Association of Cryptosporidium parvum with Suspended Particles: Impact on Oocyst Sedimentation

Abstract

The association of Cryptosporidium parvum oocysts with suspended particles can alter the oocysts' effective physical properties and influence their transport in aquatic systems. To assess this behavior, C. parvum oocysts were mixed with various suspended sediments under a variety of water chemical conditions, and the resulting settling of the oocysts was observed. Direct microscopic observations showed that oocysts attached to suspended sediments. Settling column and batch experiments demonstrated that oocysts are removed from suspension at a much higher rate when associated with sediments. The rate of oocyst sedimentation depended primarily on the type of sediment with which the oocysts were mixed. Changes in background water conditions had a relatively small impact on the extent of oocyst-particle association and the resulting oocyst deposition. We believe that the ubiquitous association of C. parvum oocysts with suspended particles enhances the sedimentation of oocysts in natural waters and that this interaction should generally be considered when predicting the migration of pathogens in the environment.

Cryptosporidium Outbreak (Water Treatment Failure): North Battleford, Saskatchewan, Spring 2001

“…The oocyst stage of cryptosporidium's lifecycle presents a challenge to water treatment (Hsu et al 2001;Medema et al 1998;Peeters et al 1989;Searcy et al 2005). Similar to Giardia spp., this cyst is quite resistant to conventional water treatment.…”

“…Similar to Giardia spp., this cyst is quite resistant to conventional water treatment. The oocyst consists of a matrix of glycoproteins and sugar mol ecules that are strongly resistant to conventional disinfectants such as chlorine (O'Donoghue 1995; Searcy et al 2005). While disinfection with chlorine is very effective against bacteria, it is relatively ineffective for inactivating Cryptosporidium.…”

“…Second, microor ganisms such as Cryptosporidium and Giardia should be treated like particles, as they are best removed via flocculation and sedi mentation. Evidence exists that Cryptosporidium oocysts have an affinity towards particulates and, thus, the sedimentation process is an effective method to remove them (Medema et al 1998;Searcy et al 2005). Finally, a good disinfection procedure incor porating both oxidation and UV light is needed.…”

“…The SCU holds nearly 700,000 L of water and agitation is provided by a turbine mixer with a variable speed drive. Coagulation, flocculation, and sedimentation processes are critical for removing suspended solids from raw water (Fewtrell et al 2001;Hsu and Yeh 2003;Searcy et al, 2005). Settling rates for North Battleford's SCU are typically 10 to 20%.…”

“…Oocysts are very small and average 5 fm but have been known to pass through filters of 1 fm (Finch and Belosevic 2001;Medema et al 1998). However, oocysts are known to be attracted to particles and are removed by flocculation/sedimentation in conventional water treatment (Fewtrell et al 2001;Searcy et al 2005).…”