What size grease interceptor does Scottsdale require?

Two passages in the city's own published material use different numbers. The gravity-interceptor definition specifies at least 500 gallons and separation by gravity over a retention time of not less than 30 minutes. Section 31-85, IPC 1003.3.7, and the guidelines' sizing formula instead use total drainage fixture units (DFU) × 3 gallons per minute × 12 minutes without a garbage disposal, or × 17 minutes with one.

The sources do not explain how the 12/17-minute calculation meets the 30-minute definition. This page reports both passages; it does not invent an engineering explanation or treat the smaller calculation as approved. Include both requirements in the plan-review question and obtain the city's accepted sizing basis before selecting the tank.

For small units, IPC 1003.3.5.1 as amended says hydromechanical grease interceptors (grease traps) are allowed only when four or fewer fixtures are used for food preparation. The only size allowed is a 50 gallon, 100 pound unit. Any facility installing a dishwasher must install a gravity interceptor.

• Drainage fixture unit (DFU): the plumbing code's number for how much drain load a fixture puts on the system. The city worksheet lists a DFU for each fixture type.
• Retention time: how long wastewater stays in the tank so grease can float. The code sets 12 or 17 minutes for the formula and the definition requires at least 30 minutes.
• Fixture, for a trap: the code says the entire unit counts, so a three-compartment sink is one fixture, not three.
• Flow restriction valve: a valve upstream of a trap that limits flow. It must be vented, and if the trap sits below floor level it must be installed so it can be inspected and maintained.

It covers anyone designing or replacing a grease interceptor in Scottsdale: designers, plumbing contractors and the restaurant owner who pays for it. It does not apply to the kitchen of a private home, and it does not cover sand and oil interceptors at car washes, which follow a different rule on our car wash page.

The city's worksheet prints DFU values by fixture. As printed, a three-compartment sink is 9, a two-compartment sink 6, a pre-rinse sink 3, a prep sink 3, a mop sink 3, a floor sink 3, a trench drain 6 and a standard floor drain 2. A hand sink is 2. Dishwashers and glasswashers take their values from the equipment cut sheet, which the city wants included with the plumbing drawings.

Here is the arithmetic for a sample kitchen with no dishwasher and no disposal: one three-compartment sink (9), one prep sink (3), one pre-rinse sink (3), one mop sink (3) and one floor drain (2) add up to 20 DFU. Twenty times 3 gpm is 60 gpm, and 60 times 12 minutes is 720 gallons. With a garbage disposal the same kitchen uses 17 minutes and comes to 1,020 gallons. Those numbers show the printed formula only. They do not establish that either tank meets the separate 30-minute definition or that the city will approve it.

A taco stand with a three-compartment sink and one prep sink. Two fixtures, no dishwasher, so a 50 gallon, 100 pound trap is allowed.

A sit-down restaurant in a strip mall. It has a pot sink, pre-rinse sink, two prep sinks, a mop sink and a dish machine. The dishwasher alone forces a gravity interceptor, and the designer must submit a size the city accepts after resolving the retention-time difference.

The edge case: no space for a new tank. IPC 1003.3.1 as amended lets one or more interceptors go on or above the floor and upstream of an existing interceptor when space or other constraints stop a new or replacement tank. That changes a retrofit answer from "dig up the parking lot" to a design review.

1. 1List every fixture on the drawings and note which drain to the interceptor.
2. 2Look up each fixture on the city's worksheet and add the DFUs. Include hand sinks in food prep areas unless your plumbing sheet carries the city's exclusion note.
3. 3Calculate the printed DFU × 3 × 12/17 formula, then ask plan review how the separate 30-minute gravity-interceptor definition applies. Obtain that clarification before specifying the tank.
4. 4If the result is over 2,500 gallons, consider two interceptors serving separate zones. The guidelines suggest zones over series or parallel connection so they can be pumped on separate cycles.
5. 5Attach the worksheet and any dishwasher cut sheet to the permit submittal.

• "Any interceptor over 500 gallons is fine." The size comes from the formula. Five hundred gallons is only the floor.
• "A hand sink can be left off the count." The guidelines count hand sinks in food prep areas unless a note on the plumbing sheet excludes them. A floor sink that receives a fixture's discharge is not counted separately.
• "A trap saves money on a busy kitchen." With a dishwasher or more than four fixtures a trap is not allowed, regardless of the volume.

This page summarizes the code. The adopted text on Municode governs.