Lemna minor as a tool for wastewater toxicity assessment and pollutants removal agent

Abstract

Municipal effluents are of major concern for the quality of the receiving water bodies. In this study the toxicity of municipal effluents was evaluated using the bioassays with aquatic higher plant common duckweed (Lemna minor L.). The aim of the study was to evaluate the applicability of L. minor for wastewater toxicity testing and it’s efficiency in toxicants removal. Toxicity tests were performed on samples of untreated and biologically treated wastewater of small city Šilalė. Chemical analysis recorded high concentrations of nutrients and heavy metals in untreated wastewater. Wastewater treatment significantly reduced (up to 60 %) the content of nutrients (N, P), however only slight reduction (up to 10%) in the concentrations of heavy metals was observed. Both, untreated and biologically treated wastewater inhibited the relative growth rate of L. minor, affected the biomass of L. minor fronds, the content of photosynthetic pigments (chlorophyll a, b) and induced oxidative stress. Undiluted wastewater was extremely phytotoxic and led to the death of test organisms. However due to high concentrations of nutrients (N, P) treated wastewater may even stimulate the growth of L. minor and may mask the toxicity of other toxicants (such as heavy metals). It was observed that Lemna minor can be used as wastewater phytoremediation agent, as they have the capacity to remove relative high amounts of PO43- and NH4+ from the wastewater, they may significantly reduce the concentrations of NO3- in wastewater and slightly reduce the content of SO42-. L. minor removed the heavy metals from the wastewater and it was detected that the concentrations of Zn, Mn, Cu and Cd in wastewater were reduced after the growth of L. minor, however, the removal of Pb was negligible.