A temperature-variable semi-continuous anaerobic digestion method and system

Abstract

The present invention relates to the technical field of anaerobic digestion of organic waste, and relates to a variable temperature semi-continuous anaerobic digestion method and system. The method comprises: subjecting the digestate to a first set temperature for a first anaerobic digestion; raising the temperature of the digestate to a second set temperature according to a preset heating rate, and subjecting the digestate to a second set temperature for a second preset time for a second anaerobic digestion; cooling the digestate to the first set temperature, and returning to the first anaerobic digestion step. The present invention promotes the proliferation of highly efficient methanogenic microorganisms of the genus Methanosarcina by utilizing a variable temperature anaerobic digestion process, strengthens the degradation and conversion of acetic acid products during the anaerobic digestion process, and thereby improves the system's ability to withstand organic loads and produce gas, solving the problem of slow reproduction and low proportion of highly efficient methanogenic microorganisms in a mesophilic anaerobic digestion system, which leads to low gas production efficiency and poor load tolerance.

Description

Technical Field

[0001] The present invention relates to the technical field of anaerobic digestion of organic waste, and in particular to a temperature-variable semi-continuous anaerobic digestion method and system. Background Art

[0002] Anaerobic digestion is an effective method for treating organic waste. It utilizes microorganisms in an oxygen-deficient environment to convert organic matter into biogas and organic fertilizer, making it an important approach to renewable energy development. The anaerobic digestion process requires a certain temperature, most commonly at moderate or high temperatures.

[0003] In order to save energy consumption, my country's biogas projects generally adopt medium-temperature anaerobic digestion mode. However, there are problems such as weak digestion capacity and generally low gas production rate (less than 1.0m3 on average). 3 / m 3 / d) and other issues.

[0004] It has been proven that the gas production efficiency of mesophilic anaerobic digestion can be enhanced by adding biological agents, biochar and substances containing group B elements to the anaerobic digester. However, since large-scale anaerobic digestion projects generally adopt continuous / semi-continuous anaerobic digestion modes, the added substances will be continuously discharged along with the materials and therefore need to be continuously replenished, which undoubtedly increases the project operating costs. Summary of the Invention

[0005] The problem to be solved by the present invention is how to improve the load carrying capacity and volumetric gas production capacity of medium-temperature semi-continuous anaerobic digestion under low-cost conditions.

[0006] To solve the above problems, the present invention provides a temperature-variable semi-continuous anaerobic digestion method, comprising:

[0007] The first anaerobic digestion step comprises: subjecting the digestate to a first set temperature for a first anaerobic digestion;

[0008] The second anaerobic digestion step comprises: raising the temperature of the digestion liquid to a second set temperature at a preset heating rate, and performing second anaerobic digestion at the second set temperature for a second preset time;

[0009] The digestate is cooled to the first set temperature and returned to the first anaerobic digestion step.

[0010] Optionally, raising the temperature of the digestive fluid to a second set temperature according to a preset heating rate includes:

[0011] The temperature of the digestive liquid is increased to a third set temperature at a first heating rate, the digestive liquid is kept at the third set temperature for a first preset time, and then the temperature is increased to the second set temperature at a second heating rate.

[0012] Optionally, the third set temperature is 50±1°C, the first preset time is 1-7 days, the second set temperature is 55±1°C, and the second preset time is 3-5 days.

[0013] Optionally, after returning to the first anaerobic digestion step, the method further includes: cyclically performing the second anaerobic digestion step and the first anaerobic digestion step.

[0014] Optionally, the digestate is obtained by mixing anaerobic fermentation inoculum with prepared organic waste slurry, wherein the organic waste slurry is obtained by mixing organic waste with water or supernatant of returned biogas slurry, and the organic waste includes one or more of livestock and poultry manure, food waste and biomass.

[0015] Optionally, the dry matter concentration of the organic waste slurry is 3%-15%.

[0016] Optionally, the first set temperature is 35±1°C.

