Method for improving caproic acid produced by anaerobic fermentation of kitchen waste

Abstract

The invention provides a method for improving caproic acid produced by anaerobic fermentation of kitchen waste, and belongs to the field of solid waste treatment and recycling, the method comprises the following steps: mixing kitchen waste particles and domesticated anaerobic sludge to obtain a mixed substrate, and adding water to make the total solid concentration of the mixed substrate be 90 g / L; adding a methane inhibitor, waste vinasse and zero-valent iron into the mixed substrate, and carrying out anaerobic fermentation to obtain caproic acid. According to the method, waste vinasse and zero-valent iron (ZVI) are cooperatively added, the waste vinasse serves as a composite electron donor and a flora carrier to solve the problem of dependence of exogenous electron donors, and ZVI strengthens electron transfer and drives a carbon chain to prolong metabolism; the two methods start from electron supply and utilization efficiency respectively, so that the yield and selectivity of hexanoic acid are remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of solid waste treatment and resource utilization, and specifically relates to a method for improving the production of hexanoic acid by anaerobic fermentation of kitchen waste. Background Technology

[0002] With the development of society and the economy and the improvement of people's living standards, the amount of food waste has been increasing year by year, becoming a major component of urban household waste. Food waste is characterized by high water content, high organic matter content, and easy decomposition. If not disposed of properly, it can easily cause secondary pollution to the environment. Therefore, achieving the reduction, harmlessness, and resource utilization of food waste is of great social and economic significance.

[0003] Anaerobic fermentation is an effective technology for treating high-organic-matter waste. Traditional anaerobic fermentation mainly aims to produce methane and recover biogas, but methane has relatively low added value as an energy product. In recent years, extending the anaerobic fermentation product chain from short-chain fatty acids (such as acetic acid and butyric acid) to medium-chain fatty acids (MCFAs, such as caproic acid) has become a research hotspot. Caproic acid, as a platform compound, is not only a high-value chemical itself, used in fragrances, plasticizers, lubricants, and other fields, but also serves as a precursor for biofuels. Its market value and application prospects are far greater than those of methane.

[0004] The biosynthesis of hexanoic acid mainly relies on the reverse beta-oxidation (RBO) pathway, which requires a continuous electron donor to drive the acetate to extend its carbon chain. However, microbial electron transfer efficiency is limited, and the expression levels of key functional genes in the RBO pathway are low, making it difficult to achieve breakthroughs in hexanoic acid yield and selectivity. Currently, ethanol is the most commonly used electron donor in research. However, the high price of commercial-grade ethanol significantly limits the economic feasibility and industrial application of this technology. Therefore, finding inexpensive and efficient alternative electron donors and increasing the proportion of hexanoic acid in volatile acids is a key bottleneck in promoting the development of hexanoic acid production technology from food waste.

[0005] Waste distiller's grains are a byproduct of the brewing industry (including baijiu, huangjiu, beer, etc.), produced in huge quantities and difficult to process. Waste distiller's grains are rich in incompletely utilized carbohydrates, crude protein, cellulose, and hemicellulose, and contain a certain amount of residual ethanol. In addition, they are enriched with functional microorganisms such as yeast and lactic acid bacteria. However, adding waste distiller's grains alone to treat kitchen waste presents two major bottlenecks: first, the hydrolysis efficiency of large organic molecules such as proteins and polysaccharides in kitchen waste is low, leading to insufficient electron acceptor supply; second, the electron transfer efficiency of microorganisms is limited, resulting in low expression levels of key functional genes in the RBO pathway, making it difficult to achieve breakthroughs in hexanoic acid yield and selectivity.

[0006] In existing technologies, anaerobic fermentation using kitchen waste as a substrate faces two major bottlenecks: low hexanoic acid production and poor economic viability. These factors severely restrict the industrial application and promotion of this technology. Summary of the Invention

[0007] In view of this, the present invention provides a method for improving the production of hexanoic acid by anaerobic fermentation of kitchen waste, so as to solve the technical problems of low hexanoic acid production and poor economic efficiency when using kitchen waste as a substrate for anaerobic fermentation in the prior art.

