Water purification using ion-exchange resin is an old-established method for pure water production. This method can be adopted for surface treatment processes such as the plating wastewater treatment.
Ion-exchange resin is available in the form of cation or anion exchanger. Each of them can be further divided into strongly acidic, mildly acidic, strongly basic, and weakly basic resins. A tower filled with such ion-exchange resin is called an ion-exchange tower. In this tower, water runs from the top toward the bottom. During this time, the following adsorption reaction occurs in the ion-exchange tower:(R: ion-exchange resin)

In the cation-exchange tower, hydrogen ion is generated when hydrogen bonded to cation-exchange resin is replaced with cation K⁺ (Cu²⁺ or Ni²⁺, for example) in the treatment water. In the case of anion-exchange resin, hydroxide ion is generated when the hydroxide reacts with the anion A^-. Ultimately, water (H₂O) is generated by the reaction of H⁺ and OH^-.
The ion-exchange resin can remove cation and anion impurities dissolved in water by the mechanism described here.

However, a specific limit applies to how much ion-exchange resin can replace. After exceeding a certain amount of ions, the resin is no longer capable of adsorbing more ions. Therefore, it is necessary to perform the "regeneration" task to restore the adsorption ability of the resin. The following reaction occurs in the regeneration process:

As shown in [Fig.1], this series of tasks are carried out in the cycle of backwash, regeneration, and cleaning processes. The backwash step loosens up resins in the tower. To regenerate the cation-exchange resin, acids with H⁺ such as hydrochloric acid and sulfuric acid will be used. To regenerate the anion-exchange resin, alkali with OH^- such as NaOH will be used. The cleaning step eliminates residual chemicals from the regeneration process.