[0001] The present disclosure relates to the field of production of microalgae. In particular, the present disclosure relates to apparatuses and methods for producing microalgae.

BACKGROUND OF THE DISCLOSURE

[0002] Several systems are known in the art for producing microalgae. However, several of them are either very costly to acquire and/or to operate. Moreover, several proposed technologies do not allow for producing, at low costs, high quality microalgae.

SUMMARY OF THE DISCLOSURE

[0003] It would thus be highly desirable to be provided with an apparatus or a method that would at least partially solve one of the problems previously mentioned or that would be an alternative to the existing technologies.

[0004] A method for producing microalgae comprising

treating a culture medium comprising seawater and nutrients effective for feeding microalgae by means of an ultrafiltration;

introducing the treated culture medium into a photobioreactor containing microalgae and growing the microalgae so as to obtain a mixture;

harvesting at least a portion of the microalgae by treating at least a portion of the mixture by means of an ultrafiltration so as to obtain a concentrated suspension of microalgae and a liquid; and

recycling the liquid by using it, as is, as the culture medium and introducing it into the photobioreactor or by treating it by means of an ultrafiltration and then using it in the photobioreactor as the culture medium.

[0005] According to another aspect, there is provided a photobioreactor comprising : at least one elongated member comprising at least one wall defining an internal chamber adapted to receive a culture medium and microalgae and adapted to at least partially allow passage of a light therethrough;

a first end member adapted to sealingly engage a first end of the at least one elongated member, the first end member being provided with a main conduit for feeding the at least one elongated member with the culture medium, the first end member being also provided with at least one injector for injecting a gas at the first end of the at least one elongated member; and a second end member adapted to contact a second end of the at least one elongated member, the second end being provided with a gas outlet.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The following drawings represents non-limitative examples in which :

[0007] Figure 1 is a schematic representation of an apparatus for producing microalgae according to the present disclosure;

[0008] Figure 2 is schematic representation of a system that can be used, for example, for concentrating an algal biomass;

[0009] Figure 3 is a cross-section of a front view of an example of a photobioreactor according to the present disclosure;

[0010] Figure 4 is a front view of a base member of the photobioreactor of Figure 3;

[0011] Figure 5 is a cross-section of another front view of the photobioreactor of Figure 3;

[0012] Figure 6 is a cross-section of a side view of the photobioreactor of Figure 3;

[0013] Figure 7 is a front view of an example of a ramp in two separate parts that can be used in combination with the photobioreactor of Figure 3; [0014] Figure 8 is a graph showing the density of microalgae produced as a function of time for different types of lights used;

[0015] Figure 9 is a graph showing the density of microalgae produced as a function of time;

[0016] Figure 10 is a graph showing the density of microalgae produced as a function of time for two different culture media used;

[0017] Figure 1 1 is a graph showing the density of microalgae produced as a function of time for two different culture media used and for different types of microalgae;

[0018] Figure 2 is a front elevation view of an example of a securing member used that can be used in the photobioreactor of Figure 3; and

[0019] Figure 13 is a front elevation view of the photobioreactor of Figure 3.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0020] The following examples are presented in a non-limitative manner.

[0021] For example, the culture medium can be heated while being filtered. For example, the ultrafiltration can be effective for sterilizing the culture medium. The ultrafiltration can be a tangential flow filtration. The culture medium can be treated by means of an ultrafiltration with a membrane allowing the passage of molecules having a molecular weight inferior to 50 000 Daltons. The medium can be treated by means of an ultrafiltration with a membrane having apertures of about 0.02 μηη. For example, the ultrafiltration of the culture medium and the ultrafiltration of the at least a portion of the mixture can be carried out with at least one same tangential flow filtration cartridge.

[0022] For example, the photobioreactor can be a vertically extending bioreactor and wherein growing the microalgae can be carried out by injecting a gaseous mixture comprising air and C0 ₂ at a bottom portion of the photobioreactor and by illuminating the photobioreator with a LED (such as a white LED, a blue LED, red LED, organic-LED (OLED), flexible organic LED (OLED) or mixtures thereof). [0023] For example, the microalgae can be mixotrophic microalgae. The microalgae can be chosen from Isochrysis galbana, Pavlova lutheri, Nannochloropsis oculata, Chaetoceros muelleri, Skeletonema costatum, Rhodomonas salina, Tetraselmis suesica, Phaeodactylum tricornutum, and Thalassiosira weissflogii. For example, the microalgae can be Pavlova lutheri. For example, the microalgae can be Nannochloropsis oculata.

