JP2023111869A - Method for treating organic waste using hydrothermal carbonization treatment and facility for treating organic waste using hydrothermal carbonization treatment - Google Patents

Abstract

To provide a method and facility for treating organic waste that can yield dewatering object having excellent dewaterability.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 first 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 (mechanical dewaterer).SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present invention relates to a method for treating organic waste using hydrothermal carbonization, and an organic waste treatment facility using hydrothermal carbonization.

For example, Japanese Patent Laid-Open No. 2002-200000 discloses a technique related to a method for treating organic waste. Patent Document 1 describes a method for treating organic waste, which includes the steps of solubilizing the digested slurry after the methane recovery step and dehydrating the solubilized treated material with a dehydrator.

Specifically, the method for treating organic waste described in Patent Document 1 includes a methane recovery step in which an organic waste slurry is anaerobicly digested in an anaerobic digestion tank to recover methane gas, and after the methane recovery step of 150 to 200° C. at a pressure that maintains the liquid phase of the digested slurry for 15 to 60 minutes for a treatment time of 15 to 60 minutes. A solubilization step of partially decomposing by supplying an oxygen-containing gas equivalent to 5 to 25% by mass of the value, and dehydrating at least part of the treated material from the solubilization step with a dehydrator to reduce the water content to 70% by mass or less. and a composting step of aerobic fermentation of the dehydrated matter after the dehydration step to make compost.

Paragraph 0057 of the specification of Patent Document 1 states, "Therefore, the oxidized slurry from the solubilization step can be easily dehydrated with a dehydrator to a water content of 70% by mass or less." In addition, in paragraph 0063 of the specification of Patent Document 1, it is stated that "the oxidized slurry obtained in the solubilization step was dehydrated using a Nutsche test machine. A dewatered sludge with a rate of about 63% by mass was obtained,”.

Japanese Patent No. 3651836

From the description of Patent Document 1, in the organic waste treatment method described in Patent Document 1, the water content of the treated material (dewatered sludge) that can be reached in the dehydration step is at most about 60% by mass.

If both dehydration and drying can be applied as means for reducing the moisture content of an object (dehydrated object), dehydration requires less energy to obtain the same moisture content. Therefore, it is preferable to carry out a treatment to obtain an object having excellent dehydration properties. In the technique described in Patent Document 1, the obtained object (oxidation-treated slurry) has a water content of at most about 60% by mass after dehydration, which is poor in dehydration.

SUMMARY OF THE INVENTION An object of the present invention is to provide an organic waste treatment method and treatment facility that can obtain a dehydrated object with excellent dehydration properties.

The method for treating organic waste disclosed in the present application is a method for treating organic waste using hydrothermal carbonization, which is a methane fermentation process in which organic waste is anaerobicly fermented in a methane fermentation tank. and a first dehydration step of dehydrating the fermented sludge after the methane fermentation step, a sludge carbonization step of hydrothermally carbonizing the dehydrated sludge obtained in the first dehydration step, and the sludge carbonization step. and a second dewatering step of dewatering the carbonized sludge slurry with a mechanical dehydrator.

By hydrothermally carbonizing the dewatered sludge obtained in the first dewatering step, the cells of microorganisms in the dehydrated sludge are destroyed, and the intracellular components are decomposed and polymerized. As a result, a carbonized sludge slurry (substance to be dewatered) having excellent dewaterability is produced. By dehydrating the carbonized sludge slurry with excellent dewatering properties using a mechanical dehydrator, carbonized sludge (processed material) with a low moisture content can be obtained.

In the second dehydration step, the carbonized sludge slurry may be dehydrated by pressing at a gauge pressure of 0.7 MPa or more and a gauge pressure of 2.0 MPa or less.

According to this configuration, it is possible to obtain carbonized sludge (processed material) having a moisture content of 25% by mass or more and 55% by mass or less. In addition, soluble Na, K, and Cl are eluted in water in the carbonized sludge slurry obtained in the sludge carbonization step. Since these components are removed together with the dehydrated filtrate in the second dehydration step, the lower the water content of the carbonized sludge, the lower the Na, K, and Cl content in the carbonized sludge. Therefore, by obtaining carbonized sludge having a water content of 25% by mass or more and 55% by mass or less, it is possible to reduce the content of Na, K, and Cl in the carbonized sludge. As a result, it is possible to obtain carbonized sludge (processed material) with a low impurity content.

Moreover, in the second dewatering step, the carbonized sludge slurry may be dewatered without adding a flocculant to the carbonized sludge slurry.

According to this configuration, since no coagulant is used for dehydration of the carbonized sludge slurry, the cost of treating the organic waste can be reduced accordingly. In addition, carbonized sludge (processed material) with less impurity content can be obtained because no flocculant is used.

In addition, carbonized sludge having a moisture content of 40% by mass or less is obtained by the second dehydration step.

According to this configuration, it is possible to obtain carbonized sludge (processed material) with a low moisture content.

Moreover, in the first dehydration step, the fermentation-treated sludge may be dehydrated so that the obtained dehydrated sludge has a moisture content of 75% by mass or more and 90% by mass or less.

According to this configuration, the hydrothermal carbonization treatment can be stably performed, and the amount of heat supplied for the hydrothermal carbonization treatment and the capacity of the reactor for performing the hydrothermal carbonization treatment are prevented from becoming excessively large. can.

