Role of hydrochar in enhancing nitrous oxide sink in microaerobic sequencing batch reactors

Wastewater treatment plant is a significant emission source of atmospheric nitrous oxide (N₂O). The electron flow to denitrifying enzymes and NosZ modulates N₂O production and consumption. Herein, char materials including hydrochar and biochar were applied to investigate the consumption performance and mechanism of N₂O in microaerobic sequencing batch reactor (SBR) systems. Hydrochar, rich in oxygen-containing groups and with high electron exchange capacity, could greatly enhance N₂O consumption in the microaerobic SBR system (HC), averaging 2.63 and 9.64 times higher than in the control (CK) and biochar treatment (BC), respectively. At a concentration of 10 mg L^-1, HC could achieve NH₄⁺-N removal efficiencies of 92.01-99.12%, which were 3.09-8.24 times of that in CK and BC. Azospira, Magnetospirillum and Candidatus Accumulibacter were the dominant active microbes in HC. The ratio of nirS to nirK microbes increased in HC. The suppression of Nar-encoded genes and promotion of Nap-encoded genes in HC may enhance cytoplasmic NO₃^- reduction, alleviating electron competition between NO₃^- and N₂O reduction. Moreover, the genes related to electron transfer via e-pili, cell motility, ATP synthase were prominently upregulated while cox encoding cytochrome c oxidase was downregulated in HC, which could favor interspecies electron transfer with alternative acceptors like N₂O over O₂. These results provide novel insights into microbial regulation of N₂O emission reduction through enhanced N₂O consumption in wastewater treatment systems, and will be useful for developing N₂O mitigation strategies.