Water treatment method and water treatment system

Abstract

To provide a technology that achieves both stabilization of carbon dioxide and recovery of microplastics.SOLUTION: An aspect of the invention is a water treatment method that recovers microplastics from water to be treated containing the microplastics and carbon dioxide and stabilizes the carbon dioxide. The water treatment method makes algae having capacity of adsorbing and recovering the microplastics present in the water to be treated, and includes a step for recovering the microplastics from the water to be treated and stabilizing the carbon dioxide in the algae from the water to be treated. The algae secrete an adhesive substance. As for the amount of an adhesive substance secreted by the algae, the adhesive substance secreted to the outside of cells has a volume of 0.25 times or more and 100 times or less relative to the cell volume.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a water treatment method and a water treatment system. [Background technology]

[0002] In recent years, as part of measures to combat global warming, research has been progressing on technologies for fixing carbon dioxide, which is considered a cause of global warming. For example, Patent Document 1 discloses a technology for fixing carbon dioxide by carbonate chlorination.

[0003] On the other hand, in recent years, the environmental impact of microplastics, which are generated when plastics break down, has become a problem. Therefore, there is a need for technologies to recover microplastics from seawater and freshwater. One technology for recovering microplastics is the removal of microplastics using microalgae (Non-Patent Document 1). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2021-070615 [Non-patent literature]

[0005] [Non-Patent Document 1] Convertec, July 2020 issue, pages 2-5 [Overview of the project] [Problems that the invention aims to solve]

[0006] Conventional technologies cannot simultaneously sequester carbon dioxide and collect microplastics. This invention addresses the above-mentioned problems and aims to provide a technology that can achieve both carbon dioxide fixation and microplastic recovery. [Means for solving the problem]

[0007] One aspect of the present invention is a water treatment method. This water treatment method recovers microplastics and fixes carbon dioxide from water to be treated that contains microplastics and carbon dioxide, and comprises the steps of: placing algae having the ability to adsorb and recover microplastics in the water to be treated; recovering the microplastics from the water to be treated; and fixing carbon from the water to be treated into the algae, wherein the algae are algae that secrete an adhesive substance, and the amount of adhesive substance secreted by the algae is such that the volume of adhesive substance secreted outside the cell is 0.25 times or more and 100 times or less compared to the cell volume. The water treatment method according to the above embodiment may include a step of supplying carbon dioxide to the water to be treated. The water to be treated may be industrial wastewater. The sticky substance may be a polysaccharide. The algae may be at least one selected from diatoms, dinoflagellates, chlorarachnion algae, green algae, red algae, conjugating algae, Euglena algae, and cyanobacteria. The method may also include a step of recovering the algae used for recovering microplastics and fixing carbon dioxide, and a step of replenishing with new algae. The timing of recovering the algae may be set according to the growth stage of the algae.

[0008] Another aspect of the present invention is a water treatment system. This water treatment system recovers microplastics and fixes carbon dioxide from water to be treated that contains microplastics and carbon dioxide, wherein algae having the ability to adsorb and recover microplastics are present in the water to be treated, the microplastics are recovered from the water to be treated, and carbon dioxide is fixed from the water to be treated into the algae, and the algae are algae that secrete an adhesive substance, and the amount of adhesive substance secreted by the algae is such that the volume of adhesive substance secreted outside the cell is 0.25 times or more and 100 times or less compared to the cell volume. In the water treatment system of the above aspect, a step of supplying carbon dioxide to the water to be treated may be provided. The water to be treated may be industrial wastewater. The adhesive substance may be a polysaccharide. The algae may be at least one selected from diatoms, euglenoids, Chlorarachnion algae, green algae, red algae, conjugating algae, Euglena algae, and cyanobacteria. The water treatment system may include means for recovering the algae used for the recovery of microplastics and the immobilization of carbon dioxide, and means for replenishing new algae. The timing of recovering the algae may be set according to the degree of growth of the algae.

Advantages of the Invention

[0009] According to the present invention, it is possible to provide a technology that can achieve both carbon dioxide fixation and microplastic recovery.

Brief Description of the Drawings

[0010] [Figure 1] FIG. 1 is a block diagram showing a schematic configuration of a water treatment system according to an embodiment. [Figure 2] FIG. 2 is a conceptual diagram showing a state in which algae attached with microplastics are removed by a first filter. [Figure 3] FIG. 3 is a flowchart showing processes related to the recovery and replenishment of algae. [Figure 4] FIG. 4 is a diagram showing a measurement procedure for the amount of an adhesive substance secreted by algae. [Figure 5] FIG. 5 is a magnified photograph of various algae used in the examples.

Modes for Carrying Out the Invention

[0011] Hereinafter, embodiments of the present invention will be described in detail. In this specification, the notation "a~b" in the description of a numerical range represents a to b inclusive, unless otherwise specified.

[0012] [Definition of Terms] <Water to be Treated>[ In this specification, the water to be treated is water such as seawater, freshwater, or brackish water in which carbon dioxide is dissolved and microplastics are present or potentially present. Specific examples of the water to be treated include industrial water, treated wastewater, domestic wastewater, and agricultural wastewater.

