A method for delivering trace elements to anaerobic fermentation bacteria by coating additives

Abstract

The application discloses a method for delivering trace elements to anaerobic fermentation bacteria by coating additives, and the method comprises the following steps: mixing a biomass raw material to be treated and inoculated sludge in a fermentation tank; mixing hydrophobic nanosilica and a metal element ion salt solution, and stirring at a high speed to prepare a slow-release coating additive; adding the prepared slow-release coating additive into the fermentation tank; and setting a fermentation stirring rate to perform anaerobic fermentation. The method can not only gradually release trace metal elements under the action of stirring shear force to improve the biological availability of the trace metal elements, thereby being beneficial to solving the problem that part of the added trace metal element ions is precipitated by other anions in the process of being uptaken by microorganisms, but also can solve the potential heavy metal pollution problem in biogas residue; meanwhile, the method can also solve the biological toxicity problem caused by the prior art, and can effectively promote the anaerobic fermentation biochemical reaction pathway to improve the methane production.

Description

Technical Field

[0001] This invention belongs to the field of anaerobic fermentation technology, and particularly relates to a method for delivering trace elements to anaerobic fermentation bacteria through a slow-release coating additive. Background Technology

[0002] In the anaerobic fermentation of biomass waste for methanogenesis, trace elements play a crucial role. For example, many trace elements (such as Mg, Ni, Fe, Co, and Zn) are essential micronutrients for cell growth, serve as electron donors / acceptors, or synthesize necessary coenzymes or cofactors in key enzymatic methanogenesis pathways. However, the trace elements provided by the biomass waste itself to the microbial community are often insufficient to meet the needs of microbial growth, reproduction, and anaerobic fermentation biochemical reactions. Therefore, it is often necessary to add trace metal elements to the anaerobic fermentation reactor to meet the needs of the microorganisms. The initial trace element addition technology involved directly adding trace metal ion salt solutions (such as MgCl2, NiCl2, FeCl2, CoCl2, and ZnCl2) to the anaerobic fermentation reactor. However, due to the relatively active chemical properties of the added trace metal elements and the presence of sulfur ions (S) in the anaerobic fermentation reactor... 2- ), carbonate ions (CO3) 2- ), phosphate ions (PO4) 3- The presence of these metal salt solutions directly leads to a significant sedimentation of metal ions. It is estimated that the sedimentation rate of these metal ions in anaerobic fermentation reactors can reach as high as 40%. In other words, the cost loss rate of the added metal ions reaches 40%. This phenomenon significantly increases the amount of trace metals used and the production cost in industrial-scale anaerobic fermentation processes, thereby significantly increasing the technical cost of enhancing anaerobic fermentation reactions by adding trace elements. It also greatly increases the operational costs of downstream treatment of biogas residue containing a large amount of sedimented metal ions. Therefore, appropriate trace metal addition strategies should be developed to improve the bioavailability of trace metals and prevent the sedimentation of metal ions.

[0003] To achieve the above objectives, researchers have reported several schemes for adding metal ions to anaerobic fermentation reactors using chelating agents as delivery carriers, all employing slow release mechanisms. The reported chelating agents mainly include ethylenediaminetetraacetic acid (EDTA), nitrogen triacetic acid (NTA), and ethylenediaminedisuccinic acid ([S,S]-EDDS). Although using these chelating agents as delivery carriers for trace metal elements can improve the bioavailability of metal ions and ultimately increase methane production to some extent, these chelating agents have strong toxicity to the environment and human health, thus greatly limiting their large-scale application in the biogas industry. Therefore, there is an urgent need to develop eco-friendly trace element delivery carriers to minimize metal ion precipitation, improve the bioavailability of metal ions, and increase the final methane production while providing trace metal elements to anaerobic fermentation microbial communities, thereby improving the overall economic viability of the technology. Summary of the Invention

[0004] In view of the above-mentioned deficiencies of the prior art, the technical problem to be solved by the present invention is how to provide an alternative technology that can effectively supplement trace elements to anaerobic fermentation microbial communities and maximize methane production.

