CN113636734B - Method for strengthening methane production efficiency of anaerobic digestion of excess sludge by combining iron-carrying nitrogen-doped composite carbon material with thermal hydrolysis pretreatment - Google Patents

Abstract

A method for strengthening the methane production efficiency of anaerobic digestion of excess sludge by combining iron-carrying nitrogen-doped composite carbon materials with thermal hydrolysis pretreatment relates to a method for strengthening the methane production efficiency of anaerobic digestion of excess sludge by thermal hydrolysis pretreatment. The invention aims to solve the technical problems of low methane production efficiency and poor stability of the existing anaerobic digestion excess sludge. The Fe-N-C material prepared by the invention enhances the electron transfer capability of the biochar through nitrogen modification, and also creatively combines the advantages of the carbon-based material and the iron-based material, thereby further strengthening the DIET path established by the conductive material in the anaerobic digestion methane production process. The Fe-N-C material has magnetism, can be recovered by magnetic force, has low cost, high yield and stable effect, promotes the acid production rate and the extracellular electron transfer rate in the anaerobic digestion process, and promotes the yield and the productivity of methane production by anaerobic digestion.

Description

A kind of iron-loaded and nitrogen-doped composite carbon material combined with thermal hydrolysis pretreatment to strengthen the exhaustion of excess sludge Methods of Oxygen Digestion Methanogenic Efficiency

technical field

The invention relates to a method for thermal hydrolysis pretreatment to strengthen the methane production efficiency of excess sludge anaerobic digestion.

Background technique

Anaerobic digestion technology is a common process for treating urban surplus sludge. It has been widely concerned by researchers for its advantages of high organic matter load, low energy consumption, high degree of sludge reduction, and the ability to produce methane and other energy gases. However, the practical application of anaerobic digestion suffers from two key issues, namely low efficiency and poor stability.

The anaerobic digestion process is mainly divided into three stages: hydrolysis, acidification and methanogenesis. Among them, the hydrolysis stage mainly involves the cracking of extracellular polymeric substances (EPS) of excess sludge and the lysis of microbial cells. Thermal hydrolysis is the most commonly used method for pretreatment of excess sludge. By heating excess sludge to a higher temperature, EPS is broken to form small molecules that are easy to hydrolyze. At the same time, microbial cells also rupture, releasing intracellular various organic matter. The hydrolysis time of excess sludge can be greatly shortened by thermal hydrolysis, which is beneficial to the further progress of anaerobic digestion.

The acidification and methanogenesis process of anaerobic digestion are completed by different bacteria and archaea through metabolism. The sequential interaction between different types of microorganisms is a key factor for efficient anaerobic digestion, which depends on efficient interspecies electron transfer. Direct interspecies electron transfer (DIET) directly transfers electrons from electron donors such as acid-producing bacteria to electron acceptors such as methanogens through the conductive cilia of microorganisms or extracellular conductors. Since there is no longer the need to generate hydrogen gas as an electron transport carrier during the electron transfer process, and it is no longer limited by the low partial pressure of hydrogen gas, DIET is spontaneous in reaction thermodynamics while reducing the energy loss during the reaction process. The speed of the direct transfer of electrons between species is also an order of magnitude higher than that of the traditional indirect transfer of hydrogen electrons between species, which accelerates the rate of methane production during anaerobic digestion.

Carbon-based materials (granular activated carbon, powdered activated carbon, biochar, and graphite, etc.) and iron-based materials (zero-valent iron, magnetite, and ferrihydrite, etc.) are commonly used to mediate the DIET pathway in the anaerobic digestion process of methanogenesis. conductor substance. However, due to their different material properties, the mechanism of action of the two is not the same. Carbon-based materials mainly transfer electrons through the active groups on the surface of the material, and at the same time provide attachment points for microorganisms with their large specific surface area to enrich relevant functional microorganisms, thereby establishing the DIET pathway and strengthening anaerobic digestion for methane production. Iron-based materials mainly use their high electrical conductivity to directly establish electronic pathways between acid-producing bacteria and methanogens to complete the DIET process. At the same time, a small amount of iron can also be used as one of the components of the coenzyme to speed up the acid-producing process. , thereby improving the efficiency of anaerobic digestion for methane production.

Contents of the invention

The present invention aims to solve the technical problems of low methane production efficiency and poor stability of the existing anaerobic digestion surplus sludge, and provides an iron-carrying nitrogen-doped composite carbon material combined with thermal hydrolysis pretreatment to strengthen the anaerobic digestion production of surplus sludge. Methane Efficiency Methods.

