seawater desalination | Climate technology center and network (2023)

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More than 97% of the water on earth is unfit for human consumption due to its salinity. The vast majority (about 99%) is seawater, the remainder being saline groundwater (US Geological Survey, 2010). Purifying this salty water promises nearly limitless water resources for coastal human civilizations. However, cleaning seawater is expensive, energy intensive and often has major adverse impacts on ecosystems. Despite these drawbacks, desalination may be an appropriate technology choice in certain environments. Technological advances continue to reduce the economic and environmental costs of desalination (WHO, 2007).

Technological readiness: Can be used immediately (dozens of desalination plants are already in operation in Japan)

Need met: The need to reduce the impact of drought in areas prone to drought due to climate change. Demand is particularly high on small islands and other locations with limited freshwater resources.

Adaptation effects: securing water resources to cope with droughts caused by climate change

Relevant technical support from CTCN

• Saltwater purification for homes and low-cost durable housing technology for coastal areas of Bangladesh

• Identify and prioritize technologies to address water scarcity and climate change impacts in Namibia

Overview and features

There are three desalination methods:

• Evaporation: Process of obtaining fresh water by condensing the steam produced by the evaporation of sea water.
• Reverse osmosis: A process of obtaining fresh water by filtering seawater under pressure using a semipermeable membrane through which seawater cannot pass.
• Electrodialysis: A method of obtaining fresh water using a special membrane that can separate seawater into dilute and concentrate, and then extract fresh water from the dilute. It is in the experimental research phase.

Desalination plants are being introduced in the Middle East and Mediterranean, and reverse osmosis in particular is spreading rapidly.

reverse osmosis desalination plant

Source: Fukuoka Ward Water Bureau

Introduction to seawater desalination

sea ​​water desalinationIt is a process that removes salt and other components to produce pure water. About 75 million people worldwide depend on desalination, and that number is expected to rise further as population growth puts pressure on freshwater resources and millions move to coastal cities with insufficient freshwater resources ( Khawaji et al., 2008). Desalination is most commonly 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 more than 50% of the water coming from desalination sources worldwide. However, as of 2005, only 7% of desalination plants in the United States used seawater. Brackish water made up most of the spring water for desalination, with the remainder being primarily river water and runoff (Gleick et al., 2006).

Two water streams result from desalination: (1) a pure water product and (2) a highly concentrated waste stream or brine. The main desalination methods can be divided into two categories: thermal methods (Figure 1) and membrane methods (Figure 2). In heat treatment, water is vaporized with heat leaving the dissolved salts or waste stream and separated from the pure water. Membrane processes use reverse osmosis and high pressure to force salt water through very fine porous filters that trap salts, leaving pure water on one side of the membrane and the waste stream on the other. As much of Earth's water is found in seas and oceans, desalination creates an opportunity for coastal communities to access virtually unlimited sources of fresh water. Additionally, desalination techniques can be used to clean up brackish water in areas of seawater intrusion. In the face of adapting to climate change, this is also a crucial resource for areas where existing freshwater resources can no longer sustain local populations or be rehabilitated to meet freshwater needs.

Thermal desalination processes generally use heat to vaporize water, leaving behind dissolved components. The water vapor 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 multistage flash distillation (MSF). in 2005, it was reported that MSF was responsible for 36% of the world's desalination (Figure 3). MSF improves the energy efficiency of simple distillation using a series of low pressure chambers, recycling waste heat and in some cases can be operated even more efficiently using waste heat from a neighboring plant. Multiple effect evaporation (MEE) (also known as multiple effect distillation) is another thermal process that uses low pressure chambers; It is possible to achieve much higher efficiency in MEE than in MSF. However, MEE is not as popular (see Figure 3) as early designs were affected by mineral deposits. Recent projects have reduced mineral deposition and MEE is gaining in popularity (Khawaji et al., 2008; Miller, 2003). For smaller operations with volume requirements of around 3,000 m3/day, vapor compression distillation (VCD) may be a suitable thermal distillation option. VCD is a technically simple, reliable and efficient process, popular in recreational areas, industries and construction sites where adequate fresh water is not available (Miller, 2003).

