Yeast-Fouling Effects in Cross-Flow Microfiltration ... - ACS Publications

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Ind. Eng. Chem. Res. 2003, 42, 130-139

Yeast-Fouling Effects in Cross-Flow Microfiltration with Periodic Reverse Filtration Wendy D. Mores and Robert H. Davis* Department of Chemical Engineering, University of Colorado, Boulder, Colorado 80309-0424

Yeast-foulant removal via reverse filtration from cellulose acetate microfiltration membranes was observed in situ using a direct visual observation (DVO) microvideo system. The sizes of the foulant blockages increased with filtration time as cells aggregated on the membrane surface and in the suspension. Coupling of the DVO-determined fractional coverage data with the experimentally determined membrane fluxes indicates that the flow through the cleaned portions of the membranes is nearly straight-through (i.e., little to no radial flow underneath foulant blockages), and the flux through the yeast-covered fraction with backpulsing is over twice the long-term fouled membrane flux without backpulsing. Cell rupture caused by reverse filtration at transmembrane pressures of 3 psi or more led to increased irreversible fouling of the membranes, with scanning electron microscopy pictures revealing a thin blanketing layer of foulant, which is most likely the contents of ruptured cells. 1. Introduction Since the first synthetic membranes became available approximately 40 years ago, the industry has grown enormously.1 Nevertheless, the membrane market growth has lagged behind expectations,2 in large part because of membrane fouling. One effective way to combat membrane fouling is periodic reverse filtration, an in situ cleaning technique which involves reversal of the flow through a membrane periodically to remove deposited foulants. The flux can be reversed for a fraction of a second every few seconds, as in rapid backpulsing, or for longer periods (for example, 10-120 s every 5-30 min), as in backflushing or backwashing.3,4 To make periodic reverse filtration as economically attractive as possible, several groups have maximized fluxes by optimizing the backwashing or backpulsing duration, frequency, and pressure.5-8 Models to predict optimal backpulsing conditions have been proposed by a number of researchers.4,9-11 Still, there remains little direct understanding of foulant deposition and removal for filtration with backpulsing or backwashing or of the different fluxes through the fouled and cleaned portions of the membrane surface. The size and permeability of foulant deposits, along with the degree of membrane pore interconnectivity, can greatly affect the filtration flux. Ho and Zydney12,13 recently performed studies on membranes with both interconnected and straight-through pores and have found that the rate of flux decline for membranes with highly interconnected pores is much smaller than that for membranes with straight-through pores because fluid is able to flow under and around blockages in more interconnected structures. In the case of membranes with interconnected pores, the degree of radial flow underneath foulant blockages is a function not only of the ratio of the radial and axial permeabilities of the membrane but also of the ratio of the radius of the blockage and the membrane thickness.14 Ho and * To whom correspondence should be addressed. Phone: 303-492-7314. Fax: 303-492-4341. E-mail: robert.davis@ colorado.edu.

Zydney15 developed a new technique to evaluate pore connectivity which consists of covering prescribed areas of the upper and lower membrane surfaces to change the relative contributions of the radial and axial flows. Experiments using poly(vinylidene fluoride) and poly(tetrafluoroethylene) membranes gave pore connectivity data consistent with the membrane morphologies and formation procedures. Microvideo methods can also be used to make quantitative observations concerning foulant coverage on membrane surfaces. A pioneering method, termed direct observation through the membrane (DOTM), was used by Hodgson et al.16 to observe the interactions of yeast and sulfonated styrene-based ion-exchange particles with Anopore anodized-alumina (AN) microfiltration membranes. By mounting a microscope on the permeate side, Hodgson et al.16 were able to visually observe the effects of varying the cross-flow velocity and transmembrane pressure as their experiments progressed. More recently, Li et al.17,18 have used DOTM in constant-flux experiments to determine critical fluxes below which there is no foulant deposition on the membrane surface. Mores and Davis10,11,19 have used a similar technique, termed direct visual observation (DVO), wherein the microscope is mounted on the feed side, enabling viewing of membranes, such as those made of cellulose acetate (CA), that are less transparent than AN membranes. Using DVO, Mores and Davis10 observed that the foulant deposition during forward filtration and removal during reverse filtration occur in nonuniform clumps. DVO pictures were used to quantify the fraction of membrane covered in foulant during forward filtration and after single backpulses at varying backpulse pressure and shear rate.11 Observations of the membrane surfaces were made at different filtration times for cross-flow microfiltration of yeast suspensions with rapid backpulsing at varied backpulsing duration and pressure.19 Membrane filtration can be used as an alternative to centrifugation in biotechnology applications to harvest cells, clarify cell lysates, and separate proteins from fermentation broths.20 Many studies involving yeast/

