Method for promoting anaerobic fermentation of excess sludge to produce medium-chain fatty acid by using blue iron ore

Abstract

The invention belongs to the technical field of anaerobic fermentation of organic wastes, and discloses a method for producing medium-chain fatty acid by promoting anaerobic fermentation of excess sludge by using blue iron ore, which comprises the following steps: inoculating anaerobic sludge by using excess activated sludge from a municipal sewage treatment plant as a fermentation substrate, adding the blue iron ore and a methanogenesis inhibitor, adjusting the pH value, and fermenting to obtain the medium-chain fatty acid. Carrying out anaerobic fermentation in the presence of an electron donor to produce medium-chain fatty acid; due to the addition of the blue iron ore, the electron transfer capability of microorganisms in a carbon chain extension process can be enhanced, so that the yield of medium-chain fatty acid in a fermentation product is increased; the addition amount of the blue iron ore in the system is 2-20g / L. According to the method, the blue iron ore is introduced into a system for producing the medium-chain fatty acid from the sludge, the addition amount of the blue iron ore in the system is strictly controlled, the blue iron ore is used as an electron mediator, and the electron transfer efficiency of microbial fermentation in the process of producing the medium-chain fatty acid from the sludge is enhanced, so that the yield of the medium-chain fatty acid is increased; further, efficient utilization of sludge resources is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of anaerobic fermentation of organic waste, and more specifically, relates to a method for promoting the anaerobic fermentation of excess sludge to produce medium-chain fatty acids by using vivianite. Background Art

[0002] Excess sludge is the main by-product of the biological treatment process in wastewater treatment plants, which contains abundant organic matter and nutrients such as nitrogen and phosphorus. With the rapid advancement of urbanization, the amount of wastewater treatment increases year by year, and the output of excess sludge also increases year by year. According to statistics, the output of municipal sludge (calculated based on a moisture content of 80%) in China exceeded 80 million tons in 2021, and it is expected that the sludge output in China will exceed 100 million tons by 2025. At present, the main treatment methods for sludge include landfill and incineration. These treatment methods have problems such as occupying a large amount of land resources, consuming fossil energy, and causing secondary pollution. According to statistics, the treatment cost of sludge at the present stage accounts for more than 60% of the total operating cost of wastewater treatment plants. These traditional treatment methods fail to realize the resource utilization of organic matter and nutrients in sludge, wasting resources and causing an environmental burden.

[0003] The technology of anaerobic fermentation to produce medium-chain fatty acids can realize the resource utilization of organic matter in sludge, providing a green and sustainable implementation path for the goal of high-value resource treatment of sludge. Medium-chain fatty acids (monounsaturated straight-chain carboxylic acids with 6-12 carbon atoms, 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 a high energy density and low water solubility, and can be widely used as biomass fuel precursors and sterilants, etc., so the added value is relatively high. However, the yield and rate of medium-chain fatty acids in traditional technologies are still low, mainly related to the low electron transfer efficiency in the carbon chain elongation process of anaerobic fermentation, which limits the industrial application of the technology.

[0004] Multiple studies have shown that the use of material additives can promote electron transfer in the process of microbial anaerobic fermentation, thereby increasing 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 anaerobic fermentation of sludge. However, zero-valent iron has a high production cost, poor oxidation stability, demanding storage conditions, easy agglomeration, and a large amount needs to be added 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. However, sulfur elements will produce toxic hydrogen sulfide gas with a pungent odor during anaerobic fermentation, which endangers the environment and human health. Chinese Patent CN118995838A discloses a method of adding manganese ore to promote the production of medium-chain fatty acids. However, manganese is a heavy metal that endangers the environment and life health. After being added to the anaerobic fermentation of sludge, heavy metals will remain in the sludge fermentation residue, and the subsequent sludge fermentation residue is difficult to compost or be used for agriculture and greening. It can be seen that the addition of manganese will limit the subsequent treatment and resource utilization of fermented sludge.

