Giardia and Cryptosporidum in raw and finished water

The American Water System has conducted extensive monitoring of its operations since 1988. Analysis of 347 surface water samples collected between 1988 and 1993 showed that the prevalence rate of Giardia and Cryptosporidium was 53.9 percent and 60.2 percent, respectively. But because the parasite assay does not indicate viability or virulence, these results do not necessarily indicate that these water systems, were at risk from waterborne pathogens. To supplement coagulation and filtration, the average system will have to apply sufficient disinfection to reduce viable Giardia levels by 3.1 log₁₀. An analysis of existing disinfection practices shows that most systems are already applying disinfectant at a level sufficient to reduce Giardia levels. However, the proposed Disinfectants/Disinfection By-products (D/DBP) Rule may hamper the ability of water utilities to apply sufficient disinfection under current operating conditions. Careful integration of the D/DBP and the Enhanced Surface Water Treatment rule is encouraged.