Wastewater treatment system, wastewater treatment unit, and wastewater treatment method

Abstract

To provide high C / N ratio effluent suitable for biological denitrification by circulating and reusing aquaculture wastewater.SOLUTION: A pre-treatment facility 1, functioning as a wastewater treatment system for treating aquaculture wastewater W2 containing seawater discharged from an aquaculture tank 2, includes a filtration separation part 4 that filters and separates the aquaculture wastewater W2 into solids and filtered water W3, an accelerated oxidation part 5 having ozone gas supply means 51 capable of gas-liquid mixing ozone gas into the filtered water W3 and ultraviolet irradiation means 52 capable of irradiating the filtered water gas-liquid mixed with ozone gas with ultraviolet light and a foam separation part 6 that separates foam from the filtered water introduced with bubbles.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a wastewater treatment system, a wastewater treatment unit, and a wastewater treatment method for treating aquaculture wastewater containing seawater discharged from aquaculture tanks. [Background technology]

[0002] The water in aquaculture tanks gradually becomes contaminated with ammonia resulting from the excrement and feed of the reared organisms. Ammonia is highly toxic to the reared organisms and inhibits their growth, so a process called nitrification treatment is carried out to oxidize the ammonia into nitrite and nitrate using nitrifying bacteria present in the rearing water or by actively bringing the rearing water into contact with nitrifying bacteria. In land-based aquaculture water, ammonia and nitrite are highly toxic to the reared organisms, so their concentrations are used as maintenance and management indicators.

[0003] In land-based aquaculture using seawater, circulating breeding water treatment equipment (breeding water purification equipment) is commonly used, but nitric acid (nitrate), which is produced from ammonia by nitrifying bacteria, accumulates in the breeding water the longer the breeding water is used in the aquaculture tank, and its concentration increases. Nitrate and nitrate are less toxic than ammonia and nitrite, but at high concentrations they can cause a decrease in the appetite of the breeding organisms, resulting in a slower growth rate and a decline in meat quality. For this reason, nitric acid (nitrate) must be removed from the breeding water in the aquaculture tank.

[0004] Typical methods for removing nitrate (nitrates) from breeding water include water exchange, which involves diluting the water by replacing part of it; aquaponics, which uses the water for hydroponic cultivation and other methods to allow plants to absorb the nitrates as nutrients; and biological denitrification, which uses anaerobic microorganisms to reduce the nitrates in the water and release them as nitrogen gas. Among these methods, water exchange requires the preparation of artificial seawater when the water contains seawater, and consideration must be given to drainage when using seawater with high salt content. Therefore, water exchanges tend to increase costs. Furthermore, only a limited number of plants can be used with seawater, and a process of removing salt is required to prevent salt damage, making aquaponics difficult to apply.

[0005] In contrast, biological denitrification has a proven track record in wastewater treatment and has the potential to be used as a method for removing nitric acid (nitrates) from aquaculture wastewater, including seawater. To implement biological denitrification, the C / N ratio (ratio of organic carbon to nitrogen) of the water to be treated generally needs to be 5 or higher. For example, Japanese Patent Application Laid-Open Publication No. 2011-130686 (Patent Document 1) proposes an aquaculture device that purifies breeding water used for raising fish and shellfish and circulates it through a circulation path. The device includes a fish and shellfish breeding tank, a filtration device that separates and removes floating suspended solids in the breeding tank, and a membrane separation activated sludge treatment device that uses a group of microorganisms to treat and separate and remove organic matter and ammonia in the breeding water. In the aquaculture apparatus of Patent Document 1, the circulation path has a first path that introduces settleable suspended matter in the breeding tank as a first concentrated water into the membrane bioreactor, a second path that removes the supernatant from the breeding tank and introduces it into the filtration device, a third path that returns the filtrate filtered by the filtration device together with treated water treated by the membrane bioreactor to the breeding tank, and a fourth path that introduces the settleable suspended matter separated by the filtration device as a second concentrated water into the membrane bioreactor, and the membrane bioreactor is equipped with a sludge discharge path that discharges excess sludge. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-130686 Summary of the Invention [Problem to be solved by the invention]

[0007] The circulating aquaculture system of Patent Document 1 introduces high-BOD raw water into a membrane bioreactor as a nutrient source for activated sludge microorganisms by introducing settleable suspended matter discharged from a breeding tank and floating suspended matter collected separately by a filtration device into the membrane bioreactor without discharging them outside the system. In the circulating aquaculture system of Patent Document 1, floating suspended matter with a particle size of 40 micrometers or more collected by the filtration device is introduced into the membrane bioreactor as a nutrient source for the microorganisms, and the treated water (filtrate) is returned to the breeding tank. In other words, in Patent Document 1, fine organic suspended matter, such as those with a particle size of less than 40 micrometers contained in the filtrate from the filtration device, remains in the breeding tank and is neither recovered nor utilized. Because the breeding water in the breeding tank contains a large amount of fine organic suspended matter other than the settleable suspended matter and floating suspended matter, there is still room for improvement in the technology for obtaining wastewater with a high C / N ratio.

[0008] Thus, there is a need to recycle and reuse aquaculture wastewater while obtaining wastewater with a high C / N ratio that can be applied to biological denitrification. [Means for solving the problem]

[0009] The wastewater treatment system according to the present invention is a wastewater treatment system that treats aquaculture wastewater containing seawater discharged from aquaculture tanks, and is characterized by comprising: a filtration / separation unit that filters and separates the aquaculture wastewater into solids and filtrate; an accelerated oxidation unit that has an ozone gas supply means that can mix ozone gas with the filtrate and an ultraviolet light irradiation means that can irradiate the filtrate with ultraviolet light in which the ozone gas has been mixed; and a foam separation unit that performs foam separation on the filtrate introduced together with air bubbles.

[0010] The wastewater treatment method according to the present invention is a wastewater treatment method for treating aquaculture wastewater containing seawater discharged from aquaculture tanks, and is characterized by comprising: a filtration / separation step of filtering and separating the aquaculture wastewater into solids and filtrate; an accelerated oxidation step of mixing ozone gas with the filtrate obtained in the filtration / separation step in a gas-liquid state, and then irradiating the filtrate with ultraviolet light; and a foam separation step of introducing the filtrate obtained in the filtration / separation step together with air bubbles to perform foam separation.

[0011] These methods allow aquaculture wastewater containing seawater to be filtered and separated into solids and filtrate, and fine organic suspended matter contained in the filtrate can be further separated and accelerated oxidized by foam separation. This allows the fine organic suspended matter to be concentrated and recovered, and then oxidized and decomposed into organic carbon-containing substances in a form that is easily usable by microorganisms. This allows the aquaculture wastewater to be recycled and reused, yielding wastewater with a high C / N ratio that is suitable for biological denitrification.

[0012] The wastewater treatment unit according to the present invention is a wastewater treatment unit that treats aquaculture wastewater containing seawater discharged from an aquaculture tank, and is characterized by comprising an accelerated oxidation section having an ozone gas supply means for gas-liquid mixing of ozone gas with the aquaculture wastewater and an ultraviolet irradiation means for irradiating ultraviolet light onto the aquaculture wastewater with the gas-liquid mixture of ozone gas, and a foam separation section for foam separation of the aquaculture wastewater treated in the accelerated oxidation section.

[0013] According to this configuration, the aquaculture wastewater treated in the foam separation section is subjected to foam separation, so that the organic suspended matter in the aquaculture wastewater can be efficiently concentrated and recovered, and wastewater with a high C / N ratio can be obtained from the aquaculture wastewater.

[0014] Preferred embodiments of the present invention will be described below, but the scope of the present invention is not limited to the preferred embodiments described below.

[0015] In the wastewater treatment system according to the present invention, it is preferable that the foam separation section performs foam separation on the filtrate that has been oxidized in the advanced oxidation section.

[0016] According to this configuration, the filtered water that has been oxidized in the advanced oxidation section is subjected to foam separation, which makes it easy to efficiently concentrate and recover organic suspended matter in the aquaculture wastewater.

[0017] The wastewater treatment system of the present invention preferably includes a biological treatment tank having an anaerobic treatment section downstream of both the advanced oxidation section and the foam separation section, and the filtered water that has been treated in the advanced oxidation section and foam-separated together with the air bubbles in the foam separation section is introduced into the anaerobic treatment section.

[0018] According to this configuration, wastewater with a high C / N ratio, which contains organic carbon-containing substances in a form that is easily usable by microorganisms, can be introduced into the anaerobic treatment section of the biological treatment tank, and the organic suspended matter in the aquaculture wastewater can be effectively used as a carbon source for biological denitrification.

