A water stream is passed between two juxtaposed similar ion exchange membranes (AEMs or CEMs), forming an ion depletion and ion enrichment zones when an electric field is applied. As cations are selectively transferred through the CEMs, for example, anions are relocated in order to achieve electro-neutrality, resulting in the concentration drop (increase) in ion depletion (enrichment) zone. Trifurcation of the output channel allows collection of concentrated, dilute and intermediate streams, with the intermediate stream serving as input to the next stage of a serialized implementation.