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 firstorder 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.