[0017] The present invention further provides a temperature-variable anaerobic digestion system for implementing the temperature-variable semi-continuous anaerobic digestion method as described above, comprising:

[0018] An anaerobic digester, used for supplying digestate for a first anaerobic digestion and a second anaerobic digestion;

[0019] The temperature control unit is used to control the temperature of the digestive liquid in the anaerobic digester, including raising the temperature of the digestive liquid to a first set temperature, raising the temperature of the digestive liquid from the first set temperature to a second set temperature, and lowering the temperature of the digestive liquid from the second set temperature to the first set temperature.

[0020] The advantages of the variable temperature semi-continuous anaerobic digestion method of the present invention over the prior art are:

[0021] The present invention employs a variable temperature semi-continuous anaerobic digestion method, first subjecting the digestate to a first set temperature for a first anaerobic digestion step, and then raising the temperature of the digestate by heating. The increased temperature promotes the proliferation of Methanosarcina microorganisms, thereby strengthening their dominant position in the anaerobic methanogenic digestion community and increasing the proportion of Methanosarcina microorganisms in the mesophilic anaerobic digestion system, thereby improving gas (methane) production capacity. Furthermore, since Methanosarcina microorganisms have a strong ability to convert acetic acid, this method can also address the accumulation of organic acids such as acetic acid caused by increased organic loads, reduce gas production inhibition, and thus improve the load tolerance of the anaerobic digestion system.

[0022] In addition, compared with the direct combination of medium-temperature fermentation and high-temperature fermentation, the advantages of the variable temperature fermentation method are also reflected in the following: on the one hand, the gradual heating process allows microorganisms to adapt to the environment under gradually changing temperature conditions, reducing adverse effects on microorganisms, which is beneficial to the adaptability and growth of bacteria and improving digestion efficiency; on the other hand, the high temperature stage of the variable temperature digestion process can promote the reproduction of other hydrolytic microorganisms and eliminate heat-sensitive harmful bacteria; on the other hand, the present invention can effectively enhance the gas production capacity of medium-temperature anaerobic digestion without changing the original biogas project structure.

[0023] Therefore, the present invention promotes the proliferation of efficient methanogenic microorganisms of the genus Methanosarcina by utilizing a variable temperature anaerobic digestion process, strengthens the degradation and conversion of acetic acid products during the anaerobic digestion process, thereby improving the system's tolerance to organic loads and gas production capabilities, and solves the problem of slow reproduction and low proportion of efficient methanogenic microorganisms in the medium-temperature anaerobic digestion system, which leads to low gas production efficiency and poor load tolerance. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Flow chart of the variable temperature semi-continuous anaerobic digestion method according to an embodiment of the present invention;

[0025] Figure 2 The proportion of the dominant methanogenic species in the methanogenic bacteria community in Example 1 of the present invention and the control example;

[0026] Figure 3 The figure shows the gas production comparison between Example 1 of the present invention and the control example. DETAILED DESCRIPTION

[0027] For anaerobic digestion systems, improving their load capacity and volumetric gas production capacity is crucial for improving waste treatment efficiency. Load capacity refers to the amount of organic matter that can be handled per unit reactor volume or surface area. A high load capacity indicates that the reactor can handle a larger organic load and achieve higher treatment efficiency. Volumetric gas production capacity refers to the amount of biogas produced per unit reactor volume or surface area, also known as the biogas production rate. A high volumetric gas production capacity indicates that the reactor can produce more biogas and achieve higher utilization efficiency.

[0028] The main way to increase the gas production rate of the tank volume is to increase the organic load. The load tolerance of mesophilic anaerobic digestion is significantly weaker than that of thermophilic anaerobic digestion. However, thermophilic anaerobic digestion requires higher temperature conditions (usually between 50°C and 60°C), and maintaining this temperature requires more energy, resulting in higher energy consumption and increased operating costs of the system. In addition, thermophilic anaerobic digestion requires thermophilic bacteria to operate normally, and these bacteria are relatively sensitive and easily disturbed by the external environment, resulting in decreased system stability. Therefore, it is particularly important to develop a low-cost and effective method for strengthening mesophilic anaerobic digestion.

