Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR CONCENTRATING
AN AQUEOUS SUSPENSION OF MICROALGAE
The present invention relates to improvements in the field of the
production of microalgae. More particularly, the invention relates to an
improved method and apparatus for concentrating an aqueous suspension of
microalgae.
Microalgae are at the basis of the marine alimentary chain. For
many marine organisms, microalgae represent the sole source of food. The
culture of zooplankton and mollusk requires a massive production of
microalgae. It is generally admitted that the production costs of microalgae
represent about one third of the operation costs of a commercial hatchery.
Much research has been done in order to develop an alternative diet which may
totally or partially replace a natural diet consisting of feeding the marine
microorganism with natural food. These alternative diets have been proposed
in order to reduce and even to eliminate the high production costs of the
microalgae. Microalgae paste was one of the suggested alternative diets to
replace diets consisting of living microalgae. These pastes are prepared by
centrifugation or flocculation processes for obtaining concentrated suspension
of microalgae. The mayor drawback of the methods of preparing concentrated
suspension of microalgae is that the obtained microalgae have a low nutritive
value. This considerable loss is explained by the fact that even if such
techniques are efficient for concentrating and preserving the algal biomass,
they do not allow the preservation of living biological material. In fact,
when
using such methods, a rapid biochemical degradation of the microalgae occurs.
In particular, the lipidic content of the microalgae is substantially reduced.
Thus, the microalgae paste and other substitutes such as microencapsulated
lipids and microalgae powders cannot completely replace natural diets
consisting of living microalgae.
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U.S. Pat. No. 5,910,254 describes a method for dewatering an
aqueous suspension of microalgae by introducing the suspension into a bubble
column for generating a froth of bubbles and adsorbed algal cells that can be
separated from the aqueous suspension. This method permits to isolate valuable
S organic compounds from microalgae such as beta carotene, carotenoids,
glycerol and proteins, but does not maintain the integrity of the microalgae
since the latter are ruptured during the method.
U.S. Pat. No. 6,524,486 describes a method and apparatus for
separating microalgae from water without rupturing cells. Such a method
comprises three different steps (flocculation, flotation and dehydration) and
requires the use of flocculating agents.
When using flocculating agents or preservative agents, chemicals
are added to the concentrated suspension of microalgae and the effects of
these
products on the stability of the suspension are often unknown.
Many pharmaceutical and neutraceutical products are supplied
from the environment, such as animals, plants, bacteria and fungus. Also, a
plurality of new bioactive molecules have been extracted and isolated from
marine organisms. It has been estimated that about 30,000 different species of
microalgae are present in the ocean. One of the biggest challenges is thus to
facilitate the supply of these microorganisms. Even if the industrial
production
of microalgae has been required for the aquaculture for decades, recuperation
of the vegetal biomass for the eventual extraction of a new bioactive molecule
is quite recent. Since the methods used so far for extracting and isolating
microalgae from their culture mediums (centrifugation and flocculation) and
their preservation (freezing and preservatives) are known to reduce the
quality
of the obtained microalgae, it is evident that the development of new methods
is needed.
It is theref~re an object of the present invention to overcome the
above drawbacks and to provide a method and apparatus for concentrating a
suspension of microalgae without rupturing the microalgae.
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According to a first aspect of the invention, there is provided a
method of concentrating an aqueous suspension of microalgae, comprising the
step of passing the suspension of microalgae through a tangential filtering
device for partially removing water from the suspension without rupturing the
microalgae, thereby obtaining a concentrated suspension of microalgae and
filtered water.
According to a second aspect of the invention, there is provided a
method of producing a concentrated suspension of microalgae, comprising the
steps of:
a) providing a reservoir containing an aqueous suspension of
microalgae, and a tangential filtering device in fluid flow communication with
the reservoir;
b) passing the suspension from the reservoir through the
tangential filtering device to partially remove water from the suspension
without rupturing the microalgae, thereby obtaining the concentrated
suspension of microalgae and filtered water; and
c) recovering the concentrated suspension of microalgae.
According to a third aspect of the invention, there is provided an
apparatus for concentrating an aqueous suspension of microalgae, comprising:
- a reservoir adapted to contain the suspension of microalgae to
be concentrated;
- a tangential filtering device in fluid flow communication with
the reservoir, for partially removing water from the suspension without
rupturing the microalgae; and
- a pump for passing the suspension from the reservoir through
the tangential filtering device, thereby obtaining a concentrated suspension
of
microalgae and filtered water.
