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Sun-Assisted Desalination

Energy-saving process uses free heat to desalinate seawater.
December 17, 2009

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.

Spray pond: Natural sunlight is used to evaporate and concentrate the salt water in seawater.

“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.

Hold the salt: The spray pond and associated equipment operating in a prototype system in British Columbia.

The two regular streams–one now having a surplus of positive ions and the other having a surplus of negative ions–are also connected to the third saltwater stream, which is the target for final purification. The two out-of-balance streams want to become balanced again, so they essentially strip the third stream of all positive and negative ions. The end result is de-ionized water that only requires some basic chlorination or ultraviolet treatment before being piped into homes and businesses.

Sparrow, who is also chief executive of Saltworks, says the process uses low-pressure pumps to circulate the water, meaning lightweight plastic pipes can be used instead of corrosion-resistant steel. Saltworks cofounder and president Joshua Zoshi says scaling up the system should be simple because the plastics and ion-selective chemicals used are plentiful and cheap. “Our next step is to engage with industry and work with potential customers to get the technology out into the field,” Zoshi says.

Much of the research and pilot-plant funding to date has come from Canada’s National Research Council, B.C. Hydro’s Powertech Labs, and Sustainable Development Technology Canada, a federal agency that supports clean technology development through grants.

Rick Whittaker, chief technology officer at SDTC, says the company has a reasonable chance of success because the science behind it is sound and the approach is based largely on the creative integration of existing technologies. “There’s technical risk,” says Whittaker. “But we’re quite confident they can scale it up.”

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