3
Key Technological and Scientific Issues for Desalination

In order to meet the long-term objectives for cost reduction and wider applicability of desalination identified in the Roadmap, innovative ideas will need to be developed and nurtured. The Roadmap and recommendations made in this report should not restrict investment in emerging ideas and technologies but should instead serve to stimulate creative thinkers to apply their expertise and knowledge to achieve the goal of improving desalination and water purification processes and considerably lowering their costs.

Five technology areas are identified in the Roadmap: membranes, thermal technology, alternative technologies, concentrate management, and reuse and recycling. These areas clearly point in the right direction, although the environmental, economic, and social costs of energy for desalination should be included within an additional cross-cutting research area. According to one example provided in the Roadmap, electrical power accounts for 44 percent of the costs of reverse osmosis of seawater (USBR and SNL, 2003), although the exact costs will vary with plant size or the cost of electricity. The impacts of energy use will need to be examined for desalination plants to become more widely used.

While research and technological developments continue to reduce the costs of desalinated water by optimizing performance, additional cost reductions may be more difficult to achieve, especially as many current systems are already operating at high efficiencies. This chapter discusses the technological and scientific issues for desalination, according to the five technological areas in the Roadmap. For each technology area, the cost issues and technical opportunities for contributing to desalination are described, and the projects identified in the Roadmap are reviewed. Missing topics that deserve further study are presented, and some research areas are suggested to be deleted. Research topics proposed in the Roadmap that were considered appropriate are not discussed at length; thus, the amount of discussion on individual projects should not be viewed as a reflection of the panel’s priorities. These suggested revisions to the research areas itemized in the Roadmap for each of the technology areas are summarized in Tables 3-1 through 3-6.



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Review of the Desalination and Water Purification Technology Roadmap 3 Key Technological and Scientific Issues for Desalination In order to meet the long-term objectives for cost reduction and wider applicability of desalination identified in the Roadmap, innovative ideas will need to be developed and nurtured. The Roadmap and recommendations made in this report should not restrict investment in emerging ideas and technologies but should instead serve to stimulate creative thinkers to apply their expertise and knowledge to achieve the goal of improving desalination and water purification processes and considerably lowering their costs. Five technology areas are identified in the Roadmap: membranes, thermal technology, alternative technologies, concentrate management, and reuse and recycling. These areas clearly point in the right direction, although the environmental, economic, and social costs of energy for desalination should be included within an additional cross-cutting research area. According to one example provided in the Roadmap, electrical power accounts for 44 percent of the costs of reverse osmosis of seawater (USBR and SNL, 2003), although the exact costs will vary with plant size or the cost of electricity. The impacts of energy use will need to be examined for desalination plants to become more widely used. While research and technological developments continue to reduce the costs of desalinated water by optimizing performance, additional cost reductions may be more difficult to achieve, especially as many current systems are already operating at high efficiencies. This chapter discusses the technological and scientific issues for desalination, according to the five technological areas in the Roadmap. For each technology area, the cost issues and technical opportunities for contributing to desalination are described, and the projects identified in the Roadmap are reviewed. Missing topics that deserve further study are presented, and some research areas are suggested to be deleted. Research topics proposed in the Roadmap that were considered appropriate are not discussed at length; thus, the amount of discussion on individual projects should not be viewed as a reflection of the panel’s priorities. These suggested revisions to the research areas itemized in the Roadmap for each of the technology areas are summarized in Tables 3-1 through 3-6.

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Review of the Desalination and Water Purification Technology Roadmap MEMBRANE TECHNOLOGIES Semi-permeable membranes can be used to selectively allow or prohibit the passage of ions, enabling the desalination of water. Over the last 40 years, tremendous advancements have been made in the field of membrane technologies. In fact, reverse osmosis (RO) represents the fastest growing segment of the desalination market, and as of 2002, RO represented 43.5 percent of the capacity of all desalination plants greater than 0.026 mgd, approximately equal to the thermal desalination capacity (Wangnick, 2002). As noted in the Roadmap, “membranes are expected to play critical roles in formulating future water supply solutions.” Membrane technologies can be used for desalination of both seawater and brackish water, but they are more commonly used to desalinate brackish water because energy consumption is proportional to the salt content in the source water. Membrane technologies have the potential to contribute to water supplies through their use in treating degraded waters in reuse or recycling applications since membrane technology can remove microorganisms and many organic contaminants from feed water. Compared to thermal distillation processes, membrane technologies generally have lower capital costs and require less energy, contributing to lower operating costs. However, the product water salinity tends to be higher for membrane desalination (<500 ppm TDS) than that produced by thermal technologies (≤25 ppm TDS) (USBR, 2003a). Membrane technologies for desalination and water purification typically operate under one of two driving forces: pressure or electrical potential. The following pressure-driven membrane technologies are commercially available for treating impaired waters in a range of applications (Lee and Koros, 2002) (Figure 3-1). In addition to understanding the removal capabilities of the membrane process, it is important to note the typical pressure driving force ranges and separation mechanisms, because it can affect their power consumption. Reverse osmosis (RO) membranes are used for salt removal in brackish and seawater applications. RO membranes have also been shown to remove substantial quantities of some molecular organic contaminants from water (Sedlak and Pinkston, 2001; Heberer et al., 2001). RO removes contaminants by solution diffusion4 and operates under a trans-membrane pressure difference in the range of ~ 5–8 MPa. Nanofiltration (NF) membranes are used for water softening (removing primarily divalent cations), organics and sulfate removal, and some removal of viruses. NF membranes operate under a trans-membrane pressure difference in the range of 0.5–1.5 MPa. Removal is by combined sieving and solution diffusion. Ultrafiltration (UF) membranes are used for removal of color, higher weight dissolved organic compounds, bacteria, and some viruses. UF membranes also operate via a sieving mechanism under a trans-membrane pressure difference in the range of ~50–500 kPa. 4   The solution diffusion theory presumes that both the solutes and water molecules dissolve in the RO membrane material and diffuse through. Water passes based on pressure, but solute separation occurs because of a difference in diffusion rates through the RO membrane.

