Cross-flow Filtration Membrane Research Articles

Ultrahigh-throughput cross-flow filtration of solution-processed 2D materials enabled by porous ceramic membranes.

Printed electronics is a disruptive technology in multiple applications including environmental and biological sensors, flexible displays, and wearable diagnostic devices. With superlative electronic, optical, mechanical, and chemical properties, two-dimensional (2D) materials are promising candidates for printable electronic inks. While liquid-phase exfoliation (LPE) methods can produce electronic-grade 2D materials, conventional batch separation processes typically rely on centrifugation, which requires significant time and effort to remove incompletely exfoliated bulk powders, hindering the scale-up of 2D ink manufacturing. While cross-flow filtration (CFF) has emerged as a promising continuous flow separation method for solution-processed 2D nanosheets, previously demonstrated polymer CFF membranes necessitate low 2D nanosheet concentrations to avoid fouling, which ultimately limits mass throughput. Here, we demonstrate a fully flow-based, exfoliation-to-ink system for electronic-grade 2D materials using an integrated cross-flow separation and concentration system. To overcome the relatively low-throughput processing concentrations of incumbent polymer CFF membranes, we employ porous ceramic CFF membranes that are tolerant to 10-fold higher nanosheet concentrations and flow rates without compromising separation efficiency. Furthermore, we demonstrate a concentration method via cross-flow ultrafiltration, where the retentate can be directly formulated into printable inks with electronic-grade performance that meets or exceeds centrifugally produced inks. Life cycle assessment and technoeconomic analysis quantitatively confirm the advantages of ceramic versus polymer CFF membranes including reductions of 97%, 96%, 94%, and 93% for greenhouse gas emissions, water consumption, fossil fuel consumption, and specific production costs, respectively. Overall, this work presents an environmentally sustainable and cost-effective solution for the fabrication, separation, and printing of electronic-grade 2D materials.

Characterizing jamming of dilute and semi-dilute fiber suspensions in a sudden contraction and a T-junction

The clogging or jamming of particle suspensions is a ubiquitous problem, hindering the efficiency of particle–liquid and particle–particle separations. Motivated by pressure screening in the pulp and paper industry, we characterize jamming of dilute and semi-dilute mono-disperse rigid-rod suspensions passing through channels mimicking dead-end and cross-flow filtration membranes, experimentally, using particle-tracking velocimetry. We observe that jams nucleate by either bridging of isolated particles across the constriction, or by localized mechanical entanglement of the particles, i.e., flocculation. Uniquely, we observe floc-formation during acceleration into the aperture and report this as primary mechanism for jamming events. We characterized the accumulation-release cycles of the jamming event using an exponential probability distribution; this distribution is indicative of a Poisson process. For jams nucleated by single-particle bridging, the distribution is (primarily) related to the number of fibers passing through the aperture; this is similar to dry, granular materials. For floc-based nucleation events, the distribution is (primarily) related to the suspension concentration with the average time between jams decreasing inversely with the square-root of the initial suspension concentration. For the conditions tested, the distribution was insensitive to changes in constriction geometry.

Simultaneous use of a crossflow filtration membrane as microbial fuel cell anode – Permeate flow leads to 4-fold increased current densities

A new concept for the combination of membrane bioreactors and microbial fuel cells is introduced, that aims at the production of electricity for reducing the overall energy consumption of wastewater treatment. In contrast to previous approaches, the anode is integrated as microfiltration membrane in sidestream crossflow configuration. Using a stainless steel filtration membrane with G. sulfurreducens and an acetate-based synthetic medium, up to 4-fold higher current densities are achieved. In a standard setup without filtration, a membrane of filter grade 1 µm shows current densities of 5.8 A m−2 ± 0.5 A m−2 compared to >11 A m−2 when it is used simultaneously as membrane filter. With smaller pore sizes of filter grade 0.5 µm, 4.4 A m−2 ± 0.5 A m−2 in a standard setup and >15 A m−2 in a filtration setup are achieved. The permeate flow was identified as the main parameter leading to increased current densities.

