Method for strengthening anaerobic fermentation of sludge and synchronously producing medium-chain fatty acid and hydrogen

Abstract

The invention belongs to the technical field of sewage treatment, and discloses a method for strengthening anaerobic fermentation of sludge and synchronously producing medium-chain fatty acid and hydrogen. The method comprises the following steps: settling and concentrating sludge, mixing the sludge with alkaline biochar, adding potassium ferrate for reaction to obtain pretreated sludge, adjusting the pH value, mixing the pretreated sludge with an electron donor and inoculated sludge, and carrying out anaerobic fermentation to obtain fermentation liquor rich in medium-chain fatty acid and hydrogen. An alkaline biochar-potassium ferrate coupling system is utilized to crack extracellular polymeric substances and cell walls of the sludge, so that particle organic matters are driven to be dissolved, and the activity of methane bacteria is inhibited; fe2O3 derived from potassium ferrate and biochar form an iron-carbon complex in situ, and a composite technology with triple functions of pretreatment, electronic regulation and methane inhibition is formed by strengthening electron transfer efficiency and enzyme activity and promoting an organic matter acidification process and a carbon chain extension reaction, so that synchronous recovery of medium-chain fatty acid and hydrogen is realized; the sludge treatment efficiency is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of sewage treatment, and in particular to a method for enhancing anaerobic fermentation of sludge to simultaneously produce medium-chain fatty acids and hydrogen. Background Art

[0002] With the continuous expansion of sewage treatment scale, the production of residual sludge has increased sharply, and its efficient resource utilization has become a core challenge in the field of environmental engineering. Traditional sludge anaerobic fermentation technology is dominated by methane production, but methane has low economic efficiency, high storage and transportation costs, and there is a risk of methane escape triggering a greenhouse effect. In comparison, medium-chain fatty acids (such as hexanoic acid, heptanoic acid, and octanoic acid) are easy to separate and extract from fermentation broth, have high economic value, and have a large market demand (can be converted into biofuels and chemical raw materials), while hydrogen is a green hydrogen energy. Therefore, the targeted conversion of sludge organic matter into high-value-added products - medium-chain fatty acids and hydrogen, has become an emerging breakthrough in sludge resource utilization.

[0003] 60-80% of the organic matter in sludge is densely encapsulated within extracellular polymeric substances (EPS) and microbial cell walls, making it difficult to effectively hydrolyze and acidify, resulting in a shortage of electron acceptors required for the synthesis of medium-chain fatty acids. To overcome this technical bottleneck, pretreatment technology has become a key link in promoting sludge wall breakdown and organic matter release. Compared with traditional pretreatment methods such as acid / alkaline treatment, heat, or ultrasound, advanced oxidation technologies (such as Fenton and persulfate) offer significant advantages such as ease of operation and high sludge lysis efficiency. Among them, potassium ferrate pretreatment technology has the greatest potential for application – its wide adaptability to system pH and the green and pollution-free reaction products offer significant advantages over traditional Fenton reagents and persulfate oxidation systems. For example, patent CN115094095B discloses a method for promoting the anaerobic fermentation of excess sludge to produce medium-chain fatty acids and recover phosphorus. This technology uses potassium ferrate as a pretreatment technology to promote sludge dissolution and inhibit methanogen activity. The pretreated sludge is then used as a fermentation substrate for the simultaneous recovery of medium-chain fatty acids and cyanite. However, the existing technology of using potassium ferrate pretreatment to enhance sludge fermentation to produce medium-chain fatty acids still has obvious limitations: first, potassium ferrate is rapidly consumed as the reaction progresses during the pretreatment stage, making it difficult to effectively act on the core step of carbon chain extension in the subsequent fermentation process; second, under initial neutral conditions, potassium ferrate is unstable and prone to self-decomposition reactions, which reduces the reaction efficiency with sludge polymer structures; third, single potassium ferrate pretreatment often requires the addition of high doses of oxidants, which not only increases treatment costs but may also inhibit the initial metabolic activity of hydrolytic and fermentative bacteria.

