CN113636734A - Method for strengthening methane production efficiency of anaerobic digestion of excess sludge through combined thermal hydrolysis pretreatment of iron-carrying nitrogen-doped composite carbon material - Google Patents
Method for strengthening methane production efficiency of anaerobic digestion of excess sludge through combined thermal hydrolysis pretreatment of iron-carrying nitrogen-doped composite carbon material Download PDFInfo
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Abstract
A method for strengthening the methane production efficiency of anaerobic digestion of excess sludge by combining iron-carrying nitrogen-doped composite carbon material 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
Technical Field
The invention relates to a method for strengthening the methane production efficiency of anaerobic digestion of excess sludge by thermal hydrolysis pretreatment.
Background
The anaerobic digestion technology is a common process for treating urban excess sludge, and is widely concerned by researchers due to the advantages of high organic matter load, low energy consumption, high sludge reduction degree, capability of producing energy gases such as methane and the like. However, anaerobic digestion suffers from two key problems in practical applications, namely low efficiency and poor stability.
The anaerobic digestion process is mainly divided into three stages of hydrolysis, acidification and methane production. Among them, the hydrolysis stage mainly involves the breakdown of Extracellular Polymeric Substances (EPS) of excess sludge and the lysis and lysis of microbial cells. The thermal hydrolysis method is the most commonly used method for pretreating excess sludge, and by heating the excess sludge to a higher temperature, EPS is broken to generate small molecules which are easy to hydrolyze, and meanwhile, microbial cells are also broken to release various organic matters in the cells. The hydrolysis time of the residual sludge can be greatly shortened by the thermal hydrolysis method, which is beneficial to the further proceeding of anaerobic digestion.
The acidification and methanogenesis processes of anaerobic digestion are synergistically completed by different bacteria and archaea through metabolism. The sequential cooperation between different types of microorganisms is a key factor for efficient anaerobic digestion, depending on efficient interspecies electron transfer. Direct Interspecies Electron Transfer (DIET) transfers electrons from an electron donor such as an acid-producing bacterium to an electron acceptor such as a methanogen in a contacting manner directly through conductive cilia of the microorganism or an extracellular conductive substance. Because the requirement of generating hydrogen as an electron transport carrier is not needed in the electron transfer process, and the limitation of low hydrogen partial pressure is avoided, the DIET has spontaneity in reaction thermodynamics, and simultaneously, the energy loss in the reaction process is reduced. Compared with the traditional indirect transfer of the interspecies hydrogen and the electrons, the direct transfer speed of the interspecies electrons is also higher by one order of magnitude, and the generation rate of methane in the anaerobic digestion process is increased.
Carbon-based materials (granular activated carbon, powdered activated carbon, biochar, graphite, and the like) and iron-based materials (zero-valent iron, magnetite, ferrihydrite, and the like) are commonly used as conductor materials that mediate the DIET pathway in the anaerobic digestion methanogenesis process. However, the mechanism of action of the two is different due to their different material properties. The carbon-based material mainly transfers electrons through active groups on the surface of the material, and provides attachment points for microorganisms by using larger specific surface area of the carbon-based material, so that related functional microorganisms are enriched, a DIET (Diet) way is established, and anaerobic digestion is enhanced to produce methane. The iron-based material directly establishes an electronic path between acid-producing bacteria and methanogenic bacteria mainly by using higher conductivity of the iron-based material to complete a DIET process, and simultaneously a small amount of iron can be used as one of components of coenzyme to accelerate the acid-producing process so as to improve the efficiency of anaerobic digestion for producing methane.
Disclosure of Invention
The invention aims to solve the technical problems of low methane production efficiency and poor stability of the existing anaerobic digestion excess sludge, and provides a method for strengthening the methane production efficiency of the anaerobic digestion excess sludge by combining an iron-carrying nitrogen-doped composite carbon material with thermal hydrolysis pretreatment.
