Enhanced nitrification-denitrification coupling for high-rate nitrogen removal in tidal flow constructed wetlands.

Achieving efficient nitrogen removal in tidal flow constructed wetlands (TFCWs) under high hydraulic loading rates (HLRs) remains challenging because rapid hydraulic exchange may disrupt the balance between nitrification and denitrification by altering oxygen and carbon distributions. In this study, an integrated operational strategy involving substrate alkalinity, carbon availability, and hydraulic conditions was evaluated in TFCWs treating real domestic wastewater. The alkaline steel slag-zeolite substrate maintained stable NH4+-N removal (>93%) at elevated HLRs (up to 2.51 m d-1), although nitrate accumulation indicated insufficient denitrification. Increasing the influent chemical oxygen demand to nitrogen ratio to 8 markedly enhanced denitrification, resulting in total nitrogen removal efficiencies exceeding 80%. Vertical profiling further revealed that carbon limitation in deeper bed layers remained a major constraint under high loading. Microbial analyses indicated enrichment of ammonia-oxidizing archaea and Nitrospira OTUs phylogenetically affiliated with comammox-related lineages in the alkaline substrate, consistent with stable nitrification performance under high HLRs, while methanol addition was associated with a shift in denitrifying communities toward methylotrophic taxa. Overall, coordinated regulation of substrate alkalinity, carbon availability, and hydraulic operation enhanced nitrification-denitrification coupling and enabled high-rate nitrogen removal in TFCWs. These findings improve the understanding of the factors governing nitrogen transformation under intensive hydraulic loading conditions and provide practical insights for optimizing TFCWs.