A process for preparing methane by anaerobic fermentation of discarded wolfberry fruit

Abstract

The invention discloses a process for preparing methane by anaerobic fermentation of discarded wolfberry fruit, which comprises crushing the discarded wolfberry fruit into pulp, taking back the inoculated sludge and culturing under medium-temperature anaerobic conditions to reduce the background methane production; then adding the obtained discarded wolfberry fruit pulp to the inoculated sludge for anaerobic fermentation to prepare methane, and the anaerobic fermentation conditions are: the value of the organic load OL is 14-16.71g VS / L, the value of the inoculation ratio I / S is 2.4-2.5, the initial pH value is 7.72-8, and the reaction temperature is 35-37°C. In this way, a cumulative methane production of up to 430.03mL / g VS can be obtained, and the obtained discarded wolfberry fruit methane production is greater than that of other commonly used fermentation raw materials, indicating that it has more obvious advantages as an anaerobic fermentation raw material, and has greater methane production potential, which can not only explore new potential raw materials for biogas projects, but also provide new ways and technical basis for the resource utilization of discarded wolfberry fruit.

Description

Technical Field

[0001] The invention relates to the technical field of methane preparation, and in particular to a method for preparing methane by using organic waste. Background Art

[0002] At present, the annual planting area of ​​wolfberry in China is about 1.5 million mu, with an output of about 420,000 tons. With the continuous development and growth of the wolfberry industry, the amount of discarded wolfberry fruit (defective fruit, rotten fruit, naturally fallen fruit, and by-products such as residues in the processing process) is also increasing. It is rich in nutrients such as polysaccharides and organic acids. It is estimated that the annual output of discarded wolfberry fruit in China is nearly 100,000 tons. A large amount of discarded wolfberry fruit will cause environmental pollution and waste of resources.

[0003] Anaerobic fermentation to produce methane is a clean and efficient way to utilize waste resources. It can not only reduce environmental pollution and produce high-quality organic fertilizer, but also be used as energy for rural life and promote the development of ecological agriculture. However, no process for anaerobic fermentation of discarded wolfberry fruit to produce methane has been found so far. In addition, since discarded wolfberry fruit is significantly different from other existing raw materials for anaerobic fermentation to produce methane, it is impossible to directly apply the existing methane production process to discarded wolfberry fruit for methane production. Therefore, it is necessary to carry out anaerobic fermentation research on discarded wolfberry fruit to discover new potential raw materials for biogas projects, and at the same time provide new ways and technical basis for the resource utilization of discarded wolfberry fruit. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a process for preparing methane by anaerobic fermentation of discarded wolfberry fruits, which has mild preparation conditions, simple preparation steps, convenient operation and high methane yield, in view of the shortcomings of the prior art.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solution: a process for preparing methane by anaerobic fermentation of discarded wolfberry fruit, characterized in that the process is carried out according to the following steps:

[0006] Step 1, crushing the discarded wolfberry fruit into pulp, taking back the inoculated sludge and culturing it under mesophilic anaerobic conditions to reduce background methane production;

[0007] Step 2: Add the discarded wolfberry fruit pulp obtained in step 1 to the inoculated sludge for anaerobic fermentation to produce methane. The conditions for anaerobic fermentation are as follows: the organic load OL (the amount of organic matter put into the fermentation system per unit volume) is 14-16.71 gVS / L, the inoculation ratio I / S (the ratio of the inoculated sludge to the material added in the fermentation system, based on the amount of volatile substances, i.e. VS) is 2.4-2.5, the initial pH is 7.72-8, and the reaction temperature is 35-37°C.

[0008] In step 2, after the waste wolfberry fruit and inoculated sludge are added, the initial pH of the system is adjusted with 1 mol / L sodium hydroxide solution.

[0009] In step 2, 50 mg / g of sodium bicarbonate is added as a buffer based on the dry mass of the discarded wolfberry fruit to improve the buffering capacity of the fermentation system and prevent the fermentation system from becoming acidic.