Assessment and comparison of Anopore and cross flow filtration methods for the determination of dissolved iron size fractionation into soluble and colloidal phases in seawater

The two most frequently used methods for determining the size fractionation of dissolved iron (dFe) in seawater, 0.02 µm pore size Anopore membrane filtration and cross flow filtration using a 10 kDa regenerated cellulose filter, were evaluated and compared. Anopore filtration was found to produce consistent soluble Fe (sFe) concentrations in the filtrate even after > 1 L unfiltered seawater was filtered, indicating that clogging is not an issue for typical open ocean filtration volumes. Cross flow filtration (CFF) only achieved a 70% to 75% mass balance regardless of flow rate and seawater preconditioning; however, Fe losses arose largely from the colloidal (not soluble) size fraction, and Fe loss was constrained to Fe clogging in/on the CFF membrane. Both Anopore membrane and cross flow filtration methods reproducibly size fractionated dFe in seawater samples. Additionally, sFe arising from these two filtration methods were compared for the first time using seawater samples from the North Pacific and Atlantic Oceans. sFe from CFF was always lower than sFe from Anopore filtration, with the ratio of sFe in CFF/Anopore filtration averaging 68 ± 23% (n = 23). This sFe difference is attributed to a combination of the smaller effective pore size of the regenerated cellulose CFF system and the spatial variability in the concentration and composition of dFe in the 10 kDa ‐ 0.02 µm size fraction. Finally, the advantages and disadvantages of each method were reviewed to offer future users a suite of factors with which to choose their ideal ultrafiltration method.

Comparative pyrosequencing analysis of bacterial community change in biofilm formed on seawater reverse osmosis membrane

The change in bacterial community structure induced by bacterial competition and succession was investigated during seawater reverse osmosis (SWRO) in order to elucidate a possible link between the bacterial consortium on SWRO membranes and biofouling. To date, there has been no definitive characterization of the microbial diversity in SWRO in terms of distinguishing time-dependent changes in the richness or abundance of bacterial species. For bacterial succession within biofilms on the membrane surface, SWRO using a cross-flow filtration membrane test unit was operated for 5 and 100 h, respectively. As results of the pyrosequencing analysis, bacterial communities differed considerably among seawater and the 5 and 100 h samples. From a total of 33,876 pyrosequences (using a 95% sequence similarity), there were less than 1% of shared species, confirming the influence of the operational time factor and lack of similarity of these communities. During SWRO operation, the abundance of Pseudomonas stutzeri BBSPN3 (GU594474) belonging to gamma-Proteobacteria suggest that biofouling of SWRO membrane might be driven by the dominant influence of a specific species. In addition, among the bacterial competition of five bacterial species (Pseudomonas aeruginosa, Bacillus sp., Rhodobacter sp., Flavobacterium sp., and Mycobacterium sp.) competing for bacterial colonization on the SWRO membrane surfaces, it was exhibited that Bacillus sp. was the most dominant. The dominant influences of Pseudomonas sp. and Bacillus sp. on biofouling during actual SWRO is decisive depending on higher removal efficiency of the seawater pretreatment.

Assessment of cross-flow filtration for the size fractionation of freshwater colloids and particles

This research has evaluated the ability of cross-flow filtration (CFF) to perform correct size fractionation of natural aquatic colloids (materials from 1 nm to 1 μm in size) and particles (>1 μm) using scanning electron microscopy (SEM) combined with atomic force microscopy (AFM). SEM provided very clear images at high lateral resolution (ca. 2–5 nm), whereas AFM offered extremely low resolution limits (sub-nanometer) and was consequently most useful for studying very small material. Both SEM and AFM were consistent in demonstrating the presence of colloids smaller than 50 nm in all fractions including the retentates (i.e. the fractions retained by the CFF membrane), showing that CFF fractionation is not fully quantitative and not based on size alone. This finding suggests that previous studies that investigated trace element partitioning between dissolved, colloidal and particulate fractions using CFF may need to be re-visited as the importance of particles and large colloids may have been over-estimated. The observation that ultra-fine colloidal material strongly interacted with and completely coated a mica substrate to form a thin film has important potential implications for our understanding of the behaviour of trace elements in aquatic systems. The results suggest that clean, ‘pure’ surfaces are unlikely to exist in the natural environment. As surface binding of trace elements is of great importance, the nature of this sorbed layer may dominate trace element partitioning, rather than the nature of the bulk particle.

Effect of hydrodynamics and solution ionic strength on permeate flux in cross-flow filtration: direct experimental observation of filter cake cross-sections

The structure of membrane cakes formed in cross-flow filtration was investigated for conditions of variable particle size and solution ionic strength. In all cases, microscopic examination of membrane cake cross-sections revealed a stratified structure: a dense region of the colloidal deposit adjacent to the membrane with an abrupt transition to a much more porous layer near the membrane–suspension interface. The thickness and structure of individual layers varied as functions of particle size and solution ionic strength. Relative specific hydraulic resistances of individual layers were estimated from regressed transport coefficients in a model describing macroscopic transport and permeate flux. This work provides direct experimental confirmation of trends predicted from theory that couples hydrodynamic conditions and interparticle interactions to describe the permeate flux performance of cross-flow filtration membranes.

