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Home , Electrodialysis, Electrodialysis reversal, Membrane distillation, Solar desalination

E-249 04-10

Desalination Methods for Producing

*Justin K. Mechell and Bruce Lesikar s populations increase and sources of high- Source waters quality fresh drinking water decrease, many Several factors influence the selection of source Acommunities have considered using desalina- waters to feed plants: the location of the tion processes to provide when other plant in relation to water sources available, the deliv- sources and treatment procedures are uneconomical ery destination of the treated water, the quality of or not environmentally responsible. the source water, the pretreatment options available, Desalination is any process that removes excess and the ecological impacts of the concentrate dis- and other minerals from water. In most desali- charge. nation processes, feed water is treated and two streams of water are produced: • Treated fresh water that has low concentra- Seawater is taken into a desalination plant ei- tions of salts and minerals ther from the water’s surface or from below the sea • Concentrate or , which has and floor. In the past, large-capacity seawater desalina- mineral higher than that of tion plants have used surface intakes on the open the feed water sea. The feed water for desalination processes can Although surface water intake can affect and be be seawater or . Brackish water con- affected by organisms in the ocean, the issues related tains more salt than does fresh water and less than to this method can be minimized or resolved by salt water. It is commonly found in estuaries, which proper intake design, operation, and maintenance of are the lower courses of rivers where they meet the technologies. The technologies include passive sea, and aquifers, which are stores of water under- screens, fine mesh screens, filter net barriers, and ground. behavioral systems. They are designed to prevent or An early U.S. desalination plant was built in minimize the environmental impact to the sur- 1961 in Freeport, Texas. It produced 1 million gal- rounding intake area and to minimize the amount of lons per day (mgd) using a long vertical tube distil- pretreatment needed before the feed water reaches lation (LVT) process to produce water for the City of the primary treatment systems. Freeport, Texas. As technology rapidly improves, Subsurface intakes are sometimes feasible if the two other processes—thermal and —are geology of the intake site permits. When the water is becoming viable options to convert saline water to taken in from below the surface, the process causes drinkable fresh water. less damage to marine life. However, if the geology

* Extension Program Specialist, and Professor and Extension Agricultural Engineer, The Texas A&M University System 1 Figure 1. Example of a multi-stage flash (MSF) process (Source: Buros, 1990). of the site is unfavorable, a subsurface intake can tion (MED), and vapor compression distillation harm nearby freshwater aquifers. Methods of sub- (VCD). Another thermal method, solar distillation, surface intake include vertical beach wells, radial is typically used for very small production rates. wells, and infiltration galleries. technologies are prima- A major advantage to using a subsurface intake rily used in the United States. These systems treat is that the water is filtered naturally as it passes the feed water by using a gradient to force- through the soil profile to the intake. This feed the water through . The three major improves the quality of feed water, decreasing the membrane processes are (ED), elec- need for pretreatment. trodialysis reversal (EDR), and (RO). Brackish water Brackish water is commonly used as a source Thermal technologies for desalination facilities. It is usually pulled from Multi-stage flash distillation local estuaries or brackish inland water wells. Be- Multi-stage flash distillation is a process that cause it typically has less salt and a lower concentra- sends the saline feed water through multiple cham- tion of suspended solids than does seawater, bers (Fig. 1). In these chambers, the water is heated brackish water needs less pretreatment, which de- and compressed to a high temperature and high creases overall production costs. However, a disad- pressure. As the water progressively passes through vantage is that disposing of the brine from an inland the chambers, the pressure is reduced, causing the desalination location increases the cost and can raise water to rapidly boil. The vapor, which is fresh environmental concerns. water, is produced in each chamber from boiling and then is condensed and collected. Desalination technologies Two distillation technologies are used primarily Multi-effect distillation around the world for desalination: thermal distilla- Multi-effect distillation employs the same prin- tion and membrane distillation. cipals as the multi-stage flash distillation process ex- Thermal distillation technologies are widely cept that instead of using multiple chambers of a used in the Middle East, primarily because the re- single vessel, MED uses successive vessels (Fig. 2). gion’s petroleum reserves keep costs low. The The water vapor that is formed when the water boils three major, large-scale thermal processes are multi- is condensed and collected. The multiple vessels stage flash distillation (MSF), multi-effect distilla- make the MED process more efficient.

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

Vapor compression distillation solar distillation units vary greatly, the basic princi- Vapor compression distillation can function in- pals are the same. The sun provides the energy to dependently or be used in combination with an- evaporate the saline water. The water vapor formed other thermal distillation process. VCD uses heat from the process then condenses on from the compression of vapor to evaporate the feed the clear glass or plastic covering and is collected as water (Fig. 3). VCD units are commonly used to fresh water in the condensate trough. The covering produce fresh water for small- to medium-scale pur- is used to both transmit radiant energy and allow poses such as resorts, industries, and petroleum water vapor to condense on its interior surface. The drilling sites. salt and un-evaporated water left behind in the still basin forms the brine that must be dis- Solar distillation carded appropriately. is generally used for small- This practice is often used in arid regions scale operations (Fig. 4). Although the designs of where safe fresh water is not available. Solar distil-

Figure 3. Example of a vapor compression distillation (VCD) process (Source: Buros, 1990). 3 built to allow passage of ei- ther positively or negatively charged , but not both. Ions are atoms or molecules that have a net positive or net negative charge. Four common ionic molecules in saline water are , , , and car- bonate. Electrodialysis and use the driving force of an elec- Figure 4. Example of a solar still desalination process (Source: Buros, 1990). trical potential to attract and move different cations (positively charged ions) or lation units produce differing amounts of fresh anions (negatively charged ions) through a perme- water, according to their design and geographic lo- able membrane, producing fresh water on the other cation. Recent tests on four solar still designs by the side (Fig. 5). Texas AgriLife Extension Service in College Station, The cations are attracted to the negative elec- Texas, have shown that a solar still with as little as trode, and the anions are attracted to the positive 7.5 square feet of surface area can produce enough . When the membranes are placed so that water for a person to survive.

