Assembly for treating fluids, comprising a support (12) having a first and second oppositely arranged surfaces (121) for backing support of a semi permeable membrane (11), a first fluid conveying compartments (124) interposed between the first and second surfaces, a plurality of first fluid passages (126) extending from the first surface (121) and being in fluid communication with the first compartments (124), and a first duct attached to the support (12) and in fluid communication with the first compartments. The assembly comprises a second compartment (125) arranged for conveying fluid and different from the first compartment, and a second duct attached to the support (12) and configured to be in fluid communication with the second compartment (125).