Commercial Dishwasher and Grease Interceptor in a Small Cafe
A 40-seat cafe is remodeling a kitchen. The owner is installing a single-tank commercial dishwasher with a 1.5 in drain outlet and a 3-compartment sink with a 2 in drain. The existing kitchen has a 2 in floor drain connected to a 4 in building drain and a 20 gpm grease interceptor that was sized for the previous 2-compartment sink. The new dishwasher will be next to the sink, and the owner wants to pipe both the sink and dishwasher directly into the floor drain with a rubber coupling. The dishwasher has no built-in air gap. The local health department requires an indirect waste receptor. The grease interceptor is under the slab and has a 2 in inlet and outlet. The owner wants to avoid cutting concrete. The dishwasher final rinse is 180 F. The sink has a pre-rinse spray valve. The inspector has not yet visited.
1. How must the dishwasher and sink waste connect? Explain direct versus indirect waste and air gap/air break.
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The dishwasher must not be hard-piped into the floor drain with a rubber coupling. A commercial dishwasher is a cross-connection risk: its final rinse and wash water can be contaminated, and a direct connection can siphon or back up into the machine. It must discharge through an air gap or an air break to an indirect waste receptor, such as a floor sink, sized for the discharge and provided with a trap and vent. The 3-compartment sink can be directly connected through its trap to the grease interceptor, but the sink and dishwasher waste streams must be arranged so the receptor is not used as a grease trap. The floor drain is not a substitute for a properly sized indirect waste receptor unless it meets the receptor requirements. The inspector should see the air gap/air break, the receptor, and the trap before the concrete is closed.
2. Is the 20 gpm interceptor adequate? What sizing information is missing?
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The 20 gpm interceptor may or may not be adequate; the data given are insufficient. Sizing requires the capacity of each sink compartment (length x width x depth), the dishwasher discharge rate, and the adopted code's sizing method or the manufacturer's rating. A 3-compartment sink plus a commercial dishwasher and a pre-rinse spray often exceeds a 20 gpm unit, especially with simultaneous use. Do not size by the drain pipe size alone. Calculate the total fixture capacity, apply the code's retention/sizing table or formula, and compare with the existing interceptor's rated capacity and inlet/outlet size. If the existing unit is undersized, it will pass grease and clog the building drain. Options include a larger interceptor, a secondary unit, or an approved alternative; verify with the AHJ before cutting concrete.
3. What venting and accessibility issues does the under-slab interceptor create?
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An under-slab grease interceptor must be accessible for cleaning and inspection; if it is buried without a removable cover, that is a code violation. It must be vented per the adopted code, typically with a vent sized to the interceptor and connected to the building vent system, and it must have inlet and outlet baffles. The indirect waste receptor for the dishwasher needs a trap and a vent; the floor drain needs a trap primer if it is an emergency floor drain. A rubber coupling buried in concrete is not acceptable as a permanent connection. Provide cleanouts, a gas-tight cover, and a schedule for pumping. Also confirm that the dishwasher's 180 F rinse does not require a temperature-rated receptor and that the pre-rinse spray does not overwhelm the interceptor.