How big must a Mesa restaurant grease interceptor be? City Code 8-4-22(E)

The section sizes traps and interceptors by "the drainage fixture unit value." Every interceptor is at least 500 gallons and at most 2,500, with at least two chambers. Each fixture's outlet or trap size converts to a fixture-unit value:

The formula in (E)(2) has five steps. Multiply each fixture type's count by its unit value, total them, multiply the total by 3 gpm, multiply that by 12, and round up to the next interceptor size. Section (E)(3) then sets the build: at least 500 gallons and two chambers in concrete, three chambers each with a manway at 750 gallons or more, a 90 degree elbow inlet with an 18 inch down spout, a tee outlet with a threaded cover and 18 inch down spout, two-way cleanouts within 5 feet before and after, and a traffic-rated cover that doesn't say the city owns it.

• Drainage fixture unit. A code number for how hard a fixture can hit the drain, set here by outlet size.
• Double Size. The table doesn't say what a dishwasher doubles. The natural reading is its own outlet-size value, doubled. That is a reading, not text.
• Fixture, for traps. "The entire unit," so a three-compartment sink is one fixture.
• Two flow numbers. The table's lead-in converts one unit to 7.5 gpm, while the formula steps use 3 gpm. The section doesn't reconcile them; the steps in (E)(2) are what produce a size.

Any Mesa food service kitchen putting a trap or interceptor on its grease drains. Auto shops and carwashes get oil or sand interceptors under the same section, but this formula is about grease. It does not cover a kitchen with no grease-bearing fixtures.

A trap must be rated at least 50 gpm with 100 pounds of grease capacity, with a vented flow restriction valve upstream.

A small cafe. Three-compartment sink, prep sink, mop sink and one floor drain, no dishwasher. Only the two sinks are food-prep fixtures, so a trap qualifies. Adding a dishwasher, or growing to five food-prep fixtures, moves it to an interceptor.

A 27 unit kitchen. Three-compartment sink (2 inch, 3 units), prep sink (2 inch, 3), mop sink (3 inch, 5), two 3 inch floor drains (10) and a dishwasher with a 2 inch outlet (3 doubled, 6) total 27. Times 3 gpm is 81, times 12 is 972 gallons. The section lists no standard tank sizes, so it rounds up to the next tank available, and at 750 or more it needs three chambers. This example is our arithmetic, not a Mesa worked example.

The edge case. The same cafe adds a dishwasher next spring. The trap is no longer allowed, and the plan needs an interceptor.

1. 1List every grease-bearing fixture with its outlet size, and mark any dishwasher.
2. 2Decide trap or interceptor with the table above, then run the (E)(2) arithmetic.
3. 3Draw the device with chambers, manways, down spouts and cleanouts on the plumbing plans for Development Services.
4. 4Have Industrial Pretreatment review it before installation: 480-644-2484 or industrialpretreatment@mesaaz.gov.
5. 5If the calculation tops 2,500 gallons, ask Industrial Pretreatment how they want it split, since the section is silent.

• "A three-compartment sink is three fixtures." For trap counting it is one unit. For interceptor sizing its outlet size sets its value.
• "Bigger is safer." 2,500 gallons is the ceiling.
• "A 500 gallon tank is standard." It is only the floor. The calculation can come out much higher.
• "Phoenix numbers apply." Phoenix adds a 17 minute retention time for garbage disposals. Mesa's formula multiplies by a factor of 12 and does not add a 17 factor for disposals; Mesa restricts disposals instead.