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Concept
Water softening
Summary
Water softening is the removal of calcium, magnesium, and certain other metal cations in hard water. The resulting soft water requires less soap for the same cleaning effort, as soap is not wasted bonding with calcium ions. Soft water also extends the lifetime of plumbing by reducing or eliminating scale build-up in pipes and fittings. Water softening is usually achieved using lime softening or ion-exchange resins but is increasingly being accomplished using nanofiltration or reverse osmosis membranes.
Hard water
The presence of certain metal ions like calcium and magnesium, principally as bicarbonates, chlorides, and sulfates, in water causes a variety of problems.
Hard water leads to the buildup of limescale, which can foul plumbing, and promote galvanic corrosion. In industrial scale water softening plants, the effluent flow from the re-generation process can precipitate scale that can interfere with sewage systems.
The slippery feeling associated with washing in soft water is caused by the weaker attraction of the soap to the water ions when the water has been stripped of its mineral content. The surface of human skin has a light charge that the soap tends to bind with, requiring more effort and a greater volume of water to remove. Hard water contains calcium or magnesium ions that form insoluble salts upon reacting with soap, leaving a coating of insoluble stearates on tub and shower surfaces, commonly called soap scum.
The most common means for removing water hardness rely on ion-exchange resin or reverse osmosis. Other approaches include precipitation methods and sequestration by the addition of chelating agents.
Distillation and reverse osmosis are the most widely used two non-chemical methods of water softening.
Conventional water-softening appliances intended for household use depend on an ion-exchange resin in which "hardness ions"—mainly Ca2+ and Mg2+—are exchanged for sodium ions. As described by NSF/ANSI Standard 44, ion-exchange devices reduce the hardness by replacing magnesium and calcium (Mg2+ and Ca2+) with sodium or potassium ions (Na+ and K+).
Official source
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