Hydraulic characteristics and enhanced nitrogen removal performance of a fold-flow iron-based bioretention pond

Bioretention ponds often suffer from limited denitrification under variable stormwater conditions. To address this, a concave fold-flow board and sponge iron filler were introduced into a bioretention system. Three configurations - direct-flow sand-based (control), fold-flow sand-based, and fold-flow iron-based - were evaluated via tracer experiments, CFD simulation, long-term operation under different rainfall intensities, and 16S rRNA sequencing. The fold-flow structure significantly improved hydraulic performance. Under medium rainfall, the standardized variance of residence time distribution increased from 0.08-0.27 and hydraulic efficiency λ from 0.82-0.97, indicating a transition from near plug flow to mixed flow with better space utilization. The fold-flow board also created alternating aerobic-anaerobic zones favourable for nitrogen transformation. Under high rainfall, the fold-flow iron-based pond achieved removal rates of 81.66% for NH₄⁺-N, 94.11% for NO₃^--N, and 89.54% for TN. Compared with the fold-flow sand-based pond, removals increased by 1.72%, 18.55%, and 13.10%, respectively; relative to the direct-flow sand-based pond, increases were 4.99%, 40.77%, and 40.56%. Microbial analysis revealed higher relative abundances of Bacteroidota and Chryseolinea and a lower abundance of Patescibacteria in the iron-amended system, suggesting a microbial community more conducive to denitrification. Overall, the enhanced nitrogen removal is attributed to the synergistic coupling of hydraulic regulation, redox zonation, and microbial adaptation. Combining a simple fold-flow structure with low-cost sponge iron offers a promising strategy for designing or retrofitting stormwater bioretention systems.