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Grease traps (also known as interceptors, grease recovery devices and grease converters) are plumbing devices 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. However, very large amounts of oil from food production in kitchens and restaurants can overwhelm the septic tank or treatment facility, causing a release of untreated sewage into the environment. Also, high viscosity fats and cooking greases such as lard solidify when cooled, and can combine with other disposed solids to form blockages in drain pipes.
Grease traps have been used since the Victorian days, although Nathaniel Whiting obtained the first patent for a modern day grease trap in the late 1800s. They are used to reduce the amount of fats, oils and greases (FOGs) that enter the main sewers. Effectively they are boxes within the drain run that flows between the sinks in a kitchen to the sewer system. They only have kitchen waste water flowing through them and are not served by any other drainage system such as toilets. They can be made from a number of different materials; e.g. stainless steel, plastics, concrete & cast iron. They range from 35 litre capacity to 45,000 litres and above capacity. 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 devices. The most common are the types 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 the aforementioned devices, as well as overall grease retention capacity (typically 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 United States. This standard requires that grease interceptors remove a minimum of 90% of the 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 third-party testing laboratory.
The most common, passive grease traps, are small point of use units used under three compartment sinks within the kitchen. There has been little innovation in this in kitchen passive grease trap technology until recently with the introduction of a liner grease trap, which provide significant health, hygiene and safety benefits for end users. They restrict flow and remove 85-90% of the incoming FOG. Food Solids along with fats, oils, and grease are trapped and stored in these devices.
The second most common type of interceptor is the large in-ground tank, which is usually 500-2000 gallons. These units are constructed of concrete, fiberglass, or steel. By nature of their larger size, they have larger 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 wastewater enters the tank the grease free water is pushed out of the tank.
A third system type GRDs (Grease Recovery Devices), remove the surface grease automatically when trapped. The recovered grease or "brown grease" is not recyclable with the waste vegetable oil from the kitchens deep-fryers.
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 (FOG) 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. "Brown Grease" is generally not recycled and goes to landfill waste.
Restaurant and foodservice kitchens produce a lot of waste grease which is present in the drain lines from the various sinks dishwashers and cooking equipment such as combi ovens and commercial woks. If not removed the grease will congeal within the sewer and cause blockages and back-ups.
In the United States, 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 fit some kind of interceptor device to collect the grease before it enters the sewer. Additionally where FOG is a concern in the local wastewater collection system communities have set up inspection programs to ensure that these grease traps and/or interceptors are being maintained on a routine basis.
It is estimated 50% of all sanitary sewer overflows are caused by grease blockages with over 10 billion gallons of raw sewage spills annually.
When the outflow from the kitchen sink enters the grease trap, the solid food particles sink to the bottom while the 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 similar manner as with 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 the dishes before they enter the sink or dishwasher.
To try to maintain some degree of efficiency there has been a trend to specify larger and larger traps. Unfortunately, providing a large tank for the effluent to stand also means that food waste scraps also have time to settle to the bottom of the tank, further reducing the available volume and adding to the clean out problem. Also, rotting food contained within an interceptor breaks down producing toxic waste (i.e. 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 having multiple interceptors In series will separate grease much better.
Because it will have been in the trap for some time, grease collected in this way will have been contaminated and is unsuitable for further use. This kind of grease is referred to as brown grease.
Waste from passive grease traps and gravity interceptors is referred to as brown grease. Brown grease is rotted food solids in combination with fats, oils, and grease (FOG). Brown grease is pumped out of the traps and interceptors by grease pumping trucks. Unlike the collected yellow grease, the majority of brown grease ends up in landfill sites. New facilities (2012) and new technology are beginning to allow brown grease to be recycled.