Method for treating organic waste using hydrothermal carbonization, and equipment for treating organic waste using hydrothermal carbonization.

Abstract

To provide a method and facility for treating organic waste that can make the hydrothermal carbonization reaction more efficient.SOLUTION: The method for treating organic waste includes a methane fermentation step of anaerobically fermenting organic waste in a digestion tank 1 (methane fermentation tank), a first dewatering step of dewatering the fermentation-treated sludge after the methane fermentation step by a dewaterer 2, a sludge carbonization step of hydrothermal carbonization treating the dewatered sludge obtained in the first dewatering step in a hydrothermal carbonization unit 3, and a second dewatering step of dewatering the carbonized sludge slurry obtained in the sludge carbonization step by a second dewaterer 4, and in which, in the sludge carbonization step, a predetermined amount of acid is added to the dewatered sludge.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for treating organic waste using hydrothermal carbonization, and a treatment facility for organic waste using hydrothermal carbonization.

Background Art

[0002] As a technology related to a method for treating organic waste, for example, there is one described in Patent Document 1. In the method for treating organic waste described in Patent Document 1, organic waste is supplied from an organic waste source to a wet waste tank and a wet waste mixing tank, and then a first batch of the mixed wet waste is supplied to a first thermal reactor to be heated and pressurized, and a second batch of the mixed wet waste is supplied to a second thermal reactor to be heated and pressurized, and the hydrolytic process biochar sludge (BCS) is alternately discharged from the first thermal reactor (5) or the second thermal reactor (6) to a biochar cooler.

[0003] In paragraph 0041 of the specification of Patent Document 1, it is described that "With this arrangement, a compact system that optimizes the use of energy within the system itself can be realized."

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in the method for treating organic waste described in Patent Document 1, the thermal hydrolysis process in which a batch of mixed wet waste is supplied to a heat reactor and subjected to heating and pressurization is carried out under conditions of 200-250°C, 20-25 bar, and 3-5 hours. Because it is a high-temperature, high-pressure, and long-duration process, there are problems in that it is costly to perform thermal hydrolysis and the treatment takes a long time.

[0006] Therefore, the present invention aims to provide a method and equipment for treating organic waste that can improve the efficiency of the hydrothermal carbonization reaction. [Means for solving the problem]

[0007] The method for treating organic waste disclosed herein is a method for treating organic waste using hydrothermal carbonization treatment, comprising: a methane fermentation step of treating organic waste by anaerobic fermentation in a methane fermentation tank; a first dewatering step of dewatering the fermentation sludge after the methane fermentation step; a sludge carbonization step of treating the dewatered sludge obtained in the first dewatering step by hydrothermal carbonization; and a second dewatering step of dewatering the carbonized sludge slurry obtained in the sludge carbonization step, wherein a predetermined amount of acid is added to the dewatered sludge in the sludge carbonization step.

[0008] By adding a predetermined amount of acid to the dewatered sludge obtained in the first dewatering step and subjecting it to hydrothermal carbonization treatment, the acid functions as a catalyst for the decomposition of organic matter, which is the initial reaction of hydrothermal carbonization. As a result, the hydrothermal carbonization reaction can proceed even if the reaction temperature is lowered or the treatment time is shortened, making the hydrothermal carbonization treatment more efficient.

[0009] The system may further include a filtrate return step in which the hydrothermal carbonization dewatered filtrate separated from the carbonized sludge slurry in the second dewatering step is returned to the methane fermentation tank, and a separation liquid treatment step in which the fermentation treatment dewatered filtrate separated from the fermentation treatment sludge in the first dewatering step is subjected to acid coagulation and precipitation treatment using an iron-based inorganic coagulant.

[0010] With this configuration, by returning the hydrothermal carbonization dehydrated filtrate, which has a low pH, to the methane fermentation tank, the pH of the fermentation-treated dehydrated filtrate separated from the fermentation-treated sludge in the first dehydration step becomes lower, thus reducing the amount of acid added to the fermentation-treated dehydrated filtrate in the separated liquid treatment step. In addition, the amount of energy recovered increases because the organic matter contained in the hydrothermal carbonization dehydrated filtrate becomes a raw material for methane fermentation. As a result, the amount of organic matter in the treatment system decreases, and the increase in COD of the treated water is suppressed. Furthermore, by treating the fermentation-treated dehydrated filtrate with an iron-based inorganic coagulant using acid coagulation and precipitation treatment, phosphorus and heavy metals are removed, and the increase in COD of the treated water is also suppressed. As a result, the deterioration of effluent water quality can be suppressed.

[0011] The system may further include a pH measurement step for measuring the pH of the carbonized sludge slurry, and an acid addition amount adjustment step for adjusting the amount of acid added to the dewatered sludge based on the pH value of the carbonized sludge slurry obtained in the pH measurement step.

[0012] This configuration avoids problems such as insufficient acid addition preventing the hydrothermal carbonization reaction from proceeding properly, increased costs due to excessive acid addition, and adverse effects on subsequent processes.

[0013] Furthermore, the acid may be sulfuric acid.

[0014] This configuration, by using sulfuric acid as the acid, prevents an increase in chlorine concentration in the carbonized sludge. Therefore, when using carbonized sludge as a solid fuel, a deterioration in fuel quality can be prevented. Furthermore, since the nitrogen concentration and COD (chemical oxygen demand) of the treated water do not increase, the deterioration of effluent quality can be avoided, thereby reducing the environmental burden.

