Municipal Wastewater Treatment with Phragmites australis L. and Typha latifolia L. for Irrigation Reuse. Boron and Heavy Metals

摘要

In this work, we compared the performance of Phragmites australis and Typha latifolia for depurating primary-treated urban wastewater and evaluated their suitability for irrigation reuse. Macrophytes were planted in two pilot-scale constructed wetland systems (CWs) and monitored during a 2-year experiment (2002-2003). CW efficiency was evaluated in terms of both mass removal and water quality considering boron (B) and the following heavy metals: aluminium (Al), arsenic (As), beryllium (Be) manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), cadmium (Cd), lead (Pb), selenium (Se) and vanadium (V). The accumulation of the elements, both in plant tissues and sandy substrate layer, and their offtake with the macrophyte harvest were also measured. In quantitative terms, the established CW systems showed high removal efficiency for Al (96 %), Cu (91 %), Pb (88 %) and Zn (85 %), while lower efficiencies were observed for Fe (44 %), Co (31 %) and B (40 %). The sediment played a strategic role in the adsorption and accumulation of wastewater pollutants, while plants acted as phytostabilizers since element root concentrations were generally from one to two orders of magnitude higher than those observed in the other parts. The results were less favourable in terms of water quality because the high evapotranspiration counteracted the depuration process by concentrating the elements in the outflow water. Outflow water contained more B (68 %), Mn (196 %) and, in the case of CW managed with Phragmites, also Fe (73 %) than inflow water, breaking the Italian guidelines for irrigation reuse. Integrating solutions to reduce the high evapotranspiration of CWs with more efficient pre-cleaning systems are necessary to obtain better removal efficiencies that reduce the effect of ET on water quality.