Method for synthesizing hexanoic acid through carbon chain extension based on two-stage anaerobic fermentation

Abstract

The invention belongs to the technical field of sewage treatment, and discloses a method for synthesizing hexanoic acid by carbon chain extension based on two-stage anaerobic fermentation. The method comprises the following steps: carrying out alkali pretreatment on concentrated sludge; performing first anaerobic fermentation on the concentrated sludge subjected to alkali pretreatment to obtain a fermentation product; and mixing the fermentation product, caproic acid synthetic bacteria, an electron donor, a nutrient substance and a first methane inhibitor, and carrying out second anaerobic fermentation to obtain caproic acid. According to the invention, two-stage anaerobic fermentation is more beneficial to carbon chain extension reaction, and different from a slow reaction process caused by short-chain fatty acid lacking of a one-stage anaerobic fermentation system, more caproic acid is generated in the two-stage anaerobic fermentation process. Moreover, the alkali pretreatment effectively destroys the structure of extracellular polymer substances, so that more dissolved organic matters can be released, the concentration of acetic acid can be increased, more organic matrixes can be provided, and the synthesis of hexanoic acid can be improved.

Description

Technical Field

[0001] The present invention relates to the technical field of sewage treatment, and in particular to a method for synthesizing caproic acid by carbon chain extension based on two-stage anaerobic fermentation. Background Art

[0002] With the continuous increase in the discharge of industrial wastewater and domestic sewage. At present, 90% of sewage treatment processes use the activated sludge method, resulting in an increasing amount of residual sludge. Sludge contains rich organic matter and nutrients, of which organic carbon compounds account for about 60% of the dry weight; followed by organic compounds containing nitrogen and phosphorus, which account for 3.5% and 2.0% of the dry weight respectively. Therefore, promoting the resource utilization of sludge is a very promising direction, which is in line with the technical concept of "harmless resource treatment and disposal of sludge". Taking into account the sustainable development of the economy and the environment, in recent years, researchers have focused more on the direction of using waste activated sludge to produce renewable biochemicals.

[0003] Based on the high energy recovery and low environmental impact of traditional anaerobic digestion processes, researchers have found that utilizing sludge as a biochemical organic matter to convert it into high-value-added products holds broad promise, particularly in the production of caproic acid through anaerobic fermentation. However, current approaches to producing caproic acid through anaerobic fermentation suffer from low yields and difficulty disrupting sludge cell walls. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for synthesizing caproic acid by carbon chain extension based on two-stage anaerobic fermentation, so as to solve the problems of low caproic acid yield and difficulty in sludge cell wall disruption in existing fermentation schemes.

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0006] The present invention provides a method for synthesizing caproic acid by carbon chain extension based on two-stage anaerobic fermentation, comprising the following steps:

[0007] performing an alkali pretreatment on the concentrated sludge; performing a first anaerobic fermentation on the concentrated sludge after the alkali pretreatment to obtain a fermentation product;

[0008] The fermentation product, hexanoic acid-synthesizing bacteria, an electron donor, a culture medium, and a first methane inhibitor are mixed and subjected to a second anaerobic fermentation to obtain hexanoic acid.

[0009] Preferably, in the method for synthesizing hexanoic acid by carbon chain extension based on two-stage anaerobic fermentation, the pH of the concentrated sludge is 7.33-7.37, the total suspended solids concentration of the concentrated sludge is 31.81-31.85 g / L, and the volatile suspended solids concentration of the concentrated sludge is 12.03-12.07 g / L.

[0010] Preferably, in the method for synthesizing caproic acid by carbon chain extension based on two-stage anaerobic fermentation, the conditions for the alkaline pretreatment include: pH 10-14, temperature 110-130° C., and time 10-30 min.

[0011] Preferably, in the method for synthesizing caproic acid by carbon chain extension based on two-stage anaerobic fermentation, the conditions of the first anaerobic fermentation include: initial pH of 5-7, temperature of 30-40°C, rotation speed of 100-200 rpm, and time of 3-4 days.

[0012] Preferably, in the method for synthesizing caproic acid by carbon chain extension based on two-stage anaerobic fermentation, the caproic acid-synthesizing bacteria include one or more of Clostridium sensu stricto, Romboutsia, and Terrisporobacter;

[0013] The mass ratio of the concentrated sludge to the hexanoic acid-synthesizing bacteria is 1-2:9-12.

[0014] Preferably, in the method for synthesizing caproic acid by carbon chain extension based on two-stage anaerobic fermentation, the caproic acid-synthesizing bacteria are acclimated before use;

[0015] The acclimation process comprises the following steps: under anaerobic conditions, mixing caproate-synthesizing bacteria, a chain extension substrate, a culture medium, and a second methane inhibitor, and culturing;

[0016] The culture conditions include: pH value of 4-8, temperature of 20-50° C., rotation speed of 100-200 rpm, and time of 2-7 days.

[0017] Preferably, in the method for synthesizing caproic acid by carbon chain extension based on two-stage anaerobic fermentation, the electron donor includes ethanol and / or lactic acid;

[0018] The molar ratio of the electron acceptor to the electron donor in the fermentation product is 1:3-5.

[0019] Preferably, in the method for synthesizing hexanoic acid by carbon chain extension based on two-stage anaerobic fermentation, the culture medium comprises a mixture of ammonium chloride, magnesium sulfate heptahydrate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, sodium chloride, vitamin solution, and trace element solution;

[0020] Wherein, the concentration of ammonium chloride in the second anaerobic fermentation mixture is 0.23-0.27 g / L;

[0021] The concentration of the magnesium sulfate heptahydrate in the second anaerobic fermentation mixture is 0.1 to 0.4 g / L;

[0022] The concentration of the potassium dihydrogen phosphate in the second anaerobic fermentation mixture is 0.21-0.25 g / L;

[0023] The concentration of the dipotassium hydrogen phosphate in the second anaerobic fermentation mixture is 0.29-0.33 g / L;

[0024] The concentration of sodium chloride in the second anaerobic fermentation mixture is 0.6-1 g / L;

[0025] The concentration of the vitamin solution in the second anaerobic fermentation mixture is 1 to 3 mL / L;

[0026] The concentration of the trace element solution in the second anaerobic fermentation mixture is 1 to 3 mL / L.

