Organic waste treatment apparatus, method for operating the same, and method for treating organic waste

Abstract

An object of the present invention is to provide an organic waste treatment apparatus, an operation method thereof, and an organic waste treatment method capable of preventing an increase in the number of anaerobic digestion tanks and an adverse effect on a water treatment facility and recovering a large amount of resources including biogas from a sewage treatment plant even in a higher organic matter load (input of a large amount of dehydrated cake) than before by increasing the efficiency of an anaerobic digestion process.SOLUTION: The organic waste treatment apparatus of the present invention comprises an anaerobic digestion tank for producing digested sludge by methane fermentation of slurry-like organic waste with anaerobic microorganisms, a solid-liquid separation device for separating the digested sludge in the anaerobic digestion tank into concentrated sludge and separated liquid, and an electrodialysis device for separating desalted liquid by electrodialysis of the separated liquid with an ion exchange membrane, wherein the separated liquid is introduced into the anaerobic digestion tank.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an anaerobic digester that decomposes organic waste through a methane fermentation reaction in the presence of digested sludge containing anaerobic microorganisms, and more particularly to an organic waste treatment device that can increase the amount of organic waste treated per tank volume, a method for operating the same, and a method for treating organic waste. [Background technology]

[0002] Anaerobic digestion, in which sewage sludge, human waste, food waste, and other organic waste slurry is decomposed into biogas through methane fermentation in the presence of anaerobic microorganisms, has been used for a long time. In this type of anaerobic digestion process, some of the nitrogen components contained in the organic waste are released into the liquid phase of the anaerobic digester as ammonia nitrogen through the action of the anaerobic microorganisms.

[0003] Conventionally, a method for obtaining liquid fertilizer from digestive fluid generated during methane fermentation of organic waste such as human waste and septic tank sludge has been known, as described in Patent Document 1 below. The technology described in Patent Document 1 relates to a technology in which digestive fluid of organic matter is subjected to solid-liquid separation by centrifugation, separated into solid and liquid fractions in a pretreatment process, and the filtrate obtained by filtering the liquid fraction is subjected to electrodialysis to produce liquid fertilizer. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 7340655 Summary of the Invention [Problem to be solved by the invention]

[0005] In the prior art, there is a problem that the treatment efficiency in anaerobic digestion treatment needs to be improved more than ever before, but the technology disclosed in Patent Document 1 is not capable of treating high-concentration organic waste.

[0006] In view of the above-mentioned problems, the present inventors have an object to provide an organic waste treatment apparatus and an operating method thereof, and a method for treating organic waste, which can improve the efficiency of the anaerobic digestion process. [Means for solving the problem]

[0007] One embodiment of the present invention provides an organic waste treatment apparatus comprising: an anaerobic digestion tank that uses anaerobic microorganisms to perform methane fermentation of slurry-like organic waste to produce digested sludge; a solid-liquid separation device that separates the digested sludge from the anaerobic digestion tank into concentrated sludge and a separated liquid; and an electrodialysis device that electrodialyzes the separated liquid using an ion exchange membrane to separate a desalted liquid, and the separated liquid is introduced into the anaerobic digestion tank. One embodiment of the present invention provides a method for operating an organic waste treatment apparatus comprising: an anaerobic digestion tank that uses anaerobic microorganisms to perform methane fermentation of slurry-like organic waste to produce digested sludge; a solid-liquid separation device that separates the digested sludge from the anaerobic digestion tank into concentrated sludge and a separated liquid; and an electrodialysis device that electrodialyzes the separated liquid using an ion exchange membrane to separate a desalinated liquid, the separated liquid being introduced into the anaerobic digestion tank. The electrodialysis device is operated under intermittent conditions that alternate between operation for 10 minutes or less and stoppage for 0.1 minutes or more. One embodiment of the present invention provides an organic waste treatment method comprising the steps of: subjecting slurry-like organic waste to methane fermentation by anaerobic microorganisms in an anaerobic digestion tank to produce digested sludge; separating the digested sludge in the anaerobic digestion tank into concentrated sludge and a separated liquid; subjecting the separated liquid to electrodialysis using an ion exchange membrane to separate a desalted liquid; and introducing the separated liquid into the anaerobic digestion tank. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide an organic waste treatment apparatus and an operating method thereof, which make it possible to use high-concentration organic waste and increase the efficiency of anaerobic digestion treatment in an anaerobic digestion tank, and a method for treating organic waste. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a system diagram showing an overview of an organic waste treatment apparatus according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing the flow of treatment liquid in an electrodialysis device provided in the organic waste treatment device. [Figure 3] 1 is a graph showing the relationship between the number of operating days and the tank load of the anaerobic digestion tank when a normal anaerobic digestion process not equipped with a solid-liquid separation device or an electrodialysis device is operated in a transient manner in which the amount of dilution water for the dewatered cake is gradually reduced and the tank load is correspondingly increased. [Figure 4] 10 is a graph showing the relationship between the number of days of operation and the ammonia nitrogen concentration in the tank when the same operation is performed. [Figure 5] 10 is a graph showing the relationship between the number of days of operation and the methane production rate in the tank when the same operation was performed. [Figure 6] 1 is a graph showing the relationship between the number of operating days and the tank load of the anaerobic digester when an organic waste treatment device equipped with a solid-liquid separator and an electrodialysis device is used, dehydrated cake is introduced into the anaerobic digester without adding dilution water to the dehydrated cake, and the digested sludge concentration is set to approximately 80 g / L. [Figure 7] 10 is a graph showing the relationship between the number of days of operation and the ammonia nitrogen concentration in the tank when the same operation is performed. [Figure 8] 10 is a graph showing the relationship between the number of days of operation and the methane production rate in the tank when the same operation was performed. [Figure 9] This graph shows the relationship between the total amount of sewage sludge fed and the total amount of polymer gel-like substance in operation (A), in which an organic waste treatment device equipped with a solid-liquid separator and an electrical demineralizer was used, and dewatered cake was fed into an anaerobic digester without adding dilution water to the dewatered cake, the digested sludge concentration was set to approximately 20 g / L, and the peripheral speed of the agitator blades was set to 0.2 m / sec. [Figure 10] This is a graph showing the relationship between the total amount of sewage sludge input and the total amount of polymer gel-like substance in operation (B) when the same device was used, the digested sludge concentration was set to approximately 90 g / L, and the peripheral speed of the agitator blades was set to 0.2 m / sec. [Figure 11] This is a graph showing the relationship between the total amount of sewage sludge fed into the device and the total amount of polymer gel-like substance in operation (C), in which the same device was used, the digested sludge concentration was set to approximately 90 g / L, and the peripheral speed of the agitator blades was set to 0.4 m / sec. [Figure 12] This graph shows the relationship between the total amount of sewage sludge fed and the total amount of polymer gel-like substance in operation (D), in which an organic waste treatment device equipped with a solid-liquid separator and an electrical demineralizer was used, and dehydrated cake was fed into an anaerobic digester without adding dilution water to the dehydrated cake, with the digested sludge concentration set to approximately 80 g / L and operation performed with repeated stirring and cessation of stirring. [Figure 13] This graph shows the relationship between operating time and current value when an organic waste treatment device equipped with a solid-liquid separator and an electrical demineralization device is used, and the filtrate obtained by feeding dehydrated cake into an anaerobic digester and operating it is then dialyzed using an electrodialysis device. [Figure 14] This graph shows the relationship between operating time and electrical conductivity in the concentration zone of an organic waste treatment device equipped with a solid-liquid separator and an electrical demineralization device, when the filtrate obtained by feeding dehydrated cake into an anaerobic digester and operating the device is dialyzed using an electrodialysis device. [Figure 15] This graph shows the relationship between the operating time and current value of electrodialysis, based on the results of a test in which an organic waste treatment apparatus equipped with a solid-liquid separator and an electrical demineralization apparatus was used, and the filtrate obtained by feeding dehydrated cake into an anaerobic digester and operating it was then dialyzed using an electrodialysis apparatus, with the apparatus repeatedly operating and pausing. [Figure 16] 1 is a photograph showing an accumulation of gelatinous material formed on the surface of an ion exchange membrane of an electrodialysis device as a result of operating an organic waste treatment device in a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0010] The organic waste treatment device and its operating method and organic waste treatment method according to this embodiment will be described in detail below, with specific examples given. Note that the drawings used in the following description may show characteristic parts enlarged for convenience in order to make the features easier to understand, and the dimensional proportions of each component may not necessarily be the same as in reality. FIG. 1 is a system diagram showing an overview of an organic waste treatment apparatus 1 according to a first embodiment. The organic waste treatment apparatus 1 of this embodiment has an anaerobic digestion tank 3 into which organic waste OW to be treated is introduced via an introduction pipe 2. A discharge pipe 4 is connected to the bottom of the anaerobic digestion tank 3, and a solid-liquid separator 5 is connected to the anaerobic digestion tank 3 via the discharge pipe 4. The solid-liquid separator 5 takes in digested sludge contained in the bottom of the anaerobic digestion tank 3 via the discharge pipe 4 and separates it into thickened sludge and filtrate. A centrifugal separator is an example of the solid-liquid separator 5. The digested sludge is a suspension containing solids, including unreacted sludge, and residue, produced by methane fermentation of the organic waste OW introduced into the anaerobic digestion tank, as described below.