[0005] Vivianite, as an iron-phosphorus mineral with a wide natural source, has the characteristics of being non-toxic and harmless, easy to synthesize from industrial wastewater, and having weak magnetism. Currently, multiple patents (CN113023702B, CN118184042A, CN113173648A) have reported methods for recovering vivianite from sewage and sludge, confirming the wide source and simple synthesis of vivianite. However, there has been no research or patent exploring its application in promoting the production of medium-chain fatty acids by anaerobic fermentation of sludge. If a process can be developed to use vivianite to promote the production of medium-chain fatty acids by anaerobic fermentation of sludge, it can not only increase the yield of medium-chain fatty acids but also realize the resource utilization of sludge, which is of great significance for engineering applications and environmental benefits. Summary of the Invention

[0006] Aiming at the defects existing in the existing technology for the production of medium-chain fatty acids by anaerobic fermentation of sludge, the purpose of the present invention is to provide a method for promoting the anaerobic fermentation of excess sludge to produce medium-chain fatty acids by using vivianite. By introducing vivianite into the anaerobic fermentation system of sludge and strictly controlling the addition amount of vivianite in the system, using vivianite as an electron mediator to enhance the electron transfer efficiency of microbial fermentation in the process of sludge producing medium-chain fatty acids, thereby increasing the yield of medium-chain fatty acids in the process of anaerobic fermentation of sludge, and further realizing the efficient utilization of sludge resources. Compared with the existing technology, the vivianite used in the present invention has the advantages of being non-toxic and harmless, wide source, and simple synthesis, which can effectively solve the problems of low yield and slow speed of medium-chain fatty acid production by anaerobic fermentation of sludge, promote the production of medium-chain fatty acids by anaerobic fermentation, and realize the engineering application of sludge resource utilization.

[0007] To achieve the above object, according to the present invention, a method for adding vivianite to promote the anaerobic fermentation of sludge to produce medium-chain fatty acids is provided, which is characterized in that at least one of the excess activated sludge, concentrated excess activated sludge, and sludge hydrolysis acidification liquid from a municipal sewage treatment plant is used as the fermentation substrate, anaerobic sludge is inoculated into the fermentation substrate, vivianite and a methanogenesis inhibitor are added, and the initial fermentation pH value of the system is adjusted to 6.0 - 7.5, and then the fermentation system is purged with a protective gas to remove oxygen and sealed, and anaerobic fermentation is carried out with the participation of an electron donor; due to the addition of vivianite, the microbial electron transfer ability during the carbon chain elongation process can be strengthened, thereby increasing the production of medium-chain fatty acids in the fermentation products;

[0008] Among them, the addition amount of the vivianite in the system is 2 - 20 g / L; the sludge hydrolysis acidification liquid is obtained by hydrolyzing and acidifying the excess activated sludge; the carbon number of the medium-chain fatty acids is 6 - 12.

[0009] As a further preference of the present invention, the vivianite is vivianite powder or granules and is blue.

[0010] As a further preference of the present invention, the anaerobic sludge is the anaerobic fermentation tank sludge of a sewage treatment plant, or the sludge enriched with carbon chain elongation functional bacteria after domestication of anaerobic sludge.

[0011] As a further preference of the present invention, 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 adjusting the initial fermentation pH value of the system;

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

[0014] As a further preference of the present invention, the methanogenesis inhibitor is 2-bromoethylsulfonic acid sodium, and the addition amount in the system is 5 - 15 g / L.

[0015] As a further preference of the present invention, the temperature at which the anaerobic fermentation takes place is 30 - 40 °C, and the oscillating rotary speed is 100 - 180 rpm.

[0016] As a further preference of the present invention, the anaerobic fermentation is carried out in a constant temperature oscillating culture device.

