Method for collecting microalgae

Abstract

To provide a recovery method for microalgae cultured in a salt-containing culture medium using a solid-phase surface culture method, by reducing the salt concentration. [Solution] The method for recovering microalgae according to the present disclosure is a method for recovering microalgae cultured on the surface of a carrier using a first culture medium containing salt, comprising, in this order, a washing water addition step S103 in which washing water is added to the microalgae in culture from the surface side of the carrier, and a recovery step S104 in which the microalgae are recovered, wherein the salt concentration of the washing water added in the washing water addition step S103 is lower than the salt concentration of the first culture medium.

Description

Technical Field

[0001] The present disclosure relates to a method for collecting microalgae.

Background Art

[0002] In recent years, the utilization of algal biomass has attracted attention as an initiative to reduce greenhouse gas emissions. In order to utilize algal biomass in energy production or other industries, it is required to produce a large amount of biomass at low cost. For the cultivation of microalgae, a liquid culture method is mainly used, in which algae are suspended in a liquid medium and cultured with stirring. However, this culture method requires a large amount of energy for stirring the medium, dewatering the biomass, and the like.

[0003] Accordingly, for example, as a culture method capable of producing biomass at lower cost and more efficiently than conventional methods, Patent Document 1 describes a solid-phase surface culture method in which microalgae are supported on a carrier and cultured. Although the solid-phase surface culture method has various advantages, for example, it has the advantage that biomass with a low water content can be obtained because microalgae can be cultured at high density with a small amount of medium.

[0004] Further, as a method for supplying nutrient salts to microalgae during culture in solid-phase surface culture, Patent Document 1 proposes a method of supplying nutrients to microalgae by spraying the nutrient salts from the gas phase side to the microalgae being cultured on a carrier. Further, Patent Document 2 proposes culturing microalgal cells on the surface of the culture carrier by supplying water and nutrients to the culture carrier located above the water surface of the liquid medium using capillary action.

[0005] Algae, which are expected to be used as biomass raw materials, include species that inhabit a wide range of environments. When cultivating marine or brackish water microalgae, the recovered biomass contains salt. Therefore, burning salt-containing biomass can damage the furnace and potentially cause disadvantages to biomass utilization. For this reason, desalination is usually required when using algal biomass cultivated in a salt-containing medium.

[0006] Conventionally, filtration and membrane separation methods have been widely used to remove salt from biomass. Patent documents 3 and 4 also describe methods for coagulation and desalination. The method described in Patent Document 3 involves diluting the microalgae and culture medium with water during recovery to lower the salt concentration, adding a coagulant to coagulate and flocculate, and then dehydrating and desalinizing to obtain biomass. The method described in Patent Document 4 involves adding a chitosan solution and water to dried biomass, gently stirring and allowing it to stand to transfer the salt to the chitosan solution, and then separating the biomass and salt by centrifugation. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Special Publication No. 2014-516571 [Patent Document 2] Japanese Patent Publication No. 2017-99301 [Patent Document 3] Japanese Patent Publication No. 2018-93766 [Patent Document 4] Japanese Patent Application Publication No. 7-147 [Patent Document 5] Japanese Patent Publication No. 2018-157817 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] However, in order to produce dry biomass using the conventional coagulation separation method, a dewatering operation is required after desalting. Therefore, applying the above desalting method to biomass recovered by solid-phase surface culture would negate the advantage of solid-phase surface culture, which is that it yields biomass with a low moisture content. It is thought that similar problems occur with conventional desalting techniques such as filtration and membrane separation.

[0009] Thus, when cultivating algae in a salt-containing medium using solid-phase surface culture, no method has been demonstrated to date that allows for the recovery of biomass with a low salt concentration without compromising the advantages of solid-phase surface culture.

[0010] This disclosure is made in view of the circumstances described above, and aims to provide a recovery method for recovering microalgae cultured in a salt-containing culture medium using a solid-phase surface culture method, by reducing the salt concentration. [Means for solving the problem]

[0011] A method for recovering microalgae according to one aspect of the present disclosure is a method for recovering microalgae that have been supported on the surface of a carrier and cultured using a first medium containing salt, comprising, in this order, a washing water addition step of adding washing water to the microalgae during culture from the surface side of the carrier, and a recovery step of recovering the microalgae, wherein the salt concentration of the washing water added in the washing water addition step is lower than the salt concentration of the first medium. A method for recovering microalgae according to another aspect of the present disclosure is a method for recovering microalgae that have been supported on the surface of a carrier and cultured in a first medium containing salt, comprising: a second medium supply step of supplying the first medium as a second medium; and a recovery step of recovering the microalgae. The second medium contains the above, and its salt concentration is lower than that of the first medium. [Effects of the Invention]

[0012] According to this disclosure, a method is provided for harvesting microalgae cultured in a salt-containing medium using a solid-phase surface culture method, after reducing the salt concentration.

Brief Description of the Drawings

[0013] [Figure 1] This is a side view showing the overall configuration of the culture apparatus. [Figure 2A] This is a flowchart showing the recovery method of the first embodiment. [Figure 2B] This is a flowchart showing the recovery method of the second embodiment. [Figure 2C] This is a flowchart showing the details of the second medium supply step of the recovery method of the second embodiment. [Figure 3] This is a graph showing the results of Test Example 1. [Figure 4] This is a side view showing the overall configuration of the culture apparatus in Test Example 2. [Figure 5] This is a graph showing the results of Test Example 2. [Figure 6] This is a graph showing the results of Test Example 3.

Modes for Carrying Out the Invention

[0014] (1. Culture Apparatus) The microalgae culture apparatus 100 according to an embodiment of the present disclosure will be described with reference to Figure 1. The culture apparatus 100 used in the present disclosure is an apparatus for solid-phase surface culture of microalgae. The culture apparatus 100 includes a carrier 1, a support 2, a liquid medium tank 3, a pump 4, a light source 5, a water sprinkler 8, and a liquid reservoir 15. Each element is described below.

[0015] The carrier 1 is a material that can support microalgae 6 on its surface or internally, and that can permeate the culture medium 7. The carrier 1 only needs to have water absorption and water retention properties and a shape that can support microalgae 6; its shape and material are not particularly limited. In this embodiment, the carrier 1 is in the form of a sheet. As the material of the carrier 1, for example, filter paper, cloth, and sponge may be used. These materials may be used individually or in combination. To facilitate the support of microalgae 6, the material of the carrier 1 is preferably a material that does not allow microalgae 6 to easily permeate.

[0016] During the cultivation of the microalgae 6, it is desirable that the carrier 1 contains enough moisture to allow the microalgae 6 to be cultivated. In this embodiment, a portion of the carrier 1 is in contact with the supplied culture medium 7. By a portion of the carrier 1 being in contact with the supplied culture medium 7, the culture medium 7 can permeate into the carrier 1 and be supplied to the microalgae 6 it supports. Alternatively, the culture medium 7 may be supplied to the carrier 1 by dropping it onto a portion of the carrier 1.

