Abstract: A method for the anaerobic fermentation of organic waste materials in a digestion zone, which method comprises introducing into the zone algae produced externally of the zone, and/or at least part of the liquid substrate in which such algae have been grown.Apparatus for anaerobic fermentation consisting essentially of a liquid storage tank comprising an inner bag-like container made of flexible material and provided with inlet and outlet means for the introduction of feed materials and the simultaneous removal of liquid and gaseous products, an outer supporting wall or frame, and at least one layer of insulating material arranged between the outer wall of the frame and the container.