Photocatalyst suitable for methods of cultivating aquatic organisms or plants and for purifying the water used in their cultivation.

Abstract

By maintaining the catalytic effect of platinum over a long period, this product provides a photocatalyst that can sustain the functions of decomposing ammonia in aquarium water and purifying water quality. [Solution] A photocatalytic stock solution is prepared by dispersing titanium dioxide, titanium dioxide sol, oxidation catalyst, ceramic powder, and an aqueous binder in activated water activated by a photocatalyst. A platinum sol, in which platinum nanoparticles are dispersed in liquid, is then mixed with the stock solution to produce a photocatalytic dispersion. The photocatalytic dispersion is then impregnated into a porous filter material, and the catalytically active components contained in the photocatalytic dispersion—titanium dioxide, oxidation catalyst, ceramic powder, and platinum nanoparticles—are supported on the filter material.

Description

Technical Field

[0001] The present invention relates to the decomposition of ammonia in breeding water by a photocatalyst and the purification of breeding water in a circulating land-based aquaculture that circulates while purifying the breeding water of aquatic organisms such as fish, shellfish, and crustaceans.

Background Art

[0002] When culturing aquatic organisms such as fish, shellfish, and crustaceans, breeding water such as seawater is put into an aquarium. However, water quality pollution occurs due to uneaten feed and excrement, so it is necessary to replace the breeding water. When culturing on land such as in the inland area, it is difficult to replace the breeding water. Therefore, by circulating while purifying the breeding water, the replacement of the breeding water becomes unnecessary.

[0003] In such breeding water, ammonia is generated as a pollutant due to uneaten feed and excrement. Therefore, in a circulating land-based aquaculture, it is necessary to decompose and remove ammonia in the breeding water. For example, in paragraph 0126 of Patent Document 1, it is described that ammonia in water is decomposed by a photocatalyst such as Pt / TiO2 (platinum-plated titania). Further, Patent Document 2 describes that a photocatalyst of titanium dioxide having a noble metal deposited on its surface is used for a water purification treatment for decomposing ammonia.

[0004] By the way, since the Pt / TiO2 catalyst has a problem in durability, a catalyst for treating ammonia-containing water that can maintain a high ammonia decomposition activity over a long period is described in Patent Document 3. This catalyst is a catalyst used when subjecting ammonia-containing water to catalytic oxidation treatment with an oxygen-containing gas, and is obtained by supporting platinum and another noble metal other than platinum on a titania carrier. The catalyst is produced by impregnating titania in a solution containing both a platinum compound and a compound of another noble metal.

Patent Document 1

Patent Document 2

Patent Document 3

[0005] In the photocatalytic decomposition of ammonia, direct decomposition is difficult with titanium dioxide photocatalysts. Therefore, by adding co-catalysts such as platinum as described above, the generation of reactive oxygen species is promoted, and ammonia can be efficiently decomposed.

[0006] Incidentally, the purification of rearing water in recirculating aquaculture systems is carried out continuously over a long period of time. Maintaining the effect of platinum addition over a long period of time is difficult by simply adding platinum to a titanium dioxide carrier. Therefore, the present invention aims to provide a photocatalyst that can sustain the function of decomposing ammonia and purifying water quality in recirculating rearing water by maintaining the catalytic effect of platinum over a long period of time. [Means for solving the problem]

[0007] The photocatalytic dispersion for producing the photocatalyst of the present invention is a mixture of a photocatalyst stock solution, which is prepared by dispersing titanium dioxide, titanium dioxide sol, oxidation catalyst, ceramic powder, and aqueous binder in activated water activated by a photocatalyst, and a platinum sol, which is prepared by dispersing platinum nanoparticles in a liquid.

[0008] The particle size of the platinum nanoparticles is preferably 6 to 10 nm. The amount of platinum nanoparticles is adjusted according to the ammonia concentration so that the amount of platinum nanoparticles increases as the concentration of the ammonia being decomposed increases.

