Crossflow filtration of Saccharomyces cerevisiae using an unsteady jet

This paper deals with the influence of a new flow unsteadiness on the permeate flux in crossflow filtration of microbial suspension during fermentation. A pneumatically controlled valve generates intermittent jets from the main flow, leading to the formation of large vortices moving downstream along the tubular membrane. The unsteadiness does not affect the cell behaviour during fermentation and the resulting permeate flux is found twice higher than for usual filtration process.     

Flux enhancement for membrane filtration of bacterial suspensions using high-frequency backpulsing

A promising method for reducing membrane fouling during crossflow microfiltration of biological suspensions is backpulsing. Very short backpulses (0.1-1.0 s) have been used to increase the net flux for washed bacterial suspensions and whole bacterial fermentation broths. The net fluxes under optimum backpulsing conditions for the washed bacteria are approximately 10-fold higher than those obtained during normal crossflow microfiltration operation, whereas only a 2-fold improvement in the net flux is achieved for the fermentation broths. A theory is presented that is based on external fouling during forward filtration and nonuniform cleaning of the membrane during reverse filtration. The model contains an adjustable parameter which is a measure of the cleaning efficiency during backpulsing; the cleaning efficiency found by fitting the model to the experiments increases with increasing frequency and duration of the backpulses. The theory predicts an optimum backpulsing frequency, as was observed experimentally. An economic analysis shows that crossflow microfiltration with backpulsing has lower costs than centrifugation, rotary vacuum filtration, and crossflow microfiltration without backpulsing.     

Factors affecting the performance of crossflow filtration of yeast cell suspension

Factors affecting the performance of crossflow filtration were investigated with a thin-channel module and yeast cells. In crossflow filtration of Saccharomyces cerevisiae cells cultivated with YPD medium (Yeast extract, polypeptone, and dextrose) and suspended in saline, a steady state was attained within several minutes when the cell concentration was low and the circulation flow rate was high. The steady-state flux and the change in flux during the initial unsteady state were explained well by conventional filtration theory, with the amount of cake deposited and the mean specific resistance to the cake measured in a dead-end filtration apparatus used in calculation. When the circulation flow rate was lower than a critical value, a part of the channel of the crossflow filtration module was plugged with cell cake, and thus the steady-state flux was low. In crossflow filtration of suspensions of commercially available baker's yeast, the flux gradually decreased, and the flux after 8 h of filtration was lower than the value calculated by filtration theory. Fine particles contaminating the baker's yeast was responsible for the decrease. A similar phenomenon was responsible for the decrease. A similar phenomenon was observed in crossflow filtration of a broth of S. cerevisiae cells cultivated in molasses medium, which also contains such particles, had no effect of the permeation flux during crossflow filtration. (c) 1993 John Wiley & Sons, Inc.     

Influence of the flow regime on the efficiency of a gas-liquid two-phase medium filtration

An easy technique, consisting in injecting air into the liquid stream, is proposed to enhance the permeate flux in crossflow filtration of a model fluid (i.e a bentonite suspension). The injected air promotes turbulence and increases the superficial crossflow velocity that leads to a regular disturbance of the boundary layer. A systematic study of different two-phase configurations points up that the slug flow seems the most appropriate regime. The resulting permeate rate is increased up to 140%, in comparison with the usual filtration processes.     

The Effect of Gas Sparging in Cross‐Flow Microfiltration of 2,3‐Butanediol Fermentation Broth

Recovery of 2,3‐butanediol from a fermentation broth entails the separation of cells and other suspended solids as the initial step for subsequent separation stages. The aim of this work was to study the cross‐flow filtration of broth in the fermentation of 2,3‐butanediol from blackstrap molasses by Klebsiella oxytoca (NRRL B‐199). A plate type laboratory scale cross‐flow microfiltration unit with a 0.2‐μm cellulose acetate membrane was employed for this purpose. Preliminary results showed that the permeate flux would decline rapidly due to fouling caused by the natural impurities of blackstrap molasses, and modifications of the conventional cross‐flow filtration would be essential to achieve a filtration rate appropriate for practical purposes. In this work, the permeate flux was enhanced by air sparging, which scoured the membrane surface of colloidal deposits and allowed a practical filtration rate to be maintained. The average permeate flux increased by 39 % and 54 % for an air sparging rate of 0.5 L/min and 1.0 L/min respectively, in the case of an initial biomass concentration of 4.66 g/L. For an initial biomass concentration of 14.2 g/L, the flux increased by 105 % and 146 % for the gas rate of 0.5 and 1.0 L/min, respectively. It may be concluded that gas sparging is beneficial in cross‐flow filtration of thick suspensions like a fermentation broth.     