[0011] One end of a return pipe 6 is connected to the discharge side of the thickened sludge in the solid-liquid separator 5, and the other end of the return pipe 6 is connected to the bottom side of the peripheral wall of the anaerobic digestion tank 3. As one example, the position where the other end of the return pipe 6 is connected in the anaerobic digestion tank 3 is higher than the position where the discharge pipe 4 is connected in the anaerobic digestion tank 3. A discharge pipe 3A is connected to a part of the peripheral wall of the anaerobic digestion tank 3 above the connection point of the return pipe 6, and is used to discharge digested sludge to the outside of the system.

[0012] An electrodialysis device 8 is connected to the filtrate discharge side of the solid-liquid separation device 5 via a connecting pipe 7. The electrodialysis device 8 has the function of passing the filtrate from the solid-liquid separation device 5 through it to separate it into a desalted liquid and a concentrated salt waste liquid. The desalted liquid discharge side of the electrodialysis device 8 is connected to an input pipe 2 via a connecting pipe 9, and is configured so that the desalted liquid can be input into the anaerobic digestion tank 3 via the input pipe 2. In this embodiment, the anaerobic digestion tank 3 is equipped with an agitator 10 for agitating the sludge. In Figure 1, a rotating shaft 11 driven by a motor (not shown) is housed vertically downward inside the anaerobic digestion tank 3, and an agitating blade (propeller blade) 12 is attached to the lower end of the rotating shaft 11. The agitating blade 12 is housed inside the anaerobic digestion tank 3 and is positioned so that it can agitate the sludge containing anaerobic microorganisms.

[0013] The organic waste treatment apparatus 1 shown in Figure 1 can be described as comprising: an anaerobic digestion tank 3 that performs methane fermentation on organic waste in the presence of sludge containing anaerobic microorganisms; a solid-liquid separation device 5 that separates the digested sludge in the anaerobic digestion tank 3 into concentrated sludge and filtrate (separated liquid) and returns some or all of the concentrated sludge to the anaerobic digestion tank 3; and an electrodialysis device 8 that electrodialyzes some or all of the separated liquid using an ion exchange membrane and returns some or all of the desalted liquid to the anaerobic digestion tank 3.

[0014] FIG. 2 is a configuration diagram showing an example of the internal structure of the electrodialysis device 8 shown in FIG. The electrodialysis device 8 of this embodiment has a first cation exchange membrane 16 and a first anion exchange membrane 17 located on either side of a central concentration zone 15. It also has a second anion exchange membrane 18 located outside the concentration zone 15, facing and spaced apart from one of the first cation exchange membranes 16, and a second cation exchange membrane 19 located outside the concentration zone 15, facing and spaced apart from the other first anion exchange membrane 17.

[0015] A first demineralization zone 20 is formed in the region sandwiched between the first cation exchange membrane 16 and the second anion exchange membrane 18, and a second demineralization zone 21 is formed in the region sandwiched between the first anion exchange membrane 17 and the second cation exchange membrane 19. The first desalting zone 20 is surrounded by a first cation exchange membrane 16, a second anion exchange membrane 18, and peripheral walls 22 and 23 that support them. One peripheral wall 22 is formed with an inlet port 22a through which the filtrate from the solid-liquid separator 5 is introduced, and the other peripheral wall 23 is formed with an outlet port 23a through which the desalted water that has passed through the desalting zone 20 is discharged. Although not shown in Figure 2, the peripheral walls 22 and 23 form an outer wall that surrounds the periphery of the first desalting zone 20, and the first desalting zone 20 is adjacent to the concentration zone 15 via the first cation exchange membrane 16.

