CN105238822A - Combined pretreatment method for producing short-chain volatile fatty acids by enhancing anaerobic fermentation of sludge - Google Patents

Abstract

The invention discloses a combined pretreatment method for strengthening sludge anaerobic fermentation to produce short-chain volatile fatty acids and preparation of short-chain volatile fatty acids. The combined pretreatment method and the preparation method of short-chain volatile fatty acids are as follows: use the sludge from the secondary sedimentation tank of the urban sewage treatment plant as the fermentation substrate; inoculate the anaerobic activated sludge into the fermentation substrate; add nitrous acid to the fermentation system Sodium and sodium dodecylbenzenesulfonate were pretreated in acidic environment (pH=6.0) and fermented for 2 days, then adjusted to neutral and carried out anaerobic fermentation to produce short-chain volatile fatty acids, the output of the maximum volatile fatty acids is 345.5mg? COD/g? VSS, the best response time is 4 days. The invention can increase the acid production of sludge anaerobic fermentation, and can also shorten the optimum fermentation time. The acid production method increases the content of short-chain volatile fatty acids and shortens the optimal reaction time on the basis of resource utilization of sludge and reduction of environmental pollution. The invention has the advantages of high product yield, short reaction time, and simple operation of the preparation method Stable and energy-saving and consumption-reducing advantages.

Description

A joint pretreatment method for strengthening anaerobic fermentation of sludge to produce short-chain volatile fatty acids

technical field

The invention belongs to the technical field of environmental protection, and specifically relates to a combination of free nitrous acid and sodium dodecylbenzenesulfonate to pretreat sludge to produce short-chain volatile fatty acids and a preparation method thereof

Background technique

Surplus sludge is a by-product of sewage plant operation, which contains a large amount of organic matter. At the same time, the cost of treatment and disposal of excess sludge generally accounts for 50-60% of the total cost of operation and management of the entire sewage plant. Therefore, how to realize the recycling, stabilization and reduction of sludge is the research focus of scholars. In addition, there is generally a problem of insufficient carbon sources in the process of biological nitrogen and phosphorus removal in sewage treatment plants, which seriously limits the effect of biological nitrogen and phosphorus removal. If the sludge produced by the sewage plant can be anaerobically fermented to produce short-chain volatile fatty acids, and then supplemented with the produced short-chain volatile fatty acids, on the one hand, the problem of insufficient carbon sources will be solved, and at the same time, the resource utilization of sludge will be realized .

Sludge anaerobic fermentation includes three continuous processes of hydrolysis, acidification and methanation. The short-chain volatile fatty acids are the products of the acidification process and are consumed as the substrate for methanation to produce methane, carbon dioxide and other gases during the methanation process. In order to increase the accumulation of short-chain volatile fatty acids in this process, the first two reaction steps should be enhanced and the last reaction step should be inhibited. Previous pretreatments for promoting anaerobic fermentation of sludge include: heat treatment, mechanical pretreatment (microwave, ultrasonic, high pressure, etc.), chemical reagent pretreatment (alkali, acid, etc.) and biological pretreatment (enzyme, inoculation of bacteria, etc.). The above method can increase the hydrolysis rate and degradation degree of sewage, but has the disadvantages of large capital investment, unstable operation and low acid production effect. Therefore urgently need to invent a kind of pretreatment method with low price (final in situ production in sewage plant), reliable and stable operation and high acid production efficiency.

Free nitrous acid (the protonated form of nitrite) has a strong destructive effect on the extracellular polymers and cell walls of cells, thereby releasing intracellular substances and accelerating the anaerobic hydrolysis rate of sludge. However, there are still certain limitations in the extent of its damage. At the same time, sodium dodecylbenzenesulfonate is also a commonly used foaming agent and emulsifier in sewage plants. It has a certain content in sludge, and sodium dodecylbenzenesulfonate has serious effects on sludge methanation. Inhibition, so that the consumption of short-chain volatile fatty acids can be reduced. However, the use of free nitrous acid and sodium dodecylbenzenesulfonate to enhance the anaerobic fermentation of sludge to produce short-chain volatile fatty acids has not been reported so far. In addition, both free nitrous acid and sodium dodecylbenzenesulfonate can be obtained in-situ in the sewage plant, so the present invention can realize the in-situ production of short-chain fatty acids from sludge, and the invention has the advantages of low operating cost and high acid production efficiency High, reaction time period and other advantages.