[0008] To achieve the above-mentioned objectives, this invention provides a method for improving the production of hexanoic acid through anaerobic fermentation of kitchen waste, comprising the following steps: S1. Remove impurities from kitchen waste, freeze-dry it, and then pulverize it to obtain kitchen waste granules; S2. The anaerobic sludge is placed in a mixed anaerobic reactor for acclimation to obtain acclimated anaerobic sludge. S3. Mix the kitchen waste granules and the acclimated anaerobic sludge to obtain a mixed substrate. Add water to make the total solids concentration of the mixed substrate 90 g / L. The ratio of the amount of kitchen waste granules to anaerobic sludge added is 1:1-1.5 based on the mass ratio of volatile solids. S4. Add methane inhibitor, waste distiller's grains and zero-valent iron to the mixed substrate and carry out anaerobic fermentation. After the anaerobic fermentation is completed, hexanoic acid is obtained.

[0009] Preferably, the average particle size of the kitchen waste particles in S1 is less than or equal to 0.6 mm.

[0010] Preferably, the moisture content of the waste lees in S4 is 60%-70%.

[0011] Preferably, the amount of waste lees added in S4 is 3-8 g / L (based on the volume of the mixed substrate).

[0012] Preferably, the amount of methane inhibitor added in S4 is 9-10.5 g / L.

[0013] Preferably, the methane inhibitor in S4 is sodium 2-bromoethanesulfonate.

[0014] Preferably, the amount of zero-valent iron added in S4 is 5-10 g / L.

[0015] Preferably, the amount of zero-valent iron added in S4 is 7 g / L.

[0016] Preferably, the temperature of the anaerobic fermentation in S4 is 35-37°C.

[0017] Preferably, the pH value of the reaction system during the anaerobic fermentation process described in S4 is 5.5-6.0.

[0018] The core mechanism of this invention lies in reshaping the fermentation microbial community and metabolic network through multi-target synergistic intervention, thereby strongly promoting and maintaining the efficient "chain elongation" process. Specifically: First, by adding a methane inhibitor (such as sodium 2-bromoethanesulfonate), the activity of methanogenic archaea is specifically inhibited, thereby cutting off the ineffective consumption of carbon and electron flows to methane and forcing valuable reaction resources (such as short-chain fatty acid precursors and hydrogen / electrons) to be redirected to the hexanoic acid synthesis pathway. Second, waste distiller's grains containing residual ethanol and lactic acid bacteria provide ideal electron donors and carbon primers for the chain elongation process. Ethanol lactic acid, under the action of specific microorganisms, undergoes a reverse β-oxidation reaction, which efficiently condenses with short-chain fatty acids such as acetic acid and butyric acid in the system, gradually extending the carbon chain to hexanoic acid. At the same time, the addition of zero-valent iron further optimizes the reaction microenvironment. During the corrosion process, it continuously releases hydrogen, which not only stimulates hydrogen-producing and acetic acid-producing bacteria to generate more precursors, but also strengthens the hydrogen-dependent chain elongation metabolism by maintaining a low hydrogen partial pressure environment. Zero-valent iron itself, as a potential electron source, can also be directly utilized by microorganisms, enhancing the reducing power of the system. Therefore, this invention is not a simple formulation, but rather a systematic approach that precisely directs and locks metabolic flow onto the hexanoic acid synthesis pathway through a synergistic strategy of "suppressing competitors, strengthening the main force, and supplying sophisticated equipment," ultimately achieving a significant increase in hexanoic acid production.

[0019] This invention addresses the issue of dependence on and utilization of exogenous high-valent iron (ZVI) by adding waste distiller's grains and zero-valent iron (ZVI) to an anaerobic fermentation system for kitchen waste. Waste distiller's grains, as a composite functional additive, provide in-situ electron donors (derived from residual ethanol and subsequent fermentation-produced lactic acid and ethanol) and supplement functional microbial communities (such as yeast and hydrolytic bacteria). Simultaneously, ZVI provides attachment sites, promoting the enrichment of functional microbial communities, upregulating the expression of key genes in the RBO pathway (Hbd, Ech, Kat) and fatty acid biosynthesis (FAB) pathway genes (Acc, FabB), and enhancing electron transport efficiency and carbon chain elongation metabolism. Both ZVI and ZVI synergistically promote hexanoic acid synthesis from both electron donor and electron acceptor perspectives, thereby solving the problem of dependence on and utilization of exogenous high-valent electron donors.

