Optimal design for hydraulic efficiency performance of free-water-surface constructed wetlands

This study aims to provide the optimal design of different free-water-surface constructed wetlands (FWS CWs) according to hydraulic efficiency index (λ). This index is mainly determined by the position and distribution of the hydraulic retention time (HRT) curve of a wetland. HRT can be defined as the period during which each portion of fluid remains in the wetland under the influence of flow uniformity. Flow uniformity of a wetland is primarily affected by the aspect ratio of the wetland (ARw), the inlet and outlet configuration, and the obstruction designation. The important findings about these three factors in this study are as follows:1.When the ARw is greater than 5, the λ will reach 0.9 or even higher. If the project site or field area cannot meet the theoretical standard, the recommended ARw is higher than 1.88 so that the λ will be higher than 0.7.2.The best configuration of inlet and outlet is uniform–midpoint. Meanwhile, midpoint–midpoint is preferable to corner–corner.3.According to the multi-linear regression of numerical experiment results, it is suggested that the obstruction width-to-wetland width ratio (wob/ww)(wob/ww), rather than the amount of obstruction, is the most significant influential factor in the performance of hydraulic efficiency.