Managing Hydrogen Sulfide Corrosion in Concrete Sewers
Hydrogen sulfide corrosion is a biological, chemical, and operational problem that can shorten the service life of concrete sewer infrastructure. It develops when wastewater conditions allow sulfate-reducing bacteria to produce sulfide, which later becomes sulfuric acid on exposed concrete surfaces. The result can include odor complaints, structural deterioration, falling debris, and costly rehabilitation.
The risk is greatest in force mains, rising mains, inverted siphons, wet wells, and long gravity sewers with low flow or extended detention. A successful control program connects field observations, wastewater chemistry, ventilation, worker safety, and asset management instead of relying on a single chemical treatment.
Water and wastewater professionals can use regional knowledge to compare methods and understand how treatment decisions affect downstream processes. Technical resources such as the LABS of CWEA newsletters can help practitioners follow current discussions, case studies, and professional development opportunities relevant to sewer collection systems.
How sulfide damages concrete
Sulfide generation begins in the slime layer attached to the submerged sewer surface. When dissolved oxygen and nitrate are depleted, sulfate-reducing bacteria use sulfate as an electron acceptor and produce dissolved sulfide. Warm temperatures, high biochemical oxygen demand, stagnant conditions, and long detention times can accelerate the process.
As wastewater turbulence releases dissolved sulfide into the sewer atmosphere, hydrogen sulfide gas accumulates near the crown of the pipe. Moisture on the concrete surface supports sulfur-oxidizing bacteria, which convert the gas into sulfuric acid. This acid reacts with cement compounds, dissolving the binder and creating a soft, porous, gypsum-rich surface.
Early deterioration may appear as discoloration, roughness, exposed aggregate, or a sharp increase in odor. Advanced damage can reduce wall thickness, expose reinforcement, compromise manholes, and create voids that are difficult to detect without inspection. Hydrogen sulfide is also highly toxic, so corrosion control must never be separated from confined-space entry procedures and atmospheric monitoring.
Finding the highest-risk locations
A useful assessment starts with a map of conditions rather than a list of isolated odor complaints. Review sewer diameter, slope, age, material, hydraulic profile, force-main discharge points, pumping schedules, wet-well levels, and areas where wastewater becomes septic. Locations with long retention times and turbulent discharge deserve particular attention.
Field measurements can include dissolved sulfide, total sulfide, pH, temperature, dissolved oxygen, oxidation-reduction potential, flow, and headspace hydrogen sulfide. Sampling should capture changing conditions, including early morning, peak wet-weather flow, low-flow periods, and pump cycles. A single grab sample may miss the period when sulfide formation is most intense.
Inspection methods should match the suspected failure mode. Visual surveys, closed-circuit television, wall-thickness measurements, corrosion profiling, and manhole inspections can establish whether deterioration is superficial or structural. Comparing current findings with earlier inspection records also helps estimate the rate of concrete loss and prioritize renewal work.
Selecting a control strategy
The most direct approach is to prevent sulfide formation in the wastewater. Ferric salts, ferrous salts, nitrate, peroxide, oxygen, and other oxidants can change the chemistry that supports sulfide production. Each option has different effects on chemical demand, residuals, sludge production, worker exposure, storage, dosing equipment, and downstream treatment.
Chemical addition is often effective when the dose responds to measured conditions. Fixed dosing may waste product during wet weather and under-treat the system during hot, low-flow periods. Automated control can use flow, wet-well level, oxidation-reduction potential, or sulfide measurements, although instruments require cleaning, calibration, and a maintenance plan.
Physical and biological measures can complement chemical treatment. Increasing ventilation may reduce headspace concentrations but will not necessarily stop sulfide generation. Flushing, improved flow patterns, shorter detention, and oxygen transfer can reduce septic conditions where system geometry allows. At treatment plants, process changes should be evaluated across the entire facility; technical material on biological nutrient removal illustrates why upstream and downstream process interactions matter.