The iron-carrying nitrogen-doped composite carbon material combined with thermal hydrolysis pretreatment of the present invention strengthens the method for anaerobic digestion of excess sludge to produce methane, which is carried out according to the following steps:

1. Select the remaining sludge in the secondary sedimentation tank of the municipal sewage treatment plant, sieve through a 10-20 mesh screen to remove large-grained sand and gravel, let it settle for 30-40 minutes to concentrate, and remove part of the supernatant to make the remaining sludge The total suspended solids concentration TSS is 16g/L-25g/L, the volatile suspended solids VSS is 9g/L-13g/L, and the pH of the remaining sludge is 6.2-6.8 by adding an acid-base regulator;

2. Put the remaining sludge treated in step 1 into the reactor, heat it to 150°C-160°C and keep it warm for 30min-40min, then take out the remaining sludge after naturally cooling down to room temperature, and store it at 4°C for later use;

3. Pretreatment of carbon materials: Soak biochar in HCl aqueous solution with a pH of 1~2 for 24h~25h to remove ash and organic matter on the surface of activated carbon; filter, then wash with distilled water at 100°C, and place at 105°C~ Dry it in a drying oven at 110°C, put it into a weighing bottle and place it in a desiccator for standby, and obtain pretreated activated carbon;

The biochar is commercial coconut shell biochar, with a particle size of 100 mesh to 150 mesh and a specific surface area of 200m ² /g to 400m ² /g;

4. Prepare 4mol/L～8mol/L HNO ₃ aqueous solution, add the activated carbon pretreated in step 3, shake and react for 6h～7h, let stand, filter, wash with water until the pH remains unchanged, put the washed sample in a dry place Dry it in the oven at 105°C to 110°C, put it into a weighing bottle and put it in a desiccator for standby to obtain pre-oxidized biochar;

5. In order to incorporate nitrogen into the carbon material to improve its biocompatibility and increase its electron transfer capability, put the biochar pre-oxidized in step 4 into a tube furnace and raise the temperature to 600 ° _C in a pure N atmosphere. ℃～900℃, then feed the mixed gas of N ₂ and NH ₃ according to the volume ratio of 1:1, keep the temperature in the mixed gas of N ₂ and NH ₃ and the condition of 600℃～900℃ for 2h～6h, then in pure Naturally lower the temperature in _N2 atmosphere to obtain nitrogen-doped biochar; the heating and cooling rates in step 5 are both 5°C/min-10°C/min; the gas flow rate in step 5 is 0.2L/min-1L/min;

6. In order to load iron in nitrogen-doped carbon materials, so that it has the advantage of iron-based materials mediating DIET pathway in anaerobic digestion of methane, load Fe ₃ O ₄ on the surface of nitrogen-doped carbon materials by co-precipitation method: the The ultrapure water is aerated with N ₂ to remove dissolved oxygen, and then H ₂ SO ₄ is added under the condition of N ₂ aeration to adjust the pH to 1-2. Under the condition of N ₂ aeration, FeSO ₄ ·7H ₂ O and FeCl ₃ ·6H ₂ O is dissolved in it so that the concentrations of FeSO ₄ ·7H ₂ O and FeCl ₃ ·6H ₂ O are 0.075mol/L～0.08mol/L and 0.1125mol/L～0.113mol/L respectively _. Heating to 60°C-70°C constant temperature under the condition of N ₂ aeration, then adding the nitrogen-doped biochar prepared in step 5 under the condition of N ₂ aeration, magnetic stirring for 15-60 min under N 2 aeration, and aeration and stirring under N ₂ Add NaOH to adjust the pH to 9-10, stop stirring (at this time, magnetic substances will appear), age in a water bath at 60°C to 90°C and continue to expose to _N2 for 2h to 6h, wash and filter, and store at 105°C The Fe-NC material can be obtained after vacuum drying at ~110°C; the ratio of the total mass of the iron element of FeSO ₄ 7H ₂ O and the iron element of FeCl ₃ 6H ₂ O to the mass of nitrogen-doped biochar is 1: 3;

7. The inoculum sludge for anaerobic digestion and methane production is the effluent sludge from the top of the municipal sludge anaerobic fermentation tank, the total suspended solids concentration TSS is 55g/L-75g/L, and the volatile suspended solids VSS is 27g/L-38g /L, pH is 7～7.5; With the leftover sludge preserved in step 2 at 4 ℃ for use as the digested bottom mud of anaerobic digestion methanogen, according to the volume ratio of digested bottom mud and inoculation seed mud is (9～10 ): 1 for mixing, put it into an anaerobic fermentation bottle, add the Fe-N-C material obtained in step 6 to it according to the dosage concentration of 5g/L～10g/L, and carry out anaerobic digestion in a constant temperature shaking incubator to produce methane In the experiment, the temperature was 33° C. to 37° C., and the rotation speed was 120 rpm to 200 rpm until no more gas was generated in the anaerobic fermentation bottle.

In step seven, regularly collect the gas in the air bag at the upper end of the bottle mouth to measure the volume and composition, and measure the conductivity of the sludge at the end of the test.

The invention mainly modifies the biochar by ammonia nitrogen modification and loading ferric oxide, enhances the electron transfer ability of the biochar and combines it with iron-based materials, and prepares the iron-loaded nitrogen-doped composite carbon material Fe-N-C , Fe-N-C materials can simultaneously take advantage of the advantages of carbon-based materials and iron-based materials in mediating the DIET pathway in the process of anaerobic digestion and methanogenesis, efficiently transfer electrons, provide microbial attachment points and increase the activity of key acid-producing enzymes, and increase the remaining pollution. Methanogenic efficiency of mud anaerobic digestion.

