Organic wastes, such as animal manure, are fed into an elongated digester tank at one end thereof in slurry form. The slurry slowly moves to the other end of the tank for subsequent disposal. During the residence time in the tank decomposition of the waste occurs yielding methane gas and carbon dioxide. A cover over the digester collects the gas generated. A heat exchange arrangement causes transverse stirring of the slurry as it passes through the tank enhancing the decomposition process. Scum suppressors on the surface of the slurry prevent interference with the formation and release of the methane gas. The settling of solids is controlled by the use of gas jets disposed along the bottom of the tank.