In situ MRI of alginate fouling and flow in ceramic hollow fiber membranes

Unveiling the spatiotemporal dynamics of membrane fouling: A focused review on dynamic fouling characterization techniques and future perspectives

Membrane technology has emerged as a crucial method for obtaining clean water from unconventional sources in the face of water scarcity. It finds wide applications in wastewater treatment, advanced treatment, and desalination of seawater and brackish water. However, membrane fouling poses a huge challenge that limits the development of membrane-based water treatment technologies. Characterizing the dynamics of membrane fouling is crucial for understanding its development, mechanisms, and effective mitigation. Instrumental techniques that enable in situ or real-time characterization of the dynamics of membrane fouling provide insights into the temporal and spatial evolution of fouling, which play a crucial role in understanding the fouling mechanism and the formulation of membrane control strategies. This review consolidates existing knowledge about the principal advanced instrumental analysis technologies employed to characterize the dynamics of membrane fouling, in terms of membrane structure, morphology, and intermolecular forces. Working principles, applications, and limitations of each technique are discussed, enabling researchers to select appropriate methods for their specific studies. Furthermore, prospects for the future development of dynamic characterization techniques for membrane fouling are discussed, underscoring the need for continued research and innovation in this field to overcome the challenges posed by membrane fouling.

Monitoring residual fouling after cleaning of multi-fiber membrane modules fiber-by-fiber using non-invasive MRI monitoring

In this study non-invasive low field magnetic resonance imaging (MRI) technology was used to monitor fouling induced changes in fiber-by-fiber hydrodynamics inside a multi-fiber hollow fiber membrane module containing 401 fibers. Using structural and velocity images the fouling evolution of these membrane modules were shown to exhibit distinct trends in fiber-by-fiber volumetric flow, with increasing fouling causing a decrease in the number of flow active fibers. This study shows that the fouling rate is not evenly distributed over the parallel fibers, which results in a broadening of the fiber to fiber flowrate distribution. During cleaning, this distribution is initially broadened further, as relatively clean fibers are cleaned more rapidly compared to clogged fibers. By tracking the volumetric flow rate of individual fibers inside the modules during the fouling-cleaning cycle it was possible to observe a fouling memory-like effect with residual fouling occurring preferentially at the outer edge of the fiber bundle during repeated fouling-cleaning cycle. These results demonstrate the ability of MRI velocity imaging to quantitatively monitor these effects which are important when testing the effectiveness of cleaning protocols due to the long term effect that residual fouling and memory-like effect may have on the operation of membrane modules.

MRI on a new polymeric multichannel membrane for ultrafiltration

Membrane ultrafiltration in new polymeric multi-channel membranes designed for in-out filtration was investigated to get insights into structure, flow and filtration properties. The apparent novelty of the membrane concerns the geometry and configuration of the feed channels. In-situ magnetic resonance imaging (MRI) allows non-invasive and non-destructive investigations with adequate spatial and time resolution. The structure of the new polymeric membrane was measured with an in-plane spatial resolution of 35 µm/pixel revealing first the polymer density distribution over the 19-channel membrane and second the wettability of the fiber and its cavities of different dimensions. MRI was also used to answer questions about flow and consequently feed distribution in the channels. Finally, in-situ filtration of an aqueous solution of sodium alginate was observed which led to deposit formation at the channel’s inner surfaces. The kinetics of this deposit formation was quantified. Backwashing and flushing gave insight into the cleanability of the channels.

Magnetic Resonance Imaging: Time-Dependent Wetting and Swelling Behavior of an Auxetic Hydrogel Based on Natural Polymers

A time-dependent understanding of swelling characteristics and external stimuli behavior is crucial for the development and understanding of functional hydrogels. Magnetic resonance imaging (MRI) offers the opportunity to study three-dimensional (3D) soft materials nondestructively. This technique is already widely used as an image-based medical diagnostic tool and is applied here to evaluate complex structures of a hydrogel-a double network of chemically crosslinked casein enhanced with alginate-fabricated by 3D printing. When hydrogel disks immersed in four different liquid systems were analyzed, the material exhibited distinct system-dependent behavior characterized by rheological and mechanical measurements. Further material functionalization was achieved by macroscopic structuring of the hydrogel as an auxetic material based on a re-entrant honeycomb structure. MRI offers the advantage of monitoring overall changes in the area of the analyzed specimen and internal structural changes simultaneously. To assess the behavior of this complex structure, a series of short MRI measurements, each lasting 1.7 min, captured liquid diffusion and thus structural swelling behavior. A clear dependence of external and internal structural changes as a function of liquid properties causing these changes was observed. In conclusion, this approach might pave the way for prospective applications to monitor liquid diffusion into (e.g., vascularization) and swelling behavior of functional hydrogels.

