The United Nations estimates that more than two billion people live in water-stressed countries and about half the population experience severe water scarcity at least one month per year. According to the Food and Agriculture Organization, some of this water stress comes from agriculture, which uses 69 percent of the world's freshwater resources. Fertilizers and pesticides from agricultural operations also diminish freshwater resources, polluting rivers and lakes.
Climate change is also threatening freshwater resources. Dry regions are becoming dryer, experiencing more frequent droughts. Heavy, flood-inducing precipitation is becoming more frequent elsewhere, which also threatens the integrity of fresh water. As rivers overflow their banks, pollutants and bacteria mix with that water. Heavy usage of water coupled with climate change effects could cause water shortages for two-thirds of the population by 2025. As a solution, some scientists have turned to water harvesting technology.
In some places, the technology already exists to collect water from the air. For years, South American countries like Chile and Peru have used fog harvesters. Fog harvesters typically consist of a mesh wire screen, like a window screen, that collects water as fog moves through it. But the harvesters usually collect just 1 to 3 percent of passing fog. Andwhile high humidity levels make this method effective, they don't work well in deserts. That's where water harvesters come in. Water harvesters don't depend on fog, which is a cloud that reaches the ground. Instead, they depend on water vapor in the air, meaning they can even extract water from dry, desert air.
A water harvester looks pretty underwhelming. From the outside, it looks like a white box in a plastic tank. But inside, a metal-organic framework acts like a sponge, soaking up water vapor and then releasing it to condense into pure, drinkable water. The metal-organic framework is a powder, with metal and carbon-based molecules arranged with tiny air pockets between them. This gives the framework a large surface area and allows it to absorb more moisture. Scientists at the University of California, Berkeley developed the prototype water harvester in 2017, publishing their work the following year in Science Advances.
The research team put the device to the test in the desert city of Scottsdale, Arizona. There, they tested its ability to collect water in dry conditions. Relative humidity levels were as low as 5 percent during the day, but up to 40 percent at night.
At night, the metal-organic framework is open to the air, and absorbs water vapor. During the day, a cover seals the container to trap the vapor inside. Using the heat from sunlight, the vapor from the metal-organic framework warms and flows into the larger container, where it cools. A reflector on a portion of the cover ensures that sunlight only reaches the metal-organic framework, allowing the container walls to remain a cooler temperature. Water droplets form on the walls as the vapor condenses, and then collects at the bottom of the container.
From their tests, the researchers found that after one day-and-night cycle, a water harvester with a kilogram of metal-organic-framework can produce 100 grams of water. That's 3.75 ounces of water.
Half a glass of water per day won't solve the water scarcity problem, but these scientists believe their device could be scaled up fast. However, the problem is that it's expensive. The researchers tested a metal-organic framework made of zirconium, which costs over a hundred dollars per kilogram. They're now testing out a metal-organic framework made of aluminum, which would be more affordable for large-scale use. In the same Science Advances paper, the researchers reported that they tested the aluminum metal-organic framework in the lab, and found it even better at absorbing water than zirconium.
The harvester also has low energy needs, requiring only sunlight. This makes it ideal for use in places where electricity is unavailable. It also makes the harvester a more environmentally friendly solution than other designs that require the burning of fuels, for example. Currently, a handful of countries and cities use desalination to obtain fresh water, but this technology is extremely energy and cost-intensive. Water harvesters, if scaled up, could be a better alternative. And, unlike fog harvesters, it can generate water in arid conditions.
While it's possible that someday a water harvester will provide fresh water to water-scarce communities, the larger problem is how the world can better manage its water resources and address climate change.