Using Graywater and Stormwater to Enhance Local Water Supplies: An Assessment of Risks, Costs, and Benefits 

A key issue in evaluating the merits of the potential beneficial uses of greywater and stormwater pertains to what benefits are generated, and how those benefits compare to the costs. This is not a simple exercise, and it is not possible to draw broadly generalizable inferences for the following reasons: (1) many different types of benefits and costs may be relevant, (2) the types and magnitudes of benefits and costs typically very use- and site-specific, and (3) the benefits and costs may be borne by a wide range of different individuals and entities. Each of these key issues is discussed in turn.PDF] Using Graywater and Stormwater to Enhance Local Water Supplies: An  Assessment of Risks, Costs, and Benefits | Semantic Scholar

Furthermore, many of the potentially important benefits are difficult to quantify and value in monetary terms (i.e., they consist of what economists refer to as “nonmarket values”). Indeed, many of the motives for pursuing beneficial use projects for greywater or stormwater include aspirational and other values that extend beyond financial returns and water supply benefits. These facts may make it challenging to provide a viable and fair comparison of the key benefits of a stormwater or greywater project to its costs: the costs are generally identified and monetized, but many key benefits may not be readily amenable to monetization.1 This may create an unfortunate imbalance in how beneficial use projects are perceived and evaluated, unless considerable effort is applied to fully recognize, account for, and estimate the full range of important, applicable benefits.

RANGE OF POTENTIAL BENEFITS AND COSTS

The types of benefits and costs associated with graywater or stormwater use are highly diverse. Although the costs are primarily financial, the benefits may be financial, social (i.e., related to health, well-being, and quality of life),2 and environmental. The benefits may be broadly distributed or limited to the entity implementing the project.

Financial costs refer to the total out-of-pocket expense borne by whoever is paying for a graywater or stormwater project. Financial benefits may include returns to a homeowner or municipality in terms of avoided costs (e.g., reduced purchases of potable or imported water or delayed infrastructure upgrades) or other monetary returns (such as avoided fines for stormwater violations). Environmental benefits may accrue because of potential water-related savings in energy use and related emissions of carbon dioxide and other air pollutants associated with energy generation. Additionally, stormwater capture and use can reduce harm to local watersheds from otherwise poorly managed flows. Social benefits may include providing a community with aesthetic improvements due to increased green space or reducing traffic disruptions associated with periodic urban street flooding during intense rain events. Public education and increased individual awareness of local water supply and related issues may be considered as social benefits—although ones that can be particularly difficult to quantify and monetize. Box 7-1 provides an overview of many of the types of financial, social, and environmental benefits and costs that may be derived from a given graywater or stormwater application.

Many key benefits (and costs) can be highly site specific. In locations where water is relatively scarce, tapping graywater or stormwater resources is likely to reduce the demands on potable or other supplies that may be very expensive (at the margin) or associated with environmental stressors (e.g., diminishing stream flows to levels that place

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1 Various nonmarket valuation approaches may be applied on a case-specific basis to estimate many of the potential benefits. These approaches include hedonic or other revealed preference techniques and/or survey and related stated preference methods (Adamowicz et al., 1998Alcubilla and Lund, 2006Cadavid and Ando, 2013NRC, 1997Poor et al., 2007; Van Houtven, et al. 2014). The challenge is that such methods can be complicated, data intensive, and costly to apply properly, and need to be performed on a case- and site-specific basis.

2 This is sometimes referred to as the Triple Bottom Line (TBL) framework. In the field of economics, the term “social values” is used to include all the possible benefits (and costs), including both the internal, financial aspects borne by private individuals and entities, as well as the broader array of external impacts that may be reflected under the “social” and “environmental” categories used in this chapter, but this definition is not used in this report.

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