JP7493888B2 - Organic waste treatment method - Google Patents

Description

The present invention relates to a method for treating organic waste in which solid-containing organic waste is put into a fermentation tank as a substrate and subjected to anaerobic fermentation treatment.

Patent Document 1 discloses a waste treatment method in which biologically treated sludge produced by biological treatment is separated into a solid-liquid portion and a filtrate, the sludge portion is stirred and mixed with a liquid organic waste having fluidity containing organic matter decomposable by anaerobic organisms and a solid organic waste containing solid organic matter decomposable by anaerobic organisms, and subjected to methane fermentation treatment, followed by solid-liquid separation to collect the treated filtrate, and the treated filtrate is biologically treated together with the filtrate using aerobic microorganisms.

It is also disclosed that the efficiency of methane fermentation treatment can be improved by adjusting the moisture content of the total solids concentration to 5% or more and 20% or less before the methane fermentation treatment. If the total solids concentration is lower than 5%, the concentration of organic matter to be subjected to methane fermentation treatment is reduced, and the amount of methane gas generated relative to the operating energy is reduced, decreasing the efficiency of methane fermentation. If the total solids concentration is higher than 20%, the viscosity increases and the presence of solid organic matter makes it impossible to decompose the organic matter uniformly and in a short time, decreasing the efficiency of methane fermentation.

Japanese Patent Application Laid-Open No. 11-309438

However, the conventional methane fermentation treatment method described above adjusts the moisture content to between 5% and 20% before fermentation treatment, but does not adjust the total solids concentration in the fermenter to between 5% and 20%, and depending on the decomposition rate, it may deviate from this range. In particular, when the decomposition rate is high, the total solids concentration easily falls below 5%, making it difficult to stabilize the solids concentration in the fermenter.

In view of the above-mentioned problems, the object of the present invention is to provide an organic waste treatment method that can stably maintain a solids concentration in the fermentation tank suitable for fermentation by diluting the substrate introduced into the fermentation tank.

In order to achieve the above-mentioned object, a first characteristic configuration of the organic waste treatment method according to the present invention is an organic waste treatment method in which organic waste containing solids is fed as a substrate into a fermentation tank for anaerobic fermentation treatment, comprising the steps of: a first step of measuring the solid concentration of the input substrate fed into the fermentation tank; a second step of estimating a solid concentration in the tank, which is the solid concentration of the fermentation liquid obtained by subjecting the input substrate to anerobic fermentation treatment in the fermentation tank, based on the solid concentration and a predetermined decomposition rate by anaerobic fermentation treatment of the input substrate, and setting a dilution ratio of the input substrate so that the solid concentration in the tank becomes a predetermined target value; and a third step of diluting the input substrate to the dilution ratio.

Based on the solids concentration of the input substrate measured in the first step, the dilution rate of the input substrate is set in the second step so that the solids concentration in the fermenter is a predetermined target value, and in the third step the input substrate is diluted, so that the solids concentration in the fermenter is maintained at the target value. As a result, the fermenter can be made small while maintaining stable fermentation efficiency.

The second characteristic feature of the present invention is, in addition to the first characteristic feature described above, that the dilution rate is expressed by the following formula: Solid concentration in tank = Solid concentration of input substrate x (1 - Decomposition rate of input substrate)
The dilution rate is set based on the target value of the dilution rate = solids concentration in the tank divided by the target value of the solids concentration in the tank.

The solids concentration in the tank is calculated by multiplying the solids concentration of the input substrate by (1 - decomposition rate of the input substrate), and the dilution rate is calculated by dividing the solids concentration in the tank by the target solids concentration in the tank. Note that the decomposition rate of the input substrate can be determined by an empirical value measured in advance, and can also be measured periodically.

The third characteristic configuration, in addition to the first or second characteristic configuration described above, is that the target value of the solids concentration in the tank is set to a value in the range of 5-15%.

Setting the target solids concentration in the tank to a value in the range of 5-15% promotes stable fermentation.

The fourth characteristic configuration is, in addition to any of the first to third characteristic configurations described above, that a target value for the solids concentration in the tank is set so that the viscosity in the tank is equal to or lower than a predetermined target viscosity in the tank, based on a predetermined correlation between the solids concentration in the tank and the viscosity in the tank, which is the viscosity of the fermentation liquid.

