Multi-effect sludge anaerobic digestion intensification method and system based on sodium disilicate synergistic thermal hydrolysis

Abstract

The application discloses a sludge anaerobic digestion intensification method and system based on sodium disilicate and heat hydrolysis, and belongs to the technical field of sludge resource utilization and energy recovery. By innovatively introducing sodium disilicate as a chemical conditioner, combined with heat hydrolysis pretreatment of specific temperature and pressure parameters, the multiple synergistic effects of efficient sludge cell structure cracking, organic matter dissolution rate improvement and heavy metal passivation are realized. Compared with traditional technologies, the methane yield is increased by 50% to 70%, the volatile solid degradation rate is increased by 30% to 45%, the digestion cycle is shortened by 30% to 50%, and the toxic effects of inhibitory substances (such as sulfides and ammonia nitrogen) and heavy metals are significantly reduced. The method has high efficiency, environmental protection and economy, and provides a breakthrough solution for sludge energy utilization.

Description

Technical Field

[0001] The invention relates to the field of sludge treatment technology, and in particular to the energy-based treatment of sludge with high solids content and high toxicity. It improves anaerobic digestion efficiency and reduces environmental risks through chemical-physical synergistic pretreatment. Background Technology

[0002] With accelerated urbanization and industrial expansion, sludge production has surged annually. Anaerobic digestion, as a mainstream sludge resource utilization technology, can convert organic matter into methane through microbial metabolism, achieving the dual goals of energy recovery and pollution reduction. However, the organic matter in sludge is mainly encapsulated in a dense structure composed of microbial cell walls and extracellular polymeric substances (EPS). In conventional anaerobic digestion, the hydrolysis stage is extremely slow, becoming the rate-limiting step. Studies have shown that only 30%-40% of the volatile solids (VS) in untreated sludge are degraded, and the methane yield is generally below 200 mL / gVS. Although high-temperature digestion (55℃) can partially increase the reaction rate, energy consumption increases by more than 50%, and the effect on cell breakdown is limited. How to efficiently release biodegradable organic matter in sludge has become the core challenge for improving the efficiency of anaerobic digestion.

[0003] To address the structure of sludge, the industry has developed pretreatment technologies such as hot water hydrolysis, ultrasound, and ozone oxidation. Among these, hot water hydrolysis is widely used due to its relatively simple operation, but its technical shortcomings are becoming increasingly apparent: First, energy consumption and secondary pollution: Typical hot water hydrolysis requires maintaining temperatures above 160℃ and pressures above 1.0 MPa, resulting in energy consumption of 150-250 kWh per ton of sludge treated. Simultaneously, high temperatures may lead to Maillard reactions, generating recalcitrant substances such as melanoidins (accounting for 5%-15% of total organic matter), which in turn inhibits subsequent digestion. Second, the risk of heavy metal activation: Under high temperature and pressure, the activity of normally stable heavy metal ions (such as Cu, Zn, and Cr) in the sludge is significantly enhanced, with leaching rates increasing by 40%-60%, severely inhibiting methanogenic bacteria activity. For example, when Cu... 2+ When the concentration exceeds 50 mg / L, the methane yield can decrease by 30%-50%.

[0004] Chemical conditioning methods (such as alkali treatment and Fenton oxidation) improve the organic matter dissolution rate by disrupting the EPS structure, but they introduce new problems: firstly, pH fluctuations and salt accumulation. Adjusting with strong alkalis (NaOH) or strong acids (H2SO4) can easily lead to pH imbalance in the digestive system, requiring the addition of buffers. Furthermore, the addition of Fe... 2+ The Fenton process using H2O2 produces a large amount of iron sludge, increasing the burden of solid waste disposal. Secondly, it lacks economic viability. Taking municipal sludge as an example, if NaOH pretreatment is used (dosage 4%-6% dry weight), the reagent cost accounts for 35%-45% of the total treatment cost, making large-scale promotion difficult.

[0005] In recent years, researchers have attempted to enhance the effect through combined physical-chemical pretreatment, but key technological bottlenecks have yet to be overcome. Firstly, the synergistic effect is insufficient. Patent CN115611492A proposes a method for treating sludge based on sodium sulfite-assisted hot water hydrolysis. While this promotes the dissolution of organic matter and lowers the hot water hydrolysis temperature, sulfite, a known carcinogen, may cause secondary pollution and toxicity to microorganisms during the treatment process, requiring additional treatment. Secondly, the equipment becomes more complex. Patent CN108483831A uses microwave-ultrasound synergistic microbial fuel cell technology to treat residual sludge, achieving a SCOD removal rate of over 82.3% and a VS degradation rate of 33.1%, but the equipment investment cost increases by more than two times, and there are too many complex operations.