Moreover, you may further provide the filtrate return process of returning the hydrothermal carbon dehydration filtrate isolate|separated from the said carbonized sludge slurry in the said 2nd dehydration process to the said methane fermentation tank.

According to this configuration, the amount of energy recovered increases because the organic matter contained in the hydrothermal carbonization filtrate is used as a raw material for methane fermentation. In addition, since the amount of organic matter is reduced from the treatment system by that amount, an increase in the COD of the treated water can be suppressed, and deterioration of the quality of the discharged water can be suppressed. Details are as follows. If the hydrothermally carbonized dewatering filtrate is directly sent to the water treatment system for treatment, the COD load in the water treatment system may increase and the discharged water quality may deteriorate. The COD load increase in the treatment system is suppressed, and the deterioration of effluent water quality can be suppressed.

Further, the carbonized sludge slurry having a temperature of 60° C. or higher obtained in the sludge carbonization step may be supplied to the second dehydration step.

According to this configuration, the viscosity of the carbonized sludge slurry can be kept low, the carbonized sludge slurry can be easily transferred, and the dewaterability of the carbonized sludge slurry is improved.

The method further includes a filtrate return step for returning the hydrothermally carbonized dewatering filtrate separated from the carbonized sludge slurry in the second dehydration step to the methane fermentation tank, wherein the hydrothermally carbonized dewatering filtrate is returned to the methane fermentation tank. During the return, the hydrothermally dehydrated by carbonization filtrate may be subjected to nitrogen stripping to release and remove the ammonia nitrogen contained in the filtrate by carbonization as ammonia gas.

According to this configuration, although the nitrogen concentration in the methane fermentation tank may become high depending on the properties of the hydrothermal carbonization filtrate, which may inhibit microbial activity, the inhibition of microbial activity can be suppressed. In addition, the removed ammonia gas can be used, for example, as fertilizer.

The organic waste treatment facility disclosed in the present application is an organic waste treatment facility using hydrothermal carbonization, comprising a methane fermentation tank for anaerobic fermentation treatment of organic waste, and the methane fermentation tank A first dehydrator for dehydrating the fermented sludge discharged from the first dehydrator, a hydrothermal carbonization device for hydrothermally carbonizing the dewatered sludge obtained by the first dehydrator, and the carbonized sludge obtained by the hydrothermal carbonization device. and a second dehydrator for mechanically dehydrating the slurry.

By hydrothermally carbonizing the dehydrated sludge obtained in the first dehydrator, the cells of microorganisms in the dehydrated sludge are destroyed, and the components in the cells are decomposed and polymerized. As a result, a carbonized sludge slurry (substance to be dewatered) having excellent dewaterability is produced. By dehydrating this carbonized sludge slurry with excellent dewatering properties with the second dehydrator, carbonized sludge (processed material) with a low water content can be obtained.

The second dehydrator may be a filter press dehydrator.

According to this configuration, it is possible to obtain carbonized sludge (processed material) with a low moisture content.

In addition, the hydrothermal carbonization apparatus includes a heating heat exchanger for preheating the dehydrated sludge, and the dehydrated sludge preheated by the heating heat exchanger in a gas atmosphere containing no oxygen or having a low oxygen concentration, or A reactor for performing high-temperature and high-pressure treatment in a state in which oxygen is shut off, and a cooling heat exchanger for cooling the carbonized sludge slurry obtained in the reactor, wherein the temperature of the carbonized sludge slurry is reduced in the cooling heat exchanger. may be adjusted.

According to this configuration, the viscosity of the carbonized sludge slurry can be adjusted, the transfer of the carbonized sludge slurry can be facilitated, and the dewaterability of the carbonized sludge slurry can be improved. Also, the amount of carbonized sludge slurry withdrawn can be stabilized.

According to the organic waste disposal method or the organic waste disposal facility configured as described above, it is possible to obtain an object to be dehydrated with excellent dehydration properties.

1 is a block diagram showing an example of an organic waste treatment facility using hydrothermal carbonization; FIG. FIG. 2 is a diagram showing an example of a specific configuration of the hydrothermal carbonization apparatus shown in FIG. 1; 4 is a graph showing test results of dehydrating carbonized sludge slurry obtained by a hydrothermal carbonization apparatus by a mechanical dehydrator.

EMBODIMENT OF THE INVENTION Hereinafter, it demonstrates, referring drawings for the form for implementing this invention.

The organic waste to be treated in the treatment method of the present invention includes sewage sludge, food waste (food biomass) such as garbage, lignocellulosic waste such as waste paper and waste paper, agricultural residue, and livestock manure. is. These organic wastes may be treated individually or mixed. The treatment of sewage sludge will be described below as an example of an object to be treated.

As shown in FIG. 1, the organic waste treatment facility includes a digestion tank 1 as a methane fermentation tank, a first dehydrator 2, a hydrothermal carbonization device 3, a second dehydrator 4, and a curing container 5. and a separated liquid processing device 6. The digestion tank 1, the first dehydrator 2, the hydrothermal carbonization device 3, the second dehydrator 4, and the curing vessel 5 are provided in this order from the upstream side of the treatment process. The separated liquid treatment device 6 is provided downstream of the first dehydrator 2 in the treatment process.

