1

Source watersSeveral factors influence the selection of source

waters to feed desalination plants: the location of theplant in relation to water sources available, the deliv-ery destination of the treated water, the quality ofthe source water, the pretreatment options available,and the ecological impacts of the concentrate dis-charge.

SeawaterSeawater is taken into a desalination plant ei-

ther from the water’s surface or from below the seafloor. In the past, large-capacity seawater desalina-tion plants have used surface intakes on the opensea.

Although surface water intake can affect and beaffected by organisms in the ocean, the issues relatedto this method can be minimized or resolved byproper intake design, operation, and maintenance oftechnologies. The technologies include passivescreens, fine mesh screens, filter net barriers, andbehavioral systems. They are designed to prevent orminimize the environmental impact to the sur-rounding intake area and to minimize the amount ofpretreatment needed before the feed water reachesthe primary treatment systems.

Subsurface intakes are sometimes feasible if thegeology of the intake site permits. When the water istaken in from below the surface, the process causesless damage to marine life. However, if the geology

Desalination Methodsfor Producing Drinking Water

As populations increase and sources of high-quality fresh drinking water decrease, manycommunities have considered using desalina-

tion processes to provide fresh water when othersources and treatment procedures are uneconomicalor not environmentally responsible.

Desalination is any process that removes excesssalts and other minerals from water. In most desali-nation processes, feed water is treated and twostreams of water are produced:

• Treated fresh water that has low concentra-tions of salts and minerals• Concentrate or brine, which has salt andmineral concentrations higher than that ofthe feed waterThe feed water for desalination processes can

be seawater or brackish water. Brackish water con-tains more salt than does fresh water and less thansalt water. It is commonly found in estuaries, whichare the lower courses of rivers where they meet thesea, and aquifers, which are stores of water under-ground.

An early U.S. desalination plant was built in1961 in Freeport, Texas. It produced 1 million gal-lons per day (mgd) using a long vertical tube distil-lation (LVT) process to produce water for the City ofFreeport, Texas. As technology rapidly improves,two other processes—thermal and membrane—arebecoming viable options to convert saline water todrinkable fresh water.

*Justin K. Mechell and Bruce Lesikar

* Extension Program Specialist, and Professor and ExtensionAgricultural Engineer, The Texas A&M University System

E-24904-10

2

of the site is unfavorable, a subsurface intake canharm nearby freshwater aquifers. Methods of sub-surface intake include vertical beach wells, radialwells, and infiltration galleries.

A major advantage to using a subsurface intakeis that the water is filtered naturally as it passesthrough the soil profile to the intake. This filtrationimproves the quality of feed water, decreasing theneed for pretreatment.

Brackish waterBrackish water is commonly used as a source

for desalination facilities. It is usually pulled fromlocal estuaries or brackish inland water wells. Be-cause it typically has less salt and a lower concentra-tion of suspended solids than does seawater,brackish water needs less pretreatment, which de-creases overall production costs. However, a disad-vantage is that disposing of the brine from an inlanddesalination location increases the cost and can raiseenvironmental concerns.

Desalination technologiesTwo distillation technologies are used primarily

around the world for desalination: thermal distilla-tion and membrane distillation.

Thermal distillation technologies are widelyused in the Middle East, primarily because the re-gion’s petroleum reserves keep energy costs low. Thethree major, large-scale thermal processes are multi-stage flash distillation (MSF), multi-effect distilla-

tion (MED), and vapor compression distillation(VCD). Another thermal method, solar distillation,is typically used for very small production rates.

Membrane distillation technologies are prima-rily used in the United States. These systems treatthe feed water by using a pressure gradient to force-feed the water through membranes. The three majormembrane processes are electrodialysis (ED), elec-trodialysis reversal (EDR), and reverse osmosis(RO).

Thermal technologiesMulti-stage flash distillation

Multi-stage flash distillation is a process thatsends the saline feed water through multiple cham-bers (Fig. 1). In these chambers, the water is heatedand compressed to a high temperature and highpressure. As the water progressively passes throughthe chambers, the pressure is reduced, causing thewater to rapidly boil. The vapor, which is freshwater, is produced in each chamber from boilingand then is condensed and collected.