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Review of the Desalination and Water Purification Technology Roadmap FIGURE 3-1 Size ranges removed by various membrane types along the filtration spectrum. SOURCE: Pankratz and Tonner, 2003. Microfiltration (MF) membranes are used for turbidity reduction and removal of suspended solids and bacteria. MF membranes operate via a sieving mechanism under a trans-membrane pressure difference in the range of ~50–500 kPa. Electrodialysis is another membrane-based process that is important to desalination, which operates under a different driving force, applying an electrical potential to motivate ions in opposite directions to produce an ion-depleted and ion-enriched stream in each cell pair. Electrodialysis (ED) is the separation of the ionic constituents in water through the use of electrical potential and cation- and anion-specific membranes. In ED applications, hundreds of positively and negatively charged cell pairs are assembled in a stack to achieve a practical module (Lee and Koros, 2002; Strathmann, 1992). Electrodialysis reversal (EDR) operates according to the same principles, but periodically reverses the polarity of the system to reduce scaling5 and membrane clogging. Electrodialysis represents approximately three percent of worldwide desalination capacity (Wangnick, 2002). Summary of Cost Issues Desalination costs associated with the reverse osmosis process have markedly declined in recent years (Figure 1-6). These cost reductions have occurred through economies of scale and improvements in membrane technology (e.g., increased salt- 5   Scaling is the deposition of mineral deposits on the interior surfaces of process equipment or water lines as a result of heating or other physical or chemical changes.

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Review of the Desalination and Water Purification Technology Roadmap rejection, flux rate, and longevity), energy recovery devices, and reduced material costs. Considering the recent improvements in membrane-based desalination, substantial further cost savings could be more difficult to achieve, suggesting the need for a carefully developed research agenda targeted to areas that offer the most promise for cost reduction. The Roadmap provides an example of the cost breakdown for seawater desalination by RO that suggests that the largest cost reduction potential lies in capital costs (fixed charges) and energy (Figure 3-2). Continued improvements in membrane materials, permeability, and energy recovery devices could generate additional cost reductions. Substantial savings could also arise from improvements or simplifications to pretreatment systems for membrane desalination, since capital and operating costs for reverse osmosis pretreatment can represent more than 50 percent of the overall cost of a reverse osmosis system (Pankratz and Tonner, 2003). The Roadmap proposes long-term critical objectives of 50–80 percent reduction in capital and operating costs and an increase in energy efficiency of 50–80 percent. For membrane-based desalination facilities, these energy goals will not be possible with advances in existing membrane technology alone. A simplified but fundamental example can illustrate the hard limits that the technology, as it is currently practiced, is encountering. Production of a purified stream of permeate water typically involves a permeate recovery ratio (the fraction of feedwater passing through the membrane) much less than 100 percent. The salt concentration increases in the water that does not pass through the membrane (the concentrate) and requires even more driving force to produce the next increment of product water as higher permeate recovery ratios are achieved. Given the mechanical limits of membranes and the desire to avoid excessive pressure, the permeate recovery ratio is typically limited to 50 percent or less for seawater feeds (Wilf and Klinko, 1997). As an example, in a RO seawater system operating at 50 percent feedwater recovery, flux rate of 8.5 gallons per square foot per day (gfd), with a 34,000 ppm TDS seawater feed at 22°C, the required feed pressure will be about 65 bar (940 psi). If the system would utilize a 100 percent efficient pumping and energy recovery FIGURE 3-2 Cost structure for a reverse osmosis desalination of seawater. SOURCE: USBR and SNL, 2003.

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Review of the Desalination and Water Purification Technology Roadmap FIGURE 3-3 Typical reverse osmosis membrane desalination system with energy recovery. unit, the minimum energy consumption would be 6.7 kWh/1000 gal (1.77 kWh/m3).6 A typical RO system with energy recovery is illustrated in Figure 3-3. Current state-of-the-art seawater RO systems under similar conditions can operate at 8.4 kWh/1000 gal (Andrews et al., 2001); an 80 percent reduction would result in 1.7 kWh/1000 gal, which is not a realistic goal for standard RO technology. Such energy recovery approaches provide, at best, the ability to operate at the thermodynamic efficiency limit. Based on the above 6.7 kWh/1000 gal limit, this would represent a maximum optimistic reduction of 20 percent.7 To obtain further reductions in energy, a different desalination approach is required, such as the targeted ability to remove only impurities from the water, rather than passage of all of the purified water across the membrane. The Roadmap correctly states that, as noted above, technology breakthroughs could result in more efficient membrane technologies that would remove only the specific target contaminants from the water stream. This targeted removal has attractive aspects in many cases with a well-defined feed stream containing known impurities. The lower 6   This value—the energy required for high pressure pumps for reverse osmosis of seawater, Ero—was calculated as Ero =K*Pf/(Effhyd*Effmot*R) -Erec, where K is a unit conversion factor, Pf is the calculated feed pressure, Effhyd is the pump hydraulic efficiency, Effmot is the pump motor efficiency, R is the system recovery ratio (assumed here to equal 0.5), and Erec is the energy recovered through an energy recovery turbine. The required feed pressure was calculated with the above stated parameters for a multi-element membrane unit using the software package IMS by Hydranautics, which assumes the performance of commercial seawater membranes. The value for Erec= K*Pc*Efft*(1-R)/(Effmot*R), where Pc is the pressure of the concentrate stream, and Efft is the energy recovery turbine efficiency. Assuming 100 percent efficiencies and no frictional losses in the system (so that Pf= Pc), the equations can be combined into Ero=K*Pf. Actual RO operations would require additional energy to power the necessary pretreatment and auxiliary equipment. 7   Similar estimates are also derived by consideration of fundamental thermodynamic calculations based on free energies for typical feed, permeate and concentrate streams.

