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 (N O). A systematic analysis of the pathways and affecting parameters is, therefore, required for developing N O mitigation strategies. To this end, a mathematical model capable of describing different N O 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 N O production in the granular structure as well as the impacts of operating conditions on N O production. The results show that: (a) in the aerobic zone close to the granule surface where AOB contribute to N O production through both the AOB denitrification pathway and the NH OH pathway, the co-occurring HB consume N O produced by AOB but indirectly enhance the N O production by providing NO from NO reduction for the NH OH pathway, (b) the inner anoxic zone of granules with the dominance of anammox bacteria acts as a sink for NO diffusing from the outer aerobic zone and, therefore, reduces N O production from the AOB denitrification pathway, (c) operating parameters including bulk DO, influent NH , and granule size affect the N O production in the granules mainly by regulating the NH OH pathway of AOB, accounting for 34-58% of N O turnover, and (d) the competition between the NH OH pathway and heterotrophic denitrification for nitric oxide leads to the positive role of HB in reducing N O production in the autotrophic nitrogen removal granules, which could be further enhanced in the presence of a proper level of influent organics.