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] JP 2005-13909 A [Patent Document 2] JP 2019-131432 A 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, mainly consisting of methane, is collected from the digestive liquid and used as recycled energy. As the digestive liquid from methane fermentation contains nutrients for plants, its use as liquid fertilizer is being considered.

[0005] The concentration of nutrients contained in the digestive fluid is low when left as is, so when used as liquid fertilizer, it is necessary to spray a large amount, but this is not fully utilized due to the high costs of transporting and spraying the digestive fluid. One possible method to increase the concentration of the fertilizer components is to evaporate the water in the digestive fluid, but this is not desirable because it requires a huge energy load to evaporate the water and also concentrates impurities that are not actually necessary for liquid fertilizer. When subjecting the digestive fluid to coagulation treatment, it is necessary to add a large amount of chemicals, but many of these chemicals are not organic, which poses problems for a bio-circulation system (organic matter circulation 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 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, 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 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 by the discharge unit while rotating the rotating container with the opening facing downward, and attaching the solid phase to the inner wall surface of the wall by centrifugal force. Effect of the Invention

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

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

[0010] [Overview 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 an 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 by the discharge unit while rotating the rotating container with the opening facing downward, and attaching the solid phase to the inner wall surface of the wall by centrifugal force.

[0011] According to the wastewater treatment system of the present invention, the centrifugal separator can separate the solid phase and the liquid phase contained in the wastewater by utilizing the centrifugal force. Then, the filtration device can remove the solid content remaining in the liquid phase. In this case, the solid phase is removed by the centrifugal separator, and the clogging of the filtration device and the increase in the frequency of replacing the filtration membrane can be suppressed, so that the solid content can be efficiently removed 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 concentration of nitrogen component. Here, since the centrifugal separator includes the storage tank, the rotating container, and the discharge section of the above configuration, the solid phase and the liquid phase can be separated by attaching the solid phase to the inner wall surface of the rotating container. In this case, the liquid phase flows down the inner wall surface of the rotating container and returns to the storage tank again by its own weight, and only the solid phase is deposited on the inner wall surface of the rotating container by the centrifugal force, so that the solid phase and the liquid phase can be separated in a state where the solid phase is separated from the liquid phase. Since the deposited solid phase is attached to the inner wall surface of the rotating container, the solid phase attached to the inner wall surface of the rotating container can be scraped off periodically and easily collected. In addition, the centrifuge, filtration and electrodialysis devices including the above-mentioned storage tank, rotating container and discharge section can be constructed relatively compactly and inexpensively. As described above, the above-mentioned wastewater treatment system can obtain the desired liquid fertilizer by concentrating the wastewater using a physical filtration, concentration and separation method without using a large amount of chemicals. The solid matter recovered from the wall surface of the rotating container contains nitrogen and phosphorus components, and can be used as solid compost. The wastewater treatment system according to the present invention can efficiently treat even digestive fluid containing a large amount of organic matter and solid matter.

[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, the separation of solid matter becomes insufficient, and if the rotation speed is too high, it leads 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 preferably 5000 rpm or more, more preferably 5500 rpm or more, and is preferably 6500 rpm or less.

[0013] In the wastewater treatment system, the centrifugal force generated in the rotating vessel may be 2200G or more and 6800G or less. By increasing the centrifugal force in this way, the solid phase contained in the wastewater can be more reliably attached to the inner wall surface of the rotating vessel while the liquid phase can be 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 rotating vessel is preferably 3500G or more, further preferably 4200G or more, and is preferably 6000G or less.

