Environ. Sci. Technol. 2005, 39, 4166-4171
Effect of Lot Variability on Ultraviolet Radiation Inactivation Kinetics of Cryptosporidium parvum Oocysts M A N O S I V A G A N E S A N * ,† A N D SIVA SIVAGANESAN‡ Office of the Director, Water Supply and Water Resources Division, National Risk Management Research Laboratory, U.S. Environmental Protection Agency, 26 West Martin Luther King Drive, Cincinnati, Ohio 45268, and Department of Mathematics, University of Cincinnati, Cincinnati, Ohio 45221
Numerous studies have demonstrated the efficiency of ultraviolet (UV) radiation for the inactivation of oocysts of Cryptosporidium parvum. In these studies inactivation is measured as reduction in oocysts. A primary goal is to estimate the UV radiation required to achieve a high degree of inactivation. Different lots of Cryptosporidium parvum oocysts are used in these studies, and the inactivation rate may vary depending on the lot of oocysts used. The goal of this paper is to account for the error in estimating the amount of inactivation after exposure to UV radiation, and for the effect of lot variability in determining the required UV radiation. A Bayesian approach is used to simultaneously model the logistic dose-response model and the UV inactivation kinetic model. The oocysts lot variability is incorporated using a hierarchical Bayesian model. Posterior distributions using Markov Chain Monte Carlo method is used to obtain estimates and Bayesian credible interval for the required UV radiation to achieve a given inactivation level of Cryptosporidium parvum oocysts.
1. Introduction Ultraviolet radiation (UV) is one of several methods used by water utilities to inactivate Cryptosporidium parvum oocysts in drinking water. Inactivation of oocysts is measured as the reduction in infectivity in log-units (with base 10). For instance, a reduction of 3 log-units means oocyst infectivity has been reduced to one thousandth of the initial level. Previous studies using cell culture assay demonstrate that the infectivity of C. parvum oocysts may be reduced significantly by relatively low exposure to UV (
