Method for decomposing straw by solid-liquid alternation

Abstract

The application discloses a method for decomposing straw by solid-liquid alternation, which adopts a net frame hanging basket to fill in the crushed straw, and under the action of a compound microbial flora, the method utilizes the repeated alternation of solid and liquid, and determines the time and frequency of the liquid fermentation (increasing the microbial quantity) or the solid fermentation (producing enzyme) of the straw immersed in the liquid according to the quantity of the compound microbial flora in the decomposing liquid and the enzyme activity of the Lap, and then the straw is immersed in the liquid again after the solid enzyme production, so that the mass transfer contact between the enzyme and the straw substrate is strengthened, and the straw decomposition rate is greatly improved.

Description

Technical Field

[0001] This invention belongs to the field of agricultural waste straw treatment, specifically relating to a method for alternating solid-liquid decomposition of straw.

[0002] This invention falls under the strategic emerging industries catalogue, specifically category 4.3 of the bio-agriculture industry, and specifically category 4.3.3 of the key areas of bio-fertilizers, including key products and technical services. Specifically, it covers agricultural biogas production and comprehensive utilization, and crop straw return technology. Background Technology

[0003] In my country, agricultural solid waste such as straw has wide applications in two areas. First, biogasification technology, which produces biogas as an energy source while degrading straw and other waste, and the biogas slurry can be used as fertilizer. Biogasification is currently an important way to control pollution and change the rural energy structure, but its implementation coverage is still not very high. Second, composting is also an effective way to digest rural waste such as straw. However, conventional composting methods lack microorganisms with special decomposition functions and cannot effectively digest straw.

[0004] Straw is mainly composed of cellulose, hemicellulose, and lignin. The decomposition of these substances requires the synergistic action of multiple microbial species. Furthermore, because hemicellulose and lignin form a covalently bonded complex that encapsulates cellulose, cellulase has difficulty contacting it, making straw decomposition challenging. Moreover, microbial decomposition of straw requires specific enzymes produced by specific microorganisms. For example, lignin decomposition requires lignin peroxidase (Lip) or laccase (Lac), hemicellulose decomposition requires hemicellulase (such as xylase), and cellulose decomposition requires exoglucose exonuclease, endoglucose exonuclease, and β-1,4-glucanase. Although these enzymes can be produced by different fungi or bacteria, the cost of directly using these enzymes in engineering applications is too high and uneconomical due to limitations in the enzyme production environment and conditions. Conventional solid-state or liquid-state aerobic fermentation methods are prone to substrate inhibition, where the reducing sugars produced during decomposition hinder continuous enzyme production by microorganisms and affect the activity of existing enzymes.

[0005] Under natural conditions, straw decomposes in 6 months to 1.5 years. In order to make better use of the substances in straw, people usually need to use some physical, chemical or biological methods to convert the carbon and nitrogen-containing substances into bioenergy or agricultural nutrient resources. Among the above-mentioned treatment methods, biological methods are the mildest, have a high organic matter conversion rate and no secondary pollution problems.

[0006] In biological methods, the main decomposition methods are solid-state and liquid-state methods, but solid-state fermentation and liquid-state fermentation have some problems.

[0007] The characteristics of solid-state fermentation of straw are low water activity and uneven fermentation. The growth of microorganisms, the absorption of nutrients, and the secretion of metabolic products are uneven throughout the fermentation process. Although solid-state fermentation produces relatively high enzyme yields, the detection and control of fermentation parameters are difficult. Currently, the measurable or adjustable parameters for solid-state fermentation mainly include: culture medium water content, air humidity, CO2 and O2 content, pH value, temperature, and microbial growth. Temperature control is particularly challenging; generally, microorganisms decomposing straw face cell death and enzyme inactivation above 50°C. Furthermore, the gradient in nutrient concentration and the uneven distribution of carbon and nitrogen sources easily lead to a contradiction between microbial growth and enzyme decomposition. Solid-state fermentation using liquid rinsing is essentially still solid-state fermentation. Straw solid-state fermentation uses rinsing to separate the decomposition liquid, which typically has a COD concentration of around 10,000 mg / L. To achieve this concentration, with a water:straw ratio of 10:1, the liquid extraction time requires more than 15 days.

