Filtering water is an incredibly important process, but it's not as simple as it seems. As the pores in a membrane inevitably get clogged with material, it takes time and energy to clean or replace them. Now researchers have tested a new nature-inspired membrane that filters liquids using other liquids, making for a more efficient and longer-lasting membrane.
The basic concept of filter is pretty straight-forward – to remove particles from water, simply run it through a membrane full of pores that are big enough to let water molecules pass, but too small for other particles to get through. The problem there is that after a while, the particles tend to build up on the surface of the membrane and in the pores, in a process called fouling.
As this fouling builds up, the water needs to be pumped through the membrane with more force, eating up more energy in the process. Cleaning the membrane involves forcing cleaning chemicals backwards through it, but not only does that require putting the filtering process on hold, it can't get rid of all the build-up. In the end, membranes need to be replaced fairly frequently.
Enter the new filter design. Known as a liquid-gated membrane (LGM), the filter is coated with a liquid that acts like a reversible gate. This liquid fills the pores when closed, but when pressure is applied to the membrane the liquid moves to the sides, allowing the water to pass through. Other particles still can't fit through, but the liquid lining prevents them from building up on the surface or clogging the pores.
In nature, LGM-like systems can be seen in leaves, which essentially allow the plants to "breathe" in carbon dioxide.
For the new study, scientists from the Wyss Institute, Northeastern University and the University of Waterloo set out to test how well LGMs work. First they made LGMs by coating 25-mm-wide filter membranes with a widely-used liquid lubricant called perfluoropolyether. They then placed these LGMs into water polluted with nanoclay particles, which simulates the kind of wastewater produced by industrial drilling.
When compared to more conventional filters, the team found that LGMs were twice as energy efficient, and right from the start they needed 16 percent less pressure to get the water flowing. The LGMs also lasted almost three times longer before needing to be cleaned, and when they did they'd accumulated 60 percent less nanoclay in their pores, which is incredibly difficult to remove through normal cleaning techniques.
"This is the first study to demonstrate that LGMs can achieve sustained filtration in settings similar to those found in heavy industry, and it provides insight into how LGMs resist different types of fouling, which could lead to their use in a variety of water-processing settings," says Jack Alvarenga, first author of the study.
The team now plans to conduct larger-scale studies of LGMs, including filtering different kinds of liquids and particles, as they try to determine how well they might perform commercially.
The research was published in the journal APL Materials, and the filter can be seen in action in the animation below.
Source: Wyss Institute
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