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Mostafa, Simón ¹ ; Rubinato, Matteo ² ; Rosario-Ortiz, Fernando L. ³ ; Linden, Karl G. ⁴

¹ University of Colorado, Boulder
² University of Colorado, Boulder
³ University of Colorado, Boulder
⁴ University of Colorado, Boulder

Sunlight irradiation is known to contribute to microbial inactivation in natural treatment systems (NTS) and surface waters via both direct and indirect processes. Direct and endogenous photoinactivation is hindered with increasing depth due to light screening by natural and anthropogenic organic matter (OM). Additionally, indirect exogenous processes, predominantly involving singlet oxygen (¹O₂), are anticipated to be important for disinfection in NTS, with OM serving as a photosensitizer. This study investigated the influence of OM on endogenous and exogenous photoinactivation of Enterococcus faecalis upon exposure to simulated sunlight in surface waters, wastewater, and in the presence of natural organic matter (OM) isolates or a ¹O₂ sensitizer, methylene blue. The presence of OM resulted in increases inactivation rates of 17-67% compared to sensitizer-free experiments. This suggests that the role of OM as a photosensitizer for the formation of reactive intermediates is greater than its negative impact due to light screening at levels typically found in NTS. Specifically, an increase in light absorbance (particularly at shorter wavelengths) are estimated to reduce direct and endogenous processes by up to 41% under our experimental conditions while the presence of external photosensitizers enhances exogenous inactivation processes. An approach is presented to help estimate the endogenous and exogenous inactivation rates of E. faecalis as a function of depth in the presence of OM based on common optical measurements. These results are important in informing the design parameters (e.g. depth, retention time) of NTS by providing a measure of inactivation kinetics as impacted by depth in the presence of diverse types of OM.