Restaurant Grease Interceptor Bypass and Dishwasher Connection
An 80-seat restaurant is being renovated. The new kitchen has a three-compartment sink, a pre-rinse spray valve, a conveyor-type dishwasher, and four floor drains. The contractor has connected the sink and dishwasher to a new 3 in. branch that runs directly to the building drain downstream of an existing 500 gal outdoor grease interceptor. The interceptor vent pipe was capped during paving, and the dishwasher has a direct connection with no air gap or air break. Within a month, grease odors are reported in the dining room, the interceptor is pumping out every two weeks, and floor drains in the dish area back up. The owner wants to know whether the interceptor is too small or whether the piping is wrong. A plumbing inspector has been asked to review the installation.
1. Identify the code issues in the connections described and explain which wastes must be routed through the grease interceptor.
Read the worked answer
Grease-laden waste from the three-compartment sink, pre-rinse spray, and dish area must be routed to the grease interceptor before the building drain. The dishwasher connection as described is wrong: dishwashers typically require indirect waste through an air gap or air break, not a direct connection, to prevent backflow and grease backup into the machine. The capped interceptor vent is a serious defect: without venting, the interceptor traps air, slows flow, causes odors, and can siphon or back up. Floor drains in the dish area may also receive grease and should be evaluated; they need traps and trap primers if subject to evaporation. The direct branch downstream of the interceptor bypasses treatment for the sink and dishwasher, sending grease to the building drain and explaining recurring pump-outs and odors. The primary problem is piping and venting, not simply interceptor capacity.
2. Evaluate the interceptor size and venting, and outline the correction.
Read the worked answer
Interceptor sizing is based on fixture flow and grease load, not seating alone. A 500 gal outdoor interceptor may be adequate for a small restaurant, but it cannot be judged while the sink and dishwasher bypass it and the vent is capped. Restore the vent to a proper termination; route all grease-laden waste through the interceptor; install an air gap or air break for the dishwasher; provide accessible covers, baffles, and an outlet tee; and check for a solids trap. If the interceptor is undersized after calculation, add capacity or a second unit. Establish a pump-out schedule based on the grease cap and the 25% rule, and keep records. Do not use enzymes as a substitute for proper sizing and venting. The correction must address both the bypass and the vent before resizing is decided.
3. What sequence of tests and inspections should be required before the restaurant opens?
Read the worked answer
Sequence: isolate and correct the piping; restore and test the vent; install the air gap or air break; clean and inspect the interceptor; test flow from each fixture and check for backups; verify trap primers on floor drains; conduct a final inspection with access covers open; and document the pump-out and maintenance schedule. Before opening, confirm there is no cross-connection between potable water and grease waste, and check the dishwasher final rinse temperature. The inspector should witness a flow test and verify venting and the air gap. If the interceptor is replaced or resized, verify the inlet and outlet elevations and baffles. The judgement point is that odors and frequent pump-outs can be caused by a bypass and unvented interceptor even when the tank volume is adequate.