Enhanced nitrogen and phosphorus removal in stormwater via multistage bioretention systems with reed and iron scrap

Bioretention systems with submerged zones are among the most effective stormwater control measures for mitigating urban runoff pollution. However, the stability of these systems is often challenged by media composition and structural configuration. In this study, three novel bioretention systems were constructed using reed, iron scraps, and plant-derived carbon as amendment materials, and their pollutant removal performance was evaluated under simulated complex rainfall conditions. The system with a submerged reed–iron scrap structure (G2) exhibited higher efficiency and stability in nitrogen and phosphorus removal compared to the iron scrap–reed structure (G1) and the uniformly mixed structure (G0). Specifically, the G2 system achieved removal efficiencies of 64.47%–99.94% for nitrate (NO₃⁻-N), 71.09%–92.67% for ammonium (NH₄⁺-N), and 85.32%–99.34% for phosphate (PO₄³⁻-P). Meanwhile, the effluent nitrite (NO₂⁻-N), as an intermediate by-product, was significantly lower in G2 (0.08 mg/L) than in G1 (0.13 mg/L) and G0 (0.27 mg/L). Analysis of pore water chemistry, substrate characteristics, and microbial communities collectively indicated that reed, iron scrap, and biochar synergistically facilitated denitrification, while the autotrophic layer composed of iron scraps and biochar in the submerged zone served as the primary site for NO₃⁻-N removal during stormwater infiltration. The findings provide new insights into mitigating urban non-point source pollution. • Reed–iron scrap bioretention system achieved high pollutant removals from stormwater • The NO₃⁻-N, NH₄⁺-N and PO₄³⁻-P contaminations were removed by 92.67-99.94% • Reed–iron scrap system showed greater stability under varying rainfall conditions • Reed, iron scrap and biochar in the stratified system facilitated denitrification • Stratified configuration in reed–iron scrap promoted higher microbial diversity