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Microfiltration of Chlorella sp.: Influence of material and membrane pore size

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Four membranes were used to separate Chlorella sp. from their culture medium in cross-flow microfiltration (MF) experiments: cellulose acetate (CA), cellulose nitrate (CN), polypropylene (PP) and polyvinylidenefluoride (PVDF). It was found that the hydrophilic CA and CN membranes with a pore size of 1.2 µm exhibited the best performances among all the membranes in terms of permeation flux. The hydrophobicity of each membrane material was determined by measuring the angle between the water (liquid) and membrane (solid). Contact angle measurements showed that deionized (DI) water had almost adsorbed onto the surfaces of the CA and CN membranes, which gave 0.00° contact angle values. The PP and PVDF membranes were more hydrophobic, giving contact angle values of 95.97° and 126.63°, respectively. Although the pure water flux increased with increasing pore diameter (0.8 < 1.2 < 3.0 µm) in hydrophilic CA and CN membranes, the best performance in term of filtration rate for filtering a microalgae suspension was attained by membranes with a pore size of 1.2 µm. The fouled membrane pore sizes and pore blocking were inspected using a scanning electron microscope (SEM). MF with large pore diameters was more sensitive to fouling that contributed to intermediate blocking, where the size of the membrane pores is almost equivalent to that of cells.
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Membrane Water Treatment, Vol. 4, No. 2 (2013) 143-155 143
Microfiltration of Chlorella sp.: Influence of material
and membrane pore size
A.L. Ahmad, N.H. Mat Yasin, C.J.C. Derek and J.K. Lim
School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia,
Seri Ampangan, 14300 Nibong Tebal, Seberang Perai Selatan, Pulau Pinang, Malaysia
(Received August 08, 2012, Revised March 09, 2013, Accepted April 01, 2013)
Abstract. Four membranes were used to separate Chlorella sp. from their culture medium in cross-flow
microfiltration (MF) experiments: cellulose acetate (CA), cellulose nitrate (CN), polypropylene (PP) and
polyvinylidenefluoride (PVDF). It was found that the hydrophilic CA and CN membranes with a pore size
of 1.2 µm exhibited the best performances among all the membranes in terms of permeation flux. The
hydrophobicity of each membrane material was determined by measuring the angle between the water
(liquid) and membrane (solid). Contact angle measurements showed that deionized (DI) water had almost
adsorbed onto the surfaces of the CA and CN membranes, which gave 0.00° contact angle values. The PP
and PVDF membranes were more hydrophobic, giving contact angle values of 95.97° and 126.63°,
respectively. Although the pure water flux increased with increasing pore diameter (0.8 < 1.2 < 3.0 µm) in
hydrophilic CA and CN membranes, the best performance in term of filtration rate for filtering a microalgae
suspension was attained by membranes with a pore size of 1.2 µm. The fouled membrane pore sizes and
pore blocking were inspected using a scanning electron microscope (SEM). MF with large pore diameters
was more sensitive to fouling that contributed to intermediate blocking, where the size of the membrane
pores is almost equivalent to that of cells.
Keywords: cross-flow microfiltration; Chlorella sp.; flux decline; pore blocking
1.Introduction
In recent decades, various membrane processes have been developed tailored towards a wide
range of applications and their numbers will certainly increase in coming years. Common
applications include concentration, purification and fractionation processes. However, today,
membrane filtration has been used intensively in the separation and purification steps of
biotechnological processes in order to harvest micro-organisms (Rossignol et al. 1999) due to its
economical, efficient and energy-saving advantages (Hwang and Sz 2010).
Many studies of membrane separation using microfiltration (MF) membrane have been
reported on biological suspensions. Petrusevski et al. (1995) examined a tangential flow filtration
system for the concentration of algae in natural fresh water. A membrane with 0.45 µm pores was
selected for the concentration of microalgae to limit the accumulation of unwanted water
constituents and to reduce membrane fouling. Jaouen et al. (1999) first reported the effects of
Corresponding author, Professor, E-mail: chlatif@eng.usm.my
A.L. Ahmad, N.H. Mat Yasin, C.J.C. Derek and J.K. Lim
shear stresses on microalgal cell suspensions (Tetraselmis suecica) in the various pumps of
tangential MF systems. Krstic et al. (2001) reported some observations on the influence of
operation conditions, biomass structure and feed composition during cross-flow MF of Polyporus
squamosus fermentation broth. Babel and Takizawa (2010) observed the effects of feed
concentration and transmembrane pressure (TMP) on cake resistance; whereas Dizge et al. (2011)
studied the effects of membrane type and pore size on filtration flux.
Rossi et al. (2008) presented the results of the fouling phenomenon with different suspensions:
fresh biomass, stressed biomass and a suspension of Arthrospira platensis enriched in
exopolysaccharides (EPS). Certainly, fouling is the major constraint in membrane filtration. It
causes a significant decline in flux and increases TMP (Makardij et al. 1999, Pearce 2007).
Consequently, in order to improve performance and minimize the fouling phenomena, it is
necessary to know the important factors affecting membrane fouling. Hwang and Huang (2009)
stated in their article that the fouling mechanism and the performance of membrane filtration
depended on various factors: biological polymers (e.g., proteins, carbohydrates, nucleic acids),
membrane characteristics (e.g., morphology, membrane pore size, zeta potential, hydrophilic
affinity), bio-macromolecular characteristics (e.g., molecular weight of biopolymers,
configuration) and system operating conditions (e.g., filtration pressure, cross-flow velocity,
soluble microbial products (SMP) concentration). Solute adsorption and particle interception also
influenced the membrane filtration processes (Hwang and Sz 2010).
To date, there have been many publications (Petrusevski et al. 1995, Rossignol et al. 1999,
Jaouen et al. 1999, Krstic et al. 2001, Rossi et al. 2008, Babel and Takizawa 2010, Dizge et al.
2011) on membrane processes for algal filtration but the constraints of membrane filtration have
yet to be explored in detail. The study presented here focuses on the membrane performance of
cross-flow MF in the harvesting of Chlorella sp. suspensions. Membrane performance (i.e.,
permeation flux) was investigated as a function of various membrane characteristics (i.e., different
membrane materials with various pore sizes) under constant operating conditions (i.e., pressure,
cells concentration and cross-flow velocity). Four different types of membranes with pore sizes
ranging from 0.8 µm to 3.0 µm were used in the experiments. Fresh membranes were
characterized using contact angle measurements to determine the hydrophobicity of membrane
surfaces. Additionally, a scanning electron microscope (SEM) was used to inspect the membrane
surfaces before and after MF.
