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Preparation of asymmetric membranes for desalination, clarification of turbid water and biotechnological down-stream processing
Desalination, 93 (1993) 273-286 Elsevier Science PublishersB.V., Amsterdam
273
Preparation of asymmetric membranes for desalination, clarification of turbid water and biotechnological down-stream processing Cynthia L. Radiman, Hadi Sangkanparan, V.S. Praptowidodo and Oei Ban Liang KJC a on Biotechnology,
Imtitut Teknologi Bandung, Bandung (Indonesia)
SUMMARY
Clarifization of turbid water and desalination by membrane technology could be considered as one of the alternatives to overcome problems in the of ground and surface water for household needs in Indonesia. Membrane technology is also being used in the down-stream processing in biotechnology. Separation of molecules of very differing molecular sizes can be accomplished most efficiently by membrane filtration. Isolation and concentration of biotechnologically produced enzymes are good examples. In this paper the preparation of hollow fibers and flat membranes were described. Polysulfone was used as the base material. Various water clarification and biotechnological down-stream processes using the prepared membranes will also be described.
INTRODUCTION
Asymmetric membranes are prepared by techniques based on the principle of phase separation: a homogeneous polymer solution is transformed into a two-phase system - a solid continuous phase, rich in polymer, and a dispersed liquid phase, poor in polymer, forming an interconnected network of membrane pores. This technique was developed by Loeb and Sourirajan [ 11.
OOll-9164/93/$06.00
0 1993 Elsevier Science PublishersB.V. All rights reserved.
274
Though the basic principle for making hollow fiber (HF) membranes is similar to flat membranes, there is a firm distinction between them. HF membranes must be self-supporting and has a sufficient resistance to the applied pressure in order to avoid compaction. Cabasso et al. found that many parameters are involved and interfere each other in determining the characteristics of the produced HF membranes [2]. The effects of some parameters such as dope composition, type of bore precipitant and distance between spinneret and coagulation bath on the membrane characteristics were studied in this work. Those HF membranes were prepared by the dry-wet spinning method, using polysulfone as the base material. In Indonesia, the supply of clean water could not fulfill the community needs. Many people use ground and surface water for their household needs, but in certain areas, the water contains too much ferric ions, harmful compounds and is highly turbid. Besides that, Indonesia is an archipelago. in some regions near the coast, the sea water begins to penetrate into the ground water. Though the concentration of sodium chloride is not as high as in the sea water, this could cause problems for the future, especially in obtaining drinking water. Therefore, clarification of turbid water and desalination by membrane technology could be considered as one of the alternatives to overcome those problems. The set-up of HF modules was then realized in order to create a simple module with a good performance for the clarification of turbid water. Since turbid water contains several kinds of suspension and colloidal particles, the first attempt in this application was to decrease the amount of clay and iron (III) hydroxide in the turbid water. Ions of harmful metals could not be retained by the micro and ultrafiltration methods. Therefore, the preparation of composite membranes which belong to the reverse osmosis membranes was also be studied in order to obtain HF modules with better performances. Membrane technology is also being used in the down-stream processing in biotechnology. Separation of molecules of very differing molecular sizes can be accomplished most efficiently by membrane filtration. Isolation and concentration of biotechnologically produced enzymes using the prepared membranes were also carried out in this work. EXPERIMENTS
Preparation of HF membranes
HF were prepared by the dry-wet process using the equipment which scheme was shown in Fig. 1.
275
do pa
bore precipitant
Fig. 1. Scheme of equipment for HF membrane preparation.
