Method for promoting production of medium-chain fatty acids by anaerobic fermentation of residual sludge using vivianite

Abstract

The present application belongs to the technical field of organic waste anaerobic fermentation, and discloses a method for producing medium-chain fatty acids by using blue vitriol to promote residual sludge anaerobic fermentation, which uses residual activated sludge from a municipal sewage treatment plant as a fermentation substrate, inoculates anaerobic sludge, adds blue vitriol and a methanogenesis inhibitor, adjusts pH value, and carries out anaerobic fermentation to produce medium-chain fatty acids under the participation of an electron donor; due to the addition of blue vitriol, the electron transfer capacity of microorganisms in the carbon chain extension process can be strengthened, thereby improving the yield of medium-chain fatty acids in the fermentation product; the addition amount of blue vitriol in the system is 2-20 g / L. By introducing blue vitriol into the sludge medium-chain fatty acid production system and strictly controlling the addition amount of blue vitriol in the system, the present application uses blue vitriol as an electron mediator to strengthen the electron transfer efficiency of microbial fermentation in the sludge medium-chain fatty acid production process, thereby improving the yield of medium-chain fatty acids, and further realizing efficient utilization of sludge resources.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of organic waste anaerobic fermentation, and more particularly relates to a method for producing medium-chain fatty acids by promoting anaerobic fermentation of residual sludge using blue vitriol. BACKGROUND

[0002] Residual sludge is the main byproduct of the biological treatment process in wastewater treatment plants, which contains rich organic matter and nutrients such as nitrogen and phosphorus. With the rapid advancement of urbanization, the amount of wastewater treatment is increasing year by year, and the production of residual sludge is also increasing year by year. According to statistics, the production of municipal sludge (based on 80% moisture content) in China in 2021 exceeded 800 million tons, and it is estimated that the sludge production in China will exceed 100 million tons by 2025. At present, the main treatment methods of sludge include landfill and incineration, which have problems such as occupying a large amount of land resources, consuming fossil energy, and causing secondary pollution. According to statistics, the current treatment cost of sludge accounts for more than 60% of the total operating cost of the wastewater treatment plant. These traditional treatment methods have failed to realize the resource utilization of organic matter and nutrients in sludge, which not only wastes resources but also causes environmental burden.

[0003] Anaerobic fermentation to produce medium-chain fatty acids can realize the resource utilization of organic matter in sludge, and provides a green and sustainable path to achieve the goal of high-value resource treatment of sludge. Medium-chain fatty acids (monosaturated straight-chain carboxylic acids with carbon number of 6-12, such as n-hexanoic acid) are a class of widely used platform compounds. Compared with traditional anaerobic fermentation products such as ethanol and short-chain fatty acids, medium-chain fatty acids have high energy density and low water solubility, and can be widely used as biomass fuel precursors and sterilizing agents, so they have high added value. However, the yield and rate of medium-chain fatty acids by traditional technology are still low, which is mainly related to the low electron transfer efficiency of carbon chain elongation in anaerobic fermentation, limiting the industrial application of the technology.

[0004] Many studies have shown that the use of material additives can promote the electron transfer of microbial anaerobic fermentation process and thus improve the production of medium-chain fatty acids. Chinese Patent CN110734933A discloses a method of adding nano zero-valent iron to promote the production of medium-chain fatty acids by sludge anaerobic fermentation, but the production cost of zero-valent iron is high, the stability is poor, the storage condition is harsh, the aggregation is easy, and a large amount of addition is needed to achieve the promotion effect. Chinese Patent CN118291554A discloses a method of using pyrite to promote the production of medium-chain fatty acids by sludge, but the sulfur element will produce toxic hydrogen sulfide gas with unpleasant odor after anaerobic fermentation, which will harm the environment and human health. Chinese Patent CN118995838A discloses a method of adding manganese ore to promote the production of medium-chain fatty acids, but manganese is a heavy metal that harms the environment and human health, and after being added to sludge anaerobic fermentation, heavy metals will be left in the sludge fermentation residue, making it difficult to compost or utilize in agriculture and greening. Therefore, the addition of manganese will limit the subsequent treatment and resource utilization of the fermented sludge.