[0006] Sodium disilicate (Na2Si2O5), an environmentally friendly silicate, has been widely used in detergents and the ceramics industry, but its value in sludge treatment has not been fully explored. Existing research mainly focuses on the conditioning effects of diatomaceous earth or sodium monosilicate (Na2SiO3): Firstly, diatomaceous earth conditioning. Patent CN108083614B shows that a composite dewatering agent based on diatomaceous earth can improve sludge dewatering performance, but it has a low specific surface area, a complex manufacturing process, limited adsorption capacity for EPS, and a weak passivation effect on heavy metals. Secondly, sodium monosilicate modification. Studies indicate that sodium monosilicate can promote sludge dissolution by releasing OH-, but excessive addition (>3%) will lead to pH>11, requiring additional adjustment to neutrality and increasing operational complexity.

[0007] The aforementioned technical shortcomings indicate that existing pretreatment methods are inadequate in terms of efficiency, cost, and environmental risk control. In particular, for industrial sludge with high solids and heavy metal content, a solution that combines efficient decontamination, toxicity control, and low energy consumption has not yet been developed. Therefore, developing a novel chemical-physical synergistic pretreatment technology to achieve efficient sludge energy conversion through multi-mechanism coupling has become a key issue urgently needing breakthroughs in this field. Summary of the Invention

[0008] To address the problems of low organic matter leaching rate, insufficient methane yield, high risk of secondary pollution, and significant heavy metal inhibition effect in existing sludge anaerobic digestion technologies, this invention proposes an innovative chemical-physical synergistic pretreatment method and system. Figure 1 By introducing deep coupling between sodium disilicate and hot water hydrolysis, combined with dynamic parameter control and system integration optimization, this method achieves efficient dissection of sludge cell structure, elimination of biotoxicity inhibition, and comprehensive improvement in energy conversion efficiency. It aims to overcome the limitations of traditional technologies and provide an efficient, environmentally friendly, and economically feasible sludge resource utilization solution.

[0009] The core of this invention lies in constructing a multi-level synergistic system of "chemical conditioning - gradient breakthrough - bio-enhancement" (Chemical conditioning - gradient breakthrough - bio-enhancement). Figure 2 The specific technical solution is as follows:

[0010] 1. Sodium disilicate's targeted conditioning and multiple mechanisms of action

[0011] Sodium disilicate (Na2Si2O5) with a specific particle size (50-200 mesh) is added to the sludge. The dosage is dynamically adjusted according to the organic matter content of the sludge, and the calculation formula is as follows:

[0012] Sodium disilicate dosage (%) = 0.2 × sludge VS / TS ratio + 0.1

[0013] The compound exerts a synergistic effect through the following pathways: (1) Physical deconstruction: the microporous structure of sodium disilicate (specific surface area > 80 m²) 2 / g) adsorbs extracellular polymeric substances (EPS) in sludge, weakening the mechanical strength of the cell wall; (2) chemical passivation: silicon-oxygen framework (Si-O-Si) and heavy metal ions (Cu) 2+ Zn 2+ (2) Forming stable Si-OM complexes, reducing their bioavailability; (3) Electrical regulation: By adjusting the sludge Zeta potential to -15mV to -25mV, the sludge dispersibility is improved, and local coking during the hot hydrolysis process is avoided.

[0014] 2. Dynamic coupling and energy optimization of hot water hydrolysis parameters

[0015] During the hydrolysis stage, a dynamic relationship equation between temperature (T) and pressure (P) is established:

[0016] P(MPa) = 0.007 × T(°C) + 0.1

[0017] The reaction conditions are controlled at a temperature of 120-180℃, a pressure of 0.5-1.5MPa, and a time of 10-60 minutes. This equation ensures that the reaction path is tilted towards the dissolution of organic matter, avoiding excessive carbonization and the formation of recalcitrant substances such as melanoidins. Simultaneously, supercritical carbon dioxide (SC-CO2) is used as the heat transfer medium, whose diffusion coefficient is more than 10 times that of conventional heat transfer fluids, resulting in a 40% increase in heat transfer efficiency and a 25%-30% reduction in energy consumption.

