BACKGROUND: In this study, a plug-flow A(2)O (anaerobic/anoxic/oxic) reactor, with a working volume of 52.5 L, was employed to investigate the performance of biological nutrients removal and microbial population variations when treating low C/N ratio domestic wastewater. RESULTS: Results showed that TN removal was significantly affected by the shortage of carbon source while phosphorus removal was only slightly affected. The effluent soluble orthophosphate-phosphorus (SOP) concentration was lower than 0.50 mg L-1 but the TN concentration was over 20 mg L-1 when the C/N ratio was 4.43. There was denitrifying phosphorus removal in the anoxic reactor and this was enhanced by increasing the volume ratio of anoxic reactor and maintaining appropriate mixed liquor recycle rate. More than 60% of the SOP were removed in anoxic reactors by denitrifying phosphorus removal when the volume ratio of anaerobic/anoxic/oxic was 1/1.4/1.6 and the mixed liquor recycle rate was 250%. The TN concentration of effluent decreased to 11.34 mg L-1 and SOP concentration was still lower than 0.5 mg L-1 in this condition. The main microorganisms found in the process by polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) and the functional biodiversity are discussed. CONCLUSION: Traditional design and operating parameters of A(2)O are not appropriate for treating low C/N wastewater. Enhancing the denitrifying phosphorus removal ratio in an A(2)O process is an effective way to increase the removal rate of N and P from low C/N wastewater. (C) 2010 Society of Chemical Industry