Advantages of a ClO/NaClO combination process for controlling the disinfection by-products (DBPs) for high algae-laden water.

Chlorine dioxide (ClO) has been widely used in the process of preoxidation and disinfection as an excellent water treatment reagent. However, the inorganic by-products produced by ClO, such as chlorite (ClO) and chlorate (ClO) are harmful to human health, and this has become a potential problem when using ClO in drinking water treatment. In this study, ClO alone and a ClO/NaClO combination process were carried out to evaluate the algae removal efficiency of the treatment and the formation of disinfection by-products (DBPs: chlorite, chlorate, trihalomethanes and haloacetic acids) for high algae-laden water with 124.16 µg L chlorophyll a (Chl.a) content. The results show that disinfection with 1.5 mg L ClO alone results in a ClO concentration exceeding 0.7 mg L. ClO preoxidation/ClO disinfection is applicable for the control of effluent quality, but the ClO concentration still has an excessive risk when using 0.8 mg L and 0.6 mg L ClO for the two process, respectively. In the ClO/NaClO combination process, the ClO concentration is below 0.6 mg L, and trihalomethane (THM) and haloacetic acid (HAA) concentrations are lower than 60% of the maximum contaminant levels (MCLs) set by the World Health Organization (WHO). Further, the formation of ClO is more effectively controlled by NaClO preoxidation/ClO disinfection than ClO preoxidation/NaClO disinfection.