On the basis of thermal hydrolysis pretreatment of excess sludge, the present invention establishes a DIET approach in the anaerobic digestion process by preparing and adding Fe-N-C materials, accelerates the production rate of methane, and improves the efficiency of the anaerobic digestion reactor. stability.

The innovation and principle of the present invention: the innovation mainly lies in the preparation process of the Fe-N-C material in steps 3 to 6; the carbon-based material mainly transfers electrons through the active groups on the surface of the material, and at the same time provides microorganisms with a large specific surface area. point, enrich relevant functional microorganisms, and thus establish the DIET pathway; and iron-based materials mainly use their high electrical conductivity to directly establish the electronic pathway between acid-producing bacteria and methanogenic bacteria to complete the DIET process, while a small amount of Iron also serves as one of the components of the coenzyme to accelerate the acid production process, thereby enhancing the efficiency of anaerobic digestion for methane production. The Fe-N-C material prepared by the present invention creatively combines the advantages of carbon-based materials and iron-based materials, and at the same time, through nitrogen modification, the electron transfer ability of biochar is enhanced, and the conductive material is further strengthened in the process of anaerobic digestion and methane production. The DIET pathway established in . The Fe-N-C material of the present invention has magnetism, can be recovered by magnetic force, has low cost and high profit, and has a stable effect, promotes the acid production rate and extracellular electron transfer rate in the anaerobic digestion process, and improves the output and production rate of anaerobic digestion methane. Rate.

Description of drawings

Fig. 1 is the schematic diagram of the anaerobic fermentation bottle in the step seven of test one, and 1 is a sampling port, and 2 is Fe-N-C material, and 3 is an air bag;

Fig. 2 is the SEM picture of the Fe-N-C material that the step 6 of test one prepares;

Fig. 3 is the first EDS figure of the Fe-N-C material that the step 6 of test one prepares;

Fig. 4 is the second EDS figure of the Fe-N-C material that the step 6 of test one prepares;

Fig. 5 is the 3rd EDS figure of the Fe-N-C material that the step 6 of test one prepares;

Fig. 6 is the XPS figure of the Fe-N-C material that the step 6 of test one prepares;

Fig. 7 is the XRD figure of the Fe-N-C material that the step 6 of test one prepares;

Fig. 8 is a graph showing the change of cumulative methane production with anaerobic digestion time in the process of anaerobic digestion of excess sludge when adding different materials.

Detailed ways

Specific implementation mode 1: This implementation mode is a method for strengthening the anaerobic digestion of excess sludge to produce methane by combining iron-carrying nitrogen-doped composite carbon material with thermal hydrolysis pretreatment. Specifically, it is carried out according to the following steps:

1. Select the remaining sludge in the secondary sedimentation tank of the municipal sewage treatment plant, sieve through a 10-20 mesh screen to remove large-grained sand and gravel, let it settle for 30-40 minutes to concentrate, and remove part of the supernatant to make the remaining sludge The total suspended solids concentration TSS is 16g/L-25g/L, the volatile suspended solids VSS is 9g/L-13g/L, and the pH of the remaining sludge is 6.2-6.8 by adding an acid-base regulator;

2. Put the remaining sludge treated in step 1 into the reactor, heat it to 150°C-160°C and keep it warm for 30min-40min, then take out the remaining sludge after naturally cooling down to room temperature, and store it at 4°C for later use;

3. Pretreatment of carbon materials: Soak biochar in HCl aqueous solution with a pH of 1~2 for 24h~25h to remove ash and organic matter on the surface of activated carbon; filter, then wash with distilled water at 100°C, and place at 105°C~ Dry it in a drying oven at 110°C, put it into a weighing bottle and place it in a desiccator for standby, and obtain pretreated activated carbon;

The biochar is commercial coconut shell biochar, with a particle size of 100 mesh to 150 mesh and a specific surface area of 200m ² /g to 400m ² /g;

4. Prepare 4mol/L～8mol/L HNO ₃ aqueous solution, add the activated carbon pretreated in step 3, shake and react for 6h～7h, let stand, filter, wash with water until the pH remains unchanged, put the washed sample in a dry place Dry it in the oven at 105°C to 110°C, put it into a weighing bottle and put it in a desiccator for standby to obtain pre-oxidized biochar;

5. Put the pre-oxidized biochar in step 4 into a tube furnace, raise the temperature to 600°C~900°C in a pure N ₂ atmosphere, and then feed the mixed gas of N ₂ and NH ₃ according to the volume ratio of 1:1 , in the mixed gas of N ₂ and NH ₃ and the temperature of 600°C to 900°C for 2h to 6h, and then naturally lower the temperature in the pure N ₂ atmosphere to obtain nitrogen-doped biochar; the heating and cooling rates in step 5 are 5°C/min～10°C/min; the gas flow in step 5 is 0.2L/min～1L/min;

