An aerobic biological filter with floating filter media was tested using domestic wastewater to determine the optimum operating and backwashing parameters in this study. This system was designed for a small-scale joint treatment plant and 4 mm diameter polystyrene foam pellets were used as the filter media. As the backwash is the most important point for using a floating biolifter, "air shot" system, a turbulent-flow backwashing utilising the air reverse syphon and the buoyancy of the filter media, was developed for this purpose. Through the test, the optimum position of the diffuser tube and the structure of the backwashing device were determined. It was confirmed that the system could achieve effluent biological oxygen demand (BOD) of under 10 mg/l, and a nitrification rate of over 86% at a BOD loading of 0.7 kg/m(3) per day and "total-nitrogen (T-N) loading" of 0.16 kg/m(3) per day. The backwashing frequency should be as many times as possible per cycle to remove the sloughed sludge thoroughly in order to maintain good effluent quality after backwashing. (C) 2003 Elsevier Ltd. All rights reserved.