Method for enhancing efficiency of anaerobic membrane bioreactor based on nano ferric oxide-biochar composite conductive material

Abstract

The invention discloses a preparation method of a nano ferric oxide-biochar (Fe2O3-BC) composite conductive material and an application method of the nano ferric oxide-biochar (Fe2O3-BC) composite conductive material in an anaerobic membrane bioreactor (AnMBR). The composite material is prepared by mixing nano iron oxide (Fe2O3NPs) and biochar (BC) according to a predetermined mass ratio and compounding by adopting a ball milling process. Through the conductive characteristic and the porous structure of the composite material, the interspecific electron transfer efficiency of microorganisms in the AnMBR reactor is improved, and the methane yield is improved by 1.2 times; meanwhile, accumulation of microbial extracellular polymeric substances on the surface of the membrane is effectively reduced, 21.6%-65% of membrane pollution is relieved, and the total filtration resistance is reduced by 52.8%. The technology has the advantages of simple preparation process, low cost, stable operation and the like, can be widely applied to the fields of municipal sludge treatment and high-concentration organic wastewater treatment, and realizes the dual goals of improvement of anaerobic digestion efficiency and membrane pollution control.

Description

Technical Field

[0001] The present invention belongs to the field of environmental engineering and sewage treatment technology, and specifically relates to a nano-ferric oxide-biochar (Fe2O3-BC) composite conductive material for enhancing the performance of an anaerobic membrane bioreactor (AnMBR) and a preparation method thereof, which is suitable for efficient anaerobic digestion treatment and energy recovery of sludge and organic solid waste. Background Art

[0002] With the acceleration of global urbanization and the surge in population, the amount of sludge generated by sewage treatment has increased dramatically, and its efficient treatment and resource utilization have become environmental issues that need to be urgently addressed. The anaerobic membrane bioreactor (AnMBR) technology combines the dual advantages of anaerobic digestion and membrane separation, and can achieve independent control of hydraulic retention time (HRT) and solid retention time (SRT), effectively intercepting microorganisms, thereby reducing the reactor volume, improving treatment stability, reducing sludge production, and enhancing energy recovery efficiency. The existing AnMBR technology has two major technical bottlenecks: first, the microbial metabolic efficiency is low and the electron transfer rate is slow during anaerobic digestion, resulting in low methane yield; second, the deposition of microbial metabolites on the membrane surface will significantly reduce the membrane flux, increase operating energy consumption and maintenance costs.

[0003] Interspecies electron transfer (IET) is the key rate-limiting step in methane production. Studies have shown that the introduction of conductive materials can promote the direct interspecies electron transfer (DIET) process during anaerobic digestion, thereby increasing methane yield. However, existing conductive materials have obvious defects: although iron-based materials (such as nano-ferroferric oxide and nano-zero-valent iron) have excellent conductivity and multivalent redox properties, they are prone to nanoparticle agglomeration, affecting dispersibility and stability; although carbon-based materials (such as activated carbon and biochar) have good biocompatibility and porous structure, they are not conductive enough, which limits their electron transfer efficiency. The present invention innovatively selects nano-ferric oxide (Fe2O3NPs) as an iron-based material, which has higher chemical stability and better electron transfer ability, and can effectively avoid the problem of zero-valent iron being easily oxidized and ineffective. By compounding Fe2O3NPs with biochar (BC), a new conductive material with high specific surface area, multi-level pore structure and excellent biocompatibility is constructed, which significantly improves microbial attachment and interspecies electron transfer efficiency. In addition, breaking through the limitations of traditional chemical composite methods (such as co-precipitation, hydrothermal synthesis, etc.), biochar is first prepared by pyrolysis, and then low-cost ball milling technology is used to achieve efficient composite of Fe2O3NPs and BC. The process is simple, does not require a large amount of chemical reagents, and is suitable for large-scale production.

