Complementary uses of chlorine dioxide and ozone for drinking water treatment

Application of both chlorine dioxide (ClO2) and ozone (O-3) at a single drinking water treatment plant is rare in the United States but not in other countries. European utilities that use both oxidants commonly add ozone at one or more points prior to filtration and then apply ClO2 after filtration. In this mode, the oxidant demand is considerably reduced prior to ClO2 addition, thus reducing the ClO2 requirement for maintaining a concentration of 0.1 mg/L to 0.2 mg/L in the distribution system. The combined use of ClO2 and O-3 has merit in many situations, but the way in which the oxidants are sequenced (e.g. pre-ozonation followed by ClO2 treatment or vice versa) is critical in terms of finished water quality. The objectives of this paper are: (1) to review the most common water-treatment goals associated with ClO2 and O-3, and (2) to discuss the potential consequences and benefits of applying both oxidants at various points in the treatment train.