Disinfection model based on excess inactivation sites: implications for linear disinfection curves and the Chick-Watson dilution coefficient.

Current disinfection models generally are either empirical modifications of Chick's law (linear survivor curves) or hit or site models modified from the radiation literature. In this paper, a general disinfection model is developed that assumes a large number of inactivation sites. From a probabilistic model of damaged site distribution, the normalized number of surviving organisms is described as the cumulative distribution function (cdf) of the normal distribution, with the independent variable equal to a measure of damage and the mean and variance equal and determined by dose-response submodels. Submodels were developed for chemical disinfectants without disinfectant demand, ultraviolet radiation, and chemical disinfectants with first-order disinfectant demand. This infinite site model reproduces linear, shouldering, and tailing survivor curves from literature data. In addition, it predicts Chick-Watson dilution coefficients in the range observed in the literature. The infinite site model offers the interpretation that linear, shoulder, tailing, and biphasic survivor curves and the apparent Chick-Watson dilution coefficient are ramifications of the normal cdf, rather than mechanistic laws.