Effect of key design parameters on the efficiency of horizontal subsurface flow constructed wetlands

This study evaluates the effect of hydraulic loading rate (HLR), aspect ratio, granular medium size and water depth on the removal of selected contaminants in horizontal subsurface flow constructed wetlands (SSF) over a period of 3 years. Experiments were carried out in a pilot-scale SSF system comprising four pairs of lined beds of almost equal surface area (54-56 m2 each), with the following aspect ratios: 1:1, 1.5:1, 2:1 and 2.5:1. The size of the granular medium of each pair varied from coarse granitic gravel (D60 = 10 mm, Cu = 1.6) to small granitic gravel (D60 = 3.5 mm, Cu = 1.7). The beds of the pair with longest aspect ratio were made shallower (0.27 m) than the rest (0.5 m). The system was sampled from May 2001 to December 2003. The results indicate that water depth is a determining factor in the performance of the SSF. Beds with a water depth of 0.27 m in general removed more chemical oxygen demand, biochemical oxygen demand, ammonia and dissolved reactive phosphorus (DRP). The HLR was also a very important factor in controlling the efficiency of the SSF. SSF with fine gravel produced effluents of better quality than those with coarse gravel; the differences were low in comparison to the effect of water depth and HLR. Aspect ratio is not important in comparison with the other factors. Water depth causes differences in the redox status of the SSF, which in turn influences the biochemical reactions responsible for the degradation of organic matter. Finally, the relative importance of the biochemical reactions controls the efficiency of the system.