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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 in the late 1800's. The traps reduce the amount of fats, oils and greases (FOGs) that enter sewers. They comprise boxes within the drain run that flows between the sinks in a kitchen and 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.
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There are three primary types of device. The most common are those specified by ASME (American Society Of Mechanical Engineers), utilizing baffles, or a proprietary inlet diffuser.
Grease trap sizing is based on the size of the 2- or 3-compartment sink, dishwasher, pot sinks, and mop sinks. The cumulative flow rates of these devices, as well as overall grease retention capacity (in pounds or kilograms) are considered. Currently, ASME Standard (ASME A112.14.3) is being adopted by both of the National Model Plumbing Codes that cover most of the US. This standard requires that grease interceptors remove a minimum of 90% of incoming FOGs. It also requires that grease interceptors are third-party tested and certified to 90 days compliance with the standard pumping. This third-party testing must be conducted by a recognized and approved testing laboratory.
The most common passive grease traps are smaller, point-of-use units used under three-compartment sinks or adjacent to dishwashers in kitchens.
The second most common type of interceptor is the large in-ground tank, which is usually 500–2,000 US gallons (1,900–7,600 l; 420–1,670 imp gal). These units are constructed of concrete, fiberglass or steel. They have greater grease and solid storage capacities for high-flow applications such as a restaurant or hospital store. They are commonly called gravity interceptors. Interceptors require a retention time of 30 minutes to allow the fats, oils, grease and food solids to settle in the tank. As more waste water enters the tank, the grease-free water is pushed out of the tank. The rotting brown grease inside a grease trap or grease interceptor must be pumped out on a scheduled basis. The brown grease is not recycled and goes to landfill. On average 300 to 400 pounds (140 to 180 kg) of brown grease goes to landfill annually from each restaurant.
A third system type, GRDs (grease recovery devices), removes the grease automatically when trapped. The recovered grease or "yellow grease" is recycled with the waste vegetable oil from the kitchen's deep-fryers. Restaurants need not effect grease trap pumping as do restaurants with conventional grease traps or grease interceptors.
Passive grease traps and passive grease interceptors must be emptied and cleaned when 25% full. As the passive devices fill with fats, oils, and grease, they become less productive for grease recovery. A full grease trap does not stop any FOG from entering the sanitary sewer system. The emptied contents or "brown grease" is considered hazardous waste in many jurisdictions.
Restaurant and foodservice kitchens produce much waste grease which is present in the drain lines from various sinks, dishwashers and cooking equipment such as combi ovens and commercial woks. If not removed, the grease can clump and cause blockage and back-up in the sewer.
In the US, sewers back up annually “an estimated 400,000 times, and municipal sewer overflows on 40,000 occasions”. The EPA has determined that sewer pipe blockages are the leading cause of sewer overflows, and grease is the primary cause of sewer blockages. Even if accumulated FOG does not escalate into blockages and sanitary sewer overflows, it can disrupt wastewater utility operations and increase operations and maintenance requirements” 
For these reasons, depending on the country, nearly all municipalities require commercial kitchen operations to use some type of interceptor device to collect grease before it enters sewers. Where FOG is a concern in the local wastewater system, communities have established inspection programs to ensure that these grease traps and/or interceptors are being routinely maintained.
It is estimated 50% of all sewer overflows are caused by grease blockages, with over 10 billion US gallons (3.8×1010 l; 8.3×109 imp gal) of raw sewage spills annually.
Method of operation
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When the outflow from the kitchen sink enters the grease trap, the solid food particles sink to the bottom, while lighter grease and oil floats to the top. The relatively grease-free water is then fed into the normal septic system. The food solids at the bottom and floating oil and grease must be periodically removed in a manner similar to septic tank pumping. A traditional grease trap is not a food disposal unit. Unfinished food must be scraped into the garbage or food recycling bin. Milkshakes, gravy, sauces and food solids must be scraped off dishes before entering the sink or dishwasher.
To maintain some degree of efficiency, there has been a trend to specify larger traps. Unfortunately, providing a large tank for the effluent to stand also means that food waste has time to settle to the bottom of the tank, reducing available volume and adding to clean-out problems. Also, rotting food contained within an interceptor breaks down, producing toxic waste (such as sulfur gases) - hydrogen sulfide combines with the water present to create sulfuric acid. This attacks mild steel and concrete materials, resulting in "rot out", On the other hand, polyethylene has acid-resisting properties. A bigger interceptor is not a better interceptor. In most cases, multiple interceptors in series will separate grease much better.
Because it has been in the trap for some time, grease thus collected will be contaminated and is unsuitable for further use. This type of grease is called brown grease.
Disposal of trapped grease
Waste from passive grease traps and gravity interceptors is called brown grease. Brown grease is rotted food solids in combination with fats, oils, and grease (FOG). Brown grease is pumped from the traps and interceptors by grease pumping trucks. Unlike the collected yellow grease, the majority of brown grease goes to landfill sites. New facilities (2012) and new technology are beginning to allow brown grease to be recycled.
- Whitman, D. (2000). The sickening sewer crisis. U.S. News & World Report, 128(23), 16. Retrieved Friday, May 04, 2007 from the Business Source Corporate database.
- EPA. (2004). Report to Congress: impacts and control of CSOs and SSOs (EPA 833-R-04-001). Washington, DC; United State Environmental Protection Agency Office of Water, p. 4-28. http://cfpub.epa.gov/npdes/cso/cpolicy_report2004.cfm
- Mero, C. & Wilkerson, J. (2007). Reduce Sewer Congestion. Water Environment & Technology, 19(7), 44-52.
- A112.14.3 Grease Interceptors Standard and A112.14.6 FOG (Fats, Oils, & Greases) Disposal Systems Standard, American Society of Mechanical Engineers (ASME)
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