2. Materials and methods
2.1 Microalgal suspensions
The green microalga, Chlorella sp., that was used in this study was cultivated in Bold’s Basal
Medium (BBM) in a batch culture. Chlorella sp. was chosen as a model alga because it is
commonly found in natural water and easily cultured in the laboratory (Petrusevski et al. 1995).
Cell concentration was determined using a hemocytometer with an optical microscope and was
correlated with the absorbance at 600 nm measured using a Shimadzu UV-1601 spectrophotometer
(USA). This method has been described in our previous research (Ahmad et al. 2011). The fresh
cultures were taken on day 9 (cell density reached 4.86 × 109 cells/ml) because this day was the
point at which the cells had reached their maximal electronegative strength (Ahmad et al. 2012,
Lim et al. 2012). In order to compare the performances of the membranes, all experiments were
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Microfiltration of Chlorella sp.: Influence of material and membrane pore size
Table 1 Main characteristic of the membranes
Membrane material Pore diameter (µm) Manufacturer
Cellulose acetate (CA)
0.8
1.2
3.0
Sartorius
Sterlitech
Sterlitech
Cellulose nitrate (CN)
0.8
1.2
3.0
Sartorius
Sartorius
Sartorius
Polypropylene (PP) 0.8 Milipore
Polyvinylidenefluoride (PVDF) 0.8 Milipore
carried out at the same cell concentration level. The size of the particles in suspension was
measured using a CILAS model 1180 (France) laser diffraction-based particle size analyzer. The
measurements were performed at a wavelength of 830 nm at scattering angle ranged from 0° to
45° under wet mode and particle sizes ranging from 0.04 to 2500 µm were obtained.
Approximately 60 × 109 cells per sample were used to achieve the required obscuration of 4–6 %,
and each sample was measured in triplicate. The shape of the algal cells was observed using light
microscopy.
2.2 Analytical methods
In this work, the surface hydrophobicities of the newly commercialized membranes were
analyzed by measuring the contact angle of DI water on the different membrane materials. The
contact angle measurements were then analyzed with a computer software program, Optical
Contact Angle SCA 15 (Germany). A detail of the method of contact angle measurement is
provided elsewhere (Ahmad et al. 2010). The result of all measurements was the average of at
least 10 single measurements (two angles per drop). Thus, a total of 20 angle measurements were
made for each membrane sample.
The surface morphologies of the fresh and fouled membranes were observed using a Carl Zeiss
Supra 35VP SEM (Germany). The non-conducting membrane samples were coated with gold
before visualization. Based on the SEM images, the pores of the fresh membrane were clearly
visible.
2.3 MF membranes
Four commercially available MF membranes, with different nominal pore size and 47 mm in
diameter of circular membrane supplied by Milipore, Sartorius and Sterlitech, were used for
comparison in this work. These membranes and their characteristics are summarized in Table 1.
The membranes were immersed in DI water overnight before use in the experiments. This
preparative step was done to remove any trace quantities of chemicals on the membrane surface.
2.4 Experimental procedures
The experiments were performed in a specially fabricated module for flat circular membranes,
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A.L. Ahmad, N.H. Mat Yasin, C.J.C. Derek and J.K. Lim
Fig. 1 Schematic diagram of cross-flow microfiltration system
with a cross-flow configuration and an effective area of 7.07 × 10-4 m
2. The setup of this
cross-flow filtration test rig is illustrated in Fig. 1. The operating conditions were constant for all
the experiments. The TMP and cross-flow velocity (CFV) were maintained at 100 kPa and 0.13
ms-1, respectively. A stirrer in the suspension tank was used to ensure that the cells were evenly
distributed in the feed suspension. The suspension was continuously recycled throughout the
filtration module by a Masterflex model 7553-79 peristaltic pump (US). Both the concentrated
retentate and the permeate were recirculated back to the suspension tank in all experiments. The
permeate was collected in a filtrate receiver that was returned back to the suspension tank in order
to keep the cell concentration constant.
Upon placement of a membrane in the module, DI water was circulated in the test rig at a TMP
of 100 kPa and a CFV of 0.13 ms-1 for 5 min before microfiltration experiment with microalgae
cells to measure the pure water flux (Jo) for each fresh membrane. Jo across a clean membrane can
be expressed in terms of volume of permeate per unit time per unit membrane surface area
(lh-1m-2). Generally, Jo through a porous membrane in pressure driven processes is directly
proportional to the applied hydrostatic pressure, according to Darcy’s law (Mulder 1996). Darcy’s
law states that the solvent passage through the membrane is a function of the applied pressure as
follows.
m
TM
oR
P
J.
(1)
where η is the solvent viscosity, Rm is the hydrodynamic resistance of the membrane and PTM is the
TMP. Rm is a membrane constant and does not depend on the feed composition or on the applied
pressure (Mulder 1996).
During cell separation, membrane performances were evaluated according to the permeate flux,
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Microfiltration of Chlorella sp.: Influence of material and membrane pore size
J (lh-1m-2). It was calculated by dividing the permeate volume, ΔV, collected in time period, Δt, by
the effective surface area of the membrane, Ae, as follows
e
A
tV
J
/ (2)
The filtration of the microalgae suspension in all the experiments was performed until the
filtration flux reached a steady state (at least 1 h). After each experiment, the system was rinsed
with DI water and a new membrane was placed for the next experiment. All the experiments were
conducted in triplicates for each membrane.
3. Results and discussion
3.1 Characterization of Chlorella sp.
Chlorella sp. is a unicellular green alga. It is the strain most favored by researches because
easily available and easily cultured in the laboratory (Petrusevski et al. 1995). Individual Chlorella
sp. cells are spherical in shape and loosely aggregated (Fig. 2).
The particle size distribution of Chlorella sp. is important as it affects the performance of
cross-flow filtration. As shown in Fig. 3, one peak was observed in the particle sizes distribution
ranged from 2 to 8 µm with a mean size diameter of 3.67 µm as determined by laser diffraction
technique.
