Filtration characteristics and structure of cake in crossflow filtration of bacterial suspension

JOURNAL Or FERMENTATION AND BIOENGINEERING Vol. 78, No. 6, 455-461. 1994

Filtration Characteristics and Structure of Cake in Crossflow Filtration of Bacterial Suspension

TAKAAKI TANAKA, KEN-ICHI ABE, HIROYUKI ASAKAWA, H I D E T O S H I YOSHIDA, AND K A Z U H I R O NAKANISHI*

Department of Biotechnology, Faculty of Engineering, Okayama University, Tsushima-Naka, Okayama 700, Japan

Received 24 June 1994/Accepted 21 September 1994

A suspension of various microorganisms was cross-filtered and the filtration characteristics were studied. In crossflow filtration of Corynebacterium glutamicum, an ellipsoidal-shaped bacterium, the experimental permeation flux was in agreement with the calculated value based on the filtration theory, using the specific resistance measured in dead-end filtration and the amount of cake per unit filtration area. The cells deposited on the membrane in the same manner as in dead-end filtration. On the other hand, in crossflow filtration of Bacillus species, all of which are rod-shaped cells, the cells in the cake formed on the membrane were oriented toward the direction of the circulation flow. This arrangement of the cells increased the specific resistance of the cake, which made the flux lower than the calculated value, using the specific resistance measured in dead-end filtration. Furthermore, we clarified that the degree of the cell arrangement was dependent on the operational conditions in the crossflow filtration of rod-shaped cells.

Crossflow filtration has been used to separate microbial cells from broths in fermentation processes (1-4). It has many advantages over conventional filtration and centrifugation. In particular, the permeate does not contain the microbial cells when an appropriate membrane is used and the scale-up of crossflow filtration is achieved with ease. However, the method of determining the suitable operational conditions has not yet been established, since factors affecting the permeation flux, such as operational conditions, medium components and properties of microorganisms, have not been sufficiently clarified. In crossflow filtration, the microbial cake formed on the membrane usually governs the resistance to permeation. Thus, analysis of the structure of the cake is crucial to clarify the mechanism of crossflow filtration. Several papers have been reported which deal with the relationship between the permeation flux or resistance to filtration and structure of the microbial cake. The medium components sometimes affect the cake structure (5, 6). Nagata et al. studied the cause for the decline of flux during crossflow filtration of Bacillus poly- myxa broth and clarified that magnesium ammonium phosphate precipitate formed during steam sterilization of the medium increased the resistance of the microbial cake (5). The increase in cake resistance due to the exis- tence of fine particles in the medium was also pointed out in the crossflow filtration of baker 's yeast cells cultivated in molasses (6). Some antifoams such as polypropy- lene glycol reduce the permeation flux in crossflow filtration fermentation broths (7). The shape, size and structure o f the cell wall or cell membrane also affect the structure of microbial cake. Nakanishi et al. studied in particular the effect of size and shape of microorganisms on the specific resistance of microbial cake in dead-end filtration (8). Hodgson et al. recently reported that the extracellular matrix of marine bacterium considerably affects the cake structure in dead-end filtration (9). How- ever, few studies have been reported on the structure of

* Corresponding author.

the microbial cake during crossflow filtration where shear stress is parallel to the membrane surface.

We have studied the permeation behavior in the crossflow filtration of Saccharomyces cerevisiae cells and Escherichia coli ceils suspended in 0.90/00 NaCI solution, particularly focusing on the relationship between the structure of the cake and the permeation behavior. In the crossflow filtration of the suspension of S. cerevisiae cells, which are almost spherical in shape, the cells deposited in a manner similar to that in the dead-end filtration (10). On the other hand, in the case of E. coli cells which are rod-shaped, we found that the cells deposited toward the direction of circulation flow by shear stress during crossflow filtration, while they deposited at random during dead-end filtration (11).

In this study, we cross-filtered suspensions of various bacteria such as Corynebacterium glutamicum, Bacillus subtilis, Bacillus brevis, and Bacillus cereus in 0.90/00 NaC1 solution as a model system to clarify in particular the effect of the shape of bacteria on the cake structure and permeation flux. We also performed dead-end filtration of the bacteria and compared cake structures with those formed in crossflow filtration.

MATERIALS AND METHODS

Microorganisms An ellipsoidal bacterium, C. glutamicum, and three rod-shaped bacteria, B. subtilis IFO 3009, B. brevis IFO 3331, and B. cereus IFO 13494, were used.