By setting the viscosity inside the tank below a specified target viscosity, the substrate inside the tank is properly stirred and mixed, promoting stable fermentation.

The fifth characteristic configuration is that, in addition to the fourth characteristic configuration described above, the target viscosity in the tank is set to a value of 30,000 mPa·s or less.

If the target viscosity in the tank is 30,000 mPa·s or less, the substrate in the tank will be properly stirred and mixed.

As explained above, the present invention provides an organic waste treatment method that can stably maintain a solids concentration in the fermentation tank suitable for fermentation by diluting the substrate added to the fermentation tank.

FIG. 1 is an explanatory diagram of an organic waste treatment system to which the organic waste treatment method according to the present invention is applied. FIG. 1 is an explanatory diagram of an organic waste treatment apparatus to which the organic waste treatment method according to the present invention is applied. 1A is a plan view of the feed mechanism, FIG. 1B is a side view of the feed mechanism, FIG. 1C is a cross-sectional view of FIG. 1B along line A, FIG. 1D is a cross-sectional view of FIG. 1B along line B, and FIG. 1E is a cross-sectional view of FIG. 1B along line C. An explanatory diagram of the organic waste treatment method according to the present invention.

The organic waste treatment method according to the present invention will be described below with reference to the drawings.

[Description of organic waste treatment system to which organic waste treatment method is applied]
An organic waste treatment system to which the organic waste treatment method of the present invention is applied is shown in Fig. 1. This is an organic waste treatment device that treats organic waste such as municipal waste with anaerobic microorganisms, and recovers the energy contained in the organic waste in the form of biogas such as methane gas and carbon dioxide.

Combustible waste such as paper waste, kitchen waste, and resin collected from ordinary households is crushed into small pieces using a crusher or other equipment, and a pre-processing step is carried out to separate organic solid waste suitable for organic waste treatment from resin, inorganic matter, metals, and other waste that is not suitable for organic waste treatment. Organic waste that is not suitable for organic waste treatment is treated in a garbage incineration facility, and organic waste that is suitable for treatment is put into an organic waste treatment device and treated with methane fermentation.

The methane gas contained in the biogas produced during organic waste treatment is recovered as renewable energy, and the organic waste treatment residue is incinerated after undergoing residue treatment such as dehydration. The dehydrated filtrate produced during the dehydration of the organic waste treatment residue is biologically treated using the activated sludge method, and the excess sludge produced during this biological treatment can also be used as a material suitable for organic waste treatment.

[Explanation of organic waste treatment device]
The overall configuration of the organic waste treatment device is shown in Figure 2. The organic waste treatment device 10 is configured with a fermenter 20, a feeding mechanism 30, a stirring mechanism 40, a gas exhaust section 50, a fermentation liquid exhaust port 60, a mechanism for discharging material unsuitable for fermentation 70, etc., and is an apparatus in which the material suitable for fermentation, that is, organic waste containing solid matter, which is the material to be treated, is directly fed into the fermenter 20 and subjected to anaerobic fermentation treatment without solubilization treatment using a separate solubilization treatment tank or the like.

The fermentation tank 20 is a vertical treatment tank formed into a cylindrical shape using steel plates, and a jacket 22 that serves as a flow path for the heat medium is provided on the side wall, and the heat medium supplied from the heat medium supply section 23 passes through the jacket 22 and is discharged from the heat medium discharge section 21. Warm water is preferably used as the heat medium, and for example, warm water of about 57°C is supplied to the jacket 22 to heat the fermentation liquid in the tank to a temperature of about 55°C suitable for fermentation.

A disk-shaped lid 25 is removably fixed to the top of the fermentation tank 20 by a flange so that the tank is sealed. The lid 25 supports the stirring mechanism 40 and is connected to a pipe equipped with a biogas exhaust valve V3 that functions as a gas exhaust section 50 for extracting the biogas generated by the anaerobic fermentation process, and is also provided with a viewing window 26 for visually checking the inside of the tank.

A funnel-shaped section 24 that gradually narrows downward is formed at the lower end of the side wall of the fermentation tank 20, and a mechanism 70 for discharging materials unsuitable for fermentation is provided at the center of the bottom.