3.2 MF performances
Test runs using DI water were used to determine initial membrane performances prior to the
MF of microalgae cells. Fig. 4 shows the average permeation flux of DI water that was determined
experimentally for each material with various pore sizes. CA and CN membranes had the highest
water fluxes among all the membranes. These results are related to the hydrophobicity of the fresh
membranes, which will be discussed in the next section. CA and CN membranes with large pore
diameters (3.0 µm) exhibited the highest water fluxes because filtration flux increases with
increasing pore diameter in the absence of fouling. This result can be expected because a higher
filtration is caused by increasing pore diameter although in different types of membranes. A similar
observation was made by Rossignol et al. (1999) when filtering marine microalgae with
microfiltration and ultrafiltration membranes.
3.3 Effect of membrane material
Membrane fouling can be characterized by an initial rapid decrease in flux, followed by a long
and gradual flux decline (Field et al. 1995). The performances of four different MF membrane
materials are shown in Fig. 5. All the membranes had pore sizes measuring 0.8 µm. A significant
flux decline was observed for the first 10 min, after which a gradual reduction eventually
stabilized into the steady state permeation flux. It was observed that the significant flux decline
was due to the deposition of microalgae cells on the membrane surface that led to membrane
fouling. Despite being made from different materials, the CA and CN membranes showed similar
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A.L. Ahmad, N.H. Mat Yasin, C.J.C. Derek and J.K. Lim
Fig. 2 Optical observation of Chlorella sp.cells
permeate flux patterns. After 50 min of operation, the steady state permeation fluxes for both
membranes were still very similar and were the highest values attained. The PP membrane ranked
a close second while the PVDF membrane had the lowest steady state flux. Flux decline is clearly
affected by the membrane material and literature (Ho and Zydney 1999, Dizge et al. 2011)
suggests that it can also be affected by porosity and roughness of the surface.
Before conducting the MF experiments, the hydrophobicity of each membrane material was
determined by measuring the angle between the water (liquid) and membrane (solid). Ahmad et al.
(2010) reported that a material is deemed hydrophobic if the value of the contact angle is greater
than 90°. Thus, the smaller the receding angle, the less hydrophobic, or more hydrophilic the
membrane surface.
Before conducting the MF experiments, the hydrophobicity of each membrane material was
determined by measuring the angle between the water (liquid) and membrane (solid). Ahmad et al.
(2010) reported that a material is deemed hydrophobic if the value of the contact angle is greater
than 90°. Thus, the smaller the receding angle, the less hydrophobic, or more hydrophilic the
membrane surface.
The shapes of the DI water droplets on PP and PVDF membrane surface are shown in Fig. 6.
Of the four membranes used in this study, the PVDF membrane had the most hydrophobic surface
with a contact angle value of 126.63°.The PP membrane was less hydrophobic with a contact angle
value of 95.97°. These angles are much larger than the 0.00° values of the CA and CN membranes
(Figures are not shown) that indicate that DI water had almost adsorbed on those surfaces. This
means that the CA and CN membranes had the greatest performances due to their hydrophilic
nature that allowed them to adsorb DI water much more quickly than the hydrophobic membranes.
In addition, DI water droplet is easier to immerse in the membrane and the membrane wettability
rate is faster in the hydrophilic surfaces.
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Microfiltration of Chlorella sp.: Influence of material and membrane pore size
Conversely, the PVDF membrane’s large contact angle is responsible for its lower permeability.
Jung et al. (2006) confirmed that the rate of flux decline for the hydrophobic membrane was
significantly greater than for the hydrophilic membrane. This phenomenon can be further
explained by PVDF’s low surface tension values (25 dynescm-1) and lack of active groups in its
surface chemistry for the formation of “hydrogen-bonds” with water or aqueous solutions (Ahmad
et al. 2010).
The significance of the contact angle was also discovered by Gekas et al. (1992) and Jonsson
and Jonsson (1995). They found that when comparing membranes of different materials, the lower
the receding angle, the higher the relative flux and flux recovery at the end of filtration. They also
found that the measurements of the receding angle had to be supplemented with measurements of
surface roughness and porosity because more open membranes have a higher apparent
hydrophilicity due to a higher porosity.
However, the opposite result was observed by Dizge et al. (2011) when a CA membrane with a
pore size of 0.45 µm showed the most rapid decline in flux compared to polyethersulfone (PES),
mixed ester (ME) and polycarbonate (PC) membranes. They also stated that the initial rapid drop
of the flux could not be due to the porosity of the membrane, as suggested by Gekas et al. (1992),
but rather to the irregular and rough surface of the CA membrane. Nevertheless, according to
several authors, the rapid flux decline or the lower permeability performance was attributed to the
hydrophobic nature of the membrane (Masselin et al. 2001, Vaisanen et al. 2002).
In terms of steady state permeation flux, the hydrophilic membranes in this study seem to be
better choices than the hydrophobic membranes. Thus, the CA and CN membranes need to be
compared using another membrane characteristic: pore size.
3.4 Effect of pore size
The effects of the different pore sizes on the permeate fluxes of the CA and CN membranes can
be seen in Fig. 7. At the beginning of filtration, flux increased with increasing membrane pore size
(CA 0.8 < CA 1.2 < CA 3.0 and CN 0.8 < CN 1.2 < CN 3.0). This implies that most membrane
fouling occurred at the entrances of the membrane pores or on the membrane surfaces early in the
filtration period. However, after 6 min of operation, the flux was observed to be higher for both
CA and CN membranes with a pore size of 1.2 µm (CA 0.8 < CA 3.0 < CA 1.2 and CN 0.8 < CN
3.0 < CN 1.2). The decrease in flux for the membranes with a pore size of 3.0 µm could be the
result of pore blocking with microalgae cells.
Table 2 Blocking phenomena in filtration
Type of blocking Description
Complete blocking Particle size is larger than that of the membrane pore; particles will deposit on
the membrane surface to block the entrances of the membrane pores completely.
Intermediate blocking The diameter of the particles is almost the same as that of the membrane pores;
particles may deposit on the pores entrances or migrate into the pores.
Standard blocking Particle sizes are smaller than those of the membrane pores; particles are
deposited onto the internal pore walls, leading to a decrease in the pore volume.
Cake filtration
This is similar to complete blocking. However, when the concentration of the
slurry is high enough, particles may deposit on the membrane surface or on the
deposited particle layer to form a filter cake.