[0018] 3. System Integration and Function Enhancement Module

[0019] The design includes an integrated system with the following innovative modules: (1) Microwave pre-radiation unit: Before adding sodium disilicate, the sludge is irradiated with 200-500W microwave for 2-10 minutes to destroy the colloidal structure of the sludge using high-frequency electromagnetic waves, thereby increasing the penetration efficiency of subsequent chemical reagents by more than 50%; (2) Gradient heating reactor: A jacketed multi-stage heating structure is adopted to achieve a precise heating rate of 3-8℃ / min, avoiding the denaturation of organic matter caused by sudden temperature changes; (3) Heavy metal adsorption bed: Modified zeolite or biochar (pore size 2-5nm) is filled in the anaerobic digestion unit to adsorb residual heavy metal ions through ion exchange and surface complexation. Figure 3 This reduces the leaching rate to below 20%.

[0020] This invention offers the following original contributions compared to existing technologies: First, it utilizes dynamic parameter adaptation technology. For the first time, it establishes a quantitative relationship between sodium disilicate dosage and temperature-pressure equations, achieving precise matching between pretreatment conditions and sludge characteristics, thus avoiding reagent waste or incomplete reactions. Second, it employs a multi-mechanism synergistic effect. Through the triple action of sodium disilicate—physical adsorption, chemical passivation, and electrical regulation—combined with SC-CO2 heat transfer and microwave pre-radiation, a cascaded enhancement effect of "chemical-physical-biological" is formed. Third, it provides comprehensive toxicity control. From heavy metal complexation in the pretreatment stage to heavy metal adsorption in the digestion stage, it systematically blocks the inhibition of methanogenic bacteria by toxic substances, resolving the contradiction of "breakthrough-inhibition" in traditional technologies.

[0021] This invention is particularly applicable to the following challenging sludge treatment scenarios: (1) High solids sludge: municipal or industrial sludge with a solids content of 5%-20% requires multiple dilutions using traditional methods, while this invention can treat it directly; (2) Heavy metal sludge: electroplating and metallurgical sludge with a total Cu, Zn, and Pb content > 500 mg / kg, where decomposition and detoxification are completed simultaneously; (3) Industrial sludge: papermaking and petrochemical sludge containing complex components such as lignin and oils, where the VS degradation rate still reaches over 70%.

[0022] Typical implementation process includes: (1) sludge characteristic analysis (VS / TS, heavy metal content); (2) calculating the amount of sodium disilicate added according to the dynamic formula, and performing microwave pre-radiation and stirring mixing; (3) controlling the hot water hydrolysis reaction based on the TP equation, and using SC-CO2 to enhance heat transfer; (4) adding methanogenic bacteria enrichment agent in the anaerobic digestion stage, and using adsorption bed for continuous purification.

[0023] Through the systematic integration of the above-mentioned technical solutions, this invention has achieved a leap from "single solution" to "multi-effect synergy" in sludge treatment, providing a new technical path that is both innovative and practical for sludge energy conversion. Attached Figure Description

[0024] Figure 1 Process flow diagram;

[0025] Figure 2 System structure diagram;

[0026] Figure 3 Diagram of the microscopic mechanism of sodium disilicate complexing heavy metals. Detailed Implementation

[0027] The technical solution of the present invention will be described in detail below with reference to embodiments and comparative examples, but the scope of protection of the present invention is not limited thereto.

[0028] Example 1: Municipal Sludge Treatment

[0029] This embodiment addresses the residual sludge (82% moisture content, VS / TS ratio 0.68, Cu content 280 mg / kg) from a municipal wastewater treatment plant. A dynamic formula was used to calculate the sodium disilicate dosage to be 0.24% of the dry weight (corresponding to a 150-mesh particle size). After pre-irradiation with a 300W microwave for 5 minutes to disrupt the sludge's colloidal structure, uniform mixing was achieved by stirring at 120 rpm for 25 minutes. Subsequently, hydrolysis was performed in supercritical CO2 heat transfer medium (160℃, 1.22 MPa, 30 minutes). The reaction path was precisely controlled using a temperature-pressure coupling equation to avoid excessive carbonization of organic matter. The pretreated sludge was then introduced into a continuous stirred anaerobic reactor (CSTR), with 3% Methanosarcina bacteria added. The reactor was reacted at 38℃ for 15 days, and a built-in biochar adsorption bed (3 mm particle size) was used for continuous adsorption of heavy metal ions. The final methane yield reached 418 mL / gVS, a 99% improvement over traditional hot water hydrolysis. The VS degradation rate was 83%, the Cu leaching rate was reduced to 19%, and the digestion cycle was shortened by 40%. This method requires no acid or alkali adjustment throughout the process. The sludge pH is stabilized at 7.8-8.2 through the buffering effect of sodium disilicate and CO2 stripping, completely avoiding the salt accumulation problem of traditional alkali treatment.

