Abstract: This pump has two end chambers (2, 3) in which are housed two pistons (4, 5) which pump sea water towards an reverse osmosis membrane (7), and a central chamber (8) in which a sliding block is housed, activated by displacement of the pistons (4 and 5) performing the piloting of a directional valve (9). This pump alternatively provokes the inversion of the external pressure intake towards the front areas of the end chambers (2, 3) and the inversion of the pressure intake of the water rejected by the membrane towards the rear areas of said chambers (2, 3).