[0017] Support 2 is equipment that supports carrier 1. Support 2 only needs to be able to hold carrier 1 in a predetermined position in a state in which microalgae 6 can be carried on carrier 1, and its specific shape and material are not limited. In this embodiment, as shown in Figure 1, support 2 is a base that can hold the microalgae-carrying surface 13 (surface) of carrier 1 at a predetermined angle, and includes an inclined portion 12 having a predetermined angle θ with respect to the horizontal plane. The positional relationship between the microalgae 6 on the carrier 1 and the light source 5 can be adjusted by appropriately setting the shape and installation location of the support 2. In this embodiment, the positional relationship between the microalgae 6 on the carrier 1 and the light source 5 can be adjusted by appropriately changing the angle θ of the inclined portion 12 of the support 2.

[0018] The material of the support 2 is not particularly limited, but a material with high corrosion resistance is preferred. Also, from the viewpoint of retaining moisture in the carrier 1, the inclined portion 12 is preferably impermeable to water. Examples of materials having such properties include non-metallic materials such as polyvinyl chloride resin and acrylic resin, and metallic materials such as stainless steel and titanium. The surface shape of the inclined portion 12 of the support 2 may be flat, angular, or uneven, but is not limited to these.

[0019] The liquid culture medium tank 3 is a container for storing the culture medium 7. The material of the liquid culture medium tank 3 may be, but is not limited to, polyvinyl chloride, polyethylene, plastic, or glass. The shape of the liquid culture medium tank 3 may be cylindrical or rectangular, but its shape and size are not particularly limited. The liquid culture medium tank 3 may have an opening for supplying the culture medium 7, and may have a lid that can be opened and closed to separate it from the external environment. The opening may be, for example, a part for connecting a tubular accessory.

[0020] Furthermore, a stirrer for the culture medium 7 may be installed inside the liquid culture medium tank 3, and measuring instruments such as a thermometer and a pH meter may also be installed. Other components besides the culture medium 7 may also be placed in the liquid culture medium tank 3. The solution filled in the liquid culture medium tank 3 is not particularly limited, and the solution may be water or wastewater discharged from various industries, and nutrients may be added to reuse the culture medium after cultivation. Such nutrients may be components necessary or useful for the growth of microalgae 6, such as nitrogen or phosphorus.

[0021] Pump 4 supplies the culture medium 7 stored in the liquid culture medium tank 3 to the carrier 1. Pump 4 can adjust the amount of culture medium 7 supplied to the carrier 1. The type of pump 4 is not limited, but it may be, for example, a vacuum pump, a centrifugal pump, or a rotary pump. Pump 4 is particularly preferred when the culture medium contact portion of the carrier 1 is located vertically above the liquid culture medium tank 3. The culture apparatus 100 is not limited to a configuration that includes pump 4. Also, when the culture medium 7 is being supplied to the carrier 1 by pump 4, the supply of culture medium 7 can be stopped by temporarily stopping the operation of pump 4.

[0022] Light source 5 supplies light to the microalgae 6 being cultured on carrier 1. The wavelength of light emitted from light source 5 includes wavelengths (350-700 nm) that the microalgae 6 can use for photosynthesis. Examples include LEDs, fluorescent lamps, and incandescent light bulbs. Instead of using light source 5, the culture apparatus 100 may be installed outdoors and sunlight may be used.

[0023] The watering device 8 is a device that can spray (add) cleaning water from the surface side of the carrier 1. The specific configuration of the watering device 8 is not limited to having a configuration that allows cleaning water to be added from the surface side of the carrier 1 in the cleaning water addition step S103 described later. Furthermore, the watering device 8 is not necessarily required depending on the addition method in the cleaning water addition step S103 described later.

[0024] The liquid reservoir 15 is a portion capable of temporarily storing the culture medium 7. For example, the culture medium 7 is supplied to the carrier 1 when at least a portion of the upper end of the carrier 1 comes into contact with the culture medium 7 inside the liquid reservoir 15.

[0025] (2. Subjects to be cultured) Next, the microalgae 6 and culture medium 7 that are the subject of cultivation in this disclosure will be described below. Algae refer to all organisms that perform oxygen-evolving photosynthesis, excluding land plants. Among algae, those that are small, ranging in size from 1 μm to several hundred μm, are called microalgae. Microalgae inhabit diverse aquatic environments, from freshwater to seawater, and reproduce by performing photosynthesis. The microalgae 6 covered by this disclosure are species that can grow in salt-containing culture media. The microalgae 6 covered by this disclosure may be, for example, marine or brackish water species, or species that can grow in culture media with a salinity of 3% or higher. An example of a marine or brackish water species is the NS001C strain described in Patent Document 5. The microalgae 6 may be, for example, algae belonging to cyanobacteria, green algae, Euglena algae, diatoms, true eyespot algae, etc., or other algae that can grow in salt-containing culture media. An example of other algae that can grow in salt-containing culture media is the NS001C strain that can grow in coke oven wastewater described in Patent Document 4.

[0026] In this disclosure, the culture medium 7 is not particularly limited as long as it contains salt and can be used to grow microalgae 6 by cultivation. In this embodiment, the culture medium 7 may be artificial seawater, F / 2 medium, ESM medium, K medium, etc., and contains macronutrients such as nitrogen, phosphorus, potassium, and calcium, and trace metals such as iron, manganese, and zinc as nutrients. In addition, industrial wastewater can be used as long as it contains the above-mentioned components. The salt concentration of the culture medium 7 used to grow microalgae 6 by cultivation is preferably 3% or higher.

[0027] (3. Culture method) A method for culturing microalgae 6 using the culture apparatus 100 of this embodiment will be described. First, a carrier 1 bearing microalgae 6 is placed on the surface of a support 2. Next, culture medium 7 is supplied from the top of the carrier 1, and the culture medium 7 flows into the inside of the carrier 1 and the support installation surface 14 (hereinafter referred to as the liquid phase side).

[0028] A light source 5 is installed so that light is irradiated onto the microalgae-supported surface 13 (gas phase side) of the carrier 1, and cultivation is performed by irradiating with light. The location of the cultivation apparatus 100 is not particularly limited and may be indoors or outdoors. Depending on the microalgae 6 used for cultivation, it is preferable to install the cultivation surface (microalgae-supported surface 13) in an atmosphere with a CO2 concentration higher than that of the atmosphere, from the viewpoint of promoting the growth of the microalgae 6. The method of supporting the microalgae 6 and the method of supplying the culture medium 7 in the above cultivation method will be explained below.

