Evaluation of the process performance of comammox-like nitrospira dominant down-flow hanging sponge reactor with reduced nitrous oxide emissions

Nitrification/denitrification mitigates excess nitrogen in wastewater and reduces nutrient pollution in recipient surface waters but emits substantial amounts of nitrous oxide (N2O). Complete ammonia-oxidizing (comammox) bacteria provide novel opportunities to mitigate N2O emissions from wastewater treatment systems. In this study, a down-flow hanging sponge (DHS) reactor with low-strength ammonia-based synthetic wastewater was used to culture comammox bacteria, to study the microbial community structure, and to assess the nitrogen removal performance. The results showed a high NH4+-N removal efficiency of 98 f 4 % and complete nitrification during the entire experimental period. 16S rRNA gene sequencing and metagenomic analysis showed that comammoxlike Nitrospira dominated the DHS-retained sludge, and that comammox-like Nitrospira and ammonia-oxidizing archaea may have coexisted symbiotically. The dissolved N2O emissions per NH4+-N removed from the DHS reactor were much lower than those from conventional activated sludge processes, indicating that the DHS reactor could be effective in reducing N2O emissions during wastewater treatment.