A method for simultaneously producing medium-chain fatty acids and hydrogen by enhancing anaerobic fermentation of sludge

Abstract

The present application belongs to the technical field of sewage treatment, and discloses a method for simultaneously producing medium-chain fatty acids and hydrogen by enhancing sludge anaerobic fermentation. The sludge is concentrated and mixed with alkaline biochar, and then potassium ferrate is added for reaction to obtain pretreated sludge. After adjusting the pH, the sludge is mixed with an electron donor and inoculated sludge for anaerobic fermentation to obtain a fermentation liquor rich in medium-chain fatty acids and hydrogen. The alkaline biochar-potassium ferrate coupling system is used to break down the extracellular polymeric substance and cell wall of the sludge, drive the dissolution of particulate organic matter, and inhibit the activity of methanogens. Fe2O3 derived from potassium ferrate forms an iron-carbon composite with biochar in situ, and by enhancing the efficiency of electron transfer and enzyme activity, the acidification process of organic matter and the carbon chain extension reaction are promoted, forming a composite technology with three functions of "pretreatment-electron regulation-methane inhibition", realizing the simultaneous recovery of medium-chain fatty acids and hydrogen, and improving the sludge treatment efficiency.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of sewage treatment, and particularly relates to a method for simultaneously producing medium-chain fatty acids and hydrogen by strengthening sludge anaerobic fermentation. BACKGROUND

[0002] With the continuous expansion of sewage treatment scale, the production of excess sludge has increased dramatically, 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 cost, and the risk of causing greenhouse effect due to methane emission. 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 large market demand (can be converted into biofuels and chemical raw materials), and hydrogen is a green hydrogen energy. Therefore, the directional conversion of sludge organic matter into high-value products, medium-chain fatty acids and hydrogen, has become a new breakthrough in sludge resource utilization.

[0003] 60-80% of the organic matter in sludge is tightly wrapped in extracellular polymeric substances (EPS) and microbial cell walls, making it difficult to effectively hydrolyze and acidify, and thus causing insufficient supply of electron acceptors required for the synthesis of medium-chain fatty acids. In order to break this technical bottleneck, pretreatment technology is a key link to promote sludge wall breaking and organic matter release. Compared with traditional acid / alkali treatment, heat or ultrasonic pretreatment methods, advanced oxidation technology (such as Fenton, persulfate) shows significant advantages such as simple operation, high sludge cracking efficiency, etc. Among them, potassium ferrate pretreatment technology has more application potential - it has wide adaptability to system pH, and the reaction products are green and pollution-free, which has obvious advantages over traditional Fenton reagent and persulfate oxidation system. For example, patent CN115094095B discloses a method for promoting the production of medium-chain fatty acids and phosphorus recovery by anaerobic fermentation of excess sludge, which uses potassium ferrate as a pretreatment technology to promote sludge dissolution and inhibit the activity of methanogens, and then uses the pretreated sludge as a fermentation substrate to simultaneously recover medium-chain fatty acids and blue iron ore. However, the existing technology for using potassium ferrate pretreatment to strengthen sludge fermentation to produce medium-chain fatty acids still has obvious limitations: first, potassium ferrate will be rapidly consumed during the pretreatment stage, making it difficult to effectively act on the carbon chain extension core step in the subsequent fermentation process; second, under initial neutral conditions, potassium ferrate is unstable and prone to self-decomposition, reducing the reaction efficiency with the sludge polymer structure; third, single potassium ferrate pretreatment often requires high dosage of oxidizing agent, which not only increases the treatment cost, but also may inhibit the initial metabolic activity of hydrolysis and fermentation bacteria.

