Why does the floor sink back up when the dish machine runs?

A floor sink is the open end of an indirect drain on purpose. Maricopa County's construction guide says "Indirect waste lines require a minimum 1-inch air gap above the flood level of an approved floor sink or drain." That gap keeps sewage from reaching the machine, and it also means any restriction downstream has nowhere to go but up and out of the receptor. The FDA Food Code annex states the goal: "The drainage system must be designed and installed properly to prevent the backup of sewage and the possible contamination of foods or food-contact surfaces in the establishment."

1. 1The receptor itself. Pull the basket or strainer and look at the sink body. Food solids, labels and lime scale pile up here first. Maricopa's guide says to "Design and size drain lines & receptors to prevent overflows and splashing," so an undersized or half-blocked receptor can overflow even when the pipe behind it is fine.
2. 2The indirect line. The guide says "Indirect drain lines should not exceed 10 linear feet to an approved floor sink or drain" and that "each fixture will require a separate rigid drain line." A long run, a sag or a shared line can leave the burst with too little room.
3. 3The pattern. If only that floor sink spills and it drains normally between cycles, look at the sink's own trap and line. If the prep sink, other floor drains or the hand sink rise too, the restriction is farther down: the branch line, the interceptor inlet or the building drain.
4. 4The interceptor. Phoenix's FOG manual says dishwashers must go to a gravity interceptor: it is required for "any food service establishment with a food waste disposal unit (garbage disposal) or dishwasher regardless of the number of fixtures." A full or damaged interceptor pushes water back toward the kitchen. Floor sinks backing up from the interceptor covers that case.

Do not remove the air gap, seal the drain line into the floor sink or run the machine into a bucket workaround that becomes permanent. Maricopa's guide is direct: "A direct connection may not exist between the sewerage system and any drains originating from equipment in which food, portable equipment, or utensils are placed, except if otherwise required by the applicable plumbing code." Skip enzyme or chemical products too; they do not fix a restriction, and Phoenix prohibits using them to dissolve grease in an interceptor.

The usual order is the receptor trap, then the branch line with a cable or a jet, then a camera if it comes back. For a kitchen that cannot close, we schedule line jetting before open or after close. We inspect and repair grease interceptors, but a licensed hauler does the pumping. See commercial kitchen drain overflowing for the grease side, and call if it spills during service. Emergency calls are answered 24/7, and you get the price up front before work starts.