Sunlight Inactivation of Waterborne Viruses: Mechanisms, Modeling, and Application to Surface Waters and Wastewater Treatment

Sunlight inactivation of microorganisms is a natural process that can occur in any sunlit water and has implications for microbial ecology, the fate of microbial contaminants in the environment, and natural wastewater treatment systems. This work focuses on sunlight inactivation of waterborne viruses, which present a special challenge in health-related water microbiology given that waterborne human viruses are important etiologies of disease, and are difficult to remove and inactivate in water.;There are three proposed sunlight inactivation mechanisms for viruses: the direct- and indirect-endogenous mechanisms, which require absorption of photons by virus components, and the exogenous mechanism, which involves reaction between the virus and exogenously produced reactive intermediates formed by photochemical reactions. All three mechanisms are affected by water quality. Natural organic matter, for example, is found in most aquatic environments and is capable of both attenuating sunlight---which would decrease sunlight exposure and therefore inactivation rates---and photosensitizing production of reactive intermediates---which could increase inactivation rates of viruses susceptible to the exogenous mechanism.;The first goal of this dissertation was to better understand the mechanisms of sunlight inactivation of select bacteriophage and human viruses in surface waters containing natural organic matter. Given the two main modes by which damage is delivered to viruses in sunlit surface waters---through direct absorption of photons (the direct- and indirect-endogenous mechanisms) and contact with reactive molecules formed by sensitizers in the water column (the exogenous mechanism)---a better understanding of sunlight inactivation mechanisms can help us predict how environmental conditions (e.g., sunlight irradiance, light attenuation, water quality, depth, mixing) can affect observed inactivation rates.;The second part of this dissertation focuses on a case study of wastewater irrigation practiced in Accra, Ghana, with goals to better understand the health risks associated with wastewater irrigation in Accra, and to determine whether small, farmer-dug ponds can contribute to disinfection of irrigation water. To provide data that can be used to refine quantitative microbial risk assessment models of wastewater-fed agriculture in Accra, irrigation water samples were analyzed for concentrations of fecal indicator microorganisms (human-specific Bacteroidales, E. coli, enterococci, thermotolerant coliform, and F+ and somatic coliphages) and two human viruses (adenovirus and norovirus genogroup II). E. coli concentrations in all samples exceeded recommended limits set by the World Health Organization, human viruses were detected in 75% of samples analyzed, and virus concentrations were quantified in 60% of samples. Indicator organism and virus concentrations were compared as part of an analysis of differential stability of fecal indicator organisms and pathogens in the environment, and the appropriateness of assumptions used in quantitative microbial risk assessment to relate indicator organism concentrations to those of pathogens.;After determining indicator organism and pathogen concentrations in Accra irrigation water, we investigated the ability of a farmer-developed intervention (small, on-farm ponds) to disinfect wastewater before use in vegetable irrigation. Results indicated that sunlight inactivation dominated the removal of two bacteria (E. coli and enterococci) and two bacteriophages (F+ and somatic coliphages) in these ponds, and that the ponds can contribute to the multi-barrier approach to reducing health risks related to wastewater irrigation. On-farm pond design and management recommendations, as well as challenges, are also discussed. (Abstract shortened by UMI.).