Method for efficiently removing nitrogen and phosphorus pollutants in biogas engineering

Abstract

The invention discloses a method for efficiently removing nitrogen and phosphorus pollutants in biogas engineering, which adopts a'one-pot method 'to prepare magnesium-phosphorus modified hydrothermal carbon and coproduce carbon quantum dots, and fully utilizes the'pore adsorption-metal collaboration-chemical precipitation' functions of the magnesium-phosphorus modified hydrothermal carbon on nitrogen and phosphorus. The function of low-concentration nitrogen and phosphorus is further utilized by enhancing photosynthesis of microalgae through carbon quantum dots, the magnesium-phosphorus modified hydrothermal carbon is applied to a biogas slurry purification stage and an anaerobic fermentation biogas production stage, and the magnesium-phosphorus modified hydrothermal carbon is added in the anaerobic fermentation stage of a biogas project to remove nitrogen and phosphorus in situ; in the biogas slurry purification stage, magnesium-phosphorus modified hydrothermal carbon is firstly added to remove high-concentration nitrogen and phosphorus, then microalgae coupling carbon quantum dots are inoculated to enhance nitrogen and phosphorus removal, and the effect of multi-section collaborative cascade pollution reduction of a biogas project is achieved.

Description

Technical field:

[0001] The present invention relates to the technical field of resource utilization of farming and breeding wastes, and in particular to a method for efficiently removing nitrogen and phosphorus pollutants in a biogas project. Background technology:

[0002] The current large-scale development of the livestock and poultry farming industry also generates large amounts of manure waste. To address the accumulation of livestock and poultry manure, biogas projects have been widely promoted in my country. However, the biogas slurry produced after biogas fermentation suffers from high emissions, high concentrations of nitrogen and phosphorus pollutants, excessive heavy metals, and an imbalance in the carbon-nitrogen ratio. Traditional water treatment processes struggle to meet emission standards. Meanwhile, large and medium-sized biogas projects generate large and concentrated volumes of biogas slurry. However, due to the separation of farming and livestock production, the slurry cannot be directly utilized in nearby farmland, and the high cost of long-distance transportation makes its disposal challenging. If discharged directly without proper treatment, the excessive nitrogen and phosphorus content can cause eutrophication of water bodies and disrupt the ecological balance.

[0003] Currently reported biochars for nitrogen and phosphorus removal from biogas slurry are primarily magnesium salt-modified biochars, such as magnesium salt-impregnated pig manure biochar (CN 115999506 A) and magnesium salt-impregnated straw biochar (CN 115715972 A). These biochars are first pyrolyzed at 450–700°C and then further modified. This process is complex and energy-intensive. Hydrothermal carbonization technology offers milder reaction conditions, is highly tolerant of feedstock, and eliminates the need for pre-drying, reducing energy consumption by approximately 30–50%. Further modification with acids, bases, and metal salts can enhance nitrogen and phosphorus adsorption, but this process is still primarily a two- or multi-step process. There is an urgent need to simplify the carbonization steps and improve nitrogen and phosphorus removal efficiency. More importantly, nitrogen and phosphorus removal from biogas slurry is a "post-process" process, requiring further research. Summary of the invention:

[0004] The purpose of the present invention is to provide a method for efficiently removing nitrogen and phosphorus pollutants in a biogas project. The method adopts a "one-pot method" to prepare magnesium-phosphorus modified hydrothermal carbon and co-produce carbon quantum dots, fully utilizing the "pore adsorption-metal synergy-chemical precipitation" function of magnesium-phosphorus modified hydrothermal carbon on nitrogen and phosphorus, and the carbon quantum dots enhance the photosynthesis of microalgae to further utilize the low-concentration nitrogen and phosphorus function. The magnesium-phosphorus modified hydrothermal carbon is applied not only to the biogas slurry purification stage, but also to the anaerobic fermentation biogas production stage. In the anaerobic fermentation stage of the biogas project, magnesium-phosphorus modified hydrothermal carbon is added to remove nitrogen and phosphorus in situ. In the biogas slurry purification stage, magnesium-phosphorus modified hydrothermal carbon is first added to remove high-concentration nitrogen and phosphorus, and then microalgae coupled with carbon quantum dots are inoculated to enhance nitrogen and phosphorus removal, thereby achieving the effect of coordinated cascade pollution reduction in multiple stages of the biogas project, and solving the problems of low nitrogen and phosphorus removal efficiency and high energy consumption in the prior art.

