APPARATUS AND METHOD FOR THE PRODUCTION OF MICROALGAE

Abstract

This apparatus (30) for the production of microalgae comprises a vessel (1) intended to receive a culture medium of the microalgae, and a lighting device (6) intended to be positioned in the vessel inside the culture medium. The lighting device (6) is configured to emit light at least in a range of wavelengths useful for the photosynthesis of the microalgae. The apparatus comprises a control system (15) for automatically controlling a power supply of the lighting device (6) so as to adjust the output light intensity of the lighting device (6) according to the concentration of microalgae in the vessel (1).

Claims

1. An apparatus for the production of microalgae comprising: a vessel intended to receive a culture medium of the microalgae; a lighting device intended to be positioned in the vessel inside the culture medium, the lighting device being configured to emit light at least in a range of wavelengths useful for the photosynthesis of the microalgae; and a control system for automatically controlling a power supply of the lighting device so as to adjust the output light intensity of the lighting device according to the concentration of microalgae in the vessel.

2. The apparatus according to claim 1, wherein the control system comprises at least one light sensor configured to receive light emitted by the lighting device and that has passed through the culture medium received in the vessel.

3. The apparatus according to claim 2, wherein the control system comprises a processing unit configured to control a power supply of the lighting device so as to adjust the light intensity received by the light sensor at a predefined value of light intensity according to the concentration of microalgae in the vessel.

4. The apparatus according to claim 2, wherein the control system comprises a processing unit configured to determine the concentration of microalgae in the vessel by comparison between the output light intensity of the lighting device and the light intensity received by the light sensor from the lighting device.

5. The apparatus according to claim 2, wherein the walls of the vessel are transparent, the light sensor being configured to receive a total light corresponding to the light emitted by the lighting device and that has passed through the culture medium received in the vessel, and the ambient light emitted by external light sources useful for the photosynthesis of the microalgae, the control system comprising a processing unit configured to control a power supply of the lighting device so as to adjust the output light intensity of the lighting device according to the ambient light received by the light sensor and the concentration of microalgae in the vessel.

6. The apparatus according to claim 2, wherein the lighting device is positioned centrally inside the vessel, while the light sensor is positioned on a peripheral wall of the vessel, preferably on the outer face of the peripheral wall of the vessel.

7. The apparatus according to claim 1, wherein the lighting device comprises at least one LED (Light-Emitting Diode) emitting white light, preferably with a CCT (Color Correlated Temperature) of between about 3000K and 5000K.

8. The apparatus according to claim 1, comprising a harvesting device configured to ensure filtration of microalgae from a volume of the culture medium and allow the remaining culture medium to return in the vessel.

9. The apparatus according to claim 8, wherein the harvesting device comprises a membrane pump, a filtration membrane with a first pore size and a cup with a second pore size less than the first pore size.

10. The apparatus according to claim 1, comprising a heating device intended to adjust the temperature of the culture medium received in the vessel.

11. The apparatus according to claim 1, characterized in that it comprises an aeration and mixing device intended to ensure homogeneity of temperature, homogeneity of microalgae distribution and gas exchange in the culture medium received in the vessel.

12. A method for the production of microalgae, comprising: filling a vessel with a culture medium of the microalgae; activating a lighting device positioned in the vessel inside the culture medium, the lighting device being configured to emit light at least in a range of wavelengths useful for the photosynthesis of the microalgae; and automatically controlling a power supply of the lighting device so as to adjust the output light intensity of the lighting device according to the concentration of microalgae in the vessel.

13. The method according to claim 12, wherein the automatic control of the power supply of the lighting device is performed by adjusting the light intensity received by at least one light sensor at a predefined value of light intensity according to the concentration of microalgae in the vessel, wherein the light sensor is arranged to receive light emitted by the lighting device and that has passed through the culture medium received in the vessel.

14. The method according to claim 13, wherein the concentration of microalgae in the vessel is determined by comparison between the output light intensity of the lighting device and the light intensity received by the light sensor from the lighting device.

15. The method according to claim 13, wherein the walls of the vessel are transparent, the light sensor being configured to receive a total light corresponding to the light emitted by the lighting device and that has passed through the culture medium received in the vessel, and the ambient light emitted by external light sources useful for the photosynthesis of the microalgae, wherein the power supply of the lighting device is controlled so as to adjust the output light intensity of the lighting device according to the ambient light received by the light sensor and the concentration of microalgae in the vessel.

16. The method according to claim 12, wherein the automatic control of the power supply of the lighting device so as to adjust the output light intensity of the lighting device according to the concentration of microalgae in the vessel is performed during a growing phase of the microalgae and/or during a harvesting phase of the microalgae.