[0015] Furthermore, in the first dewatering step, the fermentation-treated sludge may be dewatered using an iron-based inorganic flocculant.

[0016] With this configuration, even if the hydrothermal carbonization dehydrated filtrate containing sulfates is returned to the methane fermentation tank, the iron ions contained therein react with sulfide ions produced by sulfate-reducing bacteria to form iron sulfide, thereby suppressing the generation of hydrogen sulfide.

[0017] Furthermore, in the sludge carbonization process, the amount of sulfuric acid added to the dewatered sludge may be 20 to 140 g-H2SO4 / kg-DS.

[0018] This configuration allows for more efficient hydrothermal carbonization.

[0019] The organic waste treatment facility disclosed in this application is an organic waste treatment facility using hydrothermal carbonization treatment, comprising: a methane fermentation tank for anaerobic fermentation treatment of organic waste; a first dewatering machine for dewatering the fermentation treatment sludge discharged from the methane fermentation tank; a hydrothermal carbonization device for hydrothermal carbonization treatment of the dewatered sludge obtained from the first dewatering machine; and a second dewatering machine for dewatering the carbonized sludge slurry obtained from the hydrothermal carbonization device, wherein the hydrothermal carbonization device has an acid adding device for adding a predetermined amount of acid to the dewatered sludge.

[0020] By adding a predetermined amount of acid to the dewatered sludge obtained in the first dewatering machine and subjecting it to hydrothermal carbonization treatment, the acid functions as a catalyst for the decomposition of organic matter, which is the initial reaction of hydrothermal carbonization. As a result, the hydrothermal carbonization reaction can proceed even if the reaction temperature is lowered or the treatment time is shortened, making the hydrothermal carbonization treatment more efficient.

[0021] The hydrothermal carbonization apparatus includes a reactor that processes the dewatered sludge at high temperature and pressure in a gas atmosphere that does not contain oxygen or has a low oxygen concentration, or in a state where oxygen is blocked. The dewatered sludge is intermittently supplied to the reactor by a dewatered sludge supply pump, and the acid addition device may add the acid to the dewatered sludge while the dewatered sludge supply pump is in operation.

[0022] According to this configuration, during the operation of the dewatered sludge supply pump, the entire amount of acid is surely transferred to the reactor, so that it is possible to prevent the acid from remaining in the pipe from its injection port to the reactor during the stop of the dewatered sludge supply pump. Therefore, it is possible to avoid corrosion occurring in the pipe.

[0023] Further, the hydrothermal carbonization device may further include pH measurement means for measuring the pH of the carbonized sludge slurry, and the acid addition device may adjust the amount of acid added to the dewatered sludge based on the pH value of the carbonized sludge slurry obtained by the pH measurement means.

[0024] According to this configuration, it is possible to avoid the situation where the hydrothermal carbonization reaction does not proceed sufficiently due to insufficient addition of acid, the cost increases due to excessive input of acid, or an adverse effect on the subsequent treatment occurs.

[0025] Also, the acid may be sulfuric acid.

[0026] According to this configuration, by using sulfuric acid as the acid, the chlorine concentration in the carbonized sludge does not increase. Therefore, when the carbonized sludge is used as a solid fuel, it is possible to prevent a decrease in fuel quality. Also, since the nitrogen concentration and COD (chemical oxygen demand) of the treated water do not increase either, by avoiding deterioration of the discharged water quality, the environmental load can be reduced.

Advantages of the Invention

[0027] According to the method for treating organic waste or the treatment equipment for organic waste having the above configuration, the hydrothermal carbonization reaction can be made more efficient.

Brief Description of the Drawings

[0028] [Figure 1] It is a block diagram showing an example of a treatment equipment for organic waste using hydrothermal carbonization treatment. <0000!05> [Figure 2] It is a diagram showing an example of the specific configuration of the hydrothermal carbonization device shown in FIG. 1. [Figure 3]This graph shows the relationship between the amount of sulfuric acid added to the dewatered sludge obtained by dewatering fermented sludge and the moisture content of the carbonized sludge obtained by dewatering the carbonized sludge slurry after hydrothermal carbonization treatment of the dewatered sludge. [Modes for carrying out the invention]

[0029] Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings.

[0030] The organic waste treated in the treatment method of the present invention includes sewage sludge, human waste sludge, agricultural community wastewater sludge, septic tank sludge, food waste such as kitchen waste (food-derived biomass), lignocellulosic waste such as recycled paper and waste paper, agricultural residues, and livestock manure. These organic wastes may be treated individually or in combination. Below, the treatment of sewage sludge will be described as an example.

[0031] As shown in Figure 1, the organic waste treatment facility comprises a digester 1 as a methane fermentation tank, a first dewatering machine 2, a hydrothermal carbonization device 3, a second dewatering machine 4, a curing container 5, and a separation liquid treatment device 6. The digester 1, first dewatering machine 2, hydrothermal carbonization device 3, second dewatering machine 4, and curing container 5 are installed in this order from the upstream side of the treatment process. The separation liquid treatment device 6 is installed downstream of the first dewatering machine 2 in the treatment process.

