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 scale, 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, that is, trade places with, 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

Methods for water softening

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.

Since Ca²⁺ and Mg²⁺ exist as nonvolatile salts, they can be removed by distilling the water, but distillation is too expensive in most cases (rainwater is soft because it is, in effect, distilled).

Ion-exchange resin devices

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 exhibits ion-exchange properties; these minerals are widely used in laundry detergents.

How it works

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 (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.

Regeneration

The resin's capacity is gradually exhausted and eventually it contains only divalent ions, Mg²⁺ and Ca²⁺ for cation exchange resins, and SO₄^2- 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.

In industrial scale water softening plants, the effluent flow from re-generation process can precipitate scale that can interfere with sewerage systems.

Other methods

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 EDTA.

Physical conditioners

Physical conditioners claim that subjecting water to a magnetic field or radio waves provide similar benefits to water softening. While physical conditioning does not remove calcium and magnesium and therefore cannot claim to soften water in the traditional sense, there are opinions that "physical conditioning can make water seem softer." One claim is that "the effect of the physical conditioning, or presence of zinc, is to cause the calcium salts to precipitate differently such that they are less encrusting." If true, this would require many chemistry and physical chemistry textbooks to be rewritten.

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 litres (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 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 the 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, which would have the added benefit of helping to lower blood pressure, although costly. However, this should be done carefully as elevated potassium levels are dangerous and can lead to complications such as cardiac arrhythmia, although a person with normal kidney function would have to consume a large amount of potassium to develop hyperkalemia.

See also

• Ion exchange
• Water purification
• Descaling agent
• Desalination

References

External links

• Wilkes University hard water site
• Riverside County CA water softener restrictions
• 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