Method for enhancing anaerobic digestion gas production efficiency of urban sludge by coupling static magnetic field with oily sludge biochar

Abstract

The invention discloses a method for strengthening anaerobic digestion gas production efficiency of urban sludge by coupling a static magnetic field with oily sludge biochar, relates to a method for anaerobic digestion gas production of urban sludge, and aims to solve the technical problem that the anaerobic digestion gas production efficiency is influenced due to high cost and single metal element of the existing magnetized biochar. The method comprises the following steps: pyrolyzing oil sludge at the bottom of an oil storage tank to obtain oil-containing sludge biochar; urban sludge is inoculated and then put into an anaerobic digestion reactor, meanwhile, oil-containing sludge biochar is added, anaerobic digestion is conducted in a static magnetic field after even mixing, and anaerobic digestion gas production treatment of the urban sludge is completed. Oily sludge biochar is coupled with a static magnetic field, so that the electrochemical characteristics in the system are enhanced, electron transfer is accelerated, and rapid conversion of organic matters and improvement of methane yield are realized. The method can be used in the urban sludge treatment field.

Description

Technical Field

[0001] The invention relates to a method for producing gas by anaerobic digestion of urban sludge, and belongs to the technical field of anaerobic digestion of urban sludge. Background Art

[0002] The rapid urbanization and population growth have accelerated the construction of wastewater treatment plants (WWTPs) to meet increasingly stringent discharge standards, which has produced a large amount of inevitable byproducts - urban sludge. Urban sewage sludge contains high concentrations of toxic substances such as heavy metals and complex organic matter. Direct treatment will cause environmental risks and health risks including soil, air and water pollution. Therefore, there is an urgent need to further find economical and effective sludge treatment methods.

[0003] In recent years, anaerobic digestion (AD) has attracted more and more attention as a green and low-carbon technology. On the one hand, AD can convert various organic matter in sewage sludge into renewable bioenergy methane, replace fossil fuels, and realize the resource utilization of sludge. On the other hand, AD can reduce the volume and weight of sludge and achieve sludge reduction. However, the slow hydrolysis process and low biogas production seriously limit its large-scale application. Under this condition, the addition of biochar overcomes the shortcomings of unstable operation, slow hydrolysis efficiency and low methane production of the AD system, improves the buffering capacity of the system, supports the synergistic effect of the microbial community, and accelerates direct interspecies electron transfer (DIET). This requires the development of high-quality biochar with multiple characteristics such as high conductivity, rich functional groups, large specific surface area and porous structure. Therefore, in recent years, the use of "magnetization" to load zero-valent iron or magnetite onto biochar to improve the gas production efficiency of AD has attracted much attention. However, this "magnetization" method increases the cost of the material, and there is also the problem of a single metal element, which affects its gas production efficiency during anaerobic digestion. Summary of the invention

[0004] The present invention aims to solve the technical problem that the existing magnetic biochar has high cost and single metal element which affects the gas production efficiency of anaerobic digestion, and provides a method for coupling oily sludge biochar with static magnetic field to enhance the gas production efficiency of anaerobic digestion of urban sludge.

[0005] The method of the present invention for enhancing the gas production efficiency of anaerobic digestion of urban sludge by coupling the static magnetic field with oily sludge biochar comprises the following steps:

[0006] 1. Preparation of oily sludge biochar: After the oil sludge at the bottom of the oil storage tank is naturally air-dried, it is ground and sieved to obtain oil sludge powder; the oil sludge powder is placed in a magnetic boat, and then the magnetic boat is placed in a tubular furnace, and under nitrogen protection, the temperature is increased to 500-700°C at a heating rate of 5-10°C / min and maintained for 2-3 hours for pyrolysis; the pyrolysis product is ground through a 100-mesh sieve to obtain oily sludge biochar;

[0007] 2. Obtaining anaerobic digestion substrate: let the sludge in the secondary sedimentation tank of the urban sewage treatment plant settle statically, and after the supernatant and the urban sludge are separated, the supernatant is discharged; the volatile matter content (VS) in the urban sludge is measured, and then the inoculated mud is inoculated into the urban sludge to obtain the anaerobic digestion substrate; the amount of inoculated mud added must meet the ratio of the volatile matter content in the inoculated mud to the volatile matter content in the urban sludge of 1: (3 to 3.5);

[0008] 3. Adding oily sludge biochar: Add the anaerobic digestion substrate into the anaerobic digestion reactor, and then add the oily sludge biochar prepared in step 1 into the anaerobic digestion reactor, and shake and mix evenly;

