Potential of nitrous oxide conversion in batch and down-flow hanging sponge bioreactor systems

Nitrous oxide (N₂O), an important intermediate or by-product in the nitrogen cycle is among the priority greenhouse gas to be mitigated. It can be produced from the aerobic nitrification processes but whether converted under aerobic conditions remains unclear. This study set up the batch and down-flow hanging sponge (DHS) reactor experiments to investigate the aerobic conversion potential of N₂O. The results of batch assays indicated that the significant decrease of N₂O along with oxygen consumption was attributed to the biological reactions. After a 270-d operation of the DHS reactor with N₂O as the sole substrate, it could achieve a removal rate of 6.99 g N g^-1 VSS d^-1 with high N₂O efficiency (~95%). In the reactor, nitrate was accumulated in the effluent up to 7.8 mg N L^-1 at the loadings < 1 mmol N₂O-N d^-1, whereas gaseous N₂ turned out to be the end product at the loadings > 2 mmol N₂O-N d^-1. The cloning library analysis of 16S rRNA gene sequence revealed a complex community structure, comprising abundant Nitrososphaera gargensis and Nitrospira spp. developed in the DHS system. The quantitative PCR analysis further showed that the bacterial and archaeal ammonia-oxidizing populations could greatly increase with the reactor operations, suggesting an important role in the N₂O-converting consortia. Overall, these results suggested the feasibility of N₂O conversion in the bioreactor systems under aerobic conditions.