For convenience, hard water is classified as
Temporary hardness is due to the presence of soluble bicarbonates of calcium, magnesium and iron in water.
Permanent hardness is caused by the presence of the chlorides and sulphates of calcium and magnesium in the water as chlorides and sulphates.
Temporary hardness can be removed by:
When the soluble bicarbonates are converted into insoluble carbonates which are precipitated and CO2 is evolved.
Ca (HCO3)2 CaCO3 + H2O + CO2
Mg (HCO3)2 MgCO3 + H2O + CO2
Fe (HCO3)2 = FeCO3 + H2O + CO2
(4FeCO3 + 6H2O + 4Fe(OH)3 + 4CO2)
While CaCO3 and MgCO3 form white powdery precipitate, iron is precipitated as slimy brown precipitate of Fe(OH)2. Magnesium bicarbonate hardness is not fully removed by boiling, as MgCO3 is appreciably soluble in water compared to CaCO3.
Temporary hard water is softened for industrial purposes by adding the calculated quantity of slaked lime which precipitates the insoluble carbonates.
Ca(HCO3)2 + Ca(OH)2 2CaCO3 + 2H2O
Mg(HCO3)1 + Ca(OH)2 MgCO3 + CaCO3 + 2H2O
The MgCO3. which is appreciably soluble, then reacts with further quantity of Ca(CH)2 to form insoluble Mg (OH)2
MgCO3 Ca(OH)2 = Mg(OH)2 CaCO3
Mg(OH)2 is sparingly soluble and hence removed.
Slaked lime must be added in extremely calculated quantities, as any excess of Ca(OH)2 present in the softened water would again cause hardness. The mixing of lime and water is done in large settling tanks. After the precipitates have settled down, the clear soft water is pumped to reservoirs for distribution.
Permanent hardness of water is caused by the presence of the chlorides and sulphates of calcium and magnesium in the water. While temporary water hardness can be removed by boiling or adding lime, permanent hardness cannot be removed by those processes since such methods do not precipitate the calcium or magnesium of hard water.
Permanently hard water is softened by the addition of Na2CO3 to water, when insoluble CaCO3 and MgCO3 precipitate. The insoluble carbonates can then be removed by filtering through a water treatment filter process.
CaSO4 Na2CO3 = CaCO3 Na2SO4
MgCl2 Na2CO3 = MgCO3 2NaCl
It must be noted that along with the chlorides and sulphates of calcium and magnesium (permanent hardness), there are also present the bicarbonates in some quantities (temporary hardness. Hence method or methods capable of removing both types of water hardness is adopted.