Continuous methanogenic digestion of organic solid waste

Abstract

The present application provides a kind of continuous micro-aerobic digestion methane production method of organic solid waste, it relates to microorganism digestion methane production.The method comprises the following steps: organic solid waste and anaerobic sludge are loaded into reactor for anaerobic digestion;After anaerobic digestion reaction is stable, start to continuously import air or oxygen into reactor, maintain the dissolved oxygen content in reactor in 0.20-0.35 mg / L range;When the daily gas production of reactor decreases compared with the daily gas production of previous day, start to supplement organic solid waste into reactor, daily feed quantity is calculated according to hydraulic retention time, daily feed quantity and discharge quantity are equal, and the gas produced in reactor is collected by exhaust port.By the above method, continuous gas production in the same reactor can be realized, which overcomes the defect that when two-phase digestion is used, the reaction substance needs to be transferred after micro-aerobic aeration is completed, which increases the operation complexity and is not suitable for continuous digestion methane production.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of microbial digestion for methane production, and in particular to a continuous micro-aerobic digestion method for organic solid waste. BACKGROUND

[0002] Anaerobic digestion (AD) is one of the important biomass energy utilization technologies, which can convert solid organic matter into dissolved organic matter, and convert the substances contained in waste into biogas for combustion or power generation, which has great advantages in economy and environment. The general anaerobic digestion process can be divided into four stages, namely hydrolysis, acidification, hydrogen and acetic acid production, and methane production stage. In the hydrolysis stage, high molecular organic matter is converted into small molecular organic matter, so that it can be utilized by bacteria; in the acidification stage, small molecular organic matter produced in the previous stage is converted into volatile acid under the action of acidification bacteria; in the hydrogen and acetic acid production stage, the product of acid-producing bacteria is converted into acetic acid, hydrogen and carbon dioxide by acetic acid bacteria; in the methane production stage, acetic acid, hydrogen, carbon dioxide, formic acid and methanol are converted into methane, carbon dioxide and new cell material.

[0003] Current studies have shown that the supply of trace oxygen can accelerate the hydrolysis rate of enzymes in anaerobic digestion microorganisms and improve the relative abundance of microbial populations, so the micro-aerobic digestion technology is increasingly applied in digestion for methane production. However, the existing methane production process is usually divided into two stages of pretreatment and anaerobic digestion, that is, a pretreatment device is added before the anaerobic digestion device, and a micro-aerobic environment is formed by aeration in the pretreatment device to promote the hydrolysis of the digestion substrate. For example, patent document CN105152506A discloses a method for high-temperature micro-aerobic-anaerobic digestion of organic sludge. The organic sludge is first subjected to a high-temperature micro-aerobic digestion process, after the high-temperature micro-aerobic digestion is completed, the digested sludge is discharged from the discharge port and enters the pH adjustment tank through the pipeline, the alkali solution is added into the pH adjustment tank to adjust the pH of the material in the tank, the sludge is left in the tank for 3-5h to release the dissolved oxygen in the sludge, and then the sludge is pumped into the anaerobic digestion reactor and left for 6-20 days to complete the anaerobic digestion process. In this method, the reaction is two-phase digestion, and the reaction material needs to be transferred after the micro-aerobic aeration is completed, which increases the complexity of the operation, and this method is not suitable for continuous digestion for methane production.

[0004] Therefore, how to provide a continuous micro-aerobic digestion method for organic solid waste for methane production has become a technical problem to be solved by those skilled in the art. SUMMARY

[0005] Therefore, the present application aims to overcome the defects in the prior art that the reaction substance needs to be transferred after the micro-aerobic aeration is finished, which increases the complexity of operation and is not suitable for continuous digestion and methane production, thereby providing a continuous micro-aerobic digestion and methane production method for organic solid waste.

[0006] To achieve the above-mentioned object, the present application provides the following technical solutions.

