Pressure effects in cross-flow microfiltration of suspensions of whole bacterial cells

Abstract

The effect of Trans-Membrane Pressure (TMP) on permeate flux during cross-flow microfiltration of bacterial cell suspensions in tubular ceramic membranes is studied experimentally. Continuous filtration experiments with suspensions of whole bacterial cells (Mycobacterium M156) show a dramatic permeate flux decline with increasing TMP. During the very early stages of the filtration process, a linear relationship between permeate flux and TMP is observed, suggesting an initial surface sorption of cells on the membrane surface. At longer times, the permeate flux vs. TMP data exhibit a critical pressure beyond which the permeate flux declines with increasing trans-membrane pressure. This is interpreted in terms of the formation of a compressible cake, whose permeability can be described through the Carman-Kozeny equation. IntroductionThe application of conventional methods (such as dead-end filtration, sedimentation, and centrifugation) for the separation of cells from liquid streams in pharmaceutical applications in a continuous production line poses many practical difficulties, mainly because of the highly hydrated and glutinous character and of the low density of biological cells. An alternative method involves the use of cross-flow membrane filtration [1] in which the filtrate passes through the membrane by virtue of a pressure differential (the TransMembrane-Pressure or TMP). In cross-flow microfiltration, the product to be filtered is passed over a tubular or flat porous surface. The flowrate of the product stream across the surface tends to maintain the direction of flow and controls the thickness of the deposited cell layer. The cell suspension to be filtered is usually pumped at a velocity in the rage 1-8 m/s parallel to the surface of the membrane and with a TMP between 0.1-0.5 MPa. The liquid permeates through the membrane and the feed emerges in a more concentrated form at the exit of the module. The partially concentrated retentate can be recycled into the reactor while the permeate is removed for further processing. Figure 1 is a schematic description of the principle behind the operation of a crossflow microfiltration membrane.While the cross-flow mode is a significant improvement over dead-end filtration, permeate flux still decreases with time, even when the feed concentration is kept constant. Typical product fluxes of 0.138 · 10\4 m3 sec\1 m\2 to 0.415 · 10\4 m3 sec\1 m\2 are observed; these do not compare very favorably to threshold flux values of between 0.2 · 10\4 m3 sec\1 m\2 and 1.11 · 10\4 m3 sec\1 m\2 which have been quoted as necessary for cross-flow microfiltration to compete with such established operations as centrifugation and precoat filtration [2,3]. Still, the advantage of continuous operation afforded by cross-flow filtration has resulted in a sustained interest for this technology. Applications of cross-flow filtration include water and sewage treatment [4], harvesting of plant and animal cells [5], food processing [6] and dewatering of mineral slurries [7]. Both submillimetre and ...

Microfiltration conditions modify Lactobacillus bulgaricus cryotolerance in response to physiological changes

“…1). This decrease was explained by an increased concentration in cells and cell residues, which, when deposited on the membrane surface, led to a decrease in TMP [17] due to additional resistance to the permeate flux through the membrane [4]. This behavior differed from that observed by Carrère et al [18], who reported a faster reduction of the permeate flux at the beginning of the process followed by a constant and stable value.…”

“…The same behavior was previously described during microfiltration of Lactococcus lactis ssp. lactis ATCC19435 [6], Mycobacterium M156 [17] and Bacillus subtilis [19]. Finally, condition CF5 (3.25 m/s and 0.1 MPa) was characterized by an intermediate behavior.…”

Modeling the influence of slurry concentration on Saccharomyces cerevisiae cake porosity and resistance during microfiltration

“…According to Redkar and Davis15 and to Tanaka et al,10 the initial transient flux decline in the cross‐flow mode follows the dead‐end filtration law. Numerous yeast microfiltration experiments performed on membranes of pore size less than 1 μm confirm the cake filtration mode and the possibility of using the Carman–Kozeny equation to model the cake permeability/hydraulic resistance 4, 6–8, 21–24…”

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