Sludge anaerobic acidogenesis strengthening method based on pyrohydrolysis and potassium permanganate synergistic degradation of melanoids

Abstract

The invention discloses a sludge anaerobic acidogenesis strengthening method based on pyrohydrolysis and potassium permanganate synergistic degradation of melanoids. The method comprises the following steps: (1) screening sludge to remove impurities, and then carrying out gravity settling concentration; (2) pyrohydrolysis treatment: high-temperature and high-pressure treatment, pressure relief and cooling; (3) adding potassium permanganate for oxidation, and performing ultrasonic dispersion; (4) anaerobic fermentation: inoculating activated sludge, and controlling fermentation conditions; and (5) separating and extracting the short-chain fatty acid. Through the synergistic effect of pyrohydrolysis and potassium permanganate, melanoids generated in the pyrohydrolysis process are degraded in a targeted mode, the inhibition effect of melanoids on anaerobic microorganisms is eliminated, and meanwhile efficient dissolution of organic matter in sludge is promoted. Nano-manganese dioxide generated by oxidizing potassium permanganate serves as an electron transfer medium, the metabolic activity of acid-producing bacteria is further enhanced, and the acid pickling recovery rate is larger than or equal to 90%. According to the method, the yield of short-chain fatty acid is increased by 65% (as high as 456mg / g VS), high efficiency and environmental friendliness are achieved, and an innovative solution is provided for sludge resourceful treatment.

Description

Technical Field

[0001] The invention belongs to the technical field of sludge resource treatment and environmental protection, and particularly relates to a method for enhancing the acid production performance of sludge anaerobic fermentation through thermal hydrolysis combined with potassium permanganate pretreatment. Background Art

[0002] With the acceleration of urbanization and the widespread use of sewage treatment facilities, the amount of residual sludge generated by global sewage treatment plants has exceeded 100 million tons annually and continues to grow at an average annual rate of 5%. Sludge is rich in organic matter, pathogens, and heavy metals. If improperly handled, it can easily cause secondary pollution (such as greenhouse gas emissions, soil acidification, and groundwater contamination), seriously threatening the ecological environment and human health. Traditional sludge disposal technologies (such as landfill, incineration, and composting) are increasingly unable to meet the needs of sustainable development due to high costs, resource waste, and high carbon emissions. Against this backdrop, sludge resource utilization technology has become a research hotspot, among which anaerobic fermentation has attracted much attention due to its ability to convert organic matter in sludge into high-value-added products (such as short-chain fatty acids and biogas).

[0003] Short-chain fatty acids (SCFAs), as one of the main products of anaerobic fermentation, are important precursors for the synthesis of bioplastics and biofuels, and their market demand continues to grow. However, approximately 60% to 70% of the organic matter in sludge exists in the form of polymers inside microbial cells or is encapsulated in extracellular polymers (EPS). These substances are difficult to be directly decomposed by hydrolytic enzymes due to the physical barriers of semi-rigid cell walls and EPS, resulting in the hydrolysis rate of traditional anaerobic fermentation becoming the rate-limiting step of the entire process. During the direct fermentation of sludge without pretreatment, its industrial application faces significant technical bottlenecks due to the dual constraints of low SCFA yield and long fermentation cycle.

[0004] In order to break through the bottleneck of hydrolysis, physical, chemical and biological pretreatment technologies have been widely studied. Among them, thermal hydrolysis technology has become the mainstream method because it can effectively destroy the sludge floc structure and cell wall. Through high temperature (120-180℃) and high pressure (0.5-2.0MPa) treatment, the release rate of intracellular organic matter in sludge can be increased by more than 50%, and the yield of SCFAs is significantly increased. However, high temperature environment will induce amino compounds (such as proteins) in sludge to undergo Maillard reaction with reducing sugars to produce melanoidins. These substances have complex aromatic structures and antioxidant properties, are difficult to be degraded by microorganisms, and will inhibit anaerobic fermentation through the following pathways: (1) Competition for electron donors: The quinone group in melanoidins can act as an electron acceptor, competing with acid-producing bacteria for electrons, inhibiting the accumulation of volatile fatty acids; (2) Toxicity effect: Melanoidins and their degradation intermediates (such as furan compounds) are cytotoxic to acid-producing bacteria, reducing their metabolic activity; (3) Shielding effect: Melanoidins are adsorbed on the surface of sludge particles, hindering the contact between microorganisms and substrates. In addition, the thermal hydrolysis process also releases difficult-to-biodegrade organic matter such as humic acid, further exacerbating the problem of fermentation inhibition.

