Effects of clogging on hydraulic behavior in a vertical-flow constructed wetland system: A modelling approach

Abstract

The hydrological behavior of subsurface flow constructed wetlands changes with the operation time due to the gradual clogging of the matrix pores. However, studies on changes in hydraulics associated with the clogging process in constructed wetlands are still inadequate, with the variations of internal flow field that cause changes of hydraulic behavior still unknown. Conservative tracer tests and Computational Fluid Dynamics (CFD) software Fluent 6.3 were combined in this study to analyze the effect of clogging on hydraulic behavior in a vertical-flow constructed wetland experimental system. As the porosity of the experimental system decreased from 0.3669 to 0.316 caused by clogging, both the nominal hydraulic retention time and the actual average retention time decreased sharply, with the latter dropping even faster, indicating that the reduction of the pore volume caused by clogging is not the only reason for the decrease in the actual hydraulic retention time. During the experimental period, the system had a serious short-circuiting and mixing phenomenon, and the dead zone area was rather large. The overall hydraulic efficiency was poor. According to the CFD simulation results, which were verified by Nash-Sutcliffe efficiency factor (NSE), the flow rate in the experimental system increased with the operation time. Correspondingly, the time required to pass through the matrix pores decreased, which contributed to an additional drop in hydraulic retention time, along with the decrease of porosity.