Search

Ultraviolet (UV) disinfection provides chemical free inactivation of a wide range of microorganisms. One primary mechanism of UV inactivation of bacteria is through DNA damage, which inhibits bacterial replication and reproduction. The ability of bacteria to repair DNA damage raises concerns about the post-treatment safety of UV treated water, especially if bacteria repair and regrow under favorable conditions. A well-known repair process is photorepair, which can lead to photoreactivation of inactivated cells. To address this concern, UV disinfection processes can be optimized by selecting specific wavelengths and UV doses to prevent repair and regrowth in various water treatment contexts.Emerging UV technology, such as UV LED devices, can be tailored to emit specific wavelengths to prevent microbial repair. In this study, UV-LEDs with nominal wavelengths 265, 280, and 285 nm and combined 265 + 285 nm were applied in collimated beam tests to quantify inactivation and photoreactivation kinetics of E. coli. At UV doses above 25 mJ/cm2, which were not evaluated in previous UV disinfection studies, the inactivation efficacy was highest for 265 + 285 nm, followed by 265 nm, 285 nm, and 280 nm. Both time-based and photorepair fluence-based photoreactivation were quantified. After 40 mJ/cm2, photoreactivation was greater for 265 + 285 nm than 265 nm. This was unexpected because the contribution of 285 nm in the combined wavelength disinfection was expected to suppress repair due to protein damage. Inactivation kinetics and regrowth kinetics may impact accurate quantification of photoreactivation. Molecular detection methods are needed to measure DNA damage repair to distinguish regrowth from repair.Photorepair is a concern after UV disinfection for point of use and decentralized community water treatment. A quantitative microbial risk assessment (QMRA) algorithm was developed to estimate probability of infection associated with pathogenic E. coli in community drinking water systems where solar photorepair light exposure can occur. QMRA is a powerful tool for estimating public health risk of exposure when lacking data. Photoreactivation was determined as a function of effective photorepair fluence from sunlight and a pathogen dose-response of E. coli O157:H7 was used to estimate probability of infection. The objective of the QMRA was to determine contamination, UV treatment, light exposure, and water container conditions where the acceptable risk of 1 in 10,000 is exceeded in water consumption scenarios with and without photoreactivation. For concentrations less than 5.7 × 105 CFU/L, the acceptable risk is not exceeded for point of use consumption without photoreactivation. For concentrations 104 CFU/mL and less, photorepair light exposures less than 30 minutes will not exceed the acceptable risk. Thus, 30 minutes is proposed as a conservative bottled water shelf life due to photorepair, but this estimate depends on solar and water container material characteristics that affect photorepair fluence calculations and is complicated by solar disinfection effects. The QMRA supports UV disinfection as an effective treatment for immediate consumption as well as for disinfection at communal water taps. The QMRA approach could be expanded to quantifying pathogen photoreactivation in other UV disinfection applications.

Abstract: The introduction of drinking water safety plans in Alberta has resulted in increased attention towards water-based, opportunistic pathogens like Legionella pneumophila, which grow in premise plumbing biofilms and cannot be controlled through drinking water treatment alone. Currently, there are neither specific standards nor regulatory guidelines to control Legionella spp. in Canadian drinking water systems and current culture-based detection is limited by inaccuracy, 10 day incubation periods, and an inability to measure VBNC cells. Thus, it was the objective of this project to improve upon the poor reliability of L. pneumophila detection by examining viability assays that correlate with amoeba infectivity data, which could be coupled with a novel IMS-FCM detection system, in hopes of providing a better, timely estimate of public health risk. Experiments using the novel IMS-FCM detection system revealed sensitivity to L. pneumophila below the recommended action level guidelines given by Public Health Ontario (

Abstract: The inactivation fluence of a target wastewater plant in Alberta, Canada was determined in the laboratory and was found to be close to that reported by the target wastewater plant. The effect of inactivation fluence, effective reactivation fluence, temperature, and river water on the percent photoreactivation of total coliforms was investigated under both indoor and outdoor conditions. A concept, namely the ‘effective reactivation fluence’, based on weighting the spectral fluence rate by the action spectrum for photoreactivation, is introduced. Higher inactivation fluence and effective reactivation fluence decreased the percent photoreactivation of total coliforms, while higher temperature increased it. Also, the percent photoreactivation of total coliforms decreased on increasing the percent river water. In addition, the effect of various covers on the percent photoreactivation of total coliforms was studied representing higher percent photoreactivation of total coliforms by cutting off the UV-B portion of sunlight.

This thesis was written to study photoreactivation in different physiological conditions of the vegetative cell as well as the photoreactivation of the two morphological states of the Azotobacter cell: the vegetative cell and the cyst.