METHOD FOR PROMOTING ANAEROBIC FERMENTATION BY USING CARBONY ELEMENTS

Abstract

The present disclosure provides a method for promoting anaerobic fermentation by using carbonyl iron and relates to the technical field of biological fermentation. The method comprises the following steps: adding carbonyl iron to a fermentation liquid for anaerobic fermentation. The present disclosure applies carbonyl iron to anaerobic fermentation, which improves the efficiency of anaerobic fermentation and thus increases methane production. On the one hand, carbonyl iron can accelerate the degradation of complex organic substances by promoting microbial iron respiration, and on the other hand, it can supply microorganisms with the iron necessary for their growth and promote both hydrolytic acid-producing bacteria and methanogenic bacteria.

Description

A method for promoting anaerobic digestion using carbonyl iron Technical Field

[0001] The invention relates to the technical field of biological fermentation, in particular to a method for promoting anaerobic digestion by utilizing carbonyl iron. Background Art

[0002] Anaerobic digestion utilizes the synergistic metabolism of multiple microorganisms to simultaneously process agricultural waste, producing methane-rich clean energy and nutrient-rich biogas residues and liquids that can be used as fertilizer, thereby replenishing and balancing the energy and organic matter consumed in production and daily life. However, long-term anaerobic digestion systems are prone to the accumulation of intermediate metabolites due to a mismatch between the physicochemical properties of the materials and the metabolic needs of the microorganisms. This inhibits microbial life, leading to low digestion system efficiency and poor stability, thus hindering the efficient resource utilization of agricultural waste.

[0003] Chicken manure, due to its high nitrogen content and high putrefaction, is one of the raw materials most susceptible to inhibition during anaerobic digestion of livestock and poultry manure. During the anaerobic digestion process, large amounts of fatty acids and ammonia nitrogen are readily produced. The neutralization of these two factors makes it difficult to diagnose and regulate the resulting inhibition through conventional methods such as pH. Consequently, anaerobic digestion projects using chicken manure as a substrate often experience stagnant gas production while maintaining a normal pH. In this situation, ensuring the metabolic balance of the anaerobic digestion microbial population, optimizing the efficiency of the anaerobic digestion process, and improving process stability are key to maintaining efficient anaerobic treatment of chicken manure.

[0004] Summary of the Invention

[0005] The purpose of the present invention is to provide a method for promoting anaerobic digestion using carbonyl iron to solve the problems existing in the above-mentioned prior art. The present invention applies biochar coupled with carbonyl iron to anaerobic digestion, which can improve the anaerobic digestion effect and thus increase methane production.

[0006] To achieve the above object, the present invention provides the following solutions:

[0007] The present invention provides a method for promoting anaerobic digestion by utilizing carbonyl iron. The method comprises: adding carbonyl iron into fermentation liquid for anaerobic digestion.

[0008] Furthermore, the amount of carbonyl iron added is 6 wt % of the volatile solids of the substrate added to the fermentation liquid.

[0009] Furthermore, the substrate is animal excrement, kitchen waste or waste vegetables.

[0010] Furthermore, biochar is added to the fermentation liquid.

[0011] Furthermore, the amount of biochar added is 2 wt% of the volatile solids of the substrate added to the fermentation liquid.

[0012] Furthermore, the substrate is animal excrement, kitchen waste or waste vegetables.

[0013] Iron is an important component of cellular enzymes and a trace element necessary for the growth and metabolism of anaerobic microorganisms. It is crucial for the biosynthesis of methane. Iron is widely distributed in nature, has good electrical conductivity and strong ferromagnetism, and is a relatively active metal. In the prior art, the application of iron elements in anaerobic digestion systems mainly uses elemental iron such as nano-iron, which mainly utilizes the reducing properties of zero-valent iron to maintain a low redox potential. The excellent electrical conductivity enables iron to promote the electron transfer process between syntrophic fatty acid oxidizing bacteria and hydrogen-consuming methanogens, thereby accelerating the degradation of VFAs. Carbonyl iron (Fe(CO)5) powder is a powdered reduced elemental iron powder that is widely used in iron nutrition fortification and iron supplements. Carbonyl iron powder is a pentacarbonyl iron complex synthesized from iron block raw material and carbon monoxide under high temperature and high pressure conditions. It has an average particle size of 4.8μm and has the characteristics of high purity, fine particles, high surface activity, etc., and has an onion-shaped structure. Our research has found that carbonyl iron powder is more stable than elemental zero-valent iron. Compared to magnetite, carbonyl iron, in its zero-valent state, is more reactive. Carbonyl iron can simultaneously provide a partial carbon source and iron for microbial growth. Microbial absorption of the iron in carbonyl iron is better than that of nano-iron. Furthermore, carbonyl iron has a higher bioaffinity than nano-iron, making it more tolerant to anaerobic microorganisms and exhibiting a greater affinity for anaerobic microorganisms. Furthermore, the unique chemical structure and physicochemical properties of carbonyl iron also lend it excellent electron transfer efficiency. Therefore, considering the potential for trace element supplementation and the promotion of interspecies electron transfer in microorganisms, the application of carbonyl iron in anaerobic digestion systems has significant potential for enhancing methanogenesis.