[0004] In the process of carbon chain extension metabolism, functional bacteria use ethanol as an electron donor and acetic acid as an electron acceptor to gradually convert sludge acidification products (such as acetic acid, propionic acid, etc.) into butyric acid and valeric acid through reverse beta oxidation and fatty acid synthesis pathway, and further generate caproic acid, heptanoic acid, 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 adding conductive materials (such as biochar, zero-valent iron, etc.) can improve 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 in an anaerobic reaction system to promote the production of medium-chain fatty acids. This technology selects sludge concentrated in the secondary sedimentation tank after anaerobic reactor cultivation as the substrate, mixes the inoculum and the substrate, and adds ethanol and biochar in an anaerobic environment to produce high-concentration medium-chain fatty acids. For another example, patent CN117142454A discloses a fruit peel biochar, its preparation method and application in strengthening residual sludge methane production. This technology uses fruit peel (banana peel) pyrolysis carbonization to prepare biochar, and then uses the strong alkaline advantage of biochar to first pretreat the residual sludge, improve the hydrolysis rate of the residual sludge, break the barrier of macromolecular substance dissolution, and provide more organic substrates for the subsequent anaerobic digestion process. At the same time, the biochar added in the pretreatment stage can still strengthen the effect of anaerobic digestion in the subsequent stage. However, the existing technical system has significant defects: on the one hand, conductive materials (such as zero-valent iron, biochar) only play the role of electron transfer medium, have little effect on sludge lysis, and cannot block the competitive consumption of key intermediate products (H2 and acetic acid) by methanogens; on the other hand, in order to inhibit methanogenic activity, high-dose inhibitors (such as 2-bromoethyl sulfonic acid sodium, BES) need to be continuously added, which not only greatly increases the operation cost, but also may cause ecological toxicity risk.

[0005] Although the above two types of technologies (pretreatment, carbon chain extension regulation) can significantly improve the efficiency of medium-chain fatty acid biosynthesis in sludge anaerobic fermentation system, 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. Further, the former requires a high dose of oxidants (such as potassium ferrate dosage can reach 0.25-0.5 g / g TSS), and the latter needs to supplement methanogenic inhibitors (such as BES dosage reaches 5-10.5 g / L), further increasing the cost of sludge treatment. Based on this, developing a new composite technology with the functions of "pretreatment-electronic regulation-methane inhibition" will become an important technical path for realizing the green and low-carbon transformation of sludge resourceization process. SUMMARY

[0006] The application aims to provide a method for simultaneously producing medium-chain fatty acids and hydrogen by strengthening sludge anaerobic fermentation, solve technical bottlenecks such as slow release rate of particulate organic matter and low concentration of acidification products in the original sludge without pretreatment or any accelerant in the anaerobic fermentation process, and overcome problems such as high cost and limited regulation dimension of a single treatment technology.

[0007] In order to achieve the above-mentioned application purposes, the application provides the following technical solutions.

[0008] The application provides a method for simultaneously producing medium-chain fatty acids and hydrogen by strengthening sludge anaerobic fermentation, 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 6.0-7.5 to obtain conditioned sludge.

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

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

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

[0014] Preferably, the temperature of the mixing is 18-28 DEG C; the mixing time is 3-6 min; the mixing process further comprises stirring; and the stirring speed is 200-400 rpm.

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

[0016] Preferably, the temperature of the reaction is 25-35 DEG C; the reaction time is 24-48 h; the reaction process further comprises rapid stirring and slow stirring; the speed of the rapid stirring is 200-400 rpm; the rapid stirring time is 3-6 min; and the speed of the slow stirring is 100-150 rpm.

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

[0018] Preferably, the preparation method of the inoculated sludge is mixing concentrated sludge, sodium 2-bromoethyl sulfonate and an electron donor, and performing anaerobic fermentation to obtain the inoculated sludge.

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

[0020] Preferably, the temperature of the anaerobic fermentation is 32-38℃; the time of the anaerobic fermentation is 8-12 days; the anaerobic fermentation further comprises stirring; the stirring speed is 100-150 rpm.

[0021] According to the technical solution, compared with the prior art, the present application has the following beneficial effects:

[0022] 1) The present application utilizes the strong oxidizing property and superimposed alkaline effect of the alkaline biochar-potassium ferrate coupling system to efficiently break down sludge extracellular polymeric substances and cell walls in the pretreatment stage, drive granular organic matter dissolution, and effectively inhibit methanogen activity; at the same time, Fe2O3 derived from potassium ferrate and biochar form an iron-carbon composite in situ through physical and chemical interactions, which significantly promotes the organic matter acidification process and carbon chain extension reaction (short-chain fatty acid upgrading to medium-chain fatty acid) in the fermentation stage by strengthening electron transfer efficiency and enzyme activity, forming a new composite technology with triple functions of “pretreatment-electronic regulation-methane inhibition”, which strengthens the simultaneous recovery of medium-chain fatty acid and hydrogen during sludge anaerobic fermentation, reduces sludge treatment cost, realizes the simultaneous recovery of gas and liquid products, and improves sludge treatment efficiency.