[0004] The Fe2O3-BC composite conductive material provided by the present invention can effectively improve the methane yield of AnMBR and alleviate membrane fouling, providing an engineering solution with greater commercial prospects for anaerobic digestion of organic solid waste. Summary of the Invention

[0005] This invention addresses the problems of low methane yield and severe membrane fouling in existing AnMBR (AnMBR) technology by providing a method for preparing an Fe2O3-BC composite conductive material and its application in AnMBR systems. This technology promotes microbial electron transfer in anaerobic membrane bioreactors, enriches functional microbial flora, and alleviates membrane fouling. It can effectively improve anaerobic digestion efficiency, reduce membrane module cleaning frequency, and reduce service life, thereby enhancing the overall performance of the anaerobic membrane bioreactor.

[0006] In order to achieve the above objectives, the present invention provides a method for preparing a highly dispersible Fe2O3-BC composite conductive material. By optimizing the mass ratio of Fe2O3NPs and BC and adopting a simple and inexpensive ball milling process, a composite material with excellent conductivity and stable dispersibility is obtained.

[0007] The preparation method of the nano-Fe2O3-BC composite conductive material specifically includes the following steps: selecting agricultural waste as a biochar precursor, and obtaining raw material particles with a particle size of 0.5 to 2 mm after crushing and screening. The obtained biomass raw material is placed in a tubular furnace, heated to 400 to 600°C at a heating rate of 5 to 10°C / min under nitrogen protection, maintained for pyrolysis for 1 to 2 hours, and ground through an 80 to 100 mesh sieve after natural cooling to obtain BC for use. BC, Fe2O3NPs material and agate balls are placed in a ball mill in a certain proportion and ball milled for a period of time to obtain the Fe2O3-BC composite conductive material.

[0008] Furthermore, the biochar raw material is corn cob, corn stalks or rice straw.

[0009] Furthermore, the particle size of the Fe2O3NPs is 20 to 50 nm.

[0010] Furthermore, the mass ratio of the Fe2O3NPs to BC is 1:5 to 1:30.

[0011] Furthermore, the mass ratio of the material mixture to the agate balls is 1:50 to 1:150.

[0012] Furthermore, the ball milling speed is 200-400 rpm, the ball milling time is 5-8 hours, and the rotation direction is automatically switched every 30 minutes during the ball milling process to ensure uniform mixing.

[0013] The method of using the Fe2O3-BC composite conductive material to enhance the methane production efficiency of the anaerobic membrane bioreactor and alleviate membrane fouling specifically includes the following steps: During the startup phase of the anaerobic membrane bioreactor, waste activated sludge, inoculated sludge and membrane assembly are sequentially added to the AnMBR, and the Fe2O3-BC composite conductive material is used as an efficiency enhancer. Nitrogen is then continuously introduced into the reactor for 10 to 30 minutes to ensure that the oxygen in the system is completely discharged. The reactor is sealed to start the operation of the semi-continuous completely mixed AnMBR. A mechanical stirring device is provided in the AnMBR, and the water outlet of the membrane assembly is connected to a water outlet pump through a pipeline. A pressure gauge is provided on the pipeline between the water outlet of the membrane assembly and the water outlet pump, and the gas outlet of the AnMBR is connected to a gas collection bag.

[0014] Furthermore, a water bath layer is provided outside the anaerobic membrane bioreactor to maintain the internal temperature at 35-37° C., and the stirring speed is controlled at 70-140 rpm.

[0015] Furthermore, the dosage of the Fe2O3-BC composite conductive material is 2 to 8 g / L.

[0016] Furthermore, the ratio of Fe2O3NPs to BC in the composite conductive material is 1:25 to 3:25.

[0017] Furthermore, the membrane component material is an immersed hollow fiber membrane of polyvinylidene fluoride (PVDF), polyethersulfone (PES), polysulfone (PS), polypropylene (PP), etc.

[0018] Furthermore, the average pore size of the membrane assembly is 0.1 to 0.2 μm, and the effective filtration area is 0.1 to 0.2 m 2 .

