Enhancing reoxygenation and pollutant removal via riffle-pool sequence mimicry in constructed wetlands

Low dissolved oxygen (DO) levels often limit the purification efficiency of constructed wetlands. This study takes inspiration from natural riffle-pool sequences, where turbulent flow in shallow riffles enhances aeration while calm pools promote sedimentation. A bionic shallow-deep water structure was designed and tested under different topographic and culvert conditions. Controlled experiments were conducted on a 1 m× 6 m physical model (April-June 2024) under steady hydraulic conditions. Without culverts, the riffle-pool terrain formed a stable flow pattern: slow movement in pools (0.02–0.04 m/s) aided sedimentation, and faster flow in riffles (0.12–0.15 m/s) improved oxygen renewal. This setup established a DO gradient from well-oxygenated riffles (6.8–7.5 mg/L) to anoxic pool bottoms (3.0–3.5 mg/L), leading to high removal rates of NH₄⁺-N (77.5–79.0%) and TN (64.5–66.0%). In contrast, the presence of culverts disrupted this synergy and reduced removal rates by 15–20%. The optimal configuration combines riffle-pool topography with the absence of culverts, offering an effective solution to oxygen limitation and a scientific basis for designing efficient constructed wetlands. However, further validation under dynamic loading conditions and full-scale system testing is required to confirm field applicability.