Water softeners are ion exchangers .
In the softener, hard water flows through a tank filled with synthetic resin beads. Millions of sodium ions are loosely attached to each bead. When the resin becomes saturated with hardness ions, it must be regenerated in order to work again. During regeneration, salt water from a brine tank flows through the resin and it exchanges hardness ions for sodium ions. During the backwash cycle of regeneration, accumulated hardness ions and excess brine go down the drain.