10.1021/ie020421k CCC: $25.00 © 2003 American Chemical Society Published on Web 12/06/2002

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membrane systems have been performed in efforts to improve fluxes in biological systems.21-24 In some cases, researchers have used rapid backpulsing to increase the fluxes for yeast systems.3,4,9,19 Cell response to applied forces in these studies is a factor of the strength and elasticity of the cell wall, the arrangement of the molecules comprising the cell wall, and genetic factors dealing with composition and assembly.25 Though cell mechanical properties have been investigated using micropipet aspiration, osmotic swelling/shrinking, cell poking, cell compression, and atomic force microscopy, there is still poor understanding of cell-wall mechanical properties,25-28 making it difficult to predict the responses of cells to applied forces.29 While cells are often assumed to remain intact in membrane applications, there is a possibility of cell rupture, particularly with rapid backpulsing. Cell rupture can lead to severe membrane fouling by the intracellular contents. In this paper, we examine fouling of CA membranes by yeast cells and cleaning of the fouled membranes by periodic reverse filtration. DVO is used to view the membrane surface before and after backwashes and backpulses at different filtration times. Using DVO pictures and the measured overall fluxes through the membrane, the different fluxes through the cleaned and uncleaned membrane fractions are investigated. Because the CA membranes have interconnected pores, at least some lateral flow underneath the fouled membrane fractions is expected. The effects of periodic reverse filtration and retentate recycling on yeast cell rupture are also investigated, with the fouling effects of the ruptured cell debris explored in the filtration experiments. Scanning electron microscopy (SEM) pictures of the fouled membrane surfaces are further used to explore the results of cell rupture. 2. Materials and Methods The microfiltration membranes are supported CA (pore size ) 0.2 µm; cat. no. A02SP04700, Micron Separations, Inc., Westborough, MA). Each membrane was used once and then discarded. The foulant is Saccharomyces cerevisiae (Fleischmann’s active dry yeast) from King Soopers retail market. The yeast was washed and dyed using techniques described previously.11 Yeast concentrations of 0.05 and 0.1 g/L (based on dry weight after washing) were used. The filtration apparatus and DVO system have been described previously.10 In most experiments, the retentate was returned to the feed line. In select cases, however, the retentate was discarded. The membrane surface was directly viewed from the feed side using a Nikon Labophot microscope. Quantitative measurements of the foulant coverage were made using the public-domain NIH Image program (developed at the U.S. National Institute of Health and available on the Internet at http://rsp.info.nih.gov/nih-image/). The acrylic device housing the membrane is composed of one channel, 2.35 cm wide by 2.35 cm long by 0.08 cm high. DVO pictures shown were taken within a 9 mm2 square area centered 1 cm from the feed inlet and an equal distance from the two sides of the membrane; the entire membrane surface was also monitored to confirm that this small area is representative of the entire membrane. In some instances, the membranes were carefully removed from the module at the end of the experiment, and SEMs (Stereoscan 250 MK3, Cambridge) were made at high magnification.

