Morning Sewer Odor in Second-Floor Offices
A six-story mixed-use building has offices on floors 2–4 and a restaurant on the ground floor. Occupants of two second-floor offices report an intermittent 'rotten egg' odor on weekday mornings, usually 08:00–10:00. The restrooms and the restaurant dining room have no complaints. A janitor closet between the two offices contains a floor drain that is used only for mop water. The restaurant's 1,500 L grease interceptor is pumped monthly and has a measured headspace H2S of 8 ppm. The 150 mm sanitary stack runs inside a chase beside the offices and terminates on the roof; the 100 mm vent stack is 1.5 m away. A blower for the building's dedicated outdoor air system maintains the offices at −12 Pa relative to outdoors. An H2S meter reads 0.2 ppm in one office and 0.0 ppm in the other. A smoke test of the sanitary drain is proposed. The building owner wants a low-cost fix and doubts the grease interceptor is involved because it is 20 m away.
1. Identify the most probable odor pathway and explain why the grease interceptor may still be involved even though it is 20 m away. Use the pressure and plumbing details.
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The most probable pathway is the janitor-closet floor drain. At −12 Pa the offices are under negative pressure relative to outdoors, so air is drawn into the building through openings rather than pushed out. If the floor-drain trap has evaporated from infrequent use, the drain becomes an open connection to the sanitary drain system, and sewer gas is pulled into the closet and then into the adjacent offices. The intermittent morning pattern fits HVAC startup and morning wastewater flows, which pressurize the drain system and increase gas movement.
The grease interceptor is not ruled out by distance. Its 8 ppm headspace H2S shows active sulfide generation, and its gas can communicate through the shared building drain and stack. The interceptor is a generator, while the dry trap is the entry point. Two offices with different readings are also consistent with a local pathway: one office may be closer to the closet or more negatively pressurized. The smoke test is useful but should not be the first step; a dry trap can be checked and corrected in minutes. If the trap is wet and the odor persists, smoke testing or tracer gas can confirm a broken drain or vent connection.
2. Propose a stepwise diagnostic and a permanent corrective action, including how to verify success.
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First, inspect the janitor-closet floor drain and measure the trap seal. If dry, add water and recheck odor and pressure within 24 hours. At the same time, verify that the sanitary and vent stacks are intact in the chase, that the vent terminates above the roof and is not blocked, and that the restaurant interceptor vent is not discharging indoors. Record pressure differentials in both offices and the closet. If the odor persists after the trap is primed, use a smoke test or tracer gas with the HVAC operating, and isolate the interceptor to see whether odor changes.
Permanent correction should include a trap primer or a scheduled water charge for the floor drain, repair of any broken drain or vent, and rebalancing of the HVAC to avoid excessive negative pressure. The interceptor should be checked for proper venting and pumping frequency; 8 ppm headspace is not unusual, but a blocked vent or undersized interceptor can worsen gas migration. Verify success with two weeks of occupant logs, repeated H2S and pressure readings, and a follow-up smoke test if needed. A low-cost fixed water charge is acceptable only if the trap is the sole pathway; otherwise it will not solve a vent or interceptor problem.