In this paper, Quantitative Polymerase Chain Reaction (qPCR), Illumina MiSeq sequencing and N-15 stable isotopic tracing methods were applied to study the effects of influent organic loading, BOD5/TN and NO2 -N/ NH4-N ratios on the combined nitrogen removal through denitrification and anammox. The results show that, the total nitrogen removal was above 90% at the optimum reaction condition: organic loading 0.04 kg/m(3) center dot day, BOD5/TN 0.2, and NO2-N/NH4-N 1.0. Approximately 9.43% of the nitrogen removal occurred through anammox process when BOD5/TN was 0.1, and increased to 21.46% when BOD5/TN was 0.2. The anammox accounted for 10% when NO2-N/NH4-N was 0.5, and that increased to 20.72% when NO2-N/NH4-N was 1.0-1.5. hzsA was directly proportional to the influent BOD5/TN. A positive correlation between BOD5/TN and anammox microorganisms existed. Similarly, there was a positive corre-lation between NO2-N/NH4-N and denitrification microorganisms. Therefore, it is feasible to regulate the influent to improve nitrogen removal efficiency. (C) 2016 Elsevier Ltd. All rights reserved.