In order to remove the divalent ion

In order to remove the divalent ions, various methods have
been widely applied as a means of effective water softening:
chemical precipitation, ion exchange process, nanofiltration,
reverse osmosis, and electromembrane systems such as
electrodialysis, electrodialysis reversal, and electro-deionization
reversal. However, in case of the chemical precipitation,
the choices of additional chemicals are restricted for the
purpose of drinking water.