A numerical study of the effect of wetland shape and inlet-outlet configuration on wetland performance

摘要

The hydraulic efficiency of wetlands for wastewater treatment was investigated as a function of wetland shape and vegetation density using a 2D depth-averaged numerical model. First, the numerical model was calibrated and validated against field data and then was applied to 8 hypothetical wetlands of rectangular and elliptical shape and different aspect ratio (i.e. 1:1-4:1). The vegetation density was varied from 0 to 1000 stems/m(2). The effect of inlet-outlet configuration was analyzed by simulating the hydraulic response of wetlands with different alignment of the flow inlet and outlet and wetlands with multiple inlets. The resulting Residence Time Distributions (RTDs) were derived from numerical simulations of the flow field and the temporal evolution of the outlet concentration of a passive tracer injected at the inlet. The simulated velocity field demonstrated that wetland shape can have significant impact on the size of dead zone areas, which is also reflected in the RTD. Efficiency metrics associated with detention time and degree of mixing improved for an elliptical shape compared to a rectangular shape. An ellipse shape improved the wetland performance by reducing the area of dead zones at the corners, and thereby increasing the effective wetland volume contributing to the treatment process. Configurations in which inlet and outlet were located at opposite corners of the wetland, and wetlands with multiple inlets produced smaller dead zones, which reduced the variance of the RTD. The simulation results also revealed an interesting threshold behavior with regard to stem density. For stem density above 300 stems/m(2), which is typical of treatment wetlands, the model predictions were not sensitive to the exact value of stem density selected, which simplifies the parameterization of models. This quantitative analysis of the effect of wetland shape, inlet-outlet configuration and vegetation density can help engineers to achieve more efficient and cost-effective design solutions for wastewater treatment wetlands. (C) 2017 Elsevier B.V. All rights reserved.