A bioreactor has a biofilm that receives a gas through a supporting membrane and another biofilm attached to an inert support. The first biofilm is aerated through the membrane and provides nitrification. The other biofilm has an anoxic or anaerobic zone and provides denitrification. A module useful in the bioreactor has cords potted in at least one potting head. Optionally, some or all of the cords have a gas transfer membrane. The module may provide inert supports, active gas transfer supports or a combination of both types of support. Multiple modules may be assembled together into a cassette, the cassette providing inert supports, active supports or a combination. The module or cassette may have an aerator for mixing or biofilm control.