Protein fouling of uf membranes: Causes and consequences

Use of ultrafiltration/diafiltration for the processing of antisense oligonucleotides

Ultrafiltration/diafiltration (UF/DF) has been the hallmark for concentrating and buffer exchange of protein and peptide-based therapeutics for years. Here we examine the capabilities and limitations of UF/DF membranes to process oligonucleotides using antisense oligonucleotides (ASOs) as a model. Using a 3 kDa UF/DF membrane, oligonucleotides as small as 6 kDa are shown to have low sieving coefficients (<0.008) and thus can be concentrated to high concentrations (≤200 mg/mL) with high yield (≥95%) and low viscosity (<15 centipoise), provided the oligonucleotide is designed not to undergo self-hybridization. In general, the oligonucleotide should be at least twice the reported membrane molecular weight cutoff for robust retention. Regarding diafiltration, results show that a small amount of salt is necessary to maintain adequate flux at concentrations exceeding about 40 mg/mL. Removal of salts along with residual solvents and small molecule process-related impurities can be robust provided they are not positively charged as the interaction with the oligonucleotide can prevent passage through the membrane, even for common divalent cations such as calcium or magnesium. Overall, UF/DF is a valuable tool to utilize in oligonucleotide processing, especially as a final drug substance formulation step that enables a liquid active pharmaceutical ingredient.

New insights into the structure of membrane fouling by biomolecules using comparison with isotherms and ATR-FTIR local quantification

The objective of this paper was to propose a deepened analyze of a microfiltration membrane fouling by two biomolecules: a protein (Bovine Serum Albumin) and a peptide (Glutathione). In addition to an analysis of flux decline, the mass of biomolecules accumulated on the membrane during filtration was quantified and compared to adsorption experiments, using Fourier Transform Infra Red spectroscopy in Attenuated Total Reflection mode (ATR-FTIR). It was demonstrated that the same quantity of accumulated biomolecules on the apparent membrane area can generate totally different flux declines because of different fouling mechanisms. On the one hand, Glutathione can adsorb in the whole porous media of the membrane, penetrating through the pores, modifying the hydrophilicity at low concentrations and generating pore constriction at high concentrations. On the other hand, BSA organize a dense irreversible fouling in the first minutes of filtration containing a quantity equivalent to more than 45 monolayers, leading to pore blocking and pore constriction. This structure is resistant to rinsing and NaOH cleaning. Then a reversible fouling, containing a quantity equivalent to more than 90 monolayers is settled. The above structure can be removed with an intensive water rinsing and corresponds to a rather porous cake leading to a low resistance to water permeation, whereas the intermediate structure can only be removed with chemical cleaning and has a higher impact on water permeation. The original approach detailed in this paper allowed to go deeper in the understanding of the membrane fouling by soft matter, not detailed in previous papers.

Clarification Processes of Orange Prickly Pear Juice (Opuntia spp.) by Microfiltration

In this study, fresh orange prickly pear juice (Opuntia spp.) was clarified by a cross-flow microfiltration (MF) process on a laboratory scale. The viability of the process-in terms of productivity (permeate flux of 77.80 L/h) and the rejection of selected membranes towards specific compounds-was analyzed. The quality of the clarified juice was also analyzed for total antioxidants (TEAC), betalains content (mg/100 g wet base), turbidity (NTU) and colorimetry parameters (L, a*, b*, Croma and H). The MF process permitted an excellent level of clarification, reducing the suspended solids and turbidity of the fresh juice. In the clarified juice, a decrease in total antioxidants (2.03 TEAC) and betalains content (4.54 mg/100 g wet basis) was observed as compared to the fresh juice. Furthermore, there were significant changes in color properties due to the effects of the L, a*, b*, C and h° values after removal of turbidity of the juice. The turbidity also decreased (from 164.33 to 0.37 NTU).

S-Layer Ultrafiltration Membranes

Monomolecular arrays of protein subunits forming surface layers (S-layers) are the most common outermost cell envelope components of prokaryotic organisms (bacteria and archaea). Since S-layers are periodic structures, they exhibit identical physicochemical properties for each constituent molecular unit down to the sub-nanometer level. Pores passing through S-layers show identical size and morphology and are in the range of ultrafiltration membranes. The functional groups on the surface and in the pores of the S-layer protein lattice are accessible for chemical modifications and for binding functional molecules in very precise fashion. S-layer ultrafiltration membranes (SUMs) can be produced by depositing S-layer fragments as a coherent (multi)layer on microfiltration membranes. After inter- and intramolecular crosslinking of the composite structure, the chemical and thermal resistance of these membranes was shown to be comparable to polyamide membranes. Chemical modification and/or specific binding of differently sized molecules allow the tuning of the surface properties and molecular sieving characteristics of SUMs. SUMs can be utilized as matrices for the controlled immobilization of functional biomolecules (e.g., ligands, enzymes, antibodies, and antigens) as required for many applications (e.g., biosensors, diagnostics, enzyme- and affinity-membranes). Finally, SUM represent unique supporting structures for stabilizing functional lipid membranes at meso- and macroscopic scale.