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A.L. Ahmad, N.H. Mat Yasin, C.J.C. Derek and J.K. Lim
Hwang and Lin (2002) described four kinds of blocking phenomena in filtration: 1) complete
blocking, 2) intermediate blocking, 3) standard blocking and 4) cake filtration (Table 2). Because
the particle size of cells suspension was ranging from 2 to 8 µm with mean size diameter of
microalgae cells is 3.67 µm (Fig. 3), the blocking of the 3.0 µm pores falls under the category of
intermediate blocking. Microalgae cells could settle on other surface cells that were already
blocking some pores or they could also directly block some membrane areas. In complete blocking,
the cells can still block the entrances of the membrane pores completely, but the larger cells cannot
block the internal pore walls as they have decreased in diameter, allowing only the solution from
microalgae cells to pass through the membrane. This phenomenon can be proved via the
characterization of the membrane surfaces before and after MF.
Fig. 3 Particle size distribution of Chlorella sp. (wavelength = 830 nm)
Fig. 4 Pure water flux of membranes (TMP = 100 kPa; CFV = 0.13 ms-1)
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Microfiltration of Chlorella sp.: Influence of material and membrane pore size
Fig. 5 Effect of membrane material on microfiltration performance (TMP = 100 kPa; CFV = 0.13 ms-1)
(a) (b)
Fig. 6 Optical images of a water droplet on the membrane surface: (a) PP 0.8 µm and (b) PVDF 0.8 µm
There are a number of methods to characterize the membrane pore structure. SEM is one of the
most popular methods that provide two-dimensional images of surfaces. The SEM images for the
fresh and fouled CA membranes with pore sizes of 1.2 and 3.0 µm are presented in Fig. 8 and Fig.
9. Figs. 8(a) and 9(a) show SEM images of the fresh membrane surfaces. Although the CA
membrane has a very rough surface with irregular pores, it has a relatively flat surface with surface
pore sizes greater than the nominal pore size of the membrane. Comparing the SEM images shown
in Figs. 8(b) and 9(b), the cells were deposited on the membrane surface, leading to a reduction in
the pore diameter; therefore, increasing the filtration resistance. However, a few cells in Fig. 8(b)
were carried by the retentate across the membrane surfaces and did not foul the interior of the
pores. The rapid flux decline is mainly due to the membrane pore size reduction at the pore
entrances. Thus, the mean pore size of the fouled membrane was reduced to only 30-60% of the
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A.L. Ahmad, N.H. Mat Yasin, C.J.C. Derek and J.K. Lim
original size due to the fouling of the cells. The opposite image was observed in Fig. 9(b) where
the cells adsorbed onto the membrane pore walls, therefore, clogging the interior of the pores and
thus, the membrane resistance is increased quickly and lowest filtration rate could be obtained.
Because the size of the membrane pores was almost equivalent to that of cells, a few cells may
have migrated into the membrane pores, whereas others deposited on the pore entrances. This
phenomenon can also be classified as intermediate blocking of cells (Hwang and Lin 2002). This
blocking would be transform into cake filtration after 10 min of MF and larger cake resistance
occurred, thus, resulting in a reduction of permeate flux compared to the membranes with pores
size of 1.2 µm. In addition, the microfiltration with large pore diameters was more sensitive to
fouling (pore clogging), which probably increased with the presence of cell fragments and debris
that produced a cake layer that caused irreversible fouling (Rossi et al. 2004).
(a)
(b)
Fig. 7 Effect of pore size on microfiltration performance for (a) CA; and (b) CN membranes
(TMP = 100 kPa; CFV = 0.13 ms-1)
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Microfiltration of Chlorella sp.: Influence of material and membrane pore size
(a) (b)
Fig. 8 SEM images of CA membrane with pore size of 1.2 µm at 60 min of filtration: (a) fresh
membrane; and (b) fouled membrane
(a) (b)
Fig. 9 SEM images of CA membrane with pore size of 3.0 µm at 60 min of filtration: (a) fresh
membrane and (b) fouled membrane
4. Conclusions
The flux decline behavior of microalgae separation in cross-flow MF was studied using four
different membrane materials: CA, CN, PP and PVDF. The reduction in permeate flux was
dependent on the type of membrane material. The hydrophilicity of the membrane, determined
from contact angle measurements, was also an important factor affecting flux. The CA and CN
membranes, with contact angles of 0.00° had the greatest performances in terms of steady state
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A.L. Ahmad, N.H. Mat Yasin, C.J.C. Derek and J.K. Lim
permeation flux. The PP and PVDF membranes had contact angle values of 95.97° and 126.63°,
respectively. CA and CN membranes with three pore sizes were compared to evaluate the
efficiency of these hydrophilic membranes. Although the pure water flux increased with increasing
the pore size diameter (0.8 < 1.2 < 3.0 µm), optimum performances were obtained from
membranes with a pore size of 1.2 µm when filtering a foulant suspension. The increase in
membrane pore diameter in MF promotes pore plugging that contributes to intermediate blocking.
The pore blocking on the membrane surfaces were observed using SEM images. All these
membrane characteristics led us to conclude that the hydrophilic CA and CN membranes with a
pore size of 1.2 µm exhibited the best performances in term of permeation flux.
Acknowledgements
The research described in this paper was financially supported by USM Research University
(RU) Grant (Grant No. 1001/PJKIMIA/814060) and (Grant No. 1001/PJKIMIA/811165), USM
Postgraduate Research Grant (Grant No. 1001/JKIMIA/8044014) and USM Membrane Science
and Technology Cluster. The authors are very grateful to Mr. Rashid Selamat for assisting with the
SEM measurements. N.H. Mat Yasin gratefully acknowledges Universiti Malaysia Pahang (SLAB
2011) for the scholarship.
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continuous separation microalgae/culture medium: compared performances of cross-flow microfiltration
and ultrafiltration”, Aquacultural Engineering, 20, 191-208.
Vaisanen, P., Bird, M.R. and Nystrom, M. (2002), “Treatment of UF membranes with simple and formulated
cleaning agents”, Trans IChemE Part C, 80, 98-108.
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... Membrane filtration performance and the fouling mechanism depend on the characteristics of the feed solution, the properties of the membrane, and the operating conditions of the system [21]. Rossi et al. [16] previously performed critical flux and limiting flux studies to minimize membrane fouling during the long-term filtration of microalgal cultures. ...
... There are many different methods available for characterizing the membrane structure. One of the most commonly used characterization methods that enables the visualization of the membrane surface is SEM examination [21]. SEM images of clean membranes (left) and fouled membranes (right) can be seen from Figure 9. ...