The influence of the pore size, the foaming temperature and the viscosity of the continuous phase on the properties of foams produced by membrane foaming

Membrane foaming is a new method of foaming. To enlarge the knowledge about the influencing factors and to know how to vary the structure of the resulting foam, different factors were evaluated. A whey protein solution with 10% protein was foamed as a model solution by means of a tubular cross-flow filtration membrane. The pore size of the membrane was varied. The smaller the pore size, the smaller the bubbles produced. As a result, the foam firmness increases and less drainage was observed when smaller pore sizes were applied. An important factor is that the added amount of gas must be stabilised as completely as possible in the foam. In order to achieve this, both the process and the product parameters were varied. Raising the foaming temperature increased the quantity of stabilised gas. The whey proteins then diffuse faster to the bubble surfaces and stabilise these by unfolding and networking reactions to prevent the coalescence of the bubbles. The product parameter viscosity was found to influence the foaming result in such a way that up to a viscosity of 40 mPa s the incorporated gas bubbles are stabilised by the higher viscosity. At viscosities higher than 40 mPa s it is difficult to incorporate in the bubbles, and the foam structure becomes coarser due to increased coalescence at the pores of the membrane. The foam stability is enhanced with higher viscosities.

On-Line Crossflow Filtration Coupled to Column Liquid Chromatography for the Determination of Triazines in Environmental Waters

Characterisation and selectivity of planar crossflow filtration membranes of 47 mm diameter for sampling and sample clean-up of environmental waters containing triazines and high levels of humic substances were investigated. The filtrate is collected in a sample loop, introduced to a column liquid chromatographic (CLC) separation system and quantified by UV-absorbance at 220 nm. Seven membranes, both cellulosic- and polymer types were investigated using samples with 10–80 mg/l of commercial humic acid. Triazine herbicides were chosen as model compounds together with other compounds of environmental concern such as p-nitrophenol and p-chlorophenol. The investigated membranes were found to interact in the filtration process. This led to large differences in terms of selectivity due to differences in pore size, hydrophobicity, porosity, and chemical structure of the membrane. The size and shape of the characteristic matrix front present in the obtained chromato-grams were depending on the type of membrane used. In addition to these matrix effects, some of the membranes resulted in analyte recovery values of less than 100%. Repeatability and stability using nylon and PVDF membranes were evaluated as well as flux variances. An application with the optimised conditions in the CFF-LC-UV system for real surface water from a local creek was carried out with a resulting LOD of 1–5 μ.g/l (S/N = 3) using a 100 μl injection volume.

Effective Permeability of Multi-Channel Cross-Flow Filtration Membranes from Permeate Flux Measurements using the Boundary Integral Method.

The problem of fluid transport inside ceramic, porous, multi-channel microfiltration membranes has been studied experimentally and computationally. The transport of permeate in a seven (star-shaped) channel tubular membrane is modeled according to Darcy’s equation for flow in porous media; the resultant Laplace equation is solved numerically on the complex, multi-connected geometry of interest using the Boundary Integral Method (BIM). Based on this model, we propose an efficient and novel procedure for determining the effective permeability (Km) of the membrane module by comparing computational results with on-line experimental measurements of permeate flux at various levels of Trans-Membrane Pressure (TMP). Besides determining Km, the proposed approach can be used to identify alternative designs and operating conditions in membrane modules of arbitrary geometrical complexity.

An intercomparison of cross-flow filtration techniques used for sampling marine colloids: Overview and organic carbon results

An intercomparison was conducted between 14 different cross-flow filtration (CFF) systems. Each CFF membrane had a 1000 nominal molecular weight cut-off, and five different manufacturers' membranes were tested. The goal of this exercise was to examine whether the different CFF systems were behaving in a well-defined and operationally reproducible manner in marine applications. Surface seawater from Woods Hole and mid-depth waters from Hawaii were prefiltered (< 0.2 μm) and subsamples were processed according to individual protocols. The main criterion for this intercomparison was the agreement of bulk organic carbon (OC) content of the permeate and colloidal fractions. OC blanks ranged from < 10 to > 100 μM. The variation in OC blanks between CFF systems, even of a single type, suggested that cleaning and handling protocols were critical. One of the primary features of this intercomparison was the large range in apparent retention characteristics of the different CFF systems when used on natural seawater samples. In both settings, the quantity of colloidal material retained by the CFF systems followed the order: Amicon > Filtron ≥ Osmonics > Membrex. For example, in the Hawaii sample, the Amicon CFF system retained on average 43% colloidal OC, while the Filtron, Osmonics and Membrex retained < 4%. Variations within a factor of 2–5 were found within a single membrane type. Other results from this intercomparison suggested that these same relative retention characteristics hold for Fe and Al, organic nitrogen and organic phosphorus, optical properties, and colloid standards. Time-series samples of permeate showed a generally increasing OC concentration with time or concentration factor which must be taken into account when interpreting CFF data. We recommend that considerable care be taken in quantifying CFF blanks and in assessing the CFF cut-off for each system prior to use in marine applications. Time-series sampling and the use of standard molecules in controlled experiments are encouraged in order to further our understanding of the behavior of natural compound assemblages during CFF processing.

