Fog harvesting: What is it?
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Water is everywhere. It covers more than 70% of our planet, makes up over half of our body mass and it's nearly three quarters of our brains. We use it for everything from drinking to washing, growing food, generating power and keeping cities running. What happens when there isn't enough of it?

In the Atacama Desert in Chile, researchers have been harvesting water from the air using giant nets to trap fog. It's a technique that's been used on a small scale in different parts of the world, kind of like how tiny mist droplets are caught on the undersides of leaves or between blades of grass in the early morning. But this study from Chile has found that it could be scaled up to meet the entire water demand of some of the world's driest cities.

So in this episode, you're going to hear how fog catching works and how fog catchers are used in different parts of the world to help mitigate water shortages. I'm Hannah Gelbart, and this is What in the World from the BBC World Service. To go through this one drop at a time, I'm speaking to the BBC science correspondent, Vic Gill. Hello. Hi there. How are you doing, Hannah? Good, thank you. Can you break it down for us? How do fog catchers work?

It's actually really simple. It's a net to catch fog fundamentally. It's something that's been used for several decades on a small scale, mainly kind of rural settings where you can set it up somewhere where there's a reliable source of fog, somewhere on the coast maybe that's mountainous where you get sea fogs that roll in and where there's a need for water. And essentially it's a net, a really fine mesh net that's strung up between poles and

And the fog passes through. That's really rich in moisture. And the droplets form on this fine mesh. And you can capture those droplets and pipe them away and store them in tanks. So that's fog water harvesting.

What is the mesh made out of? Usually plastic, something that the droplets will form on and run off readily. So actually a kind of fine plastic mesh is usually what's used. And how much water can you actually catch using this method?

It depends on how much fog you get. Right. So this is something that has been used mainly just on a small scale where it's needed. So there might be a small rural community. There's quite a big, I think one of the biggest setups is actually in Morocco. That's a small rural community that has set it up to basically fulfill their own needs. But on average, you can get about 2.5 litres per

per square metre of mesh per day in the area of South America where this research is focused on the Chilean coast in northern Chile where it's very, very dry. But it's a technique that's used quite a lot in South America because if you picture that continent, you have this sort of strip of coastline along the Pacific. So you have this huge ocean body and you get these sea fogs

So you have these perfect conditions where the warm air moves over the colder sea surface and it creates this fog. And then just because of the prevailing winds and the weather conditions, that gets swept onto shore. And because you have these high mountains, you have places where the fog kind of meets the land. So you have naturally very regularly foggy places.

So it depends on how much mesh you have and how big your fog harvesting system is. Are fog catchers expensive to make? Where are they mostly used apart from in South America? You mentioned also Morocco. They're used on very kind of as needed scales at the moment. So it tends to be there's quite a few experimental setups. They've tested different materials.

to see what reliably forms the most droplets and is the most efficient fog collector. And where they're used, where they're needed, it tends to be small farms or little communities that are collecting and storing their own water supplies. This research is focused in northern Chile, where actually on the edge of the Atacama Desert there,

there's a real need for water. There are cities where most of the water supply comes from underground and there's a huge amount of pressure on those water supplies. And in this city where this research is focused in Alto Ospicio, which is where the researchers were looking at mapping where fog capturing could work on a very big scale. It's a city that gets about less than five millimetres of rain per year, so it barely rains. There's a lot of pressure on their water supply and

And what they've done is they've combined satellite imagery of these huge Pacific sea fogs that move over this very dry mountain city. They've mapped what they call blue zones around the city. And they've said that if you could set up large scale fog collection devices there, so on a much larger scale than just a few nets strung between poles, they're talking about 17,000 square meters of mesh. So, you know, whole fields of these fog collection devices there.

If you strung up 17,000 square metres of mesh around Alto Aspicio, you could produce 300,000 litres of water every day. And that currently is the amount of water that is delivered by truck to slums and shantytowns in the city to people who don't have a direct and secure water supply. So that could be an absolute game changer for people who do not have ready access to clean water. So this is what these scientists are calling for is essentially...

to upscale this technology where it's really needed. This might be a really basic question, but when the fog is collected as water, is it ready and safe to drink? If it comes from the sea, would it be particularly salty? If it comes from a city, could it have pollution or be contaminated?

It's a really interesting point because this isn't saltwater. This is water from the atmosphere that sort of passed over the sea and the conditions that the sort of difference between the hot and the cold causes this moisture to condense into a cloud. And then the weather sort of moves it inland. So this is this is freshwater droplets that gather on these nets if you catch it.

