This study incorporates three currently known nitrous oxide (N2O) production pathways: ammoniumoxidizing bacteria (AOB) denitrification, incomplete hydroxylamine (NH2OH) oxidation, and heterotrophic denitrification on intracellular polymers, into a mathematical model to describe N2O production in an anaerobic/oxic/anoxic (AOA) process for the first time. The developed model was calibrated and validated by four experimental cases, then evaluated by two independent anaerobic/aerobic (AO) studies from literature. The modeling results displayed good agreement with the measured data. N2O was primarily generated in the aerobic stage by AOB denitrification (67.84-81.64%) in the AOA system. Smaller amounts of N2O were produced via incomplete NH2OH oxidation (15.61-32.17%) and heterotrophic denitrification on intracellular polymers (0-12.47%). The high nitrite inhibition on N2O reductase led to the increased N2O accumulation in heterotrophic denitrification on intracellular polymers. The new model was capable of modeling nitrification-denitrification dynamics and heterotrophic denitrification on intracellular polymers in the AOA system. (C) 2016 Elsevier Ltd. All rights reserved.