Beneath the Surface: New insights into removal processes in the depths of Slow Sand Filters

Slow sand filtration is one of the oldest water treatment technology widely applied globally across various scales from centralised systems to point-of-use treatment. The major functions of these filters include turbidity removal, disinfection and production of biologically stable drinking water. SSFs operate on the principle of percolation of water through a fine sand bed at low rates of 0.1-0.3 m/h without backwashing. This fosters the growth of a thick biofilm layer called the Schmutzdecke on the filter surface, and thinner biofilms in the remaining filter bed.

SSFs to date are operated as “black boxes” with limited understanding of the underlying processes contributing to treatment. This research aimed to unravel the physical-chemical and biological processes involved in disinfection and removal of biological stability parameters (dissolved organic carbon (DOC) and ammonium (NH4+) and by contributing to the development of new design rules for modern SSFs. Moving beyond the traditional focus on the Schmutzdecke, considerable attention was given to understanding the role of the entire filter system in removing enteric pathogens, DOC and NH4 +. The insights from the depth-specific investigation in both full-scale SSFs at a Dutch drinking water utility and experimental filters in the laboratory yielded two main conclusions.....