Comparative effects of chlorine and UV LED disinfection on drinking water and biofilms, with chlorine causing greater shifts in microbial community composition

Microorganisms in drinking water systems can form biofilms that promote microbial regrowth, and reduce disinfection efficiency, potentially increasing human exposure and health risks. The 16S rRNA gene was sequenced to compare the yearlong impacts of slow-sand filter effluent disinfection by chlorination versus UV light-emitting diodes (LEDs) on bacterial communities in water and biofilm samples at a water treatment plant. Significant differences were observed between sample types (water and biofilm) and sample sources (filter influent, filter effluent, chlorine effluent, and UV LED effluent). Dominant phyla that were observed are commonly found in water treatment systems, including Proteobacteria, Actinobacteria, Bacteroidetes, and Firmicutes, with notable detection of the Polaromonas genus for this rural, mountain setting. Biofilms were significantly different from bulk water in terms of beta diversity and abundance of taxa present. The microbial community in biofilms after chlorination differed significantly from biofilms after filtration and biofilms after UV LED disinfection, while those after filtration did not differ from UV LED disinfection, highlighting the particular significance of chlorine in impacting downstream biofilm microbiomes. This difference in selective pressure due to disinfection was also seen for genera containing potential opportunistic pathogens. Among environmental variables, DOC and precipitation were significantly associated with water sample microbial community structure; and significant relationships were observed with some of the most abundant taxa and/or differentially abundant taxa including Legionella, Escherichia, Shigella, and Pseudomonas. Understanding differences in selective pressure after UV-LED as compared to chlorine disinfection can inform development of disinfection practices to optimize disinfection and minimize human health risks.