Toxicity of heavy metals to duckweed-based wastewater treatment ponds with different depth

Purpose - The main purpose of this study is to investigate the bioaccumulation pattern and fate of heavy metals in duckweed-based wastewater treatment ponds with different depth. Design/methodology/approach - Three replicates of four reactors were used in this experiment. The reactors were randomly distributed on the bench and filled with 50 percent diluted sewage for the control reactors whereas diluted sewage mixed with 5 mg Pb/l and 5 mg Zn/l was used for the treatment reactors. The reactors were stocked with Lemna gibba at 1,000 g fresh weight per each square meter. The culture tanks were exposed to temperature range of 21-25 degrees C and light regime of 16 hours light using halogen lamps and 8 hours dark. The light intensity was maintained around 200 mu Em(-2)S(-2). The experiment extended for 30 days. Regular monitoring of the growth performance of duckweed was carried out with subsequent analysis of dry matter, heavy metals, phosphorous and nitrogen content. Composite samples from the water phase were subjected to the analysis of ammonia, nitrite, nitrate, orthophosphate and sulfate concentration. The analysis of sediment and biofilm were carried out at the end of the experiment. Interpretation of results was carried out using one-way analysis of variance. Findings - Statistical analysis showed a significant reduction in the growth rate of duckweed within the first five days exposure time. After five days exposure, the growth rate in the treatments returned to the normal growth till day 15 after which the growth became significantly lower in the small and medium scale ponds. The results revealed that zinc is more bio-available than lead and both metals are mostly precipitated in the sediment probably as sulfides. Practical implications - The results help in enhancement of heavy metals removal in a small anaerobic pretreatment unit before entering to the pond system by sulfate addition. Originality/value - The results of the study confirm the positive effect of pond depth in reducing the heavy metal toxicity to the duckweed-based wastewater treatment ponds.