[0009] The photocatalyst consists of titanium dioxide, an oxidation catalyst, ceramic powder, and platinum nanoparticles, which are catalytically active components contained in a photocatalytic dispersion, supported on a porous filter material. As a method for producing this photocatalyst, a photocatalytic dispersion is produced by mixing titanium dioxide, titanium dioxide sol, an oxidation catalyst, ceramic powder, and an aqueous binder into activated water activated by the photocatalyst and dispersing them in a photocatalyst stock solution, then mixing in a platinum sol in which platinum nanoparticles are dispersed in liquid to produce a photocatalytic dispersion. The photocatalytic dispersion is then impregnated into a porous filter material, and the catalytically active components contained in the photocatalytic dispersion, namely titanium dioxide, an oxidation catalyst, ceramic powder, and platinum nanoparticles, are supported on the filter material.

[0010] In a recirculating land-based aquaculture system that circulates water while purifying it, a photocatalyst is housed in a translucent cylindrical container, with catalytic active components including titanium dioxide, an oxidation catalyst, ceramic powder, and platinum nanoparticles supported on a porous filter material. Multiple such containers are placed in the circulation path, and light is irradiated onto the containers through which the aquatic water flows to purify the water while decomposing the ammonia in it.

[0011] In the container, the rearing water flows vertically, and adjacent containers are connected such that the directions of rearing water flow are opposite to each other. In the upstream container, the rearing water flows upward, and in the downstream container, the rearing water flows downward.

[0012] As a method for cultivating aquatic organisms or plants using the aforementioned photocatalyst, a photocatalyst, which has catalytically active components including titanium dioxide, an oxidation catalyst, ceramic powder, and platinum nanoparticles supported on a porous filter material, and aquatic organisms or plants are placed at separate locations in a water flow path, and while irradiating the photocatalyst with light, the photocatalyst is brought into contact with water to activate the water, and the activated water is brought into contact with the aquatic organisms or plants. [Effects of the Invention]

[0013] According to the present invention, by using a platinum sol in which platinum nanoparticles are dispersed, the platinum nanoparticles are uniformly and highly dispersed on titanium dioxide and filter media without aggregation, allowing the platinum nanoparticles to be supported over a wide area. As a result, the function of platinum as a co-catalyst in photocatalysis can be fully exercised, and moreover, ammonia decomposition and water purification by photocatalysis can be sustained. [Brief explanation of the drawing]

[0014] [Figure 1] Schematic diagram of a circulating aquaculture apparatus according to an embodiment of the present invention. [Figure 2] Diagram showing a container containing a photocatalyst within a water treatment device. [Figure 3] (a)~(c) Diagrams showing the change in ammonia concentration when ammonia in artificial seawater is decomposed by photocatalysis. [Figure 4] This diagram shows the change in ammonia concentration when high concentrations of ammonia in artificial seawater are decomposed by photocatalysis. [Figure 5] (a)~(c) Diagrams showing the change in ammonia concentration when ammonia in tap water is decomposed by photocatalysis. [Modes for carrying out the invention]

[0015] The following describes an embodiment of the photocatalyst used for purifying rearing water according to the present invention. Figure 1 shows a schematic diagram of the recirculating aquaculture apparatus of this embodiment. The recirculating aquaculture apparatus is a closed-loop land-based aquaculture apparatus that circulates purified rearing water, and is equipped with a rearing tank 1, a sedimentation tank 2, a foam separation device 3, a water treatment device 4, a circulation pump 5, and a sterilization device 6, forming a circulation path 7 through which rearing water from the rearing tank 1 returns to the rearing tank 1. In the circulation path 7, the sedimentation tank 2, foam separation device 3, water treatment device 4, circulation pump 5, and sterilization device 6 are arranged in order from the upstream side, and each piece of equipment is connected by piping such as pipes.

[0016] In the breeding tank 1 for breeding aquatic organisms such as fish, shellfish, and crustaceans, breeding water such as seawater, artificial seawater, fresh water, and suitable environmental water suitable for breeding aquatic organisms is put in. The breeding water is discharged from the bottom of the breeding tank 1.