[0029] The method for supporting the microalgae 6 on the carrier 1 is not particularly limited. For example, the microalgae 6 can be supported by applying or spraying a culture solution containing the microalgae 6 onto the carrier 1. Alternatively, the culture solution can be concentrated by centrifugation or other means to apply a paste-like form of microalgae 6. Furthermore, the rate of support of the culture medium 7 can be accelerated by placing a highly absorbent material on the side of the carrier 1 opposite to the microalgae-supporting surface 13 (the support installation surface 14).

[0030] The method of supplying the culture medium 7 to the carrier 1 is not particularly limited. For example, the culture medium 7 may be dropped from the top of the carrier 1, or the culture medium 7 may be supplied into the carrier 1 by capillary action from the surface of the carrier 1 that contacts the culture medium. If the surface of the support 2 is at an angle with respect to the horizontal plane (the angle θ between the surface of the support 2 and the horizontal plane is greater than 0), the culture medium 7 supplied to the top of the carrier 1 can flow down to the bottom of the carrier 1 by gravity. In this case, the culture medium 7 used for culturing flows out from the bottom of the carrier 1 and is stored in the liquid culture medium tank 3. This culture medium 7 can be reused after adjusting its components as needed, and the culture medium 7 can be recycled and used. In addition, new culture medium may be added to the culture medium 7 in the liquid culture medium tank 3, and the culture medium 7 in the liquid culture medium tank 3 can be discarded if it deteriorates due to use.

[0031] (4. Collection Method) A method for recovering the microalgae 6 grown in the above culture method will now be described. As stated above, the method for recovering the microalgae 6 of this disclosure can be suitably used to recover the microalgae 6 cultured in a salt-containing medium. The method for recovering microalgae 6 according to this disclosure is characterized by having a washing water addition step S103 or a second culture medium supply step S101 in which washing water is added to the microalgae 6 in culture from the surface side of the carrier.

[0032] This step reduces the salinity (biomass salinity) contained in the recovered biomass. The recovery method, which includes the washing water addition step S103, the second culture medium supply step S101, and the recovery step S104 for recovering the microalgae 6, is described below.

[0033] In the following, for explanatory purposes, the culture medium used for the growth of microalgae 6 in the above cultivation method will be referred to as the first medium. The salinity of the first medium is not limited, but it is preferably 3% or higher.

[0034] (Recovery method of the first embodiment) Figure 2A shows a flowchart of the recovery method according to the first embodiment. As shown in Figure 2A, the recovery method of the first embodiment comprises a second culture medium supply step S101, a culture medium supply stop step S102, a washing water addition step S103, and a recovery step S104. Note that the recovery method only needs to include the washing water addition step S103 and the recovery step S104, and for example, it does not need to include both the second culture medium supply step S101 and the culture medium supply stop step S102, or it may not include either one of them.

[0035] The recovery method of the first embodiment will be described. In the second culture medium supply step S101, the culture medium 7 supplied to the carrier 1 is changed from the first culture medium to the second culture medium. For example, by changing the culture medium 7 stored in the liquid culture medium tank 3 to the second culture medium, the circulating culture medium can be replaced with the second culture medium. The salt concentration of the second medium is lower than that of the first medium. The second medium may have the same composition as the washing water added to the carrier 1 in the washing water addition step S103 described later, or it may have a different composition. It is preferable that the second medium does not contain salt. Since the salt concentration of the second culture medium is lower than that of the first culture medium, the salt concentration of the culture medium 7 contained in the carrier 1 can be reduced in the second culture medium supply step S101, and the biomass salt concentration of the culture medium brought in during biomass recovery can be reduced.

[0036] In the culture medium supply termination process S102, the supply of culture medium 7 is stopped. In the culture medium supply cessation step S102 of the first embodiment, the supply of the second culture medium to the carrier 1 is stopped. This reduces the amount of water (amount of the second culture medium) contained in the carrier 1. In the culture medium supply cessation step S102, it is preferable that, for example, the amount of water contained in the carrier is reduced by 50% or more. If the recovery method does not include the second culture medium supply step S101 described above, the culture medium supply cessation step S102 stops the supply of the first culture medium to the carrier 1. That is, in the culture medium supply cessation step S102, if the culture medium supplied to the carrier is the first culture medium, the supply of the first culture medium is stopped, and if it is the second culture medium, the supply of the second culture medium is stopped.

[0037] The method for measuring the amount of water contained in the carrier 1 is not limited, but it can be determined, for example, by measuring the weight of the carrier 1 before and after the culture medium supply stoppage step S102.

[0038] In the culture medium supply stop step S102, operations such as stopping the operation of the pump 4 or removing tubes connected to the pump 4 may be performed. Furthermore, from the viewpoint of reliably reducing the amount of moisture contained in the carrier 1, it is preferable to wait for a certain period of time (for example, until the flow of water from the culture medium stops from the bottom of the carrier) between stopping the supply of the culture medium 7 and adding the washing water in the culture medium supply stop step S102. Additionally, the amount of moisture contained in the carrier 1 can be reliably reduced by using a water absorber or the like. As in the recovery method of the first embodiment, by including a culture medium supply stop step S102 before the washing water addition step S103, the rate at which the carrier 1 absorbs the washing water added in the washing water addition step S103 is increased, and the amount of water that can be added in a certain period of time can be increased. Therefore, the salinity of the microalgae 6 recovered in the recovery step S104 described later can be efficiently reduced.

[0039] The washing water added in step S103 only needs to have a salinity of less than or equal to that of the first culture medium. Examples of washing water to be added include tap water and industrial water, but are not limited to these. As washing water to be added, culture media for freshwater microalgae and wastewater discharged from industry can also be used. The lower the salinity of the washing water added in step S103, the better the effect of reducing the biomass salinity. Therefore, it is preferable to use ultrapure water, ion-exchanged water, distilled water, etc., as washing water with a lower salinity. In the first embodiment, the salinity of the washing water added in the washing water addition step S103 is lower than the salinity of the first culture medium. Furthermore, if there is a second culture medium supply step S101 as in the first embodiment, it is preferable that the salinity of the washing water added to the carrier 1 in the washing water addition step S103 described later be less than or equal to the salinity of the second culture medium.

[0040] The washing water is added to the microalgae 6 being cultured on the carrier 1 from the surface side (gas phase side) of the carrier. By adding the washing water to the microalgae 6 being cultured from the surface side of the carrier, the salt on the surface side of the carrier (microalgae supporting surface 13 side) can be moved to the support installation surface 14 side, and the salt concentration contained in the microalgae 6 recovered in the recovery step S104 described later can be efficiently reduced.

[0041] As a method for adding washing water, for example, by installing a watering device 8 on top of the carrier 1 and support 2 of the culture apparatus 100, washing water can be sprayed (added) from the surface side of the carrier. This watering device 8 may be a mist generator or an automatic watering device, but is not limited to these. In addition, the addition of washing water can be substituted by spraying using a sprayer, adding using a wash bottle or pipette, etc., without using the watering device 8. The addition method does not have to be limited to the above, as long as it is added from the surface side of the carrier to the microalgae 6, but it is preferable that the water pressure is such that the microalgae 6 do not detach from the carrier 1.

