It comprises the substitution of a recirculation valve (9) of the check-valve type by a free piston (104), having a stroke which is limited by an inlet stop (106) and an outlet stop (107). Additionally, the disclosed device can be associated to a damping valve (110) of the flap type which is hinged to one side and which has a small through orifice (111). Alternatively, the free piston (104) and the inlet and outlet stops (106, 107) can be substituted by a membrane (108) which is fixed at its periphery and displaceable at its central part towards the inlet conduit (1') and towards the outlet conduit (21). Furthermore, in order to eliminate the hydraulic ram shocks at the inlet side of the desalting plant during the changer over operation of the supply chambers (5, 5'), a bypass valve (500) is mounted between the inlet conduit (1') and the outlet conduit (21) so that water can flow through said valve when the inlet valve (61) closes the supply of water to be desalted to both supply chambers (5, 5') simultaneously. Fig 1 and 13.