An anaerobic sequence batch process carried out in a single vessel. The biological reactions occur under anaerobic conditions and the vessel is operated on a fill-and-draw basis in a sequential manner. When waste is entering the reactor, the vessel is mixed by biogas or liquid recirculation. Waste feeding continues until the reactor is filled to its predetermined full liquid level. The anaerobic reaction proceeds with intermittent or continuous mixing. Mixing is then discontinued, allowing the biomass to settle under quiescent conditions resulting in the formation of a low suspended solids supernatant. After sufficient time in the settling cycle, supernatant is withdrawn from the reactor lowering the reactor contents to the predetermined lower liquid level. Gas or liquid recirculation mixing is then begun again along with waste feeding. Excess biomass is wasted from the reactor on a periodic basis. The capacity of the reactor depends on the number of feeding-clarification-wasting cycles that can be achieved in a day.