Depending on regional water quality, post-treatment in the home is often worthwhile. The main target is hardness: the sum of alkaline earth ions, among which calcium causes the greatest trouble. Together with carbonate present in virtually all drinking water, calcium forms calcium carbonate, or lime, which crystallises as white scale on pipes, fittings and heat-transfer surfaces in boilers, dishwashers and washing machines, and can ultimately damage household installations.
Water with high lime content is called hard; low lime content, soft. Two ion exchange approaches reduce hardness: complete softening and partial softening. Both rely on the same strong-acid cation resin exchanging calcium and magnesium for sodium or potassium, differing in how much hardness is left in the finished water and how much salt the cycle consumes.
Beyond appliance protection, softened water noticeably improves the taste of coffee and tea, and reduces soap and detergent consumption.