Wetlands have been proven to be efficient and cost-effective ecological treatment systems for municipal and domestic wastewater. It is essential to understand the hydrodynamic characteristics and the related contaminant transport process to optimize the treatment efficiency in free water surface wetlands. Thirty-six tracer experiments were conducted under different water depths and wetland configurations, such as installing static obstacles and dynamic disturbances. The particle image velocimetry and a novel color-concentration transform method were developed to reveal spatiotemporal flow velocity and residence time distribution. The flow fields are categorized into short-circuiting, circulation flow, dead zones, and the corresponding contaminant transport phenomena are flow advection, shear dispersion, and eddy diffusion. The flow circulation dominates the formation of the dead zone and decreases contaminant dissipation. The flow path could have effectively meandered, and the dead zone and short-circuiting could be reduced by increasing the length of the obstacles. The improved flow field is close to the plug flow, indicating enhanced hydraulic performance and treatment efficiency. The dynamic disturbance reflects the movement of fish in wetlands and provides momentum flux to promote the dissipation of pollutants in the circulation field and dead zone, alleviating the deterioration of water quality caused by pollutant accumulation. The findings of this study may provide a critical reference for the optimal design of wetlands.
Keywords: Constructed wetlands; Contaminant transport; Flow circulation; Hydrodynamics; Residence time distribution; Tracer experiments.
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