[0005] Vivianite is a widely available iron phosphate mineral in nature, which is non-toxic and harmless, easy to synthesize from industrial wastewater, and has weak magnetism. Currently, several patents (CN113023702B, CN118184042A, CN113173648A) have reported methods for recovering vivianite from wastewater and sludge, confirming the characteristics of wide source and simple synthesis of vivianite. However, there is no research or patent exploring its application in promoting the production of medium-chain fatty acids by sludge anaerobic fermentation. If a process for promoting the production of medium-chain fatty acids by sludge anaerobic fermentation using vivianite can be developed, it will not only improve the yield of medium-chain fatty acids, but also realize the resource utilization of sludge, which is of great significance for engineering application and environmental benefits. SUMMARY

[0006] In view of the defects in the existing technology for producing medium-chain fatty acids by sludge anaerobic fermentation, the purpose of the present application is to provide a method for promoting the production of medium-chain fatty acids by residual sludge anaerobic fermentation using vivianite. In this method, vivianite is introduced into the sludge anaerobic fermentation system, and the addition amount of vivianite in the system is strictly controlled. Vivianite is used as an electron mediator to strengthen the electron transfer efficiency of microbial fermentation in the process of producing medium-chain fatty acids by sludge, thereby improving the yield of medium-chain fatty acids in the process of sludge anaerobic fermentation, and realizing the efficient utilization of sludge resources. Compared with the prior art, the vivianite used in the present application has the advantages of non-toxicity, wide source, simple synthesis, etc., which can effectively solve the problems of low yield and slow speed of sludge anaerobic fermentation for producing medium-chain fatty acids, promote the production of medium-chain fatty acids by anaerobic fermentation, and realize the engineering application of sludge resource utilization.

[0007] In order to achieve the above-mentioned purpose, according to the present application, a method for adding blue vitriol to promote sludge anaerobic fermentation to produce medium-chain fatty acids is provided, characterized in that at least one of residual activated sludge from a municipal sewage treatment plant, concentrated residual activated sludge, and sludge hydrolysis acidification liquid is used as a fermentation substrate, anaerobic sludge is inoculated into the fermentation substrate, blue vitriol and a methanogenesis inhibitor are added, and the initial fermentation pH value of the system is adjusted to 6.0-7.5, then the fermentation system is purged with protective gas to exclude oxygen and sealed, and anaerobic fermentation is carried out under the participation of an electron donor; due to the addition of blue vitriol, the electron transfer capacity of microorganisms in the carbon chain elongation process can be strengthened, thereby improving the yield of medium-chain fatty acids in the fermentation product.

[0008] In the method, the amount of blue vitriol added in the system is 2-20 g / L; the sludge hydrolysis acidification liquid is obtained by hydrolysis and acidification of residual activated sludge; and the carbon number of the medium-chain fatty acids is 6-12.

[0009] As a further preferred embodiment of the present application, the blue vitriol is blue vitriol powder or particles, which is blue.

[0010] As a further preferred embodiment of the present application, the anaerobic sludge is anaerobic fermentation tank sludge of a sewage treatment plant, or sludge in which a carbon chain elongation functional bacteria genus is enriched after domestication of anaerobic sludge.

[0011] As a further preferred embodiment of the present application, the electron donor is selected from ethanol, lactic acid, methanol, amino acids, sugars, hydrogen, and carbon monoxide.

[0012] When the electron donor is selected from ethanol, lactic acid, methanol, amino acids, and sugars, the electron donor is added to the system before the initial fermentation pH value of the system is adjusted.

[0013] When the electron donor is selected from hydrogen and carbon monoxide, the electron donor is continuously or intermittently introduced into the system during the anaerobic fermentation process.

[0014] As a further preferred embodiment of the present application, the methanogenesis inhibitor is 2-bromoethyl sodium sulfonate, and the amount of the methanogenesis inhibitor added in the system is 5-15 g / L.

[0015] As a further preferred embodiment of the present application, the temperature of the anaerobic fermentation is 30-40℃, and the rotation speed of the oscillation rotation is 100-180 rpm.

[0016] As a further preferred embodiment of the present application, the anaerobic fermentation is carried out in a constant-temperature oscillation culture device.

