Investigation of trihalomethane formation after chlorine dioxide preoxidation followed by chlorination of natural organic matter

Chlorine dioxide (ClO2) is widely used as powerful oxidant to remove taste, odor and pathogenic microorganisms with chlorine during water treatment. In this study, the formation of trihalomethanes (THMs) was investigated in sequential ClO2 and chlorination processes. During the study, water samples were collected from Terkos Lake water (TLW) and Büyükçekmece Lake water (BLW) in Istanbul. To understand the relationship between the formation of THMs and the characteristics of natural organic matter (NOM), changes in the molecular weight (MW) of NOM fractions before and after ClO2 oxidation were characterized with the ultrafiltration technique. The NOM fraction with MW <1 kDa is the dominant fraction among all the fractionated water samples. ClO2 oxidized NOM to more hydrophilic and smaller organic fractions as seen in the NOM fraction with MWCO <1 kDa. ClO2 preoxidation reduced THM concentrations but produced chlorite (ClO2 –) and chlorate (ClO3 –). The increase in ClO2 doses enhanced the reduction percentages of THMs during subsequent chlorination in water samples without or with bromide application. Bromine incorporation was higher in ClO2ClO2 preoxidized TLW and BLW samples. As a result, the findings of this study demonstrated that ClO2 preoxidation prior to chlorination is an applicable strategy for control of THMs in water treatment.