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Review of the Desalination and Water Purification Technology Roadmap operating pressures possible with such an approach would also result in lower operating costs.8 Selective contaminant removal would reduce the amount of mass of chemicals in the concentrate that then must be properly disposed. However, this approach runs the risk of not producing as pure a water product, since unrecognized contaminants that are not targeted for removal may remain in the treated water. This aspect is a significant public health concern when dealing with degraded waters from diverse sources. Review of Research Directions The membrane research areas and projects identified in the Roadmap for improving the efficiency and cost of desalination are appropriate but incomplete. The Roadmap identifies a significant portion of the research areas critical to improving membrane technologies in desalination. However, there are some areas that are not included in the Roadmap, and some of the existing topics should be expanded. The table of research topics included in the Roadmap has been modified (Table 3-1) to highlight these missing topics and summarize the suggested revisions. Sensor Development/Membrane Integrity To address the “national need” of providing safe water, the project to develop an on-line viral analyzer should be expanded to include pathogens as a broader definition of potentially harmful biological contaminants in water. The integrity of the membranes and membrane system is also a critical research area that should be included. Even a tiny area of defects in the membrane surface of an otherwise perfect barrier to pathogens can allow a number of organisms to pass across the barrier into the product water. In cases involving long storage time, some non-parasitic organisms could multiply to an unsafe level of pathogens in the product water. Integrity verification of RO/NF membranes is expected to become an important issue in the future as potential sources of water for desalination (including seawater) are facing contamination by municipal and agricultural discharge. Tailorable Membrane Selectivity In order to ensure sustainability and adequate water supplies, it is important to develop the ability to design in selectivity as well as permeability. Tailorable membrane selectivity would facilitate reliable removal of specific contaminants if and when they are identified in a given source water. This technology would enable undesirable components to be removed at some acceptable cost in terms of permeability and contribute to water supply and reuse options. Membrane Fouling Efforts to mitigate membrane fouling should be expanded to include the development of fouling-resistant elements and systems and appropriate indicators of fouling. 8   Since RO/NF operation is based on applying pressure higher than the osmotic pressure difference between the feed and the permeate, if only selective ions are rejected, the osmotic pressure of the permeate is closer to the osmotic pressure of the feed; thus, lower feed pressures would be required for the same permeate flux rate.

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Review of the Desalination and Water Purification Technology Roadmap Membranes can be fouled by any number of organic or inorganic materials, including microbial biomass, such as algae or bacteria. Harsh cleaning agents decrease the life of a membrane element and contribute significantly to membrane system operating costs. Therefore, the development of fouling-resistant membrane surfaces and elements would be beneficial, leading to longer membrane life spans and reduced operating costs from both cleaning and pretreatment to reduce fouling. Given widely different feed water qualities and membrane configurations, it would be difficult to develop a membrane surface that is completely resistant to all types of fouling; thus, module restoration will also be necessary. Therefore, improved methods of cleaning and restoring fouled membrane modules rather than disposing of them is an important priority for research. Membrane Operating Costs Reduction of operating and maintenance costs is imperative to the goal of reducing the costs of desalination. Specifically, reducing the use of pre- and posttreatment chemicals and improving cartridge filter design in order to reduce replacement rate are two areas for potential cost savings related to membrane processes. The selection of pretreatment methods is based on the feedwater quality, membrane material, module configuration, recovery, and desired effluent quality (Taylor and Jacobs, 1996). It would be advantageous to reduce the need for pretreatment by improving the membrane materials or configuration, including the use of backwashable MF or UF as prefilters. For example, advances in membrane configurations could improve the hydrodynamics of the system by increasing the cross-flow velocity or introducing dean vortices in the module to minimize concentration polarization and thus the need for removal of particulates upstream of the module (Belfort et al., 1994). Posttreatment is an important cost component and should also be addressed. RO- and NF-treated permeate tends to be corrosive because of reduced pH, calcium, and alkalinity. The corrosive tendency of desalted water can be reduced by the addition of lime or soda ash and/or by the addition or removal of CO2. The amount of chemicals added for posttreatment can be reduced by developing membranes with selective ion rejection (e.g., to specifically reduce boron, which can be hazardous in agricultural applications) or through application of integrated processes to optimize the overall treatment scheme. Membrane Process Design Further reductions in manufacturing costs of membrane desalination facilities should be explored, such as designing equipment to utilize less expensive materials and improving configurations to reduce element costs. Membrane process design should specifically include integrated membrane (Glueckstern et al., 2002) and hybrid membrane/non-membrane components. Integrated membrane systems utilize two membrane technologies, either including membrane pretreatment or using two different membrane types for salinity reduction, thereby improving the efficiency of the plant. Strategically designed hybrid membrane systems, such as membrane-thermal systems, may decrease energy consumption and/or control water quality, depending on the quality of the feedwater (Ludwig, 2003). These membrane/thermal desalination hybrid plants may offer greater flexibility when determining the final salt content and overall energy consumption of the system. Opportunities remain for process optimization in integrated membrane and hybrid desalination systems.

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Review of the Desalination and Water Purification Technology Roadmap TABLE 3-1 A summary of the committee’s recommendations for research topics for membrane technologies. National Need → Technology Area ↓ Provide Safe Water Ensure Sustainability/ Ensure Ad equate Supplies Keep Water Affordable Membrane Technologies Smart membranes Contain embedded sensors Disinfection treatment 2020: sense contaminant differential across the membrane, automatically change performance and selectivity Sensor development Model compounds for organics Online viral/pathogen/pyrogen analyzer Micro/in-situ/ built-in EPS sensor to detect biofilms; particulate fouling sensor Integrity verification Membrane research Completely oxidant resistant Operate over a range of pH’s (enable either mechanical/chemical cleaning) Adjust removal capability based on feed water quality and removal needs (2014—pharmaceuticals removal based on molecular weight, hydrophilicity) Biofilm-resistant surfaces Develop high integrity membranes & systems Mechanistic/fundamental approach to membrane design CFD of feed channel Conduct research to gain understanding of molecular-level effects Design-in permeability/selectivity Develop understanding of whole system (based on current knowledge) Develop model of optimization Research sensitivity of parameters for model Develop fundamental understanding of fouling mechanisms Understand how to mitigate fouling Understand biofouling Optimize operational controls Develop fouling resistant elements/ systems Develop indicators for fouling Develop performance restoration of fouled membrane Basic research to improve permeability Minimize resistance Model/test non spiral configurations Improve methods or develop new methods of reducing/recovering energy Integrate membrane and membrane system designs Reduce membrane operating/maintenance costs Reduce consumption of pretreatment and posttreatment chemicals Improve cartridge filter design to reduce replacement rate Reduce manufacturing costs through design Identify or develop less expensive materials for membranes and filtration systems, including corrosion resistant materials Improve configuration to reduce elements cost NOTE: These recommendations are presented as revisionsto the “research areaswith the greatest potential” as identified in theRoadmap. The table has been reproduced in the same format that appears in the Roadmap, and italicized topics indicate additional promising research areas suggested in this report. SOURCE: Modifiedfrom USBR and SNL, 2003.

