Analysis of contaminant dispersion in constructed wetland flows with absorptive boundaries

Abstract

Pollution in wetland flows poses a significant challenge to ecosystem preservation, underscoring the importance of wetland restoration. Constructed wetlands have emerged as a widely embraced solution for contaminant mitigation. In this study, we focus on the contaminant removal process within the wetland flows to contribute to wetland restoration. A constructed wetland with ecological floating and bottom absorptive beds has been employed to enhance the capacity of contaminant removal within wetland flows. However, the environmental dispersion in constructed wetland flows is further complicated by the presence of irreversible first-order absorptive boundaries at the upper and bottom beds. Therefore, our work focuses on comprehending the expansion processes of contaminant clouds under absorptive boundaries through a multiscale analysis to demonstrate the removal effects of constructed wetlands. The findings highlight significant deviations in the spatial distribution of contaminant clouds, particularly in the vertical dimension, attributed to absorptive effects. To analyze the impact of removal mechanisms on the dispersion patterns of contaminant clouds, variations in the vertical concentration peak are examined. A predictive model is developed to estimate the position of the concentration crest in the vertical direction based on changes in absorptive ratios between the upper and bottom boundaries. Furthermore, in order to explore the key factor on removal capacity and efficiency within constructed wetland flows, we look into the process of contaminant depletion by examining variations in removed mass and local absorptive and removal efficiency along the streamwise direction in response to deviations in absorptive strengths at the upper and bottom beds. The ratios of the two boundary removal intensities play a significant role in the absorptive dynamics within wetland flows, particularly during the early stages of cloud expansion. By adjusting the absorption ratio between two absorptive beds to 1:1 with a certain total depletion strength in wetland flows, the enhancement ratio of depletion efficiency can increase by nearly 30%. It has been proven that employing identical absorption strengths at two absorptive beds is an economical and efficient means to amplify the absorption efforts within designed constructed wetland flows. This study provides guidelines for the design of constructed wetlands aimed at enhancing their capacity for sewage treatment.