The first is the Hydromechanical. These units rely on the principles of heat and gravity to separate the FOG from the waste water. Normally these units are carbon steel or plastic. Hydromechanical grease traps control the flow of water allowing the hot water to cool. The baffle catches the FOG and keeps it inside the grease trap. These grease traps must be cleaned manually and often. The sizing of the Hydromechanical units are expressed in poundage and in gallons per minute (GPM) flow. Sizes are normally 8lb/4GPM, 20lb/10GPM, 30lb/15GPM, 40lb/20GPM, 50lb/25GPM, 70lb/35GPM and 100lb/50GPM units.
The second type of grease trap is Gravity. This type of grease trap uses compartments of two or more and uses the larger amount of water flowing to slow down the water inside the trap. This allows for FOG to become buoyant and to separate. The physical size of Gravity grease traps compared to the Hydromechanical is large. These grease traps are more efficient, allowing 90% of the FOG to be collected out of the water. The sizing of the Gravity units are expressed in the gallons it will hold.
The third is an Automatic Grease Removal Unit (AGRU). This type of grease trap utilizes mechanical and electrical components to actually skim FOG out of the water. These systems are smaller yet highly efficient. Most AGRU units can eliminate 99% of FOG from water. Most have an internal collection device for food particles, a heating element and a external FOG collection unit. The AGRUs are more expensive than the the other two, but one can clearly see that they are lower maintenance and high efficiency. Sizing of these units varies greatly and can come in sizes small enough to place under a sink or large enough to accommodate an entire restaurant.