Effects of ozonation on disinfection byproduct formation and speciation during subsequent chlorination

Ozone has been widely used for drinking water treatment recently. This study was conducted to investigate the effect of dosing ozone on the formation potentials and speciation of disinfection by-products (DBPs, brominated DBPs in particular) during subsequent chlorination. Trihalomethanes (THMs), trihaloacetic acids (THAAs), dihaloacetic acids (DHAAs), dihaloacetonitriles (DHANs), chloral hydrate (CH)and trichloronitromethane (TCNM) were included. The results showed that the yields of THMs, THAAs and DHAAs reached the maxima at 1.83, 0.65 and 0.56μM, respectively, corresponding to an ozone dose approximately at 2mgL^-1. The formation potentials of CH and TCNM increased, while that of DHAN decreased, with the increase of ozone dose up to 6mgL^-1. The bromide incorporation factor values of THMs, THAAs, DHAAs and DHANs increased from 0.62, 0.37, 0.45 and 0.39 at O₃=0mgL^-1 to 0.89, 0.65, 0.62 and 0.89 at O₃=6mgL^-1, respectively. It indicated that the use of ozone as a primary disinfectant may cause a shift to more brominated DBPs during subsequent chlorination, and the shift may be more evident with increased ozone dose. The total percentage of brominated DBPs (as bromide) reached the maximum value of 55% at 2mgL^-1 ozone dose.