[0030] Example 2: Electroplating sludge treatment

[0031] For a high-heavy-metal sludge (75% moisture content, VS / TS ratio 0.45, Zn+Pb total 1200 mg / kg) from an electroplating plant, this embodiment adopts an incremental dosing strategy, increasing the sodium disilicate dosage to 2.0% of the dry weight (100 mesh particle size), combined with 500W microwave radiation for 8 minutes to enhance the heavy metal dissociation efficiency. After the mixed sludge is adjusted to pH 9.5, it enters a multi-stage gradient heating hot water hydrolysis tank (180℃, 1.36 MPa, 15 minutes), utilizing the high diffusivity of supercritical CO2 to accelerate the dissolution of organic matter. The anaerobic digestion stage uses an upflow anaerobic sludge blanket (UASB) reactor, adding 5% of toxic Methanothrix bacteria agent, and operating at 45℃ for 12 days, while simultaneously achieving dynamic retention of heavy metals through a modified zeolite adsorption bed (2 nm pore size). The results showed that the methane yield reached 372 mL / g VS, which was 91% higher than that of sodium disilicate treatment alone. The Zn+Pb leaching rate was only 8%, the sulfide concentration was controlled at 42 mg / L (below the limit of 50 mg / L), and there was no scaling in the system, making it suitable for long-term continuous operation.

[0032] Example 3: Sludge Treatment in the Paper Industry

[0033] This embodiment treats high-lignin sludge from a paper mill (70% moisture content, VS / TS ratio 0.52, lignin content 18%). Sodium disilicate (0.20% of dry weight, 200 mesh particle size) was added according to the formula. Microwave irradiation at 250W for 10 minutes promoted the depolymerization of the lignin-cellulose complex. The mixed sludge naturally rose to 9.2 without artificial pH intervention, followed by stepwise hot hydrolysis (140℃, 1.08MPa, 45 minutes) to avoid lignin condensation reaction under mild conditions. Anaerobic digestion employed a two-stage process: the first 5 days were run at 50℃ to activate thermophilic bacteria, and the temperature was lowered to 42℃ for the next 15 days with the addition of 2% compound bacterial agent (Methanosarcina:Methanothrix = 3:1). The final methane yield reached 345 mL / gVS, the lignin degradation rate was 65%, and the VS degradation rate was 71%. Compared with traditional NaOH treatment, the lignin degradation rate is increased by 44%, and there is no salt accumulation throughout the process (12.5 g / L in Comparative Example 3), the sludge dewatering performance is improved by 30%, and the filter cake moisture content is reduced to below 65%.

[0034] Comparative Example 1: Traditional Hot Water Hydrolysis Treatment

[0035] The municipal sludge was treated in the same manner as in Example 1, but without the addition of sodium disilicate. Direct hot hydrolysis (160℃, 1.22MPa, 30 minutes) was performed, with anaerobic digestion conditions identical to Example 1. The methane yield was only 210 mL / gVS, and the Cu leaching rate was 81%. Additional heavy metal precipitant was required, increasing the overall cost by 40%.

[0036] Comparative Example 2: Treatment with Sodium Disilicate Alone

[0037] The electroplating sludge was treated in the same manner as in Example 2. 2.0% sodium disilicate was added, and the mixture was stirred. Without hot water hydrolysis, it was directly subjected to anaerobic digestion (45°C, 12 days), without hot water hydrolysis or an adsorption bed. The methane yield was less than 200 mL / gVS, and the heavy metal leaching rate was 62%, failing to meet emission standards.

[0038] Comparative Example 3: Combined hydrolysis of NaOH and water

[0039] The papermaking sludge was treated in the same manner as in Example 3. 4% NaOH was added to adjust the pH to 12, and the mixture was stirred. Then, hot hydrolysis was performed (140°C, 1.08 MPa, 45 minutes). Before anaerobic digestion, the sludge was neutralized to pH 7.0 with HCl. The strong alkaline treatment resulted in a salt accumulation of 12.5 g / L, making subsequent dehydration difficult, and the lignin degradation rate was only 45%.

