Abstract: An apparatus for the anaerobic digestion of organic waste material such as manure in liquid condition include bringing the waste material in an acidification space with a roof collecting gas generated thereby and one or more openings below said roof giving access from said space to a methane fermentation space so that the gas below the roof pushes the liquid to said latter space. A discharge of gas from below said roof to the outside allows liquid to flow back through said openings into the acidification space. Below said roof there may be a separate wall defining a hydrolysis space, into which the waste material is fed before flowing to the acidification space. The whole of the device or considerable parts of the walls and ducts thereof may be made of flexible gastight material so as to be easily folded or wound up for transport to the site of use so as to be inflatable to sustain its operating shape by a higher than atmospheric pressure of the gas therein.