A pilot-scale modified Orbal oxidation ditch with internal recycle of the nitrified liquor was developed to significantly enhance total nitrogen (TN) removal through simultaneous nitrification/denitrification (SND) coupled with pre-denitrification under the aerated-anoxic condition. Long-term operational results showed that the highest TN removal efficiency was achieved at very low dissolved oxygen (DO) of 0.15-0.25 mg/L and oxidation-reduction potential (ORP) of 10-40 my within the outer channel. Appropriate internal recycle ratio played a significant role for increasing nitrates reduction within the outer channel, however, excessively high recycle flow deteriorated nitrogen removal instead because of diluted carbon source. At recycle ratio of 9.4, TN removal efficiency further increased to about 87% at optimal DO and pre-denitrification was responsible for TN removal improvement according to nitrogen mass balances. Molecular biology analysis revealed the correlation between variations in nitrogen removal and changes in bacterial community composition. (C) 2013 Elsevier B.V. All rights reserved.