Desalination and Membrane Technologies: Federal Research and Adoption Issues
Released: 2015-01-02T00:00:00

A Primer on Desalination

Interest in using desalination technologies to treat seawater, brackish water, wastewaters, and contaminated sources has increased globally and in the United States, as costs have fallen and pressure to develop drought-proof water supplies has grown. Adoption of desalination, however, remains constrained by financial, environmental, regulatory, and social factors. At issue is the role Congress establishes for the federal government in desalination, particularly in desalination research and development and the federal regulatory environment for desalination projects. Also of congressional interest is what role desalination may play in meeting water demands.

Desalination processes generally treat saline or impaired waters to produce a stream of freshwater, and a separate, saltier stream often called waste concentrate or brine . The availability and regulation of disposal options for waste concentrate can limit adoption in some locations; this is a particular challenge for large-scale inland facilities. For seawater desalination, the impacts of intake facilities on marine life also often are raised as concerns.

Desalination's attractions are that it can create a new source of freshwater from otherwise unusable waters, and that this source may be more dependable and drought-proof than freshwater sources that rely on annual or multi-year precipitation, runoff, and recharge rates. Another significant application of desalination technologies is for treatment of contaminated waters or industrial water or municipal wastewater. Some communities and industries use desalination technologies to produce drinking water that meets federal standards, to treat contaminated water supplies to meet disposal requirements, or to reuse industrial wastewater. Many of the technologies developed for desalination also can produce high-quality industrial process water. For many of these applications, there may be few technological substitutes that are as effective and reliable as desalination technologies.

There are multiple desalination methods. Two common categories of desalination technologies—thermal (e.g., distillation) and membrane (e.g., reverse osmosis)—are the most common, with reverse osmosis technologies dominating in the United States. For more information on traditional and emerging technologies, see Appendix A .

Desalination treatment costs have dropped in recent decades, making the technology more competitive with other water supply augmentation and treatment options. Electricity expenses vary from one-third to one-half of the cost of operating desalination facilities. A rise in electricity prices could reverse the declining trend in desalination costs; similarly, drops in electricity costs improve desalination's competitiveness. Costs and cost uncertainties remain among the more significant challenges to implementing large-scale desalination facilities, especially seawater desalination plants. Desalination's energy intensity also raises concerns about the greenhouse gas emissions emitted and desalination's usefulness as part of a climate change adaptation strategy. Substantial uncertainty also remains about the environmental impacts of large-scale desalination facilities. Social acceptance and regulatory processes also affect the technologies' adoption and perceived risks. Research and additional full-scale facilities may resolve uncertainties, alleviate concerns, and contribute to cost reductions and options for mitigating environmental impacts.

To date, the federal government primarily has supported research and development, some demonstration projects, and select full-scale facilities (often through congressionally directed spending). The federal government also may support construction of municipal desalination facilities through loans or other credit assistance provided through programs of the U.S. Environmental Protection Agency (EPA). For most municipal desalination facilities, local governments or public water utilities (often with state-level involvement and federal construction loans) have been responsible for planning, testing, building, and operating desalination facilities, similar to their responsibility for treating freshwater drinking water supplies.

During recent Congresses, legislative proposals have identified a range of different potential federal roles in desalination. Desalination issues before the 114 th Congress may include how to focus federal research to produce results that provide public benefits, at what level to support desalination research and projects, and how to provide a regulatory context that protects the environment and public health without unnecessarily disadvantaging these technologies.

Recent Congressional Consideration

The 113 th Congress authorized a new federal credit assistance program, the Water Infrastructure Finance and Innovation Act (WIFIA, 33 U.S.C. §3901), as part of P.L. 113-121 . The portion of the program administered by EPA may be used in financing a range of water projects, including desalination projects.

The Water Desalination Act of 1996, as amended (42 U.S.C. §10301), authorized the main desalination research and demonstration outreach program of the Department of the Interior, which is carried out by the Bureau of Reclamation. The 112 th Congress extended through FY2013 its annual authorization of appropriations at $3 million. Bills in the 113 th Congress proposed another extension, but the 113 th Congress did not extend the authorization of appropriations. Although an extension was not enacted, the 113 th Congress provided appropriations for these desalination research and demonstration activities (e.g., $1.75 million for FY2015) and for operation and maintenance of the Department of the Interior's operation of the Brackish Groundwater National Desalination Research Facility ($1.15 million for FY2015).

