Abstract: An apparatus for treating waste water includes a revolvable disc assembly partially submerged in a tank containing waste water. The disc assembly includes a plurality of similar lightweight nonmetallic discs, each disc comprising a plurality of similar disc sections. Adjacent discs of the disc assembly engage each other and each disc has a surface configuration that cooperates with the surface configuration of the next adjacent disc to define a plurality of concentric passages between the discs. A plurality of elongate rigid rods secure the disc through yieldable joints to a pair of circular end support members and to circular intermediate support members.