Grease interceptor

Grease traps (also known as grease interceptors, and grease recovery devices) are plumbing devices designed to intercept greases and solids before they enter the sanitary system.

Types

There are three primary types of devices. The most common are the types specified by ASME. Utilizing baffles inside small 50 US Gallon tanks, they restrict flow and remove 85-90% of the incoming fat, oil, and grease (FOG). The second most common type of interceptor 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.

Uses

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. 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. For this reason, 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.

Method of operation

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 weir 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.

Because it will have been in the trap for some time, the 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.

The essential difference between a GRD and a traditional trap or interceptor is that the GRD constantly removes the captured grease into a separate container and thus maintains its efficiency. A good GRD will have means of preventing the food scraps from entering the tank ( a strainer basket ) and a means of regularly flushing out the fine silts which would otherwise collect in the bottom of the tank. Properly installed in the correct kitchen environment a GRD will continuously give high levels of efficiency ensuring the sewers remain clear with no blockages or back-ups. The recovered grease is often suitable for recycling in to bio-diesel and thus has some value. This kind of grease is called yellow grease.

References

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Further reading

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See also

* American Society of Mechanical Engineers
* Plumbing and Drainage Institute ( of America )