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Feed spacers in reverse osmosis systems generate complex fluid flows that limit computational fluid dynamics (CFD) simulations to small length and time scales. That limits our ability to simulate mineral scaling and other membrane fouling phenomena, which occur over longer length and time scales. Thus motivated, we develop a reduced model that replaces the CFD simulation of the velocity field with an analytical model that mimics spacers. This focuses the remaining numerical effort on simulating the advection–diffusion equation governing solute transport. We motivate and validate the model with CFD simulations and bench-scale experiments of spacer filaments in three different arrangements, including cases of unsteady vortex shedding. We show that the model produces a roughly 10,000-fold speedup compared to CFD, and accurately reproduces CFD predictions of not only the average and maximum concentrations, but also the local concentration distribution along the membrane. We also demonstrate the model for simulating a feed channel with a length-to-height ratio of 200. The model provides a simple testbed for exploratory studies of multispecies transport, precipitation, and membrane fouling phenomena for which simulating spacers is often prohibitive.

Biofouling is an ongoing challenge for water treatment membrane processes. Reducing biofilm growth on the membrane surface or on the polymeric feed spacer will reduce operation, maintenance, and module replacement costs. Laser‐induced graphene (LIG) is a low cost, environmentally friendly, electrically conductive carbon material shown to have antibiofouling properties. Here it has been shown that an electrically conductive LIG‐coated polypropylene (PP) feed spacer has both antimicrobial and antifouling effects under a low electrical current, and when implemented into a spiral wound membrane module reduced biofilm growth on both the membrane and the spacer components. The antibacterial property of the LIG spacer is tested using Pseudomonas aeruginosa and the brackish water Rheinheimera sp. as model organisms. Using a voltage of 12 V, P. aeruginosa is completely inactivated in 10 h, while a dynamic accumulation assay employing Rheinheimera sp. showed significant reduction (p < 0.05) in bacterial adhesion compared to an uncoated spacer. The spacer is incorporated into a spiral wound reverse osmosis (RO) membrane module, and reduced biofouling is observed on both the membrane and LIG spacers components using brackish water and 12 V. This study demonstrates the feasibility of electrically conductive feed spacer components in spiral wound RO membrane modules. [ABSTRACT FROM AUTHOR]

In this study, the impact of ferric ions on gypsum crystallization and NF membrane performance was investigated to mitigate iron-accelerated gypsum scaling. Gypsum scaling in the presence of ferric ions led to a significant decline in the water flux, but a relatively higher reversible fouling ratio was observed compared to that under individual gypsum and iron-oxide scaling. Ferric ions influenced the crystallization of gypsum on NF membranes by serving as additional nucleation sites, leading to accelerated gypsum scaling. Various mitigation methods (e.g., feed spacers and cleaning processes) were evaluated to effectively control iron-accelerated gypsum scaling. The introduction of spacers exhibited a high mitigation efficiency by enhancing the mass transfer of scalant ions due to changes in the water channel hydrodynamics, but the overall efficiency decreased over time. To enhance the mitigation efficiency, different cleaning methods (e.g., CO 2 oversaturated solution, HCl, and NaCl flushing) were conducted with feed spacers. A CO 2 saturated solution demonstrated high cleaning efficiency owing to the high turbulence induced by CO 2 bubbles, whereas HCl flushing showed negligible efficiency. NaCl flushing exhibited the highest cleaning efficiency through mono/divalent ion-exchange effects on iron-gypsum scaling. This study provides comprehensive insights into iron-accelerated gypsum scaling mitigation strategies for nanofiltration membranes, highlighting the impact of coexisting ferric compounds on gypsum scaling. [Display omitted] • The impact of gypsum, iron oxide, and combined scaling on NF performance was investigated. • The presence of ferric ion significantly accelerated gypsum crystallization on NF membrane. • Feed spacers effectively mitigated scaling but had limited efficiency over a long period. • CO 2 oversaturated flushing exhibits high mitigation efficiency through bubble formation. • NaCl flushing demonstrated superior mitigation efficiency via ion-exchange effects. [ABSTRACT FROM AUTHOR]

