Performance of a constructed wetland as an upstream intervention for stormwater runoff quality management

In most developing countries, stormwater runoff has had significant impacts on aquatic environment by directly causing pollution of receiving water and reduction in treatment performance of wastewater treatment plants. With increasing encroachment on natural wetlands in Uganda, constructed wetlands offer a feasible option for the environment to cope up and buffer the impact of pollutants from the ever-increasing urban masses. This study investigated the performance efficiencies of three configurations (varied by the substrate used) of microcosm wetlands to remove physicochemical parameters from stormwater runoff in Uganda. The parameters monitored included chemical oxygen demand (COD), total suspended solids (TSS), total nitrogen (TN), and total phosphorus (TP). Hydraulic retention times (HRTs) of 2, 4, 6, and 8 days were studied. The mean concentrations of the physicochemical parameters in the runoff were 219.4 ± 12.8 mg/L COD, 77.4 ± 8.3 mg/L TSS, 9.0 ± 0.4 mg/L TN, and 1.6 ± 0.1 mg/L TP. Configuration A, vegetated with cattail (Typha latifolia) and bulrush (Scirpus lacustris), achieved maximum COD removal of 75.9% (HRT = 6 days), TN removal of 72.8% (HRT = 8 days), and TP removal of 62.8% (HRT = 8 days). Configuration C, the control, with no substrate, achieved the highest TSS removal of 75.6%. The results suggest that vegetated microcosm constructed wetlands can potentially be used to pre-treat stormwater within the catchment. However, an upstream sedimentation process unit is required to enhance their performance and to avoid premature clogging of the wetlands by TSS. The pre-treated stormwater reduces pollutant load into wastewater treatment plants and consequently better raw water quality for water treatment plants.