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

Abstract

The invention discloses a sludge anaerobic digestion strengthening method and system based on sodium disilicate synergistic thermal hydrolysis, and belongs to the technical field of sludge recycling and energy recovery. Sodium disilicate is innovatively introduced as a chemical conditioner, and thermal hydrolysis pretreatment with specific temperature and pressure parameters is combined, so that multiple synergistic effects of efficient cracking of a sludge cell structure, improvement of the dissolution rate of organic matters and heavy metal passivation are realized. Compared with the prior art, the methane yield is increased by 50%-70%, the volatile solid degradation rate is increased by 30%-45%, the digestion period is shortened by 30%-50%, and the toxic influence of inhibitory substances (such as sulfide and ammonia nitrogen) and heavy metal is remarkably reduced. The method has the advantages of high efficiency, environmental protection and economy, and provides a breakthrough solution for sludge energy regeneration.

Description

Technical Field

[0001] The invention relates to the technical field of sludge treatment, in particular to the energy treatment of high-solid content and high-toxicity sludge, and improves the anaerobic digestion efficiency and reduces environmental risks through chemical-physical synergistic pretreatment. Background Art

[0002] With the acceleration of urbanization and the expansion of industrial scale, the output of sludge has increased sharply year by year. According to statistics, the annual output of municipal sludge in my country has exceeded 60 million tons (wet basis), and the total amount of industrial sludge exceeds 40 million tons, which contains a large amount of pollutants such as organic matter, pathogens and heavy metals. As the mainstream sludge resource technology, anaerobic digestion 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 wrapped in a dense structure composed of microbial cell walls and extracellular polymers (EPS). In the conventional anaerobic digestion process, the hydrolysis stage is extremely slow and becomes the rate-limiting step. Studies have shown that only 30%-40% of volatile solids (VS) are degraded in untreated sludge, and the methane yield is generally less than 200mL / gVS. Although high temperature digestion (55°C) can partially increase the reaction rate, the energy consumption increases by more than 50%, and the effect on cell cracking is limited. How to efficiently release biodegradable organic matter in sludge has become a core challenge to improve the efficiency of anaerobic digestion.

[0003] In order to break the sludge structure, the industry has developed pretreatment technologies such as thermal hydrolysis, ultrasound, and ozone oxidation. Among them, thermal hydrolysis is widely used because of its relatively simple operation, but its technical shortcomings are becoming increasingly apparent: First, energy consumption and secondary pollution problems: Typical thermal hydrolysis needs to maintain a high temperature of more than 160°C and a pressure of more than 1.0MPa, and the energy consumption for treating each ton of sludge is as high as 150-250kWh. At the same time, high temperature may cause the Maillard reaction to produce difficult-to-degrade substances such as melanoidins (accounting for 5%-15% of the total organic matter), which in turn inhibits the subsequent digestion process. The second is the risk of heavy metal activation: Under high temperature and high pressure environments, the activity of originally stable heavy metal ions (such as Cu, Zn, Cr) in the sludge is significantly enhanced, and the leaching rate can be increased by 40%-60%, which seriously inhibits the activity of methanogens. 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) increase the dissolution rate of organic matter by destroying the EPS structure, but they introduce new problems: one is pH fluctuation and salt accumulation. Strong alkali (NaOH) or strong acid (H2SO4) conditioning can easily lead to pH imbalance in the digestive system, and additional buffers need to be added. 2+ / H 2 O 2The Fenton process will produce a large amount of iron sludge, increasing the burden of solid waste disposal. Second, it is not economical. Taking municipal sludge as an example, if NaOH pretreatment (dosage 4%-6% dry weight) is used, the cost of the reagent accounts for 35%-45% of the total treatment cost, which is difficult to promote on a large scale.