[0023] 2) The synergistic effect of alkaline biochar and potassium ferrate in the present application reduces the potassium ferrate dosage by 60-80% (from the conventional 0.25-0.5 g / g TSS to 0.1-0.2 g / g TSS), and the dosage of biochar is also reduced by 60-85% (the conventional dosage is 20 g / L), while the BES inhibitor is zero-dosed. Moreover, alkaline biochar is derived from waste biomass, which is more economical than materials such as citrus peels (orange peels, banana peels) and livestock and poultry manure, and realizes “waste treatment with waste”.

[0024] 3) The content of n-hexanoic acid in the fermentation product obtained by the present application can reach 10495 mg COD / L, and the hydrogen production can reach 284 mL / L of sludge. The yield of medium-chain fatty acids is increased by more than 200% compared with single technology, and the hydrogen production is increased by more than 250% compared with single technology. BRIEF DESCRIPTION OF DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description.

[0026] Figure 1 Figure 1 is a schematic diagram of the process flow for the simultaneous production of medium-chain fatty acids and hydrogen by enhanced sludge anaerobic fermentation according to Embodiment 1.

[0027] Figure 2 Figure 2 shows the yield of medium-chain fatty acids in the anaerobic fermentation stage as a function of fermentation time for Embodiment 1 and Comparative Examples 1-3.

[0028] Figure 3 Figure 3 shows the hydrogen production rate in the anaerobic fermentation stage as a function of fermentation time for Embodiment 1 and Comparative Examples 1-3.

[0029] Figure 4 Figure 4 shows the SCOD content in the supernatant of the pretreated sludge as a function of reaction time in the pretreatment stage for Embodiment 1 and Comparative Examples 1-3.

[0030] Figure 5 Figure 5 shows the three-dimensional fluorescence spectrum (EEM) of the supernatant of the pretreated sludge as a function of reaction time in the pretreatment stage for Embodiment 1 and Comparative Examples 1-3.

[0031] wherein Control represents the blank group, AlkBC represents the alkali biochar group, Ferrate represents the potassium ferrate group, and AlkBC-Ferrate represents the alkali biochar-potassium ferrate group.

[0032] Figure 6 Figure 7 is a mechanism verification for Embodiment 1: under the condition that ethanol is the electron donor and acetic acid is the electron acceptor, the effect of different iron-carbon forms on the production of medium-chain fatty acids in the carbon chain extension step.

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

[0034] The present application provides a method for the simultaneous production of medium-chain fatty acids and hydrogen by enhanced sludge anaerobic fermentation, comprising the following steps:

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

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

[0037] Mixing the conditioning sludge, electron donor and inoculation sludge for anaerobic fermentation to obtain a fermentation liquor rich in medium-chain fatty acids and hydrogen.

[0038] In the present application, the sludge is preferably residual sludge of a secondary sedimentation tank of a municipal wastewater treatment plant and / or primary sedimentation tank sludge of a municipal wastewater treatment plant, and is further preferably residual sludge of a secondary sedimentation tank of a municipal wastewater treatment plant.

[0039] In the present application, the content of total suspended solids in the concentrated sludge is preferably 24-30 g / L, further preferably 25-28 g / L, and more preferably 25.29 g / L.

[0040] In the present application, 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, and is further preferably fruit peel, and more preferably banana peel; the temperature of high-temperature carbonization is preferably 550-700℃, further preferably 550-650℃, and more preferably 600℃; the time of high-temperature carbonization is preferably 1-3 h, further preferably 1-2 h, and more preferably 2 h.