Membrane desalination processes use high pressure to force water molecules through very small pores (holes), trapping salts and other larger molecules. Reverse osmosis (RO) is the most widely used membrane desalination technology, accounting for 46% of global desalination capacity in 2005 (Figure 3). The name of the process comes from the fact that pressure is used to propel water molecules across the membrane in a direction opposite to the way they would naturally move due to osmotic pressure. Since osmotic pressure must be overcome, the energy required to propel water molecules across the membrane is directly related to salt concentration. Therefore, RO was most commonly used for low salinity brackish water and accounted for only 10% of seawater desalination worldwide in 1999 (Khawaji et al., 2008). However, energy efficiency and RO savings have improved significantly with the development of more durable polymeric membranes, improvement of pretreatment steps and implementation of energy recovery devices. In many cases, RO is now more cost-effective than thermal methods for treating seawater (Miller, 2003; Greenlee et al., 2009).

About 90% of the 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/energy) and other emerging technologies (Figure 3).

Electrodialysis (ED) uses electricity to remove ions from water. Unlike the thermal and membrane methods described above, ED cannot be used to remove uncharged molecules from well water (Miller, 2003). It is also possible to desalinate water by freezing it at temperatures slightly below 0°C, but it involves complicated steps to separate the solid and liquid phases and is not commonly practised. However, in a cold climate, natural freeze-thaw cycles have been used to purify water at a cost competitive with RO (Miller, 2003; Boyson et al., 1999). Interest in capturing solar energy has led to significant advances in solar distillation processes. Hybrid desalination, combining thermal and membrane processes and generally operated in parallel with a power generation plant, is a promising new technology that has been successfully implemented (Ludwig, 2004; Mahmed, 2005). Nanofiltration (NF) membranes cannot reduce the salinity of seawater to potable levels, but they have been used to treat brackish water. NF membranes are a popular pretreatment step associated with RO (Greenlee et al., 2009).

Advances in desalination technology have been incremental, leading to steady improvements in energy efficiency, durability, and reduced operation and maintenance for many technologies. However, new technologies in research and development can lead to great 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).

sea ​​water desalinationit is most effective when implemented in water sectors with strong water policies, well-defined availability and demand for water resources, and strong technical expertise. In terms of budget and local demand for freshwater resources, there are several options for desalination plants, water purification approaches and potential sources of energy for desalination (eg alternative energy such as wind). The properties of salt water, such as salinity, temperature, general pollution level, etc., have a big impact on the choice of technology. For example, membrane processes are best suited for brackish water, which typically has lower salt concentrations. Pretreatment (eg microfiltration of algae from seawater) may be required before starting desalination processes, as well as advanced separation processes for the waste stream (including cooling if necessary). Environmental impact monitoring and assessment should also be carried out, depending on the size of the facility and waste disposal methods.

Benefits of seawater desalination

• Desalination can significantly support adaptation to climate change, primarily through water supply diversification and resilience to water quality degradation. Diversifying the water supply can provide alternative or supplemental sources of water when current water resources are insufficient in quantity or quality. Desalination technologies also offer resilience to water quality degradation, as they can typically produce very pure product water, even from highly contaminated water sources.
• One of the main challenges in adapting to climate change is building resilience to the reduction in per capita freshwater availability. Both short-term droughts and longer-term climate trends with less precipitation can lead to reduced per capita water availability. These climate trends are running parallel to population growth, land use change and depletion of water tables; Therefore, a rapid decline in per capita freshwater availability is likely.
• Access to an adequate supply of safe water for domestic, commercial and industrial purposes is essential for health, well-being and economic development (WHO, 2007), and desalination can provide access to water to potentially arid or arid areas. abundance of salt water that was previously unusable.
• It provides safe drinking water due to the high quality of the outgoing water. It can also provide water for other sectors such as B. Industries that require very pure water sources such as pharmaceuticals.

Disadvantages of seawater desalination

• Major disadvantages of current desalination processes include cost, energy requirements and environmental impact. Environmental impacts include the disposal of the concentrated waste stream and the impact of the inflow and outflow on local ecosystems. These are discussed in more detail in Implementation Barriers (see below).
• Despite these disadvantages, the use of desalination is widely expected to increase in the 21st century, mainly for two reasons. Research and development will continue to make desalination less energy intensive, more cost-competitive and more environmentally friendly. Growing demand: Population growth, economic development and urbanization are leading to an ever-increasing demand for water supplies on the coast and in other regions with access to saline waters.
• The large energy demands of current desalination processes will contribute to greenhouse gas emissions and could delay efforts to mitigate climate change.