The method for strengthening the anaerobic digestion methanogenesis efficiency of excess sludge by combining the iron-carrying nitrogen-doped composite carbon material with the thermal hydrolysis pretreatment is carried out according to the following steps:
firstly, selecting excess sludge of a secondary sedimentation tank of a municipal sewage treatment plant, sieving the excess sludge by a screen with 10-20 meshes to remove large-particle gravels, standing and precipitating the excess sludge for 30-40 min to concentrate, removing part of supernatant to ensure that the total suspended solid concentration TSS of the excess sludge is 16-25 g/L and the volatile suspended solid VSS is 9-13 g/L, and adding an acid-base regulator to ensure that the pH value of the excess sludge is 6.2-6.8;
secondly, putting the excess sludge treated in the first step into a reaction kettle, heating to 150-160 ℃, preserving the heat for 30-40 min, naturally cooling to room temperature, taking out the excess sludge, and preserving at 4 ℃ for later use;
thirdly, pretreating the carbon material: soaking the biochar in an HCl aqueous solution with the pH value of 1-2 for 24-25 h to remove ash and organic matters on the surface of the activated carbon; filtering, washing with distilled water at 100 ℃, drying in a drying oven at 105-110 ℃, filling into a weighing bottle, and placing in a dryer for later use to obtain pretreated activated carbon;
the biochar is commercial coconut shell biochar, the particle size of the biochar is 100-150 meshes, and the specific surface area of the biochar is 200m2/g～400m2/g；
Fourthly, 4 mol/L-8 mol/L HNO is prepared3Adding the activated carbon pretreated in the third step into the aqueous solution, carrying out oscillation reaction for 6-7 h, standing, filtering, washing with water until the pH value is unchanged, putting the washed sample into a drying oven, drying at 105-110 ℃, putting into a weighing bottle, and putting into a dryer for later use to obtain preoxidized biochar;
fifthly, in order to dope nitrogen element into the carbon material to improve the biological affinity and increase the electron transfer capacity of the carbon material, the biological carbon pre-oxidized in the step four is put into a tube furnace and is added with pure N2Heating to 600-900 ℃ in the atmosphere, and then introducing N according to the volume ratio of 1:12And NH3Mixed gas of (2) in N2And NH3The mixed gas is kept at the constant temperature of 600-900 ℃ for 2-6 h and then is added with pure N2Naturally cooling in the atmosphere to obtain nitrogen-doped biochar; the heating and cooling rates in the fifth step are both 5-10 ℃/min; the gas flow in the fifth step is 0.2L/min-1L/min;
sixthly, loading iron element in the nitrogen-doped carbon material to enable the nitrogen-doped carbon material to have the advantage of mediating DIET (Diet) path of iron-based material in anaerobic digestion methane production, and loading Fe on the surface of the nitrogen-doped carbon material by a coprecipitation method3O4: passing ultrapure water through N2Aerating to remove dissolved oxygen, then adding N2Adding H under the condition of aeration2SO4Adjusting the pH to 1-2 at N2FeSO is mixed under the condition of aeration4·7H2O and FeCl3·6H2Dissolving O in the solution to make FeSO4·7H2O and FeCl3·6H2The concentration of O is 0.075 mol/L-0.08 mol/L and 0.1125 mol/L-0.113 mol/L respectively, in N2Heating to 60-70 ℃ under the aeration condition, keeping the temperature constant, and then carrying out N2Adding the nitrogen-doped biochar prepared in the fifth step under the aeration condition, and continuously aerating N2Magnetically stirring for 15-60 min under N2Adding NaOH to adjust the pH value to 9-10 under the aeration and stirring conditions, stopping stirring (magnetic substances appear at the moment), carrying out water bath at 60-90 ℃ and continuously aerating N2Aging for 2-6 h under the condition of (1), washing with water, filtering, and vacuum drying at 105-110 ℃ to obtain the Fe-N-C material; the FeSO4·7H2Iron element of O and FeCl3·6H2The ratio of the total mass of the iron element of O to the mass of the nitrogen-doped biochar is 1: 3;
seventhly, inoculating seed sludge for anaerobic digestion and methane production is municipal sludge anaerobic fermentation tank top effluent sludge, the total suspended solid concentration TSS is 55-75 g/L, the volatile suspended solid VSS is 27-38 g/L, and the pH is 7-7.5; and (3) taking the residual sludge to be used preserved at the temperature of 4 ℃ in the step two as the digested bottom sludge for anaerobic digestion methane production, mixing the digested bottom sludge and the inoculated seed sludge according to the volume ratio of (9-10) to 1, putting the mixture into an anaerobic fermentation bottle, adding the Fe-N-C material obtained in the step six into the anaerobic fermentation bottle according to the adding concentration of 5 g/L-10 g/L, carrying out an anaerobic digestion methane production experiment in a constant-temperature oscillation incubator at the temperature of 33-37 ℃ and the rotating speed of 120-200 rpm until no gas is generated in the anaerobic fermentation bottle.
And seventhly, collecting gas in the gas bag at the upper end of the bottle mouth periodically to measure the volume and the components, and measuring the sludge conductivity when the test is finished.
According to the invention, the biochar is modified mainly by ammonia nitrogen modification and ferroferric oxide loading methods, the electron transfer capacity of the biochar is enhanced, and the biochar is combined with an iron-based material to prepare the iron-loaded nitrogen-doped composite carbon material Fe-N-C, wherein the Fe-N-C material can simultaneously exert the advantages of a DIET (Diet mediated approach) way of the carbon-based material and the iron-based material in the process of producing methane by anaerobic digestion, electrons are efficiently transferred, microorganism attachment points are provided, the activity of key acid-producing enzymes is improved, and the efficiency of producing methane by anaerobic digestion of residual sludge is improved.
