Wastewater treatment performance and gaseous emissions in MFC-CWs affected by influent C/N ratios

Constructed wetlands with microbial fuel cells (MFC-CWs) are widely acknowledged to be efficient in pollutant removal, and have been reported to be beneficial in reducing specific gaseous emissions. However, as a critical parameter that directly addresses, the behavior of carbon (C) and nitrogen (N), the impact of the influent C/N ratio on the comprehensive performance of MFC-CWs remains unknown. Herein, the impact of C/N ratios (i.e., 1, 5, 10, 15) on pollutants (COD, symbolscript NO3--N, NO2--N, TN and TP) removal, multiple gaseous (CO2, CH4, N2O and NH3) emissions and electricity generation of MFC-CWs were evaluated, and the internal mechanisms were explored. Increasing the C/N ratio enhanced COD removal, and the MFC-CWs achieved superior symbolscript TN, and TP removal efficiencies at C/N ratios of 5 and 10. The higher C/N ratio was generally accompanied by greater CO2, CH4, and NH3 fluxes. Nevertheless, the lowest global warming potential (GWP) was observed at C/N of 5 due to the lowest N2O flux having been obtained. The functional gene analysis results showed that the decrease in N2O flux was attributed to an increase in the nosZ/(nirS symbolscript nirK) ratio, and the CH4 flux was regulated by the abundance of pomA and mcrA genes. The highest power density (152.54 mA/m3) and current intensity (649.03 mW/m3) were also observed in MFC-CWs with a C/N ratio of 5, mainly due to the increase in electrogenic bacteria (e.g., Proteobacteria and Firmicutes). Consequently, the influent C/N ratio of 5 was optimal for treating wastewater and generating electricity efficiently, and also attenuating the corresponding GWP in MFC-CWs.