Friday, September 5, 2008

Water Softening

A water softener reduces the calcium or magnesium ion concentration in hard water. 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 adhere to the surfaces of pipes and heat exchanger surfaces. 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 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.

Water softeners are typically used when water is supplied from wells. Usually public water systems have low hardness so individual consumers need not have their own water softening equipment.

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 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.

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, many jurisdictions prohibit such release and require users to dispose of the spent brine at an approved site or to use a commercial service company.