Deionized water was first filtered through each membrane at a forward transmembrane pressure of ∆Pf ) 3 psi for 2 min to determine the clean membrane flux, Jo. The suspension was then filtered through the membrane for t ) 1.5-2 h while reversing the flow for tb ) 0.4-10 s at a reverse transmembrane pressure of ∆Pb ) 1-12 psi after every forward-filtration period of duration tf ) 10-60 s and transmembrane pressure ∆Pf ) 3 psi. In some cases, the membranes were backwashed with deionized water after the 1.5-2 h of filtration with backpulsing. Individual backwashes of the following durations (in seconds) were employed: tbw ) 0.1, 0.1 (0.2), 0.1 (0.3), 0.1 (0.4), 0.2 (0.6), 0.4 (1), 0.2 (1.2), 0.8 (2), 1 (3), 1 (4), 2 (6), 6 (12), and 288 (300). Each number in parentheses is the cumulative backwash duration up to that point. Backwash pressures of ∆Pbw ) 1-12 psi were used. Water was run over the membrane surface during each backwash. After each backwash, the permeate line was closed and water was run over the membrane for 10 s, after which the permeate line was re-opened and the recovered clean-water flux (Jr) was determined. Some experiments were performed using a phosphate buffer saline (PBS) buffer to suspend the yeast cells instead of deionized water. PBS buffer was prepared by adding 32 g of sodium chloride (cat. no. S271-3, Fisher Scientific, Fair Lawn, NJ), 0.8 g of potassium chloride (cat. no. BP366-500, Fisher Scientific), 5.76 g of sodium phosphate (cat. no. S374-500, Fisher Scientific), and 0.96 g of potassium phosphate (cat. no. P285-500, Fisher Scientific) to 4 L of deionized water. All experiments were done at room temperature (2225 °C) using a forward transmembrane pressure of ∆Pf ) 3 psi and a recirculation flow rate of 1.6 mL/s. The corresponding nominal wall shear rate is 640 s-1, the Reynolds number is 68 (laminar flow), and the average superficial fluid velocity over the membrane is 8.5 cm/ s. 3. Results and Discussion 3.1. Periodic Backwashing Experiments. Experiments were first conducted by filtering 0.1 g/L of washed yeast through 0.2 µm CA membranes, with backwashing for 10 s after every 60 s of forward filtration. Figure 1 depicts forward and reverse membrane fluxes for one typical experiment with periodic backwashing, as well as the flux decline for an experiment without backwashing (solid line). Flux data were taken every 2 s during forward filtration and every 1 s during reverse filtration. Forward fluxes are given at the beginning (Jf,i, open circles) and end (filled circles) of each forward-filtration period; likewise, reverse filtration fluxes are given at the beginning (filled squares) and end (open squares) of each backpulse period. The next flux through the membrane, 〈J〉, is calculated as

∫tt +t Jf dt + ∫tt+t+t +t Jb dt c

〈J〉 )

c

f

c

c

t f + tb

f

f

b

(1)

where Jf is the positive flux during forward filtration, Jb is the negative flux during reverse filtration (backwashing), tf is the duration of the forward-filtration period between backwashes, tb is the backwash duration, tc is the time at the start of the cycle, and t is the filtration time. When backwashing was not used, the

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single cells (≈5 µm in diameter). The sizes of the foulant blockages increased over time as cells aggregated on the membrane surface or cell aggregates formed in the suspension were deposited. After 60 min of filtration, the foulant blockages had grown to aggregates with diameters of up to 100 µm (Figure 3). After 2 h of filtration, there remained only small (≈5-10 µm in diameter) patches of uncovered membrane. The overall forward fluxes may be expressed as the sum of the fluxes through the covered and uncovered portions of the membrane surface using the f values from Figure 3:

Jf ) fJc + (1 - f)Ju

Figure 1. Dimensionless forward and reverse fluxes as a function of filtration time. A suspension of 0.1 g/L washed yeast in water was filtered at room temperature (22-24 °C) through a 0.2 µm CA membrane at ∆Pf ) 3 psi and γ ) 640 s-1, with the retentate recycled back to the feed. Membranes were backwashed at ∆Pb ) 3 psi for tb ) 10 s after every tf ) 60 s of forward filtration. Forward-filtration fluxes are given at the beginning (O) and end (b) of each forward-filtration period; reverse-filtration fluxes are given at the beginning (9) and end (0) of each backwash period. The net fluxes calculated using eq 1 are given by ×’s. The solid line depicts the flux decline when backwashing was not used.