Financial requirements and costs of seawater desalination

A recently published overview of desalination costs showed that costs are highly site specific and costs per treated volume can vary widely. Factors reported to have the greatest impact on the cost per m3 include: energy costs, facility size and salinity/TDS content of the spring water (Karagiannis and soldiers, 2008). Capital costs for construction also play a role, of course, but they are almost exclusively site-specific.

Membrane desalination costs drop sharply with decreasing salt concentration. Seawater contains on average about 35,000 mg/l of TDS; Brackish water can be treated much more cheaply with 1000-10000 mg/L (Greenlee et al., 2009). The cost per volume for RO desalination of brackish water generally ranges from US$0.26 to US$0.54/m3 for large plants producing 5,000 to 60,000 m3/day and is much higher (US$0.78 to US$ 1.33 USD/m3) for mills producing less than 1,000 m3/day. The cost per volume of seawater RO is US$0.44-1.62/m3 for plants producing more than 12,000 m3/day (Karagiannis and soldiers, 2008).

Thermal processes (generally for seawater desalination) are subject to the same economies of scale. The cost of thermal desalination plants has been reported as US$2-2.60/m3 for 1000-1200 m3/day and US$0.52-1.95/m3 for plants producing more than 12,000 m3/day (Karagiannis and soldiers, 2008).

Adaptation strategies to climate change must take into account not only future climate predictions, but also future technological developments. The costs associated with desalination continue to gradually decline as technological efficiency improves. As mentioned above, it is also possible that new technologies will be developed to greatly reduce the cost of desalination.

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Hybrid systems using evaporation and reverse osmosis have attracted a lot of attention recently due to their low cost, now below US$ 0.5 per m3 of fresh water produced. On the other hand, high oil prices and scarce metal supplies can increase operating and construction costs, making it difficult to keep water extraction costs low in the future.

Shuqaiq Water Production Project, Saudi Arabia (completed in 2010)

An 850,000 kW power plant and 178,000 m3/day of seawater for freshwater plants was recently built in Shuqaiq (a city near the Red Sea border of Yemen) and started supplying Saudi Power and Water Company in 2010 within a twenty-year contract. Total project cost: approximately $2 billion

Mesaido Industrial District Power and Water Project Water Production Project, Qatar (completed in 2010)

This involved building a new 2,000 MW power plant in the Mesaido industrial district near Doha, Qatar. Total project cost: approximately $2.3 billion

Institutional and organizational requirements for seawater 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 main needs identified include the inadequacy of:

• Evaluation of specific information and data sources for desalination
• Technical abilities
• financial resources for research
• national policies on long-term planning and establishment of institutional infrastructure for the management and operation of desalination

Training requirements and formal education for desalination are also discussed in detail.

Until recently, little information was available on the institutional aspects of desalination. A World Bank project helped to define key institutional issues related to desalination and provided recommendations for implementation. These issues include how and when desalination should be integrated into broader water policies, how desalination can be integrated into energy policies and energy co-production, the role of private companies, and how desalinated water is distributed and charged (WHO, 2007) (WHO, 2007). World Bank, 2005; DHV Water and BRL Ingénierie, 2004). Many of the recommendations for developing desalination relate to solving broader problems in the water sector. Desalination requires a significant economic investment; Therefore, inefficiencies, waste and low-level balances in the water sector can be amplified when desalination is implemented (WHO, 2007; DHV Water and BRL Ingénierie, 2004). Key recommendations for governments studying the development of desalination include:

• Develop a potable water policy using an integrated water resource management (IWRM) approach to accurately determine the potential, demand and consumption of renewable freshwater resources. Only when the suitability of conventional water resources is understood should the development of unconventional (eg saline) water resources be pursued (DHV Water and BRL Ingénierie, 2004).
• Implementation of water conservation and demand management in all sectors. Key methods include reducing unbilled water in piping systems, using only limited targeted subsidies, and avoiding groundwater pollution (World Bank, 2005; DHV Water and BRL Ingénierie, 2004).
• Consider desalination in combination with other unconventional water sources, including reuse of treated wastewater, cross-border water imports, rainwater harvesting, and microcatchments (DHV Water and BRL Ingénierie, 2004).

The World Bank issues a warning to those who believe that desalination is a panacea: “It may be preferable not to engage in large-scale desalination unless the underlying weaknesses of the water sector are addressed… Desalination must.” remain a last resort and should only be used after careful consideration of cheaper alternatives in terms of managing supply and demand” (World Bank, 2005).

