Water softening removes the hardness from feedwater to prevent hard water scale from coating evaporator tubes. Over time, hard water will degrade a distillation unit's capacity. Water softening of feedwater removes calcium and magnesium ions, the cause of water hardness, and replaces them with sodium ions.

Water softeners should be sized for salt brine regeneration after 12 hours of operation. The regeneration process involves rinsing the water softener's cation-based resin with a salt brine solution. The concentrated sodium ions push the calcium and magnesium ions away from the resin and to drain, beginning the softening process all over again.

In some applications, secondary water softeners are installed after a carbon filter to remove ammonium ions from the feedwater. The cation resin does not attract ammonium ions as readily as calcium and magnesium ions but, if not removed, the ammonium ions will produce high-conductivity distillate. Therefore, a primary water softener is used to remove calcium and magnesium while a secondary softener or softeners remove the ammonium. Because the secondary units operate in a biocide-free environment, they are typically designed for hot water sanitization.