[0032] (Digestor and methane fermentation process) Digestor 1 is a tank for anaerobic fermentation treatment of sewage sludge. The methane fermentation process is a process in which sewage sludge is treated with anaerobic fermentation in digester 1. The solid content of the sewage sludge introduced into digester 1 is, for example, 3 to 9% by mass. Digestor 1 is operated at a temperature of approximately 30 to 42°C for a residence time of approximately 15 to 30 days in the medium-temperature fermentation process, and at a temperature of approximately 50 to 60°C for a residence time of approximately 7 to 20 days in the high-temperature fermentation process.

[0033] Anaerobic fermentation of sewage sludge generates digester gas in digester tank 1. The digester gas is a biogas consisting of approximately 60% methane and 40% carbon dioxide by volume. The generated digester gas is removed from digester tank 1 and used as fuel for heating digester tank 1 and hydrothermal carbonization equipment 3, or as fuel for power generation equipment (not shown). In other words, by treating sewage sludge with anaerobic fermentation, the energy contained in the sewage sludge can be recovered as digester gas (gas energy).

[0034] The fermentation residue of sewage sludge after anaerobic fermentation treatment, i.e., the fermented sludge, is discharged from the digester tank 1 to the outside.

[0035] (First dehydrator and first dehydration process) The fermentation-treated sludge discharged from the digester 1 to the outside is supplied to the first dewatering machine 2. The first dewatering machine 2 is a machine that dewaters the fermentation-treated sludge discharged from the digester 1. The first dewatering step is a step in which the fermentation-treated sludge after the methane fermentation step is dewatered. The solid matter concentration of the fermentation-treated sludge is, for example, 1.5 to 5% by mass. The first dewatering machine 2 may be a belt press dewatering machine, a filter press dewatering machine, a centrifugal dewatering machine, a screw press dewatering machine, a belt concentrator, or a centrifugal concentrator.

[0036] The fermentation-treated sludge supplied to the first dewatering machine 2 may be dewatered using an iron-based inorganic flocculant, or it may be dewatered using an iron-based inorganic flocculant and a polymer flocculant in combination. As an iron-based inorganic flocculant, for example, inorganic flocculants such as polyferric sulfate, ferric chloride, and ferrous sulfate can be used. As a polymer flocculant, for example, cationic polymer flocculants, anionic polymer flocculants, nonionic polymer flocculants, and amphoteric polymer flocculants can be used, but here it is preferable to use a cationic polymer flocculant.

[0037] The first dewatering machine 2 (first dewatering step) dewaters the fermentation-treated sludge, for example, to a dewatered sludge with a water content of about 80% by mass. The fermentation-treated dewatered filtrate (digested dewatered filtrate) separated from the fermentation-treated sludge by the first dewatering machine 2 is sent to the separation liquid treatment device 6.

[0038] (Separation liquid processing apparatus and separation liquid processing process) The fermentation-treated dewatered filtrate (digested dewatered filtrate) separated from the fermentation-treated sludge is supplied to the separation liquid treatment device 6. The separation liquid treatment device 6 is a device that processes the fermentation-treated dewatered filtrate (digested dewatered filtrate) separated from the fermentation-treated sludge in the first dewatering machine 2. The separation liquid treatment process is a process of processing the fermentation-treated dewatered filtrate (digested dewatered filtrate) separated from the fermentation-treated sludge in the first dewatering process. The treated water obtained from the separation liquid treatment device 6 is returned to the water treatment equipment (not shown).

[0039] The separation liquid treatment device 6 is, for example, a device that performs coagulation and sedimentation treatment on the fermentation treatment dewatered filtrate (digestion dewatered filtrate). The separation liquid treatment step is, for example, a step of coagulation and sedimentation treatment on the fermentation treatment dewatered filtrate (digestion dewatered filtrate). The coagulation and sedimentation treatment is preferably an acidic coagulation and sedimentation treatment. Acidic coagulation and sedimentation treatment involves adjusting the pH of the water to be treated as necessary, injecting an iron-based inorganic coagulant, and performing coagulation and sedimentation treatment under acidic conditions. By performing acidic coagulation and sedimentation treatment, pollutants contained in the fermentation treatment dewatered filtrate (digestion dewatered filtrate) are coagulated and sedimented, thereby removing phosphorus and heavy metals, and suppressing the rise in COD of the treated water. As a result, deterioration of the effluent quality can be suppressed. As an iron-based inorganic coagulant, for example, inorganic coagulants such as polyferric sulfate, ferric chloride, and ferrous sulfate can be used. In addition, the precipitate (sedimented sludge) from the coagulation and sedimentation treatment may be mixed with a carbonized sludge slurry. This reduces the amount of settled sludge to be treated and allows for effective use as carbonized sludge (fuel). Furthermore, the treatment of the fermentation-treated dehydrated filtrate (digested dehydrated filtrate) is not limited to coagulation and sedimentation, but may also be coagulation and flotation separation, or coagulation and filtration.

[0040] (Hydrothermal carbonization equipment and sludge carbonization process) The dewatered sludge is supplied to the hydrothermal carbonization apparatus 3. The hydrothermal carbonization apparatus 3 is a device that adds a predetermined amount of acid to the dewatered sludge obtained in the first dewatering machine 2 and performs hydrothermal carbonization treatment. The sludge carbonization process is a process in which a predetermined amount of acid is added to the dewatered sludge obtained in the first dewatering process and performs hydrothermal carbonization treatment. Hydrothermal carbonization treatment refers to a process in which a material to be treated, which contains water, is carbonized by high temperature and high pressure treatment in a gas atmosphere that does not contain oxygen or has a low oxygen concentration, or in a state where oxygen is blocked.