[0020] The present invention has the following beneficial effects: Multiple synergistic mechanisms: This can be summarized as the synergistic effect of the electron donor supply from waste distiller's grains and the electron acceptor synergistic effect of ZVI. Waste distiller's grains not only provide residual ethanol as a direct electron donor, but their fermentable sugars can also ferment in situ within the system to produce ethanol, achieving a slow and continuous release of electron donors. ZVI, by increasing the specific surface area for microbial attachment, significantly enriches hydrolytic bacteria, promoting the hydrolysis of macromolecular organic matter such as proteins and polysaccharides in food waste, generating more short-chain fatty acids (acetic acid, butyric acid) as electron acceptors in the RBO pathway. The two form a "supply and acceptance" balance, solving the problem of insufficient electron donors or acceptors when added alone, improving the degradation efficiency of complex organic matter in food waste, and increasing hexanoic acid production.

[0021] Significantly reduced costs: This invention utilizes waste distiller's grains as an alternative to expensive commercial ethanol and lactic acid as electron donors, significantly reducing raw material costs. The addition of ZVI optimizes the acid-producing effect of waste distiller's grains and avoids the characteristics of high-frequency replenishment of zero-valent iron (ZVI), reducing the total process cost by 45%-50%. Furthermore, it simultaneously treats both food waste and waste distiller's grains, avoiding resource waste and secondary pollution, thus meeting the needs of environmental protection and the circular economy. This lays an economic foundation for the industrial application of the technology.

[0022] Significantly Improved Hexanoic Acid Yield and Selectivity: The key to this invention lies in the synergistic effect of distillery waste and zero-valent iron (ZVI), which constructs a highly efficient and stable electron donor-acceptor balance system. Distillery waste not only provides abundant residual ethanol and fermentable sugars, enabling continuous electron donor release, but also introduces functional microbial communities, strengthening the microbial basis for carbon chain elongation. Meanwhile, ZVI significantly enhances the driving force for precursor formation and carbon chain elongation reactions by promoting the hydrolysis of macromolecular organic matter, releasing hydrogen, and optimizing electron transfer efficiency. The synergistic effect of these two components achieves a significant increase in hexanoic acid yield and selectivity without relying on high-valent exogenous electron donors. Attached Figure Description

[0023] Figure 1 The VFA yield is shown in the diagram for 13 days of fermentation using the methods provided in Specific Examples 1-3. Figure 2 The VFA yield graphs for 13 days of fermentation are provided for specific comparative examples 1-3. Figure 3 The diagram shows the electron donor yield after 13 days of fermentation for each group of methods provided in specific embodiments 1-3. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0025] Example 1 This embodiment provides a method for improving the production of hexanoic acid during the anaerobic fermentation of kitchen waste, including the following steps: S1. Remove bones, plastics and other impurities from the kitchen waste, homogenize it using a high-speed blender (12000 rpm, 3 min), freeze dry (-50℃, 72 h, vacuum degree <10 Pa), then pulverize it through a 30-mesh sieve (particle size ≤0.6 mm), and store it at -20℃ and relative humidity <30% to obtain kitchen waste granules. S2. Waste distiller's grains are partially dehydrated by plate and frame filter press (pressure 0.3MPa), resulting in a moisture content of 60% for the waste distiller's grains. S3. Place the flocculent anaerobic sludge in a 5L completely mixed anaerobic reactor (CSTR) for acclimatization for 14 days to carry out pre-cultivation and acclimatization, and obtain acclimatized anaerobic sludge. S4. Select 100-mesh industrial-grade zero-valent iron (ZVI) (purity ≥98%). First, soak it in 1mol / L hydrochloric acid for 30 minutes to remove the surface oxide layer, rinse it with deionized water until neutral, and vacuum dry it for later use to avoid oxidation and failure. S5. Mix the kitchen waste granules and the acclimated anaerobic sludge to obtain a mixed substrate. Add water to make the total solids concentration of the mixed substrate 90g / L. The ratio of the amount of kitchen waste granules to anaerobic sludge added is 1:1 based on the mass ratio of volatile solids. S6. Add sodium 2-bromoethanesulfonate, waste distiller's grains, and zero-valent iron to the mixed substrate. Based on the volume of the mixed substrate, the amount of waste distiller's grains added is 5 g / L, the amount of sodium 2-bromoethanesulfonate added is 10.5 g / L, and the amount of zero-valent iron added is 7 g / L. After sealing, purge with nitrogen for 2 min to remove oxygen, and carry out anaerobic fermentation at 35°C. During the anaerobic fermentation process, the pH value of the reaction system is 5.5. S7. During the fermentation cycle, samples are taken periodically to test the composition and concentration of volatile fatty acids (VFAs), especially the yield and selectivity of hexanoic acid, to obtain hexanoic acid.