Cultivation All cells were cultivated in a medium containing 0.5%0 yeast extract (Difco Laboratories, Detroit, MI, USA), 0.5% Polypepton (Nippon Phar- maceutical Co., Tokyo), 0.5% NaC1, and 1% glucose. The pH of the medium was adjusted to 7.0 with 1 N NaOH. The microorganisms were cultivated at 30°C with reciprocal shaking (120 strokes per minute). The cells were inoculated into 100 cm 3 of the medium contained in a 500-cm 3 shaking flask as a seed culture. The seed culture of C. glutamicum was incubated for 10 h, while

455

456 TANAKA ET AL. J. FERMENT. BIOENG.,

those o f other bacteria were incubated for 24 h. Then, 8 cm 3 of the seed culture was inoculated into 400 cm 3 of the medium contained in a 2,000-cm 3 shaking flask as the main culture. C. glutamicum was cultivated for 14 h, while other bacteria were cultivated for 24 h. Af ter cultivation, the cells were collected by centr i fugat ion and washed with 0.9%o NaC1 solution. They were then suspended in 0.9%o NaC1 solut ion and used for the follow- ing experiments.

Membrane A microfl l t rat ion membrane, C045 (Toyo Roshi Co. Ltd . , Tokyo) , was used for both dead- end and crossflow fil tration experiments. The membrane was a screen-type filter with a nominal pore size of 0.45 [Lm.

Measurement of specific resistance The specific resistance was measured at 20°C in dead-end filtration. A cell suspension in 0 . 9 ~ NaC1 solut ion was filtered with the aid of pressure f rom a ni trogen gas cylinder. A module with a fil tration area of 38.1 cm 2 and a depth of 7 cm was used for the measurement of the specific resistance by the unsteady-state method. The specific resistance of the microbial cake was calculated from the change in the permeate volume with t ime as described elsewhere (8). In the measurement by the steady-state method, a module with a fi l tration area of 7 .07cm 2 and a depth of l cm was used to at tain steady-state fil tration in a short time.

The dependence of the specific resistance on the transmembrane pressure was evaluated with the compressibility index (8). The compressibi l i ty index, n, is defined as the slope of a linear par t of a plot of logar i thm of specific resistance, a, versus logar i thm of t ransmembrane pressure, ~P.

Crosstlow filtration Crossflow filtration was performed at 20°C with a thin-channel type module made of po lycarbona te having a fi l tration area of 24cm 2 (2 .4cm in width and 10cm in length). The channel depth can be changed and usually it was set to 2.5 mm. A cell suspension was circulated with a ro tary pump (RM10, N a k a m u r a Metal Co. , Osaka) equipped with a variable-speed drive (Ringcorn RXM-400, Shimpo Indus- try Co. , Kyoto). The permeate was returned to the reser- voir tank to keep the cell concentra t ion constant . Crossflow fil tration was per formed for various dura- tions. Af ter the experiment, we measured the weight of the cake formed on the membrane . The circulat ion flow rate, the t ransmembrane pressure and the cell concentra- t ion were usually 30 cm3/s, 49 kPa and 5 k g / m 3 in wet weight, respectively. The average linear velocity of circulat ion flow was 0.5 m/s .

To examine the effect of filtration condit ions, the circulat ion flow rate, the channel depth, the t ransmembrane pressure, and the cell concentra t ion were changed in the range of 10-60cm3/s, 1.5-5.5 mm, 29-98 kPa, and 1.25- 10 k g / m 3, respectively. The linear velocity of the circulation flow ranged f rom 0.2 to 1.0 m/s .

Evaluation of resistance of cake and resistance caused by membrane plugging The resistance caused by membrane plugging, Rp, was measured as follows. The total resistance to the permeat ion during crossflow filtra- t ion, Rt, is calculated using Eq. 1,

R t : AP/(/~. J) (1)

where J is the volumetric permeat ion flux, and l~ is the viscosity o f the permeate. The value of It was 1.01 × 1 0 - 3 p a . s . Af ter the fi l tration, the cake was wiped out with a sponge and the permeat ion flux of 0.9%o NaC1

solut ion was measured using the wiped membrane at the same filtration condit ions. The membrane resistance after crossflow filtration, Rm" , which includes the effect of plugging of membrane pores, was calculated from the permeat ion flux. Since Rt is the sum of the resistance of the cake, Rg, and Rm', Rg was calculated using Eq. 2.

Rg=Rt-Rm" (2)

Rp was calculated using Eq. 3.

