An apparatus to treat an influent solution comprising ions to obtain a selectable ion concentration in an effluent solution. The apparatus comprises an electrochemical cell comprising a housing comprising first and second electrodes. A water-splitting ion exchange membrane is between the first and second electrodes, the membrane comprising ananion exchange surface facing the first electrode, and an cation exchange surface facing the second electrode, or vice versa. The housing also has an influent solution inlet and an effluent solution outlet with a solution channel that allows influent solution to flow past both the anion and cation exchange surfaces of the water-splitting ion exchange membrane to form the effluent solution. A variable voltage supply is capable of maintaining the first and second electrodes at a plurality of different voltages during an ion exchange stage.