NOVELTY - Operating a biogas plant (9), comprises fermenting agricultural manure (1), fermentation mash partially out-gassed in previous fermentation process, sewage sludge, and/or wastes from food production as fermentation substrate in another fermentation process or post-fermenting in a further fermentation process, separating a thick phase (3) from manure, previous partially out-gassed fermentation mash, sewage sludge or food production waste in a first step from a thin phase (4). The thick phase is introduced in the fermentation chamber of the biogas plant.    USE - The method is useful for operating a biogas plant.    ADVANTAGE - The method avoids the discharge of final fermentation residues on agricultural land.    DETAILED DESCRIPTION - Operating a biogas plant (9), comprises fermenting agricultural manure, fermentation mash partially out-gassed in previous fermentation process, sewage sludge, and/or wastes from food production as fermentation substrate in another fermentation process or post-fermenting in a further fermentation process, separating a thick phase (3) from manure, previous partially out-gassed fermentation mash, sewage sludge or food production waste in a first step from a thin phase (4). The thick phase is introduced in the fermentation chamber of the biogas plant. The separated thin phase is supplied to a culturing device with fast growing aquatic plants, and at least a portion of the biomass from aquatic plant regularly removable from the culturing device is supplied to the fermentation chamber of the biogas plant filled with the thick phase. INDEPENDENT CLAIMS are also included for:    (1) a device for operating the biogas plant, comprising a separation means for physical separation of the thick phase and the thin phase from agricultural manure, or previous out-gassed fermentation substrate, a transport- and a mixing device for additional filling of the biogas plant with aquatic plants or with an additional substrate generated from aquatic plants and a biogas auxiliary substance, where the biogas plant is attached to the culturing device for aquatic plants; and    (2) a biogas auxiliary substance made of mixture of biomass of aquatic plants and a biogas booster, where the biogas booster is made of fatty acid ester, fats, oils, distiller grains, fat-containing-, sugar- or alcohol traces or yeast containing residues from food production or trace elements.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic process flow of the method (Drawing includes non-English language text).    Agricultural manure (1)    Thick phase (3)    Thin phase (4)    Aquatic biomass (7)    Biogas plant (9)