Restaurant Kitchen Drainage and Grease Interceptor Review
A 90-seat restaurant is being built in an existing suburban shell. The kitchen will have a three-compartment sink, a pre-rinse sink with a spray hose, a conveyor dish machine, a mop sink, two floor drains, and a hand sink. The tenant's plumber proposes to connect the dish machine drain directly to a new exterior grease interceptor, run the three-compartment sink through a 2-inch trap to the same interceptor, and terminate the interceptor vent as a 2-inch pipe stubbed 6 inches above the roof surface. The spray hose at the pre-rinse sink hangs below the sink rim. The general contractor wants to omit the dish-area floor drain to save slope and to connect the mop sink directly to the sanitary drain without a trap primer. The local health department has not yet inspected. The building official asks you to review the proposed kitchen drainage before rough-in approval. No plumbing fixtures are installed yet.
1. Identify the cross-connection and indirect-waste issues at the dish machine and pre-rinse sink, and state the corrections you would require before rough-in.
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Separate the potable-supply cross-connection from the drainage issue. The pre-rinse spray hose hangs below the sink rim, so the hose bibb needs an approved backflow preventer, such as a vacuum breaker, or the hose must be relocated so the outlet stays above the flood-level rim with an air gap. Do not rely on the user to keep the hose raised.
The dish machine should not be directly connected to the grease interceptor if the code requires indirect waste for dishwashing equipment. Require an air gap or air break discharging to a trapped and vented receptor, sized for the machine's discharge, before the waste reaches the interceptor. A direct connection can allow sewage or grease-laden waste to back up into the machine. The interceptor is for grease-laden waste; it is not a substitute for indirect-waste protection. Verify local health-department requirements because some jurisdictions require the dish machine to bypass the interceptor or use separate pretreatment.
2. Evaluate the proposed grease interceptor connections and venting, including whether all the listed fixtures must discharge to it.
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Interceptor sizing is not established by a 1,000-gallon label alone; it must match the fixtures and the local pretreatment rules. The three-compartment sink and pre-rinse sink are the usual grease-laden contributors. The dish machine may be excluded or separately treated, depending on local health and pretreatment requirements, and floor drains may or may not be directed to the interceptor. Sending every kitchen drain to one interceptor can overload it, create odors, and make maintenance difficult.
The interceptor must be accessible for cleaning and properly vented. A 2-inch vent stubbed 6 inches above the roof may meet a minimum termination height, but the vent must still be sized for the connected drainage and installed so it cannot be blocked by roof features. Confirm that the vent is not serving as the only vent for fixtures that require a separate vent. Require the interceptor to be tested for watertightness and inspected before backfill.
3. The contractor proposes no dish-area floor drain and no trap primer at the mop sink. How do you evaluate those omissions, and what inspection sequence do you require?
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A missing dish-area floor drain is not automatically a code violation unless the code or health department requires a floor drain in that location. The judgment call is whether washdown, spills, or equipment leaks will create a sanitary nuisance. If no floor drain is installed, require written health-department approval and a cleanable route for spills.
The mop sink does not normally need a trap primer because it is used regularly; trap primers are for traps subject to evaporation. However, the mop sink hose bibb needs backflow protection if a hose can reach below the rim. For any floor drain that is installed, provide a trap-seal primer or other approved means if the trap can evaporate.
Inspection sequence: rough-in inspection after drainage and venting are complete but before concealment; a water or air test of the drainage system; final inspection after fixtures, backflow devices, and the interceptor are installed and accessible.