Wastewater treatment system and wastewater treatment method

Abstract

To provide a wastewater treatment system capable of obtaining a desired liquid fertilizer by concentrating wastewater such as digestive fluid by a physical filtration concentration separation system without using a large amount of chemicals.SOLUTION: A wastewater treatment system includes: a centrifugal separator that separates organic matter-containing wastewater into a solid phase and a liquid phase by a centrifugal force; a filtration device that filters the liquid phase separated by the centrifugal separator to remove solids; and an electrodialysis device that electrodialyzes filtrate filtered by the filtration device to obtain a concentrated solution in which nitrogen components are concentrated. The centrifugal separator includes: a storage tank that stores the wastewater; a rotating container that includes a wall portion, is disposed above the storage tank, is provided with an opening, and rotates at a predetermined speed; and a discharge unit that discharges the wastewater stored in the storage tank toward an inner wall surface of the rotating container. The centrifugal separator discharges the wastewater from the storage tank toward an inner wall surface of the wall portion by the discharge unit while rotating the rotating container with the opening facing downward, and separates the solid phase from the liquid phase by adhering the solid phase to the inner wall surface of the wall portion by the centrifugal force.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a wastewater treatment system and a wastewater treatment method. [Background technology]

[0002] Techniques relating to the treatment of digestive fluid produced by methane fermentation of organic waste have been disclosed (see, for example, Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-13909 [Patent Document 2] Japanese Patent Application Publication No. 2019-131432 Summary of the Invention [Problem to be solved by the invention]

[0004] Recently, organic waste such as livestock waste and food waste has been fermented, and biogas, primarily composed of methane, is recovered from the digestate and used as recycled energy. Because the digestate from methane fermentation contains nutrients for plants, its use as liquid fertilizer is being considered.

[0005] Because the concentration of nutrients contained in the digestive fluid is low when left as is, it needs to be sprayed in large quantities when used as liquid fertilizer, but the costs of transporting and spraying the digestive fluid are high, so this method is not fully utilized. One possible method to increase the concentration of fertilizer components is to evaporate the water in the digestive fluid, but this requires a huge energy load to evaporate the water and is undesirable because it also concentrates impurities that are not actually necessary for liquid fertilizer. When subjecting the digestive fluid to coagulation treatment, large amounts of chemicals need to be added, but many of these chemicals are not organic, which poses challenges for a biorecycle system (organic matter recycle system).

[0006] Therefore, one of the objectives is to provide an effluent treatment system that can obtain the desired liquid fertilizer by concentrating effluent such as digestive fluid using a physical filtration, concentration, and separation method without using large amounts of chemicals. [Means for solving the problem]

[0007] The wastewater treatment system according to the present invention includes a centrifuge that separates wastewater containing organic matter into a solid phase and a liquid phase by centrifugal force, a filtration device that filters the liquid phase separated by the centrifuge to remove solids, and an electrodialysis device that electrodialyzes the filtrate filtered by the filtration device to obtain a concentrate with a concentrated nitrogen component. The centrifuge includes a storage tank that stores the wastewater, a rotating container that includes a wall, is disposed above the storage tank, has an opening, and rotates at a predetermined speed, and a discharge unit that discharges the wastewater stored in the storage tank toward the inner wall surface of the rotating container. The centrifuge separates the solid phase from the liquid phase by rotating the rotating container with the opening facing downward, using the discharge unit to discharge the wastewater from the storage tank toward the inner wall surface of the wall. [Effects of the Invention]

[0008] According to the wastewater treatment system, the desired liquid fertilizer can be obtained by concentrating wastewater such as digestive fluid using a physical filtration, concentration and separation method without using a large amount of chemicals. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a block diagram showing the configuration of a waste liquid treatment system according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing a part of the centrifugal separator. [Figure 3] FIG. 3 is a schematic perspective view showing a part of the filtration device. [Figure 4] FIG. 4 is a flowchart showing typical steps in a wastewater treatment method using the wastewater treatment system. [Figure 5]FIG. 5 is a diagram showing an example of a material balance in a wastewater treatment method using the wastewater treatment system according to the first embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010] [Outline of the embodiment] The wastewater treatment system according to the present invention includes a centrifuge that separates wastewater containing organic matter into a solid phase and a liquid phase by centrifugal force, a filtration device that filters the liquid phase separated by the centrifuge to remove solids, and an electrodialysis device that electrodialyzes the filtrate filtered by the filtration device to obtain a concentrate in which nitrogen components are concentrated. The centrifuge includes a storage tank that stores the wastewater, a rotating container that includes a wall, is disposed above the storage tank, has an opening, and rotates at a predetermined speed, and a discharge unit that discharges the wastewater stored in the storage tank toward the inner wall surface of the rotating container. The centrifuge separates the solid phase from the liquid phase by discharging the wastewater from the storage tank toward the inner wall surface of the wall using the discharge unit while rotating the rotating container with the opening facing downward.

