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Desalination: A National Perspective 4 State of the Technology Desalination technologies and their application have evolved substantially over the past 50 years. The five key elements of a desalination system (see Figure 4-1), for either brackish water or seawater desalination, are as follows: Intakes—the structures used to extract source water and convey it to the process system; Pretreatment—removal of suspended solids and control of biological growth, to prepare the source water for further processing; Desalination—the process that removes dissolved solids, primarily salts and other inorganic constituents, from a water source; Post-treatment—the addition of chemicals to the product water to prevent corrosion of downstream infrastructure piping; and Concentrate management—the handling and disposal or reuse of waste residuals from the desalination system.1 Depending on the source water and the desalination technology used, specific elements may vary in their importance in the overall system. For example, inland brackish groundwater desalination facilities will use wells and pumps to bring the source water to the facility, and these systems may need little or no pretreatment. In contrast, seawater reverse osmosis (RO) desalination may use more elaborate intake structures, depending on the specific site conditions, and may require extensive pretreatment. The state of the technology for each of these elements and 1 The broader term concentrate management (as opposed to concentrate disposal) is used throughout the report in recognition that waste management options are not limited only to disposal or discharge. However, it is worth noting that few economically viable concentrate reuse applications currently exist and that nearly all desalination concentrate in the United States is disposed rather than beneficially reused.
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Desalination: A National Perspective FIGURE 4-1. Key elements of a desalination system. Although shown here for a membrane-based system, these steps also describe the major components of non-membrane systems. SOURCE: Modified from Buros et al. (1980). their current technical barriers are discussed in this chapter. The focus of the chapter is on technologies that are commercially available or in the late stages of development, although some emerging technologies that are still in the early phases of research and development are discussed in boxes. FEEDWATER INTAKE OPTIONS Desalination facilities require a reliable supply of feedwater. Feedwater quantity and quality vary based on the specifics of the site and often determine the feasibility of siting a plant at a given location. Intake designs can affect feedwater quality and the environmental impacts of a desalination facility at a given site. Current technologies and issues with desalination intakes are discussed in this section. Brackish water desalination facilities can utilize feedwater from surface water sources or wells. Inland desalination plants use intake technology that is no different from traditional water-treatment plants dependent on surface water or groundwater, and this technology is well developed. Therefore, these technologies will not be described in detail here. There are important environmental issues, however, associated with
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Desalination: A National Perspective sustainable brackish groundwater withdrawals for inland systems that are discussed further in Chapter 5. Seawater desalination intakes generally fall into one of two major categories—surface intakes located above the seafloor and subsurface intakes located beneath the seafloor or sandy beach. Surface intakes, often called open intakes, are in direct communication with the ocean or sea, where feedwater can be taken directly from the surface or below the surface through submerged intakes. Subsurface intakes take their feed-water from below the floor of the ocean using naturally occurring sand and geologic formations to provide filtration. Subsurface intakes can be horizontally drilled from central wells, slant drilled from onshore beaches, or excavated to create infiltration beds. Subsurface intake options can produce higher-quality feedwater and thereby reduce the pretreatment necessary for membrane desalination systems. In contrast, thermal seawater desalination systems require less pretreatment than RO and require only coarse screens to protect the process equipment. Thermal desalination plants commonly use surface (open) intakes. Design engineering, equipment procurement, and construction spending on intakes and outfalls are estimated to total 5 to 7 percent of capital costs for RO and thermal desalination plants (GWI, 2006a). Other costs, such as monitoring and permitting, may add to the overall costs of the intakes. There are several factors affecting the final cost for constructing and operating an intake system; among these are the type of intake being used, the type of coastal conditions, and the distance from the intake to the plant itself. The following section focuses on the latest design and engineering options for coastal intakes. Surface Intakes Thermal seawater desalination and large seawater RO facilities (>38,000 m3/day; >10 million gallons per day [MGD]) predominantly use open-water intakes. Screens are added to the intake structures to reduce the number of marine organisms taken in with the source water (referred to as entrainment; see also Chapter 5). Application of screen technology to the power industry has existed since the early twentieth century. Early screens included a front-end “trash rack” consisting of fixed bars to prevent large debris from entering the water intake system. Traveling screens are rotated and washed intermittently with a high-pressure wash. Alternative screen technology includes modified traveling screens with fish handling systems, fine-mesh screens, cylindrical wedge wire screens, fish net barriers, louvers, angled traveling screens, and velocity caps (California Water Desalination Task Force, 2003).
