Sequential use of UV-LEDs irradiation and chlorine to disinfect waterborne fungal spores: Efficiency, mechanism and photoreactivation.

In this work, sequential applications of light-emitting diodes (UV-LEDs) with two wavelengths and chlorine (Cl) were performed for fungal spores disinfection: UV-Cl, Cl-UV, UV/Cl-UV, UV-UV/Cl, Cl-UV/Cl-Cl. Overall comparisons of the sequential processes with respect to the inhibitory effect on photoreactivation were also evaluated. According to the evaluation of culturability and membrane permeability, inactivation of fungal spores by UV was not enhanced by prior or post exposure to Cl, but in the UV/Cl process with pre or post UV treatment, the inactivation efficiency was greatly enhanced. Take P. polonicum for example, pre-treatments by UV and UV (40 mJ/cm) caused the log count reduction (LCR) of 1.05 log and 0.95 log, then the followed UV/Cl and UV/Cl at the same UV fluence caused additional LCR of 1.80 log and 2.00 log. The permeabilization of P. polonicum was also accelerated in the processes of UV/Cl-UV and UV-UV/Cl, especially at the wavelength of 280 nm. In the sequential processes, especially those containing UV/Cl or at the wavelength of 280 nm, could promote the formation of intracellular reactive oxygen species (ROS), thus leading to more severe damage to the spores as reflected in the culturability reduction, membrane permeability and inhibition of photoreactivation.