Performance Evaluation of a Full-Scale Constructed Wetland for Treating Stormwater Runoff

The performance of a stormwater wetland consisting of a retention pond followed by a horizontal subsurface flow (SSF)-constructed wetland is evaluated during 11 stormwater events. Samples collected from the inlet and outlet of the wetland are analyzed for total suspended solids (TSS), chemical oxygen demand (COD), total nitrogen (TN), ammonium (NH4+-N), nitrate (NO3--N), PO43-, total phosphorus (TP), and trace metals. Despite the high variability in inflow concentrations, high removal efficacies for TSS (84.3%) and COD (79.2%) are observed. The overall nitrogen removal is satisfactory with a removal efficiency (RE) of 53.5% for TN, 56.5% for NH4+-N, and 76.5% for NO3--N. Lower inflow concentrations limit the TP removal (29.5%) and can allow previously sorbed phosphorus to be released from sediments into the water column. In addition, retention pond is limited in removing dissolved and fine fractions. Lower concentrations of heavy metals are observed compared to influent, and the average RE is 67.2% for Zn, 73.2% for Cu, 41.7% for Cr, 7.1% for Cd, 44.1% for Ni, and 60.6% for Pb, respectively. Relatively high levels of Zn in the roots of Phragmites australis are observed, followed by Cu, Ni, and Cr. Mean concentrations of TSS, TN, and TP are all moderately correlated to inflow intensity during stormwater events in contrast to dissolved nutrients. Flow intensity and effluent concentrations of TSS, TN, TP, and PO43+ are also significantly positively correlated.