A key issue in evaluating the merits of the potential beneficial uses of graywater 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 are 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.
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 graywater 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 graywater 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.
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
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., 1998; Alcubilla and Lund, 2006; Cadavid and Ando, 2013; NRC, 1997; Poor 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.
special status fish species at risk, creating a large carbon footprint from seawater desalination). In such settings, the benefits of graywater or stormwater use may include both financial benefits and an array of benefits to the broader human and natural community. These same benefits would not be experienced in a location with abundant conventional water supplies.
Benefits and costs are also highly dependent on the specific type of application (i.e., end use) and the scale of the application. Graywater is typically reused through simple systems at the household scale, but large multi-residential properties and even regional systems are possible (see Table 2-1 and Box 2-1) and involve more costly treatment and dual-distribution systems. Stormwater applications can also vary widely in scale, ranging from household to neighborhood to regional levels. The scale will impact relative costs (e.g., because of economies of scale) and also the level and types of benefits generated.
When examining the costs and benefits of graywater and stormwater use projects, who bears the costs and who realizes the benefits are additional important considerations. For example, when a project generates significant external benefits (such as environmental benefits) that are not fully captured by those who bear the costs, how should the costs be equitably shared? In some cases, subsidies may provide a financial incentive for providing broader benefits. The distribution of the benefits and costs must address concerns about affordability, social equity (fairness), and environmental justice.
This section presents what is known about the financial costs of graywater and stormwater projects. Ideally, financial costs should account for the full costs during the life cycle of the project, including initial installation (e.g., equipment, construction, permitting), annual operation and maintenance (O&M) over the full course of the project’s effective operating lifetime, any capital replacement required during the asset’s effective life, and any decommissioning and subsequent replacement costs at the end of useful life. These costs can be presented as total annualized costs (with capital expenses converted to equivalent amortized annual values, and added to annual O&M expenses) or as a total “present value” of all project costs in current dollars. However, there is a dearth of well-documented data on the full costs of such projects, particularly their life-cycle costs, because most are new with minimal data about long-term maintenance costs and performance effectiveness.
It is important to include all costs, regardless of who bears them, and to keep track of who bears which costs. For example, a household may bear much of the expense of installing a graywater system that taps its laundry machine and diverts the effluent to a subsurface drip system irrigating its garden. However, the local municipality and/or a local nongovernmental organization may provide subsidized support for these activities, and these expenses must be included in a full accounting of financial costs.
Factors That Affect Financial Cost
Several key factors impact the costs of graywater and stormwater beneficial use projects. These factors include whether treatment is provided, the type and level of treatment, and the scale of the application (including size of storage). For simple, household-scale, laundry-to-landscape systems (Box 2-4) where untreated graywater is conveyed without additional pumps to a subsurface irrigation system, capital costs are fairly minimal (primarily labor), with little if any O&M costs. In a multi-residential apartment building, graywater reuse for toilet flushing requires treatment (see Chapter 6) as well as some mechanical pumping and more elaborate piping and storage investments. Treatment costs may vary depending on the water quality objectives, which may be governed by regulations that vary from state to state or even by locality (see Chapter 8). Costs will also be influenced by whether the application is part of a new development or a major renovation effort, or a retrofit into an existing structure or established neighborhood. Retrofit on-site capture and use projects are typically considerably more expensive than installations in new developments.
Stormwater projects will also vary widely in their financial costs, depending on the scale, location, and the type and extent of conveyance and treatment (if any) that may be necessary. When stormwater is collected and infiltrated into local groundwater basins in the same general proximity in which it is collected, the costs will be less, all else being equal, than when stormwater is harvested at one location and pumped a considerable distance to another location where it can be more suitably put to direct beneficial use. Stormwater infiltration projects may often require the purchase of land, which varies widely in price. The capture and use of stormwater in some states may also require the purchase of water rights from downstream users.
Summary of Cost Data
This section summarizes the limited available data on the financial costs of graywater reuse and stormwater capture and use at the household and neighborhood scales. Most available data are focused on the household scale. Some data on regional-scale stormwater capture for groundwater recharge are also included, although such project costs would be highly
site specific. Monetary values reported in this chapter have been adjusted to 2014 U.S. dollars, unless stated otherwise.3
Household-scale Graywater Reuse
Graywater system costs at the household scale vary widely depending on the size and complexity of the system. Information on installing do-it-yourself laundry-to-landscape systems is widely available online, and some water utilities provide subsidies for material costs. San Francisco Public Utilities Commission (SFPUC, 2012) reports that laundry-to-landscape systems cost a few hundred dollars for a self-installed system and $1,000-$2,000 for a professionally installed system. SFPUC has offered a $112 subsidy toward a $117 graywater kit that can be installed by a homeowner without plumbing skills and without a permit (P. Kehoe, SFPUC, personal communication, 2014). Costs increase if a pump or filtration system is also needed and with professional installation (see Box 7-2). Nonmonetary costs arise when homeowners devote their own time to installing the system (which may be valued in monetary terms based on a prevailing wage rate).
