Water softeners usually use the principle of ion exchange. In a water softener unit, water passes through an ion exchange column containing specific resins that adsorb calcium and magnesium ions in the water while releasing the same amount of sodium ions or other soluble salts. In this way, the calcium and magnesium ions in the water are "exchanged" into relatively soluble sodium salts.

With the use of water softening devices, the exchange resin is gradually absorbed by calcium and magnesium ions and loses its ability to soften water. In order to maintain the performance of the water softener device, regular regeneration is required, that is, the exchange resin is flushed with high-concentration brine (salt solution) to flush away the calcium and magnesium ions adsorbed on the resin, so that the resin regains the ability to soften water.