[0027] Preferably, in the method for synthesizing hexanoic acid by carbon chain extension based on two-stage anaerobic fermentation, the first methane inhibitor comprises sodium 2-bromoethanesulfonate;

[0028] The concentration of the first methane inhibitor in the second anaerobic fermentation mixture is 8-12 g / L.

[0029] Preferably, in the method for synthesizing caproic acid by carbon chain extension based on two-stage anaerobic fermentation, the conditions of the second anaerobic fermentation include: initial pH of 4-8, temperature of 20-50°C, rotation speed of 100-200 rpm, and time of 2-14 days.

[0030] It can be seen from the above technical solutions that compared with the prior art, the present invention has the following beneficial effects:

[0031] (1) The method of the present invention performs alkaline pretreatment on concentrated sludge, which greatly promotes the cell fragmentation (lysis of cell wall structure) and dissolution of extracellular polymers in the sludge, so that more proteins are decomposed into amino acids by proteases during the carbon chain extension reaction process, further optimizes the construction of the sludge organic matrix, provides more sufficient organic matrix for the subsequent carbon chain extension process to synthesize hexanoic acid, improves the synthesis and conversion efficiency of the carbon chain extension functional bacteria, promotes the biological resource conversion of sludge in a more efficient direction, and provides an important theoretical and practical basis for the future realization of industrial production.

[0032] (2) In the scheme of the present invention for synthesizing caproic acid by carbon chain extension of two-stage anaerobic fermentation using short-term pre-fermented sludge as the substrate, the caproic acid production of the two-stage experimental group under the action of caproic acid synthesizing bacteria was 156.07 mg COD / g VSS, which was 3.05 and 2.51 times that of the one-stage control group, respectively. Therefore, the effect of the two-stage anaerobic fermentation on the synthesis of caproic acid by carbon chain extension was much higher than that of the one-stage anaerobic fermentation. In addition, the acetic acid in the two-stage fermentation sludge experimental group reached 124.11 mg COD / g VSS, which was 6.50 times that of the one-stage reactor, providing more sufficient substrate for the subsequent anaerobic fermentation reaction. This shows that compared with the one-stage anaerobic fermentation, the two-stage anaerobic fermentation has higher economic utilization and excellent environmental friendliness. The caproic acid produced by the two-stage anaerobic fermentation is much higher than that of the one-stage anaerobic fermentation. In addition, the scheme of the present invention greatly promotes the breakage of cells in the sludge and the dissolution of extracellular polymeric substances, which is of great significance for achieving the reduction, harmlessness and resource utilization of sludge. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for describing the embodiments or the prior art.

[0034] Figure 1 This is a comparison chart of the acid production results in the methods described in Example 1 and Comparative Examples 1 to 5;

[0035] Figure 2 This is a comparison of the three-dimensional fluorescence spectra of soluble organic matrix (DOM), loosely attached extracellular polymeric substances (LB-EPS) and tightly attached extracellular polymeric substances (TB-EPS) by the methods described in Example 1 and Comparative Example 1; wherein, a is Comparative Example 1, Control DOM in a is the control group of soluble organic matrix in Comparative Example 1, Control LB in a is the control group of loosely attached extracellular polymeric substances in Comparative Example 1, and Control TB in a is the control group of tightly attached extracellular polymeric substances in Comparative Example 1; b is Example 1, APDOM in b is the experimental group of alkali pretreatment of soluble organic matrix in Example 1, APLB in b is the experimental group of alkali pretreatment of loosely attached extracellular polymeric substances in Example 1, and APTB in b is the experimental group of alkali pretreatment of tightly attached extracellular polymeric substances in Example 1. DETAILED DESCRIPTION

[0036] The present invention provides a method for synthesizing caproic acid by carbon chain extension based on two-stage anaerobic fermentation, comprising the following steps:

[0037] performing an alkali pretreatment on the concentrated sludge; performing a first anaerobic fermentation on the concentrated sludge after the alkali pretreatment to obtain a fermentation product;

[0038] The fermentation product, hexanoic acid-synthesizing bacteria, an electron donor, a culture medium, and a first methane inhibitor are mixed and subjected to a second anaerobic fermentation to obtain hexanoic acid.

[0039] In the present invention, the concentrated sludge is preferably obtained in the following manner: the excess sludge is allowed to stand, the supernatant is discharged, and the concentrated excess sludge is concentrated; and the concentrated excess sludge is passed through a 40-mesh sieve to obtain concentrated sludge.

[0040] In the present invention, the source of the excess sludge is preferably the secondary sedimentation tank of a sewage treatment plant.

[0041] In the present invention, the standing conditions include: the temperature is preferably 0-10°C, more preferably 2-6°C, more preferably 4°C; the time is preferably 10-24h, more preferably 20-24h, more preferably 24h.

[0042] In the present invention, the pH of the concentrated sludge is preferably 7.33-7.37, more preferably 7.34-7.36, and more preferably 7.35; the total suspended solids concentration of the concentrated sludge is preferably 31.81-31.85 g / L, more preferably 31.82-31.84 g / L, and more preferably 31.83 g / L; the volatile suspended solids concentration of the concentrated sludge is preferably 12.03-12.07 g / L, more preferably 12.04-12.06 g / L, and more preferably 12.05 g / L.

[0043] In the present invention, the reagent used in the alkali pretreatment is preferably NaOH mother liquor.

[0044] In the present invention, the NaOH mother liquor preferably includes sodium hydroxide and hydrochloric acid.

[0045] In the present invention, the concentration of the sodium hydroxide in the NaOH mother liquor is preferably 3 to 5 mol / L, more preferably 3.5 to 4.5 mol / L, and even more preferably 4 mol / L.

[0046] In the present invention, the mass fraction of the hydrochloric acid in the NaOH mother liquor is preferably 10 to 14%, more preferably 10 to 12%, and even more preferably 12%.

[0047] In the present invention, the mass ratio of sodium hydroxide in the NaOH mother liquor to the total suspended solids in the concentrated sludge is preferably 0.05 to 0.1:1, more preferably 0.07 to 0.1:1, and even more preferably 0.09:1.

[0048] In the present invention, the conditions for the alkali pretreatment include: pH is preferably 10-14, more preferably 11.9-12.1, more preferably 12; temperature is preferably 110-130°C, more preferably 115-125°C, more preferably 120°C; time is preferably 10-30 min, more preferably 15-25 min, more preferably 20 min.

[0049] In the present invention, sterilization is performed during the alkali pretreatment.

