Underwater skimming improves retention and degradation of Cryptosporidium oocysts in slow sand filters

Cryptosporidium oocysts are resilient protozoan pathogens that resist conventional disinfection, posing significant challenges to drinking water quality. Filtration processes like slow sand filters (SSFs) effectively remove these oocysts, but limited data exist on their fate in SSFs, particularly following maintenance practices such as skimming. This study examined the spatial and temporal distribution of inactivated Cryptosporidium parvum oocysts in pilot-scale SSF operated under two skimming regimes: dry skimming and underwater skimming. The underwater skim approach offers benefits in terms of production volume gains and reduced downtime, but pathogen removal has not been comprehensively assessed using this approach. Across two 4-day dosing periods, oocyst breakthrough was lower under UWS (UWS: 7.6 % vs DS: 47.0 % of filtrate samples were positive for oocysts). In addition, core samples were collected at six time points to track oocyst retention and vertical migration. In both underwater skim and dry skim slow sand filters, most oocysts were captured in the top 100 mm of the filter, gradually moving downward over time. Notably, underwater skim filters retained more oocysts in the upper layers than dry skim filters, resulting in lower breakthrough frequency. Although skimming did remove some oocysts in both regimes, the majority were rendered undetectable in situ through processes such as predation, enzymatic digestion, and natural decay—evidenced by the increasing proportion of oocyst-like bodies and their near-complete absence from the filtrate. Thus, underwater skim is a viable alternative to dry skim for Cryptosporidium removal, sustaining filter performance by trapping oocysts in the upper layers and maintaining similar rates of oocyst degradation. These insights support improved SSF maintenance strategies that enhance pathogen removal.