[0019] In the wastewater treatment system according to the present invention, the biological treatment tank preferably has an aerobic treatment section downstream of the anaerobic treatment section, and has return means for returning the supernatant of the wastewater aerobically treated in the aerobic treatment section to a tank for storing the aquaculture wastewater, and the returned supernatant is preferably treated in at least one accelerated oxidation section before reaching the aquaculture tank.

[0020] With this configuration, the treated water from the biological treatment tank can be returned to the aquaculture tank after undergoing accelerated oxidation treatment, making it difficult for microorganisms from the biological treatment tank to be mixed into the aquaculture tank. Therefore, the aquaculture wastewater can be reused while maintaining a growth environment for the microorganisms in the aquaculture tank.

[0021] The wastewater treatment system according to the present invention preferably includes a buffer tank for storing the aquaculture wastewater between the aquaculture tank and the filtration / separation unit, and the return means returns the supernatant to the buffer tank.

[0022] With this configuration, treated water from the biological treatment tank is returned to the buffer tank, and then passes through the filtration and separation section, advanced oxidation section, and foam separation section before returning to the aquaculture tank. Therefore, treated water that has been filtered, advanced oxidation, and foam separation is returned to the aquaculture tank, making it difficult for microorganisms from the biological treatment tank to be mixed into the aquaculture tank. Furthermore, this type of circulation system allows advanced oxidation treatment to be performed in a single advanced oxidation section, making it easy to create a simple system.

[0023] In the wastewater treatment system according to the present invention, it is preferable that the filtration separation unit has a backwashing mechanism, and backwash wastewater from the backwashing mechanism is further introduced into the anoxic treatment unit.

[0024] According to this configuration, the solid matter separated in the filtration separation section can also be used as an organic carbon source.

[0025] In the wastewater treatment system according to the present invention, the filtration separation section is preferably a fibrous filtration device in which the fibrous filtration material is compressed to form a fibrous filtration layer during filtration, and the compressed fibrous filtration material expands during backwashing.

[0026] According to this configuration, backwash wastewater containing a high concentration of solids captured in the filtration separation section can be easily obtained, and organic carbon sources can be efficiently recovered from the aquaculture wastewater.

[0027] In the wastewater treatment system according to the present invention, it is preferable that the foam separation section further includes a gas supply means for supplying air bubbles.

[0028] This configuration makes it easier for the foam separation section to perform foam separation more efficiently.

[0029] Further features and advantages of the present invention will become more apparent from the following description of exemplary and non-limiting embodiments, which is given with reference to the drawings. [Brief explanation of the drawings]

[0030] [Figure 1] 1 is an overall schematic diagram showing a wastewater treatment system according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram illustrating details of a pretreatment unit (wastewater treatment unit) according to an embodiment of the present invention. [Figure 3] 1 is a schematic diagram showing a filtration / separation unit of a wastewater treatment system according to an embodiment of the present invention, illustrating a filtration / separation process. [Figure 4] FIG. 2 is a schematic diagram showing a filtration / separation unit of a wastewater treatment system according to an embodiment of the present invention, illustrating a gas scrubbing step. [Figure 5] FIG. 2 is a schematic diagram showing a filtration separation unit of a wastewater treatment system according to an embodiment of the present invention, illustrating a backwashing process using backwash water. [Figure 6] 5A and 5B are diagrams illustrating a movable guide of a filtration separation unit according to an embodiment of the present invention. [Figure 7] FIG. 2 is an overall schematic diagram showing a wastewater treatment system according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0031] Embodiments of a wastewater treatment system and a wastewater treatment unit according to the present invention will be described with reference to the drawings. Below, a wastewater treatment system 10 as a first embodiment of the wastewater treatment system according to the present invention and a pretreatment unit 7 as a first embodiment of the wastewater treatment unit according to the present invention will be described with reference to Figs.

[0032] (1) Wastewater treatment system of the first embodiment The wastewater treatment system 10 purifies and reuses aquaculture wastewater W2, which contains seawater and is discharged from the aquaculture tank 2, and comprises two compartments: a pretreatment facility 1 and a biological treatment tank 8. The pretreatment facility 1 removes solids and suspended matter from the aquaculture wastewater W2 and returns it to the aquaculture tank 2, while also recovering the removed solids and suspended matter. The biological treatment tank 8 uses the solids and suspended matter recovered by the pretreatment facility 1 as a nutrient source for the microorganisms in the activated sludge and removes nitric acid (nitrates) from the aquaculture wastewater W2 through biological treatment.

[0033] [Overall configuration of pretreatment facility 1] The pretreatment facility 1 includes a filtration / separation unit 4, an advanced oxidation unit 5, and a foam separation unit 6. Separated water W6, from which solids and suspended matter have been removed, is returned to the aquaculture tank 2. Meanwhile, the pretreatment facility 1 recovers concentrated wastewater W4 containing suspended organic matter. That is, the pretreatment facility 1 is configured to recover concentrated wastewater W4 with a high C / N ratio from the aquaculture wastewater W2 with a low C / N ratio in a circulating purification treatment system for the aquaculture wastewater W2. In this embodiment, the culture water W1 in the aquaculture tank 2 is stored in a first buffer tank 31 via a strainer to prevent the intake of cultured organisms, and this is used as the aquaculture wastewater W2 and the water to be treated in the pretreatment facility 1. Concentrated wastewater W4 with a high C / N ratio is accumulated in a second buffer tank 32.

[0034] [Breeding water and aquaculture wastewater] In the culture water W1 of the aquaculture tank 2, crustaceans such as shrimp and crabs, fish, and other organisms (not shown) are raised. Typically, the culture water W1 of the aquaculture tank 2 receives sufficient oxygen supply to the organisms by aeration or other means, and the dissolved oxygen (DO) is at a supersaturated level. Therefore, the aerobic microorganisms (bacteria) coexisting in the aquaculture tank utilize available organic carbon-containing substances in the culture water W1 as a nutrient source, consuming carbon through respiration and releasing it as carbon dioxide from the culture water W1. The aquaculture wastewater W2 contains organic carbon-containing substances such as feed residues, organism waste, organism surface mucus, microbial cells and colonies such as biofilms, carcasses, exoskeletons, and pigments leached from these. These contain a lot of carbon, but as mentioned above, organic carbon-containing substances in a form that is easily usable by microorganisms are consumed by the microorganisms and their concentrations in the culture water W1 decrease, so it is thought that, with the exception of some feed residues, dead bodies, and water-soluble pigments, organic carbon-containing substances that are difficult for microorganisms to decompose are likely to remain in the culture wastewater W2. On the other hand, the majority of nitrogen is nitric acid (nitrate) produced by microorganisms (nitrifying bacteria) in the culture water W1, and most exists in a water-soluble form. As a result, the amount of usable organic carbon in the culture water W1 and the culture wastewater W2 is extremely small compared to the amount of nitrogen.

[0035] The present inventors focused on the size of the turbidity composed of organic matter in the aquaculture wastewater W2 and found that fine organic suspended matter with a particle size of 30 μm or less in the aquaculture wastewater W2 is difficult for microorganisms to decompose and is therefore difficult to use as a nutrient source for microorganisms, while coarse solid matter exceeding 30 μm in the aquaculture wastewater W2 is easily utilized as a nutrient source for microorganisms. This is because the relatively coarse particles are primarily composed of feed residues, biological waste products, exoskeletons, and carcasses of reared organisms, and are therefore easily utilized as a nutrient source for microorganisms in the aquaculture wastewater W2 and for microbial communities in biological denitrification. On the other hand, the fine organic suspended matter is protected by strong cell walls, such as the microbial bodies, or is the residue remaining after only the parts that are easily utilized as a nutrient source for microorganisms are utilized, and is therefore considered to be difficult for microorganisms to utilize as a nutrient source. Furthermore, if the coarse solid matter remains in the rearing water W1, it will gradually be decomposed by microorganisms, ultimately leaving behind fine, persistent organic suspended matter.

[0036] The present inventors have conducted extensive research into an apparatus and method for recovering the fine organic suspended matter from the aquaculture wastewater W2 and using the fine organic suspended matter as a nutrient source for microorganisms in biological denitrification. The present inventors have discovered that by applying an advanced oxidation section 5 and a foam separation section 6 to a circulating aquaculture wastewater treatment system, the fine organic suspended matter can be oxidatively decomposed into a form that is easily usable by microorganisms and recovered in high concentrations, thereby completing the present invention.

[0037] Next, the configuration of each part of the pretreatment facility 1 will be described.

