Desalination of seawater
Desalination of seawater is the process of removing salt and impurities from seawater to produce fresh, potable water. This essential technique is becoming increasingly relevant due to the rising demand for freshwater in arid regions, where natural sources are scarce. Desalination can occur naturally through the hydrologic cycle, where evaporation from the ocean leads to rain, but human-made methods are needed to meet the growing demand.
Several techniques are employed in desalination, including distillation, reverse osmosis, and freezing. Distillation involves heating seawater to create vapor, which is then condensed back into liquid water, while reverse osmosis uses pressure to push saltwater through a membrane, leaving impurities behind. Freezing is a less common method that takes advantage of the fact that freshwater freezes before saltwater, allowing for the collection of ice crystals.
Despite its benefits, desalination is energy-intensive and can be costly, making it more viable in areas with abundant energy resources. As technology advances, the efficiency and accessibility of desalination processes may improve, potentially playing a crucial role in addressing global water scarcity.
Desalination of seawater
Desalination of water is an often-disputed subject where climate change is concerned. Scientists are unsure whether desalination will have a major effect on Earth’s climate.
Background
The removal of salt from seawater is an ages-old process and a multimillion-dollar industry. According to a 2018 United Nations study, sixteen thousand plants operate across 177 countries, and as the need remained ever present, this number was only expected to grow. The demand for freshwater, especially in arid regions, drives people to create and implement new and more effective ways to remove salt from water. Desalination occurs naturally as part of the hydrologic cycle. The sun evaporates water from the ocean. The vapor, condensed by cooler air in the atmosphere, forms rain clouds. The rain from these clouds reaches the ground as pure liquid water. Earth’s ecosystems depend on this process.
All artificial desalination processes are based on the natural hydrologic cycle. For the most part, the energy requirements to desalinate seawater are heavy, making the process expensive. Still, it is estimated that 30 percent of the world’s irrigated areas suffer from problems that prevent crops from flourishing as they would if freshwater were available. The need for desalinated water for human and crop consumption has remained critical in the Middle East and other regions where freshwater is not abundant.
Distillation
The most fundamental form of desalination is distillation, one of the earliest forms of water treatment. Ancient mariners used this process to convert seawater into drinking water on long voyages. By heating salt water, capturing the vapors, and letting them condense back into a liquid, they removed salt and other impurities. The same process is used to separate alcohol from fermented grains.
Passive Vacuum Technology
Passive vacuum technology is used to decrease the energy requirements of desalination. By elevating a container, a partial vacuum can be created by the difference between air pressure inside and outside the container. The vacuum, in turn, allows water directed through the container to evaporate at a lower temperature, making it feasible to heat the water to its evaporation point with solar power. The temperature requirement for this system is less than for other methods, in part because it represents a closed system, so heat and vapors remain within it.
As pure water evaporates from salt water, the salinity of the remaining water increases, thereby decreasing its evaporation rate. Fresh salt water needs to replace the remaining brine at a rate equal to the rate of evaporation to maintain the overall salinity of the system. A tube-in-tube heat exchanger is used to inject new salt water while simultaneously drawing off the concentrated brine. The freshwater produced is reconstituted to its liquid state through a series of condenser coils similar to that of a moonshiner’s still. The freshwater is delivered to a storage tank, while the concentrated brine can be sent to a solar tank and condensed further.
This method has the advantage of using far less energy than do typical solar evaporators, and the vacuum effect produced within the enclosed system makes a pump unnecessary. Internal pressure is developed within the closed system that is sufficient to push water through the system. It is a continuous system, yet it has its shortfalls as well. The system needs to be cleaned and restarted on a periodic basis, lest the noncondensable gasses produced from the process build up and destroy the vacuum.
Reverse Osmosis
Another method of desalinating seawater is reverse osmosis. A system has four major steps: pretreatment, pressurization, membrane separation, and post-treatment stabilization. In this method, saltwater is pumped into an enclosed system, building pressure that forces it through a water-permeable membrane. The membrane prevents dissolved salts and other impurities from passing through it, thereby purifying the water. The resulting brine is pushed through the pressurized side of the reactor and stored. The concentrated salts are then discharged in an effort to minimize the pressure build-up. Without discharging the build-up of salts and other impurities, more energy would have to be input into the system to achieve purification. The main energy expenditure of reverse osmosis is thus in the initial pressurization of the feedwater through the membrane.
The key advantage to reverse osmosis is its simplicity. The only difficulties that the process presents are in minimizing the need to clean the membrane by producing enough clean feedwater to keep the system running at top capacity and in the need to remove particulates in that feedwater by pretreating the water. The technology is viable for regions with a readily available supply of groundwater or seawater.
The Purity of Ice
Yet another way to desalinate water that is seldom used is freezing. This technique is based on the simple fact that freshwater will freeze before salt water. Allowing water to freeze and crystallize, separating the crystals from the salty slurry, and then applying heat, freshwater may be obtained from salt water. Northern states and territories are the most likely to find this method viable, because they experience more months of cold weather and harsher climates than do more southern states. The harsh climate can be harnessed to freeze water without energy input.
This method has its benefits. At freezing temperatures, scaling—the build-up of elements other than salt within the water—is minimized. Also, equipment does not suffer the corrosion it does when water vapor is involved. Corrosion is a major drawback in methods involving heating the water that, in general, does not exist when freezing is involved.
Icebergs have been considered as a potential source of a massive supply of freshwater because of their purity. Icebergs contain water that is almost as pure as distilled water. They are abundant and can be easily procured. Large towing ships could, in theory, remove an iceberg from the polar region and tow it to an area where the already desalinated water within the iceberg could be easily thawed and put to use. Because Arctic icebergs are irregularly shaped, Antarctic icebergs are more suitable for transport. A suitable iceberg must be shaped correctly and must weigh somewhere around 91 million metric tons to retain enough frozen pure water by the time it reaches its destination. The drawbacks of this method include the requisite time, erosion of the iceberg during transport, financial concerns, and the uncertainty of the ecological and climatic effects of removing icebergs from an already decaying environment.
Context
The advantages of desalination are many and varied. In regions where freshwater is not readily available, desalination and purifying water could provide an economic and population boom. Arid regions, such as Asia and the Middle East, could be irrigated, and crops could be sown to increase the food supply. Desalination will play a part in the future of humankind, and it remained up to humans to create new and improved methods of providing freshwater. By the 2020s, more countries were prioritizing reducing the amount of energy used, and subsequently carbon emissions produced, in the desalination process, including through renewable power. At that point, Saudi Arabia had become a leader in supplying desalination needs through solar-powered plants. An area that continued to prove of particular concern in terms of water scarcity solutions was Yemen, which additionally faced ongoing armed conflict. Different methods are required to desalinate water in different regions of the world, but fresh, potable water remained a necessity everywhere.
Key Concepts
- distillation: use of evaporation and to remove solutes from a liquid; one of the earliest forms of artificial desalination
- passive vacuum technology: method that utilizes gravity and atmospheric pressure, rather than pumps, to create a vacuum, which enables evaporation to occur at lower temperatures, requiring less energy
- reverse osmosis: forced passage of a liquid through a membrane to remove solutes
Bibliography
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