NOVELTY - The method comprises subjecting a waste stream comprises organic substances to an anaerobic degradation process in which a reactor biogas comprising the methane is produced from the organic substances, transferring a dissolved methane from a liquid effluent to a gas phase, and producing energy using the methane. The liquid effluent comprises the dissolved methane. The methane is subjected to an oxidation reaction using a unit, where a reaction energy released by the oxidation is recovered or the methane is oxidized using a flare (22).    USE - The method is useful for recovering methane from a liquid effluent of a reactor.    ADVANTAGE - The method is capable of effectively and economically recovering the methane from liquid effluent of the reactor with reduced energy and improved productivity, and without causing explosion.    DETAILED DESCRIPTION - The method comprises subjecting a waste stream comprises organic substances to an anaerobic degradation process in which a reactor biogas comprising the methane is produced from the organic substances, transferring a dissolved methane from a liquid effluent to a gas phase, and producing energy using the methane. The liquid effluent comprises the dissolved methane. The methane is subjected to an oxidation reaction using a unit, where a reaction energy released by the oxidation is recovered or the methane is oxidized using a flare (22). The effluent from the reactor is passed through a collection channel, which comprises the liquid effluent and the gas phase such as air and has openings via which the liquid effluent is introduced into the collection channel and contacted with the gas phase. The collection channel methane is transferred from the effluent to the gas phase. The gas phase and the effluent: are passed through the collection channel; and are introduced into a collection sump (3) in which the gas phase is separated from the effluent. The effluent is introduced into a gas stripping device, where the methane is stripped from the effluent and is transferred from the liquid effluent to the gas phase. The transferring step comprises subjecting the liquid effluent to aeration for forming a gas phase comprising air and methane. The gas phase comprising air and methane is used as combustion air for combusting the biogas obtained from the anaerobic degradation process. The gas phase is introduced into a combustion air inlet tank, which comprises an inlet (42) for ambient air via which the inlet ambient air is introduced in the combustion air inlet tank at times when there is insufficient combustion air provided by the gas phase comprising air and methane for combusting all the biogas that is to be used for energy production. The tank further comprises an outlet (40) for combustion air via which the outlet of the combustion air is directed to a combustion unit, where the biogas is combusted for producing energy. The effluent is subjected to vacuum extraction, where the methane is transferred from the effluent to the gas phase. The gas phase comprising the transferred methane is combined with biogas that is collected as the gas phase from the reactor and used for energy recovery or is flared off. An INDEPENDENT CLAIM is included for a reactor system for preparing biogas by anaerobic degradation of organic substances.    DESCRIPTION OF DRAWING(S) - The diagram shows a reactor system.    Reactor (1)    Collection sump (3)    Flare (22)    Outlet (40)    Inlet. (42)