Abstract: An improved reactor for treatment of wastewater having an influent line for wastewater, and effluent line for treated wastewater, and an aerobic bed fluidized by wastewater from the influent line. The bed is located between the influent line and the effluent line and contains particulate media seeded with a layer of bacteria. The improved reactor consists of an internal particulate media cleaning means located between the bed and an outlet to the effluent line for creating turbulence which shears excess biomass from the particulate media within the cleaning means and permits controlled exit of the excess biomass from the cleaning means and the reactor while maintaining cleaned media within the reactor. The internal particulate media cleaning means has at least one cylindrical screen with a longitudinal passage therethrough. The screen is respectively engaged to an inner periphery of the reactor above and below an outlet to the effluent line by gaskets fixed on opposite ends of the screen.