Wastewater Infrastructure Corrosion: Causes, Mechanisms, and Prevention

Corrosion is the primary cause of premature failure in wastewater infrastructure assets worldwide. Unlike the visible, external corrosion that affects above-ground metallic structures, the corrosion that destroys concrete sewers and treatment plant structures is largely driven by biological processes occurring in the dark, humid environment inside the infrastructure — making it difficult to detect, easy to underestimate, and expensive to remediate once advanced. A thorough understanding of corrosion mechanisms is the prerequisite to developing effective prevention strategies.

Hydrogen Sulfide: The Root Cause

The corrosion of concrete sewer infrastructure begins with hydrogen sulfide (H₂S), a colorless, toxic gas produced by sulfate-reducing bacteria (SRB) metabolizing sulfate and organic sulfur compounds in the anaerobic zones of wastewater streams. SRB, primarily species of the genus Desulfovibrio, use sulfate as an electron acceptor in their respiratory metabolism, reducing it to sulfide, which at sewer pH values below 8 exists predominantly as dissolved H₂S gas.

H₂S partitions between the liquid and gas phases of the sewer according to Henry’s Law, with the gas-phase concentration dependent on liquid temperature, pH, flow velocity (which influences turbulence and gas transfer rates), and total dissolved sulfide concentration. In the atmosphere above the waterline — the crown zone of gravity sewers, the upper walls of manholes, and the above-waterline surfaces of wet wells — H₂S concentrations range from parts per billion to tens of parts per million depending on system characteristics and wastewater composition.

The Microbial Corrosion Mechanism

The destruction of concrete by hydrogen sulfide occurs through a two-stage microbial process. In the first stage, H₂S absorbed into the moisture film on concrete surfaces reacts with cement hydration products, primarily portlandite (Ca(OH)₂), to form calcium sulfide. This reaction is chemical, not biological, and proceeds wherever H₂S contacts moist concrete. The product, calcium sulfide, is soluble and is gradually leached from the concrete surface, but the initial concrete degradation from this stage alone is relatively slow.

The accelerated deterioration that makes microbially induced corrosion (MIC) so destructive begins in the second stage, when sulfur-oxidizing bacteria (SOB) — primarily acidophilic species of the genera Thiobacillus and Acidithiobacillus — colonize the concrete surface and begin oxidizing absorbed sulfide compounds through sulfuric acid as a metabolic byproduct. These bacteria thrive at extremely low pH values (below 2.0), conditions that would be lethal to most other organisms, and are capable of generating H₂SO₄ at concentrations sufficient to dissolve all calcium-bearing phases in concrete at rates up to 10 millimeters per year in severe cases.

Galvanic and Electrochemical Corrosion in Steel Components

While concrete sewer structures face predominantly biological corrosion, steel components — including manhole frames and covers, pipe joints, force main pipelines, and structural reinforcement in concrete — are subject to electrochemical corrosion processes. In wastewater environments, these include galvanic corrosion (where dissimilar metals in electrical contact in the presence of an electrolyte drive current flow that oxidizes the more active metal), differential aeration corrosion (where variations in dissolved oxygen concentration in the surrounding soil or water create electrochemical potential differences), and microbially influenced corrosion of steel (where the metabolic products of bacteria in contact with steel surfaces catalyze electrochemical corrosion reactions).

Prevention Strategies

Effective corrosion prevention in wastewater infrastructure requires a multi-barrier approach addressing both the generation and the exposure of corrosive species. Source control — reducing H₂S generation through chemical dosing of the wastewater with iron salts, nitrate, or oxygen — decreases the H₂S load available for conversion to sulfuric acid. Ventilation management — maintaining adequate air flow in sewers to dilute H₂S concentrations below the threshold for SOB colonization — delays the onset of biological corrosion. And protective coating systems — creating a physical barrier between the concrete substrate and the corrosive atmosphere — prevent the contact necessary for both chemical and biological corrosion to occur.

For long-term asset protection, spray-applied polyurea coatings offer the most durable and maintenance-free barrier available for concrete sewer structures. Their seamless application, crack-bridging flexibility, and resistance to both H₂SO₄ and the mechanical abrasion of wastewater solids provide comprehensive protection against the multiple failure mechanisms that threaten concrete infrastructure in sewer environments.