Water softening (water softener) 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.
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.
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 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.
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.
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.
-wikipedia-