[0016] The second desalting zone 21 is surrounded by a first anion exchange membrane 17, a second cation exchange membrane 19, and peripheral walls 24, 25 that support them. One peripheral wall 24 has an inlet 24a through which the filtrate from the solid-liquid separator 5 is introduced, and the other peripheral wall 25 has an outlet 25a through which the desalted water that has passed through the desalting zone 21 is discharged. Although not shown in Figure 2, the peripheral walls 24, 25 form an outer wall that surrounds the periphery of the second desalting zone 21, and the second desalting zone 21 is adjacent to the concentration zone 15 via the first anion exchange membrane 17.

[0017] A first demineralization zone 20 is disposed facing one side of the second anion exchange membrane 18, and a first recovery zone 27 surrounded by a peripheral wall 28 is formed on the other side of the second anion exchange membrane 18 so as to face the second anion exchange membrane 18. A discharge pipe 29 is connected to the first recovery zone 27, and an extension pipe 30 connected to this discharge pipe 29 allows the concentrated salt effluent to be discharged from the first recovery zone 27 to the outside of the system.

[0018] A second demineralization zone 21 is disposed facing one side of the second cation exchange membrane 19, and a second recovery zone 33 surrounded by a peripheral wall 32 is formed on the other side of the second cation exchange membrane 19 so as to face the second cation exchange membrane 19. A discharge pipe 34 is connected to the second recovery zone 33, and an extension pipe 30 connected to this discharge pipe 34 enables the concentrated salt effluent to be discharged from the second recovery zone 33 to the outside of the system. The concentration zone 15 is surrounded by a first cation exchange membrane 16 and a first anion exchange membrane 17 and peripheral walls 35, 36 that support them. A discharge pipe 37 is connected to the concentration zone 15, and an extension pipe 30 connected to the discharge pipe 37 allows the concentrated salt effluent to be discharged from the concentration zone 15 to the outside of the system.

[0019] 2 is a diagram showing the schematic configuration of electrodialysis device 8, and does not show details of peripheral walls 22, 23, 24, 25, 28, and 32, but the device is configured so that liquid can be filled in each of concentration zone 15, demineralization zones 20 and 21, and recovery zones 27 and 33. As shown in FIG. 2, electrodialysis device 8 is configured so that an anode is disposed on the inner side of peripheral wall 28 and a cathode is disposed on the inner side of peripheral wall 32, and electrodialysis can be performed by passing a direct current through the liquid in electrodialysis device 8 from power sources connected to each electrode.

[0020] In this embodiment, the organic waste OW to be treated includes organic matter and substances with high nitrogen content obtained during sewage treatment, and includes organic waste in the form of a normal-concentration slurry. Specifically, this includes sewage sludge that has had its water content reduced and its solid content increased in a concentration process, which is a pretreatment process before the digestion process during sewage treatment. It may also include dehydrated sewage sludge cake. Here, the dehydrated cake refers to sewage sludge that has been dehydrated and has a moisture content of about 65 to 85%, but it may also have a higher moisture content, such as about 80 to 90%. Dehydrated cake with a lower moisture content may also be added during the treatment process.

[0021] The anaerobic digester 3 is a tank where the organic waste OW is fermented into methane in the presence of sludge containing anaerobic microorganisms housed inside the tank. The sludge containing anaerobic microorganisms contains acid-producing bacteria and methanogenic archaea. In the anaerobic digester 3, the organic components are biodegraded by the anaerobic microorganisms and converted into biogas through the steps of solubilization → degradation to smaller molecules → organic acid production → methane production. Non-biodegradable organic matter in the organic waste and polymer gel-like substances (refractory soluble organic matter) produced by solubilization become components of the digested sludge along with the proliferating microorganisms. In addition, the nitrogen components of the organic matter are biodegraded into ammonia nitrogen by acid-producing bacteria.

[0022] The concentration of digested sludge in anaerobic digester 3 is adjusted by the amount of thickened sludge withdrawn from the system through solid-liquid separator 5. Although the efficiency of methane fermentation increases with increasing digested sludge concentration, an extremely high concentration increases viscosity, hindering stirring / mixing of the digested sludge in anaerobic digester 3. Therefore, it is preferable to adjust and maintain the digested sludge concentration at around 50 to 90 g / L, at which stirring / mixing is mechanically easy, and more preferably at around 70 to 90 g / L. The solids retention time in the anaerobic digester 3 is also determined by the amount of thickened sludge withdrawn from the system through the solid-liquid separator 5. Although the efficiency of methane fermentation increases with the reaction time, an extremely long solids retention time increases the required volume of the anaerobic digester 3. Therefore, it is preferable to adjust and maintain the reaction time at the conventional level of approximately 20 to 30 days. The digested sludge concentration is the mass of solids and residue, including unreacted sludge, per liter of total digested sludge (suspension).

[0023] There are no particular restrictions on the form of the solid-liquid separator 5. However, since it is preferable that the filtrate (separated liquid) supplied to the downstream electrodialysis device 8 contains few suspended solids, microfiltration membranes or ultrafiltration membranes made of hollow fibers or flat membranes that hardly discharge these are particularly suitable. This form is also preferable because the membrane partially removes the persistent soluble components that are produced as the organic components of the organic waste OW decompose. There is no particular upper limit on the concentrated sludge concentration, but because an extremely high concentration increases viscosity and causes significant pressure loss in the liquid delivery pump, a maximum of around 90 g / L is preferred. In light of the above, the digested sludge concentration can be selected within a range of 50 to 90 g / L, and in consideration of operation in a practical-scale apparatus, a range of 60 to 80 g / L is preferable. The concentrated sludge concentration is the mass of solids and residue, including unreacted sludge, per 1 L of the total concentrated sludge concentration (suspension).

[0024] There are no particular limitations on the form of the electrodialysis device 8. By the desalination treatment in the electrodialysis device 8, the ammonia nitrogen in the separated liquid is electrically removed together with other dissolved salts, and transferred to the concentrated effluent discharged from the concentrated salt layer. The salt concentration in the concentrated salt layer discharged from the electrodialysis device 8 is preferably such that the polyvalent ions, such as calcium and phosphate, contained in the raw water do not become supersaturated and deposit as sparingly soluble precipitates on the ion exchange membrane.