[0042] The amount of washing water added in washing water addition step S103 is not limited, but from the viewpoint of sufficiently reducing the biomass salinity, for example, the amount of washing water added is limited to the amount of washing water added to the algae culture area on carrier 1 (1 m²). 2 It is preferable to add 5L or more to the amount.

[0043] In the recovery step S104, the method for recovering the microalgae 6 being cultivated on the carrier 1 is not particularly limited. For example, the microalgae 6 can be recovered by scraping them off the surface of the carrier 1 using a spatula, scraper, or the like. Alternatively, the microalgae 6 may be recovered by detaching them from the carrier 1 by suction. Furthermore, the carrier 1 bearing the microalgae 6 can also be removed from the culture device 100 and the microalgae 6 recovered from there.

[0044] The salinity of the biomass recovered in the recovery process is preferably 3% or less, more preferably 2% or less, and even more preferably 1% or less, in terms of chlorine concentration relative to the dry weight of the biomass. The salinity of the biomass can be determined, for example, by measuring the chlorine concentration as follows. The biomass recovered during the recovery process is placed in a 15 mL centrifuge tube and dried overnight at 105°C. The salinity of the resulting dried biomass is analyzed by combustion ion chromatography. The biomass salinity is calculated by converting the obtained analytical values ​​to salinity per unit dry weight of the recovered biomass.

[0045] According to this first embodiment, in solid-phase surface culture, biomass recovered by a method having a washing water addition step S103 can have a reduced salt concentration compared to untreated biomass without the above step. Furthermore, as in the first embodiment, by including at least one of the second culture medium supply step S101 and the second culture medium supply step S103 before the washing water addition step S103, even more salt can be removed.

[0046] Next, the recovery method of the second embodiment will be described. Figure 2B shows a flowchart of the recovery method in the second embodiment. Furthermore, Figure 2C shows the breakdown of the second culture medium supply step S101 in the second embodiment. The recovery method of this embodiment 2, shown in Figure 2B, comprises a second culture medium supply step S101, a culture medium supply stop step S102, and a recovery step S104. Note that the culture medium supply stop step S102 is optional.

[0047] As in the second embodiment, in solid-phase surface culture, the salinity of biomass recovered by a method comprising a second culture medium supply step S101 and a recovery step S104 can be reduced to approximately the same level as the salinity of biomass recovered by the method according to the first embodiment (a method comprising a washing water addition step S103 and a recovery step S104).

[0048] In the second embodiment, first, the second culture medium supply step S101 is performed. The second culture medium supply step S101 in the second embodiment is a step of changing the culture medium 7 supplied to the carrier 1 from the first culture medium to the second culture medium, similar to the first embodiment. For example, by changing the culture medium 7 stored in the liquid culture medium tank 3 and the liquid reservoir 15 to the second culture medium, the culture medium 7 that can be supplied to the carrier 1 can be replaced with the second culture medium. The salinity of the second culture medium is lower than that of the first culture medium.

[0049] Next, the recovery process S104 is performed to recover the microalgae 6 being cultivated on the carrier 1. As mentioned above, the method of recovering the microalgae is to scrape off the microalgae 6 from the surface of the carrier 1 using a spatula, scraper, etc., but is not limited to these methods.

[0050] Furthermore, in the recovery method of the second embodiment, from the viewpoint of more reliably reducing the salt concentration contained in the microalgae 6 recovered in the recovery step S104, it is preferable that the second culture medium supply step S101 includes the first culture medium removal step S101A and the second culture medium circulation step S101B, as shown in Figure 2C.

[0051] Step S101A, the first culture medium removal step, is a step in the culture method to remove the first culture medium held in the carrier 1. For example, this step can be performed by appropriately combining processes such as stopping the supply of the first culture medium to the carrier 1, removing the culture medium 7 (first culture medium) stored in the liquid culture medium tank 3 and the liquid reservoir 15, or aspirating the first culture medium held in the carrier 1. This makes it possible to reduce the amount of first culture medium held in the carrier 1, and as a result, the amount of salt derived from the first culture medium held in the carrier 1 and the microalgae 6 can be reduced. Therefore, the salt concentration that is carried over when the culture medium 7 is changed to the second culture medium in a subsequent step can be reduced. It is preferable that the amount of first culture medium contained in the carrier is reduced by 50% or more.

[0052] The subsequent second culture medium circulation step S101B is a step in which the second culture medium is supplied to the carrier 1 and circulated inside the carrier 1. For example, the second culture medium is introduced into the liquid culture medium tank 3 and the liquid reservoir 15, and the pump 4 is activated to supply the second culture medium to the carrier 1, pass it through the inside of the carrier 1, and circulate it for a predetermined time. The specific time for supplying and circulating the second culture medium to the carrier 1 is not limited and is set appropriately depending on the size and type of carrier 1.

[0053] This effectively removes salts attached to or encapsulated in the microalgae 6 from inside the carrier 1, i.e., from the liquid phase side. As a result, it is possible to reduce the salt concentration held by the microalgae 6 recovered in the recovery step S104.

[0054] Furthermore, some types of microalgae 6 are highly sensitive to salt concentration (salinity) stress. When using such microalgae 6, the first medium removal step S101A described above may be omitted, and only the second medium circulation step S101B may be performed. Salt concentration stress refers to the stress that algae experience when exposed to an environment with a salinity outside the optimal range. For example, it refers to the stress that algae experience when exposed to a salinity environment that is lower (or higher) than the optimal range.

[0055] Here, it is also possible to perform the culture medium supply stop step S102 after the second culture medium supply step S101. The culture medium supply stop step S102 in the second embodiment may be the same in content as the culture medium supply stop step S102 in the first embodiment. In the supply stop step S102 of the second embodiment, the supply of the second culture medium is stopped. This makes it possible to reduce the amount of water (amount of second culture medium) contained in the carrier 1. As for methods of stopping the culture medium supply, as described above, there are methods such as stopping the operation of the pump 4 or removing tubes etc. connected to the pump 4, but are not limited to these. In order to reliably reduce the amount of second culture medium contained in the carrier 1, it is preferable to wait for a predetermined time after stopping the supply of culture medium 7 until the next recovery step S104. The specific value of the predetermined time is not limited, but in the culture medium supply stop step S102, it is preferable to set a time such that, for example, the amount of water contained in the carrier (amount of second culture medium) decreases by 50% or more compared to before the start of the culture medium supply stop step S102.

