A method for enhancing anaerobic digestion of sludge by sulfate radical pre-treatment

Abstract

This invention belongs to the technical field of sludge anaerobic treatment agents, specifically relating to a method for enhancing anaerobic digestion of sludge through sulfate free radical pretreatment. The method includes the following steps: (1) adding compound bacteria to the sludge to be treated and fermenting it under anaerobic conditions to obtain fermented sludge; (2) adding activated carbon loaded with zero-valent iron to the fermented sludge; (3) continuing to add sodium persulfate to the fermented sludge; shaking to obtain pretreated sludge; (4) adding the pretreated sludge and inoculated sludge to an anaerobic digestion reactor and performing anaerobic digestion and gas production under mesophilic conditions. The method of this invention significantly increases the total methane production compared to the original sludge, reduces sludge CST, significantly improves organic matter leaching and anaerobic digestion efficiency, and also improves dewatering performance, providing convenience for subsequent sludge treatment.

Description

Technical Field

[0001] This invention belongs to the technical field of sludge anaerobic treatment agents, specifically relating to a method for enhancing anaerobic digestion of sludge through sulfate free radical pretreatment. Background Technology

[0002] During anaerobic digestion of sludge, the rate-limiting hydrolysis stage results in low overall gas production efficiency. Therefore, sludge pretreatment is necessary to break down sludge flocs and cell walls, allowing organic matter to dissolve, accelerating hydrolysis and subsequent stages, and thus improving anaerobic digestion efficiency. However, existing pretreatment methods have certain limitations.

[0003] Urban sludge is a byproduct of wastewater treatment. Besides organic matter and water, sludge contains a large number of pathogens, parasite eggs, heavy metals, and other toxic and harmful substances, and is accompanied by foul odors. Improper disposal can cause secondary environmental pollution. With the increasing sludge production in my country, how to economically, environmentally, and sustainably dispose of sludge has become an urgent problem to solve. Anaerobic digestion can reduce and render sludge harmless, and also produce biogas for resource recovery, showing broad application prospects. The anaerobic digestion process is generally divided into four stages: hydrolysis, acidification, acetylation, and methanogenesis. The hydrolysis stage is the rate-limiting stage of anaerobic digestion, restricting the efficiency of sludge anaerobic digestion; therefore, pretreatment technologies are needed to accelerate hydrolysis.

[0004] Common pretreatment technologies include alkaline pretreatment, ozone pretreatment, Fenton's reagent (ferrous ions plus hydrogen peroxide) oxidation pretreatment, ultrasonic pretreatment, heating, and microwave pretreatment. However, these pretreatment technologies all have certain limitations. For example, alkaline pretreatment causes the sludge pH to deviate from the suitable pH range for anaerobic digestion, requiring readjustment; ozone and Fenton's reagent pretreatment have certain stability and safety concerns; and ultrasonic, heating, and microwave pretreatment are relatively expensive.

[0005] Therefore, the purpose of this invention is to provide a novel pretreatment method: sulfate radical oxidation technology, thereby improving the efficiency of anaerobic digestion of sludge and gas production. Summary of the Invention

[0006] The purpose of this invention is to provide a method for enhancing anaerobic digestion of sludge through sulfate radical pretreatment, so as to improve the efficiency of anaerobic digestion and methanogenesis, while also improving sludge dewatering performance.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A method for enhancing anaerobic digestion of sludge through sulfate radical pretreatment includes the following steps: (1) Add compound bacteria to the sludge to be treated and ferment it under anaerobic conditions to obtain fermented sludge; (2) Add activated carbon loaded with zero-valent iron to the fermentation sludge; (3) Add sodium persulfate to the fermented sludge; shake to obtain pretreated sludge; (4) Add the pretreated sludge and the inoculated sludge into the anaerobic digestion reactor and carry out anaerobic digestion and gas production under mesophilic conditions.

[0008] Preferably, the complex microorganisms include Clostridium butyricum, Streptococcus thermophilus, and Saccharomyces cerevisiae.

