Desalination—removing dissolved minerals from seawater, brackish groundwater, or treated wastewater—appears at first glance to be the ideal answer to freshwater shortages. What could be more attractive than harnessing the planet’s seemingly inexhaustible 1.34 quadrillion (that’s 15 zeros) megaliters of seawater? Oceans are drought-proof. In theory, so is wastewater.
In the Persian Gulf, where seawater normally contains a lip-puckering 45 grams of salt per liter, thermal desalination is king. This method vaporizes water to purify it. Pretty much everywhere else, desalination is done by reverse osmosis, which purifies water by forcing it through membranes. Both methods cut down the salt to a drinkable 0.5 gram per liter.
But there’s a price. As long as the cost of desalination continues to depend on the cost of energy, these technologies won’t help much of the energy-starved developing world that needs them the most. And desalination isn’t just energy intensive; there’s also the problem of the toxic sludge it leaves behind. In this business, brine—the concentrated result of purifying water—might as well be a four-letter word. Throwing the brine back into the ocean can kill fish and smaller denizens of the food chain.
Several contenders might make history of these concerns. New methods in the pipeline reduce desalination’s energy demands in innovative and intriguing ways. Further off are technologies that could turn desalination’s Achilles’ heel into a source of strength: In the future, desalination might just be powered by the very stuff it filters out.