A grease trap is a small hydromechanical unit, usually inside the kitchen, for kitchens with up to four grease-bearing fixtures. A grease interceptor is a large underground tank outside, at least 500 gallons in Phoenix, Tempe and Mesa. A commercial dishwasher pushes a kitchen from a trap up to an interceptor.
Grease trap (hydromechanical interceptor)
Phoenix's FOG manual calls a grease trap a hydromechanical grease interceptor: a small unit, usually installed indoors. Phoenix approves it only for food service establishments with up to four plumbing fixtures. The manual's example is a three-compartment sink, a prep sink and a mop sink all piped to one trap. Scottsdale describes traps as typically under 100 gallons, plumbed directly to the three-compartment sink.
Traps are rated by flow and grease capacity. Phoenix approves four sizes, from 20 gpm and 40 lb up to 50 gpm and 100 lb. Tempe and Mesa set their floor at 50 gpm and 100 lb.
Grease interceptor (gravity interceptor)
Interceptors are much larger, installed outdoors, and built as two or three chambers. Phoenix, Tempe and Mesa all set the smallest size at 500 gallons. Tempe caps them at 2,500 gallons. In Phoenix, a 500-gallon unit needs two manways and two compartments. Other businesses need other kinds of interceptors, such as the ones on commercial laundry drains; see whether a commercial laundry needs a lint interceptor.
What forces the bigger tank
- More than four fixtures. Phoenix and Tempe both require an interceptor past four.
- A commercial dishwasher. Tempe, Mesa and Phoenix all send dishwasher wastewater to an interceptor, never a trap.
- A food waste disposal. Phoenix requires an interceptor for any kitchen with one. Tempe does not permit garbage disposals or food grinders at all, and Mesa bars new grinders for commercial kitchens.
For Mesa, a fixture means the whole unit, so a three-compartment sink counts once.
Upkeep is different too
Phoenix wants traps fully pumped and cleaned every 30 days, never past 90, and interceptors at least every three months. Tempe uses the same monthly and quarterly split. Chandler adds that even the smallest device holds at least 25 gallons of foul liquid waste, which is why most owners hire a pumper.
What to do
Count the fixtures before you sign a lease on a former retail space, because a slab with no grease line means trenching. Our commercial dishwasher and grease trap page covers the dishwasher rule, and what size grease interceptor a restaurant needs covers the math. City-by-city rules sit in the Valley grease trap and interceptor rules table. HQ gives free estimates on commercial kitchen plumbing.
Related: what an automatic grease removal device is, when an indoor trap has to become an outdoor interceptor, and what installing an interceptor involves.
Running a smaller menu? See whether a coffee shop, bakery or bar needs a grease trap.
Can a small unit under the sink handle a three-compartment sink? See the answer.