Corrosion is the rusting of metal in pipes or tanks due to the corrosive action of water, waste-water or soil. Scales are deposits caused by the deposition of calcium carbonate on the inner surface of the water or waste-water treatment equipment pipes or tanks.
Stable water is the water, which tends to be neither corrosive nor scale-forming. Corrosive, also known as aggressive or unstable, water will tend to corrode (rust) metal in the pipes or tanks it passes through.
Scale-forming water will tend to deposit calcium carbonate scale on the inner surfaces of these pipes or tanks. The above is more characteristic in water treatment plant, but for the waste-water, it involves other things as shall be discussed soon.
Causes of Corrosion in waste-water Treatment Plant
Bacteria in the slime under flowing sewage convert sulphates in the sewage into sulphides. Sulphides in the liquid make their way to the surface of the sewage and released into the sewer atmosphere as hydrogen sulphide (H2S) gas.
Read Also : Water and Waste-water Purification Process
H2S gas in atmosphere makes contact with slime in the crown of the sewer, which contains more bacteria. Bacterial action converts H2S gas to sulphuric acid which causes corrosion in the crown of the pipe and this corrosion is also called crown corrosion.
If pipe material is of corrodible nature, sulphuric acid attacks the pipe material and causing ultimate failure.
Fig.: The Chemical Actions during Corrosion
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