited in the drain (mostly water) in the primary separation basin. The water in the trap (from the drains) cools the grease to room temperature, solidifying it. Because the grease is lighter than water, it floats to the top and is “trapped.” The food solids, which are heavier than water, sink to the bottom.
A crossover pipe located in the middle of the wall separating the primary from the secondary separation compartment allows the cleaner water to flow into the secondary separation basin. The “primary” achieves a majority of the separation, while some of the grease still suspended in the water makes its way through the crossover and is captured in the “secondary” creating a second separation.
The outgoing line leaves the secondary and travels up until it establishes the normal liquid level, then it will flow downward to the sanitary sewer system. It should be noted that a functioning grease trap should always maintain a normal liquid level. This does not indicate that the trap is “full,” (or full of grease). A functioning grease trap should be filled to the normal liquid level with water, grease, and food solids.