[0007] The present application provides a continuous micro-aerobic digestion and methane production method for organic solid waste, comprising the following steps:

[0008] (1) loading organic solid waste and anaerobic sludge into a reactor for anaerobic digestion;

[0009] (2) after the anaerobic digestion reaction is stable, continuously supplying air or oxygen into the reactor, and maintaining the dissolved oxygen content in the reactor within the range of 0.20-0.35 mg / L;

[0010] (3) when the daily gas production of the reactor decreases compared with that of the previous day, supplementing organic solid waste into the reactor, the daily feeding amount in the first week is calculated according to the hydraulic retention time of 90-110 days, the daily feeding amount in the second week is calculated according to the hydraulic retention time of 40-60 days, the daily feeding amount in the third week is calculated according to the hydraulic retention time of 20-40 days, and the daily feeding amount in the fourth week and the following weeks is calculated according to the hydraulic retention time of 10-30 days, the daily feeding amount is equal to the daily discharging amount, and the gas produced in the reactor is collected through the gas outlet.

[0011] Further, in step (3), the daily feeding amount in the first week is calculated according to the hydraulic retention time of 100 days, the daily feeding amount in the second week is calculated according to the hydraulic retention time of 50 days, the daily feeding amount in the third week is calculated according to the hydraulic retention time of 30 days, and the daily feeding amount in the fourth week and the following weeks is calculated according to the hydraulic retention time of 20 days.

[0012] Further, in step (2), the continuously supplying air or oxygen into the reactor is started after the anaerobic digestion reaction is performed for 1-2 days.

[0013] Further, in step (2), the air or oxygen is supplied into the liquid phase in the reactor and / or into the gas phase in the reactor.

[0014] Further, in step (1), the organic solid waste and the anaerobic sludge are mixed according to the C / N ratio of 20-30.

[0015] Further, in step (1), the temperature in the reactor is adjusted to 30-55℃.

[0016] Further, in step (1), the pH value in the reactor is adjusted to 6.5-7.8.

[0017] Further, in step (1), the solid content in the reactor is adjusted to 5%-15%.

[0018] Further, in step (1), after the organic solid waste and anaerobic sludge are loaded into the reactor, the stirring device of the reactor is started, and the rotating speed is 50-100 r / min.

[0019] Further, the organic solid waste comprises at least one of straw, excrement, and kitchen waste.

[0020] The technical scheme of the present application has the following advantages:

[0021] The method for continuously producing methane from organic solid waste by micro-aerobic digestion provided by the present application first loads organic solid waste and anaerobic sludge into a reactor for anaerobic digestion to start the reaction, and typical organic substances in the waste, such as cellulose, are decomposed into cellobiose and glucose by cellulase, starch is decomposed into maltose and glucose, and protein is decomposed into short peptides and amino acids, and the small molecules after decomposition can be absorbed and utilized by microorganisms for further decomposition, and the pH starts to show a downward trend; after the anaerobic digestion reaction is stable, continuous aeration is started to provide a micro-aerobic environment in the reactor and improve the hydrolysis efficiency of the organic solid waste, and the hydrolysis rate is further improved, which is specifically manifested in that the gas production gradually increases, the methane content gradually increases, and the pH further decreases; when the daily gas production of the reactor decreases compared with that of the previous day, the organic solid waste is started to be supplemented into the reactor, and in order to prevent the microbial community in the reactor from being replaced on a large scale due to the feeding, the daily feeding amount can be calculated according to the hydraulic retention time (HRT) of the digestion tank, and the HRT gradually decreases, because the replacement and development of the microbial community in the reactor need a certain time, and an ideal microbial community can be obtained by gradually changing the HRT for domestication. The above method can realize continuous gas production in the same reactor, and overcomes the defect that the reaction material needs to be transferred after the micro-aerobic aeration is completed when two-phase digestion is used, which increases the operation complexity. BRIEF DESCRIPTION OF DRAWINGS

[0022] In order to more clearly illustrate the specific embodiments of the present application or the technical scheme in the prior art, the drawings needed in the following specific embodiment or prior art description will be briefly introduced. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0023] Figure 1is a structural schematic diagram of a reactor for continuous micro-aerobic digestion and methane production in an alternative embodiment of the present application;

[0024] Figure 2 is a schematic diagram of the position of air or oxygen inlet of the reactor in an alternative embodiment of the present application;