Applicant has found quite surprisingly that by using a tangential
filtering device for partially removing water from the aqueous suspension of
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microalgae, it is possible to concentrate the suspension of microalgae without
rupturing the microalgae.
The expression "microalgae in the concentrated suspension
obtained have a reproductive potential which is maintained for a period of at
least 25 days" as used herein means that over a period of 25 days, the
reproductive potential of the microalage permits a constant growth of a
culture
of these microalgae.
In the method according to the first aspect of the invention, the
suspension prior to being concentrated can have a concentration ranging from 1
to 500 x 106 cells/mL and preferably from 1 x 106 to 50 x 106 cells/mL. In the
method according to the second aspect of the invention, the suspension prior
to
being concentrated can have a concentration ranging from 1 to 100 x 106
cells/mL and preferably from 1 x 106 to 30 x 106 cells/mL. The suspension
prior to being concentrated according to the methods of the invention can
originate from a fresh culture of microalgae.
The concentrated suspension obtained according to the method as
defined in the first aspect of the invention can have a concentration ranging
from 2 to 30 x 101° cells/mL and preferably from 2 x 106 to 1-0 x
101° cells/mL.
The concentrated suspension obtained according to the method as defined in
the second aspect of the invention can have a concentration ranging from
1 x 106 to 30 x 101° cells/mL and preferably from 2 x 106 to 10 x
101° cells/mL.
The concentrated suspension obtained according to the methods
of the invention can be from 2 to 1000 and preferably from 100 to 800 times
more concentrated than the suspension prior to concentration.
The filtered water obtained according to the methods of the
invention can be used for the culture of microalgae.
The method as defined in the second aspect of the invention can
further include prior to step (c):
b') recycling the concentrated suspension obtained in step (b) to
the reservoir and then repeating step (b).
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Preferably, step (b') is repeated until the suspension obtained
reaches a desired concentration. The desired concentration can range from 1 x
106 to 30 x 101° cells/mL and preferably from 2 x 106 to 10 x
101° cells/mL or
can be from 4 to 1000 and preferably from 100 to 800 times more concentrated
than the suspension prior to concentration. During step (b) or (b'), a fresh
suspension of microalgae can be added into the reservoir. Step (c) can be
carried out by recovering the concentrated suspension of microalgae from the
reservoir. Preferably, step (c) is carried out by recovering the concentrated
suspension of microalgae from the reservoir and from the tangential filtering
device.
The method according to the first aspect of the invention can
further comprise the step of recovering the concentrated suspension of
microalgae. The methods of the invention are preferably continuous methods.
In the methods of the invention, the step of passing the
suspension through the tangential filtering device can be an ultrafiltration.
In the methods of the invention and in the apparatus according to
the third aspect of the invention, the tangential filtering device can
comprise a
cartridge containing a plurality of spaced-apart parallel tubular members,
wherein the tubular members have porous walls with pores of a predetermined
molecular weight cut-off.
In the methods of the invention and in the apparatus according to
the third aspect of the invention, the tangential filtering device can
comprise a
plurality of tangential filtration cartridges arranged in fluid flow
communication with one another or in parallel relationship to one another.
Preferably, the tangential filtration cartridges each contain a plurality of
spaced-apart parallel tubular members, wherein the tubular members have
porous walls with pores of a predetermined molecular weight cut-off.
The molecular weight cut-off of the pores of the tubular member,
in the methods of the invention and in the apparatus according to the third
aspect of the invention, can range from 1000 to 100000 Daltons and preferably
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from 5000 to 20000 Daltons. The tubular members are preferably hollow
fibers. The tubular members can define a total filtration surface ranging from
0.03 to 300 m2, preferably from 5 to 130 m2 and even more preferably from 10
to 25 m2.
In the methods of the invention, the suspension passing through
the tangential filtering device can have a flow rate ranging from 1 to 5000,
preferably from 100 to 1000 and more preferably from 250 to 500 L/hour. The
pressure of the suspension passing through the tangential filtering device can
range from 1 to 150 psi and preferably from 5 to 25 psi. The tangential
filtering
device can be disposed vertically and the suspension is passed therethrough
upwardly or they can be disposed horizontally.
