Can an under-sink grease interceptor serve a 3-compartment sink?

It can in some cities, if the kitchen is small, the unit meets the city's minimum rating, and nothing else on the line is off limits. Scottsdale's guidelines even describe grease traps as small devices located inside the kitchen and directly plumbed to the 3-compartment sink. The catch is the size rule, which changes from city to city.

### What counts as an under-sink unit?

There is no code term for it. It is a marketing label for a compact hydromechanical trap. Schier, a manufacturer, says on its GB2 product page that the compact design makes it a fit for under-the-sink installations, and its spec sheet says the unit is designed for indoor, on-floor, below-grade or low-profile under sink installations. Whether a given model is allowed is up to each city.

### What do the Valley cities say?

The Scottsdale guidelines we read are based on the amended 2021 plumbing code and were last modified in January 2023, so ask the city whether the size rule has moved. Scottsdale writes its size as 50gal/100lb, which reads as the same 50 gpm and 100 pound rating the other cities use.

For a wider look at how the Valley cities compare, see the Glendale, Peoria, Gilbert and Surprise page and the dishwasher page.

### What does Phoenix say about the sink itself?

Phoenix's manual says small hydromechanical interceptors are approved for food service establishments with up to four plumbing fixtures. Its typical layout is a three-compartment sink, a prep sink and a mop sink all routed to one trap. It also says no food waste disposals or dishwashers can go to a trap.

Phoenix's amendment to the 2024 plumbing code then gives a table by fixture count. One fixture needs a trap rated at 20 gpm with at least 40 pounds of grease capacity. Two need 25 gpm and 50 pounds. Three need 35 gpm and 70 pounds. Four need 50 gpm and 100 pounds. Phoenix approves specific models, and its approved trap list uses the same four ratings. The device has to be certified by the Plumbing and Drainage Institute (PDI) or by IAPMO, and made of stainless steel or polymer.

### Is a 3-compartment sink one fixture or three?

This is the question that decides the trap size. The cities do not all say it the same way.

Where is it written down? Scottsdale says that for sizing a trap, a fixture means the entire unit, and a three-compartment sink is considered a unit. Mesa's code says the same thing: "a fixture means the entire unit, e.g., a three (3) compartment sink is considered one unit."

Phoenix's manual does not say how compartments are counted. Its example puts the three-compartment sink in a list of up to four fixtures. Tempe counts kitchen fixtures such as sinks and floor drains connected to the grease waste line. If your layout is close to the four-fixture limit, ask the city how it counts before you order anything.

What else counts toward the four? Tempe's wording says sinks and floor drains count. Phoenix's manual says inspectors count every grease bearing fixture, including sinks, floor sinks, floor drains, mop sinks and dishwashers. Mesa's code ties the trap to any kitchen with four or fewer fixtures used for food preparation. A prep sink, a mop sink and a floor drain can use up the count quickly. When a fifth fixture shows up, the kitchen has crossed into gravity interceptor territory. See what size grease interceptor a restaurant needs.

Does Mesa require a trap at a three-compartment sink? Mesa's code says a grease trap shall be installed whenever a three-compartment sink is required by Maricopa County. The county's food establishment construction guide says a three-compartment sink is required in all food service facilities. So in Mesa, a food business should expect at least a trap, and an interceptor if it has a dishwasher or more than four fixtures. Mesa's code also says users shall not install or replace a garbage grinder that sends liquefied waste to the sewer, except for domestic sources. Maricopa County says grinders are not permitted on three-compartment sinks.

A small trap is rated by flow, in gallons per minute, plus how many pounds of grease it holds. PDI, the trade body for drainage product makers, says these units are tested at their rated flow, and that with an expected maximum flow of 20 gpm you would use a 20 gpm size.

### How is the flow figure worked out?

Cities that use a fixture count skip the math. If a city has no table, or you want to check a unit, there is a volume method. It appears in a sizing guide by consultant Ken Loucks that the City of Lakeport, California hosts. It works like this:

1. 1Multiply length, width and depth of one compartment in inches, then divide by 231 to get gallons.
2. 2Multiply by the number of compartments.
3. 3Multiply by 0.75, since a sink is not filled to the brim.
4. 4For a one-minute drain time, that number is the flow in gpm.

The guide's own example is a three-compartment sink with each compartment 18 x 24 x 12 inches. That works out to about 50 gpm for a one-minute drain time, or about 25 gpm with a two-minute drain time. It says to use a one-minute period when the interceptor is within 20 feet of the fixtures or has indirectly connected fixtures, and it says a two-minute period is a common complaint of owners because fixtures drain slowly.