[0025] Figure 3 is a curve diagram of pH change with reaction time under different daily aeration rates in the experimental example of the present application;

[0026] Figure 4 is a curve diagram of biogas yield change with reaction time under different daily aeration rates in the experimental example of the present application;

[0027] Figure 5 is a curve diagram of biogas production rate change with reaction time under different daily aeration rates in the experimental example of the present application;

[0028] Figure 6 is a curve diagram of methane content change with reaction time under different daily aeration rates in the experimental example of the present application;

[0029] Figure 7 is a curve diagram of methane yield change with reaction time under different daily aeration rates in the experimental example of the present application;

[0030] Figure 8 is a curve diagram of methane production rate change with reaction time under different daily aeration rates in the experimental example of the present application.

[0031] Reference signs:

[0032] 1 - feed inlet; 2 - discharge outlet; 3 - air inlet; 4 - exhaust outlet; 5 - stirring device. DETAILED DESCRIPTION

[0033] The following examples are provided to better further understand the present application, and are not limited to the best mode, and do not constitute a limitation on the content and protection scope of the present application, and any person under the inspiration of the present application or the combination of the present application with other prior art features, any product same or similar to the present application, falls within the protection scope of the present application.

[0034] The specific experimental steps or conditions not indicated in the examples can be carried out according to the conventional experimental steps described in the literature in the art or the operation or conditions. The raw materials or instruments used are conventional products that can be obtained by market purchase, including but not limited to the raw materials or instruments used in the examples of the present application.

[0035] As Figure 1 and Figure 2As shown, as an optional embodiment of the present application, the continuous micro-aerobic digestion and methane production method of organic solid waste provided by the present application is carried out in a reactor (such as a digestion tank), and the reactor is provided with a feeding port 1, a discharging port 2, an air inlet 3, and an exhaust port 4. The feeding port 1 is used for feeding the reaction material into the reactor, the discharging port 2 is used for discharging the digestion product in the reactor, the air inlet 3 is used for feeding air or oxygen into the reactor, and the exhaust port 4 is used for collecting the gas produced by digestion. In addition, the reactor is also provided with a stirring device 5 and a dissolved oxygen meter (not shown in the figure). The stirring device 5 is used for fully stirring the reaction material to make the reaction more uniform, and the dissolved oxygen meter is used for detecting the dissolved oxygen content (DO) in the reactor.

[0036] The continuous micro-aerobic digestion and methane production method of organic solid waste provided by the present application comprises the following steps:

[0037] (1) loading the organic solid waste and anaerobic sludge into the reactor for anaerobic digestion;

[0038] (2) after the anaerobic digestion reaction is stable, continuously feeding air or oxygen into the reactor to maintain the dissolved oxygen content in the reactor within the range of 0.20-0.35 mg / L;

[0039] (3) when the daily gas production of the reactor decreases compared with that of the previous day, supplementing the organic solid waste into the reactor. The daily feeding amount in the first week is calculated according to the hydraulic retention time of 90-110 days, the daily feeding amount in the second week is calculated according to the hydraulic retention time of 40-60 days, the daily feeding amount in the third week is calculated according to the hydraulic retention time of 20-40 days, and the daily feeding amount in the fourth week and the following weeks is calculated according to the hydraulic retention time of 10-30 days. The daily feeding amount is equal to the daily discharging amount, and the gas produced in the reactor is collected by the exhaust port.

[0040] In step (1):

[0041] The organic solid waste includes at least one of straw, excrement, and kitchen waste.

[0042] The anaerobic sludge can come from a sewage treatment plant, a breeding farm, etc., and contains rich microbial flora.

[0043] The organic solid waste and the anaerobic sludge are mixed according to the C / N ratio of 20-30.

[0044] The temperature in the reactor is adjusted to 30-55℃, and preferably 37℃.

[0045] The pH value in the reactor is adjusted to 6.5-7.8, and preferably 7.6.

[0046] The solid content in the reactor is adjusted to 5%-15%, and preferably 7.06%.

