Explain Some Points regarding to Hardness

1) Hardness removal using lime and soda is based on the solubility reactions of calcium carbonate, CaCO₃ and magnesium hydroxide, Mg(OH)₂  

2) Precipitate Ca⁺² as CaCO₃ and Mg⁺² as Mg(OH)₂ 

3) pH must be raised to 10.3 for Ca⁺² removal and to about 11 for Mg⁺² removal

4) Remove Ca-CH, Mg-CH and Ca-NCH and Mg-NCH in that order because the cost of removal increases in the same order

5) Leave as much Mg hardness as possible and as much NCH as possible, but Mg Conc. of above 40 mg/l as CaCO₃ causes problems 

6) It is a common practice to use a minimum of 20 mg/l as CaCO₃ of excess lime to achieve reasonable removal.