Multi-effect distillationMulti-effect distillation employs the same prin-

cipals as the multi-stage flash distillation process ex-cept that instead of using multiple chambers of asingle vessel, MED uses successive vessels (Fig. 2).The water vapor that is formed when the water boilsis condensed and collected. The multiple vesselsmake the MED process more efficient.

Figure 1. Example of a multi-stage flash distillation (MSF) process (Source: Buros, 1990).

Vapor compression distillationVapor compression distillation can function in-

dependently or be used in combination with an-other thermal distillation process. VCD uses heatfrom the compression of vapor to evaporate the feedwater (Fig. 3). VCD units are commonly used toproduce fresh water for small- to medium-scale pur-poses such as resorts, industries, and petroleumdrilling sites.

Solar distillationSolar desalination is generally used for small-

scale operations (Fig. 4). Although the designs of

solar distillation units vary greatly, the basic princi-pals are the same. The sun provides the energy toevaporate the saline water. The water vapor formedfrom the evaporation process then condenses onthe clear glass or plastic covering and is collected asfresh water in the condensate trough. The coveringis used to both transmit radiant energy and allowwater vapor to condense on its interior surface. Thesalt and un-evaporated water left behind in the stillbasin forms the brine solution that must be dis-carded appropriately.

This practice is often used in arid regionswhere safe fresh water is not available. Solar distil-

Figure 2. Example of a multi-effect distillation (MED) process (Source: Buros, 1990).

Figure 3. Example of a vapor compression distillation (VCD) process (Source: Buros, 1990).

3

lation units produce differing amounts of freshwater, according to their design and geographic lo-cation. Recent tests on four solar still designs by theTexas AgriLife Extension Service in College Station,Texas, have shown that a solar still with as little as7.5 square feet of surface area can produce enoughwater for a person to survive.

Membrane technologiesA membrane desalination process

uses a physical barrier—the mem-brane—and a driving force. The drivingforce can be an electrical potential,which is used in electrodialysis or elec-trodialysis reversal, or a pressure gradi-ent, which is used in reverse osmosis.

Membrane technologies often re-quire that the water undergo chemicaland physical pretreatment to limitblockage by debris and scale formationon the membrane surfaces. Table 1(page 5) details the basic characteristicsof membrane processes.

Electrodialysisand electrodialysis reversal

The membranes used in electro-dialysis and electrodialysis reversal are

built to allow passage of ei-ther positively or negativelycharged ions, but not both.Ions are atoms or moleculesthat have a net positive ornet negative charge. Fourcommon ionic molecules insaline water are sodium,chloride, calcium, and car-bonate.

Electrodialysis andelectrodialysis reversal usethe driving force of an elec-trical potential to attractand move different cations(positively charged ions) or

anions (negatively charged ions) through a perme-able membrane, producing fresh water on the otherside (Fig. 5).

The cations are attracted to the negative elec-trode, and the anions are attracted to the positiveelectrode. When the membranes are placed so that

Figure 4. Example of a solar still desalination process (Source: Buros, 1990).

Figure 5. Example of an electrodialysis process showing the basicmovements of ions in the treatment process (Source: Buros, 1990).

4

5

Membraneprocess

Membranedrivingforce

Typicalseparationmechanism

Operatingstructure(pore size)

Typicaloperatingrange, (µm)

Permeatedescription

Typicalconstituentsremoved

Microfiltration Hydrostaticpressure

difference orvacuum in open

vessels

Sieve Macropores(> 50 nm)

0.08–2.0 Water +dissolvedsolutes

TSS, turbidity,protozoanoocysts andcysts, somebacteria and

viruses

Ultrafiltration Hydrostaticpressuredifference

Sieve Mesopores(2–50 nm)

0.005–0.2 Water +small

molecules

Macromolecules,colloids,

most bacteria,some viruses,

proteins

Nanofiltration Hydrostaticpressuredifference

Sieve +solution/diffusion +exclusion

Micropores(< 2 nm)

0.001–0.01 Water +very smallmolecules,ionic solutes

Smallmolecules,

somehardness,viruses

Reverseosmosis

Hydrostaticpressuredifference

Solution/diffusion +exclusion

Dense(< 2 nm)

0.0001–0.001

Water +very smallmolecules,ionic solutes

Very smallmolecules,

color,hardness,sulfates,

nitrate, sodium,other ions

Dialysis Concentrationdifference

Diffusion Mesopores(2–50 nm)

-- Water +small

molecules

Macromolecules,colloids,

most bacteria,some viruses,

proteins

Electrodialysis Electromotiveforce

Ion exchangewith selectivemembranes

Micropores(< 2 nm)

-- Water +ionic solutes

Ionizedsalt ions

Table 1. General characteristics of membrane processes (Source: Metcalf and Eddy, 2003).

some allow only cations to pass and others allowonly anions to pass, the process can effectively re-move the constituents from the feed water that makeit a saline solution.