As a preliminary study the parameters selected were composition of polymer solutions, type of bore precipitant and distance between spinnerette and coagulation bath. The dope (polymer solution) consisted of a mixture of polysulfone (PSF) and polyethyleneglycol (PEG) with Mn = 1,000 in N,Ndimethylacetamide (DMAc) as the solvent. They were stirred up for about 24 hours at room temperature and then filtered to remove existing aggregates in the dope. They were then put in the oven at 50” C for about one hour to remove air bubbles. After cooling at room temperature, the dope was ready for the spinning process. Characterizationof membranes A certain number (+ 10 - 15) of about 45 cm length of fibers were potted at both ends in a polyethylene tubing using epoxy resin. Those bundles were then used for the characterization of membranes using an apparatus shown in Fig. 2. Pure water permeability was calculated by using the equation L=
J A.AP where Lp =permeability coefficient, J =permeation rate (L/s), A =area (m2) of fiber bundle, estimated from the measurement of the average inner diameter of HF using a scaled loupe and the length of HF, and AP=Trans membrane pressure = l/2 (Pi + P,). P
276 prraaurr
feed
bundle
Fig. 2. Apparatus for characterizationof
of
HP
membranes.
Rejection coefficients were determined towards dextran solutions. Dextrans T-500 and T-2000 which have M, of 500,000 and 2,000,000 respectively were used. The rejection coeffkients were calculated from the equation r
=
1 -- 5
x 100%
! 5 I where r = rejectron coefficient, CP=concentration of permeate, and Cf = concentration of concentrate. Concentration of dextrans were determined spectrophotometrically by fen01 - sulphuric acid method. The absorption of light was measured at 490 nm. A calibration curve using glucose standards is needed for the calculation. Determination of turbidity
Turbidity in water was measured in NTU (nephelometric turbidity unit) using a mixture of hidrazine sulphate and hexamethylenetetramine as .:* standard. The wavelength used was 420 run. Determination of metal ion content in water
To evaluate the efficiency of clarification of turbid water, the content of
277
certain metal ions such as Fe3+, Mg’+, Ca2+, Mn4+ and Na were determined by AAS (Atomic Absorption Spectroscopy). Analysis of membrane structure
The structures of membranes were observed from their cross-section using SEM (scanning electron microscopy). The samples were prepared by breaking the fibers at the temperature of liquid nitrogen, then attached to aluminium support with a double surface Scotch tape. The mounted specimen was then coated with gold. Construction of HF modules f 1300 - 1500 fibers of 1 m length were put into a PVC cylinder which
has been completed with inlet and outlet. The ends of the bundle were fixed to the PVC by epoxy resin. This module was then combined with a prefilter which retained coarse particles before entering the HF module. Preparation of composite membranes
Two kinds of procedures using different solutions were chosen for the preparation of composite membranes, i.e. : Procedure I 1st solution: 2nd solution:
:
aqueous solution of 6.8 % (w/w) polyethyleneimine (PEI). 0.5 % (w/v) toluene diisocyanate (TDI) in hexane
Procedure II : 1st solution: aqueous solution of 2% (w/w) m-phenylenediamine (MPD). 2nd solution: 0.1 % (w/v) trimesoylchloride (TMC) in hexane. A freshly prepared PSF membrane was put in PEI or MPD solution for 1 minute and drained for 1 minute. The membrane was then put in TDI or TMC solution for 1 minute and drained for 10 minutes. In procedure I, the membrane was then dried in the oven at 120°C for 10 minutes.
278 RESULTS AND DISCUSSIONS
Eflects of spinning conditions on membrane characteristics Eflect of polymer concentrations on the quality of HF It is known that the viscosity of solution plays an important role in the spinning process of HF. Therefore, at the beginning of our work, PSF concentrations were varied between 16 and 20%. Fig. 3 shows that the optimum condition for HF preparation was obtained by using a PSF concentration of 18%. This concentration was used afterwards.
I
‘8,
j+ 300 L
i
’
90
rdext 54:
P 200
80
_.-.
i
.T-2000 (99
70
t
I
I
I
1
16
18
20
,
Fig. 3. Effect of PSF concentration on membrane characteristics.
Eflect of spinneret distance from the coagulQtionbath and type of bore precipitant on membrane characteristics
Table I showed that the hydraulic permeability coefficient of HF decreased as the distance between spinneret and coagulation bath increased. Aptel et al. found the same result and explained that it was due to the small change in the shape of the micropores formed in the skin. On emerging from TABLE I Effect of spinneret distance from coagulation bath on membrane characteristics (PSF=18%, PEG=l8%, applied prcssure=O.f5atm, bore precipitant: water) Spinneret distance from coagulation bath (m) 9 20 30
LP of water (m3/h. atm.m2) 358 245
144
Dext. T-500 L r(S) rl 63 56 54
56 77 99
Dext. T-2000 L” 42 43 45
7 (%) 72 96 94
279
Fig. 4. SEM photo of HG membrane prepared with a spinneret-coagulation bath distance of 9 cm.
