LAUSR

Abstract Competition for electrons among different steps of denitrification on intracellular polymers (X STO ) plays a significant role in nitrous oxide (N 2 O) accumulation in the biological nitrogen removal process. In this work, this electron competition was considered in a mathematical model to predict N 2 O production in anaerobic/anoxic/oxic sequencing batch reactors (A 2 O-SBR) for the first time. The affinity constant for intracellular polymers of heterotrophs ( K STO ) that was used in previously published models was divided into four affinity constants ( K STO , 1 , K STO , 2 , K STO , 3 and K STO , 4 ) to represent the ability of each denitrification reductase to compete for intracellular polymers. The improved model was calibrated and validated using experimental data from three independent A 2 O-SBR systems. The results demonstrated that the modeling predictions strongly agreed with the measured data from all experimental tests under various operational conditions. The modeling results indicated that N 2 O accumulation resulted from the more rapid decline of the N 2 O reduction rate than the nitrite reduction rate for the inadequate X STO in these A 2 O-SBR systems. The modeling results also suggested that distinguishing affinity constants for intracellular polymers during the four-step denitrification felicitously described a different X STO distribution in each reduction step, thereby better predicting nitrogen dynamics and N 2 O production in A 2 O processes than the published model. The improved model is therefore a preferable tool to gain insight into N 2 O accumulation in A 2 O processes.