The electrodialysis reversal process functions asdoes the electrodialysis process; the only differenceis that in the reverse process, the polarity, or charge,of the electrodes is switched periodically. This rever-sal in flow of ions helps remove scaling and otherdebris from the membranes, which extends the sys-tem’s operating life.

Reverse osmosisReverse osmosis uses a pressure gradient as the

driving force to move high-pressure saline feedwater through a membrane that prevents the saltions from passing (Fig. 6).

There are several membrane treatmentprocesses, including reverse osmosis, nanofiltration,ultrafiltration, and microfiltration. The pore sizes ofthe membranes differ according to the type ofprocess (Fig. 7).

Because the ROmembrane has suchsmall pores, the feedwater must be pretreatedadequately before beingpassed through it. Thewater can be pretreatedchemically, to preventbiological growth andscaling, and physically,to remove any sus-pended solids.

The high-pressurefeed water flows throughthe individual mem-brane elements. Thespiral RO membrane ele-ment is constructed in aconcentric spiral patternthat allows alternatinglayers of feed water andbrine spacing, RO mem-brane, and a porousproduct water carrier(Fig. 8). The porousproduct water carrierallows the fresh water toflow into the center ofthe membrane elementto be collected in theproduct water tube.

To enable eachpressure vessel to treatmore water, the individ-ual membrane elementsare connected in series (Fig. 9). After the waterpasses through the membrane elements within thepressure vessels, it goes through post treatment. Posttreatment prepares the water for distribution to thepublic.

Concentrate management optionsBoth thermal and membrane desalination

processes produce a stream of brine water that has ahigh concentration of salt and other minerals or

6

Figure 7. Range of nominal membrane pore sizes for reverse osmosis (RO),nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF) (Source: Metcalfand Eddy, 2003).

Figure 6. Basic components of a membrane treatment process.

chemicals that were either removed during the de-salination process or added to help pretreat the feedwater. For all of the processes, the brine must be dis-posed of in an economical and environmentallyfriendly way.

Options for discharging the brine include dis-charge into the ocean, injection through a well into asaline aquifer, and evaporation. Each option has ad-vantages and disadvantages. In all cases, the brinewater should have a minimal impact on the sur-

rounding water bodies or aquifers. Specific consid-erations for the water quality include saline concen-tration, water temperature, dissolved oxygenconcentrations, and any constituents added as pre-treatment.

SummaryAs high-quality freshwater resources decrease,

more communities will consider desalination ofbrackish and salt water to produce drinking water.All desalination technologies have advantages anddisadvantages based on site-specific limitations andrequirements. Small-scale desalination of brackishwater using solar stills is a promising method in re-mote locations where good-quality water for drink-ing and cooking is unavailable. For more widespread

7

Figure 8. Cutaway view of a spiral reverse osmosis membrane element (Source: Buros, 1990).

Figure 9. Cross section of a pressure vessel with three membrane elements (Source: Buros, 1990).

implementation, desalination processes need tech-nological improvements and increased energyefficiency.

ReferencesBuros, O. K., The ABC’s of Desalting,International Desalination Association.Topsfield, Massachusetts. 1990.

Krishna, Hari J., Introduction to DesalinationTechnologies, Texas Water DevelopmentBoard. 2004.

Metcalf and Eddy,Wastewater Engineering:Treatment and Reuse. McGraw-Hill, Inc.,New York. 2003.

Pankratz, Tom, Desalination TechnologyTrends, CH2M Hill, Inc. 2004.

This material is based on work supported by theNational Institute of Food and Agriculture, U.S. Department of Agriculture,

under Agreement No. 2009-34461-19772 and Agreement No. 2009-45049-05492.