[0005] To achieve the above objectives, the present invention provides a method for delivering trace elements to anaerobic fermentation bacteria, the method comprising:

[0006] Step 1: Mix the biomass raw materials to be treated with the inoculated sludge in a fermenter for anaerobic fermentation;

[0007] Step 2: Mix hydrophobic nano-silica with a metal element ion salt solution and stir at high speed within the range of 18,000 to 22,000 rpm to prepare a slow-release coating additive.

[0008] Step 3: Add the slow-release coating additive prepared in Step 2 to the anaerobic fermenter, set the fermentation stirring rate, and carry out anaerobic fermentation.

[0009] Step 4: After the anaerobic fermentation is completed, the resulting biogas residue will be processed downstream.

[0010] Furthermore, the method also includes step five, the recovery of hydrophobic nano silica: after fermentation is completed, the fermentation liquid is discharged and left to stand for 1 day. When the hydrophobic nano silica completely floats on the surface of the fermentation liquid, the hydrophobic nano silica in the fermentation liquid is recovered, and then washed, dried, and sealed for storage for reuse.

[0011] Preferably, in step one, the biomass raw material to be treated is kitchen waste. First, plastics and other non-biodegradable components are removed from it, and then it is homogenized using a mixer to obtain kitchen waste slurry, which is used as raw material for subsequent anaerobic fermentation.

[0012] Preferably, in step one, the ratio of volatile solids in the inoculated sludge to the biomass raw material to be treated is 0.71.

[0013] Preferably, in step one, the fermentation is batch fermentation or semi-continuous anaerobic fermentation, and the fermentation temperature is 37-55℃.

[0014] Preferably, in step two, the metal element ion salt solution is a salt solution of metal element ions and deionized water, wherein the metal element is a trace metal element essential for microbial growth and metabolism, and the metal element ion is selected from Mg, Ni, Fe, Co, and Zn ions. The metal element ion salt solution is a MgCl2, NiCl2, FeCl2, CoCl2, or ZnCl2 solution.

[0015] Preferably, in step two, the metal element ion salt solution has a concentration of 5-10 mg / L.

[0016] Preferably, in step two, each 20 mL of metal element ion salt solution corresponds to 6.667 g of hydrophobic nano-silica.

[0017] Preferably, in step two, the particle size range of the slow-release coating additive is 1.5–40 μm, and the average particle size is 4.564 ± 2.951 μm.

[0018] Preferably, in step two, the high-speed stirring speed is 20,000 rpm, and the stirring time is 30 to 60 seconds, preferably 45 seconds.

[0019] Preferably, in step two, the amount of the slow-release coating additive added ranges from 10 to 30 g / L of the reactor working volume, and more preferably, the amount added is 20 g / L of the reactor working volume.

[0020] Preferably, in step three, the fermentation stirring rate is 400-1200 rpm, more preferably 600-800 rpm, so that the slow-release coating additive gradually breaks down under the action of stirring shear force and slowly releases the trace metal element ions inside, thus supplementing the anaerobic fermentation microbial community with trace elements.

[0021] Preferably, in step four, the downstream processing of the obtained biogas residue can be carried out by using fertilizer or raw material technology to convert it into useful energy or resources. The biogas residue can be composted into organic fertilizer, or it can be dehydrated and used as raw material for gasification or pyrolysis processes to produce biochar and combustible gas.

[0022] Technical effect

[0023] This invention provides an eco-friendly trace element delivery carrier, namely a slow-release coated metal element ion additive made by combining nano-silica with a metal element ion salt solution. This additive not only gradually releases trace metal elements under stirring shear force to improve their bioavailability, thus addressing the problem of partial precipitation of added trace metal ions by other anions during microbial uptake, but also solves the potential heavy metal pollution problem in biogas residue. Furthermore, it overcomes the biotoxicity issues caused by existing technologies (using chelating reagents for delivery carriers) and effectively promotes anaerobic fermentation biochemical reaction pathways to increase methane production. Specifically,

[0024] 1. Compared with the existing method of "using chelating reagents as a carrier for the delivery of trace metal element ions", the advantages of this invention are that it is non-biotoxic, environmentally friendly, uses inexpensive raw materials, does not increase the cost burden, and can solve 100% of the pollution and ecotoxicity problems caused by the enrichment of chelating reagents in biogas residue.