(Digestion tank and methane fermentation process)
The digestion tank 1 is a tank for anaerobic fermentation treatment of sewage sludge. The methane fermentation step is a step of anaerobic fermentation treatment of sewage sludge in the digestion tank 1 . The solid matter concentration of sewage sludge introduced into the digestion tank 1 is, for example, 3 to 9% by mass. The digestion tank 1 is operated at a temperature of about 30 to 42° C. for a retention time of about 15 to 30 days for mesophilic fermentation, and at a temperature of about 50 to 60° C. for a retention time of about 7 to 20 days for high temperature fermentation.

Digestion gas is generated in the digestion tank 1 by anaerobic fermentation of sewage sludge. Digestion gas is a gas (biogas) containing about 60% by volume methane and about 40% by volume carbon dioxide. The generated digestion gas is taken out from the digestion tank 1 and used as fuel for heating the digestion tank 1 and the hydrothermal carbonization device 3, or used as fuel for power generation equipment (not shown). That is, by subjecting sewage sludge to anaerobic fermentation treatment, the energy possessed by sewage sludge can be recovered as digestion gas (gas energy).

Fermentation residue of sewage sludge after the anaerobic fermentation treatment, that is, fermentation sludge is discharged from the digestion tank 1 to the outside.

(First dehydrator and first dehydration step)
Fermented sludge discharged from the digestion tank 1 to the outside is supplied to the first dehydrator 2 . The first dehydrator 2 is a machine for dehydrating the fermented sludge discharged from the digestion tank 1 . The first dewatering step is a step of dewatering the fermented sludge after the methane fermentation step. The solid matter concentration of the fermentation sludge is, for example, 1.5 to 5% by mass. The first dehydrator 2 is a belt press dehydrator, a filter press dehydrator, a centrifugal dehydrator, a screw press dehydrator, a belt concentrator, a centrifugal concentrator, or the like.

In the first dehydrator 2 (first dehydration step), it is preferable to dehydrate the fermentation-treated sludge so that the moisture content of the dewatered sludge is 75% by mass or more and 90% by mass or less. By the first dehydrator 2 (first dehydration step), the fermented sludge becomes dehydrated sludge having a water content of about 80% by mass, for example.

In the first dehydrator 2 (first dehydration step), an iron-based inorganic coagulant may be used for dehydration, or an iron-based inorganic coagulant and a polymer coagulant may be used in combination. As the iron-based inorganic flocculant, for example, inorganic flocculants such as polyferric sulfate, ferric chloride, and ferrous sulfate can be used. As the polymer flocculant, for example, in addition to cationic polymer flocculants and anionic polymer flocculants, nonionic polymer flocculants and amphoteric polymer flocculants can be used. It is preferable to use a cationic polymer flocculant. In the first dehydrator 2 (first dehydration step), dehydration is preferably performed by adding 0.5 to 2.5% by mass of a polymer flocculant to the amount of solids in the fermentation sludge. By the first dehydrator 2 (first dewatering step) for dehydrating the fermented sludge under the above preferable conditions, the fermented sludge becomes dehydrated sludge having a water content of about 75 to 90% by mass. More preferably, the fermentation-treated sludge is dehydrated so that the moisture content of the dehydrated sludge is 75% by mass or more and 85% by mass or less.

If the water content is less than 75% by mass, the fluidity of the dehydrated sludge is reduced, and the pressure loss of the piping through which the dehydrated sludge is transferred and the heating heat exchanger 8 described later increases, resulting in transfer of the dehydrated sludge. There is a possibility that the hydrothermal carbonization treatment in the reactor 7, which will be described later, cannot be stably performed.

If the water content exceeds 90% by mass, the water content of the dehydrated sludge is high, so there is a risk that a larger amount of heat will need to be supplied in the subsequent hydrothermal carbonization treatment, and the capacity of the reactor 7, which will be described later, will be increased. There is a risk that the need will arise.

In the first dehydrator 2 (first dehydration step), a flocculant is added to the fermented sludge so that the moisture content is in the above preferable range, dehydrated to form dehydrated sludge, and then hydrothermally carbonized, which will be described later. The hydrothermal carbonization treatment in the reactor 7 can be stably performed, and the amount of heat supplied to the reactor 7 and the capacity of the reactor 7 can be suppressed from becoming excessively large. The dehydration of the fermentation sludge by the first dehydrator 2 (first dehydration step) is not limited to dehydration of the fermentation sludge so that the moisture content falls within the above preferable range.

The dehydrated filtrate (digested dehydrated filtrate) separated from the fermentation-treated sludge by the first dehydrator 2 is sent to the separated liquid treatment device 6 .

(Separated liquid treatment device and separated liquid treatment process)
A dehydrated filtrate (digested dehydrated filtrate) separated from the fermented sludge is supplied to the separated liquid treatment device 6 . The separated liquid treatment device 6 is a device for treating the dehydrated filtrate (digested dehydrated filtrate) separated from the fermented sludge by the first dehydrator 2 . The separated liquid treatment step is a step of treating the dehydrated filtrate (digested dehydrated filtrate) separated from the fermented sludge in the first dehydration step. Treated water obtained in the separated liquid treatment device 6 is returned to a water treatment facility (not shown).