Enzyme recovery during gas/liquid two-phase flow microfiltration of enzyme/yeast mixtures

The effect of a gas/liquid two-phase flow on the recovery of an enzyme was evaluated and compared with standard crossflow operation when confronted with the microfiltration of a high-fouling yeast suspension. Ceramic tubular and flat sheet membranes were used. At constant feed concentration (permeate recycling) and transmembrane pressure, the results obtained with the tubular membrane were dependent on the two-phase flow pattern. In comparison with single-phase flow performances at the same liquid velocity, the enzyme transmission was maintained at a high level with a bubble flow pattern but it decreased by 70% with a slug flow, whatever the flow rate ratio. Identical results were obtained with flat sheet membranes: for the highest flow rate ratio, the enzyme transmission was reduced by 70% even though the permeate flux was improved by 240%. During diafiltration experiments with the tubular membrane, it was found that a bubble flow pattern led to a 13% higher enzyme recovery compared to single-phase flow conditions, whereas with a slug flow the enzyme recovery was strongly reduced. With bubble flow conditions, energy consumption was minimal, confirming that this flow pattern was the most suitable for enzyme recovery.     

Transient and stationary operating conditions on performance of lactic acid bacteria crossflow microfiltration

A filtration rig equipped with a tubular alumina membrane was used to study the performance of crossflow microfiltration of Lactobacillus helveticus. Experiments were performed at constant permeation flux. High cell concentrations and fast transient conditions to the stationary J adversely affected permeability. Membrane fouling was due to a fast irreversible layer formation and to a reversible cell cake. This microbial deposit characteristics were dependent on the ratio permeation flux/wall shear stress, J/tau(w). Fouling was faster and more severe when J/tau(w) was greater than a critical value of 1.15 L(-1) . h(-1) . m(-2) . Pa(-1). The disordered structure of this cell cake seemed to lead to a macromolecule deposit between the cells which adversely affected the membrane permeability. (c) 1996 John Wiley & Sons, Inc.     

Crossflow filtration of baker's yeast with periodical stopping of permeation flow and bubbling

The periodical stopping of permeation flow was applied to increase the permeation flux in crossflow filtration of commercially available baker's yeast cell suspension. The permeation flux after 3 h filtration in the crossflow filtration increased to 8 x 10(-5) m(3) /m(2) s (290 L/m(2) h) from 2 x 10(-5) m(3)/m(2) s (72 L/m(2) h) by applying the periodical stopping of permeation. Introduction of air bubbles during the stopping period of permeation further increased the flux.(c) John Wiley & Sons, Inc.     

Bovine serum albumin-hemoglobin fractionation: significance of ultrafiltration system and feed solution characteristics

This work investigates the fractionation of similar molecular weight proteins bovine serum albumin (69 kD) and bovine hemoglobin (67 kD) by ultrafiltration. Three different membranes, viz. regenerated cellulose, poly(sulfone) and surface modified poly(acrylonitrile), each with a nominal molecular cutoff rating of 100 kD, were examined. The experiments were conducted in dead end, crossflow and vortex flow filtration modes and the separation was studied as a function of feed pH and ionic strength. Under similar system hydrodynamics, the surface modified poly(acrylonitrile) membrane displayed the highest resolution with minimum membrane fouling. The separation could be improved further by operating at low applied pressure (40 kPa) and high mass transfer (> 20 × 10^–6 m/s) in a vortex flow module. Under these conditions, the highest separation factor of 40 was obtained at the pI of hemoglobin.     