[0005] The present invention is achieved through the following technical solutions:

[0006] A method for efficiently removing nitrogen and phosphorus pollutants in a biogas project, the method comprising the following steps:

[0007] 1) Preparation of magnesium-phosphorus modified hydrothermal carbon and carbon quantum dots by a one-pot method: mixing agricultural waste with a 0.2-3M Mg(OH)2-KH2PO4 solution at a solid-liquid ratio of 1 g:(10-20) ml, and subjecting the mixture to a hydrothermal reaction at 180°C to 280°C for 0.5-8 h, preferably at 200°C to 260°C for 0.5-1 h. After the reaction, the solid and liquid are separated, and the solid phase is dried to obtain magnesium-phosphorus modified hydrothermal carbon. The liquid phase is dialyzed through a 1000 Da dialysis bag and freeze-dried to obtain carbon quantum dot powder; the magnesium-phosphorus molar ratio in the Mg(OH)2-KH2PO4 solution is 0.8-1.2;

[0008] 2) During the anaerobic fermentation stage of the livestock and poultry manure raw material inoculated with sludge, adding the magnesium-phosphorus-modified hydrothermal carbon obtained in step 1) to remove nitrogen and phosphorus in situ, wherein the amount of magnesium-phosphorus-modified hydrothermal carbon added is 10 to 30 wt% of the mass of the volatile component (VS) of the raw material;

[0009] 3) adding the magnesium-phosphorus modified hydrothermal carbon obtained in step 1) to the fermented biogas slurry in step 2) at a solid-to-liquid ratio of 1 to 30 g / L to adsorb and remove nitrogen and phosphorus;

[0010] 4) Finally, microalgae are inoculated into the biogas slurry obtained in step 3) and carbon quantum dots are added at a ratio of 1 to 10 mg / L, and carbon dioxide is introduced and cultured under light to achieve efficient cascade removal of nitrogen and phosphorus in the biogas project.

[0011] In step 1), the farming waste is selected from coconut shell powder, tobacco stalk powder, biogas residue, etc.

[0012] Preferably, when coconut shell is used as the substrate, the solid-liquid ratio is 1:10 g / ml, the concentration of Mg(OH)2-KH2PO4 solution is 0.6 M, and the reaction conditions are 200°C for reaction for 0.5 h; when tobacco stalks are used as the substrate, the solid-liquid ratio is 1:20 g / ml, the concentration of Mg(OH)2-KH2PO4 solution is 0.6 M, and the reaction conditions are 260°C for reaction for 0.5 h; when biogas residue is used as the substrate, the solid-liquid ratio is 1:20 g / ml, the concentration of Mg(OH)2-KH2PO4 solution is 0.6 M, and the reaction conditions are 220°C for reaction for 1 h.

[0013] The beneficial effects of the present invention are as follows:

[0014] 1) The "one-pot" method of preparing magnesium-phosphorus-modified hydrothermal carbon and co-producing carbon quantum dots in the present invention is simpler and has lower energy consumption than other modification methods. It can also simultaneously produce two types of products for pollution reduction: magnesium-phosphorus-modified hydrothermal carbon and carbon quantum dots.

[0015] 2) The method for efficiently removing nitrogen and phosphorus pollutants in biogas projects adopted by the present invention makes full use of the "pore adsorption-metal synergy-chemical precipitation" function of magnesium-phosphorus modified hydrothermal carbon on nitrogen and phosphorus, and carbon quantum dots enhance microalgae photosynthesis. Magnesium-phosphorus modified hydrothermal carbon is applied not only to the biogas slurry purification stage, but also to the anaerobic fermentation biogas production stage. Magnesium-phosphorus modified hydrothermal carbon is added in situ to remove nitrogen and phosphorus in the anaerobic fermentation stage of the biogas project. Magnesium-phosphorus modified hydrothermal carbon is first added to remove high-concentration nitrogen and phosphorus in the biogas slurry purification stage, and then microalgae are inoculated with carbon quantum dots to enhance nitrogen and phosphorus removal, achieving the effect of multi-stage pollution reduction in biogas projects. Compared with traditional A / O biological denitrification processes, Fenton and other multi-stage treatment processes, the present invention only adds carbon (carbon) materials, and microalgae can be used for value-added purposes such as feed after nitrogen fixation, which is more sustainable. Compared with pyrolysis and step-by-step modification methods, the "one-pot method" preparation process of the present invention is simple and has lower energy consumption. Compared with the traditional struvite method, the same amount of nitrogen and phosphorus is removed with less magnesium-phosphorus reagent consumption. Description of the drawings:

[0016] Figure 1 The results of in-situ nitrogen and phosphorus removal by adding magnesium-phosphorus modified hydrothermal carbon during the high-nitrogen anaerobic fermentation stage in Example 2 are shown. Here, a) shows the changes in ammonia nitrogen removal under different hydrothermal carbons; b) shows the changes in orthophosphate removal under different hydrothermal carbons.

