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