Magnetic Resonance Imaging as a tool for quality control in extrusion-based bioprinting

Bioprinting is gaining importance for the manufacturing of tailor-made hydrogel scaffolds in tissue engineering, pharmaceutical research and cell therapy. However, structure fidelity and geometric deviations of printed objects heavily influence mass transport and process reproducibility. Fast, three-dimensional and nondestructive quality control methods will be decisive for the approval in larger studies or industry. Magnetic Resonance Imaging (MRI) meets these requirements for characterizing heterogeneous soft materials with different properties. Complementary to the idea of decentralized 3D printing, magnetic resonance tomography is common in medicine, and image data processing tools can be transferred system-independently. In this study, we evaluated a MRI measurement and image analysis protocol to jointly assess the reproducibility of three different hydrogels and a reference material. Critical parameters for object quality, namely porosity, hole areas and deviations along the height of the scaffolds are discussed. Geometric deviations could be correlated to specific process parameters, anomalies of the ink or changes of ambient conditions. This strategy allows the systematic investigation of complex 3D objects as well as an implementation as a process control tool. Combined with the monitoring of metadata this approach might pave the way for future industrial applications of 3D printing in the field of biopharmaceutics. This article is protected by copyright. All rights reserved.

Membrane Fouling Phenomena in Microfluidic Systems: From Technical Challenges to Scientific Opportunities

The almost ubiquitous, though undesired, deposition and accumulation of suspended/dissolved matter on solid surfaces, known as fouling, represents a crucial issue strongly affecting the efficiency and sustainability of micro-scale reactors. Fouling becomes even more detrimental for all the applications that require the use of membrane separation units. As a matter of fact, membrane technology is a key route towards process intensification, having the potential to replace conventional separation procedures, with significant energy savings and reduced environmental impact, in a broad range of applications, from water purification to food and pharmaceutical industries. Despite all the research efforts so far, fouling still represents an unsolved problem. The complex interplay of physical and chemical mechanisms governing its evolution is indeed yet to be fully unraveled and the role played by foulants' properties or operating conditions is an area of active research where microfluidics can play a fundamental role. The aim of this review is to explore fouling through microfluidic systems, assessing the fundamental interactions involved and how microfluidics enables the comprehension of the mechanisms characterizing the process. The main mathematical models describing the fouling stages will also be reviewed and their limitations discussed. Finally, the principal dynamic investigation techniques in which microfluidics represents a key tool will be discussed, analyzing their employment to study fouling.

Structural Characterisation of Deposit Layer during Milk Protein Microfiltration by Means of In-Situ MRI and Compositional Analysis

Milk protein fractionation by microfiltration membranes is an established but still growing field in dairy technology. Even under cross-flow conditions, this filtration process is impaired by the formation of a deposit by the retained protein fraction, mainly casein micelles. Due to deposition formation and consequently increased overall filtration resistance, the mass flow of the smaller whey protein fraction declines within the first few minutes of filtration. Currently, there are only a handful of analytical techniques available for the direct observation of deposit formation with opaque feed media and membranes. Here, we report on the ongoing development of a non-invasive and non-destructive method based on magnetic resonance imaging (MRI), and its application to characterise deposit layer formation during milk protein fractionation in ceramic hollow fibre membranes as a function of filtration pressure and temperature, temporally and spatially resolved. In addition, the chemical composition of the deposit was analysed by reversed phase high pressure liquid chromatography (RP-HPLC). We correlate the structural information gained by in-situ MRI with the protein amount and composition of the deposit layer obtained by RP-HPLC. We show that the combination of in-situ MRI and chemical analysis by RP-HPLC has the potential to allow for a better scientific understanding of the pressure and temperature dependence of deposit layer formation.

In situ measurement of deposit layer formation during skim milk filtration by MRI

Filtration and separation via membranes are key processes in food processing. One major application of membrane filtration is in the dairy industry, aiming for the separation of different milk proteins. The various chemical components of milk possess different physiochemical properties and can be used most effectively in food processing if they are separately available and remain in their native state. Microfiltration of skim milk allows a fractionation of the milk proteins casein and whey by size. A deposit is formed on the membrane surface mainly but not exclusively by micellar casein proteins during filtration. Membrane pore blockage by whey proteins and fouling occur during membrane filtration, negatively affecting the yield of the whey protein fraction. Skim milk filtration and the deposit layer formation were measured time and spatially resolved by in situ magnetic resonance imaging (MRI). The nature of the fouling layer was investigated during dead-end filtration in ceramic hollow fiber membranes. MRI was used to further clarify the influence of operating conditions on separation and filtration mechanisms that are responsible for growth of the fouling layer and its reversibility. The MRI measurements were analyzed for a detailed description of skim milk filtration by modeling the signal intensity distribution.

Polyoxometalate-based high-spin cluster systems: a NMR relaxivity study up to 1.4 GHz/33 T

Paramagnetic polyoxometalates are of special interest with regard to their application as alternative contrast agents in nonhuman magnetic resonance imaging which is increasingly used in materials science and process engineering.