[0038] [Configuration of filtration separation section] The filtration / separation section 4 filters and separates the aquaculture wastewater W2 into solids and filtrate W3. During filtration, the aquaculture wastewater W2 is sent from the first buffer tank 31 to the filtration / separation section 4 by the pump P1, and the solids accumulate in the filtration / separation section 4 until they are removed by backwashing. The filtrate W3 is sent to the advanced oxidation section 5 of the pretreatment unit 7.

[0039] In this embodiment, the fibrous filtration device shown in Figures 3 to 6 is used as the filtration separation unit 4. As shown in Figure 3, the filtration separation unit 4 according to this embodiment includes an outer cylinder 41, a fibrous filtering material 42 arranged inside the outer cylinder 41, a fixing plate 43 for fixing the upper end of the fibrous filtering material 42, a movable guide 44 for fixing the lower end of the fibrous filtering material 42, and a guide pole 45 erected inside the outer cylinder 41 in the longitudinal direction of the outer cylinder 41.

[0040] In detail, the outer cylinder 41 has a first opening 41A located at the upper end side and a second opening 41B located at the lower end side, and an upper end 42A of the fibrous filtration material 42 is fixed to and suspended from a fixed plate 43 located on the first opening 41A side inside the outer cylinder 41. A lower end 42B of the fibrous filtration material 42 is fixed integrally to a movable guide 44, and the fibrous filtration material 42 contracts or expands as the movable guide 44 moves up and down inside the outer cylinder 41. During filtration, aquaculture wastewater W2 is supplied from the second opening 41B of the outer cylinder 41, causing the movable guide 44 to move upward, and during backwashing, backwashing water is supplied from the first opening 41A, causing the movable guide 44 to move downward.

[0041] As shown in FIG. 3 , during filtration, the filtration separation unit 4 of this configuration supplies aquaculture wastewater W2 through the second opening 41B of the outer cylinder 41, moves the movable guide 44 upward, and filters the aquaculture wastewater W2 through the fibrous filter material 42 compressed by the movable guide 44. This configuration performs so-called "upward filtration," compressing the fibrous filter material 42 with the upward water flow against gravity to form a filtration layer. Therefore, the thickness of the filtration layer can be easily controlled by balancing the water flow of the aquaculture wastewater W2 and the weight of the movable guide 44, making it easy to achieve stable filtration performance. Furthermore, excessive compression of the fibrous filter material 42 can be prevented. Because the upper end 42A of the fibrous filter material 42 is fixed to the fixed plate 43 and the lower end 42B is fixed to the movable guide 44, the surface formed by the movable guide 44 allows the fibrous filter material 42 to be compressed approximately uniformly from the lower end 42B side. Furthermore, even if the fibrous filtering material 42 is violently shaken during backwashing, the lower end side is fixed, so the core strings and fibers of the fibrous filtering material 42 are less likely to become entangled.

[0042] In this embodiment, a guide pole 45 extending in the vertical direction, which is the longitudinal direction of the outer cylinder 41, is provided inside the outer cylinder 41, and the movable guide 44 moves in the vertical direction along the guide pole 45. The guide pole 45 is erected at approximately the center of the horizontal cross section of the outer cylinder 41. The guide pole 45 is made of a long pipe and is fixed inside the outer cylinder 41 with its upper end closed. In this embodiment, the lower end of the guide pole 45 is fixed inside the outer cylinder 41 so as to communicate with the gas ejection means 46. The material of the guide pole 45 is not particularly limited, and it may be made of a metal pipe such as stainless steel, a resin pipe, or the like.

[0043] As shown in Fig. 6, the movable guide 44 has a guided hole 44A at approximately the center, through which a guide pole 45 is inserted. In this embodiment, the lower ends 42B of the five fibrous filtering materials 42 are attached and fixed to five lower end fixing portions 44B arranged around the guided hole 44A of the movable guide 44. The upper fixing plate 43 also has five upper end fixing portions for fixing the upper ends 42A of the fibrous filtering materials 42, and is provided with holes through which the guide pole 45 is inserted, similar to the guided holes 44A of the movable guide 44. The upper ends 42A of the five fibrous filtering materials 42 are attached and fixed to these upper end fixing portions. The fixing plate only needs to be able to fix the upper ends of the fibrous filtering materials 42, and is formed with openings or through-holes that are larger than the movable guide 44, so that water can easily pass through.

[0044] The outer cylinder 41 is provided with a lower stopper portion 45A that restricts downward movement of the movable guide 44. In this embodiment, the lower stopper portion 45A is provided as a protruding portion on the guide pole 45, and is formed so that its outer diameter is larger than the diameter of the guided hole 44A of the movable guide 44. The presence of the lower stopper portion 45A makes it possible to prevent the fibrous filtering material 42 from stretching excessively during backwashing.

[0045] Furthermore, an upper stopper portion (not shown) is provided to restrict the upward movement of the movable guide 44. In this embodiment, similar to the lower stopper portion 45A, the upper stopper portion is provided as a protruding portion on the guide pole 45. The protruding portion serving as the upper stopper portion is also formed so that its outer diameter is larger than the diameter of the guided hole 44A of the movable guide 44. By providing the upper stopper portion, it is possible to prevent the fibrous filtering material 42 from being excessively compressed.

[0046] In this embodiment, the movable guide 44 is a perforated plate made of metal such as stainless steel or aluminum, or plastic. In this embodiment, the perforated plate is a circular perforated metal plate having a guided hole 44A in the center and an outer diameter smaller than the inner diameter of the outer cylinder 41. The perforated plate preferably has an opening diameter of 1 to 8 mm and an opening ratio of 15 to 60% in the portion other than the guided hole 43A. The opening diameter is more preferably 1 to 6 mm, and 2 to 4 mm is particularly preferably. The opening ratio is more preferably 20 to 50%, and 30 to 40% is particularly preferably. The thickness of the perforated plate is preferably 0.5 to 8 mm, more preferably 1 to 5 mm, and particularly preferably 1 to 3 mm. In the portion other than the guided hole 43A, the weight per unit area is preferably 1 to 10 kg / m 2 The preferred range is 3 to 8 kg / m 2A stainless steel perforated metal plate having a central through-hole serving as the guided hole 44A with a diameter of 80 mm (when the guide pole 50 has a diameter of 27.5 mm), a diameter of 144 mm (when the outer cylinder 41 has an inner diameter of 165 mm), a thickness of 1.5 mm, and an opening diameter (diameter) of 3 mm (across the entire area other than the guided hole 44A) and a pitch of 4 mm can be used. Examples of perforated metal plates include punched metal. Such a movable guide 44 will not immediately descend under its own weight after the passage of the aquaculture wastewater W2 through the filtration process, preventing excessive tension from being applied to the fibrous filtration material 42. Furthermore, the appropriate weight of the movable guide 44 facilitates balancing the weight of the movable guide 44 with the water flow of the aquaculture wastewater W2, making it easier to form a filtration layer in which the fibrous filtration material 42 is compressed to a uniform thickness, resulting in stable filtration performance. In this embodiment, the diameter of each of the multiple openings (excluding the guided hole 44A) provided in the movable guide 44 is smaller than the clearance (average shortest distance) between the guided hole 44A and the guide pole 45 and the clearance (average shortest distance) between the inner diameter of the outer tube 41 and the outer diameter of the movable guide 44.

[0047] The fibrous filtering material 42 may have a length of 500 to 1500 mm and a diameter of 50 to 150 mm. In this embodiment, five fibrous filtering materials 42 are used, but 1 to 20, preferably 3 to 10, may be used depending on the inner diameter of the outer tube 41, etc. In this embodiment, a string-like contact material is used, in which fine fibers are woven into a braided shape around a core string. The core strings at both ends of the string-like contact material are fastened to the upper end fixing portion of the fixed plate and the lower end fixing portion 44B of the movable guide 44, respectively. In the filtration / separation unit 4 of this embodiment, the Biocord (registered trademark) PP series (made of polypropylene resin fiber, length approximately 1000 mm, diameter approximately 100 mm) manufactured by TBR Corporation is used, but this is not limiting, and various types of fibrous filtering materials 42 may be used.

[0048] The lower end of the outer cylinder 41 is further provided with a gas ejection means 46 that can supply gas (compressed air) to the compressed fibrous filter material 42 from the second opening 41B side of the outer cylinder 41 after the filtration process. The gas supplied by the gas ejection means 46 passes through the fibers of the fibrous filter material 42 while growing bubbles, allowing the fibrous filter material 42 to stretch simply by the action of the gas alone. The stretching direction of the fibrous filter material 42 is opposite the direction of travel of the bubbles, and the fibrous filter material 42 is loosened by the gas and naturally stretches. In this way, sudden and excessive stress is not applied to the fibrous filter material 42 when it stretches, preventing damage to the fibrous filter material 42. After the fibrous filter material 42 has stretched and a certain amount of contaminants has fallen off, it is then possible to perform backwashing with backwash water, which reduces pressure loss and the amount of backwash water used. In the backwashing process after the fiber filter material 42 is stretched, the flow direction of the backwashing water is downward, so the flow direction of the backwashing water and the settling direction of the pollutants are the same, and the pollutants can be smoothly and easily discharged from the system.