flux declined from a clean membrane flux of Jo ) 3500 ( 300 LMH (L/m2-h) to a fouled membrane flux of Js ) 510 ( 80 LMH after 7200 s of filtration. When periodic backwashing was used, a net flux after 7200 s of filtration of 〈J〉 ) 1100 ( 200 LMH was achieved, about twice that of the fouled membrane flux. Errors are (1 standard deviation for three experiments. The forward flux declined during the first 60 s to 80% of its initial flux as particles accumulated on the membrane surface and blocked the pores. The reverse flux increased in magnitude 80% during the first backwash (t ) 60-70 s) as foulants were pushed off and swept away from the membrane surface. As the experiment progressed, the membrane became irreversibly fouled by the yeast, and both the forward and reverse fluxes decreased. The flux at the beginning of each forward-filtration period decreased from Jf,i/Jo ) 1 at t ) 0 s to Jf,i/Jo ) 0.44 at t ) 7200 s, with only a slight flux decline during each period of forward filtration. The net flux reached a nearly steady value after approximately 6500 s of filtration time. Figures 2 and 3 show DVO pictures of the membrane surface during the experimental run depicted in Figure 1. Pictures during the initial 60 s of forward filtration and after the first 10 s backwash (delayed slightly while the membrane comes back into focus after the backwash) are shown in Figure 2. Yeast buildup during forward filtration is quite large, but the majority of the foulant was removed by the backwash. In Figure 3, pictures are given directly (1) before and (2) after backwashes at t ) 1, 5, 10, 30, 60, and 120 min. The fraction of the membrane covered with foulant, f, is given for each picture taken before and after a backwash. The membrane surface coverage increases with time (Figure 4), with backwashing becoming less effective as foulants are irreversibly bound. Initially, the yeast deposition on the membrane surface is mostly

(2)

where Ju is the flux through the uncovered fraction of the membrane and Jc is the flux through the yeastcovered fraction of the membrane. For initial estimates, the experimentally determined clean-membrane flux of Jo ) 3500 LMH was used for the flux through the uncovered fraction of the membrane, and the fouled membrane flux without backwashing of Js ) 510 LMH (Figure 1) was used for the flux through the covered fraction of the membrane. Using these values in eq 2, together with DVO measurements for f, the DVO-based fluxes at the beginning of forward-filtration cycles are compared to the measured fluxes in Figure 4. The DVObased fluxes from eq 2 increasingly underpredict the experimental data as the filtration time increases. This finding indicates that either the flux through the cleaned fraction of the membrane is higher than the clean-membrane flux (because of lateral flow of liquid under blocked pores), the flux through the fouled fraction of the membrane is greater than the fouled membrane flux (because of a smaller cake when backwashing is used), or a combination of these two possibilities. To better understand the fluxes through the covered and uncovered fractions of the membrane, the experimental data were fitted to a model presented by Ho and Zydney.14 They make use of Darcy’s law and the continuity equation to predict the flow through a single cylindrical region covered either with a central blockage or an annular blockage. Three dimensionless parameters appear in the model: the permeability ratio K, the dimensionless fouling layer permeability β, and the fractional pore blockage f. The permeability ratio is defined by

K)

()

Kz rb K r δm

2

(3)

where Kz and Kr are the membrane permeabilities in the axial and radial directions, respectively, rb is the blockage radius, and δm is the membrane thickness. At large K values (K g 1000), there is little to no radial fluid flow through the membrane. As K is decreased, radial fluid flow under the foulant blockage increases, reducing the effects of the blockage on the membrane flux. The dimensionless fouling layer permeability is defined as the ratio of the foulant layer permeability to the membrane permeability, or equivalently the ratio of the membrane resistance (Rm) to the foulant layer resistance (Rf). Using Darcy’s law for resistances in series, the dimensionless foulant layer permeability is

β)

1/Jo Rm ) Rf 1/Jc - 1/Jo

(4)

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Figure 2. DVO pictures of the surface of the membrane during the first cycle of the experiment described in Figure 1. A picture of the cleaned membrane was taken as soon as it came into focus after the backwash (t ) 71 s).

where it is assumed that internal membrane fouling is negligible, so that the membrane resistance remains equal to its initial value. The fractional pore blockage, f, is the fraction of membrane covered by foulant and can be determined from DVO pictures. Figure 5 is a plot of the permeate flux at the beginning of the forward portion of each cycle versus the fractional coverage of the membrane by yeast foulant. The experimental data are taken from Figure 4 and shown as symbols, whereas the solid curves represent the central blockage model of Ho and Zydney14 for β ) 0.17. This β value corresponds to the dimensionless foulant permeability calculated from eq 4 using Jc ) Js ) 510 LMH (the fouled membrane flux without backwashing) as a lower bound. The apparent K values for the choice β ) 0.17 are between 10 and 100, and they decrease weakly with increasing fractional surface coverage. However, eq 3 indicates that decreasing K values imply decreasing blockage size, but the DVO pictures (Figure 3) show that the typical blockage size just after each backpulse increases as time progresses and the fractional coverage increases. Thus, the limit β ) 0.17 does not provide a very good fit of the data. The dashed lines in Figure 5 are for the limit K f ∞, for which the flow is essentially straight through the membrane pores (i.e., no pore interconnectivity or radial fluid flow). In this limiting case, the flow through the uncovered portion of the membrane is assumed to be equal to the clean membrane flux, Ju ) Jo ) 3500 LMH. The best-fit value of the dimensionless permeability is then β ) 0.5, which from eq 4 corresponds to fluxes through the covered fraction of approximately Jc ) 1200 LMH. The model fit is better in this case than the case for the solid lines, and so the data are adequately described by eq 2 with Ju ) Jo ) 3500 LMH and Jc ) 1200 LMH. If internal membrane fouling occurred, so