[0013] <Microplastics> In this specification, "microplastics" refers to particles with a maximum length of 0.1 μm or more and 5000 μm or less. However, the plastics present (or potentially present) in the water to be treated may include not only microplastics but also plastic particles smaller than 0.1 μm or larger than 5000 μm. Furthermore, in reality, the majority of microplastics (for example, 80% or more, 90% or more, and 95% or more of the total number of particles) are, for example, 0.1 μm or larger, 0.5 μm or larger, 1 μm or larger, 2 μm or larger, 3 μm or larger, 4 μm or larger, 5 μm or larger, 6 μm or larger, 7 μm or larger, 10 μm or larger, 50 μm or larger, 100 μm or larger, 500 μm or larger, 1000 μm or larger, 2500 μm or larger; 2500 μm or smaller, 1000 μm or smaller, 500 μm or smaller, 100 μm or smaller, 50 μm or smaller, 10 μm or smaller, 9 μm or smaller, 8 μm or smaller, 7 μm or smaller, 6 μm or smaller, 5 μm or smaller, 4 μm or smaller, and 3 μm or smaller (maximum length portion). As is well known, microplastics include primary microplastics (plastics manufactured at a micro size: for example, used in facial cleansers, fabric softeners, and capsules for slow-release fertilizers) and secondary microplastics (larger plastics that have been broken down into micro-size particles in the natural environment).

[0014] <Algae with microplastic adsorption and recovery capabilities> In this specification, "algae having microplastic adsorption and recovery ability" refers to algae that can reduce the microplastic concentration in treated water by a predetermined amount (e.g., 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95%) or more compared to the microplastic concentration in treated water without algae. Here, examples of algae that secrete sticky substances include diatoms and brown algae belonging to the Stramenopiles, dinoflagellates belonging to the Alveolata, Chlorarachnion algae belonging to the Rhizaria, green algae and red algae belonging to the Archaeoplastida, conjugating algae, Euglena algae belonging to the Excavata, and cyanobacteria belonging to the Bacteria. Here, microalgae are known to release various viscous substances extracellularly. The adhesive substances are typically polysaccharides, such as agarose and porphyran in red algae like tengusa and conjugating algae, and alginic acid and fucose-containing polysaccharides in brown algae like kelp. Incidentally, among the many types of diatoms, Skeletonma tropicum is particularly suitable. Furthermore, cyanobacteria and green algae are suitable because of their excellent growth rate. In addition, Euglena is suitable because it has flagella and can actively adsorb microplastics.

[0015] Other types of algae include those with physical structures that capture microplastics (e.g., porous structures, uneven structures) (e.g., brown algae); and algae that are charged with the opposite charge to microplastics. For example, microalgae vary in shape and size, but some are porous algae with a large surface area, and others form filamentous colonies. These structures also have the function of trapping microplastics. Unlike algae with sticky substances, these types of algae are less susceptible to structural changes due to environmental changes, and are therefore superior in that they can stably retain the captured microplastics.

[0016] For removing microplastics, one type of algae selected from the above-mentioned types may be used, or a combination of two or more specific types of algae may be used. For example, by combining diatoms with algae that grow faster than those diatoms (e.g., cyanobacteria, green algae), a sufficient microplastic adsorption effect can be obtained even in the early stages of cultivation. Furthermore, by combining diatoms with algae larger in size (for example, dinoflagellates and brown algae), it is possible to obtain sufficient adsorption effects for larger microplastics that are difficult to adsorb by diatoms alone.

[0017] Here, the size of the algae is not particularly limited. However, given that the size of the microplastics to be treated is between 0.1 μm and 5000 μm, it is preferable that the algae be 5000 μm or larger (for example, the size of the algae in groups or clusters). However, the size of the algae may be made to depend on the size of the microplastics present in the treated water. In this case, the expected sizes of the algae are, for example, 0.1 μm or larger, 1 μm or larger, 2 μm or larger, 5 μm or larger, 10 μm or larger, 50 μm or larger, 1000 μm or larger, 2500 μm or larger, 500 μm or smaller, 1000 μm or smaller, 2500 μm or larger, 500 μm or smaller, 250 μm or smaller, 100 μm or smaller, 50 μm or smaller, 25 μm or smaller, 20 μm or smaller, 10 μm or smaller, 5 μm or smaller, and 1 μm or smaller. Here, "size" refers to the largest diameter portion (for example, the longest diameter portion in the case of rod-shaped algae). Although algae of various sizes exist in the system, "size" here refers to the average size of 100 randomly selected algae.

[0018] The amount of adhesive substance secreted by algae is preferably such that the volume of adhesive substance secreted outside the cell is between 0.25 and 100 times the cell volume. Within this range, it is possible to provide a means for the stable recovery of microplastics over a long period of time. The method for measuring the volume of adhesive substance is as follows: Add 10 μL of microalgae culture solution to a glass slide. Add 10 μL of India ink diluted 5 times, mix the India ink and microalgae culture solution thoroughly, cover with a coverslip, and measure the cell volume of the microalgae and the volume of extracellular mucilage under a microscopic environment. Following the method of Kishimoto et al. {Kishimoto N., Ichise S., Suzuki K., Yamamoto C.: Analysis of long-term variation in phytoplankton biovolume in the northern basin of Lake Biwa. Limnology 14: 117-128(2013)}, each alga was approximated by an elliptical cylinder, ellipse, rectangular prism, or a combination thereof, and the cell volume was calculated. For the volume of extracellular mucilage, the volume including the portion not stained with India ink was calculated, and the volume of extracellular mucilage was determined by dividing this by the cell volume. Figure 4 shows the above procedure. In addition, although algae of various sizes exist in the system, the "quantity" referred to here is the average value of the volumes of 100 randomly obtained algae.