[0041] As shown in Figure 2, the hydrothermal carbonization apparatus 3 includes an acid addition device 13 for adding acid to the dewatered sludge, and for example, a heating heat exchanger 8, a reactor 7, and a cooling heat exchanger 9. The hydrothermal carbonization apparatus 3 may further include a pH measuring means 14. However, the configuration of the hydrothermal carbonization apparatus 3 is not limited to this.

[0042] The heating heat exchanger 8 is a heater that preheats the dewatered sludge obtained from the first dewatering machine 2 before supplying it to the reactor 7. Dewatered sludge with a moisture content of approximately 80% by mass is supplied to the heating heat exchanger 8 from the first dewatering machine 2 by the dewatered sludge supply pump 12.

[0043] The acid addition device 13 includes a chemical tank containing acid and a pump that supplies acid from this tank. The device may be configured to supply acid from this tank to the piping connecting the heating heat exchanger 8 and the reactor 7, or to supply acid to the reactor 7. The acid addition device 13 may also consist of a valve for adjusting the amount of acid added. When dewatered sludge is intermittently supplied to the reactor 7 by the dewatered sludge supply pump 12, it is preferable to add acid to the dewatered sludge while the dewatered sludge supply pump 12 is in operation. Alternatively, the addition of acid to the dewatered sludge may be started after a predetermined time has elapsed since the start of operation of the dewatered sludge supply pump 12, and the operation of the dewatered sludge supply pump 12 may be stopped after a predetermined time has elapsed since the stop of acid addition. By adding acid while the dewatered sludge supply pump 12 is in operation in this way, the entire amount of acid is reliably transferred to the reactor 7, thus preventing the acid from remaining in the piping from the inlet to the reactor 7 while the dewatered sludge supply pump 12 is stopped. Therefore, corrosion can be avoided in the piping and the heating heat exchanger 8.

[0044] As for acids, sulfuric acid, hydrochloric acid, nitric acid, or organic acids can be placed in the chemical tank and used for hydrothermal carbonization treatment. However, using hydrochloric acid increases the chlorine concentration in the carbonized sludge, which may affect the fuel quality when the carbonized sludge is used as solid fuel. Using nitric acid or organic acids increases the nitrogen concentration or COD (chemical oxygen demand) of the treated water, which may worsen the quality of the effluent and increase the environmental burden. For this reason, sulfuric acid is preferred over hydrochloric acid, nitric acid, or organic acids for use in the hydrothermal carbonization treatment in this invention.

[0045] Reactor 7 is a container for high-temperature, high-pressure treatment of dewatered sludge in a gas atmosphere that does not contain oxygen or has a low oxygen concentration, or in a state where oxygen is blocked. For example, reactor 7 treats dewatered sludge at high temperature and high pressure in a gas atmosphere with an oxygen concentration of 5% by volume or less. The high-temperature, high-pressure treatment is started with the atmosphere purged with an inert gas, or even if the atmosphere at the start is air (oxygen concentration 21% by volume), the oxygen is quickly consumed by the oxidation of easily decomposable organic matter in the material being treated, and no air is supplied thereafter. Therefore, except immediately after the start, the oxygen concentration in the gas phase inside reactor 7 during the reaction is maintained at approximately 0% by volume. Reactor 7 has an agitator 7a. The carbonized sludge slurry inside reactor 7 and the dewatered sludge supplied to reactor 7 are mixed and stirred by the agitator 7a. A cylindrical jacket 7b is provided on the outer circumference of reactor 7. The heat transfer oil heated in the heat transfer oil heat exchanger 10 is circulated and supplied to the jacket 7b by the heat transfer oil circulation pump 11. The carbonized sludge slurry inside reactor 7 is indirectly heated by a circulating heat transfer oil. Note that other heat transfer fluids may be used instead of the aforementioned heat transfer oil.

[0046] In hydrothermal carbonization, the acid functions as a catalyst for the decomposition of organic matter, which is the initial reaction of hydrothermal carbonization. Therefore, even if the temperature of the dewatered sludge in the reactor 7 (reaction temperature) is lowered or the residence time of the dewatered sludge (treatment time) is shortened, the hydrothermal carbonization reaction will proceed, making the hydrothermal carbonization process more efficient. Conventional hydrothermal carbonization is carried out at high temperature, high pressure, and for a long time, such as a reaction temperature of 200-250°C, a pressure (gauge pressure) of 2.0-2.5 MPa, and a residence time of 180-300 minutes, to obtain carbonized sludge with a low water content. In contrast, by adding a predetermined amount of acid to the dewatered sludge, the hydrothermal carbonization reaction proceeds even at a reaction temperature of around 180°C, a pressure (gauge pressure) of around 1.5 MPa, and a residence time of around 120 minutes. This reduces the cost of thermal hydrolysis and shortens the treatment time. Furthermore, high-temperature and high-pressure treatment of dewatered sludge means treating the dewatered sludge at a temperature of 160°C to 250°C, and at a gauge pressure of 0.6 MPa to 3 MPa inside the reactor 7.

[0047] The dewatered sludge supplied into reactor 7 is processed at the above pressure and temperature for the above processing time to become a carbonized sludge slurry.

