inland desalination: challenges and research needs

Brady, Kottenstette, Mayer, Hightower46

JOURNAL OF CONTEMPORARY WATER RESEARCH & EDUCATIONUCOWR

Inland Desalination:Challenges and Research Needs

Patrick V. Brady, Richard J. Kottenstette, Thomas M. Mayer, Mike M. Hightower

Sandia National Laboratories

UNIVERSITIES COUNCIL ON WATER RESOURCESJOURNAL OF CONTEMPORARY WATER RESEARCH & EDUCATION

ISSUE 132, PAGES 46-51, DECEMBER 2005

Drought and population growth haveboth contributed to water scarcity inmany inland areas of the United States,

especially the Southwest. This has focused attentionon inland desalination of subsurface brackish watersand wastewaters. Inland desalination will differfrom seawater desalination because the byproductbrine (concentrate) produced during reverse osmosistreatment cannot be disposed of in the ocean.Moreover, inland brackish waters and wastewatersdiffer in composition from seawater, the formerbeing dominated by calcium, carbonate and sulfaterather than sodium and chloride. Concentratemanagement for inland desalination will have toaddress basic salinity and sustainability issues, whilesmaller plant size and water pumping costs may leadto increased expenses for inland desalination plants.

Background

The demand for freshwater in many regions ofthe world has outstripped supply. More than 50percent of countries in the world will likely facewater stress or water shortages by 2025, and by2050, as much as 75 percent of the world’spopulation could face water scarcity (UnitedNations, 2003). Despite limited supplies, economicgrowth in the Southwest U.S. has increased thedemand for water, which has led to unsustainablewater management practices including ground watermining, subsidence, and reductions in surface andground water quality and availability.

In the United States, the economic future of thearid Southwest will demand some combination ofwater conservation, recycling, and the creation of“new water.” One source of “new water” is

desalinated brackish surface and ground water. Asshown in Figure 1, much of the United States, andparticularly the Southwest, contains extensivebrackish ground water resources. Because itunderlies more easily-accessible and higher-qualityfresh water resources, it has remained largelyuntapped. However, as fresh water supplies becomemore limited, desalination of these brackish waterresources has become more common.

Desalination research and development effortshave greatly improved desalination performance andcosts. By the late 1990s, there were more than 12,500desalination plants in operation in the world, whichgenerated more than six billion gallons of fresh waterper day and accounted for approximately 1 percentof the world’s daily production of drinking water. Inthe next 20 years, over $20 billion will be spent todevelop new desalination facilities worldwide, whichwill double the volume of freshwater being generatedthrough desalination (Martin-Lagardette, 2001).Many of the newer systems are being used inlandfor both brackish water desalination and water reuse.By 2002, about 500 desalination systems were inoperation in about forty states in the U.S.

Desalination Trends and Needs inthe Southwest

Desalination in the Southwest and other inlandareas takes many forms including:

• Enhancing domestic water supplies. Manysouthwestern water districts are evaluatingbrackish groundwater desalination to supplementthe limited freshwater supplies and to providewater for industrial and municipal use.

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JOURNAL OF CONTEMPORARY WATER RESEARCH & EDUCATION

• Fossil energy production. Large volumesof saline or brackish water are commonlyco-produced in oil and gas production (roughlyten times as much water is produced as oil inmany fields). Oil companies in New Mexico(NM), Colorado (CO), and Texas (TX) areevaluating the treatment and desalination ofoil and gas produced water for supplementingriver flows during drought, rehabilitatingrangeland, and cooling water for power plants.

• Treatment of impaired surface water.Many of the river systems in the Southwestsuffer from salt buildup caused by surfacerunoff, irrigation practices, urban uses, andevaporation. Desalination of these impairedrivers will become increasingly important tomeet more stringent water quality standardsfor domestic and ecological-based totalmaximum daily load (TMDL) requirements.

• Industrial and domestic water pretreatmentand reuse. As water conservation and reusebecome increasingly more important,desalination-based water and wastewatertreatment technologies could meet water qualitystandards for water reuse.

