Wastewater from a food-manufacturing plant with a low concentration of organic matter below 100 mg/l TOC was first treated at 37 degrees C in an anaerobic fluidized-bed reactor (AFBR) or in an upflow anaerobic sludge blanket (UASB). The TOC removal efficiency in both reactors decreased from 85% to 65% as the influent TOC concentration decreased from 100 to 35 mg/l at a hydraulic retention time (HRT) of 6 h. Treatment at an HRT of 4 h resulted in an effluent TOC concentration of 11 to 15 mg/l. The concentration of suspended solids in the effluent could be reduced to 20 mg/l, which corresponded to 7% of that of the influent. The effluent from both reactors was then treated anaerobically in a fixed-bed reactor system. The TOC concentration and optical density (OD) of the effluent from the aerobic treatment were reduced to 5 mg/l and 0.005, respectively, at an HRT of 2 h. When anaerobically or aerobically treated effluent was passed over an activated carbon column, the effluent TOC concentration was reduced to 2 to 3 mg/l. The conductivity of 1.3 mS/cm in raw wastewater, which was not removed through the above treatments, was reduced to 0.001 mS/cm on an ion-exchange resin column. An effluent quality corresponding to that of ultra-pure water for industrial use was finally attained by the treatment in this multi-step system.