The Fog Collectors: Harvesting Water From Thin Air

by Renee Cho | March 7, 2011

Today nearly two people in ten have no source of safe drinking water according to the U.N. Millions of people, most of them children, die from diseases associated with inadequate water supply, sanitation, and hygiene each year. But in some desert areas, where there is very little rain, fog and dew are abundant sources of humidity that are being harvested to produce fresh water.

Fog or dew collection is an ancient practice. Archaeologists have found evidence in Israel of low circular walls that were built around plants and vines to collect moisture from condensation. In South America’s Atacama Desert and in Egypt, piles of stones were arranged so that condensation could trickle down the inside walls where it was collected and then stored.

FogQuest, a Canadian non-profit, uses modern fog collectors to bring drinking water and water for irrigation and reforestation to rural communities in developing countries around the world. Fog collectors can be used in regions and deserts that receive less than one millimeter (about the thickness of a paper clip) of rain each year, but to work, they require fog and light winds.

Fog, a cloud that touches the ground, is made of tiny droplets of water—each cubic meter of fog contains .05 to .5 grams (half the weight of a paper clip) of water. Fog collectors look like tall volleyball nets slung between two poles, but they are made of a polypropylene or polyethylene mesh that is especially efficient at capturing water droplets. When the fog rolls in, the tiny droplets of water cling to the mesh, and as more and more cluster together, they drip into a gutter below that channels the water to a water tank. Fog collectors, which can also harvest rain and drizzle, are best suited to high-elevation arid and rural areas; they would not work in cities because of the space constraints and water needs of an urban environment.

Fog collection projects have used from 2 to 100 fog collectors, and depending on the location, each panel can produce 150 to 750 liters of fresh water a day during the foggy season. In the village of Chungungo, Chile where annual precipitation is less than 6 centimeters, 100 fog collectors produced 15,000 liters of water a year for ten years. Harvested fog water meets the World Health Organization’s drinking water standards.

The hills above Lima, Peru receive about 1.5 centimeters of rain each year, but fog from the Pacific Ocean moves in from June to November. FogQuest’s project in the village of Bellavista here is producing 2271 liters of water a day with seven fog collectors. Not only do they supply ample drinking water, they provide enough water for villagers to have gardens and grow tara trees which produce tannins that are sold for leather treatment. Eventually, the trees will become self-sustaining, collecting their own fog water, reforesting the area and replenishing the groundwater.

FogQuest was founded in 2000 by Sherry Bennett and Robert Schemenauer, an atmospheric scientist who has been working on fog collection for over 20 years. The all-volunteer organization gets funding from grants, donations and membership fees. When a fog collection project is proposed, FogQuest first assesses the conditions of the location to make sure there is enough fog. Since the work of building and maintaining the fog collectors depends on the locals themselves, FogQuest needs a local partner and a community willing to pitch in. If these conditions are met and there is funding available, the organization builds a small fog collector, costing from $75 to $200, to see how much water can be harvested. If all goes well, larger fog collectors of approximately 40 square meters, which can produce 200 liters per day, can be set up for $1000 to $1500 each. The system is completely passive, requires no energy inputs, and can last ten years provided it’s taken care of.

Schemenauer’s first fog collectors were developed in El Tofo, Chile in 1987, which led to a fog collection project in the village of Chungungo, Chile in 1992. Since then, small and large fog collection projects have been established in Peru, Ecuador, Guatemala, Cape Verde Islands, Eritrea, South Africa, Yemen, Oman, Ethiopia, Israel, and Nepal. FogQuest is currently evaluating the potential of a project in Tanzania, and will soon construct 15 large fog collectors in Sidi Ifni, Morocco.

Scientists in Australia are developing an entirely different fog collection strategy modeled after the Stenocara beetle of the Namib Desert. The Namib Desert in southwest Africa is one of the driest places on earth, receiving less than 2 centimeters of rain annually, but night and morning fog from the Atlantic Ocean are the lifeblood of the desert’s flora and fauna. When the fog sweeps in, the dime-sized Stenocara beetle clambers up the dunes, sticks his rear end into the air and faces his back to the fog. The configuration of hydrophilic (water-loving) bumps and hydrophobic (water-repellent) troughs between the bumps on his shell collect the moisture and channel water droplets right into the beetle’s mouth.

Inspired by nature’s design, scientists Chiara Neto, Stuart Thickett, and Andrew Harris from the University of Sydney have invented a synthetic surface using a combination of chemistry and structure. The surface is composed of two polymer layers: the top is hydrophilic, while the bottom is hydrophobic, which makes water droplets detach as soon as they get large enough. Similar surfaces inspired by the Stenocara have collected up to 10 liters of water per square meter every hour. The Australian scientists’ goal is to manufacture an equally efficient surface that could be used for local water harvesting—every house could have part of its roof coated with the surface to channel condensation into a rainwater tank. Prototypes have shown that this technology is several times more productive than mesh harvesting methods and could be scaled up to work in urban environments. Because each cubic meter of Australia’s coastal air holds 30 grams of water, this invention could be a boon for a country that has suffered severe droughts in recent years and is considered the driest inhabited continent on earth.

Meanwhile Chinese scientists are studying the structure of spider silk to learn why it is so effective at collecting water from the air. Under an electron microscope, Lei Jiang and Yong Zhao from the Chinese Academy of Sciences in Beijing and colleagues observed that spider silk fibers change structure when they come into contact with water. The fibers form hydrophilic spindle knots while the joints in between the knots remain smooth, so condensing water droplets slide along the smooth surfaces and coalesce into bigger drops at the knots. The scientists dipped nylon into a polymer solution; when it was stretched out, small polymer droplets formed, which became spindle knots once they dried. The scientists hope to use this research to develop synthetic materials that can direct and control water droplets more efficiently, which could potentially be used to make FogQuest’s fog collectors even more effective.

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