Removal of Hardness of Water

Removal of Temporary Hardness

Temporary hardness present in the water can be removed by using the following methods: -

By heating:

Salts like calcium bicarbonate and magnesium bicarbonate, when dissolved in water causes temporary hardness. While heating, soluble salts of calcium and magnesium (bicarbonates) change into insoluble salts. The chemical equations are as follows:

Calcium bicarbonate → Calcium carbonate + Carbon dioxide + Water
Ca(HCO₃)₂ → CaCO₃ + CO₂ + H₂O

Magnesium bicarbonate → Magnesium carbonate + Carbon dioxide + Water
Mg(HCO₃)₂ → MgCO₃ + CO₂ + H₂O

Treating with lime water:

The temporary hardness of water can be removed by treating it with lime water. The chemical equations are as follows:

Calcium Bicarbonate + Calcium Hydroxide → Calcium Carbonate + Water
Ca(HCO₃)₂ + Ca(OH)₂ → 2CaCO₃ + 2H₂O

Magnesium Bicarbonate + Calcium Hydroxide → Calcium Carbonate + Magnesium Carbonate + Water
Mg(HCO₃)₂ + Ca(OH)₂ → CaCO₃ + MgCO₃ + 2H₂O

Removal of Permanent Hardness

Permanent hardness cannot be removed by heating only. Soluble salts of calcium and magnesium are turned to insoluble salts by the following methods.

Treatment with washing soda

Soluble salts of calcium and magnesium change into insoluble salts due to the treatment with sodium carbonate (washing soda).

Calcium Chloride + Sodium Carbonate → Calcium Carbonate + Sodium Chloride
CaCl₂ + Na₂CO₃ → CaCO₃ + 2NaCl

Magnesium Sulphate + Sodium Carbonate → Magnesium Carbonate + Sodium Sulphate
MgSO₄ + Na₂CO₃ → MgCO₃ + Na₂SO₄

Permutit Process:

Permutit is the trade name for an artificially prepared sodium zeolite or sodium aluminium orthosilicate obtained by fusing together sodium carbonate silica and alumina. Its composition resembles Na₂Al₂SiO₈ or Na₂Z, where, Z stands for Zeolite = Al₂SiO₈. It possesses the higher quality of reactivity with calcium and magnesium ions forming insoluble calcium or magnesium zeolites.

2Na₂Z + Ca⁺⁺ → CaZ₂ +4Na

Thus by passing hard water through a cylinder packed with permutit, all the ions of soluble salts of calcium and magnesium responsible for the hardness of water are retained and replaced by sodium ions. In due course, however, the permutit is exhausted or losses its capacity of reacting with these salts by having given up all its sodium ions. This can be regenerated by soaking in a 10% solution of common salt when the reverse change takes place and calcium and magnesium ions pass into solution and sodium zeolite is formed again.

CaZ₂ + 2NaCl → Na₂Z + CaCl₂
MgZ₂ + 2NaCl → Na₂Z + MgCl₂

The resulting calcium and magnesium chlorides are washed out and the regenerated permutit is used over again.