An electrodialysis unit 8 comprises a cathode 68, an anode 70, a membrane 71 between the cathode 68 and the anode 70, a cathode flow path 72 for water flow along the membrane 71 on the cathode side, an anode flow path 74 for water flow along the membrane 71 on the anode side, and a reaction zone formed between the membrane 71 and the cathode 68 where the cathode 68 faces the anode 70, wherein the cathode flow path 72 is arranged for laminar flow in the reaction zone and wherein the electrodialysis unit 8 comprises flow conditioning elements 64 arranged to promote laminar flow in the incoming water flow to the cathode flow path 72.