Excess Lime-softening:
In this process, lime is added in excess. Magnesium temporary hardness is removed when an equivalent amount of lime is added. Magnesium carbonate is produced, which does not precipitate in cold water. When pH is raised to 11 with excess lime, the insoluble magnesium hydroxide is produced, which has a solubility of about 10 mg/l.
Mg(HCO3)2 + Ca(OH)2 → CaCO3 + MgCO3 + 2H2O
MgCO3 + Ca(OH)2 → Mg(OH)2 + CaCO3
The high pH level produces good disinfection as a by-product and thus, chlorination may not be required after excess lime softening. Carbonation is necessary to remove the excess lime and reduce the pH after such softening.