Drinking water treatment. Softened water flowing from the top 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. This forms the cloudy water seen here, with the enlarged sand grains settling to the bottom of the reactor. The softened water flows over the top as more water is added from the bottom. Sodium ions are used to replace the calcium ions. Photographed in France.