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Abstract

Safe drinking water is essential to humans, thus natural raw water requires necessary treatment, especially removing pathogenic microorganisms for disinfection. In addition to conventional chemical disinfectant, ultraviolet (UV) radiation has been increasingly used for water disinfection. Recently a new UV source - UV light-emitting diodes (UV-LEDs) - has emerged with many special features, which is believed to be a promising alternative to conventional UV lamps for water disinfection. This research focused on two special features of UV-LEDs, multiple wavelengths and pulsed irradiation, to explore the effect and potential on water disinfection. UV-LEDs in different UV wavelength ranges were combined in various manners to investigate the effect of multiple wavelengths on microorganisms inactivation in water. The results showed the effect of UV-LEDs multiple wavelengths depends on the wavelength combinations among UVA (315 – 400 nm), UVB (280 – 315 nm) and UVC (200 – 280 nm), the manner to apply different wavelengths (e.g. simultaneous, sequential), as well as different types of microorganisms (e.g. bacteria, virus). Combinations of UVC/UVB always achieved additive effect on microorganisms inactivation due to the same photochemical reactivation induced by UVC/UVB on DNA that follows the Second Law of Photochemistry. However, combining UVA with UVC/UVB simultaneously or applying UVA after UVC/UVB reduced the inactivation of bacterium E. coli due to DNA repair and photoreactivation effect of UVA. A special wavelength combination was developed by applying UVA as pretreatment followed by UVC inactivation, which achieved dramatic inactivation improvement and significant reactivation reduction on E. coli. The effects and mechanisms of this special combination were thoroughly investigated and revealed in this research. The effect of UV-LEDs pulsed irradiation was examined by applying pulsed irradiation with various pulsed patterns (frequency and duty rate) on different microorganisms in pure water and wastewater. Comparable inactivation were obtained by UV-LEDs continuous irradiation and various pulsed irradiation on all the four microorganisms examined, which clarified the role of pulsation on UV disinfection. The findings in this research promote a better understanding on UV disinfection and are of considerable significance to take full advantage of UV-LEDs for water disinfection.