Effects of bromide on inactivation efficacy and disinfection byproduct formation in photocatalytic inactivation

The high effectiveness of TiO₂-UVA system in bacterial disinfection has gained attention to use this technology in water treatment on different source waters, including desalinated sea water and grey water. However, source waters containing a significant level of bromide could have different chemical pathways during TiO₂-UVA disinfection process because reactive bromine species could be formed. To illustrate the water safety from the Br-TiO₂-UVA system, this study investigated bacterial inactivation efficiency and disinfection byproduct (DBP) formation under different pH, TiO₂ dosages and bromide concentrations in a laboratory setting. At a high bromide concentration (65 mg/L, equivalent to the concentration of natural sea water), the bacterial inactivation rate increased 2 times at pH 5 and more than 5 times at pH 8. However, a significant increase of brominated DBPs, which were considered more carcinogenic and toxic than chlorinated DBPs, was observed. The bacterial inactivation pattern was shifted from the "shoulder-log" to the "log-tail" under different bromide concentrations, suggesting the existence of bromide altered the bacterial inactivation mechanisms. We also observed that the inactivation kinetics of Br-TiO₂-UVA system was greatly influenced by water pH. Our laboratory experiments demonstrated that bromide could improve the performance of photocatalytic inactivation, but it also could reduce the water safety by generating a higher level of brominated-DBPs in treated water. Water engineers should pay attention to the brominated DBP formation when applying TiO₂-UVA photocatalysis on source waters with a significant level of bromide.