Water softener systems use a process called ion exchange to remove calcium and magnesium, which are the trademarks of hard water. Overly hard water can make household cleaning difficult, as it often leaves scum on kitchen and bathroom fixtures, dulls clothing, and leaves spots on dishes. Water softening must be used with care, however, because it exchanges sodium for the minerals that give water hard characteristics, and it may be harmful for people on low-sodium diets. Sodium produced by softening water can also be harmful to plants. The different types of water softener systems all utilize ion exchange, but they vary in the amount of input they require from the operator.
Hard water originates from groundwater sources, such as aquifers, because calcium, magnesium, and other minerals dissolve into the water from the surrounding rocks. The calcium and magnesium cause the characteristics that are referred to as hard. Hardness is measured in grains per gallon (GPG), with one grain of hardness equal in weight to one kernel of wheat. Any water with greater than 1 GPG of calcium and magnesium is considered hard, with greater than 10.5 GPG being classified as very hard.
Chemicals are sometimes added to laundry detergent to soften water used for washing clothes, but all water softener systems that treat the water used by an entire house use ion exchange. An ion is an electrically-charged molecule, and different ions may have different charge strengths. Water softeners contain an exchange medium, which starts out coated in positively-charged sodium ions. The calcium and magnesium ions in hard water are also positively-charged, but they have a stronger charge than the sodium ions. This means that when water passes through the exchange medium, the calcium and magnesium ions are captured by the medium, while the sodium ions are freed from it, because the calcium and magnesium ions have a stronger attraction.
Eventually, the exchange medium in a water softener system becomes completely coated in calcium and magnesium ions. When this happens the softener is backwashed with a sodium chloride solution, a process known as recharging. The calcium and magnesium ions form compounds with the chlorine and are washed away, while the sodium ions again become attached to the exchange medium.
Different types of water softener systems vary according to how much work the user has to perform in order to recharge them. The most popular type of home water softeners are automatic softeners, also sometimes called fully automatic softeners, which use a timer set by the owner to automatically recharge the system. Another type of water softener that operates automatically is demand initiated regeneration (DIR), which determines when the system needs recharged based on the amount of water that has been used.
Both manual and semi-automatic systems require more input from the user. A semi-automatic water softener requires the owner to manually tell the softener when to recharge. Manual water softeners require the user to set all the parameters for recharging the softener, including when the system is recharged and for how long.