Obstacles to seawater desalination

• Effects of concentrated waste streams on ecosystems and effects of seawater abstraction on aquatic life;
• Disposing of the waste stream resulting from desalination can have negative environmental impacts due to its high concentration of salts and trace chemicals, although this is improving with recent technological advances.
• Desalination techniques are relatively expensive and energy intensive, although there are increasing opportunities to use renewable energy such as B. Solar or wind powered desalination coupling
• Developing countries, which generally have the greatest freshwater needs, may not be able to deploy desalination because the best opportunities for its implementation lie in well-managed water sectors with clean water policies.
• Optimal use requires training, regular maintenance and access to spare parts, which can be a limiting factor in remote and smaller communities.

The environmental impacts of desalination must be weighed against those of increased use of freshwater sources (eg groundwater depletion, diversion of surface water flows) (Gassan, 2007). Although RO product water is almost entirely pure, it is possible that some potentially worrisome compounds are leaking into the product water; Pre-treatment or post-treatment processes can be used to address the few compounds that are not well removed by RO (e.g. boron). For a 20-page description of Environmental Impact Assessment (EIA) procedures for desalination projects, see the World Health Organization Guide (WHO, 2007)

Key issues for the proliferation of these technologies include reducing the cost of freshwater production, stabilizing plant performance, and introducing simple procedures for operating, maintaining, and managing the plant.

Opportunities for seawater desalination

By securing new water resources, these technologies can enable stable water supplies for homes and industries.

Desalination gives utilities access to almost unlimited water resources in many arid areas. However, as discussed briefly in Section E, implementing desalination can sometimes exacerbate the problems of an underperforming water sector (WHO, 2007; World Bank, 2005). Therefore, the best opportunities for implementation lie in well-functioning water sectors, with well-defined water policies, well-characterized water resource availability and demand, technical expertise and relatively little waste and inefficiency. Opportunities for desalination are greatest when:

• Freshwater resources are insufficient to meet demand (water stress or water scarcity)
• It provides benefits in adapting to climate change in water stressed areas through diversifying water sources and reducing pressure on fresh water sources.
• For membrane systems, an abundant supply of low salt/TDS brackish water is available; or in thermal systems, the population is on a coast with an adjacent facility (such as a power plant) that provides ample waste heat
• Consumers refuse to reuse treated effluents
• Technological advances are continually reducing the economic and environmental impact of desalination
• It has the potential to provide an almost unlimited water supply if sustainable methods of energy use and safe drainage are employed.

Implementation Considerations*

Technological Maturity: 3-4
Initial investment: 3-5
Operating costs: 3-5
Implementation period: 2-3

* This description of the customization technology provides an overall assessment of four dimensions related to technology implementation. It presents an indicative rating scale from 1 to 5, as follows:
Technological Maturity: 1 - in early stages of research and development to 5 - fully mature and widespread
Initial investment: 1 - very low cost, to 5 - very high cost needed to implement the technology
Operating costs: 1 - very low/no cost, to 5 - very high operating and maintenance costs
Deployment timeline: 1 - Very quick to deploy and reach desired capacity, to 5 - Significant time investment required to build and/or reach full capacity

This classification is for guidance only and should be seen in relation to the other technologies included in this guide. More specific costs and timelines should be identified as relevant to the specific technology and geography.

Examples of seawater desalination

Case 1. Maintenance of electrical and piping systems for seawater desalination plants (Tuvalu)

• Tuvalu has a tropical marine climate and the capital, Funafuti, has an annual rainfall of just 3,000 mm. As the island is formed on a coral reef, a stable water source is scarce. As a result, residents depend on rainwater for nearly all of their domestic needs, and chronic water shortages leave them vulnerable to long-term droughts. Therefore, finding safe sources of water other than rainwater is critical.

• Based on this situation, Japan provided a seawater desalination plant through assistance granted to Grassroots Human Security Projects in the form of the Seawater Desalination Plant Donation (1999) and Facilities Improvement Plan of Funafuti Island Water Supply (2006) as a form of cooperation to provide equipment for a stable water supply.

• Currently, however, the electrical switchboard and the seawater pump are not working, and public works department staff responsible for water works and technicians, including private sector staff, are having difficulty carrying out repairs. For this reason, technicians from Japan are sent to train the personnel of the Public Works Department in the proper repair, maintenance and operation of the seawater desalination plant.

Outono 2. Uminonakamichi Nata Meerwasserentsalzungsanlage (Fukuoka District Waterworks Agency)

• Fukuoka Prefecture has a limited amount of land suitable for dams and few major rivers, and a decrease in annual precipitation leads to frequent droughts. Demand for water has rapidly increased with urbanization and a growing population, creating challenges in finding new water resources.

