Grease Interceptor Sizing for a Restaurant Tenant Build-Out
A restaurant tenant is building out a 2,400 sq ft space with a 90-seat dining room and a full commercial kitchen. Fixtures draining to the grease interceptor include a three-compartment sink (each compartment 18 in x 24 in x 12 in deep), a pre-rinse sink, a hand sink, and a floor drain in front of the dish machine. The dish machine is a single-tank conveyor rated at 60 gallons per hour with a 40-gallon fill. The building's existing exterior interceptor is a 750-gallon concrete unit shared with a neighboring tenant. The contractor proposes connecting all kitchen fixtures to the existing unit without changes. The local code requires a fixture-unit-based or retention-time-based sizing method and a minimum 30-minute retention at peak.
1. Evaluate whether the existing 750-gallon interceptor is likely adequate and describe the sizing logic you would apply.
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Start with the compartment sink capacity: 18 x 24 x 12 in = 5,184 cubic inches = 22.5 gallons per compartment. A three-compartment sink is typically sized on the largest compartment filled to the overflow, so about 22.5 gallons, and many codes apply a factor for the number of compartments and a drain-down time. The dish machine adds its 40-gallon fill and 60 gph rated flow. The pre-rinse sink adds its own capacity. Using the common retention-time method, required volume = total fixture discharge during a peak period x 30 minutes retention. If the combined kitchen discharge at peak is, say, 25 to 30 gpm, a 30-minute retention requires 750 to 900 gallons - the existing 750-gallon unit is at or below the threshold and is shared with another tenant, so it is likely undersized. The correct answer is to compute the peak discharge from all connected fixtures, apply the code's sizing formula, and compare to the existing unit; sharing with a neighbor further erodes capacity. Recommend a dedicated properly sized interceptor or a documented variance, not a straight connection.
2. Identify the code and operational issues with connecting the hand sink and the floor drain to the interceptor.
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Hand sinks are not typically required to drain to a grease interceptor and in many codes are prohibited from doing so because the interceptor is a grease-laden waste system and a hand sink is a clean-waste fixture; routing it there wastes capacity and can create odor and backup problems. A floor drain in front of the dish machine may receive grease-laden spillage and is often allowed or required to connect, but its trap and the interceptor must be vented properly. Also check that the interceptor is vented, that the outlet is protected against grease carryover, and that the fixture connections use the required air gaps or air breaks where the code requires them (for example, a pre-rinse sink with a spray valve needs an air gap or vacuum breaker). Finally, confirm the interceptor is accessible for pumping and that the tenant has a maintenance schedule, since an undersized or unmaintained unit will fail inspection regardless of sizing.