NOVELTY - Biological wastewater treatment by biosludge comprising anaerobic microorganisms, comprises supplying the wastewater (1) below the biosludge which is present in a reactor vessel (3), during which the waste water flows through the biosludge from bottom to top; discharging the waste water above the biosludge; degassing the biosludge with the waste water dissipated from the reactor vessel; and at least partially recycling into the reactor vessel.    USE - The method is useful for biological wastewater treatment by biosludge comprising anaerobic microorganisms (claimed).    ADVANTAGE - The reduction in degradation performance of the reactor vessel is counteracted by degassing biological sludge in a most efficient manner.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a plant for biological, anaerobic wastewater treatment comprising a reactor vessel, which contains bio-sludge comprising anaerobic microorganisms, and comprises at least one inlet in the lower part, for the waste-water to be purified, an overflow for the discharge of the waste water at least in the upper part, and at least one separator for separating at least the biogas resulting in wastewater treatment, from the purified wastewater. The wastewater discharged from the reactor vessel, is supplied to a hydrocyclone (7), and the obtained heavy parts are at least partially recycled to the reactor vessel; the wastewater discharged from the reactor vessel, is supplied through a sieve, and the accumulating waste materials are at least partly recycled into the reactor vessel; or the wastewater discharged from the reactor vessel, is supplied for degassing the entrained biosludge by a centrifugal pump and the degassed bio-sludge is supplied in a subsequent treatment unit of the wastewater and at least partly recycled into the reactor vessel.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic representation of the plant for waste water treatment.    Waste water (1)    Reactor vessel (3)    Hydrocyclone (7)    Low-shear pump (11)