A fixed-bed biofilm reactor with sludge discharger and backwashing system was used as an on-site pilot facility to treat polluted stream water. The annual mean influent concentrations of biochemical oxygen demand (BOD) and total suspended solid (TSS) were 73.9 and 81.5 mg/l, and the removal efficiencies were 87.3 and 86.8%, respectively. The sludge discharger and backwashing system periodically removes influent TSS and excess biofilm, and maintains a constant effluent concentration (BOD: 6-10 mg/l, TSS: 5-11 mg/l). Overall, this system has the potential for long-term water quality improvement of polluted streams without bed clogging. (C) 2004 Elsevier B.V. All rights reserved.