[0008] The characteristics of liquid fermentation of straw include high water activity, relatively easy oxygen supply control, and relatively uniform fermentation. However, with the accumulation of decomposition products, it is very susceptible to product inhibition, leading to reduced enzyme activity and difficulty in sustaining decomposition. The COD concentration of the decomposition liquid is generally around 8000 mg / L, and it usually takes 5-7 days to reach this concentration. Since the conditions required for cell growth and enzyme production are exactly opposite—for example, cell growth requires sufficient carbon and nitrogen sources, while ligninase can only be produced under nitrogen-limited conditions—it is difficult to dynamically control the growth of microorganisms and enzyme production during liquid fermentation.

[0009] To increase the speed at which straw is completely decomposed by microorganisms, it is necessary to find a suitable method that can ensure sufficient microbial growth and the production of enough enzymes, thereby rapidly completing the decomposition of straw, based on the conditions for microbial growth and enzyme production. Summary of the Invention

[0010] To address the aforementioned deficiencies in the existing technology, this invention provides a method for alternating solid-liquid decomposition of straw.

[0011] The object of this invention is achieved in the following manner: A method for alternating solid-liquid decomposition of straw involves treating crushed straw with alkaline ozone, then preparing a decomposition solution with a weight ratio of straw: compound microbial agent: water = 1:0.05:(10-20). The alkaline-ozone-treated straw is placed in a basket, which is then submerged in the decomposition solution. The bottom of the solution is aerated, causing the compound microbial cell count to increase rapidly. Once a set value is reached, the basket is lifted out of the decomposition solution, leaving the straw in a solid-state fermentation state. The microbial cells attached to the straw consume carbon and nitrogen sources, leading to a carbon- and nitrogen-limited state, which stimulates the compound microorganisms to produce enzymes. Once the Lip enzyme activity reaches a set value, the basket is submerged back into the decomposition solution. The process of immersing and lifting the basket in the solution is repeated based on the compound microbial cell count and Lip enzyme activity in the solution. The process is stopped when the straw decomposition reaches 50% or more, or when the COD of the decomposition solution is below 5000 mg / L.

[0012] The set value for the amount of complex microbial cells in the decomposition liquid when the basket is lifted and removed is >10. 8 The enzyme activity was set to >10 u / mg when the basket was submerged again in the decomposition solution.

[0013] The conditions for lifting the basket out of the decomposition solution again are: the basket has been submerged in the decomposition solution for more than 4 hours, and the total number of compound microorganisms is >10. 8 The condition for re-immersing the basket in the decomposition solution is enzyme activity > 10 u / mg.

[0014] If the temperature inside the straw exceeds 50°C, start the submersible pump to extract the liquid from the bottom of the decomposition solution and then pour it back onto the liquid surface or the straw.

[0015] When the COD concentration in the decomposition solution is greater than 20,000 mg / L, release part of the decomposition solution and replenish it with an equal amount of clean water and nitrogen source.

[0016] Each time, half of the total amount of decomposition liquid is released.

[0017] The compound microbial agent includes white rot fungi, Trichoderma viride, and Aspergillus niger, which are mixed in a ratio of 1:0.5:0.1.

[0018] The white-rot bacteria in the compound microbial agent were cultured in PDA medium at 28°C for 48 hours, and then acclimatized in acclimatization medium at 30°C for 72 hours. The acclimatization culture medium contained carbon source glucose or brown sugar, nitrogen source urea, ammonium sulfate and soy milk, inorganic salt magnesium sulfate and dipotassium hydrogen phosphate, compound vitamins, alkaline ozone treatment solution and fly ash leachate, with a carbon-nitrogen ratio of 1:1-2:1.