Contents of the invention

The technical problem to be solved by the present invention is to overcome the defects of limited efficiency and output of short-chain volatile fatty acids and long fermentation time in the prior art, and provide a product with high yield, simple preparation method, energy saving and consumption reduction, low operating cost, and the use of free fatty acid A method for the combined pretreatment of nitric acid and sodium dodecylbenzenesulfonate to strengthen anaerobic fermentation treatment sludge to produce short-chain volatile fatty acids.

The invention provides a method for using free nitrous acid and sodium dodecylbenzene sulfonate to jointly pretreat and strengthen sludge anaerobic fermentation to produce short-chain volatile fatty acids. The specific steps are as follows:

(1) Using the sludge from the secondary sedimentation tank of the urban sewage treatment plant as the fermentation substrate;

(2) inoculating anaerobic activated sludge into the fermentation substrate as the inoculum;

(3) Add nitrite and sodium dodecylbenzenesulfonate to the fermentation system and control the reaction pH=6.0 for 2 days of pretreatment.

(4) After the pretreatment is completed, adjust pH=7.0 to carry out anaerobic fermentation to produce short-chain volatile fatty acids.

In the above preparation method, preferably, in step (1), after the sludge is naturally precipitated at 4° C. for 24 hours, the supernatant is removed to obtain the fermentation substrate; the sludge is mainly the remaining sludge from the secondary sedimentation tank of the urban sewage treatment plant.

The above-mentioned preparation method, preferably, the inoculated sludge in the step (2) is taken from the inoculated sludge in a UASB reactor in Changsha, which is rich in anaerobic fermentation microorganisms, and the mass ratio of the inoculated sludge to the fermentation substrate It is 1:9.

In the above preparation method, preferably, in step (3), after adding sodium nitrite to the fermentation substrate, the concentration of free nitrous acid is controlled to be 0-3.08 mgHNO ₂ -N/L, and the pH is 6±0.1. The quality of sodium dodecylbenzenesulfonate is controlled at 0.02g/gDS. The dosage of free nitrous acid is the key parameter of sludge fermentation in this system. When the concentration of free nitrous acid is controlled at 0-1.54mgHNO ₂ -N /L, the content of short-chain volatile fatty acids in sludge fermentation broth increased with the increase of free nitrous acid. When the concentration of free nitrous acid exceeds 1.54mgHNO ₂ -N/L, the content of short-chain volatile fatty acids in the fermentation broth decreases with the increase of free nitrous acid concentration. Therefore, the optimal dosage of free nitrous acid in this joint pretreatment is 1.54mgHNO ₂ -N/L.

The above-mentioned preparation method, preferably, the pretreatment step in step (3) is to stir the fermentation substrate, inoculum, free nitrous acid and sodium dodecylbenzenesulfonate mechanically for 48 hours, and control the rotating speed to be 100～130rpm/ min, and further determine that the rotating speed is 120rpm/min.

In the above preparation method, preferably, the stirring temperature is 20±0.1°C.

In the above preparation method, preferably, the anaerobic fermentation temperature is 30-40°C, further, the anaerobic fermentation temperature is 33-37°C.

In the above preparation method, preferably, the time of anaerobic fermentation is 3-8 days.

The innovation point of the present invention is:

Free nitrous acid is the protonated form of nitrite (NO ₂ ^- ), which has a strong destructive effect on the extracellular polymers and cell walls of microorganisms in sludge, thereby accelerating the release of intracellular substances. Sodium dodecylbenzenesulfonate is a chemical surfactant, which has a strong inhibitory effect on methanogenic bacteria in the sludge anaerobic fermentation system, and can inhibit the growth of methanogenic bacteria. The protonated form, and sodium dodecylbenzene sulfonate itself has a certain content in the sludge. The invention combines free nitrous acid and sodium dodecylbenzenesulfonate to strengthen the accumulation of short-chain volatile fatty acids. The combination of free nitrous acid and sodium dodecylbenzenesulfonate can better promote the dissolution of proteins and polysaccharides, and then be used by acid-producing bacteria, and because sodium dodecylbenzenesulfonate can inhibit the activity of methanogenic bacteria , reducing the consumption of short-chain volatile fatty acids. In turn, the short-chain volatile fatty acids are accumulated and the fermentation time is shortened. The amount of short-chain volatile fat produced by combined effect or FNA alone and sodium dodecylbenzenesulfonate alone pretreatment is larger.

