AWWA WQTC60657

Research was conducted at Metropolitan (at a demonstration-scale plant) on the effects of chlorine dioxide (or its byproduct chlorite) on bromate and chlorate formation and disinfection (CT) credit during subsequent ozonation. Pre-oxidation with 0.5 or 1.0 mg/L chlorine dioxide just prior to ozone addition resulted in reduced bromate formation and increased Giardia inactivation credit from ozone. The addition of chlorite only (0.7 mg/L) reduced bromate formation and disinfection credit. The results suggest that the presence of chlorite reacts rapidly with ozone, reducing the amount of ozone available for either bromate formation or disinfection credit. Alternatively, the addition of chlorine dioxide in these tests appeared to satisfy more of the oxidant demand than that exerted by the chlorite produced, resulting in a net benefit in bromate control and increased disinfection. In these tests, chlorite was oxidized to chlorate, which needs to be considered if chlorate is regulated in the future (currently there is an action level of 0.8 mg/L for chlorate in California). In additional tests, low-pH ozonation (6.5) reduced bromate formation below its maximum contaminant level of 10 µg/L. Combinations of chlorine dioxide pre-oxidation and low-pH ozonation further reduced bromate formation. This paper discusses the relative merits of each approach for controlling bromate formation and meeting disinfection goals. Includes 13 references, figures.