[0009] 4. Anaerobic digestion under a static magnetic field: Select sheet magnets and fix them on both sides of the anaerobic digestion reactor to apply a static magnetic field to the anaerobic digestion reactor; fill the anaerobic digestion reactor with nitrogen to expel the air in the reactor, then seal the reactor, carry out anaerobic digestion for 20 to 30 days, and collect the generated methane gas; during the anaerobic digestion, the temperature of the reactor is controlled at 36 to 38°C, and stirred 4 to 5 times a day, each stirring time is 1 to 1.5 hours, to complete the anaerobic digestion gas production treatment of urban sludge.

[0010] Furthermore, the oil sludge at the bottom of the oil storage tank described in step 1 contains water, petroleum hydrocarbons and metal components;

[0011] Furthermore, the oily sludge biochar obtained in step 1 contains Fe and trace elements in addition to carbon;

[0012] Furthermore, the content of Fe in the oily sludge biochar obtained in step 1 is 5 to 8 mg / g;

[0013] Furthermore, the trace elements in the oily sludge biochar obtained in step one are Ni, Cu, Zn, Cr and Mg.

[0014] Furthermore, the oily sludge biochar obtained in step one has a Ni content of 0.008-0.010 mg / g, a Cu content of 0.012-0.017 mg / g, and a Zn content of 0.018-0.03 mg / g.

[0015] Furthermore, the inoculated mud in step 2 is mud taken from an anaerobic digestion reactor that operates well.

[0016] Furthermore, the amount added in step three is calculated based on the oily sludge biochar concentration of 2.5 to 3.5 g / L.

[0017] Furthermore, in the static magnetic field described in step 4, the magnetic force of the magnet is 40 to 50 mT.

[0018] Furthermore, the stirring in step 4 is carried out at a stirring speed of 180 to 300 rpm.

[0019] The raw material of the present invention, oily sludge, is a common waste in the petroleum industry, which is generated during the exploration, production, transportation, storage and refining of crude oil. It contains water, petroleum hydrocarbons, metals and other components. A large amount of hydrocarbons in the oily sludge are converted into biochar during the pyrolysis process; a large amount of iron in the biochar has the characteristics of magnetite and can enrich functional bacteria such as iron-reducing bacteria, thereby strengthening the hydrolysis of organic matter; the rich trace elements Ni, Cu and Zn in the biochar provide necessary trace elements for the metabolic activities of microorganisms, promote the activity of enzymes, accelerate the transformation of organic matter, and improve the anaerobic digestion of organic matter in urban sludge.

[0020] The present invention utilizes oily sludge biochar coupled with a static magnetic field to strengthen the electrochemical characteristics of the system, which helps accelerate electron transfer, achieve rapid conversion of organic matter and increase methane production / yield. Compared with conventional biochar, the use of oily sludge biochar reduces the input of conventional biochar magnetization process, and realizes the two-way resource utilization of urban sludge and oily sludge. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a graph of cumulative methane production during the anaerobic digestion cycle of urban sludge in Example 1 and Comparative Example 1;

[0022] Figure 2 It is a graph of cumulative methane production during the anaerobic digestion cycle of urban sludge in Example 1 and Comparative Example 2;

[0023] Figure 3 It is a graph of cumulative methane production during the anaerobic digestion cycle of urban sludge in Example 1 and Comparative Example 3;

[0024] Figure 4 This is a graph showing the effect of different oily sludge biochar dosages on the cumulative methane production of urban sludge AD in Example 1 and Examples 2 and 3;

[0025] Figure 5 Graph showing the effect of different magnetic field intensities on the cumulative methane production of urban sludge AD in Example 1 and Example 4;

[0026] Figure 6 This is a graph showing the effect of different rotation speeds on the cumulative methane production of urban sludge AD in Example 1 and Examples 5 and 6. DETAILED DESCRIPTION

[0027] The beneficial effects of the present invention are verified by the following examples.

