In this study, a novel technology was developed to achieve efficient partial nitrification at moderately low temperature, which would save the aeration cost and have the capacity to treat a wide range of ammonium-rich wastewaters with low chemical oxygen demand-to-nitrogen (COD:N) ratios. At pH of 7.1-7.4 and a sludge retention time (SRT) of greater than 100 days, a laboratory-scale intermittently aerated sequencing batch reactor (IA-SBR) was operated at about 20 degrees C to treat synthetic wastewater containing readily biodegradable COD of 100-300 mg L-1 and ammonium-nitrogen (NH4+-N) of 300 mg L-1 During the aeration periods, DO concentrations were controlled at less than 0.2 mg L-1, and the aeration was terminated when on-line NH4+-N measurement reached 20 mg L-1. During 180 days of operation, the mass ratio of nitrite-N (NO2--N) to the total oxidized nitrogen was over 90% in the effluent. Molecular analyses show that ammonium oxidizing bacteria (AOB) represented up to 10% of the total bacterial population in the sludge biomass, and the ratio of AOB to nitrite oxidizing bacteria (NOB) was generally over 100. The IA-SBR quickly recovered after non-operation for 47 days. The three main factors enabling long-term stable partial nitrification in this study were: enrichment of AOB in the start-up period: controlled oxygen-limited conditions and the intermittent aeration strategy. (C) 2011 Elsevier B.V. All rights reserved.