[0017] The sedimentation tank 2 stores the breeding water discharged from the breeding tank 1 and precipitates heavy solids such as the excrement and uneaten feed of the aquatic organisms in the breeding water. The solids accumulated at the bottom of the sedimentation tank 2 are discharged by opening the opening and closing valve.

[0018] The foam separation device 3 receives the breeding water from which solids have been removed from the sedimentation tank 2. The foam separation device 3 generates bubbles by a bubble generator and adsorbs and concentrates suspended substances such as proteins and lipids in the breeding water onto the bubbles. The suspended substances are separated by floating. The suspended substances containing the bubbles collected at the upper part are discharged, and the suspended substances are removed from the breeding water.

[0019] The water treatment device 4 receives the breeding water from which suspended substances have been removed from the foam separation device 3. The water treatment device 4 contains the photocatalyst according to the present invention. In the water treatment device 4, ammonia contained in the breeding water is removed, organic substances are decomposed, and the water quality is purified. That is, by the photocatalyst irradiated with light, ammonia in the breeding water is decomposed into nitrogen and hydrogen at room temperature, and the generated active oxygen oxidizes and decomposes organic substances and inactivates bacteria and viruses. Thus, by purifying the breeding water, the water quality suitable for the growth of aquatic organisms can be maintained.

[0020] The circulation pump 5 is installed between the water treatment device 4 and the sterilization device 6, generates a water flow in the circulation path 7, and discharges and supplies the breeding water to each facility. The sterilization device 6 sterilizes the breeding water by ultraviolet irradiation.

[0021] Next, a photocatalyst suitable for decomposing ammonia used in the circulating aquaculture device will be described. This photocatalyst is one in which titanium oxide, an oxidation catalyst, ceramic powder, and platinum nanoparticles, which are catalytic active components, are supported on a porous filter medium.

[0022] As a method for producing this photocatalyst, a photocatalyst stock solution is prepared by dispersing titanium dioxide, titanium dioxide sol, oxidation catalyst, ceramic powder, and an aqueous binder in activated treated water, and this stock solution is mixed with a platinum sol in which platinum nanoparticles are dispersed in a liquid to produce a photocatalyst dispersion. Then, the photocatalyst dispersion is impregnated into a porous filter material, and the catalytically active components contained in the photocatalyst dispersion—titanium dioxide, oxidation catalyst, ceramic powder, and platinum nanoparticles—are supported on the filter material.

[0023] The photocatalyst stock solution is identical to the composite functional photocatalyst dispersion described in Japanese Patent Publication No. 5082034. First, activated water is produced, and this activated water is used to produce the composite functional photocatalyst dispersion (the photocatalyst stock solution mentioned above).

[0024] To produce activated water, 100 parts by weight of pure water is mixed with 2.5 parts by weight of titanium dioxide fine powder (Teika Co., Ltd.'s TKP-101 photocatalytic titanium dioxide (anatase crystal, crystallite size 6 nm)), 2.5 parts by weight of titanium dioxide sol (Teika Co., Ltd.'s TKS-203 photocatalytic titanium dioxide (anatase crystal aqueous sol, crystallite size 6 nm)), and 3.0 parts by weight of an aqueous binder (acrylic resin emulsion adhesive: Nippon Shokubai Co., Ltd.'s Uble E-11). The mixture is then thoroughly stirred at room temperature until the fine powder is uniformly dispersed to prepare a photocatalytic dispersion.

[0025] Next, an inorganic porous material having open bubbles (artificial zeolite: granulated into particles of about 3 to 10 mm in size) is immersed in the photocatalytic dispersion prepared in this way, and the mixture is stirred as needed once the fine powder in the dispersion has settled. An appropriate immersion time is about 3 to 24 hours, after which the inorganic porous material is removed from the liquid, and any attached water droplets are dried off to obtain a photocatalyst for producing activated treated water. As the inorganic porous material, one having open bubbles is preferred, and suitable materials include glass powder foam made by fusing glass powder (commercially available products include those made by Atoma Co., Ltd.), natural zeolite, or artificial zeolite (manufactured by Chubu Electric Power Co., Inc., Kyushu Electric Power Co., Inc., etc. from waste generated from combustion furnaces of thermal power plants).