Comparing options for sewer operators
No treatment works equally well in every collection system. The right choice depends on whether the primary objective is odor reduction, sulfide prevention, worker protection, preservation of concrete, or a combination of these goals. Operators should also account for chemical delivery access, emergency response, electrical classification, and the consequences of a dosing interruption.
| Control approach | Primary effect | Advantages | Key limitations |
|---|---|---|---|
| Ferric or ferrous salts | Binds dissolved sulfide and changes sulfide chemistry | Familiar technology; can provide rapid reduction | Chemical storage, residual solids, and dosing costs |
| Nitrate addition | Interrupts sulfate-reducing conditions | Effective in force mains and septic zones | Ongoing product demand; downstream nitrogen impacts |
| Peroxide or other oxidants | Oxidizes sulfide | Fast response and strong odor control potential | Safety, decomposition, and chemical handling requirements |
| Oxygen or air injection | Raises dissolved oxygen and redox conditions | Can reduce septic conditions continuously | Energy use, equipment maintenance, and limited reach |
| Flow and detention changes | Reduces time available for sulfide formation | May lower long-term chemical dependence | Often constrained by existing grades and capacity |
| Protective lining or coating | Separates concrete from corrosive conditions | Extends asset life after rehabilitation | Requires proper surface preparation and quality control |
A pilot program can reveal performance before a full-scale installation. Establish baseline sulfide and odor conditions, define a target, operate the control for a representative period, and compare results across multiple flow and temperature conditions. Cost evaluation should include capital work, chemicals, labor, monitoring, residual handling, and avoided repairs.
Protecting and rehabilitating concrete
Chemical control is most valuable when it is paired with an assessment of the concrete itself. A pipe may continue to function hydraulically while losing structural capacity at the crown. Engineers should determine whether the substrate is sound, whether reinforcement has corroded, and whether groundwater infiltration or external loading adds risk.
Rehabilitation options include cementitious liners, polymer-based coatings, cured-in-place systems, segmental liners, and full replacement. Product selection should reflect the expected pH, sulfide concentration, moisture conditions, hydraulic abrasion, cleaning practices, and installation access. Surface preparation is critical: coatings applied over weak, contaminated, or damp concrete may fail prematurely.
Manholes and wet wells often need special attention because they combine turbulence, confined geometry, and concentrated headspace gas. Repair specifications should address substrate preparation, thickness, adhesion, holiday testing, curing, and inspection records. A warranty is useful, but documented quality assurance is more important than a product name alone.
Building a practical monitoring program
A sustainable program assigns responsibility for sampling, instrument maintenance, chemical inventory, alarm response, and inspection scheduling. Trend data should be reviewed alongside pump runtime, flow, temperature, rainfall, and chemical consumption. This makes it easier to distinguish a process change from an instrument problem or an unusual loading event.
Operators should define action levels before an alarm occurs. For example, a rising headspace concentration may trigger ventilation checks and worker-protection measures, while increasing dissolved sulfide at a force-main discharge may trigger a dosing review. Alarm management should include escalation contacts and procedures for loss of telemetry, empty tanks, failed pumps, or blocked injection points.
Training remains essential. Staff need to understand where hydrogen sulfide can accumulate, how quickly conditions can change, and why odor is not a reliable measure of safety. Regular refreshers, calibration records, confined-space planning, and coordination among operations, engineering, maintenance, and safety teams turn monitoring data into dependable risk reduction.
Recommendations for a durable program
A collection system can make measurable progress by combining targeted controls with disciplined asset management:
- Establish a sulfide and corrosion baseline at force-main discharges, wet wells, manholes, and known low-flow locations.
- Use seasonal and operational data to adjust chemical dosing rather than relying solely on fixed daily rates.
- Pair odor and gas monitoring with concrete inspections so corrosion is addressed before structural damage becomes severe.
- Select rehabilitation materials based on verified exposure conditions, surface preparation requirements, and quality-control testing.
- Review worker-safety procedures whenever hydrogen sulfide readings, ventilation patterns, or access conditions change.
The strongest programs also document what happens after each intervention. Record chemical rates, sulfide measurements, odor reports, inspection findings, maintenance events, and costs in a format that supports future design decisions. Over time, this record can show which controls provide durable value and where capital renewal should take priority.
Managing hydrogen sulfide corrosion in concrete sewers is a continuing operational responsibility, but it can be approached systematically. By combining wastewater chemistry, reliable monitoring, appropriate treatment, structural assessment, and staff training, agencies can protect workers, reduce nuisance odors, and extend the useful life of critical collection assets.
LABS of CWEA gives professionals across the Los Angeles Basin a place to exchange practical experience through technical presentations, workshops, facility tours, and peer connections. Engage with the organization’s resources and professional events to strengthen local corrosion-control practice and help advance the water environment profession.