Plan Review: Grease Interceptor and Fixture Count
A 90-seat restaurant submits plans with a 1,000-gallon exterior concrete grease interceptor, a 2-inch kitchen drain, and a 3-compartment sink with a 1-1/2-inch indirect waste. The plan shows the interceptor 60 feet from the building, with a 2-inch line at 1/8 inch per foot, two cleanouts, and a vent at the interceptor. The fixture-unit count on the plan totals 46 for the whole building. The designer cites the seat count to justify the interceptor size and states that the 2-inch line is adequate because the sink is the only fixture discharging to it. The jurisdiction requires interceptor sizing by the uniform plumbing code method using fixture units and drainage load.
1. Review the interceptor sizing and the drain line. What is wrong, and what would you require?
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Interceptor sizing. Sizing by seat count alone is not the required method here; the jurisdiction requires the fixture-unit/drainage-load method. That method uses the total drainage fixture units discharging to the interceptor, the drainage load in gallons per minute, and a retention time, then applies a safety factor. A 1,000-gallon interceptor may be adequate or may be grossly oversized, but the plan does not demonstrate it. The correct response is to require a sizing calculation showing fixture units, gpm, retention time, and the code's required capacity, and to confirm the interceptor is listed and accessible for pumping.
The 2-inch line at 1/8 inch per foot. A 2-inch drain at 1/8 inch per foot is at the minimum slope and is acceptable only if the load is within the capacity of a 2-inch line at that slope. The stated 46 fixture units for the building is not the same as the load on this branch; the answer must separate total building load from the load on the interceptor branch. If the branch carries only the 3-compartment sink, 2-inch may be fine - but the plan must show the branch load, not the building total.
Distance and cleanouts. 60 feet with two cleanouts is long; cleanouts must be at the interceptor inlet and outlet and at changes of direction, and the line must be accessible for rodding. A vent at the interceptor is required and is shown - that part is correct.
2. The 3-compartment sink discharges through a 1-1/2-inch indirect waste. Is this acceptable?
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The indirect waste is the right idea - a sink in a food area should not be directly connected to the drainage system - but the details must be checked:
- Air gap vs. air break. An indirect waste must terminate with an air gap or air break above the receptor. A 1-1/2-inch indirect waste into a floor sink or receptor must have the correct air gap (typically twice the pipe diameter, and not less than 1 inch) above the flood-level rim of the receptor.
- Receptor sizing. The receptor must be sized for the discharge rate of the sink compartments and must be trapped and vented. A 1-1/2-inch indirect waste from a 3-compartment sink may exceed the capacity of a small floor sink.
- Compartment discharge. The compartments must drain separately or through a common outlet sized for the combined flow; the sink must not discharge to the interceptor through a line smaller than required.
- Grease-laden waste only. The interceptor must receive only the grease-laden waste. If the plan routes other fixtures to it, the sizing and the required retention time change.
The correct answer approves the indirect-waste concept, requires the air gap to be dimensioned on the plan, requires the receptor to be sized and vented, and requires the interceptor sizing calculation before approval.