Disinfection byproducts and their toxicity in wastewater effluents treated by the mixing oxidant of ClO2/Cl2

Mixing oxidant of chlorine dioxide (ClO2) and chlorine (Cl-2) often applied in water disinfection. Two secondary wastewater effluents at different ammonium-N levels (0.1 and 1.6 mg N L-1) were treated with the mixing oxidant (ClO2/Cl-2) to evaluate the formation of disinfection byproducts (DBPs) and the associated cytotoxicity of treated wastewaters. The total chlorine concentrations of ClO2 and Cl-2 were maintained at 10 mg L-1 as Cl-2 with varied mixing ratios of ClO2 to Cl-2. The formation of 37 halogenated DBPs, including nitrogenous, brominated and iodinated analogues, and 2 inorganic DBPs (chlorite and chlorate) was examined. The sum concentrations of the halogenated DBPs were reduced remarkably with decreasing Cl-2 percentages, but each individual DBP group had distinct features. The regulated trihalomethanes reduced the most when ClO2 was present in chlorination, but decreasing Cl-2 percentage from 70% to 30% was not quite effective to reduce the formation of iodinated trihalomethanes, haloacetic acids and haloacetontriles in low ammonium-N wastewater. The bromine and iodine substitution factors tend to increase with decreasing Cl-2 percentages, indicating that destruction of DBP precursors by ClO2 favored bromine and iodine incorporation. Additionally, decreasing Cl-2 percentages in the mixing oxidant (ClO2/Cl-2) was often accompanied with lower chlorate formation but higher chlorite formation. The toxicity of treated wastewaters was evaluated through two approaches: the calculated cytotoxicity based on the concentrations of detected DBPs and the experimental cytotoxicity using the Chinese hamster ovary (CHO) cells. The calculated cytotoxicity decreased with decreasing Cl-2 percentages, with haloacetonitriles and haloacetaldehydes as predominate contributors. However, the experimental cytotoxicity tests showed that treatment of high ammonium-N wastewater with ClO2/Cl-2 exhibited considerable higher (> 3 times) cytotoxicity potency than using single disinfectant. (C) 2019 Elsevier Ltd. All rights reserved.