Effects of UV Irradiation by Light Emitting Diodes on Heterotrophic Bacteria in Tap Water

Ultraviolet light emitting diodes ( UV ‐ LED s) are small mercury‐free devices that can be installed at the point of use ( POU ) of water for disinfection. Considering that heterotrophic bacteria are of concern in drinking water systems, we applied a flow‐through UV ‐ LED apparatus to dechlorinated tap water, and determined the heterotrophic plate count ( HPC ) in samples after UV ‐ LED exposure ( UV +) compared to samples without UV ‐ LED application ( UV −). The UV + and UV − samples were maintained at 20°C to track HPC profiles during storage for 7 days. It was confirmed that UV + samples showed negative HPC or lower HPC than UV − for 5 days of storage after the flow‐through test. HPC bacteria formed colonies with different morphological characteristics, and yellow colonies were closest to Novosphingobium sp., with 99% identity, while white and pale pink colonies were closest to Methylobacterium sp., with 99–100% identity, based on 16S rRNA gene sequences. White colonies became dominant in UV +, indicating that UV ‐ LED exposure can select UV ‐resistant species such as Methylobacterium . This study shows the effects of UV ‐ LED application on HPC bacteria in tap water and implies that future research is required on the significance and impacts of microbial selection by UV ‐ LED exposure.