Heavy metal removal in duckweed and algae ponds as a polishing step for textile wastewater treatment

Untreated textile wastewater is a typical source of heavy metal pollution in aquatic ecosystems. In this study, the use of algae and duckweed ponds as post-treatment for textile wastewater has been evaluated under the hypothesis that differing conditions such as pH, redox potential and dissolved oxygen in these ponds would lead to different heavy metal removal efficiencies. Two lab-scale systems each consisting of three ponds in series and seeded with algae (natural colonisation) and duckweed (Lemna minor), respectively, have been operated at a hydraulic retention time of 7 days and under two different metal loading rates and light regimes (16/8 h light/darkness and 24 h light). Cr removal rates were 94% for the duckweed ponds and 98% for the algal ponds, indifferently of the metal loading rate and light regime. No effect of pond type could be demonstrated for Zn removal. Under the 16/8 light regime, Zn removal proceeded well (∼70%) at a low metal loading rate, but dropped to below 40% at the higher metal loading rate. The removal efficiency raised back to 80% at the higher metal loading rate but under 24 h light regime. Pb, Cd and Cu all showed relatively similar patterns with removal efficiencies of 36% and 33% for Pb, 33% and 21% for Cd and 27% and 29% for Cu in the duckweed and the algal ponds, respectively. This indicates that both treatment systems are not very suitable as a polishing step for removing these heavy metals. Despite the significant differences in terms of physico-chemical conditions, differences in metal removal efficiency between algal and duckweed ponds were rather small.