[0011] The wastewater treatment system of the present invention can separate the solid and liquid phases contained in the wastewater using centrifugal force with a centrifuge. Then, the filtration device can remove the solids remaining in the liquid phase. In this case, the centrifuge removes the solid phase, thereby preventing clogging of the filtration device and increasing the frequency of filter membrane replacement, thereby enabling efficient removal of solids from the liquid phase over a long period of time. Then, the electrodialysis device can concentrate the nitrogen component to obtain a liquid fertilizer with a desired nitrogen concentration. Here, the centrifuge includes the storage tank, rotating vessel, and discharge section configured as described above, and can separate the solid and liquid phases by adhering the solid phase to the inner wall surface of the rotating vessel. In this case, the liquid phase flows down the inner wall surface of the rotating vessel and returns to the storage tank under its own weight, while the solid phase is deposited on the inner wall surface of the rotating vessel by centrifugal force, allowing the solid and liquid phases to be separated while the solid phase is separated from the liquid phase. Since the deposited solid phase adheres to the inner wall surface of the rotating vessel, it can be easily recovered by periodically scraping off the solid phase from the inner wall surface of the rotating vessel. Furthermore, the centrifuge, filtration, and electrodialysis devices including the above-described storage tank, rotating vessel, and discharge section can be constructed relatively compactly and inexpensively. As described above, the above-described wastewater treatment system can concentrate and treat wastewater using a physical filtration, concentration, and separation method without using large amounts of chemicals to obtain the desired liquid fertilizer. Furthermore, the solids recovered from the wall of the rotating vessel contain nitrogen, phosphorus, and other components, and can therefore be used as solid compost. The wastewater treatment system of the present invention can efficiently treat digestive fluids containing large amounts of organic matter and solids.

[0012] In the wastewater treatment system, the rotation speed of the rotary vessel may be 4000 rpm (rounds per minute) or more and 7000 rpm or less. If the rotation speed is too low, separation of solids will be insufficient, and if the rotation speed is too high, it will lead to energy loss. By increasing the rotation speed in this way, the solid phase contained in the wastewater can be more reliably attached to the inner wall surface of the rotary vessel while the liquid phase can be returned to the storage tank. Therefore, the solid phase and the liquid phase can be separated more efficiently. The rotation speed of the rotary vessel is 5000 rpm or more, preferably 5500 rpm or more, and preferably 6500 rpm or less.

[0013] In the wastewater treatment system, the centrifugal force generated in the rotary vessel may be 2200 G or more and 6800 G or less. By increasing the centrifugal force in this manner, the solid phase contained in the wastewater can be more reliably adhered to the inner wall surface of the rotary vessel while the liquid phase is returned to the storage tank. On the other hand, if the centrifugal force is too high, it leads to energy loss. Therefore, the solid phase and the liquid phase can be separated more efficiently. The centrifugal force generated in the rotary vessel is preferably 3500 G or more, further preferably 4200 G or more, and is preferably 6000 G or less.

[0014] In the wastewater treatment system, the periphery of the wall portion may be disposed inside the storage tank when viewed from the discharge direction of the discharge portion. This allows the liquid phase that drips down from the periphery of the wall portion to be reliably returned to the storage tank. Therefore, the solid phase and the liquid phase can be separated more efficiently.

[0015] In the wastewater treatment system, the filtration device may include a tubular ultrafiltration membrane. Alternatively, tangential flow filtration may be performed, in which the liquid phase flows through the ultrafiltration membrane in a direction parallel to the surface of the ultrafiltration membrane. The permeate that permeates the ultrafiltration membrane may be recovered as filtrate and sent to an electrodialysis device. Components that do not permeate the ultrafiltration membrane are recovered as retentate. The filtration device circulates the wastewater to separate the retentate from the permeate. This allows for the removal of most of the solids from the liquid phase, which still contains a relatively large amount of solids, in a short period of time, resulting in a permeate with a low solids content, or in some cases, a permeate with almost no solids. Furthermore, the permeate that permeates the tubular ultrafiltration membrane is easily recovered, and the retentate that does not permeate the ultrafiltration membrane is easily extracted. Therefore, solids can be efficiently removed from the liquid phase. The retentate recovered by the filtration device has a higher nitrogen concentration than the wastewater and can be used as liquid fertilizer.

