Access to clean drinking water is one of the pressing problems of our times. 71 percent of the earth’s surface is covered with water, yet one out of three people don’t have access to drinking water. This is because up to 97 percent of the Earth’s water is in the oceans, saturated with salt and undrinkable. Most of Earth’s drinkable freshwater is either locked up in ice caps or deep underground. Accessible freshwater resources are unevenly distributed around our planet and fast running out. For communities not situated close to these sources, of which there are many, the threat of Day Zero events — the day when a town’s water supply gets shut off — is ominous.
Current state of global desalination
Removing salt from seawater is widely regarded as the best way to solve this drinking water crisis. Desalination is after all a natural process triggered by the sun; seawater evaporates to form clouds and precipitates as rain.
Industrially, there are two routes for desalination of seawater; thermal treatment and membrane separation. However, both of these require centralized plants that are expensive to set up. They also rely on fossil fuels and are extremely energy intensive. Nevertheless, the desalination industry is booming. The number of active desalination plants in operation worldwide has doubled since the early 2000s. Left unchecked, these will spew out 500 million tonnes of CO2 per year by 2040!
On a local scale, both approaches yield brine — super salty water that is dumped back into the ocean. With thermal desalination plants, up to 75 percent of the water leaves as brine. With membrane technology, we are looking at a 50-50 split between freshwater and brine. This is a huge problem as detailed in this recent review article on the state of global brine production. Brine being denser than water, sinks to the sea bed. There, it cuts off oxygen supply and wreaks havoc on coastal ecosystems. It gets worse when you factor in that brine may also contain heavy metals and chemicals.
Despite these drawbacks, desalination plants are used extensively in rich countries like Saudi Arabia. However, there is consensus that unless and until we figure out how to deal with brine and move on from fossil fuels to power desalination plants, it is not the long term answer to our freshwater problem.
Trends in thermal desalination tech
What if instead of using fossil fuels to boil millions of gallons of seawater or pumping it through membranes, we get the Sun to do the work? This idea has been around for a while – purifying seawater by harnessing solar energy. One exciting implementation of this idea is the Solar Dome in the city of Neom in Saudi Arabia. UK based Solar Water Plc in association with Cranfield University developed a glass-steel dome structure that facilitates thermal desalination without fossil fuels. This first-of-its-kind thermal desalination system uses a series of mirrors to concentrate sunlight onto the dome to boil seawater within. The process is extremely efficient, yielding 30,000 cubic meters of freshwater per hour at an estimated 34 cents per cubic meter. It is also 100 percent carbon neutral, as the source of heat to boil water is the Sun, and not fossil fuels.
So what’s the catch? Brine. As desalination proceeds, the hypersaline brine sinks to the bottom of the dome. It is claimed that the brine will be extracted and put to commercial use in lithium batteries, grit for roads, fertilizer or detergents. Irrespective of its envisioned use, one would imagine that frequent removal of brine will add to the overall cost of freshwater production. Only time will tell if the city’s intended plan for brine disposal is sustainable. Other efforts to harness solar energy to purify water over the years have not taken off because the materials used either cause secondary pollution, or do not last in the real world. Specifically, evaporation rates reduce gradually at first, but worsen with fouling and salt precipitation.
Trends in membrane desalination tech
There has been progress in membrane technology too. A membrane with micro sized pores capable of removing 99.9 percent salt from seawater in minutes recently made the news. This system designed by researchers from Korea relies on a difference in vapor pressure between seawater and freshwater. Water vapor diffuses through and condenses on one side, while salt is trapped within the membrane. Initial performance data looks promising, but the technology has only been demonstrated at the lab scale.
The fibers that form the membrane are made using specialty polymers through a technique called co-axial electrospinning, which is typically difficult to scale. Nevertheless, the electrospun membrane itself is water repellant, which is a big deal. Wet membranes cause salt to seep through into the output freshwater, reducing efficiency. The membrane offers stable desalination for 30 days, which is a big improvement over similar membranes that show reduced efficiency in as little as two days. Another desalination technology worth keeping eyes on is EnergyX’s metal organic framework (MOF) composite membranes, which are potentially more scalable.
