Selectable ion concentrations with electrolytic ion exchange

An apparatus (20) treats an influent solution (70) comprising ions to obtain a selectable ion concentration in an effluent solution (80). The apparatus (20) comprises an electrochemical cell (22) comprising a housing (25) comprising first and second electrodes (40, 45). A water-splitting ion exchange membrane (100) is between the first and second electrodes (40, 45), the membrane (100)comprising an anion exchange surface (46) facing the first electrode (40), and an cation exchange surface (48) facing the second electrode (45), or vice versa. The housing (25) also has an influent solution inlet (30) and an effluent solution outlet (35) with a solution channel (52) that allows influent solution (70) to flow past both the anion and cation exchange surfaces (46, 48) of the water-splitting ion exchange membrane (100) to form the effluent solution (80). A variable voltage supply (50) is capable of maintaining the first and second electrodes (40, 45) at a plurality of different voltages during an ion exchange stage.