[0016] In the wastewater treatment system, the filtration device may include a viscosity sensor that measures the viscosity of the liquid phase circulating through the filtration device and a flow meter that measures the flow rate of the liquid phase circulating through the filtration device. The filtration device may be controlled to stop when the viscosity of the liquid phase measured by the viscosity sensor reaches a predetermined value or when the flow rate of the liquid phase measured by the flow meter falls below a predetermined value. As filtration by the filtration device progresses, the permeated liquid is recovered and the retentate liquid is circulated, increasing the viscosity of the retentate liquid and slowing its flow rate. In tangential flow filtration, a slow flow rate of the circulating retentate liquid increases the likelihood of membrane clogging. Therefore, in the circulation loop of the filtration device, a flow meter is placed, for example, downstream of the ultrafiltration membrane to measure the flow rate of the retentate liquid. When the flow rate of the retentate liquid falls below a predetermined value, filtration is stopped. However, effluent contains a wide variety of organic matter and solids, and its properties are not constant. For example, if the effluent is a digested liquid discharged after methane fermentation, the digested liquid contains a wide variety of substances, such as straw and rice husks, and its properties are unstable. Therefore, even if the flow rate of the circulating non-permeated liquid is above a predetermined value, the viscosity of the non-permeated liquid may become higher than expected. If this condition continues, excessive pressure may be applied to the ultrafiltration membrane, potentially damaging it. Therefore, a viscosity sensor is installed downstream of the ultrafiltration membrane in the circulation loop of the filtration device to measure the viscosity of the non-permeated liquid. Filtration is stopped when the viscosity exceeds a predetermined value. When filtration is stopped, the fact and the reason for the stop may be indicated by a display or lamp. If the properties of the non-permeated liquid are stable, controlling the filtration device based solely on the flow rate or viscosity is unlikely to cause problems. However, because the properties of the effluent are unstable, it is preferable to control the filtration device based on the flow rate and viscosity. By managing the viscosity and flow rate of the liquid phase in this way, reliable filtration in a short time is possible while preventing damage to the ultrafiltration membrane. Therefore, solids can be more appropriately removed from the liquid phase.

[0017] The wastewater treatment system may further include a metal ion removal device containing a chelating resin, which is disposed before the electrodialysis device. This removes metal ions from the filtrate before treatment with the electrodialysis device, preventing clogging of the electrodialysis device. Removing calcium ions, in particular, can prevent clogging. This helps prevent damage to the electrodialysis device and frequent replacement of components.

[0018] The electrodialysis device has a layered structure in which cation exchange membranes and anion exchange membranes are alternately arranged, with deionization compartments and concentration compartments disposed between them. Electrodes are installed at both ends of the layered structure. When a liquid is supplied to the deionization compartment and a voltage is applied to the electrodes, ions are attracted to the electrodes, causing nitrogen ions to collect in the concentration compartment. As a result, nitrogen components are concentrated in the concentrated liquid discharged from the concentration compartment, and a deionized liquid is discharged from the deionization compartment. The nitrogen concentration of the concentrated liquid is, for example, 5 to 15 times higher than that of the effluent, making it suitable for use as liquid fertilizer.

[0019] The wastewater treatment method according to the present invention includes a centrifugation step of separating organic matter-containing wastewater into a solid phase and a liquid phase by centrifugal force, a filtration step of filtering the separated liquid phase to remove solids after the centrifugation step, and an electrodialysis step of electrodialyzing the filtrate obtained after the filtration step to obtain a concentrated liquid with concentrated nitrogen components. In the centrifugation step, the wastewater is discharged from a storage tank that stores the wastewater toward the inner wall surface of a rotating vessel that rotates at a predetermined speed with its opening facing downward, and the solid phase is caused to adhere to the inner wall surface by centrifugal force, and the liquid phase is recovered in the storage tank, thereby separating the solid phase from the liquid phase.

[0020] According to this wastewater treatment method, the desired liquid fertilizer can be obtained by treating the wastewater inexpensively without requiring large-scale facilities. The wastewater treatment method according to the present invention can inexpensively and efficiently treat even digested liquid containing a large amount of organic matter and solids.

[0021] [Specific example of embodiment] Next, an example of a specific embodiment of a wastewater treatment system according to the present invention will be described with reference to the drawings. The following description will be given using an example of digested liquid, which is wastewater discharged from a methane fermentation tank used for biogas power generation. However, the present invention is not limited to this example and can be applied to a treatment system and method for treating wastewater containing organic matter. Furthermore, the digested liquid is not limited to digested liquid obtained by methane fermentation. In the following drawings, the same or corresponding parts are designated by the same reference numerals, and their description will not be repeated.

[0022] (Embodiment 1) An explanation will be given of an effluent treatment system according to Embodiment 1 of the present invention. Fig. 1 is a block diagram showing the configuration of an effluent treatment system (digestive fluid treatment system) according to Embodiment 1 of the present invention.

[0023] Referring to FIG. 1, a digestive fluid treatment system 11 according to a first embodiment treats digestive fluid containing organic matter produced as a result of methane fermentation to obtain a desired liquid fertilizer. The digestive fluid treatment system 11 includes a centrifuge 12, a filtration device 13, a metal ion removal device 14, and an electrodialysis device 15. The centrifuge 12 separates the digestive fluid containing organic matter into a solid phase and a liquid phase by centrifugal force. The digestive fluid separated by the filtration device 13 is processed through the centrifuge 12, the filtration device 13, the metal ion removal device 14, and the electrodialysis device 15 to produce a liquid fertilizer with a desired concentration. The digestive fluid treatment system 11 may be configured by separately arranging the centrifuge 12, the filtration device 13, the metal ion removal device 14, and the electrodialysis device 15, or may be configured as an integrated device incorporating the above-mentioned multiple types of devices.