Fig. 5. SEM photo of HF membrane prepared with a spinneret-coagulation bath distance of 20 cm.
Fig. 6. SEM photo of HF membrane prepared with a spinneret-coagulation bath distance of 30 cm.
Fig. 7. SEM photo of HF membrane prepared with a spinneret-coagulation bath distance of 50 cm.
280
the spinneret, the viscous liquid relaxes from shear flow and is elongated under the effect of its own mass, since the spinneret is at a distance above the coagulation bath. Aptel also showed that an elongation ratio of only 1.4 resulted in a three-fold decrease in the permeability. It appears that the rejection coefficients towards dextrans become larger when the distance increases. It could be explained qualitatively by the same picture of the microstructure of the skin. The retention of a given polydisperse solute will tend to increase when the elongation ratio of the pores goes up. If we look at the cross-sections of those prepared membranes (Figs. 49, two layers of finger-like macrovoids are found. This indicates that the outer surface layer was not yet solidified when the fibers enter the water bath. Longer distance between spinneret and coagulation bath results in thicker inner layer. As shown by Fig. 7, a certain distance (in this case the distance = 50 cm) could produce one layer of finger-like macrovoids. It means that the solidification process has been completed before entering the coagulation bath. It shows that a desired morphology could be obtained by controlling the distance between spinneret and coagulation bath. E#ect of type of bore precipitant on membrane characteristics
A mixture of DMAc and water 30/70 (v/v) was used as bore precipitant instead of water. As shown by Table II, this mixture increased the permeability of the produced membrane. Table II Effect of type of bore precipitant on membrane characteristics (PSP=18%,PEG=18%,appliedpressure=0.35atm.Spinneretcoagulationbathdistance=30cm) Bore
precipitant
LP of water (m3/h atm m2) l
DMAc/H20 30/70 H&
Dext. T-SC@
Dext. T-2000
l
173 144
Lp
T(%)
Lp
7 (W)
80 54
76 90
62 45
79 94
The skin is formed by gelation and the porous sublayer is the result of liquid - liquid phase inversion by nucleation and growth. The microstructure of the skin will then depend on the ratio of solvent outflow to non-solvent inflow, i.e. to the ratio of the chemical potential difference of the non-
281
Fig. 8. SEM photo of HF membrane prepared with DMAc/&O
as bore precipitant.
solvent and the solvent at the spinning solution/coagulation fluid interface. If the non-solvent contains a certain amount of solvent (in this case it is the DMAc), the diffusion of solvent become slower. Consequently, the surface overconcentration of polymer is smaller and the evolution of the interface composition reaches a value which produces a less dense membrane skin. In fact, the permeability of membranes with DMAc/H,O as bore precipitant is higher and the rejection coefficients towards dextrans are lower than those with water as the precipitant. Fig. 8 shows the cross-section of HF membrane prepared with a mixture of DMAc and water as bore precipitant. It seems that the type of bore precipitant does not affect the structure of the produced membranes. Applications of HF modules for the clarification of turbid water
The results of clarification of turbid water are shown in Table III. It is shown that HF membranes could retain’all the colloidal particles which are responsible for the turbidity of water. Since the permeability is large enough, the prepared HF modules are suitable for the clarification of turbid water in household scale, especially if the water contains too much ferric ions. Concerning other ions such as Mn and Mg ions, the prepared HF modules have a zero filtering efficiency towards them. Those ions have smaller
282
TABLE III Clarification of turbid water by I-IF modules Feedl Permeate
LP of Turbidity permeate (NTU) (m3/h atm m2) l
Feed Permeate Filtering Efficiency ( W) Feed Permeate Filtering Effkiency (%)
Fe3+ @pm)
Mn4+ @pm)
Mg2+ zIl2+ @pm) @pm)
90 0
5.84 0.8
5.22 5.22
14.38 14.38
1.15 0.45
100
86
0
0
61
87.4 0
26.1 0.83
5 5
8 3.9
100
97
0
51
l
246
315
dimensions than the pores of HF membranes, so they pass through the membrane. To overcome this problem, composite RO membranes should be prepared. Since the particles are retained at the inner surface of the fibers, it is expected that the permeability will decrease after a certain time. To minimize the fouling effect of membranes, a washing procedure should be carried out during the clarification of turbid water. This is done by flushing the membrane with clean water from the opposite direction. Fig. 9 shows the effect of back-washing on the permeability of HF modules. It seems that a periodical back-washing could extend the life-time of membranes, though it could not reach the initial permeability.