The financial costs of household-scale graywater systems increase with complexity, especially if they are professionally installed and/or must be professionally maintained. Some systems that are more complex than laundry-to-landscape kits can still be installed by homeowners with basic plumbing and electrical skills, reducing out-of-pocket installation costs but requiring investment of the homeowner’s time. SFPUC (2012) reports that a simple whole-house graywater system with a tank and pump, but no filtration costs between $500 and several thousand dollars, depending on the extent of plumbing work and whether the system is professionally installed. Adding a sand filter system (but no disinfection) increases per household costs into the range of $5,000 to $15,000 (SFPUC, 2012).4 Although cost data are limited, one whole-house graywater collection system for irrigation with professionally installed dual plumbing (installed during construction) and homeowner-installed tank and pump costs $2,300 for a five-bedroom house, with the bulk of that cost ($1,700) being associated with the dual plumbing system (L. Roesner, Colorado State University, personal communication, 2015). A smaller home would have lower plumbing installation costs. The costs of retrofitting dual plumbing in an existing home would be substantially higher.
Neighborhood and Regional Graywater Reuse
The costs associated with graywater systems naturally increase as they are scaled up to neighborhoods and regions, although economies of scale may occur. In multi-residential settings, graywater is commonly used for both landscape irrigation and toilet flushing. The costs for graywater reuse at the neighborhood scale depend on several factors, including the type and scale of graywater treatment, the extent of dual-plumbing required (including whether dual collection for irrigation or dual collection and distribution for toilet flushing is used), and whether the plumbing is installed as part of a new construction project or as part of a retrofit into an existing building. Extremely limited cost data are available on neighborhood-scale graywater projects.
Hodgson (2012) breaks down capital and O&M costs for 14 different graywater system sizes (50-5,000 gpd; 190-190,000 1pd) and for various disinfectant schemes (see Table 7-1). For example, for a 1,000 gpd (3,800 1pd) system (capable of treating enough graywater to provide toilet flushing for about 150 to 160 people in a dormitory-type setting),5 total storage and treatment costs (plumbing not included) amount to $4.39 per 1,000 gallons (Kgal). As shown in Table 7-1, initial and annual operating costs increase with capacity, but units over 100 to 300 gpd (380 to 1,100 1pd) achieve economies of scale that significantly reduce the cost per unit of water (Hodgson, 2012). These costs are comparable to or lower than potable water rates in many cities,6 and they are significantly lower if combined wastewater charges are considered. However, the plumbing cost, which is omitted but may be the most expensive component, especially for a retrofit application, would also need to be taken into account. An analysis of costs of graywater use in a mixed-use development ranging from 40,000 to 500,000 ft2 (3,700 to 46,000 m2) revealed that installation of dual collection and distribution plumbing during building construction represented 77 to 93 percent of the capital costs for the project (RMC Water and Environment, 2012), although Cordery-Cotter and Sharvelle (2014) reported lower plumbing costs for a small dormitory-scale project (see Box 7-3).
An additional potential effect of wide-scale graywater use (if not accompanied by population growth) within existing development is the reduction of wastewater discharges to sewer systems, which may increase costs for wastewater utilities if reduced flows impede the in-sewer transport of wastes to the wastewater treatment facility and cause other problems in the sewer network. Reuse of graywater not only reduces the combined wastewater flow but also increases concentrations because most of the wastewater constituents must still be conveyed for treatment (see Box 3-3). Reduced flow decreases flow velocity in wastewater collection systems, which results in increased residence time, potentially
TABLE 7-1 Cost of Graywater Treatment and Storage Systems of Varying Scale, Including a Coarse Filter and Chlorine Disinfection System (Plumbing Excluded)
|System Size (gpd)||Capital||Annual Chemical and Energy||Annual Maintenance||$/Kgal|
NOTES: Costs have been adjusted to 2014 dollars. The costs of dual plumbing and distribution plumbing during new construction projects (omitted in this table) have been reported to represent 77 to 93 percent of the cost for a large commercial graywater project (RMC Water and Environment, 2012), although the committee was unable to find detailed data on plumbing costs across a range of project types. Life-cycle costs are reported by Hodgson based on a 3 percent discount rate and a 10-year effective lifetime.
SOURCE: Hodgson (2012).
causing solids sedimentation, anaerobic conditions (septicity), odor and corrosion problems, and increased frequency of sewer blockages.
These effects on wastewater flows are already resulting from increased indoor water conservation from water saving devices. By comparison, the impact of graywater reuse within existing development is anticipated to be minor because of the challenge of retrofitting buildings for indoor nonpotable reuse. In new development, however, wastewater collection systems can be designed to accommodate reduced flows where significant graywater reclamation and reuse is planned. A full accounting of costs would consider increased wastewater system costs associated with widespread graywater implementation, although such an assessment has not been conducted. Widespread potable water use reductions, if they arise across the community, might result in the need for utilities to increase their water and wastewater rates to cover their fixed costs, potentially offsetting financial savings enjoyed by residents.
Harvesting Roof Runoff
Household roof runoff harvesting systems typically consist of rain barrel or cistern collection systems (see Chapter 6). The cost of a residential-scale rain barrel is estimated to be in the range of $60 to $100. Some of this cost may be provided through utility subsidies. Rain barrels vary in size and material, with a typical barrel holding up to 35 gallons (130 liters) of roof runoff (Pitt et al., 2011). O&M costs are negligible.