[0049] The gas ejection means 46 includes a gas supply source 46A, such as an air compressor, disposed outside the outer cylinder 41; a gas outlet 46B that ejects the gas supplied from the gas supply source 46A from the second opening 41B side inside the outer cylinder 41; and an air valve 46C located between the gas supply source 46A and the gas outlet 46B. In this embodiment, the gas outlet 46B is a plurality of openings formed in a cylindrical body disposed at a midpoint on the lower end side of the guide pole 45. As shown in FIG. 4, the gas supplied from the gas supply source 46A is introduced into a portion of the guide pole 45 below the lower stopper portion 45A and ejected from the gas outlet 46B into the outer cylinder 41. The connection between the cylindrical body in which the gas outlet 46B is formed and the guide pole 45 directly above it is closed.

[0050] In this embodiment, gas is injected from a cylindrical body located at the lower end of the guide pole 45. This allows the gas to be injected from approximately the center of the outer cylinder 41, making it easier for the air bubbles to come into uniform contact with the fibrous filter material 42. Furthermore, the gas outlet 46B opens horizontally, which prevents sludge and other contaminants from entering and clogging the gas outlet 46B compared to an upward opening. Furthermore, this configuration eliminates the need for a separate component for gas injection (such as an aeration pipe) inside the outer cylinder 41, thereby achieving space savings. It is even more preferable for the cylindrical body to be made of a wedge wire screen. A wedge wire screen is a screen in which wedge wires with an inverted triangular cross section are arranged at equal intervals, forming slits between adjacent wedge wires. By arranging the slits of this wedge wire screen on the outside of the cylindrical body and injecting gas, large contaminants do not enter the interior of the cylindrical body, further suppressing clogging of the gas outlet 46B.

[0051] As described above, the filtration / separation unit 4 supplies the stored aquaculture wastewater W2 to the second opening 41B as shown in Fig. 3 during filtration, but is configured to be able to supply backwash water from the first opening 41A by a backwashing mechanism as shown in Fig. 5 during backwashing of the fibrous filter material 42. That is, the filtration / separation unit 4 has a backwashing mechanism, and during backwashing, the filtration / separation unit 4 discharges backwashing wastewater W7 from the second opening 41B. The backwashing wastewater W7 is temporarily stored in the second buffer tank 32 and is introduced into the anaerobic treatment unit 81 of the biological treatment tank 8 together with concentrated wastewater W4, which will be described later.

[0052] As shown in Figures 3 to 5, the filtration / separation unit 4 includes a liquid or gas valve and a liquid-transport pump P1 in the piping that serves as the fluid flow path. The flow paths can be switched by switching each valve. In this embodiment, each of these valves is an electrically controlled valve, electrically connected to a control device (not shown) and controlled by the control device. In this embodiment, the liquid-transport pump is also electrically connected to the control device, and its operation is controlled by the control device while monitoring the water levels of the first buffer tank 31, the second buffer tank 32, and the anoxic treatment section 81 of the biological treatment tank 80 (described later). The safety valve S at the top of the outer cylinder 41 automatically opens when the internal pressure of the flow path exceeds a certain level, releasing the fluid to the outside. The pump P1 is generally constantly operating except when the air valve is open. The filtration / separation unit 4 is operated by switching the opening and closing of each valve, switching between filtration operation and backwashing operation. In Figures 3 to 5, open valves are indicated by white outlines, and closed valves are indicated by black outlines. The piping through which wastewater flows is indicated by thick lines.

[0053] The filtering / separation unit 4 is only required to remove coarse solids, and suspended matter of, for example, 30 μm or less (preferably about 5 μm or less) may be contained in the filtered water W3. The solids separated by the filtering / separation unit 4 include, for example, feed residues, biological waste, molted skins and carcasses of reared organisms, etc.

[0054] [Configuration of the advanced oxidation unit] The advanced oxidation unit 5 performs an advanced oxidation process (AOP) on the filtered water W3. As shown in FIG. 2 , the advanced oxidation unit 5 includes an ozone gas supply means 51 capable of mixing ozone gas with the filtered water W3 in a gas-liquid state, and an ultraviolet irradiation means 52 capable of irradiating ultraviolet light onto the filtrate W3 containing the gas-liquid mixture of ozone gas. Advanced oxidation is a treatment method that combines ozone treatment with ultraviolet light to generate hydroxyl radicals with strong oxidizing power, which then completely decompose organic matter. The advanced oxidation unit 5 of the present invention can be a conventional advanced oxidation treatment device that combines ozone gas treatment with ultraviolet light irradiation. In the present invention, the ozone gas supply means 51 and ultraviolet light irradiation means 52 are provided in this order, and ultraviolet light is irradiated onto the filtered water W3 containing the gas-liquid mixture of ozone gas. Because hydroxyl radicals have an extremely short lifespan, this type of advanced oxidation treatment allows the hydroxyl radicals to disappear before the advanced oxidation-treated water is returned to the aquaculture tank 2. Furthermore, the advanced oxidation unit 5 makes it difficult for ozone and hydrogen peroxide, which are harmful to the reared organisms, to remain in the treated water, and the system can be used in a circulating aquaculture wastewater treatment system.

[0055] As shown in FIG. 2, the ozone gas supply means 51 is composed of an ozone gas generator 51A, an ejector 51B, and ozone gas flow rate adjustment valves (51C, 51D). The ejector 51B mixes ozone gas with the filtered water W3. The ozone gas generator 51A can generate high-concentration ozone using, for example, a silent discharge ozonizer and PSA concentrated oxygen. The flow path from the filtration / separation section 4 to the ultraviolet irradiation means 52 branches into two paths. One path has an ejector 51B, and the other path is configured so that the filtered water W3 flows directly through it. The amount of ozone gas mixed is adjusted by balancing the concentration of the ozone gas supplied by the ozone gas generator 51A and the flow rate controlled by the valves (51C, 51D) of these two branch paths. The ejector 51B used in this embodiment is a type of vacuum pump that generates a vacuum using the Venturi effect and mixes two types of fluids using differential pressure. The gas-liquid mixing device for ozone gas is not limited to an ejector, and for example, ozone gas may be directly mixed into the filtered water W3 using a compressor, etc. The amount of ozone gas mixed into the filtered water W3 subjected to ultraviolet irradiation means 52 is not particularly limited, but the gas-liquid supply ratio (liquid / gas) is set taking into consideration the concentration of fine organic suspended matter to be decomposed, the target processing amount, the amount of ozone generated by the ozonizer, etc., and can be set to a condition such as 0.4 to 0.6 (preferably 0.5).

[0056] Next, ultraviolet light is irradiated onto the filtered water W3 containing the ozone gas mixed with the liquid by the ultraviolet light irradiation means 52. The ultraviolet light irradiation means 52 can be a conventionally known ultraviolet treatment device used for sterilizing water. Specifically, various types of ultraviolet light irradiation devices can be used, such as a device that has an ultraviolet lamp installed in the water pipe of the filtered water W3 and irradiates ultraviolet light from the inside, or a device like an ultraviolet tube that irradiates ultraviolet light from the outside of the water pipe. The ozone in the filtered water W3 is decomposed by ultraviolet light with a wavelength of 254 nm to form hydroxyl radicals ( · To generate UV-rays (OH), it is preferable to use a low-pressure mercury lamp (ozone-less type or ozone-generating type) that emits UV-rays at a wavelength of 254 nm, but light sources such as UV-LEDs with short wavelengths can also be used.

[0057] The ozone gas mixed with the filtrate water W3 is decomposed by ultraviolet light to generate hydroxyl radicals, which oxidize and decompose the organic suspended matter in the filtrate water W3, including persistent substances, due to the powerful oxidizing action of the hydroxyl radicals. As a result, the organic suspended matter is reduced in molecular weight and made hydrophilic, and can become organic carbon-containing substances in a form that is easily utilized by microorganisms. Furthermore, the organic matter (organic carbon) that has become water-soluble through the oxidation treatment in the advanced oxidation unit 5 is then returned to the aquaculture tank 2. Because this organic matter (organic carbon) is water-soluble, it is easily consumed as a nutrient source by the aerobic microorganisms in the breeding water W1 in the aquaculture tank 3.