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

[0015] In the above wastewater treatment system, the filtration device may include a tubular ultrafiltration membrane. A tangential flow filtration method may be performed in which the liquid phase is passed through the inside of the ultrafiltration membrane and the liquid is passed in a direction parallel to the surface of the ultrafiltration membrane. The permeated liquid that has permeated the ultrafiltration membrane may be collected as a filtrate and sent to an electrodialysis device. The components that do not permeate the ultrafiltration membrane are collected as a non-permeated liquid. The filtration device circulates the wastewater to separate the non-permeated liquid from the permeated liquid. This allows the majority of the solids to be removed from the liquid phase that still contains a relatively large amount of solids in a short period of time, and a permeated liquid with a low solid content, or in some cases, a permeated liquid containing almost no solids, can be obtained. In addition, the permeated liquid that has permeated the tubular ultrafiltration membrane is also easily collected, and the non-permeated liquid that does not permeate the ultrafiltration membrane is easily extracted. Therefore, the solids can be efficiently removed from the liquid phase. The non-permeated liquid collected by the filtration device has a higher nitrogen concentration than the wastewater and can also 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 flowmeter that measures the flow rate of the liquid phase circulating through the filtration device. 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 flowmeter becomes less than a predetermined value, the filtration may be controlled to stop. As the filtration by the filtration device progresses, the permeated liquid is collected and the non-permeated liquid is circulated, so that the viscosity of the non-permeated liquid increases and the flow rate slows. In tangential flow filtration, if the flow rate of the circulating non-permeated liquid is slow, clogging of the membrane is likely to occur. Therefore, in the circulation loop of the filtration device, for example, a flowmeter is disposed downstream of the ultrafiltration membrane to measure the flow rate of the non-permeated liquid. When the flow rate of the non-permeated liquid becomes less than a predetermined value, the filtration is stopped. However, the wastewater contains a wide variety of organic matter and solids, and the properties are not constant. For example, when the effluent is a digestive liquid discharged after methane fermentation, the digestive 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 equal to or higher than a predetermined value, the viscosity of the non-permeated liquid may become higher than expected. If such a state continues, excessive pressure may be applied to the ultrafiltration membrane, which may damage the ultrafiltration membrane. 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, and filtration is stopped when the viscosity becomes equal to or higher than a predetermined value. When filtration is stopped, the fact and the reason for stopping filtration may be displayed by a display or a lamp. If the properties of the non-permeated liquid are stable, there is a low possibility of problems occurring even if the filtration device is controlled based only on the flow rate or viscosity, but since 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 removed more appropriately 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. In this way, metal ions contained in the filtrate are removed before treatment with the electrodialysis device, and so-called clogging of the electrodialysis device can be prevented. In particular, clogging can be prevented by removing calcium ions. This makes it easy to avoid damage to the electrodialysis device and frequent replacement of parts.

[0018] The electrodialysis device has a laminated structure in which cation exchange membranes and anion exchange membranes are alternately arranged, with desalting compartments and concentrating compartments arranged between them. Electrodes are installed at both ends of the laminated structure. When liquid is supplied to the desalting compartment and a voltage is applied to the electrodes, ions are attracted to the electrodes, and nitrogen ions gather in the concentrating compartment. As a result, the concentrated liquid discharged from the concentrating compartment contains concentrated nitrogen components, and the desalted liquid is discharged from the desalting compartment. The nitrogen concentration of the concentrated liquid is, for example, 5 to 15 times higher than that of the effluent, and the concentrated liquid can be suitably used as a liquid fertilizer.

[0019] The wastewater treatment method according to the present invention includes a centrifugation step of separating wastewater containing 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, and an electrodialysis step of electrodialyzing the filtrate obtained after the filtration step to obtain a concentrated liquid in which nitrogen components are concentrated. In the centrifugation step, the wastewater is discharged from a storage tank that stores the wastewater toward the inner wall surface of a wall of a rotating container that rotates at a predetermined speed with its opening facing downward, the solid phase is attached to the inner wall surface of the wall by centrifugal force, and the liquid phase is collected in the storage tank, thereby separating the solid phase from the liquid phase.

[0020] According to such a wastewater treatment method, it is possible to obtain a desired liquid fertilizer 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 digestive fluid containing a large amount of organic matter and solids.

[0021] [Specific Example of the Embodiment] Next, an example of a specific embodiment of the wastewater treatment system according to the present invention will be described with reference to the drawings. In the following, a digestive liquid, which is wastewater discharged from a methane fermentation tank used for biogas power generation, will be described as an example, but the present invention is not limited thereto and can be applied to a treatment system and a treatment method for treating wastewater containing organic matter. Furthermore, the digestive liquid is not limited to the digestive liquid produced 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) A description will be given of a wastewater treatment system according to a first embodiment of the present invention. Fig. 1 is a block diagram showing a configuration of a wastewater treatment system (digestive fluid treatment system) in the first embodiment of the present invention.