[0017] Figure 2 The methane production after adding magnesium-phosphorus modified hydrothermal carbon during the high-nitrogen anaerobic fermentation stage in Example 2; a) Cumulative methane production at different hydrothermal carbon additions; b) Daily methane production at different hydrothermal carbon additions;

[0018] Figure 3 The figures show the nitrogen and phosphorus removal by adding different magnesium-phosphorus-modified hydrothermal carbons to the biogas slurry in Example 3 and the characterization of the carbon structure; wherein, a, the nitrogen and phosphorus removal by adding different magnesium-phosphorus-modified hydrothermal carbons to the biogas slurry; b, Fourier infrared spectra of different magnesium-phosphorus-modified hydrothermal carbons before and after the adsorption of nitrogen and phosphorus (in the figure, MgPR+NP represents the magnesium-phosphorus-modified biogas residue hydrothermal carbon after the adsorption of nitrogen and phosphorus; MgPR represents the magnesium-phosphorus-modified biogas residue hydrothermal carbon before the adsorption of nitrogen and phosphorus, MgPT+NP represents the magnesium-phosphorus-modified tobacco stem hydrothermal carbon after the adsorption of nitrogen and phosphorus, and MgPC+NP represents the magnesium-phosphorus-modified coconut shell hydrothermal carbon after the adsorption of nitrogen and phosphorus); c, scanning electron microscope image of the magnesium-phosphorus-modified biogas residue hydrothermal carbon after the adsorption of nitrogen and phosphorus; d, X-ray diffraction pattern of the magnesium-phosphorus-modified biogas residue hydrothermal carbon before and after the adsorption of nitrogen and phosphorus (MgPR+NP represents the magnesium-phosphorus-modified biogas residue hydrothermal carbon after the adsorption of nitrogen and phosphorus). Specific implementation method:

[0019] The following is a further description of the present invention, but not a limitation of the present invention.

[0020] Example 1: One-pot preparation of magnesium-phosphorus modified hydrothermal carbon and co-produced carbon quantum dots

[0021] (a) Coconut shell powder was mixed with a 0.6 M Mg(OH)2-KH2PO4 solution (magnesium-phosphorus molar ratio of 1) at a solid-liquid ratio of 1:20 (g / ml). The mixture was reacted at 200°C for 0.5 h. After the reaction, the solid and liquid were separated and the solid phase was dried to obtain magnesium-phosphorus-modified coconut shell hydrochar (denoted as MgPC). The liquid phase was dialyzed through a 1000 Da dialysis bag and freeze-dried to obtain the corresponding carbon quantum dot powder. The modified hydrochar was denoted as MgC, with Mg(OH)2 replacing Mg(OH)2-KH2PO4 and other properties remaining unchanged.

[0022] (b) Tobacco stem powder was mixed with 0.6 M Mg(OH)2-KH2PO4 (magnesium-phosphorus molar ratio of 1) solution at a solid-liquid ratio of 1:20 (g / ml) and reacted at 260°C for 0.5 h. After the reaction, the solid and liquid were separated and the solid phase was dried to obtain magnesium-phosphorus modified tobacco stem hydrothermal charcoal (denoted as MgPT). The liquid phase was dialyzed with a 1000 Da dialysis bag and freeze-dried to obtain the corresponding carbon quantum dot powder.

[0023] (c) The biogas residue powder was mixed with 0.6 M Mg(OH)2-KH2PO4 (magnesium-phosphorus molar ratio of 1) solution at a solid-liquid ratio of 1:20 (g / ml), and the mixture was reacted at 220°C for 1 h. After the reaction, the solid and liquid were separated, and the solid phase was dried to obtain magnesium-phosphorus modified biogas residue hydrothermal carbon (denoted as MgPR). The liquid phase was dialyzed through a 1000 Da dialysis bag and freeze-dried to obtain the corresponding carbon quantum dot powder.