That is one example with one sink size. A smaller sink gives a smaller number. Use your own sink's dimensions, not the guide's.

### What happens if the trap is rated too low?

PDI says the trap's vented flow control keeps the designed flow from being exceeded, so grease removal is not compromised, but drain-down times get longer and can be unworkable in a busy kitchen. Zurn says that if the maximum recommended flow is exceeded, efficiency decreases considerably. So a low rating shows up as slow sinks, standing water and, in the end, overflow. See why a grease trap keeps overflowing.

### Where do DFUs fit?

DFU stands for drainage fixture unit, a code number for how hard a fixture can hit the drain. Cities use DFUs to size gravity tanks, not small traps. Phoenix's worksheet counts a sink with food waste at 3 DFUs. Scottsdale's worksheet lists a 3-compartment sink at 9 DFUs, a 2-compartment sink at 6 and a prep sink at 3. Those numbers feed the gravity tank formula. They are not a trap rating. PDI notes that sizing a hydromechanical trap by DFUs can lead to peak flows beyond the size chosen, which stretches drain-down times. The sizing worksheet page walks through the gravity math.

### What is a real product's rating?

Manufacturers list flow and grease capacity together. Schier's current GB2 product page lists 35 gpm and 119 pounds, with a 20 gallon liquid capacity. An older Schier spec sheet dated 2022 lists 35 or 50 gpm settings with 130 and 127 pounds. Ratings change when a unit is recertified, so read the current listing.

Here is what that means for the city rules above. A 35 gpm rating is below the 50 gpm minimum that Scottsdale, Mesa and Tempe name. It matches the Phoenix table for three fixtures. That is arithmetic on the numbers in the documents, not an approval of any model. Each city decides which models it accepts.

Rating is only half of it. The install has a few conditions of its own.

• How the sink drains. Maricopa County's guide says all three compartments should drain indirectly to a floor sink with an air gap. Zurn's care sheet warns that indirect piping to one of its units could cause standing water in the floor sink, or flooding, if a line blockage occurs. Ask the plan reviewer where the trap sits relative to the floor sink.
• The flow control. The plumbing code says grease interceptors shall have devices to control the rate of flow, and that the flow control shall be vented and end at least 6 inches above the flood rim level, or follow the manufacturer's instructions. Mesa and Scottsdale say the valve goes upstream of the trap and must be vented. Zurn says to place it beyond the last connection and as close as possible to the underside of the lowest fixture, and its sheet limits how far below the sink the flow control can sit. See the venting page.
• Clearance. Zurn says the space above the unit has to be greater than the height of the removable baffle, so the cover and baffle can come out for cleaning.
• Cleanout. Phoenix's amendment requires a two-way cleanout on the discharge side of every hydromechanical interceptor.
• Grouping. Zurn says one interceptor serving several fixtures is recommended only where the fixtures are close together, each individually trapped and back-vented.
• Service. Phoenix says hydromechanical units must be fully pumped and cleaned every 30 days unless the Director grants a variance, and none may go more than 90 days without maintenance. Tempe says monthly. A trap under a sink has to be reachable for that.

### What should you do before you buy a trap?

Work through this list.

1. 1Count every fixture that will drain to the trap, then compare it to the city's limit of four.
2. 2Rule out a dishwasher or food waste disposal on that line. A dishwasher sends a kitchen to a gravity interceptor in all four cities. Phoenix says the same for disposals, Tempe does not permit them, and Mesa restricts new ones.
3. 3Get the city's approved list and minimum rating. Phoenix requires pre-approval of every device.
4. 4Work out the flow for your sink using the volume method, and check it against the rating.
5. 5Read the maker's install sheet for the flow control, vent and clearances.
6. 6Submit plans. Phoenix says all interceptors installed or replaced go through plan review, and a like-for-like swap of a hydromechanical trap can be submitted over the counter. The permit page covers each city.

If you are opening a coffee shop, bakery or bar, our page on those kitchens may help. For a trap that is being moved outside, see the indoor trap to outdoor interceptor page.

Call a licensed plumber before the trap is bought. The fixture count, the flow control, the vent, the cleanout and the plumbing plan all have to agree, and the plan reviewer wants them drawn. A plumber can install or replace the trap and its plumbing parts under permit. Our plumbers are licensed and insured, we give free estimates with upfront pricing, and we run 24/7 emergency service if a grease line backs up during service. Start at our commercial plumbing page.