Desalination A National Perspective (2008) / Chapter Skim
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4 State of the Technology
4 State of the Technology
Pages 59-107
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From page 59... ...
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)
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From page 60... ...
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.
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From page 61... ...
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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From page 62... ...
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.
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From page 63... ...
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.
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From page 64... ...
Horizontal directionally drilled (or slant-drilled) wells, shown in Figure 4-3, are increasingly being considered for use in large seawater desalination facilities.
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From page 65... ...
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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From page 66... ...
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.
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From page 67... ...
Chemicals such as ferric chloride or polyelectrolytes are added to enhance the coagulation of suspended solids prior to settling and filtration (Table 4-1)
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From page 68... ...
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 Mak kawi, 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 В-ethyl phenyl ketocyclo- 25 Andijani et al., 2000 hexylamino hydrochloride NOTE: The types and concentrations of pretreatment chemicals vary with plant design and source water conditions.
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From page 69... ...
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.
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From page 70... ...
The Tampa Bay pretreatment system depicted in Figure 4-6, the first of its kind for seawater desalination, is not a proven pretreatment technology. Because pretreatment is a key step to successful operation of seawater desalination plants, it is critical that new pretreatment approaches be tested systematically before implementation.
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From page 71... ...
is a mature technology and has been proven effective for seawater desalination plants with surface water intakes. MF and UF pretreatment systems for RO are likely to gain more popularity due to the superior quality of water that such systems can produce, although their robustness under such conditions remains unproven.
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From page 72... ...
Current online thermal 3 capacity 20 Current online membrane capacity 15 10 5 0 1950 1960 1970 1980 1990 2000 2010 Year FIGURE 4-7. Cumulative global capacity of installed desalination plants for thermal and membrane technology.
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From page 73... ...
. The dramatic growth in membrane and thermal desalination processes over the past four decades is shown in Figure 4-7.
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From page 74... ...
In any reversible desalination process, the same energy is needed to desalt water, and this energy is independent of the technology or device employed and the exact mechanism of desalination. Thus, all desalination systems share a theoretical minimum work (available energy)
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From page 75... ...
State of the Technology 75 V2 1 W= V1 − V2 ∫Π V1 os dV (2) The osmotic pressure is a function of the activity of water, and it decreases with increasing salinity of the salt solution.
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From page 76... ...
For both brackish water and seawater, membrane processes can reduce salinity in the product water to levels less than 500 ppm TDS. Reverse Osmosis The RO process uses semipermeable membranes and a driving force of hydraulic pressure, in the range of about 1,000 to 8,300 kilopascals (kPa)
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From page 77... ...
For seawater RO, the specific energy usage is typically about 3-7 kWh/m3 with energy recovery devices (Alonitis et al., 2003; Miller, 2003; see Table 4-2)
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From page 78... ...
However, larger facilities may group pumps, membranes, and energy recovery into process or pressure centers to lower capital costs and improve operating costs. b Product water quality for RO is a design variable.
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From page 79... ...
State of the Technology 79 TABLE 4-3 Comparison of Predominant Brackish Water Desalination Processes Brackish water RO ED/EDR NF Operating <45 <43 <45 temperature (°C) Pretreatment High Medium High requirement Electrical energy 0.5-3 ~0.5 kWh/m3 <1 3 use (kWh/m )
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From page 80... ...
Another limitation in RO desalination is the relatively low recovery rate in seawater and brackish water desalination (up to about 60 percent and 50-90 percent, respectively) , which results in large volumes of concentrate.
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From page 81... ...
. For example, demonstration testing is under way on a dual RO system with intermediate chemical precipitation to further enhance recovery in brackish water desalination by addressing scaling concerns (Williams et al., 2002)
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From page 82... ...
Potential benefits include the efficient use of low-grade heat or solar energy, small footprint, and low capital costs compared to conventional thermal desalination methods. Membrane Distillation In membrane distillation, saltwater is warmed to enhance vapor production, and the vapor is exposed to a membrane that can pass water vapor but not liquid water.
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From page 83... ...
