Collaborators at the University of Waterloo and Northeastern University have joined forces for a study conducted by the Wyss Institute for Biologically Inspired Engineering at Harvard University to compare the transmembrane pressure between a standard filter and a liquid gated membrane filter.
According to the study, the use of liquid gated membranes filter nanoclay particles out of water with twofold higher efficiency, nearly threefold longer time-to-foul, and a reduction in the pressure required for filtration over conventional membranes, offering a solution that could reduce the overall cost, electricity costs and emissions output for the filtering of municipal and industrial wastewater, as well as other industries such as oil and gas.
“Our new approach harnesses dynamic and responsive control over a highly sensitive and reversible gating mechanism, which we can now apply toward many diverse applications,” said study co-author Joanna Aizenberg, in a Harvard announcement.
According to to the study, the typical method of processing water by passing it through a membrane with pores sized to filter particles larger than water molecules can be prone to clogging by the very materials it is designed to filter out. As a result, the process requires more energy to force water through a partially clogged membrane and frequent membrane replacement, both of which increase water treatment costs.
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How liquid gates membranes work
The liquid gated membranes mimic nature’s use of liquid-filled pores to control the movement of liquids, gases and particles through biological filters, using the lowest possible amount of energy. “Much like the small stomata openings in plants’ leaves allow gases to pass through,” the study reports. Each liquid gated membrane is coated with a liquid that acts as a reversible gate, filling and sealing its pores in the “closed” state.
“When pressure is applied to the membrane,” the study continues, “the liquid inside the pores is pulled to the sides, creating open, liquid-lined pores that can be tuned to allow the passage of specific liquids or gases, and resist fouling due to the liquid layer’s slippery surface. The use of fluid-lined pores also enables the separation of a target compound from a mixture of different substances, which is common in industrial liquid processing.”
The research team tested the liquid gated membranes on a suspension of bentonite clay in water to mimic the wastewater produced by drilling activities. They infused 25-mm discs of a standard filter membrane with perfluoropolyether, a type of liquid lubricant, to convert them into liquid. They then placed the liquid under pressure to draw water through the pores but leave the nanoclay particles behind, and compared their performance to untreated membranes.
The untreated membranes displayed signs of nanoclay fouling faster than the liquid gated membranes, with treated membranes able to filter water three times longer than the standard membranes.
“Less frequent backwashing could translate to a reduction in the use of cleaning chemicals and energy required to pump backwash water, and improve the filtration rate in industrial water treatment settings,” the study found.
The liquid gated membranes had a 60% reduction in the amount of nanoclay that accumulated within their structure during filtration, and required 16% less pressure to initiate the filtration process, reflecting further energy savings.
Wyss Institute at Harvard University has just posted a video animation that first compares the transmembrane pressure between a standard filter and a liquid gated membrane filter. The second part of the animation shows the tendency of each system to clog up due to fouling.