[0017] Through the above technical solution conceived by the present invention, compared with the prior art, the following

[0018] beneficial effects can be obtained:

[0019] (1) The present invention uses the excess activated sludge from urban sewage treatment plants as the 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). An inoculum, a methanogenesis inhibitor, an electron donor, and vivianite are added to the anaerobic fermentation reactor, and the addition amount of vivianite 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 ultimately increase the production of medium-chain fatty acids in sludge anaerobic fermentation. Vivianite is a substance that is widely distributed in nature, can be synthesized and recovered from industrial wastewater, is easy to synthesize in the laboratory, and is inexpensive. When added to the sludge anaerobic fermentation system and the addition amount of vivianite in the system is strictly controlled to be 2 - 20 g / L, it can act as an electron mediator to promote the electron transfer of anaerobic fermentation microorganisms in the sludge, accelerate the microbial reaction efficiency, and increase the production of medium-chain fatty acids.

[0020] (2) The present invention overcomes technical prejudice. In the prior art, the use of material additives (such as biochar materials, zero-valent iron, Fe3O4, etc.) to promote electron transfer in the process of microbial anaerobic fermentation is often applicable to both the systems for methane production and short-chain fatty acid production and the systems for medium-chain fatty acid production, and can promote the production of corresponding products. The prior art has reported that vivianite will inhibit the metabolic effects of related microorganisms in anaerobic fermentation for methane production and short-chain fatty acid production, while the present invention discovers that in the system for medium-chain fatty acid production, using vivianite can instead promote the metabolic process of medium-chain fatty acid-producing microorganisms and increase the production of medium-chain fatty acids. Of course, the addition amount of vivianite in the system needs to be strictly controlled to be 2 - 20 g / L.

[0021] (3) Vivianite is suitable for long-term storage and does not require oxygen isolation and low-temperature preservation. Compared with zero-valent iron, vivianite has the advantages of simple manufacture, not easy to agglomerate, strong biocompatibility, stable properties, being able to be stored in the air environment for a long time, simple storage conditions, and easy recovery; compared with materials such as manganese ore and pyrite, vivianite does not contain toxic and harmful elements such as heavy metals and sulfur, is green and pollution-free to the human body and the natural environment, and the sludge fermentation residue containing vivianite can also be used as a fertilizer in the agriculture and forestry industries. It can be seen that the present invention's addition of vivianite to promote the anaerobic fermentation of excess sludge to produce medium-chain fatty acids is an economically friendly sludge resource utilization technology. Description of the Drawings

[0022] Figure 1 is a process flow schematic diagram of the method for promoting the anaerobic fermentation of excess sludge to produce medium-chain fatty acids using vivianite in the present invention.

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

[0024] Figure 3It is the ethanol and volatile fatty acid concentrations in the system at the end of the reaction on the 20th day of the reaction for the groups adding different concentrations of vivianite in Examples 1-4 and Comparative Examples 1-3. Detailed implementation manners

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

[0026] In the method for promoting anaerobic fermentation of sludge to produce medium-chain fatty acids by adding vivianite in the present invention, in actual operation, it can be: using the excess activated sludge from a municipal sewage treatment plant as a fermentation substrate and 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 cause a carbon chain elongation reaction in the system and produce medium-chain fatty acids), adjusting the pH, and finally purging the fermentation system with nitrogen to remove oxygen and then sealing it with a rubber stopper and a lid, and placing it in a constant temperature shaking culture device for fermentation. During operation, a gas chromatograph can be used to regularly detect liquid samples, and the reactor is stopped when the product concentration is stable. The vivianite added during the fermentation process can strengthen the microbial electron transfer ability during the carbon chain elongation process, thereby increasing the production of medium-chain fatty acids (the medium-chain fatty acids in the product are mainly n-caproic acid).

