Is Desalination the Answer to Global Water Shortages?
Sunday, 08 December 2013 00:00  |  Written by Aaron Lada, Ph.D. | Article

Clean Drinking Water by Jenny DowningThe success of any human settlement hinges on the availability of fresh water. Due to a variety of factors, many areas are experiencing water shortages that threaten to become more widespread. Desalination, the process of removing salt from seawater, is capable of providing fresh water to arid locations. Yet its exorbitant cost and potential for environmental harm currently prevents exploitation of this technology. Is there hope that desalination will be the answer to the world’s water problems?

A Worsening Water Crisis
Less than 1% of the Earth’s water is fresh and available for use. Our poor stewardship of water resources has caused even this scant supply to dwindle, while at the same time demand is rising. Indeed, global water consumption is doubling every 20 years, and projections estimate that by 2025, demand for water will exceed the supply by 56%. Factors such as population growth, climate change, and increased demand by industry and agriculture have made water a valuable resource that often dictates financial and security issues. Thus tapping into the saltwater of the oceans—which cover more than two-thirds of the planet’s surface—as a source for fresh water appears to be an attractive solution.

How It Works
There are two main approaches to desalination: distillation and membrane filtration. Based on methods used for thousands of years, distillation involves boiling sea water to produce steam—purified water vapor. The steam is collected in a separate container and cooled so it will condense back into water.

Newer technologies such as reverse osmosis and electrodialysis use filters to desalinate water. In reverse osmosis the filter permits water to cross while excluding salt. Pressure is applied to a tank of seawater forcing water through the membrane; the desalinated water is collected on the other side.

Electrodialysis uses an electric current instead of pressure, and salt can move across its filter. The current induces a charge on the salt molecules, and electrodes holding the opposite charge are placed on the other side of the membrane. Attraction to the electrodes pulls the salt across, leaving purified water behind.

Desalination plants are located near an ocean and use large pipes to bring in the water, which is first pretreated to remove particulate matter, kill pathogens and bring it to the appropriate pH. All desalination methods produce a concentrated waste product composed of the salts found in seawater and chemicals used in the process. Disposal methods for the concentrate include dumping it back in the ocean, injecting it into deep underground wells, storing it in above-ground evaporation ponds, and zero-liquid discharge procedures that produce a solid waste product.

The Associated Costs
All the current desalination methods require large amounts of energy, with distillation being the most energy intensive. This makes desalination very expensive, with approximately 20-35% of the price due to energy costs. Generally, desalination is two to four times as expensive as treating fresh water, but this varies by location based on the quality of water treated, method used and types of energy available. Desalination is sometimes the only choice for producing potable water in desert and arid regions, and while wealthy countries can afford the cost, it isn’t yet an option in poorer areas.

One way to reduce the cost is to treat brackish water—groundwater with a much lower salt content than seawater. Since there is less salt to remove, less energy is required.

Another cost-saving measure is to locate desalination plants together with thermoelectric power plants that use seawater to cool their generators. The power plant would preheat the seawater that could then be desalinated at a lower cost, since less energy would be needed.

Environmental Issues with Desalination
There are several ways in which desalination can harm the environment. But even treatment and overuse of fresh water can negatively impact ecosystems—including draining lakes dry and preventing rivers from reaching their final destinations.

Specific Concerns:

  • Large amounts of energy are required and if fossil fuels are used, it increases greenhouse-gas emissions.
  • Disposing of the concentrate back into the ocean can harm organisms and disrupt ecosystems. Other disposal methods haven’t yet been proven as safe alternatives.
  • Aquatic organisms are killed while pumping in seawater.
  • Building a large desalination facility impacts the local ecology.
  • More people might be attracted to move to fragile coastal areas if a reliable source of water is available.
  • Seawater intake pipes could leak and contaminate groundwater.

The Future of Desalination
Currently, there are more than 12,000 desalination plants in 120 countries producing less than 1% of fresh water consumed globally. Yet, according to a Natural Resources Defense Council report (pdf), desalination isn’t the answer to water-shortage issues caused by climate change. In fact, the report concludes that water conservation methods would be more effective at addressing shortages than desalination.

But desalination technology is improving; in the last ten years advances in reverse osmosis have led to significant energy reductions. The concept of biomimicry, which uses designs and techniques found in nature to solve modern problems, has led to new reverse-osmosis filters that take cues from how individual cells move water across their membranes. Water can either move directly across the cell membrane or pass through protein channels called aquaporins. It is more efficient for water to cross through an aquaporin, and an internal positive charge repels salts, making them selective. A company, aptly named Aquaporin, is developing membranes with similarly designed selective channels that allow water to cross the membrane more easily, requiring less pressure and thus less energy.

As long as the global population and temperature continue to increase, water shortages will become more common. Desalination can effectively produce fresh water, but its high price and potential to harm the environment has kept it from becoming a major supplier. The world needs to address its water problems, and currently conservation—not desalination—appears to be the best course.

Additional resources:
Water Woes:  The Impending H2O Crisis
The National Resource Defense Council’s Guide to Water Conservation(pdf)
The World Water Organization
New Desalination Technologies
Fresh Water Supplies at Risk, Part 4: Water Conservation
Biomimicry: Designing Technology Based on Nature’s Wisdom

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