Hard water is caused by dissolved calcium and magnesium in the water. Hard water interferes with laundering, washing dishes, bathing, and personal grooming. It also affects appliances. For example, scale builds up in water heaters, increasing the costs of heating water and reducing the life of the appliance. Hard water, when used with soap, causes soap deposits that will not dissolve.
Water is softened by passing through a bed of ion-exchange resin.
The softening process exchanges calcium and magnesium ions in the water for sodium ions in the resin. Approximately 15 mg of sodium are added per gallon for each grain of hardness reduced. When the sodium is exhausted, the softener needs to be regenerated. However, if there is oxidized iron or iron bacteria in the water, the ion exchange resin becomes coated or clogged and loses its softening ability. The size of a water softener depends on the hardness of water before softening, the quantity of water to be softened, and length of time between recharging.