A restaurant has to move from an indoor grease trap to an outdoor interceptor when the kitchen outgrows what a trap may serve: more than four fixtures or any commercial dishwasher. Phoenix adds garbage disposals. Scottsdale and Gilbert can also require the change after a remodel, an addition or a grease blockage.
What forces a restaurant to replace a grease trap with an interceptor?
Five things show up in Valley codes. Which ones apply depends on your city.
### Does adding a fifth fixture force the change?
Yes, in the cities that publish a fixture limit. A grease trap is a small unit, usually indoors. Phoenix's FOG manual says it is approved for food service establishments with up to four plumbing fixtures. Gravity interceptors are required for establishments with more than four.
The other cities say the same thing in their own words:
- Tempe. A trap can be installed when there are four or fewer kitchen fixtures connected to the grease waste line. An interceptor is required when there are over four.
- Mesa. Code 8-4-22 allows a trap only when there are four or fewer fixtures used for food preparation.
- Scottsdale. The amended 2021 IPC allows a trap only with four or fewer fixtures used for food preparation, and only one trap size.
Each city counts differently, so a fifth sink may or may not tip you over. More on that below.
### Does a commercial dishwasher force the change?
In the four cities above, yes, whatever your fixture count. Phoenix says no dishwasher can discharge to a hydromechanical unit. Tempe says a facility with a commercial grade dishwasher must install an interceptor. Mesa says any facility installing a dishwasher shall install a grease interceptor. Scottsdale says the same. Our page on dishwashers and grease traps has the detail.
### Does a garbage disposal force the change?
In Phoenix, yes. The FOG manual requires a gravity interceptor for any food service establishment with a food waste disposal unit, regardless of the number of fixtures. In other cities the answer is often that you cannot have one. Tempe's code says users shall not install or replace a garbage grinder or disposal, apart from domestic sources. Mesa's code carries the same sentence for grease traps.
### Does a remodel or addition force the change?
It can. Scottsdale's guidelines list the cases where traps, interceptors and separators shall be installed. Two of them fit an existing restaurant:
- 1An existing facility that has no interceptor or separator, when it has additions, alterations or remodeling.
- 2An existing facility with no interceptor or separator that has caused a blockage or is contributing to grease build-up in the city sewer.
The same section adds a rule for kitchens that already have an interceptor. If additions, alterations or remodeling will increase the volume going to it, the guidelines say the interceptor or separator "shall be sized for the new volume." So a bigger kitchen can mean a bigger tank, not just a first tank.
Gilbert's code reaches the same place from a different direction. The owner of every newly constructed, remodeled or converted commercial facility with one or more grease generating activities must install a grease removal device at least as large as the adopted plumbing code requires.
### Can a sewer blockage or an inspector force the change?
Yes. Gilbert's code says that when the director notifies you that the sewer line serving your business has a grease blockage, a history of blockage, or accelerated line maintenance resulting from grease disposal, you must install a grease removal device. If you already have one, you may be required to install a new or more effective device.
Phoenix works through inspections. The FOG manual says the inspector checks whether any interceptor parts are missing or the interceptor has rotted out and needs replacing. If something needs fixing, the inspector issues a requirement with a time frame. Appendix B then tells you to bring the inspection report to plan review when a device is installed because of an inspector's requirement. Extensions can be granted based on business needs if the request is in writing.
Tempe's inspection list points the same way. Grease control devices must be in good condition, with no holes, missing or broken parts or severe corrosion. A trap that fails that test needs repair or replacement.
How do you know if your kitchen is close to the limit?
You are close to the limit if you have three or four fixtures on the grease line, or if you plan a dishwasher or a disposal. Count first, because every city has its own idea of what counts, and the difference can decide whether a new sink is fine or a problem.
### What counts as a fixture?
Phoenix lists grease bearing fixtures as floor sinks, floor drains, mop sinks, trough sinks, trough drains, pre-wash sinks, prep sinks, pot sinks, can wash sinks, dishwashers and garbage disposals in the kitchen prep or wash area. Hand wash sinks do not apply. The manual's own example of a trap kitchen is a three-compartment sink, a prep sink and a mop sink. That is three fixtures. Add a floor sink and a pot sink and you are at five.
