Biological corrosion of sewers and sewage treatment plants constitutes a serious problem and its effects result in the loss of billions of dollars a year. It is generally acknowledged that corrosion of concrete caused by microorganisms is associated with their production of sulphuric acid as the main metabolic product.

The activity of the bacteria leads to the formation of the sulphur cycle in the system, and consequently to the formation of sulphuric acid, whose presence causes corrosion of concrete. Moreover, the presence of other organisms, such as nitrifying bacteria, fungi and organic acids also contributes to the degradation of the concrete.

Durability of concrete structures can be reduced by a lot of different mechanisms, including chemical and physical ones. Concrete at a sewage treatment plant may be exposed to simultaneous (concurrent) effect of these mechanisms causing unexpectedly rapid degradation. Concrete at different parts of a sewage treatment plant may be exposed to sulphuric acid produced by microorganisms, sulphates and chlorides – containing water, to permanent submersion in water and constant flow, as well as to changeable temperatures of freezing and thawing.

Occurrence of such conditions may cause problems to concrete, including cement dissolution, macro- and micro-cracks, as well as chipping and corrosion of the reinforcing steel (Coating, 2011).

Biological corrosion at sewage treatment plants is caused by corrosive secretions (metabolic products) of microorganisms. The conditions prevailing there are very good for rapid development of bacteria. Treatment plants are hardly ever completely filled with sewage and as a result there is a lot of space above the water line for bacterial development and growth and accumulation of gas products from sewage decomposition. Because of the unusual rate of bacterial proliferation, enormous quantities are formed in a short time, causing a huge potential threat to the facility.

Hydrogen sulfide formation in drainage systems often is a result of the wastewater transport to large centralized treatment plants. This results in long transport distances and requires pumping the wastewater through pressure pipes. In the pressure pipes, there is no reaeration of the wastewater as in the gravity pipes, which lead to anaerobic conditions and sulfide production because of microbial activity [Danish Environmental Protection Agency, 1988].

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