[0004] In the process of carbon chain extension metabolism, the functional bacteria use ethanol as an electron donor and acetic acid as an electron acceptor, and gradually convert the sludge acidification products (such as acetic acid, propionic acid, etc.) into butyric acid and valeric acid through reverse β-oxidation and fatty acid synthesis pathways, and further generate hexanoic acid, heptanoic acid, and octanoic acid, accompanied by the release of a large amount of hydrogen. However, the carbon chain extension efficiency of this process is limited by the electron transfer rate between microorganisms. Studies have shown that the addition of conductive materials (such as biochar, zero-valent iron, etc.) can improve the interspecies electron transfer efficiency by constructing an electron transfer network, thereby strengthening the carbon chain extension reaction. For example, patent CN110819662A discloses a method for adding biochar to an anaerobic reaction system to promote the output of medium-chain fatty acids. This technology selects the concentrated sludge in the secondary sedimentation tank after anaerobic reactor cultivation as a substrate, mixes the inoculum and the substrate, and adds ethanol and biochar under anaerobic conditions to produce high concentrations of medium-chain fatty acids. For example, patent CN117142454A discloses a fruit peel biochar, its preparation method, and its application in enhancing methanogenesis in excess sludge. This technology uses pyrolysis and carbonization of fruit peels (banana peels) to produce biochar. The biochar then pre-treats the excess sludge, leveraging its strong alkalinity to increase its hydrolysis rate, break down barriers to the dissolution of macromolecules, and provide more organic substrate for subsequent anaerobic digestion. Furthermore, the biochar added during the pre-treatment stage can still enhance anaerobic digestion in subsequent stages. However, existing systems have significant drawbacks. First, conductive materials (such as zero-valent iron and biochar) act only as electron transfer mediators, have little effect on sludge lysis, and are unable to block competitive consumption of key intermediates (H2 and acetic acid) by methanogens. Second, to inhibit methanogenic activity, high doses of inhibitors (such as sodium 2-bromoethylsulfonate, BES) must be continuously added exogenously. This significantly increases operating costs and the brominated compounds present potential ecotoxicity risks.

[0005] Although the above two types of technologies (pretreatment and carbon chain extension regulation) can significantly improve the efficiency of medium-chain fatty acid biosynthesis in sludge anaerobic fermentation systems, their effects are limited to the release of intracellular organic matter at the front end, or focus on the regulation of electron transfer in the subsequent carbon chain extension step. Furthermore, the former requires a higher dosage of oxidant (such as potassium ferrate dosage of up to 0.25-0.5g / g TSS), and the latter requires the supplementation of methane inhibitors (such as BES dosage of 5-10.5g / L), further increasing the cost of sludge treatment. Based on this, the development of a new composite technology with the triple functions of "pretreatment-electronic regulation-methane inhibition" will become an important technical path to achieve a green and low-carbon transformation of the sludge resource utilization process. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for enhancing the anaerobic fermentation of sludge to simultaneously produce medium-chain fatty acids and hydrogen, so as to solve the technical bottlenecks such as slow release rate of particulate organic matter and low concentration of acidification products in the anaerobic fermentation process of raw sludge without pretreatment or addition of any promoter, and at the same time overcome the problems of high cost and limited control dimension of single treatment technology.

[0007] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0008] The present invention provides a method for simultaneously producing medium-chain fatty acids and hydrogen by enhancing anaerobic fermentation of sludge, comprising the following steps:

[0009] The sludge is settled to obtain concentrated sludge;

[0010] The concentrated sludge and alkaline biochar are mixed to obtain sludge containing alkaline biochar; potassium ferrate is added to the sludge containing alkaline biochar to react to obtain pretreated sludge; the pH of the pretreated sludge is adjusted to 6.0 to 7.5 to obtain tempered sludge;

[0011] The conditioned sludge, electron donor and inoculated sludge are mixed and anaerobic fermented to obtain fermentation liquid rich in medium-chain fatty acids and hydrogen.

[0012] Preferably, the total suspended solids content in the concentrated sludge is 24-30 g / L.

[0013] Preferably, the mass concentration of alkaline biochar in the alkaline biochar-containing sludge is 3 to 8 g / L; and the pH of the alkaline biochar-containing sludge is 7.9 to 8.8.

[0014] Preferably, the mixing temperature is 18-28° C.; the mixing time is 3-6 min; the mixing process also includes stirring; and the stirring speed is 200-400 rpm.

[0015] Preferably, the mass ratio of the potassium ferrate to the total suspended solids in the concentrated sludge is 0.1-0.2:1.

[0016] Preferably, the reaction temperature is 25-35°C; the reaction time is 24-48 hours; the reaction process also includes rapid stirring and slow stirring; the speed of the rapid stirring is 200-400 rpm; the time of the rapid stirring is 3-6 minutes; the speed of the slow stirring is 100-150 rpm.

[0017] Preferably, the electron donor is ethanol or lactic acid.

[0018] Preferably, the method for preparing the inoculum sludge is: mixing concentrated sludge, sodium 2-bromoethylsulfonate and an electron donor, and performing anaerobic fermentation to obtain the inoculum sludge.

