A bench-scale experiment using an innovative integrated oxidation ditch process with vertical circle was carried out to remove carbonaceous substrates and ammonia nitrogen. The new process not only reduces land area of about 50% and energy consumption by 40% comparing with the conventional oxidation ditch with intrachannel clarifier, but also it has the characteristics of compact configuration, simple operation and easy maintenance. In this new system, the clarifier does not affect the hydrodynamic characteristics of the mixed liquid in the ditch, and the sludge can automatically return to the down ditch. The nitrification process and carbonaceous organic biodegradation could be performed well. At HRT of 12-20 h, and temperature of 10-30 degreesC, the average concentrations of COD, SS, and NH4+-N in the effluent were 21, 15.2, and 1 mg/l, respectively, when the average concentrations of COD, SS, and NH4+-N in the influent were averaged 420, 152, and 59.2 mg/l, respectively. (C) 2003 Elsevier Ltd. All rights reserved.