Desalination Innovation Could Make Water Filtration More Cost-Effective

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Desalination Innovation Could Make Water Filtration More Cost-Effective



This 3D model of a polymer desalination membrane shows water -- the silver channels, moving from top to bottom -- avoiding dense spots in the membrane and slowing flow. [Credit: Ganapathysubramanian research group/Iowa State University and Gregory Foss/Texas Advanced Computing Center.]

Biological membranes let the right stuff into cells while keeping the wrong stuff out. And, as researchers noted in a paper just published by the journal  Science , they are remarkable and ideal for their job.

But they're not necessarily ideal for high-volume, industrial jobs such as pushing saltwater through a membrane to remove salt and make fresh water for drinking, irrigating crops, watering livestock or creating energy.

Can we learn from those high-performing biological membranes? Can we apply nature's homogenous design strategies to manufactured, polymer membranes? Can we quantify what makes some of those industrial membranes perform better than others?

Researchers from Iowa State University, Penn State University, the University of Texas at Austin, DuPont Water Solutions and Dow Chemical Co. – led by Enrique Gomez of Penn State and Manish Kumar of Texas – have used transmission electron microscopy and 3D computational modeling to look for answers.

Iowa State's Baskar Ganapathysubramanian, the Joseph C. and Elizabeth A. Anderlik Professor in Engineering from the department of mechanical engineering, and Biswajit Khara, a doctoral student in mechanical engineering, contributed their expertise in applied mathematics, high-performance computing and 3D modeling to the project.

The researchers found that creating a uniform membrane density down to the nanoscale of billionths of a meter is crucial for maximizing the performance of reverse-osmosis, water-filtration membranes. Their discovery has just been published online by the journal  Science  and was the cover paper of the Jan. 1 print edition.

Working with Penn State's transmission electron microscope measurements of four different polymer membranes used for water desalination, the Iowa State engineers used the Stampede2 supercomputer at the Texas Advanced Computing Center (TACC) to predict water flow through 3D models of the membranes, allowing detailed comparative analysis of why some membranes performed better than others.


Reprinted with permission from AAAS.

"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."

The  Science  cover image was created by Greg Foss, a visualization expert and artist at TACC, in collaboration with the Iowa State researchers. Foss also contributed to the manuscript illustrations.

In the cover image (above and right), 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; and 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.

Most telling are the silver lines showing water moving around dense spots in the membrane.

"We're showing how water concentration changes across the membrane." Ganapathysubramanian said of the models which required high-performance computing to solve. "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."

Khara added, "The simulations themselves posed computional challenges, as the diffusivity within an inhomogeneous membrane can differ by six orders of magnitude"

So, the paper concludes, the key to better desalination membranes is figuring out how to measure and control at very small scales the densities of manufactured membranes. Manufacturing engineers and materials scientists need to make the density uniform throughout the membrane, thus promoting water flow without sacrificing salt removal.

It's one more example of the computational work from Ganapathysubramanian's lab helping to solve a very fundamental yet practical problem.

"These simulations provided a lot of information for figuring out the key to making desalination membranes much more effective," said Ganapathysubramanian, whose work on the project was partly supported by two grants from the National Science Foundation.



Biological membranes can achieve remarkably high permeabilities, while maintaining ideal selectivities, by relying on well-defined internal nanoscale structures in the form of membrane proteins. Here, we apply such design strategies to desalination membranes. A series of polyamide desalination membranes—which were synthesized in an industrial-scale manufacturing line and varied in processing conditions but retained similar chemical compositions—show increasing water permeability and active layer thickness with constant sodium chloride selectivity. Transmission electron microscopy measurements enabled us to determine nanoscale three-dimensional polyamide density maps and predict water permeability with zero adjustable parameters. Density fluctuations are detrimental to water transport, which makes systematic control over nanoscale polyamide inhomogeneity a key route to maximizing water permeability without sacrificing salt selectivity in desalination membranes.