[0024] For example, the culture medium can be prepared by filtering seawater and mixing the filtered seawater with nutrients effective for feeding microalgae thereto.

[0025] For example, the microalgae can have been inoculated into the photobioreactor before introducing the culture medium therein.

[0026] For example, the sequence of treating the culture medium, introducing the treated culture medium into a photobioreactor, harvesting at least a portion of the microalgae, and recycling the liquid can be carried out more than once.

[0027] For example, the sequence of treating the culture medium, introducing the treated culture medium into a photobioreactor, harvesting at least a portion of the microalgae, and recycling the liquid can be carried out two, three or four times.

[0028] For example, the photobioreactor can comprise a first securing member adapted to secure the at least one elongated member to the first end member, the first securing member comprising at least one portion adapted to contact the at least one elongated member and at least one portion adapted to be connected to the first end member. The at least one elongated member can be a cylinder comprising a transparent material. The at least one portion adapted to contact the at least one elongated member can contact the cylinder and can be secured around the cylinder by means of a fastener. The at least one portion adapted to be connected to the first member can comprise at least one aperture for receiving a connector adapted to be connected at one end to the at least one portion adapted to be connected to the first member and, at another end, to be connected to the first end member. For example, the at least one portion adapted to contact the at least one elongated member can comprise two semi-circular portions connected together at one end by a hinge and adapted to be connected at another end by the fastener. For example, the at least one portion adapted to be connected to the first member can be a peripheral edge connected to the at least one portion adapted to contact the at least one elongated member. The peripheral edge can be provided with at least two apertures adapted each to receive and abut a connector adapted to be connected at one end to the peripheral edge and, at another end, to be fixed to the first end member. The connector can be a fastener adapted to be substantially parallel to the cylinder.

[0029] The photobioreactor can further comprise a second securing member adapted to secure the at least one elongated member to the second end member. The second securing member can comprise at least one portion adapted to contact the at least one elongated member and at least one portion adapted to be connected to the second end member. For example, the at least one elongated member can be a cylinder comprising a transparent material, and wherein the at least one portion adapted to contact the at least one elongated member contacts the cylinder and can be secured around the cylinder by means of a fastener, the at least one portion adapted to be connected to the second end member comprises at least one aperture for receiving a connector adapted to be connected at one end to the at least one portion adapted to be connected to the second member and, at another end, to be connected to the second end member. For example, the at least one portion adapted to contact the at least one elongated member can comprise two semi-circular portions connected together at one end by a hinge and adapted to be connected at another end by the fastener.

[0030] For example, the at least one portion adapted to be connected to the second member can be a peripheral edge connected to the at least one portion adapted to contact the at least one elongated member, the peripheral edge being provided with at least two apertures adapted each to receive and abut a connector adapted to be connected at one end to the peripheral edge and, at another end, to be fixed to the second end member. The connector can be a fastener adapted to be substantially parallel to the cylinder. [0031] For example, the at least one elongated member can be a vertically extending cylinder comprising a transparent material, the first end member being a base member disposed below the cylinder and the second end member being a cover disposed on top of the cylinder. The base member can define at least one cavity having a downward tapering frusto-conical shape for receiving the at least one elongated member. The main conduit can be substantially centered at a bottom portion of the cavity. The base member can define at least one cavity having a downward tapering conical shape for receiving the at least one elongated member. The main conduit can be substantially disposed at an apex of the conical shape.

[0032] For example, the injector can be disposed at a bottom portion of the frusto-conical shape or the conical shape.

[0033] For example, the injector can be disposed at a bottom portion of the frusto-conical shape or the conical shape and above the main conduit.

[0034] For example, the injector can be disposed at a bottom portion of the frusto-conical shape or the conical shape and above the main conduit. The injector can be disposed substantially at right angle with respect to the cylinder.

[0035] For example, the photobioreactor can comprise a plurality of elongated members, each member can be sealingly engaged with the first end member and contacting the second end member. The main conduit can be effective for feeding the elongated members and the photobioreactor can comprise a plurality of injectors for injecting a gas at the first end of the elongated members. Each of the elongated members can be provided with one of the injectors.