[0028] Example 1: The method of this example of enhancing the gas production efficiency of anaerobic digestion of urban sludge by coupling the static magnetic field with oily sludge biochar is carried out according to the following steps:

[0029] 1. Preparation of oily sludge biochar: After the oily sludge at the bottom of the petroleum storage tank is naturally air-dried, it is ground and sieved to obtain oily sludge powder; the oily sludge powder is placed in a magnetic boat, and then the magnetic boat is placed in a tubular furnace, and under nitrogen protection, the temperature is increased to 600°C at a heating rate of 5°C / min and maintained for 2 hours for pyrolysis; the pyrolysis product is ground through a 100-mesh sieve to obtain oily sludge biochar, recorded as BC600, and placed in a polyethylene bottle for standby use; in addition, the pyrolysis temperature is changed to 500°C to prepare oily sludge biochar BC500; the pyrolysis temperature is changed to 700°C to prepare oily sludge biochar BC700;

[0030] 2. Preparation of anaerobic digestion substrate: Take the sludge from the secondary sedimentation tank of the urban sewage treatment plant, store it in a refrigerator at 4℃, and let it settle for more than 1 hour. After the supernatant and the urban sludge are separated, discharge the supernatant. Repeat this process for 5 times until there is no obvious separation of the sludge, and then obtain the concentrated urban sludge; shake the concentrated sludge evenly to determine the volatile matter content (VS), and then inoculate the inoculated sludge at a ratio of 3:1 between the VS of the urban sludge and the VS of the inoculated sludge, mix evenly, and obtain the anaerobic digestion substrate; the inoculated sludge is the sludge taken from the anaerobic digestion reactor that is operating well;

[0031] 3. Adding oily sludge biochar: Add the anaerobic digestion substrate into the anaerobic digestion reactor, and then add the oily sludge biochar BC600 prepared in step 1 into the anaerobic digestion reactor. The addition concentration of the oily sludge biochar is 3.0 g / L, and shake to mix evenly;

[0032] 4. Anaerobic digestion under a static magnetic field: Two sheet magnets with similar magnetic force were selected and fixed on both sides of the anaerobic digestion reactor, and a static magnetic field was applied to the anaerobic digestion reactor. The magnetic force of the magnet was 45mT. Nitrogen was filled into the anaerobic digestion reactor for aeration for 5 minutes to expel the air in the reactor. The reactor was then sealed and anaerobic digestion was carried out for 51 days. The generated methane gas was collected and the methane production was measured. During the anaerobic digestion, the anaerobic digestion reactor was placed in a warm water bath to control the temperature of the anaerobic digestion reactor at 37°C, and stirred 4 times a day. Each stirring time was 1 hour and the stirring speed was 200rpm to complete the anaerobic digestion gas production treatment of urban sludge.

[0033] The specific surface area and composition analysis of the oily sludge biochar BC600, BC500 and BC700 prepared in step 1 of this embodiment are shown in Table 1.

[0034] Table 1 Specific surface area and composition of oily sludge biochar BC600, BC500 and BC700 prepared in step 1 of Example 1

[0035]

[0036]

[0037] As can be seen from Table 1, the oily sludge biochar prepared using the sludge at the bottom of the oil storage tank contains not only iron, but also trace elements Ni, Cu, and Zn, which provide necessary trace elements for the metabolic activities of microorganisms, promote enzyme activity, accelerate the transformation of organic matter, and improve the anaerobic digestion of organic matter in urban sludge.

[0038] Comparative Example 1: This comparative example is different from Example 1 in that no static magnetic field is applied in step 4, and the rest is the same as Example 1.

[0039] The cumulative methane production of Example 1 and Comparative Example 1 was compared and tested. Figure 1 As shown, by comparing Example 1 and Comparative Example 1, it is found that the coupled application of the static magnetic field can significantly increase the cumulative methane production of anaerobic digestion of urban sludge compared to the addition of oily sludge biochar alone.

[0040] Comparative Example 2: This comparative example is different from Example 1 in that no oily sludge biochar is added during anaerobic digestion, and only a static magnetic field is applied. The specific operation method is as follows:

[0041] 1. Preparation of anaerobic digestion substrate: This step is the same as step 2 of Example 1;

[0042] 2. Anaerobic digestion: Add anaerobic digestion substrate into the anaerobic digestion reactor, select two sheet magnets with similar magnetic force and fix them on both sides of the anaerobic digestion reactor, apply a static magnetic field to the anaerobic digestion reactor, and the magnetic force of the magnet is 45mT; fill the anaerobic digestion reactor with nitrogen for aeration for 5 minutes to exhaust the air in the reactor, and then seal the reactor for anaerobic digestion. The operation time is 51 days, and the generated methane gas is collected and the methane production is measured; during the anaerobic digestion, place the anaerobic digestion reactor in a warm water bath to control the temperature of the anaerobic digestion reactor at 37°C, and stir it 4 times a day, each stirring time is 1 hour, and the stirring speed is 200rpm, to complete the anaerobic digestion gas production treatment of urban sludge.