Cities such as El Paso, Las Vegas, Phoenix, andTucson are all considering desalination plant optionsto supplement or improve water supplies in their area,while cities such as Scottsdale, AZ, and Ft. Stockton,TX are currently operating desalination facilities. Amid-sized city like Alamogordo, NM, which has apopulation around 30,000, is planning to construct anapproximately 10 million gallon per day desalination

plant to supplement its fresh surface and groundwaterresources. This reflects the fact that desalination oflocally available brackish water in many water-poorregions of the U.S. is becoming cost competitive withtransporting fresh water from greater distances.

To lower the overall cost of desalination, significantinvestment in research and development of newdesalination technologies must be implemented in thecoming decades. The Desalination and WaterPurification Technology Roadmap developed by theU.S. Bureau of Reclamation and Sandia NationalLaboratories is currently being updated to help identifyfuture desalination research objectives and goals forthe U.S. between 2008 and 2020 (U.S. Bureau ofReclamation 2003). In addition, the Tularosa BasinNational Desalination Research Facility has recentlybeen constructed outside of Alamogordo, NM (SandiaNational Laboratories 2002). Its purpose is to pilottest new and promising inland desalinationtechnologies. In the discussion below, we highlight afew of the research efforts for inland desalinationthat are presently being conducted at Sandia in linewith the Roadmap.

Inland Desalination Concerns

There are at least three major concerns for inlanddesalination. These include variable and site-specificwater composition, concentrate disposal (since it isimpossible to put saline concentrate into a largereservoir like the ocean), and inefficient economiesof scale (since most inland plants will not be as largeas their seawater counterparts).

Chemical CompositionThe chemical composition of inland brackish

waters set sharp constraints on the level and cost ofwater recovery. They also point to where technologicaladvances are needed. Table 1 compares thecompositions of several brackish inland waters toseawater. It should be noted that most inland saltwaters are enriched in calcium and depleted insodium relative to seawater (Powder River Basincoal bed methane water is an exception and will bedescribed later). Silica levels are often higher in inlandwaters. Bicarbonate levels are broadly similarreflecting equilibrium with dissolved carbon dioxideand possibly calcium carbonate salts. Unlikeseawater, the dominant anion in inland waters tendsto be sulfate as opposed to chloride.

Figure 1. General Location of Saline Ground WaterResources in the United States (Krieger et al., 1957)



Each water source has been considered fortreatment to drinking water quality. The brackishgroundwater from Tularosa, NM is a Ca-SO4 typewater whose chemistry reflects interaction withsubsurface gypsum deposits. Las Vegas, NVbrackish groundwater is a Na-SO4 type water thatformed through interaction with gypsum and Ca-rich clays – in parallel with evaporativeconcentration. Waters in the Hueco Bolson of Texasare a dilute version of the Tularosa Basin, NMwaters. In Yuma, AZ, the water of the ColoradoRiver and an agricultural drain contain concentratedwater high in Na, Cl and SO4. Produced waters fromoil and gas operations also possess a wide range ofcompositions. Typically, there is organic carbon andsulfur – sulfide or sulfate depending upon the

oxidation state of the fluid. Some coal bed methanewaters are so dilute as to be potable – the only hazardbeing their high Na/Ca ratio which leads to soildeterioration that reduces use for irrigation.Wastewater has been subjected to direct use byhumans and can potentially contain pharmaceuticallyactive compounds – in addition to bacterial and viralcontaminants, and the simple salts (especially nitrate)that are concentrated through general use of water.

