The sizing of a grease trap was not a very difficult task at one time, but as sewer lines all across the United States are getting older, more and and more cities are implementing stronger requirements for the use of grease traps. A grease trap, or grease interceptor, functions to prevent the excessive introduction of grease and other organic matter into local sewer systems. Some of these systems are well over a hundred years old, so many communities are taking steps to extend the usable life of their fragile infrastructure.
Determine what exactly needs to be routed to the grease trap, as stipulated in your local sewer code. Typically a grease trap would connect to a scullery sink and dishwashers, but items such as preparation sinks, pre-rinse washing sinks and floor drains or troughs may also need to be connected.
Measure the interior dimensions of all sinks that are to be connected to the grease trap and note them down for calculation.
Calculate your drainage load for sinks, using a calculator or spreadsheet. Drainage loads are calculated by the PDI (Plumbing and Drainage Institute) method and are determined by GPM (gallon per minute) flow rate. The loads can be calculated easily using the sink bowl dimensions.
Using your computer, input the noted dimensions and perform the following calculations to acquire a required pound capacity for each sink.
Length x Width x Height x Number of Bowls = Total Cubic Inches Total Cubic Inches ÷ 231 = GPM GPM x .75 = Pound capacity of grease trap needed for sinks
Repeat this calculation for each sink and combine all sums to acquire a pound rating for total sink volume.
Acquire all manufacturer's specification sheets for the other equipment that will be connected to the grease trap. Obtain the GPM rate for each piece of equipment and perform the following calculation.
GPM x .75 = Pound capacity of grease trap needed
Add the pound capacities for all sinks and other pieces of equipment to determine the sum total required pound capacity. This is your total pound capacity requirement for your grease trap.