[0058] [Configuration of foam separation section] The filtrate W3 that has undergone advanced oxidation treatment in the advanced oxidation unit 5 is then subjected to foam separation in the foam separation unit 6. The filtrate W3 is introduced into the foam separation unit 6 together with air bubbles. In the present invention, stable air bubbles can be generated because the water to be treated is aquaculture wastewater W2 containing seawater, making it possible to remove organic suspended matter by foam separation. In this embodiment, the foam separation unit 6 has a foam separation tower 61 and a gas supply means 62. The gas supply means 62 introduces air bubbles into the filtrate W3 to facilitate foam separation, and is composed of a gas generator 62A, an ejector 62B, and gas flow rate adjustment valves (62C, 62D). The ejector 62B mixes air bubbles 63 into the filtrate W3. As with the ozone gas supply means 51 described above, the ejector 62B is located on one side of the branched flow path, and the amount of bubbles introduced is adjusted by opening and closing valves 62C and 63C with the other flow path that does not have the ejector 62B. The gas supplied by the gas supply means 62 may be any gas that does not affect the reared organisms, such as compressed air, but air or oxygen is preferred because using gases other than air or oxygen can lead to a decrease in the dissolved oxygen content (DO). The bubble diameter of these gases is not particularly limited, but bubbles of about 100 μm to 300 μm may be introduced. The treatment conditions in the foam separation section 6 are not particularly limited, but for example, when the internal volume of the foam separation tower 61 is 150 L, the bubble supply amount is set to 0.5 to 10 m. 3 / hour (preferably 1.0 to 5.0 m 3 / hour), circulation flow rate is 1.0 to 10.0 m 3 / hour, (preferably 2.0 to 8.0 m 3 / hour, more preferably 3.0 to 7.0 m 3 / hour) and a gas-liquid supply ratio (liquid / gas) of 0.3 to 0.7 (preferably 0.4 to 0.6).

[0059] The filtrate W3, which has undergone advanced oxidation in the advanced oxidation unit 5, is introduced below the foam separation tower 61, where foam separation is performed after the bubbles 63 are mixed with the gas and liquid. The filtrate W3, which is foam-separated in the foam separation unit 6, contains a large amount of organic suspended matter, for example, of approximately 30 μm or less. The organic suspended matter separated in the foam separation unit 6 includes, for example, microbial aggregates containing strong microbial cell walls, as well as semi-solid components that cannot be captured by conventional filtration, such as mucus on the surface of living organisms. The organic suspended matter 64 in the filtrate W3 is adsorbed onto the surfaces of the bubbles 63 rising in the foam separation tower 61, and the organic suspended matter 64 is separated together with the bubbles 63. Concentrated wastewater W4 is discharged from the top of the foam separation tower 61. In the foam separation unit 6, the organic suspended matter 64 is discharged together with a small amount of water between the bubbles 63, resulting in concentrated wastewater W4 with a high concentration of organic suspended matter 64. The concentrated wastewater W4 is temporarily stored in the second buffer tank 32. The wastewater W5 stored in the second buffer tank 32 includes the concentrated wastewater W4 obtained by foam separation in the foam separation section 6, as well as the backwash wastewater W7 from the filtration separation section 4. The amount of dissolved oxygen in the wastewater W5 in the second buffer tank 32 may be reduced by aeration with nitrogen using a nitrogen gas supply means (not shown).

[0060] In this embodiment, as shown in Figures 1 and 2, the foam separation section 6 foam-separates the filtrate W3 that has been oxidized in the advanced oxidation section 5, and a pretreatment unit 7 is provided with the advanced oxidation section 5 and the foam separation section 6, in this order. When the advanced oxidation section 5 is provided upstream of the foam separation section 6, as in this configuration, suspended solids (carbon content) in the wastewater can be concentrated and efficiently recovered. In detail, the inventors presumed the following mechanisms (A) to (D) in the pretreatment unit 7 and decided to provide the advanced oxidation section 5 upstream of the foam separation section 6. (A) When advanced oxidation is performed first, the protein molecules contained in the organic suspension are decomposed, and the number of molecular ends with polar groups increases. As a result, the protein molecules are more likely to adsorb to bubbles due to their zeta potential, improving the separation efficiency of the organic suspension during foam separation. (B) The organic suspended solids are broken down into smaller molecules by the accelerated oxidation treatment, improving the fluidity of the concentrated wastewater W4 after foam separation. As a result, the treatment efficiency of foam separation is improved. (C) If advanced oxidation is performed prior to foam separation, the concentrations of ozone and hydroxyl radicals generated by advanced oxidation decrease during foam separation. (D) When the foam separation section 6 is equipped with the gas supply means 62, a large amount of gas is introduced, and therefore the ozone and hydroxyl radicals introduced in the advanced oxidation section 5 are diluted by the gas.

[0061] The separated water W6 separated in the foam separation section 6 is returned to the aquaculture tank 2 either directly or via an activated carbon filter 21. The separated water W6 has been largely freed of fine organic suspended matter that is difficult for microorganisms to utilize, and contains easily decomposable water-soluble organic matter produced by the advanced oxidation treatment in the advanced oxidation section 5. This water-soluble organic matter can be used as a nutrient source for nitrifying bacteria to perform nitrification in the rearing water W1 of the aquaculture tank 2.

[0062] The concentrated wastewater W4 discharged from the foam separation section 6 has stable bubbles 63 containing seawater with organic suspended matter 64 attached thereto, and moves slowly through the piping. For this reason, as shown in Figure 1, a small amount of washing seawater may be flowed through the piping to wash away the organic suspended matter 64. When a certain amount of wastewater W5 has been stored in the second buffer tank 32, it is introduced into the biological treatment tank 8.

[0063] In this embodiment, the rearing water W1 in the aquaculture tank 2 is configured in a circulation system so that solids and organic suspended matter are removed from the rearing water W1 through the first buffer tank 31, the filtration separation section 4, the pre-treatment unit 7 (advanced oxidation section 5, foam separation section 6) and the activated carbon filter 21, and the water is returned to the aquaculture tank 2.

[0064] [Biological treatment tank and wastewater treatment system equipped with biological treatment tank] Next, we will explain a wastewater treatment system 10 equipped with a biological treatment tank 8 downstream of the pretreatment facility 1. The biological treatment tank 8 biologically treats wastewater W5, which includes concentrated wastewater W4 and backwash wastewater W7 recovered by the pretreatment facility 1, to remove nitric acid (nitrates). The wastewater treatment system 10 includes at least the pretreatment facility 1, the biological treatment tank 8, a liquid delivery means (flow path piping and pump P2) for delivering the wastewater W5 obtained in the pretreatment facility 1 to the biological treatment tank 8, and a return means 9 for returning the treated liquid (supernatant W8) from the biological treatment tank 8.

[0065] As shown in FIG. 1, the biological treatment tank 8 includes an anaerobic treatment section 81 that performs anaerobic treatment, an aerobic treatment section 82 that performs aerobic treatment, and a settling section 83 that settles activated sludge from the contents treated in the aerobic treatment section 82.

[0066] That is, the wastewater treatment system 10 includes an anaerobic treatment section 81 after both the aforementioned advanced oxidation section 5 and foam separation section 6. The concentrated wastewater W4 that has been treated in the advanced oxidation section 5 and foam-separated together with air bubbles in the foam separation section 6 is introduced at least into the anaerobic treatment section 81.

[0067] In this embodiment, wastewater W5 stored in the second buffer tank 32 is introduced into the anaerobic treatment unit 81. The wastewater W5 is wastewater with a high C / N ratio, including concentrated wastewater W4 and backwash wastewater W7. In the anaerobic treatment unit 81, nitrate and nitrite (nitrate nitrogen) contained in the wastewater W5 are reduced to nitrogen gas by anaerobic bacteria under anaerobic conditions. The nitrogen gas is released into the atmosphere, consuming and removing the nitrate. The wastewater from which nitrate has been removed is introduced from the anaerobic treatment unit 81 into the aerobic treatment unit 82.

[0068] In the aerobic treatment unit 82, aerobic treatment is performed by aeration. In the present invention, the main functions of the aerobic treatment unit 82 are to restore dissolved oxygen (DO), remove trace amounts of toxic gases (e.g., hydrogen sulfide) that may be generated in the anaerobic treatment unit 81, and re-oxidize and neutralize harmful nitrite nitrogen that may be generated in the anaerobic treatment unit 81. In a typical biological treatment unit (activated sludge treatment unit), the purpose is to decompose suspended and dissolved organic matter in the water using microorganisms, releasing carbon as carbon dioxide, immobilizing the microbial bodies, and allowing them to settle as sludge, and to oxidize ammonia, nitrite, and other substances in the water to nitrate (nitrification). However, in the wastewater treatment system of the present invention, ammonia is generally removed from the culture water W1 in the aquaculture tank 2, and the above-mentioned functions are the main functions.