Fig. 9. Effect of back-washing on the permeability of an HF module.
283
Characterizationand applicationsof compositemembranes Table IV shows the rejections of several salts by two kinds of composite membranes. TABLE IV
Characteristics of composite membranes (PSF=lS%,
applied pressure=20
Typeof composite PEI - TDI MPD - TMC
atm. Salt concentration in the feed=250
Lp of water
(m3/h.atm.m2) 36 19
ppm)
TofN+ (W)
7of Ca2+
T of M2+
(96)
@)
87.6 63.3
86.5 64.3
91.6 62.7
Figs. 10 and 11 show the cross-sections of both type of membranes. They are similar to those of PSF flat membranes. Since the composite membranes could retain some of the metal ions in the water, the previous HF module was combined with the composite membrane and used for the clarification of turbid water. It means that the permeate from HF module was used as the feed of composite membrane. The results are shown in Table V. TABLE V
Clarification of turbid water by using HF module and composite membrane Type of membrane
Lp of water (m3/h.atm.m2)
HF microfiltration
0.547
RO (MPD-TMC) RO (TDI-PEI)
0.013 0.004
7 of turb
7 of Fe3+
7 of Mn4+ (%)
7Of Mg2+ (%)
7 of Zn2+ (%)
(S)
(%)
loo
loo
16
21
0
89.7 96.8
89.5 85.2
73.1 88.5
From this table it could be seen that the composite membranes could retain about the half of Mg and Zn ion content, but pass all the Mn ions. However, in order to increase the filtering efficiency, the composition of composites should be studied more in detail to find the appropriate compositions for the clarification of turbid water.
Fig. 10. SEM photo of PEI-TDI composite membrane.
Fig. 11. SEM photo of MPD-TMC composite membrane.
Ultra_filtration of penicillinacylase The possibility of using PS flat membranes to concentrate biotechnologitally produced enzymes was also investigated. As a preliminary study, fermentation broth containing penicillin acylase was used as feed. It was found that at 1 atm and 25”C, a rejection of 91% and a flux of 0.05 m3/ m2 h could be obtained. Under this condition the enzyme could be concentrated almost three times, but the formation of gel layer on the surface of membranes was still observed even though a Millipore pellicone cassette with a tangential flow had been used. The optimization of this enzyme concentration is still under investigation. l
285 CONCLUSIONS
From this work it can be concluded that hollow fibers can be used for the clarification of turbid water which contains too many ferric ions. HF modules witha permeability coefficient about 0.3 m3/m2 atm hare suitable for a household scale. To increase the filtering efficiency of other harmful ions, composite HF membranes should be used. By using suitable conditions, membranes could also be used to concentrate penicillin acylase. l
l
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the Japan International Cooperation Agency and the Inter University Centre on Biotechnology for all the financial support of this research.
REFERENCES 1 2 3 4
S. Loeb and S. Sourirajan, Advan. Chem. Ser., 38 (1962) 117. I. Cabasso, E. Klein and J.K. Smith, J. Appl. Polym. Sci., 20 (1976) 2377. P. Aptel, N. Abidine, F. Ivaldi and J.P. Lafaille, J. Membr. Sci., 22 (1985) 199-215. R.E. Kesting, Synthetic Polymeric Membranes, 2nd ed., Wiley, New York, 1985.