Membrane technologies A membrane desalination process uses a physical barrier—the mem- brane—and a driving force. The driving force 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 chemical and physical pretreatment to limit blockage by debris and scale formation on the membrane surfaces. Table 1 (page 5) details the basic characteristics of membrane processes.

Electrodialysis and electrodialysis reversal The membranes used in electro- Figure 5. Example of an electrodialysis process showing the basic dialysis and electrodialysis reversal are movements of ions in the treatment process (Source: Buros, 1990).

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

Membrane Membrane Typical Operating Typical Permeate Typical process driving separation structure operating description constituents force mechanism (pore size) range, (µm) removed

Microfiltration Hydrostatic Sieve Macropores 0.08–2.0 Water + TSS, turbidity, pressure (> 50 nm) dissolved protozoan difference or solutes oocysts and vacuum in open cysts, some vessels bacteria and viruses

Ultrafiltration Hydrostatic Sieve Mesopores 0.005–0.2 Water + Macromolecules, pressure (2–50 nm) small colloids, difference molecules most bacteria, some viruses, proteins

Nanofiltration Hydrostatic Sieve + Micropores 0.001–0.01 Water + Small pressure solution/ (< 2 nm) very small molecules, difference + molecules, some exclusion ionic solutes hardness, viruses

Reverse Hydrostatic Solution/ Dense 0.0001– Water + Very small osmosis pressure diffusion + (< 2 nm) 0.001 very small molecules, difference exclusion molecules, color, ionic solutes hardness, sulfates, nitrate, sodium, other ions Dialysis Diffusion Mesopores -- Water + Macromolecules, difference (2–50 nm) small colloids, molecules most bacteria, some viruses, proteins

Electrodialysis Electromotive exchange Micropores -- Water + Ionized force with selective (< 2 nm) ionic solutes salt ions membranes some allow only cations to pass and others allow Reverse osmosis only anions to pass, the process can effectively re- Reverse osmosis uses a pressure gradient as the move the constituents from the feed water that make driving force to move high-pressure saline feed it a saline solution. water through a membrane that prevents the salt The electrodialysis reversal process functions as ions from passing (Fig. 6). does the electrodialysis process; the only difference There are several membrane treatment is that in the reverse process, the polarity, or charge, processes, including reverse osmosis, , of the is switched periodically. This rever- ultrafiltration, and microfiltration. The pore sizes of sal in flow of ions helps remove scaling and other the membranes differ according to the type of debris from the membranes, which extends the sys- process (Fig. 7). tem’s operating life.

5 Because the RO membrane has such small pores, the feed water must be pretreated adequately before being passed through it. The water can be pretreated chemically, to prevent biological growth and scaling, and physically, to remove any sus- Figure 6. Basic components of a membrane treatment process. pended solids. The high-pressure feed water flows through the individual mem- brane elements. The spiral RO membrane ele- ment is constructed in a concentric spiral pattern that allows alternating layers of feed water and brine spacing, RO mem- brane, and a porous product water carrier (Fig. 8). The porous product water carrier allows the fresh water to flow into the center of the membrane element to be collected in the product water tube. To enable each pressure vessel to treat Figure 7. Range of nominal membrane pore sizes for reverse osmosis (RO), more water, the individ- nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF) (Source: Metcalf and Eddy, 2003). ual membrane elements are connected in series (Fig. 9). After the water chemicals that were either removed during the de- passes through the membrane elements within the salination process or added to help pretreat the feed pressure vessels, it goes through post treatment. Post water. For all of the processes, the brine must be dis- treatment prepares the water for distribution to the posed of in an economical and environmentally public. friendly way. Options for discharging the brine include dis- Concentrate management options charge into the ocean, injection through a well into a Both thermal and membrane desalination saline aquifer, and evaporation. Each option has ad- processes produce a stream of brine water that has a vantages and disadvantages. In all cases, the brine high concentration of salt and other minerals or water should have a minimal impact on the sur-

6 Figure 8. Cutaway view of a spiral reverse osmosis membrane element (Source: Buros, 1990). rounding water bodies or aquifers. Specific consid- implementation, desalination processes need tech- erations for the water quality include saline concen- nological improvements and increased energy tration, water temperature, dissolved efficiency. concentrations, and any constituents added as pre- treatment. References Buros, O. K., The ABC’s of Desalting, Summary International Desalination Association. As high-quality freshwater resources decrease, Topsfield, Massachusetts. 1990. more communities will consider desalination of Krishna, Hari J., Introduction to Desalination brackish and salt water to produce drinking water. Technologies, Texas Water Development All desalination technologies have advantages and Board. 2004. disadvantages based on site-specific limitations and Metcalf and Eddy, Wastewater Engineering: requirements. Small-scale desalination of brackish Treatment and Reuse. McGraw-Hill, Inc., water using solar stills is a promising method in re- New York. 2003. mote locations where good-quality water for drink- Pankratz, Tom, Desalination Technology ing and cooking is unavailable. For more widespread Trends, CH2M Hill, Inc. 2004.

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

7 This material is based on work supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2009-34461-19772 and Agreement No. 2009-45049-05492.