0229220 Hernandez The last several decades, numerous disinfection studies have been executed to determine the inactivation response of many different types of microorganisms exposed to various disinfectants. EPA's current disinfection standards, which regulate the performance of public water treatment systems, have been built upon such studies (AWWA, 1999). Of notable concern in the water treatment field is the fact that current federal disinfection standards are generalized criteria that must be applied to a wide range of source waters, each with their own characteristics. While both the scientific and regulatory communities clearly recognize that disinfection performance can be significantly impacted by source water characteristics, only recently have disinfection studies been chartered (Dow, 2001) to determine the impact of common water quality parameters (e.g. turbidity and natural organic matter (NOM) content) on the inactivation of "emerging" bacterial pathogens, bacterial spores, and protozoan (oo)cysts by ozone. The main engineering "process" variables affecting the disinfection performance of water treatment works are disinfectant dose, mixing regime, and residence (contact) time - these parameters form the basis of the modern "CT" concept, which was first introduced by Chick and Watson (Gyurek and Finch, 1998) nearly a century ago. The CT concept was originally derived from observations of vegetative bacterial cells (i.e. not spores) and has been widely accepted as a robust engineering tool for disinfection system designs. Over the last generation, conventional CT disinfection models used for chlorine have been extended to include O3. The engineering of ozone (O3) disinfection systems for public water supplies has received increasing scientific and regulatory attention because of the potential for chlorine to form by-products, which present significant human health risks. While ozone is gaining popularity in full-scale disinfection applications, its germicidal abilities have been studied for many years. O3 appears to be more effective than chlorine-based disinfectants against the most oxidant-resistant microorganism physiologies - bacterial spores and protozoan (oo)cysts. However, ozone is extremely reactive and the kinetics of ozone-associated disinfection reactions is so rapid, that understanding ozone inactivation mechanisms with different microorganisms is extremely challenging (Elovitz et al., 2000). Ozone can induce the aggregation of organic particulate matter (Chandrakanth, 1996), but because of culturing artifacts, its potential for inducing coagulation of bacteria, has not been documented. Motivation and Research Needs. Modern disinfection engineering remains rooted in a CT response model that is predominantly based on the observations of culturing dispersed vegetative bacterial cells from synthetic waters. While conventional CT models have extended some reasonable protection for water supplies using chlorine, their predictive capacity needs to be more comprehensive in order to leverage the disinfection capabilities and advantages of ozone. Projected increases for ozone use provide motivation to study its coagulation effects and disinfection efficacy against emerging pathogens, with a focus on its inactivation mechanisms against resilient physiologies.