[0019] The expanded culture medium for the compound microbial agent consists of 10 g / L brown sugar, 100 ml / L soy milk, 2 g / L urea, 2 g / L ammonium sulfate, 2 g / L wheat bran, and 900 ml / L alkaline ozone treatment solution.

[0020] The size of the crushed straw should not exceed 5cm.

[0021] Compared to existing technologies, this invention utilizes a complex microbial community and a repeated alternating solid-liquid process to improve the straw decomposition rate. Practical results show that compared to simple liquid fermentation, solid-state fermentation, or solid-state leaching, this method shortens the straw decomposition time and increases the decomposition rate. Conventional methods typically take at least 30 days to achieve a 50% straw decomposition rate, while this invention achieves 50% decomposition in just 6 days. Furthermore, the COD content of the decomposition liquid reaches 22,000 mg / L after six days using the alternating solid-liquid decomposition method, compared to only around 10,000 mg / L for conventional methods after six days. This represents a straw decomposition rate more than double that of conventional methods.

[0022] This invention can easily achieve large-scale, mass decomposition and processing of agricultural waste—straw. The resulting residue can be further fermented into alcohol through ethanol fermentation, or into biogas, i.e., biogas slurry, through anaerobic fermentation, and has broad application prospects. Attached Figure Description

[0023] Figure 1 It is an invention process flow diagram. Detailed Implementation

[0024] As shown in Figure 1, a method for alternating solid-liquid decomposition of straw involves first treating the pulverized straw with alkaline ozone. The thickness of the pulverized straw is no more than 5 cm, preferably 5 cm. For details of the alkaline ozone treatment, please refer to Chinese Patent 200710139494.1. Then, a decomposition solution is prepared according to a weight ratio of straw:compound microbial agent:water = 1:0.05:10. The prepared decomposition solution is placed in a steel tank or cement pool. The alkaline-ozone-treated straw is then loaded into a wire mesh basket, which is submerged in the decomposition solution. At this point, the bottom of the decomposition solution is aerated to ensure sufficient nutrients and oxygen supply, leading to a rapid increase in the amount of compound microorganisms. When the amount of compound microorganisms reaches a set value of 10... 8 After the concentration of microorganisms reaches 10 μL / mL, the basket is lifted out of the decomposition liquid. The straw is in a solid-state fermentation state. At this time, a large number of microorganisms adhere to the straw, and due to the rapid growth of the microorganisms, they consume carbon and nitrogen sources, entering a carbon- and nitrogen-limited state, thereby stimulating the complex microorganisms to produce a large amount of enzymes. After the Lip enzyme activity reaches the set value of 10 U / mg, the basket is submerged back into the decomposition liquid. The process of immersing the basket in the liquid and lifting it out of the liquid is repeated according to the amount of complex microorganisms and Lip enzyme activity in the decomposition liquid. The conditions for lifting the basket out of the decomposition liquid again are: the time of immersion of the basket in the decomposition liquid is greater than 4 hours and the amount of complex microorganisms is greater than 10 μL / mL. 8 If the number of microorganisms per mL is less than 10 after the basket has been submerged in the decomposition solution for 4 hours, the total number of microorganisms in the complex will not reach 10.8 If the number of microorganisms is 10 / mL, continue soaking the straw until the total number of microorganisms reaches 10. 8 The basket can be lifted out of the decomposition solution again only when the Lip enzyme activity is >10 u / mg. The basket is then re-immersed in the decomposition solution when the Lip enzyme activity is >10 u / mg. The basket immersion and lifting process continues until the straw decomposition reaches 50% or more, or the COD of the decomposition solution is below 5000 mg / L. The criterion for reaching 50% straw decomposition is: the COD concentration of the second portion released from the decomposition solution is greater than 20000 mg / L. Considering maximizing efficiency in a relatively short time, the decomposition reaction can be terminated at this point, significantly improving the usability of straw biomass. In practice, extending the decomposition time to maximize the decomposition effect until the COD of the decomposition solution is below 5000 mg / L is generally time-consuming. Considering time, cost, and effectiveness, this termination condition is generally not recommended.

