Water softening is the reduction of the concentration of calcium, magnesium, and other ions in hard water. These “hardness ions” can cause a variety of undesired effects including interfering with the action of soaps, the 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 in which hardness ions are exchanged for sodium ions. Water softening may be desirable where the source of water is hard. However, hard water also conveys some benefits to health by providing dietary calcium and magnesium and reducing the solubility of potentially toxic metal ions such as lead and copper.
Water softening methods mainly rely on the removal of Ca2+ and Mg2+ from a solution or the sequestration of these ions, i.e. binding them to a molecule that removes their ability to form scale or interfere with detergents. Removal is achieved by ion exchange and by precipitation methods. Sequestration entails the addition of chemical compounds called sequestration (or chelating) agents.
Ion-exchange materials contain sodium ions (Na+) that are electrostatically bound and that readily are replaced by hardness ions such as Ca2+ and Mg2+. Ion exchange resins are organic polymers containing anionic functional groups to which the Na+ is bound. Minerals called zeolites also exhibit ion-exchange properties; these minerals are widely used in laundry detergents.
The water to be treated passes through a bed of the resin. Negatively-charged resins absorb and bind metal ions, which are positively charged (2RNa(s)+M2+(aq)=R2M(s)+2Na+(aq)(M=Mg/Ca)). The resins initially contain univalent (1+) ions, most commonly sodium, but sometimes also hydrogen (H+) or potassium (K+). Divalent calcium and magnesium ions in the water replace these univalent ions, which are released into the water. The “harder” the water, the more hydrogen, sodium or potassium ions are released from the resin and into the water.
Resins are also available to remove carbonate, bi-carbonate and sulphate ions which are absorbed and hydroxyl ions released from the resin. Both types of resin may be provided in a single water softener.
The resin’s capacity is gradually exhausted and eventually it contains only divalent ions, Mg2+ and Ca2+ for cation exchange resins, and SO42- for anion exchange resins. At this stage, the resin must be regenerated. If a cationic resin is used (to remove calcium and magnesium ions) then regeneration is usually effected by passing a concentrated brine, usually of sodium chloride or potassium chloride, or hydrochloric acid solution through them. For anionic resins, regeneration typically uses a solution of sodium hydroxide (lye) or potassium hydroxide. The salts used for regeneration are released into the soil or sewer.