Method for producing volatile fatty acids from sweet potato peel and sweet potato peel acid hydrolysis liquid and fermentation liquid prepared according to the method

Abstract

This invention provides a method for producing volatile fatty acids from sweet potato peels, as well as a sweet potato peel acid hydrolysate and fermentation broth prepared according to the method. It relates to the field of organic solid waste resource utilization technology. The method includes: mixing sweet potato peels with an inorganic acid solution of 1%~2% by mass at a material-to-liquid mass-to-volume ratio of 1:4.4~5.7 g / mL, and stirring the mixture at 80℃~120℃ for 20~40 minutes to obtain a sweet potato peel acid hydrolysate. The inorganic acid includes at least one of hydrochloric acid and sulfuric acid. The sweet potato peel acid hydrolysate is then anaerobically fermented with residual sludge from a municipal wastewater treatment plant under conditions of pH 6~10 to obtain a fermentation broth containing volatile fatty acids.

Description

Technical Field

[0001] This application relates to the field of organic solid waste resource utilization technology, and in particular to a method for producing volatile fatty acids from sweet potato peel as raw material, and sweet potato peel acid hydrolysate and fermentation broth prepared according to the method. Background Technology

[0002] China is the world's largest producer of sweet potatoes. The deep processing of sweet potatoes generates a large amount of byproducts, with sweet potato peels being the main byproduct, accounting for 20% to 30% of the total processed raw materials. Sweet potato peels are rich in dietary fiber, starch, anthocyanins, polyphenols, flavonoids, and other active ingredients, possessing high resource potential. Currently, the utilization rate of sweet potato peels is low; apart from a small amount used as animal feed, most are discarded directly, resulting in resource waste.

[0003] The anaerobic fermentation of waste biomass to produce volatile fatty acids (VFAs) is currently a research hotspot in the field of solid waste resource utilization. VFA-rich fermentation broth can be directly reused as a high-quality liquid carbon source in nitrogen and phosphorus removal processes in urban and industrial wastewater treatment plants, replacing traditional commercial carbon sources such as methanol or sodium acetate. It can also be further purified and separated to produce high-purity acetic acid, propionic acid, and other basic chemical products, showing broad application prospects. Existing technologies have disclosed various cases of acid-producing fermentation of organic solid waste, such as using citrus peels, distiller's grains, straw, kitchen waste, grass biomass, soybean residue, and mushroom residue as substrates, followed by physical, chemical, or biological pretreatment, and then fermenting under anaerobic conditions to produce acid.

[0004] However, existing technologies still face two major bottlenecks: First, plant-derived waste is rich in structural polysaccharides such as cellulose and hemicellulose. If parameters such as acid concentration, reaction temperature, and reaction time are not properly controlled during pretreatment, either the cell wall structure cannot be sufficiently destroyed, resulting in insufficient release of soluble organic matter, or the dehydration reaction of hemicellulose, pentose and hexose degradation products of cellulose will be triggered, generating furan-like microbial activity inhibitors such as furfural and 5-hydroxymethylfurfural (HMF), thereby reducing substrate conversion efficiency. Second, different biomass has significant differences in physicochemical characteristics and composition, requiring different pretreatment and fermentation conditions. Finding the optimal treatment conditions based on its basic characteristics, increasing the proportion of VFA in the fermentation broth, and increasing the mass proportion of acetic acid in VFA are the key focuses of process control. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a method for producing volatile fatty acids from sweet potato peel, as well as a sweet potato peel acid hydrolysate and fermentation broth prepared according to this method.

[0006] As a first aspect of the present invention, the present invention provides a method for producing volatile fatty acids from sweet potato peel, comprising: mixing sweet potato peel with an inorganic acid solution of 1% to 2% by mass at a mass-to-volume ratio of 1:4.4 to 5.7 g / mL, stirring and reacting at 80°C to 120°C for 20 to 40 minutes to obtain a sweet potato peel acid hydrolysate, wherein the inorganic acid includes at least one of hydrochloric acid and sulfuric acid; and anaerobic fermenting the sweet potato peel acid hydrolysate with residual sludge from a municipal wastewater treatment plant under conditions of pH 6 to 10 to obtain a fermentation broth containing volatile fatty acids.

[0007] As a second aspect of the present invention, the present invention provides a sweet potato peel acid hydrolysate, wherein the SCOD of the sweet potato peel acid hydrolysate is 55~132 g / L; the COD equivalent of the total sugar in the sweet potato peel acid hydrolysate is 71%~93%, and the sweet potato peel acid hydrolysate is prepared by the following method: sweet potato peel is mixed with an inorganic acid solution of 1%~2% by mass at a material-to-liquid mass-to-volume ratio of 1:4.4~5.7 g / mL, and stirred at 80℃~120℃ for 20~40 minutes to obtain the sweet potato peel acid hydrolysate, wherein the inorganic acid includes at least one of hydrochloric acid and sulfuric acid.

[0008] As a third aspect of the present invention, the present invention provides a fermentation broth for producing volatile fatty acids from sweet potato peel, which is prepared by the above-mentioned method for producing volatile fatty acids from sweet potato peel as raw material, wherein the chemical oxygen demand equivalent of the volatile fatty acids in the fermentation broth is 50% to 60%; and the mass percentage of acetic acid in the volatile fatty acids is 79% to 90%.

[0009] The technical solution provided by this invention has at least one of the following beneficial effects:

[0010] (1) In this invention, volatile fatty acids are produced from waste sweet potato peels, realizing the high-value utilization of agricultural waste, effectively improving the resource utilization rate of sweet potato peels, and having both environmental and economic benefits.