Empirical correlations for mass transfer coefficient and friction factor are often used in process models for reverse osmosis (RO) membrane systems. These usually involve four dimensionless groups, namely Reynolds number (Re), Sherwood number (Sh), friction factor (f), and Schmidt number (Sc), with the associated coefficients and exponents being obtained by fitting to experimental data. However, the range of geometric and operating conditions covered by the experiments is often limited. In this study, new dimensionless correlations for concentration polarization (CP) modulus and friction factor are presented, which are obtained by dimensional analysis and using simulation data from computational fluid dynamics (CFD). Two-dimensional CFD simulations are performed on three configurations of spacer-filled channels with 76 combinations of operating and geometric conditions for each configuration, covering a broad range of conditions encountered in RO membrane systems. Results obtained with the new correlations are compared with those from existing correlations in the literature. There is good consistency in the predicted CP with mean discrepancies less than 6%, but larger discrepancies for pressure gradient are found among the various friction factor correlations. Furthermore, the new correlations are implemented in a process model with six spiral wound modules in series and the predicted recovery, pressure drop, and specific energy consumption are compared with a reference case obtained by ROSA (Reverse Osmosis System Analysis, The Dow Chemical Company). Differences in predicted recovery and pressure drop are up to 5% and 83%, respectively, highlighting the need for careful selection of correlations when using predictive models in process design. Compared to existing mass transfer correlations, a distinct advantage of our correlations for CP modulus is that they can be directly used to estimate the impact of permeate flux on CP at a membrane surface without having to resort to the film theory.

Adjustment of feed spacers holds the potential to effectively address the inherent trade-off between permeate flux, anti-fouling capabilities, and feed channel pressure (FCP) drop, consequently enhancing the filtration performance of spiral wound modules (SWM). In this study, a novel automated spacers design approach based on response surface methodology and multi-objective genetic algorithm was successfully developed and experimentally verified, for the first time. By establishing the correlation between spacer parameters and key performance parameters of nanofiltration SWM (average wall shear, specific energy consumption, etc.), the design method facilitated the screening and optimization of the optimal spacer parameters by manipulating the variables spacer parameters to locally modify the spacer shapes and then to improve filtration performance. Subsequent simulations and experiments both demonstrated that, in contrast to the traditional spacer, the optimized spacer exhibited remarkable efficacy in striking a balance between membrane performance and energy consumption. Specifically, flux attenuation was reduced by 8 %, and minimum wall shear increased by 57.9 %, resulting in a significant reduction in membrane fouling. This report presents a small-scale and practical approach to spacer's local design, offering a valuable solution to address the inherent constraints posed by the interplay among permeation, anti-fouling, and FCP drop. [Display omitted] • An automated object-oriented and accurate spacer design method is developed. • The method is derived from response surface methodology and multi-objective genetic algorithms. • The optimized NF process improved filtration performance comprehensively. [ABSTRACT FROM AUTHOR]

In this proof of concept study, 3D printed feed spacers with complex geometries based on triply periodic minimal surfaces (TPMS) were designed and tested in reverse osmosis (RO) and ultrafiltration (UF) processes. The spacers showed flux enhancement of 15.5% and 38% in brackish water RO and UF tests with sodium alginate solution, respectively, in comparison to a commercial feed spacer. Moreover, lower feed channel pressure drop was also observed for the TPMS spacers. Biofouling tests were performed and the membranes were characterized using total organic carbon (TOC) and fluorescence microscopy. The TPMS spacers yielded a reduction in biofouling when compared to commercial feed spacers. Fouling patterns on the membranes were visualized for the different spacers using crystal violet stain, which also revealed a significantly reduced biofilm deposition using the TPMS spacers. The TPMS-based feed spacers have shown great promise in enhancing both RO and UF membrane processes, both in terms of flux enhancement and fouling reduction. [ABSTRACT FROM AUTHOR]

