"Impact of environmental and process conditions on the microbial ecology and performance of full-scale slow sand filters in drinking water treatment"
Slow sand filters (SSFs) are commonly used for treating drinking water, effectively removing contaminants such as particles, organic matter, and microorganisms. However, the prokaryotic community ecology within these filters and the factors influencing its composition remain largely unclear. In this study, we analyzed the top sand layer of SSFs, known as the Schmutzdecke (SCM), along with the influent and effluent water from full-scale SSFs at four drinking water treatment plants (DWTPs) in the Netherlands. These plants use SSFs as the final step in their treatment to produce unchlorinated drinking water. Two DWTPs treat surface water after dune infiltration, whereas the other two treat reservoir-stored surface water. The biomass in the SCM of all SSFs ranged from 1.0 × 108 to 7.7 × 108 16S rRNA gene copies per gram of wet sand. The effluent water biomass was consistently lower than the influent water across all SSFs, indicating the effectiveness of the SSF process in reducing biomass load in the effluent. Key biological and chemical parameters showed that pretreatment with dune infiltration produced more biologically stable drinking water compared to reservoir storage. Beta diversity analysis and differently abundant taxa revealed distinct prokaryotic communities in the SCM and water samples, suggesting adaptation of the prokaryotes to their specific matrix type and environment rather than merely the accumulation of microorganisms in the sand from the influent water. The genera SWB02, Gemmata, Pedomicrobium, Nitrospira, and mle1-7 were more enriched in the SCM than in the water, while the genus Candidatus Omnitrophus was enriched in the water samples. Moreover, our findings show that each DWTP hosts a unique prokaryotic community composition in both the SCM and water samples, and that upstream treatment directly or indirectly, by affecting the biological stability of the SSF influent water, were likely to be responsible for the development of these unique communities. The results also showed that SSFs act as polishing filters when treating dune-infiltrated surface water or as filters that significantly change the prokaryotic community and biological stability of the water when treating reservoir-stored surface water.