A series of these shafts are placed in a rectangular tank so that the discs are immersed to approximately 40% of their diameter. The discs are spaced so that, during submergence, wastewater can enter the separations between the corrugated surfaces, providing the microorganisms with a food source. When rotated out of the tank, the liquid trickles out of the plastic media and is replaced by air, providing a source of oxygen to satisfy the respiratory requirement of the microorganisms. The rotation of the discs and the biological activity on them maintains a constant thickness of biologic growth. Growth sloughed from the discs is separated from the flow in the secondary clarifiers.