In the ion-exchange softening process, once the resin can no longer soften water, it must be?

In the ion-exchange softening process, the resin used to remove calcium and magnesium ions from water can become saturated over time, losing its effectiveness. When this happens, regeneration is required to restore the resin's ability to soften water.

Regeneration involves a chemical process where the resin is treated with a concentrated salt solution, usually sodium chloride. This salt solution replaces the calcium and magnesium ions accumulated on the resin with sodium ions, enabling the resin to continue to soften incoming water. This cycle of exchange maintains the softening capability of the resin, ensuring that the water supply remains softened and free of hardness.

The terminology associated with the other potential answers does not accurately describe the process needed to restore the resin's functionality. While "renewed," "recatalyzed," and "recharged" could hint at different maintenance processes in various contexts, the specific term used in ion exchange technology is "regenerated," reflecting the function of renewing the active sites on the resin specifically through ion exchange actions with a brine solution.