Abstract: A Class B biosolids sludge digestion process whereby feed sludge is introduced to a completely-mixed, continuously- or intermittently fed and drawn, sealed, anaerobic digester, and sludge is drawn off at a rate such that the average mean cell residence time for the sludge in the digester is greater than or equal to 10 days, and less than 15 days. The temperature of the sludge is maintained in the thermophilic range to produce treated sludge having: a volatile solids content that is at least 38% less than the volatile solids content of the feed sludge, a reduced fecal coliform content that is at least a 2 log (base 10) reduction from the feed sludge, and where there is no more than 2,000,000 most probable number/gram total solids (dry weight basis) of fecal coliform in the treated sludge.