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Desalination: A National Perspective Impingement, defined as the pinning and trapping of fish or other larger organisms against the screens of the intake structures, can cause severe injury and death to organisms. State-of-the-art intake systems have been developed to greatly reduce impingement. For example, some intake screens can be backflushed with compressed air. These screens have no moving parts, operate with a very low velocity (to mitigate impingement), and are generally referred to as “passive screens.” A Ristroph screen is a modified traveling screen with water-filled lifting buckets that collect impinged organisms and transport them to a bypass, trough, or other protected area. Similarly, fish baskets consisting of framed screen panels can be attached to the vertical traveling screens. Fish that are removed are typically returned to the water via sluiceway or pipeline. Fine-mesh screens have a mesh size of 5 mm or less and are designed to exclude larger eggs, larvae, and juvenile fish from the intakes. Cylindrical wedge wire screens will exclude organisms larger than the nominal screen opening of 0.5 to 10 mm. Their cylindrical shape dissipates the velocity, allowing organisms to escape the flow field, although adequate countercurrent flow is needed to transport organisms away from the screen (California Water Desalination Task Force, 2003). An emerging screen technology that is currently in development to address impingement and entrainment is described in Box 4-1. Louvers are a series of vertical panels placed perpendicular to the intake approach flow. They create a new velocity field that carries fish away from the intake and toward a fish bypass system. Louvers rely primarily on a fish’s ability to recognize the new flow field and swim away. They have been successful in reducing impingement but are not effective against the entrainment of eggs and larvae (California Water Desalination Task Force, 2003). Shipboard seawater desalination approaches that situate the water treatment facility in the deeper ocean far from environmentally sensitive coastal areas could also reduce impingement and entrainment. One recent approach uses telescoping source water intakes to bypass the photic zone, where most marine organisms reside. This deeper water also contains fewer suspended solids, thus reducing the pretreatment required. Subsurface Intakes Coastal subsurface intakes include beach wells, radial wells, horizontal directionally drilled (also called slant-drilled) wells, and infiltration galleries. By taking advantage of the natural filtration provided by sediments, subsurface seawater intakes can reduce the amount of total organic carbon and total suspended solids, thereby reducing the pretreat-
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Desalination: A National Perspective BOX 4-1 Emerging Technologies to Reduce Impingement and/or Entrainment A Marine Life Exclusion System has been designed to reduce impingement as well as entrainment. This water-permeable barrier (see Figure 4-2) is spread around the intake structure, preventing aquatic organisms from approaching the water intake point. The curtain is either suspended by flotation billets and anchored in place, or integrated into existing shoreline intake structures. Sealed against the seafloor and shoreline structures, it completely surrounds the intake structure, preventing targeted planktonic and neustonic organisms from entering the system. Because the surface area of the curtain is large compared to an intake screen, the water velocity through the curtain is up to 98 percent less than the velocity near the intake structure. Low water velocity enables even small fish larvae to drift away from the boom. This technology has primarily been used in riverine environments, although it is currently being tested in marine settings to examine its durability, susceptibility to fouling, and cleaning requirements (McCusker et al., 2007; Mirant Lovett, 2006; San Francisco Bay Conservation and Development Commission, 2005). FIGURE 4-2. Curtain designed to reduce intake velocities and minimize impingement and entrainment. SOURCE: McCusker et al. (2007) ment required for membrane-based desalination systems and lowering the associated operations and maintenance costs. Pumping from subsurface intakes may also under some conditions dilute the seawater with less saline groundwater, thereby reducing the total dissolved solids (TDS) in the intake water. Vertically drilled beach wells are typically used for small (<19,000 m3/day; <5 MGD) systems where the local hydrogeology (e.g., aquifer transmissivity) will permit it. Beach wells have been used effectively in
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Desalination: A National Perspective the Caribbean and Mediterranean and are the intakes of choice for proposed plants in Hawaii. They pose minimal environmental concerns because benthic communities remain undisturbed and entrainment and impingement of marine organisms are eliminated. In most cases, no further pretreatment is needed. One of the potential disadvantages of beach wells is that deep wells may result in lower water temperature and thus higher viscosity; hence, higher pressure (and increased energy) will be required to pump the water through the RO membranes. Care should also be taken to ensure that the source water withdrawals do not cause deleterious effects on local aquifers. Horizontal directionally drilled (or slant-drilled) wells, shown in Figure 4-3, are increasingly being considered for use in large seawater desalination facilities. Although more expensive to construct than beach wells, they can minimize shoreline structures. Slant-drilled wells are under study in Dana Point, California, for example. They are also currently in use at several seawater RO plants in Spain, including the facility at San Pedro del Pinatar, which has a capacity of over 170,000 m3/day (Peters et al., 2006). An alternative approach recently used at the Fukuoka, Japan plant is a seabed infiltration gallery (Figure 4-4). This intake system requires isolating a section of beach so that sand can be removed to the desired depth. Varying grades of small rock and gravel are placed into the excavation and perforated pipes are installed to convey source water to the plant. The rock and gravel are covered with the same sand material that was excavated from the beach, before the ocean is allowed to resume its FIGURE 4-3. Slant-drilled well concept. SOURCE: Adapted from Richard Bell (2006).