[0050] In the present invention, nitrogen is injected before the first anaerobic fermentation to ensure an anaerobic environment. The time for injecting nitrogen is preferably 5 to 20 minutes, more preferably 8 to 12 minutes, and even more preferably 10 minutes.

[0051] In the present invention, the conditions for the first anaerobic fermentation include: the initial pH is preferably 5-7, more preferably 5.7-6.3, and more preferably 6; the temperature is preferably 30-40°C, more preferably 30-35°C, and more preferably 30°C; the rotation speed is preferably 100-200 rpm, more preferably 110-130 rpm, and more preferably 120 rpm; the time is preferably 3-4 days, more preferably 3.5-4 days, and more preferably 4 days.

[0052] In the present invention, the hexanoic acid-synthesizing bacteria preferably include one or more of Clostridium sensu stricto (strain number: CICC 20464, Latin name: Bacillus fusiformis), Romboutsia (Chinese name: White Seaside Dragon Bagel Bacteria, Latin genus name: Romboutsia, Latin species name: lituseburensis, original number: YB101), and Terrisporobacter (strain number: CICC 6006, Chinese name, Ginseng Terrisporobacter, Latin name: Bacillus ginsengihumi), further preferably include Clostridium sensu stricto or Romboutsia, and more preferably include Clostridium sensu stricto.

[0053] In the present invention, the source of the hexanoic acid-synthesizing bacteria is preferably: excess sludge from Shanxi Zhengyang Wastewater Purification Co., Ltd.

[0054] In the present invention, the mass ratio of the concentrated sludge to the hexanoic acid-synthesizing bacteria is preferably 1-2:9-12, more preferably 1-2:9-10, and even more preferably 1:9.

[0055] In the present invention, the hexanoic acid-synthesizing bacteria are preferably acclimated before use.

[0056] In the present invention, the acclimation process preferably includes the following steps: under anaerobic conditions, the caproate-synthesizing bacteria, the chain extension substrate, the culture medium, and the second methane inhibitor are mixed and cultured.

[0057] In the present invention, the chain extension substrate preferably comprises a mixture of sodium acetate and ethanol.

[0058] In the present invention, the amount of sodium acetate used in the acclimation process is preferably 4-6 g / L, more preferably 4.8-5.3 g / L, and more preferably 5 g / L.

[0059] In the present invention, the amount of ethanol used in the acclimation process is preferably 10 to 30 mL / L, more preferably 19 to 24 mL / L, and even more preferably 20 mL / L.

[0060] In the present invention, the culture medium preferably includes: a mixture of ammonium dihydrogen phosphate, sodium bicarbonate, mineral solution, and vitamin solution.

[0061] In the present invention, the amount of ammonium dihydrogen phosphate used in the acclimation process is preferably 3.3-3.8 g / L, more preferably 3.4-3.7 g / L, and even more preferably 3.6 g / L.

[0062] In the present invention, the amount of sodium bicarbonate used in the acclimation process is preferably 5.2-5.7 g / L, more preferably 5.3-5.6 g / L, and even more preferably 5.4 g / L.

[0063] In the present invention, the amount of the mineral solution used during the acclimation process is preferably 3-6 mL / L, more preferably 4-5 mL / L, and even more preferably 5 mL / L.

[0064] In the present invention, the dosage of the vitamin solution during the acclimation process is preferably 7 to 13 μL / L, more preferably 8 to 12 μL / L, and even more preferably 10 μL / L.

[0065] In the present invention, the second methane inhibitor preferably includes sodium 2-bromoethane sulfonate.

[0066] In the present invention, the dosage of the second methane inhibitor during the acclimation process is preferably 5 to 15 g / L, more preferably 8 to 12 g / L, and even more preferably 10 g / L.

[0067] In the present invention, it is preferred that nitrogen gas does not need to be added during the culture process.

[0068] In the present invention, the culture conditions include: the pH value is preferably 4 to 8, more preferably 5 to 7, and more preferably 6; the temperature is preferably 20 to 50°C, more preferably 30 to 40°C, and more preferably 30°C; the rotation speed is preferably 100 to 200 rpm, more preferably 110 to 150 rpm, and more preferably 120 rpm; the time is preferably 2 to 7 days, more preferably 5 to 7 days, and more preferably 7 days.

[0069] In the present invention, the electron donor preferably includes ethanol and / or lactic acid, and more preferably ethanol.

[0070] In the present invention, the molar ratio of the electron acceptor to the electron donor in the fermentation product is preferably 1:3-5, more preferably 1:3-4, and even more preferably 1:3.

[0071] In the present invention, the electron acceptor in the fermentation product is preferably acetic acid.

[0072] In the present invention, the culture medium preferably comprises a mixture of ammonium chloride, magnesium sulfate heptahydrate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, sodium chloride, vitamin solution, and trace element solution.

[0073] In the present invention, the concentration of ammonium chloride in the second anaerobic fermentation mixture is preferably 0.23 to 0.27 g / L, more preferably 0.24 to 0.26 g / L, and even more preferably 0.25 g / L.

[0074] In the present invention, the concentration of the magnesium sulfate heptahydrate in the second anaerobic fermentation mixture is preferably 0.1 to 0.4 g / L, more preferably 0.1 to 0.3 g / L, and even more preferably 0.2 g / L.

[0075] In the present invention, the concentration of potassium dihydrogen phosphate in the second anaerobic fermentation mixture is preferably 0.21 to 0.25 g / L, more preferably 0.22 to 0.24 g / L, and even more preferably 0.23 g / L.

[0076] In the present invention, the concentration of the dipotassium hydrogen phosphate in the second anaerobic fermentation mixture is preferably 0.29 to 0.33 g / L, more preferably 0.3 to 0.32 g / L, and even more preferably 0.31 g / L.

[0077] In the present invention, the concentration of sodium chloride in the second anaerobic fermentation mixture is preferably 0.6 to 1 g / L, more preferably 0.7 to 0.9 g / L, and even more preferably 0.8 g / L.

[0078] In the present invention, the concentration of the vitamin solution in the second anaerobic fermentation mixture is preferably 1 to 3 mL / L, more preferably 1 to 2 mL / L, and even more preferably 1 mL / L.

[0079] In the present invention, the concentration of the trace element solution in the second anaerobic fermentation mixture is preferably 1 to 3 mL / L, more preferably 1 to 2 mL / L, and even more preferably 1 mL / L.

