The sustainability of autotrophic granular system performing partial nitritation and anaerobic ammonium oxidation (anammox) for complete nitrogen removal is impaired by the production of nitrous oxide (N2O). A systematic analysis of the pathways and affecting parameters is, therefore, required for developing N2O mitigation strategies. To this end, a mathematical model capable of describing different N2O production pathways was defined in this study by synthesizing relevant mechanisms of ammonium‐oxidizing bacteria (AOB), nitrite‐oxidizing bacteria, heterotrophic bacteria (HB), and anammox bacteria. With the model validity reliably tested and verified using two independent sets of experimental data from two different autotrophic nitrogen removal biofilm/granular systems, the defined model was applied to reveal the underlying mechanisms of N2O production in the granular structure as well as the impacts of operating conditions on N2O production. The results show that: (a) in the aerobic zone close to the granule surface where AOB contribute to N 2O production through both the AOB denitrification pathway and the NH 2OH pathway, the co‐occurring HB consume N 2O produced by AOB but indirectly enhance the N 2O production by providing NO from NO 2 − reduction for the NH2OH pathway, (b) the inner anoxic zone of granules with the dominance of anammox bacteria acts as a sink for NO2− diffusing from the outer aerobic zone and, therefore, reduces N2O production from the AOB denitrification pathway, (c) operating parameters including bulk DO, influent NH4+, and granule size affect the N2O production in the granules mainly by regulating the NH2OH pathway of AOB, accounting for 34–58% of N2O turnover, and (d) the competition between the NH2OH pathway and heterotrophic denitrification for nitric oxide leads to the positive role of HB in reducing N2O production in the autotrophic nitrogen removal granules, which could be further enhanced in the presence of a proper level of influent organics.
- Anaerobic ammonium oxidation (anammox)
- Mathematical modeling
- Nitrous oxide (N2O)
- Partial nitritation