[0017] By the above technical scheme conceived by the present application, compared with the prior art, the following advantages can be achieved

[0018] Advantages:

[0019] (1) The present application uses the excess activated sludge of municipal sewage treatment plant as fermentation substrate (of course, concentrated excess activated sludge or sludge hydrolysis acidification liquid obtained by hydrolysis acidification of excess activated sludge can also be used), inoculum, methanogenesis inhibitor, electron donor are added to the anaerobic fermentation reactor, and blue vitriol is added, and the addition amount of blue vitriol in the system is strictly controlled to be 2-20 g / L, which can improve the electron transfer efficiency of microorganisms in sludge anaerobic fermentation, and finally improve the yield of medium-chain fatty acids in sludge anaerobic fermentation. Blue vitriol is a widely distributed substance in nature, which can be synthesized and recovered from industrial wastewater, easily synthesized in the laboratory, and affordable. When added to the sludge anaerobic fermentation system and the addition amount of blue vitriol in the system is strictly controlled to be 2-20 g / L, it can act as an electron mediator, promote the electron transfer of anaerobic fermentation microorganisms in the sludge, speed up the reaction efficiency of microorganisms, and improve the yield of medium-chain fatty acids.

[0020] (2) The present application overcomes the technical bias. In the prior art, the use of material additives (such as bio-carbon materials, zero-valent iron, Fe3O4, etc.) to promote the electron transfer of microbial anaerobic fermentation process is often suitable for both methanogenesis and short-chain fatty acid production systems, and can promote the production of corresponding products. The prior art has reported that blue vitriol can inhibit the metabolic action of related microorganisms in anaerobic fermentation for methanogenesis and short-chain fatty acid production, while the present application finds that the use of blue vitriol in the production of medium-chain fatty acids can promote the metabolic process of microorganisms producing medium-chain fatty acids and improve the yield of medium-chain fatty acids. Of course, the addition amount of blue vitriol in the system needs to be strictly controlled to be 2-20 g / L.

[0021] (3) Blue vitriol is suitable for long-term storage and does not need to be stored in an oxygen-free low-temperature environment. Compared with zero-valent iron, blue vitriol has the advantages of simple manufacturing, non-agglomeration, strong biocompatibility, stable properties, long-term storage in air environment, simple storage conditions, easy recovery, etc.; compared with manganese ore, pyrite and other materials, blue vitriol does not contain heavy metals and toxic and harmful elements such as sulfur, and is green and harmless to the human body and the natural environment. In addition, sludge fermentation residues containing blue vitriol can also be used as fertilizer in the agricultural and forestry industries. Therefore, the present application of adding blue vitriol to promote the production of medium-chain fatty acids by anaerobic fermentation of excess sludge is an economical and friendly sludge resource technology. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 is a process flow diagram of the method for producing medium-chain fatty acids by anaerobic fermentation of excess sludge using blue vitriol according to the present application.

[0023] Figure 2 is a structural schematic diagram of the anaerobic fermentation reactor and the constant-temperature shaking culture device used in the examples.

[0024] Figure 3Ethanol and volatile fatty acids concentration in the system at the end of the reaction at day 20 of the reaction for the different concentrations of the vivianite group added in examples 1-4 and comparative examples 1-3. DETAILED DESCRIPTION

[0025] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and should not be used to limit the present application. In addition, the technical features involved in each embodiment of the present application described below can be combined with each other as long as they do not conflict with each other.

[0026] In the present application, the method for promoting sludge anaerobic fermentation to produce medium-chain fatty acids by adding vivianite, in actual operation, can be as follows: using residual activated sludge from a municipal sewage treatment plant as the fermentation substrate, adding it into an anaerobic reactor, inoculating anaerobic sludge, adding vivianite, then adding a methanogenesis inhibitor (to avoid methanogenesis), then adding an electron donor (to make the system undergo carbon chain elongation reaction and produce medium-chain fatty acids), adjusting pH, and finally purging the fermentation system with nitrogen to remove oxygen and sealing it with a rubber plug and cover, and placing it in a constant-temperature shaking culture device for fermentation. During operation, gas chromatography can be used to regularly detect liquid samples, and the reactor can be stopped when the product concentration is stable. The vivianite added during fermentation can strengthen the electron transfer capacity of microorganisms during carbon chain elongation, thereby increasing the yield of medium-chain fatty acids (the main medium-chain fatty acid in the product is n-hexanoic acid).

