New Restaurant Grease Interceptor - Odour, Trap Loss and a Municipal Notice
During the renovation of a 120-seat restaurant, the owner installed a 2,000 L outdoor gravity grease interceptor 9 m from a three-compartment sink, a pre-rinse spray and a conveyor dishwasher. The interceptor is connected to the building with 75 mm pipe laid at 1/8 in/ft; no flow-control fitting was installed. A 50 mm vent from the interceptor runs up the outside wall and terminates 0.9 m below a rooftop make-up air intake and level with the kitchen windows. Four floor drains and a mop sink connect to the same branch. Since opening, staff report sewer odour in the kitchen, gurgling at the mop sink and visible water-level drops in the floor-drain traps overnight. The municipality has found grease in the downstream manhole and issued a notice requiring corrective action. The interceptor is pumped monthly, but the hauler reports only a thin grease mat and mostly water.
1. Explain what is causing the odour and the dropping trap seals, and why the greasy symptoms are not simply a plugged line.
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The pattern is trap-seal loss, not blockage. Gurgling means air is being pulled through a trap; odour and the overnight level drop mean the seal is being siphoned or evaporating. A blocked line would show slow drainage and backing up at the lowest fixture, which is not the complaint.
Two mechanisms, both consistent with the installation:
- Venting. The interceptor is vented to outdoors only. The branch from the fixtures to the interceptor needs its own vent connection taken off downstream of the last fixture and upstream of the interceptor, and the interceptor needs a vent that terminates clear of air intakes, doors and windows. With the vent terminating 0.9 m under a make-up air intake, the relief point is effectively unusable - and whatever does vent goes straight back into the building.
- Hydraulic surge without a flow control fitting. Draining a three-compartment sink and a pre-rinse spray dumps a large slug into 75 mm pipe. The slug fills the branch and the interceptor inlet, creating a vacuum behind it that siphons the nearby mop sink and floor-drain traps (self-siphonage). The same velocity destroys the retention time the interceptor needs.
Also check the trap arms for correct size and developed length, and that no floor drain has been converted to a 'dead' connection that is no longer vented.
2. What does the missing flow-control fitting do, and how would you size and correct the installation?
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What it does: a flow-control fitting is a restricted orifice installed on the interceptor inlet, accessible for cleaning (usually under a cleanout cover), sized to pass no more than the interceptor's rated capacity. It slows the discharge so the interceptor holds the flow long enough for grease to float and solids to settle instead of being scoured straight through to the sewer.
Why it explains the evidence: a thin mat and grease at the manhole are the classic signs of an interceptor being pushed beyond its rated flow. High inlet velocity re-suspends the float layer and carries it out. Monthly pumping cannot compensate for continuous carry-over.
Sizing and correction:
- Establish the design flow from the connected fixtures (three-compartment sink, dishwasher, pre-rinse spray, mop sink, floor drains), using the fixture capacity/flow ratings the code assigns, not the pipe size.
- Confirm the 2,000 L unit gives adequate retention at that flow (gravity interceptors are commonly sized around 30 minutes at design flow). If it does not, add a second unit in parallel or replace it.
- Install the flow-control fitting at the inlet, sized to the interceptor rating, with a cleanout.
- Verify the 75 mm branch slope and that the run is as short and direct as practical; long shallow runs let grease coat the pipe and restrict it further.
- Clean the branch and interceptor before proving the fix, and document the pump-out for the municipality.
3. Draft the technical content of the response to the municipal notice, including the venting correction and the termination requirements.
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Structure the response as: cause, correction, verification, maintenance.
1. Cause (state it plainly). No flow-control fitting meant the interceptor was hydraulically overloaded, so retention time was lost and grease was carried to the sewer. The interceptor vent terminated below a make-up air intake and level with operable windows, which is both a code violation and the source of the kitchen odour.
2. Venting correction.
- Take a vent off the fixture branch upstream of the interceptor and carry it to the building venting system.
- Re-terminate the interceptor vent so it rises above the roof surface by at least the code minimum and is separated from windows, doors and air intakes by the required distance (verify locally - commonly 1 m; more if the intake is close). Never terminate under an intake, in a soffit, or at window height.
- Confirm every floor drain and the mop sink have correctly sized, unobstructed vent connections.
3. Hydraulic correction. Install the flow-control fitting at the rated capacity, upsize the branch if the run and grade cannot be improved, and pump/clean the interceptor and branch.
4. Verification. Re-inspect the downstream manhole at 30 and 90 days, submit pump-out records and a grease-hauler contract, and add strainers to the sink outlets and a written FOG (fats, oils, grease) procedure for staff. Sign and date the report.