[0029] Existing technologies, such as adding biological agents, biochar, and substances containing group B elements to anaerobic digesters, require continuous replenishment, increasing project operating costs. While combining mesophilic and thermophilic anaerobic fermentation can increase biogas yield, the combined systems require material transfer and connection, including pipelines, valves, and other equipment, which undoubtedly increases system complexity and operational risks. Furthermore, combining the two fermentation systems requires control and adjustment of different operating parameters, including temperature, pH, and feed rate, requiring more sophisticated monitoring and adjustment, increasing operational complexity.

[0030] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0031] See also Figure 1 As shown, a temperature-variable semi-continuous anaerobic digestion method according to an embodiment of the present invention comprises:

[0032] S1, subjecting the digestate to a first anaerobic digestion at a first set temperature;

[0033] S2, raising the temperature of the digestate to a second set temperature at a preset heating rate, and performing a second anaerobic digestion at the second set temperature for a second preset time;

[0034] S3, cooling the digestive fluid to the first set temperature, and returning to step S1.

[0035] It should be understood that the above method relies on the anaerobic digestion system, and increasing the organic load of the anaerobic digestion system is the main way to increase the volumetric gas production capacity of the anaerobic digestion tank (or digester, etc.). The increase in organic load can easily lead to the accumulation of products of volatile organic acids such as acetic acid in the anaerobic digestion system, thereby forming gas production inhibition.

[0036] Methanosarcina (Latin name: Methanosarcina) microorganisms have a strong ability to convert acetic acid, making them highly efficient methanogens, crucial for improving the load-bearing capacity of anaerobic digestion systems. This genus can survive and produce methanogens between 25-55°C. Efficient methanogens reproduce more slowly at mesophilic temperatures (around 30°C), making it difficult for them to form dominant colonies, which can easily lead to acidification and gas production failure in the entire system. However, this genus reproduces more rapidly at 55°C.

[0037] This embodiment employs a variable temperature semi-continuous anaerobic digestion method, first subjecting the digestate to a first anaerobic digestion at a first set temperature, for example, a mesophilic anaerobic digestion. The digestate temperature is then raised by heating. This increase in temperature promotes the proliferation of Methanosarcina microorganisms, thereby strengthening their dominant position within the anaerobic methanogenic digestion community and increasing the proportion of Methanosarcina microorganisms in the mesophilic anaerobic digestion system, thereby improving gas (methane) production capacity. Furthermore, because Methanosarcina microorganisms have a strong ability to convert acetic acid, this method can address the accumulation of organic acids such as acetic acid caused by increased organic loads, reduce gas production inhibition, and thereby improve the load tolerance of the anaerobic digestion system.

[0038] In addition, this embodiment adopts a variable temperature fermentation method rather than a direct combination of medium temperature fermentation and high temperature fermentation. On the one hand, the gradual temperature increase process allows the microorganisms to adapt to the environment under gradually changing temperature conditions, reducing adverse effects on the microorganisms, which is beneficial to the adaptability and growth of bacteria and improves digestion efficiency; on the other hand, the high temperature stage of the variable temperature digestion process can promote the reproduction of other hydrolytic microorganisms and eliminate heat-sensitive harmful bacteria; on the other hand, this embodiment can effectively enhance the gas production capacity of medium temperature anaerobic digestion without changing the original biogas project structure.

[0039] In summary, this embodiment utilizes the variable temperature anaerobic digestion process to promote the proliferation of efficient methanogenic microorganisms of the genus Methanosarcina, enhance the degradation and conversion of acetic acid products during the anaerobic digestion process, thereby improving the system's tolerance to organic loads and gas production capabilities, and solving the problem of slow reproduction and low proportion of efficient methanogenic microorganisms in the mesophilic anaerobic digestion system, which leads to low gas production efficiency and poor load tolerance.

[0040] It should be noted that in this embodiment, in step S3, after the temperature of the digestate is cooled to the first set temperature, the process returns to step S1, i.e., the first anaerobic digestion continues at the first set temperature. Thereafter, the first anaerobic digestion can continue, or after returning to step S1, steps S2 and S3 can be continued, thereby achieving cyclic temperature-variable fermentation. Whether to cycle and the number of cycles can be determined based on actual conditions.

