Illustration of a next-generation water desalination membrane. Water molecules are shown in white and red and sodium and chlorine ions are shown in green and purple. Image appears courtesy of David Cohen-Tanugi via MIT News.
Scientists from MIT have designed a next-generation water desalination membrane that could greatly improve our ability to extract drinkable water from the sea.
Material scientists from the Massachusetts Institute of Technology (MIT) have designed a next-generation water desalination membrane that could greatly improve our ability to extract drinkable water from the sea.
The membrane is composed of a one-carbon atom thick sheet of graphene. Their research about this new form of nanotechnology was published on July 5, 2012 in the journal Nano Letters.Water scarcity is a growing problem in many parts of the world. Less than 1% of the world’s freshwater is accessible for human use. Most of the water on earth is seawater. Although desalination technology exists that can extract drinking water from seawater, it is energy intensive and costly to use.
David Cohen-Tanugi and Jeffrey Grossman are scientists from the Department of Materials Science and Engineering at MIT. They were interested in determining if graphene, a pure carbon substance, could be used for water desalination. Graphene is already being used in other technological applications such as DNA sequencing. However, its potential for use in water desalination is just beginning to be explored.Cohen-Tanugi and Grossman built a molecular computer model to examine how well graphene could purify seawater. Graphene is produced in a sheet of bonded carbon atoms that are arranged into a hexagonal, honeycomb-shaped structure. They found that with the correct pore size in the nanometer range, a graphene membrane could effectively filter out large salt molecules while allowing smaller water molecules to flow quickly through the pores. Because graphene membranes are much thinner than conventional membranes, their use in desalination plants would have the potential to greatly reduce the amount of energy needed for water purification.
Unfortunately, engineers have not yet found a way to punch holes in the graphene with the degree of precision that this new desalination technology would require. Hopefully, they will soon.
David Cohen-Tanugi, a PhD candidate at MIT and lead author of the study, commented on the study’s findings in a video interview with MIT News. He said:
The enhanced water permeability of nanoporous graphene could be an important advantage over existing water desalination technology. So while there is still a lot of work to be done on this topic, we are very encouraged by our existing results and we’re excited to see the role that nanoporous grapheme could play in the future of global water resources.
The research was funded by an MIT Energy Initiative and a John S. Hennessy Fellowship to David Cohen-Tanugi. This research was cited by the Smithsonian as one of the top five most surprising scientific milestones of 2012.
Bottom line: Material scientists from the Massachusetts Institute of Technology (MIT) have designed a next-generation water desalination membrane that could greatly improve our ability to extract drinkable water from the sea. The membrane is composed of a one-carbon atom thick sheet of graphene. Their research about this new form of nanotechnology was published on July 5, 2012 in the journal Nano Letters.
Bridget Scanlon on groundwater depletion and solutions:
Heather Cooley on reasons for and against desalination
Amy Zander led a scientific assessment on turning salt water to fresh
Deanna Conners comes to EarthSky as an Environmental Scientist who holds a Ph.D. in Toxicology and a M.S. in Environmental Studies. Her interest in toxicology stems from having grown up near the Love Canal Superfund Site in Western New York. The aim of her current work is to provide high-quality scientific information to the public and decision makers and to build cross-disciplinary partnerships to help solve environmental problems. She writes about earth science and nature conservation for the EarthTrekker Blog, and views blogging as a valuable tool to help make science accessible to more people. In her free time Deanna enjoys getting outdoors to snowboard, surf and photograph the wonders of nature.
Bridget Scanlon and other scientists say we need to know where our water resources are, and what options society can leverage to use these resources most effectively.
Earth’s human population has reached 7 billion and is still climbing. Scientists say the depletion of groundwater around the world is a serious issue. They say we need to know where our water resources are, and what options society can leverage to use these resources most effectively. Bridget Scanlon is a Senior Research Scientist at the Bureau of Economic Geology at the University of Texas at Austin. Her research team works to assess water resources and offer sustainable solutions. She spoke with EarthSky’s Jorge Salazar. This interview is part of a series, made possible in part by the Bureau of Economic Geology at the University of Texas at Austin.Will we have enough water for 9 billion people, projected by the year 2050?
I think we’ll have enough water for that population.
But you say there’s a hidden cost of water in everything we use and consume. Tell us about that.
That’s right. When you ask them how much water they use, a lot of people think of the household usage for laundry and showering and things like that. But our food choices involve a lot more water.
What do you mean?
People may have heard of the carbon footprint. Similarly there’s a water footprint. The water footprint represents how much water is required to produce whatever product we’re talking about. It could be a car, or a steak or whatever. It represents the amount of virtual water that’s embodied in that product.
The highest water footprint is for beef, followed by other types of meats. The lowest is for vegetables and some fruits. A kilogram of steak would represent about 15,000 liters of water, a cup of tea, maybe 35 liters.
The problem with our increasing human population is that dietary preferences are also changing around the globe. When I visit China and India, I see a lot more ice cream and dairy products. They’re moving toward a more western type diet, which is a more water-intensive diet.
Tell us about water used for food production. We understand irrigation is a special concern of yours. Is the way we irrigate our fields sustainable?
I would say it is sustainable in some regions but not in most. Currently, about 90 percent of global freshwater resources is consumed by irrigated agriculture. Water shortages are most likely to occur where irrigation is most widespread.