[0010] The discarded wolfberry fruits in step 1 are first frozen at -20°C to keep them fresh, or they can be prepared immediately after being retrieved, without the need for freezing.

[0011] Among them, the characteristics of discarded wolfberry fruit and inoculated sludge are as follows:

[0012]

[0013] Anaerobic fermentation mainly consists of four stages: hydrolysis, acidification, acetification and methanogenesis. Hydrolysis is the process by which large molecular organic polymers in the substrate (such as carbohydrates, proteins and fats) are reduced to smaller substances (such as monosaccharides, amino acids and fatty acids). During the acidification process, acidogenic bacteria further decompose the hydrolyzed raw materials to produce NH3, CO2, H2, H2S, organic acids, short-chain fatty acids and some other trace by-products. In the acetification stage, organic acids such as propionic acid and butyric acid can be oxidized and decomposed by microorganisms to produce acetic acid and H2; at the same time, H2 and CO2 can also be converted into acetic acid under certain conditions. The last stage is the methanogenesis process. Methanogens can use the final products of the previous steps as substrates to produce methane, which mainly includes two pathways: acetic acid nutrition (1) and hydrogen nutrition (2). The reactions are as follows:

[0014] CH3COOH →CH4 + CO2 (1)

[0015] CO2 + 4H2 →CH4 + 2H2O (2)

[0016] The present invention uses discarded wolfberry fruit as a raw material, and obtains a methane yield of up to 430.03 mL / g VS by optimizing the anaerobic fermentation process parameters. The obtained methane yield of discarded wolfberry fruit is greater than that of other commonly used fermentation raw materials, indicating that using it as an anaerobic fermentation raw material has more obvious advantages and greater methane production potential. It can not only explore new potential raw materials for biogas projects, but also provide a new approach and technical basis for the resource utilization of discarded wolfberry fruit. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a bar graph showing the effect of OL on methane production from discarded wolfberry fruits;

[0018] Figure 2It is a bar graph showing the effect of I / S on the methane production of discarded wolfberry fruit;

[0019] Figure 3 It is a bar graph showing the effect of initial pH on methane production from discarded wolfberry fruit;

[0020] Figure 4 Response surface diagram and contour diagram for the interaction between I / S and OL;

[0021] Figure 5 The response surface diagram and contour diagram of the interaction between OL and initial pH;

[0022] Figure 6 Response surface diagram and contour diagram of the interaction between initial pH and I / S. DETAILED DESCRIPTION

[0023] The present invention is further described below by means of specific implementation modes in conjunction with the accompanying drawings, and the implementation process is as follows:

[0024] 1. Prepare experimental materials such as discarded wolfberry fruit and inoculated sludge; prepare experimental reagents such as sodium bicarbonate, sodium hydroxide, and ultrapure water; prepare instruments such as an oven, a muffle furnace, a pH meter, an electronic balance, and a methane production potential tester.

[0025] Experimental materials: Wasted wolfberry fruits were collected from Nuomuhong Farm in Qinghai Province and stored at -20℃. The discarded wolfberry fruits were crushed into pulp during the experiment. The inoculated sludge was collected from an agricultural biogas digester of a certain agricultural limited liability company in Qinghai Province, which used cow dung as raw material and operated stably. After collection, it was cultured under anaerobic conditions at medium temperature (35±1)℃ for 7 days to reduce the background methane production. The raw material characteristics are shown in Table 1.

[0026]

[0027] 2. The drying method and the calcination method were used to determine the total solids (baking in an oven at 105°C for 6 h) and volatile solids (calcining in a muffle furnace at 575°C for 2 h) of discarded wolfberry fruits and inoculated sludge.