On the basis of the thermal hydrolysis pretreatment of the excess sludge, a DIET (Diet) way is established in the anaerobic digestion process by preparing and adding Fe-N-C materials, so that the methane generation rate is accelerated, and the stability of the anaerobic digestion reactor is improved.
The innovation point and the principle of the invention are as follows: the innovation mainly lies in the preparation flow of the Fe-N-C material in the steps from three to six; the carbon-based material mainly transfers electrons through active groups on the surface of the material, and provides attachment points for microorganisms by using a larger specific surface area of the carbon-based material, so that related functional microorganisms are enriched, and a DIET (Diet pathway) is established; the iron-based material directly establishes an electronic path between acid-producing bacteria and methanogenic bacteria mainly by using higher conductivity of the iron-based material to complete a DIET process, and simultaneously a small amount of iron can be used as one of components of coenzyme to accelerate the acid-producing process so as to improve the efficiency of anaerobic digestion for producing methane. The Fe-N-C material prepared by the invention creatively combines the advantages of the carbon-based material and the iron-based material, and simultaneously enhances the electron transfer capability of the biochar through nitrogen modification, 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.
Drawings
FIG. 1 is a schematic view of an anaerobic fermentation flask in step seven of test one, wherein 1 is a sampling port, 2 is Fe-N-C material, and 3 is a gas collecting bag;
FIG. 2 is an SEM image of the Fe-N-C material prepared in step six of experiment one;
FIG. 3 is a first EDS map of Fe-N-C material prepared in step six of run one;
FIG. 4 is a second EDS map of the Fe-N-C material prepared in step six of run one;
FIG. 5 is a third EDS map of the Fe-N-C material prepared in step six of experiment one;
FIG. 6 is an XPS plot of the Fe-N-C material prepared at step six of experiment one;
FIG. 7 is an XRD pattern of Fe-N-C material prepared in step six of experiment one;
FIG. 8 is a graph showing the cumulative yield of methane during the anaerobic digestion of excess sludge to produce methane as a function of anaerobic digestion time with different materials added.
Detailed Description
The first embodiment is as follows: the embodiment is a method for strengthening the methane production efficiency of anaerobic digestion of excess sludge by combining iron-carrying nitrogen-doped composite carbon material with thermal hydrolysis pretreatment, which is specifically carried out according to the following steps:
firstly, selecting excess sludge of a secondary sedimentation tank of a municipal sewage treatment plant, sieving the excess sludge by a screen with 10-20 meshes to remove large-particle gravels, standing and precipitating the excess sludge for 30-40 min to concentrate, removing part of supernatant to ensure that the total suspended solid concentration TSS of the excess sludge is 16-25 g/L and the volatile suspended solid VSS is 9-13 g/L, and adding an acid-base regulator to ensure that the pH value of the excess sludge is 6.2-6.8;
secondly, putting the excess sludge treated in the first step into a reaction kettle, heating to 150-160 ℃, preserving the heat for 30-40 min, naturally cooling to room temperature, taking out the excess sludge, and preserving at 4 ℃ for later use;
thirdly, pretreating the carbon material: soaking the biochar in an HCl aqueous solution with the pH value of 1-2 for 24-25 h to remove ash and organic matters on the surface of the activated carbon; filtering, washing with distilled water at 100 ℃, drying in a drying oven at 105-110 ℃, filling into a weighing bottle, and placing in a dryer for later use to obtain pretreated activated carbon;
the biochar is commercial coconut shell biochar, the particle size of the biochar is 100-150 meshes, and the specific surface area of the biochar is 200m2/g～400m2/g；
Fourthly, 4 mol/L-8 mol/L HNO is prepared3Adding the activated carbon pretreated in the third step into the aqueous solution, carrying out oscillation reaction for 6-7 h, standing, filtering, washing with water until the pH value is unchanged, putting the washed sample into a drying oven, drying at 105-110 ℃, putting into a weighing bottle, and putting into a dryer for later use to obtain preoxidized biochar;
fifthly, putting the biological carbon pre-oxidized in the step four into a tubular furnace, and adding pure N2Heating to 600-900 ℃ in the atmosphere, and then introducing N according to the volume ratio of 1:12And NH3Mixed gas of (2) in N2And NH3The mixed gas is kept at the constant temperature of 600-900 ℃ for 2-6 h and then is added with pure N2Naturally cooling in the atmosphere to obtain nitrogen-doped biochar; step by stepIn the fifth step, the heating and cooling rates are both 5-10 ℃/min; the gas flow in the fifth step is 0.2L/min-1L/min;
