The potentials of 3D-printed feed spacers in reducing the environmental footprint of membrane separation processes

Membrane-based separation technologies play a pivotal role in attenuating the freshwater shortage worldwide. Spiral wound membrane (SWM) modules which involve several membrane envelopes glued together and separated by feed spacers have been used widely in these technologies. The primary role of feed spacers in the SWM module is to create the flow channel and enhance surface mixing near the membrane surface to improve mass transfer. However, they cause higher feed channel pressure (FCP) drop, create localized dead zones, and often lead to increased fouling/biofouling. Many studies have utilized 3D printing technologies to improve the performance of feed spacers. In this article, a comprehensive review of the historical evolution of 3D printing of feed spacers and their role in reducing the environmental footprint of membrane separation processes is provided. The influence of the 3D-printed feed spacer's design on the hydraulic performance of membrane filtration, fouling/biofouling, and FCP drop are also extensively discussed. 3D printing showed great potential in the fabrication of innovative feed spacers for different membrane filtration processes. During the last few years, great advancements in 3D printing technologies led to faster and higher precision printers with microscale resolution (>1 μm). Novel feed spacers capable of enhancing mass transfer, reducing FCP drop, and minimizing biofouling became now possible. Such spacers can potentially reduce the energy consumption of the filtration process, thus, their environmental footprint. Nevertheless, 3D printing material toxicity and higher fabrication energy requirements are still environmentally concerning. Future studies should consider the chemical stability, toxicity, flexibility, and affordability of 3D-printed feed spacers as well as their integration into real membrane modules. Once these concerns are addressed, 3D-printed spacers will slowly move from the prototyping stage to replace the commercially available spacers and be entirely accepted for large-scale manufacturing.