Toxicity of heavy metals to duckweed-based wastewater treatment ponds with different depth Saber A. El-Shafai, Fayza A. Nasr and Fatma A. El-Gohary Water Pollution Control Department, National Research Center, Dokki, Egypt, and N. Peter van der Steen and Huub J. Gijzen Environmental Resources Department, UNESCO-IHE Institute for Water Education, Delft, The Netherlands Abstract 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 21-258C and light regime of 16 hours light using halogen lamps and 8 hours dark. The light intensity was maintained around 200 m Em 2 2 S 21 . 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. Keywords Heavy metals, Water, Biochemistry Paper type Research paper The current issue and full text archive of this journal is available at www.emeraldinsight.com/1477-7835.htm This research was supported by the UNESCO-IHE Institute for Water Education and the World Laboratory Scholarship (T-1 Project). The authors wish to thank the World Laboratory for partial funding of this research. They also wish to thank G.F. Kruis, P. Heerings, F. Wiegman and Don at the UNESCO-IHE Institute for their help during the lab analysis of this work. Toxicity of heavy metals 313 Management of Environmental Quality: An International Journal Vol. 17 No. 3, 2006 pp. 313-322 q Emerald Group Publishing Limited 1477-7835 DOI 10.1108/14777830610658728