The separated liquid processing device 6 is, for example, a device that coagulates and sediments the dehydrated filtrate (digested dehydrated filtrate). The separated liquid treatment step is, for example, a step of subjecting the dehydrated filtrate (digested dehydrated filtrate) to a coagulating sedimentation treatment. The treatment of the dehydrated filtrate (digestion-dehydrated filtrate) is not limited to coagulation sedimentation treatment, and may be coagulation flotation separation treatment, coagulation filtration treatment, or the like.

(Hydrothermal carbonization device and sludge carbonization process)
Dewatered sludge is supplied to the hydrothermal carbonization device 3 . The hydrothermal carbonization device 3 is a device for hydrothermally carbonizing the dewatered sludge obtained by the first dehydrator 2 . The sludge carbonization step is a step of hydrothermally carbonizing the dewatered sludge obtained in the first dewatering step. Hydrothermal carbonization is a process of carbonizing an object containing water 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 shut off. say.

As shown in FIG. 2, the hydrothermal carbonization device 3 includes, for example, a heating heat exchanger 8, a reactor 7, and a cooling heat exchanger 9. In addition, the structure of the hydrothermal carbonization apparatus 3 is not limited to this.

The heating heat exchanger 8 is a heater for preheating the dehydrated sludge obtained by the first dehydrator 2 before the dehydrated sludge is supplied to the reactor 7 . The heating heat exchanger 8 is supplied with dehydrated sludge having a water content of about 80% by mass from the first dehydrator 2, for example.

The reactor 7 is a vessel for high-temperature and high-pressure treatment of dehydrated sludge in a gas atmosphere containing no oxygen or with a low oxygen concentration or in a state in which oxygen is shut off. As an example, the reactor 7 treats the dehydrated sludge at high temperature and high pressure in a gas atmosphere with an oxygen concentration of 5% by volume or less. The high-temperature and high-pressure treatment is started in a state of being replaced with an inert gas, or even if the air (oxygen concentration 21% by volume) is used at the start, the easily decomposable organic matter in the object to be treated is quickly oxidized. Since oxygen is consumed and air is not supplied thereafter, the oxygen concentration in the gas phase inside the reactor 7 during the reaction is kept at approximately 0% by volume except immediately after the start. The reactor 7 has an agitator 7a. The carbonized sludge slurry in the reactor 7 and the dehydrated sludge supplied to the reactor 7 are mixed and stirred by the stirrer 7a. A tubular jacket 7 b is provided around the outer periphery of the reactor 7 . The heat medium oil heated by the heat medium oil boiler 10 is circulated and supplied to the jacket 7 b by the heat medium oil circulation pump 11 . The carbonized sludge slurry in the reactor 7 is indirectly heated by the circulating heat transfer oil. In addition, another heat medium may be used instead of the heat medium oil.

The carbonized sludge slurry in the reactor 7 is indirectly heated by thermal oil to a temperature of 200° C., for example. The pressure inside the reactor 7 is the sum of the pressure corresponding to the sub-critical water corresponding to the temperature inside the reactor 7 and the pressure due to the gas generated by the decomposition of the components of the object to be treated. The temperature of the carbonized sludge slurry in the reactor 7 is not limited to 200°C. The temperature of the carbonized sludge slurry in the reactor 7 may be any temperature within the range of 160°C to 220°C. The pressure inside the reactor 7 is set at a gauge pressure of about 0.6 MPa to about 3 MPa. High-temperature and high-pressure treatment of the dewatered sludge means treatment of the dewatered sludge at a temperature of 160° C. or higher and 220° C. or lower and a gauge pressure of 0.6 MPa or higher and a gauge pressure of 3 MPa or lower in the reactor 7 .

The treatment time of the dewatered sludge, that is, the residence time of the carbonized sludge slurry in the reactor 7 is, for example, 3 hours. The residence time of the carbonized sludge slurry in the reactor 7 is not limited to 3 hours. The residence time of the carbonized sludge slurry in the reactor 7 may be within the range of 2 hours to 5 hours.

The dehydrated sludge supplied into the reactor 7 is treated at the above pressure and temperature for the above treatment time to form a carbonized sludge slurry. The hydrothermal carbonization treatment destroys the cells of microorganisms in the dewatered sludge, and causes decomposition and polymerization of intracellular components. As a result, a carbonized sludge slurry with excellent dewaterability is produced.

The cooling heat exchanger 9 is a cooler for cooling the carbonized sludge slurry obtained by hydrothermal carbonization in the reactor 7 . In the cooling heat exchanger 9, the temperature of the carbonized sludge slurry is adjusted to a region suitable for handling. By adjusting the temperature of the carbonized sludge slurry, the viscosity of the carbonized sludge slurry can be adjusted, the transfer of the carbonized sludge slurry can be facilitated, and the dewaterability of the carbonized sludge slurry can be improved. .