Tackling desalination with aerogels
It is clear that over the years, there has been significant efforts to make desalination affordable, energy efficient, carbon neutral, and brine free. However, there is no single technology available today that ticks all of these boxes.
A cleverly designed material engineered from dairy waste may soon enable solar energy desalination without brine build up.
Prof. Raffaele Mezzenga and his team at ETH Zurich used whey, a readily available byproduct of the dairy industry, to create an aerogel with an energy harvesting top surface, and a super absorbing underside.
Synthesis of the aerogel occurs in two stages. First, a gel is made with whey and second, the gel is freeze-dried. To form the energy harvesting and water repellent top surface, the researchers added polydopamine to whey before forming the gel. Freeze-drying the two types of gels placed one on top of the other is what gives the unique two-layered aerogel structure necessary for desalination.
When placed on the surface of seawater, capillary action draws water into the aerogel, while the top surface harvests energy from the sun to confine heat and evaporate seawater. As the aerogel is less dense than water, it floats, ensuring the top evaporator surface is always exposed to sunlight. Channels that run all the way along the height of the hybrid aerogel allow rapid water transport to the hot top surface, where it evaporates.
How is this brine free?
As pure water evaporates from the surface, the salt that remains undergoes two pathways, Prof. Raffaele notes. Some of the salt stays in the aerogel structure and some diffuse back into water due to osmotic pressure. There is a tradeoff between these two phenomena, the exact ratio of which depends on the composition of the aerogel.
The protein chosen in this case adsorbs pollutants from seawater, and not mineral salt. This way, as desalination proceeds, most of the salt is diffused back into seawater. However, the process of releasing salt back into seawater is gradual and occurs over a considerable area. This allows plenty of time for the salt to be diluted again. This is in stark contrast to the practice of ‘dumping brine’ which harms coastal ecosystems near desalination plants.
The two-layer design of the aerogel allows seawater purification through thermal distillation and adsorption of contaminants. After many cycles, some salt does get adsorbed onto the aerogel’s structure. This changes its color from black to white, after which it needs to be replaced. All of the materials involved are cheap, biodegradable, and do not cause secondary pollution.
Importantly, the aerogel synthesis process is easy to scale up. In fact, the team prepared a 20 centimeter diameter aerogel, placed it on seawater, and exposed it to real sunlight. It worked as expected with water droplets forming in as little as 1 hour!
Beyond desalination
Applications are not limited to desalination however, as they found that the aerogel could remove heavy metals, organic contaminants, bacteria, and even viruses extremely efficiently. They even went on to demonstrate how it could make water from three real world water bodies — Lake Constance, Red Sea, and Siwa Salt Lakes — potable!
Compared to previous efforts at desalination with aerogels, this attempt by Prof. Raffaele’s team stands out in its choice of materials; using a byproduct of the dairy industry to create value in a completely different domain is a win-win for stakeholders and the environment.
All things considered, the hybrid aerogel approach to desalination seems to negate the criticisms aimed at existing desalination methods. The process is carbon neutral, as fossil fuels are not burnt to boil water or pump it through membranes. Nor are large quantities of brine accumulated, as salt is gradually released into running seawater, diluting and spreading it over a substantial area in the sea.
The fact that the primary ingredient involved, whey, is a readily available waste product of the dairy industry is a bonus. “The quantity of water that can be desalinated by placing these aerogels on the surface of seawater is limitless”, says Prof. Raffaele.
Thoughts
As promising as this seems, there are a few possible challenges when implementing this at a large scale.
For instance, in the open sea, unpredictable weather events can result in strong winds and huge waves. Will there be potentially expensive engineering challenges to ensure the aerogels and the accompanying freshwater collection units stay in place? Further, freshwater yield is directly related to the area of seawater coverage. Does this mean more and more of the sea’s surface needs to be covered with aerogel over time?
Even so, this study is proof that well thought out material design can make seawater potable using virtually no energy, emitting no carbon, and without harming marine ecosystems.
Desalination may be the long term answer to our freshwater problem after all — just not how we implemented it thus far.