[0056] Furthermore, the salinity of the recovered microalgae 6 can be reduced by performing the second culture medium supply step S101 and the culture medium supply stop step S102 multiple times. That is, after performing the (first) second culture medium supply step S101 and culture medium supply stop step S102, repeating the same steps (second culture medium supply step S101 and culture medium supply stop step S102) a second time, a third time, etc., the salinity of the carrier 1 can be gradually reduced, thereby reducing the salinity of the recovered microalgae 6. In this case, it is preferable that the salinity of the newly supplied second culture medium is less than or equal to the salinity of the recycled second culture medium. That is, it is preferable that the salinity of the second culture medium (2-2 culture medium) supplied in the second second culture medium supply step S101 is less than or equal to the salinity of the second culture medium (2-1 culture medium) supplied in the first second culture medium supply step S101.

[0057] In the second embodiment, the culture medium supply cessation step S102 reduces the amount of water contained in the carrier 1 (amount of first culture medium and / or second culture medium), thereby reducing the amount of water attached to or encapsulated in the microalgae 6. As a result, the salt concentration contained in the recovered microalgae 6 can be efficiently reduced.

[0058] By using the recovery method according to the second embodiment, the salinity of the recovered biomass can be reduced while the microalgae 6 remain on the carrier 1. Therefore, the reduction in the amount of recoverable biomass can be suppressed, and biomass can be recovered efficiently. Furthermore, since it is possible to slow down the changes in the components of the culture medium 7 held by the microalgae 6, this method can be suitably applied even when using microalgae 6 that are sensitive to rapid fluctuations in salinity (salt concentration stress). [Examples]

[0059] The following describes the test examples provided in this disclosure in detail, but these test examples are not intended to limit the scope of this disclosure.

[0060] <1. Test Example 1: Confirmation of Desalination Effect> [culture] Multiple transparent plastic containers (Styrofoam rectangular case type 9, AS ONE Corporation) were prepared, each containing a PVA sponge (Bell Eater FB(D), AION Corporation) as a carrier, and these were used as culture cases.

[0061] For the culture medium (first medium), a reinforced seawater medium prepared as follows was used. 35 g of Marine Art SA-F (Tomita Pharmaceutical Co., Ltd.) was dissolved in 1 L of distilled water and autoclaved (121°C, 30 minutes) to prepare artificial seawater. Separately, one bag of Daigo IMK medium for 100 L (Shioya MS Co., Ltd.) was dissolved in 100 mL of distilled water and filtered in a clean bench to prepare an IMK stock solution. 2 mL of the IMK stock solution was added to 1 L of artificial seawater to prepare the reinforced seawater medium (first medium).

[0062] Each 140 mL of the prepared reinforced seawater culture medium was placed in the culture case and supplied to the PVA sponge. The microalgae used were the NS001C strain described in Patent Document 5. The samples for Comparative Example a, Example a, Example b, and Example c were obtained by suction filtering 30 mL of the pre-culture solution of the NS001C strain, which had been pre-cultured in liquid culture, and then supporting the microalgae on glass fiber filter paper (GF / F, Whatman, Φ47 mm) and culturing them. Three of these glass fiber filter papers, each supporting the microalgae, were placed on the PVA sponge of the culture case with the algae-supported side facing upwards. These culture cases were placed in a transparent sealed culture chamber and cultured for two days. The transparent sealed culture chamber was placed on a shelf in a laboratory at room temperature of 25°C, and a mixed gas with a flow rate adjusted by a gas mixer to achieve a CO2 concentration of 2% was passed through the container. For culture illumination (light source), multiple straight-tube LED lights were installed in parallel on the top surface of a sealed container, and light was irradiated using a timer set to 14 hours of light and 10 hours of dark. The photosynthetic photon flux density on the top surface of the sponge in the culture case was approximately 600 μmol / m². 2 It was / s.

[0063] [Biomass collection] After culturing, the culture cases were removed, and each culture case was subjected to the following treatment (Table 1), after which the biomass was recovered from the glass fiber filter paper.

[0064] [Table 1]

[0065] In comparative example a, only the recovery process was performed, and no treatment was carried out before the recovery of the microalgae.

[0066] In Example a, a process simulating the cessation of culture medium supply and a washing water addition process were carried out before the recovery process. Specifically, a sponge containing culture medium (first medium) was removed from the culture case along with the glass fiber filter paper on the sponge, and placed on a plastic plate positioned at a 60-degree angle to the ground. It was left in this state for 3 minutes, allowing the culture medium (first medium) contained in the sponge to drain from below. Then, using a micropipette, 5 mL of ultrapure water (washing water) was gently added to the microalgae on the glass fiber filter paper for each sheet of filter paper. Converting the amount of washing water added in Example a to the amount of water per unit area, it was 5.2 L / m². 2 That was the case.

[0067] In Example b, the process was basically the same as in Example a, but the amount of ultrapure water added was changed from 5 mL to 10 mL per sheet of filter paper. The amount of washing water added per unit area in Example b was 10.4 L / m². 2 That was the case.

[0068] In Example c, before the recovery process, a process simulating the supply of the second culture medium, a process simulating the cessation of culture medium supply, and a process simulating the addition of washing water were performed. Specifically, the sponge containing the culture medium, along with the glass fiber filter paper placed on top of it, was removed from the culture case and placed at a 60-degree angle to the ground. Next, 500 mL of ultrapure water (second culture medium) was added to the top of the sponge and allowed to flow downwards, thereby replacing the liquid culture medium inside the sponge with ultrapure water (second culture medium). After that, it was left for 3 minutes, and as in Example b, 10 mL of ultrapure water (water) was gently added to the microalgae using a micropipette.

[0069] In Examples a, b, and c, the operation of leaving the sponge on a plastic plate at a 60-degree angle to the ground for 3 minutes simulates the operation of stopping the pump 4 in the culture apparatus (culture medium supply stop step). In Example c, the operation of letting 500 mL of ultrapure water flow downwards from the top of the sponge simulates the operation of changing the circulating culture medium (first medium) to ultrapure water (second medium) in the culture apparatus (second medium supply step).

[0070] In all of Comparative Example a, Examples a, b, and c, the biomass (microalgae) was recovered by scraping the microalgae from the glass fiber filter paper with a small spatula.

[0071] As comparative example b, biomass recovered from liquid culture was prepared. 30 mL of the culture solution of strain NS001C prepared by liquid culture was transferred to a 50 mL centrifuge tube and centrifuged at 25°C and 2,000 × g for 10 minutes. After centrifugation, the supernatant was removed, and 10 mL of ultrapure water was added to the precipitate to suspend the microalgae. The mixture was then centrifuged again at 25°C and 2,000 × g for 10 minutes. After centrifugation, the supernatant was removed, and the precipitate was recovered as biomass.

[0072] [Chlorine concentration measurement] The biomass collected as described above was used to measure the chlorine concentration (salinity) within the algae and the chlorine concentration derived from the culture medium brought in during collection (hereinafter referred to as the "introduced chlorine concentration") as follows.

