Heavy metals removal and accumulation in H-SSF systems receiving domestic and industrial wastewaters

Heavy metals, like Ag, As, Ba, Cd, Cr, Hg, Pb and Se are often present in the wastewaters of many commercial, industrial and agricultural activities, as well as in domestic ones. In Italy, legal limits are set for heavy metals in wastewaters discharged into surface water bodies (D. Lgs 152/06), with particularly stringent limits applying when the reclaimed wastewaters are for reuse (D. M. 185/03). Chemical treatments (precipitation, co-precipitation and chemical oxidation) are quite often adopted in order to greatly reduce heavy metals concentrations in the final effluent and meet the corresponding legal limits. In the last decades, many studies have been carried out on the metals removal capacity of natural systems like constructed wetlands, in particular horizontal subsurface flow (H-SSF) systems ( Gambrell 1994; Kadlec and Knight, 1996; Vymazal and Krasa 2003; Ranieri 2004; Garcia et al., 2006; Paredes et al., 2006; Vymazal et al., 2006, Lesage et al., 2007; Vymazal et al., 2009). This paper investigates the removal capacity of H-SSF systems for the main heavy metals in urban wastewaters. It reports and discusses experimental data collected during a campaign carried out on a pilot plant built in Ferrara, Northern Italy, which is fed with the secondary effluent of the town’s wastewater treatment plant (WWTP). This plant treats domestic as well as industrial (mainly from the petrochemical pole) wastewaters. Al, Pb, Cr, Cu, Fe, Mn, Ni, V, Zn and Ti removal rates were analysed in two H-SSF beds, which differ by shape, hydraulic retention time, presence of plants and water depth. Moreover, metal accumulation in rhizomes, leaves and bed biomass was investigated