sixthly, passing the ultrapure water through N2Aerating to remove dissolved oxygen, then adding N2Adding H under the condition of aeration2SO4Adjusting the pH to 1-2 at N2FeSO is mixed under the condition of aeration4·7H2O and FeCl3·6H2Dissolving O in the solution to make FeSO4·7H2O and FeCl3·6H2The concentration of O is 0.075 mol/L-0.08 mol/L and 0.1125 mol/L-0.113 mol/L respectively, in N2Heating to 60-70 ℃ under the aeration condition, keeping the temperature constant, and then carrying out N2Adding the nitrogen-doped biochar prepared in the fifth step under the aeration condition, and continuously aerating N2Magnetically stirring for 15-60 min under N2Adding NaOH to adjust the pH value to 9-10 under the conditions of aeration and stirring, stopping stirring, and continuously aerating N in water bath at 60-90 DEG C2Aging for 2-6 h under the condition of (1), washing with water, filtering, and vacuum drying at 105-110 ℃ to obtain the Fe-N-C material; the FeSO4·7H2Iron element of O and FeCl3·6H2The ratio of the total mass of the iron element of O to the mass of the nitrogen-doped biochar is 1: 3;
seventhly, inoculating seed sludge for anaerobic digestion and methane production is municipal sludge anaerobic fermentation tank top effluent sludge, the total suspended solid concentration TSS is 55-75 g/L, the volatile suspended solid VSS is 27-38 g/L, and the pH is 7-7.5; and (3) taking the residual sludge to be used preserved at the temperature of 4 ℃ in the step two as the digested bottom sludge for anaerobic digestion methane production, mixing the digested bottom sludge and the inoculated seed sludge according to the volume ratio of (9-10) to 1, putting the mixture into an anaerobic fermentation bottle, adding the Fe-N-C material obtained in the step six into the anaerobic fermentation bottle according to the adding concentration of 5 g/L-10 g/L, carrying out an anaerobic digestion methane production experiment in a constant-temperature oscillation incubator at the temperature of 33-37 ℃ and the rotating speed of 120-200 rpm until no gas is generated in the anaerobic fermentation bottle.
The second embodiment is as follows: the first difference between the present embodiment and the specific embodiment is: and step one, selecting the excess sludge of a secondary sedimentation tank of a municipal sewage treatment plant, sieving the excess sludge by a 15-mesh screen to remove large-particle sand stones, standing and precipitating the excess sludge for 30min for concentration, removing part of supernatant to ensure that the total suspended solid concentration TSS of the excess sludge is 20g/L and the volatile suspended solid VSS is 10g/L, and adding an acid-base regulator to ensure that the pH value of the excess sludge is 6.5. The rest is the same as the first embodiment.
The third concrete implementation mode: the present embodiment differs from the first or second embodiment in that: and step two, putting the excess sludge treated in the step one into a reaction kettle, heating to 155 ℃, preserving the heat for 30min, naturally cooling to room temperature, taking out the excess sludge, and preserving at 4 ℃ for later use. The others are the same as in the first or second embodiment.
The fourth concrete implementation mode: the difference between this embodiment mode and one of the first to third embodiment modes is: soaking the biochar in an HCl aqueous solution with the pH value of 1-2 for 24 hours to remove ash and organic matters on the surface of the activated carbon; filtering, washing with 100 deg.C distilled water, oven drying in 105 deg.C drying oven, placing into weighing bottle, and drying in a dryer to obtain pretreated active carbon. The rest is the same as one of the first to third embodiments.
The fifth concrete implementation mode: the fourth difference between this embodiment and the specific embodiment is that: step four, 6mol/L HNO is prepared3And (3) adding the activated carbon pretreated in the third step into the aqueous solution, carrying out oscillation reaction for 6 hours, standing, filtering, washing with water until the pH value is unchanged, putting the washed sample into a drying oven, drying at 105 ℃, putting the sample into a weighing bottle, and putting the sample into a dryer for later use to obtain the preoxidized biochar. The rest is the same as the fourth embodiment.
The sixth specific implementation mode: the fifth embodiment is different from the fifth embodiment in that: step five, putting the biological carbon pre-oxidized in the step four into a tubular furnace, and adding pure N2Raising the temperature to 800 ℃ in the atmosphere, and then introducing N according to the volume ratio of 1:12And NH3Mixed gas of (2) in N2And NH3The mixed gas is kept at the constant temperature of 800 ℃ for 5 hours and then is added with pure N2Naturally cooling in the atmosphere to obtain nitrogen-doped biochar; the heating and cooling rates in the fifth step are both 8 ℃/min; and the gas flow in the fifth step is 0.8L/min. Other phases of the detailed descriptionThe same is true.