In addition, both free nitrous acid and sodium dodecylbenzenesulfonate can be obtained from sewage plants. Free nitrous acid can be recovered from the sludge fermentation broth, and sodium dodecylbenzene sulfonate is commonly used as a foaming agent and emulsifier, and then sodium dodecylbenzene sulfonate will be transferred to the sludge system. Utilizing free nitrous acid and sodium dodecylbenzene sulfonate to strengthen anaerobic fermentation of sludge to produce short-chain volatile fatty acids can realize the resource utilization of sludge. In addition, the produced rich short-chain volatile fatty acids can be refluxed after treatment. The influent of the sewage treatment plant is used as an external carbon source to enhance biological nitrogen and phosphorus removal, improve the efficiency of sewage treatment, and realize the recycling of energy.

In the present invention, the free nitrous acid mainly destroys the extracellular polymer and cell wall of the sludge, thereby accelerating the hydrolysis process of the sludge, and the concentration of the free nitrous acid further affects the accumulation of short-chain volatile fatty acids. The experimental results show that when the concentration of free nitrous acid is 0-1.54mgHNO ₂ -N/L, with the increase of the concentration of free nitrous acid, the accumulation of short-chain volatile fatty acids will also increase, but when the concentration of free nitrous acid When the concentration exceeds 1.54mgHNO ₂ -N/L, free nitrous acid can inhibit the accumulation of short-chain volatile fatty acids. In this joint invention, the amount of sodium dodecylbenzene sulfonate is selected as 0.02g/gDS, and the combination of the two produces a synergistic effect, which greatly increases the accumulation of short-chain volatile fatty acids. And the maximum production of short-chain volatile fatty acids was 334.5mgCOD/gVSS, and the best fermentation time was 4 days.

Compared with the prior art, the present invention has the advantages of:

(1) The present invention uses the red by-product sludge in the operation process of the sewage plant as the fermentation substrate to carry out anaerobic fermentation to produce short-chain volatile fatty acids, thereby realizing resource utilization, reduction and stabilization of sludge. In addition, the short-chain volatile fatty acids produced by sludge fermentation can be applied to influent water to increase the content of organic carbon in influent water. The invention is a circular economic model for urban sewage treatment plants.

(2) The present invention uses free nitrous acid and sodium dodecylbenzene sulfonate to jointly pretreat sludge to prepare short-chain volatile fatty acids. The maximum content of short-chain volatile fatty acids prepared under the joint action was significantly higher than that prepared separately. And the optimal fermentation time under the joint action is also significantly shorter than that of the two alone.

(3) Both free nitrous acid and sodium dodecylbenzenesulfonate can be obtained in the sewage plant without additional addition. The application of combined fermentation technology has a certain effect on improving and optimizing the existing sludge treatment system Guiding significance.

detailed description

The present invention will be further described below in conjunction with specific preferred embodiments, but the protection scope of the present invention is not limited thereby.

Example

All materials and instruments used in the following examples are commercially available. The sludge was taken from the remaining sludge in the secondary settling tank of Changsha No. 2 Sewage Treatment Plant. The inoculated anaerobic sludge contained microorganisms such as Proteobacteria, Bacteroidetes and Actinobacteria.

Example 1

(1) Preparation of fermentation substrate: take the remaining sludge (water content 98.9%, pH=7.0) from the sewage treatment plant and place it in a refrigerator at 4° C. for natural precipitation for 24 hours. After discharging the upper liquid, the remaining sludge is obtained as the fermentation substrate.

(2) Pretreatment: Add sodium nitrite and sodium dodecylbenzenesulfonate to the fermented substrate obtained in step (1), control the concentration of free nitrous acid to be 0.51mgHNO ₂ -N/L, twelve The concentration of sodium alkylbenzene sulfonate is 0.02g/gDS, pH=6.0 and maintained for 2 days, during which the temperature is controlled at 20±0.1°C, and the rotational speed of sludge mechanical stirring is 120rpm/min.

(3) Anaerobic fermentation: On the 3rd day of fermentation, adjust the pH in the reactor to 7.0, fill with nitrogen to drive oxygen for 15 minutes, and control the temperature at 35±1°C for anaerobic fermentation to produce short-chain volatile fatty acids. When the total time was on the 4th day, the amount of short-chain volatile fatty acids produced reached the maximum and was 284.1 mgCOD/gVSS.

