New membrane filtering technology helps address water scarcity issues

The membrane could be used in advanced water treatment systems and other applications.

With the increase in population and industrial developments, water demands have also increased, leading to more severe water scarcity. This necessitates the use of nontraditional water sources, including wastewater, seawater, and brackish water.

In recent years, membrane technologies have emerged as the premier tool for water reuse and desalination due to their high energy efficiency, ease of operation, and compact design. Some of them – for example, desalination by reverse osmosis (RO) and wastewater treatment by membrane reactors – are already applied in industries at scale.

Despite their widespread implementation, RO systems have experienced long-standing limitations in performance related to membrane materials. Current membranes also poorly remove low molecular weight neutral solutes and are sensitive to degradation from oxidants used in water treatment.

Now, researchers at the University of Colorado Boulder (CU Boulder) have developed a new membrane water filtration system based around air bubbles that can help address water scarcity issues around the world.

Generally, membrane filters rely on pressure to force water through a sieve to separate water from unwanted particles and contaminants. The new membrane system uses a tiny layer of air bubbles to distill the water rather than sieve it. According to researchers, this change makes the new system more permeable and better at removing unwanted impurities than the common reverse osmosis systems.

Current state-of-the-art membranes are subject to a trade-off between water permeability and water-salt selectivity, where a gain in water permeability results in a loss in salt rejection. In contrast, the team observed membranes that rely on a gas-liquid phase change can increase permeability without sacrificing water-salt selectivity by decreasing the thickness of the air layer.

For proof-of-concept testing, the team fabricated air-trapping membranes using porous anodic aluminum oxide (AAO) substrates modified with a controlled hydrophobic coating.

Since separation occurs due to a gas-liquid phase change, researchers observed near-complete rejection of dissolved solutes, including sodium chloride, boron, urea, and N-nitroso dimethylamine. Membranes fabricated with sub-200-nm-thick air layers showed water permeabilities that exceed those of commercial membranes without sacrificing salt rejection. Also, they found the air-trapping membranes tolerate exposure to chlorine and ozone oxidants.

The team’s membrane technology could be used in advanced water treatment systems and other applications. “These can be utilized to purify water to a very high degree when it comes to desalination of seawater and in wastewater reuse efforts,” Assistant Professor Anthony Straub said in a statement. “We also have ongoing work with NASA to use these membranes to recycle water during space exploration and research missions.”

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