The fermentation unsuitable material discharge mechanism 70 is a mechanism for discharging fermentation unsuitable materials such as sand, shells, and metals mixed in the fermentation liquid from the fermentation tank 20, and is composed of a cylindrical section 73 that protrudes downward from the bottom of the fermentation tank 20 and has a smaller diameter than the fermentation tank 20, and double damper mechanisms 71, 72 attached to the cylindrical section 73. With the lower damper 72 closed, the upper damper 71 is opened to take in the fermentation unsuitable materials, and then the upper damper 71 is closed and the lower damper 72 is opened to remove the fermentation unsuitable materials from the tank.

When sand, metal, shells, and other materials unsuitable for fermentation that are mixed in with the material to be treated gradually settle in the fermentation liquid and accumulate at the bottom, there is a risk that the effective fermentation volume will decrease and the fermentation efficiency will decline. However, by accumulating the materials unsuitable for fermentation in the cylindrical section 73 formed at the bottom and discharging them outside the tank via the double damper mechanism 71, 72, it is possible to avoid a decrease in the fermentation volume without requiring a large power source such as a pump device and while maintaining an anaerobic state.

The stirring mechanism 40 is provided to stir the organic waste put into the fermentation tank 20 within the tank, and is composed of a stirring motor 45 installed on the top of the lid 25, a vertically oriented rotating shaft rotatably supported by bearings on the lid 25, a stirring shaft 41 connected to the rotating shaft via a coupling 43, and stirring blades 42, 44, and 46 fixed to the stirring shaft 41. The stirring shaft 41 is installed so as to coincide with the axis of the fermentation tank 20.

The upper end of the agitator shaft 41 is connected to a rotating shaft supported by the lid 25 via a coupling 43, and the lower end of the agitator shaft 41 is a free end that is not supported by a shaft. Therefore, when maintenance is required, the agitator shaft 41 can be pulled out from the upper end of the fermenter 20 together with the agitator blades 42, 44, and 46 by removing the lid 25 from the fermenter 20 without disassembly or removing the sludge from the tank.

The sedimentation promotion blade 42 located at the top is composed of two flat blade pieces 42a fixed to a sleeve 42b fitted and fixed to the agitator shaft 41 so that they form a central angle of 180°. It is positioned so that it crosses the liquid level of the fermentation liquid in the fermentation tank 20, preferably so that the liquid level is located in the vertical center of the blade surface, and its tip extends to a position close to the inner wall surface of the fermentation tank 20.

Below the settling promotion blade 42, three dispersion blades 44 are provided in the vertical direction.
The dispersion blade 44 is composed of two flat blade pieces 44a fixed to a sleeve fixed to the stirring shaft 41 so as to form a central angle of 180°, and the radial length of each blade piece 44a is shorter than that of the settling promotion blade 42, and is set to a length in the range of 40 to 90%, preferably 50 to 70%, of the inner diameter of the fermentation tank 20. The flat blade pieces 44a can efficiently stir the vertical middle part of the fermentation liquid, and can separate light and heavy matters.

A scraping blade 46 is provided below the dispersion blade 44 .
The scraping blade 46 is fixed to the agitator shaft 41 at the bottom position of the fermentation tank 20 and is equipped with a lower dispersion blade piece 46a which disperses the material to be treated in the fermentation liquid, and a scraping blade piece 46c which is fixed to the tip of the lower dispersion blade piece 46a and scrapes the materials unsuitable for fermentation that have settled in the fermentation liquid toward the fermentation unsuitable material discharge mechanism 70.

As shown in Figures 2 and 3(a) to (e), the feeding mechanism 30 is a mechanism for feeding organic waste into the fermentation tank 20, and is composed of a cylindrical casing 31 (see Figure 3(e)) with a rectangular cross section that is inserted with its tip facing downward and flange-fixed to a flange pipe 36 formed on the wall of the fermentation tank 20 in an inclined position at a predetermined inclination angle θ with respect to the horizontal plane, a pressing body 32 with a rectangular cross section that is installed inside the cylindrical casing 31, and a hydraulic drive mechanism 33 that drives the pressing body 32 back and forth along the axis of the cylindrical casing 31.

