Apparatus and method are disclosed

Apparatus and method are disclosed for an anaerobic process in three stages having progressively higher temperatures up to 75° 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° 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. The thermophilic stage contains a micron-size filter upon which a bacterial slime forms and which passes a sediment-free, treated effluent. Sterilized effluent from the fourth stage passes through the underlying heat-exchange compartment having the form of a truncated cone for heat exchange to the bottoms of the three concentric flow chambers.