Index >> Applications of Microbial Interactions >> Microbially Induced Concrete Corrosion

It is found that aerobic heterotrophs, and neutrophilic and acidophilic sulphur oxidisers are the dominant microbes. There are also SRB, al1a.erobic heterotrophs, nitrate reducing bacteria, and ammonia oxidising bacteria present in-some of the samples.

The corrosion of concrete pipes is a consequence of a cyclic process caused by microbial sulphur metabolism. Two types of sulphur metabolism are involved in the cycle of sulphur in the environment. One is an anaerobic process in which H2S is produced by anaerobic bacteria, the other is an aerobic process in which the H2S is oxidised to elemental sulphur (S) or sulphuric acid (H₂SO₄), This cyclic process exists as a natural method for the cycling of sulphur compounds in the environment and may also exist in sewage collection systems.

The sulphide produced eventually partitions into HS- and H₂S. The H₂S is a gas and evolves into the headspace of the sewer pipes, reaching the crown of the pipe. The crown of the pipe is exposed to an aerobic environment which supports the growth of sulphur oxidising bacteria. The sulphur oxidising bacteria grow on and within the concrete of the pipe, oxidising the H2S present and producing H₂SO₄, The sulphuric acid dissolves the CaOH and CaCO₃ in the cement binder, thus causing corrosion of the concrete pipes.

There have been only a few species of thiobacilli (the largest genera of organisms that oxidise H₂S to H₂SO₄) described by researchers. These are T. navellus, T. thioparus, T. neopolitanus, T. intermedius and T. thiooxidans  The first four organisms are important for establishing the acidic conditions necessary for corrosion to occur, while the acid loving T. thiooxidans grows in conditions of very low pH and produces H 4 in copious amounts, thus lowering the pH even more.