Abstract: A bio-reactor having a downflow self-cleaning configuration includes a liquid deflecting means generally concentric with a cylindrical tank. The liquid deflecting means is located just below the surface of liquid in the tank and is configured to direct liquid radially outwardly towards an inner surface of the tank. The liquid flows downwardly adjacent the inner surface, then inwardly toward the center of the tank and upwardly adjacent the center of the tank. The liquid circulates in a toroidal configuration to stir buoyant media pellets within the tank. A variation of the design incorporates upwelling injected diffused air to induce the same toroidal circulation pattern.