[0043] The cumulative methane production of Example 1 and Comparative Example 2 was compared and tested. Figure 2 As shown, by comparing Example 1 with Comparative Example 2, it is found that the coupled application of oily sludge biochar can significantly increase the cumulative methane production of anaerobic digestion of urban sludge compared to the application of a static magnetic field alone.

[0044] Comparative Example 3: This comparative example is different from Example 1 in that no oily sludge biochar is added during anaerobic digestion, and no static magnetic field is applied. The specific operation method is as follows:

[0045] 1. Preparation of anaerobic digestion substrate: This step is the same as step 2 of Example 1;

[0046] 2. Anaerobic digestion and methane production: The anaerobic digestion substrate is added to the anaerobic digestion reactor, and the reactor is filled with nitrogen. After aeration for 5 minutes until all the air is discharged, anaerobic digestion is carried out. The operation time is 51 days, and the generated methane gas is collected and the methane production is measured. During the anaerobic digestion, the anaerobic digestion reactor is placed in a warm water bath to control the temperature of the anaerobic digestion reactor at 37°C, and stirred 4 times a day, each stirring time is 1 hour, and the stirring speed is 200rpm, to complete the anaerobic digestion and gas production treatment of urban sludge.

[0047] The cumulative methane production of Example 1 and Comparative Example 3 was compared and tested. Figure 3 As shown, by comparing Example 1 and Comparative Example 2, it was found that the anaerobic digestion system of static magnetic field coupled with oily sludge biochar had significantly improved cumulative methane production compared with the system without applying static magnetic field and adding oily sludge biochar.

[0048] Example 2: This example is different from Example 1 in that the addition concentration of oily sludge biochar in step 3 is 2.0 g / L, and the rest is the same as Example 1.

[0049] Example 3: This example is different from Example 1 in that the addition concentration of oily sludge biochar in step 3 is 4.0 g / L, and the rest is the same as Example 1.

[0050] The static magnetic field strength and stirring speed of Example 1 and Examples 2 and 3 are the same, and the added concentration of oily sludge biochar is different. The cumulative methane production of Example 1 and Examples 2 and 3 is compared. Figure 4 As shown, the cumulative methane production of Example 1 is much higher than that of Examples 2 and 3, indicating that appropriately increasing the concentration of oily sludge biochar can increase methane production, but excessive oily sludge biochar concentration will inhibit the promotion effect. Therefore, 3.0 g / L biochar is the optimal design concentration.

[0051] Example 4: This example is different from Example 1 in that the magnetic force of the magnet in step 4 is 90 mT, and the rest is the same as Example 1.

[0052] The oily sludge biochar concentrations in Example 1 and Example 4 are the same, the stirring speed is the same, and the static magnetic field strength is different. The cumulative methane production of Example 1 and Example 4 is compared. Figure 5As shown, the cumulative methane production of Example 1 is much higher than that of Example 4, indicating that increasing the intensity of the static magnetic field is not conducive to the generation of methane, and a weak magnetic intensity of 50 mT is more conducive to anaerobic digestion.

[0053] Example 5: This example is different from Example 1 in that the stirring speed in step 4 is 100 rpm, and the rest is the same as Example 1.

[0054] Example 6: This example is different from Example 1 in that the stirring speed in step 4 is 400 rpm, and the rest is the same as Example 1.

[0055] The oily sludge biochar concentrations of Example 1 and Examples 5 and 6 are the same, the magnetic field strength is the same, and the stirring speed is different. The cumulative methane production of Example 1 and Examples 5 and 6 is compared. Figure 6 As shown, the cumulative methane production of Example 1 is much higher than that of Examples 5 and 6, indicating that appropriately increasing the stirring speed is beneficial to the generation of methane, while too fast a stirring speed is not conducive to the microorganisms fully utilizing the organic matter, resulting in a decrease in methane production. Therefore, 200 rpm is the optimal stirring speed.

[0056] The present invention utilizes the common waste oily sludge in the petroleum industry for pyrolysis, and a large amount of hydrocarbons therein are converted into high-value-added carbonaceous materials, and the iron element therein is converted into a magnetic substance, so that the biochar has magnetic properties and is enriched with functional bacteria such as iron-reducing bacteria. At the same time, the rich trace elements therein are also beneficial to the metabolism of microorganisms. Under the action of the coupled magnetic field, the synergistic effect of the magnetic biochar and the magnetic field can be brought into play, the hydrolysis of sludge organic matter is enhanced, the conversion of organic matter is promoted, and the gas production efficiency of the system is improved.

