Butyl Sealant in the Joint
What happens in the joint

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Grease traps or grease interceptors are tanks used to separate grease and water to prevent grease from entering municipal sewer systems, which eliminates costly repairs and clean out operations. There are several  ways to prevent grease from escaping a grease trap/interceptor and entering a municipal system - using a tank configuration with both a ninety degree elbow in the partition wall and an outlet tee and/or priming the tank with clean water.

Partition Wall Passage
The most common tank configuration to keep grease in the tank is using a ninety degree elbow as the passage through the partition wall from Compartment A to Compartment B (see drawing below) and has a tremendous effect on how much grease passes through to the second compartment. A simple ninety degree PVC pipe is extended down within twelve inches off the floor and protects against grease from entering the second compartment. Using a ninety partition wall passage is the most common method used by municipalities to prevent grease from exiting a grease trap.

Priming a Grease Trap
Priming a grease trap is not typical practice but is considered the gold star method of keeping grease in the tank. Before placing a new grease trap into operation or after pumping a grease trap, the tank is primed with clean water. Priming the tank involves filling both compartments of the tank with approximately twelve inches of clean water. The water level should be at or above the partition wall passage between compartments, in this case above the end of the ninety degree partition passage pipe.

When the grease hits the water in a primed tank, it is given the chance to cool, separate, and float to the top of the tank in Compartment A,  preventing grease from passing to the second compartment (Compartment B). Without the clean water added during priming, grease would settle into the bottom of Compartment A and pass through the partition wall into Compartment B. Once in the second compartment, some or all of the grease would make its way into the municipal sewer system, depending on the second compartment configuration.

As a worst case scenario where 100% grease is poured into the first compartment without priming before operation, approximately 2.2 gallons of grease would escape into the second compartment.

Using an Outlet Tee
The outlet tee and the second compartment should be viewed only as emergency overflow or secondary protection and should be used in all grease traps. When an outlet tee is installed and extended downward twelve inches off the inside floor, of that 2.2 gallons of grease that entered the second compartment in our worst case scenario, only 0.14 gallons of grease would actually exit the tank, thus dramatically reducing the amount of grease entering the municipal sewer system.

When all three methods - priming, a ninety degree passage in the partition wall, and outlet tee - are used together, they eliminate any grease from exiting the tank. If priming is not an option as it is not as common in most municipalities, the partition wall ninety and outlet tee reduce the amount of grease exiting the tank to near zero.
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