_-~
1-
i_-=
273
Preparation of asymmetric membranes for desalination, clarification of turbid water and biotechnological down-stream processing Cynthia L. Radiman, Hadi Sangkanparan, V.S. Praptowidodo and Oei Ban Liang KJC a on Biotechnology,
Imtitut Teknologi Bandung, Bandung (Indonesia)
SUMMARY
Clarifization of turbid water and desalination by membrane technology could be considered as one of the alternatives to overcome problems in the of ground and surface water for household needs in Indonesia. Membrane technology is also being used in the down-stream processing in biotechnology. Separation of molecules of very differing molecular sizes can be accomplished most efficiently by membrane filtration. Isolation and concentration of biotechnologically produced enzymes are good examples. In this paper the preparation of hollow fibers and flat membranes were described. Polysulfone was used as the base material. Various water clarification and biotechnological down-stream processes using the prepared membranes will also be described.
INTRODUCTION
Asymmetric membranes are prepared by techniques based on the principle of phase separation: a homogeneous polymer solution is transformed into a two-phase system - a solid continuous phase, rich in polymer, and a dispersed liquid phase, poor in polymer, forming an interconnected network of membrane pores. This technique was developed by Loeb and Sourirajan [ 11.
OOll-9164/93/$06.00
0 1993 Elsevier Science PublishersB.V. All rights reserved.
274
Though the basic principle for making hollow fiber (HF) membranes is similar to flat membranes, there is a firm distinction between them. HF membranes must be self-supporting and has a sufficient resistance to the applied pressure in order to avoid compaction. Cabasso et al. found that many parameters are involved and interfere each other in determining the characteristics of the produced HF membranes [2]. The effects of some parameters such as dope composition, type of bore precipitant and distance between spinneret and coagulation bath on the membrane characteristics were studied in this work. Those HF membranes were prepared by the dry-wet spinning method, using polysulfone as the base material. In Indonesia, the supply of clean water could not fulfill the community needs. Many people use ground and surface water for their household needs, but in certain areas, the water contains too much ferric ions, harmful compounds and is highly turbid. Besides that, Indonesia is an archipelago. in some regions near the coast, the sea water begins to penetrate into the ground water. Though the concentration of sodium chloride is not as high as in the sea water, this could cause problems for the future, especially in obtaining drinking water. Therefore, clarification of turbid water and desalination by membrane technology could be considered as one of the alternatives to overcome those problems. The set-up of HF modules was then realized in order to create a simple module with a good performance for the clarification of turbid water. Since turbid water contains several kinds of suspension and colloidal particles, the first attempt in this application was to decrease the amount of clay and iron (III) hydroxide in the turbid water. Ions of harmful metals could not be retained by the micro and ultrafiltration methods. Therefore, the preparation of composite membranes which belong to the reverse osmosis membranes was also be studied in order to obtain HF modules with better performances. Membrane technology is also being used in the down-stream processing in biotechnology. Separation of molecules of very differing molecular sizes can be accomplished most efficiently by membrane filtration. Isolation and concentration of biotechnologically produced enzymes using the prepared membranes were also carried out in this work. EXPERIMENTS
Preparation of HF membranes
HF were prepared by the dry-wet process using the equipment which scheme was shown in Fig. 1.
275
do pa
bore precipitant
Fig. 1. Scheme of equipment for HF membrane preparation.
As a preliminary study the parameters selected were composition of polymer solutions, type of bore precipitant and distance between spinnerette and coagulation bath. The dope (polymer solution) consisted of a mixture of polysulfone (PSF) and polyethyleneglycol (PEG) with Mn = 1,000 in N,Ndimethylacetamide (DMAc) as the solvent. They were stirred up for about 24 hours at room temperature and then filtered to remove existing aggregates in the dope. They were then put in the oven at 50” C for about one hour to remove air bubbles. After cooling at room temperature, the dope was ready for the spinning process. Characterizationof membranes A certain number (+ 10 - 15) of about 45 cm length of fibers were potted at both ends in a polyethylene tubing using epoxy resin. Those bundles were then used for the characterization of membranes using an apparatus shown in Fig. 2. Pure water permeability was calculated by using the equation L=
J A.AP where Lp =permeability coefficient, J =permeation rate (L/s), A =area (m2) of fiber bundle, estimated from the measurement of the average inner diameter of HF using a scaled loupe and the length of HF, and AP=Trans membrane pressure = l/2 (Pi + P,). P
276 prraaurr
feed
bundle
Fig. 2. Apparatus for characterizationof
of
HP
membranes.