Description

[0042] Features and advantages of the invention will become apparent from the following description of an embodiment of an apparatus and a method for the production of microalgae according to the invention, this description being given merely by way of example and with reference to the appended drawings in which:

[0043] FIG. 1 is a perspective view of an apparatus for the production of microalgae according to the invention;

[0044] FIG. 2 is a cross section along the plane II of FIG. 1;

[0045] FIG. 3 is a cross section along the line III-III of FIG. 2; and

[0046] FIG. 4 is a graph showing the evolutions, as a function of time during the growing phase, both of the output light intensity of the lighting device of the apparatus of FIGS. 1 to 3 and of the concentration of microalgae in the vessel of the apparatus, in an illustrative example of a production cycle of Spirulina (Arthrospira platensis) by means of the apparatus.

[0047] As shown in FIGS. 1 to 3, the apparatus 30 according to the illustrative embodiment of the invention comprises a cylindrical vessel 1, intended to contain an alkaline culture medium of the microalgae, and a cover 2 located on the upper part of the vessel 1. The cover 2 is intended to house and protect technical components needed for the culture of the microalgae. The cover 2 is designed to close the vessel 1 so as to lower evaporation and protect the culture against contamination. The apparatus also has a general power supply system 23 (of 230V) and an electric transformer 20 housed in the cover, which is intended to power supply several components of the apparatus at a lower voltage (24V or 12V). The apparatus 30 is provided with a main switch button 18 for switching the apparatus ON/OFF.

[0048] In an advantageous manner, the walls of the vessel 1 are transparent so that photosynthesis of the microalgae can occur with the light from both internal light sources positioned in the vessel and external light sources. As a variant, the walls of the vessel 1 may not be transparent, in which case the apparatus 30 involves photosynthesis of the microalgae with the light of internal light sources only. In this embodiment, the vessel 1 is made of glass, which has the advantages of being an inert material, resistant to culture medium characteristics, offering a weakly adhering surface to microalgae accumulation, easy to wash and sustainable. Of course, transparent materials other than glass may also be considered for the vessel 1.

[0049] The apparatus 30 comprises a lighting device 6 which is fixed on a lower part 3 of the cover 2 in such a way that, when the cover 2 is fitted to the vessel 1, the lighting device 6 is immersed in the culture medium received in the vessel 1. In this embodiment, the lighting device 6 comprises an outer transparent cylindrical tube 12 hermetically closed at the bottom, and an inner transparent cylindrical tube 21 placed inside the outer tube 12. A ribbon of LEDs 14 is fixed onto the inner tube 21 so that the LEDs 14 are distributed along the tubes 12, 21, in the volume defined between the two tubes. In this example, the cylindrical tubes 12 and 21 are advantageously made of PMMA (poly(methyl methacrylate)) compatible with alimentary purpose.

[0050] In this illustrative embodiment, the lighting device 6 is positioned at the center of the vessel 1 when the cover 2 is fitted to the vessel 1. In this way, the light emitted by the lighting device 6 is distributed homogeneously in the culture medium received in the vessel 1 and can reach the microalgae optimally for their photosynthesis. The wavelengths of the LEDs 14 are adapted to the pigment specificities of the microalgae to be produced in the apparatus. For example, in the case where the apparatus 30 is used for the production of Spirulina (Arthrospira platensis), the LEDs 14 are advantageously white LEDs with a CCT of 4100 K, so that all Spirulina pigments are involved in photosynthesis.

[0051] The apparatus 30 further comprises a control system for automatically controlling a power supply of the lighting device 6 so as to adjust the output light intensity of the lighting device according to the concentration of microalgae in the vessel 1. As visible in FIGS. 1 to 3, the control system comprises a light sensor 15, positioned on the outer face 22A of the peripheral wall 22 of the vessel 1, and a processing unit 19, housed in the cover 2. For example, the light sensor 15 may be a silicon NPN epitaxial planar phototransistor, sensitive to visible light much like the human eye, having a peak sensitivity at 570 nm and an angle of half sensitivity of 20°.

[0052] In accordance with the invention, the processing unit 19 is configured to control the power supply of the lighting device 6 so as to adjust the light intensity received by the light sensor 15 at a predefined value of light intensity depending on the concentration of microalgae in the vessel 1. In this example, the processing unit 19 is further configured to activate the lighting device 6 according to a daily ON/OFF cycle. According to a specific example, the characteristics of the lighting device 6 are the following: 60 LEDs per meter; 14.4 W/m; and the control parameters of the lighting device 6 are: ON/OFF 16 hours/8 hours between 6 am and 10 pm and output light intensity adapted according to the concentration of microalgae in the vessel 1 during the ON period.

