NOVELTY - The device comprises: fermenters (1) to receive a substrate (2) to be percolated, where the substrate is arranged on a floor surface of the fermenters; a percolate container (3), where the fermenters and the percolate container are connected with each other such that the percolate within the fermenters seeping through the substrate makes its way into the percolate container, from which it is directed back to the fermenters as necessary; and a thermal reservoir and/or a percolate collector that are partially located below the floor surface of the fermenters.    USE - The device is useful for producing biogas (claimed).    ADVANTAGE - The device is compact and energy-efficient, creates synergistic effects by its design, and produces the biogas without any heat losses.    DETAILED DESCRIPTION - The device comprises: fermenters (1) to receive a substrate (2) to be percolated, where the substrate is arranged on a floor surface of the fermenters; a percolate container (3), where the fermenters and the percolate container are connected with each other such that the percolate within the fermenters seeping through the substrate makes its way into the percolate container, from which it is directed back to the fermenters as necessary; and a thermal reservoir and/or a percolate collector that are partially located below the floor surface of the fermenters. The thermal reservoir is formed by the percolate container. The percolate container is partially positioned below the fermenters in a base area. The percolate collector is positioned: between the fermenters and the percolate container; below the floor surface of the fermenters; and adjacent to the percolate container. The fermenters include: the floor surface that is partially inclined in sections such that the percolate seeping through the substrate flows toward a wall and/or a corner of the fermenters; and outlets to let out the percolate. The fermenters are positioned adjacent to one another, below which a common percolate container is positioned. The device further comprises: intermediary walls positioned within the percolate container such that the percolate finding its way into the percolate container is fed meanderingly through the percolate container between a percolate inlet and a percolate outlet; and a quantity of modules, where the modules include the fermenters, a percolate container collector section and a percolate container section. The outlets of the fermenters feed into the percolate collector that extends along the fermenters and ends at the percolate inlet of the percolate container. Each outlet of the fermenters ends at the pertinent percolate container collector section. The percolate collector is formed as a collecting line and/or as a sand trap (5).    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic sectional view of a device for producing biogas.    Fermenters (1)    Substrate (2)    Percolate container (3)    Base (4)    Sand trap. (5)