With softening, only the ions of the water hardness are exchanged. With decarbonisation, on the other hand, the hydrogen carbonate salts of the calcium and magnesium, the so-called carbonate hardeners, are removed. This process results in partial demineralisation of the water.

Decarbonisation occurs via filtration using a weakly acidic cation exchanger loaded with hydrogen ions. Carbon dioxide results from decarbonisation and can be removed in a spray tower. In many cases, decarbonisation is preceded by softening.
Regeneration primarily occurs with hydrochloric acid.