Over 97% of the water on earth is unsuitable for human consumption due to its salinity. The vast majority (about 99%) of this is seawater, with most of the remainder consisting of saline groundwater (US Geological Survey, 2010). Purification of this saline water holds the promise of nearly unlimited water resources for human civilizations in coastal regions. However, purification of seawater is expensive, energy intensive and often has large adverse impacts on ecosystems. Despite these drawbacks, desalination can be an appropriate technological choice in certain settings. Technological advancements continue to decrease the economic and environmental costs of desalination (WHO, 2007).
Desalination is the removal of sodium chloride and other dissolved constituents from seawater, brackish waters, wastewater, or contaminated freshwater. Approximately 75 million people worldwide rely on desalination and that number is expected to grow as freshwater resources are stressed by population growth and millions more move to coastal cities with inadequate freshwater resources (Khawaji et al., 2008). Desalination is most widely used in arid regions; more than half of the world’s desalination capacity (volume) is located in the Middle East and North Africa. Seawater accounts for over 50% of desalination source water worldwide. However, as of 2005 in the United States, only 7% of desalination plants used seawater. Brackish waters made up the majority of source waters for desalination, with most of the remainder consisting of river waters and wastewaters (Gleick et al., 2006).
Two streams of water result from desalination: (1) a pure product water and (2) a high-concentration waste stream or brine. The principal desalination methods fall into two categories: thermal processes (Figure 1) and membrane processes (Figure 2).
Figure 1: A diagram of water distillation, the most simple thermal desalination process. Here, a flame is applied to a beaker containing salt water; the water evaporates leaving the salts behind. The water vapor then travels up and into the adjacent tube, where it condenses and drips into the flask as pure liquid water. Modern thermal processes (MSF, MEE, VCD, etc.) yield much greater energy efficiency than simple distillation (source: Filters Fast LLC, 2005).
Thermal desalination processes generally use heat to evaporate water, leaving dissolved constituents behind. The water vapour is then condensed and collected as product water. Distillation is the simplest of these thermal processes and the energy efficiency of this simple process has been greatly improved (Foundation for Water Research, 2006). The most common thermal desalination process today is multi-stage flash (MSF) distillation; in 2005, MSF was reported to account for 36% of desalination worldwide (Figure 3). MSF improves on the energy efficiency of simple distillation by utilizing a series of low-pressure chambers, recycling waste heat and, in some cases, can be operated at even greater efficiency by utilising the waste heat from an adjacent power plant. Multiple-effect Evaporation (MEE) (also known as multiple-effect distillation) is another thermal process that utilizes low-pressure chambers; it is possible to achieve much greater efficiency in MEE than in MSF. However, MEE is not as popular (see Figure 3) because early designs were plagued by mineral scaling. Newer designs have reduced mineral scaling and MEE is gaining in popularity (Khawaji et al., 2008; Miller, 2003). For smaller operations with volume needs around 3000 m3/day, vapour compression distillation (VCD) can be an appropriate thermal distillation option. VCD is a technically simple, reliable and efficient process that is popular for resorts, industries and work sites where adequate freshwater is unavailable (Miller, 2003).
Membrane desalination processes utilize high pressure to force water molecules through very small pores (holes) while retaining salts and other larger molecules. Reverse osmosis (RO) is the most widely used membrane desalination technology, and represented 46% of global desalination capacity in 2005 (Figure 3). The name of the process stems from the fact that pressure is used to drive water molecules across the membrane in a direction opposite to that they would naturally move due to osmotic pressure. Because osmotic pressure must be overcome, the energy needed to drive water molecules across the membrane is directly related to the salt concentration. Therefore, RO has been most often used for brackish waters that are lower in salt concentration and, in 1999, only accounted for 10% of seawater desalination worldwide (Khawaji et al., 2008). However, the energy efficiency and economics of RO have improved markedly with development of more durable polymer membranes, improvement of pretreatment steps, and implementation of energy recovery devices. In many cases, RO is now more economical than thermal methods for treating seawater (Miller, 2003; Greenlee et al., 2009).
About 90% of global volume capacity for desalination is represented by the four thermal and membrane processes discussed above. Other desalination processes include electrodialysis, freezing, solar distillation, hybrid (thermal/membrane/power), and other emerging technologies (Figure 3).
