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1 Introduction Growing concern over local water scarcity and challenges in meeting future water demand has led to heightened interest in desalination tech- nology. In early 2003, the U.S. Bureau of Reclamation and Sandia Na- tional Laboratories completed a technology planning activity (the De- salination and Water Purification Technology Roadmap or Roadmap) intended to serve as a strategic pathway for future desalination and water purification research (USBR and Sandia National Laboratories, 2003). In the fall of 2002, at the request of the Bureau of Reclamation, the Na- tional Research Councilâs (NRCâs) Water Science and Technology Board initiated an independent assessment of the Roadmap (NRC, 2004b). NRC (2004b) concluded that in order for desalination technolo- gies to provide safe, reliable, sustainable, and cost-effective water supply for water utilities in the United States, current and anticipated challenges need to be identified and a national research agenda developed. How- ever, the report noted that additional work was needed to build upon the Roadmap and provide thorough, critical analyses of current technologies and research objectives to develop a strategic research agenda for desali- nation. This report seeks to address these objectives. SALINE WATER AS A WATER SUPPLY ALTERNATIVE The Earth contains a vast amount of water, but much of it is too salty for human use without advanced treatment. Nearly all of the Earthâs wa- ter is found in the worldâs oceans, while only about 2.5 percent exists as freshwater (see Table 1-1). Much of the freshwater is bound as glaciers and permanent snow, leaving only a small fraction of useable freshwater to meet the worldâs human water demands and to satisfy environmental needs. As some of the demands for freshwater continue to grow, the availability of new supplies from traditional freshwater sources continues 13
14 Desalination: A National Perspective to decline. Therefore, communities are increasingly looking toward more saline waters, such as brackish groundwater or seawater, or otherwise âimpairedâ waters to address water supply needs. There are many ways to define the salinity (salt concentration) ranges for fresh and saline waters. Water with greater than 2,000 to 3,000 mg/L total dissolved solids (TDS) is considered too salty to drink (Freeze and Cherry, 1979) or to grow most crops. The World Health Organiza- tion considers water with TDS concentrations below 1,000 mg/L to be generally acceptable to consumers, although it notes that acceptability may vary according to circumstances (WHO, 2003). The U.S. Environ- mental Protection Agency (EPA) notes that drinking water with TDS greater than 500 mg/L can be distasteful (USEPA, 1979). Brackish water has a salinity between that of fresh- and seawater. In more than 97 per- cent of seawater in the world the salinity is between 33,000 and 37,000 mg/L (Stumm and Morgan, 1996), although the Persian Gulf has an av- erage TDS of 48,000 mg/L (Pankratz and Tonner, 2003). Water with sa- linity greater than that of seawater is called brine (USGS, 2003). As noted in Table 1-1, nearly 1 percent of the worldâs water exists as brackish or saline groundwater. In most inland cases, groundwater salin- ity results from the dissolution of minerals present in the subsurface, pos- sibly concentrated further by evapotranspiration. Coastal aquifers form another class of brackish water, which is created from the natural mixing of seawater with groundwater that is discharging to the ocean (see also Chapter 5). The thickness of this brackish mixing zone is sometimes in- creased by coastal groundwater pumping. Brackish groundwater exists at elevations less than 305 m (1,000 feet) across much of the conterminous United States (Feth, 1965) (Figure 1-1) and almost certainly at TABLE 1-1 Major Stocks of Water on Earth Amount Percentage of 6 3 Location (10 km ) World Water Ocean 1338.0 96.5 Glaciers and permanent snow 24.1 1.74 Groundwater (brackish or saline) 12.9 0.94 Groundwater (fresh) 10.5 0.76 Ground ice/permafrost 0.30 0.022 Freshwater lakes 0.091 0.007 Freshwater stream channels 0.002 0.0002 SOURCE: Shiklomanov, 1993.
Introduction 15 FIGURE 1-1. Depth to brackish groundwater (greater than 1,000 mg/L total dis- solved solids) in the conterminous United States. SOURCE: Generalized from Feth (1965). comparable depths in Hawaii and Alaska. Both coastal and inland com- munities are increasingly considering brackish groundwater as a possible water supply resource. Desalination processes generally treat seawater and brackish waters to produce freshwater (i.e., the desired product stream) and a separate saltier concentrate stream. Several approaches can be used to desalinate saline water sources at the municipal scale. The earliest commercial plants used mostly large-scale thermal evaporation or distillation of sea- water. Major facilities were first built in the Persian Gulf region, where excess or inexpensive energy was available and where natural sources of freshwater are relatively scarce. Beginning in the 1970s, plants were in- stalled that used pumps and membranes to produce freshwater, applying the natural biological process of osmosis in reverse. Significant advances in reverse osmosis technology have been achieved in recent years that have reduced the water production costs of desalination. Worldwide, the online capacity1 for desalination now exceeds 37 million cubic meters of water per day (30,000 acre-feet per day or 10,000 million gallons per 1 In this report, online capacity includes desalination plants that have been con- firmed by Global Water Intelligence (GWI, 2006b) to be online and those that are âpresumed online.â These online capacity totals do not include plants that were confirmed to be offline, under construction, decommissioned, or âmothballedâ or those that were presumed by GWI to be offline.