[0014] The research and application of biochar in anaerobic digestion systems has reached a mature stage. Biochar can significantly enhance methane production by adsorbing and removing inhibitors, providing attachment space for microorganisms, and promoting interspecies electron transfer. Due to its excellent carbon skeleton and stability, biochar, when coupled with powdered iron in anaerobic digestion systems, helps maintain the reactivity of iron and promotes its effective dispersion. Furthermore, the potential difference between iron and carbon forms a weak galvanic cell within the digestion system, forming a microelectrolysis system that promotes the degradation of organic matter and provides sufficient substrate for methanogens. Therefore, coupling carbonyl iron and biochar in anaerobic digestion systems has great potential for enhancing methane production.

[0015] The present invention discloses the following technical effects:

[0016] (1) On the one hand, carbonyl iron can accelerate the degradation of complex organic matter by promoting microbial iron respiration. On the other hand, it can provide microorganisms with the necessary iron supplement for growth, and has a promoting effect on both hydrolytic acidifying bacteria and methanogens.

[0017] (2) The coupled addition of biochar and carbonyl iron to anaerobic digestion has a synergistic effect, which can further promote the production of induced enzymes and the synthesis of intermediate metabolites required by microorganisms, shortening the adaptation period of microorganisms. Due to the potential difference between iron and carbon, biochar and carbonyl iron may form a microelectrolysis system within the anaerobic digestion system, acting as anode and cathode to donate and accept electrons, synergistically promoting electron transfer in the mutualistic methanogenesis pathway.

[0018] (3) The addition of biochar will reduce the pH value of the fermentation liquid, while the addition of carbonyl iron will increase the pH value of the fermentation liquid. The coupled addition of biochar and carbonyl iron has a more significant effect on improving the pH.

[0019] (4) Biochar and carbonyl iron can regulate the soluble salt concentration by adjusting the conductivity of the fermentation liquid, thereby improving the activity of anaerobic digestion microorganisms.

[0020] (5) Biochar and carbonyl iron can also be used for anaerobic digestion of substrates such as food waste. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0022] Figure 1 shows the daily methane production during anaerobic digestion;

[0023] Figure 2 shows the cumulative production of methane during anaerobic digestion;

[0024] Figure 3 shows the pH changes during anaerobic digestion;

[0025] Figure 4 shows the conductivity change of anaerobic digestion liquid. DETAILED DESCRIPTION

[0026] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0027] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. The intermediate value within any stated value or stated range, and each smaller range between any other stated value or intermediate value within the stated range, is also encompassed within the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0028] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0029] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments described herein without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the description of the invention. The description and examples are intended to be illustrative only.

[0030] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0031] The method of the present invention is applicable to perishable organic matter such as animal manure, restaurant kitchen waste, and waste vegetables. The present invention uses chicken manure as a substrate to illustrate a method of promoting anaerobic digestion by coupling biochar with carbonyl iron.

[0032] Example 1

[0033] 1. Experimental Materials

[0034] The substrate used for anaerobic digestion was chicken manure collected from a large-scale livestock farm, with total solids (TS) and volatile solids (VS) contents of 24% and 15%, respectively. The inoculum was obtained from a continuously stirred anaerobic digester operating under mesophilic conditions (36 ± 1°C) in the laboratory, with a moisture content of 97%. Carbonyl iron powder was purchased from Beijing Ruidong Surface Source Environmental Protection Technology Co., Ltd., with a particle size of 1 to 3 μm. Biochar was prepared by pyrolysis of waste fruit wood at a final temperature of 550°C and a residence time of 2 hours, and then pulverized to a particle size of 0.3 to 0.45 mm.

