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 producing high-quality and sustainable drinking water, relying on chemical, physical, and microbial processes to remove nutrients, organic matter, and pathogens. Despite numerous studies on the physical and chemical mechanisms in SSFs, the microbial processes and dynamics remain poorly understood. This study bridges this knowledge gap by investigating the spatial and temporal dynamics of prokaryotic communities within SSFs, by analysing different depths and the top layer, the Schmutzdecke (SCM), over time in full-scale SSFs from different drinking water treatment plants in The Netherlands. Utilising 16S ribosomal RNA gene-targeted amplicon sequencing and quantitative PCR, we observed a horizontally uniform prokaryotic community at each depth at all analysed SSFs, suggesting effective influent water and nutrient distribution, regardless of filter size or influent inlet design. Vertically, however, the prokaryotic composition varied significantly, with the SCM showing higher biomass and diversity compared to the deeper layers. This study identified a core prokaryotic community, including the families Nitrospiraceae, Pirellulaceae, Nitrosomonadaceae, Gemmataceae, and Vicinamibacteriaceae, consistent across various depths and SSFs, and in the SCMs of different ages. Their presence suggests a central role in supporting key biological processes in SSFs such as organic matter degradation and nitrification. Additionally, the relative abundance of archaea increased with sand depth in all SSFs, suggesting their adaptation to 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. Our findings highlight the presence of spatially distinct microbial communities at various depths of SSFs, suggesting the removal of specific compounds in distinct sand layers. Moreover, 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 ensures reliable SSF performance. It also implies possibilities for earlier SSF operational restart after cleaning than is conventionally done, but with continuous monitoring of water quality parameters to ensure microbial safety. These findings lay the groundwork for future research to focus on these microorganisms and their functional potential.