How to do a better job of cleaning salt water? Keeping desalination tools dry

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The stream of hot, high salinity water is on one side of the membrane and a stream of pure, cool water is on the other. the heated water evaporates, leaving the salt behind and forming a vapor, which turns into a distilled liquid form when it meets the stream of cold water on the other side of the membrane. Credit: Allyson McGaughey.

Growing up in Seattle, Allyson McGaughey, Ph.D. of USC Viterbi School of Engineering. ’21, has never been confronted with the daily reality of drought. In the increasingly hot and drier Los Angeles desert, however, the water scarcity has been exposed, heightening the urgency of finding alternative water solutions.

In research published in the Journal of Membrane Science, McGaughey, in coordination with Amy Childress, USC Emeritus Professor Viterbi Gabilan, revealed new ideas on how best to design water purification processes, for example treating wastewater in a treatment facility of water, using membrane distillation (MD). MD is a process that separates salt from water using a thin, dry, porous membrane. Moderate temperature differences cause the water to move from one side to the other.

To understand this better, think of a spaghetti strainer, but with much, much smaller holes. A stream of water poured through the strainer will be “cleaned” of some material in the water that is too large to fit through the holes in the strainer (like the pores of a membrane), leaving a “clean” stream. on the other side of the strainer. However, anything smaller than these holes, like the salt dissolved in the water in our pasta, can still pass. To purify even more, what if we could only recover steam, or pure water vapor? Now imagine a colander that only allows steam, not liquid water, to pass through the holes. Then even dissolved salts cannot pass. Using a highly hydrophobic (water-fearful) membrane that does just that, MD can be used to extract pure, desalinated water from contaminated streams.

According to the researchers, the success of membrane distillation relies heavily on membrane designs that can reduce or eliminate moisture build-up in the membrane. According to the researchers, if a membrane becomes wet, it can lose its effectiveness, compromising the quality of the treated water. To that end, McGaughey, now a postdoctoral fellow at Princeton University, studied how best to design membranes so that they don’t get excessively wet and successfully treat water, removing salt and contaminants and creating pure or high quality flow.

Among their main findings, McGaughey said, reducing the size of the membrane pores or increasing the thickness of the membrane itself can increase water resistance and delay or prevent contamination of the flow of water. ‘purified water.

The membranes are generally made of a hydrophobic or water resistant synthetic material with pores of 0.1 to 0.5 microns. McGaughey said that while other processes are generally more energy efficient than membrane distillation – for example, a process called reverse osmosis – in the case of saltier water streams, these more typical processes require pressure. great for forcing water molecules through the membrane. . Thus, making them less practical for dealing with very salty streams.

In contrast, membrane distillation allows more saline water to be purified more efficiently than with reverse osmosis and allows scientists to purify saltier wastewater which is usually removed because it cannot be effectively cleaned by traditional methods. water treatment.

The problem, McGaughey said, is that membranes that filter wastewater can get excessively wet. “In reverse osmosis, we use dense non-porous membranes so that only water molecules pass through, but in membrane distillation there are holes in the membranes that can allow contamination if they get wet,” she declared.

Optimizing membrane distillation to increase membrane water resistance

Desalination is inherently an expensive and energy intensive process due to the chemical properties of salt and water. Salt easily dissolves in water, creating bonds that are very difficult to break, the researchers said.

“If we had a choice, we wouldn’t desalinate at all,” said McGaughey, “but we need this water more and more. “

With membrane distillation, McGaughey said that a heated saline stream is placed on one side of a dry membrane and a stream of fresh, pure water is on the other. The temperature difference between the two streams is the driving force that moves the water from side to side. In order to separate pure water from salt and other contaminants, the water molecules in the salt stream change from a liquid to a vapor gas due to heat.

Inside the dry pores of the membrane there is a small air space that allows the collection of vapor, which occurs when salt water heats up and evaporates, passing through the membrane while leaving the salt behind. behind. Since the air gap is small, little heat is needed to turn salt water into vapor, which means you can use solar power to heat the salt liquid. The vapor represents the purified water or distillate, which on the other side of the membrane, is cooled – by cold water – and returns in liquid form.

The membrane’s resistance to liquid water, or wetting resistance, is essential to ensure that the distillate stream is actually purified rather than contaminated. When the membrane is wet, the liquid water mixes from the wastewater or saline stream with the stream of purified water, creating an output of inferior quality, perhaps even a water outlet that would not meet drinkability standards. .

Trying to understand how a membrane loses its wetting resistance at a fundamental level and how this can be avoided through the hydrophobicity of the membrane material and pore size is essential, the researchers said.

“We have membranes that are working now, but when you go up to extremely high salinities and you get salt precipitation on the surface of the membrane, that’s still a big challenge,” McGaughey said.

New challenges in water supply

“Managing high salinity waste streams is a major challenge, for example industrial waste streams,” McGaughey said.

“This [membrane distillation] will never be more energy efficient than reverse osmosis, but it can use solar thermal energy or low quality “waste” heat, which means it can rely on green energy. This means less carbon emissions than the electricity we use to drive reverse osmosis and it can also achieve higher salinity fluxes, ”she said.

Instead of a single process being a stand-alone solution, McGaughey said membrane distillation could be a complement to reverse osmosis, for example something you can use downstream (later in the process of treating l ‘water), after reverse osmosis treatment.

“Membrane distillation could be used on the waste salt water stream that comes out of reverse osmosis to maximize the use of available water,” she said.

McGaughey also said membrane distillation could also have applications in rural and non-electrified areas.


A student’s mission to protect the world’s most precious resource


More information:
Allyson L. McGaughey et al, Wetting indicators, modes and tradeoffs in membrane distillation, Journal of Membrane Science (2021). DOI: 10.1016 / j.memsci.2021.119947

Provided by the University of Southern California

Quote: How to clean salt water better? Keeping Desalination Tools Dry (2021, November 2) retrieved November 2, 2021 from https://phys.org/news/2021-11-job-salty-desalination-tools.html

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