Prof. Hassan A. Arafat
Center for Membrane and Advanced Water Technology, Khalifa University, Abu Dhabi, United Arab Emirates
Feed spacers constitute an indispensible component of membrane modules. In addition to their main function as space holders for feed within the module, they also provide mechanical support to the membrane and induce feed turbulence, thus supressing both temperature and concentration polarization phnomena. However, commercial spacers have also been shown to constitute the point of origin for fouling within a membrane module, which is initiated at the dead zones created at the intersection of spacers filaments. These dead zone provide suitable sites for the deposition of organics, attachment of microbes for biofouling and nucleation for scaling.
In recent years, additive manufacturing, also known as three-dimensional (3D) printing, has been gaining momentum in several applications including aerospace, automotive industry and the medical field. Likewise, 3D printing has also gained attention as a promising fabrication pathway for several components of membrane-based systems for desalination and water treatment. The unique benefit of 3D printing over conventional manufacturing processes lies in its ability to fabricate structures with complex geometries that can be optimized for fluid flow, heat transfer, etc., based on the targeted application. Taking advantage of the advancements in 3D printing, several research groups around the world have designed and tested 3D-printed feed spacers with complex geometries for a range of membrane applications, including reverse osmosis (RO), ultrafiltration (UF) and membrane distillation (MD). The initial reported results from these groups were rather promising.
One class of complex geometries that were tested as designs for feed spacers are triply periodic minimal surfaces (TPMS). These geometries can be described mathematically such that they have no self-intersecting or enfolded surfaces. “Triply periodic” means that the structure can be patterned in the 3D space and “minimal surface” means that it locally minimizes surface area for a given boundary such that the mean curvature at each point on the surface is zero. Our research group has studied the use of these shapes as spacer designs for membrane applications over the last 4 years. We theorized that the interconnected maze-like pathways of TPMS structures would enhance turbulence through the feed channel, while the perfectly smooth minimal surface would minimize pressure drop, as well as reduce the available locations for the attachment of foulants. Our studies were carried out for both pressure-driven (RO and UF) and heat-driven membrane processes (MD). Selected TPMS spacer designs were fabricated via Selective Laser Sintering (SLS). We found that the use of TPMS spacers not only resulted in enhanced mass transfer through the membranes, but it also led to significantly curtailed fouling and pressure drop in the feed channels. The ability of the TPMS spacers to improve flux while maintaining low pressure drop translates to lower energy footprint of the membrane system. Likewise, their ability to reduce scaling and biofouling and to improve the efficiency of membrane cleaning will lead to significant reduction in the use of chemicals, which has an impact on both process and environmental costs.
Our finding, as well as those of other groups, open up new avenues for further application of 3D printing of feed spacers in a wide range of membrane-based water (and wastewater) applications. Still, technical and economic aspects of the 3D printing process have to be managed carefully, but the potential for this application is massive.