Search

The article discusses the global water crisis and the role of membrane technology, particularly reverse osmosis (RO), microfiltration (MF), and ultrafiltration (UF), in addressing water challenges by 2050. Membrane technology has evolved to provide safe, sustainable drinking water by treating unconventional water sources like seawater, brackish groundwater, and wastewater. The article highlights the advancements in RO membranes for desalination, the importance of MF and UF membranes for pathogen treatment, and the potential of other membrane technologies like electrodialysis (ED) and membrane distillation (MD) in water treatment by 2050. [Extracted from the article]

Membrane fouling is a major drawback of reverse osmosis (RO) membrane filtration as it negatively affects the quantity and quality of the treated water and the lifespan of the membrane. Spacers play a pivotal role in the hydrodynamics of the membrane channel thus influencing membrane fouling. Building on our previous work (Xie et al., Journal of Membrane Science, 2014, 453:92-99) where a series of novel sinusoidal spacers were built, we investigate here the spacers' ability to mitigate foulant deposition onto membrane surfaces. Experiments were performed and a three-dimensional computational fluid dynamics (CFD) model was built. The results of experiments with humic acid and calcium ions (a surrogate foulant recipe) showed that certain sinusoidal spacers had less fouling than mesh spacers. Furthermore, the fouling on membranes diminished as the tortuosity (ratio of amplitude to wavelength of the sinusoidal pattern) increased. CFD simulations explored the development of fouling over time and the CFD results were compared with scanned images of fouled membranes. The simulation results matched well with the experimental results and suggested that wall shear stress alteration by the spacers could be a mechanism for fouling reduction. [ABSTRACT FROM AUTHOR]

• CFD was used to investigate membrane systems with 3D spacers that are directly printed onto the membrane surface. • CP was studied to help understand the hydrodynamic influences of spacers. • Spacer geometries were optimized through pressure drop and CP studies. • Membranes with 3D printed spacers can reduce CP and improve permeate flux. In the recent decade, various printing technologies have been under development for producing spacers in reverse osmosis (RO) and nanofiltration (NF) modules. One manufacturing technology directly attaches discrete objects or features to the feed-side membrane surface. The novelty of this method is that tangential flow over the membrane is much less obstructed than with conventional feed spacers, and thus thinner spacers can be built. Within the same cylindrical module volume, more layers can be packed and more membrane surface area can be added to achieve higher permeate flow rates. The research goal of this paper is to design efficient models that help us discover the best parameters for discrete-object spacers. Spacer heights and spacer patterns are directly related to longitudinal pressure drop and concentration polarization (CP). Computational fluid dynamics (CFD) is useful in helping expedite the process of discovering the best designs with both low pressure drop and low CP. Simulations investigated spacer shapes including regular cylinders, elliptical cylinders, and airfoils. Ellipses with a length:width ratio of 2.4 while maintaining a between-feature distance of 6 mm were optimal for minimizing pressure drop. Spacer heights ranging from 200 µm to 500 µm were simulated to discover the height to achieve the same pressure drop as a 30 mil (0.762 mm) conventional spacer. Simulation results indicate that discrete-object spacers with elliptical design can greatly increase water productivity in a spiral-wound module by increasing packing capacity. These designs also reduce CP by 7% by improving the hydrodynamics, compared to empty channels. [ABSTRACT FROM AUTHOR]

Abstract: In bench-scale tests of seawater reverse osmosis desalination it is important to carefully consider osmotic pressure effects and determine the extent of concentration polarization so that sources of flux variation—whether from fouling, compaction, or osmotic pressure changes—can be properly assessed. Rigorous modeling of concentration polarization is difficult because of the complex geometries and flow regimes in RO modules; typically, concentration polarization must be measured. However, concentration polarization measurement usually requires knowledge of membrane permeability, which can vary from coupon to coupon. In this study a method is presented to determine both the membrane permeability and the concentration polarization regime in a single test. The key to the test is to allow the salt concentration to vary over time in a predictable way and extract parameters from a model fitted to the flux data. The usefulness of this technique is highlighted by evaluating results from several seawater experiments. It was found that specific flux decline in the experiments was caused by changes in osmotic pressure and membrane compaction. RO fouling by seawater organic-matter was not significant for the several seawater samples tested. [Copyright &y& Elsevier]

Forward osmosis (FO) and pressure retarded osmosis (PRO) are the two operational modes for osmotically driven membrane processes (ODMPs). ODMPs have gained increasing popularity in the laboratory over the years; however, OMDPs have not been applied in very many cases at full scale because they are still emerging technologies that require further development. Computational fluid dynamics (CFD) modeling coupled with solute transport evaluation provides a tool to study hydrodynamics and concentration polarization in FO and PRO. In this study a series of models were developed to predict water flux. The simulation results of empty-channel (with no feed spacer) membrane cells were verified by comparison with experimental results, showing that CFD simulation with solute transport is a reliable tool. Ensuing 2D and 3D models were built to study the impact of feed spacers on the velocity and concentration distribution inside the flow channels, and investigate whether the presence of spacers would enable enhancement of water flux. The results showed that spacers could change the concentration and velocity profile and they could reduce or enhance water flux depending on the inlet flow velocity and distance between the membrane and spacer. [ABSTRACT FROM AUTHOR]

Creating membranes with engineered surface features has been shown to reduce membrane fouling and increase flux. Surface feature patterns can be created by several means, such as thermal embossing with hard stamps, template-based micromolding, and printing. It has been proposed that the patterns create enhanced mixing and irregular fluid flow that increases mass transfer of solutes away from the membrane. The main objective of this paper is to explore whether enhanced mixing and improved mass transfer actually do take place for reverse osmosis (RO) membranes operated in laminar flow conditions typical of full-scale applications. We analyzed velocity, concentration, shear stress, and concentration polarization (CP) profiles for flat, nanopatterned, and micropatterned membranes using computational fluid dynamics. Our methods coupled the calculation of fluid flow with solute mass transport, rather than imposing a flux, as has often been done in other studies. A correlation between Sherwood number and mass-transfer coefficient for flat membranes was utilized to help characterize the hydrodynamic conditions. These results were in good agreement with the numerical simulations, providing support for the modeling results. Models with flat, several line and groove patterns, rectangular and circular pillars, and pyramids were explored. Feature sizes ranged from zero (flat) to 512 μm. The ratio of feature length, between-feature distance, and feature height was 1:1:0.5. Results indicate that patterns greatly affected velocity, shear stress, and concentration profiles. Lower shear stress was observed in the valleys between the pattern features, corresponding to the higher concentration region. Some vortices were generated in the valleys, but these were low-velocity flow features. For all of the patterned membranes CP was between 1% and 64% higher than the corresponding flat membrane. It was found that pattern roughness correlated with boundary layer thickness and thus the patterns with higher roughness caused lower mass transfer of solute away from the surface. Rather than enhancing mixing to redistribute solute, the patterns accumulated solute in valleys and behind surface features. Despite the elevated CP, the nominal permeate flux increased by as much as 40% in patterned membranes due to higher surface area compared to flat membranes. The advantageous results seen in other studies where patterns have helped increase flux may be caused by the additional surface area that patterns provide. • CFD was used to investigate membrane systems with surface features. • CP was studied to help understand the hydrodynamic influences of patterning. • Patterned membranes increased CP compared to flat membranes. • The additional surface area of patterned membranes compensates for the CP increase. [ABSTRACT FROM AUTHOR]