(A) Indoor grease traps should be appropriately sized, as determined by the following steps:
(1) Determine cubic content of the fixture. Multiply length by width by depth. When a fixture has multiple compartments, the user must add all the cubic content together to get the total cubic content;
(2) Determine capacity in gallons. One gallon equals 231 cubic inches;
(3) Determine actual drainage load. The fixture is normally filled to about 75% of capacity with water. The items being washed displace about 25% of the fixture content. Thus, the actual drainage load equals 75% of the fixture capacity;
(4) Determine the time that is required to completely drain the fixture. In general, good practice dictates a one-minute drainage period. However, where conditions permit, a two-minute drainage period is acceptable. Flow rate equals actual drainage load;
(5) Determine the correct size grease trap based on the flow rate determined by the chart in division (A)(6) below. Select a properly sized grease trap. If the size that is determined by this chart is not available, select the next larger sized grease trap; and
(6) Sizing of grease trap.The selections of standard certified grease traps applicable to various sized fixtures commonly used in domestic, commercial and industrial installation listed below are based on the formula provided above, and are commonly used when a grease trap is needed.
Fixture Compartment Size (Inches) | Number of Compartments | Drainage Load (Gallons) | One-Minute Drainage Load | Two-Minute Drainage Load |
18x12x8 | 1 | 5.6 | 7 | 4 |
16x14x8 | 1 | 5.8 | 7 | 4 |
20x18x8 | 1 | 9.3 | 10 | 5 |
18x18x8 | 2 | 16.8 | 20 | 10 |
20x18x8 | 2 | 18.7 | 20 | 10 |
30x20x8 | 1 | 15.5 | 20 | 10 |
18x20x12 | 1 | 14.0 | 15 | 10 |
24x20x12 | 1 | 18.7 | 20 | 10 |
22x20x12 | 2 | 34.3 | 35 | 20 |
24x24x12 | 2 | 44.9 | 50 | 25 |
18x20x12 | 3 | 42.1 | 50 | 25 |
22x20x12 | 3 | 51.4 | 70 | 35 |
18x18x18 | 3 | 56.8 | 70 | 35 |
24x20x18 | 3 | 84.2 | 100 | 70 |
20x20x18 | 4 | 93.5 | 100 | 75 |
(B) Outdoor grease traps should be appropriately sized, as determined by the Manning Formula described below.
(1) The Manning Formula. Interceptor size (in gallons) is equal to flow rate multiplied by the sum of the fixture rating and the discharge rate from any mechanical washer (i.e. dishwashers, glass washers, laundry machines and the like) and multiplied by the 24-minute retention time. (Interceptor=[(Fr)x(R)+(D)]xT)
(2) Flow rate. Flow rate is measured in GPM and determined based on the slope, pipe texture and pipe diameter. The following rates are pre-calculated.
(a) One-half foot pipe equals eight-tenths GPM per fixture.
(b) One-foot pipe equals five GPM per fixture.
(c) One and one-half foot pipe equals 15 GPM per fixture.
(d) Two-foot pipe equals 33 GPM per fixture.
(e) Two and one-half foot pipe equals 59 GPM per fixture.
(f) Three-foot pipe equals 93 GPM per fixture.
(2) Fixture ratings. Fixture ratings of grease-laden waste stream 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.
(a) Two-, three-, and four-compartment sinks have a fixture rating of one.
(b) One- and two-compartment meat prep sinks have a fixture rating of three- quarters.
(c) Pre-rinse sinks have a fixture rating of one-half.
(d) One- and two-compartment vegetable prep sinks have a fixture rating of one-quarter.
(e) Can wash/mop sinks have a fixture rating of one-quarter.
(3) Discharge rate. The discharge rate from dishwashers, laundry machines, glass washers and the like must be added to the Manning Formula calculation because of their potential for introducing large quantities of water down the discharge rate in GPMs, or you can call the manufacturer directly.
(4) Twenty-four-minute retention time. Twenty-four-minute retention time is a pre- calculated amount of time that engineers have determined to be necessary for grease to separate from water.
(Ord. passed 9-13-2021)