Increased formation of brominated disinfection by-products and toxicities during low-H2O2-mediated ozonation of reclaimed water

Reusing reclaimed water requires stringent disinfection but inevitably generates disinfection by-products (DBPs). H2O2/O-3 treatment is an efficient and environmentally benign disinfection method. For the first time, our bioassay results elucidate that low H2O2/O-3 ratio (molar) treated water increased unignorable toxicity effect compared to that of the high H2O2/O-3 ratio. To clarify this finding, individual organic brominated DBPs (Br-DBPs), bromate, and adsorbable organic bromine (AOBr) were considered due to their potential risk. Organic Br-DBPs were mainly generated from ozone-induced pathways. Individual organic Br-DBPs were not the primary concern in this scenario because they are typically only produced in observable quantities at bromide concentrations exceeding 500 mu g/L, and even then, they often remain below detection limits when treated with H2O2/O-3. On the contrary, both bromate and AOBr were detectable at low H2O2/O-3 ratios. Furthermore, bromate is produced from HOBr and bromine radicals induced by HO center dot. Moreover, bromate formation was promoted because of increased HO center dot formation, particularly at H2O2/O-3 ratios <0.24. To prevent HO center dot-induced pathways from being dominant, higher H2O2/O-3 ratios (>0.48) were required. Toxicity assays revealed that AOBr-included organic extracts of ozonated reclaimed water induced more toxic effects. The toxicity induced by the organic fraction resulted from its decreased oxidation level, which was, in turn, driven by the increased formation of bromate. Enhanced toxicity effects were observed when cells were exposed to a bromate and organic extract mixture. It indicates that both the AOBr and bromate present in low-H2O2-O-3-treated reclaimed water pose potential risks, and their coexistence further elevates these risks. Increasing the H2O2/O-3 ratio markedly decreased the generation of intracellular oxidative substances and oxidative damage. In conclusion, when treated with H2O2/O-3, shifting from HO center dot-induced pathways to ozone-induced pathways by a relatively high H2O2/O-3 ratio decreased the amounts of DBPs produced and controlled the toxic effects to ensure the safety of ozonated reclaimed water.