Plans for desalination—removing salt from seawater—date back to 4th century B.C. from ancient Greek philosopher Aristotle, according to livescience.com. Action was put into place around 200 A.D. when sailors desalinated seawater with boilers on their ships.
But the longstanding practice has not gained as much traction in the U.S. as it has globally. There are nearly 20,000 desalination plants worldwide, according to the International Desalination Association (IDA). Most of the global production capacity is in the Middle East.
But there are investments underway, including California approving $34 million in grants for eight desalination projects in 2018. Also, in December 2018, the U.S. Department of Energy (DOE) announced it would spend $100 million to establish an energy-water desalination hub to address water security issues in the U.S. It will focus on early-stage research and development for more efficient and less expensive desalination technologies including manufacturing challenges, and for treating nontraditional water sources for multiple end-use applications, according to the DOE.
Outside of the Middle East, reverse osmosis is the most popular of several desalination methods. Thermal evaporation is the most commonly used desalination method there. In reverse osmosis, seawater is pumped at a speed three times slower than a fish swims, in order to protect the marine ecosystem. The water is then filtered. High-pressure pumps send the salt water through a superfine membrane, equivalent to 100 elephants standing on a manhole cover.
For every 2 liters of pumped saltwater, 1liter is pure and then enriched with minerals and distributed in the drinking water network. The other liter, called brine, is twice as salty and, after being treated and diluted, is returned to its natural environment.
Here are three major desal challenges, as outlined by University of Arizona professor Robert Glennon, Ph.D., at The Real Truth About Health conference in 2015.
- “It’s very expensive,” Glennon said. “The membranes you use in reverse osmosis are very high-tech, costly. They’re prone to fouling and they require frequent replacement.”
- “It’s very energy-intensive. Pushing that water at high pressure through the membranes. The more energy you use, the more water you use to produce the energy and we’re in this cycle,” he added.
- Finally, what do you do with the salt that’s left over? “It doesn’t go away. It’s all concentrated into a brine stream,” Glennon said, noting challenges of plants in California and Tampa Bay. The brine disrupts marine life and can kill sea organisms.
The Carlsbad Desalination Plant is the largest desalination plant in the Western Hemisphere. The California plant, 35 miles north of San Diego, is a $1 billion plant that generates up to 56,000 acre-feet of water per year, according to the Mercury News. Ocean desalination costs are between $2,000 and $2,500 per acre-feet. One acre-foot per year is approximately 893 gallons per day. Brackish water does not have as much salt as seawater and its desalination costs are half that, and has fewer environmental impacts.
“But we want to be protective of the environment and provide water at reasonable cost,” said Richard Mills of the Department of Water Resources to the Mercury News. “That’s been the challenge for desalination, in terms of why we can’t just build a lot of plants anywhere.”
It is cheaper to desalinate water in places such as Saudi Arabia and the United Arab Emirates because of low energy prices combined with economy of scale at their large facilities, according to a Bloomberg report.
What’s Next?
Here are some ways desalination could become a more affordable option in the future:
- Colocating reverse osmosis facilities with existing water-cooled coastal power plants can lead to lower costs and lower pressure needed, thanks to less viscous water (per nationaldriller.com).
- Many small-scale plants are using solar power, per the Environmental and Energy Study Institute (EESI), and working on plans for municipal-scale solar-powered plants, but the first one is planned for 2021 in Saudi Arabia.
- Engineers at Massachusetts Institute for Technology ame up with a method called shock electrodialysis that “uses an electrically driven shockwave within a stream of flowing water, which pushes salty water to one side of the flow and fresh water to the other, allowing easy separation of the two streams.”
DESALINATION STATISTICS
- 19,744—Number of desalination plants worldwide
- 99—more than 99.7 million cubic meters per day, the global capacity of commissioned desal plants
- 150—Number of countries where desal is practiced
- 300—More than 300 million people globally who rely on desal for some or all daily needs
(source: International Desalination Association)
The May issue of Pumps & Systems magazine focused on clean water. Desalination removes salts and minerals from water to make it clean. But, it has its challenges. Here, Danfoss High Pressure Pumps Business Development and Marketing Director Anders Hogh-Skov answers some desalination questions.
What are the challenges with desalination today?
“The process of desalination is very energy demanding.”
What are the benefits for areas that use desalination?
“The clearest answer is quite simple: Clean and drinkable water. More and more places face scarcity in water supplies and desalination offers a safe solution to ensure fresh water to areas where water scarcity is experienced, e.g. for hotels, cities or within marine. The source to ensure freshwater is endless (sea water) and with a desalination process, you can create as much fresh water as your system to produce. Think about the alternatives of transporting the same amount of water. Then think about how much water a small city would use and the benefit of desalination over transportation seems obvious.”
Are there particular desalination approaches (thermal, reverse osmosis, electrical) that seem to be working better than others?
“In recent years, the natural choice of desalination for seawater is reverse osmosis. It is compared to thermal desalination a much more environmental and energy efficient solution. This is also the reason that most desalination plants are being built around the technology of reverse osmosis.”
What are the best types of equipment for desalination in 2019?
“A major hurdle with reverse osmosis is that even though it is much less energy consuming than many thermal processes, it still requires a lot of energy, especially in the high-pressure pumps. It is therefore critical to use the most efficient high-pressure pumps for sea and brackish water reverse osmosis (SWRO like axial piston pumps, for example.”
What do you see in desalination for the year going forward?
“Desalination is here to stay and a constant search for more and more energy efficient solutions has shaped the past and will also shape the future. The yearly demands are fluctuating due to political and economical instabilities, but desalination is expected to grow significantly both short and long term as shortage of fresh drinkable water is expected to continue increasing.”