Sewage treatment and process tanks

Biogenic corrosion

In sewers and tanks, bacteria reduce the sulphates present in the wastewater, producing hydrogen sulphide, which is released into the structure's atmosphere. On the walls above the water line, other bacteria oxidise it into sulphuric acid, which attacks the cementitious matrix.

It is a process that does not stop by lowering pollutant concentrations: it is intrinsic to how the plant works. In the air-liquid exchange zones, degradation can reach several millimetres a year.

The wet-dry cycle

Tanks with variable levels subject the fluctuation band to continuous saturation and drying cycles, which concentrate salts and amplify the attack. It is the zone that fails first and must be designed to stricter criteria than the rest of the structure.

The strategy

On new works it is worth combining concrete made waterproof in the mass, which limits absorption and therefore the penetration of aggressive agents, with an acid-resistant coating in critical zones. DIN 4030 classifies water aggressiveness and guides the choice.

On existing works the intervention always starts by removing degraded concrete and repairing the section to UNI EN 1504, before any protection. Joints must be treated separately: they are the point where the coating is interrupted.