[0025] 2. Compared with existing methods, the trace elements in this method are added to the fermenter in a protected film form and are gradually released into the fermentation liquid under the action of stirring shear force, thereby greatly reducing the sedimentation of metal element ions. Based on this, the advantages of the method of the present invention are that the bioavailability of metal element ions is higher, the bioavailability of trace metal element (e.g., Mg) ions is increased by 18.2%-39.1%, the purchase cost of trace metal elements is saved, and the subsequent treatment cost of biogas residue can be reduced, which helps to improve economic benefits.

[0026] 3. The method of this invention supplements the anaerobic fermentation microbial community with trace elements, effectively promoting the anaerobic fermentation biochemical reaction pathway and increasing the amount of methane produced by anaerobic fermentation by 41%, greatly increasing the economic benefits of the technology. Compared with existing methods, the advantages of this method are that the raw materials used (hydrophobic nano silica) are easy to separate and recyclable, and will not cause heavy metal or chemical pollution in the biogas residue, saving subsequent biogas residue treatment costs. It is highly practical and suitable for large-scale application. Attached Figure Description

[0027] Figure 1 This is a photograph of the granular coating additive prepared in Example 1 of the present invention;

[0028] Figure 2 This is a statistical chart of the particle size range of the granular coating additive prepared in Example 1 of the present invention;

[0029] Figure 3The following diagram illustrates the change in the percentage of Mg ions released from the coating additive in Example 2 of the present invention over time under different stirring rates.

[0030] Figure 4 The methane yield corresponding to different stirring rates in Example 2 of the present invention is shown;

[0031] Figure 5 The bioavailability of Mg ions at different fermentation time points under different treatment conditions is shown in Example 3 of the present invention;

[0032] Figure 6 The diagram illustrates the method of the present invention for supplementing anaerobic fermentation microbial communities with trace element Mg using a slow-release coating additive. Detailed Implementation

[0033] The technical solution of the present invention will be described in detail below through embodiments.

[0034] Example 1

[0035] A method for delivering trace elements to anaerobic fermenting bacteria, the method comprising:

[0036] Step 1: Collect kitchen waste. First, remove plastics and other non-biodegradable components, then homogenize it using a mixer to obtain kitchen waste slurry, which will be used as raw material for subsequent anaerobic fermentation. Anaerobic fermentation sludge is collected from the anaerobic fermentation reactor of the wastewater treatment plant. After releasing its internal organic components before fermentation, it is used as inoculum sludge for anaerobic fermentation in this method.

[0037] The physicochemical properties of the kitchen waste slurry and the pretreated inoculated sludge are shown in Table 1.

[0038] Table 1 Physicochemical properties of food waste slurry and pretreated inoculated sludge

[0039]

[0040] Note: Mean ± standard error; a Dry basis; b Wet substrate.

[0041] Step 2: Weigh 10 mg of MgCl2 powder using an analytical balance, transfer it to a beaker, add 1 L of deionized water, and stir evenly with a glass rod to promote dissolution, preparing a 10 mg / L MgCl2 salt solution. Add 333 g of hydrophobic nano-silica to the solution and transfer it to a stirrer. Start the stirrer, select a speed of 20,000 rpm, and a stirring time of 45 seconds to finally form a granular slow-release coating additive.

[0042] Step 3: Add the slow-release coating additive prepared in Step 2 to the anaerobic fermenter, set the fermentation stirring speed to 700 rpm, so that the coating additive gradually breaks under the action of stirring shear force and slowly releases the trace metal element ions inside, supplementing the anaerobic fermentation microbial community with trace elements and carrying out anaerobic fermentation.

[0043] Step 4: After the anaerobic fermentation experiment is completed, the obtained biogas residue is processed downstream. It can be processed into useful energy or resources using fertilizer or raw material technologies. The biogas residue can be composted into organic fertilizer, or it can be dehydrated and used as raw material for gasification or pyrolysis processes to produce biochar and combustible gas.

[0044] Step 5: Drain the fermentation broth and let it stand for 1 day. When the hydrophobic nano-silica is completely floating on the surface of the fermentation broth, recover the hydrophobic nano-silica from the fermentation broth, then wash it with water, dry it, and seal it for storage, so that it can be used again to prepare coating additives as delivery carriers for trace metal elements.

