The goal of this study was to investigate the optimal pH and temperature allowing enhanced nitrification in communities of nitrifying bacteria in full-scale wastewater treatment plants (WWTPs). Although maximum nitrification of activated sludge in WWTPs was achieved using similar optimal conditions, nitrification performance was uniquely influenced by simultaneous variations in temperature and pH. These results led to development of different model equations predicting the combined effects of temperature and pH on nitrification rates at WWTPs, reflecting the propensity of nitrifying bacterial communities to flourish under different conditions. Different end products of nitrification resulted not only from certain factors such as pH and temperature, but from differences in proportions of nitrifying bacteria among total bacteria. Therefore, proper adjustment of optimal pH and temperature in nitrifying bacteria communities can provide an implementable approach to improve nitrification performance of WWTPs, incorporating those specific conditions shown to favor the acclimatization of certain nitrifying bacteria. (C) 2014 Elsevier B.V. All rights reserved.