The feasibility of achieving nitritation and phosphorus removal using bio-augmentation was investigated in a continuous-flow anaerobic/oxic (A/O) reactor treating sewage. The results indicated that nitritation could be quickly start-up, and reconstructed with an increase in the nitrite accumulation rate (NAR) from 1% to 89% within 15 days by using bio-augmentation and controlling DO at 0.96 mg/L. Biological phosphorus removal could be achieved with the average phosphorus removal efficiency of 96.43% when the NAR was maintained above 78.60%. Meanwhile, sludge settleablity was good with a sludge volume index (SVI) of between 62 and 102 mL/g even under high NAR. After nitritation and biological phosphorus removal were achieved, this A/O reactor has the potential to supply appropriate influent for the anammox UASB reactor. (c) 2013 Published by Elsevier Ltd.