[0019] The cell growth rate of the algae used for microplastic removal is preferably 250% or more, more preferably 300% or more, and even more preferably 400% or more. Because the cell growth rate of the algae is within the above range, adhesive substances are secreted rapidly after culturing, allowing the microplastic adsorption effect to be exerted early after culturing. Here, the cell growth rate is calculated according to the following conditions and formula. <Culture conditions> Algae were cultured in 200 ml of culture medium (f / 2, however, sodium nitrate was changed to 750 mg / L, which is 10 times the normal concentration). Table 1 shows the components of culture medium (f / 2). Table 2 shows the components of f / 2 metals contained in culture medium (f / 2).

[0020] [Table 1]

[0021] [Table 2]

[0022] (Cell growth rate) Each algae is cultured using the culture medium described above. The cell count before culture is, for example, 5,000 to 20,000 cells / ml, and typically 10,000 cells / ml. The absorbance at a wavelength of 490 nm is measured using a UV-Vis spectrophotometer 6 hours and 3 days after the start of culture, respectively. The cell growth rate is calculated according to the following formula. Cell growth rate = (absorbance measured after 3 days) / (absorbance measured after 6 hours) × 100 Table 3 shows the cell growth rates obtained for eight types of algae.

[0023] [Table 3]

[0024] (Water treatment system) Figure 1 is a diagram showing an overview of a water treatment system 10 according to the first embodiment. As shown in Figure 1, the water treatment system 10 is an example of an industrial wastewater treatment system that includes a water treatment tank 30.

[0025] An inlet pipe 40 and a discharge pipe 50 are connected to the water treatment tank 30. The system is configured so that water to be treated (for example, industrial wastewater discharged from factories or businesses) containing microplastics and carbon dioxide flows into the water treatment tank 30 through the inlet pipe 40. The inlet pipe 40 is equipped with a flow control pump 42. The flow control pump 42 controls the flow velocity of the water to be treated flowing through the inlet pipe 40.

[0026] In the water treatment tank 30, algae with the ability to adsorb and recover microplastics grow in the water to be treated, which is introduced from outside the system. These algae recover microplastics contained in the water to be treated, and carbon dioxide contained in the water to be fixed.

[0027] Specifically, microplastics contained in the treated water are adsorbed onto the adsorbent substances secreted by the algae mentioned above. By removing the algae to which the microplastics have been adsorbed using a removal method such as a filter, the microplastics are removed from the treated water. Furthermore, carbon dioxide contained in the treated water is absorbed and fixed by algae during photosynthesis. Algae grow through a growth process that involves absorbing carbon dioxide, synthesizing organic matter, and increasing the number of cells. By removing the algae that have fixed carbon dioxide, carbon dioxide is removed from the treated water.

[0028] The water treatment tank 30 is configured to allow the addition of algae stored in the algae reserve tank 60 as needed. The algae stored in the algae reserve tank 60 are algae that have not yet been used for microplastic recovery and carbon dioxide fixation, and it is preferable that they are algae that have just germinated or are in the early stages of growth.

[0029] The water treatment tank 30 is equipped with various mechanisms to ensure the environment necessary for algae growth, as described below.

[0030] <Mechanism for irradiating algae with light> The water treatment tank 30 is configured to irradiate the algae inside with light. The light irradiated onto the algae inside the water treatment tank 30 is not limited to sunlight, but may also be artificial lighting such as LEDs, fluorescent lamps, or incandescent lamps that include light with wavelengths suitable for algae growth. When using artificial lighting, the light may be continuously (24 hours), but the irradiation time may also be set appropriately throughout the day (for example, 10-12 hours) to match the growth and resting periods of the algae. By adjusting the irradiation time of artificial lighting, the growth of algae can be further promoted depending on the type of algae. Furthermore, the artificial lighting is not limited to being installed above the water treatment tank 30, but may also be installed inside the water treatment tank 30. By installing the artificial lighting inside the water treatment tank 30, more algae inside the water treatment tank 30 can be illuminated compared to when the artificial lighting is installed outside the water treatment tank 30. As a result, algal growth in the water treatment tank 30 is further promoted, and microplastic recovery and carbon dioxide fixation can be performed more efficiently.

[0031] The water treatment tank 30 is equipped with a stirring mechanism 32. By operating the stirring mechanism 32, the algae and the water to be treated in the water treatment tank 30 are stirred, and the algae are dispersed throughout the water to be treated in the water treatment tank 30. Specific examples of stirring by the stirring mechanism 32 include stirring algae and the water to be treated with a water flow generated by a pump, propeller, or stirring bar (for example, a magnetic stirrer); stirring algae and the water to be treated with a gas such as air or carbon dioxide; and stirring algae and the water to be treated using a shaker that shakes the entire water treatment tank 30.