[0041] In some embodiments, raising the temperature of the digestive fluid to a second set temperature at a preset heating rate includes:

[0042] The temperature of the digestive liquid is increased to a third set temperature at a first heating rate, the digestive liquid is kept at the third set temperature for a first preset time, and then the temperature is increased to the second set temperature at a second heating rate.

[0043] In this embodiment, the temperature of the digestive fluid is gradually increased according to a preset heating rate, and after staying at a specific temperature for a certain period of time, the temperature is raised to a preset temperature. In this way, on the one hand, the gradual heating process allows microorganisms to gradually adapt to different temperature environments, promotes the adaptability and growth of microorganisms, and is conducive to the stability and proliferation of bacterial populations; on the other hand, by staying at the third set temperature for a certain period of time, the microorganisms can better carry out metabolic activities at this temperature, improve gas production efficiency, and increase methane production. Staying in a high temperature environment for a period of time can also promote the elimination of harmful bacteria, reduce adverse effects on the system, and ensure the stability and efficiency of the digestion process. In addition, it is also beneficial to help the system adapt to temperature changes, improve system stability, and ensure the normal operation of the system.

[0044] In some embodiments, the third set temperature is 50±1°C, the first preset time is 1-7 days, the second set temperature is 55±1°C, and the second preset time is 3-5 days.

[0045] At temperatures around 50°C, thermophilic microorganisms are more active and their gas production efficiency is relatively high. For example, some gas-producing bacteria and archaea are more active at this temperature, which can effectively promote methane production. In addition, 50°C is also relatively effective in eliminating some harmful bacteria, which is beneficial for maintaining system stability. In order to further activate some high-temperature-tolerant gas-producing bacteria, the temperature is further raised to 55±1°C. Because at a high temperature of around 55°C, some specific gas-producing microorganisms may exhibit better metabolic activity, thereby further increasing methane production and improving gas production efficiency. Staying at a temperature of around 50°C for 1-7 days and then staying at around 55°C for 3-5 days can gradually stabilize the gas production process and ensure the continuity and stability of gas production.

[0046] In some embodiments, the first heating rate is preferably 1-3°C, and the second heating rate is preferably 1-5°C.

[0047] The heating rate should not be too high or too low. The first heating rate is 1-3°C, which can gradually increase the internal temperature of the system, activate the metabolic activity of microorganisms, and provide them with a more suitable growth environment. The second heating rate is 1-5°C, which can increase the activity of gas-producing bacteria while ensuring system stability, further increasing biogas production.

[0048] In some embodiments, the first set temperature is 35±1° C. At this temperature, mesophilic anaerobic digestion can be performed.

[0049] This example develops a temperature-shifting anaerobic digestion mode in a semi-continuous anaerobic digestion system, where the temperature varies between 35°C and 55°C, utilizing a high temperature environment to promote the proliferation of efficient methanogens and the degradation and conversion of substrates.

[0050] In some of the embodiments, natural cooling is used for cooling, that is, the temperature is reduced from the second set temperature of about 55°C to the first set temperature of about 35°C by natural cooling. This is not only energy-saving, environmentally friendly, and easy to implement, but also has a stable cooling effect. More importantly, the reproduction rate of the efficient methanogen Methanosarcina microorganisms will not fluctuate greatly due to sudden cooling. The stable cooling process helps to maintain the stability of the internal environment of the system and reduce the adverse effects of sudden temperature fluctuations on the microbial community. At the same time, the stable cooling process also helps to control the generation and emission of harmful gases in the anaerobic digestion system, reduce the generation of malodors, and improve the air quality of the surrounding environment.

[0051] It should be noted that, in some other implementations, the temperature can of course be lowered by manual control, making the temperature lowering adjustable.

[0052] In some embodiments, the digestate is obtained by mixing anaerobic fermentation inoculum with a prepared organic waste slurry, and the organic waste slurry is obtained by mixing organic waste with water or the supernatant of returned biogas slurry, and the organic waste includes one or more of livestock and poultry manure, food waste and biomass.

