Raw water treatment with Cl2 and ClO2: comparison of two major class of disinfection by-products formation

Epidemiological studies have demonstrated the cellular toxicity of trihalomethanes (THMs) and haloacetic acids (HAAs) that usually form during water treatment with chemical chlorine disinfectants such as chlorine and chlorine dioxide. THMs and HAAs formation potential of organic precursor in a surface water source for water treatment with Cl-2 and ClO2 was studied with reference to key determinants. It was observed that the concentration of THMs and HAAs formed during ClO2 treatment was significantly lower than that formed during Cl-2 treatment concentration that formed during chlorination. HAAs, dominated with bromo-HAAs were predominantly formed with a negligible amount of THMs during ClO2 treatment. The combined total of HAAs and THMs formed during chlorine dioxide treatment was only 10-21% of that formed for chlorination. Among the HAAs species, dibromoacetic acid accounted for 86% of total HAAs during chlorine dioxide treatment whereas dichloroacetic acid (36.7%) followed by dibromoacetic acid (20.4%) and bromochloroacetic acid (16.9%) were major constituent formed during chlorination. Species shift towards more toxic bromo-HAAs may pose a concern on shifting water treatment from chlorine to chlorine dioxide, particularly for bromide-containing water.