Free clean water is a dream longed for by many people living in dry climates, but as climate change worsens and droughts increase, access to drinkable water will become increasingly difficult. So where will they turn to for drinking water?
Part of the answer could lie in thin air, according to researchers from MIT. Alina LaPotin and colleagues have developed a device powered by the Sun that they claim grabs and condenses clean water from dry air. A description of the prototype is published in the journal Joule.
“In areas where water scarcity is a problem, it’s important to consider different technologies which provide water, particularly as climate change will exacerbate many water scarcity issues,” Alina LaPotin, from the Massachusetts Institute of Technology, told New Scientist.
The device uses a difference in temperature to move water from the air into adsorbent materials in the device, before condensing the water back out and into a container. Overnight, when there is no Sun to heat the device, water from the surrounding air gets drawn onto the surface of the adsorbent layer, from which the water can be harvested. As the Sun rises and heats the thermal plate on top, the difference in temperature from the exposed plate and the shaded underside draws water out of the adsorbent material (desorption), condensing it into a container.
While this method has been proposed before, its water-capture abilities were too limited to warrant widespread use. This time the researchers improved on the technology by making it a dual-stage device, adding a second stage of adsorption-desorption. Furthermore, the researchers have tried to move away from specialist materials and employ more widely available options.
Still, the system requires further adjustments to boost production and lower costs before it can be deployed on a large scale. Currently, the device produces 0.8 liters of water a day – a marked improvement over previous iterations, but still under the 2.5 liters needed per day for a human to survive. Furthernore, depending on the availability of solar energy, humidity, and temperature, water production can be lowered below the 0.8 liters.
Fog harvesting requires 100 percent humidity to draw water out of the air and is currently employed in a few coastal deserts, but the conditions necessary for performance are limiting. The dual-stage improvements can work in humidities as low as 20 percent, which means it could be used in far drier climates.
Producing enough water to sustain a population is becoming ever more difficult for many nations across the globe. With areas such as California experiencing some of its driest years in recorded history, even wealthy areas may be impacted by a lack of freshwater in the coming years.
“This is an interesting and technologically significant work indeed,” said Guihua Yu, a professor of materials science and mechanical engineering at the University of Texas at Austin, said in a statement. “It represents a powerful engineering approach for designing a dual-stage AWH device to achieve higher water production yield, marking a step closer toward practical solar-driven water production."
The great Maya city of Tikal transported zeolites for water filtration thousands of years before other cultures learned or adopted the idea, archaeologists have found. The filtration was probably much better than anything known to the Europeans who conquered the area 1,500 years later.
The Corriental reservoir was one of Tikal’s sources of drinking water. Dr Kenneth Tankersley of the University of Cincinnati found crystalline quartz and zeolite when digging at the reservoir. Neither are local to the area and would have had to be brought a long way by the standards of a people who had no beasts of burden.
No one would carry these materials so far without a good reason.
Tankersley observed the quartz/zeolite combination would have removed multiple pathogens from the water supply, including heavy metals, nitrogen-rich compounds, and bacteria. The last raises the tantalizing possibility the Maya had a germ theory of disease two millennia before Pasteur, but bacterial removal was more likely a beneficial side effect of efforts to remove other impurities. “This system would still be effective today and the Maya discovered it more than 2,000 years ago," Tankersley said in a statement.
The porous limestone on which Tikal sat, in what is now northern Guatemala, does not lend itself to wells for storing water during the dry season, so access to clean water would have been particularly essential.
In Scientific Reports, Tankersley proposes a source for the minerals and even explains how people might have come to recognize their value. A decade ago co-author Professor Nicholas Dunning reported volcanic rock known as tuff, rich in quartz and zeolite, in a scarp. “It was bleeding water at a good rate,” he said. “Workers refilled their water bottles with it. It was locally famous for how clean and sweet the water was."
The tuff deposit probably produced just as good water thousands of years ago, and some long-lost Maya scientists identified the ingredients that made it filter so effectively.
The minerals at Dunning’s site match those at Corriental, but nothing similar was found at other Tikal sites, including two other reservoirs the team excavated, indicating it was mined and transported the 30 kilometers (18 miles) to the city. That may seem a short distance today but was back then a long trek.
The minerals first appear at the reservoir around 2,200 years ago, and were replenished after flash floods until the city was abandoned more than 1,000 years later. Even older water filtration systems have been found in Egypt, South Asia and Greece, but Tikal’s version was probably superior. Zeolite is used to purify water today because its pores are well sized to filter out microbes such as cyanobacteria, and its ions bind to heavy metals.
The Aztecs and Incas transported water to their cities from clean springs, but the landscape in which many Maya lived made this unviable. Instead, the Maya built thousands of reservoirs – Tikal alone had five – only a few of which have been excavated. Consequently, we do not know how widespread zeolite use was.
The volcanoes of the region gave the Maya these minerals, but they also left high concentrations of mercury in the area. Human and natural activity washed this into water supplies, where it polluted Tikal’s other reservoirs and may have contributed to the city's collapse, the team previously showed, while the zeolites gave Corriental protection.
