Desalination: Worth its Salt?

Jan. 17, 2019

Desalination plants provide fresh water to thirsty populations around the world. And in the face of global water scarcity, this technology is considered increasingly valuable. However, the brine that these treatment processes discharge can also be detrimental to the environment according to a new study.

For every liter of freshwater output, desalination plants produce about 1.5 liters of brine (depending on the feedwater salinity and desalination technology). A recent UN-backed study has determined that nearly 16,000 desalination plants worldwide pump out 142 million cubic meters (5 billion cubic feet) of salty brine every day, 50% more than previous estimates, to produce 95 million cubic meters of fresh water.

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“Around 1.5 to 2 billion people currently live in areas of physical water scarcity, where water resources are insufficient to meet water demands, at least during part of the year. Around half a billion people experience water scarcity year-round,” Dr. Vladimir Smakhtin, a co-author of the paper and the director of UNU-INWEH told “There is an urgent need to make desalination technologies more affordable and extend them to low-income and lower-middle income countries. At the same time, though, we have to address potentially severe downsides of desalination—the harm of brine and chemical pollution to the marine environment and human health.”

Brine is comprised of about 5% salt. Seawater is typically only about 3.5% salt. Brine often includes toxins and heavy metals which can accumulate in the environment and have negative effects on plant and animal species. According to marine scientists, brine is denser than seawater and often sinks to the ocean floor, where it can lower oxygen levels in seawater with devastating impacts on aquatic ecology.

To mitigate the effects, facilities can dilute brine with seawater before releasing it and take care to distribute it where ocean currents are strongest to encourage dissipation. In other cases, they can also evaporate brine into crystalized form.

Desalination technologies have evolved tremendously within the past decades and increased in both energy and water efficiency. Thermal desalination methods have been surpassed by membrane-based processes such as reverse osmosis (RO), with significantly lower brine discharge. According to a recent Wired magazine article, “With thermal, 75 percent of the water you bring in might leave as brine. With RO, it’s more 50-50 freshwater to wastewater.”

Not surprisingly, many of the coastal plants that are producing the most brine use older technology. The UN study found that of the 173 countries operating desalination plants, only four nations—Saudi Arabia, the United Arab Emirates, Kuwait, and Qatar—produce 55% of global desal brine. Some of this can be attributed to higher intake water salinity and the operation of older, less efficient desalination technologies.

What are your impressions? What strategies can you suggest for mitigating the environmental effects of brine discharge?
About the Author

Laura Sanchez

Laura Sanchez is the editor of Distributed Energy and Water Efficiency magazines.

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