GIARDIA AND CRYPTOSPORIDIUM IN RAW AND FINISHED WATER

The American Water System has conducted extensive monitoring of its op erations since 1988. Analysis of 347 surface water samples collected b etween 1988 and 1993 showed that the prevalence rate of Giardia and Cr yptosporidium was 53.9 percent and 60.2 percent, respectively. But bec ause the parasite assay does not indicate viability or virulence, thes e results do not necessarily indicate that these water systems were at risk from waterborne pathogens. To supplement coagulation and filtrat ion, the average system will have to apply sufficient disinfection to reduce viable Giardia levels by 3.1 log(10). An analysis of existing d isinfection practices shows that most systems are already applying dis infectant at a level sufficient to reduce Giardia levels, However, the proposed Disinfectants/Disinfection By-products (D/DBP) Rule may hamp er the ability of water utilities to apply sufficient disinfection und er current operating conditions. Careful integration of the D/DBP and the Enhanced Surface Water Treatment rule is encouraged.