[0027] The following examples later use the method for promoting anaerobic fermentation of excess sludge to produce medium-chain fatty acids by adding vivianite, including the following steps:

[0028] (1) Obtain excess activated sludge from the secondary sedimentation tank of a municipal sewage treatment plant, perform gravity sedimentation to remove the supernatant to reduce the moisture content and increase the concentration of excess sludge, and obtain concentrated excess activated sludge (the relevant parameters of the concentrated excess activated sludge used in the following examples and comparative examples are shown in Table 1 below). The concentrated excess sludge is used as a fermentation substrate (of course, the concentrated excess activated sludge can also be pretreated by hydrolysis and acidification with reference to the hydrolysis and acidification process known in the prior art, and the sludge hydrolysis acidification liquid obtained after pretreatment hydrolysis fermentation and acidification is used as a fermentation substrate).

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

[0030] Index parameter Concentrated residual activated sludge Anaerobic inoculation 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) Add vivianite (blue in color), excess sludge, inoculum, methanogenesis inhibitor, and electron donor into the fermentation reactor. Among them, the added concentration of vivianite is 2 - 20 g / L, and the added inoculum is taken from the anaerobic fermentation tank sludge or the sludge obtained by preliminary domestication and enrichment of carbon chain elongation functional bacteria from the anaerobic reactor sludge. Among them, 5 - 15 g / L of methanogenesis inhibitor is added to make the anaerobic fermentation proceed in the direction of acid production and avoid methanogenesis. 80 - 200 mM of electron donor is added. The addition of the electron donor is to carry out a carbon chain elongation reaction under the action of functional microorganisms in the sludge together with the short-chain fatty acids produced as electron acceptors during the anaerobic fermentation process, thereby producing medium-chain fatty acids.

[0032] The vivianite used in the subsequent examples was purchased commercially. The anaerobic inoculation sludge used in the subsequent examples and comparative examples was the same batch of anaerobic sludge enriched with carbon chain elongation functional bacteria obtained by domesticating the anaerobic fermentation tank sludge from the sewage treatment plant according to the existing process. The relevant parameters are shown in Table 1. After sequencing, it can be known that the carbon chain elongation functional bacteria contained therein include Clostridium_sensu_stricto_12, Sporanaerobacter, Corynebacterium, Acetoanaerobium, Caproiciproducens, Oscillibacter, etc.

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

[0034] (4) Introduce pure nitrogen into the anaerobic fermentation reactor to remove oxygen, ensure the anaerobic environment in the reactor, and ensure the functional activity of anaerobic fermentation microorganisms.

[0035] (5) Place the reactor in a constant temperature shaking culture device to ensure suitable fermentation conditions for microorganisms. Control the temperature at 30 - 40 °C and the rotation speed at 100 - 180 rpm. Continuously operate within 0 - 20 days and use a gas chromatograph to monitor the medium-chain fatty acid production every 2 days. Stop the reactor operation at the 20th day. At this time, the reactions in each reactor have reached a stable state.

[0036] The following are specific examples:

[0037] Example 1:

[0038] Prepare multiple reactors. During subsequent feeding, control the total volume (i.e., working volume) of the reaction system in each reactor before the start of fermentation to 150 mL. See Figure 1, 2 g / L of vivianite was added to each reactor, 120 ml of concentrated excess activated sludge, 30 mL of inoculated and acclimated anaerobic sludge were added, and 10 g / L of 2-bromoethylsulfonate was added as a methanogenesis inhibitor, and 100 mM ethanol was added. The initial fermentation pH was adjusted to 7.0, and then high-purity nitrogen was inserted into the reactor through the gas pipeline to blow off oxygen for about 5 minutes, and then the reactor was sealed with a rubber stopper and an aluminum lid, 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. It was run for 20 days, and the concentration of medium-chain fatty acids was measured with a gas chromatograph every 2 days. The reactor operation was stopped on the 20th day (at this time, the substrate and product had reached a steady state and the reaction was basically completed). The average concentration of the medium-chain fatty acid n-caproic acid produced finally in the parallel reactors was 2.37 g / L, and the electron transfer efficiency was 48.47%.

