Evaluation of the detoxification efficiencies of coking wastewater treated by combined anaerobic-anoxic-oxic (A2O) and advanced oxidation process

Coking wastewater contains many types of toxic and hazardous pollutants that have serious toxic effects on human beings as well as aquatic organisms. However, few studies have evaluated the detoxification efficiencies of the treatment processes that are extensively performed in operational coking wastewater treatment plants (WWTPs). This study investigates the detoxification efficiencies of a combined anaerobic-anoxic-oxic (A2O)-ozonation and A2O-Fenton oxidation process in two coking WWTPs using an acute immobilization test for Daphnia magna, acute toxicity test for adult zebrafish, embryo toxicity test for zebrafish and the comet assay. The raw coking wastewaters displayed high acute daphnia and fish toxicity, zebrafish embryo toxicity and genotoxicity. The A2O processing unit effectively removed acute and embryo toxicity, but not genotoxicity. In addition, the A2O effluent quality did not meet the integrated wastewater discharge standard in China (GB18918-2002). The ozonation and Fenton oxidation units used as post-treatments in these two plants not only treated the coking wastewater to the discharge standard but also reduced the genotoxicity. However, the final effluents still showed potential genotoxicity after high dilution. The results suggest that the discharge of treated coking wastewater probably poses potential risks to human health and the environment even if it met regulatory standards.