[0036] For example, the photobioreactor can comprise a plurality of the elongated members. Each member can be sealingly engaged with the first end member and contacting the second end member. The main conduit can be effective for feeding the elongated members and the photobioreactor can comprise a single injector for injecting a gas at the first end of the elongated members. [0037] For example, the photobioreactor can comprise two, four or six elongated members.

[0038] For example, the photobioreactor can further comprise a lighting system comprising:

a housing ;

a plurality of lighting elements effective for growing microalgae, the elements being connected to the housing and the housing is adapted to be fixed to the first member.

[0039] For example, the housing can comprise a bottom member, a top member and side members, the lighting elements can be connected at one end to the bottom member and at another end to the top member, the lighting elements being can be disposed in such a manner to illuminate at least one elongated member.

[0040] The photobioreactor can comprise a plurality of elongated members and a plurality of base members, each base member can have at least two elongated members connected thereto. The elongated members can be aligned and disposed one after the other. The base members can be aligned in a parallel manner and a lighting system can be disposed between two consecutives base members. The lighting systems can be parallel to the base members and the elongated members.

[0041] For example, the methods and apparatuses whose goal is to produce micro-algal biomass in the form of concentrate intended, for example, for the extraction of ingredients (oil, pigments or other substances) or of products for aquaculture (micro-algae for fodder and for conditioning water) can comprises various steps and components. The method and apparatuses can comprise from top to bottom (Figure 1 ): (1 ) a system for the preparation of the culture medium using ultrafiltration cartridges with hollow fibers (tangential flow filtration) that allow the culture medium to be filtered and sterilized in a single stage; (2) a vertical tubular photobioreactor (PBR) that allows the production of microalgae according to a semi-continuous manner in a growth chamber isolated from the outer environment and whose design favors the capture of light and the use of nutrients, with integration of a lighting system with electroluminescent diodes (LED) tested and developed specifically for a PBR; (3) a system (as defined in CA 2,448,184, hereby incorporated by reference in its entirety) for concentrating the algal biomass; and (4) the recycling of the seawater (permeate) issued from the concentration of cultures in the production. In one embodiment, the system (1 ) and the system (3) are the same or substantially the same.

1- System for the preparation of the culture medium

[0042] The function of the system is to filter and sterilize the culture medium before the introduction into the PBRs. The culture medium is made, for example, from pre-filtered seawater in a bag down to 1 μιη and warmed to which are added the nutrients indispensable for the growth of algae. The nutrients can be, for example, commercially available in concentrated solution (solution f/2, furnished by Fritz). The natural or artificial seawater used for the preparation of the culture medium can be first conditioned (filtered and warmed) in a reservoir with an adequate volume . The solution of nutrients can then be added to the reservoir (11) from which it is brought by a pump (12) to the tangential ultrafiltration cartridge or cartridges (13). The retentate can be returned to the storage reservoir (1 1) by means of the pipe (14) whereas the permeate, which constitutes the culture medium ready to be used, is directed to the PBRs by means of the feed pipe (15) (Figure 2). The ultrafiltration cartridges used with the system can be the Romicon™ brand supplied by Koch Membrane systems Inc. The system can be equipped with three cartridges that can be used one, two or three simultaneously as needed.

[0043] When carrying out a method using the apparatus shown in Figure 2, the following steps and/or elements can be used.

1.1- Conditioning of the seawater [0044] This optional step allows the seawater to be pre-filtered and warmed before directing it to the ultrafiltration system. It can be carried out in a closed circuit in the reservoir (1 1) by a circulation pump and by a filter system with bags having a porosity of 10 and 1 pm (16). For example, this step can be realized in the night preceding the use of the system in such a manner that the seawater is ready for the production at the beginning of the next day.

1.2- Preparation of the culture medium

[0045] In this stage the nutrients can be added to the reservoir (1 1 ) containing the conditioned seawater. The solution of seawater containing the nutrients can then be circulated for approximately 15 minutes in a closed circuit in the reservoir (1 1 ) by the pump (12) through the ultrafiltration cartridges (13). This operation can allow the solution of nutrients to be homogenized before its use. The output and the operating pressures of the ultrafiltration system can be adjusted in accordance with recommendations of the manufacturer of the ultrafiltration cartridges. Likewise, their maintenance and storage can be made in the short term and the long term in accordance with the recommendations of the manufacturer.

