Method for efficient removal of nitrogen and phosphorus pollutants in biogas engineering

Abstract

This invention discloses a method for the efficient removal of nitrogen and phosphorus pollutants in biogas projects. It employs a "one-pot method" to prepare magnesium-phosphorus modified hydrothermal char and co-produce carbon quantum dots. This fully utilizes the "pore adsorption-metal synergy-chemical precipitation" function of magnesium-phosphorus modified hydrothermal char for nitrogen and phosphorus, and the function of carbon quantum dots to enhance microalgal photosynthesis and further utilize low-concentration nitrogen and phosphorus. The magnesium-phosphorus modified hydrothermal char 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 char is added for in-situ removal of nitrogen and phosphorus. In the biogas slurry purification stage, magnesium-phosphorus modified hydrothermal char is first added to remove high-concentration nitrogen and phosphorus, followed by inoculation with microalgae coupled with carbon quantum dots to enhance nitrogen and phosphorus removal, achieving a multi-stage synergistic and tiered pollution reduction effect in biogas projects.

Description

Technical fields:

[0001] This invention relates to the field of resource utilization technology of agricultural and livestock waste, specifically to a method for the efficient removal of nitrogen and phosphorus pollutants in biogas projects. Background technology:

[0002] Currently, while livestock and poultry farming has achieved large-scale development, it has also generated a large amount of manure waste. To address the problem of livestock and poultry manure accumulation, my country has widely promoted biogas projects. However, biogas slurry produced after fermentation has problems such as large discharge volumes, high concentrations of nitrogen and phosphorus pollutants, excessive heavy metals, and an imbalanced carbon-nitrogen ratio. Traditional water treatment processes are insufficient to meet discharge standards. On the other hand, large and medium-sized biogas projects produce large and concentrated volumes of biogas slurry, but due to the separation of crop farming and livestock breeding, the slurry cannot be directly utilized in nearby farmland, and long-distance transportation costs are too high, posing many challenges to the disposal of biogas slurry. If discharged directly without proper treatment, the excessive nitrogen and phosphorus will cause eutrophication of water bodies and disrupt the ecological balance.

[0003] Currently reported biochar for nitrogen and phosphorus removal from biogas slurry is mainly magnesium salt-modified biochar, such as magnesium salt-impregnated pig manure biochar (CN 115999506 A) and magnesium salt-impregnated straw biochar (CN 115715972 A). These biochars are obtained through pyrolysis at 450–700℃ followed by further modification, a complex and energy-intensive process. Hydrothermal carbonization technology offers milder reaction conditions, strong compatibility with raw materials, eliminates the need for raw material drying pretreatment, and reduces energy consumption by approximately 30–50%. Further modification with acids, alkalis, and metal salts can enhance nitrogen and phosphorus adsorption; however, this process still primarily involves two or more steps, necessitating simplification of the carbonization process and improved nitrogen and phosphorus removal efficiency. More importantly, nitrogen and phosphorus removal from biogas slurry is a "post-treatment" process requiring further research. Summary of the Invention:

[0004] The purpose of this invention is to provide a method for the efficient removal of nitrogen and phosphorus pollutants in biogas projects. This method employs a "one-pot" process to prepare magnesium-phosphorus modified hydrothermal char and co-produce carbon quantum dots. It fully utilizes the "pore adsorption-metal synergy-chemical precipitation" function of magnesium-phosphorus modified hydrothermal char for nitrogen and phosphorus, and the function of carbon quantum dots to enhance microalgal photosynthesis and further utilize low-concentration nitrogen and phosphorus. The magnesium-phosphorus modified hydrothermal char 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 char is added for in-situ nitrogen and phosphorus removal. In the biogas slurry purification stage, magnesium-phosphorus modified hydrothermal char is first added to remove high-concentration nitrogen and phosphorus, followed by inoculation with microalgae coupled with carbon quantum dots to enhance nitrogen and phosphorus removal. This achieves a multi-stage synergistic cascade pollution reduction effect in biogas projects, solving the problems of low nitrogen and phosphorus removal efficiency and high energy consumption in existing technologies.

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

[0006] A method for efficient removal of nitrogen and phosphorus pollutants in biogas projects, the method comprising the following steps:

[0007] 1) One-pot method for preparing magnesium-phosphorus modified hydrothermal carbon and co-producing carbon quantum dots: Agricultural waste is mixed with 0.2–3 M Mg(OH)₂-KH₂PO₄ solution at a solid-liquid ratio of 1 g:(10–20) ml. The mixture is then subjected to a hydrothermal reaction at 180℃–280℃ for 0.5–8 h, preferably at 200℃–260℃ for 0.5–1 h. After the reaction, the solid and liquid phases are separated. 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 livestock and poultry manure raw material inoculation sludge, magnesium phosphorus modified hydrothermal carbon obtained in step 1) is added to remove nitrogen and phosphorus in situ. The amount of magnesium phosphorus modified hydrothermal carbon added is 10 to 30 wt% of the mass of the volatile components (VS) of the raw material.

[0009] 3) Add the magnesium-phosphorus modified hydrothermal carbon obtained in step 1) to the biogas slurry after fermentation in step 2) at a solid-liquid ratio of 1-30 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. Carbon dioxide is introduced for light cultivation to achieve efficient nitrogen and phosphorus removal in biogas engineering.

