Solar-Powered Nano Mesh Harvests Water from Fog

Fog harvesting provides regions that have few lakes and rivers with another significant source for freshwater. “Many areas take more water out of the ground than is replenished, so scientists are looking to the atmosphere for water-scarcity solutions,” said Thomas Schutzius, assistant professor of mechanical engineering at the University of California, Berkeley. “There is actually a lot of water available in the air and one way to capture that water is through fog harvesting, where the tiny droplets of water in the fog are collected from blowing wind using meshes or fabric.”

However, in urban and industrial settings, there is the problem of air pollution, where fog droplets become contaminated with dangerous levels of organic pollutants, many of which are hazardous to human health, making that water unsafe to drink.

To solve this problem, two mechanical engineers—Schutzius and Ritwick Ghosh, a research scientist at the Max Planck Institute for Polymer Research—teamed up to devise a totally passive fog-harvesting system that also simultaneously removes harmful contaminants from the water, an advancement that could help provide millions of people worldwide with access to safe drinking water. 

Nanoengineered steel mesh

Previous research shows it is possible to treat, to a limited degree, contaminated fog by using a steel mesh coated with metal oxide nanoparticles such as titanium dioxide. Schutzius and Ghosh decided to take this work to the next level by building a fully passive system where the nanoparticle coatings continue to treat the water 24/7, without requiring constant exposure to ultraviolet light (sunlight).

For their prototype they used a nanoengineered steel mesh coated with a special solar-powered polymer consisting of photocatalytic metal oxide nanoparticles such as titanium dioxide, which have the unique ability to stay reactive after they have been exposed to sunlight—a process called photocatalytic memory. 

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“These coatings not only worked when we were shining ultraviolet light or sunlight on them directly, but they also remained quite reactive afterward, even with no light,” said Schutzius. “This is a highly attractive capability when harvesting and purifying something like fog. The surface can be activated when it is sunny and stays reactive when it is foggy.”

94 percent of pollutants neutralized

Tests in the lab showed that the polymers were effective in collecting droplets of water on the mesh, which then trickle down into a container where interaction with titanium dioxide broke down the molecules of many of the organic pollutants contained in the droplets, making them harmless. The researchers then tested the mesh outside in direct sunlight. The system was able to collect 8 percent of the water from an artificially created fog and broke down 94 percent of the organic compounds that had been added to it, including extremely fine diesel droplets and the chemical bisphenol A, a hormonally active agent. 

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Once installed, the technology needs little or no maintenance. In addition, no additional energy is required to maintain its operation apart from a small but regular dose of UV to regenerate the catalyst in the polymer. “Half an hour of sunlight is enough to re-activate the system for an additional 24 hours,” added Schutzius.

This fully passive, solar-powered approach to collecting and treating water is “a first in its field,” according to Schutzius. He is currently in discussions with potential collaborators about doing more field tests with the new technology. In addition to harvesting drinking water from fog, this technology could also be used to recover water used in cooling towers. 

“In … cooling towers, steam escapes up into the atmosphere,” said Schutzius. “In the U.S., [it takes] a great deal of fresh water to cool power plants. It would make sense to capture some of this water before it escapes and ensure that it is pure in case you want to return it back to the environment.”

Mark Crawford is a technology writer in Corrales, N.M.