What Is Desalination and Why Is It Important?
Desalination is the process of removing salts and other minerals from water, making it suitable for human consumption, irrigation, or industrial uses. To separate out these contaminants, water is either distilled through thermal desalination or pumped through a membrane that holds back impurities but allows water to pass through.
Desalination allows communities to use a broader variety of water sources than conventional techniques, improving the resilience of water-stressed communities. About 30% of water in the United States comes from underground water reserves known as aquifers. Much of the water in these aquifers, especially in coastal regions, is too brackish (salty) for human use. If we were able to use these reserves, the brackish groundwater available in the U.S. would satisfy nearly 800 years of use at current rates, according to a U.S. Geological Survey report.
Accessing brackish water sources will become increasingly important. As sea levels rise, existing freshwater aquifers for coastal communities are at an increasingly high risk of seawater intrusion. Desalination is an important potential tool to help these communities treat contaminated water.
How Does Desalination Work?
Seawater, brackish groundwater, treated wastewater, agricultural runoff, and even water generated as a by-product from oil and gas extraction can all be made fit for human use through desalination.
Most treatment options to remove salts from water are either thermal desalination or membrane-based technologies, although other technologies are also under development.
Thermal desalination systems heat water so that it evaporates into steam, leaving behind impurities, which then condenses back into a liquid for human use. Yet these phase changes (from liquid to steam and back to liquid) mean that thermal desalination is more energy intensive than modern membrane technologies. Some of the thermal energy applied to evaporate the water can be recovered as the steam condenses back into liquid water, but a significant amount of heat ends up being wasted.
Large desalination facilities sometimes use technologies such as combined heat and power and waste heat reuse to achieve higher efficiencies. However, even the most efficient thermal desalination processes typically use more energy than alternatives like reverse osmosis.
Thermal desalination is the more popular—and sometimes only—option for applications that are beyond the capabilities of membranes. Thermal systems can treat water to a very high water purity (necessary for some industrial applications) and treat input water with a particularly high salt content, which membranes can't handle.
Membrane-based desalination describes a class of technologies in which saline water passes through a semipermeable material that allows water through but holds back dissolved solids like salts.
Reverse osmosis is the most common membrane technology—more than two-thirds of desalination facilities nationwide are brackish water reverse osmosis facilities, according to a report by the U.S. Bureau of Reclamation.
Osmosis refers to the natural movement of water from an area of high salt concentration to an area of low salt concentration. Reverse osmosis uses high-energy pumps to reverse that process, pressurizing water across one side of the membrane, leaving behind highly concentrated salty brine on the other.
Saltier water requires more pressure to force the water through the membrane, which can end up rupturing the membrane. The strength of the membrane thus creates an upper limit on the salinity of water that can be treated. Therefore, water with much more salt content than seawater typically cannot be purified with reverse osmosis technologies.
The U.S. Department of Energy's Industrial Technologies Office is investing in research, development, and demonstration opportunities for cost-effective, efficient desalination technologies.