Ozone disinfection of waterborne pathogens and their surrogates: A critical review

Viruses, Giardia cysts, and Cryptosporidium parvum oocysts are all major causes of waterborne diseases that can be uniquely challenging in terms of inactivation/removal during water and wastewater treatment and water reuse. Ozone is a strong disinfectant that has been both studied and utilized in water treatment for more than a century. Despite the wealth of data examining ozone disinfection, direct comparison of results from different studies is challenging due to the complexity of aqueous ozone chemistry and the variety of the applied approaches. In this systematic review, an analysis of the available ozone disinfection data for viruses, Giardia cysts, and C. parvum oocysts, along with their corresponding surrogates, was performed. It was based on studies implementing procedures which produce reliable and comparable datasets. Datasets were compiled and compared with the current USEPA Ct models for ozone. Additionally, the use of non-pathogenic surrogate organisms for prediction of pathogen inactivation during ozone disinfection was evaluated. Based on second-order inactivation rate constants, it was determined that the inactivation efficiency of ozone decreases in the following order: Viruses >> Giardia cysts > C. parvum oocysts. The USEPA Ct models were found to be accurate to conservative in predicting inactivation of C. parvum oocysts and viruses, respectively, however they overestimate inactivation of Giardia cysts at ozone Ct values greater than ∼1 mg min L^-1. Common surrogates of these pathogens, such as MS2 bacteriophage and Bacillus subtilis spores, were found to exhibit different inactivation kinetics to mammalian viruses and C. parvum oocysts, respectively. The compilation of data highlights the need for further studies on disinfection kinetics and inactivation mechanisms by ozone to better fit inactivation models as well as for proper selection of surrogate organisms.