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 ₂ O). A systematic analysis of the pathways and affecting parameters is, therefore, required for developing N ₂ O mitigation strategies. To this end, a mathematical model capable of describing different N ₂ 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 ₂ O production in the granular structure as well as the impacts of operating conditions on N ₂ O production. The results show that: (a) in the aerobic zone close to the granule surface where AOB contribute to N ₂ O production through both the AOB denitrification pathway and the NH ₂ OH pathway, the co-occurring HB consume N ₂ O produced by AOB but indirectly enhance the N ₂ O production by providing NO from NO ₂ ^- reduction for the NH ₂ OH pathway, (b) the inner anoxic zone of granules with the dominance of anammox bacteria acts as a sink for NO ₂ ^- diffusing from the outer aerobic zone and, therefore, reduces N ₂ O production from the AOB denitrification pathway, (c) operating parameters including bulk DO, influent NH ₄ ⁺ , and granule size affect the N ₂ O production in the granules mainly by regulating the NH ₂ OH pathway of AOB, accounting for 34-58% of N ₂ O turnover, and (d) the competition between the NH ₂ OH pathway and heterotrophic denitrification for nitric oxide leads to the positive role of HB in reducing N ₂ O production in the autotrophic nitrogen removal granules, which could be further enhanced in the presence of a proper level of influent organics.
(© 2019 Wiley Periodicals, Inc.)