NOVELTY - The arrangement of a reactor (1) comprises a gas storage (5) for separating and/or intermediately storing a gas phase such as biogas from a three-phase mixture that consists of biogas, liquid and biomass, a lockable discharging device (12) arranged at an upper end of the gas storage for withdrawing the biogas, and lockable wastewater- or recirculation pipelines, which guide the wastewater or degassed three-phase mixture and flows/opens into the gas storage and/or the reactor. The gas storage has a first connection line (15) in a center or a lower area of its vertical extension to the reactor.    USE - Arrangement of a reactor for anaerobic treatment of liquids such as wastewater through anaerobic biological conversion of organic matters and/or impurities by using biomass, where the organic compounds of the wastewater are decomposed to carbon dioxide, water and biogas such as methane in the reactor.    ADVANTAGE - The arrangement of a reactor ensures continuous and permanent anaerobic treatment of the liquids with high loading rates without requiring additional energy, and can be economically and simply manufactured.    DETAILED DESCRIPTION - The arrangement of a reactor (1) comprises a gas storage (5) for separating and/or intermediately storing a gas phase such as biogas from a three-phase mixture that consists of biogas, liquid and biomass, a lockable discharging device (12) arranged at an upper end of the gas storage for withdrawing the biogas, and lockable wastewater- or recirculation pipelines, which guide the wastewater or degassed three-phase mixture and flows/opens into the gas storage and/or into the reactor. The gas storage has a first connection line (15) in a center or a lower area of its vertical extension to the reactor such that the gas storage forms tubes communicating with the reactor containing the three-phase mixture, so that the pressure of a gas buffer formed in an upper area (17) of the gas storage corresponds to the pressure of a multi-phase column in the reactor at the height of a phase limit between the gas phase and liquid phase or solid-liquid phase in the gas storage. The first connection line forms a lockable inlet for supplying gases separated from the three-phase mixture in the reactor or for supplying the three-phase mixture from the reactor to the gas storage. The gas storage has a second connection line below the first connection line to the reactor. The medium such as water, biomass and/or two- or three-phase mixture are reversibly conductible from the gas storage into the reactor over the second connection line. The reactor has an intermediate separator, at which the gas-loaded three-phase mixture is removable from the reactor or through which the gas is separatable from the three-phase mixture. The intermediate separator is connected to the gas storage with the first connection line. The gas storage has a level meter, a relaxation control valve, and a drain-control valve at the discharging device for withdrawing the biogas. A further lockable connection line is arranged between the head region of the reactor and the gas storage. The reactor has a three-phase separation device, and an inlet system, over which the wastewater and/or the recirculating medium is introducible into a reactor sump. An INDEPENDENT CLAIM is included for a method for increasing an operating pressure of biogas from a reactor for anaerobic treatment of liquids such as wastewater through anaerobic biological conversion of organic matters and/or impurities by using biomass.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic representation of an arrangement of a reactor with gas storage.    Reactor (1)    Gas storage (5)    Discharging device (12)    First connection line (15)    Upper area. (17)