The feasibility of implementing a novel strategy to start up and stably operate a partial nitritation (PN) process in an internal-loop airlift reactor was investigated. The results suggested that the PN process was successfully set up through initial complete nitrification followed by adjusting the alkalinity-to-ammonium ratio (Alk/NH4+-N), also known as the 'initial complete nitrification and subsequent partial nitrification' (CNin-PNsub) strategy. A high nitrite accumulation ratio of approximately 98% and a maximum nitrite production rate of 0.91 kg N m(-3) day(-1) were attained under the conditions of 1.0 mol mol(-1) Alk/NH4+-N, 1.24 +/- 0.71 mg L-1 dissolved oxygen, 3.99 +/- 2.29 mg L-1 free ammonia and 0.69 +/- 0.30 mg N L-1 free nitrous acid. The stabilized effluent with an NO2--N/NH4+-N ratio of 1.15 +/- 0.06 was suitable for subsequently feeding the anaerobic ammonium oxidation (ANAMMOX) process. The modified Stover-Kincannon model was used to analyze the kinetic parameters, and the saturation constant (K-B) and maximum ammonium utilization rate (U-max) were determined to be 5.8 and 5.4 kg m(-3) day(-1), respectively. (C) 2013 Elsevier B.V. All rights reserved.