Abstract: Biodegradable organic wastes are crushed and then fed to a first cell A of a treatment unit in which they are mixed with liquid recirculated from the discharge end of that unit to form a sludge. After oxygenation in cell A, and following the establishment of an anaerobic atmosphere in that cell by the introduction of carbon dioxide, part of the sludge passes by gravity into a lower-level second cell B for prefermentation with evolution of carbon dioxide. Subsequently, part of the contents of cell B is transferred -- again by gravity -- to a third cell C, at a still lower level, where fermentation with generation of methane takes place. The methane production is continued thereafter in a fourth cell D at the lowest level, the latter containing a potash solution in which the accompanying carbon dioxide is dissolved while the methane is recovered.