Grease traps are designed to take advantage of the fact that grease, or animal fats and vegetable oils, are less dense than water. As grease and water don't mix, these fats and oils tend to float on top of water. When kitchen water flows through a grease interceptor, a system of baffles traps the grease at the top and fills downward, forcing the water down into the sewer line.
Most passive grease interceptors were designed to trap or intercept the grease before it can clog or foul sewer or septic systems, but leave cleaning to the user. Manual removal of the grease is necessary to keep the traps from becoming filled up and, in the process, less efficient at keeping grease from the wastewater lines. Grease traps are generally required by law to be cleaned when the grease sludge captured reaches 25 percent of their volume.
Traditional grease traps aren't designed to stop 100 percent of the grease to reach the drain. A major factor in how much grease bypasses the trap not controlled by design is the intake of water into the system. During periods of high water usage, particularly busy automatic dishwashing and sink draining periods, the fast and hot outflow of wastewater allows grease to bypass the grease trap. It is because of factors like these that even a well maintained grease trap allows 15 percent of grease to pass into the sewer or septic systems.