Ozone is a commonly applied disinfectant and oxidant in drinking water and has more recently been implemented for enhanced municipal wastewater treatment for potable reuse and ecosystem protection. One drawback is the potential formation of bromate, a possible human carcinogen with a strict drinking water standard of 10 μg/L. The formation of bromate from bromide during ozonation is complex and involves reactions with both ozone and secondary oxidants formed from ozone decomposition, i.e., hydroxyl radical. The underlying mechanism has been elucidated over the past several decades, and the extent of many parallel reactions occurring with either ozone or hydroxyl radicals depends strongly on the concentration, type of dissolved organic matter (DOM), and carbonate. On the basis of mechanistic considerations, several approaches minimizing bromate formation during ozonation can be applied. Removal of bromate after ozonation is less feasible. We recommend that bromate control strategies be prioritized in the following order: (1) control bromide discharge at the source and ensure optimal ozone mass-transfer design to minimize bromate formation, (2) minimize bromate formation during ozonation by chemical control strategies, such as ammonium with or without chlorine addition or hydrogen peroxide addition, which interfere with specific bromate formation steps and/or mask bromide, (3) implement a pretreatment strategy to reduce bromide and/or DOM prior to ozonation, and (4) assess the suitability of ozonation altogether or utilize a downstream treatment process that may already be in place, such as reverse osmosis, for post-ozone bromate abatement. A one-size-fits-all approach to bromate control does not exist, and treatment objectives, such as disinfection and micropollutant abatement, must also be considered.

The formation of total organic halogen (TOX), carbonaceous disinfection byproducts (DBPs) (trihalomethanes (THMs) and haloacetic acids (HAAs)), and nitrogenous DBPs (trichloronitromethane (TCNM) or chloropicrin, haloacetonitriles (HANs), and nitrosamines) was examined during the chlorination or chloramination of intracellular organic matter (IOM) extracted from Microcystis aeruginosa, Oscillatoria sp. (OSC), and Lyngbya sp. (LYN). The percentage of unknown TOX (22–38%) during chlorination indicated that the majority of DBPs were identified among THMs, HAAs, TCNM, and HANs. Bromide was readily incorporated into DBPs with speciation shifting slightly from dihalogenated species to trihalogenated species. During formation potential testing with chloramines, nitrosamine yields from IOM were measured for N-nitrosodimethylamine (NDMA, 10–52 ng/mg_C), N-nitrosopyrrolidine (NPYR, 14 ng/mg_C), N-nitrosopiperidine (NPIP, 3.7–5.5 ng/mg_C), and N-nitrosomethylethylamine (NMEA, 2.1–2.6 ng/mg_C). When IOM was added to a natural water matrix, the nitrosamine yields were not realized likely due to competition from natural organic matter. Ozonation increased NDMA and NMEA formation and reduced NPYR and NPIP formation during subsequent chloramination. In addition, ozone oxidation of IOM formed detectable concentrations of aldehydes, which may contribute to DBP formation. Finally, bioluminescence-based test results showed that >99% of the IOM extracted from OSC and LYN was biodegradable. Therefore, a biological treatment process could minimize this source of DBP precursor material during drinking water treatment.

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.

The reduction of ultraviolet (UV) absorbance at 254 nm (UV₂₅₄) and true color were identified as appropriate surrogates to assess the oxidation of six pharmaceuticals (i.e., carbamazepine, meprobamate, dilantin, primidone, atenolol, and iopromide) during ozonation of wastewater. Three tertiary-treated wastewaters were evaluated during oxidation with ozone (O₃) and O₃ coupled with hydrogen peroxide (O₃/H₂O₂). The correlation between pharmaceutical oxidation and removal of UV₂₅₄ was dependent upon the reactivity of each specific compound toward ozone, as measured by the second-order rate constant (k′′O₃). Oxidation of compounds with k′′O₃ > 10³ M⁻¹ s⁻¹ correlated well (R² > 0.73) with UV₂₅₄ reduction between 0−50%. Oxidation of compounds with apparent k′′O₃ < 10 M⁻¹ s⁻¹ resulted primarily from hydroxyl radicals and correlated well (R² > 0.80) with the UV₂₅₄ reduction of 15−65%. The removal of true color also correlated well (R² > 0.85) with the oxidation of pharmaceuticals during the ozonation of two wastewaters. These correlations demonstrate that UV₂₅₄ reduction and true color removal may be used as surrogates to evaluate pharmaceutical oxidation in the presence or absence of dissolved ozone residual during advanced wastewater treatment with O₃ or O₃/H₂O₂. The use of online UV₂₅₄ measurements would allow wastewater utilities to optimize the ozone dose required to meet their specific treatment objectives.