Nitrous oxide production in autotrophic nitrogen removal granular sludge: A modeling study

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.