The tip FE of the cylindrical casing 31 is inserted and fixed into a flange pipe 36 provided on the side wall of the fermentation tank 20 so that it is immersed in the fermentation liquid inside the fermentation tank 20. In this embodiment, the flange pipe 36 is set to an inclination of about 30° with respect to the horizontal plane, and the inclination of the cylindrical casing 31 is also set to an inclination of about 30° with respect to the horizontal plane.

Furthermore, a hopper mechanism 35 that supplies organic waste from the tip of the cylindrical casing 31 to the push-in feeding mechanism 30 is provided on the upper part of the push-in feeding mechanism 30. The hopper mechanism 35 has a rectangular cylindrical storage section formed by four side walls 35a that hang down from an upper opening 35d, which is an input section equipped with a lid body 35c that can be opened and closed, and a sloped wall section 35b that gradually narrows toward the lower end is formed and is connected to a pair of opposing walls of the side walls 35a that are arranged to sandwich the axis of the cylindrical casing 31 in a plan view.

For the sake of convenience, the cover 35c is indicated by a broken line in Fig. 3(a). One end of the cover 35c is attached to the side wall 35a via a hinge mechanism h so as to be able to be opened and closed. Reference numeral 35h in Fig. 3(c) denotes an inspection window.

The material suitable for fermentation is introduced into the storage section from the upper opening 35d, and is dropped into the cylindrical casing 31 through the opening 31a formed in the cylindrical casing 31 of the push-in introduction mechanism 30 from the lower end opening of the inclined wall section 35b.

In other words, an opening 31a is formed in the upper part of the cylindrical casing 31 that constitutes part of the feeding mechanism 30, and the longitudinal direction of the opening 31a is shaped to follow the axis of the cylindrical casing 31. The lower ends 35u (see FIG. 3(a)) of the inclined wall portion 35b are arranged so as to be connected to the longitudinal edges of the opening 31a without any steps.

The tip FE of the cylindrical casing 31 is formed to extend further toward the tip side than the maximum advance position of the pressing body 32. Therefore, when the pressing body 32 is driven backward, the organic waste pressed and compacted by the pressing body 32 remains at the tip FE of the cylindrical casing 31, and a sealing function is exerted. As a result, a large amount of fermentation liquid in the fermenter 20 does not flow into the cylindrical casing 31, and leakage of fermentation gas into the cylindrical casing 31 can be prevented.

The attachment position of the hopper mechanism 35 to the cylindrical casing 31 is set so that the liquid level of the fermentation liquid in the fermenter 20 is near the connection position CA of the hopper mechanism 35 to the wall of the cylindrical casing 31 (see FIG. 3(b)).

The hopper mechanism 35 is configured with a square cylindrical storage section with side walls 35a hanging down from a rectangular upper opening, and a funnel-shaped section whose lower end is composed of an inclined wall section 35b that tapers toward the cylindrical casing 31. In addition to the fermentation-suitable materials selected from combustible garbage, which is organic solid waste, that are fed into the storage section , human waste, septic tank sludge, and even sewage sludge are also fed in, and while they are piled up inside, they are mixed and fed into the fermentation tank 20 by the pushing-in feeding mechanism 30.

Of the side wall 35a of the hopper mechanism 35 connected to the cylindrical casing 31, a front wall portion 35e on the side in the advancing direction of the pressing body 32 is formed in an inclined posture toward the advancing direction of the pressing body 32 at the connection portion with the cylindrical casing 31 (see Figures 3(b) and (d)).

When the organic waste is pressed inside the cylindrical casing 31 by the pressing body 32, a consolidation stress acts on the front wall 35e and the pressing body 32 when it is consolidated between the front wall 35e connected to the cylindrical casing 31. At this time, if the front wall 35e on the advancing side of the pressing body 32 is formed in an inclined position toward the advancing direction of the pressing body 32 at the connection part with the cylindrical casing 31, part of the organic waste near the top of the pressing body 32 escapes into the space formed by the inclined front wall 35e, so a sudden consolidation action is avoided and the waste is smoothly pressed and supplied.

A rear wall portion 35f of a side wall 35a of the hopper mechanism 35 connected to the cylindrical casing 31 on the retreating direction side of the pressing body 32 is located on the retreating direction side of the pressing body 32 with respect to the opening 31a.