[0005] In response to the negative effects of melanoidins, the existing technology mainly blocks their production or enhances their degradation by adding chemical reagents. For example, patent CN113461283A proposes adding sulfite before thermal hydrolysis to inhibit the Maillard reaction. However, sulfite itself has strong reducing properties and may remain in the sludge to form sulfide, which not only poisons acid-producing bacteria, but also causes the emission of malodorous gases such as H2S, posing a risk of secondary pollution. Another solution (patent CN117985914A) uses ferrous ions to activate the oxidative degradation of melanoidins, but it has poor targeting to complex melanoidin molecules, insufficient degradation efficiency, and ferrous ions are easily oxidized to Fe 3+ Other methods, such as ozone oxidation and Fenton's reagent, can degrade melanoidins, but they are expensive or produce harmful byproducts, making them difficult to apply on a large scale.

[0006] At the same time, potassium permanganate (KMnO4) as a strong oxidant has shown unique potential in sludge pretreatment. Its oxidizing ability comes from MnO4 -The efficient electron capture property of potassium permanganate can selectively attack organic matter rich in electron-donating groups (such as phenolic hydroxyl groups and double bonds). Since the above groups are widely present in melanoidin molecules, potassium permanganate can theoretically target the degradation of melanoidin, while destroying the EPS structure and promoting the dissolution of organic matter. Studies have also shown that manganese dioxide (MnO2) particles generated by potassium permanganate oxidation can be adsorbed on the surface of acid-producing bacteria, acting as an electron transfer medium to accelerate extracellular electron transfer, thereby improving acid production efficiency. However, there are two major defects in single potassium permanganate pretreatment: one is excessive addition of oxidant: in order to achieve effective degradation, 0.2-0.3g / gTS of potassium permanganate needs to be added, which significantly increases the treatment cost; the second is that the reaction conditions are limited: oxidation alone is difficult to completely break down the sludge cell wall, and the efficiency of organic matter release is limited.

[0007] Therefore, how to achieve efficient melanoidin degradation and maximize organic matter release through a combination of technologies while avoiding secondary pollution has become a key issue that needs to be addressed in this field. This invention combines thermal hydrolysis with potassium permanganate oxidation to fully utilize their synergistic effects: thermal hydrolysis preferentially destroys the sludge's physical barrier and releases organic matter, while potassium permanganate targets and oxidizes the subsequently generated melanoidins. This ultimately overcomes the dual bottlenecks of existing technologies and provides an efficient and environmentally friendly solution for sludge resource utilization. Summary of the Invention

[0008] The present invention provides a method for enhancing anaerobic acid production from sludge based on the synergistic degradation of melanoidins by thermal hydrolysis and potassium permanganate, which solves the problems of low efficiency of sludge organic matter release and melanoidin inhibition through staged pretreatment. The method first uses high-temperature and high-pressure thermal hydrolysis to destroy the physical barrier of the sludge and release intracellular organic matter; then, the melanoidins generated during the thermal hydrolysis process are selectively oxidized by potassium permanganate to eliminate their toxic effects on microorganisms; at the same time, the oxidation byproduct manganese dioxide is used as an electron transfer medium to accelerate the metabolism of acid-producing bacteria, and ultimately achieve efficient generation of short-chain fatty acids. The core of the present invention lies in the synergistic mechanism of thermal hydrolysis and potassium permanganate, which not only breaks through the limitations of a single technology, but also reduces processing costs by resource-based utilization of by-products.

[0009] The specific implementation steps of the present invention are:

[0010] 1. Sludge concentration and pretreatment

[0011] After filtering the raw sludge through a 20-25 mesh screen to remove large particles, it is then gravity-sedimented for 24-48 hours at a low temperature of 3-5°C. The concentrated sludge reaches a total solids concentration of 15-40g / L. Low temperatures inhibit microbial activity, preventing premature degradation of organic matter and ensuring efficient subsequent treatment.

[0012] 2. Thermal hydrolysis co-pretreatment

[0013] The concentrated sludge is transferred to a sealed thermal hydrolysis reactor and treated at 120-160°C and 0.5-1.5 MPa for 15-30 minutes. The high temperature and pressure rupture the sludge cell walls and disintegrate the EPS structure, releasing approximately 60% of the intracellular organic matter (such as proteins and polysaccharides). After depressurization, the reactor is rapidly cooled to 45-60°C to prevent the Maillard reaction from continuing.