For larger cistern-based household systems, data from Pitt et al. (2011) for two homeowner-constructed rainwater harvest systems indicate an upfront cost of about $1,500 to $1,600, although costs vary with the extent of new collection infrastructure required and the inclusion/exclusion of treatment and pumps. One such system is located in Montana and entails a 2,500-gallon (9,500 liter) storage tank drawing water collected from a 925-square-foot (86 m2) roof, including a first flush diverter and a pump. The storage tank cost $900 and the gutters cost $380, comprising the majority of the $1,500 initial cost. The second system, located in Portland, Oregon, and approved by the city for all household water uses, includes a 1,500-gallon (5,700 liter) tank with filters ($900), ultraviolet (UV) disinfection ($380), backflow prevention device ($130), and other components, for a total cost of $1,600. Labor costs for installation (either by homeowner or contractor) are not included.
Several commercial enterprises provide roof runoff capture systems for residential buildings. No published costs are available, but these systems generally cost up to about 10 times the cost of the above homeowner-built systems (Pitt et al., 2011). The advantages of the commercial systems are the vendor’s relationships with local regulators and knowledge of the regulations, professional design and installation, and greater confidence concerning safety issues. Because local and regional regulations pertaining to rainwater harvesting and its use vary greatly throughout the country, the extra service provided by the commercial suppliers of these systems may be beneficial.
Rooftop stormwater harvesting may be applied in larger contexts, such as commercial buildings or building complexes. For example, the Frankfurt Airport in Germany installed six 26,000-gallon (100-m3) cisterns to capture rooftop runoff for toilet flushing and outdoor irrigation. The system cost $109,000 and saves 26 million gallons (98 million liters) per year for an effective cost of $4.19/Kgal (Pitt et al., 2011). O&M costs are not available.
Los Angeles Department of Water and Power (2015) reports that the highest costs per unit of water captured are associated with “on-site direct use” (Figure 7-1). Reported costs ranged from approximately $3,200/acre foot (AF) to nearly $14,000/AF, with a mean of more than $7,000/AF (or $21/Kgal) captured.
Neighborhood Stormwater Capture and Use
Cost estimates for a wide range of stormwater capture or groundwater recharge projects at household and neighborhood scales are provided by Los Angeles Department of Water and Power (LADWP) in Figure 7-1. LADWP (2015) reports the cost thresholds for projects identified for investments (less than $1,100/AF [$3.40/Kgal] for infiltration or less than $1,550/AF [$4.80/Kgal] for direct use), which are generally competitive with other new water supply options available in southern California (see Figure 7-2). The cost threshold for infiltration is lower because it does not reflect an analysis of how much of the stormwater is recoverable, and the actual cost per acre foot of recovered water is likely to be higher.
Of the alternatives, infiltration projects were significantly lower in costs than the stormwater tank capture and use projects, and neighborhood (“subregional”) scale infiltration projects are reported to be the most economical of the alternatives (approximately $1,000/AF [$3.10/Kgal] average). Neighborhood-scale tank storage and use projects (“subregional direct use”) were considerably more costly, ranging from approximately $1,200 to nearly $7,000/AF captured, with a median of $3,300/AF ($10/Kgal) considering life-cycle costs (LADWP, 2015). By comparison, imported water through the State Water Project costs in the range of $450 to $1,300/AF (or $1.40 to $4.00/Kgal) depending on pricing “tier” and treatment. The costs of imported waters, however, are increasing at a rate greater than general inflation, and the water is not always available for communities wishing to acquire additional supplies. Other new supply options such as desalination or water reuse typically cost between $950 and $2,200/AF ($2.90 to $6.75/Kgal) or more, as shown in Figure 7-2 (Raucher and Tchobanoglous, 2014; Sunding, 2013).
Regional Stormwater Capture
The costs of large centralized stormwater capture projects are highly location specific, and land acquisition requirements and the suitability of existing flood water capture and conveyance systems for a stormwater capture project significantly affect overall costs. Regional-scale stormwater capture and recharge projects may be more cost-effective where existing flood management basins and related facilities can be upgraded rather than developing new facilities with associated land acquisition costs.
Existing publicly available cost data for regional stormwater capture are limited to southern California, so the full range of costs in other locations may not be represented. In an analysis of 118 stormwater projects in southern California, West et al. (2014) observed clear economies of scale among the larger stormwater capture projects (Figure 7-3). LADWP (2010) provides details on 11 large-scale stormwater capture and recharge projects, which were designed to provide an estimated 26,000 AF/year (32,000 m3/year) of additional groundwater recharge (essentially doubling the region’s prior stormwater capture and recharge levels), at a combined capital cost of $251.9 million (assumed to be reported in 2010 dollars). LADWP (2010) estimates life-cycle costs at $60-$300/AF ($0.18-$0.92/Kgal) for these projects. By comparison, LADWP (2015) analyzed 35 large centralized stormwater capture projects and reported a range of life-cycle costs from approximately $100/AF to $4,200/AF, with an average of approximately $1,000/AF (or $3.10/Kgal). Compared to other new water alternatives in Figure 7-2, these costs are very reasonable. A more case-specific benefit-cost comparison for enhancement of a single, large-scale, regional stormwater capture and recharge project is provided in Box 7-4. The neighborhood-scale example in Chapter 2, Box 2-6, is illustrative of how community amenities and other nonmonetized co-benefits influence decision making for a major investment in a new stormwater capture system.
