A pair of Southern California entrepreneurs is now testing a deep-sea device they say could radically change the way salt is separated from seawater, vastly reducing the cost by using far less energy than present-day systems

The desalination device, called DEMWAX, would rely on the intense pressures found hundreds of feet deep to push seawater through a collection of filters, then pump purified drinking water to the surface.

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“Underwater desalination is using the natural pressure in a body of water to drive the desalination process,” said Michael S. Motherway, president of DXV Water Technologies in Tustin.

For now, however, the company is starting small. Motherway and the inventor of the device, chief executive officer Diem X. Vuong, recently installed a suitcase-sized version at the bottom of the San Joaquin Reservoir in Newport Beach.

The device will purify the treated wastewater in the reservoir to test the maintenance requirements of the system, although the water it yields will not be put into the county’s water supply.

Most of today’s desalination systems rely on coastal intake pipes that pull in seawater, often destroying the eggs and larvae of marine creatures in the process.

And the plants consume enormous quantities of electricity. The energy is needed to push the seawater through banks of tightly wound, reverse-osmosis filters, squeezing it through microscopic holes to screen out salts, bacteria, viruses and other impurities.

For both those reasons, seawater desalination often meets with strenuous objections from environmental activists.

The DXV device does the same job, minus the heavy electrical load, Motherway says.

The weight of the seawater above means that down deep, the ocean exerts a crushing force.

At a depth of about 850 feet, Motherway envisions erecting a series of specially designed filters, and using that crushing force to take the place of electrical power.

The arrays, looking something like accordion files, would be tethered across acres of ocean floor, though engineers would have to be careful to ensure the bottom was sandy and barren to avoid interference with sea life.

A “snorkel” would make contact with the lower-pressure air at the surface, allowing the system to use the difference in pressure to help draw water through the filters below.

Because the water would be drawn weakly over a huge area, rather than strongly from a small area, they say the arrays would present no danger to eggs, larvae or other marine life, and would not boost leftover brine to high levels — two effects of traditional desalination systems that bother environmental activists.

“We’re trying to be environmentally friendly,” Vuong said.

Such a plant is perhaps 10 to 15 years away, Vuong said, though a working demonstration version could come much sooner, depending on how successful the company is at attracting private investors.

“I think we’re going to have a pilot plant in the ocean within a year,” Motherway said. “We’re getting funding to do that.”

In the meantime, small versions of the system could be used to purify fresh water in lakes or reservoirs.

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