that Ju < Jo, then a value of Jc > 1200 LMH for the flux through the covered portion of the membrane would be required. 3.2. Effects of Yeast Cell Rupture. A typical SEM picture of the surface of a membrane used in one of the experiments described above is shown in Figure 6a. The surface is blanketed in some areas by a thin fouling layer which is presumably the remains and contents of ruptured cells. The vigorous filtration conditions with periodic permeate reversal apparently caused cell rupture, changing the composition of the foulant suspension. Indeed, prior work on microbial-cell rupture has shown that rupture in homogenizers occurs because of large compressive forces or shear stresses, especially when rapid variations occur because of cavitation, turbulence, or large pressure gradients as cells pass through a release valve.30-32 In the hopes of eliminating fouling due to ruptured cells, some experiments were conducted in which the retentate was discarded because the retentate may have contained broken cells and their contents caused irreversible fouling. Figure 7 shows the normalized flux versus filtration time for an experimental run under the same conditions as those in Figure 1 but with the retentate discarded. While the net flux for the backwashing experiment in Figure 1 (retentate recycled to the feed) declines from 〈J〉/Jo ) 0.66 at t ) 0 s to 〈J〉/Jo ) 0.32 at t ) 7200 s, that in Figure 7 (retentate not recycled) declines only from 〈J〉/Jo ) 0.76 to 0.66. DVO pictures of the membrane surface just before and after a backwash show the much smaller degree of foulant coverage when the retentate is discarded (Figure 8) rather than recycled (Figure 2). The surface of the membrane used in Figures 7 and 8 is shown in Figure 6b to be covered only in whole yeast cells; the blanketing substance depicted in Figure 6a is absent when the

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Figure 3. DVO pictures of the membrane for several different cycles of the experiment described in Figure 1. Cumulative filtration times and fractions of the membrane covered with foulant (f) are given. Pictures of the membrane directly (1) before and (2) after a backwash are shown.

retentate is discarded and most likely accounts for the majority of the irreversible fouling when the retentate is recycled. Moreover, the contents of broken cells can cause internal fouling within the membrane pores, which may account for a portion of the lower long-term flux when the retentate is recycled. In the experiments described above, the yeast cells were suspended in deionized water. The absence of phosphate in the suspending solution can lead to an unstable membrane potential,33 increasing the likelihood of cell rupture. Experiments were, therefore, repeated using PBS buffer to suspend the yeast cells, but similar results were obtained when deionized water was used. The cell rupture can be caused by stresses imposed by the peristaltic feed pump, by shear stress as the feed moves through the membrane module and tubing, or by transient but potentially large stresses from switching between forward and reverse filtration. To discern

whether the feed pump and membrane module were the major causes of cell rupture, the experiments of Mores and Davis11 were repeated. In these experiments, a foulant suspension was filtered for 1.5 h using ∆Pf ) 3 psi and γ ) 640 s-1, after which single backwashes were used to remove the remaining reversibly bound foulant. A recovered flux was measured after each single backwash. While Mores and Davis11 did not use rapid backpulsing during the initial 1.5 h of filtration, backpulsing at ∆Pb ) 3 psi for 0.4 s after every 10 s of forward filtration was used in the present work. The retentate was returned to the feed tank in both cases. The recovered fluxes (filled points) and model fits (solid lines) from Mores and Davis11 are shown in Figure 9. The recovered fluxes obtained by single backwashes after filtering with rapid backpulsing for 1.5 h are given by the open points. Single backwash pressures of ∆Pbw ) 1 psi (circles), 3 psi (squares), and 12 psi (diamonds) were used. Errors are (1 standard deviation. The flux

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Figure 4. Measured forward-filtration fluxes (O) and fractional surface coverage (b) versus filtration time for the conditions of Figure 1. Measurements were made at the beginning of each forward-filtration period. Fluxes calculated using eq 2 with Ju ) Jo ) 3500 LMH and Jc ) Js ) 510 LMH are depicted by open squares.

