The performance of a pilot test for biogas upgrading has been tested under different process conditions to produce oxygen-enriched air for aqueous media application. The selectivity of the polysulfone hollow fibres far N-2/O-2 permeation allows the production of an oxygen-enriched air stream in the 30-50% range and an inert gas (nitrogen between 82-98%) by operating the pilot unit at different stage-cut values in a single pass mode. Membrane area requirements for a medium size plant of 100 Nm(3) h(-1) are discussed according to a simple perfect mixing model for gas permeation.