[0019] Furthermore, the membrane bioreactor is operated in a semi-continuous filtration mode, the sludge retention time of the activated waste sludge in the membrane bioreactor is 30 to 50 days, the hydraulic retention time is 10 to 30 days, and the volatile solid concentration of the sludge is operated at 12 to 18 g / L.

[0020] Beneficial effects of the present invention:

[0021] The Fe2O3-BC composite conductive material is introduced into the anaerobic membrane bioreactor to simultaneously achieve the enhancement of anaerobic digestion production capacity (1.2 times) and the slowdown of membrane fouling (relieve flux by 21.6% to 65%). The preparation process of the Fe2O3-BC composite conductive material is simple and low-cost, and the appropriate addition can achieve significant treatment benefits. The present invention solves the problems of easy agglomeration of traditional iron-based materials and poor conductivity of carbon-based materials by regulating the loading amount and dispersion of Fe2O3NPs; the porous structure and surface functional groups of biochar are used to provide an ideal attachment environment for microorganisms, while the composite nano-iron-based material enhances the efficiency of electron transfer. The technology described in the present invention can be widely used in the fields of municipal sludge treatment, high-concentration organic wastewater treatment, etc., and has significant advantages such as low energy consumption, high efficiency, and stable operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematic diagram of the structure of the anaerobic membrane bioreactor used;

[0023] Figure 2 The preparation flow chart and scanning electron microscope image of the Fe2O3-BC composite conductive material in Example 1;

[0024] Figure 3 This is the electrochemical test diagram (It diagram) of the Fe2O3-BC composite conductive material in Example 1;

[0025] Figure 4 Graph showing cumulative methane production changes of the anaerobic membrane bioreactors R1 and R2 in Example 2;

[0026] Figure 5 Graph showing the changes in transmembrane pressure difference and membrane flux of the anaerobic membrane bioreactors R1 and R2 in Example 2;

[0027] Figure 6 This is a graph showing changes in membrane resistance of the membrane module after contamination of the anaerobic membrane bioreactor in Example 2;

[0028] Figure 7 This is a diagram showing the changes in EPS components on the surface of the membrane assembly of the anaerobic membrane bioreactor in Example 2. DETAILED DESCRIPTION

[0029] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present application belongs.

[0030] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular also includes the plural. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0031] In order to enable those skilled in the art to better understand the technical solutions in this specification, the following is a clear and complete description of the technical solutions of the present invention in conjunction with the embodiments of the present invention and the accompanying drawings. The embodiments described are only some embodiments of the present invention. Based on the embodiments of the present invention, other embodiments obtained by researchers in this field without any creative work are within the scope of protection of the present invention.

[0032] Example 1:

[0033] A method for preparing a Fe2O3-BC composite conductive material comprises the following steps:

[0034] (1) Corn stalks were washed with deionized water, dried at 60-80°C for 24 h, and chopped. The stalks were then placed in a tube furnace and heated to 400-600°C at a heating rate of 5-10°C / min under a nitrogen atmosphere (flow rate of 1 L / min). The pyrolysis was carried out at this temperature for 1-2 h. After the reaction was completed, the furnace was cooled to room temperature. The product was removed and ground through an 80-100 mesh sieve. The resulting biochar was stored in a desiccator for later use.

[0035] (2) Weigh the BC prepared above and mix it with Fe2O3NPs (purity>99%, average particle size 30nm) in a mass ratio of 25:1 and mark it as Fe1BC 25 .

[0036] (3) The mixed material was placed in a 500 mL agate ball mill, and agate balls with a diameter of 8 mm were added (the mass ratio of material to ball was 1:100). The mixture was ball milled at 300 rpm for 6 h in a planetary ball mill, and the rotation direction was switched every 30 min to ensure mixing uniformity.

[0037] (4) The ball-milled product was sieved through an 80-100 mesh sieve and sealed in a light-proof and moisture-proof brown glass bottle to obtain a Fe2O3-BC composite conductive material.

[0038] (5) Other composite materials (such as Fe3BC 25 、Fe5BC 25 ) can be prepared by adjusting the mass ratio of Fe2O3NPs to BC (3:25 and 5:25, respectively) and following the same steps.

