Evaluation of removal efficiency for acute toxicity and genotoxicity on zebrafish in anoxic–oxic process from selected municipal wastewater treatment plants

The anoxic–oxic (A/O) process has been extensively applied for simultaneous removal of organic contaminants and nitrogen in wastewater treatment. However, very little is known about its ability to remove toxic materials. Municipal wastewater contains various kinds of pollutants, some of which have recalcitrant genotoxicity and may cause potential threat to environment, and even can lead to extinction of many species. In this study, we have selected three municipal wastewater treatment plants (WWTPs) employing anoxic–oxic (A/O) process to evaluate their ability to remove acute toxicity and genotoxicity of wastewater. Mortality rate of zebrafish (Danio rerio) was used to evaluate acute toxicity, while micronucleus (MN) and comet assays were used to detect genotoxicity. Results showed that in this process the acute toxicity was completely removed as the treatment proceeded along with decrease in chemical oxygen demand (COD) (<50 mg L^?1) in the effluent. However, in these treatment processes the genotoxicity was not significantly reduced, but an increase in genotoxicity was observed. Both MN and comet assays showed similar results. The eliminated effluent may pose genotoxic threaten although its COD level has met the Chinese Sewage Discharge Standard. This study suggests that further treatment of the wastewater is required after the A/O process to remove the genotoxicity and minimize the ecotoxicological risk.