[0053] The organic waste of this embodiment can be one or a mixture of biomass such as livestock and poultry excrement, food waste or straw, etc. After the organic waste is mixed with water, etc., it is mixed with anaerobic fermentation inoculum to obtain digestion liquid, and anaerobic fermentation can be carried out.

[0054] In some embodiments, the organic waste slurry has a dry matter concentration (TS) of 3% to 15%.

[0055] Within a certain range, increasing the dry matter concentration of the organic waste slurry can provide more organic matter for microbial metabolism, thereby increasing gas production and methane production. However, an excessively high TS may increase the viscosity of the waste slurry, potentially affecting the adequate mixing of the waste slurry with microorganisms, water, and gas, thereby reducing mass transfer efficiency within the reactor. Furthermore, an excessively high TS may also limit the rate at which microorganisms degrade organic matter, thereby reducing gas production efficiency and methane production. In this embodiment, the TS of the organic waste slurry is designed within the above range to ensure the stability and efficiency of the anaerobic digestion process.

[0056] In some embodiments, the volume ratio of the anaerobic fermentation inoculum to the organic waste slurry is 1:1-3.

[0057] An appropriate amount of anaerobic fermentation inoculum can provide a rich active microbial population, help to quickly start the anaerobic digestion process, and promote the degradation of organic waste and methane production.

[0058] In this embodiment, by mixing the inoculum and the waste slurry in the above ratio, the startup time of the reactor can be accelerated, the stabilization time of the system operation can be shortened, and the degradation efficiency of organic matter can be improved, thereby increasing the biogas production.

[0059] In some embodiments, after returning to the first anaerobic digestion step, the process further includes: cyclically performing step S2 and step S1.

[0060] That is, after the digestate undergoes the first anaerobic digestion at the first set temperature, it is heated to the second set temperature and undergoes the second anaerobic digestion, then cooled to the first set temperature, continues the first anaerobic digestion, then heated to the second set temperature for the second anaerobic digestion, and then cooled, and the cycle continues.

[0061] This embodiment adopts a cyclic temperature change process. By repeatedly performing temperature change treatment, different types of microbial communities can be activated, the growth and metabolic activity of microorganisms can be promoted, and the metabolic pathways of microorganisms can be optimized, thereby increasing the degradation efficiency of organic matter and improving biogas production. At the same time, constantly changing temperature conditions can also promote the growth and reproduction of different types of microorganisms, improve the stability of the system, reduce the impact of external environmental changes on the system, and help maintain the diversity and stability of the microbial community within the system. In addition, multiple fermentation treatments also promote the complete degradation of organic matter, reduce the accumulation of residues, and thus reduce the amount of waste residue after treatment.

[0062] An embodiment of the present invention further provides a temperature-variable anaerobic digestion system for implementing the temperature-variable semi-continuous anaerobic digestion method as described in any one of the above items, comprising:

[0063] An anaerobic digester, used for supplying digestate for a first anaerobic digestion and a second anaerobic digestion;

[0064] The temperature control unit is used to control the temperature of the digestive liquid in the anaerobic digester, including raising the temperature of the digestive liquid to a first set temperature, raising the temperature of the digestive liquid from the first set temperature to a second set temperature, and lowering the temperature of the digestive liquid from the second set temperature to the first set temperature.

[0065] In addition, it should be understood that the variable temperature anaerobic digestion system also includes a feeding unit, a gas collection unit, a digestion liquid analysis unit, etc., so that the variable temperature semi-continuous anaerobic digestion method can be effectively implemented to improve the efficiency and gas production of organic waste treatment.

[0066] The system of this embodiment can perform cyclic temperature control between 35°C and 55°C, which can not only significantly increase the proportion of Methanosarcina in the anaerobic system, but also promote the hydrolysis of the substrate and the conversion of volatile acid substances in the intermediate products, which is of great significance for promoting the development of renewable energy and carbon emission reduction.

[0067] The present invention is further described below with reference to specific embodiments.