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Desalination: A National Perspective FIGURE 4-4. Seabed filtration system. SOURCE: Missimer (1994). normal shoreline. The benefits of this approach are that the source water is nearly all seawater and not diluted by freshwater aquifer contributions and that it greatly mitigates entrainment and impingement (Wright and Missimer, 1997). Although proven technically successful in Japan, theenvironmental impacts will need to be better understood and, if needed, mitigated. PRETREATMENT Pretreatment is generally required for all desalination processes. Pretreatment ensures that constituents in the source water do not reduce the
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Desalination: A National Perspective performance of the desalination facility. Thermal processes require pretreatment to avoid scaling and to control corrosive constituents of the source water. Some removal of sand or gritlike suspended solids may also be necessary to avoid pipe erosion. In membrane desalination, pretreatment involves these considerations as well as further pretreatment to remove suspended solids of both biological and mineral origin to avoid membrane fouling. Biological growth may need to be inhibited by a disinfectant or biocide. Pretreatment is a critical step in seawater and brackish water membrane desalination systems that utilize feedwater from surface water sources, because the suspended and colloidal particles, organisms, and natural organic matter need to be removed before the feedwater reaches the membranes. Indeed, proper pretreatment of feedwater is the most important factor in the successful operation of an RO plant, and pilot testing of the pretreatment process is a critical part of plant design. Brackish water desalination systems that treat groundwater require very minimal, if any, pretreatment to remove particulates because the water typically contains very low concentrations of suspended solids and organic matter. Nevertheless, brackish groundwater may require pretreatment to remove selected constituents such as dissolved iron, manganese, and sulfides, which, if oxidized, create particulates that can foul RO membranes (USBR, 2003). The quality of source water available at a particular site will also affect the extent of pretreatment needed for membrane desalination. Source water quality will depend on local site factors such as source water depth, turbidity, boat traffic, oil contamination, nearby outfalls, wind conditions, tides, and the influence of runoff. As discussed previously, subsurface seawater intakes, aquatic filter barriers, and deep ocean water intakes can greatly reduce the need for pretreatment. Due to permitting regulations and available land, however, desalination plants cannot always be sited where they will have the lowest pretreatment costs. Furthermore, because the United States employs more rigorous accounting of environmental costs, siting options in the United States may lead to greater pretreatment and greater pretreatment residuals handling needs compared to global standards of practice. The most common pretreatment processes are discussed below. Scaling and Corrosion Control Scaling is caused by the precipitation of minerals, such as calcium carbonate, from solution. Calcium sulfate scaling can be controlled via temperature control or through pretreatment by nanofiltration to remove the calcium ions. Acidification of the feedwater can prevent calcium carbon-
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Desalination: A National Perspective ate or magnesium hydroxide formation and scaling. Finally, the use of chemical antiscalants such as sodium hexametaphosphate or polymeric acids can sequester the cations that can lead to scaling problems (Table 4-1). Corrosion can be reduced by removing corrosive gases in pretreatment. Carbon dioxide can be controlled through acidification, and oxygen can be controlled with an oxygen scavenger such as sodium bisulfate or ferrous sulfate. Alternatively, corrosion can be controlled in some systems through the formation of a protective film within the system by adding zinc orthophosphate (Table 4-1). Conventional Solids Removal Methods Conventional solids removal methods such as coagulation and sedimentation followed by media filtration are still the predominant pretreatment processes for seawater RO. Chemicals such as ferric chloride or polyelectrolytes are added to enhance the coagulation of suspended solids prior to settling and filtration (Table 4-1). Traditional gravity flow filtration has been successfully used at many seawater RO plants around the world. At Point Lisas, Trinidad, gravity filters with greater-than-normal depth proved to be successful in pretreating seawater that encounters severe spikes in turbidity due to the intake location in a ship turning basin (Jacangelo and Grounds, 2004). These are mature technologies, although novel approaches to conventional filtration continue to be examined. For example, at Tampa Bay, an upflow dual sand process was installed that had previously only been used for industrial and wastewater applications (see Box 4-2). Nevertheless, there is a need to improve the quality and stability of influent to RO membranes; thus, other pretreatment options continue to emerge. Microfiltration and Ultrafiltration Microfiltration (MF) and ultrafiltration (UF) membranes are increasingly being used in the pretreatment processes for membrane desalination. Water molecules and salts are free to pass through, and water is pushed (or pulled) through the membrane at very low pressures. Particles larger than the membrane pore size (0.03-10 m for MF and 0.002-0.1 m for UF) are removed. Membranes are commercially available in flat-sheet, tubular, hollow-fiber, and spirally wound configurations. Among the benefits of MF/UF pretreatment compared to conventional pretreat-