[0009] Preferably, the dosage of the compound bacteria in the sludge to be treated is (1-3)×10⁻⁶. 8 CFU / mL.

[0010] Preferably, the ratio of viable counts of Clostridium butyricum, Streptococcus thermophilus, and Saccharomyces cerevisiae in the sludge to be treated is (1.3-1.5):1:(0.4-0.6).

[0011] Preferably, the compound bacteria are fermented under anaerobic conditions at 30-35℃ for 36-48 hours.

[0012] This invention adds a fermentation treatment before zero-valent iron (ZV) activation of persulfate. The invention uses a compound of *Clostridium butyricum*, *Streptococcus thermophilus*, and *Saccharomyces cerevisiae* at a live cell ratio of (1.3-1.5):1:(0.4-0.6). By limiting the dosage and using anaerobic fermentation conditions, the pH of the sludge can naturally decrease to a reasonable value, creating optimal conditions for ZV activating persulfate without the need for additional acid adjustment. Analysis shows that *Clostridium butyricum* secretes extracellular hydrolytic enzymes to break down large organic molecules into soluble small molecules, and the fermentation produces short-chain fatty acids, causing the pH to decrease. The low pH environment also promotes the dissolution of organic matter. After this pre-fermentation, the subsequent sulfate free radical oxidation is more efficient in destroying sludge flocs and cell walls.

[0013] Preferably, the method for preparing zero-valent iron supported on activated carbon includes the following steps: S1: Clean the activated carbon, soak it in dilute hydrochloric acid, wash it until neutral, and dry it to obtain pretreated activated carbon. S2: Dissolve FeSO4·7H2O in water to prepare a solution; add pretreated activated carbon to the solution and stir under nitrogen protection to obtain a mixed solution; S3: Add NaBH4 aqueous solution dropwise to the mixture while stirring; continue stirring after the addition is complete; S4: After the reaction is complete, the solid is separated by a magnet. The solid is washed with water and anhydrous ethanol in sequence and dried to obtain activated carbon loaded with zero valent iron.

[0014] Preferably, coconut shell charcoal and wood charcoal are mixed in a mass ratio of (2-3):1 to obtain activated carbon.

[0015] Existing activated carbon-supported zero-valent iron (ZFe) methods often use activated carbon from a single source, making it difficult to achieve both high loading rates and rapid diffusion. This invention uses a mixture of coconut shell charcoal and wood charcoal at a mass ratio of (2-3):1 as a carrier to prepare activated carbon-supported ZFe via liquid-phase reduction. Coconut shell charcoal provides a high specific surface area and microporous structure, uniformly anchoring ZFe particles and preventing agglomeration; wood charcoal provides abundant mesopores, offering rapid diffusion channels for persulfate and organic matter. The combination of these two materials forms a multi-level microporous-mesoporous channel structure, significantly improving the utilization rate of ZFe active sites, enhancing persulfate activation efficiency, and increasing the rate of sulfate free radical generation. Simultaneously, the compounded activated carbon remaining in the anaerobic digestion system can also act as a conductive medium, promoting direct interspecies electron transfer between acid-producing and methanogenic bacteria, further enhancing methanogenesis efficiency.

[0016] Preferably, the concentration of zero-valent iron in activated carbon-supported zero-valent iron in the fermentation sludge is 10-50 mmol / L.

[0017] Preferably, the concentration of sodium persulfate in the fermentation sludge is 10-50 mmol / L.

[0018] Preferably, in the fermentation sludge, the molar ratio of zero-valent iron to sodium persulfate is (0.25-2):1.

[0019] sulfate free radicals Advanced oxidation technology is an emerging oxidation technology. Sulfate radicals have strong oxidizing properties and can directly destroy sludge flocs and cell walls, causing organic matter to dissolve and thus accelerating hydrolysis and subsequent stages. Sulfate radicals can be generated by gamma rays, heating, ultraviolet light, or by transition metal activation of persulfate. Among these activation methods, transition metal activation is the most stable and economical. The equation for zero-valent iron activation of persulfate is as follows: ; .

