Yes, in practice. The 2024 IPC, which Phoenix, Mesa and Tempe use, requires an oil separator at car washing facilities, and a sand interceptor handles the grit. Phoenix lists carwashes among the businesses that need pretreatment devices, and its sewer code sets how the interceptor is approved, cleaned and recorded.
What the plumbing code requires
The 2024 International Plumbing Code (IPC) is the base code in Phoenix, Mesa, Chandler and Tempe. Phoenix also adopted the 2024 Uniform Plumbing Code as a second path, and Scottsdale uses the 2021 IPC. The interceptor rules sit in Chapter 10, Section 1003.
Section 1003.1 sets the purpose. Interceptors and separators "shall be provided to prevent the discharge of oil, grease, sand and other substances harmful or hazardous to the public sewer." A car wash produces all three of the first items on that list.
Section 1003.4 names car washes directly. It requires oil separators at "car washing facilities," along with repair garages that have floor or trench drains, into which "oil-bearing, grease-bearing or flammable wastes shall be discharged before emptying into the building drainage system or other point of disposal."
### How big the oil separator has to be
Section 1003.4.2.2 covers places "Where automobiles are serviced, greased, repaired or washed." The separator needs a capacity of not less than 6 cubic feet for the first 100 square feet of area it drains, plus 1 cubic foot for each additional 100 square feet. A listed and labeled separator can be used instead of a field design.
If it's designed to the code rather than listed, Section 1003.4.2.1 adds two dimensions. The separator must be at least 2 feet deep below the invert of the discharge drain, and the outlet must have at least an 18-inch water seal.
### What the sand side has to do
Section 1003.5 covers sand and similar interceptors for heavy solids. They "shall be designed and located so as to be provided with ready access for cleaning, and shall have a water seal of not less than 6 inches." The code gives no sizing table for the sand chamber, so the city and the design engineer decide it.
Two more rules apply to every interceptor. Section 1003.9 says interceptors must be vented so they don't become air bound. The section right after it requires access for service and maintenance, and says upkeep means "periodic removal of accumulated grease, scum, oil, or other floating substances and solids deposited in the interceptor or separator."
Here is the short version.
| IPC section | What it controls at a car wash |
|---|---|
| 1003.1 | Purpose: keep oil, grease and sand out of the sewer |
| 1003.4 | Oil separator required at car washing facilities |
| 1003.4.2.1 | Separators built to the design rules: 2 ft depth below discharge invert, 18 in outlet seal |
| 1003.4.2.2 | Capacity: 6 cu ft for first 100 sq ft drained, plus 1 cu ft per extra 100 sq ft |
| 1003.5 | For the sand side: ready access for cleaning, 6 in water seal |
| 1003.9 | Vented so the unit does not become air bound |
| 1003.10 | Access for service, and periodic removal of oil and solids |
What Valley cities add
The plumbing code covers the device. The city sewer code covers what you may discharge and how you keep the device working. That second layer is the one inspectors check after opening day.
### Phoenix
Phoenix is direct about it. Its due diligence guide for developers says "Wastewater pretreatment devices (such as interceptors for grease, oil, & sand) are required for laundries, food service establishments, vehicle repair shops, carwashes, dental offices" and other listed uses. The same guide says review of construction plans for businesses discharging process wastewater into the sanitary sewer is required.
City Code Section 28-12 is the rule behind it. It says interceptors must be installed and operated by laundries, service stations, auto repair shops, carwashes and other facilities "when the Director determines they are necessary for the proper handling of liquid wastes containing grease or oil in excessive amounts, or any flammable wastes, solids, or other harmful ingredients." That condition matters on paper. At a car wash it's almost always met, since wash water carries grit and oil by design.
Section 28-13 says interceptors "must be of a type and capacity approved by the Director," and the Department keeps a list of approved units. They must be free of obstructions and "readily and easily accessible for cleaning and inspection." If the lid sits in an active traffic area, the Director can temporarily close it to traffic to inspect the device.
Section 28-10 adds the plan review step. Where the Director requires pretreatment, the user submits plans, and "No construction of such pretreatment facilities may begin until the user receives the Director's written approval." The finished device can't go into service until the city inspects it.