Figure 6. SEM pictures of 0.2 µm CA membranes after 7200 s of filtration at room temperature (22-24 °C) using 0.1 g/L of washed yeast, ∆Pf ) ∆Pb ) 3 psi, γ ) 640 s-1, tf ) 60 s, and tb ) 10 s (a) with and (b) without retentate recycle.

Figure 5. Normalized forward-filtration flux as a function of fractional surface coverage for the central blockage model of Ho and Zydney.14 Measured fluxes are given by open circles. The solid curves are for β ) 0.17 and (top to bottom) K ) 1, 10, 100, and ∞, while the dashed curves are for K f ∞ and (top to bottom) β ) 0.75, 0.50, 0.25, and 0.

after the initial 1.5 h of filtration is much higher when backpulsing was used, with a flux increase from Js/Jo ) 0.16 ( 0.02 to 〈J〉/Jo ) 0.39 ( 0.03. However, backpulsing appears to increase the irreversible fouling of the membranes, because the recovered fluxes increase minimally when the single backwash pressure was ∆Pbw ) 1 or 3 psi and only to Jr/Jo ) 0.55 ( 0.02 when ∆Pbw ) 12 psi. When rapid backpulsing was not used during the initial 1.5 h of filtration, flux recovery via the single backpulses is quite high, with recovered fluxes reaching Jr/Jo ) 0.95 ( 0.02 using ∆Pbw ) 12 psi. Apparently, rapid backpulsing ruptures some of the cells, and the resulting cell debris and released intracellular content cause irreversible fouling on the membrane surface and within the membrane pores. The membrane surfaces were examined using SEM to observe whether the cells were ruptured and to better understand why the membranes were more irreversibly fouled when rapid backpulsing with ∆Pb ) 3 psi was

Figure 7. Dimensionless forward and reverse fluxes versus filtration time for the same conditions as those in Figure 1 but with the retentate discarded. Forward-filtration fluxes are given at the beginning (O) and end (b) of each forward-filtration period; reverse-filtration fluxes are given at the beginning (9) and end (0) of each backwash period. The net fluxes calculated using eq 1 are given by ×’s.

used. In Figure 10, two magnifications of membrane surfaces are shown at the end of experiments in which rapid backpulsing was not (Figure 10a) and was (Figure

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Figure 8. DVO pictures of the membrane for two different cycles of the experiment described in Figure 7. Cumulative filtration times and fractions of the membrane covered with foulant (f) are given. Pictures of the membrane directly (1) before and (2) after a backwash are shown.

Figure 10. SEM pictures of 0.2 µm CA membranes fouled by 0.05 g/L of washed yeast after 1.5 h of forward filtration using retentate recycle (a) without and (b) with rapid backpulsing followed by a total of 300 s of backwashing. Experiments were run at room temperature (22-24 °C) with tf ) 10 s, tb ) 0.4 s, ∆Pf ) ∆Pb ) ∆Pbw ) 3 psi, and γ ) 640 s-1. Two magnifications (1500× and 3000×) are shown.

Figure 9. Dimensionless recovered flux after single backwashes of varying cumulative duration. Suspensions of 0.05 g/L of washed yeast in water were filtered through 0.2 µm CA membranes at room temperature (22-24 °C) for 1.5 h both with (open symbols) and without (closed symbols) rapid backpulsing using tf ) 10 s, tb ) 0.4 s, ∆Pf ) ∆Pb ) 3 psi, and γ ) 640 s-1. After 1.5 h of forward filtration, membranes were cleaned via single backwashes using ∆Pbw ) 1 psi (b, O), 3 psi (9, 0), and 12 psi ([, ]) and recovered fluxes were measured after each backwash. Fits from Mores and Davis11 are given for the no-backpulsing cases. Errors are (1 standard deviation for three repeats.

