NOVELTY - System for sewage treatment comprises a buffer tank (10) for storing sewage to be treated, a bioreactor tank (30) for receiving sewage to be treated from of the buffer tank, in which the sewage is treated by microorganisms in the bioreactor tank under anaerobic conditions, and a filter tank (50) for receiving sewage from the bioreactor tank. The filter tank comprises a filter medium (52) for filtering wastewater. The bioreactor tank comprises an inlet opening (8) for wastewater in a lower region.    USE - The system is useful for sewage treatment (claimed).    ADVANTAGE - The system: is energy efficient; and increases the efficiency of biogas production.    DETAILED DESCRIPTION - System for sewage treatment comprises a buffer tank (10) for storing sewage to be treated, a bioreactor tank (30) for receiving sewage to be treated from of the buffer tank, in which the sewage is treated by microorganisms in the bioreactor tank under anaerobic conditions, and a filter tank (50) for receiving wastewater from the bioreactor tank. The filter tank comprises a filter medium (52) for filtering sewage. The bioreactor tank comprises an inlet opening (8) for sewage in a lower region. An inflow direction of sewage that is upward such that dimensions of the inlet opening and a pumping rate of wastewater in the bioreactor tank are adjusted for generating a laminar flow in the bioreactor tank. The bioreactor tank has a drain pipe. At least one end portion of the drain pipe extends inside the bioreactor tank upwards and has an upwardly directed inlet opening. The inlet opening of the drain pipe is arranged in an upper portion of the operating region of bioreactor tank which is filled with wastewater. An INDEPENDENT CLAIM is also included for treating sewage, comprising conducting sewage to be treated into a buffer tank, passing the sewage from the buffer tank to a bioreactor tank in which the sewage is treated by microorganisms under anaerobic conditions and conducting the sewage from the bioreactor tank into a filter tank, where sewage with an upward inflow is introduced through an inlet opening into a lower region of the bioreactor tank such that laminar upward flow of wastewater is generated in the bioreactor tank from the inlet opening and wastewater from the bioreactor tank is drained through a drain pipe. At least an end portion of the drain pipe extends within the bioreactor tank upwards and has an upwardly directed inlet opening, and the inlet opening of the drain pipe is arranged in an upper portion of the operating region of the sewage which is filled in bioreactor tank.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic representation of the system for wastewater treatment.    Inlet opening (8)    Buffer tank (10)    Bioreactor tank (30)    Filter tank (50)    Filter medium (52)