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Desalination: A National Perspective TABLE 4-1 Reported Dosing Concentrations of Pretreatment Chemical Additives in Reverse Osmosis and Multistage Flash Desalination REVERSE OSMOSIS DESALINATION Chemical Additive Reported Dosing (mg/L) References Biocide Chlorine 0.5-6 Abart, 1993; Redondo and Lomax, 1997; Morton et al., 1996; Woodward Clyde Consultants, 1991 Chlorine removal Sodium bisulfite 3-19 Morton et al., 1996; Redondo and Lomax, 1997; Woodward Clyde Consultants, 1991 Coagulants Ferric chloride 0.8-25 Baig and Kutbi, 1998; Woodward Clyde Consultants, 1991 Polyelectrolyte 0.2-4 Ebrahim et al., 1995; DuPont, 1994; Hussain and Ahmed, 1998 Antiscalants Sulfuric acid 6.6-100 Al-Shammiri et al., 2000; Morton et al., 1996; Al-Ahmad and Aleem, 1993 Sodium hexametaphosphate (SHMP) 2-10 Al-Ahmad and Aleem, 1993; Al-Shammiri et al,. 2000 FilmTec, 2000 Polyacrylic acid 2.9 Woodward Cycle Consultants, 1991 Phosphonate 1.4 Al-Shammiri et al., 2000 MULTISTAGE FLASH DISTILLATION Biocide Chlorine 0.25-4 Iman et al., 2000; Shams El Din and Makkawi, 1998; Khordagui, 1992; Abdel-Jawad and Al-Tabtabaei, 1999 Hypochlorite 2 Burashid, 1992 Antiscalants Polyphosphate 2.2-2.5 Hamed et al., 2000; Abdel-Jawad and Al-Tabtabaei, 1999 Polycarboxylic acid 1.5-2 Hamed et al., 1999 Polyphosphonate 1-3 Hamed et al., 1999, 2000 Antifoaming agents Polypropylene glycol 0.035-0.15 Imam et al., 2000 Corrosion control Sodium bisulfite Not given Imam et al., 2000 Ferrous sulfate 1 - 3 Shams El Din and Makkawi, 1998 B-ethyl phenyl ketocyclohexylamino hydrochloride 25 Andijani et al., 2000 NOTE: The types and concentrations of pretreatment chemicals vary with plant design and source water conditions. Thus, some or all of these chemicals may not be used at all, or they may only be used intermittently. Some added chemicals can be recovered or removed (e.g., chlorine). SOURCE: Adapted from Lattemann and Höpner (2003).
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Desalination: A National Perspective BOX 4-2 Pretreatment Changes at the Tampa Bay Seawater Desalination Plant The largest seawater desalination plant in the United States (95,000 m3/day or 25 MGD) is located in the Tampa Bay region of Florida. The Tampa Bay Seawater Desalination project obtains its source water from a “once-through” cooling system at the Tampa Electric Company (TECO) Big Bend Power Station, which withdraws its cooling water from Tampa Bay. After coming online in March 2003, the plant experienced performance problems that significantly stemmed from an inadequate pretreatment system. The desalination plant did not include any additional screens beyond those that existed within the power plant, and the upflow sand filter system was inadequate to produce pretreated seawater adequate to sustain reverse osmosis process operation. Although the warmer water from the power plant requires less energy for the desalination process, higher water temperature introduces greater potential for biological growth and pretreatment challenges. The original pretreatment process (Figure 4-5) resulted in severe premature fouling of the cartridge filters after 3-4 days of operation and premature membrane cleaning immediately following a 2-week acceptance test. FIGURE 4-5. Original pretreatment design for the Tampa Bay Water Desalination Plant. SOURCE: Adapted from figure courtesy of Tampa Bay Water. The new design (Figure 4-6) is a much more robust treatment process that includes significant changes to the head works of the plant. Changes include the addition of 1/16-inch traveling screens for debris removal, ferric chloride as a coagulant, the use of chlorine dioxide for biological growth control, mechanical chemical mixing and coagulation basins to achieve better floc formation, and extended flocculation zones to aggregate and settle suspended particles up
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Desalination: A National Perspective Assessment of Desalination Process Technology The major desalination technologies currently in use are generally efficient and reliable, but the cost and energy requirements remain high. Ongoing research efforts are motivated by the need to reduce cost or to overcome operational limits of a process, such as reducing membrane fouling or increasing energy efficiency. Although existing desalination technologies will continue to see incremental improvements, the current technologies are relatively mature, and the practical limits of further energy savings through advances in RO membranes is approximately 15 percent. Thus, alternatives to the major desalination technologies continue to be investigated to enhance or