[0032] <Mechanism for supplying carbon dioxide to treated water in a water treatment tank> The water treatment tank 30 is equipped with a gas inlet pipe 36 for supplying carbon dioxide to the water to be treated inside the tank. A gas flow rate control pump 38 is provided on the gas inlet pipe 36. The gas flow rate control pump 38 supplies carbon dioxide or a gas containing carbon dioxide (for example, air) to the water to be treated contained in the water treatment tank 30. By supplying air to the water to be treated contained in the water treatment tank 30, the amount of sticky substance secreted by algae in the water treatment tank 30 increases, and consequently, the amount of microplastics adsorbed and recovered by the algae can be increased. It is preferable to install the discharge port of the gas inlet pipe 36 at the bottom of the water treatment tank 30. This allows for the bubbling of carbon dioxide discharged from the gas inlet pipe 36, thereby increasing the carbon dioxide concentration of the water to be treated, while simultaneously agitating the algae and the water to be treated without requiring a dedicated stirring mechanism. The supply of carbon dioxide to the water to be treated may be carried out continuously to saturate the carbon dioxide concentration in the water to be treated. Alternatively, as shown in Figure 1, a gas sensor 34 capable of measuring the carbon dioxide concentration in the water to be treated in the water treatment tank 30 may be provided, and the gas flow rate discharged from the gas introduction pipe 36 may be adjusted using a gas flow rate adjustment pump 38 according to the carbon dioxide concentration measured by the gas sensor 34.

[0033] Next, preferred recovery conditions for a method in which microplastics are recovered from treated water containing microplastics and carbon dioxide in the water treatment tank 30, and carbon dioxide is fixed.

[0034] The optimal algal concentration in the water treatment tank 30 varies depending on the microplastic concentration and size in the treated water, the carbon dioxide concentration in the treated water, and the type of algae used. This condition can be determined, for example, by performing the model experiment described in the example.

[0035] The optimal recovery time within the water treatment tank 30 varies depending on the microplastic concentration and size in the treated water, the carbon dioxide concentration in the treated water, the type of algae used, and the target reduction levels for microplastic concentration and carbon dioxide concentration. These conditions can be determined, for example, by performing the model experiment described in the example.

[0036] The system is configured such that microplastics used for microplastic recovery and carbon dioxide fixation in the water treatment tank 30 are recovered through the discharge pipe 50, and treated water from which carbon dioxide has been removed is discharged outside the system. The discharge pipe 50 is equipped with a first filter 52a and a second filter 52b.

[0037] The first filter 52a is responsible for removing algae to which microplastics are attached from the water to be treated. The mesh opening or pore size of the first filter 52a is not particularly limited as long as it can remove algae to which microplastics are attached, but for example it is 5 μm. Figure 2 is a conceptual diagram showing how algae with microplastics attached are removed by the first filter 52a. As shown in Figure 2, by using the first filter 52a with an adjusted mesh opening or pore size, algae with microplastics attached are separated from algae without microplastics attached by substances secreted by the algae. By passing the water to be treated through the first filter 52a, primary treated water from which microplastics have been removed is obtained.

[0038] The second filter 52b is responsible for removing algae that have passed through the first filter 52a. The mesh size or pore size of the second filter 52b depends on the type of algae used, but is, for example, 5 μm. The second filter 52b separates the algae that have fixed carbon dioxide from the primary treated water. In other words, by passing the primary treated water through the second filter 52b, secondary treated water is obtained from which the algae that have fixed carbon dioxide have been removed.

[0039] Filters used in the first filter 52a and the second filter 52b include chemical fiber filters, natural fiber filters, metal filters such as metal meshes, and filters in the form of threads or paper.

[0040] <Agitation control> The stirring mechanism 32 may be used to continuously stir the algae and the water to be treated in the water treatment tank 30 without stopping during the algae growth process. However, as described below, stirring may also be performed intermittently in accordance with the growth and condition of the algae.

[0041] <<Stirring Control Method 1>> A flow sensor (not shown) is provided to measure the flow rate of the water to be treated introduced from the inlet pipe 40, and the stirring speed of the stirring mechanism 32 is changed according to the flow rate measured by the flow sensor. In this case, the stirring speed may be made proportional to the flow rate. Alternatively, the stirring speed may be set in steps according to the flow rate.

[0042] <<Stirring control method 2>> A camera (not shown) is used to monitor the dispersion of algae in the water treatment tank 30. Based on the obtained image, it is determined whether or not algae have settled at the bottom of the water treatment tank 30. If it is determined that algae have settled at the bottom of the water treatment tank 30, the stirring speed is increased.

[0043] <Algae harvesting and replenishment> The timing for collecting microplastics and replenishing algae used for carbon dioxide sequestration may be determined based on the algal growth stage or the filter's capacity.

[0044] <<Algae removal timing 1>> A camera (not shown) is used to image the state of algae in the water treatment tank 30. The camera may also be installed above the water treatment tank 30 to image the algae in the water being treated. Alternatively, the camera may be installed inside the water being treated in the water treatment tank 30 to image the algae in the water being treated.