Federal Desalination Research

Research Agenda

Several reports since 2000 have aimed to inform the path forward for U.S. desalination research. The first was the 2003 Desalination and Water Purification Technology Roadmap produced by the Bureau of Reclamation and Sandia National Laboratories at the request of Congress. The National Research Council then reviewed the roadmap in a 2004 report, Review of the Desalination and Water Purification Technology Roadmap , which called for a strategic national research agenda. To this end, the National Research Council (NRC) convened a Committee on Advancing Desalination Technology. That NRC committee published its own assessment in 2008, Desalination: A National Perspective . The NRC concluded that research should focus on reducing desalination costs and that substantial further cost savings were unlikely to be achieved through incremental advances in the commonly used technologies, like reverse osmosis. Consequently, the report recommended that federal desalination research funding be targeted at long-term, high-risk research not likely to be attempted by the private sector that could significantly reduce desalination costs. It also recommended a line of research on minimizing or mitigating the environmental impacts of desalination. The NRC specifically identified for federal investment research that had potential for widespread benefit and that the private sector had little willingness to perform. (See "National Research Council 2008 Desalination Research Recommendations" box for more details.)

In 2010, the Water Research Foundation, WateReuse Foundation, and Sandia National Laboratories published a report on how to implement the 2003 roadmap. The report identified research agendas for a range of topics—membrane and alternative technologies, concentrate management, and institutional issues such as energy cost reduction and regulatory compliance.

Evolution of Federal Research Funding

No single federal agency has responsibility for all federal desalination and membrane research; instead numerous agencies and departments are involved in research based on their specific missions. In FY2005, FY2006, and FY2007, federal desalination research totaled $24 million, $24 million, and $10 million, respectively. (These are the most recent comprehensive data on federal desalination funding.) The Bureau of Reclamation was responsible for half or less of that spending at $12 million, $11 million, and $4 million, respectively. Other agencies and departments with spending on desalination research included the Army, National Science Foundation, Office of Naval Research, U.S. Geological Survey, and four of the Department of Energy's National Laboratories. Sandia National Laboratory has had the largest role among the national laboratories. In FY2005 and FY2006, much of the federal desalination research was congressionally directed to specific sites and activities. The level of funding fell after FY2006, when the appropriations process began to include less congressionally directed spending.

The optimal level and type of federal support for desalination research is inherently a public policy question shaped by factors such as fiscal priorities and views on the appropriate role of the federal government in research, industry development, and water supply. Federal support for desalination research raises questions, such as what should be the respective roles of federal agencies, academic institutions, and the private sector in conducting research and commercializing the results, and should federal research be focused on basic research or promoting the use of available technologies? In addition to federal and private research activities, some states, such as California and Texas, also have supported desalination research.

In 2008, the National Research Council recommended a federal desalination research level of roughly $25 million, but recommended that the research be targeted strategically, including being directed at the research activities described above. The NRC drew the following conclusion:

There is no integrated and strategic direction to the federal desalination research and development efforts. Continuation of a federal program of research dominated by congressional earmarks and beset by competition between funding for research and funding for construction will not serve the nation well and will require the expenditure of more funds than necessary to achieve specified goals.

No recent authoritative estimate of all federal desalination spending is available. Data for Reclamation indicate a reduction in federal desalination spending. Reclamation's desalination funding has declined from $11 million in FY2006 to between $2 million and $4 million in recent years, with FY2015 funding at $3 million. While some traditional avenues for federal desalination research may be receiving less support, new avenues of support may be opening. Two of these are the Advanced Research Projects Agency-Energy (ARPA-E) and the National Science Foundation's Urban Water Engineering Research Center, which was initiated in 2011.

Desalination Adoption in the United States

Desalination and membrane technologies are increasingly investigated and used as an option for meeting municipal and industrial water supply and water treatment demands. The nation's installed desalination capacity has increased in recent years, reflecting the technology's growing competitiveness and applications and increasing demands for reliable freshwater supplies. As of 2005, approximately 2,000 desalination plants larger than 0.3 million gallons per day (MGD) were operating in the United States, with a total capacity of 1,600 MGD. This represents more than 2.4% of total U.S. municipal and industrial freshwater withdrawals, not including water for thermoelectric power plants. Two-thirds of the U.S. desalination capacity is used for municipal water supply; industry uses about 18% of the total capacity.