Evaluation of an Osmonics® spiral-wound cross-flow filtration system for sub-μm sampling of Cd, Cu and Ni in seawater

Abstract A 1-kDa (1 kDa = 1000 daltons) Osmonics spiral-wound cross-flow filtration (CFF) membrane has been evaluated for sampling Cd, Cu and Ni in 1-kDa fractions of prefiltered seawater. Size-fractionated samples were collected as part of a CFF intercomparison experiment (Buesseler et al., 1996-this issue) and in the Gulf of Maine. Mass-balance results averaged 93 ± 6% (Cd), 81 ± 19% (Cu), and 99 ± 18% (Ni) for the intercomparison experiment and 56 ± 17% (Cd), 119 ± 38% (Cu), and 91 ± 6% (Ni) for the Gulf of Maine samples. Differences in mass balance between these studies may be related, in part, to removal of sub-μm particles and associated trace metals during prefiltration. An Amicon CFF system showed higher (2–12 ×) colloidal concentrations for Cu and Ni compared to Filtron and Osmonics systems; all CFF systems showed similar Cd, Cu and Ni mass-balance results. A model based on particle impaction with a filter shows the potential for adsorption of sub-μm particles by a 1-kDa Osmonics CFF membrane and conventional (Nuclepore) filters.

Internal fouling of microporous cross-flow filtration membranes with dilute latex suspensions

The cross-flow filtration of latex with a modal particle diameter of between 1.8 and 3 μm on two membranes nominally rated at 0.45 and 1.2 μm pore size has been studied. Characterization by a Coulter porometer suggested that the average membrane pore diameters were 0.57 and 1.8 μm. The full pore distribution of the finer membrane contained pores apparently of diameters below the size of the challenge suspension. However, in both cases particles penetrated the membrane and no filter cake was formed. This caused internal membrane fouling. Scanning electron micrographs showed that the membranes were, in fact, of cellular structure with pores of diameter 5 times that of the characterization tests and displaying a high degree of interconnectivity of the internal cells. The highly interconnected nature of the pore network inside the membrane made individual pore blockage unlikely and provided the means by which high filtration efficiencies could be achieved when filtering particles smaller than the diameter of the surface openings of the membrane.

Development of a pilot‐scale microfiltration harvest for the isolation of physostigmine from Streptomyces griseofuscus broth

AbstractA pilot‐scale microfiltration harvest was developed for the isolation of physostigmine from a mycelial fermentation broth. Complete permeation for this extracellular secondary metabolite was obtained during initial laboratory screening with a variety of crossflow filtration membranes but poor filtration flux made process feasibility uncertain. Modification of the fermentation medium improved fluxes from 10–20 dm3 m−2 h−1 to 60–80 dm3 m−2 h−1 which significantly reduced membrane area requirements, making the process feasible for larger scale implementation. A commercially‐available ceramic membrane with 0·2 μm pores was employed for the scale‐up of this step to the pilot plant; this membrane was found to provide flux similar to that of polymeric membranes and had superior regeneration characteristics. Optimum performance at bench scale was obtained using 70 kPa transmembrane pressure and 4·0 m s−1 crossflow velocity. The mycelial broth had a very high suspended solids concentration (30% (v/v)) which limited us to a two‐fold concentration before starting the diafiltration washes. The process was scaled‐up to the 200–400 dm3 scale, with good reproducibility and excellent membrane regenerability observed in a series of five experiments. An increase in membrane loading did result in a decreased average flux at the larger scale (47·6 dm3 m−2 h−1), which indicated the importance of this parameter. A centrifugal pump was substituted for a positive displacement pump upon scale‐up and performed well if provided with the appropriate suction head. Scale‐up in general was straightforward, provided proper attention was given to the hydraulic design of the system.

Study of the Fouling Effect of Antifoam Compounds on the Crossflow Filtration of Yeast Suspensions

Abstract The fouling effect of different antifoams and compounds of antifoams on crossflow filtration membranes is studied by on-line measurement of the permeation flow during the concentration of suspensions of baker's yeasts in reverse osmosis water (initial yeast concentration, 9 g/L dry matter; concentration factor, 6). The experiments are carried out on three types of tubular mineral microfiltration membranes, two types of flat-sheet organic microfiltration membranes, and one ultrafiltration mineral membrane. The compounds tested are principally silicone oils, silica + silicone based antifoams, and a modified organic antifoam (modified polyalcoxyester). The presence of 100 ppm of any of these compounds