[0011] (2) In this invention, by controlling parameters such as acid concentration, reaction temperature and reaction time during the acid hydrolysis of sweet potato peel, the cell wall structure of sweet potato peel can be destroyed under mild conditions, so that the SCOD of sweet potato peel acid hydrolysate reaches 55.3~132.3g / L and the total sugar content increases to 71%~93%. At the same time, through the synergistic regulation of parameters, the generation of furfural and HMF is effectively inhibited, which alleviates the problem of low fermentation efficiency caused by insufficient release of organic matter or excessive inhibitors in the prior art.

[0012] (3) In this invention, by anaerobic fermentation of sweet potato peel acid hydrolysate and residual sludge from urban sewage treatment plant, the COD equivalent ratio of VFA in fermentation liquid is increased to 50%~60%, of which the mass ratio of acetic acid is 79%~90%, which is significantly higher than the existing process's COD equivalent ratio of VFA of 30%~50% and the mass ratio of acetic acid of 30%~65%, thus achieving targeted high yield of acetic acid and greatly improving the application value of the product. Attached Figure Description

[0013] Figure 1 This is an overall flow chart of the method for producing volatile fatty acids from sweet potato peel according to the present invention;

[0014] Figure 2 The component distribution of soluble organic matter in sweet potato peel hydrolysate under different hydrochloric acid concentrations and reaction times is shown in the figure.

[0015] Figure 3 The compositional distribution of furan inhibitors (furfural, HMF) in sweet potato peel acid hydrolysate under different hydrochloric acid concentrations and reaction times;

[0016] Figure 4 The compositional distribution of soluble organic matter in sweet potato peel hydrolysate at different temperatures and reaction times is shown in the diagram.

[0017] Figure 5 The compositional distribution of furan inhibitors (furfural, HMF) in sweet potato peel acid hydrolysate at different temperatures and reaction times;

[0018] Figure 6 The component distribution of soluble organic matter in sweet potato peel acid hydrolysate is shown in the figure for different mass-volume ratios of the liquid and reaction times.

[0019] Figure 7 The compositional distribution of furan inhibitors (furfural, HMF) in sweet potato peel acid hydrolysate under different mass-volume ratios of feed and reaction times;

[0020] Figure 8 Bar chart showing the distribution of organic components in fermentation broth at different pH values;

[0021] Figure 9 This is a graph showing the component distribution of volatile fatty acids in the fermentation broth at pH 6.

[0022] Figure 10 This is a distribution diagram of volatile fatty acids in the fermentation broth at pH 8.

[0023] Figure 11 This is a distribution diagram of volatile fatty acids in the fermentation broth at pH 10. Detailed Implementation

[0024] The embodiments of this application will now be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of this application. In the following detailed description, numerous specific details are set forth to provide a thorough understanding of the embodiments of this application for ease of explanation. However, it will be apparent that one or more embodiments may be implemented without these specific details. Furthermore, descriptions of well-known structures and technologies are omitted in the following description to avoid unnecessarily obscuring the concepts of this application.

[0025] In the process of realizing the concept of this invention, it was discovered that sweet potato peel is rich in starch, hemicellulose and protein components. If conventional alkaline heating pretreatment is used, the reducing sugars such as glucose and xylose generated by the hydrolysis of starch and hemicellulose will undergo a violent Maillard non-enzymatic browning reaction with the free amino acids released by the protein. The heterocyclic intermediates and high molecular weight melanoidins generated by the reaction are difficult to be degraded by microorganisms and have biotoxicity, which will hinder the subsequent fermentation efficiency.

[0026] To avoid the formation of persistent and toxic byproducts from the Maillard reaction that could hinder subsequent fermentation, acidic heat treatment has become the preferred pretreatment route for sweet potato peel components. However, its direct application to the sweet potato peel system still faces several insurmountable technical bottlenecks, mainly in the following three aspects:

[0027] First, while dilute acid pretreatment is an effective means of disrupting cellulose structure and promoting the dissolution of organic matter, acid concentration, reaction temperature, and reaction time are key parameters affecting the treatment effect during acidic heat treatment. If the acid concentration is too low, the temperature is insufficient, or the time is too short, the acidification of the cell wall structure is incomplete, resulting in limited release of soluble chemical oxygen demand (SCOD) and total fermentable sugars, failing to meet the substrate requirements for subsequent fermentation. Conversely, if the acid concentration is too high, the temperature is too high, or the time is too long, although the release of organic matter increases, it will exacerbate the excessive degradation of cellulose and hemicellulose, leading to the rapid formation of furan inhibitors such as furfural and 5-hydroxymethylfurfural (HMF). These inhibitors are toxic to acid-producing microorganisms; when their concentration exceeds the tolerance threshold of the microorganisms, it will lead to reduced metabolic activity and hinder the subsequent VFA synthesis process. Related studies have shown that when the furfural concentration reaches 0.3–1.2 g / L and the HMF concentration reaches 0.6 g / L or higher, it has an inhibitory effect on microbial fermentation to varying degrees. Different microorganisms have varying tolerance thresholds to furfural and HMF. For example, in *Clostridium butyricum*, furfural concentrations above 0.3 g / L and HMF concentrations above 0.6 g / L significantly inhibit butyric acid fermentation; fermentation is completely inhibited when furfural concentration reaches 1.2 g / L. In sweet potato peel acid hydrolysate, the acid hydrolysis process itself generates such inhibitors, which, if left uncontrolled, will affect subsequent fermentation efficiency.