[0019] Preferably, the volume ratio of the seeding sludge to the tempered sludge is 1:8-10; the ratio of the molar amount of the electron donor to the total volume of the seeding sludge and the tempered sludge is 0.12-0.16 mol:1L.

[0020] Preferably, the temperature of the anaerobic fermentation is 32-38° C.; the time of the anaerobic fermentation is 8-12 days; the anaerobic fermentation process also includes stirring; the stirring speed is 100-150 rpm.

[0021] It can be seen from the above technical solutions that compared with the prior art, the present invention has the following beneficial effects:

[0022] 1) The present invention utilizes the strong oxidizing properties and superimposed alkaline effect of the alkaline biochar-potassium ferrate coupling system to efficiently break down sludge extracellular polymers and cell walls in the pretreatment stage, drive the dissolution of particulate organic matter, and effectively inhibit the activity of methanogens; at the same time, Fe2O3 derived from potassium ferrate and biochar form an iron-carbon complex in situ through physical and chemical reactions. During the fermentation stage, this complex significantly promotes the organic matter acidification process and carbon chain extension reaction (short-chain fatty acids are upgraded to medium-chain fatty acids) by enhancing electron transfer efficiency and enzyme activity, forming a new composite technology with the triple functions of "pretreatment-electronic regulation-methane inhibition", strengthening the anaerobic fermentation of sludge to achieve the simultaneous recovery of medium-chain fatty acids and hydrogen, reducing the cost of sludge treatment, achieving the simultaneous recovery of gas and liquid products, and improving the sludge treatment efficiency.

[0023] 2) The synergistic effect of alkaline biochar and potassium ferrate in this invention reduces potassium ferrate dosage by 60-80% (from the conventional 0.25-0.5g / g TSS to 0.1-0.2g / g TSS), and the biochar dosage by 60-85% (conventionally requiring 20g / L), while eliminating the need for BES inhibitor. Furthermore, alkaline biochar is economically sourced from waste biomass, such as fruit peels (orange and banana peels) and livestock and poultry manure, achieving a "waste-to-waste" approach.

[0024] 3) The fermentation product obtained by the present invention contains up to 10,495 mg COD / L of n-hexanoic acid, and produces up to 284 mL / L of sludge. This increases medium-chain fatty acid production by over 200% and hydrogen production by over 250% compared to single technologies. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] 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 describing the embodiments or the prior art.

[0026] Figure 1 This is a schematic diagram of the process flow for simultaneously producing medium-chain fatty acids and hydrogen by enhanced anaerobic fermentation of sludge according to Example 1;

[0027] Figure 2 The yield of medium-chain fatty acids in the anaerobic fermentation stage of Example 1 and Comparative Examples 1 to 3 changes with fermentation time;

[0028] Figure 3 The hydrogen yield in the anaerobic fermentation stage of Example 1 and Comparative Examples 1 to 3 changes with fermentation time;

[0029] Figure 4 The SCOD content in the supernatant of the pretreated sludge in the pretreatment stage of Example 1 and Comparative Examples 1 to 3 changes with reaction time;

[0030] Figure 5 The three-dimensional fluorescence spectrum (EEM) of the pretreated sludge supernatant in the pretreatment stage of Example 1 and Comparative Examples 1 to 3 changes with reaction time;

[0031] Among them, Control represents the blank group, AlkBC represents the alkaline biochar group alone, Ferrate represents the potassium ferrate group alone, and AlkBC-Ferrate represents the alkaline biochar-potassium ferrate group;

[0032] Figure 6 To support the mechanism of Example 1: the effects of different iron-carbon forms on the production of medium-chain fatty acids in the carbon chain extension step under the conditions of ethanol as the electron donor and acetic acid as the electron acceptor;

[0033] Among them, R0-R1 represent the blank group, biochar group, ferrate-reduced particle group, ferrate-reduced particle-biochar complex group, Fe(OH)3 group, and biochar-Fe(OH)3 complex group, respectively. DETAILED DESCRIPTION

[0034] The present invention provides a method for simultaneously producing medium-chain fatty acids and hydrogen by enhancing anaerobic fermentation of sludge, comprising the following steps:

[0035] The sludge is settled to obtain concentrated sludge;

[0036] The concentrated sludge and alkaline biochar are mixed to obtain sludge containing alkaline biochar; potassium ferrate is added to the sludge containing alkaline biochar to react to obtain pretreated sludge; the pH of the pretreated sludge is adjusted to 6.0 to 7.5 to obtain tempered sludge;

[0037] The conditioned sludge, electron donor and inoculated sludge are mixed and anaerobic fermented to obtain fermentation liquid rich in medium-chain fatty acids and hydrogen.

