Zero hardness of water is achieved by

Explanation:

Water softening: 

• It is the process of hardness removal from the water. It is caused by multivalent cation and affects water quality.
   

Lime soda method: 

• It is a water softening method in which lime and soda ash are added to the water, which causes the precipitation of multivalent cation as CaCO3.
• Precipitation of CaCO3 occurs only when the pH of water is greater than 9, so in case of less pH alkalinity is added to the water. In this process small amount of Ca2+ and Mg2+ precipitates very late, which will create incrustation in the pipe, so to avoid this recarbonation is done to dissolve back this small amount of cation.
• Due to this, the method does not give zero hardness.
   

Ion Exchange Process: 

• Ion-exchange resin, (zeolite) exchanges one ion from the water being treated for another ion that is in the resin (sodium is one component of softening salt, with chlorine being the other).
• Zeolite resin exchanges sodium for calcium and magnesium. It can produce water with zero hardness.