Financial benefits include the monetary (i.e., cash flow) savings or earnings that accrue to those that implement or otherwise benefit from a beneficial use of graywater or stormwater. There may also be monetary and nonmonetary benefits and costs that accrue to parties other than those that implement the project, and these impacts also need to be properly reflected in a comprehensive assessment of total benefits and costs. Environmental and social benefits are described later in the chapter.
Financial Benefits from Household-level Applications
For a typical household-level application such as the use of graywater or on-site captured stormwater for landscape irrigation, the financial benefits primarily include the savings the user realizes if these practices lower their potable
water use (assuming there is not an offsetting water utility rate increase). Where wastewater charges are linked to metered potable use, lower potable use may also translate into savings on wastewater charges as well. If the household is on a private well or septic system, they may save some of the operating and maintenance expense associated with running their own on-site supply and wastewater systems, although the effect of diverting graywater on the life of septic systems remains unclear.
Calculating the potable water savings from graywater or stormwater use requires information on end uses (irrigation or additional uses, such as toilet flushing), seasonal irrigation requirements of landscape, and graywater or stormwater availability. The discussion in Chapter 3 presents a scenario analysis of potential water savings based on conservation irrigation of turfgrass in a medium-density development in six U.S. regions, but many other scenarios exist.
The committee’s analysis showed potential for 9 to 19 percent reduction in water use from whole-house graywater for conservation irrigation and 13 to 26 percent potable water savings where graywater is used for both toilet flushing and irrigation (see Chapter 3). This analysis assumes that the availability of a low- or no-cost water supply does not change household water use—an assumption that remains untested (see Box 3-2 for discussion of other assumptions and uncertainties of the analysis). Water savings are lower for laundry-to-landscape irrigation systems, with only 4 to 8 percent water savings in the committee’s analysis (see Table 3-2). The amount of graywater from washing machines depends considerably on the volume of water used per load, with a typical front-loading washer using 20 gallons (SFPUC, 2012). Water savings will also depend on the fill volume settings that the user selects, the number of loads per week, and the number of uses where the drain water is diverted to the sewer or septic system (e.g., when diapers have been washed). Systems that incorporate bathtub, shower, and bathroom sink graywater may provide water savings up to 19,000 gallons (72,000 liters) per household per year (assuming 2.5 persons per household).
Actual financial benefits are determined by the extent to which graywater actually offsets potable water irrigation demand. In locations where irrigation is only needed during one-half of the year, financial benefits would only accrue during periods when irrigation is needed. Therefore, the highest financial benefits from graywater systems are associated with year-round (or near-year-round) water demand, such as for toilet flushing and laundry or for irrigation in the Southwest or central United States (see Chapter 3). Example analyses of financial benefits from household-scale graywater use for irrigation and dormitory use for toilet flushing are outlined in Boxes 7-2 and 7-3.
Additional financial benefits may be derived from using graywater as a drought-proof irrigation source. For example, using graywater for landscape irrigation may enable households to maintain plantings in drought periods when water use curtailments would otherwise preclude outdoor watering. Savings may be realized by avoiding the need to replace landscape vegetation after droughts.
Water savings from stormwater capture projects will depend on local climate conditions, the size of the capture tank(s) (see Chapter 3), and the degree to which the beneficial use of stormwater offsets potable water demand. When used for conservation irrigation, two 35-gallon (130-liter) rain barrels generated potable water savings of 1 to 5 percent in the committee’s scenario analysis, and a 2,200-gallon (8,300 liter) tank for irrigation resulted in savings of 4 to 21 percent, depending on the region of the country. When both irrigation and toilet flushing were considered, the larger tank resulted in potential potable water savings from 5 to 28 percent. In all cases, financial benefits depend on the user actually reducing potable water demands. In addition, financial benefits may be decreased if potable water rate adjustments are needed because of reduced demand to cover fixed utility costs. Financial benefits from rain barrels and cisterns are described in Boxes 7-5 and 7-6.
Financial Benefits of Neighborhood- and Regional-scale Applications
Water savings from neighborhood- and regional-scale graywater reuse projects depend heavily on the capacity of the system to offset potable water demand, in addition to local and imported water rates. Larger building-wide graywater systems often see economies of scale over smaller systems. Hodgson (2012) compared graywater reuse in multi-residential buildings against local water rates across eight major U.S. cities and found that in the case of a 1,000 gpd (3,800 1pd) system, graywater reuse provided water savings that would cover treatment and storage costs in 0.7 to 4.6 years. However, Hodgson (2012) does not include plumbing costs, which would extend payback periods (perhaps considerably), given that retrofit plumbing costs may represent 80 to 90 percent of project costs (RMC Water and Environment, 2012).