6. Aerate the ultrapure water with N ₂ to remove dissolved oxygen, then add H ₂ SO ₄ under the condition of N ₂ aeration to adjust the pH to 1~2, and add FeSO ₄ ·7H ₂ under the condition of N ₂ aeration O and FeCl ₃ ·6H ₂ O are dissolved in it so that the concentrations of FeSO ₄ ·7H ₂ O and FeCl ₃ ·6H ₂ O are 0.075mol/L～0.08mol/L and 0.1125mol/L～0.113mol/L respectively. Heating to a constant temperature of 60°C to 70°C under the condition of _N2 aeration, and then adding _the nitrogen-doped biochar prepared in Step 5 under the condition of N2 aeration, stirring magnetically for 15min to 60min under _N2 aeration continuously _, and then Add NaOH under air and stirring conditions to adjust the pH to 9~10, stop stirring, age in a water bath at 60°C~90°C and continue to expose to _N2 for 2h~6h, wash with water, filter, and vacuum dry at 105°C~110°C Afterwards, the Fe-NC material is obtained; the ratio of the total mass of the iron element of FeSO ₄ 7H ₂ O and the iron element of FeCl ₃ 6H ₂ O to the mass of nitrogen-doped biochar is 1:3;

7. The inoculum sludge for anaerobic digestion and methane production is the effluent sludge from the top of the municipal sludge anaerobic fermentation tank, the total suspended solids concentration TSS is 55g/L-75g/L, and the volatile suspended solids VSS is 27g/L-38g /L, pH is 7～7.5; With the leftover sludge preserved in step 2 at 4 ℃ for use as the digested bottom mud of anaerobic digestion methanogen, according to the volume ratio of digested bottom mud and inoculation seed mud is (9～10 ): 1 for mixing, put it into an anaerobic fermentation bottle, add the Fe-N-C material obtained in step 6 to it according to the dosage concentration of 5g/L～10g/L, and carry out anaerobic digestion in a constant temperature shaking incubator to produce methane In the experiment, the temperature was 33° C. to 37° C., and the rotation speed was 120 rpm to 200 rpm until no more gas was generated in the anaerobic fermentation bottle.

Specific embodiment two: the difference between this embodiment and specific embodiment one is: select the residual sludge of the secondary settling tank of the municipal sewage treatment plant in the step one, remove the large-grain sand and gravel through 15 mesh sieves, let stand and settle for 30min Concentrate, remove part of the supernatant so that the total suspended solids concentration TSS of the remaining sludge is 20g/L, the volatile suspended solids VSS is 10g/L, and the pH of the remaining sludge is 6.5 by adding an acid-base regulator. Others are the same as the first embodiment.

Specific embodiment 3: The difference between this embodiment and specific embodiment 1 or 2 is that in step 2, take the remaining sludge treated in step 1 and put it into the reactor, heat it to 155 ° C and keep it warm for 30 minutes, and then cool it down to room temperature naturally The remaining sludge was taken out and stored at 4°C until use. Others are the same as those in Embodiment 1 or 2.

Embodiment 4: The difference between this embodiment and Embodiment 1 to 3 is that in step 3, the biochar is soaked in an aqueous HCl solution with a pH of 1 to 2 for 24 hours to remove ash and organic matter on the surface of the activated carbon; filter, and then Wash with distilled water at 100°C, dry in a drying oven at 105°C, put into a weighing bottle and place in a desiccator for standby to obtain pretreated activated carbon. Others are the same as one of the specific embodiments 1 to 3.

Specific embodiment five: the difference between this embodiment and specific embodiment four is: in step four, prepare 6mol/L _HNO3 aqueous solution, add the activated carbon after step three pretreatment, shake and react for 6h, let stand, filter, wash with water to pH Until the same, put the washed sample in a drying oven to dry at 105°C, put it into a weighing bottle and put it in a desiccator for later use to obtain pre-oxidized biochar. Others are the same as in Embodiment 4.

Specific embodiment six: the difference between this embodiment and specific embodiment five is: in step five, put the biochar pre-oxidized in step four into a tube furnace, heat up to 800°C in pure _N2 atmosphere, and then follow 1 : The volume ratio of 1 is passed into the mixed gas of _N2 and _NH3 , the mixed gas of _N2 and _NH3 is kept at a constant temperature of 5h under the condition of 800°C, and then the temperature is naturally lowered in pure _N2 atmosphere to obtain nitrogen-doped biochar; The heating and cooling rates in Step 5 are both 8° C./min; the gas flow rate in Step 5 is 0.8 L/min. Others are the same as in the fifth embodiment.

