Water Softeners contain an ion exchange resin that exchanges sodium ions (Na+) for calcium (Ca++) and Magnesium (Mg++) ions.  When the sodium is depleted, the resin needs to regenerated.  This is done by backflushing the resin with a concentrated salt (sodium chloride) solution.  Sodium ions displace the calcium and magnesium ions, which are washed down the drain, and the resin is ready for another softening cycle.

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