Grease Interceptor Sizing and Installation Guidelines

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Grease protection is an essential element for restaurants, cafes, catering facilities, commissaries, hotels, cafeterias, convenience stores, full service grocery stores, schools, hospitals, and food manufacturing plants. Grease interceptors are installed on “gray” water drain lines and are designed to remove fats, oils and grease (FOG) from wastewater. The retained FOG should be regularly removed or pumped out of the interceptor. The interceptor must be cleaned whenever 25 percent of any chamber becomes filled with FOG or solids.  See the City’s Grease Interceptor Installation and Sizing Policy.


  • Hydromechanical Grease Interceptors (HGIs) can be located inside or outside the facility and are required to have an approved type of vented flow restrictor. Flow restrictors slow the flow of water entering the grease interceptor. The total capacity of the fixtures discharging into an HGI, in gallons, shall not exceed two and one-half (2 ½) times the certified gallons-per-minute flow rate of the interceptor.
  • Gravity Grease Interceptors (GGIs) are generally installed in the ground outside the facility, upstream from the “black” water (sanitary waste) drain line, and are at least 500 gallons in capacity. In existing buildings like the downtown corridor, space for a GGI usually does not exist. Breaking down the fixtures into smaller groups and running them through an appropriately sized HGI is the best solution.
  • A Drainage Fixture Unit (DFU) is a unit of measure for the load-producing effects on a plumbing system from different kinds of plumbing fixtures. Things like produce prep sinks and hand washing sinks do not need to be connected to the grease device. If they are not plumbed into the device then they should not be included in the sizing calculation.

Examples of DFU Calculations

  • Table 1 on the next page (“Determining DFUs”) lists the number of DFUs per fixture.
  • Table 2 (“Fixture Equivalents”) lists the number of DFUs per pipe diameter.

Example 1: Restaurant with 40 seats, serves 120 meals per hour*:
3-comp sink (9 DFUs) 2-comp food prep sink (6 DFUs)
mop sink (3 DFUs)
3 hand washing sinks (3 DFUs)
2 floor drains (4 DFUs)
dishwasher w/ 2” pipe to floor sink (2”drain line for 4 DFUs)
Total = 29 DFUs

Example 2: Restaurant with 40 seats, serves 40 meals per hour* or less:
3-comp sink (9 DFUs)
2-momp food prep sink (not connected)
mop sink (3 DFUs)
hand wash sink (not connected),
pre-rinse sink (3 DFU’s)
+ dishwasher (not connected)
Total = 15 DFUs

Example 3: A commissary kitchen; facility is used by a catering company that makes food to be delivered to clients. Because it is a catering kitchen, the number of meals served per hour is not limited to onsite seating and is only limited by the amount and capacity of the kitchen. Because of this, it is required to discharge through a properly sized gravity grease interceptor.

Sizing example for site;
3-comp sink (9 DFUs)
2-comp prep sink (6 DFUs)
8 feet of trench drain (16 DFUs)
4 floor drains (8 DFUs)
Mop sink (3 DFUs)
Dishwasher pre-rinse sink (3 DFUs)
Dishwasher (4 DFUs)
Total = 49 DFUs
Table 5 of this policy indicates a 1,250 gallon GGI unit would be required.

*Meals per peak hour is determined by multiplying the number of seats by 60, and dividing by the estimated time (minutes) it takes for a patron to eat.

  • Table 4 on the next page (“HGI Sizing Chart”) is used to determine the size of HGI needed, if that is the type of grease interceptor being installed. In Example 1, based on 29 DFUs a 75 gallon per minute (gpm) unit would be required. In Example 2, a 50 gpm unit would be required.
  • Table 5 (“GGI” Sizing Chart”) is used to determine the size of GGI needed. Example 1 would require a 100 gallon unit. Example 2 would require a 750 gallon unit.
Table 1
Type of Fixture # of DFUs Comments
3-compartment sink 9
2-compartment sink Use floor sink criteria based upon drain size or number of sinks, whichever is larger Each compartment is 3 DFUs.
Floor sinks DFUs based upon sink drain size*

See table 2 below or section 702.1 in the UPC.

*Floor sinks that receive only ice machine and cooler condensate are not counted.

Mop sink 3 If cooking meat, then new mop sinks must be connected to grease protection.
Wok sink 3
Floor drains 2
Trench drains 2 DFUs per lineal foot of drain
Soup Kettle 2 DFUs per lineal foot of trench drain
Braziers 2 DFUs per lineal foot of trench drain
Steam tables Use floor sink or trench drain criteria, whichever is appropriate.
Dishwasher pre-rinse sink 3
Dishwashers Use floor sink criteria
Food waste disposers, including pulpers Use next larger size of GGI than would otherwise be required FOG bearing food waste disposers can only discharge to properly sized GGIs


Table 2

Table 3

Fixture Unit Equivalents from section 702.1 of the UPC

Pipe Size, GPM, Maximum DFU Count

Drain Size in Inches


Pipe Size, Inches

Max. Full Pipe Flow (gpm)

Max. DFU Count






























Table 4 Table 5
Hydromechanical Grease Interceptor (HGI) Sizing Chart Gravity Grease Interceptor (GGI) Sizing
DFUs(1) HGI Flow (gpm) DFUs(1) GGI Volume
8 20 8 500 gallons
10 25 21 750 gallons
13 35 35 1,000 gallons
20 50 90 1,250 gallons
35 75 172 1,500 gallons
172 100 216 2,000 gallons
216 150 307 2,500 gallons
342 200 342 3,000 gallons
428 250 428 4,000 gallons
576 350 576 5,000 gallons
720 500 720 7,500 gallons
2112 10,000 gallons
2640 15,000 gallons

(1) The maximum allowable number of DFUs that can be connected to the grease interceptor.
The information in the above tables is from section 702.0 and tables 7-5, 10-2 and 10-3 of the Uniform Plumbing Code.

The following flow chart can be used to determine what type of grease interceptor is allowed, whether or not a garbage disposal is allowed, and what the next steps of permitting might be.

Questions? Contact Environmental Services / Site Development  (253) 591-5760

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