Investigating spatial and temporal dynamics in microbial community composition of multiple full-scale slow sand filters in drinking water treatment
Slow sand filters (SSFs) are essential for sustainable drinking water treatment, relying on chemical, physical, and microbial processes to remove nutrients, organic matter, and pathogens. Despite their importance, the spatial and temporal dynamics of microbial ecology remain poorly understood. This study investigated prokaryotic communities across sand filter depths and over time in the top layer, the Schmutzdecke (SCM), of different full-scale SSFs in the Netherlands using 16S ribosomal RNA gene-targeted amplicon sequencing and quantitative PCR. Horizontally, SSFs exhibited a uniform prokaryotic community at each depth sampled, regardless of filter surface dimension or influent inlet design, suggesting even influent water and nutrient distribution. Vertically, however, the prokaryotic composition varied significantly, with the SCM showing higher biomass and diversity compared to the deeper layers that were more similar to each other. A core prokaryotic community, including the families Nitrospiraceae, Pirellulaceae, Nitrosomonadaceae, Gemmataceae, and Vicinamibacteriaceae was consistently present across all depths, SSFs and SCMs of different ages. The persistence of these taxa suggests a central role in supporting key processes in SSF such as organic matter degradation and nitrification.  Additionally,  the relative abundance of Archaea increased with sand depth in all SSFs, suggesting their adaptation to the lower-nutrient conditions found in deeper layers. Analysis of the SCM over time showed that after scraping, the prokaryotic community gradually adapted, with minimal biomass increase during the first 3.6 years, eventually evolving into a mature, diverse, and even prokaryotic community. Overall, this study highlights the presence of spatially distinct microbial communities at various depths of SSFs, suggesting the removal of specific compounds in distinct sand layers. The persistence of a core prokaryotic community across different SSFs, SCM maturation stages, and even after disturbances like scraping, demonstrates that the biology in SSFs is resilient and likely ensuring reliable SSF performance. These findings lay the groundwork for future research to focus on these microorganisms and their functional potential.