Deionised water is produced by

Deionised water, also known as demineralised water, is produced through the ion-exchange resin process. This method involves passing raw water through beds of ion-exchange resins, which are organic polymers designed to swap specific ions [1]. Cation exchange resins replace positively charged ions like calcium and magnesium with hydrogen (H+) ions, while anion exchange resins replace negatively charged ions like sulfates and chlorides with hydroxyl (OH-) ions. When these H+ and OH- ions combine, they form pure water (H2O). In contrast, the Calgon and Permutit processes are primarily used for water softening (removing hardness) rather than complete deionization. Clark's process (often misspelled as Dark's) is a traditional method for removing temporary hardness using lime. Therefore, the ion-exchange resin process is the standard industrial method for producing high-purity deionized water.

Sources

1. [1] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 5: Environmental Pollution > S.r5.7. Control Measures: > p. 106