A reactor for removing impurities from aqueous solutions, wherein the liquid is passed through series of plateshaped reaction electrodes. The reactor comprises one more interconnected units (A, B, C, D) with a series of plateshaped electrodes (1, 2) valves (8) and holes (5, 6) in the plates for redirecting the liquid flow into and through the series of reaction electrodes (1, 2). A control system consists of a number of sensors (9, 10, 11) at the liquid inlet (3) of the reactor for measuring the conductivity of the treated liquid, the organic contents of the liquid and the flow of the liquid, and the transferring the output commands from a processor (7) to valve (8) for redirecting the liquid flow and for activating or deactivating the electrode unit or units (A, B, C, D) in dependence on the measured parameters.