Water softening
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A water softener reduces the calcium or magnesium 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. 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 used when water is supplied from wells. Public water systems are also susceptible to hard water, although this is much less common.
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[edit] 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 always positively charged. The resins initially contain univalent 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 sodium or potassium ions which are released into the water. The "harder" the water, the more 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."[1]
[edit] Regeneration
As these resins become converted to their Ca2+ form they gradually lose their effectiveness and must be regenerated. This is done by passing a concentrated brine solution through them, causing the above processes to be reversed. This is a drawback, since most of the salt used for regeneration gets flushed out of the system and may be released into the soil or sewer. This can be damaging to the environment, especially in arid regions. For this reason, 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. Canada, as an example, allows regeneration flushes throughout the entire country.
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 should be used to minimize the impact of water softeners on the environment.
[edit] See also
[edit] References
- ^ Michael H. Bradshaw, G. Morgan Powell (October 2002). Sodium in Drinking Water. Kansas State University. Retrieved on April 3, 2007.
