Desalination, the process that turns salt water into drinkable fresh water, is held back by the expense and inefficiency of its methods. But recent discoveries have brought scientists closer to unlocking its full potential.

Where is the Water?

Of the Earth’s water supply, 97.5% is salt water. This leaves a mere 2.5% of fresh water, most of which is frozen in glaciers and the polar ice caps. In all, only .007% of the water in the world is available for drinking.1

How can we solve the water crisis when we have barely a drop of the planet’s supply to work with? One possible answer: Make more fresh water. Innovators have long seen the potential for abundant drinking water in our vast oceans. Desalination can be as simple as evaporating salt water and capturing the clean condensation. In contrast, it can be as complex as the leading desalination method, reverse osmosis. Reverse osmosis presses salt water through a membrane to filter out salt and other minerals.2

The Problem

Crisis solved, right? Oceans cover 70% of Earth’s surface, so there’s no shortage of salt water to purify.1 Unfortunately, desalination is expensive and requires a great deal of energy. Just imagine the amount of energy it would take to boil and evaporate billions of gallons of salt water per day. Similarly, reverse osmosis requires a large amount of power to force huge amounts of water through a membrane.2

Additionally, there is the matter of what we do with all the salt and minerals removed from the water. Treatment facilities currently dump this briny mix back into the ocean. This damages underwater ecosystems. According to recent estimates, desalination often produces more brine than fresh water.3

On one hand, desalination is an effective way to boost our limited fresh water supply. It has been invaluable in areas where water is scarce, such as the Middle East.3 On the other hand, even the best desalination methods are held back by high monetary, energy, and environmental costs. With all this in mind, should we give up on expanding desalination efforts?

Water scientists definitely haven’t. They are constantly figuring out how to fine-tune and even revolutionize the process. And with each new discovery, the drawbacks get smaller and smaller. Below are some of the most exciting developments.

Desalination Limitations Inspire Innovations

Renewable energy sources may be the key to getting around desalination’s high energy cost. Solar panels are the perfect candidate for powering desalination facilities. They are increasingly inexpensive, and areas with little rainfall (in other words, areas that benefit most from additional water sources) have abundant sunlight. Some small facilities have already successfully implemented solar panels. Saudi Arabia will open the first municipal-scale solar-powered desalination plant in 2021.4

States such as California have created disposal regulations to minimize brine’s environmental impact. But brine may actually be a useful resource. Farid Benyahia, a chemist from Qatar University, figured out how to use brine for making baking soda and calcium chloride. Calcium chloride is used for preserving canned vegetables and tanning leather. Once refined, Benyahia’s method could be more efficient than current industrial processes.5

Above all, desalination is not a magic cure for our water crisis. It brings challenges of its own, and irresponsible use could be a disaster. But if we continue to refine the process, desalination could be a key part of the solution.

References

1. John Misachi. February 18, 2018. “What percentage of the earth’s water is drinkable.” World Atlas. https://bit.ly/2VMMe8J
2. DOE/Brookhaven National Laboratory. March 28, 2019. “Illuminating water filtration: Researchers using ultrabright X-rays reveal the molecular structure of membranes used to purify seawater into drinking water.” ScienceDaily. https://bit.ly/2H4IlE1
3. Matt Simon. January 14, 2019. “Desalination is booming. But what about all that toxic brine?” Wired. https://bit.ly/2Hcf1h8
4. Amaury Laporte. November 16, 2018. “What’s the deal with desalination?” Environmental and Energy Study Institute. https://www.eesi.org/articles/view/whats-the-deal-with-desalination
5. Erica Gies. June 6, 2016. “Desalination breakthrough: Saving the sea from salt.” Scientific American. https://bit.ly/2iAvzjD

By April Day, Staff Writer for Save the Water | March 5, 2017

What is desalination?

Desalination is the process of desalting water. Two methods of desalination are membrane processing and thermal processing. Thermal processing has been used widely in the Middle East for more than 60 years to help satisfy water needs.2 This article, however, will focus on membrane processing method, which generally requires less energy and money.