It's not touched anything on the ground. It's not sort of made contact with any surfaces. So the lead researcher on this study from Universidad Mayor in Santiago in Chile said that every time she's worked in the desert and she's done a lot of these experimental fog capturing setups, she's always drunk fog water and it is clean and ready to drink.

This is fresh water that these researchers say is drinkable. And that is what it is used for when these setups happen in kind of rural environments and small communities. People are gathering this water because it is clean and ready to drink.

Is this water, could it also be used for things like farming, washing, generating power and electricity? Yes, exactly that. So what the researchers have said, based on a collection rate of about two and a half litres per square metre of mesh, which they say is a safe assumption for the level of fog that you get around Alto Ospicio. So that's about the efficiency that they could get there.

that they would be able to supply all of the drinking water that's currently trucked in, the 300,000 litres that's currently sent in, that they could provide the annual demand for irrigation of the city's about 110 square metres of green spaces as well. And they said that there would also be some additional water for hydroponic agriculture, so urban agriculture that doesn't require soil supply.

It all depends on kind of how many mesh nets you put up around the city. But they've essentially by doing this kind of combined mapping, weather forecasting and efficiency measurements of fog harvesting. What they've shown is that there are foggy zones where you could do this and where there is space to be able to produce data.

enough drinking water, but more water on top of that so that you could, you know, improve the lives of people in these very crowded, very hot cities with more green spaces and fresh food as well. So, yeah, for an arid city where there's such demand on water and no secure supply, this could be, these scientists say, a real game changer. Yeah.

You've been hearing about Chile's ambitions to scale up fog catching, but this technique can also be used on a much smaller scale. Water Foundation is an organisation that works mainly in Africa and Latin America. Here's Monica Denemy, who works for the project. Cloudfishes are installed in locations where there is regular fog and wind. These are the two basic ingredients needed for them to work. The nets, supported on galvanised steel frames...

capture water droplets as the wind pushes the fog into contact with the structure. There are a few unique features about our cloudfisher design compared to other fog capture systems. Primarily, the 3D nets, which are more efficient in capturing water droplets than flat fabrics.

and the flexibility that's built into the design to compensate for the pressure from the wind. This means that cloudfishers are durable over a long period of time, requiring no electricity and almost no maintenance to provide drinking water to nearby communities. Another organisation that works in rural communities in places like Ethiopia, Tanzania and Haiti is Walker Water. It's built water towers to capture fog and dew. Arturo Vittori is the founder.

The tower is a lightweight vertical structure made from biodegradable and local material such as bamboo, natural fibers and it functions by passively collecting moisture from the air through condensation. Requiring no electricity or complete infrastructure, Worker Water operates entirely off-grid. Thick

Back to you. What are some of the limitations of this? Lack of fog. Right, that's a big one. Before I came to this story, I wasn't really familiar with fog harvesting and sort of, you know, the possibilities for it. And when you look at it on kind of a case-by-case basis,

When you look at the possibilities for it in an urban site in Chile, where you're right on the Pacific coast and you get these perfect weather conditions, it just looks like, you know, why isn't this city doing this? It just looks like a fantastic solution. But, you know, there are a lot of other desert cities in other parts of the world that just don't have these weather systems and are under control.

similar pressure. So this isn't, you know, this isn't a kind of panacea that's going to sort of fix all of our urban water problems. We can just catch it straight out of the air. You do need very special conditions. But along, you know, that South American coast where you get these weather systems and where you get this reliable fog,

It's something that could be put to use right away. And that's what these researchers want to see happen. They want to see a new era of fog harvesting where we're solving or at least helping some of the pressures on urban water supplies by capturing it from the clouds. And with climate change and global warming, is that also affecting the amount of water in the air, the amount of fog that might be available for this?

Yeah, that's an interesting question. At the moment, the weather system...

in Chile and along the South American coast is not being thrown out of whack by rising temperatures. So that's good news. But I think that's kind of an open question. There are some unpredictable impacts of climate change. But I guess the great thing about this is that it's not a difficult technology. It's not an expensive or complicated installation. It nets on poles. So if you have, you know, if that water is moving over your city, it's

every day in the clouds and your city is in great need of clean water then it's kind of a no-brainer it's kind of it's something that could just be installed right now and you could just start collecting that water instead of just letting it you know wash over and evaporate. Vic thank you so much for coming on the podcast. Thank you my pleasure.

That is it for today's episode. Thank you so much for joining us. I'm Hannah Gelbart. This is What In The World from the BBC World Service and we'll see you soon. Bye.