[0026] Furthermore, activated water can be obtained by immersing the photocatalyst for producing activated water in water (pure water) at room temperature and irradiating it with ultraviolet light from a mercury lamp, ultraviolet lamp, black light, etc., preferably for about 3 hours or more. In the case of sunlight irradiation, it is preferable to irradiate it with light for about 3 hours or more to obtain such activated water. In addition, photocatalysts such as zinc oxide or tungsten oxide may be used as the photocatalyst for producing activated water.

[0027] The activated water produced in this manner is used to prepare the photocatalyst stock solution. 100 parts by weight of activated water is mixed with 2.5 parts by weight of titanium dioxide fine powder (Teika Co., Ltd.'s TKP-101 photocatalyst titanium dioxide (anatase crystal, crystallite size 6 nm)), 2.5 parts by weight of titanium dioxide sol (Teika Co., Ltd.'s TKS-203 photocatalyst titanium dioxide (aqueous sol of anatase crystal, crystallite size 6 nm)), 2.5 parts by weight of ferric oxide fine powder (High Purity Chemical Laboratory Co., Ltd.'s FEO10PB (α-Fe2O3 fine powder, particle size approximately 1 μm)), 3.0 parts by weight of ceramic fine powder (Tosoh Corporation's TZ-6YS zirconia fine powder (particle size 90 nm)), and 3.0 parts by weight of an aqueous binder (acrylic resin emulsion adhesive: Nippon Shokubai Co., Ltd.'s U-Table E-11). The mixture is then thoroughly stirred at room temperature until each fine powder is uniformly dispersed to prepare the photocatalyst stock solution.

[0028] In the photocatalytic dispersion, the preferred amount of titanium dioxide fine powder is 3 to 15 parts by weight, the preferred amount of titanium dioxide sol (solids) is 0.3 to 5 parts by weight, the preferred amount of oxidation catalyst is 1 to 8 parts by weight, the preferred amount of ceramic powder is 1 to 8 parts by weight, the preferred amount of aqueous binder (solids) is 0.3 to 5 parts by weight, and the remainder is preferably water.

[0029] The oxidation catalyst is not particularly limited, but ferric oxide is preferred, and in terms of the form of ferric oxide, powder is preferred, and fine powder is even more preferred. The ceramic fine powder is not particularly limited, but zirconia (zirconium oxide) is preferred, and in particular, zirconia fine powder in which yttria is uniformly dispersed and solid-solved is preferred. The aqueous binder is not particularly limited, but for example, an acrylic resin emulsion adhesive is suitably used.

[0030] Next, regarding the platinum sol, the particle size of the platinum nanoparticles is 6-10 nm. A predetermined amount of platinum nanoparticles is added to pure water or activated water, and the mixture is thoroughly stirred at room temperature to disperse the platinum nanoparticles in the water and produce the platinum sol. The platinum nanoparticles are present in the platinum sol at a concentration of 15-50%. Here, the amount of platinum nanoparticles is adjusted according to the concentration of the ammonia to be decomposed. The amount of platinum nanoparticles is adjusted so that the higher the ammonia concentration, the higher the concentration of the platinum sol.

[0031] Next, a photocatalytic dispersion is prepared. The photocatalytic stock solution and platinum sol are mixed to produce the photocatalytic dispersion. The photocatalytic dispersion is diluted by adding activated water. The ratio of photocatalytic dispersion to activated water is 1:1.

[0032] In the production of photocatalysts, activated water is used because its use improves the functionality of both the photocatalytic dispersion and the photocatalyst itself. Compared to using water that has not been pre-activated, using activated water enhances the decontamination effects such as cleaning, purification, sterilization, and deodorization, as well as the growth effects on plants and other organisms.