10b) used during the 1.5 h of forward filtration. In Figure 10a, only a few yeast cells are seen to have deposited on the otherwise clean membrane surface. In Figure 10b, however, much of the membrane is covered in a thin fouling layer, again presumed to be the broken cell debris and intercellular content; some whole yeast cells can also be seen. By comparison of Figures 10b and 6a, it is seen that more fouling by intracellular content occurs with rapid backpulsing than with less frequent backwashing. From these results, it is concluded that backpulsing is the primary cause of cell rupture. This result is not surprising because the forces required to break yeast cells are on the order of 100 µN (Kleinig and Middelberg30), which would require shear rates on the order of 1010 s-1 (much higher than shear rates

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Figure 11. Dimensionless recovered flux after single backwashes of varying duration. Suspensions of 0.05 g/L of washed yeast in water were filtered through 0.2 µm CA membranes at room temperature (22-24 °C) for 1.5 h both with (b, 9, [, O, 0, ]) and without (×, open crosses) rapid backpulsing using tf ) 10 s, tb ) 0.4 s, ∆Pf ) 3, γ ) 640 s-1, and backpulsing pressures of ∆Pb ) 1 psi (b, O), 3 psi (9, 0), and 12 psi ([, ]). Open symbols indicate rapid backpulsing experiments in which the retentate was not recycled back to the feed tank, while closed symbols are with recycle. After 1.5 h of forward filtration, membranes were cleaned via single backwashes using ∆Pbw ) 3 psi. A fit from Mores and Davis11 is given for the no-backpulsing cases. Errors are (1 standard deviation for three repeats.

present in the feed pump, tubing, and membrane module). Instead, the rapid compression and decompression as a backpulse is applied and released are the more likely cause of cell rupture. To better understand under which backpulsing conditions yeast cells are broken, the experiments depicted in Figure 9 were repeated using different rapid backpulsing pressures. The transmembrane pressure for foulant removal by individual backwashes was kept constant at ∆Pbw ) 3 psi for comparison purposes. The results are again compared to the no backpulsing case of Mores and Davis.11 In some cases, the retentate was discarded to eliminate any feed contamination. Figure 11 shows the results of experiments in which the backpulse pressure was varied from ∆Pb ) 1 psi (circles) to 3 psi (squares) to 12 psi (diamonds), with the retentate recycled (filled points) and the retentate discarded (open points). Experimental and model (solid line) results from Mores and Davis11 are given for the no-backpulsing case with retentate recycle (×) and retentate removal (open crosses) using ∆Pb ) ∆Pbw ) 3 psi. Errors are (1 standard deviation for three experiments. While rapid backpulsing at ∆Pb ) 1 psi proves ineffectual in removing the foulant cake during the 1.5 h forward filtration, it allows the cake to be easily removed by the single backwashes, as shown by the sharp curve up to Jr/Jo ) 0.8 ( 0.2 after 1.2 s of cumulative backwashing. Backpulsing at ∆Pb ) 3 and 12 psi increases the flux during the 1.5 h filtration, but there is little or no flux recovery during the single backwashes. Irreversible fouling of the membranes is much more drastic at higher backpulse pressures. The large stresses experienced by the yeast cells during the switch between forward and reverse filtration at strong backpulse pressures (∆Pb g 3 psi) cause some cells to rupture. Once ruptured, the cell walls and

Figure 12. SEM pictures of 0.2 µm CA membranes fouled by 0.05 g/L of washed yeast after 1.5 h of forward filtration with rapid backpulsing followed by a total of 300 s of backwashing in cases where the retentate was (1) discarded or (2) recycled back to the feed tank. Experiments were run at room temperature (22-24 °C) with tf ) 10 s, tb ) 0.4 s, ∆Pf ) ∆Pbw ) 3 psi, and γ ) 640 s-1. Reverse transmembrane pressures during backpulsing of ∆Pb ) (a) 1 psi, (b) 3 psi, and (c) 12 psi were used.

intercellular material can either pass through the membrane, deposit on the membrane surface, or be carried out with the retentate. If the backpulses are long, the ruptured cells and their contents will be swept away before forward filtration begins again (as in the case of Figure 7 when tb ) 10 s). However, when the duration of reverse filtration is shorter, there is a chance that the ruptured cell material will immediately foul