Section 28-15 sets the cleaning rules. Under Section 28-15, the owner must pump out and clean a gravity interceptor often enough that the combined grease, lint, oil, sand, sludge and solids don't exceed 25 percent of its liquid volume. Outside food service, a gravity interceptor must be fully pumped and cleaned at least once every 12 months unless the Director grants a variance. A smaller hydromechanical unit is on a much shorter clock: at least once every 30 days, and never more than 90 days without maintenance. Records of every cleaning, the volume removed and where it went must stay on site for at least three years. For how that yardstick works in practice, see the quarter-full rule for grease interceptors.
### Mesa and Tempe
Mesa's plumbing plan review checklist tells designers the oil separator "shall be installed at repair garages, car washing facilities, and factories with oily and flammable liquid waste," citing IPC 1003.4. The civil sheets must "Provide required grease or sand oil interceptors and locations."
Tempe's City Code 27-22 lets its Public Works Director write rules, approved by the City Council, that require nondomestic users to install, clean, maintain, repair and allow inspection of devices such as "sand/oil interceptors, oil/water separators" at their own expense. Those rules apply where wastewater carries excessive lint, sand, FOG or other pollutants, and they set the type and capacity of each device.
Other Valley cities write their own sewer rules, and the details differ. Check with the city that serves the site before the plans are drawn.
If the wash water doesn't go to the sewer
Some sites recycle most of their water. Reclaim tanks don't remove the interceptor question, since the rinse and blowdown water still goes somewhere.
ADEQ's pollution prevention guide for car washes says that if you discharge wash water from your business, you may need a Type 3 General Wash Permit for vehicles and equipment. That permit covers wash water only. It "doesn't cover sanitary sewage discharges, vehicle lubricating oils, antifreeze, gasoline, paint, varnish, solvents, pesticides, or fertilizers." A discharge that doesn't fit the general permit needs an individual Aquifer Protection Permit.
The same ADEQ guide flags the sludge. It calls sludge the most commonly generated hazardous waste at a car wash, meaning the residue found at the bottom of "wash pits, grit traps, mud traps, and sumps." It recommends testing the sludge before disposal and using only licensed businesses to haul it. A licensed liquid waste hauler handles the pumping and disposal. That is not a plumbing job.
Scenarios we see in the Valley
The answer stays yes in almost every Arizona case; what changes is how much review and capacity you need.
New tunnel wash in Phoenix. Expect plan review of the pretreatment device, an approved unit, a sized oil separator under IPC 1003.4.2.2, a vent, and a lid where the pumper and inspector can reach it. Budget time for the Director's written approval before digging.
Converting a former gas station or auto shop. An existing separator may be reusable, but only if the city accepts its type and capacity for the new drained area. Treat it as a new review until the city says otherwise. The same thinking applies when a restaurant moves into an old retail space; see a new grease interceptor for a former retail space.
Self-serve bays added to an existing site. More drained area means more required separator capacity. Six bays drain a lot more floor than two.
Full recycle with no sewer tie. The sewer rules drop out, but ADEQ's permits and sludge rules come in.
What to do next
Start with the city, then the drawings, then the maintenance plan.
- 1Call the city's pretreatment staff early. In Phoenix, that's the Water Services Department's Industrial Pretreatment Section, listed in TRT 00385.
- 2Measure the drained area. Every bay, trench and pad that drains to the sewer counts toward separator capacity.
- 3Pick an approved unit. Choose from the city's approved list, or have the engineer design one that meets IPC 1003.4.2.
- 4Plan access and venting. Lids must be reachable, and the unit must be vented under IPC 1003.9.
- 5Set up the log. Book a licensed hauler on a schedule, and keep every manifest for three years.
Venting often trips up interceptor plans; our page on grease interceptor venting requirements walks through Chapter 9 and applies to separators too. For drains in the wash building itself, see whether commercial floor drains need a trap primer.
HQ Plumbing & Air installs and repairs sand and oil interceptors and the piping to them for Valley businesses, with free estimates and upfront pricing.