The seventh embodiment: the sixth embodiment is different from the sixth embodiment in that: in the sixth step, the ultrapure water is subjected to N2Aerating to remove dissolved oxygen, then adding N2Adding H under the condition of aeration2SO4Adjusting the pH to 1-2 at N2FeSO is mixed under the condition of aeration4·7H2O and FeCl3·6H2Dissolving O in the solution to make FeSO4·7H2O and FeCl3·6H2The concentration of O is 0.075mol/L and 0.1125mol/L respectively in N2Heating to 60 deg.C under aeration, and keeping the temperature at N2Adding the nitrogen-doped biochar prepared in the fifth step under the aeration condition, and continuously aerating N2Magnetically stirring for 40min under N2Adding NaOH to adjust pH to 9 under aeration and stirring conditions, stopping stirring, and continuously aerating in 80 deg.C water bath2Aging for 4h under the condition of (1), washing with water, filtering, and vacuum drying at 105 ℃ to obtain the Fe-N-C material. The rest is the same as the sixth embodiment.
The specific implementation mode is eight: the seventh embodiment is different from the seventh embodiment in that: and seventhly, taking the residual sludge preserved at the temperature of 4 ℃ for standby in the step two as digested bottom sludge for anaerobic digestion methane production, mixing the digested bottom sludge and the inoculated seed sludge according to the volume ratio of 9:1, putting the mixture into an anaerobic fermentation bottle, adding the Fe-N-C material obtained in the step six into the anaerobic fermentation bottle according to the adding concentration of 8g/L, performing an anaerobic digestion methane production experiment in a constant-temperature oscillation incubator at the temperature of 35 ℃ and the rotating speed of 180rpm until no gas is generated in the anaerobic fermentation bottle. The rest is the same as the seventh embodiment.
The invention was verified with the following tests:
test one: the test is a method for strengthening the anaerobic digestion and methane production efficiency of excess sludge by combining iron-carrying nitrogen-doped composite carbon material with thermal hydrolysis pretreatment, and is specifically carried out according to the following steps:
selecting secondary sedimentation tank excess sludge of a municipal sewage treatment plant, sieving the secondary sedimentation tank excess sludge by using a 10-mesh screen to remove large-particle sandstone, standing and precipitating the secondary sedimentation tank excess sludge for 30min for concentration, removing part of supernatant to ensure that the total suspended solid concentration TSS of the excess sludge is 22820mg/L and the volatile suspended solid VSS is 12147mg/L, and adding an acid-base regulator to ensure that the pH value of the excess sludge is 6.41;
secondly, putting the excess sludge treated in the first step into a reaction kettle, heating to 150 ℃, preserving the heat for 30min, naturally cooling to room temperature, taking out the excess sludge, and preserving at 4 ℃ for later use;
thirdly, pretreating the carbon material: soaking the biochar in HCl aqueous solution with the pH value of 1 for 24 hours to remove ash and organic matters on the surface of the activated carbon; filtering, washing with 100 deg.C distilled water, oven drying in 105 deg.C drying oven, placing into weighing bottle, and placing in dryer for use to obtain pretreated active carbon;
the biochar is commercial coconut shell biochar, the particle size of the biochar is 125 meshes, and the specific surface area of the biochar is 236m2/g；
Fourthly, preparing 5mol/L HNO3Adding the activated carbon pretreated in the third step into the aqueous solution, carrying out oscillation reaction for 6 hours, standing, filtering, washing with water until the pH value is unchanged, putting the washed sample into a drying oven, drying at 105 ℃, putting into a weighing bottle, and putting into a dryer for later use to obtain preoxidized biochar;
fifthly, in order to dope nitrogen element into the carbon material to improve the biological affinity and increase the electron transfer capacity of the carbon material, the biological carbon pre-oxidized in the step four is put into a tube furnace and is added with pure N2Heating to 700 ℃ in the atmosphere, and then introducing N according to the volume ratio of 1:12And NH3Mixed gas of (2) in N2And NH3The temperature of the mixed gas is kept constant for 2 hours at 700 ℃, and then the mixed gas is added into pure N2Naturally cooling in the atmosphere to obtain nitrogen-doped biochar; the heating and cooling rates in the fifth step are both 10 ℃/min; the gas flow in the fifth step is 0.4L/min;
sixthly, loading iron element in the nitrogen-doped carbon material to enable the nitrogen-doped carbon material to have the advantage of mediating DIET (Diet) path of iron-based material in anaerobic digestion methane production, and loading Fe on the surface of the nitrogen-doped carbon material by a coprecipitation method3O4: passing ultrapure water through N2Aerating to remove dissolved oxygen, then adding N2Adding H under the condition of aeration2SO4Adjusting the pH to 1 at N2FeSO is mixed under the condition of aeration4·7H2O and FeCl3·6H2Dissolving O in the solution to make FeSO4·7H2O and FeCl3·6H2The concentration of O is 0.075mol/L and 0.1125mol/L respectively in N2Heating to 65 ℃ under aeration condition, and then heating in N2Adding the nitrogen-doped biochar prepared in the fifth step under the aeration condition, and continuously aerating N2Magnetically stirring for 15-60 min under N2Adding NaOH under aeration and stirring to adjust pH to 10, stopping stirring (magnetic substance appears), and continuously aerating in 65 deg.C water bath2Aging for 2h under the condition of (1), washing with water, filtering, and drying in vacuum at 105 ℃ to obtain the Fe-N-C material; the FeSO4·7H2Iron element of O and FeCl3·6H2The ratio of the total mass of the iron element of O to the mass of the nitrogen-doped biochar is 1: 3;