[0024] Example 2: In-situ removal of nitrogen and phosphorus by adding magnesium-phosphorus-modified hydrothermal carbon during anaerobic fermentation

[0025] Chicken manure was used as the anaerobic fermentation substrate, inoculated with sludge, and magnesium-phosphorus-modified hydrochar was added. The mass ratio of sludge inoculation to volatile components of chicken manure was 3:7, and the amount of magnesium-phosphorus-modified hydrochar added was 25wt% of the mass of volatile components of chicken manure. A blank control group (CK) was set up for digestion of only inoculated sludge. Liquid samples were taken before and after fermentation to determine nitrogen and phosphorus concentrations, and the anaerobic gas production components were determined using a biodegradation test system. Figure 1 As shown in the figure, the removal of ammonia nitrogen is MgPC>MgPR>MgPT>MgC>CK, the removal of orthophosphate is MgPC>MgC>MgPR>MgPT>CK, and the cumulative methane production is MgPR>MgPT>MgC>CK>MgPC. The porous structure of MgPR can provide abundant microbial attachment sites, promote biofilm formation and accelerate substrate mass transfer efficiency, and the iron, potassium and magnesium contained in MgPR synergistically promote the activity of anaerobic microorganisms, resulting in a maximum cumulative methane production of 40.09ml / gVS( Figure 2), a 19.1% increase compared to the blank control. Furthermore, the magnesium-phosphorus components in MgPR reacted with ammonia nitrogen to form struvite precipitates, achieving an ammonia nitrogen removal rate of 537.5 mg / L, a 3.77-fold increase compared to MgC. In summary, magnesium-phosphorus-modified hydrothermal carbon, through the coupled action of "pore adsorption, metal synergy, and chemical precipitation," achieved in-situ nitrogen and phosphorus removal during the anaerobic phase while simultaneously increasing methane production.

[0026] Example 3: Adding magnesium and phosphorus to biogas slurry to modify hydrothermal carbon for nitrogen and phosphorus removal

[0027] MgPC, MgPT and MgPR obtained in Example 1 were added to the biogas slurry obtained in Example 2 at a solid-liquid ratio of 20 g / L. After adsorption for 6 h at room temperature, samples were taken for analysis. Figure 3 As shown in (a), the nitrogen and phosphorus removal rates of MgPR>MgPT>MgPC are respectively. Both are higher than the removal rates of commercial coconut shell activated carbon for biogas slurry nitrogen and phosphorus (16.89% and 17.22%). MgPR has the best ability to simultaneously remove nitrogen and phosphorus from biogas slurry, at 93.72% and 62.41%, respectively. The highest specific surface area of MgPR was 58.021 m 2 / g, followed by MgPT with 49.628m 2 / g and 27.332m 2 / g (Table 1). MgPR with high specific surface area has significant advantages in nitrogen and phosphorus removal. Compared with unmodified coconut shell hydrothermal carbon (HC), unmodified tobacco stem hydrothermal carbon (HT) and unmodified biogas residue hydrothermal carbon (HR), the 1016cm-1 new infrared spectroscopy region of magnesium-phosphorus modified hydrothermal carbon is -1 (PO key) and 563cm -1 (Mg-O bond) demonstrates the successful loading of magnesium and phosphorus ( Figure 3 b), and after adsorption of nitrogen and phosphorus, the -1 New peaks appeared at 765cm-1 (NH bond) and 765cm-1 (PO bond), among which the peak intensity of MgPR was the largest, indicating that it had the best nitrogen and phosphorus adsorption capacity. The selected MgPR was tested by scanning electron microscopy, and the surface morphology after adsorption was as follows. Figure 3 As shown in (c), the carbon surface flake structure becomes more uniform and regularly distributed, and the surface roughness is significantly reduced, indicating that struvite precursors are directionally deposited on the surface of the material during nitrogen and phosphorus adsorption and self-assemble into ordered flakes. The formation of struvite crystals is also confirmed by XRD characterization ( Figure 3 d).

[0028] Table 1

[0029]

[0030] Example 4: Biogas slurry inoculated with microalgae coupled with carbon quantum dots to enhance nitrogen and phosphorus removal

[0031] The treated biogas slurry described in Example 3 was inoculated with Chlorella vulgaris and the co-produced carbon quantum dots were added at a ratio of 10 mg / L. The light intensity was 50 μmol / s at 25°C. 2 The addition of carbon quantum dots (CQDs) to the microalgae culture medium, under a 24-hour photoperiod and 5% CO2 for 7 days, significantly promoted their growth. Compared to the control group without CQDs, the growth rates of the MgPR, MgPC, and MgPT groups increased by 17.26%, 12.05%, and 10.76%, respectively. Carbohydrate content increased by 17.20%, 11.63%, and 10.69%, respectively. Photosynthetic efficiency also increased by 27.74%, 20.36%, and 18.59%, respectively. Furthermore, the ammonia nitrogen removal rates of the biogas slurry reached 55.72%, 53.66%, and 50.12%, respectively, representing increases of 16.18%, 15.20%, and 14.33%, respectively, compared to the control group without CQDs. The ammonia nitrogen and phosphorus concentrations in the biogas slurry met the "Pollutant Emission Standards for Livestock and Poultry Farming."