[0024] Next, the configuration of the centrifugal separator 12 will be described. Fig. 2 is a schematic diagram showing a part of the centrifugal separator 12. Referring to Fig. 2, the centrifugal separator 12 includes a storage tank 21, a rotary vessel 22, and a discharge unit 23. Note that the digestive fluid treatment system 11 may include a plurality of centrifugal separators 12 depending on the amount of digestive fluid to be treated.

[0025] The storage tank 21 includes a flat bottom wall 24 and a first side wall 25 that extends upright from the periphery of the bottom wall 24. That is, the storage tank 21 has an opening 26 at its top. In this embodiment, the bottom wall 24 is disk-shaped, and the first side wall 25 is hollow cylindrical. The storage tank 21 stores digestive fluid 16 that contains organic matter.

[0026] The rotating vessel 22 is disposed above the storage tank 21, and in this embodiment, vertically above the storage tank 21. The rotating vessel 22 is attached with a gap between it and the storage tank 21. The rotating vessel 22 includes a top wall 27 having a hemispherical inner wall surface and a second side wall 28 extending downward from the periphery of the top wall 27. An opening 29 is provided in the rotating vessel 22. The interior of the rotating vessel 22 is hollow, and no screw or other components are installed. In FIG. 2, the opening 29 faces downward, but when the rotating vessel 22 is not rotating, the opening 29 may face upward. The second side wall 28 is hollow and cylindrical. The inner wall surface of the top wall 27 is smoothly connected to the inner wall surface of the second side wall 28. If the size of the rotating vessel 22 is too large, it becomes difficult to rotate the rotating vessel 22, into which the digested liquid 16 with a high solid content is discharged, at a high rotation speed. On the other hand, if the size of the rotating vessel 22 is too small, the processing volume will be small and the processing time will be long. Therefore, the size of the rotating vessel 22 is preferably, for example, a hollow cylindrical shape with an inner wall surface of the second side wall portion 28 having a diameter of 20 cm to 50 cm and a capacity of approximately 1 L to 10 L. Note that when viewed from the discharge direction indicated by arrow D1 of the discharge portion 23 described below, the periphery of the wall portion (second side wall portion 28) is located inside the storage tank 21. The rotating vessel 22 rotates, for example, in the direction indicated by arrow R1 by control. The rotation axis 17 during this rotation is indicated by a dashed line. The rotation axis 17 passes through the most recessed portion of the rotating vessel 22. In the centrifugal separator 12, a rotation speed and centrifugal force suitable for removing solids of 3 μm or larger are selected, for example. The rotation speed of the rotating vessel 22 is 4000 rpm to 7000 rpm. The centrifugal force of the rotating vessel 22 is equal to or greater than 2200 G and equal to or less than 6800 G. As an example, the rotation speed of the rotating vessel 22 is selected to be 6000 rpm, and the centrifugal force of the rotating vessel 22 is selected to be 5000 G.

[0027] The discharge unit 23 is, for example, a discharge pump, and its discharge port is disposed in the storage tank 21. The discharge unit 23 discharges the digestive fluid 16 stored in the storage tank 21 toward the inner wall surface of the rotating vessel 22. That is, in this embodiment, the discharge unit 23 discharges the digestive fluid 16 vertically upward. The digestive fluid 16 filled in the storage tank 21 is circulated, and discharge onto the inner wall surface of the rotating vessel 22 and recovery into the storage tank 21 are repeated until a predetermined time has elapsed. After the centrifugation process is completed, the liquid contained in the storage tank 21 is sent to the filtration device 13. A recovery arm (not shown) is moved along the inner wall surface of the rotating vessel 22 to scrape off solids adhering to the inner wall surface, and the adhering matter is recovered in a tank (not shown). Thereafter, the rotating vessel 22 is washed. After the recovery is completed, the storage tank 21 is filled with new digestive fluid 16, and the centrifugation process is performed. The centrifugation step, recovery of solids, washing of the rotary vessel 22, and filling of the digested fluid 16 into the storage tank 21 are batch processes, and the time required for one batch can be selected appropriately but is usually about 10 to 30 minutes. The discharge unit 23 is composed of, for example, a pump, a discharge port, and piping connecting them. In FIG. 2, the discharge unit 23 is installed inside the storage tank 21, but the storage tank 21 and the discharge unit 23 may be connected by piping or the like. The storage tank 21 may be composed of multiple tanks.

