Commercial Kitchen Grease Interceptor and Indirect Waste Review
A restaurant is remodeling its kitchen. The existing 20-gpm/40-pound grease interceptor serves a 3-compartment sink and a hand sink. The new layout adds a 2-compartment prep sink, a dishwasher, a mop sink, and two floor drains. The contractor proposes to connect the dishwasher directly to the grease interceptor with a 2-inch line, run the prep sink and floor drains to the interceptor, and keep the hand sink on the sanitary drain. The interceptor is in a 30-inch crawlspace with no vent; its outlet discharges to a 4-inch building drain. The plan shows the 3-compartment sink with an indirect waste to a floor sink, but the dishwasher has no air gap. The local code requires a grease interceptor sized by fixture capacity and prohibits direct connection of dishwashers. The inspector has not yet approved the plan.
1. Decide which fixtures must discharge to the grease interceptor and which should not, and explain the basis.
Read the worked answer
The grease interceptor should receive grease-laden waste from the 3-compartment sink, the 2-compartment prep sink, and the kitchen floor drains if they receive grease-bearing water. The dishwasher is a grease-laden fixture, but the local code prohibits a direct connection; it must discharge through an air gap or air break to an indirect waste receptor that then routes to the interceptor. The hand sink is a handwashing fixture and generally should not discharge to the grease interceptor; keeping it on the sanitary drain is correct. The mop sink is normally not a grease interceptor fixture unless the local code specifically requires it, because mop water may contain cleaning chemicals and solids that complicate grease separation. The key is to trace each fixture's waste: grease and food solids go to the interceptor, while hand sinks and possibly mop sinks stay on the sanitary system. The plan must show the indirect waste receptor and the air gap, not just a line labeled "to grease trap."
2. Evaluate the proposed 20-gpm/40-pound interceptor, its lack of venting, and its crawlspace location.
Read the worked answer
The existing 20-gpm/40-pound unit was sized for a 3-compartment sink and a hand sink. Adding a prep sink, a dishwasher, and floor drains increases the fixture capacity discharging to the interceptor, so the unit is likely undersized. The local code requires sizing by fixture capacity, so the contractor must recalculate using the capacities of all fixtures discharging to the interceptor and select a unit with adequate grease-retention capacity and flow rate. The interceptor must be vented; a grease interceptor needs a vent to relieve pressure and allow proper separation, and the absence of a vent in the crawlspace is a defect. The unit must also be accessible for cleaning and inspection. A 30-inch crawlspace may not provide the required clearance for the inlet, outlet, flow-control fitting, and vent. If the interceptor cannot be properly vented and accessed, it should be relocated or replaced with an accessible unit.
3. Identify the indirect-waste and plan-review corrections needed before approval.
Read the worked answer
The dishwasher must not be directly connected to the grease interceptor. Provide an air gap or air break discharging to a floor sink or other approved indirect waste receptor, and route that receptor to the interceptor if the local code requires grease-laden dishwasher waste to be intercepted. The 3-compartment sink already has an indirect waste to a floor sink; verify that the floor sink is properly trapped, vented, and sized for the discharge. The grease interceptor needs a flow-control fitting on the inlet, a vent, and access covers. The plan should show the inlet and outlet elevations, the vent connection, the cleanout locations, and the fixture capacities used for sizing. The hand sink should remain on the sanitary drain and should not be rerouted to the interceptor. Before approval, submit the revised isometric or plan with the interceptor sizing calculation, the air-gap detail, and the vent and access locations so the inspector can verify the design against the local code.