[0080] In the present invention, the first methane inhibitor preferably includes sodium 2-bromoethane sulfonate.

[0081] In the present invention, the concentration of the first methane inhibitor in the second anaerobic fermentation mixture is preferably 8 to 12 g / L, more preferably 9 to 11 g / L, and even more preferably 10 g / L.

[0082] In the present invention, nitrogen purging is performed before the second anaerobic fermentation, and the nitrogen purging time is preferably 8 to 12 minutes, more preferably 9 to 11 minutes, and even more preferably 10 minutes.

[0083] In the present invention, the conditions for the second anaerobic fermentation include: the initial pH is preferably 4-8, more preferably 5-7, and more preferably 6; the temperature is preferably 20-50°C, more preferably 30-40°C, and more preferably 30°C; the rotation speed is preferably 100-200 rpm, more preferably 110-150 rpm, and more preferably 120 rpm; the time is preferably 2-14 days, more preferably 10-14 days, and more preferably 14 days.

[0084] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0085] Example 1

[0086] This embodiment provides a method for synthesizing caproic acid by carbon chain extension based on two-stage anaerobic fermentation, which is recorded as TS_AP_CSB and includes the following steps:

[0087] (1) The residual sludge was taken from the secondary sedimentation tank of the sewage treatment plant (Shanxi Zhengyang Sewage Purification Co., Ltd., sampling time June 7, 2022), left to stand naturally at 4°C for 24 h, and the supernatant was discharged for concentration; after concentration, the impurities in the sludge were removed by a 40-mesh sieve to obtain concentrated sludge with a pH of 7.35, a total suspended solids concentration of 31.83 g / L, and a volatile suspended solids concentration of 12.05 g / L;

[0088] (2) A total of 100 mL of hexanoic acid-synthesizing bacteria (Clostridium sensu stricto, strain number: CICC20464, Latin name: Bacillus fusiformis, from the excess sludge of Shanxi Zhengyang Wastewater Purification Co., Ltd.) was inoculated into a 500 mL anaerobic fermentation bottle. Under anaerobic conditions, 5 g / L sodium acetate and 20 mL / L ethanol were used as chain extension substrates. The culture medium was added. The culture medium components included 3.6 g / L ammonium dihydrogen phosphate, 5.4 g / L sodium bicarbonate, 5 mL / L mineral solution (MnSO4·H2O 0.10 g, ZnSO4·7H2O 0.12 g, H3BO3 0.07 g, Na2MoO4·2H2O 0.04 g, CuSO4·5H2O0.02 g, CoCl20.04 g, water 1 L) and 10 μL / L vitamin solution (biotin 20 mg, folic acid 20 mg, pyridoxine hydrochloride 100 mg, thiamine hydrochloride 5.0 mg, riboflavin 50 mg, niacin 50 mg, calcium pantothenate 50 mg, vitamin B12 1 mg, p-aminobenzoic acid 50 mg, lipoic acid 50 mg, distilled water 1 L). In addition, 10 g / L sodium 2-bromoethanesulfonate was added as a methanogenesis inhibitor. The above additives were calculated based on the total amount during the acclimation process. During the acclimation process, the pH was controlled at around 6.0, the temperature was controlled at 30°C, the shaker speed was 120 rpm, and the acclimation was carried out for 7 days.

[0089] (3) The concentrated sludge was subjected to alkali pretreatment, and a high-concentration NaOH mother liquor (sodium hydroxide concentration was 4 mol / L, and hydrochloric acid mass fraction was 12%) was added. The mass ratio of sodium hydroxide in the NaOH mother liquor to the total suspended solids of the concentrated sludge was 0.09:1. The pH was adjusted to 12, and the sludge was treated at 120°C for 20 min. During this period, the concentrated sludge was sterilized in a high-temperature autoclave. The alkali-pretreated concentrated sludge was then placed in a fermentation bottle for anaerobic fermentation. Nitrogen was added for 10 min to ensure an anaerobic environment. The fermentation bottle was placed in a shaker for culture. The initial pH during fermentation was 6. The fermentation was carried out at 30°C and a shaker speed of 120 rpm for 4 days. The fermentation was stopped when the volatile fatty acid yield reached the highest level, and the fermentation product was obtained.

[0090] (4) The fermentation product was placed in a fermentation bottle, and domesticated hexanoic acid-synthesizing bacteria were added. The mass ratio of concentrated sludge to hexanoic acid-synthesizing bacteria was 1:9. Electron donor ethanol was added to the bottle at a molar ratio of electron acceptor acetic acid to electron donor of 1:3, and a culture medium was added. The culture medium contained 0.25 g / L ammonium chloride, 0.2 g / L magnesium sulfate heptahydrate, 0.23 g / L potassium dihydrogen phosphate, 0.31 g / L potassium hydrogen phosphate, 0.8 g / L sodium chloride, 1 mL / L vitamin solution (biotin 20 mg, folic acid 20 mg, pyridoxine hydrochloride 100 mg, thiamine hydrochloride 5.0 mg, riboflavin 50 mg, nicotinic acid 50 mg, calcium pantothenate 50 mg, vitamin B12 1 mg, p-aminobenzoic acid 50 mg, lipoic acid 50 mg, and distilled water 1 L), and trace element solution (MnSO4·H2O 0.10 g, ZnSO4·7H2O0.12 g, H3BO30.07 g, Na2MoO4·2H2O0.04 g, CuSO4·5H2O0.02 g, CoCl20.04 g, and water (1 L) (1 mL / L) were added, 10 g / L sodium 2-bromoethanesulfonate was added to inhibit methane production, and the total amount of culture medium and sodium 2-bromoethanesulfonate during the anaerobic fermentation process was adjusted to pH 6.0. Finally, the bottle cap was tightened, and after purging with high-purity nitrogen for 10 min, the bottle was placed in a shaker for anaerobic fermentation. Fermentation was carried out at 30°C and a shaker speed of 120 rpm for 14 days, and the fermentation was stopped when the hexanoic acid production reached its maximum.

