Effect of ClO2 Pretreatment on Subsequent Water Treatment Processes

The effect of chlorine dioxide ( ClO2 ) pretreatment on subsequent treatment processes (coagulation, flocculation, sedimentation, filtration, and ozonation) was studied at pilot-scale at the Upper San Leandro Water Treatment Plant near Oakland, Calif. Potential impacts of ClO2 on the distribution system were also studied at bench scale using simulated distribution system (SDS) tests. Pilot trials were conducted with one train operating without ClO2 pretreatment (Train 1) and the other with a ClO2 dose of between 0.6 and 1.0 mg∕L (Train 2). Comparison between Trains 1 and 2 showed that ClO2 pretreatment resulted in a 0.1–0.2 NTU decrease in settled water turbidity when compared to no pretreatment. ClO2 pretreatment also resulted in a small (0.01 cm−1 ) decrease in ultraviolet absorbance at 254 nm . Following sedimentation, about 60% of the applied ClO2 formed chlorite ( ClO2− ) , with 10–20% forming chlorate ( ClO3− ) . Ozonation immediately converted all residual ClO2 and ClO2− to ClO3− . There was no sig...