Feed spacer is one of the critical components of the spiral wound membrane modules, which effectively influences the overall filtration performance1,2. It has a pivotal role in ensuring the mechanical support to the membrane surface and help the fluid flow between the membrane sheets while aiding in minimizing the concentration polarization/(bio)fouling by promoting fluid unsteadiness2–4 inside the filtration channel5,6. However, inappropriate spacer designs are known to aggravate (bio)fouling and ultimately increase channel pressure drop, resulting in deteriorating the performance of the filtration system3,7−10. The formation of biofilm, which is the proliferation of bacteria within extracellular polymeric substances (EPS) matrix, is reputed to be so far the inevitable impediment in water technologies11,12. Therefore, the ability of biofilm mitigation constitutes a crucial trait to consider for designing a pertinent feed spacer. A maximal mass transfer while simultaneously reducing pressure drop and (bio)fouling development is then the ultimate engineering challenge to design an optimal feed spacer for different filtration techniques.
In the past, research efforts were focused on modestly modifying the geometric parameters of commercial spacer such as spacer thickness, filament diameter, mesh size, internal filament angle, and spacer orientation to amend the hydrodynamics conditions within the feed channel8,13−19. The commercial spacer design, which consists of two layers of non-woven quasi-cylindrical filaments, creates a narrow spacer-filled channel (low porosity) that intensifies the pressure drop17,18. Moreover, asymmetric spacer filament intersections created due to non-woven design promote local dead zones20,21, which are recognized as favorable areas for (bio)fouling growth22. An intricate hydrodynamics nature is further identified due to unsymmetrical shear stress distribution on either side of the membrane surface along the non-uniform filaments. Also, the inability to trip hydrodynamics to unsteady state (near plant operational cross-flow velocities of 0.165 m/s23) adds fundamental limitations for the commercial spacer designs9.
Attempts have been concentrated on developing novel feed spacer designs to eliminate the drawbacks of the commercial type spacer design4,22,24−28. Multilayer spacers revealed a more significant mass transfer and long-term anti-fouling propensity compared to a net-type spacer25,28. Triangular filament spacer shape was found to outperform square and circular filament spacers in terms of concentration polarization reduction24. Symmetric spacer with spherical nodes (filament intersections) demonstrated its capacity to produce high shear stress distribution and mass transfer rate22. More complex spacer designs like triply periodic minimal surfaces (TMPS)26, vibrating spacers27, turbo-spacer using rotating turbines29, and helical-type spacers4 have shown promise in reverse osmosis (RO) and ultrafiltration (UF) processes. Although these proposed novel spacer designs indicate favorable progress for enhanced filtration membranes, several constraints are still encountered and hamper their commercialization and industrial implementation. It includes the design complexity26,29, the handling difficulty27,29, the risk of membrane deformation/damage24,25,30, and the high energy requirements22,25,28.
Recently, promising novel spacers have been developed by our research group and have shown an attractive industrial interest for scale-up21,31,32. These feed spacers are characterized by the introduction of perforations (perforated spacers)31,32 or column nodes (column-type spacer)21 in their designs. Their performances were numerically and experimentally evaluated in a cross-flow UF process. The most efficient design was demonstrated with spacer having perforations only at the filament intersections31. Perforated spacer design helped to lower the pressure drop and induce more unsteadiness within the filtration channel, resulting in enhanced permeate flux production and fouling mitigation. Likewise, the column-type feed spacer led to simultaneously reduce three times the pressure drop, increase two times the specific flux, and minimize the fouling accumulation when compared to the standard symmetric spacer21.
The present study was inspired by the encouraging outcomes achieved by the perforated and column (referred to as pillar hereafter) spacers21,31. The objective was to combine their advantages and geometric features to develop a novel feed spacer design for enhanced filtration processes, which is easily manufactured in an industrial scale. Consequently, symmetric pillar feed spacer with perforations (hole-pillar spacer) is introduced as a novel design. The proposed design can induce higher channel porosity, control localized hydrodynamics, and evenly distribute the shear stress inside the filtration channel. All these design features are aimed to improve the filtration performance, reduce the pressure drop, and mitigate the (bio)fouling accumulation on the membrane surface, which is an ultimate challenge for feed spacer design used in different filtration technologies including UF, nanofiltration (NF), and RO.
Three feed spacers were in-house designed and 3D-printed: commercial, pillar, and hole-pillar spacers. We first comprehensively evaluated these spacer designs by direct numerical simulations (DNS) to study the localized hydrodynamics. Later, the effect of spacer design (commercial, pillar, and hole-pillar spacers) on the filtration performance including pressure drop measurement, permeate flux analysis, and Optical Coherence Tomography (OCT) characterization for biofilm growth, is experimentally investigated in UF cross-flow filtration for biologically active feed at two different operating pressures. Finally, potential outcomes from this study and its perspectives are discussed.