Impact of organic and inorganic matter accumulations on Escherichia coli retention in slow sand filters

Access to safe drinking water is essential for public health, making pathogen removal a key objective in water treatment. Slow sand filtration (SSF) is a widely used water treatment method due to its operational simplicity and cost-effectiveness. While biological activity is considered a key factor in SSF performance, the roles of accumulated organic and inorganic matter in pathogen removal remain unclear. This study investigated their contributions to Escherichia coli ( E. coli ) retention using sand columns from three mature SSF systems in the Netherlands. Baseline E. coli removal was assessed in all filters. In two filters, selective extractions were applied using: (i) sodium hypochlorite to eliminate organic matter, and (ii) sodium dithionite–citrate and acid ammonium oxalate to remove accumulated inorganic matter (e.g., metal (oxyhydr)oxide coatings and calcites). Results show that both organic and inorganic matter contribute to E. coli retention, with varying importance across filters. In the sand columns where organic or inorganic coatings were selectively removed, log removal of E. coli decreased, indicating their mechanistic roles in retention. In a filter with high biological activity and low metal (oxyhydr)oxide content, mechanisms like predation and biofilm formation appeared to dominate retention. Conversely, in a filter with low biological activity, retention was primarily linked to calcium carbonate and iron and manganese (oxyhydr)oxide coatings. Variations in organic and inorganic matter content reflect both the characteristics of the original sand and subsequent accumulation, particularly in the filter’s top layer (the schmutzdecke). These accumulations are shaped by source water and upstream treatment, highlighting the need to consider the full treatment train when optimizing SSF performance.