The Effect of Chlorine Demand on Estimation of the Inactivation Rate Constant

Ct (disinfectant concentration multiplied by exposure time) values for chlorine are used by the US EPA to evaluate the efficacy of disinfection of microorganisms under various drinking water treatment conditions. These Ct values are generally derived from laboratory studies in which chlorine decay is characterized by a first order decay model. The concentration of chlorine is often only measured at the initial and final exposure times. In this study, using bacterial spore inactivation data where residual chlorine was measured at least twice in between initial and final exposure times, chlorine decay models were evaluated to determine the effect on Ct calculations. Traditionally Ct is treated as a constant in estimating the rate constant of the simple Chick-Watson inactivation kinetics model. As Ct is estimated it is subject to estimation error. To account for this error, the parameters of the chlorine decay and the inactivation models were estimated simultaneously.