[0039] Example 2:

[0040] In preparing multiple reactors, the working volume was controlled to 150 ml during subsequent feeding. In each reactor, 5 g / L of vivianite was added, 120 ml of concentrated excess activated sludge, 30 mL of inoculated and acclimated anaerobic sludge were added, and 10 g / L of 2-bromoethylsulfonate was added as a methanogenesis inhibitor, and 100 mM ethanol was added. The initial fermentation pH was adjusted to 7.0, and then high-purity nitrogen was inserted into the reactor through the gas pipeline to blow off oxygen for about 5 minutes, and then the reactor was sealed with a rubber stopper and an aluminum lid, 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. It was run for 20 days, and the concentration of medium-chain fatty acids was measured with a gas chromatograph every 2 days. The reactor operation was stopped on the 20th day (at this time, the substrate and product had reached a steady state and the reaction was basically completed). The average concentration of the medium-chain fatty acid n-caproic acid produced finally in the parallel reactors was 2.68 g / L, and the electron transfer efficiency was 54.64%.

[0041] Example 3:

[0042] Prepare multiple reactors and control the working volume at 150 ml during subsequent feeding. In each reactor, add 10 g / L of vivianite, 120 ml of concentrated residual activated sludge, 30 mL of inoculated and acclimated anaerobic sludge, 10 g / L of 2-bromoethylsulfonate as a methanogenesis inhibitor, and 100 mM ethanol. Adjust the initial fermentation pH to 7.0, then use high-purity nitrogen to blow through the gas pipe inserted into the reactor for about 5 minutes to remove oxygen, seal the reactor with a rubber stopper and an aluminum lid, and connect a gas collection bag to the gas outlet. Finally, place the reactor in a constant-temperature shaking incubator at 35°C and 150 rpm. Run for 20 days, measure the concentration of medium-chain fatty acids with a gas chromatograph every 2 days, and stop the reactor operation on the 20th day (when the substrate and product have reached a steady state and the reaction is basically over). The average concentration of the medium-chain fatty acid n-hexanoic acid produced finally in the parallel reactors is 2.95 g / L, and the electron transfer efficiency is 60.36%.

[0043] Example 4:

[0044] Prepare multiple reactors and control the working volume at 150 ml during subsequent feeding. In each reactor, add 20 g / L of vivianite, 120 ml of concentrated residual activated sludge, 30 mL of inoculated and acclimated anaerobic sludge, 10 g / L of 2-bromoethylsulfonate as a methanogenesis inhibitor, and 100 mM ethanol. Adjust the initial fermentation pH to 7.0, then use high-purity nitrogen to blow through the gas pipe inserted into the reactor for about 5 minutes to remove oxygen, seal the reactor with a rubber stopper and an aluminum lid, and connect a gas collection bag to the gas outlet. Finally, place the reactor in a constant-temperature shaking incubator at 35°C and 150 rpm. Run for 20 days, measure the concentration of medium-chain fatty acids with a gas chromatograph every 2 days, and stop the reactor operation on the 20th day (when the substrate and product have reached a steady state and the reaction is basically over). The average concentration of the medium-chain fatty acid n-hexanoic acid produced finally in the parallel reactors is 2.85 g / L, and the electron transfer efficiency is 58.20%.

[0045] Comparative Example 1:

[0046] This comparative example is used as a blank control group without adding vivianite. Specifically:

[0047] Prepare multiple reactors and control the working volume to 150 ml during subsequent feeding. Without adding vivianite, add 120 ml of concentrated residual activated sludge and 30 mL of inoculated and acclimated anaerobic sludge to each reactor. Then add 2-bromoethanesulfonic acid sodium at 10 g / L as a methanogenesis inhibitor and 100 mM ethanol. Adjust the initial fermentation pH to 7.0, then use high-purity nitrogen to be inserted into the reactor through the gas pipeline to blow off oxygen for about 5 minutes, then seal the reactor with a rubber stopper and an aluminum cap, and connect a gas collection bag to the gas outlet. Finally, place the reactor in a constant temperature shaker incubator at 35 °C and 150 rpm. Run for 20 days, measure the concentration of medium-chain fatty acids with a gas chromatograph every 2 days, and stop the reactor operation on the 20th day (when the substrate and product have reached a steady state and the reaction is basically over). The average concentration of the medium-chain fatty acid n-caproic acid produced finally in the parallel reactors is 2.19 g / L, and the electron transfer efficiency is 44.75%. The caproic acid yield and electron transfer efficiency of the blank control group are significantly less than those of the above examples.