A flange pipe that serves as the fermentation liquid outlet 60 is provided at the bottom of the side wall of the fermentation tank 20, and the fermentation liquid after anaerobic fermentation treatment, i.e., the digestive liquid, is discharged.

[Explanation of organic waste treatment method]
As shown in Figure 4, combustible waste is fed into a crushing and sorting device and crushed, and a predetermined amount of organic waste selected as suitable for organic waste treatment is fed as a feed substrate into a hopper mechanism 35 of the organic waste treatment device 10 at predetermined intervals. The organic waste fed into the hopper mechanism 35 is pushed into the fermentation tank 20 by a pressing body 32, and is subjected to fermentation treatment inside the fermentation tank 20.

A portion of the sorted organic waste is sampled and measured with a measuring device to determine the solids concentration of the input substrate. If the moisture content of the input substrate is W, then there is a relationship of solids concentration TS = 1-W, and the measuring device may be any known measuring device capable of measuring solids concentration TS or moisture content. The solids concentration TS is calculated by dry weight/wet weight.

The solids concentration of the input substrate measured by the measuring device is input to the control unit, and the control unit controls the opening of a valve provided in a supply pipe for the dilution liquid so that the organic waste input to the hopper mechanism 35 is diluted at a predetermined dilution rate. Treated water treated in a biological treatment device which performs nitrification and denitrification on water separated from solids and liquids after the completion of fermentation treatment, sludge withdrawn from the biological treatment device, and even clean water are used as the dilution liquid. The dilution liquid may be directly input to the hopper mechanism 35, or may be directly input to the fermentation tank 20 by attaching a pipe for inputting the dilution liquid to the lid 25 provided on the fermentation tank 20.

The measurement interval for the solids concentration of the input substrate can be a rough interval of about 15 minutes to 24 hours. For example, the dilution rate for the next day can be set using the average value of measurements accumulated over 24 hours at 15-minute intervals. Since the hydraulic retention time (HRT) in the fermenter 20 is about 10 to 40 days, a delay of several days is not a problem.

The predetermined dilution rate refers to a dilution rate at which the solid concentration in the fermenter 20 becomes a predetermined target value, and is set based on the following formulas (1) and (2).
(1) Solid concentration in the tank = solid concentration of the input substrate × (1 - decomposition rate of the input substrate)
(2) Dilution rate = concentration of solids in the tank ÷ target concentration of solids in the tank

In other words, the organic waste treatment method in which organic waste containing solids is fed as a substrate into a fermentation tank and subjected to anaerobic fermentation treatment includes a first step of measuring the solids concentration of the substrate fed into the fermentation tank 20, a second step of setting a dilution ratio of the substrate fed into the fermentation tank 20 so that the solids concentration in the tank becomes a predetermined target value, and a third step of diluting the substrate fed into the fermentation tank 20 to the dilution ratio.

The solids concentration in the tank is calculated by multiplying the solids concentration of the input substrate by (1 - decomposition rate of the input substrate), and the dilution rate is calculated by dividing the solids concentration in the tank by the target solids concentration in the tank. Note that, since the composition of the substrate input to the fermenter 20 does not change significantly, the decomposition rate of the input substrate can be determined by applying an empirical value measured in advance, and can also be measured periodically.

The decomposition rate of the input substrate can be set from the results of operation and experiments. For example, data on the solids concentration TS of the input substrate and the solids concentration TS in the fermentation tank can be accumulated, and the decomposition rate can be calculated from the relationship, or the average decomposition rate can be calculated from the mixture ratio of food waste, paper waste, etc. in the solids concentration TS of the input substrate and the general decomposition rate.

Even if the decomposition rate in the tank fluctuates slightly, the solids concentration TS in the fermenter 20 is maintained at the target value, so that the fermentation efficiency can be stabilized even if the fermenter 20 is configured to be small.

The target value for the solids concentration in the tank is preferably set to a value in the range of 5-15%, and more preferably to a value in the range of 6-12%. If the target value for the solids concentration in the tank falls below 5%, the organic matter concentration will fall and the fermentation efficiency will decrease, and if the target value for the solids concentration in the tank exceeds 15%, smooth stirring will become difficult, organic matter will not be decomposed in a short time, and the fermentation efficiency will decrease.