The microalgae in the methods and the apparatus of the invention
can be marine or freshwater microalgae. The microalgae can be selected from
the group consisting of non-motile unicellular algae, flagellates, diatoms and
blue-green algae. The microalgae can belong to the family of Chlorophyceae,
Prasinophyceae, Bacillariophyceae, Cryptophyceae, Chrysophycea,
Haptophyceae or Cyanophyceae. The microalgae can belong to a species
selected from the group consisting of Isochrysis galbana, Monochrysis lutheri,
Chaetoceros muelleri and Nannochloropsis sp. The microalgae can have a size
ranging from 1 to 100 ~.m and preferably from 3 to 20 ~,m.
In the methods of the invention, the microalgae in the
concentrated suspension obtained can have a lipidic content which is stable
for
at least 30 days, preferably for at least 15 days and more preferably for at
least
12 days. The microalgae in the concentrated suspension can have a
phospholipid content or cholesterol content which is stable for at least 30
days,
preferably for at least 15 days and more preferably for at least 12 days. The
microalgae in the concentrated suspension obtained can have a reproductive
potential which is maintained for a period of at least 25 days. The microalgae
in the concentrated suspension obtained can have a reproductive potential
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similar to fresh microalgae for a period of at least 30 days, preferably for
at
least 15 days and more preferably for at least 12 days.
In the methods of the invention, the suspension prior to
concentration and the concentrated suspension obtained can have similar
lipidic
S contents. The suspension prior to concentration and the concentrated
suspension obtained preferably have similar phospholipid contents, similar
cholesterol contents or similar nutritive values. The nutritive value of the
microalgae in the, concentrated suspension obtained can be maintained for at
least 30 days, preferably for at least 1 S days and more preferably for at
least 12
days. Preferably, the microalgae in the concentrated suspension obtained are
alive.
In the apparatus according to the third aspect of the invention, the
reservoir can have a capacity ranging from 1 to 5000 L and preferably from
100 to 500 L. The pump can be adapted to impart to the suspension a flow rate
ranging from 1 to 5000 Llhour and preferably from 100 to 500 L/hour, or a
pressure ranging from 1 to 150 psi and preferably from 5 to 25 psi.
The cartridge in the apparatus according to the third aspect of the
invention can have a feed inlet for receiving the suspension of microalgae to
be
concentrated, a first outlet for discharging the filtered water and a second
outlet
for discharging the concentrated suspension of microalgae, wherein the tubular
members define therebetween a space in fluid flow communication with the
first outlet, each the tubular member having an inlet in fluid flow
communication with the feed inlet and an outlet in fluid flow communication
with the second outlet. The second outlet can be connected to the reservoir by
a
first conduit for recycling the concentrated suspension discharged from the
cartridge. The feed inlet can be connected to.the reservoir by a second
conduit.
Preferably, the first and second conduits are connected together by a third
conduit.
The first outlet in the apparatus according to the third aspect of
the invention is preferably connected to a drain by a fourth conduit. The
first
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conduit and the fourth conduits are preferably connected together. The second
conduit can be provided with a drain for emptying the reservoir or for
emptying
the cartridge. The first conduit can provided with a flow control device for
controlling the flow rate of the concentrated suspension discharged from the
cartridge. The second conduit can be provided with a flow control device for
controlling the flow rate of the suspension passing through the tangential
filtering device. The pump is preferably disposed between the reservoir and
the
cartridge, in the second conduit.
In the apparatus according to the third aspect of the invention,
when the tangential filtration cartridges contain a plurality of spaced-apart
parallel tubular members, each cartridge preferably has a feed inlet for
receiving the suspension of microalgae to be concentrated, a first outlet for
discharging the filtered water and a second outlet for discharging the
concentrated suspension of microalgae, wherein the tubular members define
therebetween a space in fluid flow communication with the first outlet, each
the
tubular member having an inlet in fluid flow communication with the feed inlet
and an outlet in fluid flow communication with the second outlet.
The concentrated suspension of microalgae obtained by the
methods of the invention can be useful for extracting and/or isolating
bioactive
molecules. The concentrated suspension of microalgae obtained by the methods
of the invention can also used for feeding marine organisms. The marine
organisms can be zooplanktons and preferably copepods. The marine
organisms can also be mollusks and preferably filter feeding mollusks. The
methods and the apparatus of the invention can be useful in a system for
feeding marine organisms, in a system for producing microalgae as food for
marine organisms, in a system for producing rnicroalgae as a health food, in a
system for producing microalgae as a biofuel, in a system for producing
microalgae for extracting and/or isolating bioactive molecules or in a system
for producing microalgae for pharmaceutical use.