[0073] The biomass chlorine concentration was measured as follows: The recovered biomass was resuspended in 10 mL of pure water. These biomass suspensions were placed in 15 mL centrifuge tubes, vortexed for 5 seconds, and then centrifuged at 25°C and 2,000 × g for 10 minutes. After centrifugation, the chlorine concentrations of the precipitate and supernatant were analyzed by combustion ion chromatography. The obtained analytical values ​​were converted to chlorine concentrations per unit dry weight of the recovered biomass, and these values ​​were defined as the intra-algal and introduced chlorine concentrations, respectively. The results are shown in Figure 3.

[0074] As shown in Figure 3, under all conditions of Examples a to c, the biomass chlorine concentration was lower than that of Comparative Example a. This indicates that the salt contained in the biomass recovered by culturing salt-containing microalgae largely originates from the liquid culture medium introduced during the recovery of the microalgae.

[0075] Next, when the total chlorine concentration of the recovered biomass (the sum of the chlorine contained in the algae and the chlorine concentration brought in) was converted to chlorine concentration per unit dry weight (g-chlorine / g-dry biomass), the results were 0.0386 for Comparative Example a, 0.0070 for Example a, 0.0040 for Example b, 0.0037 for Example c, and 0.0057 for Comparative Example b.

[0076] Comparing the conditions for solid-phase surface culture, a reduction in chlorine concentration was observed in Example A by 81.77%, in Example B by 89.57%, and in Example C by 90.42% compared to the untreated sample in Comparative Example A. Comparing Example A and Example B, Example B, which used a larger amount of ultrapure water, showed a lower chlorine concentration. Furthermore, comparing Example B and Example C, Example 3, which included the replacement of the culture medium in the sponge with ultrapure water, showed an even lower chlorine concentration. The overall chlorine concentration of the biomass in Examples B and C was lower than that in Comparative Example B. These results demonstrate that adding washing water to the carrier surface (gas phase side) before harvesting the microalgae during culture significantly reduces the salt content in the recovered biomass, achieving a desalination effect comparable to or better than that obtained by desalining liquid-cultured microalgae with ultrapure water. In addition to the above, it was also shown that increasing the amount of ultrapure water added from the surface and changing the circulating culture medium (first medium) to ultrapure water (second medium) can also enhance the desalination effect.

[0077] <2. Test Example 2: Confirmation of Desalting Effect in Culture Device 100> The desalting effect of the recovery method of the first embodiment of this disclosure was verified using an actual culture apparatus 100. [culture] A culture apparatus 100 as shown in Figure 4 was prepared. A support 2, which has a structure in which a liquid reservoir 15 made of the same material is connected to the upper end of a polyvinyl chloride plate, was placed on a stand at an angle θ of 70 degrees to the horizontal plane. A carrier 1 made of PVA sponge (Bell Eater EB(D), AION Corporation) was placed on the upper surface of the plate-like part of the support 2. The upper end of the PVA sponge was immersed in the liquid reservoir 15 of the support 2. Next, a liquid culture medium tank 3 with a capacity of 10 L was placed below the support 2, and the lower end of the PVA sponge was inserted into the liquid culture medium tank 3. Subsequently, a pump 4 was placed near the liquid culture medium tank 3, and the end of the braided hose connected to the suction port of the pump 4 was inserted into the liquid culture medium tank 3. The end of the braided hose connected to the discharge port of the pump 4 was also inserted into the liquid reservoir of the support 2.

[0078] The culture apparatus 100 was assembled in this manner. 10 liters of enhanced seawater culture medium 7, prepared by adding 200 mL of IMK stock solution to 9.8 L of artificial seawater, were added to the liquid culture medium tank 3.

[0079] The pump 4 was operated to pump the culture medium 7 from the liquid culture medium tank 3 to the top of the support 2. At this time, the culture medium 7 supplied to the liquid reservoir 15 was supplied to the carrier 1, flowed downward through the inside of the carrier 1, and finally flowed into the liquid culture medium tank 3. In this way, the culture medium 7 was circulated inside the carrier 1. The flow rate of the pump 4 was adjusted so that the entire surface of the carrier 1 remained moist.

[0080] Next, the microalgae 6 to be cultured were prepared. Strain NS001C was cultured in liquid culture, and approximately 5 L of the resulting culture solution was loaded onto a glass fiber filter paper 9 (GF / F, Whatman, width: 460 mm, length: 570 mm) by gravity filtration. At this time, the filtration area (the area on which the microalgae 6 were loaded) was 420 mm wide and 530 mm long. The glass fiber filter paper 9 on which the microalgae 6 were loaded was then placed on the carrier 1 of the culture apparatus 100 so that the width of the filter paper 9 was horizontal.

[0081] The entire culture apparatus 100 was covered with a vinyl sheet 10, and a mixed gas, whose flow rate was adjusted in a gas mixer to a CO2 concentration of 2%, was passed through a gas vent 11 into the interior of the vinyl sheet 10. An LED light source 5 was installed in front of the culture apparatus 100, and the timer was set to 14 hours of light and 10 hours of dark to irradiate the microalgae 6 on the glass fiber filter paper 9 placed on the carrier 1 with light. The photosynthetic photon flux density on the glass fiber filter paper 9 during light irradiation (light period) was approximately 400 μmol / m³ 2 The value was / s. The culture was performed for 2 days under the aforementioned conditions.

[0082] [Biomass collection] After cultivation, the area on the glass fiber filter paper 9 where the microalgae 6 were supported was divided into two sections vertically and horizontally, creating four compartments (width: 210 mm, height: 265 mm). Biomass was recovered in each compartment by processing under the conditions shown in Table 2 below.

[0083] [Table 2]

[0084] In Comparative Example A, no special treatment was performed.

[0085] In Example A, the culture medium supply was stopped and the washing water was added before the recovery process. Specifically, the operation of pump 4 was stopped, and after being left in that state for 3 minutes, 100 mL of ultrapure water (washing water) was gently and evenly added to the surface of the microalgae 6 using an electric pipette. The amount of washing water added per unit area in Example A was 1.8 L / m². 2 That was the case.

[0086] In Examples B and C, the treatment was basically the same as in Example A, but the amount of ultrapure water added was changed to 200 mL in Example B and 400 mL in Example C. The amount of washing water added per unit area was 3.6 L / m in Example B. 2 In Example C, the flow rate was 7.2 L / m³. 2 That was the case.

[0087] Under all conditions, biomass recovery was performed by scraping off microalgae 6 with a spatula.

