Mechanisms and Pilot-Scale Validation of Iron-Loaded Biochar-Based Tidal Flow Constructed Wetlands for Enhanced Deep Nitrogen Removal from Wastewater Treatment Plant Effluent

• • Tidal flow operation significantly enhanced NH4+-N removal (P<0.001) by achieving reoxygenation capacity up to 450 g·(m2·d)−1, far exceeding conventional subsurface flow (≤100 g·(m2·d)−1), enabling robust nitrification and creating dynamic aerobic/anoxic microzones for simultaneous nitrification-denitrification. • • Iron-loaded biochar (BC-TF) achieved an average TN removal of 86.03%, significantly higher than other groups (P<0.001), attributed to its high specific surface area (341.293 m2·g−1) and porosity (75%) that concentrate nutrients, slow-release carbon supply, and electron shuttle capability enhancing denitrification. • • Iron-loaded biochar mediated Feammox pathway (anaerobic ammonium oxidation coupled to iron reduction), directly converting NH4+-N to N2 or NO2−, and the tidal flow's idle period regenerated Fe(II) to Fe(III), ensuring long-term iron cycling and sustained performance. • • Pilot-scale validation showed tidal flow increased effluent DO to 5.50 mg·L−1 (VFW) and 5.12 mg·L−1 (HSFW), improving TN removal by >30% (P<0.001) compared to continuous flow, with effluent meeting Class V of GB 3838—2002, confirming practical feasibility.