seventhly, inoculating seed sludge for anaerobic digestion and methane production is discharged water sludge at the top of the municipal sludge anaerobic fermentation tank, the total suspended solid concentration TSS is 64322mg/L, the volatile suspended solid VSS is 31652mg/L, and the pH is 7.3; taking the residual sludge preserved at the temperature of 4 ℃ for standby use in the step two as digested bottom sludge for anaerobic digestion methane production, mixing the digested bottom sludge and inoculated seed sludge according to the volume ratio of 9:1, putting the mixture into an anaerobic fermentation bottle, adding the Fe-N-C material obtained in the step six into the anaerobic fermentation bottle according to the adding concentration of 5g/L, carrying out an anaerobic digestion methane production experiment in a constant-temperature oscillation incubator, wherein the temperature is 35 ℃, the rotating speed is 180rpm, sludge is not fed or discharged in the experiment process, the operation lasts for 20 days, and finally gas is not generated in the anaerobic fermentation bottle any more; and (3) periodically collecting the gas in the air bag 3 at the upper end of the bottle mouth to measure the volume and the components, and measuring the sludge conductivity when the test is finished.
FIG. 2 is an SEM image of an Fe-N-C material prepared in step six of experiment one, and FIG. 3 is a first EDS image of an Fe-N-C material prepared in step six of experiment one, which is a composite image of three elements; FIG. 4 is a second EDS plot of the Fe-N-C material prepared in step six of run one, elemental iron alone; FIG. 5 is a third EDS plot of Fe-N-C material prepared at step six of run one, elemental oxygen alone; the iron-containing substances deposited on the surface of the activated carbon can be clearly seen from the figure.
FIG. 6 is an XPS chart of the Fe-N-C material prepared in the sixth step of the first test, and it can be seen that the Fe component in the Fe-N-C material reaches 26.69% by mass and the N component in the Fe-N-C material reaches 3.09% by mass.
FIG. 7 is an XRD pattern of the Fe-N-C material prepared in the sixth step of the first experiment, from which it can be seen that the Fe element attached to the surface of the carbon-based material by the co-precipitation method in the sixth step is mainly Fe3O4Exist in the form of (1).
And (2) test II: the test is a comparative test, 5 groups of controls are arranged, and compared with the test I, the difference is that the material added in the step seven is replaced by the same amount of the following 5 substances for comparison respectively:
adding no material;
② non-conductive glass beads;
thirdly, commercial coconut shell biochar purchased in the third step of the first test;
fourthly, testing the nitrogen-doped biochar obtained in the fifth step of the first step;
direct purchase of magnetite Fe3O4。
The test results are shown in Table 1 and FIG. 8, wherein □ in FIG. 8 is the Fe-N-C material prepared in the first test, O is (i) no material is added, Delta is (ii) non-conductive glass beads,
commercial coconut shell biochar purchased in step three of test one, nitrogen-doped biochar obtained in step five of test one,
magnetite Fe purchased directly3O4. It can be seen that the accumulated yield of methane is improved by 44.52% under the condition of adding Fe-N-C material in the first test compared with the first to fifth tests which are respectively improved by 3.08%, 29.11%, 36.64% and 39.73% compared with the first to fifth tests, which shows that the anaerobic methane-producing efficiency of the residual sludge is hardly affected by uncharged glass beads, and commercial coconut shell activated carbon, nitrogen-doped biochar and Fe3O4The conductive property of the material improves the methane-generating performance, but the material is inferior to the Fe-N-C material prepared in the first test, and the superiority of the Fe-N-C material can be seen.
The sludge conductivity is the most intuitive index for determining whether the DIET approach is established, compared with the first comparison, the sludge conductivity of the first test and the second comparison are respectively improved by 6.8 times, -0.01 time, 3.56 times, 4.83 times and 6.67 times, which shows that the uncharged glass beads can not establish the DIET approach, and conductive Fe-N-C materials, commercial coconut shell activated carbon, nitrogen-doped biochar and Fe3O4It can be established that the sludge in the group of Fe-N-C materials prepared in the first test is subjected to DIET with the highest activity and therefore the sludge conductivity. The method has the advantages of easily available raw materials, recyclable materials, low cost and remarkable improvement on the methane production efficiency.
