An electrodialysis unit 8 comprises a plurality of cathodes 68, a plurality of anodes 70 and a plurality of membranes 71; wherein the cathodes 68 and anodes 70 are arranged alternately in an electrode stack, with membranes 71 in between each cathode 68 and anode 70; and wherein the cathode 68 and the anode 70 are each formed of a single conductive plate such that both surfaces of the cathode plates and anode plates enclosed within the electrode stack are, in use, in conductive contact with the water being treated.