Effect of pH on endogenous sunlight inactivation rates of laboratory strain and wastewater sourced E. coli and enterococci

Abstract

Understanding the influence of environmental factors like pH on solar disinfection in sunlight-dependent wastewater treatment systems can aid in improving their design. Previous research found pH to influence the solar disinfection rates of bacteria in water containing exogenous photosensitizers that facilitate photo-oxidative inactivation. However, limited research has been conducted on the role of external pH on endogenous solar inactivation processes that occur independent of exogenous photosensitizers. As such, we studied the inactivation rates of laboratory-cultured and wastewater-sourced E. coli and enterococci in sensitizer-free matrices with pH ranging from 4 to 10 under full-spectrum and UVB-filtered simulated sunlight. Elevated solar inactivation rates were observed at pH 4 for all bacterial populations evaluated, and at pH 10 for laboratory-cultured and wastewater-sourced E. coli. Dark inactivation was observed at the pH extremes for some bacteria, but did not contribute significantly to the increased inactivation rates observed under simulated sunlight at these pH, except for laboratory-cultured E. coli at pH 10. UVB light was found to play an important role in sunlight inactivation, albeit the contribution of UVB light to solar inactivation observed for Enterococcus spp. diminished at pH 4 and 5, suggesting that indirect endogenous inactivation pathways facilitated by longer wavelength light were enhanced under acidic conditions. Our findings demonstrate that external pH affects the kinetics of endogenous sunlight inactivation processes, and the results have potential to be integrated into models for predicting inactivation kinetics in sunlight-mediated treatment systems that operate over a range of pH conditions.