[0028] Next, the configuration of the filtration device 13 will be described. FIG. 3 is a schematic perspective view showing a portion of the filtration device 13. In FIG. 3, some components are shown in cross section for ease of understanding. Note that FIG. 3 does not show the outer piping through which the permeate flows. The filtration device 13 includes a tubular ultrafiltration (UF) membrane 31, as well as a circulation tank, a permeate recovery tank, and a circulation pump (not shown). The filtration device 13 is also referred to as a tubular membrane module. The ultrafiltration membrane 31 has multiple micropores 32. The ultrafiltration membrane 31 removes solids that could not be separated by the centrifugal separator 12. The micropores 32 have a diameter of, for example, 0.02 μm. The ultrafiltration membrane 31 is tubular, and a liquid phase 33 flows through the tube using a tangential flow method. The permeate that permeates the ultrafiltration membrane 31 is collected as filtrate 34. The liquid phase 33 that does not permeate the ultrafiltration membrane 31 is returned to the circulation tank. The liquid phase 33 circulates between the circulation tank and the ultrafiltration membrane 31, performing batch processing, and the permeate that permeates the ultrafiltration membrane 31 and the retentate remaining in the circulation tank are collected. After that, new liquid phase 33 is filled into the circulation tank. The retentate contains more nitrogen and phosphorus than the digested liquid 16, and is used as liquid fertilizer because it is a concentrated plant nutrient.

[0029] The filtration device 13 includes a viscosity sensor 35 and a flow meter 36. The viscosity sensor 35 and the flow meter 36 are disposed, for example, between the tubular ultrafiltration membrane 31 and the circulation tank. The viscosity sensor 35 detects the viscosity of the liquid phase 33 flowing through the ultrafiltration membrane 31. The flow meter 36 detects the flow rate of the liquid phase 33 flowing through the ultrafiltration membrane 31. The measured flow rate can be used to calculate the flow rate of the liquid phase flowing through the ultrafiltration membrane 31, allowing control based on the flow rate. The operation of the filtration device 13 is controlled according to the viscosity and flow rate of the liquid phase 33 flowing through the ultrafiltration membrane 31. Specifically, for example, if the viscosity exceeds a predetermined value (e.g., 150 mPa·s), the filtration operation of the filtration device 13 is stopped regardless of the flow rate, and if the flow rate is less than the predetermined value, the filtration operation of the filtration device 13 is stopped regardless of the viscosity. Although a flow meter is used here, a flow velocity meter may be used and the filtration may be controlled to stop when the flow velocity falls below a predetermined value, for example, below 2 m / sec.

[0030] The metal ion removal device 14 contains a chelating resin that removes metal ions from the filtrate 34. The chelating resin may be selected based on the components contained in the permeated liquid. Specifically, the metal ion removal device 14 removes calcium ions contained in the filtrate 34 from the filtrate 34. Calcium ions as metal ions have been removed from the filtrate 34 that has permeated the chelating resin. The metal ion removal device 14 is disposed after the filtration device 13 and before the electrodialysis device 15.

[0031] The electrodialysis device 15 uses electrodialysis to concentrate the nitrogen components in the filtrate 34. The nitrogen components are concentrated by the electrodialysis device 15 to obtain a liquid fertilizer with a desired concentration.

[0032] Next, a digestive fluid treatment method using the digestive fluid treatment system 11 will be described. FIG. 4 is a flowchart showing typical steps in the digestive fluid treatment method using the digestive fluid treatment system 11. Referring to FIG. 4, the digestive fluid treatment method according to the first embodiment performs a centrifugation step as step (S10). This step (S10) separates the digestive fluid into a solid phase and a liquid phase using the centrifugal separator 12. In this step, the digestive fluid 16 to be treated is introduced into the storage tank 21, and discharged by the discharge unit 23 while rotating the rotary vessel 22 at the above-mentioned rotation speed. Then, the digestive fluid 16 discharged in the direction indicated by arrow D1 in FIG. 2 flows due to its own weight along the inner wall surface of the top wall 27 and the inner wall surface of the second side wall 28 in the directions indicated by arrows D2 and D3. Since the solid phase contained in the digestive fluid 16 is heavier than the liquid phase, a large centrifugal force is applied to the solid phase, causing it to adhere to the inner wall surface of the second side wall 28 of the rotary vessel 22. Meanwhile, the liquid phase flows down the inner wall surface of the second side wall portion 28 and falls back into the storage tank 21. In this way, discharge by the discharge portion 23 continues for a predetermined time while the rotating vessel 22 is rotating. Thereafter, the centrifugal separator 12 is stopped. The liquid phase remaining in the storage tank 21, from which most of the solid phase has been removed, is transported to the next process. After the centrifugal separator 12 is stopped, the solid phase adhering to the inner wall surface of the second side wall portion 28 of the rotating vessel 22 is scraped off using a spatula or the like. Thereafter, the storage tank 21 and the rotating vessel 22 are washed, and the next digested fluid is processed. In other words, centrifugation is performed in a so-called batch process.