Concentrate DisposalThe preferred method of desalination for all new

installations is reverse osmosis (RO), a membraneprocess that extracts fresh water from salty water,thus leaving a more concentrated waste stream, orconcentrate. A critical question for inland desalination

Table 1. Compositions of Seawater and Inland Brackish Sources – (mg/L)

Composition Seawater Groundwater Groundwater Groundwater Wastewater PlantReference Tularosa Basin, NM Las Vegas, NV Hueco Bolson, TX Stream Rio

Rancho, NM

Na 10800 114 755 116 150K 390 2 72 7 25Ca 410 420 576 136 36Mg 1300 163 296 33 5.1Cl 19400 170 954 202 142NO3 NR 10 31 NR 79PO4 NR 0 NR NR 20SO4 2710 1370 2290 294 110HCO3 143 270 210 190 110SiO2 2.1 22 77 31 32TDS 35300 2630 5270 1200 640

Composition Groundwater Colorado River Agricultural Coal Bed Oil & Gas Prod.Brighton, CO Yuma, AZ Drain Methane Water Water Eddy Co., NM

Yuma, AZ Powder River, WY

Na 127 121 609 300 3430K 4 5 8 8.4 NRCa 112 80 157 32 600Mg 26 30 70 16 171Cl 100 109 596 13 4460NO3 15 0.3 3.8 NR NRPO4 NR < 0.25 < 1.0 NR NRSO4 210 276 828 2.4 2660HCO3 310 179 392 950* 488SiO2 NR 9.1 14.8 11 NRTDS 880 730 2630 850 11900

*Alkalinity reported on this sample (primarily HCO3-), NR = Not Reported

150253651142792011011032640

3430NR6001714460NRNR2660488NR11900

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is: “What to do with the concentrate?” Currently,concentrate from inland RO plants is discharged tosurface water, sewers, deep wells, land application,and/or evaporation ponds/salt processing. With theexception of deep well injection, each disposal pathincreases the salt load of surface soils and waters,thus decreasing the ambient or down-stream value ofeach through decreased soil fertility and/or decreasedwater quality. Evaporation ponds often require largeland areas and are only appropriate in arid climateswith low land values. Expensive liners are required toprevent salt seepage contamination of adjacentwaters. Sewer disposal sends the salt accumulationproblem downstream. Deep well injection is notallowed in many states, but those states that allow itrequire permits, monitoring wells, and completions indeep, contained aquifers to ensure that freshwatersupplies are not contaminated. Typically, the stateregulations that are applied to injection of relativelyinert desalination residuals were originally designedto cover the disposal of more hazardous oilfield orindustrial wastes.

Research at the University of Texas-El Paso(UTEP), Sandia National Laboratories, and theTexas Bureau of Economic Geology haveconsidered manipulating the sequence by whichevaporation occurs in evaporation ponds toselectively precipitate out low permeability mineralsthat might “self seal” ponds. Minerals that havebeen considered include calcite and sepiolite – amagnesium silicate clay mineral. A self-sealing, self-healing evaporation pond, if sufficiently impermeable,might not require a liner and may be much cheaperthan at present.

The City of El Paso, Los Alamos NationalLaboratories, and Sandia are examining thegeochemical reactions that occur when RO brinesare injected into deep subsurface formations so thatsustainable deep well injection practice might beassured. Critical unknowns being investigated includethe subsurface stability and transport of Ca-complexing anti-scalants and the compatibility of theinjected brine with the underground aquifer fluids.

An emerging technology for concentrateminimization is based on zero liquid discharge (ZLD).University of South Carolina and Sandia are workingon a combined reverse osmosis-electrodialysistreatment to recover saleable salts from ROconcentrate, which produces a solid product. OtherZLD approaches use multiple effect evaporation,

crystallizers and enhanced evaporation machineryto reduce concentrate volume (Bostjancic 1996).

Economies of ScaleMany coastal desalination plants treat large volumes

of water, often 50 million gallons per day or greater, atrelatively low costs by co-locating them with coastalpower plants to take advantage of common intake andoutfall structures and less expensive power. Thesestrategies enable coastal facilities, such as the TampaBay Desalination Facility, to maintain desalination costsas low as $2.50 per 1000 gallons of water produced.Similar facilities in inland areas may cost twice as muchto operate because of smaller plant sizes, higherconcentrate disposal costs, higher well-water pumpingcosts, and higher energy costs (U.S. Bureau ofReclamation 2002).

