Grease, the industry term for animal fats and vegetable oils, is 10 to 15 percent less dense than water. Grease also won't mix with water. As a result, fats and oils float on top of water.
When kitchen wastewater flows through a grease interceptor, the grease and oils rise to the surface inside the trap and are trapped using a system of baffles. The captured grease and oils fill the trap from the top down, displacing “clean” water out of the bottom of the trap and into sewer line.
Peer into a grease trap and youll see a mat of grease. When this mat of grease gets deep enough, the trap must be cleaned out.