Example 2

(1) Preparation of fermentation substrate: take the remaining sludge (water content 98.9%, pH=7.0) from the sewage treatment plant and place it in a refrigerator at 4° C. for natural precipitation for 24 hours. After discharging the upper liquid, the remaining sludge is obtained as the fermentation substrate.

(2) Pretreatment: Add sodium nitrite and sodium dodecylbenzenesulfonate to the fermented substrate obtained in step (1), control the concentration of free nitrous acid to be 0.77mgHNO ₂ -N/L, twelve The concentration of sodium alkylbenzenesulfonate is 0.02g/gDS, pH=6.0 and maintained for 2 days, during which the temperature is controlled at 20±0.1°C, and the rotational speed of sludge mechanical stirring is 120rpm/min.

(3) Anaerobic fermentation: On the 3rd day of fermentation, adjust the pH in the reactor to 7.0, fill with nitrogen to drive oxygen for 15 minutes, and control the temperature at 35±1°C for anaerobic fermentation to produce short-chain volatile fatty acids. When the total time was on the 4th day, the amount of short-chain volatile fatty acids produced reached the maximum and was 308.2 mgCOD/gVSS.

Example 3

(1) Preparation of fermentation substrate: take the remaining sludge (water content 98.9%, pH=7.0) from the sewage treatment plant and place it in a refrigerator at 4° C. for natural precipitation for 24 hours. After discharging the upper liquid, the remaining sludge is obtained as the fermentation substrate.

(2) Pretreatment: Add sodium nitrite and sodium dodecylbenzenesulfonate to the fermentation substrate obtained in step (1), control the concentration of free nitrous acid to be 1.54mgHNO ₂ -N/L, dodecylbenzene The concentration of sodium alkylbenzenesulfonate is 0.02g/gDS, pH=6.0 and maintained for 2 days, during which the temperature is controlled at 20±0.1°C, and the rotational speed of sludge mechanical stirring is 120rpm/min.

(3) Anaerobic fermentation: On the 3rd day of fermentation, adjust the pH in the reactor to 7.0, fill with nitrogen to drive oxygen for 15 minutes, and control the temperature at 35±1°C for anaerobic fermentation to produce short-chain volatile fatty acids. When the total time was on day 4, the amount of short-chain volatile fatty acids produced reached a maximum and was 334.5 mgCOD/gVSS.

Example 4

(1) Preparation of fermentation substrate: take the remaining sludge (water content 98.9%, pH=7.0) from the sewage treatment plant and place it in a refrigerator at 4° C. for natural precipitation for 24 hours. After discharging the upper liquid, the remaining sludge is obtained as the fermentation substrate.

(2) Pretreatment: Add sodium nitrite and sodium dodecylbenzenesulfonate to the fermentation substrate obtained in step (1), control the concentration of free nitrous acid to be 2.31mgHNO ₂ -N/L, dodecylbenzenesulfonate The concentration of sodium alkylbenzenesulfonate is 0.02g/gDS, pH=6.0 and maintained for 2 days, during which the temperature is controlled at 20±0.1°C, and the rotational speed of sludge mechanical stirring is 120rpm/min.

(3) Anaerobic fermentation: On the 3rd day of fermentation, adjust the pH in the reactor to 7.0, fill with nitrogen to drive oxygen for 15 minutes, and control the temperature at 35±1°C for anaerobic fermentation to produce short-chain volatile fatty acids. When the total time was on the 4th day, the amount of short-chain volatile fatty acids produced reached the maximum and was 309.1 mgCOD/gVSS.

Example 5

(1) Preparation of fermentation substrate: take the remaining sludge (water content 98.9%, pH=7.0) from the sewage treatment plant and place it in a refrigerator at 4° C. for natural precipitation for 24 hours. After discharging the upper liquid, the remaining sludge is obtained as the fermentation substrate.

(2) Pretreatment: Add sodium nitrite and sodium dodecylbenzenesulfonate to the fermentation substrate obtained in step (1), and control the concentration of free nitrous acid to be 3.08mgHNO ₂ -N/L, twelve The concentration of sodium alkylbenzenesulfonate is 0.02g/gDS, pH=6.0 and maintained for 2 days, during which the temperature is controlled at 20±0.1°C, and the rotational speed of sludge mechanical stirring is 120rpm/min.