[0088] [Chlorine concentration measurement] The recovered biomass was analyzed for chlorine concentration both within the algae and in the introduced chlorine, as follows. First, the recovered biomass was suspended in 500 mL of pure water. 10 mL of the suspension was taken and filtered by suction using glass fiber filter paper (GF / F, Whatman, Φ47 mm) whose weight had been measured beforehand. The glass fiber filter paper, now supported with microalgae 6, was dried, and the dry weight of the biomass contained in the suspension was measured from the change in its dry weight. 200 mL was then transferred to a 1 L centrifuge bottle and centrifuged at 25°C and 2,000 × g for 10 minutes. After centrifugation, the chlorine concentrations of the precipitate and supernatant were analyzed by combustion ion chromatography to calculate the chlorine concentrations within the algae and in the introduced chlorine, respectively. Figure 5 shows the chlorine concentrations within the algae and in the introduced chlorine per unit of dry biomass weight under each condition.

[0089] As shown in Figure 5, similar to Test Example 1, the chlorine concentration introduced accounted for the majority of the total chlorine concentration in the biomass. When the total chlorine concentration of the recovered biomass (sum of chlorine concentration in the algae and introduced chlorine) was converted to chlorine concentration per dry weight (g-chlorine / g-dry biomass), it was 0.0381 for Comparative Example A, 0.0282 for Example A, 0.0098 for Example B, and 0.0053 for Example C. Comparing the chlorine concentrations under each condition, a reduction in chlorine concentration was observed in Example A (25.9%), Example B (74.3%), and Example C (86.0%) compared to the untreated biomass of Comparative Example A. From these results, it was shown that the recovery method of this disclosure is also effective when using the culture device 100 in solid-phase surface culture. It was also shown that the greater the amount of washing water added, the greater the desalination effect.

[0090] <3. Test Example 3: Confirmation of Desalination Effect by Recovery Method of the Second Embodiment> The desalting effect of the recovery method of the second embodiment of this disclosure was verified using an actual culture apparatus 100. In the recovery method of the first embodiment, it was confirmed that the salinity of the recovered microalgae could be reduced when the washing water addition step S103 was performed. However, in the first embodiment, the washing water addition step S103 may cause some of the microalgae being cultured on the carrier to detach. Detachment of microalgae from the carrier is a factor that reduces the amount of microalgae obtained by solid-phase surface culture, and is therefore undesirable in some cases. Therefore, in Test Example 3, we verified whether a similar level of desalting effect could be obtained by changing the culture medium without adding washing water, using the recovery method of the second embodiment of this disclosure.

[0091] [culture] A culture apparatus 100 similar to that of Test Example 2 was prepared. The configuration identical to that of the culture apparatus in Test Example 2 will not be explained. A support 2, which has a structure in which a liquid reservoir 15 made of a plate of the same material is connected to the upper end of a plate made of polyvinyl chloride, was placed on a stand at an angle θ of 6 degrees with respect to the horizontal plane. A PVA sponge carrier 1 was placed on the upper surface of the plate-like part of the support 2. A liquid culture medium tank 3 with a capacity of 5L was placed below the support 2 so that the solution flowing from the lower end of the PVA sponge would accumulate in the liquid culture medium tank 3. Next, a self-priming small liquid pump 4 was placed near the liquid culture medium tank 3, and the end of a silicone tube connected to the suction port of the pump 4 was inserted into the liquid culture medium tank 3. The end of a silicone tube connected to the discharge port of the pump 4 was also inserted into the liquid reservoir 15 of the support 2.

[0092] The culture apparatus 100 was assembled in this manner. 2.5 L of artificial seawater was added to the liquid culture medium tank 3.

[0093] The pump 4 was started, and the culture medium 7 was pumped from the liquid culture medium tank 3 to the top of the support 2. At this time, the culture medium 7 supplied to the liquid reservoir 15 was supplied to the carrier 1, flowed downward through the inside of the carrier 1, and finally flowed into the liquid culture medium tank 3. In this way, the culture medium 7 was circulated inside the carrier 1. The flow rate of the pump 4 was adjusted so that the entire surface of the carrier 1 remained moist.

[0094] Next, the microalgae 6 to be cultured were prepared. Strain NS001C was cultured in liquid culture, and approximately 5 L of the resulting culture solution was loaded onto a glass fiber filter paper 9 (GF / F, Whatman, width: 460 mm, length: 570 mm) by gravity filtration. At this time, the filtration area (the area on which the microalgae 6 were loaded) was 420 mm wide and 530 mm long. The glass fiber filter paper 9 on which the microalgae 6 were loaded was then placed on the carrier 1 of the culture apparatus 100 so that the width of the filter paper 9 was horizontal.

[0095] The top of the culture apparatus 100 was covered with a polyvinyl chloride plate, and an LED fluorescent lamp was installed on the top surface of the culture apparatus 100 to irradiate the microalgae 6 on the glass fiber filter paper 9 placed on the carrier 1 with light. The photosynthetic photon flux density on the glass fiber filter paper 9 during light irradiation was approximately 300 μmol / m² / s. Culture was performed for 2 hours under the above conditions.

[0096] [Biomass collection] After cultivation, the area on the glass fiber filter paper 9 where the microalgae 6 were supported was divided horizontally into two sections and vertically into four sections, creating eight compartments (width: 210 mm, height: 13.25 mm). Biomass was recovered from each compartment under the conditions shown in Table 3 below.

[0097] [Table 3]

[0098] In Comparative Example I, the recovery process was carried out without any special treatment. In Comparative Example II, only the culture medium supply stoppage process was performed before the recovery process.

[0099] In Example I, a second culture medium supply process (first culture medium removal process and second culture medium circulation process) and a culture medium supply stop process were performed before the recovery process. Specifically, as the first culture medium removal process, the operation of pump 4 was stopped, the culture medium in tank 3 and liquid reservoir 15 was removed, and the system was left in that state for 15 minutes. Then, as the second culture medium circulation process, 2.5 L of pure water (using a graduated cylinder) was added to tank 3 and liquid reservoir 15 so as not to come into direct contact with the microalgae, the operation of pump 4 was started, and the system was left in a state of circulation of the second culture medium (pure water) for 10 minutes. The culture medium supply stop process in the second culture medium supply process was performed in the same manner as the culture medium supply stop process in the first culture medium removal process.

[0100] In Examples II and III, the same procedures as in Example I were basically followed, but the second culture medium supply step and the culture medium supply stop step were performed twice in Example II and three times in Example III.

[0101] Under all conditions, biomass recovery was performed by scraping off microalgae 6 with a spatula.

[0102] [Chlorine concentration measurement] The collected biomass was placed in a 50 mL centrifuge tube and dried overnight at 105°C. Each dried sample was then ground into a homogenized paste using a mortar and pestle, and the chlorine concentration was measured by combustion ion chromatography. The chlorine concentration per unit of dry biomass weight under each condition is shown in Figure 6.