[0025] During the above process, when the COD concentration in the decomposition liquid exceeds 20,000 mg / L, a portion of the decomposition liquid needs to be released, and an equal amount of clean water and nitrogen source needs to be added. The amount of decomposition liquid released each time should be at least half of the total volume, preferably half. By monitoring the COD concentration in the liquid decomposition liquid and the release of the decomposition liquid, the effect of product inhibition on the continuous decomposition of straw can be mitigated.

[0026] Once the microbial reproduction stabilizes or the temperature rise within the straw exceeds 50°C, the submersible pump is activated to extract the bottom liquid of the decomposition solution and then backwash it onto the surface or the straw. Specifically: when the basket is submerged in the decomposition solution, the submersible pump is activated to extract the bottom liquid and then backwash it onto the surface; when the basket is lifted out of the decomposition solution, the submersible pump is activated to extract the bottom liquid and then backwash it onto the straw. Timely backwashing through temperature monitoring lowers the fermentation temperature, preventing the temperature rise of the solid portion of the straw from exceeding the tolerance range of the microorganisms, and ensuring the environmental conditions for continuous microbial growth and enzyme production.

[0027] The compound microbial agent includes white rot fungi (number: ACCC 30942), Trichoderma viride (number: ACCC 20059) and Aspergillus niger (number: AS3.4309), and the three fungi are mixed in a ratio of 1:0.5:0.1.

[0028] The white-rot fungus in the compound microbial agent is first cultured in PDA medium at 28°C for 48 hours, and then acclimatized in acclimatization medium at 30°C for 72 hours. The acclimatization culture medium contains the carbon source glucose or brown sugar, the nitrogen sources urea, ammonium sulfate and soybean milk, the inorganic salts magnesium sulfate and dipotassium hydrogen phosphate, compound vitamins, alkaline ozone treatment solution, and fly ash leachate, with a carbon-to-nitrogen ratio of 1:1-2:1. The preparation of the alkaline ozone treatment solution is described in invention patent 200710139494.1. Any commercially available fly ash leachate can be used. *Trichoderma viride* and *Aspergillus niger* can be cultured in conventional culture media and are not within the scope of this invention; therefore, they are not described in detail here.

[0029] Traditional methods for culturing white-rot fungi typically use a carbon-to-nitrogen ratio of 20-30:1, although high-carbon cultures (80-100:1) are also used. This invention employs a different carbon-to-nitrogen ratio to expand the culture of white-rot fungi. It uses a high-nitrogen, low-carbon culture medium and broth to allow the white-rot fungi to quickly transition from the adaptation period to the logarithmic growth phase. Experimental results show that the cell mass of the white-rot fungi cultured in Lip culture can reach 3 × 10⁻⁶. 9 In addition, the required time has been shortened from about 5 days in the conventional method to 1-2 days.

[0030] After domestication and cultivation, white rot fungi were mixed with Trichoderma viride and Aspergillus niger at a ratio of 1:0.5:0.1 to form a compound microbial agent. The agent was then expanded using an expansion culture medium composed of 10 g / L brown sugar, 100 ml / L soy milk, 2 g / L urea, 2 g / L ammonium sulfate, 2 g / L wheat bran, and 900 ml / L alkaline ozone treatment solution. The preparation of the alkaline ozone treatment solution is described in invention patent 200710139494.1.