Membrane Processing for Desalination

This process requires forcing water through semipermeable membranes by using high pressure, a processed called reverse osmosis.2 What is reverse osmosis? Reverse osmosis is osmosis in reverse. Osmosis is the process by which solvents, like water, seek equilibrium, namely equality in dissolved solutes like salt. The solvent moves through a semipermeable membrane that allows some molecules to pass through but not others. Osmosis would move water without salt to water with salt.5

By contrast, reverse osmosis uses pressure to force the solvent with high concentration, water with salt, through a membrane into the solvent with a low concentration of solute, water without salt. Thus, reverse osmosis captures the solids, such as salt, on one side of the membrane and the solvent, such as water, on the other.5 Membranes are usually made out of “layered polyamides and polysulfones with a porous support material”.2 Engineering advances have cut energy use in half over the past 20 years.4

Nowadays the same technological processes that desalt water also remove most organic chemicals and most microbial contaminants. Microfiltration and ultrafiltration membranes can filter and decrease some particulate loading, thereby reducing possible damage to reverse osmosis membranes and nanofiltration membranes. A lower energy cost area of research and development is “forward osmosis technology,” which relies on a solution to better mimic natural osmosis.2 Reverse osmosis can also be used for treating wastewater to make it drinkable and may produce energy to reuse for further water treatment.5 This man-made process accelerates nature’s process of “solar desalination evaporation” to produce rain, which is the main source of freshwater on earth.8

Drawbacks of Desalination

One drawback is that both desalination processes often produce water that must be treated with calcium carbonate or limestone to reduce excessive erosion of pipes, valves, and other contact surfaces. Another drawback is that both processes produce concentrated leftover salts, called brine, and a concentrate of other chemicals that were present in contaminated water. Managing these two byproducts of the process is a challenge, more so when it comes to inland plants.2

Areas in the United States face water scarcity and underground water

Worldwide, water is becoming more scarce.6 For example, China is facing a potential water crisis, perhaps risking a predicted water shortage of “199 billion cubic metres in 2030”.3 According to US Geological Survey estimates, available freshwater represents 0.76% of the world’s total water volume, and saline groundwater represents about 1%.2 Groundwater can be found in aquifers, which are geologic formations that transmit water to wells and springs.7 While underground water offers an additional water source, much of it is brackish, “typically defined as distastefully salty”.9 Additionally, pumping underground water from aquifers can cause saltwater to replace the once fresh groundwater.2 Desalination can combat both scarcity and pumping problems.

Inland desalination of brackish groundwater can provide a cost-effective alternative to other water sources

Desalination of brackish groundwater offers a less expensive alternative to processing seawater.2 As of mid-2015, according to the International Desalination Association, at least 18,426 desalination plants were operating in 150 countries that were capable of producing 86.8 million cubic meters of water daily. In the United States, the most notable examples of reliance on desalination to secure water are Tampa, Florida and Carlsbad, California.2 Israel and Texas are both places with scarce water supplies, but have succeeded in using inland desalination. El Paso, Texas currently boasts the largest desalination plant in the world by being able to produce 30,000 acre-feet of water per year. US Geological Survey is expected to issue the first map this year that identifies brackish groundwater supplies in the United States, although there are already many smaller desalination plants that treat brackish groundwater across the nation.1,4

Focus on Testing Desalinated Water for Contaminants and Disposal of Byproducts

Importantly, desalination does not create an unlimited water supply. Water conservation is still key to water sustainability.1 Drawbacks of desalination are brine and chemical contaminants that were removed from water by the process. Inland, the brine facilities could use limited lined evaporation ponds, additional treatment, and possibly deep well injection.2 You could ask this question: “how will facilities address these issues if a local water supplier is considering desalination of groundwater to supplement your personal water supply?”

The byproduct chemical concentrates will need to meet additional requirements before disposal, “because of their probable toxicities as well as salinity” (Cotruvo, 2016).2 Save the Water is working toward a lab that will fill the critical need to test and to treat sources of water.

