Effects of single and combined UV-LEDs on inactivation and subsequent reactivation of E. coli in water disinfection. 
Ultraviolet light emitting diodes (UV-LEDs) have shown a potential to replace traditional Ultraviolet (UV) pressure lamps for water disinfection.  However, the research is not sufficient and hence, it is still difficult to make any logical conclusions.  In this work, UV-LEDs with peak emissions at 267, 275, 310 nm and combined emissions at 267/275, 267/310 and 275/310 nm were applied to a batch water disinfection system.  Under either single- or combined-wavelength situation, the inactivation efficiency, reactivation (due to photoreactivation and dark repair) after UV irradiation and electrical energy consumption were evaluated by way of the model bacterium Escherichia coli.  It was found that, the 267 nm UV-LED had the highest inactivation efficiency than other UV-LEDs.  Although reactivation occurred after 267, 275, 267/275 and 275/310 nm UV-LEDs' irradiations, it occurred to a lesser extent in dark repair than in photoreactivation, demonstrating that photo-effect is the dominant mechanism of reactivation.  In addition, decay phase was more prominent than reactivation in dark repair.  However, the irradiation by the 275 nm UV-LED showed a better persistence against reactivation which could be attributed to protein damage at 275 nm.  No synergistic effect for combined wavelengths was observed in this study.  The electrical energy consumption was lower for the 275 nm UV-LED than the other UV-LEDs which was attributed to its higher wall plug efficiency.  This study showed the variation principle between the single and combined UVB/UVC-LEDs in inactivation efficiency, inhibition of reactivation, synergistic effect and electrical energy consumption in treatment of E. coli, which is useful for the reasonable exploitation of UV-LEDs in water disinfection systems.