Desalination efficiency is also an obstacle in inlandareas. Today, desalination systems have recoveryefficiencies of 60 to 85 percent for brackish waterdesalination. Although most inland RO input streamscontain relatively high levels of calcium, bicarbonateand sulfate, the upper limit of RO extraction is oftendefined by precipitate scaling of the membranes bysilica. Many inland waters contain 20-30 mg/Ldissolved silica. Recovery efficiencies like that citedabove concentrate the silica to above 120 mg/Lwhereupon silica scales form, preventing furtherwater extraction (anti-scalants are effective forcalcium carbonate and sulfate salts, but not forsilica). The current recovery limit means that roughlya fifth of the brackish resource is not used and insteadmust be disposed of. Much higher recoveryefficiencies could be attained by developing newways to quickly and effectively lower silica from100 mg/L to 20-30 mg/L. This would halveconcentrate disposal volumes, extend the supply ofbrackish resources, and potentially reduce overalldesalination costs. Sandia and various universitypartners are researching cost-effective treatmenttechnologies to reduce silica scaling by desaturatingRO interstage streams.

Future Prospects

The development of fundamentally newapproaches to desalination to supplant the establisheddesalination technologies in the coming decades isunlikely. This is a factor of both the basicthermodynamics of desalination, as well as the



significant advances made in desalination technologyover the last 50 years (Miller 2003). In order tominimize concentrate volume, energy consumption,and the overall costs of desalinated water, significantimprovements in current technologies will have to takeplace. Fortunately, there is the potential for significantimprovements in membrane technologies, as well asadvances in engineering designs and constructionmaterials that can bring other desalination technologieswithin the reach of current needs.

Thermal DesalinationThermal phase change methods (primarily

distillation processes) will always be limited by thehigh heat of vaporization of water. Heat recoveryand recycling engineering has been advanced to avery high degree, and it is likely that advances in thisarea will be incremental. Low temperature thermalprocesses that can efficiently use low grade solar,geothermal, or industrial waste heat sources have themost potential for adoption. However, such processesare currently capital intensive, and may with RO forwidespread application. Small capacity distributedsystems using cheap, reliable, low cost materials willlikely find widespread application in remote areas andother situations where cheap land and low gradeenergy are abundant, but access to an installedinfrastructure is lacking.

Membrane ProcessesReverse osmosis and to a lesser degree,

electrodialysis are likely to remain the dominantdesalination technologies for the foreseeable future.This is primarily a reflection of the progress that hasbeen made in membrane materials and in reducingthe required energy expenditure. Any competingtechnology will have to do significantly better in orderto as the preferred water treatment option. There isthe potential for improvements of the efficiency ofmembrane processes through novel nanostructuredmaterials that take advantage of uniquethermodynamics and transport properties of water inconfined spaces. Projects currently underway atSandia are exploring membrane structures with nm-scale channels engineered to selectively pass wateror salt ions with greatly increased efficiency.Synthetic structures that mimic the function ofbiological systems in transporting water across cellwalls have the potential to increase RO membraneefficiency by more than an order of magnitude. To

take full advantage of the promise of these super-efficient membranes, we will need to develop bettermethods of preventing membrane fouling, as well asbetter designs for membrane modules that efficientlytransport water to the membrane surface, and wastesalts away from it.

Hybrid SystemsHybrid systems that take advantage of the

complementary efficiencies of different desalinationprocesses may also provide a significant improvementin water recovery, energy use, and concentrateminimization. We mentioned the example of a hybridRO-electrodialysis system above. Other approachesemploy low temperature distillation processes drivenby waste heat sources to reduce the volume of ROconcentrate and increase fresh water recovery.

Water ReuseFinally, wastewater recovery and reuse represents

the most sustainable, efficient, and in many cases,the lowest cost alternative for making limitedresources go further. This is true since the wastewaterhas a low inherent salinity and has a low osmoticpressure to overcome in RO operations. Currently, apilot plant is being operated by Montgomery WatsonHarza for Sandia in Rio Rancho, New Mexico to lookinto membrane bioreactor primary treatment ofwastewater followed by reverse osmosis polishing.These technologies for removing organic andbiological contamination, as well as dissolved salts,will allow for the substantial recycling of water andwill reduce strain on our limited resources in theface of increasing demand.