[0045] The percentage of the area occupied by algae per unit area in the captured image is calculated. If the percentage of algae exceeds a certain threshold, it is determined that the algae have grown sufficiently, and the first filter 52a and the second filter 52b are replaced. This allows for the removal of algae from the water treatment tank 30 that have reduced their ability to recover microplastics and fix carbon dioxide.

[0046] In addition, imaging may be performed using a camera at predetermined intervals, and an algal growth curve may be drawn from the proportion of the area occupied by algae per unit area in the captured images. When the cell growth reaches its final stage (for example, when the growth logarithmic curve reaches a plateau), it may be determined that the algae have grown sufficiently, and the first filter 52a and the second filter 52b may be replaced. This makes it possible to remove algae from the water treatment tank 30 that have lost their ability to recover microplastics and fix carbon dioxide.

[0047] <<Algae removal timing 2>> An absorbance meter (not shown) may be used to measure the turbidity of the water to be treated in the water treatment tank 30 as an indicator of algae density. If the measured turbidity is above the standard value, it is determined that the algae have grown sufficiently, and the first filter 52a and the second filter 52b are replaced. This makes it possible to remove algae from the water treatment tank 30 that have reduced ability to recover microplastics and fix carbon dioxide.

[0048] <<Algae removal timing 3>> If fatty acids (oils) are produced as algae grow, the concentration of fatty acids in the treated water in the water treatment tank 30 may be measured. If the measured concentration is above the standard value, it is determined that the algae have grown sufficiently, and the first filter 52a and the second filter 52b are replaced. This allows for the removal of algae from the water treatment tank 30 that have reduced microplastic recovery and carbon dioxide fixation capabilities.

[0049] <<Algae removal timing 4>> If the color of algae changes as they grow, the degree of growth can be determined by the color of the algae. Specifically, as described above, the state of the algae in the water treatment tank 30 is imaged using a camera, and it is determined whether the color of the imaged algae has changed from the color of the growing stage (proliferation stage) (for example, green) to the color of the final stage of growth (for example, brownish-red). When it is determined that the algae have reached the final stage of growth, the first filter 52a and the second filter 52b are replaced. This makes it possible to remove algae from the water treatment tank 30 that have lost their ability to recover microplastics and fix carbon dioxide.

[0050] <<Algae removal timing 5>> Algae can be recovered by replacing the first filter 52a and the second filter 52b when the velocity of the water flowing through the inlet pipe 40 or the outlet pipe 50 falls below a predetermined value. This allows for the recovery of microplastics and the fixation of carbon dioxide while maintaining the filtering efficiency of the first filter 52a and the second filter 52b above a certain level.

[0051] <Algae supplementation> It is preferable to remove algae that have grown to a certain extent at a predetermined timing, and then replenish them from the algae reserve tank 60 with algae that are in the early stages of germination or growth, in an amount equivalent to the number of algae that have been removed. This makes it possible to restore the efficiency of microplastic recovery and carbon dioxide fixation when that efficiency gradually decreases as the algae grow.

[0052] Figure 3 is a flowchart showing the process for collecting and replenishing algae. As shown in Figure 3, first, a flow control pump 42 installed in the inlet pipe 40 is started, and the water to be treated is introduced into the water treatment tank 30 from outside the system (S10). In the water treatment tank 30 into which the water to be treated has been introduced, carbon dioxide is fixed by photosynthesis of algae and microplastics are collected by the sticky substance secreted by the algae (S20). Subsequently, it is determined whether or not it is necessary to stop the system, such as when a predetermined time has elapsed since the start of operation (S30). If it is necessary to stop the system (yes in S30), the flow control pump 42 installed in the inlet pipe 40 is stopped. On the other hand, if it is necessary to continue the operation of the system (no in S30), the degree of algae growth is determined based on the image captured by the camera. The determination of the degree of algae growth may be based on the ratio of the area occupied by algae per unit area in the captured image, as described above, or it may be based on the color of the captured algae. If it is determined that algae recovery is necessary based on the determined growth stage of the algae (yes in S50), the algae are recovered by replacing the filter 52a, and then new algae equivalent to the recovered algae are replenished in the water treatment tank 30 (S60), and the process returns to S20. On the other hand, if it is determined that algae recovery is not necessary (no in S50), the process returns to S20.

[0053] ≪Algae composition≫ The algae used in this embodiment may be an algal composition. Specifically, it may be a group of the same or different species of algae. Here, it is preferable that the group of algae is stored in a container or the like in a state in which the algae can survive (for example, in a liquid culture medium). Furthermore, algae that can survive even after freeze-drying may be handled in a dried form. In addition, the composition may contain components other than algae as needed.

[0054] <Temperature management> If the temperature of the water to be treated introduced from outside the system differs from the optimal growth temperature of the algae cultured in the water treatment tank 30, the temperature of the water to be treated flowing into the water treatment tank 30 from outside the system may be adjusted. Specifically, a temperature sensor is installed to measure the temperature of the water to be treated introduced from outside the system. Based on the temperature measured by the temperature sensor, the water to be treated is heated or cooled to a desired temperature suitable for algal growth. Heating methods include heat exchange with solar energy, geothermal energy, surplus waste heat from incinerators, and heating with heaters. On the other hand, cooling methods include heat exchange with environmental water such as rivers, which does not involve energy or electricity consumption, or cooling using energy and electricity (for example, heat exchange with a refrigerant).