replace existing desalination processes or fill niche applications where mainstream technologies are inapplicable (see Boxes 4-5 and 4-6). As discussed earlier, no desalination process can overcome the thermodynamic limit of desalination in terms of energy use (see Box 4-3). Nevertheless, research on alternative desalination technologies is under way in hopes of more closely approaching the thermodynamic energy limit or finding ways to power the desalination process with less-expensive energy sources, such as low-grade heat. POST-TREATMENT Desalinated water, produced directly from either thermal or membrane processes, is significantly stripped of dissolved solids, which results in a water quality that has low hardness and alkalinity. Consequently, without proper post-treatment, this water would be corrosive to pipeline materials, including metals and concrete, and may introduce metals into drinking water and reduce the lifetime of water-system infrastructure. Current technology enables desalinated water to be made non-corrosive by adding chemicals such as calcium hydroxide (slaked lime) to increase the hardness and alkalinity and sodium hydroxide (caustic soda) to adjust the pH. Carbon dioxide is commonly used to normalize the pH. Post-treatment of water is a mature science. The water chemistry issues are generally well understood and methods of altering chemical conditions are feasible and generally available, although the exact process used will depend greatly on the particular chemistry of the desalinated water and the existing infrastructure (see also Chapter 7). CONCENTRATE AND RESIDUALS MANAGEMENT All desalination processes leave behind a concentrated salt solution that may also contain some pretreatment and process residuals (see Tables 4-1
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Desalination: A National Perspective and 5-1). Concentrate and residuals management involves waste minimization, treatment, beneficial reuse, and disposal, and conventional concentrate management approaches are described in this section. Each approach has its own set of costs, benefits, environmental impacts, and limitations (Sethi et al., 2006a). Further, state regulations may limit the concentrate management practices available at any individual site (Mickley, 2006). Because of the widely varying level of technology involved in concentrate management options and site-specific factors and regulatory considerations that limit available alternatives, the cost of concentrate management can range from a relatively small fraction of the cost of the main desalination system to as high as several times the cost of the desalination system. The state of the technology, including advantages and disadvantages, for each of the current methods of concentrate management is discussed in this section. A summary of the challenges and limitations in the current state of concentrate management methods is also provided in Table 4-5. The environmental impacts of concentrate management alternatives are discussed separately in Chapter 5. Surface Water Discharge Surface water discharge to a receiving body is the most common concentrate management practice in the United States, employed by approximately 41 percent of municipal desalination facilities greater than 95 m3/day (Mickley, 2006; Figure 4-15) and at all seawater desalination facilities of significant capacity worldwide. Direct surface water discharge of concentrate is a relatively low-energy, low-technology solution to concentrate management. Costs are generally low assuming that the length of the pipeline is reasonable and the concentrate meets the permit requirements without the need for further treatment. However, it has the potential for negative impacts on aquatic organisms (see Chapter 5) and for complex permitting requirements. The salinity of the concentrate is typically higher than that of the ambient water, but good site location, engineering practice, diffuser design, and/or dilution with additional water (or treated wastewater) prior to discharge can likely minimize most potential negative environmental effects. Multiport discharge diffusers (Figure 4-16) are being employed at some seawater desalination plants to minimize environmental impacts. Studies show that concentrate, being denser than seawater, may sink and impact benthic communities. By employing multiple outlet ports, rather than a single open pipe, the mixing and dilution of the concentrate can be accelerated, lessening potential impacts in sensitive areas (EPA, 1991).
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Desalination: A National Perspective FIGURE 4-15. Identified methods of concentrate management, based on a survey of the 234 municipal desalination plants in the United States with output greater than 95 m3/day (25,000 gallon per day). SOURCE: Data from Mickley (2006). FIGURE 4-16. Multiport diffuser for improved initial mixing of surface water concentrate discharge. SOURCE: EPA (1991). Blending and diluting the concentrate with treated wastewater or power plant cooling water is also desirable to reduce environmental impacts but is not an option for all site locations. Shipboard desalination configurations can blend the concentrate with large volumes of ocean water, taken in specifically for that purpose.
