the following design elements discussed in the previous section: (1) attenuation of contaminants of concern, (2) provision of retention time, and (3) blending (or dilution). The performance of various environmental buffers is discussed in Chapter 4.
Attenuation of contaminants can occur in certain environmental buffers (e.g., wetlands, soil aquifer treatment, riverbank filtration). In this function, an engineered natural treatment system can be used before or after an aboveground water reclamation plant. However, the role of environmental buffers in attenuation of contaminants is not well documented. As detailed in Chapter 4, contaminant attenuation has been reported for some environmental buffers. However, considering site-specific differences, environmental buffers are likely to exhibit some variability in performance with respect to contaminant attenuation.
There is no widely accepted standard for retention time in environmental barriers for potable reuse systems. The retention provided by various examples discussed in this report varies from days to more than 6 months. Retention is particularly uneven where de facto reuse is concerned. Additionally, relying on environmental buffers as the only means of lengthening response times is questionable, especially in systems with short hydraulic residence times.
For potable reuse projects implemented through groundwater recharge, blending or dilution of reclaimed water with water deemed not to be of wastewater origin can occur before application or in aquifers. For surface water augmentation, blending typically occurs in a raw drinking water reservoir. The extent of dilution varies with the different natural systems, and can range from substantial dilution (<1 percent reclaimed water) to minimal dilution (>50 percent reclaimed water). As mentioned before, the need for blending depends heavily on the nature of the process train employed for attenuation.
Currently, the use and application of an environmental buffer for potable reuse is based on regulatory guidance and current practice rather than specific scientific evidence. Sufficient science does not currently exist to determine if current guidance is, in fact, appropriately protective, overprotective, or underprotective of public health. From a public outreach perspective, environmental buffers have often been perceived as important for gaining public acceptance as they create the perception of a “natural” system and provide time to respond to potential problems should they arise (Ruetten et al., 2004). NRC (1998) described a “loss of identity” that occurs in an environmental buffer, although the committee noted that “loss of identity is an issue that seems more relevant to public relations than public health protection” (NRC, 1998).
During the past decade, extensive research on the performance of reuse operations using modern engineered systems (Ternes et al., 2003; Drewes et al., 2003b; Snyder et al., 2006c; Bellona et al., 2008) as well as those using environmental buffers (Fox et al., 2001; Laws et al., 2011; Maeng et al., 2011) has demonstrated some engineered systems can perform equally well as some existing environmental buffers in diluting and attenuating contaminants, and the proper use of indicators and surrogates in the design of reuse systems offers the potential to address many concerns regarding quality assurance (Drewes et al., 2008). This committee concludes that the practice of classifying potable reuse projects as indirect and direct based on the presence or absence of an environmental buffer is not meaningful to an assessment of the final product water quality because it cannot be demonstrated that such “natural” barriers provide any public health protection that is not also available by other means. Moreover, the science required to design for uniform protection from one environmental buffer to the next is not available.
Accordingly, although the committee does view environmental buffers as useful elements of design that should be considered along with other processes and management actions in formulating potable water reuse projects, the committee does not consider environmental buffers to be an essential element of potable reuse projects. Rather than relying on environmental buffers to provide public health protection that is poorly defined, the level of quality assurance required for public health protection needs to be better defined so that potable reuse systems can be designed to provide it, with or without environmental buffers. A more quantitative understanding of the protections provided by different environmental buffers will allow engineered natural systems to be more effectively designed and operated.
Case Studies for System Design
The role of the design elements mentioned earlier (monitoring, attenuation, retention, blending) can be illustrated using three case studies that practice drink-