The effectiveness of RAS degradation under low dose ozone and effect of ozonated sludge on a full scale anaerobic-anoxic-aerobic (A/A/O) process were examined. The ratios of chemical oxygen demand (COD) to nitrogen and COD to phosphorus of ozonated sludge were 23 and 91 respectively on average, much higher than in influent. Compared with the performance of A/A/O process alone, COD removal efficiency slightly decreased after insertion of the ozonation system. Ammonia removal became more stable, but total nitrogen and total phosphorus removal efficiency was not improved. Sludge volume index (SVI) of activated sludge reached to an optimal value of 80-120 mL/g quickly due to ozonation combined with A/A/O process which reduced the danger of sludge bulking. Overall, the combination of biological process with ozonation at a low dose rate shows promise. Compared with higher dose ozonation, low-dose ozonation can improve the performance of A/A/O effectively at a smaller cost. (c) 2013 Published by Elsevier Ltd.