Comparison of nitrogen removal performance and environmental–economic impacts in deep bed subsurface flow constructed wetland modified with different metal-based substrates

Deep bed subsurface flow constructed wetland (SSF CW) has attracted wide concern in advanced wastewater treatment, due to its lower land requirement. However, its nitrogen removal performance would be greatly limited by the increase of the anaerobic zone. In this study, deep bed SSF CWs modified with different metal-based substrates were constructed to compare the nitrogen removal performance and explore its potential mechanism. The results showed that the addition of metal-based substrates increased the removal efficiency of TN and NO3−-N by 19.8–34.8 and 5.4–61.6%, respectively. The CW filled with iron–carbon micro-electrolysis filler substrates (Fe/C-CW) showed the highest nitrogen removal efficiency. N2O emissions were decreased by 14.6–45.8% and 15N stable isotope labeling indicated that this was attributed to the denitrification activity in the anoxic zone. Fe/C-CW enhanced the anoxic reducing environment and increased denitrifying genes (nosZ), thereby promoting N2O conversion and balancing nitrogen removal with carbon emission reduction. Life cycle assessment (LCA) further demonstrates that Fe/C-CW not only substantially reduced environmental impacts but also decreases occupied area by 23.2–55.4% and total costs by 24.0–56.1%. Overall, this study shed new light on the development and design optimization of novel SSF CWs.