[0103] Figure 6 shows the total chlorine concentration of the biomass, which is the sum of the chlorine concentration in the introduced culture medium and the chlorine concentration within the algae, under all conditions. The chlorine concentration per unit dry weight (g-chlorine / g-dry biomass) was 0.0379 for Comparative Example I, 0.0368 for Comparative Example II, 0.0197 for Example I, 0.0075 for Example II, and 0.0043 for Example III. Comparing the chlorine concentrations under each condition, first, comparing the untreated biomass of Comparative Example I with the biomass of Comparative Example II, where only the culture medium supply stoppage step was performed before the recovery step, no significant difference was observed between them. On the other hand, comparing the biomass of Comparative Example I with the biomass of Examples I to III, where the second culture medium supply step was performed before the recovery step, a decrease in chlorine concentration was observed compared to the untreated biomass of Comparative Example I: 47.93% for Example I, 80.30% for Example II, and 88.71% for Example III. Comparing Examples I to III, the chlorine concentration decreased as the number of times the second culture medium supply process and the culture medium supply stop process were performed (repeated number of times) increased. Furthermore, comparing the results of Example C in Test Example 2 {total biomass chlorine concentration was 0.0053 (g-chlorine / g-dried biomass)} with the results of Test Example 3, Example III in particular showed that the chlorine concentration was reduced to a level consistent with the desalination effect shown in Example C of Test Example 2. These results indicate that, for microalgae being cultured in solid-phase surface culture, the chlorine concentration in the recovered biomass can be reduced simply by changing the circulating culture medium without performing the washing water addition process. In addition, comparing Examples I to III, it was shown that the desalination effect was greater as the number of times the second culture medium supply process, the culture medium supply stop process, and the second culture medium circulation process were performed increased. In particular, Example III, in which the culture medium supply stop process and the second culture medium circulation process were performed multiple times, showed that a desalination effect comparable to that obtained when washing water was added (Example C in Test Example 2) (the difference was 0.0010, which is practically equivalent).

[0104] As described above, the recovery method according to this embodiment makes it possible to recover seawater or brackish water microalgae cultured by solid-phase surface culture while reducing the chlorine concentration, without compromising the advantage of solid-phase surface culture, which is that it yields biomass with a low water content.

[0105] Although preferred embodiments of the present invention have been described in detail above with reference to the attached drawings, the present invention is not limited to these examples. It is clear to any person with ordinary skill in the art to which the present invention belongs that various modifications or alterations can be conceived within the scope of the technical idea described in the claims, and these are also understood to fall within the technical scope of the present invention.

[0106] (Note) The above embodiment can be understood, for example, as follows:

[0107] <1> A recovery method according to one aspect of the present disclosure is a method for recovering microalgae cultured using a first culture medium supported on the surface of a carrier and containing salt, comprising, in this order, a washing water addition step of adding washing water to the microalgae during culture from the surface side of the carrier, and a recovery step of recovering the microalgae, wherein the salt concentration of the washing water added in the washing water addition step is lower than the salt concentration of the first culture medium.

[0108] <2> the above <1> The recovery method may include a culture medium supply stop step, in which the supply of the first culture medium is stopped, before the washing water addition step.

[0109] <3> the above <1> The recovery method includes a second medium supply step in which the first medium is changed to a second medium and supplied before the washing water addition step, and the salt concentration of the second medium may be lower than that of the first medium.

[0110] <4> the above <3> The recovery method may include a medium supply stop step after the second medium supply step, in which the supply of the second medium is stopped.

[0111] <5> the above <3> or <4> In the recovery method, the salinity of the washing water may be less than or equal to the salinity of the second culture medium.

[0112] <6> A recovery method according to one aspect of the present disclosure is a method for recovering microalgae supported on the surface of a carrier and cultured in a first medium containing salt, comprising: a second medium supply step of supplying the first medium as a second medium; and a recovery step of recovering the microalgae, wherein the salt concentration of the second medium is lower than that of the first medium.

[0113] <7> the above <6> In the recovery method, the second culture medium supply step may include a first culture medium removal step of removing the first culture medium held by the carrier, and a second culture medium circulation step of permeating the carrier with the second culture medium after the first culture medium removal step.

[0114] <8> the above <6> or <7> The recovery method may include a medium supply stop step after the second medium supply step, in which the supply of the second medium is stopped.

[0115] <9> the above <8> The recovery method includes a second medium circulation step in which the second medium is permeated into the carrier, and the second medium circulation step and the medium supply stop step may be performed multiple times. [Explanation of symbols]

[0116] 1 Carrier 2 Support 3. Liquid culture medium tank 4 pumps 5 light source 6 Microalgae 7 Culture medium 8. Sprinkler system 9. Glass fiber filter paper 10. Plastic sheet 11 Gas vent 12 Slope 13 Support surface 14 Support installation surface 15 Liquid reservoir 100 Culture equipment S101 Second culture medium supply process S102 Culture medium supply termination process S103 Washing water addition process S104 Recovery Process

Claims

1. A method for recovering microalgae cultured on the surface of a carrier using a first culture medium containing salt, A washing water addition step is performed in which washing water is added to the microalgae during cultivation from the surface side of the carrier, A recovery step for recovering the aforementioned microalgae, These are provided in this order, The salt concentration of the washing water added in the washing water addition step is lower than the salt concentration of the first culture medium. A method for recovering microalgae characterized by the following features.

2. The process includes a medium supply stop step, which stops the supply of the first medium, before the washing water addition step. The method for recovering microalgae according to feature 1.

3. Prior to the washing water addition step, the process includes a second medium supply step in which the first medium is changed to a second medium and supplied. The salt concentration of the second medium is lower than that of the first medium. The method for recovering microalgae according to feature 1.

4. The second medium supply step includes a medium supply stop step in which the supply of the second medium is stopped, The method for recovering microalgae according to feature 3.

5. The salinity of the washing water is less than or equal to the salinity of the second culture medium. A method for recovering microalgae according to feature 3 or 4.

6. A method for recovering microalgae that have been supported on the surface of a carrier and cultured using a first medium containing salt, A second medium supply step in which the first medium is changed to a second medium and supplied, A recovery step for recovering the aforementioned microalgae, Includes, A method for recovering microalgae, wherein the salinity of the second culture medium is lower than that of the first culture medium.

7. The second culture medium supply step is, A first medium removal step of removing the first medium held by the carrier, After the first culture medium removal step, a second culture medium circulation step is performed in which the second culture medium is permeated into the carrier, A method for recovering microalgae according to claim 6, comprising:

8. The second medium supply step includes a medium supply stop step in which the supply of the second medium is stopped, A method for recovering microalgae according to claim 6 or 7, characterized in that

9. The process includes a second culture medium circulation step in which the second culture medium is permeated into the carrier, The method for recovering microalgae according to claim 8, wherein the second culture medium circulation step and the culture medium supply cessation step are performed multiple times.

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