[0028] 3. Medium-temperature batch anaerobic fermentation experiment: The anaerobic fermentation experiment was carried out using the MultiTalent 203 fully automatic methane potential tester. The specific amount of material (waste wolfberry fruit and inoculated sludge) was calculated according to the organic load and inoculation ratio, and the waste wolfberry fruit and inoculated sludge were added and inoculated. After the addition of the materials, the initial pH of the system was adjusted with a 1 mol / L sodium hydroxide solution. In order to further prevent the acidification of the fermentation system, 50 mg / g of sodium bicarbonate was added as a buffer to improve the buffering capacity of the fermentation system based on the dry matter mass (TS) of the waste wolfberry fruit. Three parallels were set for each group of experiments; at the same time, a blank treatment was set in which only inoculated sludge was added. During the operation, the methane potential tester automatically recorded the methane production every day and collected the cumulative methane production data. The cumulative methane production of the following treatments is the value after subtracting the blank control. The experimental reaction temperature was 35.0±0.5℃, and the fermentation cycle was 30 days.

[0029] 4. Single-factor experimental design: The organic load (OL) was selected as 5, 10, 15, 20 and 25 g VS / L; the inoculation ratio (I / S) was 1.0, 1.5, 2.0, 2.5 and 3.0; the initial pH was 7.0, 7.5, 8.0, 8.5 and 9.0. In the single-factor experiment, the fixed conditions were OL 15 g VS / L, I / S 2.0, and initial pH 8.0. The anaerobic fermentation experiment was carried out using a fully automatic methane potential tester, and 3 parallel experiments were set up for each group of experiments.

[0030] 5. Response surface method optimization design: According to the results of the single factor experiment, the maximum point of the cumulative methane production in each single factor was set as the center point. With OL, I / S and initial pH as independent variables and cumulative methane production as the response value, the three-factor three-level response surface method optimization experiment was designed using the Box-Behnken method using the DesignExpert 12 software. The anaerobic fermentation experiment was carried out using a fully automatic methane potential tester. There were 17 groups of treatments in the experiment, with 3 parallels in each group.

[0031] 6. Analysis of experimental results

[0032] (1) Single factor experimental analysis

[0033] Depend on Figure 1-Figure 3It can be seen that when I / S and initial pH are constant, the cumulative methane production is the highest when OL is 15 g VS / L, and the lowest when OL is 25 g VS / L. When OL is low, there is less organic matter available for anaerobic microorganisms to decompose and utilize, so the cumulative methane production is low; when OL is too high, a large amount of soluble organic matter cannot be decomposed and utilized by microorganisms in time, and accumulates in the fermentation system, which also reduces methane production. When OL and initial pH are the same, when I / S is greater than 2.0, the cumulative methane production is high and the difference is not large; when I / S is 1.0 and 1.5, the methane production is low, which may be due to the fact that there are fewer anaerobic microorganisms that can utilize and transform organic matter in the fermentation system. When investigating the effect of initial pH on the methane production performance of discarded wolfberry fruit, it was found that when the initial pH was between 7.5 and 8.5, the cumulative methane production was higher; when the initial pH was lower, the system was easily acidified due to the high sugar content of discarded wolfberry fruit, and the cumulative methane production was lower; and when the initial pH was higher, the system was not within the optimal growth range of methanogens, which also led to a decrease in methane production. From the results of the single factor experiment, it can be seen that different OL, I / S and initial pH will affect the generation of methane. Within the range of the experimental parameter settings, OL was selected as 15 g VS / L, I / S was 2.0, and initial pH was 8.0. Under these conditions, the highest cumulative methane production of anaerobic fermentation of discarded wolfberry fruit can be obtained.

[0034] (2) Response surface regression model and variance analysis

[0035] According to the results of the single-factor experiment, the Box-Behnken method was used to design the response surface method optimization experiment with OL (10, 15, 20 g VS / L), I / S (1.5, 2, 2.5) and initial pH (7.5, 8, 8.5) as independent variables and cumulative methane production as the response value, and the experimental factor level coding table (Table 2) was obtained. Based on the design in Table 2, the specific addition amount of discarded wolfberry fruit and inoculated sludge was calculated, as shown in Table 3; at the same time, the variance analysis was performed based on Table 2 using Design-Expert 12 software, and the results are shown in Table 4.

