Context. Slow sand filtration (SSF) is widely used in drinking water treatment plants (DWPTs) as a cost-effective and safe approach to remove contaminants like particles, organic matter and microorganisms from drinking water. The removal primarily occurs in the top layer of the mature sand bed, known as the 'Schmutzdecke', and is highly affected by the microbial community.
Gap. The composition of the microbial communities of mature sand filters and their activities responsible for the removal of contaminants remain unclear. To date, almost all studies on microbial communities of sand filters have focused on the total community, without distinguishing the active microorganisms that are actually responsible for biofiltration.
Aim. i) To characterize and distinguish the composition of the total and the active microbial community in the top layers of the sand filter, and their metabolic functions related to the biofiltration; ii) to compare the microbial communities assembled in the Schmutzdecke under different operating conditions.
Methods. By using 16S amplicon sequencing of both DNA and RNA from top layers of seven full-scale slow sand filters, we determined the total and the active communities, respectively.
Findings. We found that profiles of total (DNA) and active (RNA) communities were distinct in most sand filters, and these community differences related to SSF operating conditions, including sand type, sand grain size, filtration rate, and age of the Schmutzdecke. The community difference was found to be the most pronounced in younger filters composed of coarser sand, suggesting that previous microbial ecology studies of young coarse sand filters based on total communities have misrepresented the potential metabolic function. In addition, the SSF operating conditions were shown to affect the composition of both total and active microbial communities.
Utilization. Our findings provide insight into the microbial communities of the slow sand filters which can help in improving biofiltration performance.
Graphical abstract.