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Review of the Desalination and Water Purification Technology Roadmap Membrane Bioreactors An important opportunity for membrane processes in water reuse applications is in membrane bioreactors (MBRs). MBRs have grown in use and applicability in recent years, and are now used for municipal and industrial wastewater treatment applications. Water treated by MBRs routinely meets reuse standards for certain feedwaters (Manem and Sanderson, 1996; Rittman, 1998); however, further research could increase the applicability of MBRs to a wider range of feedwater qualities. The long-term operation of a MBR is a function of the performance of the membranes, which depends on the membrane material, operational parameters, flux characteristics and module configuration. This important membrane application is further discussed in the reuse section of this chapter. Priorities Among the membrane technology areas identified in the Roadmap and those additional areas suggested by this committee (see Table 3-1), several have been identified as the highest priority research topics within this category. These topics were identified as those most likely to contribute substantially to the objectives set by the Roadmapping Team, with regard to improved energy efficiency, reduced operating costs, and high quality water. The priority topics are: Improving membrane permeability (in order to operate at a lower feed pressure for a given module cost) while improving on or maintaining current salt rejections. Improving or developing new methods for reducing energy use or recovering energy (e.g., improving the efficiency of high pressure pumps). Improving pretreatment and posttreatment methods to reduce consumption of chemicals. Developing less expensive materials to replace current corrosion resistant alloys used for high pressure piping in seawater RO systems. Developing new membranes that will enable controlled selective rejection of contaminants. Improving methods of integrity verification. Developing membranes with improved fouling-resistant surfaces. THERMAL TECHNOLOGIES Approximately one-half of the world’s installed desalination capacity uses a thermal distillation process to produce fresh water from seawater. Thermal processes are the primary desalination technologies used throughout the Middle East because these technologies can produce high purity (low TDS) water from seawater and because of the lower fuel costs in the region. Three thermal processes represent the majority of the thermal desalination technologies in use today.

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Review of the Desalination and Water Purification Technology Roadmap Multi-Stage Flash Distillation (MSF) uses a series of chambers, each with successively lower temperature and pressure, to rapidly vaporize (or “flash”) water from bulk liquid brine. The vapor is then condensed by tubes of the inflowing feed water, thereby recovering energy from the heat of condensation. Despite its large energy requirements, MSF is among the most commonly employed desalination technologies. MSF is a reliable technology capable of very large production capacities per unit. Multi-Effect Distillation (MED) is a thin-film evaporation approach, where the vapor produced by one chamber (or “effect”) subsequently condenses in the next chamber, which exists at a lower temperature and pressure, providing additional heat for vaporization. MED technology is being used with increasing frequency when thermal evaporation is preferred or required, due to its lower power consumption compared to MSF. Vapor Compression (VC) is an evaporative process where vapor from the evaporator is mechanically compressed and its heat used for subsequent evaporation of feed water. VC units tend to be used where cooling water and low-cost steam are not readily available. (Pankratz and Tonner, 2003) Three other thermal techniques—solar distillation, membrane distillation, and freezing—have been developed for desalination, although they have not been commercially successful to date (Buros, 2000). In brief, solar distillation uses the sun’s energy to evaporate water from a shallow basin, which then condenses along a sloping glass roof. In membrane distillation, salt water is warmed to enhance vapor production, and the vapor is exposed to a membrane that can pass water vapor but not liquid water. Freezing technologies use ice formation under controlled conditions in the source water, initially eliminating salt from the ice crystals and allowing the brine to be rinsed away. As noted in the Roadmap, thermal seawater distillation processes employed in the Middle East are mature technologies that may not have broad application in the United States. While thermal desalination is not expected to displace membrane-based desalination as the predominate desalination technology in the United States, thermal technologies have substantial potential and should be considered more seriously than they have been to date. For example, thermal technologies can be built in conjunction with other industrial applications, such as electric power generating facilities, to utilize waste heat and lower overall costs while providing other significant process advantages, such as high-quality distillate even in seawater applications. Summary of Cost Issues Wangnick (2002) notes that energy use represents 59 percent of the typical water costs from a very large thermal seawater desalination plant (Figure 3-4). The other major expense comes from capital costs. Thus, cost reduction efforts would be most effective if they were focused on these areas. For example, research efforts to develop less-costly corrosion-resistant heat-transfer surfaces could reduce both capital and energy costs. The most significant cost reduction opportunities for thermal desalination may be found in the area of energy management by utilizing “new” sources of heat or energy to accomplish evaporation or through the use of existing energy sources during off-peak periods for thermal desalination purposes.