[0036] As shown in Table 2, the cumulative methane production in the 17 experimental groups ranged from 155.09 to 423.92 mL / g VS, of which 11 groups had cumulative methane production above 300 mL / g VS, and the highest value was 423.92 mL / g VS. The cumulative methane production of each experimental group was set as the response value Y , different levels of coding values ​​OL( A )、I / S( B ), initial pH( C ) as the independent variable, and establish the fitting equation. Bring it into Design-Expert 12 software to establish the multivariate quadratic regression equation as follows:

[0037] Y =385.74-24.04 A +71.73 B +3.14 C +40.12 AB +14.67 AC -30.57 BC -54.38 A 2 -34.02 B 2 +39.22 C 2 (1)

[0038]

[0039]

[0040] From Table 4, we can see that the model F The value is 22.32, P <0.01, indicating that the regression effect of the entire model is significant. According to the software calculation results, R 2 is 0.9663, indicating that this model can explain 96.63% of the response value variation. P =0.8103>0.05, there is no significant difference, indicating that the model fit is good, and the residual of the model may be caused by random errors. R 2 0.8527 and adjusted R 2 The difference of 0.9230 is less than 0.2, and the sufficient precision of 16.0148 > 4 indicates that the model can be used to navigate the design space. In summary, this regression model and equation can analyze and predict the cumulative methane production of discarded wolfberry fruit.

[0041] By various factors P The value is known, A The response value is significantly affected. B Very significant impact, C The impact is not significant; AB The interactive effect of AC The interaction effect was not significant. BC The interactive effect was significant; A 2 , C 2 The curved surface effect is very significant. B 2The surface effect is significant. The absolute value of the coefficient of each factor in the regression model can reflect the strength of the factor's influence on the response value. From formula (1), it can be seen that the absolute value of the coefficient of the three influencing factors is B > A > C , indicating that the overall influence of each factor on the cumulative methane production is in the following order: I / S>OL>initial pH.

[0042]

[0043] Note:* P <0.05Significant difference, ** P <0.01Very significant difference

[0044] (3) Response surface optimization analysis

[0045] The response surface methodology was used to optimize the cumulative methane production of discarded wolfberry fruit, and the response surface and contour map were drawn to investigate the effects of the interaction between different factors on the cumulative methane production ( Figure 4-Figure 6 ). The sensitivity of the response value to the interaction of factors is determined by the steepness of the slope of the response surface. That is, the steeper the slope, the more sensitive the interaction; the gentler the surface, the smaller the impact of the interaction between factors. At the same time, the contour map can obtain the maximum response value in the interaction.

[0046] Figure 4 The response surface and contour plots of the interaction between OL and I / S. Figure 4 a It can be seen that the slope of the response surface between OL and I / S is large, indicating that OL and I / S have a high interaction effect in anaerobic fermentation. When OL is constant, the cumulative methane production increases with the increase of I / S; when I / S<2.0, the cumulative methane production decreases with the increase of OL. From the contour map, it can be seen that when OL is 12 g VS / L ~18 g VS / L and I / S is 2.0~2.5, the cumulative methane production of anaerobic fermentation of abandoned wolfberry fruit is higher ( Figure 4 b).

[0047] Figure 5 The response surface and contour plots between OL and initial pH. When OL interacted with initial pH, the cumulative methane production showed a trend of first increasing and then decreasing. When OL>18, the cumulative methane production did not change significantly with the change of initial pH ( Figure 5 a). This is because, like the fermentation system of other fruit and vegetable waste, the sugar content of discarded wolfberry fruit is high. When OL is high, the anaerobic fermentation system is prone to acidification. The initial pH alone cannot neutralize the acid produced during the fermentation process, thereby inhibiting the production of methane. Figure 5b It can be seen that when the OL was between 12 g VS / L and 16 g VS / L and the initial pH was between 7.7 and 8.3, the cumulative methane production of anaerobic fermentation of discarded wolfberry fruit was higher.

