Investigation of Factors that Limit Efficacy of Ultraviolet Disinfection in Water and Wastewater Treatment

This research has been undertaken to contribute to our understanding of factors that limit and hinder ultraviolet (UV) disinfection efficacy, specifically the presence of flocs, and self-aggregation, regrowth and reactivation of microorganisms (Chapters 1 and 2). Chapter 4 aimed to provide an in-depth understanding of the effect of particles and flocs on UV disinfection by using a well-defined and well-controlled synthetic system that simulates the bioflocculation process. Results indicated that particle size had significant effects on Escherichia coli (E. coli) inactivation at high UV doses, and larger particles protected bacteria more than smaller particles. Moreover, the size of the constituent particles of flocs did not make a significant difference on inactivation levels. In Chapter 5, the bioflocculation process was monitored with a dynamic particle analyzer (DPA 4100) and UV inactivation data were assessed in light of the floc characteristics determined with the particle analyzer. Less circular and more porous flocs formed when the degree of bioflocculation was increased and larger flocs had a more heterogeneous structure. Irradiation experiments confirmed that floc size alone cannot explain how much E. coli are protected from UV light in the presence of particles and flocs. In Chapter 6, the DPA 4100 and an inverted microscope (Nikon Eclipse Ti) were used to examine the extent of UV-induced self-aggregation of E. coli after exposure to low-pressure and medium-pressure UV irradiation. All low-pressure UV doses tested yielded significant increases in particle size following UV exposure, indicating self-aggregation. In the medium-pressure UV experiments, only a dose of 80 mJ/cm2 had a significant impact on the formation of aggregates upon UV exposure. In Chapter 7, the regrowth issue was addressed by studying regrowth in the absence of reactivation. Results showed that percent regrowth of E. coli and indigenous wastewater bacteria were higher after UV disinfection at 40 mJ/cm2 than at 15 mJ/cm2. Regrowth of E. coli was observed even in phosphate-buffered saline with no added nutrients indicating that lysis of UV-damaged cells may provide nutrients for surviving bacteria. Overall, this research underscores the importance of addressing the aforementioned issues for improving UV disinfection performance and better protecting public health.