Rejection coefficients were determined towards dextran solutions. Dextrans T-500 and T-2000 which have M, of 500,000 and 2,000,000 respectively were used. The rejection coeffkients were calculated from the equation r
=
1 -- 5
x 100%
! 5 I where r = rejectron coefficient, CP=concentration of permeate, and Cf = concentration of concentrate. Concentration of dextrans were determined spectrophotometrically by fen01 - sulphuric acid method. The absorption of light was measured at 490 nm. A calibration curve using glucose standards is needed for the calculation. Determination of turbidity
Turbidity in water was measured in NTU (nephelometric turbidity unit) using a mixture of hidrazine sulphate and hexamethylenetetramine as .:* standard. The wavelength used was 420 run. Determination of metal ion content in water
To evaluate the efficiency of clarification of turbid water, the content of
277
certain metal ions such as Fe3+, Mg’+, Ca2+, Mn4+ and Na were determined by AAS (Atomic Absorption Spectroscopy). Analysis of membrane structure
The structures of membranes were observed from their cross-section using SEM (scanning electron microscopy). The samples were prepared by breaking the fibers at the temperature of liquid nitrogen, then attached to aluminium support with a double surface Scotch tape. The mounted specimen was then coated with gold. Construction of HF modules f 1300 - 1500 fibers of 1 m length were put into a PVC cylinder which
has been completed with inlet and outlet. The ends of the bundle were fixed to the PVC by epoxy resin. This module was then combined with a prefilter which retained coarse particles before entering the HF module. Preparation of composite membranes
Two kinds of procedures using different solutions were chosen for the preparation of composite membranes, i.e. : Procedure I 1st solution: 2nd solution:
:
aqueous solution of 6.8 % (w/w) polyethyleneimine (PEI). 0.5 % (w/v) toluene diisocyanate (TDI) in hexane
Procedure II : 1st solution: aqueous solution of 2% (w/w) m-phenylenediamine (MPD). 2nd solution: 0.1 % (w/v) trimesoylchloride (TMC) in hexane. A freshly prepared PSF membrane was put in PEI or MPD solution for 1 minute and drained for 1 minute. The membrane was then put in TDI or TMC solution for 1 minute and drained for 10 minutes. In procedure I, the membrane was then dried in the oven at 120°C for 10 minutes.
278 RESULTS AND DISCUSSIONS
Eflects of spinning conditions on membrane characteristics Eflect of polymer concentrations on the quality of HF It is known that the viscosity of solution plays an important role in the spinning process of HF. Therefore, at the beginning of our work, PSF concentrations were varied between 16 and 20%. Fig. 3 shows that the optimum condition for HF preparation was obtained by using a PSF concentration of 18%. This concentration was used afterwards.
I
‘8,
j+ 300 L
i
’
90
rdext 54:
P 200
80
_.-.
i
.T-2000 (99
70
t
I
I
I
1
16
18
20
,
Fig. 3. Effect of PSF concentration on membrane characteristics.
Eflect of spinneret distance from the coagulQtionbath and type of bore precipitant on membrane characteristics
Table I showed that the hydraulic permeability coefficient of HF decreased as the distance between spinneret and coagulation bath increased. Aptel et al. found the same result and explained that it was due to the small change in the shape of the micropores formed in the skin. On emerging from TABLE I Effect of spinneret distance from coagulation bath on membrane characteristics (PSF=18%, PEG=l8%, applied prcssure=O.f5atm, bore precipitant: water) Spinneret distance from coagulation bath (m) 9 20 30
LP of water (m3/h. atm.m2) 358 245
144
Dext. T-500 L r(S) rl 63 56 54
56 77 99
Dext. T-2000 L” 42 43 45
7 (%) 72 96 94
279
Fig. 4. SEM photo of HG membrane prepared with a spinneret-coagulation bath distance of 9 cm.
