NOVELTY - Plant (10) comprises: a mixing- and crushing device (30) for incorporating a mixture of a solid biomass and a fermentation liquid and for mixing and crushing the mixture to a biomass mash; a fermentor (40) connected to the mixing- and crushing device for incorporating the biomass mash, and to promote the fermentation of the biomass mash; and a phase separation device (60) connected to the fermentor to accommodate the fermented biomass mash, which is designed to separate the fermented biomass mash into a liquid phase and a solid phase.    USE - The plant is useful for processing biomass and for producing biogas.    ADVANTAGE - The plant: is energy efficient; has continuous operation, increased biogas yield per ton of biomass, improved utilization of the given capacity of a fermentor, an increased methane yield of up to 13-19%; and includes biomass mash with residence time, which is independent of the hydraulic residence time in the fermentor.    DETAILED DESCRIPTION - Plant (10) comprises: a mixing- and crushing device (30) for incorporating a mixture of a solid biomass and a fermentation liquid and for mixing and crushing the mixture to a biomass mash; a fermentor (40) connected to the mixing- and crushing device for incorporating the biomass mash, and to promote the fermentation of the biomass mash; and a phase separation device (60) connected to the fermentor to accommodate the fermented biomass mash, which is designed to separate the fermented biomass mash into a liquid phase and a solid phase. The phase separation device is connected to the mixing- and crushing device such that the liquid phase in the mixing- and crushing device, is used as fermentation liquid for producing the biomass mash. An INDEPENDENT CLAIM is also included for processing biomass and producing biogas, comprising: mixing the solid biomass and fermentation liquid, crushing the obtained mixture to a biomass mash in the mixing- and crushing device; feeding the biomass mash into the fermentor and subjecting the mash for fermentation; feeding the fermented biomass mash from the fermentor into the phase separation device, which is connected to the fermentor, and separating the fermented biomass mash into the liquid phase and solid phase; and producing the biomass mash by using the liquid phase as the fermentation liquid in the mixing- and crushing device.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic representation of the mixing- and crushing device of the biogas plant.    Plant (10)    Feeding- and pre-crushing device (20)    Mixing- and crushing device (30)    Fermentor (40)    Phase separation device (60)