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Why is concentrated salt solution used to regenerate a water softener?
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ion-exchange reaction
Figure 1: Chemical structure of cation exchanger. The exchangeable ions are marked +. The whole structure is permeated by solvent molecules, usually water (not shown).
A water softener uses an ion-exchange resin to remove hardness by trading sodium ions for calcium and magnesium ions. Over time, the resin becomes saturated with these hardness-causing ions and loses its effectiveness.12
To regenerate the system, a concentrated solution of common salt is passed through the column. The high concentration of sodium ions in this brine forces a reversal of the exchange process: the excess sodium ions displace the accumulated calcium and magnesium ions from the resin. Once these hardness ions are flushed away with water, the resin is recharged with sodium ions and is ready to soften water again.12