Desalination Methods for Producing Drinking Water

E-249 04-10 Desalination Methods for Producing Drinking Water *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 desalination plants: the location of the tion processes to provide fresh water 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- salts and other minerals from water. In most desali- charge. nation processes, feed water is treated and two streams of water are produced: Seawater • 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 brine, which has salt and floor. In the past, large-capacity seawater desalina- mineral concentrations 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. 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 membrane—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 distillation (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. Membrane distillation 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 pressure gradient to force- through the soil profile to the intake. This filtration feed the water through membranes. The three major improves the quality of feed water, decreasing the membrane processes are electrodialysis (ED), elec- need for pretreatment. trodialysis reversal (EDR), and reverse osmosis (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 energy 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 evaporation 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 solution that must be dis- Solar distillation carded appropriately. Solar desalination 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 ions, 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 sodium, chloride, calcium, and car- bonate. Electrodialysis and electrodialysis reversal 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 electrode. When the membranes are placed so that water for a person to survive. Membrane technologies A membrane desalination process uses a physical barrier—the membrane—and a driving force. The driving force can be an electrical potential, which is used in electrodialysis or electrodialysis reversal, or a pressure gradient, 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 diffusion + molecules,

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