Partial nitrification to nitrite is technically feasible and economically favourable, especially when wastewaters contained high ammonium concentrations or low C/N ratios. Partial nitrification can be obtained by selectively inhibiting nitrite-oxidizing bacteria (NOB) through appropriate regulation of the pH, temperature and dissolved oxygen (DO) concentrations. The effect of pH, DO levels and temperature on ammonia oxidation rate and nitrite accumulation was investigated in order to determine the optimal conditions for partial nitrification of synthetic wastewater with high ammonia concentration. The experiments performed at low DO levels to lower the total oxygen needed in the nitrification step, which means great saving in aeration. During the start-up stage pH and DO were set at 7.0-7.4 and 0.5 mg/l, respectively. The reactor was operated until complete partial nitrification was achieved. The effect of pH, DO on partial nitrification was studied, as pH was kept at 6.5, 7.5, 8.5, 9.5 and DO at 0.5 +/- 0.2, 1.5 +/- 0.2 and 2.5 +/- 0.2 mg/l, and temperature at 30 degreesC. The influence of temperature on k(a) value was studied by keeping pH = 7.5, DO = 1.5 mg/l and temperature was controlled at 12, 20 and 30 degreesC, respectively. The results showed that partial nitrification to nitrite was steadily obtained and the optimal operational parameters were pH = 7.5, DO = 1.5 mg/l, T = 30 degreesC based on ammonia oxidation rate and nitrite accumulation rate. The maximum k(a) was achieved and to be 115.1 x 10(-3) mg NH4+-N (mg VSS h)(-1) under this condition. (C) 2003 Published by Elsevier Ltd.