[0014] 3. Potassium permanganate targeted oxidation

[0015] Powdered potassium permanganate (0.05-0.1g / g TS) is added to the thermally hydrolyzed sludge, stirred for 20-40 minutes, and simultaneously subjected to ultrasonic treatment (energy density 0.1-0.5W / mL, duration 10-30 minutes). The ultrasonic cavitation effect promotes contact between potassium permanganate and melanoidins, and its selective oxidation mechanism preferentially attacks electron-donating groups such as phenolic hydroxyl groups and double bonds in the melanoidin molecules, achieving a degradation efficiency exceeding 60%. The resulting manganese dioxide (MnO2) is ultrasonically dispersed into 50-100nm particles, which evenly adhere to the surface of the acid-producing bacteria.

[0016] 4.Anaerobic fermentation to enhance acid production

[0017] Mix the oxidized sludge with anaerobic activated sludge (from sewage treatment plants or breweries) at a volatile suspended solids (VSS) ratio of 1:2 to 1:10, with an inoculation concentration of 8 to 10 kg VSS / m 3 After nitrogen is passed through the container to remove oxygen, seal the container. Ferment at 25-40°C with stirring (80-120 rpm) for 6-10 days. Manganese dioxide acts as an electron acceptor, accelerating extracellular electron transfer in acid-producing bacteria and promoting the accumulation of SCFAs such as acetate and propionate.

[0018] 5. Product separation and by-product recovery

[0019] After fermentation, the supernatant is centrifuged and short-chain fatty acids are extracted (total proportion ≥ 75%). The remaining manganese dioxide particles are recovered by pickling with 0.5-1.0 mol / L dilute sulfuric acid or hydrochloric acid, with a recovery rate of ≥ 90%, to avoid heavy metal residues.

[0020] The innovation and synergy mechanism of the present invention are:

[0021] First, thermal hydrolysis and oxidation work synergistically. Thermal hydrolysis prioritizes the destruction of the sludge's physical structure, releasing organic matter and inducing melanin production. Potassium permanganate then targets and degrades melanin, forming a synergistic "wall-breaking-detoxification" chain and avoiding the problem of excessive oxidant addition in traditional technologies.

[0022] Second, it selectively degrades melanoidins. Potassium permanganate precisely attacks the active groups in melanoidins through an electron transfer mechanism. The degradation products are small molecule carboxylic acids that can be directly utilized by acid-producing bacteria. The UV254 value is reduced from 2.79 to 0.87, eliminating the inhibitory effect by 70%.

[0023] Third, resource utilization of manganese dioxide. The nano manganese dioxide generated by oxidation not only acts as an electron transfer medium to increase the acid production rate, but can also be recycled through acid washing to reduce treatment costs (saving about 15% of chemical costs per ton of sludge).

[0024] Compared with the existing technology, the technical effects of the present invention have significant advantages: first, the acid production efficiency is high: the SCFAs yield is as high as 456 mg / g VS, which is 65% higher than the single thermal hydrolysis treatment (276 mg / g VS); second, the fermentation cycle is short: shortened to 4 to 5 days (the traditional method requires 7 to 9 days); third, there are fewer difficult-to-degrade substances: the UV254 value is reduced by 68.7%, and the humic acid concentration is reduced by 55%; fourth, it is environmentally friendly: the manganese dioxide recovery rate is ≥90%, and there is no risk of secondary pollution.

[0025] The present invention improves the acid production efficiency of anaerobic fermentation of sludge to the industry-leading level through multi-stage coordinated treatment, while solving the problems of melanin inhibition and by-product pollution, providing an efficient, economical and environmentally friendly technical path for sludge resource utilization. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a process flow chart of the present invention. DETAILED DESCRIPTION

[0027] The present invention will be further described below with reference to specific embodiments. It should be understood that the embodiments described are only a portion of the embodiments of the present invention, rather than all of them. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are intended to fall within the scope of protection of the present invention.

[0028] In the following examples, materials and processing techniques, unless otherwise specified, are conventional commercially available materials or conventional processing techniques in the art.