[0028] Secondly, controlling the selectivity of products during fermentation is quite difficult. VFAs are a collective term for a class of organic acids, including acetic acid, propionic acid, butyric acid, and other components. Different components have varying values ​​and functions in industrial applications. Acetic acid, due to its excellent biodegradability and chemical activity, is a high-value component in denitrification processes and chemical synthesis. However, existing biomass fermentation technologies for VFA production generally suffer from difficulties in targeted control. Related studies show that VFAs typically account for 30%–50% of the dissolved organic matter (COD equivalent) in the fermentation broth, with concentrations mostly ranging from 0.5 to 20.0 g / L. The remainder consists of unconverted residual sugars, proteins, or other intermediate metabolites. Acetic acid typically constitutes 30%–65% of the VFA by mass, which not only reduces the economic value of the product but also increases the cost of subsequent separation and purification.

[0029] More importantly, different biomass exhibit significant differences in physicochemical and component characteristics, requiring varying pretreatment and fermentation conditions. Therefore, it is essential to identify the optimal treatment conditions based on the substrate characteristics to achieve the best VFA yield. Given the unique component structure of sweet potato peel, current technologies do not provide technical insights into pretreatment followed by anaerobic fermentation to produce VFA, and further lack technical solutions for pretreatment parameters and fermentation environments specific to sweet potato peel as a substrate.

[0030] Therefore, it is necessary to establish a fermentation regulation method that uses sweet potato peel as a substrate, selects the optimal process conditions that are suitable for the characteristics of sweet potato peel, balances the substrate release efficiency and the amount of inhibitor formation, thereby significantly increasing the proportion of VFA in the fermentation broth after anaerobic fermentation and selectively enriching acetic acid components.

[0031] Through extensive experimental exploration and condition optimization, this invention proposes for the first time a new method for the resource utilization of sweet potato peel through dilute acid pretreatment and anaerobic fermentation, aiming to fill the technological gap in this field and achieve the efficient conversion of sweet potato peel into high-value-added VFA.

[0032] As a first aspect of the present invention, the present invention provides a method for producing volatile fatty acids from sweet potato peel as raw material, comprising steps S1 to S2:

[0033] Step S1: Mix sweet potato peel with an inorganic acid solution of 1%~2% by mass at a mass-to-volume ratio of 1:4.4~5.7 g / mL, and stir at 80℃~120℃ for 20~40 minutes to obtain sweet potato peel acid hydrolysate. The inorganic acid includes at least one of hydrochloric acid and sulfuric acid.

[0034] Step S2: The acid hydrolysate of sweet potato peel and the residual sludge from the urban sewage treatment plant are anaerobically fermented under a pH of 6-10 to obtain a fermentation broth containing volatile fatty acids.

[0035] In step S1 of the above method, the sweet potato peel is preferably sweet potato peel powder, which is obtained by air-drying sweet potato peel to a moisture content of 5% to 15% and then pulverizing it through a 1mm sieve. The air-drying and sieving processes increase the contact area between the sweet potato peel and the inorganic acid solution, which is beneficial to improving the reaction efficiency.

[0036] In step S1, pretreatment with inorganic acids can selectively hydrolyze hemicellulose and some amorphous cellulose components, effectively disrupting the complete structure of the lignocellulose complex and promoting the dissolution and release of soluble sugars and organic matter.

[0037] The preferred inorganic acid is hydrochloric acid. Pretreatment with hydrochloric acid helps maintain the high activity of acid-producing bacteria, thereby significantly improving fermentation efficiency.

[0038] It should be noted that the core mechanism of inorganic acid pretreatment of sweet potato peel is: H + Catalytic cleavage of β-1,4-glycosidic bonds in cellulose and hemicellulose promotes the dissolution of soluble sugars and organic matter. Based on this mechanism, this application provides sufficient theoretical support for the application of acids from hydrochloric acid to inorganic acids such as hydrochloric acid and sulfuric acid: hydrochloric acid and sulfuric acid, as strong acids, can completely ionize to provide high concentrations of H+. + It uses highly efficient catalytic bond breaking.

[0039] The mass-to-volume ratio of sweet potato peel to hydrochloric acid solution is 1:4.4~5.7 g / mL, for example, 1:4.4, 1:5, 1:5.7, etc. This condition ensures sufficient β-glycosidic bond cleavage while avoiding excessive hydrolysis. Theoretically, a lower mass-to-volume ratio indicates a higher proportion of sweet potato peel in the reaction system, which is more conducive to increasing SCOD. However, experiments have shown that when the mass-to-volume ratio is below the lower limit of 1:4.4 g / mL, the sweet potato peel powder rapidly absorbs water and swells, easily floating and overflowing the reactor. Furthermore, the increased viscosity of the system leads to uneven stirring, which in turn hinders the release of soluble organic matter. The aforementioned mass-to-volume ratio range of 1:4.4~5.7 g / mL avoids the problems of reactor overflow and uneven stirring while ensuring efficient SCOD release.