The possible advantages of the use of membrane distillation are that it has a small footprint relative to other thermal desalination technologies, lower capital costs, and the ability to use lowgrade heat sources. Possible disadvantages include difficulty in maintaining the hydrophobicity of the membrane over long periods due to fouling and membrane degradation, the large enthalpy of vaporization required for the phase change of water transported across the membrane, and poor rejection of volatile feedstream contaminants (Peng et al., 2005; USBR, 2004)
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From page 84... ...
For NF, the typical energy usage is lower than that for RO, depending on the feedwater characteristics and the product water quality objectives. Similar to RO, energy recovery is possible using typical energy recovery devices.
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From page 85... ...
3 20 15 10 Affordable 5 Desalination Coalition 1.62 0 1975 1980 1985 1990 1995 2000 2005 2010 Year FIGURE 4-11. Seawater reverse osmosis energy use trend.
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From page 86... ...
Although the pressure exchanger offers higher efficiency than the indirect device group, the choice of energy recovery device for a specific plant design depends on a number of factors. For example, if energy is a critical issue to overall operating costs, the higher-efficiency pressurework exchangers often will be the device of choice.
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From page 87... ...
will also reduce the pressure required to produce practical water fluxes. Through a simple mass and energy balance calculation (see Appen BOX 4-6 Research to Improve Membrane Fouling Resistance, Flux, and Selectivity Current research efforts, including those in the rapidly growing field of nanotechnology, have the potential to advance technologies for water and wastewater treatment as well as desalination.
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From page 88... ...
88 Desalination: A National Perspective While these experimental observations are promising, no studies have been carried out so far that demonstrate rejection of salt by such nanotube membranes. It is also not clear at this time how such membranes will perform with seawater and brackish waters, where fouling can be an important factor.
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From page 89... ...
Although these improvements would still provide a cost savings to the desalination process, an improvement in energy savings beyond 15 percent appears to be a significant challenge. Improvements in module design that enable operation at higher fluxes appear to have the greatest potential for reducing the overall operating costs of desalination because the capital costs and energy costs per cubic meter of permeate produced would simultaneously be reduced (see Box 4-6)
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From page 90... ...
. Thermal Desalination Processes Thermal distillation was the earliest method used to desalinate seawater on a commercial basis, and thermal processes have been and continue to be a logical regional choice for desalination in the Middle East for several reasons.
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From page 91... ...
BOX 4-7 Overview of Thermal Desalination Processes Three primary thermal desalination processes have been commercially developed: • Multistage flash (MSF) distillation, a forced circulation process, is by far the most robust of all desalination technologies and is capable of very large production capacities per unit.
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From page 92... ...
. Other nonhybrid thermal desalination approaches, including solar stills and freezing, have been developed for desalination, although they have not been commercialized to date (Buros, 2000)
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From page 93... ...
The potential for mineral-scale deposition in a thermal desalination plant is an economic optimization issue, not a limitation of the process. Thermal technologies, including variations of MSF's forced circulation configuration, can work with supersaturated salt solutions and are used in brine concentrators for minimizing the volume of desalination concentrate.
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From page 94... ...
The waste energy from this exhaust steam is ideal for use by thermal desalination processes. In contrast, condensing turbines have a cool exhaust steam under vacuum conditions.
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From page 95... ...
Thermal desalination plants incorporated into this process could therefore produce the water used to dilute the acid which in turn produces the heat required for the thermal desalination process. The location of low-grade and/or waste heat resources near saline water sources and large consumers of water, including industry, has not been investigated, and research on opportunities to utilize low-grade and/or waste heat could yield economical applications of existing thermal desalination technology in the United States.
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From page 96... ...
One hybrid approach blends the product water from parallel RO and thermal desalination processes, which enables the RO membranes to operate with higher permeate TDS (Ludwig, 2004) and which can reduce the replacement costs of RO membranes by up to 40 percent (Hamed, 2005)
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From page 97... ...
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)
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From page 98... ...
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.
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From page 99... ...
FIGURE 4-16. Multiport diffuser for improved initial mixing of surface water concentrate discharge.
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From page 100... ...
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)
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From page 101... ...
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.
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From page 102... ...
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.
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From page 103... ...
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.
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From page 104... ...
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.
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From page 105... ...
. For thermal evaporation applications to become more viable, improvements are needed that reduce capital costs and/or energy usage.
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From page 106... ...
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.
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From page 107... ...
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.
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