Apparatus for treating a fluid, which comprises at least one pair of electrode layers power supplied to different polarities, delimiting between them a passage chamber for a first fluid flow containing ionized particles and exerting hydraulic pressure on at least one surface of such electrode layers. Each electrode layer comprises a metal conduction layer electrically connected to a power supply and at least one layer of graphite having a wet surface directed towards the passage chamber and a dry surface compressed, by the hydraulic pressure exerted by the first fluid contained in the passage chamber, directly in adherence against the metal conduction layer, in order to distribute the current of the power supply from the metal conduction layer to the layer of graphite.