Abstract: Effluent to be treated is introduced into the top of a first bioreactor 10 through an inlet pipe 16 and, after passing through second and third bioreactors 11 and 12, passes through a settling device 17, the treated effluent then flowing over a depth control weir 18 into an outlet pipe 19. The three bioreactors 10, 11 and 12 and the settling device 17 are housed in a tank having three sets of vertically-staggered transverse partitions 21, 22, 23 and a transverse settling tank partition 24. Each set of staggered partitions 21, 22 and 23 are arranged so that the bioreactors are interconnected in series, the effluent preferably being constrained to pass downwardly through each matrix 13, 14, 15 in turn against an upward discharge of very fine air bubbles from aeration panels 28 which extend under substantially the whole of each matrix. Aerobic bacteria feeds on nutrients in the effluent and grows on the roughened walls of the cross-flow matrix and the air bubbles promote sloughing of the bacteria.