To softenize the hard water by the ex-change way.

Ion exchange resins are organic polymers containing anionic functional groups to which the divalent cations (Ca++) bind more strongly than Valentino cations (Na+). Inorganic materials called zeolitesalso exhibit ion-exchange properties. These minerals are widely used in laundry detergents. Resins are also available to remove carbonate, bi-carbonate and sulfate ions which are absorbed and hydroxide ions released from the resin.

When all the available Na+ ions have been replaced with calcium or magnesium ions, the resin must be re-charged by eluting the Ca2+ and Mg2+ ions using a solution of sodium chloride or sodium hydroxidedepending on the type of resin used. For anionic resins, regeneration typically uses a solution of sodium hydroxide (lye) or potassium hydroxide. The waste waters eluted from the ion-exchange column containing the unwanted calcium and magnesium salts are typically discharged to the sewage system