Understanding Microbially Induced Corrosion in Sewer Systems

Microbially induced corrosion (MIC) is the dominant deterioration mechanism in concrete gravity sewer infrastructure. The process destroys more infrastructure, more quickly, than any other single failure mechanism in the wastewater sector — and yet it is preventable through the correct application of protective coating systems and source control measures. Understanding MIC at the microbiological and chemical level is essential for engineers and infrastructure managers who need to make technically defensible decisions about prevention and rehabilitation strategies.

The Microbial Communities Involved

MIC in sewers involves a consortium of microbial communities operating in sequence and often in spatial separation. In the submerged zone of gravity sewers, the primary actors are sulfate-reducing bacteria (SRB), obligate anaerobes that require the absence of oxygen and the presence of sulfate as a terminal electron acceptor for their respiratory metabolism. The most studied and prevalent genera in sewer SRB communities are Desulfovibrio, Desulfobacter, and Desulfonema.

These organisms form dense biofilms on the interior surface of sewer pipes in the submerged zone, metabolizing organic carbon in the wastewater and reducing sulfate to sulfide. The hydrogen sulfide produced partitions between the liquid phase (where it exists as HS⁻ and S²⁻ at higher pH and as dissolved H₂S gas at lower pH) and the sewer gas phase above the waterline. The rate of H₂S production is governed by available sulfate concentration, organic carbon concentration (biological oxygen demand), temperature, retention time, and biofilm surface area — all parameters that vary considerably between sewer systems and between different locations within the same system.

In the crown zone, where the sewer atmosphere contacts the concrete surface, a different microbial community operates. Initial colonization by neutrophilic sulfur-oxidizing bacteria such as Thiobacillus thioparus begins the acidification process, reducing crown surface pH from the near-neutral level of the original concrete to values in the range of 4-6. As pH decreases, these neutrophilic organisms are replaced by increasingly acidophilic species including Acidithiobacillus thiooxidans (formerly Thiobacillus thiooxidans), which thrives at pH values below 1 and produces H₂SO₄ at its maximum rate under these conditions. This biological succession produces an accelerating corrosion rate as the most aggressive organisms progressively dominate the crown biofilm community.

Spatial Patterns of MIC in Sewer Systems

MIC does not occur uniformly throughout a sewer system. Conditions that favor H₂S production — high sulfate concentration, warm wastewater temperatures, low dissolved oxygen, long retention times, and high turbidity organic loads — are characteristic of specific system configurations. Force mains (pressure sewers) are particularly vulnerable because their enclosed, anaerobic conditions and elevated wastewater temperatures promote sulfide accumulation during transmission. The highest H₂S concentrations and most rapid biological corrosion typically occur at the downhill end of force mains, where accumulated sulfide-laden wastewater is released to gravity sewer with turbulent energy transfer to the sewer atmosphere.

Manholes downstream of force main discharge points, lift station wet wells, and any gravity sewer segment with reduced flow velocity (flat grades, frequent infiltration causing high water levels, or downstream hydraulic constraints) are locations of elevated MIC risk. Systematic risk mapping based on sewer configuration, flow monitoring data, and wastewater composition analysis allows infrastructure managers to target limited inspection and rehabilitation resources to the highest-risk assets in their inventory.

Prevention and Mitigation

MIC prevention operates on three levels: reducing sulfide generation in the wastewater, controlling H₂S transfer to the sewer atmosphere, and protecting concrete surfaces from the H₂SO₄ produced by crown-zone SOB communities. Source control through iron salt dosing reduces dissolved sulfide concentration by precipitating iron sulfide; nitrate dosing suppresses SRB activity by providing an alternative electron acceptor that aerobic and facultative organisms preferentially use; and oxygen injection into force mains can eliminate the anaerobic conditions required for SRB activity entirely in segments where oxygen demand is manageable.

Surface protection through polyurea or acid-resistant epoxy coatings eliminates the contact between H₂SO₄ and concrete by providing an impermeable, chemically resistant barrier. For structures where MIC is already advanced, complete removal of deteriorated concrete to sound substrate, followed by application of a primer and thick-film polyurea coating, restores structural integrity and provides decades of protection against further biological corrosion.