1.3- Ultrafiltration of the culture medium

[0046] Once the solution of nutrients contained in the reservoir (1) has been well homogenized it can be transferred into the PBR(s). The permeate exits throught the pipe (15) of the system that has been previously connected in an aseptic manner to the entrance of a PBR(s). The filtration permeate (ultrafiltered culture medium ready to be used) can be directed to the PBR(s) while activating the control valves that permit the feed to the reservoir (1 1 ) to be cut off and opening the feed to the PBR(s). The desired quantity of culture medium can thus be pumped into the PBR(s). The feed pump is then stopped and the permeate exit is disconnected from the PBR(s) and connected to the following one and so forth until all the PBR(s) have been filled. The system is then drained, and then cleaned and sterilized by circulating a solution of sodium hypochlorite with pH 1 1 through the reservoir (17). The system is then stored with all its openings hermetically closed in order to avoid contamination until its next use.

[0047] The filtration cartridges used in the system for the preparation of the culture medium can have a porosity permitting the passage of molecules with a molecular weight lower than 50 000 Daltons (about the range of ultrafiltration membranes with a porosity of approximately 0.02 μιη). Thus, a culture medium treated on such a membrane presents characteristics suitable for the culture of microalgae due to the fact that it substantially does not contain any undesirable organism (foreign microalgae, ciliates, bacteria, viruses). Furthermore, the membranes can be very selective, which allows the homogeneity to be maintained from one lot to the other with the sole source of fluctuation being the origin of the seawater (natural, artificial, recycled). Optionally, the permeate can be further treated by means of an UV treatment to further ensure sterility.

[0048] The system described here can be equipped with three ultrafiltration cartridges. They can be used one, two or three at a time, conferring to the system polyvalence at the level of its production capacity. The use of three cartridges can simultaneously allow approximately 2,500 l/h of culture medium to be treated.

[0049] The advantages of the system for the preparation of the culture medium with tangential ultrafiltration cartridges over conventional systems (pasteurization, UV, ozone) are:

1. Filtration and sterilization of the culture medium in a single stage;

2. Elimination of undesirable organisms (substantially 100% effective);

3. Easy operation and maintenance;

4. Very low energy consumption;

5. Small footprint (approximately 2 m ² on the floor); 6. Mobile;

7. Polyvalent (capacity adjustable to the production requirements); and

8. Can be rapidly disassembled into its components for sanitary inspection

2- Photobioreactor (PBR)

[0050] The PBR (2) (see Figures 3, 4, 5, 6 and 13) is the growth unit (device for growing) of the microalgae. It can comprise two components: the growth chamber (21) and the lighting apparatus (23).

2.1- Growth chamber

[0051] The growth chamber (21 ) of the PBR can be a cylinder (25) (for example an acrylic cylinder) with a variable diameter and height. A growth chamber can comprise, for example six cylinders. The cylinders can be mounted on a base member (27) (for example of polyethylene) (UHMW) pierced by six cones machined in such a manner that the six cylinders are in a vessel communicating via an aperture (a main conduct (29)) traversing the base member (27) over its length and uniting the base of the cones (31 ) (downwardly tapered) (see Figures 3, 4 and 5). The base member (27) can also be provided with lateral orifices (injectors (33)) vis-a-vis each cone (31 ) for the introduction of the air-C0 ₂ mixture. This mixture supplies the carbon for the photosynthesis, contributes to the degassing of the culture by constantly bringing the culture to the surface, optimizes the capture of light by bringing the culture to the walls and induces a vertical movement of the culture that limits its fixation to the walls. The injectors (33) are connected to conduits (34) defined in the base (27). The cones (31 ) are provided with o- rings (32) that are effective for ensuring that the cylinders (25) are sealingly engaging the cones (31 ). A cover (35) (for example machined cover provided with six orifices (37)) for the evacuation of the gases unites the six cylinders at their upper ends (Figures 3, 5, 6). Removable flanges (39) provided with nuts and bolts fastener (40) (see Figures 3, 5, and 12) (securing members) for example made from stainless steel can be mounted at each end of the acrylic cylinders (25) and serve to fix them on the base member (27) and the cover (35) by means of fasteners (41) (bolts) (Figures3, 5 and) 6). Once it is mounted, the growth chamber (21) constitutes a block comprising one base member (27), one cover member (35) and six acrylic cylinders (Figure 6). As it can be seen in Figure 6, the main conduit (29) is connected to a main valve (30). The prototype constructed and tested demonstrated the following characteristics:

1. Inside diameter of the cylinders: 17.8 cm

2. Outside diameter of the cylinders: 16.5 cm

3. Length of the cylinders: 2.73 m

4. Volume of a cylinder: 58.5 L

5. Number of cylinders: six

6. Useful volume: 350 L

7. Base length: 1.50 m

8. Base width: 52 cm

9. Base thickness: 10 cm

2.2- Lighting apparatus

[0052] As it can be seen in Figures 6 and 13, the PBR can be, for example, of the vertical tubular type with external lighting (23). The growth chamber (21 ) was designed to constitute production units comprising one or several PBRs. The lighting apparatuses were developed in such a manner as to adapt to this design. The lighting apparatuses (for example ramps (43)) developed can use white and blue electroluminescent diodes (LEDs) that adapt to standard receptacles for T-12 fluorescent tubes and emit an intensity of approximately 8,000 to 10,000 lux with a wavelength of 400 to 700 nm. The LED tubes (45) can be mounted on ramps (43) that slide between the growth chambers (21 ) in notches provided to this end on the base members (see Figures 3 and 4). On Figure 7, ramps 51 and 52 are shown without the tubes for illustrative purposes. The arrangement of the tubes (45) on the lighting ramps (43) can allow one or two growth chambers to be lit in accordance with the alignment given to the tubes (left or right). A lighting ramp can comprise two half-ramps, each adapted to receive three LED tubes (45) into their receiving members (62) and thus be efficient for lighting three cylinders (25) of a given base member (comprising six cylinders). As shown in Figure 7, the first half-ramp (51 ), placed in front, can comprise a rheostat switch (53) disposed on the front post (54), that allows the quantity of light to be controlled, an electrical outlet (55) situated on the rear post (56), which allows the second half-ramp to be supplied with electricity, and an electrical feed cable (57). The second half-ramp (52) can adjust to the first one due to an alignment guide (59) situated at the upper part of the post (56) of the first half-ramp (51 ). It can be provided with an electrical plug (61 ) fixed on the front post (60) exactly at the same position as the electrical outlet (55) of the first half-ramp (51). Thus, when the two half-ramps are inserted between two base members in the notches provided to this end on the basis, they can become integral with one another by the alignment guide (59) and by the electrical connection (55) and (61). The rheostat switch (53) of the first half-ramp (51 ) can allow the unit to be controlled. As it can be seen on Figure 13, rows of cylinders (25) are disposed in an alternate manner with rows of tubes (45) that are disposed in ramps (43). A given row of cylinders (25) is thus disposed between two rows of tubes (45).

Table 1. Comparison of the biological productivity of Nannochloropsis oculata (cell/day and g of dry mater/PBR/day) obtained with a lighting with T- 5 fluorescent lights and blue LEDs.

Number cells produced / 24h Productivity (g _DW /PBR/24h)

Day (X10 ¹¹ )