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

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

[0043] In the present application, the temperature of mixing is preferably 18-28℃, further preferably 20-26℃, and more preferably 25℃; the time of mixing is preferably 3-6 min, further preferably 4-6 min, and more preferably 5 min; the mixing process further includes stirring; the stirring speed is preferably 200-400 rpm, further preferably 250-350 rpm, and more preferably 300 rpm.

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

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

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

[0047] In the present application, the preparation method of the inoculated sludge is as follows: concentrated sludge, sodium 2-bromoethyl sulfonate and electron donor are mixed, anaerobic fermentation is carried out, and inoculated sludge is obtained; the ratio of the use amount of the concentrated sludge, sodium 2-bromoethyl sulfonate and electron donor is preferably 1L:0.05mol:0.12-0.16mol, further preferably 1L:0.05mol:0.15-0.16mol, more preferably 1L:0.05mol:0.16mol; the content of the total suspended solids of the concentrated sludge is preferably 24-30g / L, further preferably 25-28g / L, more preferably 25.29g / L; the electron donor is preferably ethanol; the temperature of the anaerobic fermentation is preferably 32-38℃, further preferably 34-36℃, more preferably 35℃; the time of the anaerobic fermentation is preferably 10-15 days, further preferably 11-13 days, more preferably 12 days; the process of the anaerobic fermentation also includes stirring; the speed of the stirring is preferably 100-150rpm, further preferably 110-140rpm, more preferably 120rpm.

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

[0049] In the present application, the temperature of the anaerobic fermentation is preferably 32-38℃, further preferably 34-36℃, and more preferably 35℃; the time of the anaerobic fermentation is preferably 8-12 days, further preferably 10-12 days, and more preferably 12 days; the anaerobic fermentation further comprises stirring; the stirring speed is preferably 100-150rpm, further preferably 110-140rpm, and more preferably 120rpm.

[0050] The technical solutions in the embodiments of the present application will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all the other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0051] Embodiment 1

[0052] The present embodiment provides a method for producing medium-chain fatty acids and hydrogen simultaneously by strengthening anaerobic fermentation of sludge, and a process flow diagram is shown in Figure 1 The method comprises the following steps:

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

[0054] (2) Alkaline biochar (obtained by carbonizing banana peels at 600℃ for 2h under nitrogen atmosphere) is added to the concentrated sludge, and the mass concentration after addition is 5g / L. Then, the concentrated sludge is stirred at 300rpm for 5min at 25℃ to fully mix the concentrated sludge with the alkaline biochar. At this time, the pH of the sludge containing the alkaline biochar is 8.0. Then, potassium ferrate is added to the sludge containing the alkaline biochar, and the dosage of the potassium ferrate is 0.15g / g of total suspended solid. The potassium ferrate is fully contacted with the sludge by stirring at 300rpm for 5min at 25℃. Then, the reaction is carried out at 35℃, and the total reaction time is 48h, and the stirring speed is 120rpm. After the reaction, pretreated sludge is obtained. The pH of the pretreated sludge is 9.8 after 30min of reaction. The pH of the pretreated sludge is adjusted to 6.5 to obtain conditioned sludge.

[0055] (3) the conditioned sludge is transferred to the anaerobic fermentation tank as a fermentation substrate, inoculated sludge is added according to a volume ratio of 1:9 of the inoculated sludge to the conditioned sludge, the total volume of the inoculated sludge and the conditioned sludge to the headspace volume of the anaerobic fermentation tank is 1.2:1, nitrogen is filled into the anaerobic fermentation tank for 5 min to ensure an anaerobic environment; thereafter, ethanol is rapidly added to the anaerobic fermentation tank as an electron donor for carbon chain extension, the dosage is calculated according to 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 uniformly distributed in the sludge through rapid stirring; the anaerobic fermentation tank is sealed, the temperature is controlled at 35℃, the stirring intensity is controlled at 120 rpm, and anaerobic fermentation is performed for 12 days, thereby completing the simultaneous production of medium-chain fatty acids and hydrogen through the enhanced sludge anaerobic fermentation.