Figure 3: Global desalination capacity (volume) by process in 2010. RO: Reverse osmosis; MSF: Multi-stage flash distillation; MED: Multiple-effect distillation; ED: Electrodialysis; EDI: electrodeionisation; Other: includes freeze, nanofiltration, thermal and all other processes (source: Desalination.com, 2012).
Electrodialysis (ED) utilizes current to remove ions from water. Unlike the membrane and thermal processes described above, ED cannot be used to remove uncharged molecules from source water (Miller, 2003). It is also possible to desalinate water by freezing at temperatures slightly below 0° C, but it involves complicated steps to separate the solid and liquid phases and is not commonly practiced. However, in a cold climate, natural freeze-thaw cycles have been harnessed to purify water at costs competitive with RO (Miller, 2003; Boyson et al., 1999). Interest in harvesting solar energy has led to significant progress on solar distillation processes. Hybrid desalination combining thermal and membrane processes and usually operated in parallel with a power generation facility is a promising emerging technology that has been implemented successfully (Ludwig, 2004; Mahmed, 2005). Nanofiltration (NF) membranes cannot reduce seawater salinity to potable levels but they have been used to treat brackish waters. NF membranes are a popular pretreatment step when coupled with RO (Greenlee et al., 2009).
Progress in desalination technology has been incremental, resulting in consistent improvements in energy efficiency, durability and decreased operation and maintenance across many technologies. However, new technologies in research and development could potentially result in large improvements. These emerging technologies include nanotubes (Holt et al., 2006; Lawrence Livermore National Laboratory, 2006), advanced electrodialysis membranes (Sandia National Laboratories, 2010), and biomimetic membranes (Gliozzi et al., 2002).
Desalination can greatly aid climate change adaptation, primarily through diversification of water supply and resilience to water quality degradation. Diversification of water supply can provide alternative or supplementary sources of water when current water resources are inadequate in quantity or quality. Desalination technologies also provide resilience to water quality degradation because they can usually produce very pure product water, even from highly contaminated source waters.
Increasing resilience to reduced per capita freshwater availability is one of the key challenges of climate change adaptation. Both short-term drought and longer-term climatic trends of decreased precipitation can lead to decreased water availability per capita. These climatic trends are occurring in parallel with population growth, land use change, and groundwater depletion; therefore, rapid decreases in per capita freshwater availability are likely.
Access to an adequate supply of freshwater for drinking, household, commercial and industrial use is essential for health, well being, and economic development (WHO, 2007). In many settings, desalination processes can provide access to abundant saline waters that have been previously unusable.
The major drawbacks of current desalination processes include costs, energy requirements and environmental impacts. The environmental impacts include disposal of the concentrated waste stream and the effects of intakes and outfalls on local ecosystems. These are covered in more detail under barriers to implementation (see below).
Despite these drawbacks, the use of desalination is widely expected to increase in the 21st Century, primarily for two reasons. Research and development will continue to make desalination less energy intensive, more financially competitive, and more environmentally benign. Increasing demand: population growth, economic development and urbanization are leading to rapidly increasing demand for water supply in coastal and other regions with access to saline waters.
The large energy demands of current desalination processes will contribute to greenhouse gas emissions and could set back climate-change mitigation efforts.
A recently published review of desalination cost literature has shown that the costs are very much site-specific and the cost per volume treated can vary widely. Some of the factors reported to have the greatest influence on the cost per m3 include: the cost of energy, the scale of the plant, and the salt/TDS content of the source water (Karagiannis and Soldatos, 2008). Capital costs of construction are clearly a major consideration as well, but are almost entirely site-specific.
The cost of membrane desalination decreases sharply as the salt concentration decreases. Seawater, on average, contains about 35,000 mg/L TDS; brackish waters, at 1000-10,000 mg/L, can be treated much less expensively (Greenlee et al., 2009). The costs per volume to desalinate brackish water using RO have generally been reported to range from $0.26-0.54/m3 for large plants producing 5000-60,000 m3/day and are much higher ($0.78-1.33/m3) for plants producing less than 1000 m3/day. Cost per volume for seawater RO are reported to be $0.44-1.62/m3 for plants producing more than 12,000 m3/day (Karagiannis and Soldatos, 2008).