[0053] As visible in FIG. 1, the light sensor 15 is attached to a vertical gutter 24 extending along the height of the peripheral wall 22 of the vessel 1. More precisely, the light sensor 15 is placed under the gutter 24 and against the peripheral wall 22 of the vessel 1. In this way, the light sensor can capture total light corresponding to both light emitted by the lighting device 6 and that has passed through the culture medium received in the vessel 1, and ambient light emitted by external light sources also useful for the photosynthesis of the microalgae, such as natural sunlight during the day. In order to distinguish the light received by the light sensor 15 from the lighting device 6, on the one hand, and the ambient light received by the light sensor 15 from external light sources, on the other hand, the processing unit 19 turns off the lighting device 6 for a few seconds. At that time, the light sensor 15 receives only the ambient light emitted by external light sources. By subtracting the ambient light emitted by external light sources from the total light, the processing unit 19 computes the light emitted by the lighting device 6 and that has passed through the culture medium.

[0054] This configuration allows an optimal light utilization taking into account surrounding external light sources. In an advantageous manner, with such a configuration, the processing unit 19 can control the output light intensity of the lighting device 6 in such a way that the light intensity emitted by the lighting device 6 complements the light intensity emitted by the surrounding light sources to result in a light intensity received by the light sensor 15 adjusted to the predefined value of light intensity.

[0055] Depending on the light intensity received by the light sensor 15, the processing unit 19 pilots the output light intensity of the LEDs 14 of the lighting device 6 according to a predefined equation, where the relationship between LED intensity and microalgal concentration in the vessel corresponds to the photosynthetic needs of the microalgae at a given temperature of the culture medium. Thus, the processing unit 19 pilots the output light intensity of the LEDs 14 so that the light intensity received by the microalgae at each time is adjusted according to the photo synthetic needs of the microalgae at this time and at a given temperature of the culture medium. An exponential relationship between the output light intensity of the LEDs and the microalgal concentration is particularly suitable.

[0056] In particular, according to a particular embodiment, the light intensity received from the lighting device 6 (IR.sub.cor) is calculated by subtracting from the total light intensity received by the light sensor 15 (IR) the ambient light intensity (I.sub.amb) measured by the light sensor 15 when the lighting device 6 is turned off. This is expressed by equation (1):

IRcor=IR−Iamb  (1)

[0057] The light absorbance (A) of the culture medium is calculated from the output light intensity of the lighting device (ILED) and the light intensity received from the lighting device (IR.sub.cor) according to equation (2):

A=1°.sub.g¾AjtS.sub.co2.sub.r-(¾

[0058] The concentration of microalgae (c) is derived from the distance (1) between the lighting device 6 and the light sensor 15, and the absorptivity (e) of the microalgae solution according to equation (3):

[00001]$\begin{array}{cc}c=\frac{A}{\mathrm{.Math.}.Math.l}& \left(3\right)\end{array}$

[0059] As a variant, the relationship between the absorbance and the microalgal concentration may be obtained by a linear regression after calibration with samples having different concentrations.

[0060] The processing unit 19 is also configured to automatically regulate the temperature of the culture medium received in the vessel 1, by controlling a heating device 10 also fixed on the lower part of the cover 2 and consequently immersed in the culture medium when the cover 2 is fitted to the vessel 1. The heating device 10 is in the form of a heating resistance, preferably made of stainless steel in accordance with alimentary legislation. The heating device 10 has overheated self-controlled security. The processing unit 19 is configured to control the heating device 10 so as keep the optimum temperature of culture without cells damaging, in a cost-effective way. According to a specific example, the control parameters of the heating device 10 are: heating power adapted to keep the culture medium at 35° C.+/−1° C.

[0061] The processing unit 19 is also configured to control an aeration and mixing device 28 intended to ensure homogeneity of temperature, homogeneity of microalgae distribution and gas exchange in the culture medium received in the vessel 1. The aeration and mixing device 28 comprises an air pump 8 having an entrance antibacterial air filter, which is housed in the cover and connected to an aeration element 13 by means of a flexible tube 25. The flexible tube 25 passes through the internal space of the inner tube 21 of the lighting device 6 and is hermetically connected to the aeration element 13. The aeration element 13 extends beyond the ends of the tubes 12, 21 of the lighting device 6 so as to be immersed inside the culture medium when the cover 2 is fitted to the vessel 1. The aeration element 13 is preferably in stainless steel in accordance with alimentary legislation. The processing unit 19 is configured to control the aeration and mixing device 28 so as keep cells in suspension in the culture medium, sustain gas exchanges and in

[0062] particular CO.sub.2 supply to the photosynthetic organisms and removal of excess dissolved O.sub.2, and ensure temperature homogeneity in the culture medium. Supply of CO.sub.2 also maintains the pH of the culture medium above 10. This pH level favors growth of Spirulina (Arthrospira platensis) and prevents growth of other microbial communities. Removal of excess oxygen prevents buildup of hyperoxic conditions.

