Method for co-producing methane organic fertilizer and earthworm animal protein by solid anaerobic fermentation of agricultural and forestry residues

Abstract

A method for co-producing methane, organic fertilizer and earthworm animal protein by solid-state anaerobic fermentation of agricultural and forestry residues, first loosening the lignocellulose structure, then crushing through a 20-mesh sieve to obtain a pretreated raw material, then performing solid-state anaerobic fermentation and coupling earthworm breeding, and finally obtaining methane, organic fertilizer and animal protein. The lignocellulose raw material pretreated by twin-screw extrusion has a loose structure compared with simple crushing, and has a synergistic effect on the methane production rate of solid-state anaerobic fermentation. The method provided by the present invention can reduce the wastewater pollution problem caused by traditional wet anaerobic fermentation, and the amount of wastewater is reduced by 63.7%-82.2% compared with the traditional wet anaerobic fermentation carried out at 5% solid content.

Description

Technical Field

[0001] The invention belongs to environmental protection technology in the field of solid waste energy, fertilizer and resource utilization, and specifically relates to a method for co-producing methane organic fertilizer and earthworm animal protein by solid anaerobic fermentation of agricultural and forestry residues. Background Art

[0002] With the rapid growth of the global population, conventional fossil energy sources such as coal and oil are being depleted at an accelerated rate, posing a threat to my country's energy security. At the same time, the large amount of greenhouse gases emitted after the refining and combustion of fossil energy has exacerbated global climate problems and endangered the global ecological and environmental security. According to statistics, my country's annual agricultural waste resources will reach 970 million tons in 2019, while my country's forestry residues such as forest tending and felling, urban garden pruning branches and wood processing waste will reach about 900 million tons. It has been calculated that the annual production potential of biomethane from agricultural waste alone can reach 82.25 billion m 3 , about 106 million tons of standard coal, with a potential to reduce greenhouse gas emissions of 197 million tons of carbon dioxide each year. In addition to improper treatment methods such as dumping and landfilling, returning to the field and incineration, obtaining methane through anaerobic fermentation of biomass is currently one of the main means of harmless resource treatment of agricultural and forestry residues in my country.

[0003] Most agricultural and forestry residues are wood fiber raw materials, which are composed of three macromolecular substances, cellulose, hemicellulose and lignin, connected by hydrogen bonds or covalent bonds to form a complex and dense structural system. Under anaerobic conditions, lignocellulose degrades slowly and cannot provide sufficient nutrients for the growth of functional microorganisms in the anaerobic fermentation system, resulting in reduced methane production. Partial or complete removal of lignin and hemicellulose from lignocellulose by chemical means such as alkali treatment or acid treatment can effectively destroy the lignocellulosic structure and improve its hydrolysis efficiency, but it is easy to cause serious carbohydrate loss, resulting in waste of agricultural and forestry residue biomass resources, and the acid and alkali used in acid and alkali treatment and the treated waste liquid are easy to cause environmental pollution. On the other hand, the solid content of ordinary wet anaerobic fermentation is low, usually 2%-8%. The fermentation process produces a large amount of wastewater, which needs to be harmlessly treated, and arbitrary discharge will cause ecological pollution. These problems have also become bottlenecks restricting the large-scale promotion and application of anaerobic fermentation treatment of agricultural and forestry residues in my country.

[0004] It can be seen that developing a fast and simple pretreatment method with low carbohydrate loss and establishing an anaerobic fermentation system for high-solid content lignocellulosic raw materials to achieve the purpose of destroying the lignocellulose structure, improving the anaerobic fermentation efficiency, and reducing the cost of anaerobic fermentation wastewater treatment is one of the optional ways to solve the bottleneck problem of the current large-scale promotion of anaerobic fermentation of agricultural and forestry residue biomass.

