Kinetics and efficacy of membrane/DNA damage to Bacillus subtilis and autochthonous bacteria during UV/chlorine treatment under different pH and irradiation wavelengths

The UV photolysis of chlorine (UV/chlorine) produces a suite of radical species and has been proposed as a novel water treatment process for advanced oxidation and enhanced disinfection purposes. In this study, the kinetics and efficacy of membrane/DNA damage to B. subtilis and autochthonous bacteria during UV/chlorine treatment were investigated under different pH and irradiation wavelengths (254 nm Hg lamp vs 275 nm LEDs) with flow cytometer analysis. Compared to UV irradiation or chlorination alone, the UV/chlorine showed the enhanced kinetics and efficacy of cell membrane or DNA damage to B. subtilis. These enhancements by UV/chlorine might be primarily attributed to the in-situ generation of radical species HO center dot and Cl center dot around intracellular DNA. With pH increasing from acidic to alkaline, the UV254/chlorine showed the decreased kinetics of cell membrane or DNA damage to B. subtilis while the UV275/chlorine exhibited the increased cell membrane and DNA damage kinetics due to the higher chlorine photolysis rates. When the chlorine dose was not excess, the DNA damage of B. subtilis were highly depended on the cumulative exposures of free available chlorine (CTFAC). During UV275/chlorine in the presence of natural organic matter, the cell membrane damage also showed linear correlations with the decreases of UV absorbance at 254 nm (UVA254), and about 20-25% decrease of UVA254 indicates a sufficient cell membrane damage. Using the CTFAC values and decrease of UVA254, this study has provided the surrogate indicators for fast determining the minimal chlorine dose for sufficient inactivation of bacteria by UV/chlorine treatment.

Automated summary

UV/chlorine treatment enhances the kinetics and efficacy of cell membrane or DNA damage to B. subtilis compared to UV irradiation or chlorination alone. The in-situ generation of radical species HO center dot and Cl center dot around intracellular DNA might contribute to these enhancements. The study provides surrogate indicators for determining the minimal chlorine dose for effective inactivation of bacteria during UV/chlorine treatment by using CTFAC values and decreasing UVA254 levels.