NOVELTY - The method for the biological purification of waste water containing organic and nitrogenous compounds, comprises aerating and/or oxygenating the waste water in a lengthwise-operated reactor in the presence of biomass fixed on substrate and separating the waste water to a purified water and sludge in a post-clarification step. The separated sludge is fed back partly into the reactor. As substrate material for the microorganisms substrate particles are brought into the reactor at a quantity that allows the substrate particles to move freely in the wastewater.    USE - For the biological purification of wastewater containing organic and nitrogenous compounds.    ADVANTAGE - The method provides effective elimination of organic and nitrogenous compounds in the wastewater. Avoids the denitrification and nitrification capacity losses.    DETAILED DESCRIPTION - The method for the biological purification of waste water containing organic and nitrogenous compounds, comprises aerating and/or oxygenating the waste water in a lengthwise-operated reactor in the presence of biomass fixed on substrate and separating the waste water to a purified water and sludge in a post-clarification step. The separated sludge is fed back partly into the reactor. As substrate material for the microorganisms substrate particles are brought into the reactor at a quantity that allows the substrate particles to move freely in the wastewater. A desired simultaneous decomposition of organic and nitrogenous compounds is accomplished by meeting the aeration and flow conditions in the reactor. The decomposition comprises maintenance of a continuous aeration, formation of an aeration-free region that is adjacent to the aeration surface in perpendicular direction to the direction of flow and production of a twisting flow of the mobile reactor contents. The porous foam material particles are used as carrier particles with inner and outer surfaces facilitating the growth of microorganisms. The carrier particles are incorporated in a quantity of 15-35% with respect to the reactor volume. In the reactor a BOD5 (biological oxygen demand after 5 days) loading rate of 0.4 to 2.5 kg/m3/d and a TKN (total kjeldahl nitrogen) loading rate of 0.1 to 0.8 kg/m3 /d are set. The aeration is adjusted in such a manner that 1-4 mg per litre of dissolved oxygen content is maintained in the upwardly directed flow region. A continuous aeration is maintained over several aeration surfaces above the bottom of the reactor that run parallel to one another and extend themselves in the direction of flow. The aeration-free regions that lie between the aeration surfaces perpendicular to the direction of flow are maintained in an aeration-free state, where twisting flows perpendicularly to the direction of flow are produced. A 20-100% of the discharge, related to the reactor flow, is fed back to the inlet of the reactor and from there it is led to the downwardly directed flow.    DESCRIPTION OF DRAWING(S) - The drawing shows an aeration unit with aeration basin:    Reactor; (1)    Inlet; (2)    Outlet; (3)    Separation device; (4)    Aeration surface. (11)