[0020] Compared to other oxidation techniques (such as Fenton's reagent), sulfate radicals are more effective than hydroxyl radicals (OH radicals). It has stronger oxidizing properties; persulfate is more stable and easier to store; the reaction between zero-valent iron and persulfate can be carried out at room temperature and neutral pH, rather than under acidic conditions. When the molar ratio of zero-valent iron to sodium persulfate is increased from 0.25:1 to 1:1, the pretreatment effect gradually improves; when it is further increased to 2:1, the soluble chemical oxygen demand (SCOD) decreases instead. When the dosage exceeds 30 mmol / L, the increase in SCOD is not significant; therefore, 30 mmol / L zero-valent iron and 30 mmol / L sodium persulfate are determined to be the optimal dosage.

[0021] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows: 1. This invention utilizes sulfate radicals, which have stronger oxidizing power than hydroxyl radicals, to efficiently destroy sludge flocs and cell walls, significantly promoting the dissolution of organic matter. Persulfate exhibits high stability, is easy to store and transport, and activation with zero-valent iron allows for operation at room temperature and neutral pH without pH adjustment, making it simple to operate and cost-effective. After pretreatment using this method, the total methane yield from anaerobic digestion of sludge increases by approximately 53.6% compared to the original sludge. Simultaneously, the capillary water absorption time (CST) of sludge decreases from 281 seconds to 163 seconds, significantly improving dewatering performance and facilitating subsequent sludge dewatering and disposal. Overall, both economic efficiency and treatment efficiency are significantly improved.

[0022] 2. This invention adds a fermentation treatment before activating persulfate with zero-valent iron. This invention uses Clostridium butyricum, Streptococcus thermophilus and Saccharomyces cerevisiae in a live bacteria ratio of (1.3-1.5):1:(0.4-0.6). By limiting the dosage and using anaerobic fermentation conditions, the pH of the sludge can be naturally reduced to a reasonable value, creating optimal conditions for activating persulfate with zero-valent iron, without the need for additional acid adjustment.

[0023] 3. In this invention, coconut shell charcoal and wood charcoal are compounded at a mass ratio of (2-3):1 as a carrier, and activated carbon loaded with zero-valent iron is prepared by liquid phase reduction method. The compounding of the two forms a microporous-mesoporous multi-level channel structure, which significantly improves the utilization rate of zero-valent iron active sites, enhances the activation efficiency of persulfate, and increases the generation rate of sulfate free radicals. Attached Figure Description

[0024] Figure 1 The changes in SCOD of sludge under different doses of zero-valent iron-persulfate pretreatment.

[0025] Figure 2 Comparison of methane production between zero-valent iron-persulfate pretreatment and single zero-valent iron pretreatment, and between the control group. Detailed Implementation

[0026] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] All raw materials used in the following embodiments of the present invention are commercially available products.

[0028] The parameters of the sludge to be treated are: total solids (TS) content 16970 mg / L, volatile solids (VS) content 7830 mg / L, soluble chemical oxygen demand (SCOD) 33 mg / L, and pH 7.08.

[0029] Inoculation sludge: Digested sludge from anaerobic digesters in municipal wastewater treatment plants is used as inoculation sludge.

[0030] Coconut shell charcoal, made from coconut shell, 10-28 mesh, produced by Mulinsen Activated Carbon Jiangsu Co., Ltd., with an iodine value of 1100mg / g, a filling specific gravity of 0.6g / mL, and an average specific surface area of ​​1500m² / g.

[0031] Wood-based charcoal, made of wood, 10-28 mesh, mesopores 2-5nm, produced by Mulinsen Activated Carbon Jiangsu Co., Ltd.

[0032] Clostridium butyricum, strain number: CICC23847, China Industrial Microbial Culture Collection Center.

[0033] Streptococcus thermophilus, CICC6063, China Industrial Microbial Culture Collection Center.

[0034] Saccharomyces cerevisiae, strain number: CICC33384, China Industrial Microbial Culture Collection Center.