A new coiled hollow-fiber module design for enhanced microfiltration performance in biotechnology.

The microfiltration performance of a novel membrane module design with helically wound hollow fibers is compared with that obtained with a standard commercial-type crossflow module containing linear hollow fibers. Cell suspensions (yeast, E. coli, and mammalian cell cultures) commonly clarified in the biotechnology industry are used for this comparison. The effect of variables such as transmembrane pressure, particle suspension concentration, and feed flow rate on membrane performance is evaluated. Normalized permeation fluxes versus flow rate or Dean number behave according to a heat transfer correlation obtained with centrifugal instabilities of the Taylor type. The microfiltration performance of this new module design, which uses secondary flows in helical tubes, is significantly better than an equivalent current commercial crossflow module when filtering suspensions relevant to the biotechnology industry. Flux and capacity improvements of up to 3.2-fold (constant transmembrane pressure operation) and 3.9-fold (constant flux operation), respectively, were obtained with the helical module over those for the linear module.     

Crossflow filtration of yeast broth cultivated in molasses

A broth of yeast cells cultivated in molasses was crossfiltered with a thin-channel module. The permeation flux gradually decreased at a constant cell concentration. The flux was much lower than that obtained for yeast broth cultivated in yeast extract, polypeptone, and dextrose (YPD) medium during the filtration. The flux did not depend on the membrane pore size (0.45 to 5 mum). The steady-state flux was one-twentieth that calculated for a cake filtration mode from the amount of cake per unit filtration area and the specific resistance of the cake measured in a dead-end filtration apparatus. The lower flux was due to small particles (most of which were less than 1 mum in diameter) in the molasses. The mehanism of crossflow filtration of broths of yeast cells cultivated in molasses was clarified by analysis of the change in flux with time and observations with scanning electron microscopy. At the initial stage of crossflow filtration the yeast cells and particles from the molasses were deposited on the membrane to form the molasses were deposited on the membrane to form a cake in a similar way to dead-end filtration. After the deposition of cells onto the membrane ceased, the fine particles from molasses formed a thin layer, which had higher resistance than the cake formed next to the membrane. The backwashing method was effective to increase the flux. The flux increased low when the pore size was 0.45 to 0.08 mum, but using larger pores of 3 to 5 mum it returned almost to the bases line. (c) 1994 John Wiley & Sons, Inc.     

Microfiltration of recombinant yeast cells using a rotating disk dynamic filtration system

To develop a highly efficient cell harvest step under time constraint, a novel rotating disk dynamic filtration system was studied on the laboratory scale (0.147-ft.(2) nylon membrane) for concentrating recombinant yeast cells containing an intracellular product. The existing cross-flow microfiltration method yielded pseudo-steady state flux values below 25 LMH (L/m(2). h) even at low membrane loadings (10 L/ft.(2)). By creating high shear rates (up to 120,000(-1)) on the membrane surface using a rotating solid disk, this dynamic filter has demonstrated dramatically improved performance, presumably due to minimal cake buildup and reduced membrane fouling. Among the many factors investigated, disk rotating speed, which determines shear rates and flow patterns, was found to be the most important adjustable parameter. Our experimental results have shown that the flux increases with disk rotating speed, increases with transmembrane pressure at higher cell concentrations, and can be sustained at high levels under constant flux mode. At a certain membrane loading level, there was a critical speed below which it behaved similarly to a flat sheet system with equivalent shear. Average flux greater than 200 LMH has been demonstrated at 37-L/ft.(2) loading at maximum speed to complete sixfold concentration and 15-volume diafiltration for less than 100 min. An order of magnitude improvement over the crossflow microfiltration control was projected for large scale production. This superior performance, however, would be achieved at the expense of additional power input and heat dissipation, especially when cell concentration reaches above 80 g dry cell weight (DCW)/L. Although a positive linear relationship between power input and dynamic flux at a certain concentration factor has been established, high cell density associated with high viscosity impacted adversely on effective average shear rates and, eventually, severe membrane fouling, rather than cake formation, would limit the performance of this novel system. (c) 1995 John Wiley & Sons, Inc.     