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Further features and advantages of the invention will become
more readily apparent from the following description of preferred embodiments
as illustrated by way of examples in the accompanying drawings, in which:.
Figure 1 is a schematic representation of an apparatus for
concentrating a suspension of microalgae, according to a preferred embodiment
of the invention;
Figure 2 is a schematic representation of an apparatus for
concentrating a suspension of microalgae, according to another preferred
embodiment of the invention;
Figure 3 is a sectional elevation view of the tangential filtration
cartridge shown in Figure 1;
Figure 4 is a sectional view taken along line 4-4 of Figure 3;
Figure 5 is a graph showing the evolution of the reproductive
potential of microalgae from a concentrated suspension of microalgae obtained
according to a method of the invention; and
Figure 6 is a schematic representation of an apparatus fox
concentrating a suspension of microalgae, according to still another preferred
embodiment of the invention.
Referring first to Figure 1, there is illustrated an apparatus for
concentrating an aqueous suspension of microalgae, wherein a suspension of
microalgae contained in a reservoir 12 is supplied or conveyed via conduit 14
to the inlet 16 of a tangential filtration cartridge 18 by means of pump 20.
The
suspension of microalgae is passed through the tangential filtration cartridge
18
where it is concentrated; thereby obtaining filtered water which is discharged
vi a outlet 22 and supplied via conduit 24 to a drain (not shown), and a
concentrated suspension of microalgae which is discharged via outlet 26 and
supplied via conduit 28 to the reservoir 12 far optionally being further
concentrated. The conduit 14 is provided with a valve 30 for controlling the
flow rate of the suspension passing through the cartridge 18, and with a
manometer 32 which indicates the pressure generated by the flow rate of the
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suspension to be concentrated. A conduit 34 is connected to conduit 14 for
emptying the reservoir 12. A conduit 36 is also connected to conduit 14 for
emptying the cartridge 18. The conduit 24 is provided with a valve 38 for
controlling the flow rate of the filtered water discharged from the cartridge
18,
and with a manometer 40 which indicates the pressure generated by the flow
rate of the filtered water. The conduit 28 is provided with a valve 42 for
controlling the flow rate of the concentrated suspension discharged from the
cartridge 18, and with a manometer 44 which indicates the pressure generated
by the flow rate of the concentrated suspension.
Conduits 14 and 28 are connected together by conduit 46, and
conduits 24 and 28 are connected together by a conduit 48. Conduits 46 and 48
are used for bypassing the inlet 16 of the cartridge 18 when recovering the
concentrated suspension obtained. For recovering the concentrated suspension
obtained, filtered water is introduced into the reservoir 12 and supplied to
the
outlet 22 via conduits 14, 46, 28, 48 and 24. The filtered water is then
passed
through the cartridge 18 downwardly. The recovered concentrated suspension
is then discharged via conduit 36.
The tangential filtration cartridge 18 is provided with an outlet 74
which is connected to the conduit 24 by a conduit 52. The outlet 74 and
conduit
52 are used only for draining the cartridge 18, when the cartridge 18 is
cleaned.
Conduit 24 is connected to the reservoir 12 by a conduit 50. The conduit 50 is
used when filtered water is supplied via conduits 48 and 24 for cleaning the
apparatus. Conduits 24, 28, 34, 36, 46, 48, 50 and 52 are each provided with a
flow rate controlling valve 54.