[0035] Example 1 This embodiment provides a method for enhancing anaerobic digestion of sludge through sulfate radical pretreatment, including the following steps: (1) Add compound bacteria to the sludge to be treated and ferment it at 35℃ under anaerobic conditions for 40 hours to obtain fermented sludge; the compound bacteria include Clostridium butyricum, Streptococcus thermophilus, and Saccharomyces cerevisiae; the amount of compound bacteria in the sludge to be treated is 2×10 8 CFU / mL, the ratio of viable counts of Clostridium butyricum, Streptococcus thermophilus, and Saccharomyces cerevisiae was 1.4:1:0.5; (2) Add activated carbon loaded with zero-valent iron to the fermentation sludge, and the concentration of zero-valent iron in the fermentation sludge is 30 mmol / L; The preparation method of zero-valent iron supported on activated carbon includes the following steps: S1: Mix coconut shell charcoal and wood charcoal at a mass ratio of 2:1 to obtain activated carbon. Wash the activated carbon twice with water, soak it in 0.1mol / L dilute hydrochloric acid for 24h, wash it with water until neutral, and finally dry it at 105℃ to obtain pretreated activated carbon. S2: Dissolve FeSO4·7H2O in water to prepare a 0.1 mol / L solution; add pretreated activated carbon, wherein the mass ratio of Fe to activated carbon is 1:3, and stir at 200 rpm for 30 min under nitrogen protection to obtain a mixed solution; S3: While stirring at 300 rpm, add a 0.3 mol / L NaBH4 aqueous solution dropwise to the mixture at a rate of 3 mL / min; NaBH4 and Fe²⁺ + The molar ratio was 2:1, and stirring continued for 20 minutes after the addition was completed. S4: After the reaction is complete, the solid is separated by a magnet. The solid is washed twice with water and anhydrous ethanol, and dried in a vacuum drying oven at 60°C for 12 hours to obtain activated carbon loaded with zero valent iron. (3) Add sodium persulfate to the fermentation sludge. The concentration of sodium persulfate in the fermentation sludge is 30 mmol / L. Shake at 100 rpm for 30 min to obtain the pretreated sludge. (4) The pretreated sludge and the inoculated sludge were added to the anaerobic digestion reactor at a volume ratio of 2:1, and anaerobic digestion and gas production were carried out under mesophilic conditions of 35℃ for 20 days.

[0036] Example 2 This embodiment provides a method for enhancing anaerobic digestion of sludge through sulfate radical pretreatment, including the following steps: (1) Add compound bacteria to the sludge to be treated and ferment it at 35℃ under anaerobic conditions for 40 hours. The compound bacteria include Clostridium butyricum, Streptococcus thermophilus, and Saccharomyces cerevisiae to obtain fermented sludge. The amount of compound bacteria in the sludge to be treated is 1×10 8 CFU / mL, the ratio of viable counts of Clostridium butyricum, Streptococcus thermophilus, and Saccharomyces cerevisiae was 1.3:1:0.6; (2) Add activated carbon loaded with zero-valent iron to the fermentation sludge, and the concentration of zero-valent iron in the fermentation sludge is 30 mmol / L; The preparation method of zero-valent iron supported on activated carbon includes the following steps: S1: Mix coconut shell charcoal and wood charcoal at a mass ratio of 3:1 to obtain activated carbon. Wash the activated carbon twice with water, soak it in 0.1mol / L dilute hydrochloric acid for 24h, wash it with water until neutral, and finally dry it at 105℃ to obtain pretreated activated carbon. S2: Dissolve FeSO4·7H2O in water to prepare a 0.1 mol / L solution; add pretreated activated carbon, wherein the mass ratio of Fe to activated carbon is 1:3, and stir at 200 rpm for 30 min under nitrogen protection to obtain a mixed solution; S3: While stirring at 300 rpm, add a 0.3 mol / L NaBH4 aqueous solution dropwise to the mixture at a rate of 3 mL / min; NaBH4 and Fe²⁺ + The molar ratio was 2:1, and stirring continued for 20 minutes after the addition was completed. S4: After the reaction is complete, the solid is separated by a magnet. The solid is washed twice with water and anhydrous ethanol, and dried in a vacuum drying oven at 60°C for 12 hours to obtain activated carbon loaded with zero valent iron. (3) Add sodium persulfate to the fermentation sludge. The concentration of sodium persulfate in the fermentation sludge is 30 mmol / L. Shake at 100 rpm for 30 min to obtain the pretreated sludge. (4) The pretreated sludge and the inoculated sludge were added to the anaerobic digestion reactor at a volume ratio of 2:1, and anaerobic digestion and gas production were carried out under mesophilic conditions of 35℃ for 20 days.