The highest area of irrigated agriculture is in India, with about 40 million irrigated hectares. That’s followed by China, with about 20 million irrigated hectares, and then the U.S. with about 17 million hectares. That’s where irrigation is most widespread at the moment, which is why water shortages are most likely to occur in those areas as the century progresses.
[toggle title=”Why do food producers continue to irrigate then?” height=”auto”]
Irrigation helps us resolve spatial and temporal disconnects between water supply and water demand for crop production.
For example, in the North China Plain, they grow winter wheat despite the fact that their rain occurs in the summer. The Central Valley in California is a Mediterranean climate with winter rainfall, but they grow crops in the summer. So irrigation is used to circumvent that kind of temporal or seasonal disconnect.
Also, there are spatial disconnects. We grow a lot of our food in semi-deserts, because of high solar radiation and good soils, and because we can move water around. Those areas have a lot of positive aspects for crop production, but they may be lacking enough water to grow food. So groundwater or imported surface water or water from other sources has to be provided.
But water is a renewable resource, isn’t it?
Yes, to some extent. You can use the analogy of a bank balance, and think of the groundwater system as how much water you have in the bank. The health of your bank account depends on how much you put into the bank account versus how much you withdraw.
Similarly, the health of an aquifer depends on how much water is going back into the aquifer from rainfall and recharge, versus how much water you are pumping out of the aquifer for irrigation.
We use the term fossil groundwater to describe some aquifer systems because they were recharged about 10,000 years ago during Pleistocene times when the climate was cooler and wetter. For example, in the High Plains Aquifer in Texas and Kansas and other U.S. states, much of the recharge of that system occurred during Pleistocene times. The rate of depletion of that aquifer is up to 10 times the rate of recharge currently. It’s clearly not sustainable then to extract a lot of water from fossil groundwater.
What sorts of things can be done to solve the problem of accelerating depletion of groundwater?
Since irrigation consumes 90 percent of global freshwater resources, I think an obvious thing would be to reduce irrigated agriculture and convert irrigated agriculture to rain-fed agriculture.
An example of a way to use groundwater more sustainably is in the Central Valley in California. They have a large pipe system that pipes water from the humid region in the north where they have about 650 millimeters of rainfall a year, down to the desert-like region in the south, where they get 150 millimeters of rainfall. By piping this water into this system then and irrigating with surface water they can replenish the groundwater and recharge it. By mixing surface water irrigation with groundwater irrigation, they can make it more sustainable.
What are some other solutions?
Conserving water and in urban areas by, for example, reducing lawn watering and outside irrigation.
A lot of areas are looking at desalinating brackish water or desalinating seawater as another approach. In response to a 13-year drought, Australia constructed desalination plants in the cities around the coast. In those situations, we are borrowing from the energy side to provide more water. So there’s always a trade off.
There are projections that we will have more droughts and floods in the future with intensification of the water cycle. So it would be important that we can scalp water excesses and store them for times when we have limited water availability. For example, in California’s Central Valley, they take excess water from the State Water Project or the Federal Water Project, store a few cubic kilometers of groundwater in the aquifer – in what they term Groundwater Bank – and then remove it during droughts.
Let’s talk about water and energy for a moment. How are they linked?
We need a lot of water to cool steam electric generators for electricity production. Electricity production in the U.S. accounts for approximately 40 percent of the water we withdraw. But then we return most of that water back to the source. So, overall, here in the U.S., energy production consumes a small amount of water.
For example, here in Texas, energy production accounts for about three percent of the state’s water consumption, but it withdraws a lot more water.
The issue with energy and water is the need for reliable water sources to ensure the constant production of electricity. The problem is that, oftentimes, water needs for electricity are out of phase with water availability. That is, when we need energy the most, for example during heat waves or droughts, is when we have the least amount of water available.
Here in Texas, most of the power plants depend on surface water which is the most vulnerable to drought. So we have our highest energy demands when we have the least amount of water available in surface water sources.
Ways to get around some of this may be to use surface water and groundwater together. In other words, use surface water when it’s readily available and then switch to adding some groundwater during dry periods. Or store water in aquifers for the dry times.
Consider the city of San Antonio. It takes groundwater from the Edwards Aquifer and pipes it and stores it in a porous medi-aquifer, and then takes it out during times of drought. That avoids the problems of large-scale evaporation from surface water reservoirs also. The drought in 2011 in Texas was the most extreme drought on record. And evaporation rates in reservoirs were up by about 10 percent according to Texas Water Development Board staff. So storing water in aquifers also avoids the problem of large-scale evaporation and losses.
In closing, what would you like people to understand about our groundwater resources?
I think it’s important that people understand how much water they use and where it’s coming from, and how scarce it is in that region.
And it would be nice if ultimately food products were labeled as to whether they were grown with irrigation or not. If that happened, we could make better choices as consumers about what we do that would maybe reduce impacts on water.
Overall, I’d say we all need to understand our water system better. And then we need to do our part to try to reduce water consumption.
Article by scientist: Bridget Scanlon
EmailBridget R. Scanlon is a Senior Research Scientist at the Bureau of Economic Geology in the Jackson School of Geosciences of the University of Texas at Austin. Her research group is working to assess sustainability issues with respect to water resources within the context of climate variability and land-use change in semiarid regions. Her group…read more »