Water softening 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.
Problems with hard water 
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 build up of limescale, which can foul plumbing, and galvanic corrosion. In industrial scale water softening plants, the effluent flow from re-generation process can precipitate scale that can interfere with sewerage systems.
Soap scum 
The slippery feeling experienced when using soap with soft water occurs because soaps tend to bind to fats in the surface layers of skin, making soap molecules difficult to remove by simple dilution. In contrast, in hard-water areas the rinse water contains calcium and/or magnesium ions which form insoluble salts, effectively removing the residual soap from the skin but potentially leaving a coating of insoluble stearates on tub and shower surfaces, commonly called soap scum.
Water softening methods 
Practical means for softening water rely on ion-exchange polymers or reverse osmosis. Other approaches include precipitation methods and sequestration by the addition of chelating) agents. Devices that claim to use magnetism or electricity as a "water softening" technique are fraudulent.
Ion-exchange resin devices 
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+)."
Types of ion exchange materials 
Ion exchange resins are organic polymers containing anionic functional groups to which the dications (Ca++) bind more strongly than monocations (Na+). Inorganic materials called zeolites also exhibit ion-exchange properties. 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.
Regeneration of ion exchange resins 
When all the available Na+ ions have been replaced by calcium or magnesium ions, the resin must be re-charged by eluting the Ca2+ and Mg2+ ions using a solution of sodium chloride or sodium hydroxide depending on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 (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 sewerage system.
Reverse osmosis 
Reverse osmosis is the other major technology for water softening. It is mainly deployed on a large scale rather than for individual domestic applications. As described by NSF/ANSI Standard 44, it "reverses, by the application of pressure, the flow of water in a natural process of osmosis so that water passes from a more concentrated solution to a more dilute solution through a semi-permeable membrane. Most reverse osmosis systems incorporate pre- and post-filters along with the membrane itself."
Lime softening 
Chelating agents 
Chelators are used in chemical analysis, as water softeners, and are ingredients in many commercial products such as shampoos and food preservatives. Citric acid is used to soften water in soaps and laundry detergents. A commonly used synthetic chelator is ethylenediaminetetraacetic acid (EDTA).
Distillation and rain water 
Since Ca2+ and Mg2+ exist as nonvolatile salts, they can be removed by distilling the water. Distillation is too expensive in most cases. Rainwater is soft because it is naturally distilled during the water cycle of evaporation, condensation and precipitation.
Health effects 
Effects of sodium 
For people on a low-sodium diet, the increase in sodium levels (for systems releasing sodium) in the water can be significant, especially when treating very hard water. For example:
A person who drinks two liters (2 L) of softened, extremely hard water (assume 30 gpg) will consume about 126 mg more sodium (2L or 0.528 gallon x 30 gpg x 8 mg/L/gpg = 126 mg), than if unsoftened water is consumed.
This amount is significant. The American Heart Association (AHA) suggests that the 3 percent of the population who must follow a severe, salt-restricted diet should not consume more than 400 mg of sodium a day. AHA suggests that no more than 10 percent of this sodium intake should come from water. The EPA’s draft guideline of 20 mg/L for water protects people who are most susceptible. Most people who are concerned with the added sodium in water generally have one tap in the house that bypasses the softener, or have a reverse osmosis unit installed for the drinking water and cooking water, which was designed for desalinisation of sea water. Potassium chloride can also be used instead of sodium chloride, although it is more costly. However, elevated potassium levels are dangerous for people with impaired kidney function; it can lead to complications such as cardiac arrhythmia.
See also 
- Stephen Lower (July 2007). "Hard water and water softening". Retrieved 2007-10-08.
- Elmhurst College - Cleansing action of soap.
- "Water Softeners". Canadian Mortgage and Housing Corporation. Retrieved 2010-01-29.
- Filtration Facts, September 2005, U.S. Environmental Protection Administration, pp. 6-7. Accessed 6 January 2013.
- New Hampshire Dept of Environmental Services-Ion Exchange Treatment of Drinking Water
- Michael H. Bradshaw, G. Morgan Powell (October 2002). "Sodium in Drinking Water". Kansas State University. Retrieved 2007-04-03.