Experimental Study of Mass Transfer Phenomena in a Cross Flow Membrane Bioreactor: Aeration and Membrane Separation

Experimental work carried out on wastewater from a wastewater treatment plant (WWTP) showed that in a cross flow membrane bioreactor the gas/liquid transfer is highly dependent on the biomass concentration. In new biological wastewater membrane treatment processes (mostly using deep end membranes), the biomass concentration is usually about 15 g/L, which entails a decrease in the bioreactor aeration capacity by a factor of approximately four compared with clean water. The gas/liquid transfer may therefore become a limiting step in this type of process. To prevent the operating costs of the biological treatment from increasing, it is imperative that the oxygen transfer be optimized. Membrane experiments showed that the permeate flux is highly dependent on the biomass concentration and the tangential velocity in the membrane module.     

Membrane fouling in crossflow filtration: Implications for measurement of the steady state specific cake resistance

Summary When the crossflow specific cake resistance is determined from measurements of the steady state cake mass and filtrate flux, an apparent value is obtained which is equal to the true value only if membrane fouling is negligible. The apparent specific resistance can increase with increasing crossflow velocity and pure-water membrane resistance, and exhibit a minimum with respect to transmembrane pressure.     

Application of thiophilic membranes for the purification of monoclonal antibodies from cell culture media

The application of thiophilic membranes for the purification of monoclonal antibodies from hybridoma culture media was studied. Affinity filtrations were performed with membrane stacks and also in a cross flow module with a spiral filtration channel. Purification factors up to five and concentration factors of about eight could be achieved. The flux behaviour was analysed and interpreted according to existing models of filtration. The results were confirmed by scanning electron microscopy. The binding capacity of the membranes differed considerably with the mode of operation. The main component responsible for membrane fouling was identified by sodium dodecyl sulfate polyacrylamide gel electrophoresis and amino acid sequence analysis as bovine serum albumin or its fragments.     

Dean vortex membrane microfiltration and diafiltration of rBDNF E. coli inclusion bodies

Cross-flow microfiltration (CMF) and diafiltration were used to concentrate and purify recombinant Brain-Derived Neutrophic Factor (rBDNF) inclusion bodies from an E. coli cell suspension and a homogenized E. coli cell suspension (homogenate/lysate). Although these processes have been tested industrially in pilot scale with conventional linear membrane microfiltration modules, their performances were severely limited due to membrane fouling. The purpose of this work was to determine whether Dean vortex microfiltration with controlled centrifugal instabilities (Dean vortices produced in helical flow) could be used to improve filtration performance over that observed with conventional linear cross-flow microfiltration (CMF). For the microfiltration experiments with the feeds containing cell and homogenate suspensions, improvements in flux of about 50 and 70%, respectively, were obtained with the helical module as compared with that obtained with the linear module. For diafiltration with the homogenate suspension as feed, solute transport (as measured by mass) was from 100 to 40% higher after 40 and 100 min, respectively, with the helical module as compared with that obtained with the linear module. In the presence of the neutral surfactant, Tween 20, solute transport for diafiltration was at least 25 times higher during the first 10 min of operation and 100% higher after 300 min with the helical module as compared with that obtained with the linear module. Clearly, improved filtration performance, a purer and more concentrated product, and substantial savings can be expected with the new Dean vortex filters.     

Understanding and modeling alternating tangential flow filtration for perfusion cell culture