[0045] The sustained-release coating additive prepared in step two is granular, consisting of hydrophobic nano-silica particles coated with magnesium ions, such as... Figure 1 As shown: a: Naked-eye observation photograph; b: Demonstrates the excellent flowability of the coating additive; c: Scanning electron microscope image of the coating additive magnified 1000x; d: Scanning electron microscope image of the coating additive magnified 2000x.

[0046] The particle size range of the prepared coating additive was determined using a particle size analyzer, yielding a particle size range of 1.5–40 μm and an average particle size of 4.564 ± 2.951 μm. Figure 2 As shown.

[0047] Example 2:

[0048] Using the same method as step two of Example 1 above, a sustained-release coating additive coated with magnesium ions was prepared.

[0049] Using the same method as step one of Example 1 above, kitchen waste and inoculated sludge were prepared, mixed, and added to a fermentation tank. A coating additive was then added to a series of batch fermenters. For different fermenters, stirring rates of 0, 200, 400, 600, 800, 1000, 1200, and 1400 rpm were set, and the release percentage of trace metal ions at different time points was dynamically measured. The results are as follows: Figure 3 As shown; after fermentation, the methane production in the fermenters at various stirring rates was measured, and the results are as follows. Figure 4 As shown.

[0050] from Figure 3It is known that when the stirring speed is sufficiently low (0–400 rpm), the shear force generated by stirring is too small to effectively break the coating additive, and Mg ions cannot be effectively released. As the stirring speed is gradually increased, the shear force also increases, which is more conducive to the breaking of the coating additive and the effective release of internal Mg ions. Specifically, a stirring speed of 600–800 rpm is more suitable, at which point the shear force generated by stirring can maintain the slow release time of the coating additive for a longer period (approximately 10–24 hours), providing a suitable time for microbial uptake and absorption. However, when the stirring speed is too high (1000–1400 rpm), the shear force generated by stirring is too large, causing the coating additive to be released in a very short time (<30 minutes). Because the Mg ions released in such a short time cannot be completely taken up and absorbed by microorganisms, excess Mg ions and other anions (such as sulfide ions) will result in excess Mg ions and other anions (such as sulfide ions). 2- ), carbonate ions (CO3) 2- ), phosphate ions (PO4) 3- A chemical reaction occurs, causing Mg ions to precipitate and resulting in the loss of trace elements.

[0051] In summary, the experimental results show that a reference stirring rate of 600–800 rpm is suitable for the method of this invention, providing a good sustained release effect of metal ions. These results fully demonstrate that the coating additive can gradually release internal metal ions at different rates under the shear force corresponding to different stirring rates, supplementing the anaerobic fermentation microbial community in the fermenter with trace elements.

[0052] Depend on Figure 4 It is known that different stirring rates result in different methane yields. This phenomenon is mainly due to two factors. Firstly, because... Figure 3 It is known that different stirring rates result in varying degrees of rupture in the coating additive, leading to different concentrations of the released trace element Mg. Consequently, the micronutrient supplementation received by the microbial community also varies. The results show that when the stirring rate is 600–800 rpm, the coating additive can gradually rupture over a certain period, slowly releasing the internal micronutrients. Therefore, under this condition, the microbial community can obtain better micronutrient supplementation. Secondly, stirring is an important means of promoting energy and mass transfer in the anaerobic fermentation process. Too low a stirring rate (e.g., 0–400 rpm) is not conducive to the heat and mass transfer processes inside the fermenter, resulting in a slow microbial metabolic rate and a reduced methanogenesis rate. However, too high a stirring rate (e.g., 1200–1400 rpm) can cause high shear forces that may prevent the microorganisms from growing and functioning effectively.

[0053] In summary, based on the above results, the optimal stirring rate for delivering trace metal ions using the coating additive is determined to be 600-800 rpm, and this rate was applied to a semi-continuous anaerobic fermentation experiment. Furthermore, by using a slow-release coating additive to supplement the anaerobic fermentation microbial community with trace elements, the methane production from anaerobic fermentation was effectively increased, with an increase of approximately 41%.

[0054] Example 3

[0055] The effect of adding slow-release coating additives on the delivery of trace elements was verified.