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

[0024] Next, the configuration of the centrifuge device 12 will be described. Fig. 2 is a schematic diagram showing a part of the centrifuge device 12. Referring to Fig. 2, the centrifuge device 12 includes a storage tank 21, a rotating container 22, and a discharge unit 23. Note that a plurality of centrifuge devices 12 may be included in the digestive fluid treatment system 11 depending on the amount of digestive fluid to be treated.

[0025] The storage tank 21 includes a flat bottom wall portion 24 and a first side wall portion 25 extending upward from the periphery of the bottom wall portion 24. That is, the storage tank 21 has an opening 26 at its upper portion. In this embodiment, the bottom wall portion 24 is disk-shaped, and the first side wall portion 25 is hollow cylindrical. The storage tank 21 stores digestive fluid 16 containing 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 to the storage tank 21 with a gap therebetween. The rotating vessel 22 includes a top wall portion 27 having a hemispherical inner wall surface and a second side wall portion 28 extending downward from the periphery of the top wall portion 27. The rotating vessel 22 is provided with an opening 29. The inside of the rotating vessel 22 is hollow, and no screw or the like is installed. In FIG. 2, the opening 29 faces downward, but when the rotating vessel 22 is not rotating, the opening 29 may be disposed facing upward. The second side wall portion 28 is hollow cylindrical. The inner wall surface of the top wall portion 27 is smoothly connected to the inner wall surface of the second side wall portion 28. If the size of the rotating vessel 22 is too large, it becomes difficult to rotate the rotating vessel 22, into which the digestive liquid 16 having 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 amount of processing is reduced and processing takes time. 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 about 1 L (liter) to 10 L. In addition, when viewed from the discharge direction indicated by the arrow D1 of the discharge part 23 described later, the periphery of the wall portion (second side wall portion 28) is disposed inside the storage tank 21. The rotating vessel 22 rotates, for example, in the direction indicated by the arrow R1 by control. The rotating shaft 17 during this rotation is indicated by a dashed line. The rotating shaft 17 passes through the most recessed part of the rotating vessel 22. In the centrifugal separator 12, for example, a rotation speed and centrifugal force suitable for removing solids of 3 μm or more are selected. 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 has a discharge port 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 toward the vertically upward direction. The digestive fluid 16 filled in the storage tank 21 is circulated, and discharged to the inner wall surface of the rotating vessel 22 and collected in the storage tank 21 are repeated until a predetermined time has elapsed. When the centrifugation process is completed, the liquid contained in the storage tank 21 is sent to the filtration device 13. A collection arm (not shown) is moved along the inner wall surface of the rotating vessel 22 to scrape off solid matter adhering to the inner wall surface, and the adhering matter is collected in a tank (not shown). Thereafter, the rotating vessel 22 is washed. When the collection is completed, the storage tank 21 is newly filled with the digestive fluid 16, and the centrifugation process is performed. The centrifugation process, recovery of solids, washing of the rotary container 22, and filling of the digestive fluid 16 into the storage tank 21 are batch processes, and the time required for one batch can be appropriately selected, 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 part of the filtration device 13. In FIG. 3, some members are shown in cross section in order to facilitate understanding. In FIG. 3, the illustration of the outer piping through which the permeate flows is omitted. The filtration device 13 includes a tubular ultrafiltration membrane (UF (Ultra Filtration) membrane) 31, as well as a circulation tank, a permeate recovery tank, and a circulation pump, which are not shown. The filtration device 13 is also called a tubular membrane module. The ultrafiltration membrane 31 is provided with a plurality of micropores 32. The ultrafiltration membrane 31 removes solids that could not be separated by the centrifugal separator 12. The size of the micropores 32 is, for example, φ0.02 μm. The ultrafiltration membrane 31 is tubular, and a liquid phase 33 flows inside the tube by a tangential flow method, and the permeate that permeates the ultrafiltration membrane 31 is collected as a 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, where batch processing is performed, and the permeated liquid that permeates the ultrafiltration membrane 31 and the non-permeated liquid remaining in the circulation tank are collected. After that, new liquid phase 33 is charged into the circulation