[0025] An example of the dialysis action in the electrodialysis device 8 will be described below. For example, with respect to the cation exchange membranes 16, 19 present between the electrodes, anions cannot pass through the cation exchange membranes 16, 19 and therefore accumulate on one side of the cation exchange membranes 16, 19, while cations can pass through the cation exchange membranes 16, 19. With regard to the anion exchange membranes 17, 18 present between the electrodes, cations cannot pass through the anion exchange membranes 17, 18 and are therefore deposited on one side of the anion exchange membranes 17, 18, but anions can pass through the anion exchange membranes 17, 18. To perform electrodialysis by utilizing these phenomena, each of zones 15, 20, 21, 27, and 33 is filled with liquid, and then the filtrate from solid-liquid separator 5 is introduced into first demineralization zone 20 through inlet 22a, and the filtrate from solid-liquid separator 5 is introduced into second demineralization zone 21 through inlet 24a.

[0026] The filtrate from the solid-liquid separator 5 contains NH 4+ or HCO 3- Since the filtrate supplied to the desalting zones 20 and 21 contains salts that generate NH 4+ or HCO 3- As a result, after remaining in the first desalination zone 20 and the second desalination zone 21 for a predetermined time, the treated liquid that has passed through each zone 20, 21 becomes desalinated water (desalinated liquid) and is introduced into the input pipe 2 via the connecting pipe 9 on the discharge side of the electrodialysis device 8, so that the desalinated water can be returned to the anaerobic digestion tank 3. In other words, the digested sludge in the anaerobic digestion tank 3 is separated into thickened sludge and filtrate by the solid-liquid separator 5, and the filtrate is electrodialyzed using the ion exchange membrane of the electrodialysis device 8 to extract ammonia nitrogen (NH 4+ and ammonia nitrogen, NH 4+ The concentrated salt waste liquid is collected, the concentrated salt waste liquid is discharged out of the anaerobic digestion tank 3, and the desalted liquid is introduced into the anaerobic digestion tank 3. By carrying out a series of predetermined processes, it can be said that ammonia nitrogen is removed from the digested sludge in the anaerobic digestion tank 3. Therefore, it is possible to operate the organic waste treatment device 1 of this invention even if the digested sludge concentration is increased while maintaining the ammonia nitrogen concentration in the anaerobic digestion tank 3.

[0027] During the development of the organic waste treatment device 1 of this embodiment, when anaerobic digestion sludge filtrate was desalted using the electrodialysis device 8, an insoluble gel-like substance formed, particularly on the surfaces of the anion exchange membranes 17 and 18, which significantly impaired the desalting performance. As an example, in Figure 2, the accumulation of gel-like substance formed on part of the first anion exchange membrane 17 is indicated by reference numeral 40, and the accumulation of gel-like substance formed on part of the second anion exchange membrane 18 is indicated by reference numeral 41. After a detailed investigation of this phenomenon, it was found that the formation of the aforementioned gel-like substance is caused by persistent soluble organic matter that is produced as the organic components of the aforementioned organic waste OW decompose.

[0028] Conventional electrodialysis technology has been primarily used to desalinate seawater (which contains almost no organic matter and is mainly composed of inorganic salts), and the phenomenon of producing the gel-like substance mentioned above was unknown until now. Although the details of the reaction mechanism for gel formation are still unclear, the soluble organic matter that forms the gel is a polymeric component and therefore does not pass through the pores of the ion-exchange membrane. Therefore, the polymeric components that make up the gel are thought to be electrically "compressed" on the surface of the ion-exchange membrane by a direct current, eventually converting into insoluble ultra-high molecular weight components. The phenomenon of soluble polymeric components being converted into insoluble ultra-high molecular weight components by electrical compression was also unknown until now. For example, highly colloidal components, such as dead residues contained in the filtrate, are thought to become ions, be attracted by the electric potential, and accumulate on the surface of the anion-exchange membranes 17 and 18.

[0029] Further analysis of the formation of gelatinous material that inhibits desalination in the electrodialysis device 8 and its growth on the ion exchange membrane surface revealed that the growth speed differs significantly depending on the linear flow velocity of the raw water passing through the desalination zones 20 and 21. When the linear flow velocity of the raw water is high, strong shear forces are generated near the ion exchange membrane, causing the gelatinous material to peel off. This causes the thickness of the gelatinous material to converge to a constant value during continuous operation. Significantly increasing the linear flow velocity of the raw water maintains a thin gelatinous material in the desalination zone, enabling high-speed desalination. However, because the width of the desalination zone in practical devices is only about 1 mm, high linear flow velocities result in significant pressure loss in the water pump, resulting in a sharp increase in operating costs. As a result of intensive research to solve this problem, it was found that the growth of gelatinous material can be controlled by operating the water pump continuously to continuously pass water through the electrodialysis device 8 while applying a direct current load intermittently.

[0030] The growth of gelatinous material is inhibited by halting gel growth (electrical compression of polymer components on the membrane surface) (stopping the application of DC current) by peeling off the gel surface at the linear velocity of continuous water flow. Specifically, the linear velocity of the anaerobic digestion sludge filtrate (solid-liquid separation device separated liquid) passing through the surfaces of the ion exchange membranes 17 and 18 is set to 0.1 m / sec or higher. Alternatively, the DC current load in the electrodialysis device 8 is set to operate for at most 10 minutes (operation of 10 minutes or less) and rest at least 0.1 minutes (rest of 0.1 minutes or more) (repeated intermittent operation). Intermittent operation of the electrodialysis device is preferred, and it is considered preferable that the operating time of intermittent operation is longer than the rest time. In the operating method of the organic waste treatment device 1, by achieving these operating conditions, the thickness of the gel-like substance is kept to a minimum, enabling high-speed desalination almost equivalent to operation without gel contamination. Taking into consideration the practicality of the device, the linear flow velocity of the filtrate is preferably 0.1 m / sec or more.

[0031] Furthermore, as a result of intensive research into the mechanism of generation of the persistent soluble organic matter that causes the gel-like substance, it was revealed that in addition to the previously thought mechanism of solubilization of solids by anaerobic microorganisms, the gel-like substance in question is also generated from digested sludge by the shear force caused by the rotation of the agitator blades 12 provided on the agitator 10. Although it has long been known that solids can be crushed and solubilized by extremely strong shear forces, it was previously unknown that this phenomenon could occur with the "gentle" agitation typically employed in conventional anaerobic digesters.