[0069] In the settling section 83, the contents of the aerobic treatment section 82 are separated into treated water and activated sludge. The activated sludge settles in the tank and is separated from the supernatant liquid W8 (treated water). The activated sludge in the settling section 83 is returned to the anaerobic treatment section 81 by pump P3 as returned sludge.

[0070] The supernatant W8 from the settling section 83 is returned by the return means 9 to the first buffer tank 31 in which the aquaculture wastewater W2 is stored. The return means 9 is composed of a return flow path 9A and a pump P4 installed in the return flow path 9A. The supernatant W8 returned to the first buffer tank 31 is mixed with the aquaculture wastewater W2 and returns to the aquaculture tank 2 via at least the filtration / separation section 4, the advanced oxidation section 5, and the foam separation section 6. In this way, in the wastewater treatment system 10, the treated water (supernatant W8) that has been oxidatively decomposed by the advanced oxidation section 5 is returned to the aquaculture tank 2, so that microorganisms contained in the activated sludge in the biological treatment tank 8 do not get mixed into the aquaculture tank 2, and the aquaculture tank 2 can maintain its microbial environment. Furthermore, according to the wastewater treatment system 10 of this embodiment, the treated water (supernatant W8) from the biological treatment tank 8 is subjected to oxidative decomposition treatment in the same accelerated oxidation section 5 as the filtered water W3 from the aquaculture wastewater W2 and is returned to the aquaculture tank 2, so that the accelerated oxidation section 5 can be used for both purposes, resulting in a simple system.

[0071] In this embodiment, the separated water W6 after foam separation is passed through an ozone gas removal means (activated carbon filter 21 in this embodiment) before being introduced into the aquaculture tank 2. By passing the separated water W6 through the activated carbon filter 21, even if ozone gas and hydroxyl radicals generated by the advanced oxidation unit 5 remain, they can be removed. However, the installation of an ozone gas removal means such as the activated carbon filter 21 is optional. This is because the ozone gas generated by the advanced oxidation unit 5 becomes hydroxyl radicals when exposed to ultraviolet light, and because hydroxyl radicals have a short lifespan, the ozone gas and hydroxyl radicals essentially disappear before reaching the aquaculture tank 2.

[0072] As described above, the pretreatment facility 1 and pretreatment unit 7 according to this embodiment can oxidize the organic suspended matter in the aquaculture wastewater to organic carbon-containing substances in a form that is easily utilized by microorganisms, while concentrating and efficiently recovering the organic suspended matter through foam separation. Therefore, wastewater with a high C / N ratio that is suitable for biological denitrification can be obtained from the aquaculture wastewater W2 without adding organic carbon from outside the system. Furthermore, the wastewater treatment system 10 equipped with the pretreatment facility 1 and the biological treatment tank 8 can effectively utilize the organic suspended matter in the aquaculture wastewater W2 as a carbon source to remove nitrate (nitrates) through biological denitrification.

[0073] In the pretreatment facility 1 of this embodiment, the organic suspension that has become water-soluble through the accelerated oxidation treatment becomes available to microorganisms in the rearing water W1 and serves as nutrients for nitrifying bacteria, potentially promoting ammonia decomposition by the nitrifying bacteria. Furthermore, in the pretreatment facility 1 and pretreatment unit 7 of this embodiment, the accelerated oxidation section 5 is composed of an ozone gas supply means 51 and an ultraviolet light irradiation means 52, so harmful substances such as hydrogen peroxide and chlorine-based disinfectants (e.g., sodium hypochlorite) do not remain. This makes the aquaculture easy to use for the reared organisms in aquaculture, as well as for animals and humans who eat the reared organisms.

[0074] (2) Second embodiment of wastewater treatment system Next, a wastewater treatment system 20 according to a second embodiment of the present invention will be described with reference to Fig. 7. Note that the same components as those in the wastewater treatment system 10 according to the first embodiment are denoted by the same reference numerals as those in the wastewater treatment system 10, and detailed description thereof will be omitted.

[0075] The wastewater treatment system 20 differs from the wastewater treatment system 10 of the first embodiment in the configuration of the pretreatment facility. In the pretreatment facility 11 of the wastewater treatment system 20 of the second embodiment, the order of the advanced oxidation unit 5 and the foam separation unit 6 is reversed compared to the pretreatment facility 1. Instead of treating the wastewater in the advanced oxidation unit 5 and then performing foam separation, the concentrated wastewater W4 and separated water W6 separated by foam separation in the foam separation unit 6 are each subjected to advanced oxidation in two advanced oxidation units 5A and 5B. Because ozone gas is converted into short-lived hydroxyl radicals by the ultraviolet irradiation means 52 in the advanced oxidation unit 5B and disappears, ozone gas is generally not introduced into the aquaculture tank 2. However, to ensure that ozone gas is not introduced into the aquaculture tank 2, it is desirable to provide an ozone gas removal unit at the outlet of the pretreatment facility 11. In this embodiment, because the advanced oxidation unit 5B and the aquaculture tank 2 are close to each other, it is desirable to provide an ozone removal unit between the advanced oxidation unit 5B and the aquaculture tank 2. In the first and second embodiments, an example in which the activated carbon filter 21 is provided as an ozone gas removal means is shown. However, it is sufficient if the ozone can be removed when unreacted ozone remains, and instead of or in addition to the activated carbon filter 21, the ozone may be brought into contact with an ozone decomposition catalyst.

[0076] In the pretreatment facility 11, the organic suspended matter in the aquaculture wastewater W2 can be concentrated and recovered by foam separation, while the organic suspended matter can be oxidatively decomposed into organic carbon-containing substances in a form that is easily usable by microorganisms, thereby obtaining wastewater with a high C / N ratio in a form that is easily usable by microorganisms.

[0077] (3) Wastewater treatment method Next, a wastewater treatment method using a wastewater treatment system 10 according to a first embodiment will be described with reference to FIGS. 1 to 6. The wastewater treatment method according to this embodiment treats aquaculture wastewater W2 containing seawater discharged from an aquaculture tank 2, and generally comprises a pretreatment step using a pretreatment facility 1 and a biological treatment step using a biological treatment tank 8. The pretreatment step is for obtaining wastewater with a high C / N ratio from aquaculture wastewater W2 with a low C / N ratio, and includes a filtration / separation step, an advanced oxidation step, and a foam separation step. The wastewater with a high C / N ratio obtained in the pretreatment step is treated at least anoxically in the biological treatment step, and nitric acid (nitrates) is removed.

[0078] [Filtration separation process] The filtration / separation process is a process of filtering and separating the aquaculture wastewater W2 into solids and filtrate W3. In this process, as shown in FIG. 3, the aquaculture wastewater W2 is introduced through the second opening 41B of the outer cylinder 41, and the upward flow of the aquaculture wastewater W2 causes the movable guide 44 to move upward. The upward movement of the movable guide 44 is restricted by an upper stopper (not shown), and the fibrous filter material 42 is compressed to a predetermined thickness to form a filtration layer. The upper stopper maintains the thickness of the filtration layer at a substantially constant value or greater. In this embodiment, the thickness of the filtration layer during filtration is set to a predetermined value, for example, 400 mm to 600 mm. In the filtration process, the aquaculture wastewater W2 is filtered and separated into solids and filtrate W3 by this filtration layer of predetermined thickness. The filtrate W3 that passes through the filtration layer is discharged through the first opening 41A of the outer cylinder 41 and introduced into a pretreatment unit 7, which includes an advanced oxidation section 5 and a foam separation section 6. In the filtration separation step, it is sufficient to remove relatively large solids, for example, those exceeding 30 μm (preferably exceeding 10 μm, and more preferably exceeding 5 μm). In fact, increasing the filtration accuracy is synonymous with increasing the backwash frequency, so it is important to set the backwash drainage volume within an appropriate range, taking into account the water quality and the rearing density of the target organisms. Therefore, the thickness of the filtration layer can be selected more widely than the above-mentioned range. Furthermore, suspended matter of 30 μm or less is foam-separated in the foam separation section 6, so it may be contained in the filtrate W3.

[0079] [Promoted oxidation process] The accelerated oxidation process involves mixing ozone gas with the filtrate W3 obtained in the aforementioned filtration / separation process, followed by irradiating the filtrate W3 with ultraviolet light. As shown in FIG. 2, ozone gas is mixed with the filtrate W3 obtained in the filtration / separation process using an ozone gas supply means 51. The amount of ozone gas mixed is adjusted by adjusting the amount of gas generated by the ozone gas generator 51A and the opening / closing balance of the valves 51C and 51D in the two branch flow paths. Next, ultraviolet light is irradiated onto the filtrate W3 with the ozone gas mixed with the gas / liquid using an ultraviolet light irradiation means 52. When the ozone gas is irradiated with ultraviolet light, hydroxyl radicals with strong oxidizing power are generated, which oxidize and decompose the organic suspended matter and water-soluble, persistent organic matter contained in the filtrate W3, converting the high-molecular-weight compounds into low-molecular-weight organic compounds that are easily utilized as a nutrient source by microorganisms.

