Novel Biochar/Fe-Modified Biocarriers Assisted Tidal-Flow Constructed Wetlands for Enhanced Nitrogen Removal from Eutrophic Water under Low Temperatures

Constructed wetlands are regarded as a sustainable ecotechnology for eutrophic water remediation with various advantages. However, the limited oxygen, low C/N ratios, and low temperatures post challenges to efficient nitrogen removal. In this study, novel biochar/Fe-modified biocarriers were employed in tidal-flow constructed wetlands (TF-CWs) to enhance autotrophic nitrogen removal. After 170-cycles operation, CWs-C (with quantitative discharge and mixed substrate arrangement) demonstrated the most optimal nitrogen removal performance, with low effluent nitrogen concentration of 1.45 ± 0.27 mg/L and average nitrogen removal efficiency of 78.81 ± 3.67% under low temperatures (8–16.7 °C). The tidal-flow pattern bolstered NH4+ oxidation in the drainage zone, while anaerobic functional bacteria in the flooded zone enhanced the NO3− reduction. Activity assays revealed relatively higher anaerobic functional bacterial activity in the submerged zone (3.72 ± 0.20 mg N/gVSS/day). Batch tests revealed the synergy of nitrification, Feammox, and NDFO pathway. High-throughput sequencing unraveled the enrichment of nitrifying bacteria (Nitrospira, 1.28–2.82%) and iron-based autotrophic bacteria (Caldilineaceae, Microtrichales, Ellin6067 A4b). This study demonstrated that the novel biocarriers enhanced nitrogen removal in TF-CWs and proposed an optimized operation strategy, which holds remarkable significance for practical applications of constructed wetlands in eutrophic water treatment under low temperatures.