The line from the kitchen to the interceptor stops up when grease cools and hardens on the pipe wall before reaching the interceptor, most often in flat, sagging or long runs. Phoenix's amended code requires at least 1/4 inch per foot of slope upstream of an interceptor, so slope, a full interceptor and kitchen habits are the first suspects.
What happens in that line
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." Warm, greasy water from sinks and the dish machine cools as it travels. Grease sticks to the pipe wall, the opening narrows and solids catch on it. Eventually the line stops and the kitchen backs up.
Slope is a code issue here
Phoenix amended the 2024 International Plumbing Code under Ordinance G-7397 (effective August 1, 2025). Section 704.1 now says "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 slows water, which gives grease more time to cool and stick. Other Valley cities adopt and amend codes on their own, so ask the city for the rule at your address.
Other causes
- A full interceptor. The manual says "Most types of interceptors stop working effectively when fats, oils, and solids reach 25% of the device's capacity." A full interceptor can back water into the line behind it. Why the interceptor keeps overflowing covers it.
- Habits. The manual asks kitchens to "Scrape or dry-wipe excess food and grease from pots, pans, fryers, utensils, screens and mats" and to "Install screens on all kitchen drains."
- Hot-water flushing. It moves grease down the line without removing it. Grease in the sewer lateral after the interceptor explains where it goes.
- Long runs and bends that give grease more places to catch.
What fixes it
A snake bores a hole through grease and leaves the wall coated, so grease lines usually need jetting. Hydro jetting strips the pipe wall, and a camera afterward shows whether a sag or offset is the reason it keeps returning. Hot water jetting for grease lines compares methods. A pipe that holds water because of a sag is a repair, not a cleaning.
We jet restaurant lines before open or after close, and we repair and inspect interceptors. A licensed pumper empties them, and the manual says "Cleaning must be performed by a licensed pumper." Get a free estimate if the line keeps stopping.