<p>The bio gas fermenter has a first foil (12) acting as roof covering and a second foil (11) which covers a bio mass (3) and forms a gas chamber (14) above same. A third foil (10) is provided underneath the bio mass and all the foils are rolled up together along their edge (13) like a screw. The foils are circular in plan view. The first foil can have viewing windows (16) for inspecting the bio mass. The edge area can be additionally secured to prevent unravelling.</p>