[0091] Comparative Example 1

[0092] This comparative example provides a method for synthesizing caproic acid by carbon chain extension through one-stage anaerobic fermentation of sludge without alkaline pretreatment, which is recorded as Control and comprises the following steps:

[0093] (1) The residual sludge was taken from the secondary sedimentation tank of the sewage treatment plant (Shanxi Zhengyang Sewage Purification Co., Ltd., sampling time June 7, 2022), left to stand naturally at 4°C for 24 h, and the supernatant was discharged for concentration; after concentration, the impurities in the sludge were removed by a 40-mesh sieve to obtain concentrated sludge with a pH of 7.35, a total suspended solids concentration of 31.83 g / L, and a volatile suspended solids concentration of 12.05 g / L;

[0094] (2) The concentrated sludge was placed in a fermentation bottle and a culture medium was added. The culture medium contained 0.25 g / L ammonium chloride, 0.2 g / L magnesium sulfate heptahydrate, 0.23 g / L potassium dihydrogen phosphate, 0.31 g / L potassium hydrogen phosphate, 0.8 g / L sodium chloride, 1 mL / L vitamin solution (biotin 20 mg, folic acid 20 mg, pyridoxine hydrochloride 100 mg, thiamine hydrochloride 5.0 mg, riboflavin 50 mg, nicotinic acid 50 mg, calcium pantothenate 50 mg, vitamin B12 1 mg, p-aminobenzoic acid 50 mg, lipoic acid 50 mg, distilled water 1 L), and 1 mL / L trace element solution (MnSO4·H2O 0.10 g, ZnSO4·7H2O 0.12 g, H3BO3 0.07 g, Na2MoO4·2H2O 0.04 g, 0.02 g CuSO4·5H2O, 0.04 g CoCl2, and 1 L of water) were added to 1 mL / L of the mixture. 10 g / L sodium 2-bromoethanesulfonate was added to inhibit methane production. The total amount of culture medium and sodium 2-bromoethanesulfonate during the anaerobic fermentation process was calculated. The pH was adjusted to about 6.0. Finally, the bottle cap was tightened, and after purging with high-purity nitrogen for 10 minutes, the bottle was placed in a shaker for anaerobic fermentation. Fermentation was carried out at 30°C and a shaker speed of 120 rpm for 4 days, and the fermentation was stopped when the hexanoic acid production reached its maximum.

[0095] Comparative Example 2

[0096] This comparative example provides a method for synthesizing caproic acid by carbon chain extension in one-stage anaerobic fermentation of sludge without alkaline pretreatment and the addition of caproic acid-synthesizing bacteria. This method is recorded as Control_CSB and comprises the following steps:

[0097] (1) The residual sludge was taken from the secondary sedimentation tank of the sewage treatment plant (Shanxi Zhengyang Sewage Purification Co., Ltd., sampling time June 7, 2022), left to stand naturally at 4°C for 24 h, and the supernatant was discharged for concentration; after concentration, the impurities in the sludge were removed by a 40-mesh sieve to obtain concentrated sludge with a pH of 7.35, a total suspended solids concentration of 31.83 g / L, and a volatile suspended solids concentration of 12.05 g / L;

[0098] (2) A total of 100 mL of hexanoic acid-synthesizing bacteria (Clostridium sensu stricto, strain number: CICC20464, Latin name: Bacillus fusiformis, from the excess sludge of Shanxi Zhengyang Wastewater Purification Co., Ltd.) was inoculated into a 500 mL anaerobic fermentation bottle. Under anaerobic conditions, 5 g / L sodium acetate and 20 mL / L ethanol were used as chain extension substrates. The culture medium was added. The culture medium components included 3.6 g / L ammonium dihydrogen phosphate, 5.4 g / L sodium bicarbonate, 5 mL / L mineral solution (MnSO4·H2O 0.10 g, ZnSO4·7H2O 0.12 g, H3BO3 0.07 g, Na2MoO4·2H2O 0.04 g, CuSO4·5H2O0.02 g, CoCl20.04 g, water 1 L) and 10 μL / L vitamin solution (biotin 20 mg, folic acid 20 mg, pyridoxine hydrochloride 100 mg, thiamine hydrochloride 5.0 mg, riboflavin 50 mg, niacin 50 mg, calcium pantothenate 50 mg, vitamin B12 1 mg, p-aminobenzoic acid 50 mg, lipoic acid 50 mg, distilled water 1 L). In addition, 10 g / L sodium 2-bromoethanesulfonate was added as a methanogenesis inhibitor. The above additives were calculated based on the total amount during the acclimation process. During the acclimation process, the pH was controlled at around 6.0, the temperature was controlled at 30°C, the shaker speed was 120 rpm, and the acclimation was carried out for 7 days.

[0099] (3) The concentrated sludge was placed in a fermentation bottle, and the domesticated hexanoic acid-synthesizing bacteria were added. The ratio of concentrated sludge to hexanoic acid-synthesizing bacteria was 1:9. A culture medium was added, which contained 0.25 g / L ammonium chloride, 0.2 g / L magnesium sulfate heptahydrate, 0.23 g / L potassium dihydrogen phosphate, 0.31 g / L potassium hydrogen phosphate, 0.8 g / L sodium chloride, 1 mL / L vitamin solution (biotin 20 mg, folic acid 20 mg, pyridoxine hydrochloride 100 mg, thiamine hydrochloride 5.0 mg, riboflavin 50 mg, nicotinic acid 50 mg, calcium pantothenate 50 mg, vitamin B12 1 mg, p-aminobenzoic acid 50 mg, lipoic acid 50 mg, distilled water 1 L), and trace element solution (MnSO4·H2O 0.10 g, ZnSO4·7H2O 0.12 g, H3BO3 0.07 g, Na2MoO4·2H2O 0.04 g, 0.02 g CuSO4·5H2O, 0.04 g CoCl2, and 1 L of water) were added to 1 mL / L of the mixture. 10 g / L sodium 2-bromoethanesulfonate was added to inhibit methane production. The total amount of culture medium and sodium 2-bromoethanesulfonate during the anaerobic fermentation process was calculated. The pH was adjusted to about 6.0. Finally, the bottle cap was tightened, and after purging with high-purity nitrogen for 10 minutes, the bottle was placed in a shaker for anaerobic fermentation. Fermentation was carried out at 30°C and a shaker speed of 120 rpm for 14 days, and the fermentation was stopped when the hexanoic acid production reached its maximum.