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Review of the Desalination and Water Purification Technology Roadmap FIGURE 3-4 Breakdown of typical costs for a very large seawater thermal desalination plant. SOURCE: Wangnick, 2002. As acknowledged in the Roadmap, the lack of centralized water and power planning in the United States contributes to the high cost of thermal desalination. Yet the Roadmap seems to dismiss cogeneration plants (combined water and power production), despite their notably reduced energy consumption, because they are “expensive to build and operate.” Wider application of cogeneration should be explored further, particularly as older power plants are replaced or repowered. Review of Proposed Research Directions The Roadmap does not develop a research path based on opportunities for improving thermal technologies, nor does it adequately identify areas of research in thermal technologies which might help meet the report’s objectives. Overall, the Roadmap’s Working Group appears to have lacked thermal desalination expertise, and several misleading statements are made in the Roadmap about thermal desalination. For example, the report misinforms readers by neglecting to state that the energy requirement of thermal technologies (“260 kw-hr/1000 gallons – or one quarter of the electricity consumed by the average house in a month”) can be met by “waste” heat and other low-grade energy sources. The Roadmap also states that thermal plants produce “more dilute concentrate waste.” In the case of vapor compression, this is incorrect. In the case of MSF and MED processes, the concentration factor for thermal and membrane seawater desalination is very similar, but the overall thermal desalination plant discharge may be diluted because a significant amount of cooling water may also be discharged with the concentrate. The thermal technology research areas and projects identified in the Roadmap are generally appropriate but could be expanded and in some cases revised. Additional research on the topic of hybrid technologies is proposed in the Roadmap, although the rationale is not well described. The Roadmap should emphasize that integrating membrane and thermal processes with an electric generating station to meet fluctuating

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Review of the Desalination and Water Purification Technology Roadmap in membrane technologies, MBRs could provide a higher level of treatment at comparable costs of traditional treatment, thus contributing to better public health protection in reuse applications. Examine Feasibility of Decentralized Treatment of Recycled Water Recent papers suggest that decentralized wastewater treatment using membrane-based technologies could serve to produce recycled water closer to reuse sites, reducing the need for costly distribution system infrastructure (Tchobanoglous, 2003; Gagliardo and Mallia, 2003). A feasibility study should be conducted on the topic of decentralization of water recycling facilities and also examine regulatory monitoring and permitting issues. Lessons Learned from Successes and Failures in Reuse Projects The Roadmap cites the lack of public and regulatory acceptance for indirect potable reuse projects as a key factor limiting the ability to expand water reclamation and reuse, stating that recycling and reuse suffers from an “unfair stigma.” However, the Roadmap does not appear to address this controversial issue. Several states have demonstrated regulatory acceptance of recycled water for a variety of applications from landscape irrigation to indirect potable reuse, and Arizona, California, and Florida have identified indirect potable reuse projects. The water reuse industry should examine both successful (Mills et al., 1998; Seah, 2003) and unsuccessful (Lauer and Rogers, 1998; Olivieri et al., 1998) projects using the lessons learned from those projects for future reuse efforts. Lessons can also be learned from successful and unsuccessful water reuse treatment train design. Wastewater treatment process trains are becoming more complex as additional unit processes are being added to them to meet increased water quality objectives for some reuse applications. The sequencing, placement, and integration of unit treatment processes within a wastewater treatment process train can impact process performance and economics, often determining the success or failure of a reuse application. Efforts should be made to publish both successes and failures in reuse design, so that the technical community can learn from the experiences of others and better understand the complex variables and processes involved. Research Topics to be Deleted While salinity management in agricultural watersheds is important, the project seems well beyond the scope of the original legislative mandate to focus on desalination technologies. If such a task were included in the Roadmap, other management-based strategies should be considered such as water transfers, conservation, and demand management. Therefore, for consistency with the objectives of the Roadmap, this task should be addressed elsewhere by the Bureau of Reclamation. Because constructed wetlands do not fit into a desalination- or membrane-technology-based purification strategy to ensure sustainable water supply, this item should also be deleted because it appears to be beyond the scope of the Roadmapping effort. The subject of life-cycle economics of water reuse, while valuable, contains significant overlap with the many other sections and has been moved to the section on cross-cutting technologies. Pretreatment is primarily a membrane technology issue, and for consistency these topics should be deleted from the section on reuse technologies.

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Review of the Desalination and Water Purification Technology Roadmap TABLE 3-4 A summary of the committee’s recommendations for research topics for reuse and recycling technologies. National Need → Technology Area ↓ Provide Safe Water Ensure Sustainability/Ensure Adequate Supplies Keep Water Affordable Reuse/Recycling Technologies Develop improved techniques for identification and quantification of chemical contaminants Develop a set of chemical and microbiological surrogates acceptable to the public for indirect potable reuse Develop an understanding of structure activity relationship between selective organic molecules and RO membrane materials Risk comparison between various water reuse schemes and potable water counterparts with potential uncertainty described Develop tools for risk assessments, including consideration of data needed and the potential uncertainty of the analysis Real-time sensing/monitoring/controls Monitoring wastewater particulates based upon physical characteristics Molecular based biological markers for integrity monitoring Research and development for Examine feasibility of decentralized treatment based local use of recycled water Lessons learned from successes and failures in water reuse Watershed-based salinity management strategy Constructed wetlands Develop large-scale regional characterization of subsurface injection capability of US Enhance membrane bioreactor technology for cost reduction Document the lifecycle economics of water reuse for various applications Pretreatment Filtration Biological coating (disinfectant) Research to enable prediction of migration and recovery through aquifers NOTE: These recommendations are presented as revisions to the “research areas with the greatest potential ” as identified in the Roadmap. The table has been reproduced in the same format that appears in the Roadmap, and italicized topics indicate additional promising research areas suggested in this report. Words shown in strike-out represent suggested deletions. SOURCE: Modified from USBR and SNL, 2003.