[0038] In the present invention, the sludge is preferably excess sludge from the secondary sedimentation tank of a municipal sewage treatment plant and / or sludge from the primary sedimentation tank of a municipal sewage treatment plant, and is further preferably excess sludge from the secondary sedimentation tank of a municipal sewage treatment plant.

[0039] In the present invention, the total suspended solids content in the concentrated sludge is preferably 24 to 30 g / L, more preferably 25 to 28 g / L, and even more preferably 25.29 g / L.

[0040] In the present invention, the alkaline biochar is obtained by high-temperature carbonization of waste biomass under a nitrogen atmosphere; the waste biomass is preferably fruit peel and / or livestock and poultry manure, more preferably fruit peel, and more preferably banana peel; the temperature of the high-temperature carbonization is preferably 550-700°C, more preferably 550-650°C, and more preferably 600°C; the time of the high-temperature carbonization is preferably 1-3h, more preferably 1-2h, and more preferably 2h.

[0041] In the present invention, the mass concentration of the alkaline biochar in the alkaline biochar-containing sludge is preferably 3 to 8 g / L, more preferably 4 to 7 g / L, and even more preferably 5 g / L.

[0042] In the present invention, the pH of the alkaline biochar-containing sludge is preferably 7.9 to 8.8, more preferably 7.9 to 8.5, and even more preferably 8.0.

[0043] In the present invention, the mixing temperature is preferably 18 to 28°C, more preferably 20 to 26°C, and more preferably 25°C; the mixing time is preferably 3 to 6 minutes, more preferably 4 to 6 minutes, and more preferably 5 minutes; the mixing process also includes stirring; the stirring speed is preferably 200 to 400 rpm, more preferably 250 to 350 rpm, and more preferably 300 rpm.

[0044] In the present invention, the mass ratio of potassium ferrate to total suspended solids in concentrated sludge is preferably 0.1-0.2:1, more preferably 0.12-0.17:1, and even more preferably 0.15:1.

[0045] In the present invention, the reaction temperature is preferably 25-35°C, more preferably 30-35°C, more preferably 35°C; the reaction time is preferably 24-48h, more preferably 36-48h, more preferably 48h; the reaction process also includes rapid stirring and slow stirring; the rotation speed of the rapid stirring is preferably 200-400rpm, more preferably 250-350rpm, more preferably 300rpm; the rapid stirring time is preferably 3-6min, more preferably 4-6min, more preferably 5min; the rotation speed of the slow stirring is preferably 100-150rpm, more preferably 110-140rpm, more preferably 120rpm.

[0046] In the present invention, the electron donor is preferably ethanol or lactic acid, more preferably ethanol.

[0047] In the present invention, the preparation method of the seed sludge is as follows: concentrated sludge, sodium 2-bromoethylsulfonate, and an electron donor are mixed and anaerobic fermentation is performed to obtain seed sludge; the usage ratio of the concentrated sludge, sodium 2-bromoethylsulfonate, and the electron donor is preferably 1L:0.05mol:0.12-0.16mol, more preferably 1L:0.05mol:0.15-0.16mol, and more preferably 1L:0.05mol:0.16mol; the total suspended solids content of the concentrated sludge is preferably 24-30g / L , further preferably 25-28 g / L, more preferably 25.29 g / L; the electron donor is preferably ethanol; the temperature of the anaerobic fermentation is preferably 32-38°C, further preferably 34-36°C, more preferably 35°C; the time of the anaerobic fermentation is preferably 10-15 days, further preferably 11-13 days, more preferably 12 days; the anaerobic fermentation process also includes stirring; the stirring speed is preferably 100-150 rpm, further preferably 110-140 rpm, more preferably 120 rpm.

[0048] In the present invention, the volume ratio of the seeding sludge and the tempered sludge is preferably 1:8-10, more preferably 1:8.2-9.5, and more preferably 1:9; the ratio of the molar amount of the electron donor to the total volume of the seeding sludge and the tempered sludge is preferably 0.12-0.16 mol:1L, more preferably 0.13-0.15 mol:1L, and more preferably 0.145 mol:1L.

[0049] In the present invention, the temperature of the anaerobic fermentation is preferably 32-38°C, more preferably 34-36°C, and more preferably 35°C; the time of the anaerobic fermentation is preferably 8-12 days, more preferably 10-12 days, and more preferably 12 days; the anaerobic fermentation process also includes stirring; the stirring speed is preferably 100-150 rpm, more preferably 110-140 rpm, and more preferably 120 rpm.

