The Environment Magazine

Joyce Miao
California High School, San Ramon, CA 94583

If you look around, water can be found almost everywhere. It’s in the oceans, rivers, lakes, soil, skies, and glaciers. It’s even in our own homes, supplying us with water for drinking, cooking, and all our other needs. However, this kind of clean water is scarce, considering that more than 96% of Earth’s water is contained in the oceans as salt water. Currently, more than two billion people lack easy access to clean water. Luckily, people have come up with innovative ways to convert saltwater into freshwater.

Figure 1: Membrane Desalination (source: popsci.com)

Desalination is the process of turning saline water, water that contains large amounts of dissolved salts, into freshwater. There are two main types of desalination, thermal and membrane. With thermal desalination, seawater is heated up into vapor, leaving the salt and other impurities behind, with the vapor later being collected. For membrane desalination, saltwater is pushed through several filters that remove the unwanted parts. A common type of membrane desalination is reverse osmosis.

Figure 2: Manhat’s Floating Solar Still (source: cnn.com)

Currently, desalination is very commonly done in land-based industrial desalination plants. Worldwide, 173 countries have desalination plants. While globally, the water produced only accounts for 1% of the water used, several counties heavily rely on desalination, especially in the Middle East and North Africa. The largest plant in the world is Ras Al Khair in Saudi Arabia, which produces 1,360,000 cubic meters of water per day. However, these desalination plants bring up several issues. Currently, they use large amounts of energy, often coming from fossil fuels. There have been efforts to reduce the energy used, but the problem has still not been entirely solved. These desalination plants also produce large amounts of wastewater called brine. Brine is highly concentrated with the salt from the water, and after it is released from the desalination plant, often sinks to the bottom of the ocean and causes ecological damage.

Figure 3: Layered Solar Still (source: mit.edu)

An alternative to land-based desalination plants is solar-powered desalination. One such device is a floating solar still created by the company Manhat. As the structure floats on the ocean, sunlight evaporates the water underneath it, and the water vapor is captured inside the structure, where it eventually condenses. As this structure relies on sunlight, it doesn’t require other energy such as fossil fuels. Additionally, it does not produce any brine. However, this 2.25 square meter structure can only produce 1.5 liters, or 0.0015 cubic meters,  of water per day, making it very inefficient. Another, more complex, solar-powered desalination system has been developed by researchers at MIT and  China. It is a multi-layered solar still that first uses flat panels to absorb heat. Then the heat is transferred to the bottom panel of water, which heats the water into water vapor that condenses on the bottom of the panel of water above it. The heat from the water vapor heats  the upper panel of water, which in turn, also becomes water vapor. This process repeats for every layer of the solar still. This system generates 1.5 gallons, or about 0.0057 cubic meters, of water per hour for every meter of solar collecting area. As this structure also relies on sunlight, it doesn’t need other energy sources. It is also relatively inexpensive and doesn’t produce any brine.

Another alternative to land-based desalination plants is wave-powered desalination. An example of this is the company Oneka, which has created wave-powered floating desalination buoys that are mainly built from recycled plastic bottles. When the “Iceberg” class buoys are anchored to the sea floor where the waves are more than a meter tall on average, the buoy absorbs energy from the waves. Then, they use the energy to draw in seawater, where a fourth of it goes through a reverse osmosis desalination system, which converts it to freshwater. The brine from this process is combined with the other three-fourths of the seawater and released back into the ocean. By not processing so much of the water, it means that the brine is not as concentrated, only about 30% saltier than normal seawater, and the ecological damage is minimized. On the buoy, there are solar-powered sensors that test the quality of the freshwater, and the water is sent back through pipes using the energy gained from the waves. These buoys are designed to produce 30-50 cubic meters of water per day, and should last 15-20 years, with maintenance 3-7 times a year.

Figure 4: Oneka’s “Iceberg” Class Buoy (source: newatlas.com)

More eco-friendly and energy-efficient alternatives to land-based desalination plants are still being created and developed, such as a “Glacier” class buoy from Oneka, which will produce 10 times more water than the “Iceberg” class ones. For now, these small systems can’t rival the enormous desalination plants that churn out hundreds of thousands of cubic meters per day. Still, they can help support local communities without access to reliable electricity but plenty of seawater and sun.

References

Blain, Loz. “Wave-Powered Buoys Vastly Reduce the Ecological Cost of Desalination.” New Atlas, 21 Nov. 2022, newatlas.com/good-thinking/oneka-wave-power-desalination/

Chandler, David L. “Simple, Solar-Powered Water Desalination.” MIT News | Massachusetts Institute of Technology, 6 Feb. 2020, news.mit.edu/2020/passive-solar-powered-water-desalination-0207.

“Desalination Completed.” Desalination | U.S. Geological Survey, 11 Sept. 2019, http://www.usgs.gov/special-topics/water-science-school/science/desalination. 

Lewis, Nell. “How Sunlight Could Turn Seawater into Freshwater for Coastal Communities.” CNN, 20 Sept. 2022, http://www.cnn.com/2022/09/12/middleeast/desalination-manhat-abu-dhabi-scn-spc-intl/index.html. 

“Saudi Claims The World’s Largest (RO) Desalination Plant Title, For Now.” Aquatech, 31 Mar. 2022, http://www.aquatechtrade.com/news/desalination/worlds-largest-desalination-plant#:~:text=The%20Ras%20Al%20Khair%20desalination,would%20make%20it%20the%20largest. 

Simon, Matt. “Desalination Is Booming. but What about All That Toxic Brine?” Wired, 14 Jan. 2019, http://www.wired.com/story/desalination-is-booming-but-what-about-all-that-toxic-brine/.