In multi-family dwellings where individual households pay the water bill, the financial benefits of graywater or stormwater use may not accrue to those who incur the costs of installation and maintenance (unless these costs are somehow conveyed to the households). If the benefits and costs accrue to different parties, then there may be less incentive to pursue such investments.
Neighborhood-scale stormwater groundwater replenishment projects may be associated with significant net cost savings to water utilities compared with the cost of other new water sources (Figures 7-1 and 7-2). Utilities can then rely on a greater volume of extractable local groundwater in lieu of far more expensive imported surface waters.
At a more regional scale, water supply cost savings may arise at the utility and community levels in a number of ways, including avoided or deferred large-scale capital investments in developing new (or expanding existing) water supply and/ or water treatment facilities. These benefits may arise where graywater or stormwater use offsets sufficient quantities of potable water demands such that expensive, large-scale investments in existing conventional water infrastructure can
be postponed, downsized, or avoided (e.g., see Box 2-1). Savings in O&M expenses can also be realized based on the potential offset in potable water demands.
An example of water supply cost savings arises in the context of southern California, where developing local water resources (including graywater and stormwater) can offset the amount of costly water imports via the State Water Project. Box 7-4 provides a benefit-cost analysis of one large-scale regional stormwater recharge project in the Los Angeles region, in which the $4 million in present value costs to improve an existing facility is offset by $16 million in present value benefits in the form of avoided water imports, as well as a reduced carbon footprint, and several important nonquantified social and environmental benefits.
Similar types of financial savings may be realized when stormwater or graywater management and beneficial use reduce wastewater system costs. For example, reduced volumes of graywater or stormwater inflow into combined sewer systems may defer investments in expanding wastewater conveyance or treatment plant capacity. These costs would be highly site-specific but could amount to large cost savings if major infrastructure upgrades could be avoided. A potentially significant cost savings may be realized when stormwater capture and use enables a community to cost-effectively address combined sewer overflow (CSO) or municipal separate storm sewer system (MS4) compliance issues. Many municipal CSO-related consent decrees are associated with compliance costs of $1 billion or more, not including legal fees. Therefore, opportunities exist for significant potential cost savings if beneficial use of stormwater can reduce CSO and other stormwater-related compliance issues and costs.
Another financial benefit of stormwater capture and use is associated with stormwater control credit trading. The District of Columbia has implemented a market-based stormwater control credit trading system,7 in which developers can buy credits as needed to meet local stormwater management
mandates, or sell credits when stormwater control systems are implemented that exceed regulatory requirements. The availability of stormwater credits will provide fiscal gains for those parties who can manage stormwater in a less costly manner through decentralized projects.
Beyond the range of financial costs and benefits, there is a broad array of potential social benefits that may arise from the beneficial use of graywater or stormwater, as listed in Box 7-1. Not all of these benefits and costs will apply in every application or location; thus, care should be taken to identify when and where such benefits and costs apply, and whether they may be of appreciable value in that site- and scale-specific context.
Households may opt to engage in on-site graywater or stormwater use because of other values and goals that are important to them, such as the aesthetic value enjoyed from any additional or improved landscaping or garden areas attributable to the graywater- or stormwater-supplied irrigation. These nonfinancial benefits to the implementing households may be quite important and may be sufficiently valuable to the individuals to motivate their engaging in these activities regardless of the potential for no direct financial net gain.
Widespread beneficial use of graywater or stormwater use across a community could collectively yield sufficient water savings to provide a more reliable (e.g., drought-resistant) community-wide water supply, reduce energy demands by the local water utilities, and provide more aesthetically pleasing neighborhoods, which would improve public health, enhance property values, and offer recreational opportunities (see Philadelphia case study provided in Box 7-7). Additionally, the direct hands-on involvement of citizens in water-related activities (such as managing an on-site rain barrel) may provide a higher level of awareness of the water cycle that may foster constructive engagement for local water and other resource management issues, although no research could be found that documents these changes. Regional-scale applications can yield significant levels of social beneficial value to a community, by providing a more diverse and resilient water portfolio and expanding local supplies, making these communities less vulnerable to limitations on imported water as well as droughts and climate change impacts.
TABLE 7-2 Summary of Scenario Analysis of Potential Water Savings from Conservation Irrigation with Rain Barrels and Cisterns in Six U.S. Locations
|Two 35-gallon Rain Barrels||2,200-gallon Cistern|
|Average potential household water savings (gallons/year)||Estimated potential annual cost savings ($/year)||Average potential household water savings (gallons/year)||Estimated potential annual cost savings ($/year)|
NOTE: See Chapter 3 for details of the analysis. Scenarios assume medium-density, residential development, 1,500 ft2 rooftops, and irrigation to meet the evapotranspiration deficit for turfgrass, and results are based on 1995-1999 precipitation data. Landscaped area and residential density determined by location-specific data (see Appendix A).
Environmental benefits and costs of graywater and stormwater projects are also important to consider in the overall assessment of project costs and benefits. Environmental benefits of graywater and stormwater could include greenhouse gas reductions if on-site water use results in significant energy savings. Beneficial use of stormwater can result in significant environmental benefits related to improved water quality and hydrology, although adverse environmental impacts of graywater and stormwater are also possible.
