Applying UV absorbance and fluorescence indices to estimate inactivation of bacteria and formation of bromate during ozonation of water and wastewater effluent

Abstract

Ozone is an effective oxidant and disinfectant commonly used for elimination of micropollutants and inactivation of resistant microbes. However, undesirable oxidation/disinfection byproducts such as bromate might form during ozonation. In this study, the UV absorbance and fluorescence indices were applied as surrogate indicators for predicting the inactivation of bacteria and formation of bromate during ozonation of water and wastewater effluents. The inactivation efficiencies of lab-cultured Escherichia coli (E. coli) and autochthonous bacteria were measured by plating (for E. coli only) and flow cytometry with fluorescence staining. During ozonation of E. coli spiked into wastewater effluents (∼10⁶ cell/mL), the priority of inactivation efficiency determined by different cell viability methods were in the order of CFU > membrane damage > DNA damage. Approximately, 99% membrane damage and/or 90% DNA damage are conservatively supposed as an indicator for sufficient bacterial inactivation as well as degradation of antibiotic resistance genes. The related required O₃ dosing thresholds for sufficient inactivation of E. coli and autochthonous bacteria refer to ∼0.6 O₃/DOC (g/g), ∼50% decrease of UVA254, ∼60% decrease of UVA280, or ∼80% decrease of humic-like fluorescence. Within the range of 10⁶-10⁸ cell/mL, the bacterial concentration did not have significant effects on the required thresholds of the specific O₃ doses or spectroscopic indicators required for bacterial inactivation. The addition of 50 mM tert-BuOH as ·OH scavenger increased the required specific ozone doses but decreased the losses of spectroscopic indicators necessary for sufficient bacterial inactivation, and also suggested that the membrane/DNA damages for bacterial inactivation were mainly attributed to the direct O₃ attacks. The bromate concentration was determined using ion chromatography with MS/MS detection. The results showed that when O₃ was dosed at the required thresholds for sufficient bacterial inactivation, bromate formation could usually be suppressed below 10 μg/L. The present work supports that it is possible to reach a balance between bacterial inactivation and bromate formation.

Keywords: Bacterial inactivation; Bromate; Flow cytometry; Humic-like fluorescence; Ozonation; UV absorbance.