Referring to Figure 2, three tangential filtration cartridges
18A,18B,18C are identical to the tangential filtration cartridge 18 shown in
Figure 1 and are arranged in paxallel relationship to one another. An aqueous
suspension of microalgae contained in the reservoir 12 is supplied via a
common conduit 14' and then via conduits 14A, 14B and 14C to the inlets 16 of
tangential filtration cartridges 18A, 18B and 18C by means of pump 20, for
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being concentrated. The suspension of microalgae is then passed through the
tangential filtration cartridges 18A, 18B and 18C where it is concentrated,
thereby obtaining filtered water which is discharged via outlets 22 and
supplied
via conduits 24A, 24B and 24 C to a common conduit 24' and then to a drain
(not shown). The concentrated suspension of microalgae obtained is discharged
via outlets 26 of cartridges 18A, 18B and 18C, and supplied via conduits 28A,
28B and 28C to a common conduit 28' and then to the reservoir 12 for
optionally being further concentrated. The conduits 14A, 14B and 14C are
provided with valves 30A, 30B and 30C for controlling the flow rate of the
suspension passing through the cartridges 18A, 18B and 18C, and with
manometers 32A, 32B and 32C which indicate the pressure generated by the
flow rate of the suspension to be concentrated. Conduit 34 is connected to
conduit 14' for emptying the reservoir 12. Conduits 36A, 36B and 36C are
connected to conduits 14A, 14B and 14C for emptying the cartridges 18A, 18B
and 18C. The conduits 24A, 24B and 24C are provided with valves 38A, 38B
and 38C for controlling the flow rate of the filtered water discharged from
cartridges 18A, 18B and 18C, and with manometers 40A, 40B and 40C which
indicate the pressure generated by the flow rate of the filtered water. The
conduit 28A is provided with a valve 42A for controlling the flow rate of the
concentrated suspension discharged from the cartridge 18A. The conduits 28A,
28B and 28C are provided with manometers 44A, 44B and 44C which indicate
the pressure generated by the flow rate of tree concentrated suspension
discharged from cartridges 18A, 18B and 18C.
Conduits 14' and 28' are connected together by conduit 46', and
conduits 24A, 24B and 24C are connected to conduit 28' by a combination of
conduit 48' with conduits 48A, 48B and 48C. Conduits 46' and 48' are used for
bypassing the inlets 16 of the cartridges 18A, 18B and 18C when recovering
the concentrated suspension obtained. For recovering the concentrated
suspension obtained, filtered water is introduced into reservoir 12 and
supplied
to the outlets 22 of cartridges 18A, 18B and 18C via conduits 14', 46°,
28', 48',
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48A, 48B, 48C, 24A, 24B and 24C. The filtered water is then passed through
the cartridges 18A, 18B and 18C downwardly. The recovered concentrated
suspension is then discharged via conduits 14A, 14B, 14C, 36A, 36B and 36C.
The cartridges 18A, 18B and 18C have respective outlets 74A,
74B and 74C which are connected to conduits 24A, 24B and 24C by conduits
52A, 52B and 52C, respectively. The outlets 74A,74B,74C, and conduits
52A,52B,52C are used only as draining means when cleaning the cartridges
18A, 18B and 18C. The conduits 24A, 24B and 24C are connected to the
reservoir 12 by a conduit 50'. The conduit 50' is used when filtered water is
supplied via conduits 48A, 48B, 48C, 24A, 24B and 24C for cleaning the
apparatus. Conduits 14', 14A, 14B, 14C, 24A, 24B, 24C, 28', 34, 36A, 36B,
36C, 46', 48A, 48B, 48C, 50', 52A, 52B and 52C are each provided with a
control flow rate valve 54.
As shown in Figures 3 and 4, the tangential filtration cartridge 18
comprises a housing 56 provided with inlet 16 for receiving the aqueous
suspension of microalgae to be concentrated, outlet 22 for discharging
filtered
water, outlet 26 for discharging the concentrated suspension of microalgae
obtained and outlet 74 for draining the cartridge 18 when the latter is
cleaned.
The cartridge 18 further comprises a plurality of hollow fibers 58 arranged in
spaced-apart parallel relationship inside the housing 56. The hollow fibers 58
are formed of a porous material and are supported by lower and upper
apertured plates 60 and 62. The fibers 58 define therebetween a space 64
(shown in Figure 4) in fluid flow communication with outlets 22 and 74. Each
fibre 58 has an inlet 66 in fluid flow communication with an inlet chamber 68
which in turn is in fluid flow communication with. the inlet 16 of the housing
56, and an outlet 70 in fluid flow communication with an outlet chamber 72
which in turn is in fluid flow communication with the outlet 26 of the
housing.
The inlets 66 and outlets 70 of the hollow fibers 58 register with the
apertures
formed in plates 60 and 62.