[0033] Next, a filtration step is carried out as step (S20). In this step (S20), after the centrifugation step, the liquid phase 33 with a reduced solid content is filtered to further remove the solids. Specifically, the liquid phase 33 obtained in step (S10) is passed through a tube formed by an ultrafiltration membrane 31. The permeated liquid that passes through the micropores 32 to the outside of the ultrafiltration membrane 31 becomes a filtrate 34 that does not contain solids. In other words, this step removes fine solids that cannot be removed by centrifugation. This filtrate 34 is recovered and transported to the next step. Note that the liquid phase 33 that does not permeate the ultrafiltration membrane 31 even after circulating for a predetermined time remains in the circulation tank. Although the non-permeated liquid remaining in the circulation tank has lower nitrogen and phosphorus concentrations than the concentrated liquid described below, it has more concentrated nutrients than the digested liquid 16 and can therefore be effectively used as liquid fertilizer.

[0034] Next, a metal ion removal step is carried out as step (S30). In this step (S30), the filtrate 34 is passed through a metal ion removal device 14 containing a chelating resin to remove metal ions, specifically calcium ions, from the filtrate 34.

[0035] Next, in step (S40), an electrodialysis step is carried out. In this step (S40), the filtrate 34 filtered after the filtration step is electrodialyzed by the electrodialysis device 15 to obtain a concentrated liquid in which the nitrogen components are concentrated. In this way, a liquid fertilizer with a desired concentration is obtained.

[0036] Figure 5 shows an example of the material balance in a digested fluid treatment method using the digested fluid treatment system 11. The total nitrogen concentration (TN) of the digested fluid is low, necessitating the application of a large amount when used as fertilizer. The digested fluid contains a large amount of suspended solids (SS), e.g., 20,000 to 50,000 mg / L, making it difficult to use a conventional centrifuge. However, the centrifugal separator 12 of the digested fluid treatment system 11 has a simple structure and can separate a liquid with a high solid content into a solid phase and a liquid phase by rotating the rotary vessel 22 at a high rotation speed. This significantly reduces the amount of solids in the digested fluid, e.g., to approximately 1 / 10. Furthermore, because the solids are removed by centrifugation before the filtration process using the ultrafiltration membrane 31, clogging of the ultrafiltration membrane 31, a decrease in flow rate during the filtration process, and early termination of the filtration process due to increased viscosity are avoided, thereby removing most of the solids remaining in the recovered liquid. For example, the solids concentration (SS) can be reduced to approximately several tens of mg / L. In Figure 5, "cake volume" refers to the amount of solids recovered from the wall of the rotary vessel 22 of the centrifugal separator 12. The TN of the concentrated liquid discharged from the electrodialysis device 15 is concentrated several to ten times that of the digested liquid. In the example shown in Figure 5, the TN of the concentrated liquid is 27,000 mg / L (2.7%), which is a suitable nitrogen concentration for use as liquid fertilizer. The TN and total phosphorus (TP) concentrations of the desalinated water are significantly lower than those of the digested liquid, so even if the desalinated water is treated at a sewage treatment facility, it does not place a heavy burden on the facility.

[0037] The total phosphorus concentration is highest in the solids recovered from the centrifuge 12, followed by the retentate discharged from the filter 13, and lowest in the TP of the concentrate discharged from the electrodialysis device 15. The TN of the concentrate discharged from the electrodialysis device 15 is higher than the TN of the solids recovered from the centrifuge 12 and the retentate discharged from the filter 13, making it suitable for use as a liquid fertilizer with a high nitrogen content.

[0038] In the digestive fluid treatment system 11, the centrifugal separator 12 can separate the digestive fluid 16 into solid and liquid phases using centrifugal force. The filter 13 can then remove solids remaining in the liquid phase. In this case, the solids are removed by the centrifugal separator 12, which prevents clogging of the filter 13 and reduces the need to replace the ultrafiltration membrane 31 frequently. This allows for efficient removal of solids from the liquid phase over a long period of time. The electrodialysis device 15 then concentrates nitrogen components to obtain liquid fertilizer with a desired nitrogen concentration. The centrifugal separator 12 includes the storage tank 21, rotating vessel 22, and discharge section 23 described above. The solid and liquid phases can be separated by adhering the solid to the inner wall surface of the rotating vessel 22. The liquid phase flows down the inner wall surface of the rotating vessel 22 and returns to the storage tank 21 due to its own weight. Only the solid phase is deposited on the inner wall surface of the rotating vessel 22 due to centrifugal force. This allows for separation of the solid and liquid phases while separating the solid from the liquid. The accumulated solid phase adheres to the inner wall surface of the rotating vessel 22, and can be easily recovered by periodically scraping off the solid phase adhering to the inner wall surface of the rotating vessel 22. Furthermore, the centrifugal separator 12, filtration device 13, and electrodialysis device 15, which include the storage tank 21, rotating vessel 22, and discharge section 23, can be constructed relatively compactly and inexpensively. As described above, the digested fluid treatment system 11 does not require large-scale equipment, and the desired liquid fertilizer can be obtained by treating the digested fluid inexpensively. Liquid fertilizer can be obtained without using large amounts of chemicals for treating the digested fluid. Furthermore, the solid content recovered from the inner wall surface of the rotating vessel 22 contains nitrogen, phosphorus, and other components, and can therefore be used as solid compost.