[0048] Comparative Example 2:

[0049] Prepare multiple reactors and control the working volume to 150 ml during subsequent feeding. Add 1 g / L of vivianite, 120 ml of concentrated residual activated sludge and 30 mL of inoculated and acclimated anaerobic sludge to each reactor. Add 2-bromoethanesulfonic acid sodium at 10 g / L as a methanogenesis inhibitor and 100 mM ethanol. Adjust the initial fermentation pH to 7.0, then use high-purity nitrogen to be inserted into the reactor through the gas pipeline to blow off oxygen for about 5 minutes, then seal the reactor with a rubber stopper and an aluminum cap, and connect a gas collection bag to the gas outlet. Finally, place the reactor in a constant temperature shaker incubator at 35 °C and 150 rpm. Run for 20 days, measure the concentration of medium-chain fatty acids with a gas chromatograph every 2 days, and stop the reactor operation on the 20th day (when the substrate and product have reached a steady state and the reaction is basically over). The average concentration of the medium-chain fatty acid n-caproic acid produced finally in the parallel reactors is 2.28 g / L, and the electron transfer efficiency is 46.59%. The caproic acid yield and electron transfer efficiency are significantly less than those of the above examples.

[0050] Comparative Example 3:

[0051] Prepare multiple reactors. During subsequent feeding, control the working volume to 150 ml. In each reactor, add 50 g / L of vivianite, add 120 ml of concentrated residual activated sludge, inoculate 30 mL of acclimated anaerobic sludge, then add 10 g / L of 2-bromoethylsulfonate as a methanogenesis inhibitor, and add 100 mM ethanol. Adjust the initial fermentation pH to 7.0, then use high-purity nitrogen to insert into the reactor through the gas pipeline to blow off oxygen for about 5 minutes, then seal the reactor with a rubber stopper and an aluminum lid, and connect a gas collection bag to the gas outlet. Finally, place the reactor in a constant temperature shaker incubator at 35 °C and 150 rpm. Run for 20 days. Measure the concentration of medium-chain fatty acids with a gas chromatograph every 2 days. Stop the reactor operation on the 20th day (at this time, the substrate and product have reached a steady state and the reaction is basically over). The average concentration of the medium-chain fatty acid n-caproic acid produced finally in the parallel reactors is 2.01 g / L, and the electron transfer efficiency is 41.07%. The yield of caproic acid and the electron transfer efficiency are significantly less than those of the above examples.

[0052] Comparative Example 4:

[0053] Prepare multiple reactors. During subsequent feeding, control the working volume to 150 ml. In each reactor, add 10 g / L of zero-valent iron powder, add 120 ml of concentrated residual activated sludge, inoculate 30 ml of acclimated anaerobic sludge, then add 10 g / L of 2-bromoethylsulfonate as a methanogenesis inhibitor, and add 100 mM ethanol. Adjust the initial fermentation pH to 7.0, then use high-purity nitrogen to insert into the reactor through the gas pipeline to blow off oxygen for about 5 minutes, then seal the reactor with a rubber stopper and an aluminum lid, and connect a gas collection bag to the gas outlet. Finally, place the reactor in a constant temperature shaker incubator at 35 °C and 150 rpm. Run for 20 days. Measure the concentration of medium-chain fatty acids with a gas chromatograph every 2 days. Stop the reactor operation on the 20th day (at this time, the substrate and product have reached a steady state and the reaction is basically over). The average concentration of the medium-chain fatty acid n-caproic acid produced finally in the parallel reactors is 3.09 g / L, and the electron transfer efficiency is 63.51%.