[0031] Since the optimal environmental conditions for microbial reproduction and growth are in a liquid state, and enzyme production can only be achieved under nutrient limitations and the presence of inducers, sufficient contact between the enzyme and the substrate (straw cellulose) is required to complete the enzymatic decomposition process. In the solid state, the mass transfer between the enzyme and the substrate (straw cellulose) is poor. Therefore, this invention employs a combination of three types of microorganisms and a solid-liquid alternation method to optimize both cell growth and enzyme production: after enzyme production is achieved under nutrient limitations during the solid-state fermentation stage, the straw carrying a large amount of enzyme is then immersed in the liquid, ensuring mass transfer contact between the enzyme and the substrate, thus greatly improving the decomposition efficiency. The COD content in the decomposition liquid is highly correlated with the straw decomposition rate; the higher the straw decomposition rate, the higher the COD content in the decomposition liquid. According to the experimental results, the COD of the decomposition liquid of straw soaked in clean water for six days is only 2000-3000 mg / L; the COD of the decomposition liquid treated by conventional decomposition methods for six days reaches about 10000 mg / L; and the COD of the decomposition liquid treated by solid-liquid alternation method for six days can reach more than 22000 mg / L, which is 1-2 times higher than the COD of the traditional method.

[0032] This invention utilizes the combined use of three bacteria to repeatedly switch between liquid and solid states, maximizing the dual advantages of the liquid method (high bacterial growth and good mass transfer between enzyme and substrate) and the solid method (high enzyme production and high enzyme activity). Example

[0033] A 4m×4m×1.5m cement tank was selected as the fermentation container (the length and width of the tank can be adjusted according to the amount of straw to be processed). An aeration head connected to an external air pump was installed at the bottom of the tank, and a submersible pump connected to a distributed sprinkler system at the top was also installed at the bottom. A 4cm aperture plastic mesh frame basket was used to fill the tank with chopped straw. The mixture was prepared according to a weight ratio of straw:compound microbial agent:water = 1:0.05:10. 15t of water and 75kg of compound microbial agent were mixed evenly to form the decomposition liquid. 1.5t of straw was loaded into the plastic mesh frame basket and submerged in the decomposition liquid. Aeration was carried out at the bottom of the decomposition liquid, and the compound microbial cell count rapidly increased to 10. 8 The concentration of microorganisms was measured at 10 μg / mL. The basket was then lifted out of the decomposition solution. After consuming carbon and nitrogen sources, the microorganisms entered a carbon- and nitrogen-limited growth state, producing large amounts of enzyme until the Lip enzyme activity reached the set value of 10 u / mg. The basket was then submerged back into the decomposition solution. After 4 hours, the cell count of the microorganisms was measured to be 10 μg / mL. 8 The concentration of COD in the decomposition liquid was increased to 20,000 mg / L after 4 days. Half of the decomposition liquid was released, and then equal amounts of water, carbon source, and nitrogen source were added. The process of immersing and lifting the basket and straw continued. After 3 days, the COD concentration in the decomposition liquid reached 20,000 mg / L again, and half of the decomposition liquid was released. At this point, the straw decomposition was considered to have reached 50%, and the decomposition reaction was terminated. The decomposition liquid can be further fermented into alcohol to produce alcohol, or it can be fermented into biogas (biogas slurry) through anaerobic fermentation. The straw residue can be used to improve arable land soil. Example

[0034] A circular steel fermentation tank was selected as the fermentation container. The inner wall of the tank was treated with anti-corrosion coating. The tank was 2 meters high and 4 meters in diameter (the diameter of the fermentation tank can be adjusted appropriately according to the amount of straw to be processed, up to a maximum of 10 meters). Aeration heads and submersible pumps were arranged at the bottom of the tank, and the submersible pumps were connected to distributed return spray heads at the top of the tank. The basket was a stainless steel mesh frame basket with a 4cm aperture, used to fill the chopped straw. The method of alternating solid-liquid decomposition of straw was the same as in Example 1.

[0035] 1. The crushed straw is loaded into a wire mesh basket. The time and number of times the straw is immersed in the liquid for liquid fermentation (to increase the amount of microorganisms) or suspended for solid fermentation (to produce enzymes) are determined based on the amount of complex microorganisms and the activity of Lip enzymes in the decomposition liquid. After solid enzyme production, the straw is immersed in the liquid again to enhance the mass transfer contact between the enzyme and the straw substrate, thereby significantly improving the straw decomposition rate.

