A kinetic model for water disinfection employing UV-C radiation (lambda = 253.7 nm) was developed that is valid for clear waters as well as for a concentrated and nutritious medium. Escherichia coli was used as a test bacteria. The kinetic model is a modification of the series event inactivation mathematical description that takes into account the radiation absorption rate corresponding to the existing, viable bacteria and the radiation attenuation produced by the quasi-transparent or the translucent environment. It also explains two additional observed phenomena: (i) the effect of bacteria growth in the nutritious medium during disinfection and (ii) a further reduction in the inactivation rate that was attributed to some form of bacteria protection produced by a not well understood association of the bacteria with of the components of the concentrated culture. Comparing theoretical predictions from the model with experimental concentration versus time data, the model parameters were obtained. Predictions show good agreement with collected experimental data within the range of the explored variables. (c) 2005 Elsevier B.V. All rights reserved.