Abstract:
The invention relates to a method and apparatus for concentrating an aqueous suspension of microalgae. The suspension of microalgae is passed 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. Such a method can be use in systems for production of microalgae. An apparatus for carrying out the method according to the invention is also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 10/703,150, filed Nov. 6, 2003, which claims priority from Canadian Patent Application No. 2,411,383, filed Nov. 7, 2002. Each application is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    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. 
       BACKGROUND OF THE INVENTION 
       [0003]    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 major 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. 
         [0004]    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 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. 
         [0005]    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. 
         [0006]    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. 
         [0007]    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. 
       SUMMARY OF THE INVENTION 
       [0008]    It is therefore 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. 
         [0009]    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. 
         [0010]    According to a second aspect of the invention, there is provided a method of producing a concentrated suspension of microalgae, comprising the steps of: 
         [0011]    a) providing a reservoir containing an aqueous suspension of microalgae, and a tangential filtering device in fluid flow communication with the reservoir; 
         [0012]    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 
         [0013]    c) recovering the concentrated suspension of microalgae. 
         [0014]    According to a third aspect of the invention, there is provided an apparatus for concentrating an aqueous suspension of microalgae, comprising:
       a reservoir dimensioned 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.       
 
         [0018]    Applicant has found quite surprisingly that by using a tangential filtering device for partially removing water from the aqueous suspension of microalgae, it is possible to concentrate the suspension of microalgae without rupturing the microalgae. 
         [0019]    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 microalgae permits a constant growth of a culture of these microalgae. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    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 500×10 6  cells/mL and preferably from 1×10 6  to 50×10 6  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 100×10 6  cells/mL and preferably from 1×10 6  to 30×10 6  cells/mL. The suspension prior to being concentrated according to the methods of the invention can originate from a fresh culture of microalgae. 
         [0021]    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 30×10 10  cells/mL and preferably from 2×10 6  to 10×10 10  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 1×10 6  to 30×10 10  cells/mL and preferably from 2×10 6  to 10×10 10  cells/mL. 
         [0022]    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. 
         [0023]    The filtered water obtained in step (b) according to the methods of the invention can be used for the culture of microalgae. 
         [0024]    The method as defined in the second aspect of the invention can further include prior to step (c): 
         [0025]    b′) recycling the concentrated suspension obtained in step (b) to the reservoir and then repeating step (b). 
         [0026]    Preferably, step (b′) is repeated until the suspension obtained reaches a desired concentration. The desired concentration can range from 1×10 6  to 30×10 10  cells/mL and preferably from 2×10 6  to 10×10 10  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. 
         [0027]    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. 
         [0028]    In the methods of the invention, the step of passing the suspension through the tangential filtering device can be an ultrafiltration. 
         [0029]    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. 
         [0030]    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. 
         [0031]    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 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 m 2 , preferably from 5 to 130 m 2  and even more preferably from 10 to 25 m 2 . 
         [0032]    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. 
         [0033]    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 μm and preferably from 3 to 20 μm. 
         [0034]    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 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. 
         [0035]    In the methods of the invention, the suspension prior to concentration and the concentrated suspension obtained can have similar lipidic 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 and preferably for at least 15 days. Preferably, the microalgae in the concentrated suspension obtained are alive. 
         [0036]    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 L/hour and preferably from 100 to 500 L/hour, or a pressure ranging from 1 to 150 psi and preferably from 5 to 25 psi. 
         [0037]    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. 
         [0038]    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 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. 
         [0039]    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, first outlet for discharging the filtered water and 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. 
         [0040]    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 microalgae 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. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0041]    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: 
           [0042]      FIG. 1  is a schematic representation of an apparatus for concentrating a suspension of microalgae, according to a preferred embodiment of the invention; 
           [0043]      FIG. 2  is a schematic representation of an apparatus for concentrating a suspension of microalgae, according to another preferred embodiment of the invention; 
           [0044]      FIG. 3  is a sectional elevation view of the tangential filtration cartridge shown in  FIG. 1 ; 
           [0045]      FIG. 4  is a sectional view taken along line  4 - 4  of  FIG. 3 ; 
           [0046]      FIG. 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 
           [0047]      FIG. 6  is a schematic representation of an apparatus for concentrating a suspension of microalgae, according to still another preferred embodiment of the invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0048]    Referring first to  FIG. 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 via 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  for 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 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. 
         [0049]    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 . 
         [0050]    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 . 
