Solar Desalination Could be a Game Changer for California Farms

A solar desalination unit operating in California’s drought-stricken Central Valley uses parabolic mirrors to concentrate sunlight and distill freshwater from salty farm drainage. Photo courtesy of WaterFX.

Let’s be clear from the outset: I’m no fan of conventional desalination.

The idea of using climate-altering fossil fuels to drive an energy-intensive de-salting process that threatens coastal environments in order to produce drinking water that, in most cases, could be secured more cheaply through conservation and efficiency improvements, simply fails to pass the bar of economically sensible, environmentally sound solutions to our water problems.

But now desalination of a very different stripe is under way – not by the sea, but in California’s drought-stricken Central Valley farming region. The project is turning salty, contaminated agricultural drainage into fresh water that can be re-used to irrigate crops.

Powered not by fossil fuels, but by the sun, the technology has the potential to shift the way water is used and managed in parts of the west, where agriculture accounts for 70-80 percent of water use.

Developed by a San Francisco-based company called WaterFX, the solar desalination unit has been piloted in the Panoche Water District in Fresno County. Farmers in the area grow a wide variety of crops, including almonds, asparagus, tomatoes, pistachios, cotton, alfalfa and wheat.

The district is located on the valley’s west side, where farm drainage contains not only high levels of salt, but also selenium, a naturally occurring element that is essential in trace amounts but poisonous at high concentrations.

In the Central Valley and elsewhere in the western United States, irrigation has washed considerable quantities of salt, selenium and other contaminants out of the soil and into drainage water, which has polluted rivers and wetlands, and harmed birds and wildlife.

WaterFX’s “Aqua4” system offers a way of addressing both these critical contamination problems and mounting water shortages.

The technology uses parabolic mirrors to concentrate the sun’s energy, heating a tube that then distills fresh water out of the salty drainage. It’s an age-old process made far more efficient with modern technology. The system can produce 200 acre-feet (65 million gallons) of water per acre of solar collection area, making it, according to WaterFX, the most efficient solar desalination system available.

In addition to the solar collection, absorption and distillation equipment, the system includes a unit to store the solar thermal energy it produces, ensuring round-the-clock operation. It can also reclaim the metals and salts left behind from the distillation process.

During the drought, farmers in the Panoche and other Central Valley irrigation districts have faced cutbacks of 80 percent or more in their water deliveries from the dams and canals operated by the federal Bureau of Reclamation. The possibility of treating and reusing the water they do get to sustain more crop production has obvious appeal. And the districts are under the gun to further curtail the harmful pollution draining off their fields into the San Joaquin River.

The Panoche District has already made moves toward more sustainable production, including the institution of a tiered pricing schedule, where farmers pay more per acre-foot the more water they use. The majority of farms now use highly efficient irrigation systems, mostly drip and micro-sprinklers, to get more crop per drop.

With the ability to treat and reuse their water, farmers can stretch their limited supplies even further.  At a minimum, that could afford an effective hedge against drought.

The modular solar thermal technology converts previously unusable farm drainage into a new, local source of clean water. Photo courtesy of WaterFX.
The modular solar thermal technology converts previously unusable farm drainage into a new, local source of clean water. Photo courtesy of WaterFX.

But WaterFX has bigger plans.

“Our long term goal is to chart a new course towards water independence and reduce the need to import water from finite natural sources,” says Aaron Mandell, co-founder of WaterFX.

Success will depend, of course, on cost and scalability.

Aqua4 is both modular and moveable. A single module occupies 6,500 square feet and can treat 65,000 gallons per day. That basic formula – 10 gallons per square foot per day – is scalable to any size, says Mandell.

The Panoche District will continue piloting and assessing the solar desalination process over the coming years to determine if it is suitable for “scaling up,” district general manager Dennis Falaschi told the publication Ag Alert last month.

Meanwhile, WaterFX is actively moving to scale up. Through a project dubbed HydroRevolution, the company is organizing a community-based fundraising campaign to expand on its work with Panoche and build the Central Valley’s first commercial solar desalination plant.

The goal is to take drainage from 7,000 farm acres in Panoche and other neighboring districts and churn out 5,000 acre-feet (1.6 billion gallons) of clean water per year.

With an estimated 1 million acre-feet of irrigation drainage available for treatment and reuse in the Central Valley, it’s just possible that this new brand of desalination – solar-powered, decentralized and focused on water treatment and reuse – could give farmers and rural communities a sustainable new water source and open up new possibilities for keeping more flow in depleted rivers and streams.

Sandra Postel is director of the Global Water Policy Project, Freshwater Fellow of the National Geographic Society, and author of several books and numerous articles on global water issues.  She is co-creator of Change the Course, the national freshwater conservation and restoration campaign being piloted in the Colorado River Basin.

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Sandra Postel is director of the Global Water Policy Project and author of Replenish: The Virtuous Cycle of Water and Prosperity. From 2009-2015, she served as Freshwater Fellow of the National Geographic Society. Sandra is also co-creator of Change the Course, the national water stewardship initiative awarded the 2017 US Water Prize for restoring billions of gallons of water to depleted rivers and wetlands. The recipient of several honorary degrees, she works to bridge science, policy, and practice to promote innovative ways of securing water to meet both human and ecosystem needs.