[0027] The method for promoting residual sludge anaerobic fermentation to produce medium-chain fatty acids by adding vivianite in the following examples includes the following steps:

[0028] (1) Residual activated sludge is obtained from the secondary sedimentation tank of a municipal sewage treatment plant, and supernatant is removed by gravity sedimentation to reduce the water content and increase the concentration of residual sludge, obtaining concentrated residual activated sludge (the relevant parameters of the concentrated residual activated sludge used in each example and comparative example below are shown in Table 1). The concentrated residual sludge is used as the fermentation substrate (of course, the concentrated residual activated sludge can also be subjected to hydrolysis and acidification pretreatment according to the known hydrolysis and acidification process in the prior art, and the sludge hydrolysis and acidification liquid obtained after the pretreated hydrolysis and fermentation to produce acid is used as the fermentation substrate).

[0029] Table 1: Relevant parameters of concentrated residual activated sludge and anaerobic inoculated sludge used in examples and comparative examples

[0030] Indicator parameters Concentrated residual activated sludge Anaerobically inoculated sludge pH 6.98 6.52 SS (g / L) 18.73 10.71 VSS (g / L) 11.88 6.85 TCOD (g / L) 17.93 10.14

[0031] (2) In the fermentation reactor, add blue iron ore (blue), residual sludge, inoculum, methanogenesis inhibitor and electron donor. Among them, the concentration of blue iron ore added is 2-20 g / L, the inoculum added is taken from the anaerobic fermentation tank sludge, or the sludge enriched with carbon chain extension functional bacteria genus after preliminary domestication of the anaerobic reactor sludge. Among them, 5-15 g / L of methanogenesis inhibitor is added, which is to make the anaerobic fermentation proceed to produce acid and avoid methanogenesis. 80-200 mM of electron donor is added, and the addition of electron donor is to carry out carbon chain extension reaction with short-chain fatty acids produced in the anaerobic fermentation process as electron acceptors in the sludge under the action of functional microorganisms in the sludge, so as to produce medium-chain fatty acids.

[0032] The blue iron ore used in the examples hereinafter is commercially available. The anaerobic inoculum sludge used in the examples and comparative examples hereinafter is the same batch of anaerobic sludge enriched with carbon chain extension functional bacteria genus obtained by domesticating the sludge from the anaerobic fermentation tank of a sewage treatment plant according to the existing process. The relevant parameters are shown in Table 1. After sequencing, it is known that the carbon chain extension functional bacteria genus contained therein includes Clostridium_sensu_stricto_12, Sporanaerobacter, Corynebacterium, Acetoanaerobium, Caproiciproducens, Oscillibacter, etc.

[0033] (3) Hydrochloric acid and sodium hydroxide are used to adjust the fermentation pH value in the reactor, and the initial pH value is set to 6.0-7.5.

[0034] (4) Pure nitrogen gas is introduced into the anaerobic fermentation reactor to remove oxygen, so as to ensure the anaerobic environment in the reactor and ensure the functional activity of anaerobic fermentation microorganisms.

[0035] (5) The reactor is placed in a constant temperature shaking culture device to ensure suitable fermentation conditions for microorganisms, and the temperature is controlled at 30-40℃ and the rotation speed is controlled at 100-180 rpm. The reactor is continuously operated within 0-20 days, and the production of medium-chain fatty acids is monitored every 2 days using a gas chromatograph. The reactor is stopped at the 20th day when the reaction reaches a steady state.

[0036] The following are specific examples:

[0037] Example 1:

[0038] Prepare multiple reactors, and control the total volume (i.e. working volume) of the reaction system in each reactor before fermentation to 150 mL when subsequent feeding. See Figure 1Each reactor was fed with 2 g / L of blue vitriol, 120 ml of concentrated residual activated sludge, 30 mL of acclimated anaerobic sludge, 10 g / L of 2-bromoethyl sulfonic acid sodium as a methanogenesis inhibitor, and 100 mM of ethanol. The initial fermentation pH was adjusted to 7.0, and the oxygen was removed by sparging high-purity nitrogen gas into the reactor through a gas inlet tube for about 5 minutes. The reactor was then sealed with a rubber stopper and an aluminum cap, and a gas collection bag was connected to the gas outlet. Finally, the reactor was placed in a constant-temperature shaker incubator at 35°C and 150 rpm. The reaction was run for 20 days, and the concentration of medium-chain fatty acids was determined every 2 days using a gas chromatograph. The reaction was stopped on the 20th day when the substrate product reached a steady state and the reaction was essentially complete. The average concentration of the produced medium-chain fatty acid, n-hexanoic acid, in the parallel reactors was 2.37 g / L, and the electron transfer efficiency was 48.47%.