[0035] 2. Experimental Methods

[0036] Four 500mL sequencing batch anaerobic digestion reactors were each inoculated with 120mL of inoculum, containing 10.362g of chicken manure volatile solids, and brought to a working volume of 400mL with tap water. The four 500mL sequencing batch anaerobic digestion reactors were designated T1, T2, T3, and CK. T1 contained 2% biochar and 6% carbonyl iron powder relative to the volatile solids content of the chicken manure; T2 contained 2% biochar relative to the volatile solids content of the chicken manure; and T3 contained 6% carbonyl iron powder relative to the volatile solids content of the chicken manure. CK served as a control, without the addition of biochar or carbonyl iron powder.

[0037] The biogas produced by fermentation flows from the outlet port above the reactor through a silicone tube into an aluminum foil bag for storage. The fermentation cycle lasts for 80 days. The gas volume is measured daily, the gas composition is analyzed every three days, and the fermentation liquid is sampled every five days. The fermentation liquid is thoroughly mixed before sampling, and approximately 10 mL of sample is collected each time.

[0038] 3. Experimental Results

[0039] The daily and cumulative methane production during anaerobic digestion is shown in Figures 1 and 2. All treatments experienced a plateau period of methane production, with two daily peaks, showing generally similar trends. Methane is the end product of anaerobic digestion, and its production reflects the metabolic activity of the microorganisms within the reactor and the degradation efficiency of macromolecular organic matter in the fermentation substrate. During the early stages of fermentation, hydrolysis dominated the reactor, and most methanogenic microorganisms were in a growth plateau, requiring a period of metabolic adjustment to adapt to the new growth environment. This resulted in low methane production. The plateau period lasted 12 days, 18 days, 14 days, and 15 days for the T1, T2, T3, and CK groups, respectively. The treatment with biochar alone had the longest plateau period, while the treatment with biochar coupled with carbonyl iron powder had the shortest. This may be because during this period, the biochar primarily enriched hydrolytic and acidifying bacteria, with less activation of methanogens, resulting in excessively high concentrations of intermediate metabolites. Carbonyl iron, on the one hand, accelerates the degradation of complex organic matter by promoting microbial iron respiration, and on the other hand, provides essential iron supplementation for microbial growth, promoting both hydrolytic acidifying bacteria and methanogens. The combined addition of biochar and carbonyl iron has a synergistic effect, further promoting the production of induced enzymes and the synthesis of intermediate metabolites required by microorganisms, shortening the microbial adaptation period.

[0040] The trend in daily methane production was consistent with the typical growth curve of microorganisms. After the stagnation period, the growth of methanogenic microorganisms entered the logarithmic phase, and methane production increased linearly. Among all treatments, the T1 group had the earliest and highest daily methane production peak, reaching 491 mL on the 20th day. The T2, T3, and CK groups had peak methane production on the 28th, 22nd, and 23rd days, respectively, with peaks of 415 mL, 445 mL, and 345 mL. This indicates that biochar coupled with carbonyl iron can accelerate the onset of peak methane production in anaerobic digestion and significantly increase peak daily methane production (P < 0.01).

[0041] Cumulative methane production in the T1, T2, T3, and CK groups was 295 mL / gVS, 246 mL / gVS, 268 mL / gVS, and 224 mL / gVS, respectively. The combined or individual addition of biochar and carbonyl iron significantly (p < 0.05) increased cumulative CH4 production by 31.5%, 9.9%, and 19.4%, respectively, compared to the control. This may be because carbonyl iron and biochar act as conductive materials, participating in direct interspecies electron transfer between symbiotic microorganisms, thereby accelerating the oxidative degradation of volatile fatty acids produced by acidification and promoting CH4 production through CO2 hydrogenation. Furthermore, due to the potential difference between iron and carbonyl iron, biochar and carbonyl iron may form a microelectrolysis system within the anaerobic digestion system, acting as the anode and cathode, respectively, to donate and accept electrons, synergistically promoting electron transfer in the symbiotic methanogenesis pathway.

