In the present study, the performance of the partial nitritation (PN) process in a continuous flow internal loop airlift reactor was optimized by applying the response surface method (RSM). The purpose of this work was to find the optimal combination of influent ammonium (NH4+-N-inf), dissolved oxygen (DO) and the alkalinity/ammonium ratio (Alk/NH4+-N) with respect to the effluent nitrite to ammonium molar ratio and nitrite accumulation ratio. Based on the RSM results, the reduced cubic model and the quadratic model developed for the responses indicated that the optimal conditions were a DO content of 1.1-2.1 mg L-1, an Alk/NH4+-N ratio of 330-5.69 and an NH4+-N-inf content of 608-1039 mg L-1. The results of confirmation trials were close to the predictions of the developed models. Furthermore, three types of alkali were comparatively explored for use in the PN process, and bicarbonate was found to be the best alkalinity source. (C) 2013 Elsevier Ltd. All rights reserved.