AWWA ACE93163

There are three possible sources of chlorate ion in drinking water when liquid hypochlorite is used. One source is the raw water itself. A second source is from the decomposition of residual hypochlorous acid during disinfection. A third source is from the decomposition of OCL- feedstock solutions during storage prior to application. Based on a series of preliminary experiments, the authors believe that the major pathway by which ClO3- appears in drinking water originates from the decomposition of liquid hypochlorite during storage for use with FAC disinfection. Direct evidence in support of this hypothesis is the focus of this paper. As a result of detailed rate and mechanistic study, observations with respect to ClO3- formation can be generalized. The formation of ClO3- is more effectively lowered by diluting the initial OCl- concentration than by decreasing the temperature; however, both dilution and lower temperature can be effective in minimizing ClO3- formation in the highly basic OCl- feedstock. In addition to specific suggestions for decreasing the amount of OCl- formed in OCl- feedstock, the authors describe the available chlorine content in NaOCl solutions; half-life, reaction order, and reaction rate; rate constant determination for oxygen formation in NaOCl; analysis of commercially produced NaOCl; and the effect of metal ion on the formation of chlorate ion.