What does the term "ion exchange" primarily refer to in water treatment?

The term "ion exchange" in water treatment primarily refers to the process of swapping ions between two solutions. This method is utilized to remove undesirable ions, such as heavy metals or hardness ions like calcium and magnesium, from water by replacing them with more desirable ions, typically sodium or hydrogen ions.

In ion exchange systems, water is passed through a resin that facilitates this exchange, effectively removing impurities and improving water quality. This process is crucial in applications such as softening hard water, deionizing water, and purifying water for industrial processes.

The other options do not accurately describe ion exchange. Mixing two different types of water does not involve the exchange of ions between solutions. Removing sediment focuses on particulate matter rather than ionic composition. Changing temperature to purify water is a method associated with thermal processes and does not involve ion exchanges. Therefore, the accurate understanding of ion exchange is critical for effective water treatment solutions.