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The aqueous suspension of microalgae supplied to the tangential
filtration cartridge 18 flows through the inlet 16 and into the chamber 68,
and
enters each hollow fibre 58 through the inlet 66. A portion of the water
passes
through the pores defined in the walls of the fibers 58 and is thus filtered,
the
filtered water being discharged into the space 64. The filtered water is
discharged from the cartridge 18 through the outlet 22. The concentrated
suspension of mieroalgae exits the hollow fibers 58 through the outlets 70,
flows through the chamber 72 and is discharged from the cartridge 18 through
the outlet 26.
The apparatus schematized in Figure 6 is similar to the apparatus
schematized Figure 1. In fact, the apparatus of Figure 6 is a simplified
version
of the apparatus of Figure 1 wherein conduits 46, 48 and 50 have been removed
and wherein valve 30 of conduit 14 and valve 54 of conduit 24 have been
replaced with threeway valves 31 and 55; respectively. Moreover, a conduit 37
1 S connected to conduits 14 and 2S has been added.
The following examples given in a non-limitative manner are
focused on the methods of the invention using the apparatus schematized in
Figure 1 or Figure 6.
EXAMPLE 1
The concentration of various types of microalgae has been corned
out using the following general procedures using the apparatus schematized in
Figure 1. At the beginning of the procedure, all the valves were closed: The
reservoir 12 has been filled with an aqueous suspension of microalgae to be
concentrated. Valves 38 and 42 as well as valves 54 of conduits 24 and 28 have
been opened and the pump 20 has been turned on. Then, valve 30 has been
opened slowly until a pressure of S psi has been obtained on the manometer 32.
The cartridge 18 has been filled completely until filtered water has been
discharged into the drain. Valve 30 has been further opened until a pressure
of
20 psi has been obtained according to the manometer 32. Valve 42 has been
slowly turned off in order to generate a pressure of S-10 psi according to
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manometer 44. The suspension of microalgae is passed through cartridge 18,
discharged via conduit 28 and recycled into the reservoir 12 and eventually
passed again through cartridge 18 for further concentration. The suspension to
concentrate is circulated into the apparatus until the desired concentration
is
S obtained. When the desired concentration has been obtained, the valve 30 has
been slowly and completely turned off. Then, the pump 20 and all the opened
valves have also been turned off.
Then, the concentrated suspension of rnicroalgae has been
recovered in a container (not shown) by opening valve 54 of conduit 46, and
ZO then opening valve 54 of conduit 34 in order to empty reservoir 12. Valves
54
of conduits 34 and 46 have been closed. The reservoir 12 has been filled with
about 20 liters of the obtained filtered water or with filtered sea water. A
further container (not shown) has been disposed under the conduit 36, and
valve 54 of conduit 36 has been opened. Then, valves 54 of conduits 46 and 48
15 have been opened. The pump has been turned on and valve 38 has been
opened in order to generate a pressure lower than 10 psi on manometer 40. The
filtered water has been passed downwardly (or counter-current) through
cartridge 18 to remove all the concentrated suspension from the porous wall of
the hollow fibers of the cartridge 18. The concentrated suspension has been
20 discharged from the cartridge 18 via the conduit 36. When all the
concentrated
suspension has been removed from the cartridge, valve 38 and then valve 54 of
conduit 36 have been closed. Finally, the pump 20 has been turned off.
Finally, the apparatus schematized in Fig. 1 has been cleaned by
first opening valve 54 of conduit 34 and rinsing reservoir 12 with fresh
water.
25 Then, valve 54 of conduit 34 has been closed and the reservoir I2 has been
filed with 20 litres of fresh water. The pump 20 has been turned on and valves
54 of conduits 28 and 46 have been opened. Water has been circulated few
seconds and valve 54 of conduit 28 has been closed. Valves 54 of conduits 48
and 50 have been opened and water has been circulated through conduits 48
30 and 50 for few seconds. Valves 54 of conduits 46, 48 and 50 have then been
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closed. A drain (not shown) and conduit 36 have been connected together, and
valve 42 and valve 54 of conduit 36 have been opened. The valve 54 of conduit
46 has been opened until a pressure of 5 psi was reached on manometer 44.