[0026] After 13 days of fermentation, the yield of hexanoic acid was detected to be 10022.21±435.55 mg / L, and its selectivity in total volatile acids was 69.7%.

[0027] Example 2 This embodiment provides a method for improving the production of hexanoic acid during the anaerobic fermentation of kitchen waste, including the following steps: S1. Remove bones, plastics and other impurities from the kitchen waste, homogenize it using a high-speed blender (12000 rpm, 3 min), freeze dry (-50℃, 72 h, vacuum degree <10 Pa), then pulverize it through a 30-mesh sieve (particle size ≤0.6 mm), and store it at -20℃ and relative humidity <30% to obtain kitchen waste granules. S2. Waste distiller's grains are partially dehydrated by plate and frame filter press (pressure 0.3MPa), resulting in a moisture content of 65% for the waste distiller's grains. S3. Place the flocculent anaerobic sludge in a 5L completely mixed anaerobic reactor (CSTR) for acclimatization for 14 days to carry out pre-cultivation and acclimatization, and obtain acclimatized anaerobic sludge. S4. Select 100-mesh industrial-grade zero-valent iron (ZVI) (purity ≥98%). First, soak it in 1mol / L hydrochloric acid for 30 minutes to remove the surface oxide layer, rinse it with deionized water until neutral, and vacuum dry it for later use to avoid oxidation and failure. S5. Mix the kitchen waste granules and the acclimated anaerobic sludge to obtain a mixed substrate. Add water to make the total solids concentration of the mixed substrate 90g / L. The ratio of the amount of kitchen waste granules to anaerobic sludge added is 1:1.5 based on the mass ratio of volatile solids. S6. Add sodium 2-bromoethanesulfonate, waste distiller's grains, and zero-valent iron to the mixed substrate. Based on the volume of the mixed substrate, the amount of waste distiller's grains added is 3 g / L, the amount of sodium 2-bromoethanesulfonate added is 10 g / L, and the amount of zero-valent iron added is 5 g / L. After sealing, purge with nitrogen for 2 min to remove oxygen, and carry out anaerobic fermentation at 36°C. During the anaerobic fermentation process, the pH value of the reaction system is 5.8. S7. During the fermentation cycle, samples are taken periodically to test the composition and concentration of volatile fatty acids (VFAs), especially the yield and selectivity of hexanoic acid, to obtain hexanoic acid.

[0028] After 13 days of fermentation, the yield of hexanoic acid was detected to be 7115.51816±295.89 mg / L, and its selectivity in total volatile acids was 59.2%.

