Hydraulic performance of horizontal subsurface flow constructed wetlands for different strategies of filling the filter medium into the filter basin

A quasi-two-dimensional flow cell model of a horizontal subsurface flow constructed wetland was used to investigate how partitioning the wetland into smaller vertical sections prior to filling a heterogeneous filter medium into the filter basin altered the filter material packing and thereby the flow distribution. The flow through the filter medium was visualized using Nigrosine dye. Breakthrough curves for the flow cell were obtained using chloride tracer. Three inlet-outlet configurations were examined to assess the effects of the inlet and outlet positions on the flow since the inlet-outlet locations in addition to the filter medium heterogeneity may promote the development of preferential paths and dead zones. The filter medium packing patterns were dependent on the number of sections. If the wetland model was not partitioned prior to filling (one section), the filter material formed roughly horizontal layers with alternate layers of coarse and fine material. However, increasing the number of sections reduced the layer continuity and increased the flow distribution. This yielded longer retention times and higher hydraulic efficiency factors. The effect of the inlet-outlet configuration on the hydraulic parameters was greater when the number of sections was small. These results suggest that dividing the constructed wetland into several sections prior to filling the filter medium into the basin will improve the treatment efficiency.