Stormwater Quality Treatment and Pollutant Removal Pathways by a Floating Treatment Wetland

Abstract

A field trial study with side-by-side monitoring of two parallel stormwater retention ponds, one of which contained a Floating Treatment Wetland (FTW), has been carried out to assess the benefits of retrofitting a conventional retention pond with a FTW. Inflow and outflow event mean concentrations (EMCs) were quantified and used to assess the overall pollutant removal efficiency of each system. Pollutant accumulation in plants and sediments and water column physico-chemical parameters were monitored in both ponds in order to assess the fate of pollutants and associated removal mechanisms. The present study reveals that a pond retrofitted with a FTW would be more efficient than a conventional retention pond, exhibiting a 41% total suspended solids, 40% particulate zinc (PZn), 39% particulate copper (PCu), 16% dissolved copper (DCu) and 27% total phosphorus (TP) lower effluent EMC. Dissolved zinc (DZn) and soluble reactive phosphorus inlet EMCs were too low to differentiate the performance of either pond. The FTW moderately improved total nitrogen (TN) removal which was statistically better only from late spring to end autumn. Physical entrapment of particulate pollutants into the roots’ biofilm and subsequent settlement at the bottom of the pond appeared to be a significant removal pathway. Organic matter (OM) released by the roots and more neutral water column pH induced by the FTW likely enhanced DCu, DZn and dissolved phosphorus sorption onto OM and particles, floc formation and subsequent settlement. This resulted in increased sequestration of Cu, Zn and P in the sediment. Lower sediment Eh and water column DO generally guaranteed stable storage of Cu and Zn mainly bound to organics and sulphides. However occasional moderate Cu and Zn release is possible when favourable conditions exist for OM degradation or hydroxide reduction (over spring in the present study). Surprisingly, no P release was noticed from the reduced sediment of the FTW pond likely due to re-adsorption onto organics and/or clay minerals (i.e. Al-OH). The higher TN treatment performance measured from late spring to end autumn resulted from higher organic nitrogen (ON) removal, likely through increased particulate ON settlement through floc formation and enhanced mineralisation, and higher denitrification observed over this period. Even if pollutant accumulated in the vegetation of the FTW in spring, it is not thought to be a significant removal pathway, neither is the sorption of metals on the root surface plaques. Overall, inclusion of FTWs in conventional stormwater retention ponds has the potential to significantly increase pollutant removal for TSS, PCu, DCu, PZn and TP.