Microbial communities in slow sand filters for drinking water treatment adapt to organic matter altered by ozonation

Changing natural organic matter quality from anthropogenic activity and stricter requirements for micropollutant removal challenges existing systems for drinking water production. Ozonation of water followed by biofiltration, such as passage through a slow sand filter (SSF), is a partial solution. Biofiltration relies on biofilms (microbial communities within extracellular matrices). However, the effects of ozonation on SSF microbial communities are unknown. In this study, genome-resolved and read-based metagenomics were used to compare the microbial communities of two full-scale SSFs employing conventional pre-treatment to a 20 m² SSF operated in parallel with ozonation as additional pre-treatment. The SSF microbial community receiving ozonated water was less diverse than those receiving non-ozonated water. Families Hyphomicrobiaceae, Acetobacteraceae, Sphingomonadaceae and Burkholderiaceae were more abundant when ozone was used, as were genes for metabolism of single-carbon organic compounds. Conversely, genes for metabolism of aromatic compounds and fatty acids were less abundant. Metagenome assembled genomes associated with the non-ozonated SSFs were enriched with several glycoside hydrolases, while those associated with the ozonated SSF were enriched with genes for 1-2 carbon compound metabolism. No indications of increased microbial risk (pathogens or antibiotic resistance genes) were detected as a consequence of ozonation. This study shows how microbial communities of SSFs adapt to changes in organic matter quality, highlighting the key role of biofilters for production of safe and sustainable drinking water in a changing climate.