• In this regard, Fukuoka District has started to develop desalination plants to meet its water needs.

Autumn 3.

The flue gas-based seawater desalination pilot plant is an innovative, low-carbon intensive desalination system. This facility is the first of its kind in India. This plant uses the waste heat from the flue gas of a fossil fuel powered plant to distill seawater, rather than using steam or electricity as in traditional desalination systems. This is a proof-of-concept setup. Once the various elements of this technology are demonstrated and the efficiency of the plant is established, this system can be replicated with or without scale in other plants.

references

• UNEP-DHI Partnershipsea ​​water desalination
• Boyson, J. E., Harju, J. A., Rousseau, C., Solc, J., and Stepan, D. J. (1999) Evaluation of the natural freeze-thaw process to desalinate groundwater from the North Dakota Aquifer to supply water to Grand Forks, North Dakota.US Bureau of Reclamation Water Treatment Technology Program Report #23.
• Desalination. com (2012).What technologies are used?.
• DHV Water BV, Holland and BRL Engineering (2004)Seawater and brackish water desalination in the Middle East, North Africa and Central Asia: an overview of key issues and experiences across six countries. Report to the World Bank.
• Fast Filters LLC (2005)."A simple guide to water filtration."
• Water Research Foundation (2006)Review of the current state of knowledge: desalination for water supply. Marlow, UK.
• Gassan, C. (2007).Let's go to Verde Desal. News from the International Desalination Association IDA. July/August 2007.
• Gleick, PH, Cooley, H and Wolff, G (2006) With a grain of salt: an update on seawater desalination. in "The Water of the World: 2006-2007". Ed. by PH Happiness. island press. Washington DC.
• Gliozzi A, Relini A, and Chong PG. (2002) Structural and permeability properties of bolaform archae tetraether lipid biomimetic membranes.journal of membrane science. Vol. 206:131-147.
• Greenlee LF, Lawler DF, Freeman BD, Marrot B and Moulin P (2009) Desalination by reverse osmosis. Water sources, technology and current challenges.water surveyVol. 43(9):2317-2348.
• Hamed, O.A. (2005) Overview of Hybrid Desalination Systems - Current State and Future Prospects.TalkingVol. 186:207-214.
• HCTI (2008).about reverse osmosis.
• Holt, J.K., Park, G.H., Wang, Y., Stadermann, M., Artyukhin, A.B., Grigoropoulos, C.P., Noy, A., e Bakajin, O. (2006) Fast Mass Transport through Sub-2-Nanometer Carbon Nanotubes .ScienceVol. 312:1034-1037.
• Karagiannis, I.C. and Soldados, P.G. (2008) Literature on the costs of water desalination: review and evaluation.TalkingVol. 223:448-456.
• Khawaji, A.D., Kutubkhanah, I.K. and Wie, J.M. (2008) Advances in seawater desalination technologies.TalkingVol. 221:47–69.
• Lawrence Livermore National Laboratory (2006)Press release: Nanotube membranes offer the most cost-effective desalination possibility.
• Ludwig, H. (2004) Hybrid systems in seawater desalination - practical design aspects, current status and development prospects. Desalination Vol. 164:1-18.
• Miller, had (2003)Review of water resources and desalination technologies. Sandia National Laboratories. AREIA 2003-0800. Albuquerque, USA.
• Sandia National Laboratories (2010)membrane technologies.
• US Geological Survey (2010)The distribution of water on earth.
• WER (2007)Desalination for safe water supply: Guidance on the health and environmental aspects of desalination.Review in progress. World Health Organization. Geneva.
• World Bank (2005)Trends in the Desalination Market in the Middle East and Central Asia (Project #012). Banco Holanda Water Project.
• Kyowakid Industry (Inglês)http://www.kyowa-kk.co.jp/english/index.html
• Fukuoka District Waterworks Agency (inglês)http://www.f-suiki.or.jp/english/index.php
• Science Portal China (Japanese) (article by Mitsuyoshi Hirai, Water Reuse Promotion Center)http://www.spc.jst.go.jp/hottopics/0907water/r0907_hirai.html
• JICA Project Information: Maintenance of electrical and piping systems for seawater desalination plants (English version available)http://gwweb.jica.go.jp/km/ProjectView.nsf/4f3700b697729bb649256bf300087

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