[0037] Comparative Example 1 The difference between this comparative example and Example 1 is: (2) Iron sheets were added to the fermented sludge. The size of the iron sheets was 1cm×1cm, and the concentration of zero-valent iron in the iron sheets in the fermented sludge was 30mmol / L.

[0038] Comparative Example 2 The difference between this comparative example and Example 1 is that only coconut shell activated carbon was used. The material is coconut shell, 10-28 mesh, from Mulinsen Activated Carbon Jiangsu Co., Ltd., with an iodine value of 1100mg / g, a filling specific gravity of 0.6g / mL, and an average specific surface area of ​​1500m² / g.

[0039] Comparative Example 3 The difference between this comparative example and Example 1 is that only wood-based activated carbon was used, with a material of wood, 10-28 mesh, and a mesopore size of 2-5 nm, from Mulinsen Activated Carbon Jiangsu Co., Ltd.

[0040] Comparative Example 4 The difference between this comparative example and Example 1 is that coconut shell charcoal and wood charcoal are mixed in a mass ratio of 1:1 to obtain activated carbon.

[0041] Comparative Example 5 The difference between this comparative example and Example 1 is that the compound bacteria include Clostridium butyricum and Streptococcus thermophilus; the dosage of the compound bacteria in the sludge to be treated is 2 × 10⁻⁶. 8 CFU / mL, the ratio of viable bacteria of Clostridium butyricum to Streptococcus thermophilus was 1.4:1.

[0042] Comparative Example 6 The difference between this comparative example and Example 1 is that the compound bacteria include Clostridium butyricum, Streptococcus thermophilus, and Saccharomyces cerevisiae; the dosage of the compound bacteria in the sludge to be treated is 2 × 10⁻⁶. 8 CFU / mL, with a live count ratio of Clostridium butyricum, Streptococcus thermophilus, and Saccharomyces cerevisiae of 1:1:1.

[0043] Performance testing The sludge to be treated was processed according to the methods of Examples 1-2 and Comparative Examples 1-6. The parameters of the sludge to be treated were: total solids (TS) content 16970 mg / L, volatile solids (VS) content 7830 mg / L, soluble chemical oxygen demand (SCOD) 33 mg / L, and pH 7.08. The total methane production and sludge CST (capillary water absorption time) were calculated.

[0044] The results are shown in Table 1.

[0045] Table 1 Performance Test Results

[0046] From Table 1 and Figure 1-2 It can be seen that, Figure 1 The changes in SCOD of sludge under different doses of zero-valent iron-persulfate pretreatment were investigated. Figure 2 This study compares the methane production of zero-valent iron-persulfate pretreatment with that of single zero-valent iron pretreatment and the control group. The total methane production in Examples 1-2 increased by more than 50% compared to the original sludge, the sludge CST decreased from 281s to below 180s, organic matter leaching and anaerobic digestion efficiency were significantly improved, and dewatering performance was also enhanced, facilitating subsequent treatment.

[0047] Comparative Example 1 replaced the zero-valent iron supported on activated carbon with ordinary iron sheets, resulting in a decrease in the specific surface area of ​​iron and Fe²⁺. + The release rate slowed down, and the activation efficiency of persulfate decreased. Comparative Example 2 used only coconut shell charcoal, which was dominated by a microporous structure, and zero-valent iron easily blocked the pores and limited mass transfer. Comparative Example 3 used only wood charcoal, which had insufficient specific surface area, low zero-valent iron loading, and was prone to agglomeration. Comparative Example 4 combined coconut shell charcoal and wood charcoal in a 1:1 ratio, but the higher proportion of wood charcoal weakened the micropore-mesopore synergistic effect.

