Filter membranes are vital to various industries around the world. They are made from a variety of materials such as cellulose, graphene, and nylon, and can be used as barriers to convert seawater into drinking water, separate and process milk and dairy products, and extract pollutants from wastewater. They are an essential technology for these and other industries, but they are plagued by a fatal weakness: dirt.
Membrane fouling occurs when particles deposit on the filter over time, clogging the system and limiting its effectiveness and efficiency. Efforts to clean or remove these membranes often rely on chemical processes in which synthetic solvent is pumped into the membrane to flush the system. However, this can lead to loss of productivity and profit, as well as environmental and workplace safety issues related to waste disposal.
The solution to this challenge may soon appear. A team of researchers from the Department of Mechanical Engineering at MIT found an alternative with the support of a seed fund from the MIT Abdul Latif Jameel Water and Food Systems Laboratory (J-WAFS). Their solution was developed through a unique collaboration between researchers with expertise in fluid and structural dynamics.
The team developed a new system that uses controlled deformation to mechanically clean the membrane. Their new method is the first ever to combine membrane and machinery. Compared with traditional membrane cleaning technology, it may be cheaper, faster, and more environmentally friendly, and is expected to completely change our perception of filtration.
"Fouling is the biggest problem facing membranes. Solving it will change the rules of the game for everyone," said Dr. Omar Labban from the Department of Mechanical Engineering '20, a new paper published in the Journal of Membrane Science. The co-lead author of.
The work began when the two mechanical engineering professors saw the potential of combining their areas of expertise. Abdul Latif Jameel Water and Mechanical Engineering Professor and J-WAFS Director John Lienhard collaborated with Professor Xuanhe Zhao and George N. Hatsopoulos faculty. Lienhard is an international expert in the field of water purification and desalination, while Zhao focuses on the field of soft materials.
"Real-world problems, such as membrane fouling, essentially cross disciplinary boundaries," Lienhard said. "In this case, we are facing the problems of soft matter mechanics and membrane desalination. Our team combines these different knowledge through a reliable experimental plan to achieve a more environmentally friendly cleaning process."
The paper detailing the team’s new approach was selected by the journal as the editor’s choice article in February 2021. The co-authors of the paper Lienhard, Zhao, and Labban also joined the ranks of co-lead authors and core research team members who promoted this work, Grace Goon PhD '20 of Aeronautics and Astronautics, and Zi Hao Foo, former visiting student and current mechanical engineering Postgraduate.
Filtered "Achilles' Heel"
Fouling is the process by which particles are deposited on the surface of the film. Although it occurs in any membrane filtration system, fouling is particularly troublesome for desalination. As a process input, sea water needs to remove more than just salt. Dirt, from bacteria to organic materials and minerals, can be collected on the reverse osmosis membrane very quickly. Once the membranes are clogged, their efficiency is reduced, which limits the amount of clean water that can be produced and the purity of the final product.
Unfortunately, current cleaning solutions are not ideal. Membrane used for seawater desalination is cleaned with chemicals, which requires time, money and resources, not the operation of a filter device. Operators of desalination plants usually have to stop production in order to flush the system for several hours during each cleaning cycle. For the dairy industry, operators need to clean the membrane several times a day. The chemical mixture used in the flushing system is usually proprietary, which makes desalination costs prohibitively high in some countries and cities. The environmental impact is also great, because factories must figure out how to deal with large amounts of chemical waste without causing ecological and toxicological problems.
Work together to find a cleaner solution
Driven by efficiency, affordability and environmental sustainability, the research team sought to develop a chemical-free solution based on the principles of mechanics. Goon is a member of the core research team. He recalled the early days when the team explored various vibration methods, including a three-dimensional system, to shake the dirt layer off the membrane. From there, they continued to try to change the pressure on both sides of the membrane to weaken the bonded fragments. Eventually, they were able to peel off the layer.
Their solution relies on a phenomenon called membrane fouling interface fatigue. Through subtle pressure changes, the team was able to gradually weaken and deform the fouling adhesive layer until it can be washed away. Due to the fragility of the membrane, previous research has deviated from this approach, “but we have shown that if you can really control it correctly, you can avoid damaging your membrane,” Goon said. Most importantly, this method can be used for industry-standard spiral wound membrane modules, where closely spaced membrane layers pose challenges for other mechanical cleaning methods.
Although the traditional chemical cleaning process may need to supplement this mechanical solution, this new method can reduce the user's dependence on chemical cleaning, which benefits plant operators in many ways. The team’s calculations show that downtime for cleaning will be reduced by six times. With fewer plant shutdowns, the total amount of clean water produced by the system will increase. "You will save costs, you will run the plant more, and you will get more output. When cleaning is no longer a burden on the operator, the system will operate in a better state in the long run," Labban explained .
These improvements have brought tangible benefits to both producers and consumers. During the field study, the team explored the market potential of the technology and talked with factory operators in multiple industries. They all expressed disappointment at the cumbersomeness of the cleaning process. As far as the dairy industry is concerned, the industry is already facing shrinking profits caused by the pandemic. The team estimates that switching to mechanical cleaning can cut cleaning costs by half.
The unique intersection of membrane technology and mechanical processes simulated by this technology provides many solutions that are considered impossible in the desalination field. "Suddenly, you can achieve more than before-the impact and change you can achieve becomes even greater," Labban said of the opportunity for multidisciplinary collaboration.
The project not only brings together two majors: the Department of Mechanical Engineering and the Department of Aerospace. Gabrielle Enns, Annetoinette Figueroa, Lara Ketonen, Hannah Mahaffey, Bryan T. Padilla, and Maisha Prome also joined the team through the MIT Undergraduate Research Opportunity Program. The unique perspective brought by each team member helps foster creativity and friendship around the laboratory.
Since the interdisciplinary research team uses an out-of-the-box approach, this work is not easy to obtain traditional funding mechanisms. This is why the J-WAFS seed fund is so influential. "Without J-WAFS, this work would not have happened," Labban said. The grant enables the research team to use the challenge as the main research catalyst, rather than being limited to specific technical processes or structured results. This allows the team to freely use cross-departmental cooperation to achieve the integration of mechanics and membrane research in the name of better filtration strategies.
The current paper mainly focuses on organic pollutants, and the technology has only been evaluated for a limited number of industries. However, looking forward to the future, the team is happy to expand its research by applying the method in various fields including the energy and agriculture sectors. As long as the membranes are used, they need to be cleaned. "We are very pleased to be able to solve the main bottlenecks of membranes and desalination," Labban said. "Nothing can beat this challenge."
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