Device for multi-stage fluid treatment comprising first and second fluid treatment compartment being arranged in sequence. First compartment comprises a first cylindrical shell and a plurality of hollow fiber membranes arranged therein, which are embedded with their ends in a first and a second tube sheet bonded to the first shell at first and second ends of the first shell, such that a first outer space is formed around the hollow fiber membranes. The hollow fibers membranes pass with their ends through the tube sheets and are open at their ends. The first fluid treatment compartment has at least one opening in the first shell to permit fluid flow into or out of the first outer space. The second compartment comprises a second cylindrical shell and a plurality of fibers being arranged therein extending between a third end and a fourth end of the second shell. The fibers are at least embedded with one of their ends in a tube sheet bonded to the second shell at one of the ends of the second shell such that a second outer space is formed around the fibers which is in fluid communication with a fluid port in the second shell. The device is characterized in that first and second compartment are connected so as to form an integrated unit, wherein second end of second cylindrical shell and third end of second cylindrical shell of are fluid-tightly connected to each other such that at the transition between first compartment and second compartment a fluid path is formed between the lumina of the hollow fibers and the second outer space.Linvention concerne un dispositif de traitement de fluide à plusieurs étages comprenant un premier et un second compartiment de traitement de fluide disposés en séquence. Le premier compartiment comprend une première enveloppe cylindrique et une pluralité de membranes à fibres creuses disposées à lintérieur de celle-ci, qui sont incorporées avec leurs extrémités dans une première et une seconde feuille de tube liées à la première enveloppe au