[0048] In Comparative Example 5, the complex microbial strain lacked Saccharomyces cerevisiae and only contained Clostridium butyricum and Streptococcus thermophilus, resulting in decreased metabolic complementarity, a limited organic acid spectrum, and insufficient pH reduction. In Comparative Example 6, the ratio of the three microorganisms was adjusted to 1:1:1. The low proportion of Clostridium butyricum led to weakened acid production capacity, while the high proportion of Saccharomyces cerevisiae resulted in limited metabolic contribution under anaerobic conditions. All of these changes led to a decrease in pre-fermentation efficiency.

[0049] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for enhancing anaerobic digestion of sludge through sulfate radical pretreatment, characterized in that, Includes the following steps: (1) Add compound bacteria to the sludge to be treated and ferment it under anaerobic conditions to obtain fermented sludge; (2) Add activated carbon loaded with zero-valent iron to the fermentation sludge; (3) Continue to add sodium persulfate to the fermentation sludge; The sludge was pretreated by oscillation. (4) Add the pretreated sludge and the inoculated sludge into the anaerobic digestion reactor and carry out anaerobic digestion and gas production under mesophilic conditions.

2. The method for enhancing anaerobic digestion of sludge through sulfate radical pretreatment according to claim 1, characterized in that, The complex microorganisms include Clostridium butyricum, Streptococcus thermophilus, and Saccharomyces cerevisiae.

3. The method for enhancing anaerobic digestion of sludge through sulfate radical pretreatment according to claim 2, characterized in that, The amount of the complex bacteria used in the sludge to be treated is (1-3) x 10 8 CFU / mL.

4. The method for enhancing anaerobic digestion of sludge through sulfate radical pretreatment according to claim 3, characterized in that, In the sludge to be treated, the ratio of viable bacteria of Clostridium butyricum, Streptococcus thermophilus, and Saccharomyces cerevisiae was (1.3-1.5):1:(0.4-0.6).

5. The method for enhancing anaerobic digestion of sludge through sulfate radical pretreatment according to claim 4, characterized in that, The compound bacteria were fermented under anaerobic conditions at 30-35℃ for 36-48 hours.

6. The method for enhancing anaerobic digestion of sludge through sulfate radical pretreatment according to claim 1, characterized in that, The method for preparing zero-valent iron supported on activated carbon includes the following steps: S1: Clean the activated carbon, soak it in dilute hydrochloric acid, wash it until neutral, and dry it to obtain pretreated activated carbon; S2: Dissolve FeSO4·7H2O in water to prepare a solution; add pretreated activated carbon to the solution and stir under nitrogen protection to obtain a mixed solution; S3: Add NaBH4 aqueous solution dropwise to the mixture while stirring; continue stirring after the addition is complete; S4: After the reaction is complete, the solid is separated by a magnet. The solid is washed with water and anhydrous ethanol in sequence and dried to obtain activated carbon loaded with zero valent iron.

7. The method for enhancing anaerobic digestion of sludge through sulfate radical pretreatment according to claim 6, characterized in that, Activated carbon is obtained by mixing coconut shell charcoal and wood charcoal at a mass ratio of (2-3):

1.

8. The method for enhancing anaerobic digestion of sludge through sulfate radical pretreatment according to claim 1, characterized in that, The concentration of zero-valent iron in activated carbon-supported zero-valent iron in fermentation sludge is 10-50 mmol / L.

9. The method for enhancing anaerobic digestion of sludge through sulfate radical pretreatment according to claim 8, characterized in that, The concentration of sodium persulfate in fermentation sludge is 10-50 mmol / L.

10. The method for enhancing anaerobic digestion of sludge through sulfate radical pretreatment according to claim 1, characterized in that, In the fermentation sludge, the molar ratio of zero-valent iron to sodium persulfate is (0.25-2):1.

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