Anaerobic (non-air breathing) bacteria form in raw sewerage and produce hydrogen sulfide gas (H2S). Factors contributing to rapid bacterial growth are temperature, retention time, high BOD levels and turbulence. As inflow and infiltration are decreased, hydrogen sulfide gas is dramatically increased - flows are not diluted and gases are trapped inside generating more acid in less time.
Turbulence from force mains, drop manholes, steep grade changes and pumping stations allow hydrogen sulfide gas (H2S) to release into the atmosphere in pipes and manholes.
Hydrogen Sulfide gas (H2S) is converted into sulfuric acid (H2SO4) by the aerobic (air breathing) Thiobacillus bacteria that grow on the concrete surfaces above the waste water flows.
Sulfuric acid (H2SO4) produced by the aerobic (Thiobacillus) bacteria quickly corrodes the concrete, resulting in severe structural damage to the pipe and manholes.