[0005] In addition, the supply of high-quality protein feed resources in my country is extremely limited. According to statistics, my country's imports of soybeans for feed reached 96.52 million tons in 2021, and the imports of fish meal were 1.7294 million tons. The amino acid composition of earthworm protein is close to that of fish meal, and earthworms have the characteristics of omnivorous diet and fast growth. The solid residue left over from anaerobic fermentation is used to feed earthworms, which can obtain high-quality animal protein and organic fertilizer. Summary of the invention

[0006] Technical problem to be solved: The present invention aims at the problem that the structure of the current agricultural and forestry residue lignocellulose raw materials is firm and compact, and it is difficult to be quickly hydrolyzed as a carbon source during the anaerobic fermentation process, resulting in slow growth and metabolism of microorganisms in the anaerobic fermentation system, which ultimately affects the methane production efficiency of the anaerobic fermentation system. The present invention provides a method for the co-production of methane organic fertilizer and earthworm animal protein from solid anaerobic fermentation of agricultural and forestry residues, thereby solving the problem of low solid content in the current traditional wet anaerobic fermentation of lignocellulose raw materials and a large amount of wastewater after the fermentation, which is difficult to handle.

[0007] Technical solution: A method for co-producing methane, organic fertilizer and earthworm animal protein by solid-state anaerobic fermentation of agricultural and forestry residues. First, the lignocellulose structure is loosened, and then crushed through a 20-mesh sieve to obtain pretreated raw materials, and then solid-state anaerobic fermentation is carried out and coupled with earthworm breeding to finally obtain methane, organic fertilizer and animal protein.

[0008] The method for loosening the structure of lignocellulose is to subject the lignocellulose to a twin-screw extrusion process.

[0009] The process of the solid-state anaerobic fermentation is as follows: the pretreated raw material is fully mixed with dry cow dung and inoculum, the solid content ranges from 5% to 50%, the fermentation temperature is from 25°C to 55°C, and solid-state anaerobic fermentation is performed to produce methane.

[0010] After the solid anaerobic fermentation is completed, the obtained solid residue is mixed with water to prepare a base material for earthworm breeding.

[0011] The temperature for raising the above earthworms is 25°C.

[0012] Specifically, the agricultural and forestry residues (which can be corn straw, wheat straw and other crop straw, poplar and bamboo and other timber processing residues or a mixture of the former two) are pretreated by twin-screw extrusion, and then crushed to pass through the screen to obtain the pretreated fermentation raw materials. The pretreated fermentation raw materials are used as a carbon source, and are fully mixed with a nitrogen source in a certain proportion. After adding a proper amount of inoculum (anaerobic activated sludge), the moisture content of the fermentation raw materials in the anaerobic fermentation bottle is adjusted, and then anaerobic fermentation is carried out to obtain methane. After the fermentation is completed, the solid residue of the anaerobic fermentation is mixed with water to make a base material for earthworm feeding. After feeding earthworms, organic fertilizer and earthworm animal protein are obtained. The specific process flow is as follows: Figure 1 shown.

[0013] Among them, the twin-screw extrusion pretreatment method requires that the biomass raw material be fully soaked with water first, and then a twin-screw extrusion pretreatment device is added to obtain a twin-screw extrusion material: wherein the solid-liquid ratio of the corn stalk to water is preferably 1:20; the screw runs at a speed of 100-200 rpm, preferably 150 rpm.

[0014] Wherein, the rotation speed of the twin-screw pretreatment product crushing blade is preferably 35,000 rpm.

[0015] Wherein, the mesh number of the sieve is in the range of 5 mesh to 80 mesh, preferably 20 mesh.

[0016] The nitrogen source in the anaerobic fermentation can be livestock manure, kitchen waste or inorganic salts containing nitrogen, etc., preferably dry cow manure.

[0017] Wherein, the mass ratio of the anaerobic fermentation carbon source to the nitrogen source ranges from 1:1 to 20:1, preferably 7:3.

[0018] Wherein, the amount of the inoculant added is preferably 50% of the mass of the fermentation raw material.

[0019] The solid content of the solid anaerobic fermentation system is in the range of 5% to 50%, preferably 70%.

[0020] The moisture content of the solid anaerobic fermentation earthworm breeding base material ranges from 60% to 90%, preferably 70%.

