The formation of H2S in sewage is a consequence of the natural biological decomposition of sulphur-containing organic and inorganic matter. It forms under an aerobic condition by sulphate reducing bacteria in the slime of matured sewer and to a lesser extent by the bacteriological process in the sewage. In gravity sewers, the formation of H2S commences after oxygen originally present in sewage has been consumed by manifold biological processes. This is followed by the anaerobic decomposition with an ever-increasing formation of H2S which slowly escapes into the sewer atmosphere. The formation of H2S is supported by long sewage flows, low flow velocities high sewage temperatures.

The formation of H2S and the oxidation by bacteria living on the moist surface of the sewer occurs not only in gravity sewers but also and more severely in pressure pipelines, where, due to the absence of an atmosphere, continuously ideal conditions of the sulphate reducing bacteria prevail. The biological induced H2S corrosion has its effects only above the surface level of the sewage stream, where the sulphuric acid reacts with the lime content of cement-bound pipe materials. In general, a progression in corrosion of concrete sewers having a constant presence of Hydrogen Sulphide in the sewer atmosphere can be expected with 3 - 6 mm/y. Glazed stoneware pipes are immune to sulphuric acid.