[0100] Comparative Example 3

[0101] This comparative example provides a method for synthesizing caproic acid by carbon chain extension through one-stage anaerobic fermentation based on sludge alkaline pretreatment, which is referred to as AP and comprises the following steps:

[0102] (1) The residual sludge was taken from the secondary sedimentation tank of the sewage treatment plant (Shanxi Zhengyang Sewage Purification Co., Ltd., sampling time June 7, 2022), left to stand naturally at 4°C for 24 h, and the supernatant was discharged for concentration; after concentration, the impurities in the sludge were removed by a 40-mesh sieve to obtain concentrated sludge with a pH of 7.35, a total suspended solids concentration of 31.83 g / L, and a volatile suspended solids concentration of 12.05 g / L;

[0103] (2) The concentrated sludge was subjected to alkaline pretreatment by adding a high-concentration NaOH mother liquor (sodium hydroxide concentration of 4 mol / L, hydrochloric acid mass fraction of 12%), with the mass ratio of sodium hydroxide in the NaOH mother liquor to the total suspended solids of the concentrated sludge being 0.09:1. The pH was adjusted to 12, and the sludge was treated at 120°C for 20 min, during which sterilization was performed in a high-temperature autoclave;

[0104] (3) The concentrated sludge after alkali pretreatment was placed in a fermentation bottle, and a culture medium was added. The nutrients included 0.25 g / L ammonium chloride, 0.2 g / L magnesium sulfate heptahydrate, 0.23 g / L potassium dihydrogen phosphate, 0.31 g / L potassium hydrogen phosphate, 0.8 g / L sodium chloride, 1 mL / L vitamin solution (biotin 20 mg, folic acid 20 mg, pyridoxine hydrochloride 100 mg, thiamine hydrochloride 5.0 mg, riboflavin 50 mg, nicotinic acid 50 mg, calcium pantothenate 50 mg, vitamin B12 1 mg, p-aminobenzoic acid 50 mg, lipoic acid 50 mg, distilled water 1 L), and trace element solution (MnSO4·H2O 0.10 g, ZnSO4·7H2O 0.12 g, H3BO3 0.07 g, Na2MoO4·2H2O 0.04 g, CuSO4·5H2O 0.02 g, 0.04 g CoCl2, and 1 mL / L water) were added, and 10 g / L sodium 2-bromoethanesulfonate was added to inhibit methane production. The culture medium and sodium 2-bromoethanesulfonate were calculated based on the total amount during the anaerobic fermentation process. The pH was adjusted to about 6.0. Finally, the bottle cap was tightened, and after purging with high-purity nitrogen for 10 minutes, the bottle was placed in a shaker for anaerobic fermentation. Fermentation was carried out at 30°C and a shaker speed of 120 rpm for 14 days, and the fermentation was stopped when the hexanoic acid production reached its maximum.

[0105] Comparative Example 4

[0106] This comparative example provides a method for synthesizing caproic acid by carbon chain extension based on sludge alkaline pretreatment and one-stage anaerobic fermentation with the addition of caproic acid-synthesizing bacteria. This method is referred to as AP_CSB and comprises the following steps:

[0107] (1) The residual sludge was taken from the secondary sedimentation tank of the sewage treatment plant (Shanxi Zhengyang Sewage Purification Co., Ltd., sampling time June 7, 2022), left to stand naturally at 4°C for 24 h, and the supernatant was discharged for concentration; after concentration, the impurities in the sludge were removed by a 40-mesh sieve to obtain concentrated sludge with a pH of 7.35, a total suspended solids concentration of 31.83 g / L, and a volatile suspended solids concentration of 12.05 g / L;

[0108] (2) A total of 100 mL of hexanoic acid-synthesizing bacteria (Clostridium sensu stricto, strain number: CICC20464, Latin name: Bacillus fusiformis, from the excess sludge of Shanxi Zhengyang Wastewater Purification Co., Ltd.) was inoculated into a 500 mL anaerobic fermentation bottle. Under anaerobic conditions, 5 g / L sodium acetate and 20 mL / L ethanol were used as chain extension substrates. The culture medium was added. The culture medium components included 3.6 g / L ammonium dihydrogen phosphate, 5.4 g / L sodium bicarbonate, 5 mL / L mineral solution (MnSO4·H2O 0.10 g, ZnSO4·7H2O 0.12 g, H3BO3 0.07 g, Na2MoO4·2H2O 0.04 g, CuSO4·5H2O0.02 g, CoCl20.04 g, water 1 L) and 10 μL / L vitamin solution (biotin 20 mg, folic acid 20 mg, pyridoxine hydrochloride 100 mg, thiamine hydrochloride 5.0 mg, riboflavin 50 mg, niacin 50 mg, calcium pantothenate 50 mg, vitamin B12 1 mg, p-aminobenzoic acid 50 mg, lipoic acid 50 mg, distilled water 1 L). In addition, 10 g / L sodium 2-bromoethanesulfonate was added as a methanogenesis inhibitor. The above additives were calculated based on the total amount during the acclimation process. During the acclimation process, the pH was controlled at around 6.0, the temperature was controlled at 30°C, the shaker speed was 120 rpm, and the acclimation was carried out for 7 days.

[0109] (3) The concentrated sludge was subjected to alkaline pretreatment by adding a high-concentration NaOH mother liquor (sodium hydroxide concentration of 4 mol / L, hydrochloric acid mass fraction of 12%), with the mass ratio of sodium hydroxide in the NaOH mother liquor to the total suspended solids of the concentrated sludge being 0.09:1. The pH was adjusted to 12, and the sludge was treated at 120°C for 20 min, during which sterilization was performed in a high-temperature autoclave;

[0110] (4) The concentrated sludge pretreated with alkali was placed in a fermentation bottle, and the domesticated hexanoic acid-synthesizing bacteria were added. The mass ratio of the concentrated sludge pretreated with alkali to the hexanoic acid-synthesizing bacteria was 1:9. A culture medium was added, which contained 0.25 g / L ammonium chloride, 0.2 g / L magnesium sulfate heptahydrate, 0.23 g / L potassium dihydrogen phosphate, 0.31 g / L potassium hydrogen phosphate, 0.8 g / L sodium chloride, 1 mL / L vitamin solution (biotin 20 mg, folic acid 20 mg, pyridoxine hydrochloride 100 mg, thiamine hydrochloride 5.0 mg, riboflavin 50 mg, nicotinic acid 50 mg, calcium pantothenate 50 mg, vitamin B12 1 mg, p-aminobenzoic acid 50 mg, lipoic acid 50 mg, and distilled water 1 L), and trace element solution (MnSO4·H2O 0.10 g, ZnSO4·7H2O 0.12 g, 0.07 g H3BO3, 0.04 g Na2MoO4·2H2O, 0.02 g CuSO4·5H2O, 0.04 g CoCl2, and 1 L of water (1 mL / L), 10 g / L sodium 2-bromoethanesulfonate was added to inhibit methane production. The culture medium and sodium 2-bromoethanesulfonate were calculated based on the total amount during the anaerobic fermentation process. The pH was adjusted to about 6.0. Finally, the bottle cap was tightened, and after purging with high-purity nitrogen for 10 minutes, the bottle was placed in a shaker for anaerobic fermentation. Fermentation was carried out at 30°C and a shaker speed of 120 rpm for 14 days, and the fermentation was stopped when the hexanoic acid production reached its maximum.