[0032] In a conventional anaerobic digester, digested sludge is a low-concentration slurry (approximately 20 g / L), and the physical properties of this low-concentration slurry are similar to those of water. In contrast, in the anaerobic digester 3 of this embodiment, the digested sludge concentration reaches 50 to 90 g / L. Such a high digested sludge concentration is achieved by introducing high-concentration dewatered cake into the anaerobic digester 3 without dilution.

[0033] Because such highly concentrated digested sludge has extremely high viscosity, extremely strong shear forces are generated locally near the agitator blades 12 of the agitator 10. This shear force partially solubilizes the residues of dead microorganisms (solid, persistent components) contained in the digested sludge, causing them to transition to the liquid phase as persistent, soluble organic matter. This liquid phase transition phenomenon is presumed to occur only in the anaerobic digester 3 operated with a high digested sludge concentration of approximately 50 to 90 g / L, as in this embodiment. This is thought to be the reason why the aforementioned liquid phase transition phenomenon has not been known until now.

[0034] To prevent the generation of soluble organic matter from digested sludge due to shear forces generated by the rotation of the impeller 12, the problem can be solved by reducing the peripheral speed of the impeller 12, which is the source of the shear forces. The peripheral speed is determined based on the viscosity of the digested sludge, and specifically, it is preferable to agitate the digested sludge at a peripheral speed of the impeller 12 of 0.2 m / sec or less. By operating the organic waste treatment device 1 while maintaining these agitation conditions, it is possible to operate the organic waste treatment device 1 while preventing the deposition of gelatinous substances on the ion exchange membrane. Depending on the shape of the anaerobic digestion tank 3, the digested sludge may not be sufficiently agitated unless the agitator blades 12 rotate at a peripheral speed of 0.2 m / sec or more. In this case, it is preferable to set the product of the peripheral speed of the agitator blades 12 and the agitation time to within 30 m (e.g., 0.5 m / sec × 60 sec) and to provide an agitation pause of 0.5 minutes or more. Here, the peripheral speed of the agitator blade 12 is the speed representing the distance traveled by the tip of the agitator blade 12 in one second, and in the case of multiple agitator blades, it is the speed representing the distance traveled by the tip of the agitator blade with the longest length in one second.

[0035] According to the organic waste treatment apparatus 1 of this embodiment described above, the efficiency of the anaerobic digestion treatment in the anaerobic digestion tank 3 is increased, making it possible to operate with a higher organic matter load than conventionally. In this embodiment, the desalted liquid obtained in the electrodialysis device 8 is returned to the anaerobic digestion tank 3 and anaerobically treated while maintaining the ammonia nitrogen concentration in the anaerobic digestion tank 3. This means that the anaerobic digestion tank 3 can be operated even when the digested sludge concentration is as high as about 50 to 90 g / L.

[0036] Prior to passing the centrifuged liquid (soluble TOC concentration = approximately 1000 mgC / L), which contains the soluble, biodegradable polymeric organic matter present in the liquid phase of the digested sludge described above, through an electrodialysis device, a pretreatment experiment was conducted in which the centrifuged liquid was physically captured and removed using a coarse filtration membrane with a nominal pore size of 5 microns and a fine filtration membrane with a nominal pore size of 0.45 microns (widely used hollow fiber and flat membranes) to capture and remove the polymeric organic matter that causes gel formation. While 5-micron coarse filtration only removed about 5% of the organic matter contained in the centrifuged liquid, 0.45-micron precision filtration removed 60% of the organic matter contained in the centrifuged liquid. The captured organic matter forms a cake layer on the membrane surface, so colloidal components smaller than the membrane's nominal pore size are also captured and removed on the membrane surface. In other words, by using membrane filtrate instead of centrifuged liquid as the liquid passed through the electrodialysis device, the formation of gel on the surface of the ion exchange membrane of the electrodialysis device can be reduced to less than half of that achieved by centrifugation. Furthermore, colloidal components (gel precursors) trapped on the surface of the microfiltration membrane could be removed by periodically washing the membrane with a chemical solution (sodium hypochlorite, a common cleaning agent for microfiltration membranes). Furthermore, when a membrane filtration device was used as a solid-liquid separation device, the formation of colloidal components could be reduced.

[0037] When considering conventional anaerobic digestion methods, some of the organic components in organic waste are released into the liquid phase as soluble organic matter that is difficult to decompose. Because high concentrations of ammonia nitrogen have an inhibitory effect on anaerobic microorganisms, it is believed that there is an upper limit to the concentration of organic waste that can be input into an anaerobic digester.

[0038] Specifically, when the ammonia nitrogen concentration in the liquid phase reaches approximately 4000 mg-N / L, the growth of acetate-utilizing methanogenic archaea, a type of anaerobic microorganism, drops significantly, resulting in a decline in methane production and an accumulation of acetate in the liquid phase. If this condition continues for a long time, the pH of the liquid will drop, adversely affecting other microorganisms and ultimately causing a complete malfunction of the anaerobic digester, a condition known as rancidity. For this reason, the upper limit for the concentration of general organic waste that can be fed into an anaerobic digester was thought to be 40-60 g / L (4-6%) on a dry weight basis.

[0039] On the other hand, the dry weight equivalent of dehydrated sewage sludge cake reaches 200-250g / L (20-25%), so theoretically the ammonia nitrogen content is around 20,000mg-N / L. It was previously thought that directly decomposing organic waste with such an extremely high nitrogen concentration through methane fermentation was impossible unless the dehydrated cake was first diluted with a large amount of industrial water or similar. For this reason, there are no anaerobic digestion systems in the world that primarily treat dehydrated sewage sludge cake.

[0040] Meanwhile, anaerobic digesters, which ferment low-concentration sewage sludge in a slurry state, are installed nationwide in around 300 sewage treatment plants in Japan. With the recent trend toward recovering resources from organic waste, it is hoped that dehydrated cake brought in from outside will be fed into the anaerobic digesters installed in these sewage treatment plants to increase biogas production. However, when a large amount of dewatered sewage sludge cake is added to an anaerobic digester, the following two problems arise.

[0041] The first problem is the increase in the input flow rate to the anaerobic digester due to the addition of dilution water to avoid inhibition of ammonia nitrogen. When the input flow rate increases, the solids retention time in the anaerobic digester correspondingly decreases, significantly reducing the decomposition reaction by anaerobic microorganisms. To prevent this, it would be necessary to install additional anaerobic digestion tanks, which would significantly increase costs and would not be an option for sewage treatment plants that do not have enough space. In this respect, in this embodiment, the desalinated water treated in the electrodialysis device 8 is returned to the anaerobic digestion tank 3 for reuse, so there is no need to install an additional anaerobic digestion tank.

