Water Treatment > Quality Water Solutions > Treatment Issues > Microbial Considerations

Among the many microorganisms that can occur in drinking water, Cryptosporidium and Giardia have received a great deal of attention in recent years because they are widely distributed in surface waters, occur at many U.S. surface water plants and are hard to remove. Even plants with low Cryptosporidium and Giardia counts can have peak events at times of high runoff that can overwhelm treatment regimens and allow these organisms to enter the distribution system.

Utilities can keep these and other pathogens out of tap water by creating multiple barriers to their passage that combine disinfection with coagulation, sedimentation and filtration. Improved particulate removal strengthens the performance of disinfection. In general, well-controlled and operated filtered water systems provide better multiple barriers to a range of microorganisms. In addition to Cryptosporidium and Giardia, these include Microsporidium, which are smaller and less known but are causing concern among some researchers, as well as bacterial and viral pathogens less than 1 mm in size.

Experience at many water systems shows that turbidity and particle count reduction and chlorination together greatly reduce the threat from Cryptosporidium and Giardia. In fact, the risk of Cryptosporidium and Giardia outbreaks is thought to be greatly reduced if finished water particle counts are less than 1 per 100 mL in the 3-to-10 mm size range. (Cryptosporidium oocysts are generally 3-to-7 mm and Giardia cysts, 5 to 10 mm.)

This is a troublesome range because colloids and ultra-fine particles that fall within it can pass through conventional filters, even if filtered water turbidity is low (less than 0.1 NTU). While tighter filters, reverse osmosis and separation membranes can capture Giardia, Cryptosporidium and other organisms, throughput can fall dramatically. Coagulation can sweep these particles into a floc that allows the use of higher efficiency filtration with no loss in flow rate.