tank. The non-permeated liquid contains more nitrogen and phosphorus components 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 flow rate can be calculated from the measured flow rate, so that control can be performed 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 is greater than a predetermined value (for example, 150 mPa·s), the filtration operation in the filtration device 13 is stopped regardless of the flow rate, and if the flow rate is less than a predetermined value, the filtration operation in 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 includes 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 processing method using the digestive fluid processing system 11 will be described. FIG. 4 is a flow chart showing typical steps in the digestive fluid processing method using the digestive fluid processing system 11. Referring to FIG. 4, in the digestive fluid processing method in the first embodiment, a centrifugation step is performed as step (S10). This step (S10) is a step of separating the digestive fluid into a solid phase and a liquid phase using the above-mentioned centrifuge device 12. In this step, the digestive fluid 16 to be processed is put into the storage tank 21, and while rotating the rotating vessel 22 at the above-mentioned rotation speed, discharge is performed by the discharge unit 23. Then, the digestive fluid 16 discharged in a curved manner in the direction of the arrow D1 in FIG. 2 flows in the directions of the arrows D2 and D3 along the inner wall surface of the top wall portion 27 and the inner wall surface of the second side wall portion 28 due to its own weight. Here, since the solid phase contained in the digestive fluid 16 is heavier than the liquid phase, a large centrifugal force is applied to it, and the solid phase adheres to the inner wall surface of the second side wall portion 28 of the rotating vessel 22. Meanwhile, the liquid phase runs down the inner wall surface of the second side wall portion 28 and falls back into the storage tank 21. In this way, the discharge by the discharge portion 23 continues for a predetermined time while rotating the rotating vessel 22. After that, the centrifuge device 12 is stopped. The liquid phase remaining in the storage tank 21 is the liquid phase from which most of the solid phase has been removed, and is transported to the next process. After the centrifuge device 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 digestive liquid is processed. That is, 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 solid content. Specifically, the liquid phase 33 obtained in step (S10) is passed through a tube formed by an ultrafiltration membrane 31. Then, the permeated liquid that permeates to the outside of the ultrafiltration membrane 31 through the micropores 32 becomes a filtrate 34 that does not contain solid content. In other words, fine solid content that cannot be removed by centrifugation is removed in this step. This filtrate 34 is collected and transported to the next step. Note that the liquid phase 33 that did not permeate the ultrafiltration membrane 31 even after circulating for a predetermined time remains in the circulation tank. Although the nitrogen concentration and phosphorus concentration of the non-permeated liquid remaining in the circulation tank are lower than those of the concentrated liquid described below, the nutrients are more concentrated than the digested liquid 16, so it can 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] FIG. 5 shows an example of material balance in a digestive fluid treatment method using the digestive fluid treatment system 11. The total nitrogen concentration (TN) of the digestive fluid is low, and a large amount needs to be spread when used as fertilizer. The digestive fluid contains a large amount of suspended solids (SS), which are solids, for example, 20,000 to 50,000 mg / L, making it difficult to apply a general centrifuge. However, the centrifuge device 12 of the digestive fluid treatment system 11 described above 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 container 22 at a high rotation speed even if the liquid is supplied with a high solid content. This allows the amount of solids in the digestive fluid to be significantly reduced, for example, to about 1 / 10. In addition, since 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 stop of the filtration process due to an increase in viscosity can be avoided, and most of the solids remaining in the recovered liquid can be removed. For example, the solids concentration (SS) can be reduced to about several tens of mg / L. In addition, in Fig. 5, "cake amount" refers to the amount of solids recovered from the wall surface of the rotating 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 Fig. 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 concentration (TP) of the desalted water are significantly lower than those of the digested liquid, so that even if the desalted water is treated in a sewage treatment facility, the sewage treatment facility is not burdened with a large load.