Embodiment 7: The difference between this embodiment and Embodiment 6 is that in step 6, the ultrapure water is aerated with N ₂ to remove dissolved oxygen, and then H ₂ SO ₄ is added under the condition of N ₂ aeration to adjust the pH to 1~2, under the condition of N ₂ aeration, dissolve FeSO ₄ 7H ₂ O and FeCl ₃ 6H ₂ O in it so that the concentrations of FeSO ₄ 7H ₂ O and FeCl ₃ 6H ₂ O are 0.075mol/ L and 0.1125mol/L, heated to a constant temperature of 60°C under the condition of N ₂ aeration, then added the nitrogen-doped biochar prepared in step 5 under the condition of N ₂ aeration, and continued magnetic stirring under N ₂ for 40 min, Under the condition of N ₂ aeration and stirring, NaOH was added to adjust the pH to 9, the stirring was stopped, and the Fe-NC material was obtained after being aged in a water bath at 80°C and continuously exposed to N ₂ for 4 hours, washed with water and filtered, and vacuum-dried at 105°C . Others are the same as the sixth embodiment.

Embodiment 8: The difference between this embodiment and Embodiment 7 is that in step 7, the remaining sludge stored at 4°C in step 2 is used as an anaerobic digestion methane-producing digested bottom sludge, according to the digested bottom sludge and The volume ratio of the inoculated seed slurry is 9:1 and mixed, put into an anaerobic fermentation bottle, add the Fe-N-C material obtained in step 6 to it according to the dosage concentration of 8g/L, and carry out anaerobic fermentation in a constant temperature shaking incubator. In the digestion methane production experiment, the temperature was 35° C., and the rotation speed was 180 rpm, until no more gas was produced in the final anaerobic fermentation bottle. Others are the same as the seventh embodiment.

The present invention is verified with following test:

Test 1: This test is a method of enhancing the efficiency of anaerobic digestion of excess sludge for methane production by combining iron-carrying nitrogen-doped composite carbon materials with thermal hydrolysis pretreatment. The specific steps are as follows:

1. Select the remaining sludge in the secondary sedimentation tank of the municipal sewage treatment plant, sieve through a 10-mesh screen to remove large particles of sand and gravel, let it settle for 30 minutes to concentrate, and remove part of the supernatant to make the total suspended solids concentration of the remaining sludge TSS The volatile suspended solids VSS is 22820mg/L, and the volatile suspended solids VSS is 12147mg/L. By adding an acid-base regulator, the pH of the remaining sludge is 6.41;

2. Put the remaining sludge treated in step 1 into the reactor, heat it to 150°C and keep it warm for 30 minutes, then take out the remaining sludge after naturally cooling down to room temperature, and store it at 4°C for later use;

3. Pretreatment of carbon materials: Soak biochar in HCl aqueous solution with a pH of 1 for 24 hours to remove ash and organic matter on the surface of activated carbon; filter, then wash with distilled water at 100°C, and dry in a drying oven at 105°C Dry, put into weighing bottle and be placed in desiccator for subsequent use, obtain the activated carbon after pretreatment;

Described biochar is commercial coconut shell biochar, and its particle size is 125 meshes, and specific surface area is 236m ² /g;

4. Prepare 5mol/L HNO ₃ aqueous solution, add activated carbon pretreated in step 3, shake for 6 hours, let stand, filter, wash with water until the pH remains unchanged, put the washed sample in a drying oven and dry at 105°C , put into a weighing bottle and put it in a desiccator for standby, so as to obtain the pre-oxidized biochar;

5. In order to incorporate nitrogen into the carbon material to improve its biocompatibility and increase its electron transfer capability, put the biochar pre-oxidized in step 4 into a tube furnace and raise the temperature to 700 ° _C in a pure N atmosphere. ℃, then according to the volume ratio of 1:1, the mixed gas of N ₂ and NH ₃ is introduced, and the temperature is kept at 700 ℃ for 2 hours under the mixed gas of N ₂ and NH ₃ , and then the temperature is naturally lowered in the pure N ₂ atmosphere, and the obtained Nitrogen-doped biochar; the heating and cooling rates in step five are both 10°C/min; the gas flow rate in step five is 0.4L/min;

6. In order to load iron in nitrogen-doped carbon materials, so that it has the advantage of iron-based materials mediating DIET pathway in anaerobic digestion of methane, load Fe ₃ O ₄ on the surface of nitrogen-doped carbon materials by co-precipitation method: the The ultrapure water was aerated with N ₂ to remove dissolved oxygen, and then H ₂ SO ₄ was added under the condition of N ₂ aeration to adjust the pH to 1. Under the condition of N ₂ aeration, FeSO ₄ ·7H ₂ O and FeCl ₃ · Dissolve 6H ₂ O in it so that the concentrations of FeSO ₄ 7H ₂ O and FeCl ₃ 6H ₂ O are 0.075mol/L and 0.1125mol/L respectively, heat to 65°C under the condition of N ₂ aeration, and then Add the nitrogen-doped biochar prepared in step 5 under the condition of N ₂ aeration, continue magnetic stirring under N ₂ for 15-60 min, add NaOH under the condition of N ₂ aeration and stirring to adjust the pH to 10, and stop stirring (at this time Magnetic substances will appear), aged for 2 hours in a water bath at 65°C and continuously exposed to N ₂ , washed with water and filtered, and vacuum-dried at 105°C to obtain the Fe-NC material; the iron element of FeSO ₄ ·7H ₂ O The ratio of the total mass of iron element and FeCl ₃ 6H ₂ O to the mass of nitrogen-doped biochar is 1:3;