[0029] Example 3 This embodiment provides a method for improving the production of hexanoic acid during the anaerobic fermentation of kitchen waste, including the following steps: S1. Remove bones, plastics and other impurities from the kitchen waste, homogenize it using a high-speed blender (12000 rpm, 3 min), freeze dry (-50℃, 72 h, vacuum degree <10 Pa), then pulverize it through a 30-mesh sieve (particle size ≤0.6 mm), and store it at -20℃ and relative humidity <30% to obtain kitchen waste granules. S2. Waste distiller's grains are partially dehydrated by plate and frame filter press (pressure 0.3MPa), resulting in a moisture content of 70% for the waste distiller's grains. S3. Place the granular anaerobic sludge in a 5L completely mixed anaerobic reactor (CSTR) for acclimatization for 14 days to carry out pre-cultivation and acclimatization, and obtain acclimatized anaerobic sludge. S4. Select 100-mesh industrial-grade zero-valent iron (ZVI) (purity ≥98%). First, soak it in 1mol / L hydrochloric acid for 30 minutes to remove the surface oxide layer, rinse it with deionized water until neutral, and vacuum dry it for later use to avoid oxidation and failure. S5. Mix the kitchen waste granules and the acclimated anaerobic sludge to obtain a mixed substrate. Add water to make the total solids concentration of the mixed substrate 90g / L. The ratio of the amount of kitchen waste granules to anaerobic sludge added is 1:1.1 based on the mass ratio of volatile solids. S6. Add sodium 2-bromoethanesulfonate, waste distiller's grains, and zero-valent iron to the mixed substrate. Based on the volume of the mixed substrate, the amount of waste distiller's grains added is 8 g / L, the amount of sodium 2-bromoethanesulfonate added is 9 g / L, and the amount of zero-valent iron added is 10 g / L. After sealing, purge with nitrogen for 2 min to remove oxygen, and carry out anaerobic fermentation at 37°C. During the anaerobic fermentation process, the pH value of the reaction system is 6.0. S7. During the fermentation cycle, samples are taken periodically to test the composition and concentration of volatile fatty acids (VFAs), especially the yield and selectivity of hexanoic acid, to obtain hexanoic acid.

[0030] After 13 days of fermentation, the yield of hexanoic acid was 7750.502±480.12 mg / L, with a selectivity of 62.1%.

[0031] Comparative Example 1 This comparative example is the same as Example 1, except that S6 is not present in this comparative example, and S5 involves mixing kitchen waste particles and acclimated anaerobic sludge in a container for anaerobic fermentation.

[0032] After 13 days of fermentation, the yield of hexanoic acid was detected to be 1076.707±124.57 mg / L, and its selectivity in total volatile acids was 28.8%.

[0033] Comparative Example 2 The comparative example is the same as Example 1, except that waste distiller's grains are not added in S6 in this comparative example, and in S5, kitchen waste particles and acclimated anaerobic sludge are mixed and placed into a container for anaerobic fermentation.

[0034] After 13 days of fermentation, the yield of hexanoic acid was detected to be 3830.558±302.75 mg / L, and its selectivity in total volatile acids was 38.3%.

[0035] Comparative Example 3 This comparative example is the same as Example 1, except that ZVI is not added in S6 of this comparative example.

[0036] After 13 days of fermentation, the yield of hexanoic acid was detected to be 4315.820±521.416 mg / L, and its selectivity in total volatile acids was 41.6%.

[0037] The fermentation VFA yield and electron donor diagrams for Examples 1-3 and Comparative Examples 1-3 after 13 days of fermentation are shown below. Figure 1-3 As shown. After periodic collection of the fermentation broth, the solid phase was separated by pre-cooling and centrifugation. The supernatant was then filtered sequentially through a 0.22 μm PTFE membrane, acidified with 3% phosphoric acid solution, and diluted appropriately according to the actual concentration. After thorough mixing, the supernatant was transferred to a 2 mL chromatographic sample vial for low-temperature storage. Quantitative analysis of ethanol, lactic acid, and volatile fatty acids in the sample was performed using an Agilent 7890A gas chromatograph equipped with a flame ionization detector (FID). Chromatographic separation was performed using a DB-FATWAX UI polar capillary column (30 m × 0.25 mm × 0.25 μm), with an H2 flow rate of 40 mL / min and high-purity nitrogen as the carrier gas (constant flow mode, 3 mL / min). The injection port and detector temperatures were set to 250 ℃ and 300 ℃, respectively, with a split ratio of 1:10. The temperature program started at 50 ℃, increasing at 25 ℃ / min to 100 ℃, and then at 15 ℃ / min to 200 ℃.