[0111] Comparative Example 5

[0112] This comparative example provides a method for synthesizing caproic acid by carbon chain extension through two-stage anaerobic fermentation based on sludge alkaline pretreatment, which is denoted as TS_AP and comprises the following steps:

[0113] (1) The residual sludge was taken from the secondary sedimentation tank of the sewage treatment plant (Shanxi Zhengyang Sewage Purification Co., Ltd., sampling time June 7, 2022), left to stand naturally at 4°C for 24 h, and the supernatant was discharged for concentration; after concentration, the impurities in the sludge were removed by a 40-mesh sieve to obtain concentrated sludge with a pH of 7.35, a total suspended solids concentration of 31.83 g / L, and a volatile suspended solids concentration of 12.05 g / L;

[0114] (2) The concentrated sludge was subjected to alkali pretreatment, and a high-concentration NaOH mother liquor (sodium hydroxide concentration was 4 mol / L, and hydrochloric acid mass fraction was 12%) was added. The mass ratio of sodium hydroxide in the NaOH mother liquor to the total suspended solids of the concentrated sludge was 0.09:1. The pH was adjusted to 12, and the sludge was treated at 120°C for 20 min. During this period, the concentrated sludge was sterilized in a high-temperature autoclave. The concentrated sludge was then placed in a fermentation bottle for anaerobic fermentation. Nitrogen was filled for 10 min to ensure an anaerobic environment. The fermentation bottle was placed in a shaker for culture. The initial pH during fermentation was 6. The fermentation was carried out at 30°C and a shaker speed of 120 rpm for 4 days. The fermentation was stopped when the volatile fatty acid yield reached the highest level, and the fermentation product was obtained.

[0115] (3) The fermentation product was placed in a fermentation bottle, and electron donor ethanol was added to the bottle at a molar ratio of electron acceptor acetic acid to electron donor of 1:3, and a culture medium was added, wherein the culture medium contained 0.25 g / L ammonium chloride, 0.2 g / L magnesium sulfate heptahydrate, 0.23 g / L potassium dihydrogen phosphate, 0.31 g / L potassium hydrogen phosphate, 0.8 g / L sodium chloride, 1 mL / L vitamin solution (biotin 20 mg, folic acid 20 mg, pyridoxine hydrochloride 100 mg, thiamine hydrochloride 5.0 mg, riboflavin 50 mg, nicotinic acid 50 mg, calcium pantothenate 50 mg, vitamin B12 1 mg, p-aminobenzoic acid 50 mg, lipoic acid 50 mg, distilled water 1 L), and trace element solution (MnSO4·H2O 0.10 g, ZnSO4·7H2O 0.12 g, H3BO3 0.07 g, Na2MoO4·2H2O 0.04 g, 0.02 g CuSO4·5H2O, 0.04 g CoCl2, and 1 L of water) were added to 1 mL / L of the mixture. 10 g / L sodium 2-bromoethanesulfonate was added to inhibit methane production. The total amount of culture medium and sodium 2-bromoethanesulfonate during the anaerobic fermentation process was calculated. The pH was adjusted to about 6.0. Finally, the bottle cap was tightened, and after purging with high-purity nitrogen for 10 minutes, the bottle was placed in a shaker for anaerobic fermentation. Fermentation was carried out at 30°C and a shaker speed of 120 rpm for 14 days, and the fermentation was stopped when the hexanoic acid production reached its maximum.

[0116] After pretreatment, the sludge was fermented in one-stage and two-stage ways (AP, AP_CSB, TS_AP, TS_AP_CSB), and a blank control group (Control, Control_CSB) was set up to extend the carbon chain to synthesize hexanoic acid. The yield changes were as follows: Figure 1 As shown. Figure 1The highest caproic acid yields under the one-stage and two-stage fermentation conditions were: Control (23.70 mg COD / g VSS), AP (8.21 mg COD / g VSS), TS_AP (102.38 mg COD / g VSS), Control_CSB (51.12 mg COD / g VSS), AP_CSB (62.92 mg COD / g VSS), and TS_AP_CSB (156.07 mg COD / g VSS). The caproic acid yields of TS_AP were 1.25 times and 3.60 times that of the Control and AP groups, respectively. The caproic acid yields of TS_AP_CSB were 3.05 times and 1.52 times that of the Control_CSB and AP_CSB groups, respectively. Two-stage anaerobic fermentation significantly outperformed single-stage anaerobic fermentation in producing caproic acid. Both the Control and AP groups produced small amounts of hexanoic acid, indicating that the sludge-containing bacteria can metabolize substrates to a certain extent to synthesize small amounts of hexanoic acid. The lower hexanoic acid concentration in the AP group may be due to the effect of alkaline pretreatment on the remaining thermosporidial bacteria, resulting in a lack of bacteria suitable for carbon chain extension in the AP group. Furthermore, although the two-stage sludge pretreatment control group (TS_AP) lacked sufficient carbon chain extension bacteria, its Period I provided sufficient substrate, and its resident bacteria were able to utilize short-chain fatty acids such as acetic acid produced during the pre-fermentation phase to produce more hexanoic acid than the other two sludge-containing bacteria metabolites (Control and AP). The TS_AP_CSB and TS_AP groups both had the highest hexanoic acid production at the end of the fermentation, clearly demonstrating that two-stage anaerobic fermentation is more conducive to carbon chain extension reactions, promotes the reaction process, and alleviates the reaction lag caused by the lack of short-chain fatty acids in the one-stage anaerobic fermentation system.