7. The inoculated seed sludge for anaerobic digestion and methane production is the effluent sludge at the top of the municipal sludge anaerobic fermentation tank, the total suspended solids concentration TSS is 64322mg/L, the volatile suspended solids VSS is 31652mg/L, and the pH is 7.3; In step 2, the remaining sludge stored at 4°C for use is the digested sludge produced by anaerobic digestion and methane, mixed according to the volume ratio of the digested sludge and the inoculated seed sludge at 9:1, and put into an anaerobic fermentation bottle. Add the Fe-N-C material obtained in step 6 to it according to the dosage concentration of 5g/L, and conduct the anaerobic digestion methane production experiment in a constant temperature shaking incubator at a temperature of 35°C and a rotation speed of 180rpm. No mud will enter during the experiment. No sludge discharge continued for 20 days, and finally no gas was produced in the anaerobic fermentation bottle; the gas in the air bag 3 at the upper end of the bottle mouth was regularly collected to measure the volume and composition, and the sludge conductivity was measured at the end of the test.

Fig. 2 is the SEM figure of the Fe-N-C material prepared in step 6 of test one, and Fig. 3 is the first EDS figure of the Fe-N-C material prepared in step 6 of test one, which is a synthetic figure of three elements; Fig. 4 is test The second EDS figure of the Fe-N-C material prepared in step 6 of one is a separate iron element; Fig. 5 is the third EDS figure of the Fe-N-C material prepared in step 6 of test one, which is a separate oxygen element; The iron-containing substances deposited on the surface of activated carbon can be clearly seen in the figure.

Figure 6 is the XPS diagram of the Fe-N-C material prepared in step 6 of Test 1. It can be seen from the figure that the mass fraction of Fe element in the Fe-N-C material reaches 26.69%, and the mass fraction of N element reaches 3.09%.

Figure 7 is the XRD pattern of the Fe-NC material prepared in Step 6 of Test 1. It can be seen from the figure that the Fe element attached to the surface of the carbon-based material by co-precipitation in Step 6 mainly exists in the form of Fe ₃ O ₄ .

Test 2: This test is a comparative test, and a total of 5 groups of controls are set up. Compared with Test 1, the difference is that the materials added in Step 7 are replaced with the following 5 substances in equal amounts for comparison:

①No material added;

② non-conductive glass beads;

③ Commercial coconut shell biochar purchased in step 3 of test 1;

④ the nitrogen-doped biochar obtained in the step five of the test one;

⑤ Directly purchased magnetite Fe ₃ O ₄ .

为试验一的步骤三中购买的商用椰壳生物炭，◇为④试验一的步骤五中得到的掺氮生物炭，

为⑤直接购买的磁铁矿Fe₃O₄。可以看出相比于①不加任何材料，试验一中添加Fe-N-C材料的条件下甲烷累积产量提升了44.52％，而对比②～⑤分别较对比①提升了3.08％、29.11％、36.64％和39.73％，这说明不带电的玻璃珠对于剩余污泥的厌氧产甲烷效能几乎没有影响，而商用椰壳活性炭、掺氮生物炭和Fe₃O₄由于其导电特性都对产甲烷效能有所提升，但不如试验一制备的Fe-N-C材料，可见Fe-N-C材料的优越性。The test results are shown in Table 1 and Figure 8. In Figure 8, □ is the Fe-NC material prepared in Test 1, ○ is ① without adding any material, △ is ② non-conductive glass beads,

◇ is the commercial coconut shell biochar purchased in step 3 of test 1, ◇ is the nitrogen-doped biochar obtained in step 5 of ④ test 1,

⑤ directly purchased magnetite Fe ₃ O ₄ . It can be seen that compared with ① without adding any materials, the cumulative methane production increased by 44.52% under the condition of adding Fe-NC materials in Test 1, while comparisons ② to ⑤ increased by 3.08%, 29.11%, and 36.64% respectively compared with comparison ① and 39.73%, which shows that uncharged glass beads have almost no effect on the anaerobic methanogenesis performance of the remaining sludge, while commercial coconut shell activated carbon, nitrogen-doped biochar and Fe ₃ O ₄ all have an effect on the methanogenesis performance due to their conductive properties. It is improved, but not as good as the Fe-NC material prepared in Experiment 1, which shows the superiority of Fe-NC material.

Sludge conductivity is the most intuitive indicator to measure whether the DIET pathway is established. Compared with comparison ①, the sludge conductivity of test 1 and comparison ② to ⑤ increased by 6.8, -0.01, 3.56, 4.83 and 6.67 times, respectively , which shows that uncharged glass beads cannot establish the DIET pathway, but conductive Fe-NC materials, commercial coconut shell activated carbon, nitrogen-doped biochar and Fe ₃ O ₄ can be established, among which the Fe-NC material group prepared in Experiment 1 Sludge has the highest activity for DIET, so the sludge conductivity is also the largest. The method of this test has easy-to-obtain raw materials, recyclable materials, low cost, and significantly improved methane production efficiency.

