Grease Interceptor and Indirect Waste at a Bakery Conversion
A 1,800-square-foot bakery is being converted from a retail space. The new plumbing includes a three-compartment sink, a prep sink, a hand sink, a mop sink, a floor drain at the walk-in cooler, an ice machine, and a coffee brewer. The existing 4-inch building drain runs at 1/4 inch per foot to the municipal sewer, and there is no grease interceptor. The owner proposes a 1,000-gallon exterior concrete interceptor 12 feet from the building and 3 feet below the floor slab; the fixture branch would run 18 feet at 1/4 inch per foot. A contractor suggests a smaller 50-gpm hydromechanical unit under the three-compartment sink instead. The health department requires an air gap for the ice machine and a properly vented interceptor. The building has a 4-inch vent stack at the rear wall.
1. Which fixtures should connect to the grease interceptor, and which should be indirect wastes?
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Fixtures that carry grease or food solids - the three-compartment sink, prep sink, and the floor drain at the walk-in cooler if it receives food waste or cleaning water - should go to the grease interceptor. Hand sinks generally do not require a grease interceptor because they carry only handwashing water, though local rules may require all kitchen fixtures. The mop sink is a judgement call: if mop water contains grease or food solids, connect it; if it is only floor-cleaning water, many codes let it bypass, but the local health department may require it. The ice machine and coffee brewer must not drain to the grease interceptor. They are indirect wastes: the ice machine should discharge through an air gap or air break to a floor sink or receptor, with a 1-inch air gap or twice the drain diameter, and the coffee brewer likewise. Indirect waste protects the potable supply and keeps clear water out of the interceptor.
2. Is the proposed exterior interceptor reachable by gravity at the proposed slope? Show the calculation.
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The 1,000-gallon exterior interceptor is not reachable by gravity as proposed. An 18-foot branch at 1/4 inch per foot falls only 18 × 0.25 = 4.5 inches. The interceptor inlet is 3 feet (36 inches) below the slab. To reach it by gravity, the pipe would need about 2 inches per foot, which is far beyond a normal drain slope and would violate the fixture drain slope. Raising the interceptor, relocating it closer, or pumping are the only realistic options. A 1,000-gallon concrete unit may also be oversized for the fixture load; sizing should follow the local formula based on fixture capacity and retention time, not a guess. The interceptor must be vented and accessible for pumping.
3. Can a 50-gpm hydromechanical unit under the sink be used instead? What conditions apply?
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A 50-gpm hydromechanical unit under the three-compartment sink can be acceptable only if it is listed for the flow and grease load, approved by the local authority, accessible for cleaning, and vented. It must be installed so the sink compartment outlets and the unit are not subject to backflow, and the unit needs a vent to prevent trap seal loss. It cannot be installed where it will be concealed or where the cover cannot be removed. It may not handle the floor drain or prep sink unless it is sized for all connected fixtures. The under-sink option is often the least expensive for a small bakery, but it is not a substitute for proper sizing; if the local code requires a central interceptor for a 1,800-square-foot food establishment, the smaller unit may be rejected.