This research investigates the sunlight-mediated inactivation of pathogens in surface waters, with a primary focus on wastewater treatment pond systems. Two common fecal bacterial indicator organisms were studied: the gram-positive Enterococcus faecalis and gram-negative Escherichia coli .
We began by identifying and describing the role of oxygen, exogenous sensitizers, light intensity and light wavelength upon the sunlight-mediated inactivation of E. faecalis and E. coli. We found that, while in DI water the presence of UVB wavelengths increased inactivation of both E. faecalis and E. coli, in pond water, the presence of the UVB wavelengths did not increase the rate of E. faecalis inactivation. Pond water constituents played a dual role, as either photosensitizers increasing inactivation rates ( E. faecalis) or as light-attenuators, decreasing inactivation rates (E. coli). Inactivation rates of both E. faecalis and E. coli were correlated to dissolved oxygen concentrations, though differently. Decreasing dissolved oxygen below air saturation always decreased inactivation rates, however increasing above air saturation was not as straightforward. In waste stabilization pond water, where oxygen increases during the day due to algal photosynthesis, raising dissolved oxygen above air saturation increased inactivation of E. coli slightly, however E. faecalis inactivation remained constant.
We then investigated which ROS were involved in inactivation of E. faecalis and E. coli. Photosensitizers produce reactive oxygen species following the absorption of light and the transfer of light-energy to oxygen through a number of different pathways. Although our focus was upon exogenous mechanisms, endogenous mechanisms could not be excluded because in the presence of exogenous sensitizers damage by both exogenous and endogenous mechanisms can occur simultaneously. While we found evidence for the involvement of 1O2 in endogenous inactivation of both E. faecalis and E. coli, the importance of other species, while likely, remains unclear. As expected, no evidence was found to support a role for ROS produced exogenously in the inactivation of E. coli. A combination of quencher and D2O experiments, together with ROS measurements in pond water, provided strong evidence for the importance of exogenous 1O2 in E. faecalis inactivation. In addition, E. faecalis was significantly more sensitive than E. coli to inactivation by 1O2 produced by the synthetic sensitizer rose bengal, which followed the same pattern as their sensitivity to pond water constituents.
Three mechanisms have been proposed to describe sunlight-mediated inactivation: direct UVB damage to DNA, indirect endogenous inactivation caused by UVB light, and indirect exogenous inactivation involving all wavelengths of sunlight up to 550 nm. While the first mechanism is a likely part of sunlight-mediated inactivation, it did not dominate the inactivation of either bacteria in this study. The second mechanism, while previously attributed to UVB light, should be expanded to include UVA and visible wavelengths, as these caused E. faecalis inactivation in our DI water wavelength experiments. The third mechanism dominated E. faecalis inactivation and was driven by the UVA and visible wavelengths, though E. faecalis inactivation occurred through all three mechanisms. E. coli , on the other hand, was not subject to the third, exogenous mechanism in our microcosms. Instead E. coli inactivation was dominated by endogenous mechanisms, and these mechanisms were driven by the UVB and UVA wavelengths. A final mechanism was described that does not fit into the three mechanisms above; exogenous production of H2O2, which then crosses into cells and causes endogenous microbial damage. This mechanism was not important in our pond water microcosms because H2O 2 concentrations were low due to high scavenging by pond water constituents, but it may be important in other waters. Only E. coli appears susceptible to such a mechanism. (Abstract shortened by UMI.)
|Advisor:||Nelson, Kara L.|
|Commitee:||Ray, Isha, Sedlak, David L.|
|School:||University of California, Berkeley|
|Department:||Civil and Environmental Engineering|
|School Location:||United States -- California|
|Source:||DAI-B 72/06, Dissertation Abstracts International|
|Subjects:||Microbiology, Sustainability, Environmental engineering|
|Keywords:||International development, Lagoons, Photochemistry, Ponds, Sanitation, Stabilization ponds, Wastewater treatment|
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