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Desalination: A National Perspective Sewer Discharge Discharge of concentrate into an existing sewage collection system is the second most common concentrate management practice in the United States, employed by approximately 31 percent of surveyed municipal desalting facilities (Mickley, 2006). This method is also relatively low in cost and in energy use but retains the potential for adverse environmental impacts from elevated concentrations of salt or trace elements in the treated effluent (Table 4-5). A permit from the local sewage agency is required to ensure that potential adverse impacts on the wastewater treatment processes, if any, are within acceptable limits. Large-volume discharges are typically not practical or suitable. Subsurface Discharge Concentrate discharge via a subsurface discharge structure, such as a deep well, can occur in both inland and coastal areas. Deep-well injection is a mature technology that involves the disposal of concentrate into a deep geological formation, usually inland, that will serve to permanently isolate the concentrate from aquifers that may be used as a drinking water source. Appropriate geology with the presence of a structurally isolating and confining layer between the receiving aquifer and any overlying source of drinking water is required. Suitable formations for injection often contain water with TDS concentrations in excess of 10,000 mg/L. These conditions are determined through site-specific hydrogeologic assessments. Deep-well injection is commonly employed by desalination plants in certain parts of Florida and island applications where receiving aquifers can be found at relatively shallow depths, but it is less common elsewhere in the United States. Deep-well injection is used at 12 percent of municipal desalination facilities in the United States with output greater than 95 m3/day (Mickley, 2006). It is typically employed for larger desalination plants (e.g., >3,800 m3/day [>1 MGD]) because the costs for developing deep-injection wells are relatively high and are not largely reduced for smaller flows. For example, the typical capital cost of a 3,000-m-deep well is reported at $8.1 million for a concentrate flow of 3,800 m3/day, which decreases to only about $5.1 million for a concentrate flow of either 380 or 38 m3/day (Malmrose et al., 2004). These costs exclude any pretreatment or standby disposal system. While capital costs for well injection are about average of typical inland concentrate management methods, the annual operating costs are relatively low as a percentage of total operating costs (Mickley, 2006). For
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Desalination: A National Perspective example, the capital cost of well injection and several tens of miles of delivery pipes was 26 percent of total capital costs for the desalination project, but the operating costs are estimated to be 4 percent of the annual operating cost for the El Paso Water Utilities desalination facilities (Ed Archuleta, El Paso Water Utilities, personal communication, 2006; see also Box 5-2). If permitted by state regulations, existing wells from depleted oil and gas reservoirs could be used for concentrate injection, although their injection capacity would need to be evaluated, and the costs of transporting the concentrate may offset other cost savings. For seawater desalination, subsurface discharge involves using a beach well or percolation gallery beneath the beach or underneath the seafloor. Because mixing occurs beneath the surface and the discharge plume slowly dissipates into the surf zone, subsurface coastal discharge can be an effective way to minimize environmental impacts, although it requires specific hydrogeological conditions. Land Application Land application of brackish desalination concentrate can be used for lawns, parks, golf courses, or crop land; it is not practical for the large volume and highly saline concentrate from seawater desalination and is thus only considered for brackish water applications. Even for these applications, a TDS greater than about 5,000 mg/L in the concentrate can typically preclude spray irrigation (Mickley, 2006); thus, there is typically a need for addition of dilution water. Land application is usually practical and employed only for smaller concentrate flows and, because irrigation demands are seasonal, a second or backup disposal or storage method is also necessary for year-round operation (Malmrose et al., 2004). The key concerns with spray irrigation include the influence of concentrate on the soil and vegetation, potential contamination of groundwater, and runoff to surface water (see Chapter 5). The allowable salinity will depend on the tolerance of target vegetation, percolation rates, and the ability to meet the groundwater quality standards. In general the vegetation used is dependent on the site location; however, typically grasses that are high in water and salinity tolerance are used with concentrate discharge. Research is now under way to develop genetically modified feed crops that would tolerate and take up more salt.