[0050] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0051] Example 1

[0052] This embodiment provides a method for enhancing anaerobic fermentation of sludge to simultaneously produce medium-chain fatty acids and hydrogen. The process flow diagram is shown in FIG. Figure 1 As shown, the following steps are included:

[0053] (1) The residual sludge from the secondary sedimentation tank of a municipal sewage treatment plant was concentrated by gravity at room temperature, and the supernatant was removed to obtain concentrated sludge with a total suspended solids content of 25.29 g / L.

[0054] (2) Alkaline biochar (obtained by carbonizing banana peel at 600°C for 2 hours under nitrogen atmosphere) was added to the concentrated sludge to a mass concentration of 5 g / L. The mixture was then stirred at 300 rpm for 5 minutes at 25°C to fully mix with the concentrated sludge. The pH of the sludge containing alkaline biochar was 8.0. Potassium ferrate was then added to the sludge containing alkaline biochar at a dosage of 0.15 g / g total suspended solids. The mixture was stirred at 300 rpm for 5 minutes at 25°C to fully contact the potassium ferrate with the sludge. The mixture was then reacted at 35°C for a total reaction time of 48 hours at a rotation speed of 120 rpm. After the reaction, pretreated sludge was obtained. The pH of the sludge was 9.8 after 30 minutes of reaction. The pH of the pretreated sludge was adjusted to 6.5 to obtain tempered sludge.

[0055] (3) The conditioned sludge is transferred to the anaerobic fermentation tank as a fermentation substrate, and the inoculated sludge is added at a volume ratio of 1:9 to the conditioned sludge, wherein the ratio of the total volume of the inoculated sludge and the conditioned sludge to the headspace volume of the anaerobic fermentation tank is 1.2:1, and nitrogen is filled into the anaerobic fermentation tank for 5 minutes to ensure an anaerobic environment; thereafter, ethanol is quickly added to the anaerobic fermentation tank as an electron donor for carbon chain extension, and the addition amount is calculated based on the total volume of the inoculated sludge and the conditioned sludge so that the concentration of ethanol reaches 0.145 mol / L (6.70 g / L), and the ethanol is evenly distributed in the sludge by rapid stirring; the anaerobic fermentation tank is sealed, the temperature is controlled at 35°C, the stirring intensity is controlled at 120 rpm, and the anaerobic fermentation is carried out for 12 days, thereby completing the enhanced sludge anaerobic fermentation and the simultaneous production of medium-chain fatty acids and hydrogen.

[0056] The preparation method of the inoculum sludge in step (3) is as follows: sodium 2-bromoethylsulfonate is added to the concentrated sludge in step (1) to make the content of sodium 2-bromoethylsulfonate in the concentrated sludge 0.05 mol / L, and then nitrogen is filled for 5 minutes to ensure an anaerobic environment; ethanol is then added as an electron donor for carbon chain extension, and the amount added is calculated based on the volume of the concentrated sludge to make the concentration of ethanol reach 0.16 mol / L, the temperature is controlled at 35° C., the stirring intensity is controlled at 120 rpm, and anaerobic fermentation is carried out for 12 days to obtain the inoculum sludge.

[0057] During the anaerobic fermentation process, the medium-chain fatty acid concentration and hydrogen production in the fermentation system were detected by gas chromatography every 2 days. Figure 2 and Figure 3 As shown. Figure 2 and Figure 3 It can be seen that when alkaline biochar (5 g / L) and potassium ferrate (0.15 g / g total suspended solids) are added simultaneously for pretreatment, the medium-chain fatty acids (mainly n-hexanoic acid) produced by the fermentation of residual sludge can reach 10495 mg COD / L, and the hydrogen production reaches 284 mL / L sludge.

[0058] Comparative Example 1

[0059] This comparative example provides a method for the simultaneous production of medium-chain fatty acids and hydrogen by anaerobic fermentation of sludge. For details, see Example 1, except that step (2) is: the concentrated sludge is reacted at 35°C for 48 hours at a rotation speed of 120 rpm to obtain pretreated sludge after the reaction; and the pH of the pretreated sludge is adjusted to 6.5 to obtain tempered sludge.

[0060] During the anaerobic fermentation process, the medium-chain fatty acid concentration and hydrogen production in the fermentation system were detected by gas chromatography every 2 days. Figure 2 and Figure 3 As shown. Figure 2 and Figure 3 It can be seen that when the sludge is treated without adding any substances, the medium-chain fatty acids (mainly n-hexanoic acid) produced by the fermentation of the residual sludge are only 486 mg COD / L, and the hydrogen production is only 28 mL / L sludge.

