Physical and biological fate of protozoan (oo)cysts in slow sand filtration: A review

Cryptosporidium is a leading cause of waterborne disease globally, in part due to its chlorine-resistant oocysts, low infectious dose, and ability to cause widespread outbreaks. Multiple barriers are employed in drinking water treatment to reduce risks from Cryptosporidium and other pathogens. Slow sand filtration (SSF), a low-carbon drinking water treatment system, has widely varying log removal values (LRVs) (0.09 to >7.2-log; median 2.64-log across published studies). This has been attributed to filter maturity, sand grain size, filter depth, and hydraulic loading rates. Additional enhancement is linked to the stage of ripening of the biologically active layer (Schmutzdecke), which promotes the physical, chemical, and biological mechanisms responsible for oocyst retention. This review identifies key biological agents for oocyst degradation. Nematodes such as Caenorhabditis elegans ingest hundreds of oocysts per organism, while ciliates including Paramecium and Euplotes show high uptake rates. Rotifers (Bdelloidea, Epiphanes, Euchlanis) further internalize large numbers of oocysts. These organisms could contribute to degradation or sequestration of oocysts, reducing their presence in SSF. However, excreted oocysts can remain infective, so clarifying which fauna truly inactivate them versus those that merely immobilize them is needed. Pathogen load, temperature, and competition from other food sources also shape biological removal in SSF. Integrating physical straining and biological predation may help optimize SSF design and operation for Cryptosporidium control. Selecting or encouraging micro- and meiofauna with high ingestion or digestion capacities may further improve SSF performance, although the current evidence base remains limited and context-dependent. Future research should focus on elucidating the fate of excreted oocysts under realistic field conditions and identifying optimal operational parameters to maximize net Cryptosporidium removal.