Water Softening Plants: is the removal of calcium, magnesium, and certain other metal cations in hard water. The resulting soft water is more compatible with soap and extends the lifetime of plumbing. Water softening is usually achieved using  ion-exchange resins.Ion exchange resins are organic polymers containing anionic functional groups to which the divalent cations (Ca⁺⁺) bind more strongly than monovalent cations (Na⁺).. These minerals are widely used in laundry detergents. Resins are also available to remove carbonate, bi-carbonate and sulphate 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 hydroxide depending on the type of resin used.For anionic resins, regeneration typically uses a solution of sodium hydroxide 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.