Organic sulphates and inorganic sulphides are present in the slimes of sewerage flows and form the total sulphides present in the sewerage.
A fraction of the dissolved hydrogen sulphide is released into the sewer atmosphere as a gas.
A process of bacterial reduction occurs when the hydrogen sulphide, in combination with the other gases, such as carbon dioxide, is progressively oxidized, reducing the Ph to the point where it splits off with water molecules to form sulphuric acid.
The powerful inorganic sulphuric acid reacts with hydrated calcium sili- cates and lime in the cement structure and reduces it to a soft swelling paste.
Conditions are optimal for this process of sulphide attack when there is biological oxygen demand, presence of sulphates (Common in all water systems), higher ambient temperatures (quickening the kinetics of break- down and reduction), long detention time (flat grades – low velocities) Provision of a lining made of flexible plastic that is chemically inert when subjected to sulphuric acid attack. The lining shall be made from thermo-plastic PE or other flexible plastic, provided there are no pin-holes and that an effective way of attaching the plastic to the concrete is devised. Paint or coatings do not adhere very well to concrete - neither do glues or adhesives – problem.v