A grease trap (also known as grease interceptor, grease recovery device and grease converter) is a plumbing device (a type of trap) designed to intercept most greases and solids before they enter a wastewater disposal system. Common wastewater contains small amounts of oils which enter into septic tanks and treatment facilities to form a floating scum layer. This scum layer is very slowly digested and broken down by microorganisms in the anaerobic digestion process. Large amounts of oil from food preparation in restaurants can overwhelm a septic tank or treatment facility, causing release of untreated sewage into the environment. High-viscosity fats and cooking grease such as lard solidify when cooled, and can combine with other disposed solids to block drain pipes. Grease traps have been used since Victorian days. Nathaniel Whiting obtained the first patent for a grease trap in the late 1800s. These reduce the amount of fats, oils and greases (FOGs) that enter sewers. They are boxes within the drain run that flows between the sinks in a kitchen the sewer system. They only have kitchen waste water flowing through them, and do not serve any other drainage system, such as toilets. They can be made from many different materials, such as stainless steel, plastics, concrete & cast iron. They range from 35 liter capacity to 45,000 liters and greater. They can be located above ground, below ground, inside the kitchen or outside the building.