Chromium and Nickel Control in Full- and Small-scale Subsuperficial Flow Constructed Wetlands

A four-year research program has been carried out to the aim of evaluating the effectiveness of Cr and Ni removal from secondary effluent. Tests were performed in small-scale (10 l/h) and full-scale (150 m³/d) sub-surface flow (SSF) constructed wetlands (cw_s). Metals removal was also assessed as a function of increased clogging conditions that occur in the cw_s. Cr and Ni content were evaluated in sediments at various points and in plant tissues by sampling phragmites australis roots, stems, and leaves. Clogging was evaluated by measuring hydraulic conductivity at the same sampling points at the beginning and at the end of the experiment. Residence Time Distribution – RTD - curves were also assessed at the beginning and after 48 months by using lithium chloride as tracer. Plotted results showed an increased skewness in the RTD curves at the end of the experiment. Proportionality between increasing clogging conditions and sediment adsorption was observed, especially for nickel that was absorbed mainly in the roots.