[0063] The apparatus 30 also has a harvesting device 29, housed inside of the cover 2, configured to ensure filtration of microalgae from a given volume of the culture medium taken in the vessel 1, while allowing nutrients and water to return to the culture medium left in the vessel. The harvesting device 29 comprises a self-priming membrane pump 9 connected to an intake culture tube 11 so as to allow intake of a volume of the culture medium from the vessel 1. The harvesting device 29 also comprises a cup 16 having stainless steel bottom and cylindrical wall, which is housed inside the cover 2 and protected by a hood 5 with hinge. The cup 16 is removably held on a filter support 4 defined by the cover 2, so that the filtered microalgae can be collected easily by separating the cup 16 from the cover 2.

[0064] The configuration of the harvesting device 29 is such that, after a volume of the culture medium has been pumped from the vessel 1, the microalgae are first filtered passing through a filtering membrane having a first pore size of the order of 315 pm, which is connected to the membrane pump 9 and located above the cup 16. This step allows the culture sludge to be rejected from the microalgae. Then, the culture is transferred and filtered through the cup 16 whose bottom and wall have a second pore size less than that of the filtering membrane, notably of the order of 55 pm or 30 pm. In this way, the harvesting device 29 allows intake of the culture medium from the vessel 1 and its transfer in the cup 16 without damaging cell morphology, thus preserving the nutritive quality of the microalgae.

[0065] Here again, the processing unit 19 is configured to control the harvesting device 29. More specifically, the apparatus is provided with a harvesting button 7 on which a user can click, so that for each click the processing unit 19 triggers filtering of a given volume of the culture medium, for example 1 L of the culture medium. The filtered volume can be adjusted by clicking several times on the harvesting button 7, for example each click

[0066] corresponding to 1 L. Optionally, the apparatus 30 may comprise a harvest indicator (not shown in the figures) to indicate the right harvesting time.

[0067] As explained above, important parameters of the culture are automatically controlled by the processing unit 19, including: homogeneity of temperature; temperature of the culture medium received in the vessel 1; ON/OFF switching of the lighting device 6 and output light intensity of the lighting device 6 during the ON period as a function of the concentration of microalgae in the vessel 1; volume of the culture medium extracted by means of the harvesting device 29. In particular, since the processing unit 19 controls both the lighting device 6 and the harvesting device 29, light control can be performed at any time of the culture, allowing a wide freedom of use in terms of moment and quantities of microalgae to be harvested.

[0068] By way of example, in the case of the production of Spirulina (Arthrospira platensis) by means of the apparatus 30, a full cycle of production takes place during about 40 days long and is divided in two distinct phases, namely a growing phase and a harvesting phase. The growing phase duration is comprised between 7 and 10 days. The harvesting phase is about 30 days, which corresponds to a recommended cure time of Spirulina. As illustrated with the apparatus 30, the invention has the advantage that it ensures an optimized productivity rate at each step of the culture, both during the growing phase and during the harvesting phase. The processing unit 19 has all information, obtained from the heating device 10, the light sensor 15, the aeration and mixing device 28, the harvesting device 29, in order to adjust in real-time the light intensity emitted by the lighting device 6 and received by the microalgae according to the concentration of microalgae in the vessel 1 and the temperature of the culture medium.

[0069] In particular, at the beginning of the culture, when the concentration of microalgae is low, the output light intensity of the lighting device 6 is controlled to avoid photolysis of cells. Then, during the growing phase, the output light intensity of the lighting device 6 is continuously adapted to correspond to the photosynthetic needs of the microalgae. During the harvesting phase, after each extraction of a volume of microalgae by means of the harvesting device 29, the output light intensity of the lighting device 6 is controlled to be adjusted to the new lower concentration of microalgae remaining in the vessel 1, which can be computed by the processing unit 19.

[0070] By way of example, FIG. 4 shows the evolutions, as a function of time during the growing phase, of the output light intensity of the lighting device 6 and of the concentration of microalgae in the vessel 1, in an illustrative example of production of Spirulina by means of the apparatus 30. In this example, the output light intensity of the lighting device 6 is automatically regulated by the processing unit 19 so as to meet the photosynthetic needs of Spirulina at each time.