[0038] From the electron donors of Examples 1-3 ( Figure 3 ) and VFA ( Figure 1 The measurement results show that the addition of waste distillers' grains can produce an appropriate amount of ethanol and lactic acid to ensure chain elongation, and the addition of ZVI can produce more electron acceptors (acetic acid and butyric acid) to ensure chain elongation.

[0039] The VFA measurement results from Examples 1-3 and Comparative Examples 1-3 show that the addition of waste distillers' grains and ZVI (Comparative Example 1), or the addition of only waste distillers' grains (Comparative Example 2), or the addition of only ZVI (Comparative Example 3) all reduce the yield and selectivity of hexanoic acid.

[0040] The VFA measurement results of Examples 1-3 show that the amount of waste distillers' grains added directly affects the ethanol content, and ultimately affects the yield of hexanoic acid. Therefore, adding too much or too little waste distillers' grains is not conducive to increasing the yield of hexanoic acid. Only within the scope of protection of this invention can the final maximization of hexanoic acid yield be guaranteed.

[0041] The specific embodiments of the present invention described above do not constitute a limitation on the scope of protection of the present invention. Any other corresponding changes and modifications made in accordance with the technical concept of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for improving the production of hexanoic acid by anaerobic fermentation of kitchen waste, characterized in that, The method comprises the following steps: S1, removing sundries from the kitchen waste, and then freezing, drying and crushing the kitchen waste to obtain kitchen waste particles; S2, domesticating anaerobic sludge in a mixed anaerobic reactor to obtain domesticated anaerobic sludge; S3, mixing the kitchen waste particles and the domesticated anaerobic sludge to obtain a mixed substrate, and adding water to make the total solid concentration of the mixed substrate 90 g / L, and the mass ratio of the kitchen waste particles to the anaerobic sludge is 1:1-1.5; S4, adding a methane inhibitor, waste wine lees and zero-valent iron into the mixed substrate to perform anaerobic fermentation, and obtaining hexanoic acid after the anaerobic fermentation.

2. The method for improving production of hexanoic acid from anaerobic fermentation of kitchen waste according to claim 1, characterized in that, The average particle size of the kitchen waste particles in S1 is less than or equal to 0.6 mm.

3. The method for improving production of hexanoic acid from anaerobic fermentation of kitchen waste according to claim 1, characterized in that, The moisture content of the waste wine lees in S4 is 60%-70%.

4. The method for improving production of hexanoic acid from anaerobic fermentation of kitchen waste according to claim 1, characterized in that, The addition amount of the waste wine lees in S4 is 3-8 g / L.

5. The method for improving production of hexanoic acid from anaerobic fermentation of kitchen waste according to claim 1, characterized in that, The addition amount of the methane inhibitor in S4 is 9-10.5 g / L.

6. The method for improving production of hexanoic acid from anaerobic fermentation of kitchen waste according to claim 1 or 5, characterized in that, The methane inhibitor in S4 is 2-bromothesulfonate sodium.

7. The method for improving production of hexanoic acid from anaerobic fermentation of kitchen waste according to claim 1, characterized in that, The addition amount of the zero-valent iron in S4 is 5-10 g / L.

8. The method for improving production of hexanoic acid from anaerobic fermentation of kitchen waste according to claim 7, characterized in that, The addition amount of the zero-valent iron in S4 is 7 g / L. 9.The method of improving production of hexanoic acid from anaerobic fermentation of kitchen waste according to claim 1, characterized in that, The temperature of the anaerobic fermentation in S4 is 35-37℃.

10. The method for improving production of hexanoic acid from anaerobic fermentation of kitchen waste according to claim 1, characterized in that, The pH value of the reaction system in the process of the anaerobic fermentation in S4 is 5.5-6.0.

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