In a continuous anaerobic-anoxic-aerobic (A(2)O) process treating domestic wastewater at ambient temperatures, nitritation was achieved through a combination of short aerobic actual hydraulic retention time (AHRT) and low dissolved oxygen (DO) levels (0.3-0.5 mg/L). The nitrite accumulation rate was about 90% and ammonia removal efficiency was over 95%. With respect to total nitrogen removal, nitritation-denitritation at low DO levels of 0.3-0.5 mg/L was essentially equal to the complete nitrification-denitrification at DO levels of 1.5-2.5 mg/L with the addition of external carbon sources. Regardless of low DO operation, sludge bulking did not occur since the sludge volume index was below 150 ml/g. Real-time PCR assays showed that in response to complete and partial nitrification modes, the numbers of ammonia oxidizing bacteria population were 5.28 x 10(9) cells/g MLVSS and 3.95 x 10(10) cells/g MLVSS, respectively. Achievement of nitritation-denitritation is highly beneficial to the treatment of domestic wastewater in terms of lower carbon requirements and reduced aeration costs. (c) 2010 Published by Elsevier Ltd.