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A water softener reduces the calcium, magnesium, and to some degree manganese and ferrous iron ion concentration in hard water. These "hardness ions" cause three major kinds of problems. The metal ions react with soaps and calcium sensitive detergents, hindering their ability to lather properly and forming an unsightly precipitate— the familiar scum or "bathtub ring". Presence of "hardness ions" also inhibits the cleaning effect of detergent formulations. More seriously, calcium and magnesium carbonates tend to adhere to the surfaces of pipes and heat exchanger surfaces. The resulting scale build-up can restrict water flow in pipes. In boilers, the deposits act as thermal insulation that impedes the flow of heat into the water; this not only reduces heating efficiency, but allows the metal to overheat which, in a pressurized system, can lead to failure." ^[1] The presence of ions in an electrolyte can also lead to galvanic corrosion, in which one metal will preferentially corrode when in contact with another type of metal. The use of water softeners can aggravate this and cause sacrificial anodes in hot water heaters to corrode more quickly.

Conventional water-softening devices intended for household use depend on an ion-exchange resin in which "hardness" ions trade places with sodium ions that are electrostatically bound to the anionic functional groups of the polymeric resin. A class of minerals known as zeolites also exhibits ion-exchange properties; these minerals were widely used in earlier water softeners.

Water softeners are typically required when the source of water is a well, municipal or private.

The water to be treated passes through a bed of the resin. Negatively-charged resins absorb and bind metal ions, which are positively charged. The resins initially contain univalent hydrogen, sodium or potassium ions, which exchange with divalent calcium and magnesium ions in the water. This exchange eliminates precipitation and soap scum formation.

As the water passes through both kinds of resin, the hardness ions replace the hydrogen, sodium or potassium 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.

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. A paper by Kansas State University gives an example: "A person who drinks two liters (2L) of softened, extremely hard water (assume 30 gpg) will consume about 480 mg more sodium (2L x 30 gpg x 8 mg/L/gpg = 480 mg), than if unsoftened water is consumed." This is a significant amount, as they state: "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 500 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."^[2]

Most people who are concerned with the added sodium in the water generally have one faucet 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.

As these resins become loaded with hardness ions they gradually lose their effectiveness and must be regenerated by passing a concentrated brine or acid solution through them. Most of the salt used for regeneration gets flushed out of the system and may be released into the soil or sewer. The use of hydrochloric or sulfuric acid has been used as a regenerant. These processes can be damaging to the environment, especially in arid regions.^{[citation needed]} Some jurisdictions prohibit such release and require users to dispose of the spent brine at an approved site or to use a commercial service company.

Most water softener manufacturers provide metered control valves to minimize the frequency of regeneration. It is also possible on most units to adjust the amount of salt used for each regeneration. Both of these steps are recommended to minimize the impact of water softeners on the environment and conserve on salt use.^{[citation needed]} Using acid to regenerate lowers the pH of the regeneration waste.

In industrial scale water softening plants, the effluent flow from re-generation process can be very significant. Under certain conditions, such as when the effluent is discharged in admixture with domestic sewage, the calcium and magnesium salts may precipitate out as hardness scale on the inside of the discharge pipe. This can build up to such an extent so as to block the pipe as happened to a major chlor-alkali plant on the south Wales coast in the 1980s.^{[citation needed]}

• Ion exchange
• Water purification
• Descaling agent
• Desalination

1. ^ Stephen Lower (July 2007). Hard water and water softening. Retrieved on 2007-10-08.
2. ^ Michael H. Bradshaw, G. Morgan Powell (October 2002). Sodium in Drinking Water. Kansas State University. Retrieved on 2007-04-03.

• About hard water
• Wilkes University hard water site
• Riverside County CA water softener restrictions
• Water softener diagram
• Water softener regeneration
• Gallery of water-related pseudoscience - Chemist's site providing explanations of how alternatives to water softening, such as catalytic water conditioning, are not scientifically grounded