Optimizing hydraulic performance in free water surface wetlands: Impacts of emergent plant spatial distribution patterns

Free Water Surface Constructed Wetlands (FWS CWs) are widely used in wastewater treatment, but their hydraulic efficiency is often limited by irrational plant configurations. This study used MIKE21 numerical simulations to systematically explore the effects of plant density (0.2-0.4 m spacing), and distribution pattern (uniform/cross/non-uniform) on FWS CWs’ hydraulic performance, focusing on resolving flow-related issues limiting pollutant removal. Results showed emergent plants with 0.2 m cross-distribution achieved the highest hydraulic efficiency ( λ =1.03), significantly outperforming the uniform distribution patterns via enhanced longitudinal mixing and reduced surface short-circuiting. The non-uniform "dense inlet–sparse outlet" pattern cut flow dispersion by 28% and approached ideal plug-flow conditions ( N =108, continuous stirred tank reactors), critical for stabilizing treatment under variable pollutant loads. High-density uniform layouts prolonged hydraulic retention time but risked edge high-velocity zones; low-density ones caused severe short-circuiting (Short-circuiting index, S =0.81). Quantitative analysis revealed that a zoned configuration, which incorporated dense emergent plants at inlets, emergent plants in transition zones, and sparse planting at outlets, worked synergistically to optimize both hydraulic efficiency and operational stability. This study provides a science-based framework for FWS CW vegetation design, overcoming traditional empirical limitations and offering actionable guidance for its engineering application in sustainable wastewater treatment. • Cross-distributed emergent plants (0.2 m spacing) maximize hydraulic efficiency • Non-uniform pattern (dense inlet-sparse outlet) reduces flow dispersion by 28% • A zoned configuration with a dense inlet, transitional mid-zone, and sparse outlet optimally balances hydraulic efficiency and operational stability