This is the first study to achieve a short HRT (similar to 2 h for the A/O reactor), high MLSS (similar to 10 g L-1), and high volumetric loading (similar to 3.7 kg COD m(-3) d(-1) and similar to 0.6 kg NH4-N m(-3) d(-1)) in the A/O process for sewage treatment by enhancing biomass retention in the secondary clarifier (similar to 4 h settling time), which we refer to as the high-performance A/O process (HP-A/O) in this paper. Over 258 days of continuous operation, with a decrease in HRT from 12 to 2 h, remarkable COD (95 +/- 3%), NH4+-N (98 +/- 2%), TN (79 +/- 5%), and TP (74 +/- 10%) removals were stably achieved, while the air requirement significantly decreased by 22%. The HP-A/O process offers advantages over the conventional A/O process (68 h for A/O reactor, 35 g L-1 MLSS, and similar to 1.0 kg COD m(-3) d(-1)) for sewage treatment in terms of its lower energy consumption, smaller footprint and reactor requirements. (C) 2017 Elsevier Ltd. All rights reserved.