[0037] The total phosphorus concentration is highest in the solids recovered from the centrifugal separator 12, followed by the non-permeated liquid discharged from the filtration device 13, and the lowest in the TP of the concentrated liquid discharged from the electrodialysis device 15. In addition, the TN of the concentrated liquid discharged from the electrodialysis device 15 is higher than the TN of the solids recovered from the centrifugal separator 12 and the non-permeated liquid discharged from the filtration device 13, and therefore the concentrated liquid can be suitably used as a liquid fertilizer containing a large amount of nitrogen.

[0038] According to the digestive fluid treatment system 11, the centrifugal separator 12 can use centrifugal force to separate the solid phase and liquid phase contained in the digestive fluid 16. The filter 13 can then remove the solid content remaining in the liquid phase. In this case, the solid phase is removed by the centrifugal separator 12, and clogging of the filter 13 and an increase in the frequency of replacement of the ultrafiltration membrane 31 can be suppressed, so that the solid content can be efficiently removed from the liquid phase over a long period of time. The electrodialysis device 15 can then concentrate the nitrogen component to obtain a liquid fertilizer with a desired concentration of nitrogen component. Here, since the centrifugal separator 12 includes the storage tank 21, the rotating vessel 22, and the discharge section 23 configured as described above, the solid phase can be attached to the inner wall surface of the rotating vessel 22 to separate the solid phase and the liquid phase. In this case, the liquid phase returns to the storage tank 21 by its own weight along the inner wall surface of the rotating vessel 22, and only the solid phase is deposited on the inner wall surface of the rotating vessel 22 by centrifugal force, so that the solid phase and the liquid phase can be separated in a state where the solid phase is separated from the liquid phase. Since the accumulated solid phase adheres to the inner wall surface of the rotating vessel 22, it can be easily collected by periodically scraping off the solid phase adhered to the inner wall surface of the rotating vessel 22. In addition, the above-mentioned centrifuge device 12 including the storage tank 21, the rotating vessel 22 and the discharge part 23, the filtration device 13 and the electrodialysis device 15 can be constructed relatively compactly and inexpensively. As described above, the digestive fluid treatment system 11 does not require large-scale equipment, and the desired liquid fertilizer can be obtained by treating the digestive fluid inexpensively. Liquid fertilizer can be obtained without using a large amount of chemicals for treating the digestive fluid. The solid matter collected from the inner wall surface of the rotating vessel 22 contains nitrogen components, phosphorus components, etc., and can be used as solid compost.

[0039] In this embodiment, the rotation speed of the rotating 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 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.

[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 the discharge part 23, the periphery of the second side wall part 28, which is a wall part, is disposed inside the storage tank 21. Therefore, the liquid phase that drips down from the periphery of the second side wall part 28 can be reliably returned to the 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 inside of the ultrafiltration membrane 31, and the permeated liquid that has permeated the ultrafiltration membrane 31 is collected as a filtrate and sent to the electrodialysis device 15. The components that do not permeate the ultrafiltration membrane 31 are collected as a non-permeated liquid. The filtration device 13 circulates the digestive liquid to separate the non-permeated liquid from the permeated liquid. This makes it possible to obtain a permeated liquid with a low solid content from the liquid phase that still contains a relatively large amount of solids in a short period of time. In addition, the permeated liquid that has permeated the tubular ultrafiltration membrane 31 is also easily collected, and the non-permeated liquid that has not permeated the ultrafiltration membrane 31 is easily extracted. Therefore, the solids can be efficiently removed from the liquid phase. The non-permeated liquid collected by the filtration device 13 has a higher nitrogen concentration than the digestive liquid 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 flowmeter 36 that measures the flow rate of the liquid phase circulating through the filtration device 13. When the viscosity of the liquid phase measured by the viscosity sensor 35 becomes equal to or greater than a predetermined value, or when the flow rate of the liquid phase measured by the flowmeter 36 becomes less than a predetermined value, the filtration is controlled to stop. As the filtration by the filtration device 13 progresses, the permeated liquid is collected and the non-permeated liquid is circulated, so that the viscosity of the non-permeated liquid increases and the flow rate slows. In the tubular ultrafiltration membrane 31, the non-permeated liquid is caused to flow by a tangential flow method, so that a slow flow rate makes the membrane more likely to clog. Therefore, a flowmeter is disposed downstream of the ultrafiltration membrane 31 in the circulation loop of the filtration device 13 to measure the flow rate of the non-permeated liquid. When the flow rate of the non-permeated liquid becomes less than a predetermined value, the filtration is stopped. However, digestive fluid, which is an example of wastewater, is generated as a result of methane fermentation, and contains a wide variety of substances such as straw and rice husks, and has unstable properties. Therefore, even if the flow rate of the circulating non-permeated liquid is equal to or higher than a predetermined value, the viscosity of the non-permeated liquid may become higher than expected. If this state continues, excessive pressure may be applied to the ultrafiltration membrane 31, which may damage the ultrafiltration membrane 31. 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, and filtration is stopped when the viscosity becomes equal to or higher than a predetermined value. When filtration is stopped, the fact and the reason for stopping may be displayed by a display, lamp, etc. If the properties of the non-permeated liquid are stable, there is little possibility of problems occurring even if the filtration device 13 is controlled based only on the flow rate or viscosity, but since the properties of the digestive fluid are unstable, it is preferable to control the filtration device 13 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 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. Therefore, metal ions contained in the filtrate are removed before the treatment by the electrodialysis device 15, and so-called clogging in the electrodialysis device 15 can be prevented. In particular, clogging can be prevented by removing calcium ions. Therefore, damage to the electrodialysis device 15 and frequent replacement of parts can be easily avoided.