TABLE 1 Effect of different materials on the anaerobic methanogenesis efficiency of excess sludge
Claims (8)
1. A method for strengthening the anaerobic digestion methanogenesis efficiency of excess sludge by combining an iron-loaded nitrogen-doped composite carbon material with a thermal hydrolysis pretreatment is characterized in that the method for strengthening the anaerobic digestion methanogenesis efficiency of excess sludge by combining the iron-loaded nitrogen-doped composite carbon material with the thermal hydrolysis pretreatment is carried out according to the following steps:
firstly, selecting excess sludge of a secondary sedimentation tank of a municipal sewage treatment plant, sieving the excess sludge by a screen with 10-20 meshes to remove large-particle gravels, standing and precipitating the excess sludge for 30-40 min to concentrate, removing part of supernatant to ensure that the total suspended solid concentration TSS of the excess sludge is 16-25 g/L and the volatile suspended solid VSS is 9-13 g/L, and adding an acid-base regulator to ensure that the pH value of the excess sludge is 6.2-6.8;
secondly, putting the excess sludge treated in the first step into a reaction kettle, heating to 150-160 ℃, preserving the heat for 30-40 min, naturally cooling to room temperature, taking out the excess sludge, and preserving at 4 ℃ for later use;
thirdly, pretreating the carbon material: soaking the biochar in an HCl aqueous solution with the pH value of 1-2 for 24-25 h to remove ash and organic matters on the surface of the activated carbon; filtering, washing with distilled water at 100 ℃, drying in a drying oven at 105-110 ℃, filling into a weighing bottle, and placing in a dryer for later use to obtain pretreated activated carbon;
the biochar is commercial coconut shell biochar, the particle size of the biochar is 100-150 meshes, and the specific surface area of the biochar is 200m2/g～400m2/g；
Fourthly, 4 mol/L-8 mol/L HNO is prepared3Adding the activated carbon pretreated in the third step into the aqueous solution, carrying out oscillation reaction for 6-7 h, standing, filtering, washing with water until the pH value is unchanged, putting the washed sample into a drying oven, drying at 105-110 ℃, putting into a weighing bottle, and putting into a dryer for later use to obtain preoxidized biochar;
fifthly, putting the biological carbon pre-oxidized in the step four into a tubular furnace, and adding pure N2Heating to 600-900 ℃ in the atmosphere, and then introducing N according to the volume ratio of 1:12And NH3Mixed gas of (2) in N2And NH3The mixed gas is kept at the constant temperature of 600-900 ℃ for 2-6 h and then is added with pure N2Naturally cooling in the atmosphere to obtain nitrogen-doped biochar; the heating and cooling rates in the fifth step are both 5-10 ℃/min; the gas flow in the fifth step is 0.2L/min-1L/min;
sixthly, passing the ultrapure water through N2Aerating to remove dissolved oxygen, then adding N2Adding H under the condition of aeration2SO4Adjusting the pH to 1-2 at N2FeSO is mixed under the condition of aeration4·7H2O and FeCl3·6H2Dissolving O in the solution to make FeSO4·7H2O and FeCl3·6H2The concentration of O is 0.075 mol/L-0.08 mol/L and 0.1125 mol/L-0.113 mol/L respectively, in N2Heating to 60-70 ℃ under the aeration condition, keeping the temperature constant, and then carrying out N2Adding the nitrogen-doped biochar prepared in the fifth step under the aeration condition, and continuously aerating N2Magnetically stirring for 15-60 min under N2Adding NaOH to adjust the pH value to 9-10 under the conditions of aeration and stirring, stopping stirring, and carrying out water bath at 60-90 DEG CAnd continuously expose N2Aging for 2-6 h under the condition of (1), washing with water, filtering, and vacuum drying at 105-110 ℃ to obtain the Fe-N-C material; the FeSO4·7H2Iron element of O and FeCl3·6H2The ratio of the total mass of the iron element of O to the mass of the nitrogen-doped biochar is 1: 3;
seventhly, inoculating seed sludge for anaerobic digestion and methane production is municipal sludge anaerobic fermentation tank top effluent sludge, the total suspended solid concentration TSS is 55-75 g/L, the volatile suspended solid VSS is 27-38 g/L, and the pH is 7-7.5; and (3) taking the residual sludge to be used preserved at the temperature of 4 ℃ in the step two as the digested bottom sludge for anaerobic digestion methane production, mixing the digested bottom sludge and the inoculated seed sludge according to the volume ratio of (9-10) to 1, putting the mixture into an anaerobic fermentation bottle, adding the Fe-N-C material obtained in the step six into the anaerobic fermentation bottle according to the adding concentration of 5 g/L-10 g/L, carrying out an anaerobic digestion methane production experiment in a constant-temperature oscillation incubator at the temperature of 33-37 ℃ and the rotating speed of 120-200 rpm until no gas is generated in the anaerobic fermentation bottle.
