The objective of this study was to evaluate the conversion of full nitritation to partial nitrification processes by altering the influent ammonium concentration in a sequencing batch reactor at ambient temperature. After 150 days' operation, full nitritation and partial nitrification processes were successfully achieved when the influent NH4-N concentrations up to 400 and 720 mg/L, respectively. Meanwhile, sludge volumetric index (SVI) gradually decreased from 127.4 to 63.4 mL/g, while the average size of sludge improved from 29.5 to 195.6 mu m by the strategy of reducing settling time. Ammonium-oxidizing bacteria (AOB) were the dominant nitrifying bacteria according to the fluorescence in situ hybridization (FISH) analysis. Free ammonia (FA) and free nitrous acid (FNA) were the potential compounds for inhibiting the activity of nitrite-oxidizing bacteria (NOB). The obtained results may help to promote the development of new biological nitrogen removal processes in engineering, especially in relation to nitrogenrich wastewaters. (C) 2013 Elsevier B.V. All rights reserved.