J5 LED-B 15 LED-B % A|_ED-T5

1

I T5

2 4.89 5.19 6.2 2.09 2.21 5.7

3 4.39 5.12 16.7 1.85 2.34 26.4

4 5.16 4.42 -14.3 2.36 1.83 -22.4

5 4.57 5.80 27.1 1.99 2.40 20.4

6 4.99 4.99 0.1 2.26 2.16 -4.3

7 6.01 5.33 -1 1.3 2.49 2.33 -6.4

8 4.07 4.69 15.1 1.84 1.73 -6.3

9 6.10 5.18 -15.1 2.72 2.1 1 -22.6

10 4.72 4.78 1.2 2.09 2.39 14.7

11 4.36 3.68 -15.5 2.15 1.49 -30.4

12 5.73 5.17 -9.7 2.31 2.03 -12.2

13 3.58 4.56 27.4 1.58 1.73 9.6

14 5.1 1 4.78 -6.5 2.25 1.94 -13.9

15 5.23 4.82 -7.9 2.25 1.96 -12.6

16 3.32 4.55 37.2 1.58 1.86 17.4

Average ± SD 6.30 5.59 -11.3 3.52 2.08 -40.9

4.91 ± 4.92 ± 2.5 ± j. n R 2.0 ± -4.9 ±

. ± U. D

0.86 0.50 17.2 0.3 19.3

[0053] The tests carried out with this lighting apparatus showed that the productivity obtained with the lighting with the LEDs (white LED (□)) is comparable to lightings with fluorescent lights T-5 (Δ) and T-8 (0) and greater than fluorescent lights T-12 (o) (see Figure 8 and Table 1). Even if the lighting with the LEDs does not significantly contribute to increasing the biological productivity of the cultures, the advantages associated with the consumption of energy (reduction up to 80% of the consumption), the durability (20,000 h) and the low release of heat constitute arguments justifying their use.

2.3- Operation of the photobioreactor [0054] The preparation of a PBR (2) (see Figures 3 to 6) for production can begin with the cleaning and asepticizing of the components that will be in contact with the culture medium. The base member (27), the cover (35) as well as the cylinder(s) (25) can be cleaned at first with soap ^' , washed with fresh water, cleaned with 5% hydrochloric acid then washed again with fresh water. The cylinder(s) (25) can then be placed on the base member (27) and fixed to it with the flanges (39) (securing members). The cover (35) can then be put in place and fixed to the cylinders (25) by the flanges (39). The distribution piping (injector) of the air-C0 ₂ mixture can be put in place. The growth chamber (21 ) and all the conduits can then be filled with a solution of sodium hypochlorite at 500 ppm via the intake valve. A contact period of 1 to 12 hours can be allocated in order to asepticize the interior of the growth chamber (21 ). After the asepticizing period the solution of sodium hypochlorite can be emptied in the following manner: approximately 10 L of the solution are emptied at first. The air-C0 ₂ mixture can then be introduced. This operation allows a positive pressure to be preserved in the interior of the growth chamber (21) during the emptying. The growth chamber (21) can then be completely emptied. The two lighting half-ramps (51 and 52) can be put in place and connected to the electrical supply.

[0055] The introduction of the culture medium and seeding can be carried out as follows. At first, the intake valve (30) can be connected to the permeate discharge of the system for the preparation of the culture medium, which had been previously started. Approximately 20 L of culture medium can be introduced into the growth chamber (21). The intake valve (30) can then be closed, the feed pipe in the culture medium can be disconnected and the 20 L contained in the growth chamber (21) can be drained. This operation serves to eliminate the residues of sodium hypochlorite in the growth chamber. A volume of approximately 20 L of the culture to be produced can be introduced in an aseptic manner into the growth chamber (21) by the intake valve. The feed pipe (15) in the culture medium (permeate discharge of the system for the preparation of the culture medium) can then be connected to the intake valve (30) and the adequate volume of the culture medium is pumped into the growth chamber (21) (approximately 330 L for the growth chambers (six cylinders described in this embodiment). The lighting can be adjusted with the aid of the rheostat (53) to the base intensity (approximately 2000 lux) for the first days of growth, then adjusted upward in proportion with the increase in cellular density up to the maximum intensity, for example, on the 3rd or 4th day. The culture can attain a cellular density that allows it to be put in production, for example, between the 5th and the 7th day as a function of the species.

[0056] When the culture has attained the cellular production density (variable according to the species, the parameters of growth and the purpose of the production) the partial harvest can begin, for example, at intervals of 1 , 2 or 3 days. The volume collected in each production will be greater the longer the time between two harvests is. Thus, 30 to 35% of the volume of the growth chamber (21) can be collected for harvests at each 24 hours and 50 to 80% of the volume of the growth chamber for harvests at each 48 to 72 hours. The volume of the culture collected in each production can be replaced by the fresh culture medium treated in the system for the preparation of the culture medium and introduced via the intake valve or can be replaced by recycled culture medium.