The carbonized sludge slurry is cooled by the cooling heat exchanger 9 and then supplied to the second dehydrator 4 . If the temperature of the carbonized sludge slurry is less than 60° C. when it is supplied to the second dehydrator 4, the viscosity of the carbonized sludge slurry increases. There is a concern that the dewaterability of the slurry is lowered. Therefore, it is preferable to adjust the temperature of the carbonized sludge slurry to 60° C. or higher in the cooling heat exchanger 9, and supply the carbonized sludge slurry having a temperature of 60° C. or higher obtained in this sludge carbonization step to the second dehydration step. . As a result, the viscosity of the carbonized sludge slurry can be kept low, the carbonized sludge slurry can be easily transferred, and the dewaterability of the carbonized sludge slurry can be improved. Further, by cooling the carbonized sludge slurry to a substantially constant temperature and then passing it through the withdrawal device (the device that supplies the carbonized sludge slurry to the second dehydrator 4), the withdrawal amount can be stabilized. For example, when a single-screw pump is used as the extraction device, unless the temperature of the carbonized sludge slurry is high and stable, the flow rate may fluctuate due to expansion of the rotor and stator of the single-screw pump. can be done. Although it is preferable to use a filter press dehydrator as the second dehydrator 4, the temperature of the carbonized sludge slurry supplied to the second dehydration step is preferably 80° C. or less from the viewpoint of durability of the filter press dehydrator. preferable. That is, it is preferable to adjust the temperature of the carbonized sludge slurry to 60° C. or higher and 80° C. or lower in the cooling heat exchanger 9 and supply the carbonized sludge slurry at a temperature of 60° C. or higher and 80° C. or lower to the second dehydration step. The heat energy recovered by the cooling heat exchanger 9 may be used for preheating the oxygen-containing gas used for curing the carbonized sludge, as described later, or as a heat source for heating for other purposes. may be used. As a heat source for heating for other uses, for example, a heat source for preheating dehydrated sludge to be introduced into the reactor 7 can be mentioned.

(Second dehydrator and second dehydration step)
The second dehydrator 4 is a machine for mechanically dewatering the carbonized sludge slurry obtained in the hydrothermal carbonization device 3 . In addition, mechanical dehydration means dehydrating with a mechanical dehydrator. The second dehydration step is a step of dewatering the carbonized sludge slurry obtained in the sludge carbonization step with the second dehydrator 4 . The second dehydrator 4 is a mechanical dehydrator such as a filter press dehydrator, a centrifugal dehydrator, or a screw press dehydrator.

Although the second dehydrator 4 is not particularly limited, it is preferably a filter press dehydrator. The carbonized sludge slurry is in a state in which fine particles of carbide are dispersed in water. Therefore, a filter press dehydrator with fine filter openings and capable of obtaining a high solid content recovery rate even when pressed at high pressure is suitable as the second dehydrator 4 .

A filter press dehydrator is used as the second dehydrator 4, and the carbonized sludge slurry is preferably compressed and dewatered at a gauge pressure of 0.7 MPa or more and a gauge pressure of 2.0 MPa or less (compression pressure). By compressing and dewatering the carbonized sludge slurry with the above pressure, carbonized sludge having a water content of 25% by mass or more and 55% by mass or less can be obtained. In addition, soluble Na, K, and Cl are eluted in water in the carbonized sludge slurry obtained in the sludge carbonization step. Since these components are removed together with the dehydrated filtrate in the second dehydration step, the lower the water content of the carbonized sludge, the lower the Na, K, and Cl content in the carbonized sludge. Therefore, by obtaining carbonized sludge having a water content of 25% by mass or more and 55% by mass or less, it is possible to reduce the content of Na, K, and Cl in the carbonized sludge. As a result, it is possible to obtain carbonized sludge (processed material) with a low impurity content.

Furthermore, a filter press dehydrator is used as the second dehydrator 4, and the carbonized sludge slurry is preferably compressed and dewatered at a gauge pressure of 1.5 MPa or more and a gauge pressure of 2.0 MPa or less (compression pressure). . By compressing and dewatering the carbonized sludge slurry with the above pressure, carbonized sludge having a water content of 25% by mass or more and 35% by mass or less can be obtained. The pressing pressure of the carbonized sludge slurry by the filter press dehydrator is not limited to the above pressure. Alternatively, the carbonized sludge slurry may be dewatered using the second dehydrator 4 other than the filter press dehydrator.

Furthermore, in this second dehydration step, the carbonized sludge slurry is preferably dewatered without adding a flocculant to the carbonized sludge slurry. By not using a flocculant for dehydrating the carbonized sludge slurry, the cost of treating the organic waste can be reduced accordingly. In addition, carbonized sludge (processed product) with a low impurity content can be obtained because no flocculant is used, so it is suitable for use as a solid fuel or the like as described later. In particular, when an inorganic flocculant is used as a flocculant, the ash content in the carbonized sludge (treated material) increases, but since no flocculant is used, carbonized sludge (treated material) with a low ash content is obtained. Therefore, it is suitable for using the carbonized sludge for applications such as solid fuel. Dehydrated sludge that has undergone hydrothermal carbonization, that is, carbonized sludge slurry has excellent dewatering properties. is 25 mass % or more and 55 mass % or less of carbonized sludge. A flocculant may be added to the carbonized sludge slurry and the carbonized sludge slurry may be mechanically dewatered.

The carbonized sludge slurry becomes carbonized sludge having a low moisture content by the second dehydrator 4 (second dehydration step). As described above, carbonized sludge having a water content of 25% by mass or more and 55% by mass or less can be obtained from the carbonized sludge slurry, but the water content of the carbonized sludge is not limited to this.