Municipal use of desalination for saline water and wastewater treatment in the United States expanded from 1990 to 2010; the number of facilities rose from 98 facilities with a total capacity of 100 MGD in 1990 to 324 facilities with 1,100 MGD of capacity in 2010. By 2010, 32 states had municipal desalination facilities. Florida, California, and Texas have the greatest installed desalination capacity and account for 68% of the municipal desalination facilities. Florida dominates the U.S. capacity, with the facility in Tampa being a prime example of large-scale desalination implementation (see box); however, Texas and California are bringing municipal plants online or are in advanced planning stages. Several other efforts also are preliminarily investigating desalination for particular communities, such as Albuquerque.

The saline source water that is treated using desalination technologies varies largely on what sources are available near the municipalities and industry with the demand for the water. In the United States, only 7% of the existing desalination capacity uses seawater as its source. More than half of U.S. desalinated water is from brackish sources. Another 25% is river water treated for use in industrial facilities, power plants, and some commercial applications. Globally, seawater desalination represents 60% of the installed desalination capacity.

While interest in obtaining municipal water from desalination is rising in the United States, desalination is expanding most rapidly in other world regions, often in places where other supply augmentation options are limited by geopolitical as well as natural conditions, such as arid conditions with access to seawater. The Middle East, Algeria, Spain, and Australia are leading in the installation of new desalination capacity, with Saudi Arabia and the United Arab Emirates leading in annual production of desalinated water. Roughly 98% of the desalination capacity worldwide is outside of North America, with 65% in the Middle East.

Energy Concerns and Responses

Reducing Energy Intensity To Reduce Cost Uncertainties

The cost of desalination for municipal water remains a barrier to adoption. Like nearly all new freshwater sources, desalinated water comes at substantially higher costs than existing municipal water sources.

Much of the cost for seawater desalination is for the energy required for operations; in particular, the competitiveness of reverse osmosis seawater desalination is highly dependent on the price of electricity. Reverse osmosis pushes water through a membrane to separate the freshwater from the salts; this requires considerable energy input. Currently the typical energy intensity for seawater desalination using reverse osmosis with energy recovery devices is 3-7 kilowatt-hours of electricity per cubic meter of water (kWh/m 3 ). The typical energy intensity of brackish reverse osmosis desalination is less than seawater desalination, at 0.5-3 kWh/m 3 , because the energy required for desalination is a function of the salinity of the source water.

Uncertainty in whether electricity prices will rise or fall creates significant uncertainty in the cost of desalinated water. If electricity becomes more expensive, less electricity-intensive water supply options (which may include conservation, water purchases, and changes in water pricing) become comparatively more attractive.

Cost-effectively reducing desalination's energy requirements could help reduce overall costs. In recent decades, one of the ways that desalination cost reductions were achieved was through reduced energy requirements of reverse osmosis processes. Now the energy used in the reverse osmosis portion of new desalination facilities is close to the theoretical minimum energy required for separation of the salts from the water. Therefore, although there still is some room for energy efficiency improvements in using desalination as a water supply, dramatic improvements are not likely to be achieved through enhancements to standard reverse osmosis membranes. Instead energy efficiency improvements are more likely to come from other components of desalination facilities, such as the pretreatment of the water before it enters the reverse osmosis process, enhanced facility and system design, or the use and development of a new generation of technologies (see Appendix A ).

For example, energy efficiency advances in the non-membrane portions of water systems and the use of energy recovery technologies are reducing energy use per unit of freshwater produced at desalination facilities. Pumps are responsible for more than 40% of total energy costs at a desalination facility. Energy efficiency advances in a type of pump that is useful for smaller applications (called a positive displacement pump) have made desalination more cost-effective for some applications and locations and less sensitive to electricity price increases.

The Affordable Desalination Collaboration is an example of combining federal, state, local, private, and other financial resources to support research; funding from these sources was combined to build and operate a demonstration plant at the U.S. Navy's Desalination Test Facility in Port Hueneme, California. The plant tests the combined use of commercially available reverse osmosis technologies, energy recovery devices and practices, and other energy-efficient technologies to reduce the energy inputs required for seawater and brackish desalination. According to the researchers involved, the energy-efficient demonstration facility has the equivalent energy input per unit of freshwater produced as water imports into southern California from northern California and the Colorado River.

While this research is an example of reducing energy demand, other efforts are looking to substitute the type of energy used for desalination from fossil fuels to renewable energy or waste heat. The use of desalination as a climate change adaptation strategy is questioned because of its potential fossil fuel intensity relative to other adaptation and water supply options. Electricity price uncertainty and emissions considerations have driven some desalination proponents to investigate renewable energy supplies and co-location with power plants and industrial facilities to reduce electricity requirements and costs.

