Usually because gas is escaping a system built to hold it. Code requires the pit to have a gastight cover (bolted and gasketed under the UPC) and its own vent. A worn cover gasket, a blocked vent, grease and solids rotting in the pit, or level controls that let sewage sit too long will put sewer gas in the room.
What the code requires a sewage pit to have
Both plumbing codes used in the Valley treat a sewage pit as a sealed, vented tank.
| Requirement | International Plumbing Code (2024) | Uniform Plumbing Code |
|---|---|---|
| Sealed | Sump must be "tightly covered and vented" (712.1) | Covers with a "bolt-and-gasket-type manhole or equivalent opening" (710.10) |
| Cover | "Gastight removable cover" no more than 2 inches below the floor (712.3.2) | Access for inspection, repairs and cleaning (710.10) |
| Vent | Vented like a gravity system, sized by Table 906.5.1 (906.5.1) | Vent pipe through the roof, alone or combined where allowed, at least 1 1/2 inches (710.10) |
| Pneumatic ejector | Air relief pipe to its own vent stack through the roof (906.5.2) | Not covered here |
| Sewage level | Kept at least 2 inches below the inlet pipe (712.3.4) | Not covered here |
The vent's job is to let air in and out as the pump runs, so the pit stays at room pressure. The IPC says the pump must discharge automatically, so sewage shouldn't sit.
The usual causes, in the order to check them
Most smelly pits come down to one of five problems, and the first two cause most of them.
1. The cover isn't sealed. Gaskets dry out, bolts go missing after a service visit, and cords and conduits get pushed through holes that were never sealed. Any of those turns a gastight cover into a leaky one. Look for daylight around the lid, missing bolts and open holes around the pump cords and discharge pipe.
2. The vent is missing, blocked or wrong. Without a working vent, pressure changes push gas out through any weak point, which usually means the cover. Common problems: a vent that was never run, one tied into a vent that's too small or blocked higher up, a cap or a bird nest on the roof, or a vent cut short during a remodel. Wisconsin's health department suggests checking roof vents for leaves and nests as a first step for any sewer smell. See what a plumbing vent does and where vents have to end on the roof.
3. Grease, solids and scum have built up. Restaurants and buildings with kitchens send grease that floats and hardens in the pit. Solids settle in the bottom. That mass keeps rotting between pump cycles. OSHA notes that hydrogen sulfide comes from bacterial breakdown of human and animal waste, so a dirty pit keeps making more gas. Regular wet well cleaning is the fix. Our page on what a sewage lift station is and how to maintain it covers the parts and the routine.
4. The level controls or pumps are off. A float that's stuck or set too high lets sewage sit in the pit longer and climb toward the inlet, which the IPC says it must stay 2 inches below. A pump that's failing or airlocked leaves the pit full. An alarm that's been silenced hides both.
5. The smell isn't the pit at all. A dry floor drain in the pump room, a trap that lost its seal or a cracked drain nearby can smell the same. Refill any floor drain in the room first. See why a commercial building smells like sewer gas.
Signs the pit is the source
A few clues separate a pit problem from a drain problem somewhere else:
- The smell peaks when the pump runs or right after, as the level drops and air moves through the pit.
- It's strongest at the lid or around the pipes and cords that pass through the cover, and fades a few steps away.
- It gets worse on hot afternoons or after a weekend, when sewage has sat longer between pump cycles.
- The pump room smells but the restrooms that feed it don't. That points at the pit and its cover rather than the fixture traps.
- The high-level alarm has gone off, or someone has silenced it more than once.
If the smell is in restrooms or suites far from the pump room, the pit is less likely to be the cause. Start with the traps and vents on that floor.
Safety first: this is a confined space
Don't open the pit to take a look. OSHA's fact sheet says hydrogen sulfide collects in low, enclosed, poorly ventilated areas like manholes and sewer lines, and that before anyone enters such an area a qualified person must test the air, including for fire or explosion risk. Wisconsin lists methane in sewer gas too, and says both methane and hydrogen sulfide are flammable. Keep sparks and flames away from an open pit, and never lean in or climb down. If a worker collapses near the pit, call 911 and don't go in after them.
If the pump room smells strong, or anyone feels ill, clear the area. See when to evacuate for a sewer smell and OSHA's limits for hydrogen sulfide.
What fixing it usually involves
- Reseal the cover: new gasket, all bolts, and sealed grommets or fittings where cords and pipes pass through.
- Fix the vent: clear it, extend it, or run the vent the pit should have had, sized to the code table.
- Clean the pit: pump it out, remove grease and solids, and set a cleaning schedule based on how fast it builds up.
- Check the pumps and floats: confirm the pump cycles at the right level, the check valve holds and the high-level alarm works.
### Keep grease out of the pit
If a kitchen drains into the pit, the pit's odor problem often starts upstream. A grease trap or interceptor that's overdue for pumping lets fats pass into the building drain, where they cool and collect in the pit. Keeping the interceptor on schedule beats cleaning a pit full of hardened grease. See how often to pump a grease trap for the schedule rules, and keep enzyme or solvent products out of the drains unless your city allows them.
### Don't forget the valves
The IPC requires a check valve and a full-open valve on the pump discharge, with access to both, located above the sump cover or in an accessible pit outside the sump (Section 712.2). A check valve that doesn't close lets pumped sewage flow back into the pit, so the pump keeps moving the same sewage and the pit never fully empties. A valve buried where nobody can reach it tends to get skipped at service time.
Work on the vent, the pump or the discharge piping may need a permit in your city. A licensed plumber can tell you which parts do.
HQ Plumbing is licensed and insured, handles commercial plumbing across the Valley and offers 24/7 emergency service with free estimates. We can clear grease-clogged lines feeding the pit, camera the drain piping and repair vents and drains in the building.