What is a Grease Trap?

Simply put, a grease trap is a receptacle that kitchen wastewater flows through before entering the sanitary sewer lines. This receptacle captures, or "traps" grease. How? Grease, the industry term for animal fats and vegetable oils, is 10 to 15% less dense, or lighter than water. Grease is also immiscible with water, which is to say it does not mix with water. Thus, grease and oils float on water. When kitchen wastewater flows through a grease trap, the grease and oils rise to the surface and are trapped inside the receptacle using a system of baffles. The captured grease and oils fill the trap from the top down, displacing "clean" water out of the bottom of the trap and into the sanitary sewer line. This is why you see a grease "mat" when observing a grease trap. When a significant layer of grease has accumulated, the trap must be cleaned out.

In the United States, the three predominant grease traps found in foodservice operations are small passive grease traps, large pre-cast concrete grease traps, and Big Dipper automatic grease/oils removal systems. In some areas, these traps are referred to as grease "interceptors".

Passive grease traps date back to 1885 when the first US patent was issued. Today's large and small grease traps use the same basic operating design as the 1885 model. While they do a fine job trapping grease, removing the grease is a task left to the owner. If the trap is a small passive trap, it must be cleaned out by hand. If the grease trap is of the large, pre-cast concrete variety, it must be pumped out by a professional with a vacuum or pump truck. The single most important aspect to understand is that, as a grease trap fills, its separation efficiency diminishes. When a trap is filled to capacity with retained grease/oils, there is no separation occurring and the trap no longer works. Since grease and oils fill a grease trap from the top down, it is frequently hard to measure the depth or fullness of the grease trap, making it difficult to know when it needs to be cleaned. Failure to clean the grease trap in time creates a considerable volume of business for pipe cleaners, and headaches for sewer systems and septic fields.

If the grease trap is a Big Dipper Automatic Grease Removal System, the grease and oils are skimmed out automatically using a timer or sensor mechanism. The automatic grease removal process takes place daily in the case of timer-based systems and before the trap is 10 percent full in sensor-based systems. This automatic grease removal process relieves the owner of the burden of measuring grease levels and either hand scooping grease out of small passive traps or paying grease trap cleaning services to clean large traps