Agricultural runoff treatment by constructed wetlands filled with iron-carbon composites in winter: Performance augmentation by organic solids and denitrifying bacteria addition

Iron-carbon composite-filled constructed wetlands (Fe-C CWs) were employed to treat agricultural runoff in the winter season in this study, and organic substrates and phosphate-accumulating denitrifying bacteria were supplemented to improve the treatment performance. Fe-C CWs performed significantly better in pollutant removal than the control system filled with only gravel by effectively driving autotrophic denitrification, Fe-based dephosphorization and organic degradation. Organic substrate and functional bacteria addition further augmented the performance, and immobilized bacterial cells were more effective than free cells. Fe-C and organic substrates decreased the greenhouse gas emission fluxes of the CWs, and denitrifier inoculation alleviated N2O emission. The microbial community in the Fe-C substrates showed a very distinct distribution pattern compared to that in the gravel, with notably higher proportions of Trichococcus, Thauera and Dechloromonas. Bioaugmented Fe-C-based CWs are highly promising for agricultural runoff treatment, especially at low temperatures.