A grease trap takes wash water from grease-bearing fixtures: three-compartment and prep sinks, floor sinks and drains, mop sinks. Hand sinks, condensate and cooler drain lines stay off it. Food scraps, oil, boiling water and, in most Valley cities, dishwashers and garbage disposals do not belong.
What connects to it
Phoenix defines "grease bearing fixtures" as floor sinks, floor drains, mop sinks, trough sinks and drains, pre-wash sinks, prep sinks, pot sinks, can wash sinks, dishwashers and garbage disposals in the kitchen prep or wash area. Its plan review wants every one of them routed to a properly sized trap or interceptor. Tempe's inspectors check that three-compartment sinks, floor sinks, mop sinks, floor drains and dishwashers are plumbed to the device.
Keep in mind the device type limits the list. A hydromechanical trap in Phoenix takes only kitchens with up to four fixtures, and no dishwasher or disposal may discharge to it.
What stays off it
- Hand sinks. Phoenix says hand wash sinks do not count as grease bearing fixtures.
- Condensate and cooler drain lines. Phoenix says they are not routed to the grease waste line.
- Garbage disposals. Tempe does not permit garbage disposals or food grinders. Chandler cites the plumbing code: a food waste disposer cannot discharge to a FOG treatment device. Phoenix routes them to an interceptor instead, which is a different rule.
- Very hot or acidic water. Scottsdale bars wastewater above 140 degrees F or with a pH under 5.0 from going into an interceptor.
- Enzymes, solvents and emulsifiers. Phoenix prohibits using them to dissolve grease in the device, and Chandler says the same about additives.
What should never be poured in the drain at all
Food scraps and fryer oil. Phoenix asks kitchens to scrape pots and pans into the trash, put screens on all drains, and recycle used oil. See yellow grease vs brown grease.
What to do
Walk the kitchen and list every drain. Any drain that sees food or grease water goes on the grease line. Any drain that carries only cooler or condensate water does not. If you are not sure, a plumber can trace the lines and confirm before the inspector does. Our floor sink vs floor drain vs mop sink guide helps you identify each, and the dishwasher rule covers the biggest exception.
Garbage disposals are their own question, and the answer changes by city: see whether restaurant disposals are allowed in the Valley.