Globally, the pressure on the world’s water sources is already high. Increases in population will only add to this urgency. Technological innovations are helping, but they, too, are struggling to keep up. Water desalination is one answer, but the process, to date, has proven to be prohibitively expensive and energy-intensive, so it’s reserved for only the direst shortages.
Water purification, including desalination, is frequently accomplished using reverse osmosis, or forcing water through cartridges that contain thin-film composite polyamide membranes. These membranes trap salt and other impurities, allowing fresh water to flow through. The more membrane that can be put into the cartridges that create pressure to force the water through, the more efficient the process will be. Advances in the design and creation of membranes could help bring down the complexity and costs involved in desalination.
Some researchers are looking to additive manufacturing, or 3D printing, for its potential to create membranes of almost any geometrically complex shape or feature. Researchers at the University of Bath (England) Centre for Advanced Separations Engineering (CASE) have focused on the potential for using additive manufacturing to improve on separation membrane engineering with the hopes of creating more precise designs than current fabrication methods allow.
Currently, membranes are primarily produced by a process called “interfacial polymerization,” which coats a thin dense layer of a polymer onto the surface of a support membrane (which is typically formed by phase inversion). Limitations to control over the architecture of the membranes mean that researchers cannot rigorously control the design of the membrane, but because the method has been studied extensively and is well understood, it remains the process of choice for creating membranes.
Research into 3D printing membranes for reverse osmosis is just beginning. Previous studies conducted have demonstrated the strength of additive manufacturing techniques in making complex 3D geometries with internal features for membrane applications. The key is complete freedom in the design process.
|CASE team (L-R) Dr Darrell Patterson; Research Associates, Yen Chua & Nicholas Low; and Professor Davide Mattia. Credit: University of Bath|
Dr. Darrell Patterson, director of the Centre for Advanced Separations Engineering (CASE) at the University of Bath, told Design News that 3D-printed membranes could have a number of advantages over traditionally fabricated membranes.
“New 3D-printed designs could reduce membrane fouling by printing on patterns or nature inspired designs to reduce fouling/concentration polarization and increase permeance through the membrane,” he said. “This is only realizable once the resolution and accuracy of the 3D printers can be bettered compared to what is currently possible.”
The team at CASE set out to review from the literature the advantages and disadvantages for membrane creation using all the current methods of additive manufacturing, including photopolymerization, powder, material extrusion, and lamination based on resolution, accuracy, build size, speed, printed materials, mechanical properties, support, and cost.
The researchers noted that 3D-printing technology hasn’t yet reached the point where it’s capable of producing large-scale membranes that will be cost-competitive with existing products, but their work highlights the technology’s potential.
Going forward, additive manufacturing could be used to create membrane structures designed to reduce fouling by selectively channeling the feed towards particular parts of the membrane and membrane material that separate a sub-set of molecules. Other parts could be printed from different materials with different properties that separate a different subset of molecules all in the same sheet. It may also be easier to accommodate surface texturing of the membrane, which current manufacturing methods engage in to produce membranes less apt to clogging.
Additive manufacturing could also address the problems associated with the high pressures used in reverse osmosis, particularly when desalination of the water is necessary: up to 50-80 bar depending on the salt concentration to be removed.
“Current membranes accommodate these [pressures] well,” Patterson told Design News. “I could imagine that 3D printing in the future being printed of material that is more mechanically robust and therefore is less deformed or affected by pressure compared to current materials.”
As with other additive manufacturing applications, rapid prototyping could also be an advantage in terms of speed, efficiency and customization when compared to traditional prototyping. The CASE team’s work found that 3D printing has the capability to produce not only the membrane, but also the spacers and the entire membrane module, which may reduce the overall production time.
In addition, an AM process for membrane creation could make it possible to control the composition of two or more materials across the surface and interface during fabrication, allowing positional variations in physical properties and characteristics such as multiple alternating layers of materials or selective distribution of one material on another to improve the performance of the membrane.
The researchers’ paper, “Perspective on 3D Printing of Separation Membranes and Comparison to Related Unconventional Fabrication Techniques,” was published in the February 2017 issue of the Journal of Membrane Science.