         [0051]    Referring to  FIG. 2 , three tangential filtration cartridges  18 A, 18 B, 18 C are identical to the tangential filtration cartridge  18  shown in  FIG. 1  and are arranged in parallel 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  14 A,  14 B and  14 C to the inlets  16  of tangential filtration cartridges  18 A,  18 B and  18 C by means of pump  20 , for being concentrated. The suspension of microalgae is then passed through the tangential filtration cartridges  18 A,  18 B and  18 C where it is concentrated, thereby obtaining filtered water which is discharged via outlets  22  and supplied via conduits  24 A,  24 B 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  18 A,  18 B and  18 C, and supplied via conduits  28 A,  28 B and  28 C to a common conduit  28 ′ and then to the reservoir  12  for optionally being further concentrated. The conduits  14 A,  14 B and  14 C are provided with valves  30 A,  30 B and  30 C for controlling the flow rate of the suspension passing through the cartridges  18 A,  18 B and  18 C, and with manometers  32 A,  32 B and  32 C 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  36 A,  36 B and  36 C are connected to conduits  14 A,  14 B and  14 C for emptying the cartridges  18 A,  18 B and  18 C. The conduits  24 A,  24 B and  24 C are provided with valves  38 A,  38 B and  38 C for controlling the flow rate of the filtered water discharged from cartridges  18 A,  18 B and  18 C, and with manometers  40 A,  40 B and  40 C which indicate the pressure generated by the flow rate of the filtered water. The conduit  28 A is provided with a valve  42 A for controlling the flow rate of the concentrated suspension discharged from the cartridge  18 A. The conduits  28 A,  28 B and  28 C are provided with manometers  44 A,  44 B and  44 C which indicate the pressure generated by the flow rate of the concentrated suspension discharged from cartridges  18 A,  18 B and  18 C. 
         [0052]    Conduits  14 ′ and  28 ′ are connected together by conduit  46 ′, and conduits  24 A,  24 B and  24 C are connected to conduit  28 ′ by a combination of conduit  48 ′ with conduits  48 A,  48 B and  48 C. Conduits  46 ′ and  48 ′ are used for bypassing the inlets  16  of the cartridges  18 A,  18 B and  18 C 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  18 A,  18 B and  18 C via conduits  14 ′,  46 ′,  28 ′,  48 ′,  48 A,  48 B,  48 C,  24 A,  24 B and  24 C. The filtered water is then passed through the cartridges  18 A,  18 B and  18 C downwardly. The recovered concentrated suspension is then discharged via conduits  14 A,  14 B,  14 C,  36 A,  36 B and  36 C. 
         [0053]    The cartridges  18 A,  18 B and  18 C have respective outlets  74 A,  74 B and  74 C which are connected to conduits  24 A,  24 B and  24 C by conduits  52 A,  52 B and  52 C, respectively. The outlets  74 A, 74 B, 74 C, and conduits  52 A, 52 B, 52 C are used only as draining means when cleaning the cartridges  18 A,  18 B and  18 C. The conduits  24 A,  24 B and  24 C are connected to the reservoir  12  by a conduit  50 ′. The conduit  50 ′ is used when filtered water is supplied via conduits  48 A,  48 B,  48 C,  24 A,  24 B and  24 C for cleaning the apparatus. Conduits  14 ′,  14 A,  14 B,  14 C,  24 A,  24 B,  24 C,  28 ′,  34 ,  36 A,  36 B,  36 C,  46 ′,  48 A,  48 B,  48 C,  50 ′,  52 A,  52 B and  52 C are each provided with a control flow rate valve  54 . 
         [0054]    As shown in  FIGS. 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  FIG. 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 . 
         [0055]    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 microalgae 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 . 
         [0056]    The apparatus schematized in  FIG. 6  is similar to the apparatus schematized  FIG. 1 . In fact, the apparatus of  FIG. 6  is a simplified version of the apparatus of  FIG. 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  connected to conduits  14  and  25  has been added. 
         [0057]    The following examples given in a non-limitative manner are focused on the methods of the invention using the apparatus schematized in  FIG. 1  or  FIG. 6 . 
       Example 1 
       [0058]    The concentration of various types of microalgae has been carried out using the following general procedures using the apparatus schematized in  FIG. 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 5 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 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  30  has been slowly and completely turned off. Then, the pump  20  and all the opened valves have also been turned off. 
         [0059]    Then, the concentrated suspension of microalgae has been recovered in a container (not shown) by opening valve  54  of conduit  46 , and 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  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 hollow fibers of the cartridge  18 . The concentrated suspension has been 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. 
         [0060]    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. Then, valve  54  of conduit  34  has been closed and the reservoir  12  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  and  50  for few seconds. Valves  54  of conduits  46 ,  48  and  50  have then been 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 a 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 
       [0061]    The concentration of various types of microalgae has also been carried out using the following general procedures using the apparatus schematized in  FIG. 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  55  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 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. 
         [0062]    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 porous wall of 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. 
         [0063]    The apparatus schematized in  FIG. 6  has been cleaned and sterilized by first opening valve  54  of conduit  34  and rinsing reservoir  12  with fresh water. Then, valve  54  of conduit  34  has been closed and the reservoir  12  has been filed 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. 
         [0064]    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. 
         [0065]    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 carried out simply by emptying the reservoir  12 . 
       Example 3 
       [0066]    Using the above-mentioned general procedure for the apparatus schematized in  FIG. 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 15×10 6  cells/mL have been concentrated until a concentration of about 5×10 9  to 8×10 9  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 m 2 . 
         [0067]    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 muelleri  having an initial concentration of 12×10 6  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  FIG. 5 ). 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 th  day after the beginning of the test. As illustrated on  FIG. 5 , the microalgae of the concentrated suspension obtained maintained their reproductive potential during all the testing period. 
         [0068]    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. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Evolution of the composition of microalgae during a 12 days storage 
               
             
          
           
               
                   
                   
                 Photosynthetic 
                   
                   
               
               
                   
                 Cholesterols 
                 pigments 
                 Phospholipids 
                 Total 
               
               
                 Day 
                 (μg/mL) 
                 (μg/mL) 
                 (μg/mL) 
                 (μg/mL) 
               
               
                   
               
             
          
           
               
                 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 
               
               
                   
               
             
          
         
       
     
         [0069]    The results showed in Table 1 and  FIG. 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.