Reverse osmosis (RO) is the most widely used technology to desalinate brackish water and seawater. Significant efforts have been made in recent decades to improve RO efficiency. Feed spacer geometry design is a very important factor in RO membrane performance. In this work, correlations based on computational fluid dynamics and experimental work were applied in an algorithm to simulate the effect of different feed spacer geometries in full-scale brackish water reverse osmosis (BWRO) membranes with different permeability coefficients. The aim of this work was to evaluate the impact of feed spacers in conjunction with the permeability coefficients on membrane performance. The results showed a greater impact of feed spacer geometries in the membrane with the highest water permeability coefficient (A). Studying only one single element in a series, variations due to feed spacer geometries were observed in specific energy consumption ( S E C ) and permeate concentration ( C p ) of about 6.83% and 10.42%, respectively. Allowing the rolling of commercial membranes with different feed spacer geometries depending on the operating conditions could optimize the RO process.

Biofouling studies addressing biofouling control are mostly executed in short-term studies. It is unclear whether data collected from these experiments are representative for long-term biofouling as occurring in full-scale membrane systems. This study investigated whether short-term biofouling studies accelerated by biodegradable nutrient dosage to feed water were predictive for long-term biofouling development without nutrient dosage. Since the presence of a feed spacer has an strong effect on the degree of biofouling, this study employed six geometrically different feed spacers. Membrane fouling simulators (MFSs) were operated with the same (i) membrane, (ii) feed flow and (iii) feed water, but with feed spacers varying in geometry. For the short-term experiment, biofilm formation was enhanced by nutrient dosage to the MFS feed water, whereas no nutrient dosage was applied in the long-term experiment. Pressure drop development was monitored to characterize the extent of biofouling, while the accumulated viable biomass content at the end of the experimental run was quantified by adenosine triphosphate (ATP) measurements. Impact of feed spacer geometry on biofouling was compared for the short-term and long-term biofouling study. The results of the study revealed that the feed spacers exhibited the same biofouling behavior for (i) the short-term (9-d) study with nutrient dosage and (ii) the long-term (96-d) study without nutrient dosage. For the six different feed spacers, the accumulated viable biomass content (pg ATP.cm–2) was roughly the same, but the biofouling impact in terms of pressure drop increase in time was significantly different. The biofouling impact ranking of the six feed spacers was the same for the short-term and long-term biofouling studies. Therefore, it can be concluded that short-term accelerated biofouling studies in MFSs are a representative and suitable approach for the prediction of biofouling in membrane filtration systems after long-term operation [ABSTRACT FROM AUTHOR]

Feed spacers are used in spiral wound reverse osmosis (RO) membrane modules to keep the membrane sheets apart as well as to enhance mixing. They are beneficial to membrane performance but at the expense of additional pressure loss. In this study, four types of feed spacer configurations are investigated, with a total of 20 geometric variations based on commercially available spacers and selected filament angles. The impact of feed spacer design on membrane performance is investigated by means of three-dimensional (3D) computational fluid dynamics (CFD) simulations, where the solution-diffusion model is employed for water and solute transport through RO membranes. Numerical simulation results show that, for the operating and geometric conditions examined, fully woven spacers outperform other spacer configurations in mitigating concentration polarisation (CP). When designed with a mesh angle of 60°, fully woven spacers also deliver the highest water flux, although the associated pressure drops are slightly higher than their nonwoven counterparts. Middle layer geometries with a mesh angle of 30° produce the lowest water flux. On the other hand, spacers with a mesh angle of 90° show the lowest pressure drop among all the filament arrangements examined. Furthermore, the computational model presented here can also be used to predict membrane performance for a given feed spacer type and geometry. [ABSTRACT FROM AUTHOR]