Water Quantity and Quality Impacts
Potential environmental benefits of widespread graywater use for toilet flushing or other nonconsumptive uses would be decreased stresses on existing surface water sources, which may enhance conditions for aquatic life. As discussed in Chapter 5, the potential environmental impacts of graywater use for irrigation are limited if best management practices that prevent surface ponding are followed. Some impact to soil properties or plant health may occur depending on the graywater quality, particularly with elevated levels of sodium or boron, depending on local soil and climatic conditions (see Chapter 5). If graywater use for irrigation is implemented at a large scale in a way that uses more water for irrigation than was previously used with only potable water, then downstream flows could be impacted, which could have a negative impact on aquatic life (see Box 3-1).
Stormwater capture for beneficial use can provide significant environmental benefits and some environmental risks. Increased infiltration of stormwater for the purpose of groundwater recharge and pollution management offers major environmental benefits to surface waters, particularly in areas with combined sewer systems. Stormwater capture or infiltration can reduce and delay peak surface water flows and reduce hydromodification of urban streams (see Chapter 1, Figure 1-7). There are typically reduced deleterious effects on stream biota because of lower stream velocities and lower erosion rates. Stormwater infiltration can help reestablish natural groundwater levels and increase base flow in streams across long reaches in a watershed. An example of changing hydrology based on enhanced stormwater infiltration is shown in Box 7-8. In coastal areas, increased groundwater levels can provide enhanced groundwater discharge and reduce salt-water intrusion. Reduced stormwater flows reduce loads of phosphorus, sediment, metals, bacteria, and organic contaminants to inland and coastal waters. For combined sewer systems, reduction of stormwater flowing to the combined system can reduce combined sewer overflows and therefore reduce contamination of receiving water bodies. In some locations, these benefits are so large that they are often the driving objectives for stormwater management projects (see Chapter 1). These benefits can be quantified to assess the magnitude of contaminant reductions by various stormwater capture strategies (see Figure 7-4).
If stormwater is captured and used in a way that consumptive use of stormwater exceeds prior use of potable
water (see Chapter 3), then there may be impacts to downstream flows, which could, in turn, impact aquatic organisms in streams and estuaries. There is also the potential risk that stormwater contaminants may find their way into local groundwater, which could threaten local water supplies that are dependent on groundwater withdrawal (see Chapter 5). In addition, enhanced infiltration has the potential to cause geotechnical problems such as flooding basements (see Box 7-8) or altering movement of groundwater contaminant plumes.
Energy Footprint of Stormwater and Graywater Compared to Traditional Water Systems
Energy could be saved in the process of substituting conventional sources of water with alternative local or onsite sources. Energy production results in air and water pollution and generation of waste; therefore, energy saved results in reduced pollution and waste. Depending on the area of the United States, saving electricity can translate to either more or less avoided emissions. For example, based on the amount of fossil fuels burned in electricity generation (EPA, 2011), a MWh of electricity is responsible for about 100 kg of CO2 emissions for the electricity mix in Washington state and about 800 kg in Ohio. Furthermore, because energy production requires water (e.g., for cooling in electric power plants), energy savings can also result in water savings.
For decision making, it is important to quantify the energy intensities and environmental impacts of these alternatives and compare them to conventional supply of water. The following summarizes what is known about the energy needs of on-site graywater and stormwater use based on the currently available literature.
Energy Intensity of Water from Conventional Systems
The energy intensity of water provided to customers depends on the water source(s), pumping needs, treatment processes, and storage options. A range for embedded energy data for delivering water to customers is available from the published literature. However, the numbers may differ depending on the comprehensiveness of the energy assessment (e.g., if supply-chain effects were also considered). The energy intensity of water supplied to U.S. cities is typically around 0.9 kWh/m3 (Copeland, 2014), but it can be five times higher, as reported in Table 7-4. Systems report higher energy use when they rely upon a large percentage of imported water (pumped from a long distance), use brackish or seawater desalination, or require groundwater pumping. For example, conveying water from northern to southern California via the California State Water Project is estimated to have an energy requirement of 2.1-4.4 kWh/m3 (Spencer, 2013). Groundwater pumping in southern California requires an additional 0.3-0.8 kWh/m3 above energy needs for treatment and distribution.
Overall, pumping and treatment tend to be the largest energy uses for water systems, but other life-cycle phases and supply chains (e.g., treatment chemicals and equipment, pipes, pumps, buildings, and utility vehicle fleets) may have significant embedded energy that should be accounted for in a comprehensive assessment of the energy intensity of water. Life-cycle assessment (LCA) is a methodology that can provide comprehensive environmental analyses (e.g., Stokes and Horvath, 2009). Table 7-4 includes three examples of life-cycle energy intensities using LCA. In those examples, most of the energy is expended in conveying the water from the source to the water treatment plant and then to the customers (Stokes et al., 2014).