[0040] The mass fraction of the hydrochloric acid solution mentioned above is 1% to 2%, for example, 1%, 1.5%, 2%, etc. The mass fraction of the hydrochloric acid solution and the mass-to-volume ratio of the solution to the feed have a significant impact on the pretreatment effect: if the acid concentration is too high (mass fraction higher than 2%), it will lead to an increase in H+ in the solution. + As concentration increases, higher H +Increased acid concentration provides more catalytically active sites, accelerating the dehydration of pentose to furfural and hexose to 5-hydroxymethylfurfural. Increased acid concentration also promotes the dehydration of glucose to 5-HMF. If the acid concentration is too low (mass fraction below the lower limit of 1%), it cannot effectively break the β-1,4 glycosidic bonds of cellulose and hemicellulose in sweet potato peel, thus failing to fully destroy the cell wall structure. This results in insufficient release of soluble organic matter and total fermentable sugars, which cannot meet the substrate requirements for subsequent anaerobic fermentation.

[0041] The reaction temperature is 80℃~120℃, for example, 80℃, 100℃, or 120℃; the stirring time is 20~40 minutes, for example, 20 minutes, 30 minutes, or 40 minutes. If the temperature is too low or the time is too short, the cell wall breakdown will be insufficient, limiting the release of organic matter. If the acid concentration is too high, the temperature too high, or the time too long, it will exacerbate the formation of furan inhibitors, which is also detrimental to subsequent fermentation and acid production. The stirring speed is 100~400 rpm. This speed ensures sufficient contact between the acid solution and the sweet potato peel powder, increasing the release of soluble organic matter and total fermentable sugars; it also avoids the destruction of the acid-producing microbial aggregate structure due to excessive shear force.

[0042] Preferably, the hydrochloric acid solution has a mass fraction of 1%; the material-to-liquid mass-to-volume ratio is 1:4.4 g / mL; the stirring reaction temperature is 100°C; and the stirring reaction time is 20 minutes. Under these pretreatment conditions, the release of soluble organic matter and total sugar from the sweet potato peel hydrolysate is maximized while the concentrations of furfural and HMF are controlled below the inhibition level.

[0043] Specifically, the limitations of each process parameter in this invention are all based on Figures 2-7 The experimental trends have been determined, and the specific patterns are as follows:

[0044] like Figure 2 and Figure 3 As shown, when the hydrochloric acid concentration increased from 1% to 2%, although the SCOD and total sugar content of the acid hydrolysate remained at a high level, the concentrations of furfural and HMF in the sweet potato peel acid hydrolysate increased significantly at this concentration. This indicates that excessively high acid concentrations accelerate the excessive degradation of cellulose and the occurrence of side reactions.

[0045] like Figure 4 and Figure 5 As shown, when the reaction temperature is increased from 80℃ to 100℃, the SCOD and total sugar content increases significantly. However, when the reaction temperature continues to rise to 120℃, the SCOD and total sugar content remain relatively stable, but the concentrations of furfural and HMF in the sweet potato peel acid hydrolysate increase significantly, proving that 100℃ is the key temperature for balancing substrate dissolution and inhibitor control.

[0046] like Figures 2-5As shown, extending the reaction time to 40 min can slightly increase SCOD, but it leads to a significant accumulation of inhibitors such as HMF, indicating that within the time range of 20-40 min, a shorter reaction time is more conducive to suppressing the occurrence of side reactions.

[0047] like Figure 6 and Figure 7 As shown, when the mass-to-volume ratio of the feed solution was optimized from 1:5.7 g / mL to 1:4.4 g / mL, not only was the SCOD significantly improved, but the concentrations of furfural and HMF were also significantly reduced, proving that 1:4.4 g / mL is the preferred mass-to-volume ratio of the feed solution in this invention.

[0048] In step S2 of the above method, the sweet potato peel acid hydrolysate is adjusted to neutral (i.e., pH range of 6.8~7.2) before being mixed with the remaining sludge. This neutralization step effectively avoids the metabolic inhibition of acid-producing microorganisms by the acidic environment, ensuring the stable operation of subsequent anaerobic fermentation steps.

[0049] The waste sludge used in step S2 of the above method is waste sludge containing anaerobic fermentation and acid-producing bacteria, including but not limited to one or more types of acid-producing sludge such as waste sludge from secondary sedimentation tanks of wastewater treatment plants, anaerobic granular sludge, and anaerobic digested sludge from wastewater treatment plant sludge treatment centers. The main function of this waste sludge is to provide acid-producing microorganisms for subsequent anaerobic fermentation, converting soluble organic matter in the hydrolysate into volatile fatty acids.

[0050] The preferred volume ratio of the sweet potato peel acid hydrolysate to the residual sludge is 1:3 to 5, and the preferred concentration of the residual sludge is 20 to 40 g / L. The concentration of the residual sludge is expressed as total solids (TS).

[0051] According to an embodiment of the present invention, in step S2, the pH value of the anaerobic fermentation is 6-10; the temperature of the anaerobic fermentation is preferably 30℃-40℃; and the fermentation time of the anaerobic fermentation is preferably 6-10 days. The specified pH value effectively inhibits the activity of competing bacteria such as methanogens, ensuring the stability of the fermentation system; the temperature range of 30℃-40℃ matches the energy metabolism requirements of mesophilic anaerobic fermentation, avoiding the risks of slow reaction rates at low temperatures and inactivation of bacteria at high temperatures; and the 6-10 day fermentation cycle ensures the full conversion of organic matter into VFA.

[0052] Preferably, the pH value of the anaerobic fermentation is 8; the temperature of the anaerobic fermentation is 35°C; and the fermentation time is 7-8 days. Under the preferred conditions of pH=8, temperature 35°C, and fermentation time of 7-8 days, the system achieves the best balance between metabolic activity, acetic acid-directed enrichment efficiency, and process energy consumption, with VFA yield reaching its peak and the mass proportion of acetic acid significantly improved.

