Impact of UV irradiation at full scale on bacterial communities in drinking water

Water in a full-scale drinking water treatment plant was irradiated with ultraviolet (UV) doses of 250, 400, and 600 J/m2, and the effect on bacterial communities investigated using 16s rRNA gene amplicon sequencing, heterotrophic plate counts (HPCs), coliform, andEscherichia colicounts. The bacteria in the irradiated water were also analyzed following storage for 6 days at 7 °C, to approximate the conditions in the distribution system. The log10reduction of HPCs at 400 J/m2was 0.43 ± 0.12. Phylogenetic examination, including DESeq2 analysis, showed thatActinobacteriawas more resistant to UV irradiation, whereasBacteroideteswas sensitive to UV. PhylumProteobacteriacontained monophyletic groups that were either sensitive or resistant to UV exposure. The amplicon sequence variants (ASVs) resistant to UV irradiation had a greater average GC content than the ASVs sensitive to UV, at 55% ± 1.7 (n = 19) and 49% ± 2.5 (n = 16), respectively. FamiliesChitinophagaceae,Pelagibacteraceae,Holophagaceae,Methylophilaceae, andCytophagaceaedecreased linearly in relative abundance, with increasing UV dose (P < 0.05, Pearson’s correlation). When irradiated water was stored,Chitinophagaceae,Comamonadaceae, andFlavobacteriaceaefamilies decreased in relative abundance, whereasACK-M1,Mycobacteriaceae, andNitrosomonadaceaewere increasing in relative abundance. This suggests that the impact of UV irradiation cannot only be considered directly after application but that this treatment step likely continues to influence microbial dynamics throughout the distribution system.