Furthermore, it is preferable that the target value of the solid concentration in the tank set based on formulas (1) and (2) is adjusted so that the viscosity in the tank is equal to or lower than a predetermined target viscosity based on a predetermined correlation established between the solid concentration in the tank and the viscosity in the tank.
The target viscosity in the tank is preferably 30,000 mPa·s or less, and more preferably 20,000 mPa·s or less. If the viscosity exceeds 30,000 mPa·s, the fermentation liquid inside the fermenter 20 will not be stirred well, and the fermentation efficiency will decrease.

The viscosity in the tank is correlated with the solids concentration in the tank, and can be approximated by a higher-order function (second order or higher) with the solids concentration in the tank as a variable, or an exponential function with the solids concentration in the tank as a variable.

In the above embodiment, an example was described in which the target value of the solids concentration in the tank, which was set based on formulas (1) and (2), was adjusted so that the viscosity in the tank was equal to or lower than a predetermined target viscosity. Conversely, the target value of the solids concentration in the tank may be set so that the viscosity in the tank is equal to the target viscosity based on the correlation between the viscosity in the tank and the solids concentration in the tank, and then the dilution rate may be set based on formulas (1) and (2).

The materials suitable for fermentation that are put into the fermentation tank 20 described above include organic solid waste such as food waste and paper waste, sewage sludge, human waste, and septic tank sludge, as well as biomass such as livestock manure, agricultural residues, and food waste, and these organic wastes can be used alone or in mixtures.

The organic waste treatment device 10 to which the present invention can be applied is preferably configured to include the above-mentioned fermentation tank 20, the input mechanism 30, the stirring mechanism 40, the gas exhaust section 50, the fermentation liquid outlet 60, and the fermentation unsuitable material discharge mechanism 70, but the specific configuration of each section is not particularly limited. It is sufficient that the fermentation treatment device is capable of directly inputting organic waste containing at least solids and subjecting it to anaerobic fermentation treatment without solubilizing it using a separate solubilization treatment tank or the like.

The above-described embodiment is one aspect of the present invention, and the present invention is not limited to this description. It goes without saying that the specific structure, size, materials, etc. of each part can be appropriately modified and designed within the scope of the effects of the present invention.

10: Organic waste treatment device 20: Fermentation tank 30: Feeding mechanism 31: Cylindrical casing 32: Pressing body 36: Flange pipe 37: Flange pipe 40: Stirring mechanism 41: Stirring shaft 42: Settling promotion blade 44: Dispersion blade 36: Scraping blade 50: Gas exhaust section 60: Fermentation liquid exhaust port 70: Fermentation unsuitable material exhaust mechanism 71: Double damper mechanism 72: Double damper mechanism 73: Cylindrical section

Claims (5)

A method for treating organic waste, comprising: feeding organic waste containing solid matter as a substrate into a fermentation tank and subjecting the substrate to anaerobic fermentation treatment,
A first step of measuring a solids concentration of an input substrate to be input to the fermenter;
A second step of setting a dilution rate of the input substrate so that the solid concentration in the tank is a predetermined target value, the solid concentration being the solid concentration of the fermentation liquid obtained by anaerobically fermenting the input substrate in the fermenter , based on the solid concentration and a previously set decomposition rate by anaerobic fermentation of the input substrate;
a third step of diluting the input substrate to the dilution ratio;
A method for treating organic waste comprising the steps of: The dilution rate is calculated by the following formula: Solid concentration in tank = Solid concentration of input substrate x (1 - Decomposition rate of input substrate)
2. The organic waste treatment method according to claim 1, wherein the dilution rate is set based on a target value of the dilution rate = solid matter concentration in the tank divided by the solid matter concentration in the tank. The organic waste treatment method according to claim 1 or 2, in which the target value of the solids concentration in the tank is set to a value in the range of 5-15%. 4. An organic waste treatment method according to claim 1, wherein a target value of the solids concentration in the tank is set so that the viscosity in the tank is equal to or lower than a target viscosity in the tank, which is a predetermined target viscosity, based on a predetermined correlation between the solids concentration in the tank and the viscosity in the tank, which is the viscosity of the fermentation liquid. 5. The organic waste treatment method according to claim 4, wherein the target viscosity in the tank is set to a value of 30,000 mPa.s or less.

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