自1988年以来，美国供水系统对其运行进行了广泛的监测。对1988年至1993年间采集的347个地表水样本的分析表明，贾第虫和隐孢子虫的流行率分别为53.9%和60.2%。但是，由于寄生虫试验没有显示其生存能力或毒性，这些结果并不一定表明这些水系统存在来自水传播病原体的风险。为了补充凝血和过滤，平均系统必须进行充分消毒，以将活贾第虫水平降低3.1 log10。对现有消毒实践的分析表明，大多数系统已经在使用足以降低贾第虫水平的消毒剂。然而，提议的消毒剂/消毒- 产品（D/DBP）规则可能会妨碍水设施在当前运行条件下进行充分消毒的能力。鼓励仔细整合D/DBP和强化地表水处理规则。包括31个参考文献、表格、图表。

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 prevalent 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. Includes 31 references, tables, figures.