[0061] Comparative Example 2

[0062] This comparative example provides a method for the simultaneous production of medium-chain fatty acids and hydrogen by anaerobic fermentation of sludge, with reference to Example 1 for details, except that step (2) comprises the following steps: adding alkaline biochar (obtained by carbonizing banana peels at 600°C for 2 h in a nitrogen atmosphere) to the concentrated sludge, wherein the mass concentration after addition is 5 g / L; stirring the mixture at 25°C at a speed of 300 rpm for 5 min to fully mix the mixture with the concentrated sludge, wherein the pH of the sludge containing the alkaline biochar is 8.0; then reacting the sludge containing the alkaline biochar at 35°C for 48 h at a speed of 120 rpm to obtain pretreated sludge after the reaction; and adjusting the pH of the pretreated sludge to 6.5 to obtain tempered sludge.

[0063] During the anaerobic fermentation process, the medium-chain fatty acid concentration and hydrogen production in the fermentation system were detected by gas chromatography every 2 days. Figure 2 and Figure 3 As shown. Figure 2 and Figure 3 It can be seen that when alkaline biochar was added alone for sludge pretreatment, the medium-chain fatty acids (mainly n-hexanoic acid) produced by the fermentation of the remaining sludge were only 648 mg COD / L, and the hydrogen production was only 21 mL / L sludge.

[0064] Comparative Example 3

[0065] This comparative example provides a method for the simultaneous production of medium-chain fatty acids and hydrogen by anaerobic fermentation of sludge, with reference to Example 1 for details, except that step (2) comprises the following steps: adding potassium ferrate to the concentrated sludge at a dosage of 0.15 g / g of total suspended solids, stirring at a speed of 300 rpm at 25° C. for 5 min to allow the potassium ferrate to fully contact the sludge, and then reacting at 35° C. for a total reaction time of 48 h at a speed of 120 rpm. After the reaction, pretreated sludge is obtained; and the pH of the pretreated sludge is adjusted to 6.5 to obtain tempered sludge.

[0066] During the anaerobic fermentation process, the medium-chain fatty acid concentration and hydrogen production in the fermentation system were detected by gas chromatography every 2 days. Figure 2 and Figure 3 As shown. Figure 2 and Figure 3It can be seen that when potassium ferrate is added alone to pretreat the sludge, the medium-chain fatty acids (mainly n-hexanoic acid) produced by the fermentation of the residual sludge are 3212 mg COD / L, and the hydrogen production is 74 mL / L sludge.

[0067] In the present invention, the technical principles are as follows:

[0068] The organic matter in the sludge is densely wrapped in extracellular polymers (EPS) and microbial cell walls, making it difficult to effectively hydrolyze and acidify, which in turn causes insufficient supply of electron acceptors required for the synthesis of medium-chain fatty acids. In the pretreatment stage (the pretreatment stage refers to the process of steps (1) and (2) of Example 1 and Comparative Examples 1 to 3), the potassium ferrate reaction is activated by alkaline biochar to drive Fe(VI) to be converted into highly active high-valent iron intermediates (such as Fe(IV) / Fe(V)), and the alkali released by the alkaline biochar increases the stability of Fe(IV) / Fe(V), thereby improving the oxidation efficiency of potassium ferrate on the sludge. Furthermore, the strong oxidizing property and bactericidal effect of alkaline biochar-potassium ferrate effectively break down the EPS and cell wall barriers, increase the number of free organic molecules and the initial short-chain fatty acid production, and provide a large amount of directly usable substrates for the subsequent anaerobic fermentation process of producing medium-chain fatty acids, such as Figure 4 、 Figure 5 As shown in Table 1. Among them, Control represents the blank group, AlkBC represents the alkaline biochar group alone, Ferrate represents the potassium ferrate group alone, and AlkBC-Ferrate represents the alkaline biochar-potassium ferrate group. Figure 4 The release degree of organic matter from sludge particles to the supernatant under different conditions in the pretreatment stage was described. It was found that the SCOD content of the alkaline biochar-potassium ferrate group was significantly higher than that of the blank group, the alkaline biochar group alone, and the potassium ferrate group alone, indicating that the coupled system has more soluble carbon that can be directly utilized by hydrolysis and fermentation bacteria. Figure 5 As shown in the figure, the maximum fluorescence intensity value of the EEM spectrum of the supernatant of the coupled system is lower than or equivalent to that of the potassium ferrate group alone, which is obviously different from the SCOD trend. This intuitively shows that the alkaline biochar-potassium ferrate oxidation seriously damages the fluorescence properties of the sludge organic molecules, causing the fluorescent groups to quench, revealing that the alkaline biochar-potassium ferrate can directly destroy the complex structure of macromolecular organic matter through chemical oxidation, thereby proving that alkaline biochar enhances the oxidation capacity and efficiency of potassium ferrate on sludge organic matter. Therefore, after pretreatment, the concentration of short-chain fatty acids that can be directly used in carbon chain extension reactions in the alkaline biochar-potassium ferrate system is significantly higher than that of the blank and single-treatment groups (as shown in Table 1).