[0047] After loading the organic solid waste and anaerobic sludge into the reactor, turn on the reactor's stirring device at a speed of 50-100 r / min and continue stirring throughout the entire reaction process.

[0048] In step (2):

[0049] The anaerobic digestion reaction is considered stable when gas production begins in the reactor and the methane concentration gradually increases. This process generally takes 1 to 2 days. That is, after the anaerobic digestion reaction has been going on for 1 to 2 days, air or oxygen is continuously introduced into the reactor.

[0050] The amount and rate of air or oxygen introduced can be determined through prior experiments. The dissolved oxygen content in the reactor can be controlled within the range of 0.20 to 0.35 mg / L by using a dissolved oxygen meter installed in the reactor to reflect the dissolved oxygen level.

[0051] like Figure 2 As shown, air or oxygen can be introduced by introducing it into the liquid phase and / or gas phase in the reactor; either method or both can be used.

[0052] In step (3):

[0053] In a preferred implementation, the daily feed rate for the first week is calculated based on a hydraulic retention time (HRT) of 100 days; for the second week, it is calculated based on a HRT of 50 days; for the third week, it is calculated based on a HRT of 30 days; and for the fourth week and thereafter, it is calculated based on a HRT of 20 days. For example, if the digester has a volume of 2000 mL and an effective volume of 1600 mL, and the HRT gradually decreases from 100 days to 50 days, 30 days, and 20 days each week, then the daily feed rate for the first week is 1600 mL / 100 = 16 mL, for the second week it is 1600 mL / 50 = 32 mL, for the third week it is 1600 mL / 30 = 54 mL, and for the fourth week it is 1600 mL / 20 = 80 mL.

[0054] Example

[0055] This embodiment provides a method for continuous microaerobic digestion of pig manure to produce methanogens, in the following manner: Figure 1 The experiment was conducted in the reactor shown, which has an effective volume of 3000 mL. The specific procedures are as follows:

[0056] Charging: Pig manure from Tumote Left Banner of Hohhot and anaerobic digestion sludge from laboratory were mixed in a volume ratio of 1:3 (C / N ratio of 25:1), and were loaded into the reactor after the feeding port. The temperature in the reactor was adjusted to 37°C, the pH value was 7.6, the solid content was 7.06%, the stirring device of the reactor was started, and the rotating speed was set to 80 r / min. The material in the reactor was subjected to anaerobic digestion;

[0057] Aeration: After the anaerobic digestion reaction was carried out for 1 day, air was continuously introduced into the liquid phase of the reactor from the air inlet. The air was continuously and stably introduced into the reactor at a certain rate by a peristaltic pump, and the daily air input was controlled to be 40 mL. The dissolved oxygen content in the reactor was monitored by a dissolved oxygen meter and was maintained in the range of 0.20-0.35 mg / L.

[0058] Supplemental feeding: After 13 days of reaction, the daily gas production of the reactor decreased compared with that of the previous day. Pig manure was started to be supplemented into the reactor. The daily feeding amount in the first week was 30 mL (calculated according to HRT 100 days), the daily feeding amount in the second week was 60 mL (calculated according to HRT 50 days), the daily feeding amount in the third week was 100 mL (calculated according to HRT 30 days), and the daily feeding amount in the fourth week and thereafter was 150 mL (calculated according to HRT 20 days). The daily feeding amount was equal to the daily discharge amount. The daily discharge was discharged from the discharge port at the bottom of the reactor, and the gas produced in the reactor was collected by a gas bag connected to the exhaust port.

[0059] Comparative Example 1

[0060] This comparative example provides a continuous anaerobic digestion and methane production method of pig manure. The specific steps refer to the embodiment, and the only difference is that no air is introduced into the reactor. The dissolved oxygen content in the reactor is monitored by a dissolved oxygen meter and is maintained in the range of 0-0.05 mg / L.

[0061] Comparative Example 2

[0062] This comparative example provides a continuous micro-aerobic digestion and methane production method of pig manure. The specific steps refer to the embodiment, and the only difference is that the daily air input is controlled to be 20 mL. The dissolved oxygen content in the reactor is monitored by a dissolved oxygen meter and is maintained in the range of 0.10-0.25 mg / L.