... The surface of fouled membranes, on the other hand, is covered with numerous microalgae cells. During filtration, the main reason for the decrease in flux rate is the decrease in the amount of space at the entrance of the pores [21]. Figure 10 shows the detailed SEM images of a fouled UH050 membrane. ...
Article
The purpose of this study was to investigate the efficient harvesting of microalgal biomass through crossflow membrane filtration. The microalgal biomass harvesting experiments were performed using one microfiltration membrane (pore size: 0.2 µm, made from polyvinylidene fluoride) and three ultrafiltration membranes (molecular weight cut-off: 150, 50, and 30 kDa, made from polyethersulfone, hydrophilic polyethersulfone, and regenerated cellulose, respectively). Initially, to minimize membrane fouling caused by microalgal cells, experiments with the objective of determining the critical flux were performed. Based on the critical flux calculations, the best performing membrane was confirmed to be the UH050 membrane, produced from hydrophilic polyethersulfone material. Furthermore, we also evaluated the effect of transmembrane pressure (TMP) and crossflow velocity (CFV) on filtration flux. It was observed that membrane fouling was affected not only by the membrane characteristics, but also by the TMP and CFV. In all the membranes, it was observed that increasing CFV was associated with increasing filtration flux, independent of the TMP.
... The hydrophobicity of membranes is presented in terms of the contact angle between the water and membrane (contact angle value between liquid and solid). If the contact angle is higher than 90 • , the membrane material is considered hydrophobic [51]. In this work, both MF membranes have contact angles lower than 90 • (see Table 1). ...
... Nevertheless, the flux decline is clearly not affected by the membrane material (see Table 1). According to Ahmad et al. [51], the flux decline can be affected by the roughness and porosity of the surface. However, the selected feed flow rate was 10 mL/min, with J v improvements of 94% and 95% for the spinach and orange matrices, respectively, from 1 mL/min to 10 mL/min. ...
Article
Full-text available
Spinach and orange by-products are well recognized for their health benefits due to the presence of natural polyphenols with antioxidant activity. Therefore, the demand to produce functional products containing polyphenols recovered from vegetables and fruits has increased in the last decade. This work aims to use the integrated membrane process for the recovery of polyphenols from spinach and orange wastes, implemented on a laboratory scale. The clarification (microfiltration and ultrafiltration, i.e., MF and UF), pre-concentration (nanofiltration, NF), and concentration (reverse osmosis, RO) of the spinach and orange extracts were performed using membrane technology. Membrane experiments were carried out by collecting 1 mL of the permeate stream after increasing the flow rate in 1 mL/min steps. The separation and concentration factors were determined by HPLC-DAD in terms of total polyphenol content and by polyphenol families: hydroxybenzoic acids, hydroxycinnamic acids, and flavonoids. The results show that the transmembrane flux depended on the feed flow rate for MF, UF, NF, and RO techniques. For the spinach and orange matrices, MF (0.22 µm) could be used to remove suspended solids; UF membranes (30 kDa) for clarification; NF membranes (TFCS) to pre-concentrate; and RO membranes (XLE for spinach and BW30 for orange) to concentrate. A treatment sequence is proposed for the two extracts using a selective membrane train (UF, NF, and RO) to obtain polyphenol-rich streams for food, pharmaceutical, and cosmetic applications, and also to recover clean water streams.
... Using nonmembrane MF with membranes can ensure the life-length of the downstream membrane (Van Der Bruggen et al., 2003;Cheryan, 1989). Fig. 19.1 indicates a process flow diagram of MF system (Ahmad et al., 2013). ...
... Process flow diagram of microfiltration system(Ahmad et al., 2013). ...
Chapter
Water scarcity for industrial utilization has been appeared as a global issue in this modern era. Different types of wastewater containing heavy metals are continuously contaminating water. To resolve this contamination, various technologies have been adopted to reduce and among them membrane technologies have been considered more effective compared to conventional wastewater treatment technologies due to the isolation capability of nutrients and heavy metals at low concentration. This study demonstrated the comprehensive analysis on the numerous membrane technologies for wastewater pretreatment processes. Furthermore, recovering valuable resources such as nutrients through membrane technologies has been outlined in this chapter what could lead to economic and environmental feasibility. The overview of this chapter summarized that wastewater treatment and resource recovery via implementations of membrane technologies can play an excellent role in near future effectively for wastewater treatment, purification, and resource recovery in chemical, biochemical, food, and manufacturing industrial sector.
... Qu et al. reported in the study performed with a UF membrane that, according to the results of the SEM analysis that they conducted, the membrane's surface was covered with microalgal cells (Qu et al. 2012). Similarly, Ahmad et al. reported in the study performed with cellulose acetate MF membranes having pore sizes of between 1.2 and 3 lm that microalgal cells were accumulated on the membrane surface according to their own SEM analyses (Ahmad et al. 2013). Furthermore, they emphasised that this accumulation reduced the pore size which, in turn, resulted in an increase in filtration resistance (Ahmad et al. 2013). ...
... Similarly, Ahmad et al. reported in the study performed with cellulose acetate MF membranes having pore sizes of between 1.2 and 3 lm that microalgal cells were accumulated on the membrane surface according to their own SEM analyses (Ahmad et al. 2013). Furthermore, they emphasised that this accumulation reduced the pore size which, in turn, resulted in an increase in filtration resistance (Ahmad et al. 2013). Zhang et al. reported in their study performed with UF membranes that, based on their own SEM analyses, microalgal cells formed a caked layer on the fouled membrane surfaces (Zhang et al. 2010). ...
Article
In this study, hydrophilic and fouling-resistant polysulfone (PS) membranes were fabricated using the phase inversion method to reduce membrane fouling caused by microalgal culture. The Pluronic F-127 polymer, which is used as a hydrophilic co-polymer, was added to the membranes to improve the membrane properties. Characteristic specifications of the fabricated membranes, such as morphology, surface roughness, chemical structures and hydrophobicity/hydrophilicity, were studied using scanning electron microscopy, atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDS), attenuated total reflection-fourier infrared (ATR-FTIR) spectroscopy and contact angle devices. According to the results obtained, it was observed that, with the increase of the Pluronic F-127 concentration in the membranes, the surface roughness of the membranes decreased and hydrophilicity and permeation fluxes increased notably. Furthermore, it was observed that the addition of the Pluronic F-127 polymer into the membranes reduced reversible/irreversible membrane fouling. Additionally, a characterisation of the fouled membranes was performed for the purpose of comprehensively understanding the membrane fouling mechanism caused by microalgal culture.