[0069] Methanogens are ubiquitous competitive microorganisms in the sludge fermentation-coupled carbon chain extension system, and their removal or activity inhibition usually requires the addition of methanogen inhibitors or the use of inactivation techniques. After 12 days of anaerobic fermentation, the methane production of the alkaline biochar group and the blank group was almost equal. The potassium ferrate group alone reduced methane production to a certain extent, while the alkaline biochar coupled with potassium ferrate reduced methane production to zero (as shown in Table 1). This means that the alkaline biochar-potassium ferrate selectively inactivates methanogens while retaining key functional bacteria related to hydrolysis, acidification, and carbon chain extension. This eliminates the inhibitory effect of competitive metabolism on carbon chain extension in the subsequent fermentation stage, circumvents the large-scale addition of methanogen inhibitors, and promotes the directional conversion of substrates to medium-chain fatty acids.

[0070] During the anaerobic fermentation stage, the biochar-Fe2O3 complex formed by the physical and chemical interaction between potassium ferrate and alkaline biochar effectively promotes the acidification of organic matter and carbon chain extension by strengthening electron transfer and microbial enzyme activity. Specifically, potassium ferrate oxidation can increase the number of oxygen-containing functional groups on the surface of alkaline biochar, improve its redox properties, electron transfer capacity and binding effect with Fe-O, while the Fe2O3 generated in situ by potassium ferrate reduction will be loaded on the surface of biochar, increasing the specific surface area and surface activity of biochar. In other words, the iron-carbon coexistence matrix formed in the pretreatment stage can serve as a "growth medium" and "electron shuttle" for microorganisms, regulating microbial metabolism (such as acidification and carbon chain extension) in the sludge fermentation system by stimulating the secretion of extracellular hydrolases, promoting the expression of key functional genes in cells, and strengthening electron transfer, thereby improving the conversion efficiency of substrates to medium-chain fatty acids. The maximum EEM fluorescence intensity of the potassium ferrate-alkaline biochar coupling system after 48 h of pretreatment was lower than that of potassium ferrate alone, which can also be used to illustrate that the presence of the iron oxide-biochar complex accelerates the biodegradation of fluorescent organic matter by improving electron transfer efficiency or microbial metabolic activity ( Figure 5 ).

[0071] Figure 6The study demonstrated the effects of different iron-carbon forms on carbon chain extension, the core biological step in the production of medium-chain fatty acids, using ethanol as the electron donor and acetic acid as the electron acceptor. Six groups were included: a blank, a biochar, a ferrate-reduced pellet group, a ferrate-reduced pellet-biochar complex group, an Fe(OH)3 group, and a biochar-Fe(OH)3 complex group. It is noteworthy that the main iron forms in the ferrate-reduced products are Fe2O3 and Fe(OH)3. The specific anaerobic fermentation method is as follows: (1) Ferrate-reducing particle-biochar complex group: the pretreatment process is specifically as described in step (2) of Example 1, except that the concentrated sludge of Example 1 is replaced with water, and after potassium ferrate is added, 6 mol / L HCl is added repeatedly during the reaction process to make the pH close to the pH change in step (2) of Example 1, thereby obtaining a pretreatment solution containing ferrate-reducing particle-biochar complex; the fermentation process is specifically as described in step (3) of Example 1, except that the conditioned sludge is replaced with synthetic water (synthetic water specifically refers to the addition of anhydrous sodium acetate, ethanol, and BES to the pretreatment solution containing ferrate-reducing particle-biochar complex to achieve concentrations of 4 g / L, 0.16 mol / L, and 0.05 mol / L, respectively, and the ethanol in the synthetic water is the ethanol in Example 1), and the fermentation conditions are adjusted to anaerobic fermentation at 35°C and 120 rpm for 9 days. (2) Ferrate-reducing particle group: specifically as described in the ferrate-reducing particle-biochar complex group, except that no alkaline biochar is added. (3) Blank group: Refer to the ferrate-reduced particle-biochar complex group for details, except that alkaline biochar and potassium ferrate were not added. (4) Biochar group: Refer to the ferrate-reduced particle-biochar complex group for details, except that potassium ferrate was not added. (5) Biochar-Fe(OH)3 complex group: Refer to the ferrate-reduced particle-biochar complex group for details, except that potassium ferrate was replaced by trivalent iron salt. (6) Fe(OH)3 group: Refer to the biochar-Fe(OH)3 complex group for details, except that alkaline biochar was not added. Figure 6 After 9 days of anaerobic fermentation, the medium-chain fatty acid concentration in the blank group was 4858 mg COD / L. However, the presence of biochar, ferrate-reduced granules, and ferrate-reduced granules-biochar complex significantly increased the medium-chain fatty acid concentration to 7853, 9090, and 10200 mg COD / L, respectively. When Fe(OH)3 was used to represent the ferrate-reduced granules, medium-chain fatty acid production actually decreased. This demonstrates that Fe2O3 is a key active iron component on the biochar surface that promotes electron transfer and enhances chain extension. It also clearly demonstrates the important role of the biochar-Fe2O3 complex formed during the pretreatment stage in subsequent anaerobic fermentation. This method provides an efficient approach for sludge treatment and resource utilization, achieving both sludge reduction and carbon recovery.

