Grease Interceptor Odor and Backup at Tenant Improvement
A 90-seat restaurant tenant improvement is being built in an existing shell. The kitchen has a 3-compartment sink with a 2-in. drain, a commercial dishwasher with a 1.5-in. drain, two floor sinks, and a mop sink. All kitchen waste is routed to an existing 1,000-gal outdoor gravity grease interceptor through 75 ft of 4-in. PVC at 1/8 in./ft. The interceptor has a damaged second baffle, no vent, and is pumped every 6 weeks because of odors and backups. The dishwasher discharges directly into the 3-compartment sink drain downstream of the sink trap. The contractor wants to omit a vent and add a chemical enzyme feeder to reduce pumping. The local health department has flagged grease discharge, and the plumbing inspector has not yet approved the rough-in. The project is in New Mexico under the adopted plumbing code.
1. Identify the code and design problems with the direct dishwasher connection, the missing vent, and the damaged baffle.
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The dishwasher cannot discharge directly into the sink drain downstream of the sink trap. Dishwasher waste must pass through an air gap or air break before it connects to the drainage system, so a backup or siphon cannot pull contaminated water back toward the dishwasher. The sink and dishwasher also need their own traps and venting arranged so each fixture is protected.\n\nA grease interceptor must be vented as required by the adopted code. Omitting the vent traps odors and can cause sluggish flow or gas migration. The damaged second baffle defeats the interceptor's retention function because grease and solids short-circuit toward the outlet. An enzyme feeder is not a substitute for a sound baffle, proper venting, and correct maintenance.\n\nThe contractor should repair or replace the baffle, restore the vent, correct the dishwasher connection through an air gap or air break, and submit a revised plan before the rough-in is approved. The health department and plumbing inspector both have an interest in the grease discharge and fixture connections.
2. Estimate whether the 1,000-gal interceptor is adequate for the proposed fixtures. What information would you require?
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Volume alone does not prove adequacy. A 1,000-gal outdoor gravity interceptor may be reasonable for a small restaurant kitchen, but approval requires the adopted sizing method. I would require the fixture capacity and discharge rate for the 3-compartment sink, the dishwasher's gallons per cycle and cycle time, the mop sink and floor-sink loads, the peak meal-period flow, and the required retention time. I would also confirm the local health department's grease-interceptor sizing rule and any New Mexico amendment.\n\nThen compare the calculated peak flow and retention volume with the interceptor's actual liquid capacity after sludge and grease storage are accounted for. A 1,000-gal tank that is undersized or short-circuited by a broken baffle will back up and odor regardless of pumping frequency. If the sizing calculation shows adequate volume, the immediate problem is maintenance and baffling, not necessarily tank size. If it shows inadequate volume, the interceptor must be enlarged or the discharge must be reduced. Do not accept an enzyme feeder as the sole correction.
3. Describe the venting, air-gap, and inspection requirements you would enforce before approving the kitchen rough-in.
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Each fixture must be trapped and vented, and the grease interceptor must have the vent required by the adopted code. The interceptor vent should be sized and terminated where code allows, not inside the building. The dishwasher must discharge through a properly installed air gap or air break above the flood-level rim of the receptor; a direct downstream connection is not acceptable. The 3-compartment sink, mop sink, and floor sinks need approved air gaps or air breaks where required and must be individually protected from backflow and sewer gas.\n\nBefore approval, inspect the interceptor for watertightness, intact inlet and outlet baffles, correct inlet/outlet arrangement, and a proper flow-control fitting if one is required. Verify the 4-in. line has the required slope and cleanouts, and confirm the interceptor is accessible for pumping. Perform the required DWV test and document the air-gap and vent corrections. The health department's grease-discharge approval should be obtained along with the plumbing inspection.