Water softening pellets flowing from the bottom of a pellet reactor. Water softening is the process of removing dissolved calcium ions and other mineral ions. This avoids problems with limescale formation in domestic appliances and pipes, and soap scum in water that is used for washing. A pellet reactor uses pellets (here, fine grains of sand) as a nucleus on which the calcium carbonate and other minerals can form. Water is piped into the bottom of the reactor and flows out of the top of the reactor. Minerals deposited on the sand grains make them heavier, and they settle to the bottom of the reactor. Sodium ions are used to replace the calcium ions. Photographed in France.