Impacts of inlet–outlet configuration, flow rate and filter size on hydraulic behavior of quasi-2-dimensional horizontal constructed wetland: NaCl and dye tracer test

The aim of this study was to assess the influences of filter size, inflow rate and inlet–outlet configuration on short-circuiting, effective volume, distribution of active/dead zones, dispersion and dye propagation in simulated constructed wetlands. A subsurface quasi two dimensional flow constructed wetland was set up for chloride tracer test and dye visualization. Three levels of filter size, inflow rate and inlet–outlet configurations were involved. Totally 81 combinations were determined. Breakthrough curves for residence time distribution and images for dye tracer propagation were obtained. Results showed that the inflow rate and inlet–outlet configurations had significant effects on hydraulic behavior. Bigger filter size and low influent decreased short-circuiting and reduced dead zones, thus increasing active area. Bigger filter size enhanced plug flow by decreasing porous water velocity, reducing preferential flow. High flow rate yielded low plug flow with lower CSTRs number caused by higher linear velocities and enlarged dispersion. Inlet–outlet configuration had most influences on flow distribution. Inflow rate had larger impacts on dead zones distribution for low inlet–high outlet configuration. For practical constructed wetland design, choice of inlet–outlet configuration and inflow rate would improve the treatment efficiency. Larger porous size, lower inflow rate with bottom inlet–top outlet configuration could help improve hydraulic behavior, thus enhancing wastewater treatment.