[0063] Comparative Example 3

[0064] This comparative example provides a continuous micro-aerobic digestion and methane production method of pig manure. The specific steps refer to the embodiment, and the only difference is that the daily air input is controlled to be 60 mL. The dissolved oxygen content in the reactor is monitored by a dissolved oxygen meter and is maintained in the range of 0.30-0.40 mg / L.

[0065] Experimental Example

[0066] During the process of producing methane according to the method provided in the examples and Comparative Examples 1-3, the pH value of the reactor effluent (composite electrode method), the daily gas production of the reactor (drainage gas collection method), and the biogas composition (portable methane analyzer) were detected daily. Then the daily biogas yield, biogas production rate, methane content, methane yield and methane production rate were calculated respectively (the calculation of each index refers to the conventional calculation method in the art), and the statistical results of the pH value and the above indexes are shown in Table 1. Figures 3-8 The average values of each index under different daily aeration conditions after digestion stabilization were calculated, and the results are shown in Table 1.

[0067] Table 1 Average values of each index under different daily aeration conditions after digestion stabilization

[0068] Group Comparative Example 1 Comparative Example 2 Comparative Example 3 Example Parameter Daily ventilation 0 mL Daily ventilation 20 mL Daily ventilation 60 mL Daily ventilation 40 mL Rate of biogas production (m 3 / m 3 / d) 0.356±0.015 0.391±0.017 0.495±0.021 0.509±0.023 Biogas yield (L / g VS) 0.24±0.02 0.27±0.03 0.34±0.03 0.35±0.03 Methane content (%) 67.29±0.84 65.92±0.65 65.01±1.33 62.61±0.74 Methane production rate (m 3 / m 3 / d) 0.204±0.011 0.258±0.012 0.323±0.015 0.319±0.004 Methane yield (L / g VS) 0.164±0.017 0.177±0.017 0.222±0.023 0.218±0.007

[0069] The change of pH with time under different daily aeration conditions is shown in Figure 3 The difference in pH between each group is small at the initial stage of the reaction. At the initial stage of the digestion, the hydrolytic bacteria decompose organic matter to produce a large amount of organic acid, which causes the pH to decrease. With the progress of the reaction, a large amount of organic acid is utilized by methanogens to generate methane, and the nitrogen content in the pig manure is high, so the free ammonia formed by hydrolysis can neutralize the generated acid to some extent. Therefore, with the progress of the reaction, the pH in the reactor gradually increases and fluctuates around 7.9.

[0070] The change of biogas yield with time under different daily aeration conditions is shown in Figure 4 At the initial stage of the reaction, the biogas yield of each group fluctuates greatly, and with the progress of the reaction, the biogas yield gradually tends to be stable. It can be clearly seen that the biogas yield from low to high is: Comparative Example 1 (aeration 0 mL) < Comparative Example 2 (aeration 20 mL) < Comparative Example 3 (aeration 60 mL) < Example (aeration 40 mL), and the difference between each group is obvious. The average biogas yield of each group from low to high is 0.24±0.002, 0.27±0.001, 0.34±0.002, 0.35±0.002 (unit: L / gVS), respectively. It can be seen that the introduction of appropriate amount of air can accelerate the progress of the reaction and improve the biogas yield, and the effect is best when the dissolved oxygen content in the reactor is maintained in the range of 0.20-0.35 mg / L.

[0071] The change of biogas production rate with time under different daily aeration conditions is shown in Figure 5 The change trend is similar to that of the biogas yield, and after 20 days of reaction, it gradually tends to be stable,

[0072] The change of methane content with time under different daily aeration conditions is shown in Figure 6As shown in the figure, the methane content fluctuates significantly in the initial stage of the reaction, exhibiting an overall spiral upward trend. This stage is dominated by hydrolysis and acidification, with hydrolytic bacteria being the dominant species and methanogenic bacteria present in relatively small amounts. As the reaction progresses, the microbial community within the apparatus stabilizes, and the methane content gradually remains at a stable level. The methane content, from lowest to highest, is as follows: Example 1 (daily aeration rate 40 mL) < Comparative Example 3 (daily aeration rate 60 mL) < Comparative Example 2 (daily aeration rate 20 mL) < Comparative Example 1 (daily aeration rate 0 mL). This is because, in Example 1, as well as Comparative Examples 2 and 3, in addition to utilizing oxygen, nitrogen and other gases were introduced into the reactor, reducing the methane content.

