NOVELTY - The upflow reactor comprises a first floatation separator (10) having a gas hood (11) connected with a collection device (4), a second floatation separator (20) that is arranged over the first floatation separator and has a gas hood (21) connected with the collection device. The reactor is arranged with a device for controlling the biomass-recirculation. The gas hood comprises a discharge opening (12) with controllable cross sectional area and/or geometry. The first and second floatation separators have design different from one another.    USE - Upflow reactor with a biomass-recirculation for biological treatment of waste water.    ADVANTAGE - The upflow reactor ensures simple and efficient biological treatment of waste water.    DETAILED DESCRIPTION - The upflow reactor comprises a first floatation separator (10) having a gas hood (11) connected with a collection device (4), a second floatation separator (20) that is arranged over the first floatation separator and has a gas hood (21) connected with the collection device. The reactor is arranged with a device for controlling the biomass-recirculation. The gas hood comprises a discharge opening (12) with controllable cross sectional area and/or geometry. The first and second floatation separators have design different from one another. An edge area of the discharge opening is limited by a movable cover. The reactor comprises an actuator for the activity of the movable cover, and an electronic control. The actuators are equipped with a hydraulic drive. The edge area of the discharge opening is limited by a tube connector, which is connected with a compression device for water. The discharge opening is equipped with a hood-shaped or tubular siphon (12, 22). The siphon and the gas hood are formed in single-piece manner. The reactor consists of aerobic and/or anaerobic biomass.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic sectional view of an upflow reactor with a biomass-recirculation.    Collection device (4)    Floatation separator (10, 20)    Gas hood (11, 21)    Discharge opening (13)    Siphon. (12, 22)