[0042] The second problem is that a large amount of ammonia nitrogen is discharged from the equipment due to the decomposition of dewatered cake brought in from outside. Generally, digested sludge discharged from an anaerobic digester is separated into dewatered cake and filtrate in a subsequent sludge dewatering process. The cake is disposed of off-site, but the filtrate is returned to the wastewater treatment plant's water treatment facility. Because this filtrate contains a large amount of ammonia nitrogen, if a large amount of dewatered cake is brought in from outside, the water treatment facility will not function properly. This is because the ammonia nitrogen from the filtrate consumes a large amount of oxygen when it is biologically oxidized in the water treatment facility.

[0043] Given these circumstances, even in cases where only a small amount of dewatered sewage sludge cake is additionally treated, there are issues that limit its application in both anaerobic digesters and water treatment facilities. In this respect, in this embodiment, the desalted liquid having tap water level after passing through the electrodialysis device 8 is returned to the anaerobic digestion tank 3, so that it is possible to operate the anaerobic digestion tank 3 even when the digested sludge concentration is as high as about 50 to 90 g / L by adding high-concentration dehydrated cake as described above, and efficient anaerobic digestion treatment can be achieved.

[0044] Electrodialysis equipment using ion exchange membranes is a widely used method, including for recovering salt from seawater. Electrodialysis works by stacking layers sandwiched between two types of ion exchange membranes (cation exchange membrane and anion exchange membrane), passing raw water containing salts through the gaps (demineralization zones) and applying a direct current. The salts (ions) dissolved in the liquid are transported by direct current through the cation and anion exchange membranes to the adjacent gap (concentrated salt zone), and the raw water is desalinated. The concentrated salt wastewater discharged from the concentrated salt zone contains a high concentration of salts.

[0045] By effectively utilizing this principle, desalinated water can be obtained as described above. Even if the filtrate from the solid-liquid separator 5 contains a high concentration of salts, the accumulation of gelatinous material on the surface of the ion exchange membrane can be prevented by intermittently operating the electrodialysis device 8 as described above. This makes it possible to efficiently remove soluble salts, including ammonia nitrogen, even in the anaerobic digester 3 containing a high concentration of digested sludge. Furthermore, ultra-high concentration organic waste such as dehydrated cake can be biodegraded using a compact (high load) anaerobic digester 3 without adding a large amount of dilution water. Furthermore, by operating the agitator impeller 12 in the anaerobic digester 3 at a low speed or intermittently, it is possible to continue operation while minimizing the solubilization and generation of dead microorganism residues in the digested sludge, which are the source of the gel-like substance that adheres to the ion exchange membranes in the electrodialysis device 8. In other words, since operation can be performed while suppressing the generation of gel-like substance that adheres to the surfaces of the ion exchange membranes 17 and 18, the organic waste treatment device 1 can be operated stably for an extended period of time.

[0046] The organic waste treatment device 1 of this embodiment can prevent adverse effects on the facility and increase the methane fermentation reaction in the anaerobic digestion tank 3 without requiring the installation of an additional anaerobic digestion tank 3 or an increase in its volume, making it possible to recover large amounts of resources, including biogas such as methane gas, from a sewage treatment plant equipped with the anaerobic digestion tank 3. Therefore, it is possible to provide an organic waste treatment device, an operating method thereof, and an organic waste treatment method that can prevent adverse effects on water treatment facilities and recover large amounts of resources, including biogas, from sewage treatment plants. In addition, the present invention provides an organic waste treatment device 1, an operating method thereof, and an organic waste treatment method that can decompose organic waste through a methane fermentation reaction in the presence of digested sludge in an anaerobic digestion tank 3 and increase the amount of organic waste treated per tank volume. Furthermore, according to the organic waste treatment device 1 of this embodiment, the desalted liquid that has passed through the electrodialysis device 8 is returned to the anaerobic digestion tank 3 through the input pipe 2 for reuse, thereby minimizing the amount of wastewater discharged outside the system. [Example]

[0047] (Comparative Example 1) In a conventional anaerobic digestion process equipped only with an anaerobic digester without a solid-liquid separator or electrical demineralizer, a transient operation was conducted in which the amount of dilution water for the dewatered cake was gradually reduced and the tank load was correspondingly increased. This operation is the same as gradually increasing the sludge concentration fed into a typical anaerobic digester. As shown in Figure 3, when the tank load exceeded 7.5 g-COD / L / d, the ammonia nitrogen concentration in the anaerobic digester accumulated to over 4 g-N / L, as shown in Figure 4, and this inhibition caused a decrease in methane gas production, as shown in Figure 5. Eventually, methane gas production in the anaerobic digester almost stopped, and the methane fermentation reaction broke down. Here, a centrifugal separator was used as the solid-liquid separator.

[0048] Example 1 An experiment was conducted using an anaerobic digester equipped with a solid-liquid separator and an electrodialysis apparatus as shown in Figures 1 and 2, based on the thickened sludge treatment process of the present invention. The dewatered cake was fed into the anaerobic digester without adding dilution water, and the concentration of the thickened sludge was set to approximately 80 g / L. As shown in Figure 6, in an operational experiment using the configuration of the present invention, even when the tank load was 12 g-COD / L / d or more, the ammonia nitrogen concentration in the anaerobic digestion tank could be maintained at approximately 2.0 to 2.5 gN / L, as shown in Figure 7, and no decrease in methane gas production due to inhibition was observed, as shown in Figure 8. During continuous operation, 40% of the sludge from the anaerobic digester was collected per day by centrifugation to obtain filtrate (residual suspended solids of 100 mg / L or less). This was then subjected to electrodialysis for several hours at a voltage of 30 V using a benchtop electrodialysis device, yielding a desalted solution of tap water quality. The desalted liquid was returned to the anaerobic digester to maintain the ammonia nitrogen concentration in the anaerobic digester at a predetermined value. The anaerobic digester was equipped with a turbine-type agitator (impeller diameter / tank inner diameter = 0.3), and the thickened sludge was continuously and slowly agitated at a peripheral speed of 0.1-0.2 m / sec to treat the thickened sludge. A centrifugal separator was used as the solid-liquid separator.

