NOVELTY - Reducing emission of carbon dioxide originating from sewage treatment plants, preferably from at least one process tank of nitrification (3), denitrification (2) and/or biological phosphorus removal, in which the process tank of nitrification forms a head space above the process tank, comprises e.g. introducing a working gas into process tank of nitrification for oxidizing organic components in the process tank and for forming carbon dioxide, conducting carbon dioxide as a gas mixture with the working gas to the head space, and separating carbon dioxide from the gas mixture by separation unit.    USE - The method is useful for reducing emission of carbon dioxide originating from sewage treatment plants (claimed).    ADVANTAGE - The arrangement does not require further separation units and/or transport devices for denitrification and biological phosphorus removal.    DETAILED DESCRIPTION - Reducing emission of carbon dioxide originating from sewage treatment plants, preferably from at least one process tank of nitrification (3), denitrification (2) and/or biological phosphorus removal, in which the process tank of nitrification is sealed against atmosphere and forms a head space above the process tank, comprises (i) introducing a working gas into the process tank of nitrification for oxidizing organic components in the process tank and for forming carbon dioxide, (ii) removing carbon dioxide from the process tank, (iii) conducting carbon dioxide as a gas mixture with the working gas to the head space, (iv) conducting the gas mixture from the head space to a separation unit, (v) separating carbon dioxide from the gas mixture by the separation unit, (vi) discharging the separated carbon dioxide from the separation unit, and (vii) discharging the gas mixture freed from carbon dioxide, from the separation unit. An INDEPENDENT CLAIM is also included for an arrangement for reducing the emission of carbon dioxide originating from the sewage treatment plants, preferably from at least one process tank of nitrification, denitrification and/or biological phosphorus removal, comprising (a) at least one cover for closing the process tank of nitrification against atmosphere, which forms the head space above the process tank, (b) at least one first device for charging the process tank with the working gas, where a gas mixture of carbon dioxide and the working gas is formed in the headspace, (c) at least one second device for conveying the gas mixture to the separation unit, and (d) at least one separation unit for separating the carbon dioxide from the gas mixture, which is connected to the head space.    DESCRIPTION OF DRAWING(S) - The drawings 1A, 1B and 1C show schematic representations of the process steps of the method.    Process tank for recovery (1)    Process tank for denitrification (2)    Process tank for nitrification (3)    Secondary clarifier (4)