Water has been passed through cartridge 18 for about one minute and valve 42
has been closed. Valve 54 of conduit 24 has been opened and then valve 54 of
conduit 48 has been slowly opened until a pressure of 5 psi has been reached
on manometer 40. Water has been passing through the cartridge 18 and
discharged into the drain until a limpid water has been obtained. Valve 54 of
conduit 46 has been closed and the pump 20 has 'been turned off. Then, all the
valves of the apparatus have been opened, the apparatus has been drained and
all the valves have been closed. The reservoir has been filled with 20 litres
of
cleaning and sterilizing solution such as a solution of 200 ppm of sodium
hypochlorite. Valves 38 and 42 as well as valves 54 of conduits 24 and 28 have
been opened and the pump 20 has been turned on. Then, valve 30 has been
opened slowly until a pressure of 20 psi has been obtained on the manometer
32. The cleaning and sterilizing solution has been passed through the
cartridge
18 and then, valve 30 has been closed. The pump 20 has been turned off, all
the
valves have been opened and the apparatus has been drained and all the valves
have then been closed.
EXAMPLE 2
The concentration of various types of microalgae has also been
carned out using the following general procedures using the apparatus
schematized in Figure 6. At the beginning of the procedure, all the valves
were
closed. The reservoir 12 has been filled with an aqueous suspension of
microalgae to be concentrated. Valve 42 as well as valve 54 of conduit 25 have
been opened. Valve S5 is opened in such a manner of permitting passage from
conduit 24 to conduit 25 and the pump 20 has been turned on. Then, valve 31
has been opened slowly until a pressure of 5 psi has been obtained on the
manometer 32. The cartridge 18 has been filled completely until filtered water
has been discharged into the conduit 25. Valve 31 has been further opened
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until a pressure of 20 psi has been obtained according to the manometer 32.
Valve 42 has been slowly turned off in order to generate a pressure of 5-10
psi
according to manometer 44. The suspension of microalgae is passed through
cartridge 18, discharged via conduit 28 and recycled into the reservoir 12 and
eventually passed again through cartridge 18 for further concentration. The
suspension to concentrate is circulated into the apparatus until the desired
concentration is obtained. When the desired concentration has been obtained,
the valve 31 has been slowly and completely turned off. Then, the pump 20
and all the opened valves have also been turned off.
Then, the concentrated suspension of microalgae contained in the
reservoir 12, conduits 14 and 28, cartridge 18 and pump 20 is recovered in an
appropriate container (not shown) through conduit 34 by opening valve 54 of
the latter conduit, and then opening valve 31 in such a manner to permit
passage from the pump 20 to the cartridge 18. When a maximum amount of the
concentrated suspension has been recovered, all valves have been closed. The
reservoir 12 has been filled with about 20 liters of the obtained filtered
water or
with filtered sea water. A further container (not shown) or same has been
disposed under the conduit 36, and valve 54 of conduit 36 has been opened.
Then, valve 31 has been opened in such a manner to permit passage from the
pump 20 to the conduit 37. The pump has been turned on and valve 55 has
been opened in such a manner to permit the passage the conduit 37 to the
conduit 24, and to generate a pressure lower than 10 psi on manometer 40. The
filtered water has been passed downwardly (or counter-current) through
cartridge 18 to remove all the concentrated suspension from the hollow fibers
of the cartridge 18. The concentrated suspension has been discharged from the
hollow fibres of the cartridge 18 via the conduit 36.. When all the
concentrated
suspension has been removed from the cartridge, valve 31 has been closed and
the pump 20 has been turned off. Then, all the other valves have been closed.
The apparatus schematized in Figure 6 has been cleaned and
sterilized by first opening valve 54 of conduit 34 and rinsing reservoir 12
with
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fresh water. Then, valve 54 of conduit 34 has been closed and the reservoir 12
has been riled with at least 20 litres of fresh water. The pump 20 has been
turned on and valve 54 of conduit 36 has been opened. The valve 31 is opened
in such a manner to permit passage from the pump 20 to the cartridge 18 and
by verifying the manometer 32 in order to maintain the pressure below 10 psi.
The valve 31 is then close after few seconds. Valve 54 of conduit 25 is opened
and valve 55 is opened in such a manner to permit passage from conduit 24 to
conduit 25. Valve 31 has then been opened in such a manner to permit passage
from the pump 20 to the cartridge 18, until a pressure of 10 psi is obtained
on
manometer 32. Water has been passing through the cartridge 18 and discharged
through conduit 25 until a limpid water has been obtained. Fresh water is
further added into the reservoir 12 if needed. Finally, the reservoir is
emptied
by opening valve 42 and opening valve 31 in such a manner to permit passage
from the pump 20 to the conduit 37. Then, valve 31 is closed and the pump 20
is turned off. The valves are all opened and the apparatus is completely
drained. The valves 31 and 55 are opened in all possible manners in order to
permit draining of the cartridge 18 as well as conduits 24, 36 and 52. Then,
all
the valves are closed.