[0039] Example 2

[0040] A plurality of reactors were prepared, and the working volume was controlled to be 150 ml when subsequent feeding was performed. Each reactor was fed with 5 g / L of blue vitriol, 120 ml of concentrated residual activated sludge, 30 mL of acclimated anaerobic sludge, 10 g / L of 2-bromoethyl sulfonic acid sodium as a methanogenesis inhibitor, and 100 mM of ethanol. The initial fermentation pH was adjusted to 7.0, and the oxygen was removed by sparging high-purity nitrogen gas into the reactor through a gas inlet tube for about 5 minutes. The reactor was then sealed with a rubber stopper and an aluminum cap, and a gas collection bag was connected to the gas outlet. Finally, the reactor was placed in a constant-temperature shaker incubator at 35°C and 150 rpm. The reaction was run for 20 days, and the concentration of medium-chain fatty acids was determined every 2 days using a gas chromatograph. The reaction was stopped on the 20th day when the substrate product reached a steady state and the reaction was essentially complete. The average concentration of the produced medium-chain fatty acid, n-hexanoic acid, in the parallel reactors was 2.68 g / L, and the electron transfer efficiency was 54.64%.

[0041] Example 3

[0042] Preparation of multiple reactors, subsequent feeding volume control to 150 ml. Each reactor, the addition of 10 g / L of blue iron, the addition of concentrated remaining activated sludge 120 ml, inoculated with acclimated anaerobic sludge 30 mL, then add 10 g / L of 2-bromoethyl sulfonate sodium as methanogenesis inhibitor, the addition of 100 mM ethanol. The initial fermentation pH adjusted to 7.0, then using high-purity nitrogen gas through the gas pipe inserted into the reactor for about 5 minutes to remove oxygen, then use rubber plug and aluminum cover closed reactor, the gas bag is connected to the gas outlet. Finally the reactor was placed in a constant temperature incubator at 35℃ and 150 rpm. Run 20 days, every 2 days by gas chromatograph to determine the concentration of medium chain fatty acids, at the 20th day when the substrate product has reached a steady state, the reaction is basically over, the average concentration of the final produced medium chain fatty acid n-hexanoic acid in parallel reactors was 2.95 g / L, and the electron transfer efficiency was 60.36%.

[0043] Example 4:

[0044] Preparation of multiple reactors, subsequent feeding volume control to 150 ml. Each reactor, the addition of 10 g / L of blue iron, the addition of concentrated remaining activated sludge 120 ml, inoculated with acclimated anaerobic sludge 30 mL, then add 10 g / L of 2-bromoethyl sulfonate sodium as methanogenesis inhibitor, the addition of 100 mM ethanol. The initial fermentation pH adjusted to 7.0, then using high-purity nitrogen gas through the gas pipe inserted into the reactor for about 5 minutes to remove oxygen, then use rubber plug and aluminum cover closed reactor, the gas bag is connected to the gas outlet. Finally the reactor was placed in a constant temperature incubator at 35℃ and 150 rpm. Run 20 days, every 2 days by gas chromatograph to determine the concentration of medium chain fatty acids, at the 20th day when the substrate product has reached a steady state, the reaction is basically over, the average concentration of the final produced medium chain fatty acid n-hexanoic acid in parallel reactors was 2.95 g / L, and the electron transfer efficiency was 60.36%.

[0045] Comparative Example 1:

[0046] This comparative example is as a blank control group, no addition of blue iron, specifically:

[0047] Preparation of multiple reactors, subsequent feeding volume control to 150 ml. In each reactor, the addition of concentrated residual activated sludge 120 ml, inoculated with acclimated anaerobic sludge 30 mL, the addition of 10 g / L of 2-bromoethyl sulfonic acid sodium as methanogenesis inhibitor, the addition of 100 mM ethanol. The initial fermentation pH adjusted to 7.0, and then using high-purity nitrogen gas through the gas pipe inserted into the reactor for about 5 minutes to remove oxygen, and then closed with rubber plug and aluminum cover reactor, the gas bag is connected to the gas outlet. Finally the reactor was placed in a constant temperature incubator at 35°C and 150 rpm. Running 20 days, every 2 days by gas chromatograph to determine the concentration of medium chain fatty acids, at the 20th day when the reaction stopped (at this time the substrate product has reached a steady state, the reaction is basically finished), the average concentration of the final produced medium chain fatty acid n-hexanoic acid in parallel reactors was 2.19 g / L, the electron transfer efficiency was 44.75%, and the hexanoic acid yield and electron transfer efficiency of the blank control group were significantly less than the above examples.

