LAUSR

Abstract The treatment efficiency of constructed wetlands is strongly related to hydraulic conditions. The occurrences of a dead zone and short-circuiting flow decrease flow uniformity and might decrease hydraulic performance. This study aims to investigate the relationship between spatial flow characteristics and wetland treatment performance. A hydrodynamic and contaminant transport model conducted ten sets of simulations with different water depths and five sets of simulations with varying numbers of emergent obstructions. Each experiment had a variant number, dimension, and allocation of obstructions. A metric of flow uniformity was proposed and discussed. We demonstrated how the dead zone and short-circuiting alter flow conditions and affect the hydraulic performance and pollutant treatment efficiency by examining the flow characteristics. The order-of-magnitude of time scales between the advective transport rate and diffusive transport rate was analyzed by calculating the Peclet number. The first-order model was used to calculate the pollutant removal ratio. The results revealed that the values of advection were approximately tens to hundreds of times those of diffusion, indicating that the flow velocity dominated the dispersion behavior. The significant improvement in flow uniformity was found when emergent obstructions were installed. The index value of the dead zone was 2 to 69 times that of the short-circuiting flow, which shows that the dead zone significantly affected the uniformity of the flow. In addition, we discovered that when the coverage ratio of the dead zone was less than 50%, the hydraulic efficiency increased from 0.55 to 1.00. Nevertheless, the short-circuiting flow was found to have minor effects on the hydraulic efficiency. The new methodology for distinguishing dead zones and short-circuiting flow areas is useful for evaluating hydraulic performance directly instead of executing water quality modeling or tracer experiments.