[0005] In recent years, researchers have tried to enhance the effect through physical-chemical combined pretreatment, but have not yet broken through the key technical bottleneck. First, the synergistic effect is insufficient. Patent CN115611492A proposes a method for treating sludge based on sodium sulfite-assisted thermal hydrolysis. Although it promotes the dissolution of organic matter and reduces the thermal hydrolysis temperature, sulfite, as a recognized carcinogen, may cause secondary pollution and toxicity to microorganisms during the treatment process, requiring additional treatment. Second, the equipment is complicated. Patent CN108483831A uses microwave ultrasonic synergistic microbial fuel cell technology to treat residual sludge. The SCOD removal rate reaches more than 82.3%, and the VS degradation rate rises to 33.1%, but the equipment investment cost increases by more than 2 times, and there are too many complex operations.

[0006] Sodium disilicate (Na 2 Si 2 O 5 ) is an environmentally friendly silicate that has been widely used in detergent and ceramic industries, but its value in sludge treatment has not been fully explored. Existing research focuses on diatomaceous earth or sodium monosilicate (Na 2 SiO 3 ) conditioning effect: one is diatomite conditioning. Patent CN108083614B shows that the sludge dehydration performance can be improved by a composite dehydrating agent mainly composed of diatomite, but its specific surface area is low, the production process is complicated, the adsorption capacity of EPS is limited, and the passivation effect on heavy metals is weak. The second is sodium monosilicate modification. Studies have shown that sodium monosilicate can promote sludge dissolution by releasing OH-, but excessive addition (>3%) will lead to pH>11, which needs to be adjusted to neutrality, increasing the complexity of the operation.

[0007] The above technical defects show that the existing pretreatment methods are insufficient in terms of efficiency, cost and environmental risk control. In particular, for industrial sludge with high solid content and high heavy metal content, there is no solution that combines efficient decomposition, toxicity control and low energy consumption. Therefore, the development of a new chemical-physical synergistic pretreatment technology to achieve efficient energy conversion of sludge through multi-mechanism coupling has become a key issue that needs to be broken through in this field. Summary of the invention

[0008] Aiming at the problems existing in the existing sludge anaerobic digestion technology, such as low organic matter dissolution rate, insufficient methane yield, high risk of secondary pollution and significant heavy metal inhibition effect, the present invention proposes an innovative chemical-physical synergistic pretreatment method and system (Figure 1 ). By introducing the deep coupling of sodium disilicate and thermal hydrolysis, combined with dynamic parameter control and system integration optimization, the efficient destruction of sludge cell structure, the removal of biological toxicity inhibition and the overall improvement of energy conversion efficiency are achieved, aiming to break through the limitations of traditional technologies and provide an efficient, environmentally friendly and economically feasible sludge resource solution.

[0009] The core of the present invention is to construct a multi-level synergistic system of "chemical conditioning-gradient cracking-biological enhancement" ( Figure 2 ), the specific technical solutions are as follows:

[0010] 1. Sodium disilicate directional conditioning and multiple mechanisms of action

[0011] Add sodium disilicate (Na2SiO2) of a specific particle size (50-200 mesh) to the sludge. 2 Si 2 O 5 ), the dosage is dynamically adjusted according to the organic matter content of the sludge, and the calculation formula is:

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

[0013] The compound exerts synergistic effects through the following pathways: (1) Physical decomposition: The microporous structure of sodium disilicate (specific surface area> 80m 2 / g) adsorbs extracellular polymers (EPS) of sludge, weakening the mechanical strength of cell walls; (2) Chemical passivation: the silicon-oxygen skeleton (Si-O-Si) and heavy metal ions (Cu 2+ 、Zn 2+ (3) Electrical regulation: By adjusting the sludge Zeta potential to -15mV to -25mV, the sludge dispersibility can be improved and local coking during thermal hydrolysis can be avoided.

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

[0015] In the thermal hydrolysis stage, the dynamic correlation equation between temperature (T) and pressure (P) is established:

[0016] P (MPa) = 0.007 × T (°C) + 0.1

[0017] The reaction conditions are controlled to be 120-180°C, 0.5-1.5 MPa, and 10-60 minutes. This equation ensures that the reaction path is inclined toward the dissolution of organic matter, avoiding excessive carbonization to generate difficult-to-degrade substances such as melanoidins. At the same time, supercritical carbon dioxide (SC-CO 2) as a heat transfer medium, its diffusion coefficient is more than 10 times that of conventional heat carriers, the heat transfer efficiency is increased by 40%, and the energy consumption is reduced by 25%-30%.

