Apparatus for treating a fluid, which comprises at least one pair of electrode layers (10) power supplied to different polarities, delimiting between them a passage chamber (4) for a first fluid flow (F1) containing ionized particles and exerting hydraulic pressure on at least one surface of such electrode layers (10). Retention means (3) are provided, adapted to transmit a mechanical counter-pressure on the aforesaid pair of electrode layers (10) in order to prevent their moving away. Each electrode layer (10) comprises a metal conduction layer (11) electrically connected to the power supply means and at least one first layer of graphite (12) having a first wet surface (12A) directed towards the passage chamber (4) and a first dry surface (12B) compressed by the hydraulic pressure exerted by the fluid contained in the passage chamber (4), directly in adherence against the metal conduction layer (11), in order to distribute the current of the power supply means from the first metal conduction layer (11) to the first layer of graphite (12).