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Cities Are Running Out Of Freshwater. Here's How Science Can Help

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From fog-fishing nets to graphene filters, technology offers new routes to freshwater

If you live somewhere that provides a reliable water supply, you should consider yourself very lucky indeed. According to the World Health Organization, more than 630 million people have no access to clean water at all – that’s almost twice the population of the United States. If you are an urban dweller in a developed nation, you’re probably not using your water wisely either – as I mentioned in my book, you’ll get through between 130 and 300 liters (29 and 66 gallons) of water every single day. We drink it and bathe in it, we use it to heat our homes, manufacture products, and to grow and cook our food. And thanks to the design of our urban water systems, we flush huge volumes of the very cleanest water down the drain.

Now before you start getting worried that this is going to become YET ANOTHER toilet/sewer-related article from me, I promise that it’s not. Instead, I want to talk about the fact that the world has a finite supply of water. Our planet’s water cycle is very dynamic, so water changes form continuously. It may fall from clouds, flow from your tap, or be temporarily stored in humans, plants or even in plastic bottles. But, the total volume of water we have available is essentially fixed. Data from the US Geological Survey shows that all of the water – solid, liquid, gas – ‘on, in and above Earth’ would fill a space 1.39 billion cubic kilometers in size. But, only 2.5% of that is freshwater – that is, the salt-free version that we need to survive.

So, we’re forced to recycle it, constantly. If we put on a load of laundry, the wastewater produced by the process is carried off to a water treatment plant, where it is filtered and disinfected as quickly as possible, before being put back into the urban water supply (the solid stuff is a different matter, but you can read about that here). What municipal water treatment plants aim to do is speed up the processes that occur in nature, but in today’s modern world, cities are struggling to match supply with demand. This situation is so drastic that new predictions suggest that by 2025, half of the world’s population will be living in water-stressed areas. It’s clear that we need a major shakeup in how we manage the water cycle, and science can help.

Mesh billboards can capture tiny fog droplets in mountainous regions (Image credit: Aqualonis)

Aqualonis

In dry, mountainous areas, or those on the coast that experience a lot of fog, one solution is to use giant nets to ‘fish’ for water. The latest region to adopt this is in southwest Morocco. There, on the slopes of Mount Boutmezguida, stands a row of net ‘billboards’ that are harvesting water from the clouds.

It might sound crazy, but it really, really isn’t. The idea is based on forcing water vapor to condense, forming liquid water, on a material that makes it easy to collect. A mesh or net like this one works well because it offers a high surface area for the fog to cling onto. Over time, these tiny fog droplets merge to form water droplets, and they drip down the fibers of the mesh, until they reach a gutter along the bottom of the billboard. There, the water is collected and filtered through sand, before moving into a collection tank that can be accessed by locals.

Scientists at MIT have been working on a similar – if slightly more ‘high tech’ – project since 2013, and it takes inspiration from the Namib Beetle. By angling its wings into the wind, this awesome beetle can channel water from the desert fog along hydrophobic (water-fearing) or hydrophilic (water-loving) patterns on its body. This way, water reaches its mouth without much loss. The MIT team used this idea to design a mixed-materials net to funnel water along a predetermined path. You can read their latest paper (£) on that work here. (CONTINUED...)

But back to the Moroccan project – it came about as the result of a collaboration between a Moroccan NGO, Dar Si Hmad, the German Water Foundation, and a company called Aqualonis. The amounts yielded by these nets varies by region and season, but they can collect between 6 and 22 liters for every square meter of net surface. Right now, the Moroccan project produces more than 600 liters (that’s 160 gallons) a day – enough to support around several villages. But the hope is that, by mid-2018, their billboards will cover a total area of 1,700 m2, which should almost triple the output of the array, supporting thousands of villagers.

Fishing water from fog is one thing, but what if your city is in an arid region, where fog and rain are so rare that they make national news? That is the reality facing cities like Dubai and Doha, and they’ve resorted to extreme measures to supply water to their residents… They pump vast quantities of seawater into processing plants, and suck the salt from it.

