A Canadian startup has built a pilot desalination plant in Vancouver that uses a quarter of the energy of conventional plants to remove salt from seawater. The process relies on concentration gradients, and the natural tendency of sodium and chloride ions–the key components of salt–to flow from higher to lower salinity concentrations. If the system can be scaled up it could offer a cheaper way to bring drinking water to the planet’s most parched regions while leaving behind a much lower carbon footprint than other desalination methods.
“We’ve taken it from a benchtop prototype to a fully functional seawater pilot plant,” says inventor Ben Sparrow, a mechanical engineer who established Saltworks Technologies in 2008 to commercialize the process. “The plant is currently running on real seawater, and we’re in the final stage of expanding it to a capacity of 1,000 liters a day.”
Today most desalination plants are based on one of two approaches. One is distillation through an evaporation-condensation cycle, and the other is membrane filtration through reverse osmosis. But both options are energy-intensive and costly.
Saltworks takes a completely different approach based on the principles of ionic exchange. The process begins with the creation of a reservoir of seawater that is evaporated until its salt concentration rises from 3.5 percent to 18 percent or higher.
The evaporation is done in one of two ways: either the seawater is sprayed into a shallow pond exposed to sunlight and dry ambient air, or seawater is kept in a large tower that’s exposed to waste heat from a neighboring industrial facility. The second approach is used in the commercial-scale plant. The concentrated water is then pumped at low pressure into the company’s desalting unit along with three separated streams of regular seawater. At this point the most energy-intensive part of the process is already over.
Inside the desalting unit, which in the pilot plant is about the size of a microwave oven, specially treated polystyrene bridges connect two of the regular seawater streams to the highly concentrated stream. Positive ions (largely sodium) and negative ions (largely chloride) are drawn by diffusion through the polystyrene, which has been chemically treated to manipulate specific ions, from the concentrated steam into the weaker ones. One bridge is treated to allow only positively charged ions to pass, while the other bridge only allows negatively charged ions to pass. But both allow other ions in salt water, including magnesium, calcium, sulfate, and bromine ions, to pass through. “The negatives all flow in one direction and the positives all flow in another direction,” Sparrow says.