[0055] <Regulation of nutrients, pH, etc.> Additives that promote algal growth, such as minerals and vitamins, including nutrients like phosphorus, nitrogen, potassium, and sodium silicate, may be appropriately added to the water to be treated in the water treatment tank 30. The timing for adding each additive can be determined by measuring the concentration of each additive and adding it when the concentration falls below a predetermined value. By appropriately adding the above additives to the water to be treated, the growth of algae in the water treatment tank can be further promoted, and consequently, the efficiency of microplastic recovery and carbon dioxide fixation can be improved.

[0056] Furthermore, an acid (e.g., acetic acid) or alkali (e.g., sodium hydroxide) may be added to the treated water in the water treatment tank 30 as appropriate to adjust the pH. Specifically, the pH of the treated water may be measured, and an appropriate amount of acid or alkali may be added so that the measured pH value falls within a predetermined range, for example, 5 to 9. As a result, algal growth is further promoted in the water treatment tank 30, and microplastic recovery and carbon dioxide fixation can be performed more efficiently.

[0057] The embodiments of the present invention have been described above, but these are merely examples, and various other configurations can also be adopted.

[0058] <Methods for collecting algae> In the embodiment described above, algae are collected using a filter, but the means of collecting algae are not limited to this. By utilizing the difference in specific gravity between the water to be treated and the algae, lighter algae may be recovered from the upper part of the water to be treated. Alternatively, heavier algae may be recovered from the lower part of the water to be treated. Alternatively, the treated water in the water treatment tank 30 may be passed through a bag-shaped net to capture algae with microplastics attached, as well as algae that have grown sufficiently large in size, contained in the treated water in the water treatment tank 30. Alternatively, algae in the water treatment tank 30 may be scraped out using a dip net or similar tool.

[0059] <Water to be treated> In the embodiment described above, industrial wastewater is given as an example of the water to be treated in the water treatment tank 30, but the water to be treated is not limited to this, and treated sewage, domestic wastewater, agricultural wastewater, wastewater from waste treatment plants, wastewater from power plants, etc., may also be used as the water to be treated.

[0060] <Water treatment other than carbon dioxide fixation (water purification)> In the embodiments described above, carbon dioxide in the treated water is fixed during photosynthesis by algae, and carbon dioxide is removed from the treated water. However, there are no limitations on the compounds that can be taken up by the algal cells and removed from the treated water. For example, it is possible to remove heavy metals such as cadmium, cobalt, nickel, copper, zinc, and manganese, radioactive substances such as radioactive cesium, radioactive strontium, and radioactive iodine, phosphorus compounds such as reduced phosphorus compounds and phosphate ester compounds, potassium compounds such as potassium chloride, potassium sulfate, and potassium nitrate, and nitrogen compounds such as ammonia nitrogen, nitrite nitrogen, and nitrate nitrogen from the treated water to perform water treatment (water purification).

[0061] <Generation of useful substances> In this embodiment, not only are microplastics recovered and carbon dioxide fixed, but useful substances are also generated. Useful substances generated as algae grow include astaxanthin, beta-carotene, lutein, DHA, EPA, paramylon, wax esters, hydrogen, biodiesel, bioethanol, crostanin, and squalene. By separating and purifying these useful substances, they can be applied to various uses.

[0062] One aspect of the present invention is a water treatment method for recovering microplastics and carbon dioxide from water to be treated that contains microplastics. The water treatment method comprises the steps of placing algae having the ability to adsorb and recover microplastics in the water to be treated, recovering the microplastics from the water to be treated, and fixing carbon dioxide from the water to be treated within the algae. In the above embodiment, the process may include a step of supplying carbon dioxide to the water to be treated. The water to be treated may be industrial wastewater. The algae may be algae that secrete an adhesive substance. The adhesive substance may be a polysaccharide. The algae may be at least one selected from diatoms, brown algae, dinoflagellates, chlorarachnion algae, green algae, red algae, conjugating algae, Euglena algae, and cyanobacteria. The process may also include a step of recovering the algae used to purify the water to be treated, and a step of replenishing with new algae. Furthermore, the timing of recovering the algae may be set according to the growth stage of the algae. [Examples]

[0063] The present invention will be described below with reference to examples and comparative examples, but the present invention is not limited thereto.

[0064] <<Algae Cultivation 1>> The algae used in the experiment (see Table 4) were cultured in 1 L scale. Turbidity was measured and recorded using a turbidimeter (CO8000 Biowave). For example, for algae of the genus Skeletonema or those approximately 5-10 μm in size, 7000 cells / ml was used as a guideline. If the number of algal cells was greater than 7000 cells / ml, the culture was diluted with culture medium. Conversely, if the number was less, the supernatant was removed after 2-3 hours to adjust the concentration. Then, 19.648 ml of well-suspended culture solution was placed in 70 ml cell culture flasks (3 flasks were prepared). In addition, 19.648 ml of culture medium was placed in new 70 ml cell culture flasks as a control {4 flasks were prepared (one of which was used for calibration curve creation)}. Furthermore, approximately 20 ml of well-suspended culture solution was prepared for baseline correction when measuring absorbance with a UV-Vis spectrophotometer. Next, add 352 μl of 2 μm bead solution (5.68 × 10) to a 70 ml cell culture flask containing the culture medium. 8 Beads ( / ml) were added. The mixture was then mixed by pipetting and incubated in a 20°C artificial climate chamber for 1 day. Figure 5 shows magnified photographs of the various algae used. In the figure, the dotted line represents the cell surface, and the solid line represents the interface of the adhesive substance. Table 5 shows the amount of adhesive component calculated using the method described in the general description.