[0021] The present invention treats corn stalks by a twin-screw extrusion pretreatment method, effectively destroying the original dense wood fiber structure of corn stalks, improving the accessibility of wood fiber degrading microorganisms and their secreted hydrolytic enzymes to cellulose, thereby promoting the growth and metabolism of methanogens in the anaerobic fermentation system and promoting the increase of methane production. Under the same conditions, the methane production rate of biomass raw materials pretreated by twin-screw extrusion in solid anaerobic fermentation reached 0.138m 3 / kg VS, which is 1.76 times of the control; the solid anaerobic fermentation residue of the twin-screw pre-treated material is used for earthworm feeding, and the 30-day weight gain rate of earthworms reaches 148.6%, which is 52.8% higher than that of earthworms fed with anaerobic fermentation products of the control material; finally, the high-lignocellulosic fiber content anaerobic fermentation selected by the present invention reduces the amount of wastewater by 63.7% to 82.2% after fermentation compared with the traditional wet anaerobic fermentation. The method provided by the present invention not only improves the yield of methane from anaerobic fermentation, but also greatly reduces the impact of wastewater discharge on the environment.

[0022] Beneficial effects: 1. Compared with simple crushing, the lignocellulose raw material pretreated by twin-screw extrusion has a loose structure and has a synergistic effect on the methane production rate of solid-state anaerobic fermentation.

[0023] 2. The method provided by the present invention can reduce the wastewater pollution problem caused by traditional wet anaerobic fermentation. Compared with traditional wet anaerobic fermentation carried out at a solid content of 5%, the amount of wastewater is reduced by 63.7%-82.2%. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a process flow chart of the present invention;

[0025] Figure 2 Schematic diagram of the real-time monitoring device for gas production and methane output used in the present invention; wherein 1 is a feed inlet, 2 is a valve, 3 is a stirring motor, 4 is a sealed fermentation bottle, 5 is a water bath insulation device, 6 is a gas flow precision detector, 7 is a CO2 absorption device (containing 3 mol / L NaOH aqueous solution or CO2 absorbent), 8 is a gas flow precision detector, and 9 is a gas outlet / gas collection port for collecting methane gas;

[0026] Figure 3 It is the anaerobic fermentation methanogenesis process of pretreatment products BLC and FC.

[0027] Figure 4 The effect of moisture content on methane production by anaerobic fermentation of BLC, a product of twin-screw pretreatment. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the invention.

[0029] The experimental methods involved in the following examples are conventional methods unless otherwise specified; the reagents and materials shown are commercially available unless otherwise specified.

[0030] The chemical component analysis and detection methods of biomass in the following examples refer to the NREL / TP-510-42618 standard, and the removal rate calculation formula of the three main components in the wood fiber raw material is as follows:

[0031] Cellulose removal rate % = (mass of cellulose before reaction - mass of cellulose in solid product 1) / mass of cellulose in raw material before reaction * 100%;

[0032] Hemicellulose removal rate % = (mass of hemicellulose before reaction - mass of hemicellulose in solid product 1) / mass of hemicellulose in raw material before reaction * 100%;

[0033] Lignin removal rate % = (mass of lignin before reaction - mass of lignin in solid product 1) / mass of lignin in raw material before reaction * 100%.

[0034] Example 1: Obtaining twin-screw extrusion pretreatment products of agricultural and forestry residues (taking corn stalks as an example)

[0035] Unpretreated biomass (corn stalks, cellulose content 31.08%, hemicellulose content 23.77%, lignin content 14.07%), its cellulose, lignin and hemicellulose are intertwined and tightly combined to form a solid hydrolysis barrier. During the anaerobic fermentation process, this hydrolysis barrier reduces the accessibility of lignocellulose hydrolyzing microorganisms and their secreted degrading enzymes to the biomass raw materials. The hydrolysis efficiency of lignocellulose raw materials is extremely low, and the lack of available sugar makes the entire anaerobic fermentation system unable to maintain the metabolism and growth rate of microorganisms, and the final product methane yield is reduced. Therefore, this embodiment destroys the biomass raw material (taking corn stalks as an example) by twin-screw extrusion and crushing to obtain the pretreated product BLC, and the specific steps are as follows:

[0036] Soak corn stalks in water at a solid-liquid ratio of 1:20 for about 2-24 hours, preferably 12 hours (overnight soaking). Add the fully soaked corn stalks to the feed hopper, and the material slides into the twin-screw extruder with its own gravity and the screw, without additional liquid spraying during the period, and the screw speed is 150 rpm. With the effect of pressure during the twin-screw extrusion process, the wood fiber structure of the corn stalks is loosened and destroyed, and the originally tight structure becomes loose, and the surface area of ​​the material increases. After the product after twin-screw extrusion is dried, a small pulverizer (speed 35000 rpm, power 3.5KW) is added to crush for 2 minutes to further interrupt the longer wood fiber bundles, and the twin-screw extrusion pretreated material BLC (particle size ≤ 0.85mm) is obtained after sieving. The cellulose content in the product is 38.21%, the hemicellulose content is 12.30%, and the lignin content is 16.82%.