Rp=Rm'-gm (3)

Rm for the C045 membrane was 2× 101°m J. Scanning electron microscopy The cross section of

the cakes formed on the membrane during crossflow and dead-end filtrations was observed with a scanning elec- t ron microscope as described previously (6).

RESULTS AND DISCUSSION

Specific resistance of bacteria Table 1 shows the specific resistance of various microbial cakes measured by the unsteady-state method at 4 9 k P a . Table 1 also shows the compressibi l i ty index, n, evaluated from the dependence of the specific resistance on t ransmembrane pressure. The n values for B. subtilis, B. brevis, and B. cereus, which are all rod-shaped bacteria, were around 1, indicating that their microbial cakes were very com- pressible. On the other hand, the n value for C. glutami- cure was 0.6, indicating that its cake was harder than those of the rod-shaped cells.

We measured also the specific resistance of B. subtilis by the steady-state method. The permeat ion flux initially decreased with time and then reached a steady-state value. The steady-state continued for 2 h.

Crossflow filtration of C. glutamicum Figure 1 shows the permeat ion flux in the crossflow filtration of the suspension of C. glutamicum cells, which are ellipsoidal in shape. The permeat ion flux decrease was rapid during the first 15 min and then became gradual to reach a nearly constant value at 60rain. The weights of microbial cake per unit fil tration area after filtration for 5, 15 and 60min were 0.086, 0.161 and 0.221kg/m 2, respectively. The increase o f cake weight was small after 15 min of fil tration. In Fig. 1, the flux calculated by the fil tration theory (Eq. 4) using the specific resistance measured in dead-end filtration, a, and the weight of the cake per unit filtration area, w, is also shown.

AP J = (4)

lt(Rm + ~w)

Figure 1 shows that the experimental and calculated values were in good agreement. A n agreement between the flux observed in the crossflow filtration and that calculated using the specific resistance measured in dead- end filtration indicates that the microbial cake took a similar structure for both filtration modes. We observed

TABLE 1. Specific resistance of microorganisms

Strain Specific resistance a Compressibility index (m/kg) (--)

C. glutamicum 1.2 × 10 j3 0.6 B. subtilis 4.8 × 10 ~2 0.8 B. brevis 5.3 × 10 ~3 0.8 B. cereus 1.8 × 1013 1.2

a At the transmembrane pressure of 49 kPa.

VoL. 78, 1994 STRUCTURE OF CAKE IN CROSSFLOW FILTRATION OF BACTERIA 457

10-3 ! !

g x

c .o 1 0 -4

o,, o. 1 °OOoooooo o

10-5 I I I 0 20 40 60

Filtration time [min]

FIG. 1. Crossflow filtration of suspension of C. glutamicum cells. Circulation flow rate, 30cm3/s; transmembrane pressure, 49 kPa; channel depth, 2.5 mm; cell concentration, 5 kg/mL Sym- bols: (3, experimental values; • , calculated values.

the cake structure using a scanning electron microscope. Figure 2 shows the cross sections of the microbial cake formed in the dead-end filtration (a) and that in the crossflow filtration (b). The C. glutamicum cells deposited in the same manner in both filtration modes; the cells deposited nearly at random in the cake. In the crossflow filtration o f S. cerevisiae cells which are also ellipsoidal

10-3

10-4 X

C

.9 10 "s

lO-e

10-3

10-4 X

¢=

.o 1O.S

O.

lO-e

1 0 - 3

X 10"4 ",1

t - O

~ 10-5

lO-e

i !

(a)

~ 0 0 0 0 0 0

0 0

! !

50 100


I l l w u !

Ib)

5 0

OOoo 0 0 0 0 0 0

I I I I l I

10 20 30 40 50 60 70


[:c;. ' ~ 0 0 0 0 0

0 0 O 0

I I I I I I

0 10 20 30 40 50 60 0


FIG. 3. Crossflow filtration of suspensions of Bacillus species. Circulation flow rate, 30cm3/s; transmembrane pressure, 49 kPa; channel depth, 2.5 ram; cell concentration, 5 kg/m 3. Symbols: (3, experimental values; e , calculated values. (a) B. subtilis, (b) B. brevis, (c) B. cereus.