[0065] [Table 4]

[0066] [Table 5]

[0067] <<Microplastic Recovery Test>> Flasks obtained in the above-mentioned "Cultivation of Algae" {The above algae were cultured in these flasks, with a final concentration of 1 × 10 7The flask containing the bead solution (2 μm beads / ml) was removed from the artificial climate chamber without agitation. At this point, a precipitate was observed, as shown in Figure 9. The solution was then suspended by swirling and pipetting. Next, 50 μm cell strainers (pluriStrainer 50 μm) were attached to 50 ml tubes using connector rings and labeled (one for each cell culture flask except those used for the calibration curve). All culture flasks were then filtered under reduced pressure using a syringe through each cell strainer (the calibration curve samples were not filtered). The filtered samples were then capped and stored on the lab bench. A calibration curve was then created, and a bead dilution series was prepared for the calibration curve to estimate the bead recovery rate by algae. Specifically, the culture medium without added beads was set as bead concentration 0, and a stock concentration of 1.00 × 10⁻⁶ was used. 7 Repeated 1 / 2 dilution from beads / ml to 3.13 × 10 5 beads / ml, 6.25 x 10 5 beads / ml, 1.25 x 10 6 beads / ml, 2.50 x 10 6 beads / ml, 5.00 x 10 6 A solution of beads / ml was prepared.

[0068] <<Microplastic Recovery Measurement Test>> Absorbance measurements were performed at 267 nm, the fluorescence wavelength of the beads, using a BioSpec-Mini UV-Vis spectrophotometer (Shimadzu Corporation). To estimate the recovery rate of the algae beads, a beads dilution series was measured for a calibration curve, and a linear regression equation was obtained. Absorbance measurements were then performed on the permeate of a 50 μm cell strainer containing algae culture medium + beads solution as the sample, and the bead concentration in the permeate was calculated from the linear regression equation obtained from the calibration curve. The same procedure was repeated for the permeate of a 50 μm cell strainer containing non-cultured medium + beads solution as the control, and the bead concentration in the permeate was calculated from the linear regression equation obtained from the calibration curve. Finally, the bead recovery rate was calculated from the bead concentrations in the sample and the control. The results are shown in Table 6.

[0069]

Table 6

[0070] ≪Algal Culture 2≫ Using Skeletonema tropicum (with a target of 7000 cells / ml), in 200 ml of the following culture medium, under air addition conditions and CO2 addition (CO2 saturation concentration) conditions, respectively, it was placed in a 20°C artificial weather chamber and statically cultured for 3 days (n = 2). (Culture medium) 196.48 ml of medium Medium components 1 L of f / 2 medium (NaNO3: 75 mg, NaH2PO4: 6 mg, Vitamin B 12 : 0.5 μg, Biotin: 0.5 μg, Thiamine HCl: 100 μg, Na2SiO3·9H2O: 10 mg, f / 2 metals 1 ml (Na2EDTA 2H2O: 440 mg, FeCl3·6H2O: 316 mg, CoSO4·7H2O: 1.2 mg, ZnSO4·7H2O: 2.1 mg, MnCl2·4H2O: 18 mg, CuSO4·5H2O: 0.7 mg, Na2MoO4·2H2O: 0.7 mg / distilled water)) 3.52 ml of 2-μm bead solution (an aqueous solution in which PVC (polyvinyl chloride) beads mimicking microplastics are dispersed) (5.68×10 8 beads / ml) Initial bead concentration: 1×10 7 beads / ml

[0071] ≪Microplastic Recovery Test≫ The flask obtained in the above-mentioned ≪Algal Culture≫ {in which each of the above algae was cultured, final concentration 1×10 7The flask containing the bead (2 μm) / ml solution was removed from the artificial climate chamber without agitation. The solution was then suspended by swirling and pipetting. Next, 50 μm cell strainers (pluriStrainer 50 μm) were attached to 50 ml tubes using connector rings and labeled (one for each cell culture flask, excluding those for the calibration curve). All culture flasks were then filtered under reduced pressure using a syringe through each cell strainer (the calibration flasks were not filtered). The filtered samples were then capped and stored on the lab bench. Finally, a bead dilution series for the calibration curve was prepared. Specifically, the culture medium without added beads was set as bead concentration 0, and a stock concentration of 1.00 × 10⁻⁶ was used. 7 Diluting by half repeatedly from beads / ml results in 3.13 × 10 5 Beads / ml, 6.25 x 10 5 Beads / ml, 1.25 x 10 6 Beads / ml, 2.50 x 10 6 Beads / ml, 5.00 x 10 6 Beads / ml were prepared.