FIG. 3 is a graph showing the test results of dewatering the carbonized sludge slurry obtained by the hydrothermal carbonization device 3 by the second dehydrator 4 . A filter press dehydrator was used as the second dehydrator 4 . The second dehydrator 4 other than the filter press dehydrator may be used as long as the second dehydrator 4 can apply a pressure of at least a gauge pressure of 2.0 MPa to the carbonized sludge slurry. In this dehydration test, no flocculant was added to the carbonized sludge slurry.

The horizontal axis of the graph shown in FIG. 3 is the pressure (compression pressure, MPa (gauge pressure)), and the vertical axis is the water content (% by mass) of the carbonized sludge obtained by mechanical dewatering.

As can be seen from FIG. 3, the carbonized sludge slurry obtained in the hydrothermal carbonization device 3 is mechanically dewatered at a gauge pressure of 0.7 MPa or more and a gauge pressure of 2.0 MPa or less, resulting in a water content of 25% by mass. Carbonized sludge having a content of 55% by mass or less can be obtained. Further, by mechanically dewatering at a gauge pressure of 1.5 MPa or more and a gauge pressure of 2.0 MPa or less, carbonized sludge having a water content of 25% by mass or more and 35% by mass or less can be obtained. In order to obtain carbonized sludge having a water content of 40% by mass or less, dehydration should be performed at a gauge pressure of about 1.3 MPa or higher. When the pressure exceeded 2.0 MPa of gauge pressure, almost no decrease in moisture content was observed. It can be seen that at a gauge pressure of 0.7 MPa, about 55% by mass of carbonized sludge can be obtained. That is, at a gauge pressure of 0.7 MPa, it is possible to obtain carbonized sludge (processed material) with a sufficiently lower water content than conventional dehydrated sludge with a water content of about 60% by mass.

Table 1 shows the concentrations of Na, K, and Cl in the dewatered filtrate (hydrothermal dehydrated filtrate by carbonization) separated from the carbonized sludge slurry in the second dewatering step. Each concentration is a value obtained by converting the actually measured concentration when the dehydrated filtrate is diluted to the undiluted concentration.

Table 2 shows calculation of Na, K, and Cl concentrations in carbonized sludge obtained by compressing and dewatering carbonized sludge slurry under each pressure (compressing pressure) condition of gauge pressure 0.7 MPa, 1.5 MPa, and 2.0 MPa. It is a table showing values. As can be seen from Table 2, when the water content of the carbonized sludge is lowered by increasing the pressing pressure, the water content of the carbonized sludge decreases, and the dissolved Na, K, and Cl components flow out into the dewatered filtrate. do. As a result, the Na, K and Cl concentrations in the carbonized sludge are lowered. For example, Na is 733 mg/kg DS when the gauge pressure is 0.7 MPa, and 257 mg/kg DS when the gauge pressure is 2.0 MPa. The same applies to the concentrations of K and Cl.

(Filtrate return step)
The filtrate return step is a step of sending (returning) the dehydrated filtrate (hydrothermal carbonized dehydrated filtrate) separated from the carbonized sludge slurry in the second dewatering step to the digestion tank 1 . The hydrothermal carbonization treatment destroys the cells of microorganisms in the dehydrated sludge, and the organic matter in the dehydrated sludge is eluted into the filtrate (hydrothermal carbonization dewatering filtrate). Since the organic matter contained in the hydrothermally carbonized and dehydrated filtrate is used as a raw material for digestion gas, when the hydrothermally carbonized and dehydrated filtrate is put into the digestion tank 1, the amount of digestion gas generated increases accordingly. Digestion gas is energy recovered from sewage sludge (gas energy) that can be used as fuel. That is, by returning the dehydrated filtrate (hydrothermal carbonized dehydrated filtrate) separated from the carbonized sludge slurry in the second dehydration step to the digestion tank 1, the amount of digestion gas generated can be increased, and the amount of energy recovery can be increased. can be made

In addition, since part of the organic matter contained in the hydrothermal carbonization filtrate becomes the digestive gas, the organic matter is reduced from the treatment system accordingly. As a result, it is possible to suppress the deterioration of the quality of discharged water such as COD from the sewage treatment plant. Details are as follows. If the hydrothermally carbonized dewatering filtrate is directly sent to the water treatment system for treatment, the COD load in the water treatment system may increase and the discharged water quality may deteriorate. The COD load increase in the treatment system is suppressed, and the deterioration of effluent water quality can be suppressed.

By performing nitrogen stripping on the hydrothermally carbonized dewatering filtrate on the way back to the digestion tank 1, ammonia nitrogen contained in the hydrothermally carbonized dewatering filtrate is converted into ammonia gas. It should be released and removed. Nitrogen stripping refers to removing nitrogen from an object to be stripped. Alkali such as NaOH is added to the hydrothermally carbonized dewatering filtrate to make the pH more alkaline than about 8, and the ammoniacal nitrogen (NH ₄ —N) contained in the hydrothermally carbonized dewatering filtrate is released as ammonia gas. remove it. In order to perform nitrogen stripping, it is desirable that the temperature of the object to be stripped is relatively high. By supplying the carbonized sludge slurry at a temperature of 60° C. or higher to the second dehydration step, the hydrothermally carbonized dewatering filtrate separated from the carbonized sludge slurry in the second dehydration step is preferably kept at a relatively high temperature.