Claims

1. A method for enhancing the gas production efficiency of anaerobic digestion of urban sludge by coupling oily sludge biochar with static magnetic field, characterized in that: The method comprises the following steps:

1. Preparation of oily sludge biochar: After the oil sludge at the bottom of the oil storage tank is naturally air-dried, it is ground and sieved to obtain oil sludge powder; the oil sludge powder is placed in a magnetic boat, and then the magnetic boat is placed in a tubular furnace, and under nitrogen protection, the temperature is increased to 500-700°C at a heating rate of 5-10°C / min and maintained for 2-3 hours for pyrolysis; the pyrolysis product is ground through a 100-mesh sieve to obtain oily sludge biochar; 2. Obtaining anaerobic digestion substrate: let the sludge in the secondary sedimentation tank of the urban sewage treatment plant settle statically, and after the supernatant and the urban sludge are separated, the supernatant is discharged; the content of volatile substances in the urban sludge is determined, and then the inoculated mud is inoculated into the urban sludge to obtain the anaerobic digestion substrate; the amount of inoculated mud added must meet the ratio of the volatile substance content in the inoculated mud to the volatile substance content in the urban sludge of 1: (3 to 3.5); 3. Adding oily sludge biochar: Add the anaerobic digestion substrate into the anaerobic digestion reactor, and then add the oily sludge biochar prepared in step 1 into the anaerobic digestion reactor, and shake and mix evenly; 4. Anaerobic digestion under a static magnetic field: Select sheet magnets and fix them on both sides of the anaerobic digestion reactor to apply a static magnetic field to the anaerobic digestion reactor; fill the anaerobic digestion reactor with nitrogen to expel the air in the reactor, then seal the reactor, carry out anaerobic digestion for 20 to 30 days, and collect the generated methane gas; during the anaerobic digestion, the temperature of the reactor is controlled at 36 to 38°C, and stirred 4 to 5 times a day, each stirring time is 1 to 1.5 hours, to complete the anaerobic digestion gas production treatment of urban sludge.

2. The method of enhancing the gas production efficiency of anaerobic digestion of urban sludge by coupling static magnetic field with oily sludge biochar according to claim 1 is characterized in that: The oil sludge at the bottom of the oil storage tank described in step 1 contains water, petroleum hydrocarbons and metal components.

3. The method for enhancing the gas production efficiency of anaerobic digestion of urban sludge by coupling static magnetic field with oily sludge biochar according to claim 1 or 2, characterized in that: The oily sludge biochar obtained in step 1 contains not only carbon but also Fe and trace elements.

4. The method of enhancing the gas production efficiency of anaerobic digestion of urban sludge by coupling static magnetic field with oily sludge biochar according to claim 3 is characterized in that: The content of Fe element in the oily sludge biochar obtained in step 1 is 5-8 mg / g.

5. According to the method for enhancing the gas production efficiency of anaerobic digestion of urban sludge by static magnetic field coupling with oily sludge biochar as described in claim 3, the trace elements in the oily sludge biochar obtained in step 1 are Ni, Cu, Zn, Cr and Mg.

6. According to the method for enhancing the gas production efficiency of anaerobic digestion of urban sludge by static magnetic field coupling oily sludge biochar according to claim 5, the Ni content in the oily sludge biochar obtained in step 1 is 0.008-0.010 mg / g, the Cu content is 0.012-0.017 mg / g, and the Zn content is 0.018-0.03 mg / g.

7. The method for enhancing the gas production efficiency of anaerobic digestion of urban sludge by coupling static magnetic field with oily sludge biochar according to claim 1 or 2, characterized in that: The amount added in step three is calculated based on the oily sludge biochar concentration of 2.5 to 3.5 g / L.

8. The method of enhancing the gas production efficiency of anaerobic digestion of urban sludge by coupling static magnetic field with oily sludge biochar according to claim 1 or 2, characterized in that: The static magnetic field described in step 4 has a magnetic force of 40 to 50 mT.

9. The method for enhancing the gas production efficiency of anaerobic digestion of urban sludge by coupling static magnetic field with oily sludge biochar according to claim 1 or 2, characterized in that: The stirring in step 4 is carried out at a stirring speed of 180 to 300 rpm.

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