[0039] The preparation process and morphology of BC and Fe2O3-BC prepared in Example 1 are as follows: Figure 2 As shown. The BC particles prepared by pyrolysis have cracks and lamellar structures. After ball milling BC with Fe2O3NPs, Fe2O3NPs are evenly distributed on the surface and pores of Fe2O3-BC. SEM-EDS analysis confirmed the dispersibility of Fe and O in Fe2O3-BC, and Fe and O atoms are evenly embedded in the carbon matrix. With the increase of Fe2O3 doping ratio, the mass fraction of Fe and O elements increases. The particle size of Fe2O3-BC after ball milling is smaller than that of BC, which is conducive to increasing the contact area with microorganisms. Figure 3 As shown, the Fe3BC prepared in this embodiment 25 The electron contribution capacity and electron acceptance capacity reached 66.72 and 223.26 μmol e, respectively. - (g char) -1 , higher than Fe5BC 25 (37.39 and 219.03 μmol e - (g char) -1 ) and Fe1BC 25 (38.84 and 207.26 μmol e - (g char) -1 ). This indicates that an appropriate amount of Fe2O3NPs loading can effectively improve the electron transfer ability of Fe2O3-BC.

[0040] Example 2:

[0041] In this example, Fe3BC 25 The method is applied to sludge anaerobic membrane bioreactor to achieve simultaneous increase in methane production and alleviation of membrane fouling. The specific steps are as follows:

[0042] (1) Under experimental conditions, two groups of submerged anaerobic membrane bioreactors with identical appearance, structure and configuration were operated. One group did not add Fe3BC. 25 The control group (R1) of the conductive composite material and the other group were added with Fe3BC 25 Experimental group of conductive composite materials (R2).

[0043] (2) The influent matrix of the two reactors was waste activated sludge. Peristaltic pumps were used to complete the reactor sludge inlet, sludge outlet, and membrane filtrate settings. Aluminum air bags were used to collect the daily gas volume and measure the gas concentration.

[0044] (3) During the operation of the reactor, the water quality indicators of the sludge and membrane filtrate, the changes in CH4 production, and the changes in the TMP and membrane flux of the reactor were regularly recorded.

[0045] (4) After the experiment, the membrane resistance change of the membrane assembly was detected.

[0046] Further description, the specific structure of the anaerobic membrane bioreactor in step (1) is as follows Figure 1 As shown, it consists of a membrane assembly, a stirring device, a water bath circulation device, a peristaltic pump, a gas collection bag, a pressure sensor and an anaerobic digestion reactor. The stirring device (constant speed operation at 140rpm) is used to achieve complete mixing of the reactor, and the water bath circulation device is used to control the internal environmental temperature of the reactor to maintain at a medium temperature anaerobic environment of 37±1℃, and ensure that the effective volume of the two groups of reactors is 3.2L. The membrane assembly used is a built-in PVDF hollow fiber membrane assembly (membrane pore size 0.1μm, effective area 0.1m 2 In addition, a gas collection bag was used to collect gas to monitor methane changes, and a pressure sensor was used to record changes in membrane pressure.

[0047] In a further embodiment, the Fe3BC added to the R2 reactor in step (1) 25 The conductive composite material is the optimal proportion of the Fe2O3-BC conductive composite material prepared in Example 1, and the addition amount is set to 2-5 g / L.

[0048] Further explanation: the influent matrix of the anaerobic membrane bioreactor in step (2) is the residual sludge from the secondary sedimentation tank of the sewage treatment plant. Before use, the residual sludge is subjected to hot alkaline pretreatment, i.e., the pH is adjusted to 8-10 with 1-5 mol / L NaOH solution, and then placed in a constant temperature water bath and heated at 70-90°C for one hour, and then passed through a 100-120 mesh sieve after cooling.

[0049] In step (2), the frequency and volume of the substrate in and out are controlled manually and by a peristaltic pump. The membrane bioreactor is operated in a semi-continuous filtration mode, the sludge retention time of the activated waste sludge in the membrane bioreactor is 30 to 50 days, the hydraulic retention time is 10 to 30 days, and the volatile solids concentration of the sludge is operated at 12 to 18 g / L.

