Emerging investigators series: disinfection by-products in mixed chlorine dioxide and chlorine water treatment

Abstract

Chlorine dioxide (ClO₂) is an alternative to chlorine in water treatment due to its more limited formation of chlorinated disinfection by-products (DBPs). However, on-site generation of ClO₂ often involves the production of free chlorine, unreacted reagents or side-reaction products. The disinfectant is thus a mixture of ClO₂ and chlorine. The focus of this study was on the role of the mixed ClO₂/chlorine oxidant in the changes of the precursors of DBPs including trihalomethanes (THMs), chloral hydrate (CH), haloketones (HKs), haloacetonitriles (HANs), halonitromethanes (HNMs), chlorite and chlorate. The influence of bromide ions in water was also included. The ratio of ClO₂ to chlorine was found to significantly affect the DBPs formed, depending on the water sample's properties. In the three waters tested, THMs formation was the least at a ClO₂ to chlorine mass ratio of 1 : 0.5 or 1 : 0.8. Both decreases and increases in the formation of CH and HKs after ClO₂/chlorine pre-oxidation were observed compared with using ClO₂ alone. The formation of HANs and TCNM was the least at a ClO₂ to chlorine mass ratio of 1 : 0.5 or 1 : 0.8, although in the GM water sample no trend was apparent. In the presence of bromide, more bromine incorporation was found after ClO₂/chlorine pre-oxidation than when ClO₂ alone was used or without pre-oxidation at all. With an increase in the chlorine fraction in the mixed oxidant, less chlorite was formed; chlorate formation was enhanced instead. Therefore, the ratio of ClO₂ to chlorine needs to be monitored and adjusted to obtain the best control of DBP formation.