[0048] Comparative Example 2:

[0049] Preparation of multiple reactors, subsequent feeding volume control to 150 ml. In each reactor, the addition of 1 g / L of blue iron, the addition of concentrated residual activated sludge 120 ml, inoculated with acclimated anaerobic sludge 30 mL, the addition of 10 g / L of 2-bromoethyl sulfonic acid sodium as methanogenesis inhibitor, the addition of 100 mM ethanol. The initial fermentation pH adjusted to 7.0, and then using high-purity nitrogen gas through the gas pipe inserted into the reactor for about 5 minutes to remove oxygen, and then closed with rubber plug and aluminum cover reactor, the gas bag is connected to the gas outlet. Finally the reactor was placed in a constant temperature incubator at 35°C and 150 rpm. Running 20 days, every 2 days by gas chromatograph to determine the concentration of medium chain fatty acids, at the 20th day when the reaction stopped (at this time the substrate product has reached a steady state, the reaction is basically finished), the average concentration of the final produced medium chain fatty acid n-hexanoic acid in parallel reactors was 2.19 g / L, the electron transfer efficiency was 44.75%, and the hexanoic acid yield and electron transfer efficiency of the blank control group were significantly less than the above examples.

[0050] Comparative Example 3:

[0051] A plurality of reactors were prepared, and the working volume was controlled to be 150 ml when subsequent feeding. In each reactor, 50 g / L of blue vitriol was added, 120 ml of concentrated residual activated sludge was added, 30 mL of acclimated anaerobic sludge was inoculated, 10 g / L of sodium 2-bromoethyl sulfonate was added as a methanogenesis inhibitor, and 100 mM of ethanol was added. The initial fermentation pH was adjusted to 7.0, and high-purity nitrogen gas was inserted into the reactor through the gas inlet pipe to blow off oxygen for about 5 minutes, and then the reactor was sealed with a rubber plug and an aluminum cap, and a gas collection bag was connected to the gas outlet. Finally, the reactor was placed in a constant temperature incubator at 35°C and 150 rpm. After running for 20 days, the concentration of medium-chain fatty acids was determined every 2 days by gas chromatography, and the reactor was stopped at the 20th day (at this time, the substrate product had reached a steady state, and the reaction was basically completed). The average concentration of the produced medium-chain fatty acid n-hexanoic acid in the parallel reactors was 2.01 g / L, and the electron transfer efficiency was 41.07%. The yield of hexanoic acid and the electron transfer efficiency were significantly lower than those in the above examples.

[0052] Comparative Example 4:

[0053] A plurality of reactors were prepared, and the working volume was controlled to be 150 ml when subsequent feeding. In each reactor, 50 g / L of blue vitriol was added, 120 ml of concentrated residual activated sludge was added, 30 mL of acclimated anaerobic sludge was inoculated, 10 g / L of sodium 2-bromoethyl sulfonate was added as a methanogenesis inhibitor, and 100 mM of ethanol was added. The initial fermentation pH was adjusted to 7.0, and high-purity nitrogen gas was inserted into the reactor through the gas inlet pipe to blow off oxygen for about 5 minutes, and then the reactor was sealed with a rubber plug and an aluminum cap, and a gas collection bag was connected to the gas outlet. Finally, the reactor was placed in a constant temperature incubator at 35°C and 150 rpm. After running for 20 days, the concentration of medium-chain fatty acids was determined every 2 days by gas chromatography, and the reactor was stopped at the 20th day (at this time, the substrate product had reached a steady state, and the reaction was basically completed). The average concentration of the produced medium-chain fatty acid n-hexanoic acid in the parallel reactors was 2.01 g / L, and the electron transfer efficiency was 41.07%. The yield of hexanoic acid and the electron transfer efficiency were significantly lower than those in the above examples.

[0054] Although the effect of 10 g / L blue vitriol on promoting the production of medium-chain fatty acids was 4.5% lower than that of adding zero-valent iron powder. It is worth noting that zero-valent iron powder is expensive, and the synthesis process in the laboratory requires strict requirements, harsh reaction conditions, and the product needs to be stored at 2-8°C and needs to be stored in an inert gas to isolate oxygen. When used, it is easy to agglomerate and has poor dispersibility. The blue vitriol used in the above examples has a lower cost, is widely distributed in nature, and is easy to synthesize in the laboratory. The reaction conditions are mild, and the blue vitriol does not need to be stored at low temperature and isolated from oxygen at room temperature. It is easy to disperse and not easy to agglomerate when used.