Renewable and Alternative Energy Opportunities

The extent to which desalination technologies can be coupled with intermittent renewable or geothermal electric generation, use off-peak electricity or waste heat, and operate in areas of limited electric generation or transmission capacity but with renewable energy resources is increasingly receiving attention. Desalinating more water when wind energy is available (which requires facilities that can operate with a variable water inflow) and storing the treated water for when water is demanded can almost be viewed as a means of electricity storage and reduction of peak demand. Efforts to jointly manage water and energy supply and demand and to integrate renewable energy with desalination may bolster support for desalination. The first large-scale photovoltaic-powered reserve osmosis seawater desalination facility is in Saudi Arabia, with multiple additional facilities planned in the country before 2018. Concern over declining aquifer levels in Saudi Arabia was one driver for these investments. With demand for desalination increasing in energy-importing countries like India, China, and small islands, there may be particularly strong interest in facilities combining desalination with renewable energy. Some research also is being directed at opportunities for direct solar distillation desalination technologies, particularly smaller-scale production units in solar-abundant remote areas with available land and low-cost labor.

Health and Environmental Concerns

From a regulatory, oversight, and monitoring standpoint, desalination as a significant source of water supply is relatively new in the United States, which means the health and environmental regulations, guidelines, and policies regarding its use are still being developed. Existing federal, state, and local laws and policies often do not address unique issues raised by desalination. This creates uncertainty for those considering adopting desalination and membrane technologies.

Environmental and human health concerns often are raised in the context of obtaining the permits required to site, construct, and operate the facility and dispose of the waste concentrate. A draft environmental scoping study for a facility in Brownsville, TX, identified up to 26 permits, approvals, and documentation requirements for construction and operation of a seawater desalination facility. According to the Pacific Institute's report Desalination, With a Grain of Salt , as many as 9 federal, 13 state, and additional local agencies may be involved in the review or approval of a desalination plant in California. For example, during the Corps' process for issuing a seawater desalination facility permits for placing structures in waterways and dredging and filling in navigable waters, the U.S. Coast Guard would consult with the Army Corps of Engineers on whether an intake facility would be a potential navigation hazard and the National Oceanic and Atmospheric Administration would consult on whether intake facilities and discharge of waste concentrate may affect marine resources.

As previously noted, most states do not have policies and guidance specifically for desalination facilities, and instead deal with each project individually. California, through its State Water Resources Control Board, is working toward statewide guidance in order to improve statewide consistency in review and permitting of seawater desalination facilities.

Some of the regulatory requirements are not seen as particularly onerous; others may be seen as challenging depending on the location and size of the facility. Some stakeholders view the current permit process as a barrier to adoption of desalination. Other stakeholders argue that rigorous review and permitting is necessary because of the potential impact of the facilities on public health and the environment. Particular attention often is paid during permitting to the impingement and entrainment of aquatic species by intake structures for coastal and estuarine desalination facilities and the disposal of waste concentrate.

Evolving Drinking Water Guidelines

While the quality of desalinated water is typically very high, some health concerns remain regarding its use as a drinking water supply. The source water used in desalination may introduce biological and chemical contaminants to drinking water supplies that are hazardous to human health, or desalination may remove minerals essential for human health.

For example, boron, which is an uncommon concern for traditional water sources, is a significant constituent of seawater and can also be present in brackish groundwater extracted from aquifers comprised of marine deposits. Boron levels after basic reverse osmosis of seawater commonly exceed current World Health Organization health guidelines and the U.S. Environmental Protection Agency (EPA) health reference level. While the effect of boron on humans remains under investigation, boron is known to cause reproductive and developmental toxicity in animals and irritation of the digestive tract, and it accumulates in plants, which may be a concern for agricultural applications. Boron can be removed through treatment optimization, but that treatment could increase the cost of desalted seawater.

EPA sets federal standards and treatment requirements for public water supplies. In 2008, EPA determined that it would not develop a maximum contaminant level for boron because of its rare occurrence in most groundwater and surface water drinking water sources; EPA has encouraged affected states to issue guidance or regulations as appropriate. Most states have not issued such guidance. Therefore, most U.S. utilities lack clear guidance on boron levels in drinking water suitable for protecting public health. The National Research Council recommended development of boron drinking water guidance to support desalination regulatory and operating decisions; it recommended that the guidance be based on an analysis of the human health effects of boron in drinking water and other sources of exposure.

