Solar Desalination – Fresh Water for the Next Millennium

What to do about the future of eight billion thirsty people?

The question is forcing scientists and engineers to rethink ways of stretching Earth’s drinkable water supply, which accounts for just one percent of global water.

One popular solution is to increase large-scale purified water reclamation, otherwise known as “toilet to tap;” while this water is palatable, many feel uneasy given its name.

Desalination is another option, although most desalinated water projects today are powered by fossil fuels, which  pollute the environment and add harmful greenhouse gases to the atmosphere. Therefore, the current model actually does more harm than good.

At the Navajo Nation, the University of Arizona College of Engineering recently set up a demonstration for its solar-powered desalination bus to show how, with some ingenuity and adequate funding, cleaner desalination is possible.

The Navajo Nation, which gets less than one foot of rain annually and has groundwater contaminated with uranium and other dangerous pollutants, was a perfect place for the university’s desalination experiment.

Led by project volunteer, Professor Rudolfo Peon, the college’s challenge was to power desalination units in remote areas without using electricity.  As the engineers proved, one school bus – tricked out with a solar-powered mobile desalination unit – is all it took.

“Every community that sees the bus wants one of these solar desalination units. It will have an evaporation pump where they can dump salty water, vaporize it, and not affect the environment,” he said.

The engineering team used the best solar modules they could find to optimize energy dissemination in a small area.  The desalination bus can operate around the clock, transforming the contaminated underground brackish water to produce about 500 gallons of fresh water a day.

Peon says the machine took brackish water, with a salt concentration of 2000 to 4000 ppm (parts per million), and turned it into pure, fresh, drinkable water with a safe salt concentration between 200 to 500 ppm. The bus uses high pressure to push the salty water through membranes, thereby desalinating it and making it potable.

“It is pushing out water like squeezing out a lime… It’s pure water on one side, salt on the other side of the membrane,” Peon explains.

Solar desalination is extremely efficient because it uses a small amount of energy to make a large amount of potable water. The nano-filtration system desalinates the water without the need for heat or fuel.

The Nano Filtration approach is still a rarity

Less than one percent of all desalination plants in the world today use solar energy or any other renewables. However, regardless of whether the system employs fossil fuels or solar energy, the desalinators oftentimes dump the leftover salty water back to the sea, which can upset its eco-system.

Luckily, Peon says the solar-powered systems can solve that problem.

He identifies a few places around the globe that are improving the solar desalination model, which can separate salt from water without pumping the brackish water back to the sea.

Large scale solar desalination models are already in motion in other areas of the world.  In fact, Peon says larger projects are cheaper to implement and more efficient. At some successful large solar desalination plants, the salty water is poured on large pumps, which are then heated by the sun to evaporate the water, leaving nothing but the dried salt behind.

Unfortunately, the biggest challenge facing the solar desalination industry is cost. Fossil fuels, Peon explains, are subsidized in many ways, but clean energy is not. However, in the next two decades, Peon estimates that solar will become increasingly competitive with fossil fuels, and the industry will grow to include various renewable-powered technologies. Therefore, as subsidies for solar and renewables catch up to those for fossil fuels, he believes the world will become more environmentally friendly.

Solar desalination — the future of fresh water

As a professor and an engineer, Peon hopes to further his studies of solar desalination and use his research to help give society the most important element for human survival: potable drinking water. He believes solar desalination will be the most important source of clean water for the next millennium.

“If we do things carefully, we won’t have environmental issues,” he says, adding that arid countries and regions will need cheap solar solutions. “Otherwise we’re going to put more fuel into the atmosphere. We’re going to increase the temperature, and climate change is going to be more dramatic.”

Thomas Esqueda, associate vice president of water and sustainability at Fresno State University, hears some interesting ideas around converting brackish water; one such idea – though expensive – is to drive the refuse water to the Oakland wastewater plant, where it would eventually be returned to the ocean.  Some researchers suggest turning the brackish water into a food-grade salt product, such as salted grape leaves, or converting it into an industrial product, such as road salt.

But even if the salt was used to brine roads in the winter, rains often wash this salt off to streams, which eventually negatively impacts the environment and destabilizes local eco-systems.

Last year, the Department of Energy awarded $21 million to advance solar desalination technologies with projects nationwide.

Lately, Esqueda says there is more technological development around the subject of how to use the salt byproduct of desalination. The DOE, in particular, is seeking proposals nationwide for desalination projects in the Central Valley.

“It would do what we’ve been talking about. What are the separation techniques, the membrane, and what do you do with the salt? You’ve got to do them as a package at this point,” Esqueda said.

Recently, he was contacted by one company that wanted him to check out their product that promises dry residual salt.

“Most processes are going to give a brine, a slurry with some remaining water – you have to add more technology to get the water out,” Esqueda said. “A dry residual process to bring the slurry brine to a point of dry salt, would be the idea. But, it too would require more technology to get the water out.”

If solar desalination pans out, and researchers figure out what to do with all of the extra salt, the technology could be game changer toward a 100 percent renewable goal.

But, as the worldwide population soars in the coming decades to top 10 billion, Esqueda’s greater concern is not only to provide drinking water, but also to grow enough food to feed everyone.

Food is the biggest consumer of water.

“Yes, you’ll need more water for [the population] to drink, but the water to grow food is the necessity that people should be talking about, that’s the highest demand. That’s the question,” he said.

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Dianne Anderson covers education, health, and city government stories with an eye on legislative impacts to diverse communities. She has received awards from the American Cancer Society – Inland Empire, and the Southern Poverty Law Center. Over the years, she has reported for the Long Beach Leader and the Inland Valley Daily Bulletin and been a contributor to the Pasadena Weekly.