The ion exchange method uses spherical resin (ion exchange resin) to filter raw water, and the ions in the water will exchange with the ions fixed on the resin. The two common ion exchange methods are hard water softening and deionization. Hard water softening is mainly used as a pre-treatment procedure to reduce the hardness of water before reverse osmosis (RO) treatment. The spherical resin inside the softening machine softens water by exchanging two sodium ions for one calcium ion or magnesium ion.

Ion exchange resins use hydrogen ions to exchange cations and hydroxide ions to exchange anions; The cation exchange resin made from styrene and divinylbenzene containing sulfonic acid groups will exchange various cations (such as Na+, Ca2+, Al3+) encountered with hydrogen ions. Similarly, anion exchange resins made from styrene containing quaternary ammonium salts will exchange various anions (such as Cl -) encountered with hydroxide ions. The hydrogen ions released from the cation exchange resin combine with the hydroxide ions released from the anion exchange resin to produce pure water.

Cation and anion exchange resins can be packaged separately in different ion exchange beds, divided into so-called anion exchange beds and cation exchange beds. Cation exchange resin and anion exchange resin can also be mixed together and placed in the same ion exchange bed. Regardless of the form, when the resin exchanges hydrogen ions and/or hydroxide ions with charged impurities in water, it must undergo "regeneration". The regeneration process is exactly the opposite of the purification process, using hydrogen ions and hydroxide ions to regenerate and exchange impurities attached to the ion exchange resin.

If ion exchange method is combined with other purification water quality methods such as reverse osmosis, filtration, and activated carbon adsorption, ion exchange method will play a very important role in the entire purification system. The ion exchange method can effectively remove ions, but cannot effectively remove most of the organic matter or microorganisms. Microorganisms can attach to resin and use it as a culture medium, allowing them to grow rapidly and generate heat. Therefore, it is necessary to design and use it in conjunction with other purification methods.