You've probably heard that Cape Town in South Africa is getting closer every day to actually running out of water. Record drought paired with huge population growth is causing a crisis, and experts are predicting a "Day Zero" in the next few months — previously in April but now in July — for when the city will actually have to shut off taps because that's how low their water reservoirs are.
Cape Town isn't the only place facing water shortages, though. As climate change makes Earth more precarious, more regions are having to be careful with how they use this precious resource.
Scientists are working towards solutions, and a new article published in Nature Communications describes how a new invention in field testing is making drinking water out of dry, desert air.
Even in the driest of places, there's always water vapor around us in the atmosphere.
However, it's not always easy to get to it. We have machines that can collect water vapor from the air already. Some harvest fog and others basically cool the air until the water vapor is cold enough to become liquid and collect that water.
While this is great, it's not very energy efficient in deserts and other arid places where there's less water vapor in the air.
Another way of collecting water is by using a sorbent, which is basically a substance that absorbs molecules of another substance or gets those molecules to cling to it, which is called adsorption. When scientists use a sorbent to collect water, they saturate the sorbent with water from the air and then heat it to condense and release the water.
The team behind this new device found in a 2017 study that they can use a specific crystalline material that has metal ions linked with organic molecules. This material, called metal−organic frameworks (MOFs) are very porous with a high surface area. On cooler nights, the water vapor collects in the MOF and turns to liquid. When the sun heats the MOF up, the water vaporizes and then slips away and condenses again in a reservoir.
This device, which was the size of a tissue box, could collect about three quarts of water (2.8 liters) in one day. However, it was just a proof-of-concept, and while it got a lot of hype, it hadn't been tested in arid, desert conditions.
That's where this new device comes in.
The scientists tested this new device in the arid climate of Tempe, Arizona.
While their first device worked at 65% relative humidity, the conditions in Arizona were much drier, at about 10% relative humidity during the day and a swing to 20–40% relative humidity at night.
The device is solely powered by the sun and harvests uncontaminated water. While scientists are still working with a smaller-scale proof-of-concept, they estimate that the device could collect over 0.25 L of water per 1 kg of MOF in its daily cycle.
Other methods of harvesting water from the air require pumps and compressors, which can require maintenance and repairs.
"This has no moving parts," said MIT postdoc Sameer Rao. "It can be operated in a completely passive manner, in places with low humidity but large amounts of sunlight."
While that's pretty impressive given how little water was in the air during these studies, scientists are looking to improve the device by scaling up the production and making it more efficient.
We're not ready yet to support life in deserts with these devices, but we're making progress!