This study describes the feasibility of anaerobic treatment of a low-strength (500 mg-COD/l) synthetic wastewater using a semi-pilot-scale upflow anaerobic sludge blanket (UASB) reactor Under ambient temperature conditions (20-35 degrees C) with a hydraulic retention time (HRT) of 3 h and corresponding organic loading of 4 kg-COD/m(3)-day, 90-92% COD and 94-96% BOD reductions were achieved. Methane production was found to be about 141 l/kg-COD removed at standard temperature and pressure (STP). Because of the presence of a high concentration of active granular sludge in the lower portion of the reactor 80% reduction of COD occurred within the bed level. Morphological examination of the sludge showed that the granules contained diverse groups of microorganisms, where rod-type, Methanothrix-like, cells were dominant on the surface. (C) 1996 Elsevier Science Ltd.