Mesa and Scottsdale count differently for traps. For sizing a trap, a fixture means the entire unit, so a three-compartment sink is one unit. Tempe counts kitchen fixtures such as sinks and floor drains connected to the grease waste line.
Here is how the same kitchen can read in different cities.
| Kitchen | Phoenix | Mesa and Scottsdale | Tempe |
|---|---|---|---|
| Three-compartment sink, prep sink, mop sink | Three fixtures, trap allowed | Trap allowed | Three fixtures, trap allowed |
| Same kitchen plus a floor sink and a pot sink | Five, interceptor | Depends on how many are food prep fixtures | Five, interceptor |
| Same kitchen plus a commercial dishwasher | Interceptor | Interceptor | Interceptor |
The middle cell is the one to check with the reviewer. Mesa and Scottsdale count fixtures used for food preparation, so ask how a floor sink or pot sink is counted before you commit.
### What should you do before you add a station?
- 1Write down every fixture that drains to the grease line today.
- 2Write down what the new equipment adds: a sink, a floor drain, a dishwasher, a disposal.
- 3Compare the new total to your city's four-fixture limit.
- 4If you are near it, ask the city's pretreatment office before you buy equipment.
Step 4 matters. Equipment you cannot connect is money sitting on the loading dock.
### Does an old trap that still works get a pass?
The codes we read do not say. None of the Valley pages above states that an existing trap is grandfathered when you add equipment. Scottsdale and Gilbert tie their rules to additions, remodeling or conversion, so a kitchen that changes should expect a review. A kitchen that stays exactly as it is faces the ongoing rules: service schedule, records and condition.
What does the swap involve?
An interceptor is a different job from a trap. Here is the short version.
### Where does the tank go?
Outside. Phoenix's 2024 amendments say gravity interceptors are installed outside unless the city approves otherwise, and Tempe's page says the same for its interceptors. You need room for a tank, a path for the pipe, and access for service. Our page on where a grease interceptor can go covers the details, and lids in parking lots covers what goes on top.
### What if there is no room?
Scottsdale's guidelines, based on its amended 2021 IPC, include a relief valve. Where lack of space or other constraints prevent the installation or replacement of a grease interceptor, one or more grease interceptors are permitted to be installed on or above the floor and upstream of an existing grease interceptor. Phoenix's 2024 amendment deleted that relief. The city's stated reason is that installing interceptors upstream of an existing interceptor "does not increase the flow capacity of the existing grease interceptor." So in Phoenix, do not count on it. Where it applies, it helps a kitchen that already has an interceptor. It does not turn a trap into an interceptor.
### Can an automatic grease removal device replace the tank?
Not in Phoenix on its own. The amendment's justification says automatic grease removal devices are not allowed as a standalone device by the Water Department's Office of Environmental Programs. Read our page on automatic grease removal devices before you plan around one in any city.
### How is the new tank sized?
Phoenix, Scottsdale and Surprise all use drainage fixture units multiplied by 3 gallons per minute and a retention time of 12 minutes, or 17 with a garbage disposal. Phoenix's smallest gravity interceptor is 500 gallons. Our sizing walk-through does the math step by step, and what size grease interceptor a restaurant needs gives the overview.
### What paperwork comes with it?
Phoenix sends every interceptor installed or replaced through plan review, and a new gravity interceptor cannot go over the counter. See permits for replacing or adding an interceptor and the installation steps. If you rent, the tank may sit on the landlord's lot, and shared interceptors in strip centers explains that conversation. A tenant remodel also touches tenant improvement plumbing.
When should you call a plumber?
Call one before you buy the equipment that pushes you over the limit, not after. A licensed plumber can count your fixtures, read your city's rule, and tell you whether the plan needs a trap, a tank or a redesign of the kitchen line.
Call right away if a sewer blockage or a city notice is already in play. HQ is licensed and insured, gives free estimates with the price set up front, and offers 24/7 emergency service for commercial plumbing emergencies. You can start on our commercial plumbing page.