Schmutzdecke maturation and layers’ contribution to bacterial removal performance in slow sand filters for drinking water production

• Inoculation with local mature Schmutzdecke accelerated SSF ripening and functional resilience. • Inoculation aids early removal but may be less beneficial as filters mature. • The upper 10 cm became the dominant zone for bacterial removal as filters matured. • Active microbial communities continued evolving after total communities stabilized. • Schmutzdecke bioactivity is essential for effective E. coli inactivation. Slow sand filtration (SSF) is a sustainable and widely applied technology for drinking water treatment. However, improvements in SSF performance optimization remain constrained by incomplete understanding of the biological maturation process and the prolonged start-up (ripening) phase, particularly during seasonal fluctuations. To address these challenges, we investigated two pilot-scale SSFs operated in parallel over 24 months. A bioaugmentation strategy was tested by inoculating one new filter with functionally mature Schmutzdecke material from an operating full-scale SSF at the same DWTP site, while the other served as a control. Schmutzdecke development was characterised through biochemical profiling, microbial community analyses, and depth-resolved bacterial removal assessments. Bacterial removal efficiency reached 1.0-1.5 log 10 reduction of Escherichia coli after six months of ripening, with removal becoming increasingly dominated by bioactivity within the upper 10 cm of the filter bed. The contribution of deeper sand layers decreased progressively over time, approaching zero by month 18. Inoculation accelerated microbial community development and enhanced the filter’s functional resilience (bacterial removal) to seasonal fluctuations. Notably, the active microbial communities continued to change even after the total community had stabilised, reflecting ongoing functional adaptation. E. coli inactivation experiments with untreated versus sterilized Schmutzdecke confirmed that bioactivity within the Schmutzdecke is essential for bacterial inactivation in slow sand filters. Overall, these findings demonstrate that monitoring the active community of the Schmutzdecke, alongside the total community composition, is key to understanding SSF functioning. Specifically, greater stability and diversity in the active microbial community appear to enhance the resilience of filter performance. Furthermore, inoculation with functionally mature Schmutzdecke material from the same production site is an effective strategy to optimise SSF start-up and functional stability.