Cryptosporidium is a protozoan parasite commonly found in the waste of most mammals. Four species are recognized; C. parvum is the major species responsible for causing cryptosporidiosis, a gastrointestinal illness, in humans and animals. Cryptosporidium oocysts (parasitic "eggs") are ingested and invade the intestinal tract, where the organism completes its life cycle. Oocysts are 3-5 microns in diameter and possess a tough outer shell that makes them environmentally stable. Oocysts are shed in the feces and transmitted via the fecal-oral route.

Symptoms of cryptosporidiosis linger for two to four weeks in healthy people and include nausea, diarrhea, and cramps. Cryptosporidium is similar to the Giardia organism, but it is more resistant than Giardia and about one-half the size. Cryptosporidium is, therefore, harder to destroy and more difficult to physically remove from water supplies than Giardia.

There is no standard analytical method for Cryptosporidium; results can vary significantly among laboratories. A complicating factor is that current analytical methods do not discriminate between viable and non-viable oocysts. Due to the inherent problems associated with monitoring and analysis, continuous monitoring of surrogate Cryptosporidium parameter is required to ensure the safety of potable water supplies. Particle counting offers the greatest potential for surrogate monitoring.

All surface water supplies are susceptible to contamination with Cryptosporidium in various degrees. Water plants located on rivers are subject to fluctuations in raw water quality due to rainfall events, and intensively farmed areas are the most likely sources of Cryptosporidium. Therefore, plants located on rivers draining agricultural areas are considered to have the greatest risk of contamination. Municipal groundwater supplies are generally considered to be at much lower risk of contamination than surface water supplies.

Disinfectants such as chlorine and chlorine dioxide are not effective in inactivating the oocysts; results of recent ozonation studies are encouraging. Therefore, most plants providing disinfection alone are at considerable risk.

The most effective treatment for water plants is physical removal of the oocysts by coagulation, flocculation, sedimentation, and filtration. (Phipps & Bird emphasis)

The degree to which a plant is properly operated and optimized largely determines its effectiveness against Cryptosporidium. Typical measures may include source protection, intake shutdowns, continuous turbidity monitoring, elimination of hydraulic surges, optimized backwash procedures, media inspections, and elimination of backwash supernatant recycling.

Reprinted with the permission of the author, Nikolay Voutchkov, Malcolm Pirnie, Inc., and the American Water Works Association/ Virginia Section Newsletter, May 1995


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