... For this species, it can be argued that hydrophilic membranes, such as the ceramic membrane used in our work, are beneficial to limit foulant-membrane hydrophobic interactions involving proteins. Superior performance of hydrophilic membranes relative to hydrophobic membranes for the MF of Chlorella sp. has been reported with organic membranes at low CFV [43,52]. ...
Article
Full-text available
This study aimed to investigate the harvesting of microalgae by microfiltration (MF) on a ceramic membrane at relatively high cross-flow velocity (CFV) of interest for commercial processes. Pilot-scale harvesting was conducted with algal suspensions (Chlorella vulgaris and Tisochrysis lutea (T-Iso)) and algal supernatants (Porphyridium cruentum) to assess the effect of feedstock characteristics and understand flux decline mechanisms. In total recycle mode (C. vulgaris, 1 g/L), high steady-state permeation flux around 200 L/m2/h was achieved. Total filtration resistance was mainly due to cake resistance (Rc, 57%) and pore adsorption and blocking (Ra, 40%). The process hydrodynamic conditions seemed to have relatively little effect on Chlorella cell integrity. In concentration mode, average permeate flux decreased from 441 to 73 L/m2/h with increasing feed concentration (C. vulgaris, 0.25–1 g/L); the contribution of Rc decreased (82 to 57%), while that of Ra rose (7 to 40%). With T-Iso suspensions and P. cruentum supernatants at 1 g/L, average permeate flux was 59 and 49 L/m2/h, respectively, with predominance of Rc and Ra, respectively. Distinct fouling mechanisms were inferred to explain the superior filterability of C. vulgaris. The results show that ceramic membrane MF at relatively high CFV could be a suitable option for harvesting certain microalgae including C. vulgaris.
... Les valeurs de perméabilité à l'eau sur les deux types de media étudiés sont présentées en Figure 134. Les valeurs de perméabilités des membranes en acétate de cellulose sont du même ordre de grandeur que celles retrouvées dans la littérature (Ahmad et al., 2013 Figure 137 Réduction de la turbidité au cours de la filtration La Figure 137 présente les résultats en termes de réduction de turbidité pour chaque media filtrant. La turbidité initiale avant filtration est de 80 ± 7 NTU. ...
Thesis
Les membranes et les adjuvants de filtration sont très utilisés pour la filtration des vins. Les adjuvants de filtration forment un dépôt filtrant assurant l’efficacité du filtre. Cependant ces particules ne sont pas régénérables. L’objectif de ces travaux est de proposer un media filtrant régénérable et disponible en plusieurs granulométries, afin d’être utilisé lors des différentes étapes de l’élaboration des vins. Les Rilsan® en polyamide 11 biosourcé sont étudiés et caractérisés en tant qu’alternative aux diatomites. Les résultats montrent qu’avec la gamme de Rilsan® existante il est possible d’obtenir les différents grades nécessaires pour la filtration du vin. Les distributions de tailles de particules, la formation des dépôts ainsi que les efficacités de filtration ont été déterminées. Lors de la filtration sur précouche de Rilsan® les mécanismes de rétention suivent une loi de colmatage intermédiaire des pores. Du fait de la structure non poreuse du matériau par rapport aux diatomites, leur régénération est possible et a été étudiée par hydrocyclonage. Les non-tissés ont l’avantage d’avoir de très fortes porosités ; ce qui est un atout pour obtenir des débits de filtration importants. De nouveaux media en polyamide 11 fabriqués par Electrospinning sont testés comme une alternative aux membranes actuelles, de plus faible porosité. Les diamètres de pores obtenus de l’ordre de 1,5 μm sont encore trop élevés pour envisager une stabilisation microbiologique du vin. La faible résistance mécanique des media est un frein à leur développement pour la filtration des liquides. Les perméabilités sont peu dépendantes de la dimension des pores dans la gamme testée et dépendent principalement de l’épaisseur du matériau et d’autres paramètres de fabrication.
... Since it is recover practically all the oil behind in excess of a 1% leftover oil in the crude material (6). In increase, this strategy, significantly low moderate employable expense arch with other oil creation (7). ...
Article
Full-text available
Microalgae gives the great impression being to be a prominent source biomass for oil production that has the capacity of downright the fossil fuel. In this earth's surface is protected with 80% of water, so microalgae would truly be inexhaustible option of prospect for the environment needs. Microalgae have been suggested as a promising oil feed stock and have been called the third-generation feedstock. In this study, we talk about the unique and relational valued prospect of C. vulgaris and also highlights the biomass, oil extraction, solvent system and compound properties. The reflux extraction method was used to extract oil from C. vulgaris. The oil extraction of C. vulgaris biomass using heptane as a dissolvable at the following optimum condition, stirring rate of 700 RPM, temperature of 85°C and extraction time at 6 hours. Then the compounds present in oil were anatomized by GCMS studies.
... Large R c was determined under mixotrophic cultivation which is probably a result of the extensive development of biofilm in response to EPS or SMP production (it is described in the next section). Further, to better understand the membrane fouling and the resistance analysis, SEM method was used SEM is one of the most popular methods for providing images of membrane surfaces [36]. SEM images demonstrated that the surface and pores of the initial membrane is very clean and free of particles. ...
Article
Abstract The present study examined fouling in a microalgal membrane bioreactor under mixotrophic, heterotrophic and photoautotrophic conditions. N-enriched wastewater, containing nitrate source, was used as a nutrient source for cultivation of microalgae. The results confirmed that the membrane fouling rates increased under mixotrophic cultivation through enhanced production of carbohydrates in soluble microbial products (SMPc) and protein in extracellular polymeric substances (EPSp). The transmembrane pressure (TMP) jumping was observed under mixotrophic and photoautotrophic cultivation after 31 and 47 days of operation, respectively, while the TMP of heterotrophic cultivation did not exceed 10 kPa throughout 51 days. The highest EPSp was produced under mixotrophic condition due to high nitrogen removal rate. Also, the results of resistance analysis indicated that cake resistance was the main fouling resistance in all cultivation types and the latter result was confirmed by SEM analysis. In addition, higher protein fraction of cake layer on membrane foulants in comparison to carbohydrates fraction increased the hydrophobicity of membrane's surface in all cultivations (except heterotrophic culture). Compared to mixotrophic and photoautotrophic cultures, hydrophobic properties and cell size of heterotrophic microalgae increased and resulted in low membrane fouling rates.
