Enhancing nitrogen removal in constructed wetlands: The role of influent substrate concentrations in integrated...

Recent advancements in constructed wetlands (CWs) have highlighted the imperative of enhancing nitrogen (N) removal efficiency. However, the variability in influent substrate concentrations presents a challenge in optimizing N removal strategies due to its impact on removal efficiency and mechanisms. Here we show the interplay between influent substrate concentration and N removal processes within integrated vertical-flow constructed wetlands (IVFCWs), using wastewaters enriched with NO₃^--N and NH₄⁺-N at varying carbon to nitrogen (C/N) ratios (1, 3, and 6). In the NO₃^--N enriched systems, a positive correlation was observed between the C/N ratio and total nitrogen (TN) removal efficiency, which markedly increased from 13.46 ± 2.23% to 87.00 ± 2.37% as the C/N ratio escalated from 1 to 6. Conversely, in NH₄⁺-N enriched systems, TN removal efficiencies in the A-6 setup (33.69 ± 4.83%) were marginally 1.25 to 1.29 times higher than those in A-3 and A-1 systems, attributed to constraints in dissolved oxygen (DO) levels and alkalinity. Microbial community analysis and metabolic pathway assessment revealed that anaerobic denitrification, microbial N assimilation, and dissimilatory nitrate reduction to ammonium (DNRA) predominated in NO₃^--N systems with higher C/N ratios (C/N ≥ 3). In contrast, aerobic denitrification and microbial N assimilation were the primary pathways in NH₄⁺-N systems and low C/N NO₃^--N systems. A mass balance approach indicated denitrification and microbial N assimilation contributed 4.12-47.12% and 8.51-38.96% in NO₃^--N systems, respectively, and 0.55-17.35% and 7.83-33.55% in NH₄⁺-N systems to TN removal. To enhance N removal, strategies for NO₃^--N dominated systems should address carbon source limitations and electron competition between denitrification and DNRA processes, while NH₄⁺-N dominated systems require optimization of carbon utilization pathways, and ensuring adequate DO and alkalinity supply.