Abstract: The present invention relates to the manufacture of high calorific fuel gas through a process of anaerobic digestion involving retention of a slurry of organic material, maintained at a predetermined volume-to-interface (V/I) ratio and for a desired liquid retention time, pH and temperature to produce substantial amounts of methane gas while minimizing the production of carbon dioxide gas. When operating in the preferred range of volume-to-interface (V/I) relationships, the faster molecular diffusion rate of methane from the slurry as compared with carbon dioxide gas allows virtually all of the methane to be collected from the slurry while a considerably smaller amount of carbon dioxide gas is collected. Instead, the bulk of the carbon dioxide remains in the spent slurry which is transferred as sludge to a loop for carbon dioxide stripping. Liquid retention is controlled by a recirculation-stripping loop.