Anaerobic treatment of one single substrate, acetate, and three mixed substrates, Isomil (simulating municipal wastewater), molasses, and dog food (simulating primary sedimentation sludge) was studied in five bench-scale reactors. Biodegradability of effluent Chemical Oxygen Demand (COD) from the systems was determined under both anaerobic and aerobic conditions. Two reactors were operated under thermophilic anaerobic conditions in order to assess the effect of operating temperature on the amount and biodegradability of residual organics. The results showed that anaerobic treatment of acetate, Isomil, molasses, and dog food results in varying concentrations of residual COD in the effluent. With an influent COD of 30000 mg l(-1), the average effluent COD over approximately one year of operation varied from 380 mg l(-1) for the acetate system to 3730 mg l(-1) for the dog food (thermophilic) reactor. The results also indicated that the residual effluent COD from isomil, molasses, and dog food reactors could be removed more efficiently under aerobic conditions. On the other hand, the residual effluent COD from the acetate treating reactor is more amenable to further treatment under anaerobic conditions. The fraction of residual COD that was further treated under aerobic conditions averaged 81, 51, 67, and 61% for the Isomil, molasses (thermophilic), dog food (mesophilic), and dog food (thermophilic) systems, respectively. The average COD fractions that were further biodegradable anaerobically were 62, 39, 52, and 48% for the same reactors, respectively. On average, 49% of effluent residual COD from the acetate reactor was further removed under anaerobic conditions vs. 34% under aerobic conditions.