[0033] Next, the photocatalyst is manufactured by supporting titanium dioxide, an oxide catalyst, ceramic powder, and platinum nanoparticles, which are catalytic active components contained in the photocatalyst dispersion, onto a porous filter material. First, the filter material is arranged on a shallow tray, and the diluted photocatalyst dispersion is sprayed onto the surface of the filter material, repeating the spraying until the dispersion is no longer absorbed by the filter material. Alternatively, the filter material may be immersed in the photocatalyst dispersion to allow it to absorb the dispersion. The filter material impregnated with the photocatalyst dispersion is then dried. For example, the photocatalyst can be manufactured by leaving it in a hot air dryer set to 80-100°C for about 30 minutes.

[0034] Here, porous natural or artificial zeolite that allows rearing water to pass through is used as the filter media. Zeolite is suitable as a filter media because it has a high capacity to adsorb ammonia. The zeolite used can be of the Na type, Ca type, or Fe type. For example, the Na type is preferred for washing purposes, while the Ca type or Fe type is preferred for purification and growth. Furthermore, artificial zeolite has a much larger specific surface area than natural zeolite, and therefore a larger amount of catalyst adsorbed per unit weight, resulting in a more efficient catalyst. In the case of any type of zeolite, 50 to 200 parts by weight of photocatalytic dispersion are absorbed per 1000 parts by weight of zeolite.

[0035] As shown in Figure 2, the manufactured photocatalyst is placed in a translucent, elongated cylindrical container. The photocatalyst is placed in a fine-mesh net, and the net is packed into the container, thereby filling it with photocatalyst. Other porous materials, such as activated carbon, may also be placed in the net along with the photocatalyst.

[0036] Containers 10 and 11 filled with photocatalyst P are installed in a water treatment device 4 located in the circulation path 7. This places the photocatalyst within the circulation path 7. Inside the sealed space of the water treatment device 4, two containers 10 and 11 and an ultraviolet lamp 12 are installed. The two containers 10 and 11 are placed vertically so that they are exposed to the light of the ultraviolet lamp 12, and the rearing water passing through the containers 10 and 11 flows vertically. The upper parts of adjacent containers 10 and 11 are connected by piping so that the direction of rearing water flow is opposite to that of adjacent containers 10 and 11. The lower part of the upstream container 10 is connected to piping from the foam separation device 3, and the lower part of the downstream container 11 is connected to piping to the circulation pump 5. In the upstream container 10, the rearing water flows from bottom to top, and in the downstream container 11, the rearing water flows from top to bottom. Note that the two containers 10 and 11 are considered as one set, and multiple sets may be connected in series or parallel depending on the scale of the aquaculture and installed in the water treatment device 4.

[0037] When the rearing water, from which solids and suspended matter have been removed by the sedimentation tank 2 and the foam separation device 3, flows into the water treatment device 4, the rearing water passes through the upstream container 10 and the downstream container 11. When light from the ultraviolet lamp 12 is irradiated onto the photocatalyst, electrons and holes are generated on the surface of the titanium dioxide, and the holes react with ammonia molecules to oxidize the ammonia. In this way, the photocatalyst promotes the decomposition of ammonia in the rearing water.

[0038] Platinum nanoparticles act as co-catalysts that facilitate electron transfer, increasing the separation efficiency of electrons and holes and improving the reaction rate of holes. They also function as active sites that directly participate in the ammonia decomposition reaction, thereby improving the reaction rate of decomposition. Moreover, since the platinum nanoparticles in the sol are uniformly and highly dispersed on the surface of the filter material and the titanium oxide surface without aggregation, there are many active sites, allowing for efficient decomposition and high activation of the photocatalyst. Furthermore, because the platinum nanoparticles are widely dispersed and supported, the ammonia decomposition activity can be maintained for a long period, resulting in a photocatalyst that can sustain ammonia decomposition for an extended period.

[0039] As the rearing water flows from bottom to top within the upstream container 10, gravity causes the water to spread throughout the filter media, adsorbing and removing organic matter and impurities from the water. Furthermore, the increased contact time between the rearing water and the photocatalyst provides more opportunities for the photocatalyst to act, allowing it to more effectively decompose and remove ammonia and adsorbed organic matter and impurities. The rearing water then flows from top to bottom within the downstream container 11, during which time ammonia is also decomposed, further purifying the water. Moreover, when the purified rearing water comes into contact with the photocatalyst, water molecules are decomposed to produce hydrogen and oxygen, which then interact with water molecules, activating the rearing water.