2.4- Performance of the photobioreactor (PBR)

[0057] The PBR (2) with a vertical tubular column can be designed to operate totally in artificial light. This type of PBR can operate with photoautotrophic as well as with mixotrophic microalgae that require for their growth, in addition to a carbon (C0 ₂ ) source, a source of light in the range of wavelengths of 400 to 700 nm. The tests carried out in the pilot production allowed the concept of the system to be validated and allowed its productivity level to be established. Thus, the system operates in semi-continuous production with daily harvesting, producing microalgae with a specific growth rate of 0.3 to 0.4 d ^"1 according to the species cultivated. The cellular density in the PBR (2) can be maintained in production between 10 and 35 x 10 ⁶ cells/mL (100 to 300 mg of dry mater/I of culture) according to the species. Considering that the PBR can be hermetically closed and that the culture medium can be free of any undesirable organism, the lifetime of the cultures can greatly exceed that which is obtained by other known systems. Tests allowed cultures to be maintained in semi-continuous production without interruption for periods as long as eight months while keeping the culture free of contamination (see Figure 9). The system was tested with the following nine species of microalgae:

[0058]

• Isochrysis galbana

• Pavlova lutheri

• Nannochloropsis oculata

• Chaetoceros muelleri

• Skeletonema costatum

• Rhodomonas salina

• Tetraselmis suesica

• Phaeodactylum tricornutum

• Thalassiosira weissflogii

[0059] The person skilled in the art would clearly understand that based on the results discussed in the present document, it can be inferred that the methods and apparatuses described in the present document could be useful for producing various other marine microalgae and freshwater microalgae (such as Chlorella and Spirulina.

3- System for the concentration of the biomass

[0060] The culture harvested from the PBRs (2) can be temporarily stored in reservoirs before being treated. This treatment comprises concentrating the harvest in order to obtain a volume that can be readily stored in refrigeration and transported for delivery. This transformation particularly concerns the products intended for aquaculture that must be delivered fresh. The system that can be used for realizing this operation is a system for the concentration of algae by tangential filtration. Details concerning this system can be found in CA 2,448,184, as previously discussed. Other concentration systems or techniques can also be used. [0061] By treating the mixture obtained after growing the microalgae with an ultrafiltration system (tangential flow filtration), the cells can be substantially separated from the culture medium. The object is to produce a liquid concentrate (or a concentrated suspension of microalgae) in which the microalgae have retained their membrane integrity and their nutritive potential. Thus, the mixture can be concentrated in this manner by factors ranging from 150X to 300X according to the species and the requirements. That is to say that approximately 99.5% of the warmed and filtered fresh water used for the production of the cultures can be re-used or recycled.

4- Recycling of the concentration permeate (culture medium)

[0062] The recycling of the recovered seawater, called permeate, therefore proved to be an interesting strategy for reducing the production costs by significant savings in the cost of the seawater and of its treatment.

[0063] In order to evaluate the potential for recycling seawater or culture medium the permeate coming from a mono-specific culture of Nannochloropsis oculata after the concentration of the harvested cultures was used. The tests were made in four reservoirs of 200 L (two reference cultures in seawater and two cultures in permeate (recycled culture medium). After one week, the two series of cultures were harvested and concentrated. The permeate issued from the two experimental reservoirs (permeate or recycled seawater) was preserved and re-used as culture medium for a third time whereas the two reference reservoirs were filled with new seawater (phase II). One week later, the permeate of the experimental reservoirs was recycled a fourth time whereas the two references were again filled with new seawater (phase III).

[0064] The daily follow-up of the cellular growth in each of the reservoirs indicates an increase of algal growth when the recycled permeate is used as culture medium (see Figure 10). This entails an increase of the production (then expressed in cell/mL) as well as a reduction of the quantity of new seawater necessary for cultivating the algae.The average gain in productivity obtained during the three phases of the recycling of seawater is 33.8 ± 15.8% (Table 2). This gain can be, without being bound to such a theory, associated with the accumulation of nutrients not consumed in the different recyclings, or possibly to the presence of metabolites salted out by the microalgae or the bacteria in the culture medium which might act as growth activator. Again, without being bound to such a theory, a probable hypothesis resides in the potential explanation that Nannochloropsis oculata is nourished by mixotrophy. Thus, the different carbonized compounds salted out in the recycled culture medium can be used by the cells in addition to photosynthesis for their growth.

Table 2. Cellular density and culture productivity of Nannochloropsis oculata cultivated on new seawater and on recycled seawater (tangential ultrafiltration permeate).