(3) Anaerobic fermentation: On the 3rd day of fermentation, adjust the pH in the reactor to 7.0, fill with nitrogen to drive oxygen for 15 minutes, and control the temperature at 35±1°C for anaerobic fermentation to produce short-chain volatile fatty acids. When the total time was on day 4, the amount of short-chain volatile fatty acids produced was 268.4 mgCOD/gVSS.

Example 6

(1) Preparation of fermentation substrate: take the remaining sludge (water content 98.9%, pH=7.0) from the sewage treatment plant and place it in a refrigerator at 4° C. for natural precipitation for 24 hours. After discharging the upper liquid, the remaining sludge is obtained as the fermentation substrate.

(2) Pretreatment: Add sodium nitrite and sodium dodecylbenzenesulfonate to the fermentation substrate obtained in step (1), control the concentration of free nitrous acid to be 1.54mgHNO ₂ -N/L, dodecylbenzene The concentration of sodium alkylbenzene sulfonate is 0.02g/gDS, pH=6.0 and maintained for 1d, during which the temperature is controlled at 20±0.1°C, and the rotational speed of sludge mechanical stirring is 120rpm/min.

(3) Anaerobic fermentation: On the 2nd day of fermentation, the pH in the reactor was adjusted to 7.0, and nitrogen gas was used to drive away oxygen for 15 minutes, and the temperature was controlled at 35±1°C for anaerobic fermentation to produce short-chain volatile fatty acids. When the total time was at day 5, the amount of short-chain volatile fatty acids produced was 196.7 mgCOD/gVSS.

Example 7

(1) Preparation of fermentation substrate: take the remaining sludge (water content 98.9%, pH=7.0) from the sewage treatment plant and place it in a refrigerator at 4° C. for natural precipitation for 24 hours. After discharging the upper liquid, the remaining sludge is obtained as the fermentation substrate.

(2) Pretreatment: Add sodium nitrite and sodium dodecylbenzenesulfonate to the fermentation substrate obtained in step (1), control the concentration of free nitrous acid to be 1.54mgHNO ₂ -N/L, dodecylbenzene The concentration of sodium alkylbenzene sulfonate is 0.02g/gDS, pH=6.0 and maintained for 3 days, during which the temperature is controlled at 20±0.1°C, and the rotational speed of sludge mechanical stirring is 120rpm/min.

(3) Anaerobic fermentation: On the 4th day of fermentation, the pH in the reactor was adjusted to 7.0, and nitrogen gas was used to drive away oxygen for 15 minutes, and the temperature was controlled at 35±1°C for anaerobic fermentation to produce short-chain volatile fatty acids. When the total time was at day 5, the amount of short-chain volatile fatty acids produced was 319.8 mgCOD/gVSS.

Comparative example 1

(1) Preparation of fermentation substrate: take the remaining sludge (water content 98.9%, pH=7.0) from the sewage treatment plant and place it in a refrigerator at 4° C. for natural precipitation for 24 hours. After discharging the upper liquid, the remaining sludge is obtained as the fermentation substrate.

(2) Pretreatment: Add sodium nitrite and sodium dodecylbenzenesulfonate to the fermentation substrate obtained in step (1), control the concentration of free nitrous acid to be 0mgHNO ₂ -N/L, dodecane The concentration of sodium phenylsulfonate is 0.02g/gDS, during which the temperature is controlled at 20±0.1°C, and the rotational speed of sludge mechanical stirring is 120rpm/min.

(3) Anaerobic fermentation: On the 3rd day of fermentation, adjust the pH in the reactor to 7.0, fill with nitrogen to drive oxygen for 15 minutes, and control the temperature at 35±1°C for anaerobic fermentation to produce short-chain volatile fatty acids. When the total time was on day 8, the amount of short-chain volatile fatty acids produced reached a maximum and was 238.2 mgCOD/gVSS.

Comparative example 2

(1) Preparation of fermentation substrate: take the remaining sludge (water content 98.9%, pH=7.0) from the sewage treatment plant and place it in a refrigerator at 4° C. for natural precipitation for 24 hours. After discharging the upper liquid, the remaining sludge is obtained as the fermentation substrate.