Similarly, the demineralization (particularly the removal of the essential minerals calcium and magnesium) by desalination processes also can raise health concerns. This has prompted researchers to promote the remineralization of desalinated water prior to the water entering the distribution system in communities that are highly dependent on desalinated water. Another health-related concern is the extent to which microorganisms unique to seawater and algal toxins may pass through reverse osmosis membranes and enter the water supply, and how facilities may need to be operated differently when these organisms and algal toxins are present. Algal toxins are a consideration for desalination facilities in locations affected or potentially affected by harmful ocean algal blooms that can produce a range of substances, including neurotoxins (e.g., domoic acid). How to effectively manage desalination facilities in order to avoid public health threats from algal blooms is an emerging area of interest and research.

Some of the coastal facilities contemplated in the United States would treat estuarine water. Estuarine water, which is a brackish mixture of seawater and surface water, has the advantage of lower salinity than seawater. The variability in the quality and constituents in estuarine water, as well as the typical surface water contaminants (e.g., infectious microorganisms, elevated nutrient levels, and pesticides), may complicate compliance of desalinated estuarine water with federal drinking water standards.

Concentrate Disposal Challenges and Alternatives

Unlike desalination treatment costs, concentrate disposal costs generally have not decreased. For inland brackish desalination, significant constraints on adoption of the technologies are the uncertainties and the cost of waste concentrate disposal. For coastal desalination projects, the concentrate management options are often greater because of surface water disposal opportunities. EPA is authorized to manage the disposal and reuse of desalination's waste concentrate. The disposal option selected largely is determined by which alternatives are appropriate for the volumes and specific characteristics of the concentrate and the cost-effectiveness of the alternatives, which is largely shaped by the proximity of the disposal option and the infrastructure, land, and treatment investments required.

The dominant concentrate disposal options for municipal desalination facilities are surface and sewer disposal; in 26 of the 32 states with municipal desalination facilities, only surface and sewer discharge are used for concentrate disposal. Surface water disposal of waste concentrate is permitted on a project-specific basis based on predicted acute and chronic effects on the environment. Inland surface water disposal is particularly challenging because of the limited capacity of inland water bodies to be able to tolerate the concentrate's salinity. Limited amounts of concentrate also may be sent through sewer systems with large-volume wastewater treatment facility. As of 2010, deep well injection for municipal concentrate disposal had been used primarily in Florida. For injection, EPA generally classifies waste concentrate as an industrial waste, thus requiring that the concentrate be disposed of in deep wells appropriate for industrial waste. Desalination proponents argue that desalination's concentrate is sufficiently different from most industrial waste that it should be reclassified to increase the surface and injection well disposal opportunities. Some states have made efforts to promote the beneficial use of waste concentrate (e.g., use as liquids in enhanced oil and gas recovery) and facilitate its disposal including land application techniques. Notwithstanding these state efforts, land application and evaporation ponds are seldom and decreasingly used disposal options. While states can have such policies and programs in place, federal environmental regulations administered by EPA for the most part define the regulatory context of concentrate disposal.

Concluding Remarks

Desalination and membrane technologies are playing a growing role in meeting water supply and water treatment needs for municipalities and industry. The extent to which this role further expands depends in part on the cost-effectiveness of these technologies and their alternatives. Desalination's energy use, concentrate disposal options, and environmental and health concerns are among the top issues shaping the technology's adoption. How to focus federal desalination research and support to produce results that provide public benefits, and how to provide a regulatory context that protects the environment and public health without unnecessarily disadvantaging these technologies, are among the desalination issues before the 114 th Congress.

Appendix A. Traditional and Emerging Desalination Technologies

There are a number of methods for removing salts from seawater or brackish groundwater to provide water for municipal and agricultural purposes. The two most common processes, thermal distillation and reverse osmosis, are described below; their descriptions are followed by descriptions of some of the more innovative and alternative desalination technologies. The earliest commercial plants used thermal techniques. Improvements in membrane technology have reduced costs, and membrane technology is less energy-intense than thermal desalination (although it is more energy-intense than most other water supply options). Reverse osmosis and other membrane systems account for nearly 96% of the total U.S. desalination capacity and 100% of the municipal desalination capacity.