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Review of the Desalination and Water Purification Technology Roadmap Priorities Among the reuse and recycling research topics identified in the Roadmap and those suggested by this committee (see Table 3-4), several have been identified as those most likely to contribute substantially to the objectives set by the Roadmapping Team. The priority topics are: Developing improved techniques for identification and quantification of chemical contaminants. Examining the feasibility of decentralized treatment in order to reduce the transport and distribution system costs. Enhancing membrane bioreactor technology for cost reduction. Conducting a risk comparison between various water reuse schemes and potable water counterparts. Developing a set of chemical and microbiological surrogates for indirect potable reuse and developing a better understanding of the relationship between rejected solutes and the membrane. Developing more sensitive on-line membrane integrity monitoring systems. CONCENTRATE DISPOSAL Desalination and membrane-based water purification technologies do not eliminate the water constituents of concern. Rather, these constituents are concentrated in a fraction of the water, thus improving the water quality of the other fraction. Designing a desalination plant to minimize production of salt solids is not technologically feasible. However, the volume of water containing concentrated salts can be reduced through various technologies. The concentrate is a residual that then must be handled in a manner that minimizes environmental impacts. The Roadmap acknowledges the need for concentrate disposal by stating “finding environmentally-sensitive disposal options for this concentrate that do not jeopardize the sustainability of water sources is difficult, and, thus, next-generation desalination plants will have to be designed to minimize the production of these concentrates, or find useful applications for them.” (USBR and SNL, 2003) The concentrate from seawater desalination is generally returned to the sea where the process of dilution is used to minimize potential negative impacts. Concentrate from the desalination of brackish inland water supplies generally cannot be returned to sea because of geographical location. Various options are available for inland desalination concentrate management, including: deep well injection, pond evaporation, near zero liquid discharge (n-ZLD) and ZLD, solar energy ponds, shallow aquifer storage for future use, or return to a saline water body via pipeline. The best option for concentrate disposal must be selected on a site-specific basis based on economic and environmental considerations. The options may be constrained by trace metal contaminants such as selenium and arsenic that have ecologically toxic effects. Under these conditions, the concentrate must be handled in a manner to protect wildlife and human health. As recognized in the Roadmap (see Table 3-5), in some cases

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Review of the Desalination and Water Purification Technology Roadmap additional research will be necessary to determine the nature of the hazard posed by various concentrate disposal options in order to ultimately develop appropriate science-based concentrate disposal regulations. Consideration should be given to disposing concentrate from inland desalination plants in a manner that the concentrate will not degrade surface or ground waters; thus, discharge into surface streams should be avoided. Several current or future concentrate management alternatives are summarized below. Disposal in a Saline Water Body is a reasonable option when the desalination plant is located close to a very salty water body such as the ocean. In this case the main constraint is to meet environmental concerns. In most cases, appropriate dilution of the concentrate can be achieved to reduce or eliminate its impact on the environment, but saline disposal could become an issue if the concentrate contains elements with toxic effects on aquatic organisms or on wildlife that feed on aquatic organisms. Deep Well Injection is often the most viable option for concentrate management at inland desalination facilities when suitable geologic formations are available. Generally, suitable formations must be confined and/or isolated to prevent contamination of adjacent aquifers. Suitable formations for injection often contain water with TDS concentrations in excess of 10,000 mg/L and are located at great depths. Injection wells are expensive to develop and operate. Evaporation Ponds offer a means to reduce the concentrate volume by confining the concentrate water in a pond designed for the sole purpose of evaporating the water. This option can be expensive due to the large surface area required and the associated land and impermeable liner costs. Land costs are a function of location but the cost of liners could be reduced through technological improvements. Zero Liquid Discharge (ZLD) or near-ZLD employs evaporative/crystallization systems to remove as much water as possible to reduce the cost of concentrate disposal and to improve the options for beneficial use of the salt products. Gypsum is mined at many locations, and sodium chloride can be extracted from high salt content water bodies. Crop Irrigation is suggested in the Roadmap as a means for disposing of lower concentration desalination waste by application to salt-tolerant plants (halophytes). Solar Energy Ponds utilize concentrate from desalination plants to capture solar radiation and convert it into useable energy. In a sense, a solar energy pond is simply an evaporation pond with the opportunity to produce energy under the appropriate conditions. Solar radiation penetrating the water heats the lower dense layers of water which remain heavier than the layers above in spite of their rise in temperature. With no convection currents to disburse the heat due to the dense saline water layer topped by a less concentrated layer, the bottom layer’s temperature rises to very high levels. The stored heat from this bottom layer of hot brine is then extracted using a heat exchanger.

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Review of the Desalination and Water Purification Technology Roadmap Summary of Cost Issues Coastal desalination plants are often able to dispose of saline concentrate into the ocean or estuaries at relatively modest costs. However, costs associated with concentrate management can constitute a very large portion of the total costs of desalination at inland facilities, and this cost greatly reduces the economic feasibility of desalination technology at inland locations. Reducing the costs of concentrate handling would make available many sources of water, especially brackish groundwater, by making desalination a more amenable option for inland sites. Near-term options for reducing the costs of inland concentrate disposal include research on concentrate volume reduction, development of engineering design tools for concentrate storage for future use, deep-well concentrate injection technology, lower-cost evaporation pond liners, and concentrate salt mineral storage for future utilization as part of a sustainable-use strategy. Long-term options may include extraction of minerals for commercial use rather than disposal at landfills or through burial. Commercialization of these minerals at large inland desalination facilities may prove feasible if life-cycle analysis is used during the design phase. Currently very little information is available for use by engineering design firms conducting feasibility analysis of the commercial use options. Review of Research Directions The Roadmap contains a rather disjointed list of items related to concentrate management technologies that are limited to chemicals associated with desalination plants and do not include research on disposal techniques for waste from reuse/recycling facilities. Recommendations are provided based on the research areas proposed in the Roadmap. Several additional research topics are proposed, while others are recommended to be deleted. Table 3-5 provides a summary of the recommended changes. Silica Control/Removal for Concentrate Minimization Concentrate volume reduction during the desalination process is an important initial component of concentrate management, and concentrate chemistry must be optimized during this process. For example, silica must be removed from the concentrate stream to achieve high recovery rates and reduce concentrate volume because silica greatly limits the application of membrane technology when present in feed water at concentrations greater than 25 mg/L. Anti-scalants are ineffective at high silica concentrations. The standard technology for silica removal is precipitation with lime, but lime precipitation of silica produces large volumes of lime sludge and adds another process to the treatment train thereby increasing cost. Research is needed to find innovative methods for dealing with silica. Control Contaminant Concentrations in Evaporation Ponds Evaporation ponds hold the potential of providing wildlife habitat; however, it is important that the concentrate not contain elements which could be detrimental to wildlife. For example, the opportunity to dispose of saline agricultural drainage waters in

