An interconnector coupled to a permeate water collection tube, establishing first and second groups of membranes, has open and closed ends, and orifices arranged circumferentially and located at a peripheral area in an intermediate section of the interconnector. The orifices establish a connection with an axial chamber provided in the interconnector and oriented towards the open end. The open end has a coupling nut connected to a pressure tube extending along the inside of the water collection tube section located between the interconnector and a feed inlet end. The pressure tube extends outside the pressure container through a closing cap of the end of the pressure container and ends in a T-shaped connection including a sensor for measuring the pressure of the non-treated reject water corresponding to the first group of membranes located between the feed inlet end and an assembly point of the interconnector.