New Tech Innovation Promises to Beat the Drought With Bubbles
The difficulties with growing food crops never seem to end. Too hot, too cold, too bright, too dim, too wet, too dry—anything in excess, in seems, will make a plant keel over. In arid climates, the challenge is moderating moisture and humidity, but given the various water shortages around the world, that’s easier said than done. A new study in the journal Desalination suggests a solution that’s not quite like anything we’ve seen before: It uses bubbles of seawater to produce freshwater.
In much of eastern North America and western Europe, among other places, the challenge with plants is temperature moderation. The winters are too cold, and the summers too hot, for plants to produce as much yield as they’re capable of. So some farmers in those parts of the world use greenhouses to maintain the right temperature.
But in arid climates, like most of northern Africa, Australia, and even California, it’s moisture, or rather the lack thereof, that’s the most pressing variable. Greenhouses operators in these areas don’t care much about temperature regulation; the efforts are mostly focused on keeping the air humid and the crops moist. And that’s a problem given the chaos that is, for example, California’s years-long drought. But the one water source that’s in abundance in many of these places—saltwater—may be able to provide a solution.
Desalination—removing the salt from abundant seawater to create freshwater—is a costly and difficult process. Plans for desalination plants have popped up on the West Coast in the past few years, but they invariably cost millions, and sometimes hundreds of millions, just to build, and operating them isn’t free either. To produce an acre-foot of water (that’s enough for about two families of four for a year) costs a desalination plant, even a new high-tech one, about $2,000—twice as much as existing freshwater options. And agriculture uses an awful lot more water than a couple of families do.
The new system, proposed by engineers from Murdoch University in Australia, is an advanced sort of evaporation/condensation desalination system. These sorts of systems are normally very, very simple, working in the same basic way as, well, clouds do. Ambient heat, abundant in arid climates, is utilized to evaporate the clean freshwater from seawater, leaving behind impurities like salts. When the freshwater condenses, the same way a cold can of soda does on a hot day, you collect the condensation, and presto, you have desalinated water.
The variation in the new system is that it uses bubbles. Giant tubes with seawater at the bottom are churned up with fans and rotating porous discs, causing bubbles of seawater to rise through the tubes. Bubbles have a great deal of surface area, which means the ambient heat will evaporate their water faster. Theoretically, that means you can produce freshwater faster and more efficiently.
“We believe that the concept is applicable to arid regions worldwide, where the social benefits and the simplicity of the concept will drive sustainable development of remote people,” Mario Schmack, who coauthored the Desalination paper, said in an interview with SciDev. In the paper, the researchers estimate that a roughly 1,600-square-foot greenhouse could produce nearly 300 cubic feet of water and as much as 66 pounds of produce daily
But critics say this is not a scalable technique, that its energy use is far too high compared with competitors. The dominant method of desalination for greenhouses is a technique called reverse osmosis. “Reverse osmosis is essentially a filter,” said Charlie Paton, the founder of Seawater Greenhouse. “The differentiation is that it’s technically a membrane, which indicates that it has very very small pores with which to block the salt.” You use high-pressure pumps to force seawater through this membrane; in a good system of this sort, you recover about half as much freshwater from the seawater.
It’s also energy-efficient; the pumps can be powered by the movement of water, and a pump is not a spectacularly energy-thirsty piece of equipment to begin with. Reverse osmosis is usually suggested as the best option for arid-climate greenhouses because of its price. “There’s a sort of standard number that gets tossed around for reverse osmosis—from 60 cents up to $3 for a thousand liters” of freshwater, Paton said. That’s much, much lower than the number proposed by the Murdoch University researchers, who say their system will cost about $10 for the same amount of water.
Still, it’s a promising and interesting idea; many critics, like Paton, admit readily that they are biased because of their professional alignment with reverse osmosis. And energy use is one of those rubrics that’s more fluid than it sounds—a few advances in technology, and who knows? The bubble-evaporation method may be the cheapest, best desalination solution around.
Related stories on TakePart:
• Desalination Could Be the Solution to California's Drought
• This Map Shows How Fast the World Is Depleting Irreplaceable Groundwater Reserves
• Many of Your Groceries Are Really Just Groundwater in Disguise
Original article from TakePart