To understand how the process works, let’s take a step back and talk about why salty seawater can wreak havoc with our innards – or more specifically, our cells. Each cell in our body is effectively a blob of a watery solution surrounded by a semi-permeable membrane (sorry, biologists!). The membrane’s job is to control the flow of molecules in and out of the cell, to keep everything balanced. Freshwater has a similar composition to the cell itself, so if we take a gulp of it, any movement of molecules through the membrane pretty much balances out. But with its high salt content, seawater confuses the cells, and in an attempt to regain balance, they drive their own solution out through the membrane. This process is called osmosis, and it occurs anywhere you have two solutions of differing concentration, separated by a thin membrane.

Osmosis also provides a way to remove salt from seawater. By setting up a tank partitioned by an ultra-fine polymer membrane, and pressurizing seawater until it reaches 80 Bar (that’s 40 times higher than the pressure of your car’s tires), we can force osmosis to reverse direction. This would then leave the salt on one side of the membrane and salt-free water on the other. In Saudi Arabia, a country with almost no surface water, desalination is used to produce three billion liters of drinking water a day – enough to fill 1,200 Olympic-sized swimming pools.

But, running reverse osmosis pumps requires huge quantities of electricity, making desalination the most expensive way to clean water. According to Bloomberg, desalination plants “use about 15,000 kilowatt-hours of power for every million gallons of fresh water that’s produced”. That’s almost twice as much energy as wastewater treatment plants use to produce the same volume of water. So, to say that desalination is unsustainable is an understatement. Don’t get me wrong, it’s great to have as an emergency backup, and is a clever use of a largely untapped resource. But as an everyday option to keep taps flowing? No way.

However, a new paper (£), published in the journal Nature Nanotechnology, suggests that graphene oxide could be used as a ‘sieve’ for salty water. You might remember that graphene is a single layer of carbon atoms arranged in a honeycomb pattern, discovered in 2004 by Professors Andre Geim and Kostya Novoselov. Its properties are so extraordinary that the pair went on to win the 2010 Nobel Prize in Physics, which saw the University of Manchester become the center of the carbon research world.

Manchester researchers are behind this latest paper too, but it focuses on graphene oxide (GO) – a single layer of carbon atoms laced with oxygen. By stacking multiple layers of GO, they’ve shown that they can strip salt from seawater. They’re not the first to look to graphene oxide membranes for water treatment – other research groups have shown that it can filter nanoparticles or drug molecules from water. But the difference here is that this GO membrane seems to be able to tackle common salts, like sodium chloride (NaCl) found in seawater. Key to this was finding a way to overcome a common problem with these membranes – swelling. (CONTINUED...)

Salt (specifically NaCl) molecules are tiny. When they’re dissolved in water, their effective diameter is around 1 nanometer. That’s ~6,000 times smaller than a red blood cell! When most GO membranes are immersed in water, they swell up, stretching the gap between each layer to 1.35 nanometers. And that means the salt can sneak through, rendering the membrane useless, at least for desalination purposes.

But lead scientist Dr Rahul Nair and his colleagues found a way around the issue. They pinned the GO layers between walls of epoxy resin, to limit the swelling. The resulting membranes displayed spacing of between 0.98 and 0.64 nanometers – that’s smaller than the salt ions, but big enough for water molecules. In fact, they found that the size of the restricted layers actually forced the water through more quickly than in membranes with larger layers. Speaking to BBC News, Rahul said "When the capillary size is around one nanometer….those (water) molecules form a nice interconnected arrangement like a train. That makes the movement of water faster.”

Now that the researchers have shown that it is possible to filter salt from seawater using GO, they hope to begin testing their membrane against those already used in desalination plants. They’re still some way from commercializing these membranes though, and there are lots of questions to answer – can they be made at industrial scales with comparatively low cost? Are they durable enough to be used to treat many thousands of liters of seawater? Can they be cleaned and reused? And so on. However, with our freshwater reverses under such immense pressure, a material that could produce it from seawater, with minimal energy input, is certainly one worth pursuing.