[0056] Using the same method as step one of Example 1 above, prepare kitchen waste and inoculated sludge, mix them, and add them to the fermentation tank. Additives are then added to the fermentation tank under the following different conditions:

[0057] Set up experimental groups with different treatments:

[0058] Experimental group A1: The magnesium ion-coated slow-release coating additive prepared in step 2 of Example 1 was added to the fermenter;

[0059] Experimental group A2: An equal amount of hydrophobic nano-silica was added to the fermenter;

[0060] Experimental group A3: An equal amount of metal element ion salt solution (10 mg / L MgCl2 salt solution) was added to the fermenter;

[0061] Experimental group A4: Equal amounts of hydrophobic nano-silica and equal amounts of metal element ion salt solution were added simultaneously (in their individual forms, not as coating additives).

[0062] A semi-continuous anaerobic fermentation experiment was conducted. Selected process parameters included a fermenter working volume of 400 mL and an organic loading rate of 2.2-4.4 gVS / L. During the semi-continuous anaerobic fermentation experiment, fermentation broth samples were taken at 1, 28, and 56 days after reactor operation to determine the chemical speciation of the trace element magnesium ions. The bioavailability of magnesium ions at different fermentation time points under different treatment conditions is shown below. Figure 5 As shown.

[0063] Depend on Figure 5The results showed that on day 1 of fermentation, the bioavailability of Mg ions in A1, A3, and A4 was 92.7%, 78.4%, and 81.5%, respectively; on day 28 of fermentation, the bioavailability of Mg ions in A1, A3, and A4 was 47.9%, 40.3%, and 38.7%, respectively; and the bioavailability of Mg ions in A1, A3, and A4 was 39.5%, 28.7%, and 28.4%, respectively. Throughout the fermentation process, compared with the direct addition of trace metal element ion salt solutions, using a coating additive as a trace element delivery carrier improved the bioavailability of trace elements by 18.2%–39.1%.

[0064] The above experimental process can be summarized as follows: Figure 6 As shown.

[0065] The above results fully demonstrate that using coating additives to deliver trace metal ions helps to improve their bioavailability and reduce the sedimentation effect of metal ions.

[0066] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A method for delivering trace elements to anaerobic fermenting bacteria, the method comprising: Step 1: Mix the biomass raw materials to be treated with the inoculated sludge in a fermenter for anaerobic fermentation; Step 2: Mix hydrophobic nano-silica with a metal element ion salt solution and stir at high speed within the range of 18,000~22,000 rpm to prepare a slow-release coating additive. Step 3: Add the slow-release coating additive prepared in Step 2 to the anaerobic fermenter, set the fermentation stirring rate, and carry out anaerobic fermentation. Step 4: After anaerobic fermentation is completed, the resulting biogas residue will undergo downstream treatment. Step 5: Recovery of hydrophobic nano-silica: After fermentation is completed, the fermentation liquid is discharged and left to stand for 1 day. When the hydrophobic nano-silica is completely floating on the surface of the fermentation liquid, the hydrophobic nano-silica in the fermentation liquid is recovered. Then, it is washed with water, dried, sealed and stored for reuse. In step two, the metal element ion salt solution is a MgCl2, FeCl2, CoCl2, or ZnCl2 solution, and the metal element ion salt solution has a concentration of 10 mg / L. In step three, the fermentation stirring rate is 600-700 rpm, which causes the slow-release coating additive to gradually break down under the action of stirring shear force and slowly release the trace metal element ions inside, thus supplementing the anaerobic fermentation microbial community with trace elements.

2. The method as described in claim 1, wherein, In step one, the fermentation is batch fermentation or semi-continuous anaerobic fermentation, and the fermentation temperature is 37~55°C.

3. The method of claim 1, wherein, In step two, each 20 mL of metal element ion salt solution corresponds to 6.667 g of hydrophobic nano silica.

4. The method of claim 1, wherein, In step two, the particle size range of the slow-release coating additive is 1.5~40 μm, and the average particle size is 4.564 ± 2.951 μm.

5. The method of claim 1, wherein, In step two, the high-speed stirring speed is 20,000 rpm, and the stirring time is 30-60 seconds.

6. The method of claim 1, wherein, In step four, the obtained biogas residue is subjected to downstream processing. Fertilization or raw material processing technologies are used to convert it into useful energy or resources. The biogas residue is composted into organic fertilizer, or dehydrated and used as raw material for gasification or pyrolysis processes to produce biochar and combustible gas.

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