JPS644840B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an anaerobic digestion method that generates and recovers methane from organic wastes such as sewage sludge, municipal waste, agricultural and livestock waste, and also makes these organic wastes lightweight and stable. The amount of organic wastes such as sewage sludge, municipal wastes, and agricultural and livestock wastes being generated continues to increase these days, and the treatment and disposal of these wastes is of great concern from the perspective of environmental protection. On the other hand, with the recent tightening of energy demand, there is a desire for efficient waste treatment methods that consume less energy and, if possible, can recover energy. Against this background, anaerobic digestion methods, which generate and recover methane during the process of reducing and stabilizing organic waste, are beginning to attract attention. In the anaerobic digestion method, many types of anaerobic microorganisms act on organic matter to finally convert it into CH ₄ , CO ₂ ,
The principle is to separate substances with relatively simple structures such as H ₂ O and NH ₄ ⁺ , but in more detail,
It is understood that this decomposition reaction is caused by the following three types of microbial reactions proceeding sequentially. (1) Low-molecular-weight organic matter → Low-molecular-weight organic matter (2) Volatile acid production reaction Low-molecular organic matter → Volatile acid, alcohol H ₂ , CO ₂ , NH ₄ ⁺ (3) Methane production reaction Volatile acid, alcohol H ₂ , CO ₂ → CH ₄ CO _2This kind of decomposition reaction of organic matter generally proceeds very slowly, so standard anaerobic digestion, which has traditionally been used primarily to treat sewage sludge and human waste, In this method, the residence time in the reactor, that is, the number of days for digestion, is extremely long at 30 days, and this is cited as a drawback of the anaerobic digestion method. Conventionally, the rate-limiting reaction of the entire decomposition reaction has been thought to be the final step, the methane production reaction. In some cases, the first
It has been revealed that the molecular weight reduction reaction is the rate-determining step, and several techniques have been proposed to accelerate this reaction. These technologies can be roughly divided into physicochemical methods in which raw waste is pretreated by adding heat, chemicals (alkali, acids), etc., and microbial processes that are separate from the main digestion process, in which a powerful It can be divided into biological methods that propagate microorganisms that have the effect of reducing molecular weight, but the latter is superior from an economic standpoint. As a result of additional experiments and studies on the methods for reducing the molecular weight of products that have been proposed so far, the present inventors found that
We have confirmed that the low-molecular-weighting method using high-temperature, anaerobic low-molecular-weight bacteria that is active in the environment is extremely effective. Furthermore, conventionally, when such high-temperature low-molecular-weight bacteria were used in the low-molecular-weighting process, a high-temperature digestion method was usually employed in which the subsequent main digestion process was also maintained at a similar high temperature range. The present inventors also initially conducted experiments using a combination of high-temperature low-molecularization method and high-temperature digestion method, but the digested sludge discharged had poor solid-liquid separation properties, and the desorbed sludge after solid-liquid separation contains high concentrations of colloidal dissolved organic matter, and it has become clear that there are many difficulties in practical technology including sludge treatment and waste liquid treatment. Therefore, we forcibly cooled the effluent from the low-molecular-weighting process and heated the latter main digestion process to 20°C to 40°C.
When we switched to the mesophilic digestion method and continued operation, not only did the above-mentioned difficulties improve significantly, but we also
It was confirmed that the amount of methane generated and the rate of organic decomposition increased. As described above, the present invention solves the problems of anaerobic digestion of organic waste, and provides a method that can increase the speed of the molecular weight reduction reaction and improve the solid-liquid separability of the resulting digested sludge. It was completed based on the above experimental results. That is, the present invention divides the process into at least two parts when subjecting organic waste to anaerobic digestion, and converts the organic waste into 45
After being subjected to low-molecularization treatment at a high-temperature digestion temperature range of ℃ to 65℃, the treated product is cooled and then processed in the main digestion step of the second step.
This is an anaerobic digestion method for organic waste, which is characterized by anaerobic digestion treatment at a meso-temperature digestion temperature of 20°C to 40°C. An example of the embodiment of the present invention will be described with reference to FIG. It flows into the molecularization reactor 1. The low-molecular-weight reactor 1 may be a tank reactor, a fixed bed reactor, or a fluidized bed reactor used in conventional biological treatment. Any structure is sufficient as long as it can be contacted and maintain substantially anaerobic conditions. In addition, the spilled slurry 7
In order to utilize the microorganisms for the low-molecular-weight reaction contained in It may be returned to reactor 1. (Solid matter 9
Most of it is sent to the main digestion reactor 2, which will be described later. ) The temperature of the desorbed liquid 8 from the solid-liquid separator 4 is 20°C.
It flows into the main digestion reactor 2 which is maintained at ~40°C. As this main digestion reactor 2, various reactors such as tank type, fixed bed type, fluidized bed type, etc. can be applied like the low molecular weight reactor 1, but in any case, the anaerobic reaction is more severe than the low molecular weight process. Conditions are required, so
The structure must be designed with sufficient consideration given to its airtightness. Further, in this example, the main digestion step is a single step, but if necessary, it is possible to subdivide this into a plurality of steps, such as an acid production step and a subsequent methane production step. The digested sludge 10 from the main digestion reactor 2 may be disposed of as is, but in practice it is treated with a sedimentation concentration tank, a centrifuge, or other solid-liquid separation device 5.
It is separated into a desorbed liquid 11 and a precipitate 12. Desorption liquid 1
1 is treated separately, and the precipitate 12 is disposed of by incineration or the like. In this case, if desired, the precipitate 1
2 may be returned to the main digestion reactor 2. In the present invention, the outflow slurry 7 needs to be cooled because the temperature of the outflow slurry 7 from the depolymerization reactor 1 is considerably higher than the liquid temperature to be controlled in the main digestion reactor 2. In this case, it is reasonable and preferable to cool the heat exchanger 3 with the waste 6. Next, an example of the experimental method and results carried out to complete the present invention will be given as an example, and the effects of the present invention in comparison with conventional methods will also be clarified. Example As shown in Fig. 2, the experimental equipment includes a low-molecular-weighting tank 1' with an internal liquid volume of 3 and a main digestion tank 2' with an internal liquid volume of 6 (both equipped with a stirrer) as main equipment. These are set in constant temperature water tanks 13 and 16, respectively, so that the temperature of the liquid in the tank can be controlled individually. In FIG. 2, 3' is a heat exchanger, 14 is a pulp powder inlet, 15 is a cooling device for cooling liquid in the heat exchanger 3', and 17 is a gas meter. Thus, the waste sewage sludge 6' and pulp powder become low-molecular organic matter in the low-molecular conversion tank 1'.
The effluent slurry 7' flows into the main digestion tank 2' through the heat exchanger 3', is digested, and is discharged as slurry-like digested sludge 10'. A gas meter 17 measures the amount and composition of the methane-containing gas 18 generated in the low molecular weight tank 1' and the main digestion tank 2'. Next, regarding the experimental conditions, first, the temperature conditions were two types: the liquid temperature in the low molecularization tank 1' was 55℃ (high temperature low molecularization) and 33℃ (medium temperature low molecularization), and the main digestion The liquid temperature in tank 2' was set to 55℃ (high temperature digestion) and 33℃.
℃ (medium-temperature digestion), high-temperature low-molecularization-medium-temperature digestion (method of the present invention), high-temperature low-molecularization-high-temperature digestion (comparative example 1), medium-temperature low molecularization (comparative example 2), and medium-temperature Experiments were conducted on four methods: low molecular weight reduction and mesothermal digestion (Comparative Example 3). On the other hand, pulp powder and sewage sludge with an organic matter concentration of 2.5% were used as simulated wastes, and the supply amounts (processing amounts) of these were 45 g and 600 ml per day, respectively. The residence time in the reactor is 5
The main digester 2' was used for 10 days. Therefore, the organic load on the entire experimental apparatus (total capacity of both reactors) was approximately 6.7 Kg/m ³ ·day. Under such load conditions, each of the four types described above was operated. Table 1 shows the organic matter decomposition rate calculated from the amount of methane-containing gas 18 generated, its methane content, the amount of methane generated, and the average value of the organic matter concentration of the digested sludge 10' in each experiment. Next, the obtained digested sludge 10' was subjected to a dehydration test.
The test was carried out in accordance with the Nutsuchie dehydration test published by the Japan Sewage Works Association, 1974 edition of ``Sewage Testing Methods''. That is, 10% slaked lime and 5% FeCl ₃ are added and mixed to the dry solid content of 10' of digested sludge, and the area is
Table 1 shows the average values of resistivity determined by passing through a 35 cm ² leaf at a vacuum degree of 400 mmHg. In addition, dichromic acid of the supernatant liquid (corresponding to desorbed liquid 11 in Figure 1) centrifuged at 11500G
The average COD values were as shown in Table 1.

