MOP (muriate of potash) is used in water softening systems. Ion-exchange softeners allow for the improvement of the quality of potable and processed water. Water flows through a porous material, i.e. ion-exchange resin, where Ca and Mg ions are removed from water and stay in the ion-exchanger while Na or K ion is put in their place on its surface. When all the available sodium ions have been replaced with calcium or magnesium ions during operation, the resin must be recharged.
The application of potassium chloride is environmentally safe and preferable for potable, as it reduces the content of sodium ions in water.
Potassium Chloride - Alternative Regenerant for Softening Water