[0040] The effects of each embodiment are compared in the table below:

[0041] Table 1 Comparison and analysis of the effects of each embodiment

[0042]

[0043] This invention demonstrates significant technical advantages through a comparison of examples and comparative examples. First, methane yield is comprehensively improved. The methane yields of Examples 1-3 reach 418, 372, and 345 mL / g VS, respectively, representing an increase of 64%-99% compared to traditional hot water hydrolysis (210 mL / g VS) and 78%-91% compared to single sodium disilicate treatment (195 mL / g VS), without the salt accumulation problem associated with the NaOH combined treatment. Second, organic matter degradation is highly efficient and stable. The VS degradation rate increases to 71%-83%, an increase of 36%-60% compared to the traditional method (52%), especially maintaining a 71% degradation rate for high-lignin sludge. Simultaneously, heavy metal toxicity is deeply controlled. The heavy metal leaching rate is reduced to 8%-19%, a decrease of 76%-87% compared to Comparative Examples 1-2, without requiring additional passivation processes. Furthermore, it offers advantages in both economics and environmental friendliness. The entire process involves no acid-base neutralization steps, resulting in zero salt accumulation, a 35%-40% reduction in overall energy consumption, and reagent costs that are only 1 / 3 of those of the NaOH solution.

[0044] This invention utilizes a multi-effect enhancement mechanism of sodium disilicate synergistic thermal hydrolysis to improve energy recovery efficiency while systematically solving the problem of secondary pollution in sludge treatment, providing an innovative industrial solution for the resource utilization of highly challenging sludge.

Claims

1. A method for enhanced methanogenesis from sludge based on sodium disilicate-co-hydrolyzed hot water, characterized in that, Includes the following steps: a) Add sodium disilicate with a particle size of 50-200 mesh to the sludge. The amount added is 0.5%-3% of the dry weight of the sludge. Mix it evenly with stirring at 50-200 rpm. Before adding sodium disilicate, irradiate the sludge with microwave at 200-500 W for 2-10 minutes. b) The mixed sludge is pretreated by hot water hydrolysis, with the temperature controlled at 120-180℃, the pressure at 0.5-1.5 MPa, and the treatment time at 10-60 minutes; c) The pretreated sludge is introduced into the anaerobic digestion unit and reacted at 35-55℃ for 10-25 days. Methane gas is collected, and the anaerobic digestion unit is filled with modified zeolite or biochar with a pore size of 2-5 nm.

2. The method according to claim 1, characterized in that, The percentage of sodium disilicate added in step a) relative to the dry weight of sludge is positively correlated with the organic matter content of sludge, specifically satisfying: percentage of sodium disilicate added relative to the dry weight of sludge = 0.2 × sludge VS / sludge TS + 0.1, where sludge VS / sludge TS is 0.4-0.

8.

3. The method according to claim 1, characterized in that, In step b), the heating rate of the hot water hydrolysis pretreatment is 3-8℃ / min, and the temperature and pressure are controlled to satisfy: P=0.007×T+0.1, where T is 120-180℃ and P is 0.5-1.5MPa.

4. The method according to claim 1, characterized in that, In step c), a methanogenic bacteria enrichment agent is added to the anaerobic digestion unit. The enrichment agent contains Methanosarcina and Methanothrix bacteria, and the added volume is 1%-5% of the sludge volume.

5. The method according to claim 1, characterized in that, After mixing in step a), the pH of the sludge is adjusted to 9.0-10.5, and after hot water hydrolysis pretreatment, the pH is reduced to 7.5-8.5 by carbon dioxide stripping.

6. A sludge-enhanced methanogenesis system based on sodium disilicate synergistic thermal hydrolysis, used to implement the method described in any one of claims 1-5, characterized in that, include: Sludge mixing unit (1): Equipped with a screw feeder for quantitative addition of sodium disilicate, a variable frequency speed regulating stirring device and a pH online monitoring instrument; Hot water hydrolysis reaction unit (2): Adopts a jacketed reaction tank with multi-stage gradient heating and integrated pressure feedback control system; Anaerobic digestion unit (3): Built-in gas-liquid separator and biogas purification module, connected to a methane storage device.

7. The system according to claim 6, characterized in that, The jacketed circulating medium of the hot water hydrolysis reaction unit (2) is supercritical carbon dioxide, which is used to enhance heat transfer and organic matter dissolution.

8. The system according to claim 6, characterized in that, A microwave pre-radiation module is provided between the sludge mixing unit (1) and the hot water hydrolysis reaction unit (2), with a radiation power of 200-500 W and a radiation time of 2-10 minutes.

9. The system according to claim 6, characterized in that, The anaerobic digestion unit (3) is equipped with a heavy metal adsorption bed, filled with modified zeolite or biochar, for adsorbing and passivating heavy metal ions.

10. The system according to claim 6, characterized in that, The system is suitable for recalcitrant industrial sludge with a solid content of 5%-20% and a total Cu, Zn, and Pb content >500 mg / kg.

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