[0048] The cooling heat exchanger 9 is a cooler that cools the carbonized sludge slurry obtained by hydrothermal carbonization treatment in the reactor 7. In the cooling heat exchanger 9, the temperature of the carbonized sludge slurry is adjusted to a range suitable for handling. By adjusting the temperature of the carbonized sludge slurry, the viscosity of the carbonized sludge slurry can be adjusted, making it easier to transfer the carbonized sludge slurry and improving the dewatering properties of the carbonized sludge slurry.

[0049] (pH measuring means and pH measuring process) The pH measuring means 14 is a measuring instrument for measuring the pH of the carbonized sludge slurry obtained by hydrothermal carbonization treatment in the reactor 7. The pH measuring step is a step of measuring the pH of the carbonized sludge slurry obtained by hydrothermal carbonization treatment in the reactor 7. By adjusting the amount of acid added to the dewatered sludge based on the pH of the carbonized sludge slurry, it is possible to avoid the hydrothermal carbonization reaction not proceeding sufficiently due to insufficient acid addition, or the costs increasing or adverse effects on subsequent treatments due to excessive acid addition. For example, if the set pH of the carbonized sludge slurry is 4.0, the initial amount of acid added to the dewatered sludge can be set to, for example, 70g-H2SO4 / kg-DS. If the measured pH of the carbonized sludge slurry is around 4.0, the amount of acid added can be maintained. If the measured pH of the carbonized sludge slurry is not around 4.0, the amount of acid added to the dewatered sludge can be adjusted by increasing or decreasing the amount of acid added to the dewatered sludge by approximately 20% of the initial value. In Figure 2, the pH measuring means 14 is shown to measure the pH of the carbonized sludge slurry after cooling, but it may also measure the pH of the carbonized sludge obtained in the second dewatering step, or the pH of the carbonized sludge slurry in the reactor 7 (not shown).

[0050] (Acid addition amount adjustment process) The acid addition amount adjustment step is a step in which the amount of sulfuric acid added to the dewatered sludge is adjusted. The acid addition amount adjustment step may be performed in conjunction with the pH measuring means 14 (pH measuring step) to adjust the amount of acid supplied to the dewatered sludge based on the pH value of the carbonized sludge slurry or carbonized sludge.

[0051] (Second dehydrator and second dehydration process) The second dewatering machine 4 is a machine that dewaters the carbonized sludge slurry obtained in the hydrothermal carbonization apparatus 3. The second dewatering process is a process in which the carbonized sludge slurry obtained in the sludge carbonization process is dewatered by the second dewatering machine 4. The second dewatering machine 4 may be a filter press dewatering machine, a centrifugal dewatering machine, a screw press dewatering machine, a belt concentrator, or a centrifugal concentrator.

[0052] (filtrate return process) The filtrate return process is the process of sending (returning) the hydrothermal carbonization dehydrated filtrate separated from the carbonized sludge slurry in the second dewatering machine 4 (second dewatering process) to the digester 1. By returning the hydrothermal carbonization dehydrated filtrate, which has a low pH, to the methane fermentation tank, the pH of the fermentation-treated dehydrated filtrate separated from the fermentation-treated sludge in the first dewatering process is lowered, which reduces the amount of acid added to the fermentation-treated dehydrated filtrate in the separated liquid treatment process. In addition, the hydrothermal carbonization treatment destroys the cells of microorganisms in the dewatered sludge, causing organic matter in the dewatered sludge to dissolve into the filtrate (hydrothermal carbonization dehydrated filtrate). Since the organic matter contained in the hydrothermal carbonization dehydrated filtrate becomes the raw material for digester gas, when the hydrothermal carbonization dehydrated filtrate is introduced into the digester 1, the amount of digester gas generated increases accordingly. Digester gas is energy (gas energy) recovered from sewage sludge that can be used as fuel. In other words, by returning the dewatered filtrate (hydrothermal carbonization dewatered filtrate) separated from the carbonized sludge slurry in the second dewatering step back to the digester tank 1, the amount of digester gas generated can be increased, and the amount of energy recovered can be increased.

[0053] Furthermore, since some of the organic matter contained in the hydrothermal carbonization filtrate becomes digester gas, the amount of organic matter in the treatment system is reduced accordingly. As a result, the increase in COD of the treated water is suppressed, and the deterioration of effluent quality from the sewage treatment plant can be suppressed. For more details, see below. If the hydrothermal carbonization filtrate is sent directly to the water treatment system for treatment, the COD load on the water treatment system may increase, potentially worsening the effluent quality. However, because organic matter is decomposed by methane fermentation, the increase in the COD load on the water treatment system is suppressed, and the deterioration of effluent quality can be suppressed.

[0054] Figure 3 is a graph showing the relationship between the amount of sulfuric acid added to the dewatered sludge obtained by dewatering the fermented sludge and the moisture content of the carbonized sludge obtained by dewatering the carbonized sludge slurry after hydrothermal carbonization treatment of the dewatered sludge. The solid content of the dewatered sludge supplied to reactor 7 was 12% by mass, the temperature of the dewatered sludge in reactor 7 was 190°C, the pressure was 1.9 MPa gauge pressure, and the residence time of the dewatered sludge was 120 minutes. A filter press dewatering machine was used as the second dewatering machine 4, and dewatering was performed at a gauge pressure of 2.0 MPa.