[0037] Comparative Example 1: Obtaining the crushing pretreatment product of agricultural and forestry residues (taking corn stalks as an example)

[0038] In order to reflect the synergistic effect of the twin-screw extrusion pretreatment on the anaerobic fermentation efficiency of corn straw in Example 1, only the corn straw was coarsely pulverized to obtain the corn straw pulverized product FC with the same particle size, and the specific steps were as follows:

[0039] The corn stalks are added into a small pulverizer (rotation speed 35000 rpm, power 3.5KW) and pulverized for 2 minutes, and the pulverized products are taken out and sieved to obtain pulverized corn stalks (particle size ≤ 0.85 mm). The cellulose content of the product is 31.15%, the hemicellulose content is 26.53%, and the lignin content is 13.78%.

[0040] Example 2: Analysis of the performance of anaerobic fermentation of biomass pretreatment products to produce methane

[0041] The twin-screw extrusion pretreatment product BLC obtained in Example 1 and the crushed pretreatment product FC obtained in Comparative Example 1 were used as raw materials, respectively, and mixed with dry cow dung at a mass ratio of 7:3, and then inoculated anaerobic sludge (water content of about 85%) was added at a ratio of 50% of the mass of the mixed raw materials. Figure 2 The fermentation device shown in the figure was used to add an appropriate amount of water to the device until the water content of the anaerobic fermentation system was 70%. Nitrogen was introduced into the device for about 5 minutes and then sealed. The fermentation bottles were placed in a 37°C constant temperature water bath device, the stirring motor was turned on to set a timed stirring, and the volume of methane produced by the anaerobic fermentation system was measured in real time. The results are shown in Figure 3 The methane production rate of corn stalks after twin-screw extrusion BLC can reach 0.138m after anaerobic fermentation. 3 / kg VS, the methane production rate of corn straw crushed material FC is 0.078m 3 / kg VS. It can be seen that the twin-screw extrusion pretreatment described in Example 1 has a synergistic effect on the anaerobic fermentation of high-solid content biomass to produce methane.

[0042] Example 3: Effect of moisture content on the methane production performance of BLC raw materials pretreated by twin screw extruder

[0043] The twin-screw extrusion pretreatment product BLC obtained in Example 1 was mixed with dry cow dung at a mass ratio of 7:3, and inoculum anaerobic sludge (water content of about 85%) was added at a ratio of 50% of the mass of the mixed raw materials. The mixed materials were all placed in Figure 2 The fermentation device shown in the figure was used to add appropriate amount of water to the device until the solid content of the anaerobic fermentation system was 5%, 30% and 50% respectively. Nitrogen was introduced into the device for about 5 minutes, then sealed and placed at 37°C for regular stirring, and the volume of methane produced by the anaerobic fermentation system was measured in real time. The results are shown in Figure 4 As shown in Figure 2, the methane production rate of the anaerobic fermentation system decreases significantly with the decrease of water content. When the water content of the anaerobic fermentation system is 70%, the fermentation cycle is reduced by about 17 days, and the methane production rate reaches 0.130m 3 / kgV.S., the methane production rate of the anaerobic fermentation system with a water content of 95% (0.155m 3 / kg VS) is 84.04%. When the water content of the anaerobic fermentation system is 50%, the methane production rate is only 0.075m 3 / kg VS, the methane production rate of the anaerobic fermentation system with a water content of 95% (0.155m 3 / kg VS) of 48.03%. After the fermentation, the wastewater volume of the anaerobic fermentation system with 70% and 50% water content was reduced by about 63.7% and 82.2% respectively compared with the anaerobic fermentation system with 95% water content. In summary, setting the water content of the anaerobic fermentation system to 70% can not only ensure the methane production efficiency, but also greatly reduce the generation of wastewater after the anaerobic fermentation is completed, and reduce the subsequent harmless treatment costs.

