The Inactivation Kinetics of Monochloramine and Chlorine Dioxide on Monodispersed Hepatitis A Virus and MS2

In order to ensure virologically acceptable drinking water, the US EPA promulgated the Surface Water Treatment Rule and is preparing the groundwater disinfection rule (as well as amendments to the SWTR) to define requirements for disinfection to achieve specified degrees of virus inactivation. While free chlorine disinfection has been widely used since the early 20th century, the recent evidence that THMs and other chlorine by-products are carcinogens and cause other adverse health effects has focused attention on alternate disinfectants, including monochloramine and chlorine dioxide. Although previous studies have examined both disinfectants at high doses on inactivation of some important waterborne viruses, little information is available at realistic concentrations used in water treatment plants and at a range of pH levels. Therefore, in order to further characterize NH2C1 and C102 disinfection, inactivation kinetics were examined for two viruses: (1) HAV, a major waterborne pathogen, and (2) MS2, an indicator virus. Experiments were conducted using purified, monodispersed virus stocks in 0.01 M phosphate buffers at pH 6, 8, and 10. Disinfectant concentrations were at the realistic levels of 2.0 and 0.5 mg/l, respectively, for NH2C1 and C102. Inactivation kinetics were determined by computing the proportions of surviving viruses at carefully measured time intervals. Viruses were assayed by plaque techniques and both disinfectants were measured using the DPD colorimetric method. In order to compare inactivation data for the two viruses and the different test conditions, times to achieve a specified percent of virus inactivation as well as values for disinfectant concentration (C) x time for specified percent inactivation (T), or CT values were computed. In In previous studies inactivation data were treated as first-order in extrapolating to the times for 99.99% (4 log[10]) virus reduction. From examination of the experimental data from our experiments, it was evident that HAV and MS2 inactivation kinetics did not conform to the first-order model and were instead of the retardant die-off type. Subsequently, five alternative mathematical models were constricted and used to predict the kinetics of HAV and MS2 inactivation based in the experimental data. These models included: (a) a one-population model which assumes a decreasing disinfectant concentration over time, (b) a one hit, two-populations model assuming two subpopulations with different rate constants of inactivation, (c) a third model similar to (b) with the exception that the concentration of the disinfectant decreases over time, (d) a multistate model in which various stages of sublethal injury are assumed prior to inactivation, and finally (e) the distributive rate constant model, which is based upon a spectrum of inactivation rate constants for the viruses. The measure of fit was determined for each model using the least-squares method and the results for 2, 3, and 4 log[10] inactivation times were compared to the standard first-order regression model. The results indicate that a large discrepancy in the predicted times is found both between the various models and within the models when experiments of different sampling time points are used. Consequently, these data suggest that the assumption of first-order disinfection kinetics underestimate the time necessary for adequate reduction of viruses in drinking water.Master of Science in Public Healt