Removal of hardness from water is called softening is done by ion exchange process. The ion exchange process relies on replacement of the Calcium and Magnesium ions in the water by an equivalent number of Sodium ions. This eliminates the unwanted charcteristics of hard water because the sodium salts neither form scale nor a scum with soap but only lather. The Resin will take up the Calcium ion to the extent of what sodium ions the resin will have with it. Therefore, to give new sodium ions to the resin, the cheapest and easiest way would be to pass the solution of sodium - chloride (common salt) through the resin. The same resin can be used over and over again to produce soft water by giving periodic of sodium chloride (common salt) solution. So, recurring cost in such process is only the cost of common salt.
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