Bakery Grease Interceptor: Two Authorities, One Shared Tank
A 1,600-square-foot bakery-café is taking over a former retail bay in a strip center. The landlord's 1,500-gallon exterior grease interceptor sits 60 feet away across the parking lot and is shared with a nail salon two bays over; the lease prohibits cutting the slab. The café will install a three-compartment sink with 1-1/2-inch outlets and 14-by-14-by-12-inch compartments, a hand sink, a mop sink, two floor sinks, and a conveyor dish machine rated at 44 gallons per hour. The city pretreatment coordinator says a grease interceptor is required. The plumbing inspector says the plumbing code does not require one for this fixture count, but whatever is installed must be listed, vented, and accessible for cleaning. The clear space under the sink is 22 inches high by 30 inches wide, and the tenant wants to dump all three sink compartments at once into a point-of-use unit.
1. The plumbing inspector and the pretreatment coordinator disagree. Which requirement controls, and can the café discharge into the landlord's shared interceptor?
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Both control, and the stricter one wins. The shared tank is the wrong answer.
The plumbing code sets a construction minimum for the building. The publicly owned treatment works sewer use ordinance is a separate legal authority that sets pretreatment limits on what enters the collection system. Where they differ, compliance with the stricter requirement satisfies both, so the pretreatment coordinator's determination governs the grease question and the inspector still governs venting, trapping, and access. Get the coordinator's sizing basis in writing before ordering equipment.
On the shared tank:
- Nail salon waste, acrylic powder, hair, acetone, and solvents, is not kitchen fats, oils, and grease. Those materials foul and emulsify grease, harm the tank's separation, and are commonly prohibited discharges. The café would inherit the salon's violations.
- Adding the bakery load requires a hydraulic recalculation of the interceptor and written landlord and authority approval; the lease alone is not approval.
- 60 feet of underslab or exterior run, inaccessible for cleaning, means the café cannot maintain its share.
Recommend a dedicated, accessible interceptor sized for the café, and confirm with both authorities in writing.
2. Size the point-of-use option and explain what the 22-inch clearance and the dump-all-three-compartments request mean for the design.
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Compute the dump, then let the flow control, not the tenant, set the discharge rate.
Compartment volume: 14 x 14 x 12 = 2,352 cubic inches; divide by 231 = 10.2 gallons per compartment, about 30.6 gallons for three. At a practical 0.75 fill factor, roughly 23 gallons can leave the sink before the compartments refill.
That volume over a one-minute drain period is near 23 gpm peak if all three outlets dump simultaneously, far above the 10 to 20 gpm rating of a typical under-sink hydromechanical unit. The dish machine contributes only 44 gph, about 0.7 gpm, so the sink dominates the sizing.
The design consequences:
- A hydromechanical interceptor needs a flow control fitting on the inlet matched to its rated capacity; the tenant must drain compartments one at a time, and the sink must be labeled accordingly.
- 22 inches of height limits you to a shallow unit, which reduces retention volume, so confirm the manufacturer's gpm rating and grease-retention performance rather than assuming the largest unit that fits is adequate.
- The unit must be removable or serviceable in that 30-inch space; if the cover cannot come off for cleaning, the installation fails the accessibility requirement regardless of rating.
If the calculated peak cannot be controlled, the answer is an exterior gravity interceptor, not a larger unrated box.
3. Describe the drainage arrangement from sink to interceptor to sewer, including the indirect waste and venting requirements.
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Indirect waste first, then a trapped and vented interceptor.
- A three-compartment sink used for washing utensils and food equipment must discharge through an air gap or air break, typically into a floor sink or a funnel, not hard-piped into the drainage system. That protects the sink from backup and sewage from the sink.
- The floor sink or receptor needs its own trap and vent; the interceptor must not serve as the trap for other fixtures.
- The interceptor itself must be vented on the outlet side, with the vent sized to the downstream drain and taken up so it cannot be blocked by the cover or by grease.
- The outlet discharges to the building drain by gravity; check the developed length and slope from the under-sink location to the sewer, and confirm the interceptor's outlet elevation is above the drain invert.
- Keep the mop sink question open: mop water carries grit and cleaning chemicals, and many pretreatment programs route it to sanitary rather than through the interceptor. Confirm with the coordinator and record the answer.
- Provide a sample port or access point if the pretreatment program requires sampling, and label the interceptor with its rated capacity.