[0050] In a further embodiment, the effluent sludge and water quality indicators include, but are not limited to, total solids (TS), volatile solids (VS), pH, conductivity, dissolved SCOD concentration, ammonia nitrogen concentration, dissolved protein concentration, and dissolved polysaccharide concentration. Experiments found that the VS / TS ratio of the sludge mixed liquor in both reactor groups decreased from an initial 58-59% to 50%, while the pH remained within the methanogenic range of 7-8. The total reactor operation time was 50 days.

[0051] In a further solution, in step (3), in order to analyze the production of methane and carbon dioxide, a gas collection bag is used to collect the gas, a syringe is used to record the volume, gas chromatography is used to monitor the relative content of the gas, and the daily and cumulative production of methane and carbon dioxide are calculated.

[0052] In a further embodiment, in step (4), a graded cleaning method is used to distinguish different types of contamination in the resistance analysis. The total filtration resistance Rt after membrane contamination is composed of the membrane resistance Rm, the gel layer resistance Rg, and the cake layer resistance Rc. The four have the following relationship: Rt = Rm + Rc + Rg. After physical cleaning, it is assumed that the cake layer Rc is completely removed. At this time, the measured filtration membrane resistance is Rw = Rm + Rg. Based on the total filtration resistance Rt before physical cleaning, the cake layer resistance can be calculated as Rc = Rt - Rw. Based on the filtration resistance Rm of the new membrane, the gel layer resistance Rg = Rw - Rm can be calculated.

[0053] The calculation formula of membrane filtration resistance is shown as follows:

[0054] J=TMP / (μ*Rt)

[0055] Where, TMP is the transmembrane pressure difference (Pa); J is the membrane filtration flux (m 3 / m 2 / s); μ is the viscosity of the membrane filtrate (Pa·s). The viscosity coefficient of water at 25°C is 0.8937×10 -3 Pa·s; Rt is the total filtration resistance (m -1 ).

[0056] Figure 4 The cumulative methane production changes of the two groups of reactors R1 and R2 in Example 2 after 50 days of operation are shown. The results show that compared with R1 (control group), the addition of Fe3BC 25 The cumulative yield of the R2 reactor with composite conductive materials is higher, 522.27 mL / g VS, which is 1.2 times that of R1. This may be due to Figure 2 and Figure 3 Fe3BC shown in 25 It has high electrical conductivity and can promote electron transfer, thereby enhancing the methane production efficiency of anaerobic microorganisms.

[0057] like Figure 5 As shown in the figure, the changes in transmembrane pressure difference and membrane flux of two groups of anaerobic membrane bioreactors R1 and R2 are shown. Since the service life of the mesoporous fiber membrane component is 2 to 3 years, the impact on the transmembrane pressure difference is small. After 30 days of operation, the transmembrane pressure difference of both groups increased, but whether Fe3BC was added or not 25The composite conductive material has little effect on the transmembrane pressure difference. It can also be seen from the membrane flux change trend that in the operation of the anaerobic membrane bioreactor, the Fe3BC 25 Composite conductive materials can alleviate membrane fouling to a certain extent, about 21.6% to 65%. As time goes by, a large amount of sludge flocs may accumulate on the membrane surface, resulting in Fe3BC 25 The addition of composite conductive materials has a weaker effect on alleviating membrane flux.