Energy Intensity of Stormwater Capture Systems
Seventy-five percent of residential U.S. rooftop stormwater capture systems are utilized for irrigation purposes (Thomas et al., 2014). By replacing potable water with stormwater, rooftop systems offer opportunities to conserve local water sources while avoiding the required material and energy inputs needed to supply potable water for irrigation the traditional way. Theoretically, energy costs for pumping and treatment could be notably reduced. However, life-cycle energy demands can vary significantly based on the design and scale of the stormwater capture system and end uses. In the existing literature, empirical analyses of rooftop runoff capture systems report a greater energy demand (median is 1.4 kWh/m3) than theoretical studies (0.2 kWh/m3) (Vieira et al., 2014). For Australian conditions, 1.4-1.8 kWh/m3 median energy consumption was reported for rainwater supply to urban dwellings (Sharma et al., 2015), but depending on the pump design, values range between about 0.4 kWh/m3 and 5 kWh/m3. The energy intensity for household systems is characterized by material inputs (58 percent of total energy demand) and for agricultural systems is dominated by pumping needs (95 percent) (Ghimire et al., 2014).
In combined sewer systems, stormwater use for irrigation can reduce energy requirements associated with downstream wastewater treatment of stormwater. These savings were reported as 0.3 kWh/m3 in one study (Blackhurst et al., 2010), while another study, focusing on wastewater treatment, calculated the energy cost to be 0.6 to 1.2 kWh/m3 (Stokes and Horvath, 2010). Ultimately, these wastewater energy benefits would depend on the volume of stormwater diverted from the combined sewer system, which is likely to be less than the amount of stormwater used for on-site irrigation.
TABLE 7-4 Energy Requirements of Water from Several U.S. Cities or Regions
|City/Utility||Energy Requirement (kWh/m3)||Life-cycle Assessment-based Number?|
|New York City||0.7||yes|
|Santa Clara Valley Water District (northern California)—current water mix||1.3||no|
|Brackish water Santa Clara Valley Water District—brackish water desalination||2.6||no|
|Small utility (northern California, serving 40,000 people; 100% groundwater)||1.9||yes|
|Medium utility (southern California, serving 180,000 people; 83% imported water, 17% brackish groundwater desalination, <1% recycled)||4.9||yes|
|Large utility (northern California, serving 1.3 million people; 95% imported water, 5% local runoff collected in reservoirs, <1% recycled)||1.7||yes|
Energy Intensity of Graywater Reuse Systems
The energy footprint of graywater reuse depends on the system design (e.g., pipes, pumps, and other components), pumping needs, type and level of treatment, and the end use of water. For a graywater irrigation system that includes only storage and filtration, the primary energy demand is pumping (beyond the embedded energy in system materials). The laundry-to-landscape system typically requires no energy beyond that embedded in system components. Systems that provide extensive graywater treatment, such as membrane filters and biological treatment, will have a high energy demand.
The current literature offers limited data on graywater energy use, particularly at the household scale. In the United Kingdom, the nonpotable water needs (met from graywater) of 500 households were calculated to require the following amounts of energy by three different treatment types: 9,200 kWh for a reed bed, 197,000 kWh for membrane bioreactor (MBR), and 238,000 kWh for membrane chemical reactor (Memon et al., 2007). An economic feasibility study of MBRs treating graywater estimated energy use at 1.0-1.5 kWh/m3 (Friedler and Hadari, 2006). A California study assessed a sand-filtration decentralized treatment system that reused graywater for irrigation purposes, including ultraviolet treatment of effluents. The study found the life-cycle energy consumption to be 10.3 kWh/m3 (Shehabi et al., 2012). For comparison, Hendrickson et al. (2015) estimated the energy consumption of an office building-scale wetland system that recycles wastewater (not just graywater) for nonpotable use (toilet flushing and irrigation in a neighboring park) to be about 5.5 kWh/m3 throughout the system’s life cycle. There may also be opportunities to capture energy from blackwater when graywater is source-separated for reuse, as described in Chapter 6.
One of the challenges in promoting sound graywater and stormwater beneficial use projects is the lack of direct financial incentives for those who bear the implementation costs. For example, at the household level, a family that invests in stormwater capture and beneficial use via large cisterns may not save enough money on its water utility bills to justify the expense (i.e., the payback period may be longer than desired). As the cost of potable water increases, the financial incentives may become larger. For multi-residential projects, the building owner or developer who makes the infrastructure investments to install a graywater or stormwater use system may not directly benefit, because the cost savings from reduced water use accrue to the residents. Nonfinancial motives such as the enjoyment associated with an irrigated garden and related environmental and social values (e.g., the desire to use local water resources wisely) also create positive incentives for projects that tap graywater or stormwater for beneficial uses, and these benefits may extend to the larger community.
Ultimately, aligning the proper fiscal incentives boils down to understanding and communicating where there are important external benefits to a beneficial use project. Once the externalities are recognized, such as through a comprehensive benefit-cost analysis that embodies full social and environmental accounting, the challenge is finding and implementing mechanisms that better align who pays with who benefits (e.g., by cost-share agreements or public-sector subsidies).
