Breakthrough desalination maximizes flow for less expensive water filtration

3D model of a polymer desalination membrane

This 3D model of a polymeric desalination membrane shows the flow of water – the silver channels, moving up and down – avoiding dense spots in the membrane and slowing the flow. Credit: Image by Ganapathysubramanian / Iowa State University and Gregory Foss / Texas Advanced Computing Center research group

Nature has figured out how to make great membranes.

Biological membranes let the good things into cells while keeping the bad things out. And, as the researchers noted in an article just published by the journal Science, they are remarkable and perfect for their job.

But they are not necessarily ideal for high volume industrial jobs, such as passing salt water through a membrane to remove salt and produce fresh water for drinking, irrigating crops, watering livestock or create energy.

Can we learn from these high performing biological membranes? Can we apply nature’s homogeneous design strategies to manufactured polymer membranes? Can we quantify what makes some of these industrial membranes outperform others?

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

Baskar Ganapathysubramanian from Iowa State, Professor Joseph C. and Elizabeth A. Anderlik in Engineering from the Department of Mechanical Engineering, and Biswajit Khara, PhD student in Mechanical Engineering, brought their expertise in applied mathematics, high performance computing and in 3D modeling to the project.

Researchers have found that creating a uniform membrane density down to the nanoscale of one billionth of a meter is crucial to maximizing the performance of reverse osmosis water filtration membranes. Their discovery has just been published online by the journal Science and will be the cover for the print edition of January 1, 2021.

Working with Penn State’s transmission electron microscope measurements of four different polymer membranes used for desalination of water, engineers in the State of Iowa predicted the flow of water through models 3D membranes, allowing for a detailed comparative analysis of why some membranes performed better than others.

“The simulations found that membranes that are more uniform – that don’t have ‘hot spots’ – have uniform flow and better performance,” Ganapathysubramanian said. “The secret ingredient is less inhomogeneous.”

Take a look at the Science the cover image the Iowa state researchers created with help from the Texas Advanced Computing Center, Khara said: Red above the membrane shows water under higher pressure and with higher concentrations of salt; the gold, granular, sponge-like structure in the middle has denser and less dense areas within the salt barrier membrane; the silver channels show how the water flows; and the blue at the bottom shows water under lower pressure and with lower salt concentrations.

“You can see huge variations in the flow characteristics in 3D membranes,” Khara said.

Most telling are the silvery lines showing the water moving around the dense points of the membrane.

“We show how the water concentration changes across the membrane.” Ganapathysubramanian said of models that require high performance computing to be solved. “It’s beautiful. This hasn’t been done before because these detailed 3D measurements were not available, and also because such simulations are not easy to perform.”

Khara added: “The simulations themselves posed computational problems, since diffusivity in a non-homogeneous membrane can differ by six orders of magnitude.”

So, the article concludes, the key to better desalination membranes is figuring out how to measure and control the densities of the membranes manufactured on a very small scale. Manufacturing engineers and materials scientists need to even out density across the membrane, promoting water flow without sacrificing salt removal.

This is another example of the computational work of the Ganapathysubramanian lab helping to solve a very basic but practical problem.

“These simulations provided a lot of information to determine the key to making desalination membranes much more efficient,” said Ganapathysubramanian, whose work on the project was partly funded by two grants from the National Science Foundation.

Reference: December 31, 2020, Science.
DOI: 10.1126 / science.abb8518

The project was led by Enrique Gomez, professor of chemical engineering and materials science and engineering at Penn State University, and Manish Kumar, associate professor of civil, architectural and environmental engineering at the University of Texas at Austin.

Also, from Iowa State University: Biswajit Khara, Baskar Ganapathysubramanian; from Penn State: Tyler Culp, Kaitlyn Brickey, Michael Geitner, Tawanda Zimudzi, Andrew Zydney; from DuPont Water Solutions: Jeffrey Wilbur, Steve Jons; and Dow Chemical Co .: Abhishek Roy, Mou Paul.