Biological treatment of high-strength nitrogen wastewater by a combined anaerobic (upflow sludge bed filter, UBF)/aerobic (membrane bioreactor, MBR) system has been studied. This system exhibited high performance on the removal of organic matter and nitrogen simultaneously. Organic and nitrogen concentrations increased from 6,000 to 14,500 mg/L and 300 to 1,000 mg/L, respectively. At the internal recycle ratio of Q (Q is the influent flow rate), average removal efficiencies of organic and total nitrogen were found to be 99 and 46%, respectively with the relatively short HRT of 24 h. When operated with the insufficient alkalinity supply, the organic removal efficiency in anaerobic reactor reduced from 98 to 82%. At high influent ammonia concentration, ammonification which is the dissimilatory nitrate reduction to NH4+ was observed. In case of membrane fouling, transmembrane pressure (TMP) of the combined process was about 9 times higher than that of a unit MBR under same operation conditions. The reason of severe fouling in the combined system might be caused by increased extracellular polymeric substance (EPS) and hydrophobicity. EPS composition and SUVA were more sensitive parameters than surface charge and total EPS content, with respect to the change of internal recycle ratio 100-300%. ？ 2006.