Conclusion

Water scarcity and population growth will beimportant factors that will drive the U.S. towardsincreased water conservation and the increased useof desalination. The most readily available sourceof water for desalination is the appropriate and safeuse of recycled or “reuse” water. Other sources ofbrackish waters suitable for desalination are foundthroughout the inland part of the country.Desalination of these sources will require newefforts to lower the costs of desalination by reducingthe energy required for purification, as well as findinga cost-effective and environmentally acceptable wayof treating the concentrated salt byproduct.

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AcknowledgementsSandia is a multi-program laboratory operated by SandiaCorporation, a Lockheed Martin Company, for the United StatesDepartment of Energy’s National Nuclear SecurityAdministration under Contract DE-AC04-94AL85000.

Author Bio and Contact InformationPATRICK BRADY is a Distinguished Member of the Technicalstaff in the Geochemistry Department at Sandia NationalLaboratories. He is currently the Program Manager for the Sandiadesalination program. He has responsibility for the Desalinationand Water Purification Roadmap implementation as well as thelong range R&D and Jumpstart efforts at Sandia. His researchinterests are natural attenuation of soil contaminants, mineralsurface chemistry, geochemical kinetics, arsenic removaland water treatment. He is also an Adjunct Professor ofEnvironmental Engineering at New Mexico Institute of Miningand Technology. Patrick holds M.S. and Ph. D. degrees fromNorthwestern University. Patrick V. Brady, Sandia NationalLaboratories, PO Box 5800, MS 0754, Albuquerque, NM87185. (505) 844-7146. [email protected].

RICHARD KOTTENSTETTE is a technical staff member in theGeochemistry Department at Sandia National Laboratories. Heis an analytical chemist with over 25 years experience in fossilfuel, environmental and homeland security research. His currentposition is project leader for the Sandia desalination - JumpstartProject. This new effort, aimed at selecting and developingemerging desalination technologies, will work closely with theTularosa Basin National Desalination Research Facility inAlamogordo New Mexico. Richard J. Kottenstette, SandiaNational Laboratories, PO Box 5800, MS 0754, Albuquerque,NM 87185. (505) 845-3270. [email protected].

THOMAS M. MAYER is a technical staff member in theGeochemistry Department at Sandia National Laboratories, andis responsible for initiating and managing long range R&Dprograms in water treatment technologies, including desalinationand arsenic remediation. In this role, he has started a number ofresearch programs at Sandia and collaborating universities andcompanies on advanced membrane materials, biofilmcharacterization and control, and modeling of advancedmembrane modules. Prior to this effort, he has 25 years R&Dexperience in materials and process development formicroelectronics, micro-electro-mechanical systems, andnanostructure fabrication. He holds a Ph.D. in chemistry fromPennsylvania State University. Thomas M. Mayer, SandiaNational Laboratories, PO Box 5800, MS 0754, Albuquerque,NM 87185. (505) 844-0770. [email protected].

MICHAEL M. HIGHTOWER is a Distinguished Member of theTechnical staff in the Energy Security Center at Sandia NationalLaboratories. He is a civil and environmental engineer with morethan 25 years experience with research and development projectsincluding security and protection of critical infrastructures.Currently, Mike supports research and development projectsaddressing water and energy resource sustainability, and waterand energy infrastructure security and protection. These effortsinclude developing new water treatment and water monitoring

technologies, developing models and techniques to improve waterresource use and management, desalination and produced watertreatment, impact of water availability on energy security andreliability, and water, electric power, and natural gas infrastructuresecurity and protection. Mike holds a B.S and M.S. in civilengineering from New Mexico State University. Mike Hightower,PO Box 5800, MS 0755, Albuquerque, NM 87185. (505) 844-5499. [email protected].

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Martin-Lagardette, J.L., 2001. Desalination of Sea Water.WATER Engineering & Management, April 2001.

Miller, J. E., 2003. Review of Water Resources and DesalinationTechnologies. http://www.sandia.gov/water/docs/MillerSAND2003_0800.pdf

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inland desalination: challenges and research needs

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