[0053] Among these factors, the pH value of the fermentation system is a core factor in regulating the yield and selectivity of VFA. For example... Figure 8 As shown, the pH value of the fermentation system has a decisive impact on the total VFA yield. As the pH value increases from 5.0 to 8.0, the VFA concentration shows a significant upward trend, reaching a peak at pH=8.0; when the pH value continues to rise to 10.0, the VFA yield declines. Figures 9-11 As shown, the component distribution of the fermentation broth differs significantly under different pH conditions. Compared to pH 6.0 and pH 10.0, the acetic acid concentration of VFA is highest at pH 8.0. Therefore, pH 8.0 is a key parameter for achieving efficient conversion of sweet potato peel into high-value acetic acid during anaerobic fermentation.

[0054] As a second aspect of the present invention, the present invention provides a sweet potato peel acid hydrolysate, which is prepared by step S1 of the above-mentioned method for producing volatile fatty acids from sweet potato peel as raw material. The SCOD of the sweet potato peel acid hydrolysate is 55~132 g / L; the COD equivalent of the total sugar in the sweet potato peel acid hydrolysate is 71%~93%.

[0055] In this invention, the SCOD of the sweet potato peel acid hydrolysate is 55~132 g / L, providing sufficient organic matter reserves for the fermentation system and avoiding insufficient fermentation efficiency caused by excessively low substrate concentration. The COD equivalent of total sugar is as high as 71%~93%, indicating that the organic matter in the acid hydrolysate is mainly fermentable sugars that are easily utilized by acid-producing bacteria, and the proportion of difficult-to-degrade impurities such as proteins is extremely low. At the same time, the content of furan inhibitors (such as HMF) is far below the tolerance threshold of acid-producing microorganisms, thus avoiding the risk of fermentation toxicity from the source and providing a high-quality and safe substrate guarantee for subsequent efficient acid production.

[0056] According to embodiments of the present invention, the aforementioned COD equivalent percentage refers to the percentage of total SCOD after converting the measured mass concentration of the organic compound into COD equivalent by multiplying it by the COD conversion factor corresponding to its complete oxidation. Specifically, the COD conversion factors for protein, total sugar, and acetic acid are 1.50, 1.07, and 1.07 gCOD / g, respectively. The conversion factors for total sugar and acetic acid are the same because the amount of oxygen consumed per gram of organic compound during their complete oxidation is similar.

[0057] As a third aspect of the present invention, the present invention provides a fermentation broth for producing volatile fatty acids from sweet potato peel, which is prepared by the above-mentioned method for producing volatile fatty acids from sweet potato peel as raw material, wherein the chemical oxygen demand equivalent of the volatile fatty acids in the fermentation broth is 50% to 60%; and the mass percentage of acetic acid in the volatile fatty acids is 79% to 90%.

[0058] In this invention, the COD equivalent of volatile fatty acids in the fermentation broth reaches 50%~60%, which is significantly higher than the average level of 30%~50% in traditional acid-producing fermentation processes; the VFA concentration reaches 22.1g / L, which is higher than the VFA concentration range of 0.5~20g / L in traditional acid-producing fermentation; the acetic acid mass ratio is as high as 79%~90%, which is higher than the acetic acid mass ratio level of 30%~65% in traditional acid-producing fermentation. The product has outstanding orientation, which greatly reduces the difficulty and cost of subsequent separation and purification.

[0059] The present invention will be further described in detail below through specific embodiments, wherein the raw materials, reagents and detection methods used are as follows:

[0060] 1. Raw materials and reagents

[0061] Excess sludge: Taken from the secondary sedimentation tank of the urban wastewater treatment plant, and used after thickening treatment; the total solids (TS) of the sludge is 30±5g / L.

[0062] Sweet potato peel: taken from a sweet potato processing plant, the moisture content after air drying is 5.7%; the volatile solids (VS) content of sweet potato peel after crushing and sieving is 0.4~0.7g / g.

[0063] Reagents: Hydrochloric acid was of analytical grade and prepared into an acid solution with the corresponding mass fraction using deionized water; the pH of the fermentation system was adjusted using 1 mol / L NaOH and 1 mol / L HCl solutions.

[0064] 2. Detection Method

[0065] 2.1 SCOD

[0066] The detection was performed using the hash micro-backflow digestion-spectrophotometric method, based on Hach Method 8000 (Reactor Digestion Method).

[0067] 2.2 Furfural and 5-hydroxymethylfurfural (HMF)

[0068] High-performance liquid chromatography (HPLC) was used for detection. The chromatographic conditions were as follows: separation was performed using an Ultimate Plus-C18 column; the mobile phase was methanol-water solution; detection was performed using a UV detector at a wavelength of 277 nm; and quantification was performed by plotting a standard curve using standards. Units are expressed in mg / L.

[0069] 2.3 Total Sugar

[0070] The phenol-sulfuric acid method was used for detection, in accordance with GB / T 15672-2009 Determination of Total Sugar Content in Edible Fungi.

[0071] 2.4 Protein

[0072] The Coomassie Brilliant Blue method was used for detection, in accordance with SN / T 3926-2014 "Determination of Protein Content in Exported Dairy, Egg and Soy Products - Coomassie Brilliant Blue Method".

[0073] 2.5 Mass percentage of volatile fatty acids (VFA) and acetic acid

[0074] 1) Detection of VFAs: Gas chromatography was used for detection. After acidification and pretreatment, the sample was separated using a capillary column and detected by a flame ionization detector (FID). Quantification was performed by comparing the peak areas of each component in the standard solution with the corresponding components in the sample to determine the content of volatile fatty acids such as acetic acid, propionic acid, and butyric acid produced during the anaerobic fermentation of sweet potato peels.

