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Considering more than one-third of the global population (2.4 billion) live within 60 miles of an ocean, cost-effective sustainable desalination technologies would be tremendously valuable for freshwater production.
Industrial desalination is the process of removing salt and other impurities from seawater or other sources of non-potable water to produce fresh water suitable for consumption, agriculture, industry, or other applications. This method of producing fresh water from seawater has been gaining more attention as concerns surrounding water scarcity and climate change continue to grow.
The majority of the Earth’s surface is covered in salt water, ~97% of the Earth’s water is salt water. Out of the remaining freshwater, most of it is stored in groundwater or glaciers. All the rivers and lakes in the world only make up roughly ~0.0072% of the Earth’s entire water supply. (USGS, 2019) Many regions of the world resort to pumping fresh water from the ground, which can deplete aquifers. Saltwater is not useful for direct consumption and agriculture. Desalination is one method of taking seawater and processing it to become freshwater, effectively tapping the vast abundance of water on our planet in a useful manner. Considering more than one-third of the global population (2.4 billion) live within 60 miles of an ocean, cost-effective sustainable desalination technologies would be tremendously valuable for freshwater production in the future. (NASA, n.d.) There are multiple methods of desalination presenting different needs and tradeoffs, all of them require energy in one form or another.
Currently, reverse osmosis is the most common method used for desalination. Reverse osmosis involves passing the salt water through a semipermeable membrane, allowing water molecules to pass through while blocking out the salt and other impurities. The result is fresh water that is safe for consumption or other uses.
Thermal desalination is another common method, utilizing heat to evaporate the salt water and then condensing the resulting steam to capture the fresh water. During the evaporation process, the vast majority of impurities are left behind, so the resulting water is mostly free of salt and other minerals. This process can be repeated multiple times or combined with other methods to achieve the desired purification quality. There are several types of thermal desalination methods with key differences and tradeoffs, but they all utilize the basic mechanisms of heat and evaporation.
The MSF process involves a series of vertical chambers containing a set of evaporator tubes. salt water is heated in the first stage to produce steam, which is then passed through a series of condenser tubes where it is cooled to produce fresh water. The remaining steam from the first stage is then passed on to the second stage, where it is heated again to produce more fresh water. This process is repeated through multiple stages, with each stage operating at a lower pressure throughout the sequence. These chambers are also typically arranged to recycle the “waste heat” for each stage through thermal exchange and adjacent positioning throughout the system.
The pressure is lowered in each successive vessel, which causes the seawater to boil and generate steam which is then condensed to produce freshwater. The process is called "flash" evaporation because the seawater is rapidly heated and then depressurized, causing it to "flash" into steam.
MED utilizes a primary heat source to evaporate saltwater and collect fresh water across multiple stages that follow a decreasing temperature and pressure gradient throughout the system. From the first stage of high pressure and temperature, the freshwater is collected and condensed through a coil that is immediately routed through the second stage, acting as the heat source to evaporate additional seawater within that stage. Each stage can utilize the heat from the previous stage to evaporate feed water and recondense fresh water to provide heat for the next stage, allowing the water to be evaporated and condensed at progressively lower temperatures until the final stage. Similar to MSF, MED reuses heat from previous stages to achieve greater energy efficiency and freshwater production.
In vapor compression desalination, salt water is first pumped into a container called an evaporator which is a heat exchanger. The saltwater is heated in the evaporator, turning into steam while leaving behind the salt and other impurities. The steam then enters a compressor, which increases its pressure and temperature to become superheated steam. The hot, pressurized steam is then cooled in another heat exchanger, called a condenser, where it gives off its heat to a separate stream of cold water, which absorbs the heat allowing the steam to recondense into freshwater. This process can be arranged to repeat across multiple stages, reusing heat through multiple heat exchangers.
Vapor condensation was also a common method of desalinating water on some maritime vessels as the systems can be made to be very compact despite having higher energy requirements. Like other forms of thermal desalination, the initial heat source can be redirected to “waste heat” from an industrial process like power production or manufacturing. The mechanical compression features of the system also require energy, so this desalination method is not necessarily the most cost-effective in most applications.
Electrodialysis involves the use of passing an electric current through the saltwater in a manner designed to separate salt into its constituent ions when an electric current is present in the system. These ions are captured in brine compartments separated by multiple semi-permeable membranes configured to allow certain ions to pass through while blocking others. This process is very energy intensive however it is also highly effective for certain circumstances involving feed water with very high concentrations of salt.
Desalination is a common feature of maritime vessels and is typically found in specific areas where freshwater is so scarce that the need for freshwater justifies the costs involved with building and operating desalination assets. The ten largest desalination plants in the world are all located in the United Arab Emirates, Saudi Arabia, and Israel, where freshwater is scarce, economic productivity is high and coastal access to ocean water is present.
Industrial desalination is not the primary source of freshwater for most regions simply because direct freshwater extraction is much cheaper from groundwater, lake, and river resources. The costs of infrastructure and energy requirements make direct extraction the more favorable option. Direct groundwater extraction from deep underground can be cheaper even if that means depleting aquifers and lowering natural water tables in the process. As technology and energy sources continue to improve, desalination may become more common as a solution for addressing water scarcity around the world. Cheaper and more efficient means of desalination could also offer an alternative to unsustainable groundwater extraction.
