- Grease Traps (also known as Grease Interceptors, and Grease Recovery Devices), 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 era. They are used to reduce the amount of fats, oils and greases (FOG’s) that enter the main sewers. Effectively they are boxes within the drain run that flows between the sinks in a kitchen to the foul sewer system. They only have 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, Mild Steel, Plastics, Concrete, Cast Iron and can hold anywhere between 40 liters to 45000 liters and above. They can be located above ground, below ground, inside the kitchen or outside the building.

 

(ASME) Grease Interceptor

The most common are the types specified by American Society of Mechanical Engineers (ASME). Utilizing baffles inside small 50 US Gallon tanks, they restrict flow and remove 85-90% of the incoming fat, oil, and grease.

Fats, Oils and Grease Interceptors

The second most common type of interceptor (FOG). Is the large in ground tank usually 1000 gallons. These units are constructed of concrete, fiberglass, or steel. They have much larger grease and solid storage capacities for high flow applications such as a restaurant or grocery store. These units can be designed to remove up to 99% of the incoming FOG.

Automatic Grease Removal Units

Some systems known as Automatic Grease removal Units (AGRU), use mechanical skimmers to remove the surface grease when trapped.

- The traditional means of achieving this is with the passive grease trap (interceptor). The first patent was lodged by Nathaniel Whiting of California in the late 1800’s. The design remains pretty much unchanged. The current industry standard for passive grease interceptors is ASME A112.14.3, (or PDI-GD101).

- A grease recovery device (GRD) is a recent development which aims to separate out the grease and water and collect the grease for recycling. The current industry standard for GRD’s is ASME A112.14.4.

- Both traditional traps and GRD’s use the same physics which is that grease and oil are lighter than water and will rise to the top when the mix is allowed to stand for a time. They both feature a tank with an inverted wire at the outlet in order to allow water out but not grease. A traditional trap is designed to hold the grease within its tank constantly reducing its working volume and hence its ability to allow the required dwell time of 27 seconds or more for the grease/water separation to occur.

- The design codes for traps allow for an average efficiency of as little as 85% between cleanouts for the trap to be considered adequate. This means that on average 15% of the grease in the waste water is entering the sewer line. Eventually even with a robust cleanout regime it is likely that there will be a sewer backup.

- A problem with a traditional trap is that it must be emptied out either by scooping out or pumping all the contents and carting the effluent away to a specialist renderer or to landfill. This is a very unpleasant undertaking and is in consequence often neglected causing the same problems as if the trap were not there at all.

- 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 the time to settle out at the bottom of the tank further reducing the available volume and adding to the clean out problem.

 

Restaurant and food service kitchens produce a lot of waste grease which is present in the drain lines from the various sinks dishwashers and cooking equipment such as kombi ovens and commercial woks. This is known as "fats, oils, and grease" ( FOG ). If not removed the grease will congeal within the sewer and cause blockages and back-ups.