Water hardness is usually calculated as Ca2+ and Mg2+.
Softening resin is a special resin specially used to soften water. It reduces the concentration of calcium and magnesium by ion exchange technology, so that the hardness of water is controlled less than 50 mg/L (CaCO3).
Common softening resin is a strong acid cation exchange resin which is typically regenerated with sodium chloride. Industrial water is often softened with a gel strong acid cation exchange resin, and macroporous strong acid cation exchange resin can be used at a high temperature. For a hard water having a high carbonate content, a weak acid cation exchange resin is usually used for softening.

Demineralization is deionization, removing cations and anions in water, also called desalting; the result is reduced solution soluble solids, so it is also called deashing. The water after demineralization is similar to distilled water, but the cost is much lower.

A double bed system of cation exchange resin and anion exchange resin commonly used in the demineralization process, or a mixed bed system using mixed bed resin. The double bed system is easy to regenerate, and the mixed bed system is complicated to regenerate, but with a high processing precision, so it is often used for polishing the water after other systems.

The removed cations include Ca2+, Fe3+, Mg2+, Mn2+, K+, Na+
Anions are: CO32-, HCO3-, Cl-, NO3-, SO42-, SiO2