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.
BRUTE FORCE: THERMAL DESALINATION
How it works
Boil seawater, condense the vapor, and drink the result.
Pure water no matter what the feedstock is.
Energy intensive, enormous infrastructure, toxic brine waste.
Ocean water. The energy required is the same regardless of the water’s salt concentration, so the process is well suited for highly saline waters.
Evaporation and pumping can consume up to 80 megawatt-hours per megaliter of water produced. Its use is largely confined to the Middle East, where plants tend to use waste heat from adjacent power plants and cheap oil instead of electricity. Thermal desalination is not used in the United States: The typical U.S. energy price of $0.065 per kilowatt-hour translates to $5200/ML of water. At just over half a cent per liter, that’s cheap enough for bathing and drinking, but for irrigation or industrial uses, the price tag is staggering.
Saudi Arabia’s Shoaiba, an 880-ML-per-day power and desalination plant on the Red Sea, is the largest thermal desalination plant in the world. But even for seawater, the market-research firm Lux predicts, thermal desalination will not extend past the Middle East, which has the lion’s share of cheap oil to desalinate its very salty water.
PUSH! REVERSE OSMOSIS
How it works
Separate saltwater and freshwater with a membrane that blocks salt ions, and the freshwater rushes to the salty side by the natural process of osmosis. Reverse osmosis (RO) uses hydraulic pressure to shove water molecules in the opposite direction, with the membrane holding back the salt.
Comparatively low energy cost.
Toxic brine; can’t completely filter potentially harmful substances like boron, arsenic, lithium, and some pharmaceutical compounds. When membranes become clogged, they must be scraped and bleached or they stop working; cleaning, however, reduces the expensive membranes’ lifetime. Pretreating the water to remove the gunk slows the rate of fouling but requires a lot of real estate.
Brackish groundwater, which contains on average only 3 to 5 grams of salt per liter. RO is also increasingly being used to desalinate seawater, however.
The pressure needed to push water through the membrane is proportional to the water’s salinity. Higher pressure means higher energy cost. On average, RO demands at least 3.5 MWh/ML produced from brackish water and more for seawater.
A Carlsbad, Calif., plant slated to open in 2012 will produce more than 189 ML of drinking water per day from the Pacific Ocean, making it the biggest RO plant in the United States.