[0072] Table 1 Short-chain fatty acid content in the pretreatment stage and cumulative methane production in the anaerobic fermentation stage

[0073]

[0074]

[0075] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for simultaneously producing medium-chain fatty acids and hydrogen by enhanced anaerobic fermentation of sludge, characterized in that: The following steps are involved: The sludge is settled to obtain concentrated sludge; mixing concentrated sludge and alkaline biochar to obtain sludge containing alkaline biochar; adding potassium ferrate to the sludge containing alkaline biochar to react and obtain pretreated sludge; adjusting the pH of the pretreated sludge to 6.0-7.5 to obtain tempered sludge; The conditioned sludge, electron donor and inoculated sludge are mixed and anaerobic fermented to obtain fermentation liquid rich in medium-chain fatty acids and hydrogen.

2. The method for simultaneously producing medium-chain fatty acids and hydrogen by enhanced anaerobic fermentation of sludge according to claim 1, characterized in that: The total suspended solid content in the concentrated sludge is 24-30 g / L.

3. The method for simultaneously producing medium-chain fatty acids and hydrogen by enhanced anaerobic fermentation of sludge according to claim 2, characterized in that: The mass concentration of the alkaline biochar in the alkaline biochar-containing sludge is 3 to 8 g / L; the pH of the alkaline biochar-containing sludge is 7.9 to 8.

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4. The method for simultaneously producing medium-chain fatty acids and hydrogen by enhanced anaerobic fermentation of sludge according to claim 3, characterized in that: The mixing temperature is 18-28° C.; the mixing time is 3-6 min; the mixing process also includes stirring; and the stirring speed is 200-400 rpm.

5. The method for simultaneously producing medium-chain fatty acids and hydrogen by enhanced anaerobic fermentation of sludge according to claim 2, characterized in that: The mass ratio of the potassium ferrate to the total suspended solids in the concentrated sludge is 0.1-0.2:

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6. The method for simultaneously producing medium-chain fatty acids and hydrogen by enhanced anaerobic fermentation of sludge according to claim 1 or 5, characterized in that: The reaction temperature is 25-35° C.; the reaction time is 24-48 h; the reaction process also includes rapid stirring and slow stirring; the rapid stirring speed is 200-400 rpm; the rapid stirring time is 3-6 min; and the slow stirring speed is 100-150 rpm.

7. The method for simultaneously producing medium-chain fatty acids and hydrogen by enhanced anaerobic fermentation of sludge according to claim 6, characterized in that: The electron donor is ethanol or lactic acid.

8. The method for simultaneously producing medium-chain fatty acids and hydrogen by enhanced anaerobic fermentation of sludge according to claim 7, characterized in that: The method for preparing the seeding sludge comprises: mixing concentrated sludge, sodium 2-bromoethylsulfonate and an electron donor, and performing anaerobic fermentation to obtain the seeding sludge.

9. The method for simultaneously producing medium-chain fatty acids and hydrogen by enhanced anaerobic fermentation of sludge according to claim 3 or 5, characterized in that: The volume ratio of the seeding sludge to the conditioned sludge is 1:8-10; the ratio of the molar amount of the electron donor to the total volume of the seeding sludge and the conditioned sludge is 0.12-0.16 mol:1L.

10. The method for simultaneously producing medium-chain fatty acids and hydrogen by enhanced anaerobic fermentation of sludge according to claim 9, characterized in that: The temperature of the anaerobic fermentation is 32-38° C.; the time of the anaerobic fermentation is 8-12 days; the anaerobic fermentation process also includes stirring; the stirring speed is 100-150 rpm.

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