Sewage and concrete attack
In general terms domestic sewage does not attack concrete in direct contact with it. Untreated trade/industrial effluents may attack concrete if it contains aggressive chemicals (e.g. sulphates) and/or has a low pH (e.g. below 5.5) or a very high pH ie. very alkaline or acidic respecively.
However the upper part of enclosed sewerage systems, above the water line, can in some circumstances suffer attack. If the void above the sewage in a sewer or manhole is not well ventilated, anaerobic (oxygen deficient) conditions can prevail in the sewer. In anaerobic conditions hydrogen sulphide gas is produced in the sewage by anaerobic bacteria. The hydrogen sulphide gas collects in the void above the sewerage and combines with any moisture e.g. condensation, on the surfaces above the sewerage to produce sulphuric acid. The sulphuric acid has a low pH and thus attacks the concrete. It is not unusual in poorly ventilated enclosed sewerage systems to find significant damage to the underside of the crown of concrete pipes and/or the inner faces of upper portions of precast concrete manholes.
Damage has also been reported on the underside of concrete covers or roofs over enclosed sewage tanks.