[0075] 2) Acetic acid mass percentage: The acetic acid mass percentage was calculated based on the detection results of VFA gas chromatography. The absolute concentration of each component (acetic acid, propionic acid, butyric acid, etc.) was obtained quantitatively based on the peak area of ​​the gas chromatography. The acetic acid mass percentage (%) was calculated using the following formula:

[0076] Acetic acid mass percentage (%) = C 乙酸 / C 总VFA ×100%

[0077] Among them, C 乙酸 C represents the concentration of acetic acid in the sample (mg / L). 总VFA It is the sum of the concentrations (mg / L) of volatile fatty acids such as acetic acid, propionic acid, and butyric acid in the sample.

[0078] 2.6 Moisture content and volatile solids (VS) of sweet potato peel

[0079] According to the "Methods for Monitoring and Analysis of Water and Wastewater (Fourth Edition, Supplement)," the gravimetric method was used for determination. An appropriate amount of sweet potato peel sample was dried in an oven at 105℃ to constant weight, and the moisture content was calculated. Subsequently, the dried sample was placed in a muffle furnace at 600℃ and calcined for 2 hours. The volatile solids (VS) content was calculated by the differential gravimetric method.

[0080] 2.7 Total solids concentration (TS) of inoculated sludge

[0081] All samples were tested using the gravimetric method, in accordance with the "CJ / T 221-2023 Standard Test Methods for Urban Sludge".

[0082] Example

[0083] Preparation of sweet potato peel acid hydrolysate

[0084] Example 1

[0085] The air-dried sweet potato peels were crushed in a pulverizer and passed through a 1mm sieve. The sieved sweet potato peel powder was then mixed with a 1% hydrochloric acid solution at a mass-volume ratio of 1:5.7g / mL. The mixture was stirred and heated at 100℃ for 20 minutes to obtain a sweet potato peel acid hydrolysate.

[0086] The components of the prepared sweet potato peel acid hydrolysate were analyzed. The results showed that the SCOD of the sweet potato peel acid hydrolysate was 105.1 g / L, the COD equivalent of total sugar was 80.9%, and the protein content was 0.3%. The furfural content was 2 mg / L, and the HMF content was 71.8 mg / L.

[0087] Example 2

[0088] Sweet potato peel acid hydrolysate was prepared using the same method as in Example 1 above, except that the mass-to-volume ratio of sweet potato peel powder to hydrochloric acid solution was 1:4.4 g / mL.

[0089] The components of the prepared sweet potato peel acid hydrolysate were analyzed. The results showed that the SCOD of the sweet potato peel acid hydrolysate was 132.3 g / L, the COD equivalent of total sugar was 92.4%, and the protein content was 0.3%. The furfural content was 3.1 mg / L, and the HMF content was 105.0 mg / L.

[0090] Example 3

[0091] Sweet potato peel acid hydrolysate was prepared using the same method as in Example 1 above, except that the mass fraction of the hydrochloric acid solution was 2%.

[0092] The components of the prepared sweet potato peel acid hydrolysate were analyzed. The results showed that the SCOD was 105.3 g / L, the COD equivalent of total sugar was 80.1%, and the protein content was 0.3%. The furfural content was 4.6 mg / L, and the HMF content was 231.1 mg / L.

[0093] Example 4

[0094] Sweet potato peel acid hydrolysate was prepared using the same method as in Example 1 above, except that the reaction temperature was 120°C.

[0095] The components of the prepared sweet potato peel acid hydrolysate were analyzed. The results showed that the SCOD was 111.3 g / L, the COD equivalent of total sugar was 73.9%, and the protein content was 0.8%. The furfural content was 25.3 mg / L, and the HMF content was 977.9 mg / L.

[0096] Example 5

[0097] Sweet potato peel acid hydrolysate was prepared using the same method as in Example 1 above, except that the reaction temperature was 80°C.

[0098] The composition of the sweet potato peel acid hydrolysate prepared above was analyzed. The results showed that the SCOD of the sweet potato peel acid hydrolysate was 55.3 g / L, the COD equivalent of total sugar was 76.7%, and the protein content was 0.02%. Given that the SCOD of the sweet potato peel acid hydrolysate was only 55.3 g / L, the contents of furfural and HMF were not measured in this example.

[0099] Example 6

[0100] Sweet potato peel acid hydrolysate was prepared using the same method as in Example 1 above, except that the stirring and heating reaction time was 40 minutes.

[0101] The components of the prepared sweet potato peel acid hydrolysate were analyzed. The results showed that the SCOD (Sodium Scoliosis Dioxide) was 109.3 g / L, the COD equivalent of total sugar was 86.3%, and the protein content was 0.4%. The furfural content was 2.6 mg / L, and the HMF content was 130.2 mg / L.

[0102] Comparative Example 1

[0103] Sweet potato peels with a moisture content of 5% to 15% after air drying are crushed in a pulverizer and passed through a 1mm sieve. The sieved sweet potato peel powder is then mixed with ultrapure water at a mass-to-volume ratio of 1:5.7g / mL to obtain a sweet potato peel solution.

[0104] The composition of the sweet potato peel solution prepared above was analyzed. The results showed that the SCOD of the sweet potato peel solution was 33.7 g / L, the COD equivalent of total sugar was 37.9%, and the protein content was 1.9%. The furfural content was 0 mg / L, and the HMF content was 0 mg / L.