FIG. 2. Cross section of cake formed in crossflow and dead-end filtrations of C. glutamicum. The cakes were observed after a 15-min filtration. (a) Dead-end filtration (transmembrane pressure, 49 kPa); (b) crossflow filtration (circulation flow rate, 30cm3/s; transmembrane pressure, 49 kPa; channel depth, 2.5 ram; cell concentration, 5 kg/m3).

in shape, the experimental flux is in good agreement with the calculated one, using the specific resistance measured in dead-end filtration and the weight of cake per unit filtration area (10). The cake structures formed in the two filtration modes are also random as in the case of C. glutamicum. Thus, the cake structure of the microorganisms which are nearly spherical in shape is similar in both the crossflow and dead-end filtrations when the cells are suspended in 0.9% NaC1 solution.

Crossflow f i l t ra t i on o f Bacillus s p e c i e s We cross- filtered suspensions of B. subtilis, B. brevis and B. cereus, all of which are rod-shaped, and studied the permeat ion behavior. Figures 3a, 3b and 3c show the


TABLE 2. Resistance caused by plugging of membrane in crossflow filtration of B. subtilis

Rm (m J) 2.0x 10 l° R m' (m z) 5.3 × 10 l° Rp(m 1) 3.3×10 I° R~(m i) 9.0×1012 Rg(m i) 9.0×101"~ Rp/Rg (°~00) 0.4

Transmembrane pressure, 49 kPa; circulation flow rate, 30 cm3/s; cell concentration, 10 kg/m 3.

change in permeation flux during crossflow filtration of suspensions of B. subtilis, B. brevis, and B. cereus, respectively. At first glance, the experimental fluxes for all the rod-shaped cells showed a tendency similar to that for C. glutamicum. The flux decrease was rapid at the initial stage of crossflow filtration and then became gradual, reaching a constant value. However, the flux for the rod-shaped cells decreased to a much lower level than that for C. glutamicum. The flux after 60 min of crossflow filtration of C. glutamicum was 2 × 10 5 m/s , while that for the rod-shaped cells was lower than 1 × 10 5 m/s . The most significant difference was that in the crossflow filtration of rod-shaped cells, the experimental flux was much lower than the value calculated by Eq. 4 and the specific resistance measured in dead-end filtration as well as the weight of the cake per unit filtration

area. The experimental fluxes for B. subtilis, B. brevis and B. cereus after 120min, 60min and 60min of crossflow filtration, respectively, were about 1/10, 1/5 and 1/7 of the calculated ones. There are three possible reasons for this large difference between the experimental and calculated fluxes: (i) the change in physiological properties of cells during a long-term operation, (ii) plugging of membrane pores during crossflow filtration, and (iii) difference in specific resistance of the cakes formed in the dead-end filtration and crossflow filtration.

The specific resistance of the microbial cake may increase due to changes in the physiological properties such as autolysis. However, this effect is considered to be negligible, since the specific resistance measured by the steady-state method in dead-end filtration was constant for 2 h as described previously. Next, we examined the effect of plugging of membrane pores. We calculated the resistance caused by plugging, Rp, and that of cake, Rg in the case of crossflow filtration of the suspension of B. subtilis cells, according to the method described in Materials and Methods. As shown in Table 2, the ratio of Rp tO Rg was less than 1%. Thus, the effect of the pore plugging on the permeation flux was negligible.

We observed the structure of both the microbial cakes formed in crossflow filtration and dead-end filtration with a scanning electron microscope. Figures 4a, 4b and 4c show the cross sections of the microbial cakes formed in the crossflow filtration of B. subtilis, B. brevis and B.

FIG. 4. Cross sections of cake formed in crossflow and dead-end filtrations of Bacillus species. The cakes were observed after a 15-min filtration. (a), (b), and (c) show the cross sections of the cake formed in crossflow filtration of B. subtilis, B. brevis, and B. cereus, respectively (circulation flow rate, 30 cm3/s; transmembrane pressure, 49 kPa; channel depth, 2.5 ram; cell concentration, 5 kg/m3). (d) shows the cross section of the cake formed in dead-end filtration of B. subtilis (transmembrane pressure, 49 kPa).


= , % % .- ,--.

g

o

u m u

I I !

20 40 60

Flow rate [cm3/s]

1 *o

0 ~,

FIG. 5. Dependences of permeation flux, weight of cake, and specific resistance on circulation flow rate in crossflow filtration of B. subtilis. Measurements were made after crossflow filtration for 15 rain. Transmembrane pressure, 49 kPa; channel depth, 2.5 ram; cell concentration, 5 kg/mL Symbols: o , permeation flux; A, cake;

, specific resistance.

