The anaerobic sequencing batch reactor (ASBR) is a new high-rate anaerobic process (U.S. Patent No. 5,185,079) that has been developed at Iowa State University. The system offers attractive fundamental advantages over continuous-flow, suspended-growth anaerobic processes. As a result of batch feeding, it is possible to achieve low substrate concentrations just prior to the settling phase. This condition results in low gas production during the settle/decant cycles and excellent solids separation within the reactor. Thus no external clarifier is required. Also, the process tends to select for rapidly settling biomass granules as a result of the natural wasting of poorly settling floc during the decant cycle;Four, 12-liter ASBRs of different reactor configurations were operated at a constant temperature of 35∘C. The ASBRs were capable of achieving reductions in excess of 90% soluble COD and 75% total COD on a soluble milk substrate over a range of COD loadings from 2 to 12 g/L/day at HRTs of 48 hour, 24 hour, and 12 hour;Reactor configuration (depth-to-diameter ratio) appears to be significant in the performance of the ASBR, particularly with regard to the development of granular sludge. Different reactor configurations imposed different levels of selection pressure on the biomass. Tall, slender reactors were more effective in the development of granular sludge, but the short, stout reactors were able to achieve higher concentrations of mixed liquor suspended solids;Mixing effects were another focus of the research. The results indicate that overall COD removal differences between intermittent and continuous mixing were nil but overall methane production was higher with intermittent mixing. Intermittent mixing was also superior to continuous mixing from the standpoint of biosolids separation and retention;A mathematical model was developed for the non-steady-state ASBR process. The ASBR performance can be modeled as a completely mixed reactor (feed phase) followed by a batch reactor (react phase). The model provides minimum cycle time, effluent quality, and reactor sizing information.