Feed spacers are an essential part of spiral-wound reverse osmosis (RO) and nanofiltration (NF) membrane modules. Geometric modification of feed spacers is a potential option to reduce the impact of biofouling on the performance of membrane systems. The objective of this study was to evaluate the biofouling potential of two commercially available reference feed spacers and four modified feed spacers. The spacers were compared on hydraulic characterization and in biofouling studies with membrane fouling simulators (MFSs). The virgin feed spacer was characterized hydraulically by their resistance, measured in terms of feed channel pressure drop, performed by operating MFSs at varying feed water flow rates. Short-term (9 days) biofouling studies were carried out with nutrient dosage to the MFS feed water to accelerate the biofouling rate. Long-term (96 days) biofouling studies were done without nutrient dosage to the MFS feed water. Feed channel pressure drop was monitored and accumulation of active biomass was quantified by adenosine tri phosphate (ATP) determination. The six feed spacers were ranked on pressure drop (hydraulic characterization) and on biofouling impact (biofouling studies). Significantly different trends in hydraulic resistance and biofouling impact for the six feed spacers were observed. The same ranking for biofouling impact on the feed spacers was found for the (i) short-term biofouling study with nutrient dosage and the (ii) long-term biofouling study without nutrient dosage. The ranking for hydraulic resistance for six virgin feed spacers differed significantly from the ranking of the biofouling impact, indicating that hydraulic resistance of clean feed spacers does not predict the hydraulic resistance of biofouled feed spacers. Better geometric design of feed spacers can be a suitable approach to minimize impact of biofouling in spiral wound membrane systems. [ABSTRACT FROM AUTHOR]

The membrane research community has witnessed a significant research growth on feed spacers since 2010, both in quantity and scope. Increasingly novel spacer geometries and spacer chemistries are being reported in literature to tackle key challenges facing membrane processes, including concentration polarization, energy consumption, low flux, and fouling. This review aims to analyze the recent developments of spacer research, starting with a review of the various roles spacers can and do play in membrane systems and their increased prominence in tackling membrane fouling and scaling. We then review the continued need for novel spacer designs, discuss the contribution spacers can make to energy saving, and the necessity for new generations of spacers to facilitate the deployment of next generation, ultra-permeable membranes. Qualitative and quantitative spacer performance assessment tools are described. The key trends in spacer modifications over the period of 2010 to 2021 are categorized into: i) design modifications of both conventional and novel geometries, and ii) spacer surface modifications, including chemical coatings and new spacer materials. Finally, the future trends of spacer research are highlighted, with an emphasis on the development of functionalized feed spacers and new spacers made via 4D printing and smart materials. • Research progress in feed spacer development and enhancement is presented. • Quantitative and qualitative tools for spacer performance assessment are discussed. • Spacer improvement discussed based on novel geometries and surface modification. • Future perspective and emerging applications of feed spacers are highlighted. • Role of functionalized spacers, 4D printing and smart materials is emphasized. [ABSTRACT FROM AUTHOR]

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. [Display omitted] • Comprehensive review of 3D-printed feed spacers for membrane separation processes. • Evolution of 3D printing methods enabled the fabrication of intricate feed spacers. • Spacer's geometric alteration has large influence on mass transfer and biofouling. • Innovative feed spacers can reduce the energy consumption of membrane separation. • Future research should consider feed spacers' chemical stability and affordability. [ABSTRACT FROM AUTHOR]

Feed spacers are net-like structures present in spiral wound membrane modules (SWM) used for treatment of water and wastewater. Feed spacers require appropriate stiffness to support the membrane sheets without damaging and puncturing the membrane surfaces. They also need to be flexible enough to be rolled up around the central permeate tube forming the SWM. Polypropylene (PP) is the commercially used material for feed spacers due to its flexibility and excellent chemical resistance properties. In this paper, selective laser sintering (SLS) is used to investigate the printability of net-typed structures using PP materials to represent feed spacers. SLS processing parameters such as layer thickness, part bed temperature, energy density and scan pattern were studied and net-typed PP spacers were successfully fabricated. However, an analysis on tensile test and dimensional accuracy shows that Young’s modulus of the PP material tends to be correlated to the accuracy of the dimensions of the net-typed spacer prototypes. [ABSTRACT FROM PUBLISHER]