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Review of the Desalination and Water Purification Technology Roadmap evaporation ponds in the western San Joaquin Valley of California has been greatly impacted by the presence of selenium in the drainage water, which moves up the food chain with negative impacts on the birds utilizing the pond. Technologies are being developed to address this problem, but more work remains (Green et al., 2003). Arsenic is a common constituent in groundwater in the western United States; thus, arsenic and, to a lesser extent, selenium will likely occur in the concentrate from the desalination of brackish groundwater in the West. Research is needed to identify concentrate constituents from various water sources and determine the concentration at which deleterious impacts occur. Also, an improved understanding of the fate of these contaminants in concentrate management processes will be needed along with models for contaminant management and control. Technologies to remove these contaminants from the concentrate are also needed. Pond Liners Degradation of aquifers by water percolating from evaporation and solar energy ponds must be prevented. This can be accomplished by appropriately locating ponds where the percolate will not reach fresh water bodies or restricting water percolation using a geomembrane or chemical sealing method for controlling percolation rates. To improve the costs of evaporation ponds, research on low-cost liners and on chemical sealing of the soil is needed to reduce costs associated with this concentrate management method. There may also be opportunities for using existing dry lakebeds that have subsurface groundwater with very high TDS concentrations. Solar Energy Ponds The salt concentration in the storage zone for solar energy ponds must be 20 percent or higher. Reject water from a reverse osmosis desalination plant is not sufficiently concentrated and must be further concentrated—most likely in an evaporation pond—before it can be utilized in a solar power pond. Wider use of solar energy ponds may depend on research to improve salt concentration technologies and studies to evaluate using life-cycle economics. Concentrate as a Resource Current thinking views concentrates as an undesirable residual of desalination and water purification that requires disposal. If concentrate can be managed as a future resource, the feasibility of desalination can be greatly improved for inland sites. Evaporation ponds hold the potential for recovery of the chemical components of the concentrate that have commercial value. Gypsum, sodium chloride, and magnesium sulfate are three minerals that have commercial value. The potential of designing and operating evaporation ponds for long-term recovery of these minerals would certainly contribute to the sustainable aspects of desalination and should be explored. Solar ponds also offer the potential of using the stored heat energy for separation of useful minerals based on solubility differences. Additional work is needed to examine concentrate management designs and consider the use of ion-selective membranes for improved management and marketability of commercially valued desalination-derived salt solids. Concentrate containing 10,000 mg/L of dissolved solids is still 99 percent water. Treating it solely as a waste ignores its potential as a water resource. Concentrate could be stored underground using managed injection well systems for future recovery. Taking

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Review of the Desalination and Water Purification Technology Roadmap this approach would eliminate the very expensive concentrate processing systems. For example, desalination of slightly saline water with a TDS of 2,000 mg/L at a 75 percent recovery creates concentrate with approximately 8,000 mg/L TDS. Texas defines all groundwater with a TDS of between 3,000 and 10,000 mg/L as a “moderately saline” resource that has potential usefulness (TGPC, 2003). Currently, the injection of treated wastewater is used in California and Florida to control saline water movement into freshwater aquifers (Mills et al., 1998). Injection of concentrate for storage underground in either confined or unconfined aquifers as a future resource could greatly reduce the cost of handling this product and make it available for recovery in the future. Chemical and Hydrological Conditions for Subsurface Storage Modeling of the geochemical and hydrologic behavior of injected concentrate coupled with monitoring would be necessary for further advancement of subsurface concentrate storage. The chemistry of the concentrate must be suitable for injection into the formation. Research is needed on the potential for chemical interactions between concentrates, the formation, and resident fluids and gases. Currently, the potential for plugging both the formation and injection wells is difficult to assess, and the technologies and methods to coordinate concentrate chemistry at the surface with geologic formation chemistry are not well understood. Zero Liquid Discharge Although the Roadmap correctly identifies the often prohibitively high cost of zero liquid discharge (ZLD), it does not address the evaporative/crystallization systems that are employed or recommend specific areas where research might be undertaken. Brine concentrators (e.g., vertical tube falling film evaporators) and forced circulation evaporators may be used individually or in series to concentrate dissolved solids to very high levels. These mechanical devices have extremely high energy requirements that often eliminate them from consideration. Research to evaluate methods of improving their efficiencies and test the benefits of alternative configurations, innovative use of chemical anti-scalants/sequestrants, and alternate materials of construction could reduce their cost and increase their areas of applicability. The Roadmap should also acknowledge the incrementally high cost to move from “near-ZLD” to “ZLD.” Projects may be abandoned as economically or environmentally unfeasible because of the high cost to achieve ZLD, whereas near-ZLD (i.e., 90 percent concentrate volume reduction) is often acceptable. Crop Irrigation The salt concentration in the desalination concentrate can vary over a wide range depending upon the initial salinity of the water and the percent recovery of the process, and agricultural crops have a wide range of tolerance to soil water salinity. For example, cotton and sugar beets are examples of salt-tolerant crops whereas lettuce, strawberries, and green beans are examples of salt-sensitive crops. However, the water from the desalination plants producing the lowest concentration will usually exceed the salinity that can be used to irrigate the most salt-tolerant agricultural crops. An additional constraint to using desalination concentrate to irrigate crops is that even halophytes have an upper limit to the salt concentration tolerated in the root zone. Therefore, the excess salts must be leached from the root zone and the leachate has the possibility of degrading