[Table] As is clear from Table 1, the method of the present invention has a higher organic matter decomposition rate and higher methane generation than each comparative example, and is extremely superior in solid-liquid separation and dewatering properties of digested sludge. This was confirmed. Conventional method - 12,300 mg / Inventive method - 3,800 As described above, according to the present invention, by controlling the digestion temperature in the low molecularization process and the main digestion process, the solid-liquid separation property and dewatering property of digested sludge can be improved. Therefore, the disposal of the dehydrated cake becomes easier, the wastewater treatment of the dehydrated separated water is significantly streamlined, and the number of days required to digest organic matter can be shortened, and the decomposition rate of organic matter, amount of methane generated, etc. is equivalent to or better than the conventional method, and has the advantage that the digestion equipment is simple and inexpensive.

[Brief explanation of drawings]

FIG. 1 and FIG. 2 are system explanatory diagrams showing embodiments and examples of the present invention, respectively. 1... Low molecular weight reactor, 1'... Low molecular weight tank,
2... Main digestion reactor, 2'... Main digestion tank, 3,
3'... Heat exchanger, 4, 5... Solid-liquid separation device, 6
...Waste, 6'...Sewage sludge, 7,7'...Effluent slurry, 8...Desorbed liquid, 9...Solid matter, 10,
10'... Digested sludge, 11... Desorption liquid, 12...
Sediment, 13... constant temperature water tank, 14... pulp powder inlet, 15... cooling device, 16... constant temperature water tank,
17...Gas meter, 18...Methane-containing gas.

Claims (1)

[Claims] 1. In anaerobic digestion of organic waste,
The process is divided into at least two parts, and the organic waste is subjected to a low-molecularization process at a high temperature digestion temperature range of 45°C to 65°C in the first process, and then the treated product is cooled, and then the second process is performed. An anaerobic digestion method for organic waste, characterized in that anaerobic digestion is carried out at a meso-temperature digestion temperature of 20°C to 40°C in the main digestion step. 2. The digestion method according to claim 1, wherein the main digestion step is divided into an acid production step and a subsequent methane production step.