7

5

4

3 0

2

E 1 <

0 0

~B I I I I I

1 2 3 4 5

C h a n n e l d e p t h [ m i n i

10 8! 6

=o 4

2 u

n

0 6

FIG. 6. Dependences of permeation flux, weight of cake, and specific resistance on channel depth in crossflow filtration of B. subtilis. Measurements were made after crossflow filtration for 15 min. Cir- culation flow rate, 30 cmUs; transmembrane pressure, 49 kPa; cell concentration, 5 kg/m 3. Symbols: o , permeation flux; •, cake; [], specific resistance.

cereus, respectively. In all the figures, most of the cells oriented along the circulation flow paral lel to the membrane. On the other hand, B. sub t i l i s cells deposi ted nearly at r andom in the cake formed in the dead-end filtration as shown in Fig. 4d. The other two rod-shaped cells also deposi ted nearly at r andom in dead-end filtra- t ion (data not shown). The or ientat ion of the cells in crossflow filtration makes the cake structure more com- pact than that formed in dead-end fil tration, which in- creases the specific resistance. Thus, the experimental flux was much lower than the calculated one using the specific resistance measured in the dead-end filtration. The cell a r rangement observed in crossflow filtration of suspensions of Baci l lus species may be principal ly caused by the shear stress acting on the cake surface in the direc- t ion of the circulation flow (11). In the dead-end filtra- t ion, the cells deposi ted nearly at r andom since there is no shear acting on the cake parallel to the surface.

Effect of circulation flow rate on permeation behavior in crossflow filtration of B. subtilis suspension The circulation flow rate is one of the predominant factors affecting the behavior o f crossflow filtration. Figure 5 shows the effect of circulat ion flow rate on permeat ion flux, weight of microbial cake per unit fil tration area and specific resistance of the cake. The permeat ion flux and the weight of the microbial cake were measured after a 15-min filtration. The specific resistance of the cake was calculated using Eq. 4 and the experimental flux as well as the weight of the cake, since the effect of plugging was negligible. The weight of the cake decreased with increasing circulat ion flow rate, p robab ly due to the increase in the shear stress acting on the cake surface. On the other hand, the specific resistance increased as the circulation flow rate increased. When the circulation flow rate is low, the shear stress acting on the microbial cake is small. Thus, the structure of the cake might resemble that formed in dead-end filtration. At a high circulat ion flow rate, the cells are arranged by high shear stress, and the specific resistance is higher than that at a low circulation flow rate. Thus, the permeat ion flux did not increase even i f the circulat ion flow rate was increased as shown in Fig. 5. These phenomena should be taken into considerat ion when optimizing the circulation flow rate in crossflow filtration.

Effect of channel depth on permeation behavior in crossflow filtration of B. subtilis suspension Figure 6 shows the effect of the channel depth of the crossflow filtration on permeat ion flux, weight of the cake and specific resistance. The weight of the cake increased and the permeat ion flux decreased as the channel depth increased. The weight of the cake increased since the shear force decreased with increasing channel depth at a constant flow rate. However , the specific resistance decreased when the channel depth was 5.5 mm, probably due to the decrease in the shear stress acting on the cake.

Figure 7 shows the cross section of the cake after 15min of crossflow filtration at the channel depth o f 5.5 mm. The cells deposi ted nearly at r andom near the membrane surface. However, in the upper part of the cake, most of the cells deposi ted in the direction of the circulation flow. This variat ion of cell arrangement with channel depth in the cake was considered to be caused by the deposi t ion of the cells at the early stage of crossflow filtration nearly in the same manner as in dead-end filtration because the effect of shear force is

i ¸̧ 5¸̧ ̧

FIG. 7. Cross section of cake formed in crossflow filtration of suspension of B. subtilis cells. Circulation flow rate, 30cm3/s; transmembrane pressure, 49 kPa; channel depth, 5.5 ram; cell concentration, 5 kg/m 3.


~o % 2 .,-. y-.

o

E •

0

FIG. 8.

, , , 3

0

I i I I

o 20

2 ~o

8

o

e~ 0

40 60 80 00

Transmembrane pressure [kPa]

Dependences of permeation flux, weight of cake, and specific resistance on transmembrane pressure in crossflow filtration of B. subtilis. Measurements were made after crossflow filtration for 15 min. Circulation flow rate, 30 cm3/s; channel depth, 2.5 ram; cell concentration, 5 kg/m 3. Symbols: co, permeation flux; z~, cake; [], specific resistance.

small. As the amount of cells deposited increased, the permeation flux decreased. As a result, the effect of shear force increased relative to that of permeation flow and the cells started to deposit along the circulation flow. When the channel depth was 5.5 mm, the effect of shear force was low at the initial stage of crossflow filtration and thus random deposition continued to occur for a longer period than the case with a shallower channel.