[0039] In this embodiment, the rotation speed of the rotary vessel 22 is 4000 rpm or more and 7000 rpm or less. By increasing the rotation speed in this manner, the solid phase contained in the effluent can be more reliably attached to the inner wall surface of the rotary vessel 22 while the liquid phase is returned to the storage tank 21. Therefore, the solid phase and the liquid phase can be separated more efficiently.

[0040] In this embodiment, the centrifugal force generated in the rotating vessel 22 is equal to or greater than 2200 G and equal to or less than 6800 G. By increasing the centrifugal force in this manner, the solid phase contained in the effluent can be more reliably attached to the inner wall surface of the rotating vessel 22 while the liquid phase is returned to the storage tank 21. Therefore, the solid phase and the liquid phase can be separated more efficiently.

[0041] In this embodiment, when viewed from the discharge direction of discharge portion 23, the periphery of second side wall portion 28, which is a wall portion, is disposed inside storage tank 21. Therefore, the liquid phase that drips down from the periphery of second side wall portion 28 can be reliably returned to storage tank 21. Therefore, the solid phase and the liquid phase can be separated more efficiently.

[0042] In this embodiment, the filtration device 13 includes a tubular ultrafiltration membrane 31. The liquid phase is passed through the ultrafiltration membrane 31, and the permeate that permeates the ultrafiltration membrane 31 is recovered as filtrate and sent to the electrodialysis device 15. Components that do not permeate the ultrafiltration membrane 31 are recovered as retentate. The filtration device 13 circulates the digested liquor to separate the retentate from the permeate. This allows the permeate, which has a low solids content, to be obtained in a short period of time from the liquid phase, which still contains a relatively large amount of solids. In addition, the permeate that permeates the tubular ultrafiltration membrane 31 is easily recovered, and the retentate that does not permeate the ultrafiltration membrane 31 is easily extracted. Therefore, solids can be efficiently removed from the liquid phase. The retentate recovered by the filtration device 13 has a higher nitrogen concentration than the digested liquor and can also be used as liquid fertilizer.

[0043] In this embodiment, the filtration device 13 includes a viscosity sensor 35 that measures the viscosity of the liquid phase circulating through the filtration device 13 and a flow meter 36 that measures the flow rate of the liquid phase circulating through the filtration device 13. The filtration device 13 is controlled to stop when the viscosity of the liquid phase measured by the viscosity sensor 35 reaches a predetermined value or when the flow rate of the liquid phase measured by the flow meter 36 falls below a predetermined value. As filtration by the filtration device 13 progresses, the permeated liquid is recovered and the retentate liquid is circulated, increasing the viscosity of the retentate liquid and slowing its flow rate. Because the retentate liquid flows through the tubular ultrafiltration membrane 31 using a tangential flow method, a slow flow rate can easily cause membrane clogging. Therefore, a flow meter is placed downstream of the ultrafiltration membrane 31 in the circulation loop of the filtration device 13 to measure the flow rate of the retentate liquid. When the flow rate of the retentate liquid falls below a predetermined value, filtration is stopped. However, digested liquid, an example of wastewater, is produced by methane fermentation and contains a wide variety of substances, such as straw and rice husks, making its properties unstable. Therefore, even if the flow rate of the circulating non-permeated liquid is above a predetermined value, the viscosity of the non-permeated liquid may become higher than expected. If this condition continues, excessive pressure may be applied to the ultrafiltration membrane 31, potentially damaging it. Therefore, a viscosity sensor 35 is installed downstream of the ultrafiltration membrane 31 in the circulation loop of the filtration device 13 to measure the viscosity of the non-permeated liquid. Filtration is stopped when the viscosity exceeds a predetermined value. When filtration is stopped, the fact and the reason for the stop may be indicated by a display or lamp. If the properties of the non-permeated liquid are stable, controlling the filtration device 13 based solely on the flow rate or viscosity is unlikely to cause any problems. However, because the properties of the digested liquid are unstable, it is preferable to control the filtration device 13 based on both the flow rate and viscosity. By managing the viscosity and flow rate of the liquid phase in this way, reliable filtration can be achieved in a short time while preventing damage to the ultrafiltration membrane 31. Therefore, the solids can be more appropriately removed from the liquid phase.

[0044] In this embodiment, a metal ion removal device 14 containing a chelating resin is provided before the electrodialysis device 15. This removes metal ions contained in the filtrate before treatment with the electrodialysis device 15, thereby preventing clogging of the electrodialysis device 15. In particular, removing calcium ions prevents clogging. This makes it easy to avoid damage to the electrodialysis device 15 and frequent replacement of parts.

