Water softening

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A water softener reduces the calcium or magnesium ion concentration in hard water.

Problems with hard water

Water that has been in contact with limestone and other sediments tends to acquire dissolved ions, mainly calcium Ca²⁺ and magnesium Mg²⁺. The positive electrical charges of these ions are balanced by the presence of anions (negative ions), of which bicarbonate HCO₃^– and carbonate CO₃^2– are the most important. These ions are derived from carbon dioxide which is present in all water exposed to the atmosphere and also in groundwater.

These "hardness ions" cause two 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 precipitate out as adherent solids on the surfaces of pipes and especially on the hot heat exchanger surfaces of boilers. The resulting scale buildup 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 catastrophic failure.

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.

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 ions, which exchange with divalent calcium and magnesium ions in the water. This exchange eliminates the possibility of precipitation and soap scum formation.

As the water passes through both kinds of resin, the hardness ions replace the sodium which are released into the water. For most purposes, the low levels of salt in the treated water are innocuous. However because of the increase in sodium concentration, some people believe water softened in this way is not suitable for regular consumption.

Regeneration

As these resins become converted to their Ca²⁺ form they gradually lose their effectiveness and must be regenerated. This is accomplished by passing a concentrated brine solution though them, causing the above processes to be reversed. Herein lies one of the drawbacks of this system: most of the salt employed in the regeneration process gets flushed out of the system and may be released into the soil or drainage— something that can have damaging consequences to the environment, especially in arid regions. For this reason, many jurisdications prohibit such release and require users to dispose of the spent brine at an approved site or to use a commercial service company.

See also

• Ion exchange
• Water purification
• Descaling agent

External links

• [About hard water]
• [Wilkes University hard water site]
• [Water Softener REsource]
• [How Stuff Works]
• [Riverside County CA water softener restrictions]
• [How to size a water softener]
• ["Junk science" article on dubious water-treatment processes and products]
• [Understanding the Water Softening Process]

 

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