[0036] 2. In the culture of compound microorganisms, the high-nitrogen culture method (carbon-nitrogen ratio of 1:1 or 2:1) for white-rot fungi allows them to quickly pass through the adaptation period of fungal growth and enter the logarithmic growth phase, thus significantly increasing the cell density.

[0037] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several changes and improvements without departing from the overall concept of the present invention, and these should also be considered within the scope of protection of the present invention.

Claims

1. A method for alternating solid-liquid decomposition of straw, characterized in that: The crushed straw was treated with alkaline ozone. Then, a decomposition solution was prepared according to the weight ratio of straw:compound microbial agent:water = 1:0.05:(10-20). The compound microbial agent included white rot fungi, green Trichoderma, and black Aspergillus, and the three fungi were mixed at a ratio of 1:0.5:0.

1. The alkaline ozone-treated straw was loaded into a basket, which was then submerged in the decomposition solution. The bottom of the decomposition solution was aerated, and the number of compound microbial cells increased rapidly. After reaching the set value, the basket was lifted out of the decomposition solution. The straw was in a solid fermentation state. The growth of the microorganisms attached to the straw consumed carbon and nitrogen sources, leading to a carbon- and nitrogen-limited state, which stimulated the compound microorganisms to produce enzymes. After the Lip enzyme activity reached the set value, the basket was submerged in the decomposition solution again. The operation of immersing the basket in the liquid and lifting it out of the liquid was repeated according to the number of compound microbial cells and Lip enzyme activity in the decomposition solution. The process was stopped when the straw decomposition reached more than 50% or the COD of the decomposition solution was lower than 5000 mg / L. The set value for the amount of complex microbial cells in the decomposition liquid when the basket is lifted and removed is >10. 8 The Lip enzyme activity was set to >10 U / mg when the basket was submerged again in the decomposition solution.

2. The method for alternating solid-liquid decomposition of straw according to claim 1, characterized in that: The conditions for lifting the basket out of the decomposition solution again are: the basket has been submerged in the decomposition solution for more than 4 hours, and the total number of compound microorganisms is >10. 8 The condition for re-submerging the basket in the decomposition solution is that the Lip enzyme activity is >10 U / mg.

3. The method for alternating solid-liquid decomposition of straw according to claim 1, characterized in that: If the temperature inside the straw exceeds 50°C, start the submersible pump to extract the liquid from the bottom of the decomposition solution and then pour it back onto the liquid surface or the straw.

4. The method for alternating solid-liquid decomposition of straw according to claim 1, characterized in that: When the COD concentration in the decomposition solution is greater than 20,000 mg / L, release part of the decomposition solution and replenish it with an equal amount of clean water and nitrogen source.

5. The method for alternating solid-liquid decomposition of straw according to claim 4, characterized in that: Each time, half of the total amount of decomposition liquid is released.

6. The method for alternating solid-liquid decomposition of straw according to claim 1, characterized in that: The white-rot bacteria in the compound microbial agent were cultured in PDA medium at 28°C for 48 hours, and then acclimatized in acclimatization medium at 30°C for 72 hours. The acclimatization medium contained carbon source, nitrogen source, inorganic salt, compound vitamin, alkaline ozone treatment solution and fly ash leachate. The carbon source was glucose or brown sugar, the nitrogen source was urea, ammonium sulfate and soy milk, and the inorganic salt was magnesium sulfate and dipotassium hydrogen phosphate. The carbon-nitrogen ratio was 1:1-2:

1.

7. The method for alternating solid-liquid decomposition of straw according to claim 1, characterized in that: The expanded culture medium for the compound microbial agent consists of 10 g / L brown sugar, 100 ml / L soy milk, 2 g / L urea, 2 g / L ammonium sulfate, 2 g / L wheat bran, and 900 ml / L alkaline ozone treatment solution.

8. The method for alternating solid-liquid decomposition of straw according to claim 1, characterized in that: The size of the crushed straw should not exceed 5cm.

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