[0056] The preparation method of the inoculated sludge in step (3) is as follows: sodium 2-bromoethyl sulfonate is added to the concentrated sludge in step (1) so that the content of sodium 2-bromoethyl sulfonate in the concentrated sludge is 0.05 mol / L, then nitrogen is filled into the concentrated sludge for 5 min to ensure an anaerobic environment; thereafter, ethanol is added as an electron donor for carbon chain extension, the dosage is calculated according to the volume of the concentrated sludge, so that the concentration of ethanol reaches 0.16 mol / L, the temperature is controlled at 35℃, the stirring intensity is controlled at 120 rpm, and anaerobic fermentation is performed for 12 days to obtain the inoculated sludge.

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

[0058] Comparative Example 1

[0059] This comparative example provides a method for simultaneously producing medium-chain fatty acids and hydrogen through sludge anaerobic fermentation, which is specifically described in Example 1, except that step (2) is as follows: the concentrated sludge is reacted at 35℃ for 48 h at a stirring speed of 120 rpm, and the pretreated sludge is obtained after the reaction; the pH of the pretreated sludge is adjusted to 6.5 to obtain the conditioned sludge.

[0060] During the anaerobic fermentation process, the concentration of medium-chain fatty acids and the hydrogen production in the fermentation system are detected every 2 days using gas chromatography, and the results are shown in Figure 2 and Figure 3 It can be seen from Figure 2 andFigure 3 As can be seen, when no substance is added to treat the sludge, the remaining sludge fermentation produces only 486 mg COD / L of medium-chain fatty acids (mainly n-hexanoic acid), and the hydrogen production is only 28 mL / L of sludge.

[0061] Comparative Example 2

[0062] This comparative example provides a method for simultaneous production of medium-chain fatty acids and hydrogen by anaerobic fermentation of sludge, which specifically refers to Example 1, except that step (2) is: adding alkaline biochar (banana peel is carbonized at 600°C for 2h under nitrogen atmosphere) to the concentrated sludge, the mass concentration after adding is 5g / L, then stirring at 300rpm for 5min at 25°C to make it fully mixed with the concentrated sludge, at this time the pH of the sludge containing alkaline biochar is 8.0; then, the sludge containing alkaline biochar is reacted at 35°C for 48h, the rotation speed is 120rpm, after the reaction is completed, the pretreated sludge is obtained; the pH of the pretreated sludge is adjusted to 6.5 to obtain the conditioned sludge.

[0063] During the anaerobic fermentation process, the concentration of medium-chain fatty acids and the hydrogen production in the fermentation system are detected every 2 days using gas chromatography, and the results are shown in Figure 2 and Figure 3 As can be seen from Figure 2 and Figure 3 When the alkaline biochar is added alone to pretreat the sludge, the remaining sludge fermentation produces only 648 mg COD / L of medium-chain fatty acids (mainly n-hexanoic acid), and the hydrogen production is only 21 mL / L of sludge.

[0064] Comparative Example 3

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

[0066] During the anaerobic fermentation process, the concentration of medium-chain fatty acids and the hydrogen production in the fermentation system are detected every 2 days using gas chromatography, and the results are shown in Figure 2 and Figure 3 As can be seen from Figure 2 and Figure 3It can be seen that when potassium ferrate is added alone for sludge pretreatment, 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 of sludge.

[0067] In the present application, the technical principle is as follows:

[0068] The organic matter in the sludge is difficult to be effectively hydrolyzed and acidified because it is densely wrapped in extracellular polymeric substances (EPS) and microbial cell walls, which in turn causes a lack of supply of electron acceptors required for the synthesis of medium-chain fatty acids. In the pretreatment stage (the pretreatment stage refers to the processes of steps (1) and (2) of Example 1 and Comparative Examples 1-3), the reaction of activating potassium ferrate by alkaline biochar drives the conversion of Fe(VI) into high-activity high-valence 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. Further, the strong oxidizing property and sterilization effect of the alkaline biochar-potassium ferrate effectively break the EPS and cell wall barriers, increase the number of free organic molecules and the initial short-chain fatty acid yield, and provide a large amount of substrates that can be directly utilized for the subsequent production of medium-chain fatty acids by anaerobic fermentation, such as Figure 4 、 Figure 5 and 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 sludge particle organic matter in the pretreated sludge under different conditions in the pretreatment stage is described, and it can be found that the SCOD content of the alkaline biochar-potassium ferrate group is significantly higher than that of the blank group, the alkaline biochar group alone, and the potassium ferrate group alone, indicating that the coupling system has more dissolved carbon that can be directly utilized by hydrolysis and fermentation bacteria. As shown in Figure 5 , the maximum fluorescence intensity value of the EEM spectrum of the supernatant of the coupling system is lower than or comparable to that of the potassium ferrate group alone, which is obviously different from the SCOD trend. This directly indicates that the alkaline biochar-potassium ferrate severely damages the fluorescence properties of the sludge organic molecules, causing fluorescence quenching, revealing that the alkaline biochar-potassium ferrate can directly destroy the complex structure of macromolecular organic matter through chemical oxidation, thereby proving that the alkaline biochar enhances the oxidation ability and efficiency of potassium ferrate on sludge organic matter. Therefore, after pretreatment, the concentration of short-chain fatty acids that can be directly utilized for carbon chain extension reaction in the alkaline biochar-potassium ferrate system is significantly higher than that in the blank and single treatment groups (as shown in Table 1).

[0069] Methanogens are ubiquitous competitive microorganisms in sludge fermentation coupled with carbon chain extension system, and usually need to add methanogen inhibitors or use inactivation technology to achieve its removal or activity inhibition. 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 the methane production to a certain extent, and the alkaline biochar coupled with potassium ferrate reduced the methane production to 0 (as shown in Table 1). This means that alkaline biochar-potassium ferrate selectively inactivates methanogens, while retaining key functional bacteria related to hydrolysis, acidification, and carbon chain extension, thereby eliminating the inhibitory effect of competitive metabolism on carbon chain extension in the subsequent fermentation stage, avoiding the large addition of methanogen inhibitors, and promoting 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 of potassium ferrate and alkaline biochar effectively promotes the acidification and carbon chain extension of organic matter 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 performance, electron transfer capacity, and combination with Fe-O, while the in-situ generated Fe2O3 from 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 coexisting iron-carbon substrate formed in the pretreatment stage can serve as a "growth medium" and "electron shuttle" for microorganisms, stimulating extracellular hydrolytic enzyme secretion, promoting intracellular key functional gene expression, and strengthening electron transfer to regulate microbial metabolism (such as acidification and carbon chain extension) in the sludge fermentation system, thereby improving the conversion efficiency of substrates to medium-chain fatty acids. The maximum EEM fluorescence intensity value of the potassium ferrate and alkaline biochar coupling system after 48 h of pretreatment is lower than that of potassium ferrate alone, which can also be used to illustrate that the presence of 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 effects of different iron-carbon existing forms on the core biological step of carbon chain extension under the condition of ethanol as electron donor and acetic acid as electron acceptor were demonstrated, including 6 groups, namely blank group, biochar group, high ferrate reduction particle group, high ferrate reduction particle-biochar complex group, Fe(OH)3 group and biochar-Fe(OH)3 complex group. It is worth noting that the main iron form of high ferrate reduction product is Fe2O3 and Fe(OH)3. The specific anaerobic fermentation method is as follows: (1) high ferrate reduction particle-biochar complex group: the pretreatment process is specifically referred to in step (2) of embodiment 1, the difference is that the concentrated sludge in embodiment 1 is replaced with water, and 6 mol / L HCl is added several times during the reaction process after the addition of potassium ferrate to make the pH close to that in step (2) of embodiment 1, thereby obtaining a pretreatment liquid containing high ferrate reduction particle-biochar complex; the fermentation process is specifically referred to in step (3) of embodiment 1, the difference is that the conditioned sludge is replaced with synthetic water (synthetic water specifically refers to adding anhydrous sodium acetate, ethanol and BES to the pretreatment liquid containing high ferrate reduction particle-biochar complex to make the concentrations reach 4 g / L, 0.16 mol / L and 0.05 mol / L respectively, and the ethanol in the synthetic water is the ethanol in embodiment 1), and the fermentation conditions are adjusted to 35°C, 120 rpm anaerobic fermentation for 9 days. (2) High ferrate reduction particle group: specifically refer to high ferrate reduction particle-biochar complex group, the difference is that no alkaline biochar is added. (3) Blank group: specifically refer to high ferrate reduction particle-biochar complex group, the difference is that no alkaline biochar and potassium ferrate are added. (4) Biochar group: specifically refer to high ferrate reduction particle-biochar complex group, the difference is that no potassium ferrate is added. (5) Biochar-Fe(OH)3 complex group: specifically refer to high ferrate reduction particle-biochar complex group, the difference is that potassium ferrate is replaced by ferric salt. (6) Fe(OH)3 group: specifically refer to biochar-Fe(OH)3 complex group, the difference is that no alkaline biochar is added. Figure 6 It can be known that after 9 days of anaerobic fermentation, the concentration of medium chain fatty acids in the blank group is 4858 mg COD / L, and the presence of biochar, high ferrate reduction particles and high ferrate reduction particle-biochar complex significantly increases the concentration of medium chain fatty acids to 7853, 9090 and 10200 mg COD / L respectively. When Fe(OH)3 represents high ferrate reduction particles, the yield of medium chain fatty acids decreases, which proves that Fe2O3 is the key active iron component of biochar surface for promoting electron transfer and strengthening chain extension process, and clearly illustrates the important role of biochar-Fe2O3 complex formed in the pretreatment stage for subsequent anaerobic fermentation. This method provides an efficient idea for sludge treatment and resource utilization, and realizes sludge reduction and carbon recovery.