[0040] After the rearing water passes through the water treatment device 4, the ammonia is removed and the purified rearing water returns to the rearing tank 1. By circulating the rearing water, the ammonia concentration in the rearing water gradually decreases over time, and the water quality in the rearing tank 1 is purified. In addition, the supply of activated rearing water promotes the growth of aquatic organisms.

[0041] To verify the effectiveness of the photocatalyst used in this invention for ammonia decomposition, an experiment was conducted on the decomposition of ammonia in circulating water. Artificial seawater and an appropriate amount of ammonia water were placed in a 300L tank, and two containers containing the photocatalyst were connected to the tank as described above. The artificial seawater was prepared by adding artificial seawater mix (Doctor Aqua Wonder Salts, manufactured by Kaisui Marlen Co., Ltd.) to tap water, with a salinity of 3-3.5%. The circulating water in the tank was then circulated at a constant flow rate, for example, 36L / min, using a pump, and the ammonia decomposition ability of the photocatalyst was confirmed.

[0042] Circulating water, adjusted to a predetermined ammonia concentration, flows through two containers. Continuous irradiation of ultraviolet light from a UV lamp activates a photocatalyst, which decomposes the ammonia in the circulating water. This purified circulating water returns to the tank, and the ammonia concentration in the tank gradually decreases.

[0043] The ammonia concentration in the tank was measured using a portable spectrophotometer, and the change in concentration over time was investigated. As shown in Figures 3(a) to (c), when the ammonia concentration in the tank was 2.3 mg / L or less, the ammonia concentration decreased over time. As shown in Figure 4, when the ammonia concentration was high at 23 mg / L, the decomposition of ammonia did not proceed easily, and a slight decrease in ammonia concentration was observed. Normally, the target value for ammonia concentration management in land-based aquaculture is 0.2 to 2 mg / L. Therefore, by using this photocatalyst in land-based aquaculture, harmful substances such as ammonia can be removed, and water quality management that maintains an appropriate ammonia concentration can be carried out.

[0044] Furthermore, similar experiments were conducted using circulating tap water, in addition to seawater, for land-based aquaculture that uses tap water or well water. As shown in Figures 5(a) to (c), when the ammonia concentration in the tank was 2.6 mg / L or less, the ammonia concentration decreased over time. Typically, the target value for ammonia concentration management in land-based aquaculture is 0.2 to 2 mg / L. Therefore, by using this photocatalyst in land-based aquaculture, harmful substances such as ammonia can be removed from the rearing water, and water quality management that maintains an appropriate ammonia concentration can be performed.

[0045] Incidentally, this photocatalyst contains catalytically active components such as titanium dioxide, an oxidation catalyst, and ceramic powder. Since these are the same catalytically active components as those of the composite functional photocatalyst described in Japanese Patent Publication No. 5082034, it can also be used as a composite functional photocatalyst. Therefore, the photocatalyst of the present invention may be used for applications other than ammonia decomposition and water purification in land-based aquaculture.

[0046] When photocatalysts are used to grow plants such as vegetables in closed-system hydroponics, the photocatalysts and plants are placed at separate locations, and a water circulation path is formed so that water containing dissolved liquid fertilizer circulates between them. The same water treatment device 4 described above is placed in the water circulation path. The water treatment device 4 comprises two containers 10 and 11 containing photocatalysts P and an ultraviolet lamp 12. When water comes into contact with the photocatalyst irradiated with light such as ultraviolet light, the water is purified and activated. The activated water is supplied to the plants and comes into contact with them. This promotes plant growth.

[0047] Similarly, when using photocatalysts for cultivating aquatic organisms such as fish and shellfish, the photocatalyst and the aquatic organisms are placed at separate locations, and a water flow path is formed as a circulation path where the water treatment device 4 described above is positioned so that water circulates between them. When water comes into contact with a photocatalyst irradiated with light such as ultraviolet light, the water is purified and activated. The activated water is supplied to the aquatic organisms and comes into contact with them. This promotes the growth of the aquatic organisms.