Depending on the properties of the hydrothermal carbonization and dehydration filtrate, the nitrogen concentration in the digestion tank 1 may become high, inhibiting microbial activity. By removing ammonia nitrogen (NH ₄ —N) before entering the digestion tank 1, inhibition of microbial activity can be suppressed. Also, the removed ammonia gas can be captured with dilute sulfuric acid to produce ammonium sulfate, which can be used as a fertilizer.

(curing container and curing process)
The curing container 5 is a container for drying the carbonized sludge obtained by the second dehydrator 4 and reducing the exothermic ignitability of the carbonized sludge. The curing step is a step of drying the carbonized sludge obtained in the second dehydration step and reducing the exothermic ignitability of the carbonized sludge. The curing container 5 is a container generally called a hopper.

Carbonized sludge from the second dehydrator 4 is put into the curing container 5 . Air (oxygen-containing gas) is blown into the curing container 5 from, for example, the lower side surface of the curing container 5 . The carbonized sludge in the curing container 5 is dried and partially oxidized by contact with the blown air. Partial oxidation reduces the exothermic ignitability of the carbonized sludge.

The curing process is not limited to the process using the curing container 5 (hopper). For example, the carbonized sludge obtained by the second dehydrator 4 may be dropped onto a conveyor (curing section) surrounding it, and the curing step may be performed by passing an oxygen-containing gas such as air through the sludge. In addition, the oxygen-containing gas may be heated in advance in order to further promote drying, or the heat energy recovered by the cooling heat exchanger 9 may be used as the heat source for the heating.

In Japanese Industrial Standard JIS Z7312, the product specification of sewage sludge solid fuel is stipulated that the water content is 20% by mass or less. The moisture content of the carbonized sludge obtained by the second dehydrator 4 is adjusted to, for example, 20% by mass or less by performing the curing step. By adjusting the water content to 20% by mass or less, the carbonized sludge can be used as a solid fuel. The use of carbonized sludge is not limited to solid fuel. In the curing step, the water content of carbonized sludge is not limited to being adjusted to 20% by mass or less.

(effect)
The organic waste treatment method of the present embodiment includes a methane fermentation process in which sewage sludge (organic waste) is anaerobicly fermented in a digestion tank 1 (methane fermentation tank), and fermented sludge after the methane fermentation process. , a sludge carbonization step of hydrothermally carbonizing the dehydrated sludge obtained in the first dehydration step, and the carbonized sludge slurry obtained in the sludge carbonization step is subjected to a second dehydrator 4 (mechanical dewatering It has a second dehydration step for dehydrating with a machine).

According to the above processing method, the following effects are obtained.

By hydrothermally carbonizing the dewatered sludge obtained in the first dewatering step, the cells of microorganisms in the dehydrated sludge are destroyed, and the components in the cells are decomposed and polymerized. As a result, a carbonized sludge slurry with excellent dewaterability is produced. By dehydrating the carbonized sludge slurry with excellent dewatering properties using a mechanical dehydrator, carbonized sludge (processed material) with a low moisture content can be obtained.

In addition, since carbonized sludge (treated material) with a low water content is obtained, the energy consumption required for drying the carbonized sludge in the curing step can be reduced. In other words, it is possible to evaporate the water content of the treated material to the target water content with less input energy in subsequent steps. When carbonized sludge is used as solid fuel, the water content is adjusted to 20% by mass or less, so the effect of being able to reduce the energy consumption required for drying is very important.

In the second dehydration step, dehydration is preferably performed by pressing the carbonized sludge slurry at a gauge pressure of 0.7 MPa or more and a gauge pressure of 2.0 MPa or less. As a result, carbonized sludge having a water content of 25% by mass or more and 55% by mass or less can be obtained. In addition, soluble Na, K, and Cl are eluted in water in the carbonized sludge slurry obtained in the sludge carbonization step. Since these components are removed together with the dehydrated filtrate in the second dehydration step, the lower the water content of the carbonized sludge, the lower the Na, K, and Cl content in the carbonized sludge. Therefore, by obtaining carbonized sludge having a water content of 25% by mass or more and 55% by mass or less, it is possible to reduce the content of Na, K, and Cl in the carbonized sludge. As a result, it is possible to obtain carbonized sludge (processed material) with a low impurity content. More preferably, the carbonized sludge slurry is dewatered by squeezing it at a gauge pressure of 1.5 MPa or more and a gauge pressure of 2.0 MPa or less.

Moreover, in the second dehydration step, it is preferable to dewater the carbonized sludge slurry without adding a flocculant to the carbonized sludge slurry. According to this, since no coagulant is used for dehydration of the carbonized sludge slurry, the treatment cost of the sewage sludge (organic waste) can be reduced accordingly.

Further, by obtaining carbonized sludge having a moisture content of 40% by mass or less in the second dehydration step, the energy consumption required for drying the carbonized sludge in the curing step can be reduced. It is more preferable to obtain carbonized sludge having a water content of 35% by mass or less.

Furthermore, in the organic waste treatment method of the present embodiment, the hydrothermally carbonized dehydrated filtrate separated from the carbonized sludge slurry in the second dehydration step is returned to the digestion tank 1 (methane fermentation tank) in a filtrate return step. It has According to this, the amount of digestion gas generated can be increased, and the amount of energy recovery can be increased. In addition, it is possible to suppress the deterioration of the quality of effluent from sewage treatment plants such as COD.