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the membrane. To determine whether the cell debris immediately fouled the membrane or were carried back to the feed suspension, experiments were conducted with the retentate discarded. The flux after rapid backpulsing at ∆Pb ) 12 psi is the highest at Jr/Jo ) 0.84 ( 0.04, though the recovered flux after the single backwashes is greatest for ∆Pb ) 1 psi (Jr/Jo ) 0.97 ( 0.03 after 4 s of backwashing). The recovered fluxes for ∆Pb ) 3 and 12 psi when the retentate was not recycled are much larger those than when the retentate was recycled to the feed, indicating that ruptured cells contribute greatly to the irreversible fouling of the membranes but do not foul the membrane immediately after being ruptured. Backpulsing at ∆Pb ) 1 psi does not appear to cause many cells to break because the recovered fluxes are large for both the discarded and recycled retentate cases. Moreover, rapid backpulsing at low pressure (∆Pb ) 1 psi) appears to result in a less adhesive foulant cake because the recovered fluxes rise more quickly and reach equal to or greater values than those achieved in the no-backpulsing case. SEM pictures of the membrane surfaces are shown in Figure 12 for both the (1) discarded and (2) recycled retentate cases. In Figure 12a, when the rapid backpulsing pressure was ∆Pb ) 1 psi, there is little difference between the membrane surfaces. However, in parts b and c of Figure 12 (∆Pb ) 3 and 12 psi, respectively), the membrane is covered in a thin foulant layer when the retentate was recycled; when the retentate was discarded, the membrane surface is covered only in scattered whole yeast cells.

The conditions and effects of cell rupture were also investigated. It is hypothesized that rapid backpulsing causes some yeast cells to rupture, leading to greater irreversible fouling of the membrane. Once ruptured, the cell walls and intracellular material can either pass through the membrane, deposit on the membrane surface or within the pores, or be carried out with the retentate. Membrane surfaces and fluxes were, therefore, compared for the cases of no backpulsing, backpulsing with varying backpulse pressure, retentate recycle, and retentate discard. When the retentate was recycled to the feed and backpulse pressures of ∆Pb g 3 psi were used, though the flux after the initial 1.5 h of filtration was higher than in the no-backpulsing case, the flux decline was more severe, the flux could not be recovered via backwashing, and SEM pictures show that the membrane was blanketed in some areas by a thin fouling layer. This blanketing layer, most likely the contents of ruptured cells, is believed to account for the majority of the irreversible fouling of the membranes. The recovered fluxes for ∆Pb ) 3 and 12 psi are much greater when the retentate was discarded than when it was recycled, indicating that irreversibly fouling ruptured cells do not foul the membrane immediately after being ruptured but rather are carried out in the retentate stream. Backpulsing at ∆Pb ) 1 psi did not cause much cell rupture, but it did result in a less adhesive foulant cake because the recovered fluxes were found to increase more quickly with backpulse duration and to higher values than those in the no-backpulsing case. Acknowledgment

4. Concluding Remarks Yeast-foulant removal via periodic reverse filtration from CA microfiltration membranes was observed using a DVO microvideo system. The fluxes through the membranes declined with filtration time because of both reversible and irreversible foulant buildup on the membrane surface, with net fluxes declining to half of their original values after 2 h of filtration and fluxes during the forward-filtration periods declining to 44% of their original values. DVO pictures were used to determine the fraction of membrane covered in foulant before and after backwashes at various filtration times. Pictures show that periodic backwashing is less effective as the filtration time and irreversible fouling increase; furthermore, the sizes of the foulant blockages were observed to increase as cells aggregated on the membrane surface and in the suspension. The fractional coverage data coupled with flux measurements indicate that either the flux through the yeast-covered fraction of membrane is greater than the fouled membrane flux because of a smaller foulant cake buildup, the flux through the uncovered membrane fraction is greater than the clean membrane flux because of lateral liquid flow, or a combination of these two possibilities. The model of Ho and Zydney14 was used to better understand the fluxes through the covered and uncovered fractions of the membrane. The theory predictions using K f ∞ (i.e., a membrane with straight-through pores) and β ) 0.5 (i.e., a covered fraction flux over twice the long-term fouled membrane flux without backpulsing) give a good fit to the experimental data, even though CA membranes have interconnected pores. However, the data show some scatter, and so a small amount of lateral flow of permeate (with a slightly lower foulant layer permeability) may have occurred.

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Received for review June 3, 2002 Revised manuscript received September 26, 2002 Accepted October 10, 2002 IE020421K