[0068] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0069] Unless otherwise specified, the materials and reagents used in the following examples can be obtained from commercial sources.

[0070] Example 1

[0071] This embodiment provides a method for semi-continuous anaerobic digestion of pig manure or cow manure at a variable temperature, comprising the following steps:

[0072] The semi-continuous anaerobic digestion system is started by loading a certain amount of inoculum into the anaerobic digester. Dry pig manure or dehydrated pig manure (dried cow manure or dehydrated cow manure) is diluted with tap water to a slurry with a dry matter concentration (TS) of 8%. Then, the diluted pig manure (cow manure) slurry is added at a ratio of 1 / 2 of the total inoculum in the anaerobic digester. The variable temperature control system is activated. After the material temperature in the anaerobic digester reaches 35±1°C, this temperature is maintained for anaerobic digestion for several days. The gas production is observed daily. After the peak daily gas production, a pig (cow) manure slurry (TS = 8%) is added to the digester daily at a ratio of 1 / 30 of the total volume of the digester liquid. When the total volume of the digester liquid reaches the designed working volume, the same volume of liquid is synchronously added and removed daily to maintain the total volume of the digester liquid constant.

[0073] Gradual increase in the organic load of the anaerobic digestion system: The initial hydraulic retention time (HRT) is set to 30 days, and the feed TS is set to 8%. After running for 1-2 HRTs, if the gas production performance is stable, the feed TS is gradually increased or the HRT is lowered. The upper and lower limits of the two are 15% and 10 days, respectively, that is, the maximum feed TS does not exceed 15%, and the minimum HRT does not exceed 10 days; during this period, the discharge pH is continuously observed. When it shows a rapid decline, the current operating parameters are no longer changed.

[0074] Variable temperature anaerobic digestion: the temperature is increased from 35°C to 50°C at a rate of 3°C per day. When it reaches 50°C, the digestion temperature is kept unchanged for 3 days until the gas production stabilizes. The temperature is then increased to 55°C at a rate of 2.5°C per day. After reaching 55°C, it is kept for 5 days before entering the cooling stage. The temperature in the cooling stage is not artificially controlled (i.e., it is cooled by natural heat dissipation). The cyclic variable temperature digestion process ends when the temperature reaches 35°C.

[0075] Mesophilic anaerobic digestion stage: When the temperature reaches 35°C, mesophilic anaerobic digestion is carried out. When the gas production of the reactor or the pH value of the discharge shows a significant decrease, the temperature change cycle is repeated.

[0076] Example 2

[0077] This embodiment provides a method for semi-continuous anaerobic digestion of chicken manure at variable temperature, comprising the following steps:

[0078] The semi-continuous anaerobic digestion system is started by loading a certain amount of inoculum into the anaerobic digester, diluting dry chicken manure or dehydrated chicken manure with tap water to a slurry with a dry matter concentration (TS) of 3%, and then adding the diluted chicken manure slurry according to 1 / 2 of the total amount of inoculum in the anaerobic digester; starting the variable temperature control system, and maintaining this temperature for several days after the material temperature in the anaerobic digester reaches 35±1°C; observing the gas production every day, and after the peak of daily gas production, adding chicken manure slurry (TS=3%) at 1 / 10 of the total feed volume in the digester every day. When the total volume of the digested feed liquid in the digester reaches the designed working volume, the same volume of feed liquid is synchronously introduced and removed every day to maintain the total volume of the feed liquid in the digester constant.

[0079] Gradual increase in the organic load of the anaerobic digestion system: The initial hydraulic retention time (HRT) is set to 10 days, and the feed TS is set to 3%. After running for 1-2 HRTs, if the gas production performance is stable, the feed TS is gradually increased or the HRT is lowered, with the upper and lower limits of 6% and 5 days respectively. During this period, the discharge pH is continuously observed. When it shows a rapid decline, the current operating parameters are no longer changed.

