In a demineralisation plant, salts dissolved in water are removed by ion exchange processes. The exchange products react to water. Regeneration of the ion exchange resins is conducted with acid and lye.

Salts dissolved in water are dissociated, meaning they are separated into positively charged cations and negatively charged anions. For example, when dissolved in water, calcium hydrocarbonate (Ca(HCO3)2) is dissociated into the cation calcium (Ca2+) and the anion hydrogen carbonate (HCO3-).

In a demineralisation plant, those salts dissolved in water are removed by an ion exchange process. For this purpose, the to be demineralised water flows through vessels filled with ion exchange resin. First through a cation exchanger filled with acidic ion exchange resin, then through an anion exchanger filled with alkaline ion exchange resins. The acidic ion exchange resin in the cation exchanger has hydrogen ions (H+) attached to it, while hydroxide ions (HO-) are attached to the alkaline resin in the anion exchanger.