Two upflow anaerobic sludge blanket (UASB) reactors (total volume=8 l) were started-up at 48 h hydraulic retention time (HRT) and 20 degrees C temperature to examine the application of this technology for the treatment of dilute municipal wastewater under low temperature environments. HRT was reduced gradually to 10 h during 280 d of operation. The total and soluble chemical oxygen demand (COD) of municipal wastewater varied in the range of 350 to 500 mg/l and 150 to 300 mg/l respectively during this period. Under steady-state conditions, the removal of total COD and soluble COD ranged from 60 to 75% and 70 to 85% respectively at each HRT. Sulfate concentration in the influent varied from 50 to 100 mg/l as sulfur (S). About 70 to 80% of sulfate in the influent was reduced, resulting in a low recovery of methane because 30 to 40% of the total COD was consumed in sulfate reduction. Image analysis of sludge samples revealed an aggregation of about 50% of sludge particles in small granular forms with a mean size of about 1.5 mm.