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Desalination: A National Perspective
1
Introduction
Growing concern over local water scarcity and challenges in meeting future water demand has led to heightened interest in desalination technology. In early 2003, the U.S. Bureau of Reclamation and Sandia National Laboratories completed a technology planning activity (the Desalination 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 National Research Council’s (NRC’s) Water Science and Technology Board initiated an independent assessment of the Roadmap (NRC, 2004b). NRC (2004b) concluded that in order for desalination technologies 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. However, 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 desalination. 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 Earth’s water is found in the world’s 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 world’s 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
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Desalination: A National Perspective
to decline. Therefore, communities are increasingly looking toward more saline waters, such as brackish groundwater or seawater, or otherwise “impaired” 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 Organization 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. Environmental 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 percent 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 average TDS of 48,000 mg/L (Pankratz and Tonner, 2003). Water with salinity greater than that of seawater is called brine (USGS, 2003).
As noted in Table 1-1, nearly 1 percent of the world’s water exists as brackish or saline groundwater. In most inland cases, groundwater salinity results from the dissolution of minerals present in the subsurface, possibly 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 increased 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
Location
Amount (106 km3)
Percentage of 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.
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Desalination: A National Perspective
FIGURE 1-1. Depth to brackish groundwater (greater than 1,000 mg/L total dissolved solids) in the conterminous United States. SOURCE: Generalized from Feth (1965).
comparable depths in Hawaii and Alaska. Both coastal and inland communities 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 seawater. 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 installed 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 confirmed by Global Water Intelligence (GWI, 2006b) to be online and those that are “presumed online.” These online capacity totals do not include plants that were confirmed to be offline, under construction, decommissioned, or “mothballed” or those that were presumed by GWI to be offline.
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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 NRC’s Committee on Advancing Desalination Technology was formed to assess the status of desalination technologies and factors such as cost and implementation challenges, and to provide recommendations for action and research. This study was sponsored by the U.S. Bureau of Reclamation and the EPA. The committee was specifically tasked to address the following questions (with cross references to the chapters where the tasks are addressed):
Contributing to the nation’s water supplies. What is the potential 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 alternatives, including nontechnical options such as water conservation or market transfers of water? (See Chapter 6.)
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 theoretical limits to the efficiency of existing technologies and is there good reason to think that significant advancement can be made toward reaching those limits? (See Chapter 4.) What are reasonable long-term goals for advancing desalination technology? (See Chapter 8.)
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 desalination technology? (See Chapter 8.) What technical barriers should be resolved 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 technologies and research that could yield incremental improvements in current technologies? (See Chapter 8.)
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Desalination: A National Perspective
Practical aspects of implementation. What important issues related to implementation must be addressed to significantly improve the applicability of technology for desalination to help meet the nation’s water needs (e.g., economics, financing, regulatory, institutional, public acceptance)? (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 difficult will it be to deal with them? Are there other institutional issues? What problems, if any, may arise in ensuring public acceptability of desalination technologies? (See Chapter 7.)
Resources and roles. What order of magnitude of research funding is needed to significantly advance the field of desalination technology and what are appropriate roles for governmental and nongovernmental entities? (See Chapter 8.)
The committee’s conclusions and recommendations are based on a review 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:
Chapter 2 provides context for this report by describing the use of desalination technologies in the United States and globally and discussing major research programs—both historical and current—focused on advancing desalination technologies.
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.
In Chapter 4 the state of the science in desalination technology, including intakes, energy recovery, and concentrate management, is described. Current process and technology constraints are discussed, along with the most promising opportunities to maximize energy efficiency, considering the thermodynamic limitations.
In Chapter 5, environmental issues associated with desalination are discussed, focusing on source water acquisition, concentrate management, human health issues, and potential climate and energy concerns.
In Chapter 6, the financial and economic circumstances surrounding desalination technology are discussed and the benefits of desalination are examined. The costs of desalination are analyzed to high-
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light their major components and the largest opportunities for cost reductions. The chapter also includes a discussion of the costs of desalination relative to other water supply alternatives.
The practical aspects of implementation for water providers are described in Chapter 7, including regulatory concerns, public perception, and financing.
In Chapter 8, the committee presents two long-term goals for advancing desalination technology and develops a national research agenda to address these goals. Recommendations are offered on the implementation of this proposed research agenda, including an estimate of the federal resources necessary to support it.