The contribution of deeper layers in slow sand filters to pathogens removal

Shreya Ajith Trikannad; Doris van Halem; Jan Willem Foppen; Jan Peter van der Hoek

doi:10.1016/j.watres.2023.119994

The contribution of deeper layers in slow sand filters to pathogens removal

Water Res. 2023 Jun 15:237:119994. doi: 10.1016/j.watres.2023.119994. Epub 2023 Apr 24.

Authors

Shreya Ajith Trikannad¹, Doris van Halem², Jan Willem Foppen², Jan Peter van der Hoek³

Affiliations

¹ Department of Water Management, Delft University of Technology, Building 23 Stevinweg 1, 2628, Delft, the Netherlands. Electronic address: s.a.trikannad@tudelft.nl.
² Department of Water Management, Delft University of Technology, Building 23 Stevinweg 1, 2628, Delft, the Netherlands.
³ Department of Water Management, Delft University of Technology, Building 23 Stevinweg 1, 2628, Delft, the Netherlands; Waternet, Korte Ouderkerkerdijk 7, 1096 AC, Amsterdam, the Netherlands.

PMID: 37116371
DOI: 10.1016/j.watres.2023.119994

Abstract

Slow Sand Filtration is popular in drinking water treatment for the removal of a wide range of contaminants (e.g., particles, organic matter, and microorganisms). The Schmutzdecke in slow sand filters (SSFs) is known to be essential for pathogen removal, however, this layer is also responsible for increased head loss. Since the role of deeper layers in bacteria and virus removal is poorly understood, this research investigated the removal of E.coli WR1 and PhiX 174 at different depths of a full-scale SSF. Filter material from top (0-5 cm), middle (5-20 cm) and deep (20-35 cm) layers of an established filter was used in an innovative experimental set-up to differentiate physical-chemical and biological removal processes. In the analysis, we distinguished between removal by biological activity, biofilm and just sand. In addition, we modelled processes by a one-side kinetic model. The different layers contributed substantially to overall log removal of E.coli WR1 (1.4-1.7 log₁₀) and PhiX 174 (0.4-0.6 log₁₀). For E.coli WR1, biological activity caused major removal, followed by removal within biofilm and sand, whereas, removal of PhiX 174 mainly occurred within sand, followed by biofilm and biological activity. Narrow pore radii in the top layer obtained by micro-computed tomography scanner suggested enhanced retention of bacteria due to constrained transport. The retention rates of E.coli WR1 and PhiX 174 in top layer were four and five times higher than deeper layers, respectively (k_ret 1.09 min^-1 vs 0.26 min^-1 for E.coli WR1 and k_ret 0.32 min^-1 vs of 0.06 min^-1 for PhiX 174). While this higher rate was restricted to the Schmutzdecke alone (top 5 cm), the deeper layers extend to around 1 m in full-scale filters. Therefore, the contribution of deeper layers of established SSFs to the overall log removal of bacteria and viruses is much more substantial than the Schmutzdecke.

Keywords: Mature biofilm; One-site kinetic model; Pathogen removal; Schmutzdecke; Slow sand filters.

MeSH terms

Bacteria
Escherichia coli
Filtration / methods
Silicon Dioxide / chemistry
Viruses*
Water Purification* / methods
X-Ray Microtomography

Substances

Silicon Dioxide