[0080] Variable temperature anaerobic digestion: the temperature is increased from 35°C to 50°C at a rate of 3°C per day. When it reaches 50°C, the digestion temperature is kept unchanged for 3 days until the gas production stabilizes. The temperature is then increased to 55°C at a rate of 2.5°C per day. After reaching 55°C, it is kept for 3 days before entering the cooling stage. The temperature in the cooling stage is not artificially controlled (i.e., it is cooled by natural heat dissipation). The cyclic variable temperature digestion process ends when the temperature reaches 35°C.

[0081] Mesophilic anaerobic digestion stage: When the temperature reaches 35°C, mesophilic anaerobic digestion is carried out. When the gas production of the reactor or the pH value of the discharge shows a significant decrease, the temperature change cycle is repeated.

[0082] Example 3

[0083] This embodiment provides a method for semi-continuous anaerobic digestion of food waste at variable temperature, comprising the following steps:

[0084] The semi-continuous anaerobic digestion system is started by loading a certain amount of inoculum into the anaerobic digester. Dilute the food waste with tap water to a slurry with a dry matter concentration (TS) of 6%. Then, add the diluted food waste slurry at a rate of 1 / 2 of the total inoculum in the anaerobic digester. The warming system is activated. After the temperature of the material in the anaerobic digester reaches 35±1°C, this temperature is maintained for several days of anaerobic digestion. The gas production is observed daily. After the peak daily gas production, 1 / 40 of the total volume of food waste slurry (TS = 6%) is added to the digester daily. Once the total volume of the digested liquid in the digester reaches the designed working volume, the same volume of liquid is added and removed daily to maintain a constant total volume.

[0085] Gradual increase in the organic load of the anaerobic digestion system: The initial hydraulic retention time (HRT) is set to 40 days, and the feed TS is set to 6%. After running for 1-2 HRTs, if the gas production performance is stable, the feed TS is gradually increased or the HRT is lowered, with the upper and lower limits of 12% and 20 days respectively. During this period, the discharge pH is continuously observed. When it shows a rapid decline, the current operating parameters are no longer changed.

[0086] Variable temperature anaerobic digestion: the temperature is raised from 35°C to 50°C at a rate of 2°C per day. When it reaches 50°C, the digestion temperature is kept unchanged for 5 days until the gas production stabilizes. The temperature is then raised to 55°C at a rate of 2.5°C per day. After reaching 55°C, it is kept for 5 days before entering the cooling stage. The temperature in the cooling stage is not manually controlled, and the cyclic variable temperature digestion process ends when the temperature reaches 35°C.

[0087] Mesophilic anaerobic digestion stage: When the temperature reaches 35°C, mesophilic anaerobic digestion is carried out. When the gas production of the reactor or the pH value of the discharge shows a significant decrease, the temperature change cycle is repeated.

[0088] Control Example

[0089] The difference between this comparative example and Example 1 is that continuous mesophilic anaerobic digestion is adopted, and the steps are as follows:

[0090] An anaerobic digester is loaded with a certain amount of inoculum. Dry pig manure or dehydrated pig manure (dried cow manure or dehydrated cow manure) is diluted with tap water to a slurry with a dry matter concentration (TS) of 8%. Then, the diluted pig manure (cow manure) slurry is added according to 1 / 2 of the total inoculum in the anaerobic digester. After the material temperature in the anaerobic digester reaches 35±1°C, this temperature is maintained for anaerobic digestion for several days. The gas production is observed daily. After the daily gas production peak has passed, a pig (cow) manure slurry (TS = 8%) is added to the digester every day at a volume of 1 / 30 of the total feed volume in the digester. When the total volume of the digested feed in the digester reaches the designed working volume, the same volume of feed is synchronously added and removed every day to maintain a constant total volume of the feed in the digester.

[0091] The initial hydraulic retention time (HRT) is set to 30 days, and the feed TS is set to 8%. After running for 1-2 HRTs, if the gas production performance is stable, the feed TS is gradually increased or the HRT is decreased. The upper and lower limits of the two are 15% and 10 days, respectively, that is, the maximum feed TS does not exceed 15%, and the minimum HRT does not exceed 10 days. During this period, the discharge pH is continuously observed. If it shows a rapid decline, the current operating parameters are no longer changed.