Recovery of Phenolic Compounds from Red Grape Pomace Extract through Nanofiltration Membranes

The winemaking process generates a large amount of residues such as vine shots, stalks, grape pomace, and wine lees, which were only recently considered for exploitation of their valuable compounds. The purpose of this work was to investigate the performance of nanofiltration for the recovery of phenolic compounds, with bioactive capacity like antioxidant, from red grape pomace extract. Four membranes were compared in this study-three cellulose acetate (CA series: lab-prepared by phase inversion) and one commercial (NF90). All membranes were characterized for their hydraulic permeability and rejection coefficients to reference solutes like saccharose, glucose, raffinose, polyethylene glycol, sodium chloride, and sodium sulfate. Permeation flowrates and rejection coefficients towards total phenolics content, antioxidant activity, proanthocyanidins, glucose and fructose were measured in the nanofiltration of grape pomace extract using selected operating conditions. Among the investigated membranes, the CA400-22 exhibited the highest permeate flux (50.58 L/m² h at 20 bar and 25 °C), low fouling index (of about 23%), the lowest rejection coefficients towards the reference solutes and the best performance in terms of separation between sugars and phenolic compounds. Indeed, the observed rejections for glucose and fructose were 19% and 12%, respectively. On the other hand, total phenolics content and proanthocyanidins were rejected for 73% and 92%, respectively.

Enhancement of COD Removal from Oilfield Produced Wastewater by Combination of Advanced Oxidation, Adsorption and Ultrafiltration

The wastewater produced from the oilfield is chemically corrosive due to high salinity in combination with high temperatures. It is also rich in contaminants, such as oil, polyacrylamide, emulsions, suspended solid, etc. The density difference between the oil and water in the wastewater is low, which makes separation via gravity difficult. In this study, a combined pilot treatment is studied, which includes Fenton oxidation, settlement, activated carbon adsorption, and ultrafiltration (UF). The operational conditions of Fenton oxidation are optimized based on alleviating the fouling of the UF membrane. When the Fenton oxidation was operated at the molar ratio of H₂O₂ to FeSO₄ 3:1 and pH 2.2–2.5, the UF membrane could operate continuously for 20 h without cleaning. The membrane was fouled by the organics (oil/grease) and polymer, which can be effectively removed by composite cleaning reagent consisting of 0.1% NaOH and 0.1% sodium dodecylbenzenesulfonate (SDBS). With the UF treatment, the chemical oxygen demand (COD) of the effluent was less than 50 mg/L, which could meet the upgraded standard.

Colloidal interactions and fouling of NF and RO membranes: a review

Colloids are fine particles whose characteristic size falls within the rough size range of 1-1000 nm. In pressure-driven membrane systems, these fine particles have a strong tendency to foul the membranes, causing a significant loss in water permeability and often a deteriorated product water quality. There have been a large number of systematic studies on colloidal fouling of reverse osmosis (RO) and nanofiltration (NF) membranes in the last three decades, and the understanding of colloidal fouling has been significantly advanced. The current paper reviews the mechanisms and factors controlling colloidal fouling of both RO and NF membranes. Major colloidal foulants (including both rigid inorganic colloids and organic macromolecules) and their properties are summarized. The deposition of such colloidal particles on an RO or NF membrane forms a cake layer, which can adversely affect the membrane flux due to 1) the cake layer hydraulic resistance and/or 2) the cake-enhanced osmotic pressure. The effects of feedwater compositions, membrane properties, and hydrodynamic conditions are discussed in detail for inorganic colloids, natural organic matter, polysaccharides, and proteins. In general, these effects can be readily explained by considering the mass transfer near the membrane surface and the colloid-membrane (or colloid-colloid) interaction. The critical flux and limiting flux concepts, originally developed for colloidal fouling of porous membranes, are also applicable to RO and NF membranes. For small colloids (diameter≪100 nm), the limiting flux can result from two different mechanisms: 1) the diffusion-solubility (gel formation) controlled mechanism and 2) the surface interaction controlled mechanism. The former mechanism probably dominates for concentrated solutions, while the latter mechanism may be more important for dilute solutions. Future research needs on RO and NF colloidal fouling are also identified in the current paper.

Crossflow microfiltration of recombinant Escherichia coli lysates after high pressure homogenization

Crossflow membrane filtration was used to process recombinant Escherichia coli cell lysates containing protein inclusion bodies after high pressure homogenization. The number of passes through the high pressure homogenizer changed the viscosities and average particle sizes of the cell lysates. The different cell lysates were processed with a hollow fiber unit containing microfiltration membranes and a plate and frame unit with either ultrafiltration or microfiltration membranes. There were differences in permeate flux and protein transmission for the various membranes with the best performing membranes giving permeate fluxes greater than 60 L m(-2) h(-1) and protein transmissions greater than 90%. For a given membrane, no differences were observed between the cell lysates following homogenization with one, two, and three passes at 83 MPa. The lack of a difference between the three lysates is due to their similarities with respect to the released macromolecules and the presence of small (<0.1 microm) cell debris.

