current and future contamination, providing multiple engineering processes to remove contaminants in a water treatment plant (e.g., pre-oxidation, coagulation, filtration, and disinfection), and protecting the water quality from deterioration in the distribution system (e.g., by adding corrosion inhibitors and additional disinfectant and keeping the water under pressure to prevent contaminated water from entering the system). In places where reclaimed water is used to augment natural supplies or where source water cannot be protected from upstream discharges of water with impaired quality, the importance of the barriers at the water treatment plant or wastewater reclamation plant is correspondingly increased.

Evaluating Barrier Independence

The independence of multiple barriers is a key aspect of system reliability and safety, especially for removal of pathogens. The greater the degree of independence among different barriers, the more they can be relied upon to serve as backups for one another. The water treatment process train should incorporate multiple, independent treatment barriers of sufficient redundancy that required contaminant removal levels will be achieved even if the single most effective treatment barrier is not performing. For example, sedimentation and filtration should not be considered independent barriers if the success or failure of both depends on proper coagulation prior to the sedimentation step. Failure in this type of system contributed to the 1993 Cryptosporidium outbreak in Milwaukee. On the other hand, design of a sufficiently deep intake pipe for surface water extraction and the use of disinfection are independent barriers to microbial contamination.

Individual treatment barriers (or unit processes) should be evaluated individually and collectively with regard to their capacity for contaminant removal and their prospects for failure. Analysis of the contaminant-removal capabilities of independent barriers involves several steps. First, the contaminants of concern are identified, and reasonable maximum and target levels are determined. (The difference between target level and maximum level provides a margin of error.) Based on information on contaminant levels in the source water, the necessary contaminant reduction, usually expressed as logs of removal, is estimated. Each treatment process is evaluated for its removal capability, and this information is used to estimate the overall removal to be accomplished by the process train. Finally, an estimate is made for how much the overall removal will be compromised if the single most important independent barrier (unit process) were to fail.

Table 6-1 illustrates this analysis for a hypothetical comparison of



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