Multispecies acute toxicity evaluation of wastewaters from different treatment stages in a coking wastewater-treatment plant.

Coking wastewater contributes approximately 5% of the total discharge volume of industrial wastewaters every year in China. The toxicity of coking wastewater to aquatic organisms is still unknown. The authors evaluated the toxicity of wastewater from different treatment stages in a coking wastewater treatment plant, South China, using 5 test species belonging to different trophic levels: luminous bacteria, green alga, a crustacean, duckweed, and zebrafish embryos. The raw influent displayed the highest toxicity to the test species, with toxic units ranging from 16.2 to 1176. The toxicity in the wastewater was then gradually removed by sequential primary treatment, biological fluidized-bed treatment, and secondary clarifier treatment. The toxic unit of the final effluent was reduced to 2.26 for the green alga (Pseudokirchneriella subcapitata) and to 0 for the other 4 organisms. Quantitative analysis of metals and polycyclic aromatic hydrocarbons (PAHs) and qualitative scanning by gas chromatography-mass spectrometry showed the presence of a variety of pollutants in the coking wastewaters. Multivariate statistical analysis revealed that the toxicity in the coking wastewater was correlated to the chemical oxygen demand, total nitrogen, ammonia nitrogen, volatile phenols, sulfide, metals (Cr, As, Sb, Hg, Pb, and Ni), and ΣPAHs. Based on the results, it is required to set a safety emission limit value for the discharge of coking wastewater to protect aquatic organisms in the receiving water bodies.