Can auto shop floor drains connect to the sewer?

The 2024 International Plumbing Code (IPC) is the base code in Phoenix, Mesa, Chandler and Tempe, and Phoenix also adopted the 2024 Uniform Plumbing Code. Chapter 10 covers interceptors and separators.

Section 1003.4 of the IPC is the rule for shops. It requires oil separators "At repair garages where floor or trench drains are provided," and at car washes, factories with oily or flammable liquid waste, and hydraulic elevator pits. Oil-bearing, grease-bearing or flammable wastes go into the separator "before emptying into the building drainage system or other point of disposal."

Note the trigger. It's the drain, not the business license. A repair garage with no floor or trench drains, where spills are cleaned up dry, doesn't trip this section.

### How big the separator has to be

Section 1003.4.2.2 sizes the separator "Where automobiles are serviced, greased, repaired or washed or where gasoline is dispensed." The minimum is 6 cubic feet for the first 100 square feet of floor that drains to it, plus 1 cubic foot for each additional 100 square feet. A separator designed to the code also needs a depth of at least 2 feet below the invert of the discharge drain and an 18-inch water seal at the outlet. A listed and labeled separator can be used instead.

The same section carves out two areas. Parking garages where no servicing, repair or washing happens and no gasoline is dispensed "shall not require a separator." And areas of commercial garages used only for storing cars don't have to drain through one.

Here's how that plays out by area:

This is where shops get into real trouble. EPA's Underground Injection Control program regulates what it calls motor vehicle waste disposal wells. EPA describes a typical one as "floor drains or sinks in service bays that connect to a septic system or dry well." It adds that "any underground system that receives motor vehicle waste is considered to be a motor vehicle waste disposal well," including cesspools and catch basins.

The federal rule, 40 CFR 144.81, lists these as Class V wells that receive fluids "from vehicular repair or maintenance activities, such as an auto body repair shop, automotive repair shop, new and used car dealership, specialty repair shop." It gives the reason: the fluids may contain chemicals above drinking water limits, waste petroleum products, heavy metals and volatile organic compounds.

EPA's summary lists when these wells are banned. The first line applies everywhere: "Nationwide if the well was constructed after April 5, 2000." Older wells are banned in groundwater protection areas and other sensitive groundwater areas, and in states that have opted to ban them entirely. A state may waive the ban with a permit, but the permit requires the wastewater to meet drinking water standards at the point of injection.

Arizona now runs this program itself. The Federal Register notice approving the state's primacy says the change lets the Arizona Department of Environmental Quality (ADEQ) authorize underground injection for all wells regulated under the Safe Drinking Water Act in the state. It took effect October 15, 2025. EPA remains the permitting authority on Indian lands.

### How to tell where your drains go

EPA's own checklist asks whether all of your floor drains and sinks are connected to a municipal sewer. If the answer is yes, EPA's answer is "You are not affected by the rule." The same answer applies when every drain goes to a holding tank whose waste is disposed of off site, though EPA notes other state or federal disposal rules may still apply. If the drains go to a septic system or dry well, you may have a regulated well.

Old buildings make this harder than it sounds. EPA warns that building plans "may not necessarily reflect the actual construction of your wastewater system," and that renovations may not show up. It suggests that a plumber using dye or smoke tests may be able to help you find where your drain goes. A camera inspection does the same job from inside the pipe.

If you do find a shop drain going to a dry well, EPA lists a sewer hookup as one way out: "Contact the local sewer authority about the possibility of connecting floor drains to the sewer system." Closing the well starts with a letter. Write to your permitting authority at least 30 days before you close the well, then plug or close it the way that authority approves. In Arizona today, that authority is ADEQ.

The plumbing code says a separator is needed. The city sewer code says what may leave it.

Phoenix. City Code 28-12 says interceptors must be installed and operated by 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." Section 28-8 bans sending the sewer petroleum oil or products of mineral oil origin that can cause interference, pass-through or obstruction. It also bars pollutants with a closed cup flashpoint below 140 degrees Fahrenheit, which rules out gasoline and many solvents.

Phoenix also sets the upkeep. Under Section 28-13, the interceptor must be of "a type and capacity approved by the Director." Section 28-15 makes the owner pump and clean a gravity interceptor before combined oil, sand, sludge and solids exceed 25 percent of its liquid volume, and at least once every 12 months unless the Director grants a variance. The owner keeps records of each cleaning, the volume removed and the disposal for at least three years.

Tempe. City Code 27-22 lets the Public Works Director set rules, approved by the City Council, that require nondomestic users to install, clean, maintain, repair and allow inspection of "sand/oil interceptors, oil/water separators" and other pretreatment devices at their own expense.

Pumping the separator is a job for a licensed liquid waste hauler, not a plumber.

These four cases cover most of what we see in Valley shops.

Leasing a bay in an older strip building. Find out where the floor drain goes before you sign. If there's no separator on the line to the sewer, expect the city to ask for one during tenant improvement review.

A rural or county shop on septic. Service bay drains can't go to the septic system. EPA's alternatives include a service bay wastewater holding tank that is pumped out for proper disposal, or a sewer hookup where one is available.

Adding bays to an existing shop. More drained floor means more separator capacity under IPC 1003.4.2.2. Check the old unit against the new total area.

A dealership with a wash bay. IPC 1003.4 lists car washing facilities too, so wash bay drains also pass through a separator. Size it for the full drained floor.

For the next step, use this order:

1. 1Trace every drain. Dye test or camera the lines from each bay.
2. 2Size the separator. Measure the drained floor area and apply 1003.4.2.2, or pick a listed unit.
3. 3Get city approval. Submit the pretreatment device for review before construction.
4. 4Set up records. Schedule a licensed hauler and keep the paperwork for three years.

Related reading: a trap primer for commercial floor drains, floor sinks vs floor drains vs mop sinks, and whether a car wash needs a sand and oil interceptor. If you're buying the building, a sewer camera inspection before buying or leasing shows where the lines really run.

HQ Plumbing & Air installs and repairs oil separators, floor drains and the piping to the sewer, and handles camera inspection and hydro jetting, with free estimates and upfront pricing.