Prep and three-compartment sinks clog because they take food solids and grease all day and drain indirectly through an air gap into a floor sink. Screens and scraping keep solids out. A sink that keeps clogging after clearing points to the line behind the floor sink or the interceptor, not another quick snake.
How these sinks drain
Maricopa County's construction guide says "All three compartments should be drained indirectly to a floor sink with an air gap," and "The food preparation sink must be indirectly drained to a floor sink with an approved air gap (minimum 1 inch)." So the sink's water drops through an air gap into a floor sink and then into the building drain. A clog can sit in the sink trap, the short line to the floor sink, the floor sink trap or the line past it. Which one you have decides the fix.
What causes the repeat clogs
Phoenix's FOG manual says "FOG entering the sewer system from food preparation and dishwashing can and will harden and build up eventually leading to a blockage in the city sewer or your own line." In a kitchen that means:
- Food solids from pots and pans washed down without scraping.
- Grease that cools in the trap and line and hardens in place.
- Sinks with no strainer or a missing one.
- A full grease interceptor downstream that pushes water back.
The manual's habits are simple. "Scrape or dry-wipe excess food and grease from pots, pans, fryers, utensils, screens and mats, then dispose of it in the trash instead of putting it down the drain," and "Install screens on all kitchen drains to keep food scraps from going down the drain."
When it is the line, not the sink
If the sink clears after snaking and slows again in days or weeks, look at the line. Phoenix amended the 2024 International Plumbing Code under Ordinance G-7397 so that "where the drainage piping is upstream of a grease interceptor, the slope of the piping shall be not less than 1/4 inch per foot (2-percent slope)." A flat or sagging run holds grease. The grease line between the kitchen and the interceptor explains that stretch. Other Valley cities adopt and amend codes on their own, so check the rule for your address.
What actually fixes it
Clearing the sink, then jetting the branch and running a camera when it recurs. Jetting hot or cold works on grease; it should be done before open or after close so the kitchen keeps running. Enzyme and chemical products are not a fix. The Phoenix manual says "The use of chemicals, enzymes, proteins, bacteria, grease solvents, or emulsifiers to temporarily dissolve fats, oils, and grease is prohibited" for interceptors. We repair and inspect interceptors; a licensed pumper empties them.