Table 1 Effects of different materials on the anaerobic methanogenesis efficiency of excess sludge

Claims (8)

1. A method for enhancing the performance of excess sludge anaerobic digestion of methane with iron-carrying nitrogen-doped composite carbon material combined with thermal hydrolysis pretreatment, characterized in that iron-carrying nitrogen-doped composite carbon material combined with thermal hydrolysis pretreatment strengthens excess sludge The method of anaerobic digestion methane efficiency is carried out in the following steps: 1. Select the remaining sludge in the secondary sedimentation tank of the municipal sewage treatment plant, sieve through a 10-20 mesh screen to remove large-grained sand and gravel, let it settle for 30-40 minutes to concentrate, and remove part of the supernatant to make the remaining sludge The total suspended solids concentration TSS is 16g/L-25g/L, the volatile suspended solids VSS is 9g/L-13g/L, and the pH of the remaining sludge is 6.2-6.8 by adding an acid-base regulator; 2. Put the remaining sludge treated in step 1 into the reactor, heat it to 150°C-160°C and keep it warm for 30min-40min, then take out the remaining sludge after naturally cooling down to room temperature, and store it at 4°C for later use; 3. Pretreatment of carbon materials: Soak biochar in HCl aqueous solution with a pH of 1~2 for 24h~25h to remove ash and organic matter on the surface of activated carbon; filter, then wash with distilled water at 100°C, and place at 105°C~ Dry it in a drying oven at 110°C, put it into a weighing bottle and place it in a desiccator for standby, and obtain pretreated activated carbon; The biochar is commercial coconut shell biochar, with a particle size of 100 mesh to 150 mesh and a specific surface area of 200m ² /g to 400m ² /g; 4. Prepare 4mol/L～8mol/L HNO ₃ aqueous solution, add the activated carbon pretreated in step 3, shake and react for 6h～7h, let stand, filter, wash with water until the pH remains unchanged, put the washed sample in a dry place Dry it in the oven at 105°C to 110°C, put it into a weighing bottle and put it in a desiccator for standby to obtain pre-oxidized biochar; 5. Put the pre-oxidized biochar in step 4 into a tube furnace, raise the temperature to 600°C~900°C in a pure N ₂ atmosphere, and then feed the mixed gas of N ₂ and NH ₃ according to the volume ratio of 1:1 , in the mixed gas of N ₂ and NH ₃ and the temperature of 600°C to 900°C for 2h to 6h, and then naturally lower the temperature in the pure N ₂ atmosphere to obtain nitrogen-doped biochar; the heating and cooling rates in step 5 are 5°C/min～10°C/min; the gas flow in step 5 is 0.2L/min～1L/min; 6. Aerate the ultrapure water with N ₂ to remove dissolved oxygen, then add H ₂ SO ₄ under the condition of N ₂ aeration to adjust the pH to 1~2, and add FeSO ₄ ·7H ₂ under the condition of N ₂ aeration O and FeCl ₃ ·6H ₂ O are dissolved in it so that the concentrations of FeSO ₄ ·7H ₂ O and FeCl ₃ ·6H ₂ O are 0.075mol/L～0.08mol/L and 0.1125mol/L～0.113mol/L respectively. Heating to a constant temperature of 60°C to 70°C under the condition of _N2 aeration, and then adding _the nitrogen-doped biochar prepared in Step 5 under the condition of N2 aeration, stirring magnetically for 15min to 60min under _N2 aeration continuously _, and then Add NaOH under air and stirring conditions to adjust the pH to 9~10, stop stirring, age in a water bath at 60°C~90°C and continue to expose to _N2 for 2h~6h, wash with water, filter, and vacuum dry at 105°C~110°C Then Fe-NC material is obtained; the ratio of the total mass of FeSO ₄ 7H ₂ O iron element and FeCl ₃ 6H ₂ O iron element to the mass of nitrogen-doped biochar is 1:3; 7. The inoculum sludge for anaerobic digestion and methane production is the effluent sludge from the top of the municipal sludge anaerobic fermentation tank, the total suspended solids concentration TSS is 55g/L-75g/L, and the volatile suspended solids VSS is 27g/L-38g /L, pH is 7～7.5; With the leftover sludge preserved in step 2 at 4 ℃ for use as the digested bottom mud of anaerobic digestion methanogen, according to the volume ratio of digested bottom mud and inoculation seed mud is (9～10 ): 1 for mixing, put it into an anaerobic fermentation bottle, add the Fe-N-C material obtained in step 6 to it according to the dosage concentration of 5g/L～10g/L, and carry out anaerobic digestion in a constant temperature shaking incubator to produce methane In the experiment, the temperature was 33° C. to 37° C., and the rotation speed was 120 rpm to 200 rpm until no more gas was generated in the anaerobic fermentation bottle. 