For regional-scale projects, with relatively large budget implications, incentives may be more fiscally constrained, with altruistic values playing a lesser role. In such cases, it is important that the utility—and the broader regional community—recognize the important fiscal, social, and envi-
ronmental values that can be derived from a beneficial use project.8 It is important as well to account for the positive and negative externalities associated with these projects. For example, other communities may derive benefits when water savings or local water resource development relieves pressure on limited water resources shared by the broader area (e.g., where stormwater capture and recharge reduces regional demands for scarce and expensive imported waters). In such instances, some form of cost sharing across all beneficiaries will help incentivize projects that have a large number of beneficiaries outside of the immediate utility service area (i.e., subsidies may help internalize the positive impacts of such projects). This may take the form of suitable subsidies for projects that provide such external benefits and is evident in grants and cost-share arrangements such as those provided by the Bureau of Reclamation (e.g., Title XVI grants), the State of California (e.g., grants funded by Proposition 84), and the Metropolitan Water District of Southern California (via its local resources program).
It is important to recognize the full suite of benefits—as well as the full costs—of graywater and stormwater projects, although it may be empirically challenging to do so. A wide array of potential benefits may arise from projects that use graywater or stormwater. Some of these benefits are financial and can be readily estimated and portrayed in monetary terms, such as the value of water savings or the avoided cost of obtaining water from an alternative supply. In addition, important societal and environmental benefits may apply. These benefits may be difficult to quantify or monetize and are typically highly site- and scale-specific and dependent on the type of application. Costs for graywater and stormwater projects are also highly dependent on scale, system design, and plumbing requirements, and they are generally better understood than the benefits. Yet there is a lack of well-documented and complete cost information for many of the possible applications.
Simple household-scale graywater reuse or roof runoff capture systems can offer reasonable financial payback periods under certain water use scenarios and appropriate climate conditions. For example, considering the committee’s scenario analysis of potential water savings in medium-density, residential development, simple laundry-to-landscape graywater systems can offer payback periods as low as 2.5-6 years (not accounting for the cost of labor), with the shortest payback periods in the Southwest and central United States. These estimates assume graywater for irrigation actually offsets potable use—an assumption that remains to be demonstrated. Longer payback periods were estimated for rain barrels (5-26 years) and cisterns (14 to more than 50 years, not accounting for labor) used for conservation irrigation. The longer payback periods reflect locations where distinct wet and dry seasons do not coordinate well with irrigation demands, as in the arid Southwest. Shorter payback periods may be possible in more humid climates for households with large irrigated areas. The cost of installation (whether by contracting with a paid professional, or valuing homeowner-provided labor) greatly extends the payback period, as do water uses in which additional plumbing and treatment are required. However, in household-scale applications, it may well be the non-financial benefits that motivate households to adopt beneficial use—such as a sense of conserving resources or having outdoor irrigation water reliably available to support landscaping during times of drought.
Some neighborhood- or regional-scale stormwater capture and use projects provide financial benefits that exceed costs, sometimes by a wide margin, in addition to other social and environmental benefits, and economies of scale are evident for both tank capture and infiltration projects. The regional stormwater capture and recharge projects in southern California, for example, can pay back large dividends to the community in the form of significant water supply enhancements (i.e., avoiding the cost of expensive imported water). Based on available unit cost data, stormwater tank capture and use at the neighborhood scale tend to be much more costly than alternatives designed to recharge groundwater. Larger scale projects may also reduce stormwater-related regulatory compliance costs and provide a wide variety of highly valued societal and environmental benefits, including enhanced aesthetics, property values, and recreational opportunities.
Published cost data from larger-scale graywater projects are extremely limited, and therefore the financial benefits and cost options are difficult to assess beyond the pilot scale. Some efficiencies of scale would be associated with graywater toilet flushing systems in large, new, multi-residential developments (particularly compared to smaller retrofits), and additional incentives may be possible if such investments defer water and wastewater infrastructure expansion in densely populated urban areas.
Depending on the stormwater or graywater system design, energy savings are possible compared with conventional water supplies, but data for a sound assessment
8 One potentially useful approach is provided in Marsden (2013), which provides a water recycling evaluation framework that was applied as the basis for evaluating alternative stormwater harvesting systems in an Australian community by Dandy et al. (2014). Dandy’s work accounts for financial costs and greenhouse gas emissions, and addresses the different benefits perceived by different stakeholders.
are lacking. For decision making, it is important to quantify the life-cycle energy intensities of alternative water supplies and compare them to conventional supply of water. Energy saved could also result in reduced greenhouse gas and other pollutant emissions associated with energy production. The current literature contains little energy data for conventional and alternative systems. Conventional water systems in the United States are reported to provide water to customers at an energy cost of less than 1 kWh/m3 to almost 5 kWh/m3, depending mostly on pumping costs for conveying the water from the source to the water treatment plant. Roof runoff capture systems have been reported in a very limited number of studies to have a greater energy demand (median is 1.4 kWh/m3) in practice than in theoretical studies (0.2 kWh/ m3), and thus may not be less energy intensive than conventional drinking water systems. Where stormwater is diverted into combined sewer systems, additional energy savings of between 0.3 and 1.2 kWh/m3 may be obtained. Many potential variables (e.g., scale, pumping, treatment, material inputs) will drastically affect the life-cycle energy demands of these systems, and the effects of these variables in practice remain poorly understood.