Thermal methods (generally used to desalinate seawater) are subject to the same economies of scale. Costs for thermal desalination plants were reported to be $2-2.60/m3 for 1000-1200 m3/day and $0.52-1.95/m3 for plants producing more than 12,000 m3/day (Karagiannis and Soldatos, 2008).
Climate change adaptation strategies must consider not only future climate forecasts but also future technological development. The costs associated with desalination continue to decline incrementally as technological efficiency improves. As mentioned above, it is also possible that a new technology will be developed that greatly decreases the costs of desalination.
A World Bank report on desalination in the Middle East and Central Asia includes a chapter on capacity building (DHV Water and BRL Ingénierie, 2004). The major needs identified include the inadequacy of:
- information and data resource assessment specifically on desalination
- technical capabilities
- financial resources dedicated to research
- national policies in long-term planning and establishment of institutional infrastructures for management and operation of desalination
Training and formal education requirements for desalination are also discussed in detail.
Until recently, little information was available on institutional aspects of desalination. A World Bank project helped to define the key institutional issues related to desalination and provide recommendations for implementation. These issues include how and when desalination should be incorporated into a larger water policy, how to integrate desalination into energy policies and energy co-production, the role of private enterprise, and how to distribute and charge for desalinated water (WHO, 2007) (World Bank, 2005; DHV Water and BRL Ingénierie, 2004). Many of the recommendations for development of desalination relate to remedying broader problems in the water sector. Desalination requires substantial economic investment; therefore inefficiencies, waste, and low-level equilibria in the water sector can be compounded when desalination is implemented (WHO, 2007; DHV Water and BRL Ingénierie, 2004). Key recommendations for governments exploring development of desalination include:
- Develop a clear water policy using an integrated water resources management (IWRM) approach to determine accurately renewable freshwater resource potential, demand and consumption. Only when the adequacy of conventional water resources is understood should development of nonconventional (e.g. saline) water resources be pursued (DHV Water and BRL Ingénierie, 2004).
- Implement conservation and water demand management in all sectors. Key methods include reduction of non-revenue water in piped systems, use of only limited targeted subsidies, and prevention of groundwater pollution (World Bank, 2005; DHV Water and BRL Ingénierie, 2004).
- Consider desalination in combination with other non-conventional water sources including reuse of treated wastewater, importation of water across boundaries, rainwater harvesting and microcatchments (DHV Water and BRL Ingénierie, 2004).
The World Bank provides words of caution for those who believe that desalination is a panacea: “It may be preferable not to engage in desalination on a large scale unless the underlying weaknesses of the water sector are addressed...desalination should remain the last resort, and should only be applied after having carefully considered cheaper alternatives in terms of supply and demand management” (World Bank, 2005).
Barriers to desalination include environmental impacts. These include: effects of the concentrated waste stream on ecosystems; the impact of seawater intakes on aquatic life; and greenhouse gas emissions. However, the environmental impacts of desalination must be weighed against those of expanding use of freshwater sources (e.g. groundwater depletion, diverting surface water flows) (Gassan, 2007). Although RO product water is almost totally pure, it is possible that some compounds of possible concern could get into product water; pre-treatment or post-treatment processes can be used to address the few compounds that are not removed well by RO (e.g., boron). A 20-page description of procedures for Environmental Impact Assessment (EIA) of desalination projects can be found in the World Health Organization guidance document (WHO, 2007).
Desalination enables utilities in many water poor areas to access a nearly unlimited water resource. However, as discussed briefly in Section E, implementing desalination can sometimes exacerbate the problems of a poorly functioning water sector (WHO, 2007; World Bank, 2005). Therefore, the best opportunities for implementation are in water sectors that are functioning well, with well-defined water policy, well-characterized water resource availability and demand, technical expertise, and relatively little waste and inefficiency. Opportunities for desalination are greatest when:
- Freshwater resources are inadequate to meet demand (water stress or water scarcity)
- For membrane systems, an abundant source of brackish water with low salt/TDS concentration is available; or, for thermal systems, the population is located on a coastline with an adjacent facility (e.g., a power plant) that yields abundant waste heat
- Consumers are opposed to the reuse of treated wastewater
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