NOVELTY - The method for biogas production in a fermentation tank, comprises agitating fermentable biomass to form a floating layer (S) on biomass (M), collecting the arising biogas, supplying floating layer into lateral motion along longitudinal direction of tank and sprinkling the floating layer with a liquid percolate. The floating layer is broken apart at the end of the fermentation tank that is downstream to the direction of movement of the floating layer. The biomass to be fermented is supplied to the tank in the region of circular flow profiles. Rupture of the layer takes place periodically.    USE - Useful for the biogas production in a fermentation tank (claimed).    ADVANTAGE - Ensures continuous biogas production and enables to achieve a defined, more effective retention time of the floating layer during the conversion of the organic materials (biomass) into methane with slight operational effort.    DETAILED DESCRIPTION - The method for biogas production in a fermentation tank, comprises agitating fermentable biomass to form a floating layer (S) on biomass (M), collecting the arising biogas, supplying floating layer into lateral motion along longitudinal direction of tank and sprinkling the floating layer with a liquid percolate. The floating layer is broken apart at the end of the fermentation tank that is downstream to the direction of movement of the floating layer. The biomass to be fermented is supplied to the tank in the region of circular flow profiles. Rupture of the floating layer takes place periodically and the lateral motion is caused simultaneously around the region of rupture of the floating layer that takes place at the downstream end of the fermentation tank. For the generation of constant or periodic lateral motion of the floating layer, the more-liquid fractions of biomass that lie below the floating layer are dispersed in circular flow patterns. A retention time of the floating layer section in the fermentation tank is adjusted using the path of motion along the surface in the fermentation tank until the downstream end, namely until to the rupture of the floating layer in such a manner that an essentially complete aeration of the biomass bound in the floating layer is achieved. The retention time amounts between 30-120 days and the periodic rupture of the floating layer along with simultaneous motion of the floating layer takes place every 7 days or in day/night-sequence. An INDEPENDENT CLAIM is included for a fermentation tank for the biogas production.    DESCRIPTION OF DRAWING(S) - The drawing shows a longitudinal section of a fermentation tank.    Flow channel (2)    Base (10)    Side walls (11, 12)    Floating layer (S)    Biomass. (M)