[0048] like Figure 6 As shown in the figure, when I / S is constant, with the increase of initial pH, the cumulative methane production shows a trend of first increasing and then decreasing; when the initial pH is constant, the cumulative methane production increases with the increase of I / S ( Figure 6 a) From Figure 6 b It can be seen that when I / S and initial pH are at lower levels, the cumulative methane production is lower; when I / S is between 2.0 and 2.5, the cumulative methane production is higher. In summary, I / S has the greatest impact on the methane production performance of discarded wolfberry fruit.

[0049] The optimal process conditions were obtained through model optimization, namely OL of 14.76 g VS / L, I / S of 2.44, and initial pH of 7.82. The anaerobic fermentation experiment was repeated three times with the process parameters to verify the reliability and accuracy of the model. The average value of the cumulative methane production obtained in the experiment was 430.03 mL / g VS, which was very close to the model prediction value (425.54 mL / g VS), with a relative error of less than 5%, indicating that the verification model was effective.

[0050] The results were compared with other fermentation raw materials: the cumulative methane production of medium-temperature batch anaerobic fermentation of various types of straw was mostly between 124.77 and 258.07 mL / g VS, urban sludge was mostly between 248.49 and 340.33 mL / g VS, various types of animal feces were mostly between 282.53 and 398 mL / g VS, and kitchen waste was mostly between 326.68 and 373 mL / g VS, while the cumulative methane production of abandoned wolfberry fruit after optimization was 430.03 mL / g VS, and the cumulative methane production of other fermentation raw materials did not exceed the invention. It can be seen that compared with commonly used raw materials for anaerobic fermentation such as straw, urban sludge, kitchen waste and feces, the present invention uses abandoned wolfberry fruit with higher methane production potential, and it has more obvious advantages as an anaerobic fermentation raw material.

[0051] The present invention has been described in detail above. The above description is only a preferred embodiment of the present invention and should not limit the scope of implementation of the present invention. That is, all equivalent changes and modifications made within the scope of this application should still fall within the scope of the present invention.

Claims

1. A process for preparing methane by anaerobic fermentation of discarded wolfberry fruit, characterized in that: Follow these steps: Step 1, crushing the discarded wolfberry fruit into pulp, taking back the inoculated sludge and culturing it under mesophilic anaerobic conditions to reduce background methane production; Step 2, adding the discarded wolfberry fruit pulp obtained in step 1 to the inoculated sludge for anaerobic fermentation to produce methane, the conditions of the anaerobic fermentation are: the value of the organic load OL is 14-16.71 g VS / L, the value of the inoculation ratio I / S is 2.4-2.5, the initial pH value is 7.72-8, and the reaction temperature is 35-37°C; Among them, the inoculation ratio I / S refers to the ratio of the inoculation sludge to the material added in the fermentation system, based on the amount of volatile substances, that is, VS.

2. The process for preparing methane by anaerobic fermentation of discarded wolfberry fruits according to claim 1, characterized in that: In step 2, after the waste wolfberry fruit and inoculated sludge are added, the initial pH of the system is adjusted with 1 mol / L sodium hydroxide solution.

3. The process for preparing methane by anaerobic fermentation of discarded wolfberry fruits according to claim 1, characterized in that: In step 2, 50 mg / g of sodium bicarbonate is added as a buffer based on the dry mass of the discarded wolfberry fruit to improve the buffering capacity of the fermentation system and prevent the fermentation system from becoming acidic.

4. The process for preparing methane by anaerobic fermentation of discarded wolfberry fruits according to claim 1, characterized in that: The discarded wolfberry fruits in step 1 are first frozen at -20°C to keep them fresh, or are prepared immediately after the discarded wolfberry fruits are retrieved.

5. The process for preparing methane by anaerobic fermentation of discarded wolfberry fruits according to claim 1, characterized in that: The characteristics of discarded wolfberry fruit and inoculated sludge are as follows:

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