[0055] As can be seen from the above examples 1-4 and comparative examples 1-3, and by referring toFigure 3 It can be known that the method for producing medium-chain fatty acid by using the vivianite to strengthen the anaerobic fermentation of residual sludge can significantly increase the yield of medium-chain fatty acid n-hexanoic acid with the addition of appropriate vivianite. Of course, the addition amount of vivianite needs to be 2-20 g / L, at this time, compared with the blank control group without adding vivianite, the electron transfer efficiency can be significantly improved, and the yield of medium-chain fatty acid hexanoic acid can be increased by 22%-35% (the concentration of product n-hexanoic acid in example 2 is 2.68, which is increased by 22% compared with the concentration of product n-hexanoic acid in comparative example 1, which is 2.19; the concentration of product n-hexanoic acid in example 3 is 2.95, which is increased by 35% compared with the concentration of product n-hexanoic acid in comparative example 1, which is 2.19). When the addition amount of vivianite exceeds 20 g / L, the excessive addition of vivianite will reduce the yield of medium-chain fatty acid, especially when the addition amount of vivianite is 50 g / L, the yield of medium-chain fatty acid is lower than that of the blank control group, which indicates that the excessive addition of vivianite may affect the fermentation environment and inhibit the activity of microorganisms.

[0056] The above examples are only examples, for example, the electron donor for producing medium-chain fatty acid can also be ethanol, lactic acid, methanol, amino acids, sugars, hydrogen, carbon monoxide and other electron donors. When the electron donor is ethanol, lactic acid, methanol, amino acids and sugars, these electron donors are added to the system before the initial fermentation pH value of the system is adjusted; when the electron donor is hydrogen and carbon monoxide, the electron donor is continuously or intermittently introduced into the system during the anaerobic fermentation process.

[0057] Those skilled in the art can easily understand that the above description is only a preferred embodiment of the present application, and is not used to limit the present application, and any modification, equivalent replacement and improvement made within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. A method for promoting the production of medium-chain fatty acids from sludge anaerobic fermentation by adding vivianite, characterized in that, At least one of residual activated sludge from a municipal sewage treatment plant, concentrated residual activated sludge, and sludge hydrolysis acidification liquid is used as a fermentation substrate, anaerobic sludge is inoculated into the fermentation substrate, and blue vitriol and a methanogenesis inhibitor are added, and the initial fermentation pH value of the system is adjusted to 6.0-7.5, then the fermentation system is purged with protective gas to exclude oxygen and sealed, and anaerobic fermentation is carried out with the participation of an electron donor; due to the addition of blue vitriol, the electron transfer capacity of microorganisms in the carbon chain elongation process can be enhanced, thereby increasing the yield of medium-chain fatty acids in the fermentation product; In the formula, the methanogenesis inhibitor is sodium 2-bromoethyl sulfonate, the amount of blue vitriol added in the system is 2-20 g / L, the sludge hydrolysis acidification liquid is obtained by hydrolysis and acidification of residual activated sludge, and the carbon number of the medium-chain fatty acid is 6-12. The electron donor is selected from ethanol, lactic acid, methanol, amino acids, sugars, hydrogen, and carbon monoxide. When the electron donor is selected from ethanol, lactic acid, methanol, amino acids, and sugars, the electron donor is added to the system before the initial fermentation pH value of the system is adjusted. When the electron donor is selected from hydrogen and carbon monoxide, the electron donor is continuously or intermittently introduced into the system during the anaerobic fermentation process.

2. The method of claim 1, wherein, The blue vitriol is blue vitriol powder or particles, and is blue.

3. The method of claim 1, wherein, The anaerobic sludge is anaerobic fermentation tank sludge from a sewage treatment plant, or sludge in which a carbon chain elongation functional bacteria genus is enriched after domestication of anaerobic sludge.

4. The method of claim 1, wherein, The amount of the methanogenesis inhibitor added in the system is 5-15 g / L.

5. The method of claim 1, wherein, The temperature of the anaerobic fermentation is 30-40°C, and the rotation speed of the oscillation is 100-180 rpm.

6. The method of claim 1, wherein, The anaerobic fermentation is carried out in a constant-temperature oscillation culture device.

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