[0117] Comparison of fluorescence spectra of original sludge and alkali pretreatment (AP) on the release of extracellular polymeric substances in sludge Figure 2As shown in Figure 2, the fluorescence characteristics of dissolved organic matter in the sample can be divided into five regions. The focus of this study was on biodegradable organic matter in Region I (i.e., tyrosine-like proteins, Ex / Em 200-250 / 200-330) and Region IV (i.e., soluble microbial byproducts, Ex / Em 250-280 / 200-380). Organic matter in Regions II (i.e., tryptophan-like substances, Ex / Em 200-250 / 330-380), III (i.e., fulvic acids, Ex / Em 200-250 / 380-500), and V (i.e., humic acids, Ex / Em 250-400 / 380-500) is generally considered non-biodegradable. As can be seen, alkaline pretreatment effectively disrupted the structure of extracellular polymeric substances, significantly increasing the fluorescence intensity of the outermost dissolved organic matter layer of the extracellular polymeric substances. This indicates that alkaline pretreatment can effectively break the sludge cell walls, release more soluble organic matter, provide more available organic matter for microorganisms, increase the acetic acid concentration, and thus provide a more sufficient organic matrix for the subsequent synthesis of medium-chain fatty acids.

[0118] In summary, the present invention discloses a method for synthesizing caproic acid by carbon chain extension based on two-stage anaerobic fermentation. Based on the fact that the acid production by anaerobic fermentation of excess sludge is limited to the release and hydrolysis stage of macromolecular organic matter wrapped in cell walls and extracellular polymers, alkaline pretreatment and pre-fermentation in a two-stage process and the addition of caproic acid-synthesizing bacteria are proposed. The two-stage anaerobic fermentation is more conducive to the carbon chain extension reaction and produces a promotion process. Unlike the slow reaction process caused by the lack of short-chain fatty acids in the one-stage anaerobic fermentation system, the two-stage anaerobic fermentation process produces more caproic acid. In addition, the alkaline pretreatment effectively destroys the structure of extracellular polymer substances, which indicates that the alkaline pretreatment can release more dissolved organic matter, increase the concentration of acetic acid, and provide more organic matrix, thereby increasing the yield of caproic acid.

[0119] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for synthesizing caproic acid by carbon chain extension based on two-stage anaerobic fermentation, characterized in that: The following steps are involved: performing an alkali pretreatment on the concentrated sludge; performing a first anaerobic fermentation on the concentrated sludge after the alkali pretreatment to obtain a fermentation product; The fermentation product, hexanoic acid-synthesizing bacteria, an electron donor, a culture medium, and a first methane inhibitor are mixed and subjected to a second anaerobic fermentation to obtain hexanoic acid.

2. The method for synthesizing caproic acid by carbon chain extension based on two-stage anaerobic fermentation according to claim 1, characterized in that: The pH of the concentrated sludge is 7.33-7.37, the total suspended solids concentration of the concentrated sludge is 31.81-31.85 g / L, and the volatile suspended solids concentration of the concentrated sludge is 12.03-12.07 g / L.

3. The method for synthesizing caproic acid by carbon chain extension based on two-stage anaerobic fermentation according to claim 1 or 2, characterized in that: The conditions of the alkali pretreatment include: pH 10-14, temperature 110-130° C., and time 10-30 minutes.

4. The method for synthesizing caproic acid by carbon chain extension based on two-stage anaerobic fermentation according to claim 1, characterized in that: The conditions for the first anaerobic fermentation include: initial pH of 5-7, temperature of 30-40° C., rotation speed of 100-200 rpm, and time of 3-4 days.

5. The method for synthesizing caproic acid by carbon chain extension based on two-stage anaerobic fermentation according to claim 1, characterized in that: The hexanoic acid-synthesizing bacteria include one or more of Clostridium sensu stricto, Romboutsia, and Terrisporobacter; The mass ratio of the concentrated sludge to the hexanoic acid-synthesizing bacteria is 1-2:9-12.

6. The method for synthesizing caproic acid by carbon chain extension based on two-stage anaerobic fermentation according to claim 1 or 5, characterized in that: The hexanoic acid-synthesizing bacteria are acclimated before use; The acclimation process comprises the following steps: under anaerobic conditions, mixing caproate-synthesizing bacteria, a chain extension substrate, a culture medium, and a second methane inhibitor, and culturing the mixture; The culture conditions include: pH value of 4-8, temperature of 20-50° C., rotation speed of 100-200 rpm, and time of 2-7 days.

7. The method for synthesizing caproic acid by carbon chain extension based on two-stage anaerobic fermentation according to claim 1, characterized in that: The electron donor includes ethanol and / or lactic acid; The molar ratio of the electron acceptor to the electron donor in the fermentation product is 1:3-5.

8. The method for synthesizing caproic acid by carbon chain extension based on two-stage anaerobic fermentation according to claim 1 or 7, characterized in that: The culture medium includes a mixture of ammonium chloride, magnesium sulfate heptahydrate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, sodium chloride, vitamin solution, and trace element solution; Wherein, the concentration of ammonium chloride in the second anaerobic fermentation mixture is 0.23-0.27 g / L; The concentration of the magnesium sulfate heptahydrate in the second anaerobic fermentation mixture is 0.1 to 0.4 g / L; The concentration of the potassium dihydrogen phosphate in the second anaerobic fermentation mixture is 0.21-0.25 g / L; The concentration of the dipotassium hydrogen phosphate in the second anaerobic fermentation mixture is 0.29-0.33 g / L; The concentration of sodium chloride in the second anaerobic fermentation mixture is 0.6-1 g / L; The concentration of the vitamin solution in the second anaerobic fermentation mixture is 1 to 3 mL / L; The concentration of the trace element solution in the second anaerobic fermentation mixture is 1 to 3 mL / L.

9. The method for synthesizing caproic acid by carbon chain extension based on two-stage anaerobic fermentation according to claim 8, characterized in that: The first methane inhibitor includes sodium 2-bromoethane sulfonate; The concentration of the first methane inhibitor in the second anaerobic fermentation mixture is 8-12 g / L.

10. The method for synthesizing caproic acid by carbon chain extension based on two-stage anaerobic fermentation according to claim 1, characterized in that: The conditions for the second anaerobic fermentation include: initial pH of 4 to 8, temperature of 20 to 50° C., rotation speed of 100 to 200 rpm, and time of 2 to 14 days.

Images