[0072] <<Microplastic concentration measurement>> The absorbance of the beads at 267 nm (OD), which is the fluorescence of the beads, was measured using the UV-Vis spectrophotometer BioSpec-Mini (Shimadzu Corporation). 267 Measurements were taken. To calculate the bead concentration, a bead dilution series for the calibration curve was measured, and a calibration curve was created to obtain a linear regression equation. Then, absorbance measurements were taken from the permeate of the algae culture medium + bead solution through a 50 μm cell strainer, and the bead concentration in the permeate was calculated from the linear regression equation obtained from the calibration curve. The absorbance (OD) was measured using a UV-Vis spectrophotometer. 267 For baseline correction when measuring (the microplastic concentration), approximately 200 ml of a well-suspended culture medium without added diatoms was prepared, and baseline correction was performed. Under CO2 addition conditions, the microplastic concentration decreased to approximately 39% of the microplastic concentration under air addition conditions.

[0073] ≪Cell concentration measurement≫ Absorbance at 750 nm (OD) was measured using the UV-Vis spectrophotometer BioSpec-Mini (Shimadzu Corporation). 750 ) Measurements were taken. Furthermore, OD 750 Based on the results, calibration curves were determined using the background and measurement control dilution series, and cell concentration (turbidity) was calculated. Absorbance (OD) was measured using a UV-Vis spectrophotometer. 750 For baseline correction when measuring (), approximately 200 ml of well-suspended culture medium without added diatoms was prepared, and baseline correction was performed. The results obtained are shown in Table 7. The cell concentration under CO2 supplementation conditions is lower than that under air supplementation conditions, which is presumed to be because cell division is too rapid, resulting in a decrease in the size of each cell.

[0074] <<Cell Count Measurement>> The number of cells in cultured algae was counted using a hemocytometer. The results are shown in Table 4. The number of cells under CO2 supplementation conditions was approximately 2.4 times the number of cells under air supplementation conditions.

[0075] [Table 7]

[0076] Based on these results, it was confirmed that under CO2-added conditions, the number of cells increased significantly compared to air-added conditions, leading to an increase in carbon fixation and an increase in the amount of microplastics recovered. [Explanation of Symbols]

[0077] 10 Water treatment system, 30 Water treatment tank, 32 Agitation mechanism, 34 Gas sensor, 36 Gas inlet pipe, 38 Gas flow rate control pump, 40 Inlet pipe, 42 Flow rate control pump, 44 First shut-off valve, 46 Second shut-off valve, 50 Discharge pipe, 60 Algae reserve tank

Claims

1. A water treatment method for recovering microplastics and carbon dioxide from treated water containing microplastics and carbon dioxide, The process includes the step of introducing algae having the ability to adsorb and recover microplastics into the water to be treated, recovering the microplastics from the water to be treated, and fixing carbon dioxide from the water to the algae, A water treatment method wherein the algae are algae that secrete an adhesive substance, and the step includes a cultivation step in which the algae secrete the adhesive substance such that the volume of the adhesive substance secreted outside the algae's cells is 0.25 times or more and 100 times or less compared to the cell volume of the algae.

2. The water treatment method according to claim 1, further comprising the step of supplying carbon dioxide to the water to be treated.

3. The water treatment method according to claim 1 or 2, wherein the water to be treated is industrial wastewater.

4. The water treatment method according to any one of claims 1 to 3, wherein the adhesive substance is a polysaccharide.

5. The water treatment method according to any one of claims 1 to 4, wherein the algae is at least one selected from diatoms, dinoflagellates, chlorarachnion algae, green algae, red algae, conjugating algae, Euglena algae, and cyanobacteria.

6. A process for recovering the microplastics and the algae used for carbon dioxide fixation, The process of replenishing with new algae, A water treatment method according to any one of claims 1 to 5, comprising:

7. The water treatment method according to claim 6, wherein the timing for collecting the algae is set according to the degree of growth of the algae.

8. A water treatment system for recovering microplastics and fixing carbon dioxide from treated water containing microplastics and carbon dioxide, A water treatment system comprising: a water containing algae having the ability to adsorb and recover microplastics in the water to be treated; recovering the microplastics from the water to be treated; fixing carbon dioxide from the water to be treated into the algae; wherein the algae are algae that secrete an adhesive substance; and the water treatment system includes a cultivation means for causing the algae to secrete the adhesive substance such that the volume of the adhesive substance secreted outside the algae cells is 0.25 times or more and 100 times or less compared to the cell volume of the algae.

9. The water treatment system according to claim 8, further comprising the step of supplying carbon dioxide to the water to be treated.

10. The water treatment system according to claim 8 or 9, wherein the water to be treated is industrial wastewater.

11. The water treatment system according to any one of claims 8 to 10, wherein the adhesive substance is a polysaccharide.

12. The water treatment system according to any one of claims 8 to 11, wherein the algae are at least one selected from diatoms, dinoflagellates, chlorarachnion algae, green algae, red algae, conjugating algae, Euglena algae, and cyanobacteria.

13. A means for recovering the microplastics and the algae used for carbon dioxide fixation, A means of replenishing new algae, A water treatment system according to any one of claims 8 to 12, comprising:

14. The water treatment system according to claim 13, wherein the timing for collecting the algae is set according to the growth stage of the algae.

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