[0105] Comparative Example 2

[0106] Sweet potato peels with a moisture content of 5% to 15% after air drying were crushed in a pulverizer and passed through a 1mm sieve. The sieved sweet potato peel powder was then mixed with ultrapure water at a mass-to-volume ratio of 1:5.7g / mL. The mixture was then heated at 100℃ for 20 minutes with constant stirring to obtain a sweet potato peel solution.

[0107] The composition of the sweet potato peel solution prepared above was analyzed. The results showed that the SCOD of the sweet potato peel solution was 39.7 g / L, the COD equivalent of total sugar was 61.8%, and the protein content was 1.5%. The furfural content was 1.7 mg / L, and the HMF content was 4.2 mg / L.

[0108] Fermentation produces volatile fatty acids

[0109] Example 7

[0110] The sweet potato peel acid hydrolysate prepared in Example 2 was mixed with the residual sludge from the urban sewage treatment plant at a volume ratio of 1:4. The pH was adjusted to 8.0 with sodium hydroxide, the temperature was 35°C, the stirrer speed was 180 rpm, and fermentation was continued for 7 days to obtain the fermentation broth.

[0111] The composition of the fermentation broth prepared above was analyzed. The test results showed that the SCOD concentration of the fermentation broth was 48.4 g / L, the COD equivalent ratio of protein and total sugar was 3.2%, and the COD equivalent ratio of VFA was 53.8%; the VFA was 22.1 g / L, of which the mass ratio of acetic acid was 79.1%.

[0112] Example 8

[0113] The fermentation broth was prepared by anaerobic fermentation using the same method as in Example 7 above, except that the pH was 6.0 and the fermentation time was 8 days.

[0114] The composition of the fermentation broth prepared above was analyzed. The test results showed that the SCOD concentration of the fermentation broth was 35.6 g / L, the COD equivalent ratio of protein and total sugar was 7.3%, and the COD equivalent ratio of VFA was 43.1%; the VFA concentration was 13.5 g / L, of which acetic acid accounted for 86.2% by mass.

[0115] Example 9

[0116] The fermentation broth was prepared by anaerobic fermentation using the same method as in Example 7 above, except that the pH was 10.0 and the fermentation time was 10 days.

[0117] The composition of the fermentation broth prepared above was analyzed. The test results showed that the SCOD concentration of the fermentation broth was 36.8 g / L, the COD equivalent ratio of protein and total sugar was 6.7%, and the COD equivalent ratio of VFA was 41.4%; the VFA concentration was 12.8 g / L, of which acetic acid accounted for 87.5% by mass.

[0118] Comparative Example 3

[0119] The fermentation broth was prepared by anaerobic fermentation using the same method as in Example 7 above, except that the pH was 5.0 and the fermentation time was 6 days.

[0120] The composition of the fermentation broth prepared above was analyzed. The test results showed that the SCOD concentration of the fermentation broth was 37.9 g / L, the COD equivalent ratio of protein and total sugar was 20.1%, and the COD equivalent ratio of VFA was 19.3%; the VFA concentration was 6.4 g / L, of which acetic acid accounted for 89.9% by mass.

[0121] Tables 1 and 2 below summarize the results of the above experiments.

[0122] Table 1 Comparison of substrate release and inhibitor formation under different pretreatment conditions

[0123]

[0124] Combination Figures 2-7 As shown in Table 1, pretreatment conditions have a decisive impact on the dissolution efficiency of organic matter in sweet potato peel and the formation of inhibitors. The results of Comparative Examples 1 and 2 indicate that the SCOD release was extremely low without the addition of acid (33.7 g / L for Comparative Example 1 and 39.7 g / L for Comparative Example 2). This demonstrates that simple heat treatment cannot achieve effective hydrolysis of sweet potato peel.

[0125] A comparison of Examples 1 and 2 shows that, while maintaining the hydrochloric acid concentration at 1%, the reaction temperature at 100°C, and the reaction time at 20 min, optimizing the mass-to-volume ratio of the substrate to the solution from 1:5.7 g / mL to 1:4.4 g / mL significantly increased the SCOD of the acid hydrolysate from approximately 105.1 g / L to 132.3 g / L, and the COD equivalent of total sugars increased from 80.9% to 92.4%. This indicates that appropriately increasing the sweet potato peel content can significantly increase the release of organic matter from the substrate, especially the release of total sugars, thereby increasing the concentration of soluble organic matter in the acid hydrolysate.

[0126] like Figure 2 and Figure 3As shown, when the hydrochloric acid concentration increased from 1% to 2%, although the SCOD and total sugar content of the acid hydrolysate remained at a high level, the concentrations of furfural and HMF in the sweet potato peel acid hydrolysate increased significantly at this concentration. This indicates that excessively high acid concentrations promote the dehydration side reactions of hemicellulose and cellulose degradation products pentoses and hexoses.

[0127] In summary, the optimal pretreatment conditions determined in this invention are 1% hydrochloric acid (1:4.4 g / mL), 100℃, and 20 min in Example 2. The SCOD in the acid hydrolysate was 2.9 times higher than in Comparative Example 1, significantly enhancing the biodegradability of the substrate. Simultaneously, the COD equivalent of total sugars after pretreatment significantly increased from 37.9% in Comparative Example 1 to 90%–95%, greatly improving sugar utilization while maintaining relatively stable protein composition, which is beneficial for subsequent microbial utilization and transformation to produce VFA. Most importantly, under these preferred pretreatment conditions, the formation of microbial inhibitors—furfural and 5-hydroxymethylfurfural—was low, reducing the risk of inhibition of subsequent fermentation microorganisms. These optimal pretreatment conditions achieved a synergistic balance between high organic matter release and low inhibitor formation.