% gain

Recycled Δ Gain / % average

Seawater water permeate phase % gain standard

Phase Day (cell/mL) (cell/mL) (cell/mL) (cell/mL) gain Aver. deviation

Day -7.00

I 0 6.20 x10 ⁶ 5.60 x10 ⁶ x10 ⁵

Day

7 2.01 x10 ⁷ 2.98 x10 ⁷ 9.70 x10 ⁶ 1.03 x10 ⁷ 51.4

Day

II 0 4.70 x10 ⁶ 6.50 x10 ^s 1.80 x10 ⁶

Day

6 2.34 x10 ⁷ 3.00 x10 ⁷ 6.60 x10 ⁶ 4.90 x10 ⁶ 20.7

Day

III 0 5.60 x10 ⁶ 7.80 x10 ⁶ 2.20 x10 ⁶

Day

8 2.25 x10 ⁷ 3.13 x10 ⁷ 8.80 x10 ⁶ 6.60 x10 ⁶ 29.3 33.8 15.8

[0065] It can thus be seen that an average gain of more than 30 % (more than 51 % when recycled only once) can be obtained when using recycled seawater (or recycled culture medium). Other tests showed that a fourth recycling can be made while preserving the advantage in the productivity. A savings of approximately 60% of the consumption of seawater can be realized while re-using the concentration permeate with four recyclings of seawater, taking into account losses due to the dead volume and to manipulations. [0066] The test was then made by comparing the growth of three species (Nannochloropsis oculata, Pavlova lutheri and Isochrysis galbana) cultivated in a fresh seawater culture medium, in their respective recycled culture medium (permeate), in recycled culcture medium generated from the culture of the two other source of microalgae and from a mixture thereof of the three recycled culture medium. The results show that there is a considerable growth gain for N. oculata and P. lutheri, regardless of the provenance of the permeate when a recycled culture medium is used. In contrast thereto, /. galbana displays a significantly lower growth when it is cultivated in the permeates of every provenance (see Figure 1 1 ).

[0067] The recycling of seawater (or culture medium) stemming from the concentration of the microalgae by ultrafiltration and possibly from the centrifugation has several advantages related to:

1. Savings for the cost of seawater (pumping, transport).

2. Savings for the warming of the seawater.

3. Savings for the filtration cost.

4. Considerable gain in productivity.

5. Freedom from an external supply of seawater (gain of autonomy).

6. Reduction of risks associated with the supply source and with the transport.

7. Reduction of risks associated with the variation in the quality of the seawater.

8. Better control in general of the production parameters.

[0068] The examples of methods and apparatuses previously described represent a very significant improvement of the technology for the production of microalgae and more particularly in a confined environment by proposing:

1. A method for the preparation of the culture medium using a tangential ultrafiltration system with a small footprint that allows a culture medium with an optimal and constant quality to be prepared, thus favoring the longevity of the cultures and their productivity;

2. A photobioreactor with a vertical tubular column provided with an external lighting apparatus with LEDs that allows the optimization of the production parameters (capture of lighting and of carbon), which ensures the self-cleaning of the internal walls of the growth chamber and which allows an adequate degassing (0 ₂ ) of the culture medium;

3. An apparatus for the concentration of algal biomass that allows for the preparation of fresh products for and the feeding in aquaculture; and

4. A method for the recycling of the concentration permeate of cultures that allows significant savings for the cost of seawater and gains in biological productivity.

[0069] The examples of methods and apparatuses previously described also offer the following advantages:

1. Culture medium with a quality that is constant and controlled by ultrafiltration.

2. Increased biological productivity.

3. Lifetime of the cultures significantly increased as a consequence of the production in a closed PBR and of a system performing the treatment of the culture medium.

4. Constant and reproducible quality of the biomass as a consequence of the controlling of the growth parameters (temperature, light, carbon) and of the quality of the culture medium.

5. Capacity of producing several species of microalgae.

6. PBR that is self-cleaning by virtue of its design.

7. Reduced footprint and polyvalence.

[0070] The person skilled in the art would understand that the various properties or features presented in a given embodiment can be added and/or used, when applicable, to any other embodiment covered by the general scope of the present disclosure.

[0071] The present disclosure has been described with regard to specific examples. The description was intended to help the understanding of the disclosure, rather than to limit its scope. It will be apparent to one skilled in the art that various modifications can be made to the disclosure without departing from the scope of the disclosure as described herein, and such modifications are intended to be covered by the present document.