Abstract: Apparatus and method are disclosed for an anaerobic process in three stages having progressively higher temperatures up to 75.degree. C. for digesting raw household sewage with methane forming bacteria, in which dolomite is added to control the pH of the system within the range 6.0-7.5. After a sterilization fourth stage at 82.degree. C. or greater, the effluent is treated in a ground filter. Heat is countercurrently transferred from the sterilization stage effluent to the substrate of the digestion stages. The apparatus for carrying out this process has an underlying heat-exchange compartment and four concentric compartments, of which the innermost is the sterilization stage compartment containing a heating means and the outermost compartments are flow chambers for sequentially digesting the sewage in psychrophilic, mesophilic, and thermophilic stages, prior to sterilizing.

Abstract: Apparatus are disclosed for an anaerobic process in three stages having progressively higher temperatures up to 75.degree. C. for digesting raw household sewage with methane-forming thermophilic bacteria, in which dolomite is added to control the pH of the system within the range 6.0-7.5. After a sterilization fourth stage at 82.degree. C. or greater, the effluent is treated in a ground filter. Heat is countercurrently transferred from the sterilization stage effluent to the substrate of the digestion stages. The apparatus for carrying out this process has an underlying heat-exchange compartment and four concentric compartments, of which the innermost is the sterilization stage compartment containing a heating means and the outermost compartments are flow chambers for sequentially digesting the sewage in psychrophilic, mesophilic, and thermophilic stages, prior to sterilizing.