Massachusetts Water Resources Authority started adding sodium hypochlorite to its raw water (Wachusett Reservoir) in September 1997 to achieve compliance with the requirements of the Surface Water Treatment Rule for unfiltered surface water supplies, mainly the 3‐log C × T (concentration times time) value for Giardia inactivation. There were concerns about the chlorine dosage necessary to achieve pathogen inactivation and the effect of that dosage on the amount of disinfection by‐products (DBPs) formed. Weekly chlorine decay tests were initiated in April 1998 to gather information on chlorine decay so that the necessary parameters to produce an integrated C × T value could be developed (area under the chlorine decay curve). Wachusett Reservoir water quality is also affected by transfer of water from the Quabbin Reservoir, which has lower total organic carbon and UV254 absorbance levels. A model was developed to describe the rate constant as a function of hydroxide concentration (taking both pH and temperature effects into account through the ion product of water), UV254 absorbance, and chlorine dose only. The availability of kinetic parameters allows development of a method for calculating C × T achievement for primary disinfection. Models for DBP formation will be presented in another article. These models together allow for better determination of the necessary chlorine dosage to achieve the required C × T value and minimize DBP formation.

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