Researchers from Nanyang Technological University (NTU) and Singapore University of Technology and Design (SUTD) designed and fabricated innovative membrane feed spacers that can improve the overall performance of membrane filtration processes used in water treatments.
Conventionally made using traditional heat extrusion and welding methods, traditional feed spacers were not optimised to control membrane fouling – an inevitable phenomenon where contaminants are accumulated on the membrane surface. This is largely due to the fabrication process’ limitation in being able to produce geometrically complex designs.
Turning to 3D printing, the research team proposed a new sinusoidal design for spacers based on previous computational fluid dynamic (CFD) studies done on sinusoidal channels. By introducing perpendicular or slanted axial filaments to connect the sinusoidal transverse filaments, standalone layers of sinusoidal spacers can be fabricated and potentially integrated into spiral wound modules. Their research paper entitled “Fouling Mitigation in Reverse Osmosis Processes with 3D Printed Sinusoidal Spacers” was published in Water Research.
“The conventional mesh spacer design had inherent dead zones especially around the intersection nodes. These dead zones can promote membrane fouling, which is why new and innovative spacer designs are needed,” explained first author Koo Jing Wee, PhD student at NTU. “The sinusoidal spacers were proposed with this intention in mind because previous CFD studies showed that vortices were present near the peaks and valleys of the sinusoidal channels, which can disrupt the local dead zones within the area.”
In their study, they 3D printed conventional mesh spacer as well as their novel sinusoidal spacers using the Polyjet technique. The 3D printed spacers then underwent a series of extensive performance evaluations such as channel pressure loss, inorganic fouling and biofouling tests. Results indicated that the sinusoidal spacers successfully decreased membrane fouling by an additional 10% compared to conventional spacers for both inorganic fouling and biofouling tests. Optical coherence tomograph for inorganic fouling and membrane autopsy for biofouling also revealed interesting insights to the development of the foulant cake layer and showed the local dead zones within each spacer. Although channel pressure loss was slightly higher for the sinusoidal spacers, energy saved from reduced fouling (~165 kPa) still greatly outweighed the increased channel pressure loss (~1.5 kPa).
“In this work, we have demonstrated the superior membrane fouling capabilities of the 3D printed sinusoidal spacers compared to the conventional spacer. However, there is still untapped potential for further improvements,” highlighted principal investigator Associate Professor Chong Tzyy Haur from NTU. “Reducing the number of axial filaments or further optimisation to the sinusoidal wave amplitude can help reduce the channel pressure loss of the spacers. This will be an exciting area which can be further explored.”
“3D printing has redefined the fabrication of these complex membrane spacers, addressing challenges that were never before thought possible to solve. As we continue to leverage 3D printing for its agility and accuracy, it would also be interesting to look into innovative materials for the fabrication of spacers to enhance the antifouling properties of 3D printed spacers,” added co-author Professor Chua Chee Kai from SUTD.
Fouling mitigation in reverse osmosis processes with 3D printed sinusoidal spacers
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