[0080] [Foam separation process] The foam separation step is a step in which the filtrate W3 obtained in the filtration separation step is introduced together with bubbles 63 to perform foam separation. Before the filtrate W3 obtained in the advanced oxidation step is introduced into the foam separation tower 61, bubbles that will become bubbles for foam separation are introduced by a gas supply means 62, as shown in FIG. 2. The amount of bubbles for foam separation is adjusted by the amount of gas generated by the gas generator 62A and the balance between the opening and closing of the valves 62C and 62D of the two branch flow paths described above. The amount of bubbles introduced in the foam separation step (gas-liquid ratio) and bubble diameter are not particularly limited as long as they are optimized to facilitate the floating of the bubbles. For example, the bubble diameter may be approximately 100 μm to 300 μm. The amount of bubbles introduced in the foam separation step (gas-liquid ratio) and bubble diameter may be different from the amount of ozone gas introduced in the advanced oxidation step (gas-liquid ratio) and bubble diameter. The bubble diameter of the ozone gas in the advanced oxidation step is preferably small because the contact area between the bubbles and the liquid affects the reaction rate.

[0081] The filtered water W3 containing the gas bubbles mixed with the liquid is introduced into the lower part of the foam fractionation tower 61. Then, as the gas bubbles 63 rise inside the foam fractionation tower 61, the organic suspended matter 64 contained in the filtered water W3 is adsorbed onto the surface of the gas bubbles 63. The organic suspended matter 64 accumulates together with the gas bubbles 63 at the top of the foam fractionation tower 61 and is discharged by being pushed out from the top of the foam fractionation tower 61. The gas bubbles 63 that have adsorbed the organic suspended matter 64 become concentrated wastewater W4 in which the organic suspended matter 64 is concentrated while flowing through the piping, and the concentrated wastewater W4 is introduced into the second buffer tank 32. If necessary, the concentrated wastewater W4 is washed away with washing seawater.

[0082] In this way, the wastewater treatment method can oxidize organic suspended matter in the aquaculture wastewater into organic carbon-containing substances in a form that is easily utilized by microorganisms, while concentrating them through foam separation and efficiently recovering them.

[0083] Next, a description will be given of the solid content recovery process for recovering the solid content filtered and separated by the filtration separation unit 4. In this embodiment, the solid content is recovered as backwash wastewater W7, and is temporarily stored in the second buffer tank 32 together with the above-mentioned concentrated wastewater W4, and then introduced into the anaerobic treatment unit 81 of the biological treatment tank 8.

[0084] [Solid content recovery process] The pretreatment facility 1 periodically (for example, once an hour) stops operation of the above-mentioned series of processes (filtration separation process, advanced oxidation process, foam separation process) and washes the fibrous filter material 42 of the filtration separation section 4. The solid content recovery process is a process in which the solid content captured in the filtration separation section 4 is washed and removed from the filtration separation section 4, and the solid content is recovered. The solid content recovery process is composed of a gas washing process and a backwashing process using backwash water.

[0085] (Gas cleaning process) The gas scrubbing process is a process in which gas is sprayed onto the fibrous filter material 42 using the gas ejection means 46 to scrub the fibrous filter material 42. In this embodiment, this process is performed prior to the backwashing process. As filtration and separation by the filtration and separation unit 4 continues, solids in the aquaculture wastewater W2 accumulate on the fibrous filter material 42, gradually deteriorating its filtering performance. Therefore, after the filtration and separation process has continued for a predetermined period of time, gas scrubbing of the fibrous filter material 42 is performed. As shown in FIG. 4 , the gas sprayed toward the fibrous filter material 42 forms bubbles that rise while widening the spaces between the fibers of the fibrous filter material 42. The action of the bubbles kneads the fibrous filter material 42, gradually expanding the compressed fibrous filter material 42 and lowering the movable guide 30. In this manner, the fibrous filter material 42 is defibrated and vibrated, causing the solids captured by the fibrous filter material 42 to fall off. The solids that fall off the fibrous filter material 42 settle within the outer cylinder 41, and some are discharged through the second opening 41B of the outer cylinder 41. The second opening 41B serves as the outlet side (discharge side) in the subsequent backwashing step, and therefore can be efficiently discharged in the backwashing step.

[0086] The air bubbles that have passed through the outer cylinder 41 are discharged from the first opening 41A of the outer cylinder 41, and are released outside the system from the second buffer tank 32 via a branch pipe and an air release pipe provided above the first opening 41A.

[0087] The gas cleaning process may be performed only until the compressed fiber filtration material 42 expands, or may be continued for a predetermined period of time. The frequency and duration of the gas cleaning process are not particularly limited, but in this embodiment, the gas cleaning process is performed for approximately 15 seconds every hour. The frequency and duration of the gas cleaning process may be set appropriately depending on the properties and amount of solids adhering to the fiber filtration material 42. Furthermore, it is preferable that the outer tube 41 be made of a transparent material, since this makes it easy to adjust the frequency and duration of the gas cleaning process while visually checking the state of adhesion of solids to the fiber filtration material 42.

[0088] (Backwashing process using backwash water) The backwashing process is a process of cleaning the fibrous filtering material 42 by backwashing with backwash water, and in this embodiment, it is performed after the gas cleaning process. As shown in Fig. 5, backwashing water is supplied from the first opening 41A of the outer cylinder 41 to backwash the fibrous filtering material 42 that has been stretched in the gas cleaning process. With this configuration, the fibrous filtering material 42 is kneaded with air bubbles to cause solids to fall off to a certain extent, and then backwashed with backwashing water, thereby improving the cleaning effect.

[0089] When the filtration / separation unit 4 is applied to a normal filtration device, filtered water (treated water) is used as backwash water, but in this embodiment, the aquaculture wastewater W2 is used as backwash water. This is because in the pretreatment facility 1 of the present invention, the filtration / separation unit 4 only needs to remove most of the relatively large solids, and it is not necessary for microscopic solids to be completely removed.

[0090] As described above, the filtration / separation unit 4 has a backwashing mechanism, and backwashing wastewater W10 is introduced into the anaerobic treatment unit 81 of the biological treatment tank 8 in addition to the concentrated wastewater W4 described below. The backwashing wastewater W10 is preferably subjected to accelerated oxidation treatment in a separate accelerated oxidation unit (not shown) before being introduced into the anaerobic treatment unit 81 of the biological treatment tank 8. The time and frequency of backwashing can be appropriately set depending on the properties and amount of solids adhering to the fibrous filter material 42 and are not particularly limited. For example, backwashing can be performed approximately once per hour. In this embodiment, backwashing is performed once per hour, discharging 25 to 50 L of backwashing wastewater W7 for 30 seconds. Furthermore, given the objective of the present invention to obtain wastewater with a high C / N ratio, it is desirable to minimize the amount of backwashing water. For example, 0.5 to 2.0 vol. %, preferably approximately 1.0 vol. %, of the filtrate W3 (treated water) can be used. The backwashing process may be carried out for only a short time to discharge the solids that have fallen off and settled in the gas scrubbing process from the second opening 41B to the outside of the outer cylinder 41. The filtration separation section 4 using the fiber filtration device of this embodiment can reduce the amount of backwashing wastewater W10, making it suitable for the pretreatment facility 1 of the present invention. The outer cylinder 41 is preferably made of a transparent material, as this makes it easy to adjust the frequency and duration of the backwashing process while visually checking the state of adhesion of the solids to the fiber filtration material 42.

[0091] As described above, in the solid content recovery process, the fibrous filter material 42 is washed by the gas washing process and the backwashing process using backwash water, and the solid content of the fibrous filter material 42 is recovered as backwashing wastewater W10 in the second buffer tank 32. In this way, wastewater with a high C / N ratio that contains organic carbon-containing substances in a form that is easily usable by microorganisms can be obtained from the aquaculture wastewater.

[0092] [Other embodiments] Other embodiments of the wastewater treatment system, wastewater treatment unit, and wastewater treatment method according to the present invention will be described below. Note that the configurations disclosed in the following embodiments can be applied in combination with the configurations disclosed in other embodiments, as long as no contradiction occurs.