[0045] The wastewater treatment method according to the present invention includes a centrifugation step in which organic matter-containing wastewater is separated into a solid phase and a liquid phase by centrifugal force, a filtration step in which the separated liquid phase is filtered to remove solids after the centrifugation step, and an electrodialysis step in which the filtrate obtained after the filtration step is electrodialyzed to obtain a concentrated liquid in which nitrogen components are concentrated. In the centrifugation step, the wastewater is discharged from a storage tank 21 that stores the wastewater toward the inner wall surface of a rotating vessel 22 that rotates at a predetermined speed with its opening facing downward, and the solid phase is caused to adhere to the inner wall surface by centrifugal force, and the liquid phase is recovered in the storage tank 21, thereby separating the solid phase from the liquid phase.

[0046] According to such a wastewater treatment method, the desired liquid fertilizer can be obtained by treating the wastewater inexpensively without requiring large-scale facilities.

[0047] (Other embodiments) In the above embodiment, the metal ion removing device removes metal ions using a chelating resin, but the metal ions may be removed by other methods.

[0048] Furthermore, in the above embodiment, the top wall portion of the rotating vessel included in the centrifugal separator has a hemispherical inner wall surface, but this is not limited thereto. For example, the top wall portion may be tapered, i.e., the inner wall surfaces of the top wall portion and the second side wall portion may be prismatic, or the lower end portion of the second side wall portion may be bent inward to include a mechanism for suppressing the solid phase from falling into the storage tank.

[0049] The present invention is intended to cover a wide range of applications, including those related to the present invention, including those related to the present invention. [Explanation of symbols]

[0050] 11 digested fluid treatment system, 12 centrifuge, 13 filtration device, 14 metal ion removal device, 15 electrodialysis device, 16 digested fluid, 17 rotating shaft, 21 storage tank, 22 rotating vessel, 23 discharge portion, 24 bottom wall portion, 25 first side wall portion, 26, 29 opening, 27 top wall portion, 28 second side wall portion, 31 ultrafiltration membrane, 32 micropores, 33 liquid phase, 34 filtrate, 35 viscosity sensor, 36 flow meter.

Claims

1. A centrifugal separator that separates wastewater containing nitrogen components and organic matter into a solid phase and a liquid phase by centrifugal force; a filtration device that filters the liquid phase separated by the centrifugal separator to remove solids; an electrodialysis device that electrodialyzes the filtrate filtered by the filtration device to obtain a concentrated solution in which nitrogen components are concentrated, The centrifugal separator includes: a reservoir tank for storing the wastewater; a rotating vessel including a wall portion, disposed above the reservoir tank, having an opening, and rotating at a predetermined speed; a discharge unit that discharges the drainage liquid stored in the storage tank toward an inner wall surface of the rotary container, The centrifugal separator separates the solid phase from the liquid phase by causing the solid phase to adhere to the inner wall surface of the wall portion through centrifugal force.

2. The wastewater treatment system according to claim 1 , wherein the rotation speed of the rotary container is 4000 rpm or more and 7000 rpm or less.

3. 3. The wastewater treatment system according to claim 1, wherein the centrifugal force generated in the rotary container is equal to or greater than 2200 G and equal to or less than 6800 G.

4. The wastewater treatment system according to claim 1 or 2, wherein a peripheral edge of the wall portion is disposed inside the storage tank when viewed from a discharge direction of the discharge portion.

5. the filtration device includes a tubular ultrafiltration membrane; Tangential flow filtration is performed in which a liquid flows in a direction parallel to the surface of the ultrafiltration membrane, 3. The wastewater treatment system according to claim 1, wherein the liquid phase is caused to flow through the interior of the ultrafiltration membrane, and the permeate that has permeated the ultrafiltration membrane is recovered as filtrate and sent to the electrodialysis device.

6. The filtration device is a viscosity sensor for measuring the viscosity of the liquid phase circulating through the filtration device; a flow meter for measuring the flow rate of the liquid phase circulating through the filtration device; The wastewater treatment system of claim 5, wherein the filtration is controlled to stop when the viscosity of the liquid phase measured by the viscosity sensor becomes equal to or greater than a predetermined value or when the flow rate of the liquid phase measured by the flow meter becomes less than a predetermined value.

7. 3. The wastewater treatment system according to claim 1, further comprising a metal ion removal device including a chelating resin disposed before the electrodialysis device.

8. A centrifugal separation step of separating wastewater containing nitrogen components and organic matter into a solid phase and a liquid phase by centrifugal force; a filtration step of filtering the separated liquid phase to remove solids after the centrifugation step; an electrodialysis step of electrodialyzing the filtrate filtered after the filtration step to obtain a concentrate in which the nitrogen component is concentrated, The centrifugal separation step is a wastewater treatment method in which the wastewater is discharged from a storage tank that stores the wastewater toward the inner wall surface of a wall of a rotary container that is rotating at a predetermined speed with its opening facing downward, the solid phase is caused to adhere to the inner wall surface of the wall by centrifugal force, and the liquid phase is recovered in the storage tank, thereby separating the solid phase and the liquid phase.

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