Evaluation of ozone disinfection systems: Characteristic time T

The U.S. Environmental Protection Agency has recently promulgated drinking-water regulations, known as the surface water treatment rule (SWTR), to control viruses, Giardia cysts, Legionella, and heterotrophic bacteria. The SWTR specifies overall minimum removalJinactivation efficiencies by filtration and disinfection for Giardia lamblia and viruses and uses the CT concept to predict in-activation efficiencies. CT stands for the product of characteristic exposure time (T) and the characteristic concentration (C) in the disinfection chamber. This paper evaluates possible methods to determine the characteristic exposure time for ozone disinfection. The extent to which different measures of T (such as the hydraulic retention time, Tw and T50) actually ensure compliance with the required level of inactivation is evaluated by using segregated flow analysis and classical residence time distribution models. The analysis indicates that T10, the minimum exposure time of 90% of the water in the disinfection unit, is a good measure to estimate disinfection performance in most cases. T50 does not guarantee compliance with the SWTR in most cases.