The key to developing highly efficient spiral-wound modules is the improvement of the mass transfer mechanisms. In this study a study of the mass transfer has been carried out using a flat test cell with six permeate outlets and a rectangular feed channel. Using this experimental set-up, it has been shown that the mass transport along the membrane is not fully described by the Sherwood correlation, which describes a decreasing mass transfer with an increasing distance from the inlet. It was observed that in open channel without spacers, the slope of the Sherwood correlation is decreasing by increasing pressure. This effect is combined with regular stripes on the membrane depending on the crossflow velocity and pressure when using coloured particles. An explanation could be a mass transfer perpendicular to the flow direction due to vortices, which is not covered by the Sherwood correlation. This phenomenon was also observed in spacer-filled channels. In this case the stripes on the surface depended on the spacer geometry. Furthermore, the experiments were used to calculate the energy consumption vs. the mass transfer coefficient for different spacers. This research can be used as a foundation to develop a deeper understanding of the effect of concentration polarisation during the filtration and to evaluate a sufficient strategy for the development of a spacer. [Copyright &y& Elsevier]

In this study, process enhancements achieved by the use of 3D printed feed spacers based on triply periodic minimal surfaces (TPMS) were comparatively assessed using the two most common membrane distillation (MD) configurations: air gap (AGMD) and direct contact (DCMD). The MD performance was assessed based on the impact of spacer design (TPMS vs. commercial spacer) on flux and feed channel pressure drop, and their consequent impact on the levelized cost of water (COW). The studied spacer architectures led to a minimal improvement (≤17%) in AGMD flux, much lower than that achieved in DCMD (≤57%). Consequently, for a waste heat-operated MD process, the spacer-induced channel pressure drop became the most influential cost bottleneck on COW. Generally, the contribution of the pumping cost to the total operating cost was found to be greater for DCMD than AGMD. Thus, the COW of a waste heat operated DCMD is more sensitive to a decrease in spacer-induced channel pressure drop than in an AGMD. For an MD process operated with additional heat cost, the flux improvement achieved using TPMS spacers reduces the thermal energy component of the operating cost. Ultimately, this predominantly contributes to a lower COW. Unlabelled Image • 3D printed feed spacers in AGMD achieves minimal flux improvement over DCMD. • Feed channel pressure drop can be reduced by 50% in AGMD with 3D printed spacer. • Spacer-induced channel pressure drop is a cost bottleneck for waste-heat-operated MD. • Increased flux by a 3D printed spacer lowers an MD's COW with additional heat costs. [ABSTRACT FROM AUTHOR]

Empirical correlations for mass transfer coefficient and friction factor are often used in process models for reverse osmosis (RO) membrane systems. These usually involve four dimensionless groups, namely Reynolds number (Re), Sherwood number (Sh), friction factor (f), and Schmidt number (Sc), with the associated coefficients and exponents being obtained by fitting to experimental data. However, the range of geometric and operating conditions covered by the experiments is often limited. In this study, new dimensionless correlations for concentration polarization (CP) modulus and friction factor are presented, which are obtained by dimensional analysis and using simulation data from computational fluid dynamics (CFD). Two-dimensional CFD simulations are performed on three configurations of spacer-filled channels with 76 combinations of operating and geometric conditions for each configuration, covering a broad range of conditions encountered in RO membrane systems. Results obtained with the new correlations are compared with those from existing correlations in the literature. There is good consistency in the predicted CP with mean discrepancies less than 6%, but larger discrepancies for pressure gradient are found among the various friction factor correlations. Furthermore, the new correlations are implemented in a process model with six spiral wound modules in series and the predicted recovery, pressure drop, and specific energy consumption are compared with a reference case obtained by ROSA (Reverse Osmosis System Analysis, The Dow Chemical Company). Differences in predicted recovery and pressure drop are up to 5% and 83%, respectively, highlighting the need for careful selection of correlations when using predictive models in process design. Compared to existing mass transfer correlations, a distinct advantage of our correlations for CP modulus is that they can be directly used to estimate the impact of permeate flux on CP at a membrane surface without having to resort to the film theory. [ABSTRACT FROM AUTHOR]

