This study investigated the long-term treatment performance of a conventional activated sludge (AS) process operating at a microaerobic DO level (0.5-1.0 mg.L-1) in the aeration tank and a long settling time of > 10 h in the clarification tank for sewage treatment. The microaerobic DO conditions led to severe sludge bulking. However, good sludge/water separation and excellent pollutant removal performance (COD, 95 +/- 2%; NH4+ -N, 99 (+/-) 1%; and TN, 69 +/- 6%) were stably achieved in the microaerobic AS system during its 150 days of continuous operation. This is the first report to demonstrate that a long settling time effectively overcame the effect of severe filamentous bulking in conventional AS process, and that microaerobic DO conditions achieved excellent simultaneous nitrification and denitrification reactions in the aeration tank. The process characteristics of the microaerobic AS system differed substantially from those existing biological denitrification processes, including A/O, CANON, and OLAND processes. (C) 2016 Elsevier Ltd. All rights reserved.