The reservoir 12 has been filled with 20 litres of a cleaning and
sterilizing solution such as a 200 ppm solution of sodium hypochlorite. Valve
42 is opened and valve 55 is opened in such a manner to permit passage from
conduit 37 to conduit 24. The pump 20 has been turned on. The valve 31 is
opened in such a manner to permit passage from the pump 20 to conduit 37
until a pressure of 10 psi is obtained on manometer 40. The cleaning and
sterilizing solution has been passed through the cartridge 18 for about 10
minutes and then, conduits 25 and 36 are connected to a drain prior to open
their valves 54. When the whole has been circulated, the pump 20 has been
turned off. All the valves have been opened in all possible manners in order
to
permit a complete draining of the cartridge 18 and the conduits 36, 37 and 52.
Finally, all the valves have been closed.
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With respect to the apparatuses schematized in Figs. 1 and 6, it
should be noted that when preparing two (or more) separate batches of
concentrated suspension of microalgae within few hours (using the same of
microalgae), cleaning of the apparatuses between each batch is not absolutely
necessary. The recovering of the concentrated suspension obtained in a batch
can be carned out simply by emptying the reservoir 12.
EXAMPLE 3
Using the above-mentioned general procedure for the apparatus
schematized in Figure 1, aqueous suspensions of microalgae have been
concentrated. In particular, suspensions of two different species of
microalgae,
Isochrysis galbana and Chaetoceros muelleri, have been concentrated.
Suspensions of these microalgae varying from 300 to 1000 L have been
concentrated from 100 to 500 times. In fact, suspensions having an initial
concentration of 15 x 106 cells/mL have been concentrated until a
concentration
of about 5 x 109 to 8 x 109 cells/mL was obtained. The flow rate of the
suspension to concentrate passing through the cartridge was about 300 L/hour.
The hollow fibers of the cartridge had a total filtration surface of about 5
to
about 13 m2.
In order to evaluate the quality of the concentrated suspensions of
microalgae obtained, two tests have been performed on these suspensions.
Firstly, about 500 L of a suspension of a culture of Chaetoceros muelle~i
having an initial concentration of 12 x 106 cells/mL has been concentrated to
a
volume of 4 L. Then, the concentrated suspension has been stocked into
darkness at 4°C. Microalgae have been kept in suspension by bubbling
the
suspension. The concentrated suspension has been kept in such conditions for a
period of twelve days. Samples of the suspension have been taken every two
days to evaluate the reproductive potential of the microalgae (see Figure S).
The samples have been prepared by adding two or three drops of the
suspensions into test tubes containing a culture medium. The concentration of
these cultures has been evaluated with a particle counter until the 25'h day
after
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the beginning of the test. As illustrated in Figure 5, the microalgae of the
concentrated suspension obtained maintained their reproductive potential
during all the testing period.
Secondly, the cholesterols, photosynthetic pigments and
phospholipids contents (or lipidic content) of the concentrated suspension of
culture of Chaetoceros muelleri have been evaluated. As demonstrated in Table
1, these contents have not been affected during the 12 days storage of the
suspension. It should be noted that some of irregular variations observed in
these contents during the period of 12 days seem to occur randomly and are
probably related to the extraction and analysis procedures used. An
interesting
fact is that the phospholipid and the cholesterol contents did not vary
substantially during this period. Phospholipids and cholesterols are known to
have an important role in the structure of the cellular membrane of the
microalgae.
Table 1. Evolution of the composition of microalgae during a 12 days storage
Day CholesterolsPhotosyntheticPhospholipidsTotal
pigments
~1~~~) ~wg ~~) ~l~g
~Ng ~mI-) ~~)
0 0,265 13,252 26,063 39,580
2 2,434 14,530 28,364 45,328
5 0,979 9,992 19,030 30,001
8 0,952 11,846 31,782 44,580
12 0,793 10,538 20,146 31,477
The results showed in Table 1 and Figure 5 clearly demonstrate
that the methods of the invention permit to concentrate an aqueous suspension
of microalgae while maintaining the integrity of the cell structure.