Alternating tangential flow (ATF) filtration has been used with success in the Biopharmaceutical industry as a lower shear technology for cell retention with perfusion cultures. The ATF system is different than tangential flow filtration; however, in that reverse flow is used once per cycle as a means to minimize fouling. Few studies have been reported in the literature that evaluates ATF and how key system variables affect the rate at which ATF filters foul. In this study, an experimental setup was devised that allowed for determination of the time it took for fouling to occur for given mammalian (PER.C6) cell culture cell densities and viabilities as permeate flow rate and antifoam concentration was varied. The experimental results indicate, in accordance with D'Arcy's law, that the average resistance to permeate flow (across a cycle of operation) increases as biological material deposits on the membrane. Scanning electron microscope images of the post‐run filtration surface indicated that both cells and antifoam micelles deposit on the membrane. A unique mathematical model, based on the assumption that fouling was due to pore blockage from the cells and micelles in combination, was devised that allowed for estimation of sticking factors for the cells and the micelles on the membrane. This model was then used to accurately predict the increase in transmembane pressure during constant flux operation for an ATF cartridge used for perfusion cell culture. © 2014 American Institute of Chemical Engineers Biotechnol. Prog., 30:1291–1300, 2014     

Factors affecting membrane fouling in filtration of Saccharomyces cerevisiae in an internal ceramic filter bioreactor

Filtration of ethanol fermentation medium and broth by using symmetric and asymmetric ceramic membranes has been studied in an internal filter bioreactor. Factors studied included membrane structure and pore size, medium sterilization, and concentrations of glucose, yeast extract in the medium, yeast cell and protein in broth. The aim was to determine the main factors responsible for the decline in filtration performance during ethanol fermentation by Saccharomyces cerevisiae. Flux index (Fi) of a new concept has been developed to evaluate the degree of flux decline during the membrane fouling process. Fi was defined as the ratio of the membrane flux at certain filtration time (t?=?t) to the initial (t?=??0) flux of pure water, not the initial (t?=?+0) flux of the test fluid. Flux with sterilized medium was approximately two-fold higher than that with unsterilized medium although the reason could not be explained clearly. Glucose, interaction between glucose and yeast extract, yeast cells, and proteins in fermentation broth were found to play an important part in membrane fouling. Fi of the symmetric membrane decreased to a less extent than that of the asymmetric membrane with increasing glucose concentration. But, the result with various yeast cell concentrations turned out to be contrary. Fouling was more serious for asymmetric membrane during the filtration of fermentation supernatant. This was thought to be due to different fouling mechanisms for the two types of membrane.     

Protein fouling during constant-flux virus filtration: Mechanisms and modeling

As biomanufacturers consider the transition from batch to continuous processing, it will be necessary to re-examine the design and operating conditions for many downstream processes. For example, the integration of virus removal filtration in continuous biomanufacturing will likely require operation at low and constant filtrate flux instead of the high (constant) transmembrane pressures (TMPs) currently employed in traditional batch processing. The objective of this study was to examine the effect of low operating filtrate flux (5–100 L/m²/h) on protein fouling during normal flow filtration of human serum Immunoglobulin G (hIgG) through the Viresolve® Pro membrane, including a direct comparison of the fouling behavior during constant-flux and constant-pressure operation. The filter capacity, defined as the volumetric throughput of hIgG solution at which the TMP increased to 30 psi, showed a distinct minimum at intermediate filtrate flux (around 20–30 L/m²/h). The fouling data were well-described using a previously-developed mechanistic model based on sequential pore blockage and cake filtration, suitably modified for operation at constant flux. Simple analytical expressions for the pressure profiles were developed in the limits of very low and high filtrate flux, enabling rapid estimation of the filter performance and capacity. The model calculations highlight the importance of both the pressure-dependent rate of pore blockage and the compressibility of the protein cake to the fouling behavior. These results provide important insights into the overall impact of constant-flux operation on the protein fouling behavior and filter capacity during virus removal filtration using the Viresolve® Pro membrane.     

Effects of periodic backflushing with filtrate on filtration performance in an internal-filtration bioreactor

Periodic backflushing of tubular ceramic membrane filters with filtrate was employed to alleviate membrane fouling in a bioreactor with internal-filtration. As the model system, yeast fermentation was dealt with in this study. There existed optimum backflushing interval and time to give a maximum flux recovery. At 16 g/l of yeast cell concentration, the mean flux increased about 2.5 times by using such repeated operation cycles as consisted of 4.53 minutes for filtration, 4.5 seconds for intermission, and 40 seconds for backflushing. Effects of aeration, agitation speed, and yeast cell concentration were also investigated.