Excess
Lime
Softening - Magnesium carbonate hardness can be treated by this method. The above two methods cannot be used to remove magnesium effectively. Because, if lime is added to magnesium bicarbonate it will result in soluble magnesium carbonate.
Mg (HCO3)2 + Ca (OH2 2CaCO3 + MgCO3 + 2H20
However, at a pH of about 11,
MgCO3 + Ca (OH}2 Mg(OH)2 + CaCO3
Magnesium hydroxide has a solubility of about 10 mg/l and the remaining precipitates. Thus, excess lime is necessary to react with HCO3 - and Mg++. In addition, to raise the pH Ca(OH)2 is added at a concentration of 50mg/l.
This high pH level provides disinfection and may eliminate the chlorination procedure. Carbonation after treatment removes excess lime and reduces pH.
