Water softening is the removal of calcium, magnesium, and certain other metal cations in hard water. The resulting soft water is more compatible with soap and extends the lifetime of plumbing. Water softening is usually achieved using lime softening or ion-exchange resins.
Water Softening Methods
Practical means for softening water rely on ion-exchange polymers or reverse osmosis. Other approaches include precipitation methods and sequestration by the addition of chelating agents. Devices that claim to use magnetism or electricity as a "water softening" technique are fraudulent and often utilize pseudoscience.
Ion-exchange resin devices
Conventional water-softening appliances intended for household use depend on an ion-exchange resin in which "hardness ions" - mainly Ca2+ and Mg2+- are exchanged for sodium ions.
A person who drinks two liters (2 L) of softened, extremely hard water (assume 30 gpg) will consume about 126 mg more sodium (2L or 0.528 gallon x 30 gpg x 8 mg/grain = 126 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.[7] Most people who are concerned with the added sodium in 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, although it is more costly. However, elevated potassium levels are dangerous for people with impaired kidney function; it can lead to complications such as cardiac arrhythmia.
Hard water also conveys some benefits to health by reducing the solubility of potentially toxic metal ions such as lead and copper, which are more soluble in acidic, soft water than in alkaline, hard water.