Grease traps, also commonly known as grease interceptors, are devices used to intercept most greases and solids before they enter a municipal or private waste disposal system. Common wastewater contains small amounts of oils which enter into septic systems and municipal or public treatment facilities where they could form an unsightly and odor laden floating layer. This "scum" layer is very slowly digested and broken down by biological microorganisms. However, very large amounts of oil from food production in kitchens and restaurants can exceed the septic tank or treatment facility, causing a release of untreated sewage. Also, very thick fats and cooking greases solidify when cooled and can combine with other disposed solids to block drain pipes.