The sulphate ion occurs naturally in most water supplies and is present in wastewater as well.

Sulphate is reduced biologically under anaerobic conditions to sulphide, which in turn can combine with hydrogen to form hydrogen sulphide (H₂S).

The following generalised reactions are typical.

The H₂S accumulated in sewers can be oxidised to sulphuric acid, which is corrosive to sewer pipes.