City of Lakeport, California

Sizing Your Grease Trap or Interceptor

Determining the appropriate size of a grease trap or interceptor can be challenging. Fortunately, there are resources available to make the job a little easier. 
Depending on your specific grease capturing needs, an indoor grease trap/interceptor may be an effective measure for preventing the discharge of fats, oils or grease into the sanitary sewer system. Manufactured interceptors come in varying sizes, usually based on a flow rate of gallons per minute, or GPM. We recommend consulting with a licensed plumber when determining the size of your interceptor. But for basic guidance, the following steps could be useful in determining the appropriate size of your new indoor grease trap/interceptor:
Step 1:
Determine the cubic size of your sink(s) by multiplying its length, width, and depth together (L x W x D).
Step 2:
Convert that number into gallons using the following conversion: 1 gallon = 231 cubic feet.
Step 3:
Estimate the capacity of the sink(s) measured in Step 1. Usually, 75% of the sink(s) will be filled with water, the remaining 25% will be dishes, utensils, etc. Multiply that factor as a percentage (e.g. 75% = 0.75, 25% = 0.25, etc.) by the number you calculated in Step 2. This will also serve as your flow rate.
Step 4:
Select a trap/interceptor that is the next size higher than your calculated flow rate. Example: your calculated flow rate is 78 GPM. Available interceptors are sized for 70 and 80 GPM. The most appropriate choice is the latter, an 80 GMP device.
Additional sizing guidelines can be found in the most recent addition of the California Plumbing Code. A licensed plumber will be familiar with its provisions and can offer solutions unique to your needs.
Outdoor, in-ground or above-ground grease interceptors are ideal for restaurants and other food service facilities that produce large amounts of fats, oil, and grease during food preparation. City Code allows for the use of two methods when sizing an outdoor interceptor. The first is based on criteria defined in the California Plumbing Code. A licensed plumber can provide excellent interceptor solutions to meet your needs based on this method. The second is the application of the Manning Formula, which is described here in greater detail:
The Manning Formula:
Interceptor Size (in gallons) = Flow rate (GPM)/sink or fixture x sum of fixture Ratings + the Discharge rate from any mechanical washers (i.e. dishwashers, glass washers, laundry machines, etc.) x a 24 minute retention Time.
The summarized version being: I = [(Fr) x (R)) + (D)] x (T)
Flow rate, measured in gallons per minute (GPM),is determined based on the slope, pipe texture, and pipe diameter. The following rates are pre-calculated. Apply them to your calculation, as demonstrated in the examples below.
0.5” pipe = 0.8 GPM/fixture
1.0 “ = 5.0 GPM/fixture
1.5 “ = 15 GPM/fixture
2.0” = 33 GPM/fixture
2.5” = 59 GPM/fixture
3.0” = 93 GPM/fixture
Fixture ratings of grease-laden waste streams are pre-determined values for specific kitchen drainage points, such as sinks, wash basins, and floor drains. Essentially, these values represent factors by which you can multiply the flow rate of a drainage pipe to get the potential rate of water movement out of the fixture. Apply them to your calculation, as demonstrated in the examples below.
2,3, or 4 compartment sink = 1.0
1 or 2 compartment meat prep sink = 0.75
Pre-rinse sink = 0.5
1 or 2 compartment vegetable prep sink = 0.25
Can wash = 0.25
Mop sink = 0.25
Floor drain = 0.00
The discharge rate from dishwashers, laundry machines, glass washers, etc. must be added to your Manning Formula calculation because of their potential for introducing large quantities of water down the drain in a short amount of time. The user’s manual for these appliances should indicate the manufacturer’s discharge rate in GPMs, or you can call the manufacturer directly. Apply them to your calculation, as demonstrated in the examples below.
The 24 minute retention time is a pre-calculated amount of time that engineers have determined to be necessary for grease to separate from water. And, as always, apply it to your calculation, as demonstrated in the examples below.
Example 1: A restaurant has the following fixtures in its kitchen (all fixtures have a 1.5 inch diameter pipe):
 (1) 3-compartment pot sink
1 pre-rinse sink
(1) 2-compartment vegetable prep sink
1 dishwasher that discharges 10 GPM
Based on the Manning Formula:
                I = [(Fr) x (R) + (D)] x (T)
                = 15 GPM x [1 + 0.5 + 0.25] + 10 GPM] x 24 minutes
                = [(15 GPM x 1.75) + 10 GPM] x 24 minutes
                = 26.25 GPM + 10 GPM x 24 minutes
                =870 gallons         
Rounded up to the next size interceptor means a 1,000 gallon interceptor is required!
Example 2: A restaurant has the following fixtures in its kitchen:
At a 0.5 inch diameter pipe, a pre-rinse sink
At a 1.5 inch diameter pipe:
1 3-compartment pot sink
1 pre-rinse sink
1 meat prep sink
1 vegetable prep sink
At a 3.0 inch diameter pipe, 1 can wash
Using the Manning Formula, we get:
For the pre-rinse sink, we take 0.8 GPM x 0.5 = 0.4 GPM
For the 1.5 inch pipe diameter fixtures: 15 GPM x [1+ 0.5 + 0.75 + 0.25] = 37.5GPM
For the can wash: 93 GPM x 0.25 = 23.25 GPM
Add 0.4 GPM + 37.5 GPM + 23.25 GPM = 61.15 GPM x 24 minutes = 1,468 gallons
Rounded up to the next size means a 1,500 gallon Grease Interceptor is required!