References

1. Bansel, D.G. (2017, Feb. 5). Desalination of aquifers offers drought-weary California new hope. Retrieved from http://www.mercurynews.com/2017/02/05/desalination-of-salty-aquifers-offers-drought-weary-california-new-hope/
2. Cotruvo, J. (2016, Oct. 7). Desalination basics: Thermal and membrane technologies lead the way in this growing treatment process. Retrieved from http://www.watertechonline.com/desalination-basics-thermal-membrane/?platform=hootsuite
3. Dickey, L. (2017, Feb. 22). China’s water footprint: an example for future policymakers. China Dialogue. Retrieved from
   https://www.chinadialogue.net/article/show/single/en/9621-China-s-water-footprint-an-example-for-future-policymakers
4. Gillis, J. (2015, April 11). For drinking water in drought, California looks warily to sea. New York Times. Retrieved from https://www.nytimes.com/2015/04/12/science/drinking-seawater-looks-ever-more-palatable-to-californians.html?rref=collection%2Ftimestopic%2FDesalination.
5. Kershner, K. (2008, 8 May). How Reverse Osmosis Works.
   HowStuffWorks.com. Retrieved from http://science.howstuffworks.com/reverse-osmosis.htm.
6. Save the Water. (n.d.). 250 water facts. Retrieved from http://savethewater.org/education-resources/water-facts/
7. United States Geological Survey. (2016, Dec. 2). Saline water: Desalination: Thirsty? how ’bout a cool, refreshing cup of seawater? Retrieved from
   https://water.usgs.gov/edu/drinkseawater.html
8. United States Geological Survey. (2016, Dec. 9). Aquifers and groundwater. Retrieved from https://water.usgs.gov/edu/earthgwaquifer.html.
9. United States Geological Survey. (2017, Jan. 3). What is “brackish”? Retrieved from https://water.usgs.gov/ogw/gwrp/brackishgw/brackish.html

Increased agricultural production and global population has put pressure on our already limited water sources. As freshwater becomes a scarcer commodity around the world, companies are seeking new ways to reuse and replenish sources in a more efficient and environmentally friendly way. Methods such as desalination and reverse osmosis have played a significant role in creating new water sources. However, high-energy consumption poses a problem for a changing climate. (2) Forward osmosis is a process that is currently being explored and tested more frequently. Experts seek to discover the effectiveness of the process to produce freshwater from the dirtiest wastewaters in which reverse osmosis cannot. (1)

Currently 21 billion gallons of fresh water is desalinated worldwide each day.  The majority of that water is a result of reverse osmosis. (1) Reverse osmosis pumps pressurized water through a series of membranes to remove the salt, but this method still requires a significant amount of energy. Instead of forcing the water through the membrane, forward osmosis is pushed through naturally from a concentrated to less concentration solution, requiring less energy. Using forward osmosis a company called Oasys Water was able to turn roughly 60 percent of oil and gas wastewater into safe drinking water. This water is too saline to treat with reverse osmosis and would burst the membrane. (1)

While the nature motion of forward osmosis require less energy, a study from Massachusetts Institute of Technology claims that forward osmosis is less energy efficient than reverse osmosis because it involves two steps. First, the water is drawn into a concentrated solution called draw solution, and then a second step is required to produce purified water from the draw solution. The study states since the salt content in the draw solution is of such a higher concentration than seawater, it will always require a higher level of energy for regeneration. (3)

Since these processes are still being researched, there is an ongoing to debate on whether forward osmosis will prove to be a viable solution to the world’s water problems. So far it has proven it can clean much dirtier water than reverse osmosis can, but the question of whether it is worth the energy consumption still remains. Can the process be altered to become more energy efficient? Companies are working on different forms of forward osmosis to prove just that. Researchers are exploring numerous techniques including filtering through magnetic fields, or skimming oil based salts off the top of the draw solution in order to reduce energy consumption in the regeneration process. (1)

Using fertilizer drawn forward osmosis for agriculture is also being explored. Instead of using extra energy to regenerate the draw solution, a fertilizer is used in the first step. A diluted fertilizer solution is created after desalination that can be directly applied to crops, removing the second process of regeneration. Since the agricultural sector constitutes up 70 percent of the world’s freshwater withdrawal this could be a major asset to water conservation efforts. (2)

As the world’s access to fresh water sources continues to diminish, it is imperative that researchers continue to find more sustainable and efficient alternatives in order to provide for a consistently growing population.  Experimenting with processes like forward osmosis brings the world closer to developing improved methods of conservation and distribution of freshwater.

Sources:

1. McKenna,Phil. 29 April 2015. ”Purifying the Dirtiest Waters.” PBS. https://www.pbs.org/wgbh/nova/article/forward-osmosis/
2. Phuntsho, S., Shon, H., Hong, S., Lee, S., Vigneswaran, S., & Kandasamy, J. “Fertiliser drawn forward osmosis desalination: the concept, performance and limitations for fertigation.” Reviews In Environmental Science And Bio-Technology, 11(2), 147-168.
3. Water World. 25 July 2014.“Forward osmosis is not energy efficient, says MIT study.” Waterworld.com. https://www.waterworld.com/municipal/drinking-water/treatment/article/16215387/forward-osmosis-is-not-energy-efficient-says-mit-study