2. a kind of iron-carrying nitrogen-doped composite carbon material according to claim 1 combines thermal hydrolysis pretreatment to strengthen the method for anaerobic digestion of excess sludge to produce methane efficiency, it is characterized in that in the step 1, select the second part of the municipal sewage treatment plant The remaining sludge in the settling tank is sieved through a 15-mesh screen to remove large-grained sand and stones, left to settle for 30 minutes to concentrate, and part of the supernatant is removed so that the total suspended solids concentration TSS of the remaining sludge is 20g/L, and the volatile suspended solids The VSS is 10g/L, and the pH of the remaining sludge is 6.5 by adding an acid-base regulator. 3. a kind of iron-carrying nitrogen-doped composite carbon material according to claim 1 combines thermal hydrolysis pretreatment to strengthen the method for anaerobic digestion of excess sludge to produce methane efficiency, it is characterized in that step 2 takes the remaining Put the sludge into the reaction kettle, heat it to 155°C and keep it warm for 30 minutes, and then take out the remaining sludge after naturally cooling down to room temperature, and store it at 4°C until use. 4. a kind of iron-carrying nitrogen-doped composite carbon material according to claim 1 combines thermal hydrolysis pretreatment method to strengthen the method for anaerobic digestion of excess sludge to produce methane efficiency, it is characterized in that in step 3, biochar is mixed at pH 1 Soak in ~2 HCl aqueous solution for 24 hours to remove ash and organic matter on the surface of activated carbon; filter, then wash with distilled water at 100°C, dry in a drying oven at 105°C, put it in a weighing bottle and place it in a desiccator for later use. Get pretreated activated carbon. 5. a kind of iron-carrying nitrogen-doped composite carbon material according to claim 1 combines thermal hydrolysis pretreatment to strengthen the method for anaerobic digestion of excess sludge to produce methane efficiency, it is characterized in that _the HNO of 6mol/L is prepared in step 4 Add the activated carbon pretreated in step 3 to the aqueous solution, shake and react for 6 hours, let it stand, filter, and wash with water until the pH remains unchanged. Put the washed sample in a drying oven to dry at 105°C, put it in a weighing bottle and place it in a dry place. The device is used for standby to obtain pre-oxidized biochar. 6. The method of a kind of iron-carrying nitrogen-doped composite carbon material combined with thermal hydrolysis pretreatment according to claim 1 to strengthen the anaerobic digestion of excess sludge to produce methane efficiency, it is characterized in that in step 5, after step 4 is pre-oxidized Biochar was placed in a tube furnace, heated to 800°C in a pure N ₂ atmosphere, and then a mixed gas of N ₂ and NH ₃ was introduced according to a volume ratio of 1:1, and the mixed gas of N ₂ and NH ₃ and 800 The temperature was kept at ℃ for 5 hours, and then the temperature was naturally lowered in pure N ₂ atmosphere to obtain nitrogen-doped biochar; the heating and cooling rates in step 5 were both 8 ℃/min; the gas flow rate in step 5 was 0.8 L/min. 7. The method of a kind of iron-carrying nitrogen-doped composite carbon material combined with thermal hydrolysis pretreatment to strengthen the anaerobic digestion of excess sludge to produce methane according to claim 1, characterized in that in step 6, the ultrapure water is passed through N ₂ Aerate to remove dissolved oxygen, then add H ₂ SO ₄ under the condition of N ₂ aeration to adjust the pH to 1~2, and dissolve FeSO ₄ ·7H ₂ O and FeCl ₃ ·6H ₂ O under the condition of N ₂ aeration Make the concentrations of FeSO ₄ 7H ₂ O and FeCl ₃ 6H ₂ O in it to be 0.075mol/L and 0.1125mol/L respectively, heat to 60°C under the condition of N ₂ aeration, and then aerate in N ₂ Add the nitrogen-doped biochar prepared in step 5 under the condition of 2, continuously expose to N ₂ and magnetically stir for 40 minutes, add NaOH under the condition of N ₂ aeration and stirring to adjust the pH to 9, stop stirring, and continue to expose to N in a water bath at 80°C ₂ under the condition of aging for 4 hours, washed with water and filtered, and vacuum-dried at 105°C to obtain the Fe-NC material. 8. A kind of iron-carrying nitrogen-doped composite carbon material combined with thermal hydrolysis pretreatment method according to claim 1 to strengthen the anaerobic digestion of methane production efficiency of excess sludge, characterized in that in step 7, in step 2, at 4 ° C The remaining sludge stored for use is the digested sludge produced by anaerobic digestion and methane, mixed according to the volume ratio of digested sludge and inoculated seed sludge at 9:1, put into an anaerobic fermentation bottle, and added at a concentration of 8g/ L added the Fe-N-C material obtained in step 6 to it, and carried out anaerobic digestion methane production experiment in a constant temperature shaking incubator at a temperature of 35°C and a rotation speed of 180rpm until no more gas was produced in the anaerobic fermentation bottle.

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