[0055] In the graph shown in Figure 3, the horizontal axis represents the amount of sulfuric acid added per 1 kg of solid matter in the dewatered sludge obtained by dewatering the fermented sludge (g-H2SO4 / kg-DS), and the vertical axis represents the water content (%) of the carbonized sludge obtained by dewatering the carbonized sludge slurry after hydrothermal carbonization treatment of the dewatered sludge.

[0056] As can be seen from Figure 3, the water content of the carbonized sludge obtained by dewatering the carbonized sludge slurry after hydrothermal carbonization treatment of dewatered sludge with a sulfuric acid addition of 70 g-H2SO4 / kg-DS was 25.9%. On the other hand, the water content of the carbonized sludge obtained by dewatering the carbonized sludge slurry after hydrothermal carbonization treatment of dewatered sludge with a sulfuric acid addition of 35 g-H2SO4 / kg-DS was 34.2%. From the graph in Figure 3, it can be predicted that when the amount of sulfuric acid added to the dewatered sludge (sulfuric acid content in the dewatered sludge) exceeds 50 g-H2SO4 / kg-DS, carbonized sludge with a low water content of 30% or less can be obtained.

[0057] (Curing container and curing process) Curing container 5 is a container used to dry the carbonized sludge obtained in the second dewatering machine 4 and to reduce the heat generation and flammability of the carbonized sludge. The curing process is a process that dries the carbonized sludge obtained in the second dewatering process and reduces the heat generation and flammability of the carbonized sludge. Curing container 5 is a container commonly called a hopper.

[0058] The carbonized sludge from the second dewatering machine 4 is fed into a curing container 5. Air (oxygen-containing gas) is blown into the curing container 5, for example, from the lower side. The carbonized sludge inside the curing container 5 dries and partially oxidizes upon contact with the blown air. This partial oxidation reduces the heat-generating and flammable nature of the carbonized sludge.

[0059] Furthermore, the curing process is not limited to a process using a curing container 5 (hopper). For example, the curing process may be carried out by dropping the carbonized sludge obtained from the second dewatering machine 4 onto a conveyor (curing section) surrounding it and passing oxygen-containing gas such as air through it. In addition, the oxygen-containing gas may be preheated to further promote drying, or the heat energy recovered by the cooling heat exchanger 9 may be used as the heat source for heating.

[0060] (effect) The organic waste treatment method of this embodiment comprises a methane fermentation step in which sewage sludge (organic waste) is subjected to anaerobic fermentation in a digester 1 (methane fermentation tank), a first dewatering step in which the fermented sludge after the methane fermentation step is dewatered, a sludge carbonization step in which the dewatered sludge obtained in the first dewatering step is subjected to hydrothermal carbonization, and a second dewatering step in which the carbonized sludge slurry obtained in the sludge carbonization step is dewatered, wherein a predetermined amount of acid is added to the dewatered sludge in the sludge carbonization step.

[0061] The above processing method yields the following effects:

[0062] By adding a predetermined amount of acid to the dewatered sludge obtained in the first dewatering step described above and subjecting it to hydrothermal carbonization treatment, the acid functions as a catalyst for the decomposition of organic matter, which is the initial reaction of hydrothermal carbonization. As a result, the hydrothermal carbonization reaction can proceed even if the reaction temperature is lowered or the treatment time is shortened, making the hydrothermal carbonization treatment more efficient.

[0063] It is preferable to further include a filtrate return step in which the hydrothermal carbonization dehydrated filtrate separated from the carbonized sludge slurry in the second dewatering step is returned to the methane fermentation tank, and a separation liquid treatment step in which the fermentation treatment dehydrated filtrate separated from the fermentation treatment sludge in the first dewatering step is subjected to acid coagulation and precipitation treatment using an iron-based inorganic coagulant. By returning the hydrothermal carbonization dehydrated filtrate with a low pH to the methane fermentation tank, the pH of the fermentation treatment dehydrated filtrate separated from the fermentation treatment sludge in the first dewatering step is lowered, so the amount of acid added to the fermentation treatment dehydrated filtrate in the separation liquid treatment step can be reduced. In addition, the amount of energy recovered increases because the organic matter contained in the hydrothermal carbonization dehydrated filtrate becomes a raw material for methane fermentation. As a result, the amount of organic matter in the treatment system decreases, so the increase in COD of the treated water is suppressed. Furthermore, by subjecting the fermentation treatment dehydrated filtrate to acid coagulation and precipitation treatment using an iron-based inorganic coagulant, phosphorus and heavy metals are removed, and the increase in COD of the treated water is also suppressed. As a result, the deterioration of effluent quality can be suppressed.

[0064] Furthermore, it is preferable to dewater the fermentation-treated sludge using an iron-based inorganic coagulant in the first dewatering step. This allows the generation of hydrogen sulfide to be suppressed because, even if the hydrothermal carbonization dewatered filtrate containing sulfate is returned to the methane fermentation tank, the iron ions contained therein react with sulfide ions produced by sulfate-reducing bacteria to form iron sulfide.

[0065] Furthermore, in the sludge carbonization process described above, it is preferable that the amount of sulfuric acid added to the dewatered sludge is 20 to 140 g-H2SO4 / kg-DS. This makes the hydrothermal carbonization process more efficient.