... Solvent extraction method is a common and efficient technique for producing oil for biodiesel production since it is recovers almost all the oil and leaves behind only 0.5% to 0.7% residual oil in the raw material [8] and it is also involves the transfer of a soluble fraction from a solid material to a liquid solvent. In addition, this method has a relatively low operating cost compared with supercritical fluid extraction [9]. However, there are certain disadvantages for solvent extraction method such as poor extraction of polar lipids, long time required for extraction and hazards of boiling solvents [10]. ...
Article
Full-text available
This research aims to investigate the optimum condition of oil extraction method to extract maximum oil yield from freshwater microalgae Chlorella vulgaris. The modified soxhlet extraction method was used to identify the best solvent systems which are heptane, heptane: methanol (1:1), heptane: methanol (1:2), heptane: ethanol (1:1) and heptane: ethanol (1:2) for extracting the microalgae oil. The effect of different mixing rate (rpm), temperature (ºC) and extraction time (hours) were carried out using the optimized solvent system to evaluate the optimum condition of oil extraction. Based on the yield of oil extraction, heptane alone become the best solvent to extract the oil with the yield of 57.5%, followed by heptane: methanol (1:2), heptane: ethanol (1:1), heptane: ethanol (1:2) and heptane: methanol (1:1) with the yield of 47.5%, 44.8%, 43.2% and 41.4%, respectively. Maximum oil quantity of 61.27% was obtained after extracted the Chlorella vulgaris biomass using heptane as a solvent at the following optimal conditions: mixing rate of 600 rpm, temperature of 65 °C and extraction time of 5 hours. This study confirmed that an increasing temperature resulted in the increased of oil yield, but at higher temperature (greater than 65 °C), the oil yield was decreasing. Too high of temperature in oil extraction may cause partial decomposition of the microalgae cells and thus lowering the yield of oil extracted. © 2017, Malaysian Society of Analytical Sciences. All rights reserved.
Article
Membrane technology has emerged as a possible solution to the challenge of biomass recovery facing microalgal industry. However, fouling phenomenon hinders the development of membrane processes. In this regard, different fouling mitigation strategies have been developed, among which membrane surface modification is one of the most promising approaches to enhance fouling resistance. Among the surface-active materials that have been explored for membrane antifouling surface modification, zwitterionic materials show superior antifouling properties. This review discusses the unique structural features of zwitterionic materials giving rise to several outstanding properties such as high polarity, anti-polyelectrolyte effect, molecular self-association and super surface hydration which act synergistically to deal with an array of foulants in feed solutions. The foulant dynamics and fouling mechanism in microalgal culture processing are also highlighted and the anti-fouling mechanisms of zwitterionic materials such as physical and steric hindrances are discussed. Membrane zwitterionization approaches are critically outlined and their potential application for effective microalgal culture dewatering is discussed.
Article
Full-text available
The static adsorption test showed that hydrophobic organics adsorbed much more quickly than hydrophilic organics. In case of the effect of membrane properties on the adsorption of organic fractions, the adsorption ratio [C(t)/C(e)] was greater for the hydrophobic membrane than for the hydrophilic membrane regardless of the kind of organic fractions. The rate of flux decline for the hydrophobic membrane was significantly greater than for the hydrophilic membrane, regardless of pretreatment conditions. The pretreatment of raw water significantly reduced the fouling of the membrane. The permeate flux was rapidly declined by simultaneous pore blocking and cake formation. After this flux decline, the permeate flux decreased in the pore volume. This implies that all four mechanisms are valid when applying these filtration models. When the pretreatment coagulation applied, the kinetic constants, Ks, Ki, and Kc, showed lower values than UF alone process. Therefore, applying coagulation process before membrane filtration was found to be very effective in fouling reduction as well as critical flux increase due to the increase in particle size. For UF process alone, Ks, was lower for the hydrophobic membrane, while Ki and Kc were higher for the hydrophilic membrane. That is, the result showed that decrease in the pore volume, which was caused by the particle deposition into the internal pore, was greater with the hydrophobic membrane than with the hydrophilic membrane.
Book
Membranes playa central role in our daily life, or as indicated by one of my foreign colleagues, Richard Bowen, 'If you are tired of membranes, you are tired of life' . Biological membranes are hardly used in industrial applications, but separations with synthetic membranes have become increasingly important. Today, membrane processes are used in a wide range of applications and their numbers will certainly increase. Therefore, there is a need for well educated and qualified engineers, chemists, scientists and technicians who have been taught the basic principles of membrane technology. However, despite the growing importance of membrane processes, there are only a few universities that include membrane technology in their regular curricula. One of the reasons for this may be the lack of a comprehensive textbook. For me, this was one of the driving forces for writing a textbook on the basic principles of membrane technology which provides a broad view on the various aspects of membrane technology. I realise that membrane technology covers a broad field but nevertheless I have tried to describe the basic principles of the various disciplines. Although the book was written with the student in mind it can also serve as a first introduction for engineers, chemists, and technicians in all kind of industries who wish to learn the basics of membrane technology.
Article
Pressure driven membrane processes have been substantially used for the separation of solids and liquid. The main disadvantage of such systems is rapid fouling, causing higher energy consumption and lower flux output. The main objective of this study was to explore the influences of type and pore size of membranes on bio-fouling by biological suspensions. Cellulose acetate, polyethersulfone, mixed ester, polycarbonate (CA, PES, ME, PC) membranes with three different pore sizes (0.40–0.45, 0.22, 0.10μm) were used in cross flow microfiltration experiments. The flux decline behavior was observed with time. Permeate samples were taken for protein and carbohydrate analysis. Surface roughness of clean and fouled membranes were determined using atomic force microscopy (AFM) images. CA membrane with pore size of 0.45μm showed the most rapid decline in the flux among all membranes due to its irregular and rough surface. ME membranes yielded the greatest steady state flux value followed by PC and PES, while CA membranes had the lowest steady state flux. PC membranes had the greatest pore resistance (Rp) for membranes at all pore sizes. Concentration polarization was observed to be a significant fouling mechanism for all membranes.
