lkalinity is important in the water treatment plant because it promotes coagulation and inhibits corrosion. Here, we will be concerned with the amount of alkalinity which must be present in the finished water to prevent corrosion in the pipes of the distribution system.
Corrosive water results from low alkalinity, which tends to promote acidic water. Since coagulation uses up alkalinity, operators must sometimes add lime or soda ash to water to prevent corrosive finished water. However, there is no set amount of alkalinity which must be present in water to prevent corrosion. Instead, the relation between alkalinity and pH is usually the determining factor as to whether or not the finished water will be corrosive. The following three requirements must be met in order for treated water to be non-corrosive:
1. The phenolphthalein alkalinity, calculated as CaCO3, should not be greater than:
15 ppm + (0.4 × T. alk.)
This requirement limits the permissible pH to about 10.6 at 25°C (77°F).
2. The normal carbonate alkalinity should not exceed 120 ppm. This requirement may be met by keeping the total alkalinity within the limits suggested in the table below. These values apply to water at 25°C (77°F)