[0066] Furthermore, the organic waste treatment method of this embodiment includes a pH measurement step for measuring the pH of the carbonized sludge slurry or carbonized sludge, and an acid addition amount adjustment step for adjusting the amount of acid added to the dewatered sludge based on the pH value of the carbonized sludge slurry or carbonized sludge obtained in the pH measurement step. This makes it possible to avoid the hydrothermal carbonization reaction not proceeding sufficiently due to insufficient acid addition, or the costs increasing or adverse effects on subsequent treatments due to excessive acid addition.

[0067] The above embodiment can be modified as follows:

[0068] The above embodiment may be operated by installing multiple digesters 1 in parallel. For example, multiple digesters 1 may be installed and configured so that the fermented sludge from multiple digesters 1 is supplied to one first dewatering machine 2; multiple digesters 1 and multiple first dewatering machines 2 to which the fermented sludge from digesters 1 is supplied may be installed and configured so that the dewatered sludge from multiple first dewatering machines 2 is supplied to one hydrothermal carbonization device 3; multiple digesters 1, multiple first dewatering machines 2 to which the fermented sludge from digesters 1 is supplied and multiple hydrothermal carbonization devices 3 to which the dewatered sludge from the first dewatering machines 2 is supplied may be installed and configured so that the carbonized sludge slurry from multiple hydrothermal carbonization devices 3 is supplied to one second dewatering machine 4. Furthermore, multiple digesters 1 and first dewaterers 2 to which the fermented sludge from the digesters 1 is supplied may be installed, and the dewatered sludge from one or more of the first dewaterers 2 may be supplied to one hydrothermal carbonization device 3, while the dewatered sludge from one or more of the other first dewaterers 2 may be incinerated. In these embodiments, the filtrate return process only needs to be configured to return the hydrothermal carbonization dewatered filtrate separated from the carbonized sludge slurry in the second dewaterer 2 to at least one of the multiple digesters 1 installed.

[0069] The above embodiment includes a filtrate return step. In the present invention, the filtrate return step is not essential.

[0070] The above embodiment includes a separation liquid processing apparatus 6 and a separation liquid processing step. In the present invention, the separation liquid processing apparatus 6 and the separation liquid processing step are not essential.

[0071] The above embodiment includes a pH measuring means 14 and a pH measuring step. In the present invention, the pH measuring means 14 and the pH measuring step are not essential.

[0072] The above embodiment includes a curing container 5 and a curing process. In the present invention, the curing container 5 and the curing process are not essential.

[0073] The organic waste to be treated is not limited to sewage sludge. The present invention can treat a variety of organic wastes, including sewage sludge, human waste sludge, agricultural community wastewater sludge, septic tank sludge, food waste such as kitchen waste (food-derived biomass), lignocellulosic waste such as recycled paper and discarded paper, agricultural residues, and livestock manure. As described above, these organic wastes may be treated individually or in combination. [Explanation of symbols]

[0074] 1: Digestion tank (methane fermentation tank) 2: 1st dehydrator 3:Hydrothermal carbonization equipment 4:Second dehydrator 5:Curing container 6: Separation liquid treatment device 7: Reactor 12: Dewatered sludge supply pump 13: Acid addition device 14:pH measurement means

Claims

1. The methane fermentation process involves anaerobic fermentation of organic waste in a methane fermentation tank, A first dewatering step for dewatering the fermentation-treated sludge after the methane fermentation step, A sludge carbonization step is performed by hydrothermally carbonizing the dewatered sludge obtained in the first dewatering step by indirect heating, A second dewatering step is performed to dewater the carbonized sludge slurry obtained in the sludge carbonization step, Equipped with, In the sludge carbonization process, a predetermined amount of acid is added to the dewatered sludge. A method for treating organic waste using hydrothermal carbonization.

2. In the method for treating organic waste using hydrothermal carbonization treatment as described in claim 1, A filtrate return step is performed in which the hydrothermal carbonization dewatered filtrate separated from the carbonized sludge slurry in the second dewatering step is returned to the methane fermentation tank, A separation liquid treatment step is performed in which the fermentation treatment dewatered filtrate separated from the fermentation treatment sludge in the first dewatering step is subjected to acid coagulation and precipitation treatment using an iron-based inorganic coagulant, Furthermore, A method for treating organic waste using hydrothermal carbonization.

3. A methane fermentation step of treating organic waste with anaerobic fermentation in a methane fermentation tank, A first dewatering step for dewatering the fermentation-treated sludge after the methane fermentation step, A sludge carbonization step in which the dewatered sludge obtained in the first dewatering step is subjected to hydrothermal carbonization, A second dewatering step is performed to dewater the carbonized sludge slurry obtained in the sludge carbonization step, A filtrate return step is performed in which the hydrothermal carbonization dewatered filtrate separated from the carbonized sludge slurry in the second dewatering step is returned to the methane fermentation tank, A separation liquid treatment step is performed in which the fermentation treatment dewatered filtrate separated from the fermentation treatment sludge in the first dewatering step is subjected to acid coagulation and precipitation treatment using an iron-based inorganic coagulant, Furthermore, In the sludge carbonization process, a predetermined amount of acid is added to the dewatered sludge. A method for treating organic waste using hydrothermal carbonization.

4. A method for treating organic waste using hydrothermal carbonization treatment according to any one of claims 1 to 3, A pH measurement step for measuring the pH of the carbonized sludge slurry, An acid addition amount adjustment step is performed to adjust the amount of acid added to the dewatered sludge based on the pH value of the carbonized sludge slurry obtained in the pH measurement step, Furthermore, A method for treating organic waste using hydrothermal carbonization.