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Review of the Desalination and Water Purification Technology Roadmap TABLE 3-5 A summary of the committee’s recommendations for research topics for concentrate management technologies. National Need → Technology Area ↓ Provide Safe Water Ensure Sustainability/Ensure Adequate Supplies Keep Water Affordable Concentrate Management Technologies Develop chemical and hydrological modeling for subsurface storage and control of concentrates with TDS of <10,000 mg/L Develop technology and models for management and control of problem constituents like arsenic and selenium Develop technology for control/removal of silica Research to improve materials/methods for lining evaporation and solar energy ponds Develop science related concentrate specific regulations for dispersion modeling of mixing zones and ion imbalance for surface water discharge. Research into engineered ecology/bioengineering to discover: How to engineer disposal so that at least it does not harm ecosystems, and if possible benefits them Natural analogs to current treatment The biology of salty water, including understanding env. impacts, using bacteria for benefical treatment, etc. Create “super concentrate ” technologies – complete solidification of residuals and 100% recapture of water for ZLG and > 90% capture of water for near-ZLG Explore beneficial uses of concentrate including recreation, solar pond; cooling water; manufacturing; irrigation; farming agriculture; repair of dead-end stagnant canals; energy recovery; artificial wetlands, halophilic irrigation; aquaculture Explore storage and management of desalination salt solids for recovery and future beneficial/sustainable commercial use Decentralized (Point of Use) Treatment and recycling as a way of managing concentrate Create a “super concentrate ” technology –near-complete or complete solidification of residuals (> 90% capture of water for near-ZLD or 100% recapture of water for ZLD) Evaluate the life-cycle economics of solar energy ponds for inland desalination Cross-cutting: Develop methods of immobilizing/sequestering the concentrate stream Cross-cutting: Develop beneficial uses for the concentrate stream to improve the economics of disposal for ZLD processes. NOTE: These recommendations are presented as revisions to the “research areas with the greatest potential ” as identified in the Roadmap. The table has been reproduced in the same format that appears in the Roadmap, and italicized topics indicate additional promising research areas suggested in this report. Words shown in strike-out represent suggested deletions. SOURCE: Modified from USBR and SNL, 2003.

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Review of the Desalination and Water Purification Technology Roadmap groundwater or surface waters. Crop irrigation, therefore, is not a viable option under most cases, although research is needed to further examine the limits of this disposal option. Priorities Among the concentrate management research topics identified in the Roadmap and those additional topics suggested by this committee, several have been included among the highest priority. These topics were identified as those most likely of contributing substantially to the objectives set by the Roadmapping Team. The priority topics are: Reducing concentrate volume. Management/removal of toxic compounds such as arsenic. Utilization or storage of concentrate mineral at landlocked locations for sustainable system development; i.e., treat concentrate as resource for future mineral extraction rather than a waste for disposal. Underground storage and management of concentrates with a TDS of less than 10,000 mg/L as a water resource for future desalination. PROPOSED CROSS-CUTTING TECHNOLOGY-RELATED RESEARCH One major research area—energy—emerged in this review of the Roadmap, which has the potential to contribute broadly to all aspects of desalination, regardless of the technology chosen. Issues of energy appear throughout the previous discussions but are addressed collectively in this section and are summarized in Table 3-6. Summary of Cost Issues Energy is a main consideration of each of the technologies areas discussed previously, and finding ways to reduce the energy intensity per unit of water output is essential. The cost of energy is presented as relatively stable in the Roadmap, which assumes that moving to a large-scale desalination program will not alter the cost of energy significantly. This assumption may not be correct and deserves further exploration. The Roadmap also does not look at the larger environmental and social costs of energy, such as the contribution of fossil fuels to greenhouse gases and the concerns for energy security, which could have a substantial influence on wider implementation of desalination. In seeking sustainable water supplies through energy-intensive technologies, are concerns about water resources being replaced by concerns about the security of energy resources? A careful reexamination of our water policies will be needed to understand how much desalination is needed when traded off against other concerns about energy use, environmental impacts, and resource sustainability issues.

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Review of the Desalination and Water Purification Technology Roadmap Review of Research Directions Several energy research topics are proposed for various desalination technologies in the Roadmap, and these are compiled in Table 3-6 under the topic of cross-cutting technologies, along with additional research topics. Fossil fuels will likely be around for a long time as non-fossil alternatives presently lag in cost and in potential for large production. Therefore, research is needed to develop ways to lessen the impacts of fossil fuel. Examples include “downhole” energy production that can leave potential emissions underground at the source of the fossil fuel, carbon sequestration, and a much greater emphasis on conservation and energy recovery in end uses. Application of nuclear energy and renewable energy sources, such as biomass, solar, wind, and geothermal energy, for desalination also deserve additional research and development. Yet even with promising research advancements, renewable energy sources will need time to develop larger production capabilities in order to handle desalination energy needs. For this reason, a more careful look should be taken at the life-cycle aspects of desalination. While the Roadmap looks at the energy costs of some current desalination technologies and the environmental impacts of the by-products of the process such as brine disposal, several important life-cycle elements are missing, including the impact of a much larger proportion of water coming from energy intensive technologies. In the next few decades fossil fuels will be the most likely source of energy for desalination, and more consideration should been given to the greenhouse gas emissions (e.g., CO2) contributed by a greater reliance on desalination technologies. Complete life-cycle analyses for proposed desalination and water reuse facilities are needed to understand the future costs of increased use of desalination (compared to other new fresh water sources), including costs of water treatment, transportation, storage, and security, and all associated energy costs, including social and environmental costs. Life-cycle analyses may highlight other needed research, perhaps including research on using small decentralized solar energy facilities at the point of use regardless of the desalination technology choice.

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Review of the Desalination and Water Purification Technology Roadmap TABLE 3-6 A summary of the committee’s recommendations for research topics for cross-cutting technologies. National Need → Technology Area ↓ Provide Safe Water Ensure Sustainability/Ensure Adequate Supplies Keep Water Affordable Cross-cutting Technologies Energy conservation and energy recovery Carbon sequestration “Downhole” energy production Renewable energy sources Geothermal Solar Wind Biomass Alternative energy sources (e.g., fuel cells, nuclear) Life-cycle economics analyses for and water reuse desalination NOTE: This category does not exist in the Roadmap.