Clogging influence on metals migration and removal in sub-surface flow constructed wetlands

Chromium (Cr) and Nickel (Ni) removal from secondary effluent has been evaluated in a four year research program to determine the effectiveness of Sub-Surface Flow (SSF) Constructed Wetlands (cws). Tests were performed in small scale (10 l/h) and full scale (150 m3/d) SSF cws. Metals removal was also assessed as a function of increased clogging that occurred in the cws over the course of the study. Cr and Ni content were evaluated in sediments at various locations along the flow path and in plant tissues by sampling Phragmites australis roots, stems and leaves. Clogging was evaluated by measuring hydraulic conductivity at the same sampling locations 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; the skewness of the RTDs increased over this period. Proportionality between increasing clogging and sediment accumulation of metals was observed, especially for Ni. Adsorption to the original matrix and the accumulated sediment is a removal mechanism consistent with available data.

► cws tends to create flow short-circuiting in the medium after four year running.
► Removal of Cr and Ni in water and sites of adsorption decrease over time.
► Accumulation in the substrate was the predominant removal mechanism.
► Chromium has more mobility within the plant tissues than nickel.