Article
The objective of this study is to investigate the potential process for the removal of carbon dioxide (CO2) from flue gas using fundamental membrane contactor, which is a membrane gas absorption (MGA) system. The experiments consisted of microporous polyvinylidenefluoride (PVDF) flat sheet membrane with 0.1μm (as module I) and 0.45μm (as module II) pore size. 2-Amino-2-methyl-1-propanol (AMP) solution was employed as the liquid absorbent. The effect of AMP concentration was studied with variation in the range 1–5M. In addition, the experiments were carried out with 10%, 20%, 30% and 40% gas ratio of CO2 to N2 and pure CO2 as well. Through contact angle measurement, membranes for module I and module II were obtained with CA values of around 130.25° and 127.77°, respectively. The mass transfer coefficients for module II are lower than those of module I for 1–5M of AMP. Furthermore, the increase in CO2 concentration in the feed gas stream enhanced the CO2 flux as the driving force of the system was increased in sequence from 1M to 5M of AMP. However, after the particular percentage (40%) of CO2 inlet concentration, the CO2 fluxes seem saturated. The combination of AMP as liquid absorbent and PVDF microporous membrane in MGA system has shown the potential to remove the CO2 from flue gas. In addition, the higher AMP concentration gave higher mass transfer coefficient at low liquid flow rates.
Article
Both hydrophilic (C 30F) and hydrophobic (PA 50H and PES 50H) ultrafiltration membranes were fouled with (i) a 3.5 wt% whey protein solution or (ii) ground wood mill circulation water from an integrated pulp and paper mill. The membranes were subsequently treated with different cleaning agents (NaOH, HNO3, Ultrasil 11 and Libranone 960). For whey protein fouled membranes, the effectiveness of the cleaning protocol was a strong function of the sodium hydroxide concentration used. After treatment with Libranone 960, membranes fouled with ground wood mill water displayed a substantial increase in water permeability. The less hydrophilic the membrane surface, the greater the observed flux increase following cleaning. After a short period of cleaning with Libranone 960, the pure water permeability decreased as the surfactant desorbed from the membrane surface. No such trend was seen when cleaning with Ultrasil 11. FTIR, SEM and AFM techniques were used to investigate the nature of the membrane surface before and after fouling and cleaning. These techniques confirmed the efficacy of the Libranone 960 cleaning protocol.
Article
Effects of membrane morphology and operating conditions on the performance of cross-flow microfiltration are studied. Three kinds of membranes with the same mean pore size of 0.1μm are selected for filtration experiments. They are MF-Millipore, Durapore and Isopore membrane. The variations of cake and membrane resistance during filtration using these membranes are measured and used for blocking analysis. Although complete pore blocking occurs, Isopore membrane results in the highest filtration rate due to less cake formation. The filtration rate of Durapore membrane is lower than that of Isopore membrane. It is because, more particles deposit on the surface of Durapore membrane to form filter cake. MF-Millipore membrane results in the lowest filtration rate due to the most cake formation and a serious pore blocking. The pore structures of each membrane are modeled in order to understand the mechanisms of pore blocking. At the initial stage of filtration, a standard blocking occurs in MF-Millipore membrane, an intermediate blocking in Durapore membrane, and a complete blocking in Isopore membrane. These blocking models are transferred to cake filtration after 10min for all kinds of membranes. The pore blocking in each membrane is also observed and demonstrated by scanning electronic microscopy. Furthermore, filtration rate increases with either increasing cross-flow velocity or increasing filtration pressure.
Article
This study was carried out to investigate the membrane fouling phenomena due to algal deposition and to understand the nature of the cake deposited on the membrane while treating the algae-laden water by membrane filtration. To accomplish this, batch experiments in dead end mode were carried out using Chlorella algae to investigate the effect of feed concentration and transmembrane pressure (TMP) on cake resistance using cellulose ester and polyvinylidene difluoride (PVDF) membranes. It was found that algae can cause significant fouling of both the cellulose ester and PVDF membranes. Fouling due to algae is quite complex because these cells release extracellular organic matter (EOM) which significantly increases the resistance. The cake resistance offered by the Chlorella algae was independent of the membrane materials considered; thus the interaction between the membrane material and Chlorella cells was not important in the resistance development. It was also found that the cake deposited on the membrane was compressible in nature with a compressibility index of 0.439.
Article
Separation of micro-organisms from their culture medium is a critical point in many biotechnological and environmental applications. Cross-flow microfiltration and ultra-filtration often appear as suitable processes for this purpose. In this study, 11 commercial membranes (from Rhodia-Orelis, Miribel, France) were evaluated for harvesting the cyanobacterium Arthrospira platensis. The relative contributions of different membrane characteristics on the permeation flux and the fouling phenomenon were discussed, especially cut-off, constitutive materials and surface properties (charge, hydrophobicity). According to our results, the ultrafiltration membrane IRIS 3038 (polyacrylonitrile, 40 kDa, neutral and hydrophilic) exhibited the best performance in terms of permeation flux and cleanability. Consequently, this membrane was selected to perform A. platensis harvesting experiments with volume reduction factor up to 10. The general framework of this experimental study is the MELISSA project from European Space Agency (ESA, www.estec.esa.nl/ecls/): ‘development of auto-regenerative biological life support systems for men in space’, partly based on photosynthetic organisms such as cyanobacteria.
Article
Cross-flow microfiltration and ultrafiltration techniques have become a suitable process for the separation of micro-organisms in a variety of biotechnical applications. In this paper, eight commercial membranes (IRIS, Orelis, Miribel, France) were evaluated for the harvest-ing of two marine microalgae: Haslea ostrearia and Skeletonema costatum, both widely cultivated in western France (Région des Pays de Loire). The effects of cross-flow velocity, transmembrane pressure, concentration and the characteristics of suspensions are discussed. The ultrafiltration membrane (polyacrylonitrile, 40 kDa) proves to be the most efficient in the peculiar conditions of low pressure and low tangential velocity for a long-term operation. © 1999 Elsevier Science B.V. All rights reserved.
Book
The basic principles of the various aspects of membrane technology are reviewed. Polymers used as membrane material are surveyed and factors determining material properties described. Various preparation techniques are overviewed and the phase-inversion process is discussed in detail. Characterization techniques are included, both for porous and nonporous membranes. Types of driving forces, transport processes and concentration polarization are described together with membrane fouling. Aspects of module and process design are given together with some process calculations. (P.M.T.)