The presence of certain metal ions in water causes a variety of problems. These ions interfere with the action of soaps. They also lead to a build-up of limescale, which can foul plumbing and galvanic corrosion.
Conventional water-softening appliances intended for household use depend on an ion-exchange resin, the ion exchange resin for water softeners are used extensively in small water systems and individual homes apart from Industry. Ion-exchange resin, 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). Ion Exchange resin exchanges sodium for calcium and magnesium. The following chemical reactions show the exchange process, where R represents Resin, the exchange material.
Removal of carbonate hardness:
Ca(HCO3)2 + Na2R ——> CaR + 2NaHCO3
Mg(HCO3)2 + Na2R ——> MgR + 2NaHCO3
Removal of non-carbonate hardness:
CaSO4 + Na2R —– > CaR + Na2SO4
CaCl2 + Na2R ——> CaR + CaCl2
MgSO4 + Na2R ——> MgR + Na2SO4
MgCl2 + Na2R ——> MgR + 2NaC1
Ion exchange resin is organic polymers containing anionic functional groups to which the divalent cations (Ca++) bind more strongly than monovalent cations (Na+).
Ion-exchange does not alter the water’s pH or alkalinity. However, the stability of the water is altered due to the removal of calcium and magnesium and a slight increase in dissolved solids. For each ppm of calcium removed and replaced by sodium, total dissolved solids increase by 0.15 ppm. For each ppm of magnesium removed and replaced by sodium, total dissolved solids increase by 0.88 ppm.
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. The wastewaters eluted from the ion exchange column containing the unwanted calcium and magnesium salts are typically discharged to the sewage system in case of household units.