The typical way to purify water is to pass it through some kind of pore-based filter through which harmful or polluting particles larger than water molecules cannot pass. However, this traditional way to filter wastewater accounts for about 13 percent of all electricity consumed in the United States every year and releases considerable carbon dioxide into the atmosphere, which has led researchers to seek out better ways.
A team at the Wyss Institute for Biologically Inspired Engineering at Harvard University thinks it has found one. Researchers there collaborated with scientists at Northeastern University and the University of Waterloo to develop what are called liquid-gated membranes (LGMs) that can achieve similar or even better results to current filtration methods by using less electricity, they said.
|Liquid-gated membranes (white, on left) offer a more economical and less energy-intensive way to filter substances from liquids, as their specially coated, porous surfaces resist accumulation and can be tuned to allow particles of specific sizes to pass through. (Image source: Wyss Institute at Harvard University)|
Removing Nano-Sized Particles
The team developed LGMs that can filter nano-clay particles out of water with twofold higher efficiency, nearly threefold longer time-to-foul, and a reduction in the pressure required for filtration when compared to conventional membranes, said Jack Alvarenga, a research scientist at Wyss.
“Although membrane-based processing offers many benefits, fouling ultimately deteriorates performance, leading to decline in the permeate flux, inadvertent solute rejection, and correspondingly increased energy demands,” he told Design News. “Our technology aims to address this issue through a novel modification of existing membrane filters.”
LGMs mimic natural processes, such as how small stomata openings in plants’ leaves allow gases to pass through, he said. In the synthetic process, the membranes use liquid-filled pores to control the movement of liquids, gases, and particles through biological filters using the lowest possible amount of energy.
“The foundational work on this technology from our research group drew inspiration from the use of liquids in natural systems to control the flow of fluids,” Alvarenga said. “For example, on the surface of leaves, microscale stomata control air, water, and microbe exchanges in plants by using a liquid to reconfigure the pore opening. And in our lungs, micropores between lung air sacs are filled with a reversibly reconfigurable liquid capable of creating an open, liquid-lined pathway in response to pressure gradients.”
To filter and purify water, each LGM is coated with a liquid that acts as a reversible gate, filling and sealing the pores of the membrane when it’s in its “closed” state.
Tuning with Pressure
When pressure is applied to the LGM, the liquid inside the pores is pulled to the sides, which creates open, liquid-lined pores that researchers can tune to allow the passage of specific liquids or gases. The pores also resist fouling due to the liquid layer’s slippery surface, researchers said. Additionally, fluid-lined pores enable the separation of a target compound from a mixture of different substances—something that is common in industrial liquid processing.
Researchers tested their LGMs on a suspension of bentonite clay in water because this would mimic the wastewater produced by drilling activities in the oil and gas industry, they said.
To perform the tests, they created LGMs by infusing 25-millimeter discs of a standard filter membrane with perfluoropolyether, a type of liquid lubricant used in the aerospace industry. Then, they placed the LGMs under pressure to draw water through the pores but behind the nano-clay particles.
The final step was to compare the performance of the LGMs to untreated membranes. What they found was that untreated membranes displayed signs of nano-clay fouling much more quickly than the LGMs, researchers said.
The LGMs also could filter water three times longer than the standard membranes before requiring a procedure, or a “backwash,” to remove particles that had built up on the membrane. This is important to saving energy and money in water-filtration processes—as well as improving them—because less frequent backwashing also means a reduction in the use of cleaning chemicals and energy required to pump backwash water, researchers said.
“The filtration studies of a polydisperse nanoclay suspension reported here demonstrate that microfiltration membranes with liquid gating are capable of reducing transmembrane pressure, significantly increasing operational lifetime and cumulative throughput, improving the efficiency of backwashing processes and minimizing irreversible fouling build-up,” Alvarenga said.
A Number of Uses
The team envisions the use of LGMs in industries as diverse as food and beverage processing, biopharmaceutical manufacturing, textiles, paper, pulp, chemical, and petrochemical, they said. Researchers published a paper on their work in the journal APL Materials.
The team plans to move forward with larger-scale pilot studies with industry partners, longer-term operation of the LGMs, and tests to demonstrate how the membranes filter an even more complex mixture of substances. Ultimately, researchers aim to have comprehensive insight into the commercial viability of LGMs for various applications, as well as knowledge of how long they can last in different use cases.
Elizabeth Montalbano is a freelance writer who has written about technology and culture for 20 years. She has lived and worked as a professional journalist in Phoenix, San Francisco, and New York City. In her free time, she enjoys surfing, traveling, music, yoga, and cooking. She currently resides in a village on the southwest coast of Portugal.
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