To achieve nitritation from complete-nitrification seed sludge at room temperature of 19 +/- 1 degrees C, a lab-scale sequencing batch reactor (SBR) treating domestic wastewater with low C/N ratios was operated to investigate the control and optimization of nitrifying communities. Ammonia oxidizing bacteria (AOB) dominance was enhanced through the combination of low DO concentrations (<1.0 mg/L) and preset short-cycle control of aeration time. Nitritation Was Successfully established with NO2--N/NOx--N over 95%. To avoid the adverse impact of low DO concentrations on AOB activities, DO concentrations were increased to 1-2 mg/L. At the normal DO levels and temperatures, on-line control strategy of aerobic durations maintained the stability of nitritation with nitrite accumulation rate over 95% and ammonia removal above 97%. Fluorescence in-situ hybridization (FISH) analysis presented that the maximal percentage of AOB in biomass reached 10.9% and nitrite oxidizing bacteria (NOB) were washed out. (C) 2009 Elsevier Inc. All rights reserved.