Impact of light screening and photosensitization by surface water organic matter on enterococcus faecalis inactivation

Sunlight irradiation is known to contribute to microbial inactivation in natural treatment systems (NTS) and surface waters via both direct and indirect processes. This study investigated the influence of various types of organic matter (OM) on endogenous and exogenous photoinactivation of Enterococcus faecalis. Solar simulator exposure experiments were conducted in surface waters and wastewater and in the presence of natural OM isolates and a singlet oxygen (1O2) sensitizer, methylene blue. Presence of both natural and anthropogenic OM resulted in increases in inactivation rates of 17–67% compared to sensitizer-free experiments. This suggests that, while direct and endogenous photoinactivation is hindered with increasing depth due to light screening by OM, its role as a photosensitizer in the formation of reactive intermediates (predominantly involving 1O2) is greater than its negative impact due to light screening at levels typically found in NTS. Specifically, increases in light absorbance (particularly at shorter wavelengths) are estimated to reduce direct and endogenous processes by up to 41% under our experimental conditions; however, presence of external photosensitizers enhances exogenous inactivation processes to a greater degree. A novel approach is presented to help estimate endogenous and exogenous inactivation rates of E. faecalis as a function of depth in the presence of OM based on common optical measurements. Implications of this knowledge in informing design parameters (e.g., depth and retention time) of NTS are discussed, thus contributing to both fundamental understanding of photoinactivation processes in surface waters and improvement of treatment unit design.