The effects of UV-rich light pulses on pathogenic microorganisms in liquids
The work presented in this thesis is concerned with investigating the use of UV-rich light pulses for the inactivation of problematic microorganisms. UV radiation is an effective means of disinfecting surfaces and liquids and of reducing contamination in air. The germicidal effects are primarily due to the UV-C region of the electromagnetic spectrum, which interferes with the nuclear core of a microorganism resulting in a loss of ability to replicate and initiate infection. When UV radiation is delivered as pulses of light however, the results are even more appealing, with higher levels of microbial inactivation achieved in much shorter timescales. Drinking water and wastewater disinfection is normally provided by the use of chemicals such as chlorine. These are disadvantaged by the production of harmful chemical by-products and the resistance of certain types of microorganism to chemical treatment. The main aim of this investigation was therefore to look at the role of pulsed UV-rich light for inactivating a range of microorganisms suspended in liquid media and to determine how successful the treatment process would be as an alternative disinfection method. The results show that pulsed UV-rich light treatment is extremely effective against many types of bacteria, virus and Cryptosporidium. It was also demonstrated that the sensitivities of microorganisms to UV radiation can vary significantly depending on the cell-wall structure, growth phase, strains and nucroorgarusm specIes. Studies were also undertaken to identify electrical and biological parameters that may influence the inactivation success. It was found that high operating voltages and low pulse repetition frequencies give desirable levels of inactivation Other important factors investigated were sample depth, volume and reflection. Finally, the possible limitations to pulsed UV-rich light treatment were investigated. It was found that the success of the treatment is primarily determined by the transmittance of UV pulses through a sample. As expected, transmittance depends upon microorganism size and population. The major limitation of UV treatment is the ability of microorganisms to reactivate following exposure to visible light (photoreactivation). Studies showed photoreactivation to occur following pulsed UV-rich light treatment, only when the microorganisms do not receive sufficient UV treatment or if they are exposed to high intensities of visible light.