16 Desalination: A National Perspective day) (GWI, 2006b), although this sum represents only about 0.3 percent of total freshwater use (Cooley et al., 2006). More detail on specific processes and technologies is provided in Chapter 4. STATEMENT OF COMMITTEE TASK AND REPORT OVERVIEW In 2006, the NRCâs Committee on Advancing Desalination Technol- ogy was formed to assess the status of desalination technologies and fac- tors such as cost and implementation challenges, and to provide recom- mendations for action and research. This study was sponsored by the U.S. Bureau of Reclamation and the EPA. The committee was specifi- cally tasked to address the following questions (with cross references to the chapters where the tasks are addressed): 1. Contributing to the nationâs water supplies. What is the poten- tial for both seawater and inland brackish water desalination to help meet anticipated water supply needs in the United States? (See Chapters 3 and 5.) How do the costs and benefits of desalination compare with other al- ternatives, including nontechnical options such as water conservation or market transfers of water? (See Chapter 6.) 2. Assessing the state of technology and setting goals. What is the current state of the science in desalination technology? (See Chapter 4.) What have the recent trends been (both for seawater and for brackish water) in terms of total cost per unit of water produced and also in the energy efficiency of the process? (See Chapters 4 and 6.) Are there theo- retical limits to the efficiency of existing technologies and is there good reason to think that significant advancement can be made toward reach- ing those limits? (See Chapter 4.) What are reasonable long-term goals for advancing desalination technology? (See Chapter 8.) 3. Research strategy. Following up on a recommendation by NRC (2004b) calling for the development of a national research agenda, what research is needed to reach the long-term goals for advancing desalina- tion technology? (See Chapter 8.) What technical barriers should be re- solved with existing desalination technologies (including concentrate disposal) and what innovative technologies should be considered? (See Chapters 4 and 5.) In the long-term research agenda for desalination, what balance should be crafted between high-risk research in novel tech- nologies and research that could yield incremental improvements in cur- rent technologies? (See Chapter 8.)
Introduction 17 4. Practical aspects of implementation. What important issues re- lated to implementation must be addressed to significantly improve the applicability of technology for desalination to help meet the nation's wa- ter needs (e.g., economics, financing, regulatory, institutional, public ac- ceptance)? (See Chapters 6 and 7.) What are the true economic costs? (See Chapters 5 and 6.) What factors are likely to affect the availability of financing? What are the likely regulatory issues and how easy or diffi- cult will it be to deal with them? Are there other institutional issues? What problems, if any, may arise in ensuring public acceptability of de- salination technologies? (See Chapter 7.) 5. Resources and roles. What order of magnitude of research fund- ing is needed to significantly advance the field of desalination technol- ogy and what are appropriate roles for governmental and nongovernmen- tal entities? (See Chapter 8.) The committeeâs conclusions and recommendations are based on a re- view of relevant technical literature, briefings, and discussions at its six meetings, field trips to desalination facilities (see Acknowledgments), and the experience and knowledge of the committee members in their fields of expertise. Following this brief introduction, the statement of task is addressed in seven subsequent chapters of this report: â¢ Chapter 2 provides context for this report by describing the use of desalination technologies in the United States and globally and dis- cussing major research programsâboth historical and currentâfocused on advancing desalination technologies. â¢ Chapter 3 discusses the issues of water use and water sufficiency and addresses the potential for desalination technologies to help meet anticipated water supply needs. â¢ In Chapter 4 the state of the science in desalination technology, including intakes, energy recovery, and concentrate management, is de- scribed. Current process and technology constraints are discussed, along with the most promising opportunities to maximize energy efficiency, considering the thermodynamic limitations. â¢ In Chapter 5, environmental issues associated with desalination are discussed, focusing on source water acquisition, concentrate man- agement, human health issues, and potential climate and energy con- cerns. â¢ In Chapter 6, the financial and economic circumstances sur- rounding desalination technology are discussed and the benefits of de- salination are examined. The costs of desalination are analyzed to high-
18 Desalination: A National Perspective light their major components and the largest opportunities for cost reduc- tions. The chapter also includes a discussion of the costs of desalination relative to other water supply alternatives. â¢ The practical aspects of implementation for water providers are described in Chapter 7, including regulatory concerns, public perception, and financing. â¢ In Chapter 8, the committee presents two long-term goals for ad- vancing desalination technology and develops a national research agenda to address these goals. Recommendations are offered on the implementa- tion of this proposed research agenda, including an estimate of the fed- eral resources necessary to support it.