[0032] As can be seen from the above, using the efficient removal method for nitrogen and phosphorus pollutants in biogas projects of the present invention, magnesium-phosphorus-modified hydrothermal carbon is added during the anaerobic fermentation stage of the biogas project to remove nitrogen and phosphorus in situ. The nitrogen and phosphorus removal effects of MgPC>MgPR>MgPT, where MgPC removes 570 mg / L and 4.725 mg / L nitrogen and phosphorus, respectively; MgPR removes 537.5 mg / L and 3.15 mg / L nitrogen and phosphorus, respectively; and MgPT removes 525 mg / L and 1.575 mg / L nitrogen and phosphorus, respectively. Magnesium-phosphorus-modified hydrothermal carbon is first added during the biogas slurry purification stage to remove high-concentration nitrogen and phosphorus. The nitrogen and phosphorus removal effects of MgPR>MgPT>MgPC, where MgPR removes 93.72% and 62.41% nitrogen and phosphorus, respectively; MgPT removes 88.06% and 75.78%, respectively; and MgPC removes 69.61% and 42.35%, respectively. Then, microalgae coupled with carbon quantum dots are inoculated to enhance nitrogen and phosphorus removal, with the removal effect of MgPR>MgPC>MgPT, achieving the effect of multi-stage pollution reduction in the biogas project. Since high-concentration nitrogen and phosphorus are removed in the biogas slurry purification stage, the overall nitrogen and phosphorus removal effect of the entire process is MgPR>MgPT>MgPC.

Claims

1. A method for efficiently removing nitrogen and phosphorus pollutants in a biogas project, characterized in that: The method comprises the following steps: 1) Mixing agricultural waste with a 0.2-3M Mg(OH)2-KH2PO4 solution at a solid-liquid ratio of 1 g:(10-20) ml, subjecting the mixture to a hydrothermal reaction at 180°C-280°C for 0.5-8 h. After the reaction, separating the solid and liquid, and drying the solid phase to obtain magnesium-phosphorus modified hydrothermal carbon, dialyzing the liquid phase through a 1000 Da dialysis bag and freeze-drying the liquid phase to obtain carbon quantum dot powder; the magnesium-phosphorus molar ratio in the Mg(OH)2-KH2PO4 solution is 0.8-1.2; 2) During the anaerobic fermentation stage of the livestock and poultry manure raw material inoculated with sludge, adding the magnesium-phosphorus-modified hydrothermal carbon obtained in step 1) to remove nitrogen and phosphorus in situ, wherein the addition ratio of the magnesium-phosphorus-modified hydrothermal carbon is 10-30% by mass of the volatile components of the raw material; 3) adding the magnesium-phosphorus modified hydrothermal carbon obtained in step 1) to the fermented biogas slurry in step 2) at a solid-to-liquid ratio of 1 to 30 g / L to adsorb and remove nitrogen and phosphorus; 4) The biogas slurry obtained in step 3) is inoculated with microalgae and carbon quantum dots are added at a ratio of 1 to 10 mg / L, and carbon dioxide is introduced and cultured under light to achieve efficient cascade removal of nitrogen and phosphorus in the biogas project.

2. The method according to claim 1, characterized in that Step 1) The hydrothermal reaction conditions are: 200° C. to 260° C. for 0.5 to 1 h.

3. The method according to claim 1, characterized in that In step 1), the farming waste is selected from coconut shell powder, tobacco stalk powder, and biogas residue.

4. The method according to claim 3, characterized in that When coconut shell is used as the substrate, the solid-liquid ratio is 1:10 g / ml, the concentration of Mg(OH)2-KH2PO4 solution is 0.6 M, and the hydrothermal reaction conditions are 200°C for 0.5 h; when tobacco stalk is used as the substrate, the solid-liquid ratio is 1:20 g / ml, the concentration of Mg(OH)2-KH2PO4 solution is 0.6 M, and the hydrothermal reaction conditions are 260°C for 0.5 h; when biogas residue is used as the substrate, the solid-liquid ratio is 1:20 g / ml, the concentration of Mg(OH)2-KH2PO4 solution is 0.6 M, and the hydrothermal reaction conditions are 220°C for 1 h.

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