[0045] The wastewater treatment method according to the present invention includes a centrifugation step of separating wastewater containing 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, and an electrodialysis step of electrodialyzing the filtrate obtained after the filtration step 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 wall of a rotating container 22 that rotates at a predetermined speed with its opening facing downward, and the solid phase is attached to the inner wall surface of the wall by centrifugal force, and the liquid phase is collected in the storage tank 21, thereby separating the solid phase from the liquid phase.

[0046] According to such a wastewater treatment method, no large-scale equipment is required, and the wastewater can be treated inexpensively to obtain the desired liquid fertilizer.

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

[0048] In addition, in the above embodiment, the top wall of the rotating container included in the centrifuge device has a hemispherical inner wall surface, but this is not limited to this. For example, the top wall may be tapered, i.e., the inner wall surfaces of the top wall and the second side wall may be prismatic, or the lower end of the second side wall may be bent inwardly and provided with a mechanism for suppressing the solid phase from falling into the storage tank.

[0049] It should be understood that the embodiments disclosed herein are illustrative in all respects and are not restrictive in any respect. The scope of the present invention is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0050] 11 digestive fluid treatment system, 12 centrifuge, 13 filtration device, 14 metal ion removal device, 15 electrodialysis device, 16 digestive fluid, 17 rotating shaft, 21 storage tank, 22 rotating container, 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 for separating an organic wastewater into a solid phase and a liquid phase by centrifugal force; a filtration device for filtering the liquid phase separated by the centrifuge device to remove solids; an electrodialysis device for electrodialyzing the filtrate filtered by the filtration device to obtain a concentrated solution in which the nitrogen component is concentrated, The centrifuge device is A storage tank for storing the drainage liquid; 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 rotating container, The centrifuge device rotates the rotating container with the opening facing downward, and discharges the drainage from the storage tank toward the inner wall surface of the wall portion using the discharge section, and separates the solid phase from the liquid phase by adhering the solid phase to the inner wall surface of the wall portion using 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 rotating 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 , 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 inside of the ultrafiltration membrane, and a permeate that has permeated the ultrafiltration membrane is recovered as a filtrate and sent to the electrodialysis device.

6. The filtering device includes: 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 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 prior to the electrodialysis device.

8. a centrifugation step of separating the organic matter-containing wastewater into a solid phase and a liquid phase by centrifugal force; a filtration step after the centrifugation step, in which the separated liquid phase is filtered to remove solids; and an electrodialysis step of electrodialyzing the filtrate obtained after the filtration step to obtain a concentrate in which the nitrogen component is concentrated, The centrifugation 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 rotating container that rotates 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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