[0029] Example 1 (optimal conditions)

[0030] The sludge from the secondary sedimentation tank of a sewage treatment plant in Changsha was collected. The total solids (TS) concentration was 10.5 g / L, and the volatile solids (VS) accounted for 65%. A 25-mesh sieve (pore size 0.71 mm) was used to filter to remove impurities such as sand, gravel, and fiber. The sludge was placed in a 4°C low-temperature sedimentation tank and allowed to stand for 48 hours. After concentration, the TS concentration increased to 28.4 g / L. The concentrated sludge was pumped into a stainless steel thermal hydrolysis reactor with a filling volume of 75% of the reactor volume. The temperature was raised to 160°C at a rate of 5°C / min, pressurized to 1.2 MPa, and maintained for 30 minutes. The pressure was slowly released to normal pressure and cooled to 50°C through a circulating water cooling system. 0.1 g / gTS powdered potassium permanganate (purity 99%) was added to the sludge after thermal hydrolysis, and the stirring rate was 250 rpm for 30 minutes. Ultrasound (energy density 0.25 W / mL, frequency 28 kHz) was applied simultaneously and treated for 10 minutes. After ultrasonic treatment, MnO₂ particles measuring 50 to 80 nm were generated and uniformly suspended in the sludge. The treated sludge was pumped into an anaerobic fermenter and inoculated with anaerobic activated sludge (obtained from a sewage treatment plant anaerobic fermenter, VSS = 18 g / L) at a volatile suspended solids (VSS) ratio of 1:5. Nitrogen was aerated for 15 minutes until the dissolved oxygen concentration dropped to 0.05 mg / L, after which the fermenter was sealed. Fermentation was continued at 37°C with stirring (100 rpm) for 4 days. The fermentation broth was centrifuged at 8000 rpm for 10 minutes to separate the supernatant and residue. The supernatant was decolorized with activated carbon and then distilled. The total short-chain fatty acid yield was 456 mg / g VS, with acetic acid accounting for 52%, propionic acid 28%, and butyric acid 20%. The residue was acid-washed with 1.0 mol / L dilute sulfuric acid for 30 minutes, filtered, and the manganese dioxide recovered. The melanin degradation rate was 68.7% (UV254 value dropped from 2.79 to 0.87), the fermentation period was 4 days, and no secondary pollutants were generated; the manganese dioxide recovery rate was 92%.

[0031] Example 2 (medium dosage)

[0032] Step difference: the dosage of potassium permanganate was adjusted to 0.075 g / g TS, and the other conditions were the same as those in Example 1.

[0033] Results: SCFAs production was 423 mg / g VS (acetic acid 48%, propionic acid 30%, butyric acid 22%); melanin degradation rate was 58.3% (UV254 value 1.02); and manganese dioxide recovery rate was 89%.

[0034] Example 3 (low temperature fermentation)

[0035] Step differences: the fermentation temperature was adjusted to 25°C, and the other conditions were the same as in Example 1.

[0036] Results: SCFAs production was 312 mg / g VS (fermentation period extended to 8 days); melanin degradation rate was 52.1% (UV254 value 1.35); the activity of acid-producing bacteria decreased, and the proportion of acetic acid dropped to 45%.

[0037] Comparative Example 1 (thermal hydrolysis pretreatment only)

[0038] Difference in steps: anaerobic fermentation was directly performed after thermal hydrolysis, and the other conditions were the same as those in Example 1.

[0039] Results: SCFAs production was 276 mg / g VS (fermentation period 7 days); UV254 value was 2.79 (melanoidin was not degraded); humic acid concentration reached 120 mg / L, inhibiting the metabolism of acid-producing bacteria.

[0040] Comparative Example 2 (without pretreatment)

[0041] Difference in steps: Anaerobic fermentation is performed directly on concentrated sludge.

[0042] Results: SCFAs production was 112 mg / g VS (fermentation period 9 days); UV254 value was 0.61 (low melanoidin content, but insufficient organic matter release rate); intracellular organic matter release rate was only 18%, and acid production efficiency was extremely low.

[0043] Comparative Example 3 (Single Potassium Permanganate Treatment)

[0044] Step difference: 0.1 g / g TS potassium permanganate was directly added to the concentrated sludge. Other conditions were the same as those in Example 1.

[0045] Results: SCFAs production was 198 mg / g VS (fermentation period: 6 days); melanin degradation rate was 42.5% (UV254 value: 1.60); sludge cell walls were not fully destroyed, and the organic matter release rate was only 35%.

