Installing a restaurant grease interceptor means counting the grease fixtures, sizing the tank, getting an approved model through city plan review, then having a licensed plumbing contractor set it outside and pipe every grease fixture to it. In Phoenix, plan review comes before any digging.
What has to happen before anyone digs?
Three things: a fixture count, a size, and an approved plan. Skip one and the job stalls at plan review.
### How do you count the grease-bearing fixtures?
Start by listing every fixture that can send grease to the drain. Phoenix's fixture worksheet counts floor drains, floor sinks, trough drains, mop sinks, prep, pot and pre-wash sinks, dishwashers and garbage disposals in the kitchen prep or wash area. Hand wash sinks do not apply in Phoenix.
Scottsdale asks a simple test question about each fixture: can it possibly receive fats, oils or grease? It also notes that hand sinks in food prep areas count unless the plumbing sheet carries a note excluding them.
That count decides the type of device. In Phoenix, a kitchen with more than four fixtures, or any dishwasher or disposal, needs an outdoor gravity interceptor. The trap versus interceptor page covers the split, and the dishwasher rule explains why one dishwasher changes everything.
### How is the tank size decided?
Every Valley city that publishes a method turns the fixture count into gallons with arithmetic. Phoenix adds the drainage fixture units, multiplies by 3 gallons per minute, then multiplies by a retention time of 12 or 17 minutes. The sizing worksheet walk-through shows it step by step, and what size grease interceptor a restaurant needs gives the wider picture.
The size also drives the build. Phoenix's approved list says a 500 gallon interceptor has at least two compartments and two man-ways. At 750 gallons and larger it has at least two compartments and three man-ways. Mesa's code takes a different route: at least two chambers, and three chambers each with a manway once the calculated size reaches 750 gallons.
### What does plan review ask for?
Plan review is where most first-time owners lose weeks, so it is worth reading closely. Phoenix's manual says: "Per plumbing code all grease interceptors installed and replaced in the City of Phoenix must go through plan review." It gives the reasons: the interceptor is sized correctly, approved for use in Phoenix, and all grease bearing fixtures are plumbed to it.
For a new gravity interceptor, Phoenix's manual says plans cannot go over the counter and should be submitted electronically. It also says a certificate of occupancy is required for these plan reviews. The details of what to bring are in our Phoenix interceptor plan review code page and the permit FAQ for the Valley.
Other cities add their own asks. Scottsdale's guidelines say plans may need to be prepared and stamped by an Arizona registrant. For a gravity interceptor, Scottsdale requires site and civil drawings that show all setbacks and public utility easements, and it does not allow interceptors to be installed at risk.
Phoenix also keeps a list of approved gravity interceptors, and its note on that list says gravity interceptors are designed by a registered engineer and IAPMO certified. Pick from the approved list, not from a catalog.
How is the tank set and piped?
Once the plans are approved, the physical work follows a fairly standard order. Your tank maker's instruction sheet is the boss here. The model code itself says gravity interceptors are installed in accordance with the manufacturer's instructions, and Phoenix's amendment adds the city's own requirements.
### Where does the tank go?
Outside, unless the city approves otherwise. Phoenix's amendment to the 2024 International Plumbing Code says gravity grease interceptors shall be installed outside unless the city approves something else. The National Precast Concrete Association adds a practical point: grease-laden water should flow to the interceptor by gravity, so the tank sits lower than the building. It also says interceptors should be outdoors for easy inspection and to remove any chance of food contamination.
The location and access rules page covers setbacks, easements and truck access.
### What does the excavation and setting involve?
Precast concrete tanks and factory-built tanks each come with their own instruction sheet. To show what such a sheet looks like, here is what one manufacturer's guide says for a buried unit. That guide is for a 75 gallon-per-minute unit, not a big concrete tank, so treat it as an example of the type of steps, not a spec for yours.
- Dig the pit at least 12 inches larger than the unit on all sides and 6 inches deeper than the bottom.
- Lay a level base of well-packed, crushed stone about 1 inch in size. Native soil and sand are not approved backfill.
- Test the tank for water tightness before installation.
- Set the unit level on a solid surface that supports the whole bottom.
- Backfill evenly with the same crushed stone, compacting by hand only.
- Add a reinforced concrete pad at grade.
- Where groundwater is a risk, add an anti-flotation anchor kit.
That same guide says the tank is for open system (vented) gravity drainage and must not be pressure tested with air. Precast tanks are heavy, which the precast association notes helps them resist the buoyant forces of water, but the tank maker still decides how yours is set.
### How is it connected?
Pipe goes in and out, and the details are where the inspector looks.
- Upstream slope. Phoenix's amendment to the 2024 plumbing code sets the slope of drainage piping upstream of a grease interceptor at not less than 1/4 inch per foot, which is a 2 percent slope.
- Inlet and outlet. Mesa's code calls for inlet piping with a 90 degree elbow and a minimum 18 inch down spout, and outlet piping with a tee connection, a threaded cover and a minimum 18 inch down spout. The precast association explains why the inlet tee extends below the water line: it prevents excessive turbulence at the surface and helps retention time.
- Cleanouts. Mesa requires a two-way cleanout within 5 feet before and 5 feet after the interceptor.
- Vent. The interceptor must be vented. See the venting page for what the code says.
- Every grease fixture. Plan review checks that all grease bearing fixtures go to the interceptor.
If a former retail space has no grease line, the trenching is a big part of the job. The former retail space page and the change of use page cover that trigger.
### What about the lid?
The lid is part of the install, not an afterthought. Mesa requires a traffic rated cover that carries no wording saying the city owns it. The precast association says all interceptors should be designed for traffic loading, using at minimum the loads from ASTM C890 (A-16). If the tank sits in a drive lane, read the traffic rated lid page.
Who does the work, and what happens after it is set?
### Who is allowed to install it?
Scottsdale's guidelines say all work must be done by a licensed plumbing contractor per ROC requirements. The manufacturer's guide above says the same thing in its own words, recommending installation only by a licensed plumber. Who can pull the permit is a separate question, and the commercial permit page explains it.
### What is the first pump-out?
New tanks need a routine from day one. Phoenix requires gravity interceptors to be fully pumped out and cleaned at least once every 3 months unless the Director grants a variance. The manual adds that a new facility should monitor the interceptor with its pumping company or a city inspector and adjust the frequency to the grease that actually builds up. Cleaning must be done by a licensed pumper, and maintenance records stay on site for three years.
The precast association's maintenance guide says a new tank may need monthly or bimonthly cleaning until the maintenance company can establish a predictable level of grease buildup. For Phoenix schedules, see the pump-out schedule page and how often to pump a grease trap.
### What are the common ways an install goes wrong?
- Ordering a tank before plan review, then finding it is not on the approved list.
- Leaving a fixture off the sizing count, so the tank comes up short.
- Routing sanitary waste to the tank. The precast association says sanitary waste should not go into a grease interceptor.
Lining up the plumber and the drawings before you sign a lease that needs a tank saves the most time. That is the whole job, in order.
When should you call a plumber?
Call before you order equipment, not after. A licensed plumber can walk the kitchen, count fixtures, and tell you what the drawings need before plan review does. HQ is licensed and insured, and free estimates come with upfront pricing. If a grease line backs up and floods the kitchen floor, we offer 24/7 emergency service. For commercial work, start at our commercial plumbing page.
For access risers and collars, see whether you need a riser or extension collar.