Fig. 5. SEM photo of HF membrane prepared with a spinneret-coagulation bath distance of 20 cm.
Fig. 6. SEM photo of HF membrane prepared with a spinneret-coagulation bath distance of 30 cm.
Fig. 7. SEM photo of HF membrane prepared with a spinneret-coagulation bath distance of 50 cm.
280
the spinneret, the viscous liquid relaxes from shear flow and is elongated under the effect of its own mass, since the spinneret is at a distance above the coagulation bath. Aptel also showed that an elongation ratio of only 1.4 resulted in a three-fold decrease in the permeability. It appears that the rejection coefficients towards dextrans become larger when the distance increases. It could be explained qualitatively by the same picture of the microstructure of the skin. The retention of a given polydisperse solute will tend to increase when the elongation ratio of the pores goes up. If we look at the cross-sections of those prepared membranes (Figs. 49, two layers of finger-like macrovoids are found. This indicates that the outer surface layer was not yet solidified when the fibers enter the water bath. Longer distance between spinneret and coagulation bath results in thicker inner layer. As shown by Fig. 7, a certain distance (in this case the distance = 50 cm) could produce one layer of finger-like macrovoids. It means that the solidification process has been completed before entering the coagulation bath. It shows that a desired morphology could be obtained by controlling the distance between spinneret and coagulation bath. E#ect of type of bore precipitant on membrane characteristics
A mixture of DMAc and water 30/70 (v/v) was used as bore precipitant instead of water. As shown by Table II, this mixture increased the permeability of the produced membrane. Table II Effect of type of bore precipitant on membrane characteristics (PSP=18%,PEG=18%,appliedpressure=0.35atm.Spinneretcoagulationbathdistance=30cm) Bore
precipitant
LP of water (m3/h atm m2) l
DMAc/H20 30/70 H&
Dext. T-SC@
Dext. T-2000
l
173 144
Lp
T(%)
Lp
7 (W)
80 54
76 90
62 45
79 94
The skin is formed by gelation and the porous sublayer is the result of liquid - liquid phase inversion by nucleation and growth. The microstructure of the skin will then depend on the ratio of solvent outflow to non-solvent inflow, i.e. to the ratio of the chemical potential difference of the non-
281
Fig. 8. SEM photo of HF membrane prepared with DMAc/&O
as bore precipitant.
solvent and the solvent at the spinning solution/coagulation fluid interface. If the non-solvent contains a certain amount of solvent (in this case it is the DMAc), the diffusion of solvent become slower. Consequently, the surface overconcentration of polymer is smaller and the evolution of the interface composition reaches a value which produces a less dense membrane skin. In fact, the permeability of membranes with DMAc/H,O as bore precipitant is higher and the rejection coefficients towards dextrans are lower than those with water as the precipitant. Fig. 8 shows the cross-section of HF membrane prepared with a mixture of DMAc and water as bore precipitant. It seems that the type of bore precipitant does not affect the structure of the produced membranes. Applications of HF modules for the clarification of turbid water
The results of clarification of turbid water are shown in Table III. It is shown that HF membranes could retain’all the colloidal particles which are responsible for the turbidity of water. Since the permeability is large enough, the prepared HF modules are suitable for the clarification of turbid water in household scale, especially if the water contains too much ferric ions. Concerning other ions such as Mn and Mg ions, the prepared HF modules have a zero filtering efficiency towards them. Those ions have smaller
282
TABLE III Clarification of turbid water by I-IF modules Feedl Permeate
LP of Turbidity permeate (NTU) (m3/h atm m2) l
Feed Permeate Filtering Efficiency ( W) Feed Permeate Filtering Effkiency (%)
Fe3+ @pm)
Mn4+ @pm)
Mg2+ zIl2+ @pm) @pm)
90 0
5.84 0.8
5.22 5.22
14.38 14.38
1.15 0.45
100
86
0
0
61
87.4 0
26.1 0.83
5 5
8 3.9
100
97
0
51
l
246
315
dimensions than the pores of HF membranes, so they pass through the membrane. To overcome this problem, composite RO membranes should be prepared. Since the particles are retained at the inner surface of the fibers, it is expected that the permeability will decrease after a certain time. To minimize the fouling effect of membranes, a washing procedure should be carried out during the clarification of turbid water. This is done by flushing the membrane with clean water from the opposite direction. Fig. 9 shows the effect of back-washing on the permeability of HF modules. It seems that a periodical back-washing could extend the life-time of membranes, though it could not reach the initial permeability.
