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 ecological dynamics of prokaryotic communities within SSFs remain poorly understood. This study investigated the top sand layer, the Schmutzdecke (SCM), along with the influent and effluent water of 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 and do not apply advanced oxidation processes prior the SSF. In contrast, the other two DWTPs treat reservoir-stored surface water and incorporate ozonation or UV and activated carbon filtration as part of their treatment train. All SSFs consistently reduced biomass in the effluent compared to the influent, confirming their role in biomass load reduction. Key biological and chemical parameters showed that pretreatment with dune infiltration produced more biologically stable drinking water compared to reservoir storage. Moreover, while SSFs act as polishing filters when treating dune-infiltrated surface water, they significantly alter the prokaryotic community and biological stability of the water when treating reservoir-stored surface water. Prokaryotic communities in the SCM and water samples showed distinct compositions rather than merely the accumulation of microorganisms in the SCM from the influent water, demonstrating that SSF are active ecosystems different from water. The SCM exhibited a higher relative abundance of the genera SWB02, Gemmata, Pedomicrobium, Nitrospira, and mle1–7, while in the water samples the genus Candidatus Omnitrophus was relatively more abundant. Moreover, each DWTP hosts a unique prokaryotic profiles in both the SCM and water samples. Source water, upstream treatment and/or the biological stability of the influent water are identified as potential causes affecting the prokaryotic communities in SSFs that affect the microbial water quality of the effluent water.