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Desalination: A National Perspective TABLE 4-5. Concentrate Management Challenges and Limits Method Capital Costsa O&M Costsa Land Area Required Permitting Complexity Applicable for Large Conc. Flows Potential Environmental Impact Surface water discharge La La -- H Yes M Sewer discharge La La -- M No M Subsurface discharge (deep well injection) M-H M L M Maybe L Evaporation Pond H Lc H M No M Land Application M L H M No M-H Thermal evaporation H H L Ld No Ld L = low; M = medium; H = high; dashes indicate not applicable. a Costs are highly site-specific; general trends in relative costs are indicated; cost for surface water or sewer discharge can be higher if the distance from desalination facility to the discharge water body or sewer is large, necessitating long pipelines and/or pumping facilities. b Energy use for surface water or sewer discharge or land application can possibly be higher if the distance from desalination facility to the discharge water body, sewer, or land application site is large, possibly necessitating pumping facilities. c O&M costs for evaporation ponds can possibly be higher if a significant amount of monitoring wells and associated water quality analysis are required. d Permitting complexity and environmental impacts of thermal evaporation can possibly be higher if the feedwater-to-desalination process contains contaminants of concern that could be concentrated to toxic levels in the concentrated slurry or solids that are produced from this concentrate treatment process. e Low (L) pertains to Florida (where deep-well injection is commonly practiced) and moderate (M) pertains to other states in the United States. f Climate can indirectly influence surface water discharge by affecting the quantity of surface water available for dilution. Evaporation Ponds Evaporation ponds are a low-technology but high-cost approach to concentrate management, where the concentrate is pumped into a shallow lined pond and allowed to evaporate naturally using solar energy. In evaporating environments, the thermodynamic activity of the concentrate decreases with increasing concentration and will approach the average relative humidity of the air. At that point, effective natural evaporation will cease—that is, an evaporation pond that is not leaking will not
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Desalination: A National Perspective Possible Pre-treatment Needs Labor Needs and Skill Level (for operation) Energy Use Public Perception Concerns Climate Limitation Special Geological Requirements M L Lb H Maybef N L L Lb L N N L L M L-Me N Y L L L H Y Y L L Lb H Y Y L H H L N N evaporate to dryness in most environments. Periodic removal and drying of accumulated solids is necessary for long-term physical sustainability of a site, although some evaporation ponds are closed and sealed once the pond is filled by solids. Evaporation ponds, under suitable climatic conditions, enable operation of the desalination plant under ZLD conditions, where no liquid waste leaves the plant boundary. Evaporation ponds can be a viable option in relatively warm, dry climates with high evaporation rates, level terrain, and low land costs. They are typically practical and employed only for smaller concentrate flows and are often coupled with high-recovery desalination processes. This disposal method has high capital costs due primarily to the land acquisition costs to accommodate the large surface areas required and also the costs of impermeable liners, if needed. For example, assuming a relatively high evaporation rate of 0.1 L/h/m2, the typical capital cost of an evaporation pond is reported at $40 million for a concentrate flow of 3,800 m3/day (1 MGD), reducing to $4 million for a concentrate flow of 380 m3/day (Malmrose et al., 2004). The costs for a lower evaporation rate would be proportionally higher (e.g., four times as much for a rate of 0.025 L/h/m2). These costs exclude any solids disposal or seepage monitoring.
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Desalination: A National Perspective Thermal Evaporation A thermal evaporator (also known as a brine concentrator) can reduce desalination concentrate to a slurry of approximately 20 percent solids. Thermal evaporators are generally based on vapor compression technology (See Box 4-7). Most evaporators in operation are of the vertical-tube, falling-film type and employ a calcium sulfate seeded slurry process to prevent scaling (Mickley, 2006). Thermal evaporation uses a large amount of energy—more than 18.5 kWh/m3 of feedwater (Bond and Veerapaneni, 2007). Thermal evaporators have been used in industrial RO applications and are known to be a viable and reliable technology. When an evaporator is followed with a crystallizer or spray dryer, the concentrated slurry can be further reduced to solids that are suitable for landfill disposal (a zero liquid discharge (ZLD) approach). Like thermal evaporators, crystallizers are also driven by vapor compression. In a crystallizer, the brine enters the vapor body at an angle and swirls into a vortex. As water evaporates, the salt crystals are separated using a centrifuge or filter. Spray drying is another means of producing dry product from concentrate. In this method the concentrated salt solution is reduced to a fine spray and mixed with a stream of hot gas, which provides the heat for evaporation and carries off the moisture released from the concentrate. The resultant dry salt powder is collected in a bag filter. The crystallizer and spray drying processes are more capital-cost- and energy-intensive than the brine concentrator. The energy requirements of thermal evaporators combined with crystallizers can exceed 32 kWh/m3 (Pankratz, 2008). These thermal technologies may be coupled with other membrane-based high-recovery processing technologies, described earlier in the chapter, to reduce the overall energy requirements. In a pilot study of five inland brackish water sources, Bond and Veerapaneni (2007) were able to reduce the total energy use of desalination with ZLD to 0.45-1.9 kWh/m3 of product water by developing a process train involving two RO passes, intermediate concentrate treatment, and a brine concentrator, followed by an evaporation pond. In general, thermal evaporation-based processes are characterized with high capital costs and energy requirements. The capital and operating costs of these thermal evaporation methods can sometimes exceed the cost of the desalting facility (see Chapter 6). Additionally, once most or all of the liquid is removed from the wastes, landfilling costs can be significant. The high costs, including high-energy requirements, are a large deterrent to application of this process, particularly for large municipal applications. Thus, at the present time, ZLD concentrate management approaches are typically not considered for municipal drinking
