As the availability of clean, potable water becomes an increasingly urgent issue in many parts of the world, researchers are searching for new ways to treat salty, brackish or contaminated water to make it usable. Now a team at MIT has come up with an innovative approach that, unlike most traditional desalination systems, does not separate ions or water molecules with filters, which can become clogged, or boiling, which consumes great amounts of energy.
Instead, the system uses an electrically driven shockwave within a stream of flowing water, which pushes salty water to one side of the flow and fresh water to the other, allowing easy separation of the two streams. The new approach is described in the journal Environmental Science and Technology Letters, in a paper by professor of chemical engineering and mathematics Martin Bazant, graduate student Sven Schlumpberger, undergraduate Nancy Lu, and former postdoc Matthew Suss.
This approach is “a fundamentally new and different separation system,” Bazant says. And unlike most other approaches to desalination or water purification, he adds, this one performs a “membrane-less separation” of ions and particles.
Membranes in traditional desalination systems, such as those that use reverse osmosis or electrodialysis, are “selective barriers,” Bazant explains: They allow molecules of water to pass through, but block the larger sodium and chlorine atoms of salt. Compared to conventional electrodialysis, “This process looks similar, but it’s fundamentally different,” he says.
In the new process, called shock electrodialysis, water flows through a porous material —in this case, made of tiny glass particles, called a frit — with membranes or electrodes sandwiching the porous material on each side. When an electric current flows through the system, the salty water divides into regions where the salt concentration is either depleted or enriched. When that current is increased to a certain point, it generates a shockwave between these two zones, sharply dividing the streams and allowing the fresh and salty regions to be separated by a simple physical barrier at the center of the flow.
“It generates a very strong gradient,” Bazant says.
Even though the system can use membranes on each side of the porous material, Bazant explains, the water flows across those membranes, not through them. That means they are not as vulnerable to fouling — a buildup of filtered material — or to degradation due to water pressure, as happens with conventional membrane-based desalination, including conventional electrodialysis. “The salt doesn’t have to push through something,” Bazant says. The charged salt particles, or ions, “just move to one side,” he says.
The underlying phenomenon of generating a shockwave of salt concentration was discovered a few years ago by the group of Juan Santiago at Stanford University. But that finding, which involved experiments with a tiny microfluidic device and no flowing water, was not used to remove salt from the water, says Bazant, who is currently on sabbatical at Stanford.
The new system, by contrast, is a continuous process, using water flowing through cheap porous media, that should be relatively easy to scale up for desalination or water purification. “The breakthrough here is the engineering [of a practical system],” Bazant says.
One possible application would be in cleaning the vast amounts of wastewater generated by hydraulic fracturing, or fracking. This contaminated water tends to be salty, sometimes with trace amounts of toxic ions, so finding a practical and inexpensive way of cleaning it would be highly desirable. This system not only removes salt, but also a wide variety of other contaminants — and because of the electrical current passing through, it may also sterilize the stream. “The electric fields are pretty high, so we may be able to kill the bacteria,” Schlumpberger says.
The research produced both a laboratory demonstration of the process in action and a theoretical analysis that explains why the process works, Bazant says. The next step is to design a scaled-up system that could go through practical testing.
Initially at least, this process would not be competitive with methods such as reverse osmosis for large-scale seawater desalination. But it could find other uses in the cleanup of contaminated water, Schlumpberger says.
Unlike some other approaches to desalination, he adds, this one requires little infrastructure, so it might be useful for portable systems for use in remote locations, or for emergencies where water supplies are disrupted by storms or earthquakes.
Maarten Biesheuvel, a principal scientist at the Netherlands Water Technology Institute who was not involved in this research, says the work “is of very high significance to the field of water desalination. It opens up a whole range of new possibilities for water desalination, both for seawater and brackish water resources, such as groundwater.”
Biesheuvel adds that this team “shows a radically new design where within one and the same channel ions are separated between different regions. … I expect that this discovery will become a big ‘hit’ in the academic field. … It will be interesting to see whether the upscaling of this technology, from a single cell to a stack of thousands of cells, can be achieved without undue problems.”
The research was supported by the MIT Energy Initiative, Weatherford International, the USA-Israel Binational Science Foundation, and the SUTD-MIT Graduate Fellows Program.