5. A method for treating organic waste using hydrothermal carbonization treatment according to any one of claims 1 to 3, The acid is sulfuric acid. A method for treating organic waste using hydrothermal carbonization.

6. In the method for treating organic waste using hydrothermal carbonization treatment as described in claim 5, In the first dewatering step, the fermentation-treated sludge is dewatered using an iron-based inorganic flocculant. A method for treating organic waste using hydrothermal carbonization.

7. In the method for treating organic waste using hydrothermal carbonization treatment as described in claim 5, In the sludge carbonization process, the amount of sulfuric acid added to the dewatered sludge is 20 to 140 g-H 2 SO 4 / kg-DS, A method for treating organic waste using hydrothermal carbonization.

8. A methane fermentation step of treating organic waste with anaerobic fermentation in a methane fermentation tank, A first dewatering step for dewatering the fermentation-treated sludge after the methane fermentation step, A sludge carbonization step in which the dewatered sludge obtained in the first dewatering step is subjected to hydrothermal carbonization, A second dewatering step is performed to dewater the carbonized sludge slurry obtained in the sludge carbonization step, Equipped with, In the sludge carbonization process, a predetermined amount of sulfuric acid is added to the dewatered sludge. A method for treating organic waste using hydrothermal carbonization.

9. A method for treating organic waste using hydrothermal carbonization treatment as described in Claim 8, A filtrate return step is performed in which the hydrothermal carbonization dewatered filtrate separated from the carbonized sludge slurry in the second dewatering step is returned to the methane fermentation tank, A separation liquid treatment step is performed in which the fermentation treatment dewatered filtrate separated from the fermentation treatment sludge in the first dewatering step is subjected to acid coagulation and precipitation treatment using an iron-based inorganic coagulant, Furthermore, A method for treating organic waste using hydrothermal carbonization.

10. A method for treating organic waste using hydrothermal carbonization treatment according to claim 8 or 9, In the first dewatering step, the fermentation-treated sludge is dewatered using an iron-based inorganic flocculant. A method for treating organic waste using hydrothermal carbonization.

11. A method for treating organic waste using hydrothermal carbonization treatment according to claim 8 or 9, In the sludge carbonization process, the amount of sulfuric acid added to the dewatered sludge is 20 to 140 g-H₂SO₄ / kg-DS. A method for treating organic waste using hydrothermal carbonization.

12. A method for treating organic waste using hydrothermal carbonization treatment according to claim 8 or 9, A pH measurement step for measuring the pH of the carbonized sludge slurry, Based on the pH value of the carbonized sludge slurry obtained in the pH measurement step, an acid addition amount adjustment step is performed to adjust the amount of sulfuric acid added to the dewatered sludge. Furthermore, A method for treating organic waste using hydrothermal carbonization.

13. A methane fermentation tank for anaerobic fermentation treatment of organic waste, A first dewatering machine for dewatering the fermentation sludge discharged from the methane fermentation tank, A hydrothermal carbonization apparatus for hydrothermally carbonizing the dewatered sludge obtained in the first dewatering machine, A second dewatering machine for dewatering the carbonized sludge slurry obtained in the aforementioned hydrothermal carbonization apparatus, Equipped with, The aforementioned hydrothermal carbonization apparatus is An acid adding device for adding a predetermined amount of acid to the dewatered sludge, A reactor that processes the dewatered sludge at high temperature and high pressure in a gas atmosphere that does not contain oxygen or has a low oxygen concentration, or in a state where oxygen is blocked. It has, The dewatered sludge is indirectly heated in the reactor. A facility for treating organic waste using hydrothermal carbonization.

14. In the organic waste treatment facility using hydrothermal carbonization treatment described in claim 13, The acid is sulfuric acid. A facility for treating organic waste using hydrothermal carbonization.

15. A methane fermentation tank for anaerobic fermentation treatment of organic waste, A first dewatering machine for dewatering the fermentation sludge discharged from the methane fermentation tank, A hydrothermal carbonization apparatus for hydrothermally carbonizing the dewatered sludge obtained in the first dewatering machine, A second dewatering machine for dewatering the carbonized sludge slurry obtained in the aforementioned hydrothermal carbonization apparatus, Equipped with, The hydrothermal carbonization apparatus has an acid addition device for adding a predetermined amount of acid to the dewatered sludge. The acid is sulfuric acid. A facility for treating organic waste using hydrothermal carbonization.

16. In the organic waste treatment facility using hydrothermal carbonization treatment described in claim 15, The aforementioned hydrothermal carbonization apparatus is The reactor provides for high-temperature, high-pressure treatment of the dewatered sludge in a gas atmosphere that does not contain oxygen or has a low oxygen concentration, or in a state where oxygen is blocked. The dewatered sludge is supplied to the reactor by a dewatered sludge supply pump. The acid addition device adds the acid to the dewatered sludge while the dewatered sludge supply pump is in operation. A facility for treating organic waste using hydrothermal carbonization.

17. In an organic waste treatment facility using hydrothermal carbonization treatment according to any one of claims 13 to 16, The hydrothermal carbonization apparatus further includes a pH measuring means for measuring the pH of the carbonized sludge slurry. The acid addition device adjusts the amount of acid added to the dewatered sludge based on the pH value of the carbonized sludge slurry obtained by the pH measuring means. A facility for treating organic waste using hydrothermal carbonization.

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