Water shortages are adversely affecting more than 3 billion people around the world. The availability of fresh water has been falling dramatically because of growing pollution of water resources, population growth, the development of new territories, and the extension of deserts. According to the United Nations (UN), an estimated two-thirds of the world’s population will be living in water-stressed regions by 2025 if climate change is not addressed and fresh water is not conserved.
Even though freshwater reserves are large and renewable, the scale of their consumption exceeds their renewable flows. Additionally, fresh water makes up only 2.5 percent of the world’s water, with the rest being brackish water or saline seawater. Out of that, only 1 percent of fresh water is easily accessible, with most of it trapped in glaciers and snowfields. Therefore, the development of technologies for obtaining fresh water from saline water—and in particular, obtaining it from seawater—is critical to meet the increasing demand. This process is known as desalination.
“Freshwater management is becoming a crucial challenge throughout the world,” said Enrique Gomez, professor of chemical engineering and materials science at Pennsylvania State University (Penn State). “Shortages, droughts—with increasing severe weather patterns, it is expected this problem will become even more significant. It’s critically important to have clean water available, especially in low resource areas.”
Though desalination is critical for the supply of fresh water, it is an energy-intensive process. For example, processing one cubic meter of seawater into fresh water can consume up to 8.5 kWh/m3 of energy, about 47 percent of which is consumed by the electrical and thermal energies required in different desalination processes.
There are several techniques employed in desalination, but the foremost in terms of installed capacity and yearly growth is reverse osmosis (RO), which has plant membrane systems that typically use less energy than other desalination processes. RO processes use semipermeable membranes and applied pressure on the membrane feed side to preferentially push water to permeate through the membrane. Meanwhile, salts are rejected due to the salt molecules being too big to pass through the membrane. While RO systems tend to be cheaper, they still carry a significant cost. Nevertheless, it is expected that costs will continue to decrease with technology advancements that include enhanced efficiency, reduction in the footprint of processing plants, improvements to plant operations and optimization, more effective feed pretreatment processes, and lower-cost energy sources.
A group of researchers from Penn State, the University of Texas at Austin (UT Austin), Iowa State University, Dow Chemical Company and DuPont Water Solutions have made one such technological discovery.
The Initial Discovery
Even though membranes utilized for reverse osmosis are effective at removing salt and other impurities from water, they are still a bit of a mystery. “Despite their use for many years, there is much we don’t know about how water filtration membranes work,” said Gomez, who led the research. “We found that how you control the density distribution of the membrane itself at the nanoscale is really important for water-production performance.”
“Reverse osmosis membranes are widely used for cleaning water,” said Manish Kumar, an associate professor in the Department of Civil, Architectural and Environmental Engineering at UT Austin, who coled the research. “We couldn’t really say how water moves through them, so all the improvements over the past 40 years have essentially been done in the dark.”
The initial question was raised when researchers at DuPont, a company that produces desalination products, discovered that thicker membranes were surprisingly turning out to be more permeable. This went against the previously held industry standard that increasing thickness led to a reduction in the flow of water through the membrane. At a 2015 water summit organized by Kumar, DuPont met with the research team and decided to ascertain the underlying cause of the enhanced filtration efficacy of thicker membrane filters. DuPont collaborated with The National Science Foundation to fund the research.
Electron Microscopy of Membranes
Kumar and his team developed 3D reconstructions of the nanoscale membrane structure using state-of-the-art multimodal electron microscopes at Penn State’s Materials Characterization Lab. The microscope combines atomic-scale detailed imaging with techniques that reveal chemical composition. A high-intensity electron beam is scanned across the surface of the membrane and generates several 2D images that are combined to create a 3D image.
“This gives you information on the density of the polymer,” Kumar said. “Now you can find the density of each cubic nanometer of membrane to see how much is occupied by polymer and how much with free space.”
Iowa State’s Baskar Ganapathysubramanian, professor of engineering from the department of mechanical engineering, and Biswajit Khara, a doctoral student in mechanical engineering, were also a part of the team and were responsible for generating the 3D models by utilizing their expertise in applied mathematics and high performance computing.
The researchers mapped density variations in the membrane polymer film with a spatial resolution of approximately one nanometer—less than half the diameter of a DNA strand. According to Gomez, this technological advancement was key to understanding the role of density in membranes.
“You can see how some places are more or less dense in a coffee filter just by your eye,” Gomez said. “In filtration membranes, it looks even, but it’s not at the nanoscale, and how you control that mass distribution is really important for water-filtration performance.”
Based on the microscopic measurements of four different polymer membranes used for desalination, they modeled water flow through the membranes for a detailed comparative analysis of membrane performance. Greg Foss of the Texas Advanced Computing Center (TACC) helped visualize these simulations, and most of the calculations were performed on TACC’s supercomputer “Stampede2.”
Inefficient Membranes
It turned out that membrane density homogeneity at the nanoscale was the factor that was causing the increased membrane permeability discovered by the DuPont researchers—rather than the thickness of the membrane itself.
“The simulations were able to tease out that membranes that are more uniform—that have no ‘hot spots’—have uniform flow and better performance,” Ganapathysubramanian said. “The secret ingredient is less inhomogeneity.”
Khara provided more detail with reference to the visualization: “Red above the membrane shows water under higher pressure and with higher concentrations of salt. The gold, granular, sponge-like structure in the middle shows denser and less-dense areas within the salt-stopping membrane; silver channels show how water flows through. The blue at the bottom shows water under lower pressure and with lower concentrations of salt.
“You can see huge amounts of variation in the flow characteristics within the 3D membranes,” Khara said. Significantly, the silver lines are the ones showing water moving around dense spots in the membrane.
“We’re showing how water concentration changes across the membrane,” Ganapathysubramanian said of the models. “This is beautiful. It has not been done before because such detailed 3D measurements were unavailable, and also because such simulations are non-trivial to perform. The simulations themselves posed computational challenges, as the diffusivity within an inhomogeneous membrane can differ by six orders of magnitude.”
This means that manufacturing engineers and materials scientists need to make density uniform throughout membranes to promote increased water flow without sacrificing salt removal. The researchers estimate that membrane efficiency can consequently be increased by around 30 to 40 percent, resulting in more water filtered at the cost of less energy—a potentially substantial cost-saving improvement of current RO desalination processes.
Science has published the team’s research paper, which concludes that the key to better desalination membranes is figuring out how to measure and control the densities of manufactured membranes at a very small scale.
Future Research
The team is continuing to study the structure of the membranes, as well as the chemical reactions involved in the desalination process. The researchers are also examining how the best membranes for specific materials can be developed, such as sustainable yet tough membranes that can prevent the formation of bacterial growth.
“We’re continuing to push our techniques with more high-performance materials with the goal of elucidating the crucial factors of efficient filtration,” Gomez said.
The industrial researchers’ findings are definitely a massive step toward more efficient and cost-effective desalination processes, which will hopefully lead to further breakthroughs in providing clean drinking water where it is needed.