[0071] An illustrative example of a method for the production of Spirulina (Arthrospira platensis), by means of the apparatus 30, comprises steps as described below.

[0072] First, to initiate the growing phase, the vessel 1 of the apparatus is filled with 9 L of demineralized water. Then, the content of a first nutrient bag is added into the water contained in the vessel 1. For example, the components of the first nutrient bag are the following: NaHCOs, NaCl, KNO.sub.3, K2SO4, MgSO.sub.47H.sub.2O, NH4H2PO4, CaCh, CO(NH.sub.2).sub.2. In addition, FeSO.sub.4 is provided to feed the microalgae.

[0073] In order to activate the heating device 10 and the aeration and mixing device 28, the apparatus 30 is then switched on, by means of the main switch 18. After dissolution, an inoculation strain (1 L at 2 g/L concentration) is gently added into the culture medium received in the vessel 1. The Spirulina growing phase is started with 10 L of culture at 0.2 g/L concentration. The inoculation strain may be provided in a bottle of 1 L at a concentration of 2 g/L microalgae. It may also be provided in another form, for example concentrated under fresh Spirulina biomass in a protected atmosphere, allowing extended lifetime of the inoculation strain. During the growing phase, the microalgae concentration evolves from clear green to dark green in 7 to 10 days.

[0074] At the end of the growing phase is the beginning of the harvesting phase, corresponding to 30 days of fresh Spirulina consumption. Each day, a maximum of 30% in volume of the culture medium is harvested by clicking on the harvesting button 7, according to the need of a user. Each click allows to automatically filter 1 L of the culture medium. The remaining culture medium is recycled and returned into the vessel 1. The filtered

[0075] microalgae is available in the removable cup 16 located in the cover 2 of the apparatus 30, by opening the hinged hood 5. After a manual step allowing to press the filtered microalgae, fresh Spirulina is directly consumed or added into culinary preparations. Fresh Spirulina can be preserved during 48 hours at 4-5° C. in a refrigerator or longer preserved by freezing or drying.

[0076] To compensate the loss of nutrients in the culture medium and sustain productivity, nutrients are regularly added to the culture medium during the harvesting phase. For example, the components of a second nutrient bag needed for the 30 days of the harvesting phase are the following: NaHCOs, NaNO.sub.3, NH4FI2PO4, K2SO4, MgSCE 7i¾O. FeSCE is provided to feed or enrich the culture medium. Nutrient bags are designed to protect nutrients against oxidation and humidity. Evaporation is also compensated by adding demineralized water. After 30 days of harvesting phase, new inoculation strain and culture medium may be provided to start a new cycle.

[0077] In an advantageous manner, nutrient bag with higher concentration of one or several specific compounds (e.g. iron, zinc, selenium) may be provided in order to respond to specific nutritional deficiencies of a user. These bags support enrichment of the culture in a specific compound, resulting in higher concentration of a specific compound in fresh Spirulina.

[0078] In accordance with the invention, during both the growing phase and the harvesting phase of the production cycle, in order to avoid photo-inhibition phenomenon, the output light intensity of the lighting device 6 is automatically controlled by the processing unit 19 according to the microalgae concentration in the vessel 1. The light sensor 15 is calibrated to receive a predefined value of light intensity in accordance with specific needs of Spirulina. As illustrated in the graph of FIG. 4, at low microalgae concentration, the output light intensity of the lighting device 6 needed to achieve the predefined value of light intensity to be received by the light sensor 15 is low. Indeed, at low microalgae concentration, light absorption rate is low and the penetration of light within the culture is ensured for all the microalgae. The higher the microalgae concentration, the more the microalgae cells absorb and reflect light, via a diffraction phenomenon, such that light penetrates less within the culture. Then, together with the increase of microalgae concentration, the output light intensity of the lighting device 6 increases to achieve the predefined value of light intensity at the light sensor 15.

[0079] The invention is not limited to the examples described and shown. In particular, in the illustrative embodiment described above of an apparatus and a method according to the invention, the apparatus 30 includes a unique light sensor 15, which is cost-effective but requires high homogeneity of the microalgae concentration. As a variant, an apparatus and a method according to the invention can involve several light sensors, which are then advantageously distributed peripherally on the wall of the vessel so as to detect the light intensity for several zones of the culture medium. In addition, in the example described above, reference has been made to Spirulina (Arthrospira platensis), but it is understood that the invention is applicable to the production of any type of microalgae, for example to the production of Chlorella or other specific aliment such as kombucha. The conditions of the culture are adapted to the specific type of microalgae which is cultured, in particular the temperature of the culture medium and the wavelengths and intensity of the light emitted by the lighting device.