Yale researchers have confirmed that hydraulic fracturing – also known as “fracking” – does not contaminate drinking water. The process of extracting natural gas from deep underground wells using water has been given a bad reputation when it comes to the impact it has on water resources but Yale researchers recently disproved this myth in a new study that confirms a previous report by the Environmental Protection Agency (EPA) conducted earlier this year.
After analyzing 64 samples of groundwater collected from private residences in northeastern Pennsylvania, researchers determined that groundwater contamination was more closely related to surface toxins seeping down into the water than from fracking operations seeping upwards. Their findings were recently published in the journal Proceedings of the National Academy of Science.
Researchers also noted that shale underlying the Pennsylvania surface did not cause any organic chemicals to seep into groundwater aquifers. However, these findings may not be applicable to all locations worldwide.
“Geology across the country is very different. So if you’re living over in the New Albany-area shale of Illinois, that might be distinct from living in the Marcellus shale in Pennsylvania,” Plata explained.
Researchers from Duke University also recently gave people a reason to trust fracking companies. In a study published in Environmental Science & Technology Letters, scientists explained that hydraulic fracturing accounts for less than one percent of water used nationwide for industrial purposes. This suggested that the natural gas extraction processes are far less water-intensive than we previously thought.
It’s hoped that these studies will help people better understand the safety of fracking.
28 Sep 2015
Though applied since the 1940s, hydraulic fracturing boomed in the 1990s, according to The Geological Society of America. New technology allowed the practice to be applied to horizontal wells for extracting shale gas. Unprecedented growth followed. According to a 2014 report by FracTracker Alliance, over 1.1 million active oil and gas wells exist in the U.S.
“The rapid pace of shale gas development in the U.S. has naturally led to several gaps in knowledge about environmental impacts,” said Douglas Arent, executive director of the Joint Institute for Strategic Energy Analysis at the U.S. Dept. of Energy’s National Renewable Energy Laboratory.
Arent and colleagues recently published a paper in MRS Energy & Sustainability overviewing the developments of unconventional gas in the U.S., particularly focusing on trends in water and greenhouse gas emissions.
“If unconventional natural gas is produced and distributed responsibly, and incorporated into resilient energy systems with increasing levels of renewables, then gas can likely play a significant role in realizing a more sustainable energy future,” said Arent.
With many U.S. states experiencing droughts—the west coast especially—water resources are stressed. Fresh water is a valuable resource. Even if one removes hydraulic fracturing from the equation, other domestic, agricultural and industrial water needs abound.
A recent Stanford Univ. study found that regardless how deep a well was, amounts of water used to frack were indistinguishable. The average volume used to frack, according to the study, was 2.4 million gallons.
“Groundwater depletion—a situation in which water is withdrawn from aquifers faster than it can be replenished—is occurring in many areas where there are shale plays,” Arent et al. write. “Depletion not only reduces the quantity of available water, it can also result in an overall deterioration of water quality.”
Water quality degradation can occur in a myriad of ways, from leaking wells and poor wastewater treatment practices, to spills and toxic element accumulation in soil. Clarity regarding the sources and mechanisms of contamination are needed, followed by an examination of effective practices to eliminate risks, according to the researchers.
“Currently, best management practices to mitigate (water) quantity and quality related risks have not been established by industry and stakeholder groups,” the researchers write. Further, no uniformity exists across the country. Individual states are responsible for regulations regarding well construction, and mitigating potential risks to water quality. Often separate state regulations don’t mesh due to each state’s geological makeup.
An “analysis will be critical to establishing those (best management practices) and government regulations, where needed, which will ensure that shale gas can be responsibly and sustainably produced,” write the researchers.
Greenhouse gas emissions
Natural gas production, compared to coal production, results in half the carbon emissions per unit of energy. The researchers contend natural gas can offer greenhouse gas mitigation benefits relative to coal, if methane emissions are small.
In 2014, the Environmental Protection Agency reported methane gas emissions from fractured natural gas wells decreased by 73% since 2011.
“Significant work is needed to measure and verify methane emissions across the full production, transportation and distribution value chain,” the researchers write. “If natural gas is to help mitigate climate change, it will do so primarily by displacing coal. However, in the long term, natural gas itself…will not significantly alter long-range climate projections.”
While natural gas, according to the researchers, will play an important role in the U.S.’s energy future, renewable energies or carbon capture and storage will be needed to meet carbon mitigation goals.
“More transparent and accessible data related to water use and emissions from shale gas development and use…are essential to providing a more complete understanding of all the pathways to a decarbonized energy future,” said Arent.