Microorganisms inactivation by wavelength combinations of ultraviolet light-emitting diodes (UV-LEDs) Kai Song, Fariborz Taghipour, Madjid Mohseni ⁎ Department of Chemical and Biological Engineering, The University of British Columbia, 2360 East Mall, Vancouver, BC V6T 1Z3, Canada HIGHLIGHTS • UV-LEDs wavelength combinations were thoroughly investigated for water disinfection. • Inactivation of different types of micro- organisms, E. coli and MS2, was exam- ined. • The effect of multiple wavelengths de- pends on UV ranges and combining manners. • UVC- and UVB-LEDs combinations al- ways achieved additive inactivation ef- fect. • UVA may improve or reduce E. coli inac- tivation depending on the manner to apply UVA. GRAPHICAL ABSTRACT abstract article info Article history: Received 29 October 2018 Received in revised form 18 January 2019 Accepted 2 February 2019 Available online 8 February 2019 Editor: Ching-Hua Huang Ultraviolet light-emitting diode (UV-LED) is an emerging UV source with many special features due to the nature of semiconductor devices. One such feature is wavelength diversity that does not exist in conventional mercury based UV lamps, which provides opportunities to selectively combine multiple wavelengths for potentially addi- tional effects by UV-LEDs. In this work, the inactivation of different microorganisms in water was investigated by UV-LEDs wavelength combinations. Various wavelength combinations, including simultaneous and sequential exposures, in different UV ranges such as UVC, UVB and UVA, were examined. These combinations were applied to the inactivation of indicator bacterium E. coli and coliphage MS2 in water. The results showed the effect of UV- LEDs multiple wavelengths depends on which wavelengths (UVC, UVB and UVA) are combined and the manner that different wavelengths (simultaneous, sequential) are used. Also, different microorganisms (bacteria, virus) respond differently to wavelength combinations. Combinations of UVC/UVB always achieved additive effect on microorganisms inactivation due to the same photochemical reaction induced by UVC/UVB on DNA. 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. However, applying extended UVA exposure before UVC significantly improved E. coli inactivation. For virus MS2 inactivation, only additive effect was observed under various wavelength combinations. This study presented a comprehensive work on UV-LEDs wavelength combi- nations, which is of significance on the application of UV-LEDs for water disinfection. © 2019 Elsevier B.V. All rights reserved. Keywords: Ultraviolet light-emitting diode (UV-LED) Wavelength combinations Disinfection Inactivation effectiveness Water treatment 1. Introduction Water disinfection is an imperative part of water treatment pro- cesses to remove pathogenic microorganisms from water. A number Science of the Total Environment 665 (2019) 1103–1110 ⁎ Corresponding author. E-mail address: madjid.mohseni@ubc.ca (M. Mohseni). https://doi.org/10.1016/j.scitotenv.2019.02.041 0048-9697/© 2019 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv