Phoenix restaurants: When do you need an outdoor gravity grease interceptor instead of a grease trap?

Phoenix accepts two types of grease interceptor. The manual describes hydromechanical interceptors, "also commonly known as grease traps," as approved "for food service establishments with up to four plumbing fixtures," and adds that "no food waste disposal units (garbage disposals) or dishwashers can discharge to a hydromechanical grease interceptor." Gravity interceptors "are required for use in the City of Phoenix for food service establishments with more than four plumbing fixtures. They are also required for any food service establishment with a food waste disposal unit (garbage disposal) or dishwasher regardless of the number of fixtures."

You get a trap only if all of these are true:

• Four or fewer grease-bearing fixtures.
• No garbage disposal.
• No dishwasher.

Miss any one and it is a gravity interceptor. Gravity units are installed outside unless the authority having jurisdiction approves otherwise (2024 IPC 1003.3.7 as amended by Phoenix).

• Grease-bearing fixture: floor sinks, floor drains, mop sinks, trough sinks and drains, pre-wash, prep and pot sinks, can wash sinks, dishwashers and garbage disposals in the kitchen prep or wash area. Hand wash sinks do not apply, and cooler or condensate drain lines are not to be routed to the grease waste line.
• Fixture, for counting: the manual's sample layout counts a three-compartment sink, a prep sink and a mop sink as three fixtures. Read that as counting each unit once, not each compartment.
• DFU (drainage fixture unit): the number the plumbing code assigns each fixture for drain sizing. Gravity interceptors are sized from it.
• Hydromechanical trap rating: a size such as 50/100 means 50 gallons per minute maximum flow and 100 pounds of grease capacity.

Food service establishments that prepare food on site in the City of Phoenix, new or replacing. Other business types under Section 28-12, such as auto shops, are decided by the Water Services Director instead.

Trap: sized strictly by the count of grease-bearing fixtures, using Phoenix's amended IPC Table 1003.3.5.1 and the UPC Table 10-2 in the city worksheet.

Traps must be stainless steel or polymer and certified by PDI or by UPC/IAPMO. The Phoenix amendment also requires a two-way cleanout on the discharge side of every hydromechanical interceptor.

Gravity interceptor: sized in gallons. Add the DFUs for every grease-bearing fixture, multiply by 3 gpm, then by a 12-minute retention time, or 17 minutes if there is a garbage disposal. The smallest size allowed is 500 gallons. A 500-gallon unit needs two compartments and two manways, and 750 gallons or more needs at least two compartments and three manways.

The city's worksheet gives these DFUs: floor drain 2, floor sink 3, trough drain 2, mop sink 3, sink with food waste 3, garbage disposal 3, and a dishwasher at twice the trap arm size (4 for a 2 inch trap arm). The city's own example is three floor drains, a mop sink and a prep sink: 12 DFUs x 3 gpm x 12 minutes = 432 gallons, so it gets a 500-gallon unit. Our example using the same worksheet: add a 2 inch dishwasher (4 DFUs) and a disposal (3 DFUs) to that kitchen. That is 19 DFUs x 3 x 17 minutes = 969 gallons, so the interceptor must be at least that.

• Taqueria with a three-compartment sink, prep sink and mop sink. A hydromechanical trap works. Three fixtures sit under the four-fixture line.
• Pizza shop that adds a dishwasher in the remodel. The old trap cannot take it. The design switches to an outdoor gravity interceptor of at least 500 gallons.
• Same taqueria adds two floor drains. Floor drains are grease-bearing fixtures, so the count jumps from three to five. The answer flips to a gravity interceptor.
• Existing restaurant with a garbage disposal draining to a trap. The manual routes disposals to gravity interceptors only, so the disposal has to be rerouted to a gravity interceptor. That is a new installation and needs electronic plan review.

1. 1List every fixture that drains grease waste and fill in the city's Food Service Establishment Fixture Count Worksheet (TRT 00227, Appendix C of the manual).
2. 2Decide trap or gravity using the table above. If you add a dishwasher or disposal later, revisit the choice.
3. 3For a gravity unit, select a model from the city's approved list in Appendix A. All gravity interceptors must be designed by a registered engineer and be IAPMO certified.
4. 4Submit plans. A new gravity unit cannot be reviewed over the counter and is submitted electronically. Call 602-495-0278 at least 24 hours ahead so an Environmental Services inspector can join the review.
5. 5Locate the unit where it stays clear of obstructions and easy to reach for cleaning, then schedule the pump-out program.

• "A bigger trap fixes the fixture count." The manual sets the trap limit at four fixtures no matter how big the trap is.
• "Interceptor sizing is just gallons per fixture." Traps use fixture counts and gravity units use DFUs with the 12 or 17 minute rule.
• "Automatic grease removal devices can replace the interceptor." Phoenix's code-change justification says the Water Department does not allow automatic grease removal devices as a standalone device.
• "Enzymes let us skip pumping." Section 28-15 prohibits chemicals, enzymes and bacteria used in place of physical cleaning.