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 ion-exchange resins.
Problems with hard water:
The presence of certain metal ions in water causes a variety of problems. These ions interfere with the action of soaps. They also lead to build up of limescale, which can foul plumbing, and galvanic corrosion. In industrial scale water softening plants, the effluent flow from re-generation process can precipitate scale that can interfere with sewerage systems.
The slippery feeling experienced when using soap with soft water occurs because soaps tend to bind to fats in the surface layers of skin, making soap molecules difficult to remove by simple dilution. In contrast, in hard-water areas the rinse water contains calcium and/or magnesium ions which form insoluble salts, effectively removing the residual soap from the skin but potentially leaving a coating of insoluble stearates on tub and shower surfaces, commonly called soap scum.
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.
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 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/L/gpg = 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. 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 osmosisunit 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.