What Causes Hydrogen Sulfide In Sewer Systems
Hydrogen sulfide gas is the signature smell of failing wastewater infrastructure. It corrodes concrete, eats through copper, and at high concentrations is lethal. In buildings it usually shows up as a rotten-egg smell in a basement, a stairwell, or the floor above an ejector pit.
Short answer
Hydrogen sulfide forms when sulfate-reducing bacteria in the slime layer on pipe walls convert dissolved sulfate into sulfide, which happens wherever dissolved oxygen falls below about 1 mg/L — long force mains, lift station wet wells, ejector pits, and slow-moving building sewers. Control means one of three things: oxidize the sulfide, chemically bind it, or prevent its formation by dosing biology that outcompetes the sulfate-reducing bacteria.
Step by step
- 1
Locate the anaerobic zone
Map where flow slows or sits: wet wells, ejector pits, long force mains, oversized laterals, and grease-loaded runs. H₂S is generated where oxygen runs out, not where you smell it.
- 2
Measure before treating
Log gas readings at the pit or wet well across a full day. Peak generation typically follows peak flow by a few hours, and readings tell you whether you have a nuisance-odor problem or a corrosion and safety problem.
- 3
Fix the mechanical contributors
Dry traps, failed pit covers, missing vent extensions, and short-cycling pumps all move gas into occupied space. These are usually cheaper to correct than any chemical program.
- 4
Choose the control chemistry
Oxidize (calcium nitrate, hydrogen peroxide), bind (iron salts), or prevent (bioaugmentation). Municipal force mains typically use nitrate; building-level pits and wet wells are usually better served by continuous bioaugmentation.
- 5
Dose continuously, then verify
Metered dosing at the generation point holds the sulfide down between visits. Re-log gas readings after two weeks and adjust the dose from the data rather than from complaint volume.
The bacterial reaction
Sulfate-reducing bacteria (SRB) in the slime layer on pipe walls convert dissolved sulfate to sulfide whenever dissolved oxygen drops below ~1 mg/L. Long force mains, lift station wet wells, and ejector pits are the most common formation sites.
Why it gets worse in summer
Warmer wastewater holds less oxygen and SRB activity roughly doubles for every 18°F rise. Most odor complaints peak between June and September.
Control strategies
Three options: oxidize the sulfide (calcium nitrate, hydrogen peroxide), bind it (iron salts), or prevent it (bioaugmentation that outcompetes SRB). Calcium nitrate is the most common municipal program; bioaugmentation is more cost-effective for building-level pits.
Corrosion is the expensive part
Sulfide oxidizes to sulfuric acid on damp concrete crowns and metal surfaces, which is why untreated wet wells lose structural cover and pump hardware years early. Odor is the symptom; the repair bill is the real cost.
How Luften runs H₂S control as a service
We install and maintain dosing at the pit or wet well and service it monthly with readings and a written log — a maintained program, not a drum of chemical dropped off with instructions.
Frequently asked questions
- What concentration of hydrogen sulfide is dangerous?
- OSHA's 8-hour exposure limit is 10 ppm. Concentrations above 100 ppm can cause loss of consciousness, and the sense of smell is deadened well before that point.
- Does the treatment damage downstream plants?
- No. All three control chemistries are routinely dosed at municipal treatment facilities.
- Why does my basement smell like rotten eggs?
- Almost always an ejector pit, floor drain with a dry trap, or sewer line where flow has gone anaerobic. Restore trap seals and pit covers first, then treat the generation point.
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