[0049] In this experiment, the methane production rate in the anaerobic digester was approximately 60-70% of the COD standard tank load. This means that 60-70% of the COD of the input organic waste was converted to methane, which is the same level as methane production in a general anaerobic digester. Typical sewage sludge slurry (approximately 40 g-COD / L) is treated in an anaerobic digester with a retention time of 20 to 30 days. This corresponds to a tank load of 1.3 to 2.0 g-COD / L / d. In Example 1, a tank load 6 to 9 times higher than that obtained in the anaerobic digestion of typical sewage sludge was obtained. This means that the required volume of the anaerobic digester can be reduced to approximately 1 / 6 to 1 / 9 of that of a conventional typical anaerobic digester. In other words, even with a higher organic load (large amount of dehydrated cake input) than before, it is possible to treat organic waste without the need to install additional anaerobic digestion tanks.

[0050] (Comparative Example 2) Using the experimental equipment of Example 1 used above, an experiment was conducted in which thickened sludge with a concentration of approximately 40 g / L, which is the concentration of typical slurry-like thickened sludge, was added to an anaerobic digestion tank, the concentration of the thickened sludge was maintained at approximately 20 g / L, and the peripheral speed of the tip of the agitator blade of the anaerobic digestion tank was set to 0.2 m / sec. Operation (A) was operated continuously for more than two months.

[0051] In addition, an experiment was conducted in which operation (B) was performed continuously for more than two months, in which dewatered cake was added to maintain the concentration of concentrated sludge at 90 g / L, as in the operation using the experimental equipment of Example 1 used previously, and the peripheral speed of the tip of the agitator blade was set to 0.2 m / sec. Furthermore, an experiment was conducted in which operation (C) was performed in which the peripheral speed of the tip of the stirring blade was increased to 0.4 m / sec in operation (B) for more than two months. The amount of polymer gel-like substance produced in the anaerobic digester during each of the operation periods of Operation (A), Operation (B), and Operation (C) was calculated per input thickened sludge.

[0052] As shown in Figure 9, in operation (A), polymer gel-like substances were produced at a rate of 1.7% per concentration of thickened sludge fed in. In contrast, in operation (B), in which the concentration of thickened sludge was increased, the production rate was 5%, about three times higher than before, as shown in Figure 10. In operation (C), in which the agitation of the anaerobic digester was strengthened, the production rate reached 9.3%, as shown in Figure 11. This means that approximately 10% of the thickened sludge fed in was discharged as persistent soluble components.

[0053] Example 2 In operation using the experimental equipment of Example 1, dewatered cake was added to maintain the concentration of the concentrated sludge at 80 g-COD / L (solid concentration of approximately 80 g / L), the peripheral speed of the tip of the agitator blade was set to 0.2 m / sec, and intermittent operation (D) was carried out over a period of two months, in which agitation was repeated for 1 to 3 minutes and then stopped for 5 to 10 minutes. As shown in Figure 12, in this operation with intermittent mixing, the production of persistent soluble organic matter was about 2.1% per unit of thickened sludge input. This is almost the same value as the 1.7% in the aforementioned operation (A), which was operated at a concentration of about 40 g / L, which is the concentration of a typical slurry-like thickened sludge. This indicates that intermittent operation, which alternates between short periods of agitation and agitation cessation, can effectively suppress the generation of persistent soluble organic matter. Based on these results, it is considered preferable to operate an organic waste treatment device while repeatedly performing a cycle of agitation using a rotor blade for 3 minutes or less, e.g., 1 to 3 minutes, followed by agitation cessation for 5 minutes or more, e.g., 5 to 10 minutes. In other words, it is preferable to perform intermittent operation in which agitation and agitation cessation are repeated, and it is considered more preferable to perform intermittent operation in which the agitation cessation time is longer than the agitation time. It is also considered preferable to perform intermittent operation in which agitation using a rotor blade is performed for 1 to 3 minutes, followed by agitation cessation for 5 to 10 minutes.

[0054] (Comparative Example 3) A separated liquid (filtrate) from the concentrated sludge was obtained using the anaerobic digestion tank and solid-liquid separation device of the experimental equipment in Example 1. Using 3 L of this filtrate, a dialysis experiment was repeated four times (3 L x 4 times) using an electrodialysis device to which a voltage of 30 V was applied. As shown in Figure 13, in the first experiment, the initial current value (approximately 1.3 A) dropped rapidly due to high-speed desalination. This means that salts in the desalination zone, where the filtrate flows, rapidly moved to the concentration zone, and the sudden drop in salt concentration in the desalination zone caused a sharp increase in electrical resistance in that zone (current value = voltage / electrical resistance).

[0055] As shown in Figure 14, after 3 hours of operation, the electrical conductivity (proportional to the salt concentration) in the concentration zone increased from the initial 5 mS / cm to 18 mS / cm. This indicates that the salt concentration in the concentration zone increased three times compared to the initial value due to salts moving from the desalination zone. In the second experiment, the initial current value (approximately 0.8 A) was significantly lower than the initial value, as shown in Figure 13. This is because gel-like substances had accumulated on the surface of the anion exchange membrane, as shown in the photograph in Figure 16, and this accumulation of gel-like substances added electrical resistance.

[0056] In the third experiment, the initial current value (approximately 0.5 A) further decreased, and the increase in salt concentration in the concentration zone slowed down accordingly, as shown in Figure 13. As shown in Figure 14, even after 3 hours of operation, the electrical conductivity remained at 15 mS / cm, which corresponds to 1.5 hours of the initial operation. This means that the gel-like substance required more than twice the operation time of the initial operation. In the fourth experiment, the initial current value (approximately 0.4 A) was obtained, as shown in Figure 13, which was slightly lower than that in the third experiment. The increase in electrical conductivity in the concentration zone at this time was approximately 80% of that in the third experiment, as shown in Figure 14. The current behavior did not change much between the third and fourth experiments because the growth of the gel-like substance on the surface of the ion exchange membrane reached its upper limit.

