Water softening units use strongly acidic ion-exchange resin to replace hardness components (calcium and magnesium ions) with sodium ions. Instead of deposit-forming calcium hydrogencarbonate more soluble sodium salts like sodium hydrogencarbonate are produced. The hardness components are thus almost completely removed from the water; the residual hardness is less than 0,1 °dH.
As soon as the resin becomes depleted, it is regenerated with sodium chloride; the calcium and magnesium ions attached to the resin are replaced by sodium ions again. When using a simplex water softener only raw water is available during regeneration. If continuous soft water is required, a duplex water softening system must be selected.
Water softening is only an exchange of ions; the salt concentration in the water hardly changes.