Effect o f t ransmembrane pressure on permeat ion behavior in cross f low fi ltration o f B. subtilis suspens ion Figure 8 shows the effect of the t ransmembrane pressure on permeation flux, weight of the cake and specific resistance. The weight of the cake increased with the increase in t ransmembrane pressure. Although the initial flux was high at high t ransmembrane pressure (4.2 × 10 -4, 7.3 x 10 4 , 9.0× 10 -4 and 1.5 × 10-3m/s at 29, 49, 78 and 98 kPa, respectively), the permeation flux after 15 min of filtration for all the t ransmembrane pressure differences studied (29-98 kPa) was nearly the same.

The specific resistance evaluated from the weight of cake per unit filtration area and permeation flux at 15- min filtration increased with the increase in transmembrane pressure. The compressibility index of the cake es- timated from the dependence of specific resistance on t ransmembrane pressure was 0.6, which was smaller than the value obtained in dead-end filtration (Table 1). This discrepancy could be explained as follows.

At lower t ransmembrane pressures, the cells are arranged by the action of shear from the beginning of cell deposition onto the membrane; thus the cake shows a specific resistance much higher than that for the dead- end filtration as described previously. On the other hand, at higher t ransmembrane pressures, the cells at the initial stage of crossflow filtration in particular tend to deposit in a manner similar to that for the dead-end filtration due to the high permeation flux; therefore the specific resistance of the cake becomes close to that for the dead-end filtration. This is the reason why the compressibility factor obtained in the crossflow filtration of rod-shaped B. sub t i l i s suspension was lower than that in the dead-end filtration.

Effect o f cell concentrat ion on permeat ion behav ior in cross f low fi ltration o f B. sub t i l i s suspens ion Figure 9

6 u ! | m m m

~ x 3

o ~

o 0 2 4 6 8 10

Cell concentration [kg/m 3]

eo

~o r ~

1 o • c

'7

¢9

O.

0 2

FIG. 9. Dependences of permeation flux, weight of cake, and specific resistance on cell concentration in crossflow filtration of B. subtilis. Measurements were made after crossflow filtration for 15 rain. Circulation flow rate, 30 cm3/s; channel depth, 2.5 ram; transmembrane pressure, 49 kPa. Symbols: o, permeation flux; ~, cake; [], specific resistance.

shows the effect of the cell concentrat ion on permeation flux, weight of the cake and specific resistance. The weight of the cake increased and the permeation flux decreased with the increase in the cell concentration. How- ever, the specific resistance was nearly constant in the cell concentrat ion range studied.

The findings obtained in this study will be useful in the optimization of the operation of crossflow filtration of bacteria, especially of rod-shaped bacteria, although the effect of medium components in crossflow filtration should be further clarified.

NOMENCLATURE

J : permeation flux, m/s n : compressibility index, - - AP : t ransmembrane pressure, Pa Rg : resistance of cake, m 1 Rm : membrane resistance, m - Rm': membrane resistance after crossflow filtration, m Rp :resistance caused by plugging, m Rt : to ta l resistance, m w : weight of cake per unit filtration area, kg/m 2 ~r :specific resistance, m /kg y : viscosity, Pa . s

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4. Defrise, D. and Gekas, V.: Microfiltration membranes and the problems of microbial adhesion. A literature survey. Process Biochem. August, 105-116 (1988).

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Crossflow filtration of yeast broth cultivated in molasses. Bio- technol. Bioeng., 43, 1094-1101 (1994).

7. Kroner, K.H. , Hummel, W., V61kel, J., and Kula, M.-R.: Effects of antifoams on crossflow filtration of microbial suspensions, p. 223-232. In Drioli, E. and Nakagaki, M. (ed.), Mem- branes and membrane processes. Plenum Press, New York (1986).

8. Nakanishi, K., Tadokoro, T., and Matsuno, R.: On the specific resistance of microorganisms. Chem. Eng. Commun., 62, 187-201 (1987).

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Fell, C. J. D., and Marshall, K.C.: Cake resistance and solute rejection in bacterial microfiltration: the role of the extracellular matrix. J. Membrane Sci., 79, 35-53 (1993).

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Documents

Filtration characteristics and structure of cake in crossflow filtration of bacterial suspension