[0018] 3. System integration and function enhancement module

[0019] The design of the integrated system includes the following innovative modules: (1) Microwave pre-irradiation unit: Before adding sodium disilicate, the sludge is irradiated with 200-500W microwaves for 2-10 minutes, using high-frequency electromagnetic waves to destroy the colloidal structure of the sludge, thereby increasing the penetration efficiency of subsequent chemical reagents by more than 50%; (2) Gradient temperature rise reactor: A jacketed multi-stage heating structure is used to achieve a precise heating rate of 3-8°C / min to avoid 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 ( Figure 3 ), so that its leaching rate is reduced to below 20%.

[0020] Compared with the prior art, the present invention has the following original contributions: First, the parameter dynamic adaptation technology. For the first time, the quantitative relationship between the dosage of sodium disilicate and the temperature-pressure equation is established to achieve accurate matching of pretreatment conditions and sludge characteristics, avoiding waste of reagents or insufficient reaction; second, multi-mechanism synergistic effect enhancement. Through the triple effects of physical adsorption, chemical passivation and electrical regulation of sodium disilicate, combined with SC-CO 2 Heat transfer and microwave pre-radiation form a "chemical-physical-biological" cascade strengthening effect; third, toxicity is controlled throughout the process. From heavy metal complexation in the pretreatment stage to heavy metal adsorption in the digestion stage, the inhibition of toxic substances on methanogens is systematically blocked, solving the contradiction of "destruction-inhibition" in traditional technologies.

[0021] The present invention is particularly suitable for the following difficult sludge treatment scenarios: (1) High-solid sludge: municipal or industrial sludge with a solid content of 5%-20%. Traditional methods require multiple dilutions, but the present invention can directly treat it; (2) Heavy metal sludge: electroplating and metallurgical sludge with a total amount of Cu, Zn, and Pb >500 mg / kg, in which decomposition and detoxification are completed simultaneously; (3) Industrial sludge: papermaking and petrochemical sludge containing complex components such as lignin and oil, and the VS degradation rate is still above 70%.

[0022] The typical implementation process includes: (1) sludge characteristics analysis (VS / TS, heavy metal content); (2) calculating the dosage of sodium disilicate according to the dynamic formula, and performing microwave pre-irradiation and stirring mixing; (3) controlling the thermal hydrolysis reaction based on the TP equation, and using SC-CO2 to enhance heat transfer; (4) adding methanogen enrichment agent during the anaerobic digestion stage, and using adsorption bed for continuous purification.

[0023] Through the systematic integration of the above-mentioned technical solutions, the present invention realizes the leap from "single solution" to "multi-effect synergy" in sludge treatment, and provides a new technical path for sludge energy conversion that is both innovative and practical. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Process flow chart;

[0025] Figure 2 System structure diagram;

[0026] Figure 3 Microscopic mechanism diagram of sodium disilicate complexing heavy metals. DETAILED DESCRIPTION

[0027] The technical scheme of the present invention is described in detail below in conjunction with embodiments and comparative examples, but the protection scope of the present invention is not limited thereto.