2. The method for enhancing the anaerobic digestion methanogenesis efficiency of excess sludge through combined thermal hydrolysis pretreatment of the iron-loaded nitrogen-doped composite carbon material according to claim 1, wherein in the first step, the excess sludge in a secondary sedimentation tank of a municipal sewage treatment plant is selected, large-particle sand stones are removed through a 15-mesh screen, standing and sedimentation are carried out for 30min for concentration, part of supernatant is removed to enable the total suspended solid concentration TSS of the excess sludge to be 20g/L and the volatile suspended solid VSS to be 10g/L, and pH of the excess sludge is enabled to be 6.5 through adding an acid-base regulator.
3. A method for strengthening the methane production efficiency of anaerobic digestion of excess sludge by combining iron-carrying nitrogen-doped composite carbon material with thermal hydrolysis pretreatment is characterized in that the excess sludge treated in the step one is taken out in the step two and put into a reaction kettle, the temperature is raised to 155 ℃, the temperature is kept for 30min, the excess sludge is taken out after the temperature is naturally reduced to room temperature, and the excess sludge is preserved at 4 ℃ for standby.
4. A method for strengthening the methane production efficiency of anaerobic digestion of excess sludge by combining iron-carrying nitrogen-doped composite carbon material with thermal hydrolysis pretreatment is characterized in that in the third step, biochar is soaked in an HCl aqueous solution with the pH value of 1-2 for 24 hours to remove ash and organic matters on the surface of activated carbon; filtering, washing with 100 deg.C distilled water, oven drying in 105 deg.C drying oven, placing into weighing bottle, and drying in a dryer to obtain pretreated active carbon.
5. The method for strengthening the methane production efficiency of anaerobic digestion of excess sludge by combining iron-loaded nitrogen-doped composite carbon material with thermal hydrolysis pretreatment is characterized in that 6mol/L HNO is prepared in the fourth step3And (3) adding the activated carbon pretreated in the third step into the aqueous solution, carrying out oscillation reaction for 6 hours, standing, filtering, washing with water until the pH value is unchanged, putting the washed sample into a drying oven, drying at 105 ℃, putting the sample into a weighing bottle, and putting the sample into a dryer for later use to obtain the preoxidized biochar.
6. The method for strengthening the methane production efficiency of anaerobic digestion of excess sludge by combining iron-loaded nitrogen-doped composite carbon material with thermal hydrolysis pretreatment is characterized in that in the fifth step, biochar pre-oxidized in the fourth step is placed in a tubular furnace, and pure N is added into the tubular furnace2Raising the temperature to 800 ℃ in the atmosphere, and then introducing N according to the volume ratio of 1:12And NH3Mixed gas of (2) in N2And NH3The mixed gas is kept at the constant temperature of 800 ℃ for 5 hours and then is added with pure N2Naturally cooling in the atmosphere to obtain nitrogen-doped biochar; the heating and cooling rates in the fifth step are both 8 ℃/min; and the gas flow in the fifth step is 0.8L/min.
7. The method for strengthening the methane production efficiency of anaerobic digestion of excess sludge by combining iron-carrying nitrogen-doped composite carbon material with thermal hydrolysis pretreatment is characterized in that ultrapure water is subjected to N treatment in the sixth step2Aerating to remove dissolved oxygen, then adding N2Adding H under the condition of aeration2SO4Adjusting the pH to 1-2 at N2FeSO is mixed under the condition of aeration4·7H2O and FeCl3·6H2Dissolving O in the solution to make FeSO4·7H2O and FeCl3·6H2The concentration of O is 0.075mol/L and 0.1125mol/L respectively in N2Heating to 60 deg.C under aeration, and keeping the temperature at N2Adding the nitrogen-doped biochar prepared in the fifth step under the aeration condition, and continuously aerating N2Magnetically stirring for 40min under N2Adding NaOH to adjust pH to 9 under aeration and stirring conditions, stopping stirring, and continuously aerating in 80 deg.C water bath2Aging for 4h under the condition of (1), washing with water, filtering, and vacuum drying at 105 ℃ to obtain the Fe-N-C material.
8. The method is characterized in that in the seventh step, the residual sludge to be used in preservation at 4 ℃ in the second step is used as digested bottom sludge for anaerobic digestion and methane production, the digested bottom sludge and inoculated seed sludge are mixed according to the volume ratio of 9:1, the mixture is placed into an anaerobic fermentation bottle, the Fe-N-C material obtained in the sixth step is added into the anaerobic fermentation bottle according to the adding concentration of 8g/L, an anaerobic digestion and methane production experiment is carried out in a constant-temperature oscillation incubator, the temperature is 35 ℃, the rotating speed is 180rpm, and finally gas is not generated in the anaerobic fermentation bottle any more.
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