[0058] Figure 6 The resistance change of membrane modules of two groups of anaerobic membrane bioreactors R1 and R2 after completion was characterized. 25 The total filtration resistance (Rt) of the reactor (R2) enhanced by the addition of composite conductive materials was significantly reduced compared with the control group (R1), with a reduction of 52.8%. Among the resistance components, the cake layer resistance (Rc) was the main contributor, and Fe3BC 25 The Rc values ​​of the reactor and the control group were 0.89×10 11 m -1 and 3.38× 11 m -1 , accounting for 67.05% and 37.62% of the total resistance. In contrast, the difference in gel layer resistance (Rg) between the two groups of reactors is smaller, and Fe3BC 25 The Rg values ​​of the group and the control group were 7.51×10 10 m -1 and 5.74×10 10 m -1 In summary, Fe3BC 25 The addition of the composite conductive material reduced the total filtration resistance by 52.79% and the filter cake layer resistance by 73.52%, effectively alleviating membrane fouling. This improvement was attributed to the increased efficiency of organic matter degradation and the reduced accumulation of biomass on the membrane surface. Specifically, in the soluble EPS layer, the polysaccharide and protein concentrations in R2 decreased by 68.51% and 33.28%, respectively, compared to R1, indicating that polysaccharide degradation was more significant. For the tightly adhered EPS layer, the reduction rates of polysaccharides and proteins in R2 were 35.33% and 26.25%, respectively ( Figure 7 ). High levels of soluble EPS and tightly adhered EPS can lead to membrane pore blockage and the formation of a filter cake layer, thereby exacerbating the increase in membrane resistance. This indicates that more microbial metabolites are enriched on the R1 membrane component, while R2 is enriched due to Fe3BC 25 The addition of composite conductive materials improves biodegradability and significantly reduces the biomass on the membrane assembly, thereby effectively alleviating membrane fouling.

[0059] In summary, the Fe2O3-BC preparation process of the present invention is simple and low-cost, and its porous structure and high specific surface area optimize the microbial attachment and electron transfer efficiency. 25 The composite material has high electrical properties, which promotes the DIET process between microorganisms, strengthens the degradation of organic matter and the activity of methanogens, and achieves higher methane production. 25 By reducing biomass accumulation, membrane fouling can be slowed and the life of the membrane modules extended. The introduction of Fe2O3-BC simultaneously achieves the dual goals of increasing production and reducing pollution.

[0060] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a nano-ferric oxide-biochar (Fe2O3-BC) composite conductive material and enhancing the performance of an anaerobic membrane bioreactor (AnMBR), characterized in that: The conductive composite material Fe2O3-BC was added into the anaerobic membrane bioreactor for anaerobic digestion reaction. The conductive composite material was a highly conductive Fe2O3-BC composite material prepared by ball milling corn straw biochar (BC) and nano-ferric oxide (Fe2O3NPs).

2. The method according to claim 1, characterized in that The biochar preparation method comprises the following steps: using corn cobs, corn stalks or rice straw as one of the biomass raw materials, heating to 400-600°C at a heating rate of 5-10°C / min under nitrogen protection, maintaining pyrolysis for 1-2 hours, naturally cooling, and grinding through an 80-100 mesh sieve to obtain the biochar.

3. The method according to claim 1, characterized in that The particle size of the Fe2O3 NPs is 20 to 50 nm.

4. The method according to claim 1, wherein The mass ratio of BC to Fe2O3 NPs is 1:25 to 5:25, and the highly conductive Fe2O3-BC composite material is prepared by ball milling in a ball mill for 5 to 8 hours.

5. The method according to claim 1, wherein The dosage of the conductive material Fe2O3-BC in the AnMBR is 2-8 g / L.

6. The method according to claim 1, wherein The anaerobic membrane bioreactor has a sludge retention time of 30-50 days, a hydraulic retention time of 10-30 days, and operates at a volatile solid concentration of sludge of 12-18 g / L.

7. The method according to claim 1, characterized in that The anaerobic membrane bioreactor has a built-in hollow fiber membrane component, the membrane material is polyvinylidene fluoride (PVDF), polyethersulfone (PES), polysulfone (PS), polypropylene (PP), etc., the average membrane pore size is 0.1-0.2 μm, and the effective filtration membrane area is 0.1-0.2 m 2 .

8. Application of the composite conductive material Fe2O3-BC to enhance the anaerobic biofilm reaction efficiency according to any one of claims 1 to 8, characterized in that: This process can be used to treat organic waste such as high-concentration organic wastewater, residual sludge and food waste.

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