Trade-off between emission of nitrous oxide and production of dissolved organic nitrogen during A2O process

Nitrous oxide (N2O) and dissolved organic nitrogen (DON) are both the undesirable byproducts of the wastewater treatment due to their contributions to greenhouse gas emissions and eutrophication, respectively. Here, we used a 4-set Venn Diagram model with co-occurrence network analysis based on an anaerobic/anoxic/oxic (A2O) continuous flow reactor to trace the transformation of DON molecules. Then, metagenomic analysis with ex situ kinetic experiments established a link between N2O yield and microbial community. After long-term operation, the A2O reactor exhibited a trade-off between N2O yield and concentration of effluent DON. A lower C/N ratio in influent resulted in increased N2O yield but decreased concentration of effluent DON, whereas a higher C/N ratio had the opposite effect. Carbon source shaped the microbial community and may promote the emergence of microorganisms with smaller genome size stronger competitiveness for DON. These microorganisms can lead to elevated concentrations of DON and provide electron donors for denitrifying bacteria to enhance complete denitrification. Alphaproteobacteria maybe the key microorganism in simultaneously mitigating N2O and DON. This study indicates that N2O and DON should be managed in an integrated manner rather than being addressed individually during wastewater treatment.