Effect of pH on flux during ultrafiltration of sweet whey and buttermilk

The flux patterns for sweet whey and buttermilk were strongly influenced by pH. Increasing the pH of buttermilk from 6·6 to 8·0 tended to reduce initial flux values and reduce deposit formation on the membrane as indicated by lower values for the fouling coefficient. Flux was mostly controlled by concentration polarization. Reducing the pH below 6·6 increased the flux but caused more deposit on the membrane as indicated by high fouling coefficient values. Almost the opposite was found for sweet whey. The initial flux increased as pH increased, followed by considerable flux decline, which was linked to greater fouling of the membrane. Reducing the pH reduced the initial flux considerably but also reduced further fouling. These changes are thought to be brought about by the combined effects of pH change on the proteins and minerals, and in particular on calcium.

Conformation of adsorbed bovine serum albumin governing its desorption behavior at alumina-water interfaces

The mode of initial adsorption of bovine serum albumin (BSA) onto positively charged Al2O3 particles was studied as a function of surface coverage (theta). The adsorption isotherm of BSA exhibited saturation (theta = 1) and the existence of an inflection point at theta of 0.82. The relative numbers of ionic groups on a BSA molecule interacting with the Al2O3 surface at various theta were monitored by measuring the relative adsorption density of H+ and OH-, ([gamma(H+) - gamma(OH-)]), for BSA-adsorbed Al2O3 using potentiometric titration. The [gamma(H+) - gamma(OH-)] curves for Al2O3, BSA, and BSA-adsorbed Al2O3 at various KNO3 concentrations showed a common intersection point (cip) which was the pH giving the acid-base equivalence point, respectively. Compared with the cip's of Al2O3 (5.6) and BSA (5.2), the cip's of BSA-adsorbed Al2O3 were situated at points corresponding to more alkaline pH values over the theta range of 0.13 to 1.0. These results suggested that negatively charged groups, mainly carboxyl groups, on the BSA molecule electrostatically interacted with the Al2O3 surface. The degree of shift in the cip increased gradually with increasing theta from 0.13 to 0.70, while it decreased markedly over the theta range of 0.82 to 1.0. The variation in the cip reflected the change in the total number of ion pairs formed between BSA molecules and Al2O3. The initial rates of BSA desorption during alkali cleaning were low and almost constant over the theta range of 0.13 to 0.70, but increased markedly at theta higher than 0.82. It is suggested that the conformational changes of BSA adsorbed on Al2O3, involving changes in the relative magnitude of electrostatic interaction forces, occur discretely at theta of approximately 0.8.

Two-phase bioconversion product recovery by microfiltration I. Steady state studies

Recovery of an aqueous bioconversion product from complex, two-phase Pseudomonas putida broths containing 20% (v/v) soybean oil presents a significant challenge for downstream processing. Although not used before in multiple-phase separation for complex biotech products, crossflow filtration employing ceramic filters is one of the most attractive options which allow the design of integrated, continuous bioconversion processes. As a first attempt, we studied multichannel, monolithic ceramic membranes of different nominal pore sizes and lumen diameters under steady-state conditions. The best performance was obtained with 0.2-microm-pore/3-mm-lumen membrane, which completely rejected both cells and oil droplets from the permeate, creating a clear aqueous product stream. Although the same separation was achieved, the 50K molecular weight cut-off (MWCO) ultrafilter showed greater irreversible but similar reversible resistance, in addition to an order-of-magnitude higher membrane resistance. Larger nominal pore microfilters, such as 0.45 and 1.0 microm, experienced both cell and oil leakage even at low transmembrane pressure (10 psig). Attributed to greater shear at the same recirculation rate, smaller lumen filters did provide greater permeate flux. However, for practical purposes, the 0. 2-microm-pore/4-mm-lumen ceramic membrane was chosen for further evaluation. Transmembrane pressures up to 50 psig provided only marginal gains in filtration performance, whereas increasing shear rate resulted in linear increases in steady-state flux, presumably due to formation of shear-sensitive, complex gel/oil/cell layer near the membrane surface. A nominal shear rate of 9200 s-1 and 20 psig transmembrane pressure were chosen as optimal operating conditions. Additional studies in a clean system revealed that as low as 5% (v/v) soybean oil in deionized (DI) water resulted in an order-of-magnitude decline in steady-state permeate flux. Breakthrough of oil droplets occurred at 35 psig transmembrane pressure. The severe fouling and breakthrough phenomena disappeared in the presence of washed cells for transmembrane pressure up to 43 psig, implying an oil/cell layer coating the membrane surface, thus preventing oil penetration. Serious membrane fouling was also experienced in microfiltration of oil-free, cell-free supernatant and oil-free whole broth. Consequently, soluble proteins/surfactants were suspected to be the major membrane foulants. Interestingly, soybean oil up to 30% (v/v) enhanced the flux, presumably through complicated interactions with the major foulants. Regeneration of membrane was best achieved with protease and hot caustic/bleach treatments, supporting the hypothesized fouling mechanisms mentioned above. This work provides process and system information for batch microfiltration runs in the future, to be reported elsewhere as Part II of this work.