comparing the estimated health risk of water from potable reuse projects with conventional supplies using established tools of risk assessment.

The 1982 committee went on to say that the comparison should be made with the highest quality water that can be obtained from that locality even if that source may not be in use. In a similar vein, the 1998 committee, after concluding that planned potable reuse is viable, suggested that planned potable reuse should be, “an option of last resort—to be adopted only if all the alternatives are technically or economically infeasible” (NRC, 1998). All three committees (NRC, 1982, 1984, and 1998) also took the view that U.S. drinking water regulations were not intended to protect public health when raw water supplies were heavily contaminated with municipal and industrial wastewater.

In the committee’s judgment, current circumstances call for a reassessment of those views. First, as shown in Chapter 1, the United States has been operating near the limit of its water supply for several decades since about the time of the first study. As a result of further stress from continued population growth and climate change, this report is being written with a view to providing useful advice to the nation as it comes to terms with this new world where pristine water is ever less abundant, even as the domestic wastewater from an increasing population is discharged into the nation’s waterways. Second, as demonstrated in Chapter 2, the committee concludes that de facto reuse (i.e., when a drinking water source consists of some significant percentage of treated wastewater effluent from an upstream discharger) is becoming increasingly common in the United States. Finally, it has become evident to the committee that, in many communities, today’s drinking water regulations are already being employed to address the quality of drinking water prepared from water supplies that have substantial wastewater content (see also Chapter 10 for a discussion of regulations). Although this fact does not imply that the regulations are adequate for that charge, it does reflect a notable shift in perspectives since the prior NRC reuse reports were written.

THE RISK EXEMPLAR

Under these conditions, the committee judges that it is appropriate to compare the risk associated with potable reuse projects with the risk associated with de facto reuse scenarios that are representative of the supplies that are widely experienced today. The committee chose to construct a “risk exemplar” to examine how these comparisons might be made. The analysis in this exemplar uses the quantitative risk assessment methods originally proposed for organic chemicals by the NRC (1983) as expanded for microbial contaminants (Haas et al., 1999) and more recently updated (NRC, 2009b). Other methods recently developed to address pharmaceuticals, personal care products, and other anthropogenic contaminants (Rodriquez et al., 2007a,b; Snyder et al., 2008a; Bull et al., 2011) are also used in the analysis to address the risk of classes of contaminants for which rigorous toxicological data are lacking. In the committee’s judgment, these risk assessment techniques represent the best means available at this time for estimating the relative risk in such circumstances (see Chapter 6) and offer a method for evaluating the relative merits of various options for managing health risks from chemical and microbial contaminants in reclaimed water.

The committee chose to develop an exemplary comparison of risks associated with various potable reuse scenarios, including de facto potable reuse, modeled upon circumstances currently encountered in the United States today. Based on the discussion in Chapter 2, the committee concluded that it would be appropriate to compare the quality of the water in potable reuse scenarios with the quality of a de facto reuse scenario where a conventional water supply has an average annual wastewater content of 5 percent. This situation is commonly found among current surface water supplies (see Box 2-4). As shown in the figure in Box 2-4, there are many circumstances where de facto reuse exceeds 5 percent, and the committee discussed at length the appropriate wastewater content for use in the exemplar. In the end, 5 percent was selected as a wastewater content that can be reasonably viewed as commonplace and not exaggerated. Swayne et al. (1980) reported that more than 24 million people of the 76 million people surveyed were using drinking water supplies with a wastewater content of 5 percent or more in low-flow conditions (see figure in Box 2-4). Although no data exist, anecdotal evidence based on the population growth in urban areas suggests that wastewater content is often higher today.



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