interrupted treatment (Lippy and Waltrip, 1984), and similar treatment failures have been noted in more recent years (Herwaldt, 1991). Multiple barriers to contaminant breakthrough are seen as one way of preventing such outbreaks. In addition, increased concern over resistant viruses and protozoa has led the water treatment community to pay more attention to the concept of incorporating multiple barriers to such pathogens in the water system.

Velz (1970) employed the term "multiple barriers" for the concept of providing wastewater treatment when a receiving water is used for water supply. In conventional (non-reuse applications) water treatment, the use of multiple barriers to pathogens within a single facility was advocated by the American Water Works Association Organisms in Water Committee (1987). The multiple barriers concept has, in effect, been embodied in the federal Surface Water Treatment Rule, as well as a number of the state-specific reuse requirements noted earlier in this report. Currently, all processes that help to reduce the risk of waterborne contaminants are referred to as barriers. Thus, watershed protection programs, engineered water treatment processes such as disinfection and filtration, and maintenance of the water distribution infrastructure are all considered barriers to certain types of contamination (even though some of these might cause certain aspects of water quality to deteriorate).

Different types of contamination require different types of barriers. For instance, disinfectants aimed at microbial pathogens do not mitigate chemically based risks and in fact may exacerbate them. Likewise, activated carbon will remove many chemical contaminants but does little to remove viruses. Accordingly, each barrier must be examined separately for its efficacy for removal of each contaminant. The cumulative capability of all barriers to accomplish removal should be evaluated considering the levels of the contaminants in the source water, the nature of the expected health effect associated with the contaminants, the goals that have been set for the potable supply, and any additional safety factors.

The concept of multiple barriers is implicit in the design of many advanced wastewater treatment projects investigating the feasibility of reuse. These projects typically use several physical and chemical barriers to pathogens, which can cause problems if present at high levels for even a short time, but only one or two aimed at chemical contaminants, which must generally be present longer to affect health. One such design is the San Diego project, where a failure in the ion exchange process would probably cause the finished water to exceed the nitrate target until the ion exchange unit was repaired, but where there is no one process whose failure would prevent virus goals from being met.

For drinking water supplies in general, using multiple barriers might mean choosing the most pristine available water source, protecting it from

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