[0042] Figure 3 shows the pH changes during anaerobic digestion. All treatments showed similar trends, initially decreasing and then increasing. During the first five days of fermentation, the pH of each treatment dropped rapidly. This was due to the hydrolysis and acidification of complex organic matter, which led to a significant accumulation of organic acids within the system. At fermentation startup, the pH values ​​of the biochar-added T2 and CK groups were similar, at 7.22 and 7.24, respectively; those of the T1 and T3 groups were 7.53 and 7.33, respectively. The addition of carbonyl iron significantly increased the pH of the fermentation broth, likely due to the oxygen-absorbing corrosion of the iron in a near-neutral environment. As acidification progressed, the pH in the T1 and T3 groups decreased significantly more than that in the T1 and CK groups, with the largest decrease in the T1 group. The trend remained consistent with the T2 and CK groups. The pH in the T1 group also rebounded the earliest, beginning its upward trend after the fifth day. The T3 and CK groups began to rebound after the tenth day, while the T2 group experienced the last pH increase, beginning on the fifteenth day. By day 20 of fermentation, the pH values ​​of the T1 and T3 treatments had risen to the same level as the CK treatment and then remained steadily rising. The CK treatment experienced slight fluctuations between days 45 and 60, with the pH initially dropping to 8.06 before recovering to 8.21. The pH of the T2 treatment remained lower than that of the other treatments, but remained within the optimal range. At the end of fermentation, the pH values ​​of the T1, T2, T3, and CK treatments were 8.43, 8.28, 8.42, and 8.39, respectively. These results indicate that biochar addition decreased the pH of the fermentation broth, while carbonyl iron addition increased it. The combined addition of biochar and carbonyl iron significantly increased the pH.

[0043] The conductivity of the anaerobic digestion feed is positively correlated with the soluble salt concentration. Both excessively high and low soluble salt concentrations can negatively impact the activity of anaerobic digestion microorganisms. As shown in Figure 4, while the conductivity of all treatments exhibited fluctuations, the overall trend was initially upward and then downward. During the first five days of fermentation, the soluble salt concentration in the feed increased due to the hydrolysis of macromolecular, insoluble organic matter into a dissolved state, leading to a corresponding increase in the EC value. At fermentation startup, the conductivity values ​​for the T1, T2, T3, and CK groups were 16.87 mS / cm, 19.80 mS / cm, 16.46 mS / cm, and 15.08 mS / cm, respectively. Both the addition of biochar and carbonyl iron, both individually and in combination, increased the conductivity of the fermentation feed at startup. The effect of biochar alone was more pronounced, likely due to the release of cations into the fermentation feed by the biochar. Throughout the fermentation cycle, the average conductivity of the T1, T2, T3, and CK groups was 19.80 mS / cm, 21.17 mS / cm, 19.54 mS / cm, and 20.12 mS / cm, respectively. Overall, the addition of carbonyl iron reduced the conductivity of the fermentation broth.

[0044] Comparative Example 1

[0045] The operation of the T1 reactor is the same as that in Example 1, except that the carbonyl iron is replaced by nano-zero-valent iron.

[0046] The cumulative methane production of this comparative example was 243 mL / gVS.

[0047] Comparative Example 2

[0048] The operation of the T1 reactor was the same as that in Example 1, except that the carbonyl iron was replaced by nano-zero-valent iron, and 2 g / L of triolein was added to the reaction liquid.

[0049] The cumulative methane production of this comparative example was 292 mL / gVS.

[0050] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for promoting anaerobic digestion using carbonyl iron, characterized in that: The method comprises: adding carbonyl iron to a fermentation liquid for anaerobic digestion.

2. The method for promoting anaerobic digestion using carbonyl iron according to claim 1, characterized in that: The amount of carbonyl iron added is 6 wt % of the volatile solids of the substrate added to the fermentation liquid.

3. The method for promoting anaerobic digestion using carbonyl iron according to claim 2, characterized in that: The substrate is animal excrement, kitchen waste or waste vegetables.

4. The method for promoting anaerobic digestion using carbonyl iron according to claim 1 or 2, characterized in that: Biochar is also added to the fermentation liquid.

5. The method for promoting anaerobic digestion using carbonyl iron according to claim 4, characterized in that: The amount of biochar added is 2 wt % of the volatile solids of the substrate added to the fermentation liquid.

6. The method for promoting anaerobic digestion using carbonyl iron according to claim 5, characterized in that: The substrate is animal excrement, kitchen waste or waste vegetables.

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