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Desalination: A National Perspective water applications. Nevertheless, thermal evaporation processes are being considered for some water supply applications in inland regions where the concentrate flows are small and other methods of concentrate management are not feasible (e.g., the desalination facility at the Deuel Vocational Institution in Tracy, California, incorporates a brine concentrator). For thermal evaporation applications to become more viable, improvements are needed that reduce capital costs and/or energy usage. Development of beneficial salt reuse options and specific salt separation methods are also important to cost reduction of the overall process (Drioli et al., 2004). Some examples of beneficial reuse of the solid product include extraction of gypsum and sodium chloride by means of selective precipitation. However, the economic viability of beneficial reuse of desalination by-product salts depends on finding local markets to avoid high transportation costs (Jordahl, 2006). Residuals Management In addition to the concentrate, there are other waste streams from desalination that need to be managed. The state of the science here is relatively mature. The spent cleaning solutions are either disposed of with the concentrate or separately, usually into a sewer system. In the latter scenario, some pretreatment in the form of neutralization for pH adjustment may be required. In the pretreatment step, solids are removed. In some cases, these solids can be recombined with the concentrate discharge and disposed to the source water. However, more commonly the solids are separated with clarifiers and sent to a belt press for further dewatering. The resulting sludge is then hauled to a landfill. The use of a microfilter will reduce the volume of sludge to be settled in the clarifier as long as the mass of chemicals required to flocculate solids is reduced over conventional processes. The membranes and security filter cartridges also constitute a residual when they reach the end of their effective life. These residuals are commonly disposed in landfills. A few companies recover used membranes and clean them for further use in a different application. CONCLUSIONS AND RECOMMENDATIONS Although RO and thermal-based processes are relatively mature, opportunities exist to improve the energy efficiency and reduce costs. For membrane-based desalination, the most significant improvements can be realized through improved pretreatment and the creation of low-fouling,
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Desalination: A National Perspective high-flux (i.e., high-permeability) membranes that can operate at lower pressures. For thermal-based desalination, costs can be reduced by more effective use of low-grade and/or waste heat. Improvements in concentrate management would have both cost and environmental implications. These and other research findings are summarized in this section. RO technology is approaching the thermodynamic minimum energy value. The membrane industry has made great strides in reducing energy use for the desalination process with the commercialization of high-efficiency energy recovery devices and improvements in membrane technology. Current energy use is within a factor of 2 of the theoretical minimum value for seawater desalination. Practical and economic constraints are likely to inhibit RO energy use from decreasing more than approximately 15 percent from current values. This level of improvement would still be valuable to reducing cost and energy use, but greater returns on investment than this should not be expected. RO is the standard by which novel technologies should be assessed when specific energy use is the main consideration. Although the RO process is relatively mature, opportunities exist to further reduce the energy use by small but economically significant amounts. These opportunities include the following: Reduced fouling through pretreatment, Development of fouling-resistant membranes, Development of high-flux (i.e., high-permeability) membranes for operation at low pressure; Development of oxidant-resistant membranes; and Improving mechanical configuration of membrane modules and membrane system design. Operating the RO process at lower hydraulic pressure while maintaining high throughput is the key to reducing the specific energy for membrane-based desalination. To fully utilize the capacity of the high-permeability RO membranes and to accommodate even more permeable RO membranes in the future, it is imperative to reduce fouling and concentration polarization effects and to develop new module configurations and system designs to avoid or overcome thermodynamic restriction. Fouling can be reduced by a more robust pretreatment and by the development of fouling-resistant membranes. Pretreatment for RO desalination can be improved by replacing conventional physicochemical processes with membrane-based (UF
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Desalination: A National Perspective or MF) pretreatment. Conventional pretreatment technologies based on coagulation and sand filtration cannot always achieve sufficient removal of foulants. Membrane-based pretreatment, particularly UF, can produce water of superior quality with very low fouling potential. Such effective pretreatment is essential for efficient utilization of future high-flux membranes. Seawater desalination using thermal processes can be cost-effective when waste or low-grade heat is utilized effectively. Location of low-grade or waste heat resources near large water consumers may reduce the cost of heat energy and offset the higher specific energy requirements of thermal desalination when compared to RO. Hybrid membrane-thermal desalination approaches offer additional operational flexibility and opportunities for water production cost savings for facilities co-located with power plants. Thermal desalination technologies are themselves relatively mature; however, additional cost savings could be realized by improvements in materials, process configuration, and optimization of low-grade and wasxte heat resources. Few, if any, cost-effective environmentally sustainable concentrate management technologies have been developed for inland desalination facilities. Several methods are currently available for concentrate management (e.g., surface water discharge, sewer discharge, deep-well injection, evaporation ponds, land application, and high-recovery/thermal evaporation systems to minimize the volume of waste produced), and each method has its own set of site-specific costs, benefits, regulatory requirements, environmental impacts, and limitations. Low- to moderate-cost inland disposal options can be limited by the salinity of the concentrate and by location and climate factors. Only evaporation ponds and high-recovery/thermal evaporation systems are ZLD solutions, but their costs are high for municipal application.