[0048] Alternatively, instead of continuously circulating the water, it can be circulated intermittently. While the water remains in contact with the photocatalyst, it is activated by the photocatalyst when exposed to light. By contacting the water for an extended period, activated water is produced. When this activated water is supplied to aquatic organisms and plants, it can further promote plant growth.

[0049] Furthermore, this photocatalyst can also be used in aquaponics, which combines hydroponics and land-based aquaculture. The nitrates produced by the decomposition of ammonia in the water become nutrients for plants, which absorb these nutrients and grow. The purified water is used for aquaculture, promoting the growth of aquatic organisms.

[0050] It should be noted that the present invention is not limited to the embodiments described above, and many modifications and changes can be made to the embodiments described above within the scope of the present invention. The above photocatalyst may be used for ammonia decomposition and water purification in semi-closed recirculating land-based aquaculture or flow-through land-based aquaculture. [Explanation of Symbols]

[0051] 1. Rearing tank 2. Sedimentation tank 3 Foam separator 4. Water treatment equipment 5. Circulation pump 6 Sterilizer 7. Circulation pathway 10 Upstream container 11 Downstream container 12 UV lamps P photocatalyst

Claims

1. A photocatalytic dispersion comprising a photocatalyst stock solution obtained by dispersing finely powdered titanium dioxide, titanium dioxide sol, oxidation catalyst, ceramic powder, and aqueous binder in activated water activated by a photocatalyst for producing activated water, and a platinum sol obtained by dispersing platinum nanoparticles in a liquid, characterized in that the dispersion is produced by mixing the photocatalyst stock solution and the platinum sol.

2. The photocatalytic dispersion according to claim 1, characterized in that the particle size of the platinum nanoparticles is 6 to 10 nm.

3. The photocatalytic dispersion according to claim 1, characterized in that the amount of platinum nanoparticles is adjusted according to the ammonia concentration such that the amount of platinum nanoparticles increases as the concentration of ammonia to be decomposed increases.

4. A photocatalyst characterized in that titanium dioxide, an oxidation catalyst, ceramic powder, and platinum nanoparticles, which are catalytically active components contained in the photocatalytic dispersion according to any one of claims 1 to 3, are supported on a porous filter material.

5. A method for producing a photocatalyst, characterized in that a photocatalyst stock solution is prepared by dispersing finely powdered titanium dioxide, titanium dioxide sol, oxidation catalyst, ceramic powder and aqueous binder in activated water activated by a photocatalyst for producing activated water, and a platinum sol in which platinum nanoparticles are dispersed in liquid is mixed to produce a photocatalyst dispersion; the photocatalyst dispersion is impregnated into a porous filter material, and the catalytically active components contained in the photocatalyst dispersion, namely titanium dioxide, oxidation catalyst, ceramic powder and platinum nanoparticles, are supported on the filter material.

6. A method for decomposing ammonia contained in rearing water in a recirculating land-based aquaculture system that circulates water in a recirculating path while purifying the water used to raise aquatic organisms, characterized in that a photocatalyst according to claim 4 is contained in a translucent cylindrical container, a plurality of such containers are arranged in the recirculating path, and light is irradiated onto the container through which the rearing water flows, thereby purifying the rearing water while decomposing the ammonia in the rearing water.

7. The ammonia decomposition method according to claim 6, characterized in that, in the container, the rearing water flows in an up-and-down direction, adjacent containers are connected such that the directions of rearing water flow are opposite to each other, in the upstream container the rearing water flows upward, and in the downstream container the rearing water flows downward.

8. A method for cultivating aquatic organisms or plants, characterized by arranging the photocatalyst described in claim 4 and aquatic organisms or plants at separate positions in a water flow path, irradiating the photocatalyst with light while bringing the photocatalyst and water into contact to activate the water, and then bringing the activated water into contact with the aquatic organisms or plants.

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