Evaluating and elucidating the formation of nitrogen-contained disinfection by-products during pre-ozonation and chlorination

The effects of pre-ozonation on the formation of haloacetonitriles (HANs), trichloronitromethane (TCNM), and haloketones (HKs) during chlorination were evaluated. Ozone dose used in this study was 8.0, 10.0 and 25.0mg O3/min. Results showed high UV254 reduction (>80%) and relatively low dissolved organic carbon removal (40-70%) after ozonation, indicating that ozone might change significantly the chemical properties of natural organic matter presented in the raw water. Undesired ozonation by-products such as aldehydes and ketones were also formed during ozonation. At high ozone dose of 25.0mg O3/min, the formation of dichloroacetonitrile and bromochloroacetonitrile were reduced significantly. Chlorination of the ozonated water formed high concentration of TCNM and HKs were 8-10 and 31-48μg/L, respectively. It was also found that continuous hydrolysis at longer reaction time rapidly decreased the formation of HKs. Ozonation prior to chlorination practice exhibited a negative effect on TCNM and HKs reduction. A model based on the dissolved organic carbon and chlorine decay was developed not only for determining the reaction rate constants, e.g. formation and hydrolysis of HANs, HKs and TCNM, but also for interpreting the mechanisms of formation and hydrolysis for HANs, HKs and TCNM during the chlorination of natural organic matter.