[0046] The comparative analysis of the effects of each embodiment is shown in Table 1:

[0047] Table 1 Comparison of effects of various embodiments

[0048]

[0049] The anaerobic fermentation performance of sludge treated with thermal hydrolysis and potassium permanganate (Example 1) was significantly better than that of the single pretreatment or no pretreatment group. Example 1 achieved SCFAs production of 456 mg / g VS under optimal conditions (thermal hydrolysis 160 ° C, 1.2 MPa, potassium permanganate 0.1 g / g TS, fermentation 37 ° C), which was 65% and 130% higher than that of thermal hydrolysis alone (Comparative Example 1) and single potassium permanganate treatment (Comparative Example 3), respectively. The fermentation cycle was shortened to 4 days, the melanin degradation rate reached 68.7%, and the manganese dioxide recovery rate was 92%. After reducing the potassium permanganate dosage (0.075 g / g TS) in Example 2, the SCFAs production still reached 423 mg / g VS and the melanin degradation rate was 58.3%, showing the flexibility of the dosage of the agent; while low-temperature fermentation (Example 3, 25 ° C) caused the acid production efficiency to drop to 312 mg / g VS, and the cycle was extended to 8 days, indicating that temperature is crucial to the activity of the bacterial community. In summary, the synergistic effect of thermal hydrolysis and potassium permanganate, through the "wall breaking-detoxification-efficiency enhancement" mechanism, takes into account efficient acid production, rapid degradation of inhibitors and resource utilization of by-products, providing a more technically and economically superior solution for sludge resource utilization.

Claims

1. A method for enhancing anaerobic acid production from sludge based on the synergistic degradation of melanoidins by thermal hydrolysis and potassium permanganate, characterized in that: The following steps are involved: (1) Impurity removal and concentration: The raw sludge is filtered through a 20-25 mesh sieve to remove impurities, and gravity sedimented at 3-5°C for 24-48 hours to obtain concentrated sludge with a total solids concentration of 15-40 g / L; (2) Thermal hydrolysis: Place the concentrated sludge in a thermal hydrolysis reactor and treat it at 120-160°C and 0.5-1.5 MPa for 15-30 minutes, then release the pressure and cool it to 45-60°C. (3) Targeted oxidation: adding powdered potassium permanganate to the sludge treated in step (2) at a dosage of 0.05 to 0.1 g / g TS, stirring for 20 to 40 minutes, and simultaneously applying ultrasonic treatment at an ultrasonic energy density of 0.1 to 0.5 W / mL for 10 to 30 minutes; (4) Anaerobic fermentation: The oxidized sludge is mixed with anaerobic activated sludge at a volatile suspended solids ratio of 1:2 to 1:10, and the inoculation concentration is 8 to 10 kg VSS / m 3 , seal with nitrogen and ferment at 25-40℃ for 6-10 days; (5) Product separation: Separate the fermentation broth supernatant and extract short-chain fatty acids.

2. The method according to claim 1, characterized in that The oxidation reaction of potassium permanganate in step (3) targets and degrades melanoidins produced during the thermal hydrolysis process, and its selective oxidation mechanism is based on the electron-donating groups rich in melanoidin molecules.

3. The method according to claim 1 or 2, characterized in that The manganese dioxide generated by the potassium permanganate oxidation in step (3) is dispersed into nanoparticles by ultrasound, attached to the surface of the acid-producing microorganism as an electron transfer medium, and recovered by acid washing after the fermentation is completed, with a recovery rate of ≥90%.

4. The method according to claim 1, wherein The total solid concentration of the sludge after gravity sedimentation in step (1) is 15 to 40 g / L.

5. The method according to claim 1, wherein The temperature of the thermal hydrolysis treatment in step (2) is 120-160° C., the pressure is 0.5-1.5 MPa, and the treatment time is 15-30 minutes.

6. The method according to claim 1, characterized in that The dosage of potassium permanganate in step (3) is 0.05-0.1 g / g TS, and the ultrasonic energy density is 0.1-0.5 W / mL.

7. The method according to claim 1, characterized in that The anaerobic activated sludge in step (4) is taken from the anaerobic digester of a sewage treatment plant or the fermentation tank of a brewery, and the stirring rate during the fermentation process is 80 to 120 revolutions per minute.

8. The method according to claim 1, characterized in that The fermentation temperature in step (4) is 25-40° C., and the sludge retention time is 6-10 days.

9. The method according to claim 1, characterized in that The short-chain fatty acids in step (5) include acetic acid, propionic acid and butyric acid, and their total proportion is ≥75%.

10. The method according to claim 3, characterized in that The acid used for pickling recovery is dilute sulfuric acid or hydrochloric acid with a concentration of 0.5-1.0 mol / L.

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