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

[0055] It can be seen that by comprehensively considering Examples 1-4 and Comparative Examples 1-3 and referring toFigure 3 It can be seen that in the method for enhancing the anaerobic fermentation of excess sludge to produce medium-chain fatty acids proposed by the present invention, the production of the medium-chain fatty acid n-caproic acid can be significantly increased with the addition of an appropriate amount of 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 the addition of vivianite, the electron transfer efficiency can be significantly improved, and in particular, the production of the medium-chain fatty acid caproic acid can be significantly increased by 22%-35% (the concentration of n-caproic acid in the product of Example 2 is 2.68, which is 22% higher than the concentration of n-caproic acid in the product of Comparative Example 1, which is 2.19; the concentration of n-caproic acid in the product of Example 3 is 2.95, which is 35% higher than the concentration of n-caproic acid in the product of Comparative Example 1, which is 2.19). When the addition amount of vivianite exceeds 20 g / L, the excessive addition of vivianite will cause a decrease in the production of medium-chain fatty acids. Especially when the addition concentration of vivianite is 50 g / L, the production of medium-chain fatty acids is lower than that of the blank control group, indicating that too high an addition concentration of vivianite may affect the fermentation environment and inhibit the microbial activity.

[0056] The above embodiments are only examples. For example, the electron donors for producing medium-chain fatty acids can also be other electron donors such as ethanol, lactic acid, methanol, amino acids, sugars, hydrogen, and carbon monoxide. When the electron donors are 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 donors are hydrogen and carbon monoxide, the electron donors are continuously or intermittently introduced into the system during the anaerobic fermentation process.

[0057] It is easy for those skilled in the art to understand that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for promoting the anaerobic fermentation of sludge to produce medium-chain fatty acids by adding vivianite, characterized in that, Using at least one of excess activated sludge, concentrated excess activated sludge, and sludge hydrolysis acidification liquid from a municipal sewage treatment plant as a fermentation substrate, inoculating anaerobic sludge into the fermentation substrate, adding vivianite and a methanogenesis inhibitor, and adjusting the initial fermentation pH value of the system to 6.0 - 7.5, then purging the fermentation system with a protective gas to remove oxygen and sealing it, and performing anaerobic fermentation in the presence of an electron donor; due to the addition of vivianite, the microbial electron transfer ability during the carbon chain elongation process can be enhanced, thereby increasing the production of medium-chain fatty acids in the fermentation products; Among them, the addition amount of the vivianite in the system is 2 - 20 g / L; the sludge hydrolysis acidification liquid is obtained by hydrolyzing and acidifying excess activated sludge; the carbon number of the medium-chain fatty acids is 6 - 12.

2. The method according to claim 1, wherein The vivianite is vivianite powder or particles and is blue.

3. The method according to claim 1, wherein The anaerobic sludge is the anaerobic fermentation tank sludge of a sewage treatment plant or the sludge obtained by domesticating anaerobic sludge and enriching the carbon chain elongation functional bacteria genus.

4. The method according to claim 1, wherein 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 adjusting the initial fermentation pH value of the system; When the electron donor is selected from hydrogen and carbon monoxide, the electron donor is continuously or intermittently introduced into the system during anaerobic fermentation.

5. The method according to claim 1, wherein The methanogenesis inhibitor is sodium 2-bromoethylsulfonate, and its addition amount in the system is 5 - 15 g / L.

6. The method according to claim 1, wherein The temperature at which the anaerobic fermentation occurs is 30 - 40 °C, and the oscillating rotary speed is 100 - 180 rpm.

7. The method according to claim 1, wherein The anaerobic fermentation is carried out in a constant temperature oscillating culture device.

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