[0073] The changes in methane yield and methane production rate over time under different daily ventilation conditions are as follows: Figure 7 , Figure 8 As shown in the figure, during the initial stage of the reaction (0-10 days), the methane production of the "micro-aerobic" group (Examples, Comparative Examples 2-3) increased faster than that of the "anaerobic" group (Comparative Example 1). This indicates that micro-aerobic reaction accelerates the hydrolysis process and speeds up the reaction rate. The effect is best when the dissolved oxygen content in the reactor is maintained in the range of 0.20-0.35 mg / L.

[0074] The results above show that the dissolved oxygen content in the reactor has a significant impact on the entire digestion process during microaerobic continuous digestion. While the dissolved oxygen content in the reactor is typically controlled at 0.1–1.0 mg / L during microaerobic digestion, this invention has found that excellent gas production is not always achieved within this range. Experiments demonstrate that, in the continuous digestion process for methanogenesis, maintaining the dissolved oxygen content in the reactor within the range of 0.20–0.35 mg / L achieves both excellent gas production and stable reaction, representing a significant improvement.

[0075] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for continuous mesophilic methanogenic digestion of organic solid waste, characterized in that, The method comprises the following steps: (1) loading organic solid waste and anaerobic sludge into a reactor for anaerobic digestion, wherein the organic solid waste and the anaerobic sludge are mixed according to a C / N ratio of 20-30, the temperature in the reactor is adjusted to 30-55°C, the pH value is adjusted to 6.5-7.8, and the solid content is adjusted to 5%-15%; (2) after the anaerobic digestion reaction is stabilized, continuously supplying air or oxygen into the reactor, and maintaining the dissolved oxygen content in the reactor at 0.20-0.35 mg / L; (3) when the daily gas production of the reactor decreases compared with that of the previous day, supplementing the organic solid waste into the reactor, the daily feeding amount in the first week is calculated according to a hydraulic retention time of 90-110 days, the daily feeding amount in the second week is calculated according to a hydraulic retention time of 40-60 days, the daily feeding amount in the third week is calculated according to a hydraulic retention time of 20-40 days, and the daily feeding amount in the fourth week and the following weeks is calculated according to a hydraulic retention time of 10-30 days, the daily feeding amount is equal to the daily discharging amount, and the gas produced in the reactor is collected through a gas outlet.

2. The continuous mesophilic-methane-producing digestion method of organic solid waste according to claim 1, characterized by, In step (3), the daily feeding amount in the first week is calculated according to a hydraulic retention time of 100 days, the daily feeding amount in the second week is calculated according to a hydraulic retention time of 50 days, the daily feeding amount in the third week is calculated according to a hydraulic retention time of 30 days, and the daily feeding amount in the fourth week and the following weeks is calculated according to a hydraulic retention time of 20 days.

3. The continuous mesophilic-methane-producing digestion method of organic solid waste according to claim 1, characterized by, In step (2), the continuously supplying air or oxygen into the reactor is started after the anaerobic digestion reaction is performed for 1-2 days.

4. The continuous mesophilic-methane-producing digestion method of organic solid waste according to claim 1, characterized by, In step (2), the air or oxygen is supplied into the liquid phase in the reactor and / or into the gas phase in the reactor.

5. The continuous mesophilic-methane-producing digestion method of organic solid waste according to claim 1, characterized by, In step (1), after the organic solid waste and the anaerobic sludge are loaded into the reactor, the stirring device of the reactor is started, and the rotating speed is 50-100 r / min.

6. The continuous mesophilic-methane-producing digestion method of organic solid waste according to claim 1, characterized by, The organic solid waste comprises at least one of straw, excrement, and kitchen waste.

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