[0072] Table 1 Short chain fatty acid content of the pre-treatment stage, cumulative methane production of the anaerobic fermentation stage

[0073]

[0074]

[0075] The above only is the preferred embodiment of the present application, it should be pointed out that, for the ordinary skilled in the art, without departing from the principles of the present application, can also make a number of improvements and refinements, these improvements and refinements should also be considered as the protection scope of the present application.

Claims

1. A method for simultaneous production of medium-chain fatty acids and hydrogen gas by enhanced sludge anaerobic fermentation, characterized in that, The method comprises the following steps: The sludge is settled to obtain concentrated sludge; The concentrated sludge and the alkaline biochar are mixed to obtain sludge containing alkaline biochar; Potassium ferrate is added to the sludge containing alkaline biochar to react, and pretreated sludge is obtained; The pH of the pretreated sludge is adjusted to 6.0-7.5 to obtain conditioned sludge; The conditioned sludge, an electron donor and inoculated sludge are mixed for anaerobic fermentation to obtain a fermentation liquor rich in medium-chain fatty acids and hydrogen; The mass concentration of the alkaline biochar in the sludge containing alkaline biochar is 3-8 g / L; the pH of the sludge containing alkaline biochar is 7.9-8.8; the mass ratio of the potassium ferrate to the total suspended solids in the concentrated sludge is 0.1-0.2:1; the reaction temperature is 25-35℃; the reaction time is 24-48 h; the anaerobic fermentation time of the mixture of the conditioned sludge, the electron donor and the inoculated sludge is 8-12 days, and the temperature is 32-38℃; the electron donor is ethanol or lactic acid; the volume ratio of the inoculated sludge to the conditioned sludge is 1:8-10; and the molar amount of the electron donor to the total volume of the inoculated sludge and the conditioned sludge is 0.12-0.16 mol:1 L. The preparation method of the inoculated sludge comprises the following steps: the concentrated sludge, sodium 2-bromoethyl sulfonate and an electron donor are mixed for anaerobic fermentation to obtain the inoculated sludge. The anaerobic fermentation time in the preparation method of the inoculated sludge is 10-15 days, and the temperature is 32-38℃.

2. The method according to claim 1, wherein the method is characterized by, The content of the total suspended solids in the concentrated sludge is 24-30 g / L.

3. The method according to claim 1, wherein the method is characterized by, The mixing temperature of the concentrated sludge and the alkaline biochar is 18-28℃, the mixing time is 3-6 min, and the mixing process further comprises stirring, and the stirring speed is 200-400 rpm.

4. The method according to claim 1, wherein the method is characterized by, The reaction process further comprises 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.

5. The method according to claim 1, wherein the method is characterized by, The anaerobic fermentation process further comprises stirring; and the stirring speed is 100-150 rpm.

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