Effects of pre-oxidation by ozone, permanganate and ferrate on generation and toxicities of disinfection byproducts

When organic compounds are pre-oxidized before coagulation, the disinfection byproducts are usually generated in the following chlorination process in drinking water treatment. In this study, the effects of pre-oxidation on carbon removal and generation of disinfection byproducts were investigated by using common oxidants of ozone, permanganate and ferrate. It was demonstrated that turbidities were reduced by 35%, 37.5% and 38.3%, and dissolved organic carbon was reduced by 18.8%, 26.5% and 20.8%, respectively, with the pre-oxidation of ozone, permanganate and ferrate under optimal dosage but inorganic carbon was not affected. Turbidity, dissolved organic carbon and dissolved organic matter showed an initial decrease and then increase pattern with increase in ozone and permanganate concentrations, but a different continuously decreasing pattern was demonstrated by ferrate. The results indicated that pre-oxidation of organic compounds was incomplete and may include steps of colloid destabilization, cell lysis and partial mineralization. The removal rates of total disinfection byproducts under optimal concentrations were 31.69%, 45.36% and 39.35%, respectively. Eleven disinfection byproducts were identified and quantified, and the major species were chloroform, trichloroacetaldehyde and dichloroacetonitrile, which were either generated directly from organic compounds or converted from other species of disinfection byproducts. The highest LC50 of disinfection byproducts were dibromo- and bromochloro- acetonitriles, indicating the cytotoxicity was mostly contributed by the nitrogen-containing disinfection byproducts, which combined accounted for 79.6% to 83.4% of the total toxicity.