[0028] Example 1: Municipal Sludge Treatment

[0029] This embodiment is aimed at the residual sludge of a municipal sewage treatment plant (water content 82%, VS / TS ratio 0.68, Cu280mg / kg), and the dynamic formula is used to calculate the dosage of sodium disilicate as 0.24% of dry weight (corresponding to 150 mesh particle size). After 300W microwave pre-irradiation for 5 minutes to destroy the colloidal structure of the sludge, it is stirred at 120rpm for 25 minutes to achieve uniform mixing. Subsequently, thermal hydrolysis (160℃, 1.22MPa, 30 minutes) is carried out with the assistance of supercritical CO2 heat transfer medium, and the reaction path is accurately controlled by temperature-pressure coupling equation to avoid excessive carbonization of organic matter. The pretreated sludge is introduced into a continuous stirred anaerobic reactor (CSTR), 3% Methanosarcina inoculant is added, and the reaction is carried out at 38℃ for 15 days, and a built-in biochar adsorption bed (particle size 3mm) is continuously adsorbed for heavy metal ions. The final methane yield reached 418mL / gVS, which was 99% higher than that of traditional thermal hydrolysis. The VS degradation rate was 83%, the Cu leaching rate was reduced to 19%, and the digestion cycle was shortened by 40%. This solution does not require acid-base adjustment throughout the process. The sludge pH is adjusted by the buffering effect of sodium disilicate and CO 2 The stripping temperature is stabilized at 7.8-8.2, which completely avoids the salt accumulation problem of traditional alkali treatment.

[0030] Example 2: Electroplating sludge treatment

[0031] For the high heavy metal sludge of a certain electroplating plant (water content 75%, VS / TS ratio 0.45, total Zn+Pb 1200mg / kg), this embodiment adopts an incremental addition strategy to increase the dosage of sodium disilicate to 2.0% of dry weight (particle size 100 mesh), 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 hydrolysis tank (180℃, 1.36MPa, 15 minutes) and uses supercritical CO 2 The high diffusivity of the anaerobic sludge blanket accelerates the dissolution of organic matter. In the anaerobic digestion stage, an upflow anaerobic sludge blanket (UASB) reactor was used, 5% toxic-resistant Methanothrix inoculant was added, and it was operated at 45°C for 12 days. At the same time, a modified zeolite adsorption bed (pore size 2nm) was used to achieve dynamic retention of heavy metals. The results showed that the methane yield reached 372mL / g VS, which was 91% higher than that of single sodium disilicate treatment. The Zn+Pb leaching rate was only 8%, the sulfide concentration was controlled at 42mg / L (lower than the limit of 50mg / L), and there was no scaling in the system, which was suitable for long-term continuous operation.

[0032] Example 3: Papermaking Industry Sludge Treatment

[0033] This example treats high lignin sludge (water content 70%, VS / TS ratio 0.52, lignin content 18%) from a paper mill. The dosage of sodium disilicate is calculated as 0.20% of dry weight (particle size 200 mesh) according to the formula, and 250W microwave irradiation for 10 minutes promotes the depolymerization of lignin-cellulose complex. The mixed sludge naturally rises to 9.2 without artificial intervention of pH, and then undergoes step-by-step thermal hydrolysis (140°C, 1.08MPa, 45 minutes), using mild conditions to avoid lignin condensation reaction. Anaerobic digestion adopts a two-stage process: the first 5 days are operated at 50°C to activate thermophilic bacteria, and the temperature is lowered to 42°C for the next 15 days and 2% composite bacterial agent (Methanosarcina:Methanothrix=3:1) is added. The final methane yield reaches 345mL / gVS, the lignin degradation rate is 65%, and the VS degradation rate is 71%. Compared with the traditional NaOH treatment, the lignin degradation rate was increased by 44%, and there was no salt accumulation in the whole process (12.5 g / L in comparative example 3), the sludge dewatering performance was improved by 30%, and the moisture content of the filter cake was reduced to below 65%.

[0034] Comparative Example 1: Traditional thermal hydrolysis treatment

[0035] The municipal sludge treated in the same manner as in Example 1 was directly subjected to thermal hydrolysis (160°C, 1.22 MPa, 30 minutes) without the addition of sodium disilicate, and the anaerobic digestion conditions were the same as in Example 1. The methane yield was only 210 mL / gVS, the Cu leaching rate was 81%, and an additional heavy metal precipitant was required, which increased the overall cost by 40%.

[0036] Comparative Example 2: Single Sodium Disilicate Treatment

[0037] The electroplating sludge treated in Example 2 was treated with 2.0% sodium disilicate, stirred and mixed, and directly subjected to anaerobic digestion (45°C, 12 days) without thermal hydrolysis and adsorption bed. The methane yield was less than 200 mL / gVS, and the heavy metal leaching rate was 62%, which could not meet the emission standards.