[0044] Example 4: Feeding earthworms with solid residue from anaerobic fermentation of lignocellulose

[0045] Earthworms have the advantages of a varied diet and fast growth. Earthworms themselves have a high protein content and are a source of high-quality animal protein. At the same time, earthworm castings are rich in organic matter and are high-quality biological organic fertilizers. The solid residue of anaerobic fermentation products can be used to feed earthworms to obtain not only high-quality animal protein, but also better quality organic fertilizer. The effect of earthworm fermentation products on the growth of earthworms is evaluated by the 30-day weight gain rate of earthworms. The 30-day weight gain rate of earthworms = (weight of earthworms after 30 days of feeding - initial weight of earthworms) / initial weight of earthworms * 100%. The specific process is as follows:

[0046] After the anaerobic fermentation of the corn straw pretreatment products BLC and FC in Example 2 was completed, the obtained solid residues BLCG and FCG were placed outdoors for 2 days. Take 100g of solid residues BLCG and FCG and place them in 500mL beakers respectively, add water to adjust the humidity to about 70%, and configure 3 parallel samples of the two solid residues. Add about 20g of earthworms (Eisenia fetida, an average of about 2.00g / piece) to the feeding base, cover the beaker with 4 layers of ordinary medical gauze, and place it in a 25°C thermostat to avoid light. After 30 days of cultivation, the earthworms were collected and weighed. The average weight of the earthworms harvested from the BLCG base was 49.72±4.17g, and the weight gain in 30 days could reach 148.6%. The earthworms harvested from the FCG base were about 39.16±3.38g, and the weight gain rate in 30 days was 95.8%. It can be seen that the biomass raw materials pretreated by twin-screw extrusion and then anaerobic fermented as a substrate for feeding earthworms can provide better conditions for the growth and metabolism of earthworms.

[0047] Example 5: Performance analysis of anaerobic fermentation products and earthworm feeding base organic fertilizer

[0048] The two fermentation solid residues BLCG and FCG obtained in Example 2 and the two base materials BLCGJ and FCGJ obtained in Example 4 after being ingested by earthworms are all black in color, in a uniform powdery form, without odor, and are in accordance with the organic fertilizer industry standard. <ny t525-2021>The prescribed method was used to determine the total organic matter mass fraction, total nutrient mass fraction, heavy metal content, pH value, E. coli group count and Ascaris egg mortality of the four products. The results showed that the performance indicators of the organic fertilizer of the four solid residues basically reached or exceeded the organic fertilizer industry standards. In particular, the organic matter mass fraction and total nutrient mass fraction of the base materials BLCGJ and FCGJ obtained after earthworm ingestion were significantly higher than those of the fermented solid residues BLCG and FCG.

[0049] Table 1 Performance test of biogas residue organic fertilizer

[0050] < / ny>

Claims

1. A method for co-producing methane organic fertilizer and earthworm animal protein by solid anaerobic fermentation of agricultural and forestry residues, characterized in that: The corn stalks were soaked in water at a solid-liquid ratio of 1:20 for about 12 hours, and the fully soaked corn stalks were added to the feed hopper. The materials entered the twin-screw extruder with their own gravity and the sliding of the screw. No additional liquid was sprayed during the period. The screw speed was 150 rpm. With the effect of pressure during the twin-screw extrusion process, the wood fiber structure of the corn stalks was loosened and destroyed, and the originally tight structure became loose, and the surface area of ​​the material increased. After the twin-screw extrusion product was dried, it was added to a small pulverizer with a speed of 35,000 rpm and a power of 3.5KW. It was pulverized for 2 minutes to further break up the longer wood fiber bundles. After screening, the twin-screw extrusion pretreated material BLC was obtained, with a particle size of ≤0.85mm, a cellulose content of 38.21%, and a hemicellulose content of 12.30%. The lignin content is 16.82%; the obtained twin-screw extrusion pretreatment product BLC is mixed with dry cow dung in a mass ratio of 7:3, and then anaerobic sludge is added as inoculum at a ratio of 50% of the mass of the mixed raw materials, with a moisture content of 85%; all the mixed materials are loaded into a fermentation device, and an appropriate amount of water is added to the device until the moisture content of the anaerobic fermentation system reaches 70% for solid-state anaerobic fermentation, and methane is collected; after the solid-state anaerobic fermentation is completed, the solid residue obtained is organic fertilizer, which is mixed with water to make a base material for earthworm breeding, and the humidity is 70% and the temperature is 25°C to breed Eisenia fetida.

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