Duckweed and Algae Ponds as a Post-Treatment for Metal Removal from Textile Wastewater

Untreated textile wastewater is a typical source of heavy metal pollution in aquatic ecosystems. In this study, the use of algae and duckweed ponds (AP and DP) as post-treatment for textile wastewater has been evaluated under the hypothesis that differing conditions such as pH, redox potential and dissolved oxygen would lead to different heavy metal removal efficiencies. Two lab-scale systems, each consisting of three ponds in series and seeded with algae and duckweed respectively, have been operated at a total hydraulic retention time of 7 days and under two different metal loading rates. Cr was removed at 94–98%, indifferent to the loading rate but slightly better in AP compared to DP. Zn removal proceeded well (~70%) under low loading rates, but dropped to below 40% at the higher loading rate. No effect of pond type could be demonstrated. Pb, Cd and Cu all show relatively similar patterns with removal efficiencies varying between 17 and 36%, which indicates that neither system is very suitable, or under-designed as a polishing step for removing these heavy metals. AP seems to be better suited for removing Cr whereas the DP seem to be better in removing Cd and Pb. In absolute terms however, differences between AP and DP were fairly small. A further analysis is needed to unravel whether or not the differences between both pond types are due to differences in pH, dissolved oxygen and redox potential or rather due to sorption mechanisms.