[0057] Example 3 In the experimental apparatus used in the above-mentioned Comparative Example 3, an experiment was conducted in which 25 L of filtrate was used to continuously pass water through the electrodialysis device, while repeatedly operating and pausing the electrodialysis. As shown in FIG. 15, when electrodialysis was performed continuously, the current value decreased from an initial 0.4 A to 0.14 A after about 25 hours. This result shows that the resistance increased by about four times during the operation period (the amount of gel-like substance increased by four times the initial value). In contrast, when the operation was repeated with a 7.5-second rest period and a 22.5-second intermittent operation period, the decrease in the current value was clearly suppressed, as shown in Figure 15. As shown in Figure 15, when the pause time was increased to 15 seconds and the intermittent operation time was reduced to 15 seconds, the degree of decrease in current value was further improved. When 22.5 seconds of pause time and 7.5 seconds of operation time were repeated, the decrease in current value was suppressed to about 70-80% of the initial value. It is considered that intermittent operation of the electrodialysis device is preferable, and that when intermittent operation is performed, it is preferable that the operation time is longer than the pause time.

[0058] Furthermore, based on these results, it is believed that by operating the electrodialysis equipment with the intermittent operation time of the electrodialysis set to a maximum of approximately 20 seconds (20 seconds or less) and the intermittent rest time set to a maximum of approximately 20 seconds (20 seconds or less), it will be possible to carry out concentrated sludge treatment in the anaerobic digestion tank 3 while effectively suppressing the decline in desalination performance due to the generation of gel-like substances. [Explanation of symbols]

[0059] 1...Organic waste treatment equipment, 3...Anaerobic digestion tank, 5...Solid-liquid separation equipment, 8...electrodialysis device, 10...agitator, 12...agitating blade, 15...concentration zone, 16...first cation exchange membrane, 17...first anion exchange membrane, 18...second anion exchange membrane, 19...second cation exchange membrane, 20...first demineralization zone, 21...second desalination zone, 22, 23, 24, 25, 28, 32...perimeter wall, 40, 41...accumulation.

Claims

1. an anaerobic digester that uses anaerobic microorganisms to ferment slurry-like organic waste to produce digested sludge; a solid-liquid separator that separates the digested sludge in the anaerobic digestion tank into thickened sludge and a separated liquid; an electrodialysis device that electrodialyzes the separated liquid using an ion exchange membrane to separate a desalted liquid; The separated liquid is introduced into the anaerobic digestion tank.

2. the linear velocity of the separated liquid passing through the surface of the ion exchange membrane is 0.1 m / sec or more and 10 m / sec or less; The organic waste treatment device according to claim 1.

3. The concentrated sludge is introduced into the anaerobic digestion tank.

3. The organic waste treatment device according to claim 1 or 2.

4. The electrodialysis apparatus is operated intermittently.

3. The organic waste treatment device according to claim 1 or 2.

5. The intermittent operation has a longer operation time than the stop time, The organic waste treatment device according to claim 4.

6. The operation of the electrodialysis device is an intermittent operation that alternates between operation for 0.5 minutes or more and 10 minutes or less and stoppage for 0.1 minutes or more and 0.25 minutes or less. The organic waste treatment device according to claim 5.

7. A part or all of the concentrated sludge is a dehydrated cake of sewage sludge.

3. The organic waste treatment device according to claim 1 or 2.

8. an anaerobic digester that uses anaerobic microorganisms to ferment slurry-like organic waste to produce digested sludge; a solid-liquid separator that separates the digested sludge in the anaerobic digestion tank into thickened sludge and a separated liquid; an electrodialysis device that electrodialyzes the separated liquid using an ion exchange membrane to separate a desalted liquid; A method for operating an organic waste treatment apparatus, comprising introducing the separated liquid into the anaerobic digestion tank, A method for operating an organic waste treatment apparatus, wherein the electrodialysis apparatus is operated under intermittent conditions in which operation is repeated for 10 minutes or less and stoppage for 0.1 minutes or more.

9. The linear velocity of the separated liquid passing through the surface of the ion exchange membrane is set to 0.1 m / sec or more and 10 m / sec or less. A method for operating an organic waste treatment apparatus according to claim 8.

10. The concentration of digested sludge in the anaerobic digestion tank is set to a range of 50 to 90 g / L, the anaerobic digestion tank is provided with an agitator for the digested sludge, and the peripheral speed of the rotor blades of the agitator is set to 0.2 m / sec or less. A method for operating the organic waste treatment apparatus according to claim 8 or 9.

11. The peripheral speed is determined based on the viscosity state of the digested sludge. A method for operating an organic waste treatment apparatus according to claim 10.

12. A part or all of the organic waste to be treated is dehydrated cake of sewage sludge. A method for operating the organic waste treatment apparatus according to claim 8 or 9.

13. The stirring is performed by the rotor for 1 minute to 3 minutes, and then the stirring is stopped for 5 minutes to 10 minutes. This cycle is repeated in an intermittent operation. A method for operating an organic waste treatment apparatus according to claim 10.

14. The intermittent operation is carried out while continuing to supply water to the electrodialysis device. A method for operating the organic waste treatment apparatus according to claim 8 or 9.

15. The downtime during the intermittent operation of the electrodialysis apparatus is set to be longer than the intermittent operation time. A method for operating the organic waste treatment apparatus according to claim 8 or 9.

16. The operation time of the electrodialysis apparatus during intermittent operation is 20 seconds or less, and the rest time is 20 seconds or less. A method for operating the organic waste treatment apparatus according to claim 8 or 9.

17. In the anaerobic digester, anaerobic microorganisms are used to ferment the slurry-like organic waste into methane to produce digested sludge; Separating the digested sludge in the anaerobic digestion tank into thickened sludge and a separated liquid; subjecting the separated liquid to electrodialysis using an ion exchange membrane to separate a desalted liquid; introducing the separated liquid into the anaerobic digestion tank; A method for treating organic waste comprising the steps of:

18. introducing the concentrated sludge into the anaerobic digester; 18. The method for treating organic waste according to claim 17, comprising:

19. performing intermittent operation in the anaerobic digestion tank by repeatedly starting and stopping agitation.

19. The method for treating organic waste according to claim 17 or 18.

20. The intermittent operation has a longer agitation stop time than the agitation time.

20. The method for treating organic waste according to claim 19.

21. In the anaerobic digester, anaerobic microorganisms are used to ferment the slurry-like organic waste into methane to produce digested sludge; removing ammonia nitrogen from the digested sludge by performing a predetermined treatment on the digested sludge; A method for treating organic waste comprising the steps of:

22. The predetermined processing is Separating the digested sludge into a thickened sludge and a separated liquid; subjecting the separated liquid to electrodialysis using an ion exchange membrane to separate it into a desalted liquid and a concentrated salt waste liquid; introducing the desalted liquid into the anaerobic digestion tank; 22. The method for treating organic waste according to claim 21, comprising:

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