The best indicator of hydraulic short-circuiting and mixing of constructed wetlands

Abstract

Hydraulic residence time (HRT) distribution intuitively reflects flow pattern of constructed wetlands. HRT is able to reflect the merits of flow state clearly. Short-circuiting and mixing are the most important factors on HRT, exerting a significant effect on the flow pattern of wetlands. These two conditions must be avoided in wetlands construction and management. Therefore, it is necessary to select the best indicators measuring short-circuiting and mixing for understanding the flow pattern and the hydraulic performance of wetlands. This paper analyzes numerous indicators reflecting the degree of short-circuiting and mixing, displaying that t10 and Morril index t90/t10 are optimal indicators measuring short-circuiting and mixing, respectively, according to truncation and stability analysis based on tracer data. Data analysis showed that when water depth dropped, t10 increased and t90/t10 reduced gradually, indicating the improvement of short-circuiting and mixing. These results were fully consistent with the analysis results of hydraulic parameters, such as effective volume ratio e and the number of serial mixing tanks N. The excellent indicators of short-circuiting and mixing provide theoretical basis for reasonable design and evaluation of constructed wetlands, contributing to promote the hydraulic performance and purification effect.