Lime
Softening - This method is used for calcium hardness of carbonate form. Lime is added in equivalent amount to that of bicarbonate present and this result in the formation of insoluble calcium carbonate.
The solubility of CaCO3 at normal temperature is about 2Omg/l. The contact time in the tank is limited and usually a residual of about 40 mg/l CaCO3 results. Thus the softened water is saturated with CaCO3 which may deposit as a scale in the distribution system. This can be prevented by carbonation of CaCO3 which results in the formation of calcium bicarbonate
