Estimations of running costs revealed that anaerobic treatment of sewage at less than 20 degrees C was more economical than aerobic treatment. An anaerobic fluidized-bed reactor (AFBR) and an upflow anaerobic sludge blanket (UASB) were applied to the anaerobic treatment of sewage at 20 degrees C. Even at a hydraulic retention time (HRT) of 6 h, in both cases the quality of the effluent was 5 mg/l of BOD with a TOC removal efficiency of 80%, which would be appropriate for discharge into rivers. However, at an HRT of 3 h, TOC removal efficiency decreased to 70%, while in an activated sludge process (21 degrees C, HRT 3 h) the TOC removal efficiency was maintained at 83.8%. In order to investigate the effect of temperature on treatment, the temperature was decreased to 13 degrees C in stepwise fashion at an HRT of 6 h. In both the AFBR and UASB, temperatures above 15 degrees C did not effect the TOC removal efficiency, but the TOC concentration in the effluent at 13 degrees C increased to 12.5 mg/l in the AFBR and to 15 mg/l in the UASB. However, at an HRT of 6 h and temperatures above 15 degrees C as the standard conditions, the quality of the effluent treated anaerobically by the AFBR or UASB was almost the same as that by the activated sludge process (21 degrees C, HRT 6h).