Fig. 9. Effect of back-washing on the permeability of an HF module.
283
Characterizationand applicationsof compositemembranes Table IV shows the rejections of several salts by two kinds of composite membranes. TABLE IV
Characteristics of composite membranes (PSF=lS%,
applied pressure=20
Typeof composite PEI - TDI MPD - TMC
atm. Salt concentration in the feed=250
Lp of water
(m3/h.atm.m2) 36 19
ppm)
TofN+ (W)
7of Ca2+
T of M2+
(96)
@)
87.6 63.3
86.5 64.3
91.6 62.7
Figs. 10 and 11 show the cross-sections of both type of membranes. They are similar to those of PSF flat membranes. Since the composite membranes could retain some of the metal ions in the water, the previous HF module was combined with the composite membrane and used for the clarification of turbid water. It means that the permeate from HF module was used as the feed of composite membrane. The results are shown in Table V. TABLE V
Clarification of turbid water by using HF module and composite membrane Type of membrane
Lp of water (m3/h.atm.m2)
HF microfiltration
0.547
RO (MPD-TMC) RO (TDI-PEI)
0.013 0.004
7 of turb
7 of Fe3+
7 of Mn4+ (%)
7Of Mg2+ (%)
7 of Zn2+ (%)
(S)
(%)
loo
loo
16
21
0
89.7 96.8
89.5 85.2
73.1 88.5
From this table it could be seen that the composite membranes could retain about the half of Mg and Zn ion content, but pass all the Mn ions. However, in order to increase the filtering efficiency, the composition of composites should be studied more in detail to find the appropriate compositions for the clarification of turbid water.
Fig. 10. SEM photo of PEI-TDI composite membrane.
Fig. 11. SEM photo of MPD-TMC composite membrane.
Ultra_filtration of penicillinacylase The possibility of using PS flat membranes to concentrate biotechnologitally produced enzymes was also investigated. As a preliminary study, fermentation broth containing penicillin acylase was used as feed. It was found that at 1 atm and 25”C, a rejection of 91% and a flux of 0.05 m3/ m2 h could be obtained. Under this condition the enzyme could be concentrated almost three times, but the formation of gel layer on the surface of membranes was still observed even though a Millipore pellicone cassette with a tangential flow had been used. The optimization of this enzyme concentration is still under investigation. l
285 CONCLUSIONS
From this work it can be concluded that hollow fibers can be used for the clarification of turbid water which contains too many ferric ions. HF modules witha permeability coefficient about 0.3 m3/m2 atm hare suitable for a household scale. To increase the filtering efficiency of other harmful ions, composite HF membranes should be used. By using suitable conditions, membranes could also be used to concentrate penicillin acylase. l
l
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the Japan International Cooperation Agency and the Inter University Centre on Biotechnology for all the financial support of this research.
REFERENCES 1 2 3 4
S. Loeb and S. Sourirajan, Advan. Chem. Ser., 38 (1962) 117. I. Cabasso, E. Klein and J.K. Smith, J. Appl. Polym. Sci., 20 (1976) 2377. P. Aptel, N. Abidine, F. Ivaldi and J.P. Lafaille, J. Membr. Sci., 22 (1985) 199-215. R.E. Kesting, Synthetic Polymeric Membranes, 2nd ed., Wiley, New York, 1985.
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