The above embodiment can be modified as follows.

The above embodiment includes the separated liquid treatment device 6 and the separated liquid treatment step. In the present invention, the separated liquid treatment device 6 and the separated liquid treatment step are not essential.

The above embodiments comprise a filtrate return step. In the present invention, the filtrate return step is not essential.

The above embodiment comprises the curing container 5 and the curing process. In the present invention, the curing container 5 and the curing process are not essential.

Organic waste to be treated is not limited to sewage sludge. The present invention targets sewage sludge, food waste (food biomass) such as garbage, lignocellulosic waste such as used paper and waste paper, agricultural residue, and various organic waste such as livestock manure. can do. As described above, these organic wastes may be treated individually or mixed.

1: Digestion tank (methane fermentation tank)
2: First dehydrator 3: Hydrothermal carbonization device 4: Second dehydrator 5: Curing vessel 6: Separated liquid processing device 7: Reactor 8: Heating heat exchanger 9: Cooling heat exchanger

Claims (11)

A methane fermentation process in which organic waste is anaerobicly fermented in a methane fermentation tank;
a first dehydration step of dewatering the fermented sludge after the methane fermentation step;
a sludge carbonization step of hydrothermally carbonizing the dewatered sludge obtained in the first dehydration step;
a second dehydration step of dehydrating the carbonized sludge slurry obtained in the sludge carbonization step with a mechanical dehydrator;
comprising a
A method of treating organic waste using hydrothermal carbonization. In the method for treating organic waste using the hydrothermal carbonization treatment according to claim 1,
In the second dehydration step, the carbonized sludge slurry is dewatered by squeezing at a gauge pressure of 0.7 MPa or more and a gauge pressure of 2.0 MPa or less.
A method of treating organic waste using hydrothermal carbonization. In the method for treating organic waste using the hydrothermal carbonization treatment according to claim 1 or 2,
In the second dehydration step, the carbonized sludge slurry is dewatered without adding a flocculant to the carbonized sludge slurry.
A method of treating organic waste using hydrothermal carbonization. In the method for treating organic waste using the hydrothermal carbonization treatment according to claim 1 or 2,
Obtaining carbonized sludge having a water content of 40% by mass or less by the second dehydration step,
A method of treating organic waste using hydrothermal carbonization. In the method for treating organic waste using the hydrothermal carbonization treatment according to claim 1 or 2,
In the first dehydration step, the fermentation-treated sludge is dehydrated so that the resulting dewatered sludge has a moisture content of 75% by mass or more and 90% by mass or less.
A method of treating organic waste using hydrothermal carbonization. In the method for treating organic waste using the hydrothermal carbonization treatment according to claim 1 or 2,
Further comprising a filtrate return step of returning the hydrothermally carbonized dewatering filtrate separated from the carbonized sludge slurry in the second dehydration step to the methane fermentation tank,
A method of treating organic waste using hydrothermal carbonization. In the method for treating organic waste using the hydrothermal carbonization treatment according to claim 1 or 2,
Supplying the carbonized sludge slurry having a temperature of 60 ° C. or higher obtained in the sludge carbonization step to the second dehydration step;
A method of treating organic waste using hydrothermal carbonization. In the method for treating organic waste using the hydrothermal carbonization treatment according to claim 7,
Further comprising a filtrate return step of returning the hydrothermally carbonized dewatering filtrate separated from the carbonized sludge slurry in the second dehydration step to the methane fermentation tank,
On the way of returning the hydrothermally carbonized dehydration filtrate to the methane fermentation tank, nitrogen stripping is performed on the hydrothermally carbonized dewatered filtrate to remove the ammonia nitrogen contained in the hydrothermally carbonized dehydration filtrate. is released as ammonia gas and removed;
A method of treating organic waste using hydrothermal carbonization. a methane fermentation tank for anaerobic fermentation of organic waste;
a first dehydrator for dehydrating fermentation-treated sludge discharged from the methane fermentation tank;
a hydrothermal carbonization device for hydrothermally carbonizing dehydrated sludge obtained from the first dehydrator;
a second dehydrator for mechanically dewatering the carbonized sludge slurry obtained in the hydrothermal carbonization apparatus;
comprising
Equipment for treating organic waste using hydrothermal carbonization. In the organic waste treatment facility using the hydrothermal carbonization treatment according to claim 9,
wherein the second dehydrator is a filter press dehydrator;
Equipment for treating organic waste using hydrothermal carbonization. In the organic waste treatment facility using the hydrothermal carbonization treatment according to claim 9 or 10,
The hydrothermal carbonization device is
a heating heat exchanger for preheating the dehydrated sludge;
a reactor for high-temperature and high-pressure treatment of the dehydrated sludge preheated by the heating heat exchanger in a gas atmosphere containing no oxygen or with a low oxygen concentration or in a state in which oxygen is shut off;
a cooling heat exchanger for cooling the carbonized sludge slurry obtained in the reactor;
with
The temperature of the carbonized sludge slurry is adjusted in the cooling heat exchanger,
Equipment for treating organic waste using hydrothermal carbonization.

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