[0092] Effect Examples

[0093] The proportion of the dominant methanogenic species in the methanogenic bacteria community in Example 1 and the control example is as follows: Figure 2 As shown in the figure, the secondary vertical axis represents the P value of several dominant methanogenic species, which is the credibility of statistical analysis, and P < 0.05 indicates statistical significance. Figure 3 shown.

[0094] Depend on Figure 2 It can be seen that the proportion of Methanosarcina in the embodiment is greatly increased compared with the proportion in the control example. It can be seen that through temperature-variable anaerobic digestion, Methanosarcina can be greatly expanded (accounting for nearly 40% in methanogenic archaea, more than 1 times that of the control example), which promotes the degradation and conversion of volatile organic acid substances such as acetic acid accumulated in the system, and purifies and optimizes the anaerobic digestion environment and microbial colony structure, thereby improving the methane production efficiency and load tolerance of the anaerobic digestion system.

[0095] Depend on Figure 3 As can be seen, the variable temperature digestion group (Example 1) produced higher gas than the constant temperature control group (reference example). Under high organic loading conditions, Example 1 maintained a stable methane production of approximately 400 mL / g VS compared to the blank control group, demonstrating the anaerobic system's strong tolerance to high loads. In contrast, the control group's methane production dropped to approximately 150 mL / g VS, indicating a significant decrease in conversion capacity under high-load operation. This method, which eliminates the need to alter the existing biogas plant's structural model or add redundant facilities, has broad application prospects.

[0096] This embodiment performs variable temperature anaerobic digestion, which is more energy-saving than continuous high-temperature anaerobic digestion, and has higher methane production and substrate utilization efficiency than continuous medium-temperature anaerobic digestion. It is easy to use in engineering practice and has broad industrial application prospects.

[0097] Although the present invention is disclosed as above, the protection scope of the present invention is not limited thereto. Those skilled in the art may make various changes and modifications without departing from the spirit and scope of the present invention, and these changes and modifications will fall within the protection scope of the present invention.

Claims

1. A variable temperature semi-continuous anaerobic digestion method, characterized in that: include: The first anaerobic digestion step comprises: subjecting the digestate to a first set temperature for a first anaerobic digestion; The second anaerobic digestion step comprises: raising the temperature of the digestion liquid to a second set temperature at a preset heating rate, and performing second anaerobic digestion at the second set temperature for a second preset time; Cooling the digestate to the first set temperature by natural cooling, returning to the first anaerobic digestion step, and cyclically performing the second anaerobic digestion step and the first anaerobic digestion step; the first set temperature is 35±1°C; Raising the temperature of the digestive fluid to a second set temperature according to a preset heating rate includes: Raising the temperature of the digestive liquid to a third set temperature at a first heating rate, keeping the digestive liquid at the third set temperature for a first preset time, and then raising the temperature to the second set temperature at a second heating rate; The third set temperature is 50±1°C, the first preset time is 1-7 days, the second set temperature is 55±1°C, and the second preset time is 3-5 days.

2. The temperature-variable semi-continuous anaerobic digestion method according to claim 1, characterized in that: The digestate is obtained by mixing anaerobic fermentation inoculum with prepared organic waste slurry. The organic waste slurry is obtained by mixing organic waste with water or supernatant of returned biogas slurry. The organic waste includes one or more of livestock and poultry manure, restaurant kitchen waste and straw.

3. The temperature-variable semi-continuous anaerobic digestion method according to claim 2, characterized in that: The dry matter concentration of the organic waste slurry is 3%-15%.

4. A temperature-variable anaerobic digestion system, characterized in that: The method for implementing the temperature-variable semi-continuous anaerobic digestion method according to any one of claims 1 to 3 comprises: An anaerobic digester, used for supplying digestate for a first anaerobic digestion and a second anaerobic digestion; The temperature control unit is used to control the temperature of the digestive liquid in the anaerobic digester, including raising the temperature of the digestive liquid to a first set temperature, raising the temperature of the digestive liquid from the first set temperature to a second set temperature, and lowering the temperature of the digestive liquid from the second set temperature to the first set temperature.

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