[0128] Table 2 Comparison of core indicators of fermentation broth under different anaerobic fermentation conditions

[0129]

[0130] Depend on Figures 8-11 As shown in Table 2, the pH value of the fermentation system is the core factor in regulating the yield and selectivity of VFA.

[0131] Under conditions of pH 8.0 and continuous fermentation for 7 days (Example 7), the VFA concentration in the fermentation broth reached 22.1 g / L, with a COD equivalent ratio of 53.8% for VFA, and acetic acid accounting for 79.1% of the VFA. This result is attributed to the fact that the weakly alkaline environment effectively inhibited the metabolic activity of methanogens and other competing microbial communities during hydrolysis and acidification, forcing the metabolic flux to flow directionally towards the acetic acid synthesis pathway, while avoiding the problems of inhibited microbial activity and incomplete substrate conversion under acidic conditions. This leads to the highly selective and high-yield directional synthesis of volatile fatty acids.

[0132] In contrast, at pH 5.0 (Comparative Example 3), although the proportion of acetic acid in VFA increased to 89.9%, the total VFA yield dropped sharply to 6.4 g / L, the COD equivalent of VFA was only 19.3%, and the residual proportion of protein and total sugar was as high as 20.1%, indicating that the strongly acidic environment severely inhibited the activity of acid-producing bacteria, resulting in low substrate utilization. When the pH was increased to 10.0 (Example 9), although the VFA yield recovered somewhat (12.8 g / L), it was still significantly lower than the level at pH 8.0. At pH 6.0 (Example 8), the VFA yield was 13.5 g / L.

[0133] Therefore, pH=8.0 is the optimal balance point for anaerobic fermentation to balance microbial metabolic activity, substrate conversion efficiency and product selectivity, and it is also a key parameter for achieving efficient conversion of sweet potato peel into high-value acetic acid.

[0134] In summary, this invention successfully achieves high-value utilization of sweet potato peels through an anaerobic fermentation process following acid hydrolysis pretreatment. Example 7 shows that the COD equivalent of VFA in the fermentation broth was 53.8%; VFA was 22.1 g / L, with acetic acid accounting for 79.1% by mass. These indicators are significantly superior to traditional acid-producing fermentation processes (typically, the acetic acid mass percentage is only 30%~65%), demonstrating the stability, efficiency, and industrial application potential of this invention's process.

[0135] The above specific embodiments further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for producing volatile fatty acids from sweet potato peel, characterized in that, include: Sweet potato peel is mixed with an inorganic acid solution of 1% to 2% by mass at a mass-to-volume ratio of 1:4.4 to 5.7 g / mL, and stirred at 80℃ to 120℃ for 20 to 40 minutes to obtain sweet potato peel acid hydrolysate. The inorganic acid includes at least one of hydrochloric acid and sulfuric acid. The sweet potato peel acid hydrolysate was anaerobically fermented with residual sludge from a municipal wastewater treatment plant at a pH of 6-10 to obtain a fermentation broth containing volatile fatty acids.

2. The method according to claim 1, characterized in that, The inorganic acid is hydrochloric acid.

3. The method according to claim 1, characterized in that, The sweet potato peel is sweet potato peel powder, which is obtained by air-drying sweet potato peel to a moisture content of 5% to 15% and then pulverizing it through a 1mm sieve.

4. The method according to claim 3, characterized in that, The hydrochloric acid solution has a mass fraction of 1%; The mass-to-volume ratio of the feed liquid is 1:4.4 g / mL; The temperature of the stirring reaction is 100°C; The stirring reaction time is 20 minutes.

5. The method according to claim 3, characterized in that, After adjusting the sweet potato peel acid hydrolysate to neutral, it is then mixed with the remaining sludge.

6. The method according to claim 3, characterized in that, The pH value of the anaerobic fermentation is 6~10; The temperature for the anaerobic fermentation is 30℃~40℃; The anaerobic fermentation time is 6 to 10 days.

7. The method according to claim 6, characterized in that, The pH value of the anaerobic fermentation was 8; The anaerobic fermentation temperature is 35℃; The anaerobic fermentation time is 7-8 days.

8. The method according to claim 1, characterized in that, The volume ratio of the sweet potato peel acid hydrolysate to the remaining sludge is 1:3~5, and the concentration of the remaining sludge is 20~40g / L.

9. A sweet potato peel acid hydrolysate, characterized in that, The SCOD of the sweet potato peel acid hydrolysate is 55~132 g / L; The total sugar content of the acid hydrolysate of sweet potato peel is 71% to 93% of the COD equivalent. The sweet potato peel acid hydrolysate was prepared by the following method: Sweet potato peel is mixed with an inorganic acid solution of 1% to 2% by mass at a mass-to-volume ratio of 1:4.4 to 5.7 g / mL, and stirred at 80℃ to 120℃ for 20 to 40 minutes to obtain sweet potato peel acid hydrolysate. The inorganic acid includes at least one of hydrochloric acid and sulfuric acid.

10. A fermentation broth for producing volatile fatty acids from sweet potato peel, characterized in that, The fermentation broth is prepared by the method according to any one of claims 1 to 8, wherein the chemical oxygen demand equivalent of the volatile fatty acids in the fermentation broth accounts for 50% to 60%; The volatile fatty acids contain 79% to 90% acetic acid by mass.

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