Assessing the effects of flow distribution on the internal hydraulic behavior of a constructed horizontal subsurface flow wetland using a numerical model and a tracer study

This study was aimed at investigating the effects of flow distribution on a constructed subsurface horizontal flow wetland of 26 m length, 4 m width and one percent bed dip. Inlet configurations were selected as a variable parameter. Three different inlet flow configurations including midpoint-midpoint (A), corner-midpoint (B) and uniform-midpoint (C), with the same fixed outlet configurations, were studied. The combined use of tracer and numerical modeling for the evaluation of hydraulic characteristics of constructed wetlands in the horizontal subsurface flow system was presented. The mean retention time for each configuration was found to be 4.53, 3.24 and 4.65 days, respectively. According to tracer breakthrough curve, the effective volumes for configurations A and C were 87.5%, as compared to 62.1% for the configuration B. Retention distribution curves were useful in assessing the internal dispersion and hydraulic parameters to interpret the short-circuiting flow for each configuration setup. Finally, the best configuration of inlet-outlet layout to improve the performance of effluent treatment based on both numerical simulations and physical experiments was found to be the uniform-midpoint by all performance criteria and this was followed by midpoint-midpoint as the second best one.