(2) Pretreatment: Add sodium nitrite and sodium dodecylbenzenesulfonate to the fermentation substrate obtained in step (1), control the concentration of free nitrous acid to be 1.54mgHNO ₂ -N/L, dodecylbenzene The dosage of sodium alkylbenzene sulfonate is 0, during which the temperature is controlled at 20±0.1°C, and the rotational speed of sludge mechanical stirring is 120rpm/min.

(3) Anaerobic fermentation: On the 3rd day of fermentation, adjust the pH in the reactor to 7.0, fill with nitrogen to drive oxygen for 15 minutes, and control the temperature at 35±1°C for anaerobic fermentation to produce short-chain volatile fatty acids. When the total time was at day 6, the amount of short-chain volatile fatty acids produced reached a maximum and was 186.5 mgCOD/gVSS.

Comparative example 3

(1) Preparation of fermentation substrate: take the remaining sludge (water content 98.9%, pH=7.0) from the sewage treatment plant and place it in a refrigerator at 4° C. for natural precipitation for 24 hours. After discharging the upper liquid, the remaining sludge is obtained as the fermentation substrate.

(2) Anaerobic fermentation: the fermentation substrate obtained in step (1) is controlled at a temperature of 35±1° C., the mechanical stirring speed is 120 rpm/min, and nitrogen is filled to drive oxygen for 15 minutes, and it is directly fermented without pretreatment. When the fermentation time was 15 days, the production of short-chain volatile fatty acids reached the maximum and was only 72.1mgCOD/gVSS.

The above descriptions are only preferred embodiments of the present invention, and do not limit the present invention in any form. Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Any person familiar with the art, without departing from the spirit and technical solutions of the present invention, can use the methods and technical content disclosed above to make many possible changes and modifications to the technical solutions of the present invention, or modify them to be equivalent Variations of equivalent embodiments. Therefore, any simple modifications, equivalent replacements, equivalent changes and modifications made to the above embodiments according to the technical essence of the present invention that do not deviate from the technical solutions of the present invention still fall within the protection scope of the technical solutions of the present invention.

Claims (10)

1. a method for producing short-chain volatile fatty acids from combined free nitrous acid and sodium dodecylbenzenesulfonate pretreatment sludge, is characterized in that, comprises the following steps: (1) Using the sludge from the secondary sedimentation tank of the urban sewage treatment plant as the fermentation substrate; (2) inoculating anaerobic activated sludge into the fermentation substrate as the inoculum; (3) Add nitrite and sodium dodecylbenzenesulfonate to the fermentation system and control the reaction pH=6.0 for 2 days of pretreatment. (4) After the pretreatment is completed, adjust pH=7.0 to carry out anaerobic fermentation to produce short-chain volatile fatty acids. 2. The preparation method according to claim 1, characterized in that, in the step (1), after the sludge in the secondary sedimentation tank of the urban sewage plant is naturally precipitated for 24 hours at 4° C., after removing the supernatant Obtain the fermented substrate. 3. preparation method according to claim 1, is characterized in that, in the described step (2), inoculation sludge is taken from the inoculation sludge in a certain UASB reactor in Changsha, and the mass ratio of inoculation sludge and fermentation substrate is 1:9. 4. preparation method according to claim 1, is characterized in that, in described step (3), after adding sodium nitrite, control the pH=6.0 ± 0.1 of pretreatment stage, control the concentration of free nitrous acid to be 0.51～3.08mgHNO ₂ -N/L, the dosage of sodium dodecylbenzenesulfonate is 0.02g/gDS. 5. The preparation method according to claim 4, characterized in that the optimal concentration of the free nitrous acid is 0.77-2.31 mgHNO ₂ -N/L. 6. according to the preparation method described in any one in claim 1 to 5, it is characterized in that, described pretreatment step in described step (2) is that described fermented substrate and described sodium nitrite and dodecyl Sodium alkylbenzene sulfonate was subjected to continuous mechanical stirring for 48h. 7. The preparation method according to claim 6, characterized in that, the rotation speed of the mechanical stirring is 100-130 rpm/min; the stirring temperature is 20±0.1°C. 8. The preparation method according to claim 4, characterized in that the temperature of the anaerobic fermentation is 20-30°C. 9. The preparation method according to any one of claims 1 to 5, characterized in that, the time of the anaerobic fermentation is 3 to 8 days. 10. A short-chain volatile fatty acid prepared by the preparation method described in any one of claims 1 to 9.