In this study, the impacts of three feed spacer design parameters; thickness, porosity and architecture, on the cleaning efficiency of ultrafiltration (UF) membranes were studied, both in backwash and relaxation cleaning modes, using sodium alginate solution dosed with CaCl 2 as the process feed. Both commercial, net-type spacers and 3D-printed spacers based on triply periodic minimal surfaces (TPMS) architectures were utilized in the study. Increased spacer thickness and porosity were found to increase the cleaning resistance of the membrane, while TPMS spacers were found to yield a lower cleaning resistance than net-type spacers, even with the latter being thicker. The root causes of these observations were analyzed based on resistance analysis of the fouling layer on the membranes. To do that, overall, residual, reversible and irreversible fouling resistances were quantified at the end of a 20-cleaning cycle test. Statistical correlations were then established between these resistances and spacer properties. The results showed that the consequential impacts of spacer design on the shear stresses, created by the feed flow on the fouling layer, underscore the observed fouling mechanism and the consequent cleaning efficiency. The spacer design can impact these shear stresses through its hydraulic diameter and spacer-membrane contact area. When the spacer design leads to lower shear stresses, a cake filtration mechanism prevails, a more irreversible fouling occurs and more challenging cleaning becomes. A recently published work on combined intermediate pore blockage and cake filtration model for fouling was used to construe these observations. • Impacts of spacer properties on membrane backwash and relaxation is assessed. • Net-type spacers and triply periodic minimal surface (TPMS) spacers studied. • Statistical correlation between spacer design and fouling resistances determined. • Effect of spacer architecture on pore blockage and irreversible fouling evaluated. • Relationship between spacer design and fouling mechanism established. [ABSTRACT FROM AUTHOR]

Reverse osmosis is the leading process in seawater desalination. However, it is still an energy intensive technology. Feed spacer geometry design is a key factor in reverse osmosis spiral wound membrane module performance. Correlations obtained from experimental work and computational fluid dynamics modeling were used in a computational tool to simulate the impact of different feed spacer geometries in seawater reverse osmosis spiral wound membrane modules with different permeability coefficients in pressure vessels with 6, 7 and 8 elements. The aim of this work was to carry out a comparative analysis of the effect of different feed spacer geometries in combination with the water and solute permeability coefficients on seawater reverse osmosis spiral wound membrane modules performance. The results showed a higher impact of feed spacer geometries in the membrane with the highest production (highest water permeability coefficient). It was also found that the impact of feed spacer geometry increased with the number of spiral wound membrane modules in series in the pressure vessel. Installation of different feed spacer geometries in reverse osmosis membranes depending on the operating conditions could improve the performance of seawater reverse osmosis systems in terms of energy consumption and permeate quality. [ABSTRACT FROM AUTHOR]

Reverse osmosis (RO) is the most widely used technology to desalinate brackish water and seawater. Significant efforts have been made in recent decades to improve RO efficiency. Feed spacer geometry design is a very important factor in RO membrane performance. In this work, correlations based on computational fluid dynamics and experimental work were applied in an algorithm to simulate the effect of different feed spacer geometries in full-scale brackish water reverse osmosis (BWRO) membranes with different permeability coefficients. The aim of this work was to evaluate the impact of feed spacers in conjunction with the permeability coefficients on membrane performance. The results showed a greater impact of feed spacer geometries in the membrane with the highest water permeability coefficient (A). Studying only one single element in a series, variations due to feed spacer geometries were observed in specific energy consumption ( S E C ) and permeate concentration ( C p ) of about 6.83% and 10.42%, respectively. Allowing the rolling of commercial membranes with different feed spacer geometries depending on the operating conditions could optimize the RO process. [ABSTRACT FROM AUTHOR]