Reverse Osmosis

Reverse osmosis forces salty water through a semipermeable membrane that traps salt on one side and lets purified water through. Reverse osmosis plants have fewer problems with corrosion and usually have lower energy requirements than thermal processes.

Examples of how research advances in the traditional desalination technologies of reverse osmosis have the potential for improving the competitiveness and use of desalination are: nanocomposite and nanotube membranes and chlorine resistant membranes. Nanocomposite membranes appear to have the potential to reduce energy use within the reverse osmosis process by 20%, and nanotube membranes may yield a 30%-50% energy savings.

Membranes are susceptible to fouling by biological growth (i.e., biofouling), which reduces the performance of the membranes and increases energy use. The most widely used biocide is chlorine because it is inexpensive and highly effective. The most common membranes used in reverse osmosis, however, do not hold up well to exposure to oxidizing agents like chlorine. Advancements in chlorine resistant membranes would increase the resiliency of membranes and expand their applications and operational flexibility.

Distillation

In distillation, saline water is heated, separating out dissolved minerals, and the purified vapor is condensed. There are three prominent ways to perform distillation: multi-stage flash, multiple-effect distillation, and solar distillation. In general, distillation plants require less maintenance and pretreatment before the desalination process than reverse osmosis facilities.

While solar distillation is an ancient means for separating freshwater from salt using solar energy, research into improving the technology is increasing. In large part the interest stems from the potential application for the technology to supply freshwater to small remote settlements where saline supplies are the only source and power is scarce or expensive.

Innovative and Alternative Desalination Processes

Capacitive Deionization

Capacitive deionization desalinates saline waters by absorbing salts out of the water using electrically charged porous electrodes. The technology uses the fact that salts are ionic compounds with opposite charges to separate the salts from the water. The limiting factor for this technology is often the salt absorption capacity of the electrodes. Flow-through capacitive deionization shows promise for energy-efficient desalination of brackish waters.

Electrodialysis

Electrodialysis and capacitive deionization technologies depend on the ability of electrically charged ions in saline water to migrate to positive or negative poles in an electrolytic cell. Two different types of ion-selective membranes are used—one that allows passage of positive ions and one that allows negative ions to pass between the electrodes of the cell. When an electric current is applied to drive the ions, fresh water is left between the membranes. The amount of electricity required for electrodialysis, and therefore its cost, increase with increasing salinity of feed water. Thus, electrodialysis is less economically competitive for desalting seawater compared to less saline, brackish water.

Forward Osmosis

Forward osmosis is an increasingly used but relatively new membrane-based separation process that uses an osmotic pressure difference between a concentrated "draw" solution and the saline source water; the osmotic pressure drives the water to be treated across a semi-permeable membrane into the draw solution. The level of salt removal can be competitive with reverse osmosis, and forward osmosis membranes may be more resistant to fouling than reverse osmosis membranes. A main challenge is the selection of a draw solute; the solute needs to either be desirable or benign in the water supply, or be easily and economically separated out. Research is being conducted on whether a combination of ammonia and carbon dioxide gases or polymers can be used in the draw solution, and on the effects of marine biology on the membranes. The attractiveness of forward osmosis is that when combined with industrial or power production processes that produce waste heat, its electricity requirements can be significantly less than for reverse osmosis. Potential disadvantages of forward osmosis are a lower quantity of freshwater per unit of water treated and a larger quantity of brine requiring disposal.

Freezing Processes

Freezing processes involve three basic steps: (1) partial freezing of the feed water in which ice crystals of fresh water form an ice-brine slurry; (2) separating the ice crystals from the brine; and (3) melting the ice. Freezing has some inherent advantages over distillation in that less energy is required and there is a minimum of corrosion and scale formation problems because of the low temperatures involved. Freezing processes have the potential to concentrate waste streams to higher concentration than other processes, and the energy requirements are comparable to reverse osmosis. While the feasibility of freeze desalination has been demonstrated, further research and development remains before the technology will be widely available.

Ion Exchange

In ion exchange, resins substitute hydrogen and hydroxide ions for salt ions. For example, cation exchange resins are commonly used in home water softeners to remove calcium and magnesium from "hard" water. A number of municipalities use ion exchange for water softening, and industries requiring extremely pure water commonly use ion exchange resins as a final treatment following reverse osmosis or electrodialysis. The primary cost associated with ion exchange is in regenerating or replacing the resins. The higher the concentration of dissolved salts in the water, the more often the resins need to be renewed. In general, ion exchange is rarely used for salt removal on a large scale.