[0038] Comparative Example 3: NaOH combined with thermal hydrolysis

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

[0040] The effects of various embodiments are compared as shown in the following table:

[0041] Table 1 Comparative analysis of the effects of various embodiments

[0042]

[0043] The present invention shows significant technical advantages through the comparison of the embodiments and the comparative examples. First, the methane yield is comprehensively improved. The methane yields of Examples 1-3 reached 418, 372, and 345 mL / g VS, respectively, which is 64%-99% higher than the traditional thermal hydrolysis (210 mL / g VS) and 78%-91% higher than the single sodium disilicate treatment (195 mL / g VS), and there is no salt accumulation problem of the NaOH combined solution. Secondly, the degradation of organic matter is efficient and stable. The VS degradation rate increased to 71%-83%, which is 36%-60% higher than the traditional method (52%), especially for high lignin sludge, the degradation rate is still maintained at 71%. At the same time, the toxicity of heavy metals is deeply controlled. The heavy metal leaching rate dropped to 8%-19%, which is 76%-87% lower than that of Comparative Examples 1-2, and no additional passivation process is required. In addition, it is both economical and environmentally friendly. There is no acid-base neutralization step in the whole process, the salt accumulation is 0, the comprehensive energy consumption is reduced by 35%-40%, and the reagent cost is only 1 / 3 of the NaOH solution.

[0044] The present invention uses the multi-effect strengthening mechanism of sodium disilicate synergistic thermal hydrolysis to improve energy recovery efficiency while systematically solving the secondary pollution problem in sludge treatment, providing an innovative solution for industrialization of highly difficult sludge resource utilization.

Claims

1. A method for enhanced sludge methanogenesis based on sodium disilicate synergistic thermal hydrolysis, characterized in that: The following steps are involved: a) adding sodium disilicate with a particle size of 50-200 mesh to the sludge in an amount of 0.5%-3% of the dry weight of the sludge, and stirring at 50-200 rpm to mix evenly; b) subjecting the mixed sludge to thermal hydrolysis pretreatment, controlling the temperature to 120-180°C, the pressure to 0.5-1.5MPa, and the treatment time to 10-60 minutes; c) introducing the pretreated sludge into an anaerobic digestion unit, reacting at 35-55°C for 10-25 days, and collecting methane gas.

2. The method according to claim 1, characterized in that The dosage of sodium disilicate in step a) is positively correlated with the organic matter content of the sludge, specifically satisfying: sodium disilicate dosage (%) = 0.2 × sludge VS / TS ratio + 0.1, wherein the VS / TS ratio is 0.4-0.

8.

3. The method according to claim 1, characterized in that: The heating rate of the thermal hydrolysis pretreatment in step b) is 3-8°C / min, and the temperature and pressure are controlled to meet the following conditions: P (MPa) = 0.007 × T (°C) + 0.1, wherein T is 120-180°C 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 into the anaerobic digestion unit, wherein the enrichment agent comprises Methanosarcina and Methanothrix strains, and the added amount is 1%-5% of the sludge volume.

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

6. A system for implementing the method according to any one of claims 1 to 5, characterized in that: include: Sludge mixing unit (1): equipped with a screw feeder for quantitatively adding sodium disilicate, a variable frequency speed regulating stirring device and a pH online monitor; thermal hydrolysis reaction unit (2): adopts a jacketed reaction tank with multi-stage gradient heating and an integrated pressure feedback control system; anaerobic digestion unit (3): has a built-in gas-liquid separator and a biogas purification module, and is connected to a methane storage device.

7. The system according to claim 6, characterized in that The circulating medium in the jacket of the thermal 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 thermal hydrolysis reaction unit (2), with a radiation power of 200-500W 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 method according to claim 1, characterized in that The method is suitable for difficult-to-degrade industrial sludge with high solid content (5%-20%) and heavy metal content (the total amount of Cu, Zn and Pb is greater than 500 mg / kg).

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