[0093] In the above embodiment, an example has been described in which an ejector is used as a device for gas-liquid mixing of ozone gas with filtered water W3 in the ozone gas supply means 51. However, in the present invention, the device and method for gas-liquid mixing of ozone gas are not limited, and a mixer such as an in-line mixer may also be used. The same applies to the gas supply means 62; a mixer or the like may be used instead of or in addition to the ejector for gas bubble mixing.

[0094] In the above embodiment, an example has been described in which gas supply means 62 is further used to supply bubbles 63 for foam separation in the foam separation section 6. However, in the present invention, particularly when the advanced oxidation section 5 and the foam separation section 6 are provided close to each other, bubbles of ozone gas mixed with gas and liquid in the advanced oxidation section 5 may be used for foam separation in the foam separation section 6, and gas supply means 62 does not necessarily have to be provided. This is because, as mentioned above, seawater easily forms stable foam.

[0095] In the above embodiment, an example has been described in which the first buffer tank 31 for storing the aquaculture wastewater W2 is provided between the aquaculture tank 2 and the filtration / separation unit 4. However, the present invention is not limited to the presence or absence of a buffer tank, and the buffer tank may not be provided. Furthermore, multiple buffer tanks may be provided.

[0096] In the above-described embodiments, the wastewater treatment systems 10 and 20 are described as examples each including a biological treatment tank 8. However, in the wastewater treatment system of the present invention, the biological treatment tank 8 may or may not be included.

[0097] In the above embodiment, the biological treatment tank 8 is described as having an anaerobic treatment section 81, an aerobic treatment section 82, and a settling section 83. However, the biological treatment tank 8 is not limited to this configuration. When the present invention includes a biological treatment tank 8, the biological treatment tank 8 may have, for example, only the anaerobic treatment section 81, or may have both the anaerobic treatment section 81 and the aerobic treatment section 82. The biological treatment tank 8 may also be a biological treatment device (activated sludge treatment device) with a different configuration. As an example, a known membrane separation tank (e.g., a hollow fiber membrane separation tank) may be provided instead of the settling section 83, forming a membrane separation-type activated sludge treatment device. If the membrane separation tank uses a filter medium that blocks microorganisms, the treated water separated by the membrane separation section may be returned directly to the aquaculture tank 2.

[0098] In the above embodiment, an example has been described in which the anaerobic treatment section 81, the aerobic treatment section 82, and the sedimentation section 83 of the biological treatment tank 8 are configured as different tanks. However, the biological treatment tank 8 may be configured with one or two tanks, and these one or two tanks may be partitioned into the anaerobic treatment section, the aerobic treatment section, the sedimentation section, etc.

[0099] In the above embodiment, the returning means 9 is configured to return the supernatant liquid W8 of the wastewater aerobically treated in the aerobic treatment unit 82 to the first buffer tank 31. However, in the present invention, the returning means 9 may return the supernatant liquid W8 to another tank (for example, the aquaculture tank 2) as long as it does not contradict the object of the present invention.

[0100] In the above embodiment, the filtration / separation unit 4 is described as having a backwashing mechanism. However, the present invention is not limited to the presence or absence of a backwashing mechanism. For example, an operator may remove the filtering material in the filtration / separation unit 4 and manually collect the solids captured by the filtering material. Also, in the present invention, the solids captured in the filtration / separation unit 4 may not be collected, and only the concentrated wastewater W4 may be introduced into the biological treatment tank 8.

[0101] In the above embodiment, the filtration / separation unit 4 is described as a fiber filtration device. However, the present invention is not limited to this configuration. The filtration material of the filtration / separation unit 4 may be, for example, various filters, sand (sand filter), etc.

[0102] In the above embodiment, the filtration separation unit 4 is described as a fibrous filtration device that compresses fibrous filtration material upward to form a filtration layer. However, the present invention is not limited to this configuration of the fibrous filtration device, and may be a fibrous filtration device that compresses fibrous filtration material downward to form a filtration layer, or may be a fibrous filtration device with another configuration.

[0103] In the above embodiment, an example has been described in which filtrate W3 (filtrate from one pass) that has passed through the filtration / separation section 4 once is introduced into the advanced oxidation section 5 and the foam separation section 6. However, in the present invention, the filtration / separation section 4 may be a circulation-type filtration device, and filtrate that has passed through the filtration / separation section 4 multiple times may be introduced into the advanced oxidation section 5 and the foam separation section 6.

[0104] In the above-described embodiment of the wastewater treatment method, a configuration in which the foam separation step is performed after the advanced oxidation step has been mainly described as an example. However, in the wastewater treatment method of the present invention, the advanced oxidation step and the foam separation step can be performed in any order as long as the filtrate obtained in the filtration separation step is treated. For example, as shown in FIG. 7, the advanced oxidation step may be performed after the foam separation step.

[0105] Regarding other configurations, it should be understood that the embodiments disclosed in this specification are illustrative in all respects and that the scope of the present invention is not limited thereby. Those skilled in the art will easily understand that appropriate modifications are possible without departing from the spirit of the present invention. Therefore, other embodiments modified without departing from the spirit of the present invention are naturally included in the scope of the present invention. [Industrial Applicability]

[0106] The present invention can be used as a wastewater treatment system for aquaculture wastewater in land-based aquaculture. [Explanation of symbols]

[0107] 1: Pre-treatment facility 2: Aquaculture tank 21: Activated carbon filter 31: First buffer tank 32: Second buffer tank 4:Filtration separation section 5: Advanced oxidation section 51: Ozone gas supply means 52: Ultraviolet irradiation means 6: Foam separation section 7: Pretreatment unit (wastewater treatment unit) 8: Biological treatment tank 81: Anoxic treatment section 82: Aerobic treatment section 83: Settling section 9: Return method 10, 20: Wastewater treatment system P1~P4: Pump

Claims

1. A wastewater treatment system for treating aquaculture wastewater containing seawater discharged from an aquaculture tank, a filtration and separation unit that filters and separates the aquaculture wastewater into solids and filtrate; an accelerated oxidation unit including an ozone gas supply means capable of mixing ozone gas with the filtered water in a gas-liquid state, and an ultraviolet light irradiation means capable of irradiating ultraviolet light onto the filtered water in which the ozone gas has been mixed in a gas-liquid state; A foam separation section that performs foam separation on the filtrate water introduced together with air bubbles.

2. The wastewater treatment system according to claim 1 , wherein the foam separation section performs foam separation on the filtrate that has been oxidized in the advanced oxidation section.

3. A biological treatment tank having an anoxic treatment unit is provided downstream of both the accelerated oxidation unit and the foam separation unit, The wastewater treatment system according to claim 1 or 2, wherein filtrate treated in the advanced oxidation section and foam-separated together with the bubbles in the foam separation section is introduced into the anoxic treatment section.

4. The biological treatment tank has an aerobic treatment unit downstream of the anoxic treatment unit, a return means for returning the supernatant of the wastewater aerobically treated in the aerobic treatment unit to the tank for storing the aquaculture wastewater, The wastewater treatment system according to claim 3 , wherein the returned supernatant is treated in at least one advanced oxidation unit before being introduced into the aquaculture tank.

5. a buffer tank for storing the aquaculture wastewater between the aquaculture tank and the filtration / separation unit; The wastewater treatment system according to claim 4 , wherein the return means returns the supernatant to the buffer tank.

6. The wastewater treatment system according to claim 3 , wherein the filtration / separation unit has a backwashing mechanism, and backwash wastewater from the backwashing mechanism is further introduced into the anoxic treatment unit.

7. The wastewater treatment system according to claim 6, wherein the filtration separation unit is a fibrous filtration device in which a fibrous filtration material is compressed to form a fibrous filtration layer during filtration, and the compressed fibrous filtration material expands during backwashing.

8. The wastewater treatment system according to claim 1 , wherein the foam separation section further comprises a gas supply means for supplying bubbles.

9. A wastewater treatment unit that treats aquaculture wastewater containing seawater discharged from an aquaculture tank, an accelerated oxidation unit having an ozone gas supply means for gas-liquid mixing of ozone gas with the aquaculture wastewater, and an ultraviolet ray irradiation means for irradiating ultraviolet rays onto the aquaculture wastewater in which the ozone gas has been gas-liquid mixed; a foam separation section that performs foam separation on the aquaculture wastewater treated in the advanced oxidation section.

10. A wastewater treatment method for treating aquaculture wastewater containing seawater discharged from an aquaculture tank, comprising: a filtration separation step of filtering and separating the aquaculture wastewater into solids and filtrate; an accelerated oxidation step of mixing ozone gas with the filtrate obtained in the filtration separation step, and then irradiating the filtrate with ultraviolet light; a foam separation step of introducing the filtered water obtained in the filtration separation step together with air bubbles to perform foam separation.

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