When seawater is desalinated, the brine and wastewater also present some environmental concerns. Reintroducing higher salinity water into a concentrated area of the ocean can be problematic for some local species. The higher density of brine causes concentrations of salinity to sink and collect on the sea floor before dissolving and dissipating with the surrounding seawater which is less dense. High concentrations of salinity can disrupt dissolved oxygen levels in the water and become harmful to local wildlife. (Higgins & Gies, 2019) There are some methods of mitigating this, including mixing brine with treated wastewater or reducing the relative salinity content by diluting it with regular seawater.
One of the more interesting solutions for sustainable brine disposal is the application of “brine mining”. Brine from desalinated seawater has higher concentrations of the minerals and compounds that were formally more diluted in regular seawater. Brine mining is the industrial process of extracting valuable minerals from brine in a commercially feasible manner. (Chandler, 2019) Some minerals like lithium and potash are already extracted through brine mining operations using feed water from saline lakes and shallow subsurface water sources.
Seawater has different concentrations of minerals compared to terrestrial sources and the mineral composition of brine can widely vary across different locations. Seawater brine has been a source of salt extraction for centuries and can also be a source of magnesium and bromine extraction. (Mundy, 2022) If local conditions are favorable toward value potential, desalination facilities can be paired with brine mining and processing operations as an additional economic opportunity. Depending on the methods employed, brine mining can still result in brine and other waste products that require proper disposal.
Alternatively, some regions find desalination and water treatment infrastructure is better applied to municipal wastewater directly. Reprocessing municipal wastewater can reduce the energy requirements and brine burden as the feed water does not contain high concentrations of salt in the first place. Some other compounds and sediments must be separated but the methods employed for desalination can be applied to municipal wastewater more effectively. (Crutchik & Campos, 2021)
As the world's population continues to grow and water resources become increasingly scarce, the development of desalination technology will likely be of critical importance moving forward. According to the UN:
“Under the climate change scenario, nearly half of the world's population in 2030 will be living in areas of high water stress. In some arid and semi-arid areas, it will displace up to between 24 million and 700 million people.” (United Nations, n.d.)
Freshwater resource depletion can have devastating impacts ranging from environmental to humanitarian concerns. Lakes drying up can effectively cause local wildlife populations and ecosystem collapse. Depending on the region, failing crops due to a water shortage can lead to famine, refugee crisis, social destabilization, and possible geopolitical conflict. Environmental sustainability, economic stability, food security, and peace are dependent upon the availability of freshwater supplies.
In areas where freshwater is scarce, desalination systems can help promote economic growth by providing a reliable source of usable water for businesses and industries. This can in turn create new economic development opportunities and spur further investment in local communities. Desalination technologies can also be adapted towards recycling municipal wastewater, further improving general water conservation and resource efficiency.
The development of desalination systems can also be guided towards improving methods of reusing waste heat from industrial processes or tapping low-cost heat sources such as nuclear energy systems. Reducing direct energy demands of desalination systems could greatly improve the cost basis and efficiency of the entire process, making it more accessible to a wider range of communities and regions.
Nuclear Desalination facilities have been operating across many countries, for instance, Japan has a dozen desalination facilities paired with nuclear plants. India, China, and several other nations have developed nuclear desalination facilities and expressed interest in expanding these operations. There are many demonstration facilities planned around the world representing a growing interest in this industrial practice. (World Nuclear Association, n.d.) Large oceanside desalination facilities are not the only systems to utilize nuclear energy for desalination, as maritime vessels and inland regions also benefit from desalination systems.
Nuclear energy has been used for desalination for decades. Nuclear submarines run all their internal life support systems from nuclear power, from recycling air to making fresh drinking water the heat and electricity of a nuclear submarine’s reactor powers many different functions onboard the vessel. Conventionally maritime vessels powered from fossil fuels would have on-board desalination systems, utilizing heat from a steam system or mechanical energy from onboard engines. The historical presence of onboard desalination systems provided an easy transition for new vessels utilizing nuclear reactors.
Nuclear-powered desalination facilities have operated in some “inland” regions of the world. The Aktau nuclear power plant along the Caspian Sea in Kazakhstan had an onsite desalination facility that utilized waste heat from its operations before its decommissioning. The Caspian Sea represents just one of many large bodies of water that can develop high salinity while remaining isolated from the ocean. Communities and cities built up along these bodies of salty water may require desalination to support their local water needs. These applications may consider zero-point discharge-based solutions to the brine as they do not have the same dilution capacity as those along the ocean and thus pose a greater risk to local ecosystems.
The PWR-20 can provide district heating or process heat uses. This can also be used for a thermal desalination system. Direct thermal energy has efficiency advantages as the thermal energy does not need to be converted into electricity which results in a loss of energy in the process. In the event a reverse osmosis facility is present, the PWR-20 can provide a reliable source of electricity for routine operations. There are also circumstances where RO methods of desalination are found to be more efficient based on local conditions and resource availability. Some desalination facilities employ hybrid systems where MED and RO methods are combined when both heat and electricity sources are present. (World Nuclear Association, n.d.)
The compact design of the PWR-20 allows for vast flexibility in where they can be positioned and constructed. Delivery of the modules can occur to remote areas that might lack the infrastructure that would otherwise be required for massive construction sites of larger conventional plants.
There are many remote locations and small communities that might require a compact desalination facility with reliable power, especially if climate change and resource consumption patterns present a growing risk for water scarcity. Additionally, larger municipal desalination systems or wastewater treatment operations could benefit from local onsite power generation provided by a PWR-20.
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