[0011] Step 1) The agricultural and livestock waste is selected from coconut shell powder, tobacco straw 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 0.6M, and the reaction conditions are 200℃ for 0.5h; 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 0.6M, and the reaction conditions are 260℃ for 0.5h; 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 0.6M, and the reaction conditions are 220℃ for 1h.

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

[0014] 1) The one-pot method of the present invention for preparing magnesium-phosphorus modified hydrothermal carbon and producing carbon quantum dots is simple and energy-efficient compared with other modification methods, and can simultaneously obtain two types of products for pollution reduction: magnesium-phosphorus modified hydrothermal carbon and carbon quantum dots.

[0015] 2) This invention employs a highly efficient method for removing nitrogen and phosphorus pollutants from biogas projects. It fully utilizes the "pore adsorption-metal synergy-chemical precipitation" function of magnesium-phosphorus modified hydrothermal carbon for nitrogen and phosphorus, as well as the enhancement of microalgal photosynthesis by carbon quantum dots. This method applies 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 for in-situ nitrogen and phosphorus removal. In the biogas slurry purification stage, magnesium-phosphorus modified hydrothermal carbon is first added to remove high concentrations of nitrogen and phosphorus, followed by inoculation with microalgae coupled with carbon quantum dots to enhance nitrogen and phosphorus removal, achieving a multi-stage, tiered pollution reduction effect in biogas projects. Compared to traditional A / O biological denitrification processes and Fenton multi-stage treatment processes, this invention only adds carbon materials, and the nitrogen fixed by microalgae can be used for value-added purposes such as feed, resulting in better sustainability. Compared to pyrolysis and stepwise modification methods, the "one-pot" preparation process of this invention is simpler and consumes less energy. Compared to the traditional struvite method, less magnesium-phosphorus reagent is consumed to remove the same amount of nitrogen and phosphorus. Attached image description:

[0016] Figure 1 This section describes the in-situ nitrogen and phosphorus removal process using magnesium-phosphorus modified hydrothermal char during the high-nitrogen anaerobic fermentation stage in Example 2. Specifically, a) changes in ammonia nitrogen removal under different hydrothermal char types; and b) changes in orthophosphate removal under different hydrothermal char types.

[0017] Figure 2 This refers to the methane production after adding magnesium-phosphorus modified hydrothermal char during the high-nitrogen anaerobic fermentation stage in Example 2; where a) is the cumulative methane production with different amounts of hydrothermal char added; and b) is the daily methane production with different amounts of hydrothermal char added.

[0018] Figure 3 This document describes the nitrogen and phosphorus removal performance of hydrothermal char modified with different magnesium-phosphorus compounds in Example 3, and the characterization of the char structure. Specifically: a) nitrogen and phosphorus removal performance of hydrothermal char modified with different magnesium-phosphorus compounds; b) Fourier transform infrared (FTIR) spectra of different magnesium-phosphorus modified hydrothermal chars before and after nitrogen and phosphorus adsorption (MgPR+NP represents magnesium-phosphorus modified hydrothermal char after nitrogen and phosphorus adsorption; MgPR represents magnesium-phosphorus modified hydrothermal char before nitrogen and phosphorus adsorption; MgPT+NP represents magnesium-phosphorus modified tobacco hydrothermal char after nitrogen and phosphorus adsorption; MgPC+NP represents magnesium-phosphorus modified coconut shell hydrothermal char after nitrogen and phosphorus adsorption); c) Scanning electron microscope (SEM) image of magnesium-phosphorus modified hydrothermal char after nitrogen and phosphorus adsorption; d) X-ray diffraction patterns of magnesium-phosphorus modified hydrothermal char before and after nitrogen and phosphorus adsorption (MgPR+NP represents magnesium-phosphorus modified hydrothermal char after nitrogen and phosphorus adsorption). Detailed implementation method:

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

[0020] Example 1: Preparation of magnesium-phosphorus modified hydrothermal carbon and co-production of carbon quantum dots by "one-pot method"

[0021] (a) Coconut shell powder was mixed with a 0.6M Mg(OH)₂-KH₂PO₄ solution (magnesium-phosphorus molar ratio of 1) at a solid-liquid ratio of 1:20 (g / ml), and reacted at 200℃ for 0.5 h. After the reaction was completed, the solid and liquid phases were separated, and the solid phase was dried to obtain magnesium-phosphorus modified coconut shell hydrothermal carbon (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 hydrothermal carbon obtained by replacing Mg(OH)₂-KH₂PO₄ with Mg(OH)₂ while keeping other parameters unchanged was denoted as MgC.

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

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

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

[0025] Chicken manure was used as the anaerobic fermentation substrate, and sludge was inoculated with magnesium-phosphorus modified hydrothermal char. The mass ratio of sludge inoculum to volatile components of chicken manure was 3:7, and the amount of magnesium-phosphorus modified hydrothermal char added was 25 wt% of the volatile components of chicken manure. A blank control group (CK) was set up, inoculated only with sludge for digestion. Liquid samples were taken before and after fermentation to determine nitrogen and phosphorus concentrations, and the anaerobic gas composition was determined using a biodegradation testing system. Figure 1 As shown, the removal rates of ammonia nitrogen were MgPC > MgPR > MgPT > MgC > CK, the removal rates of orthophosphate were MgPC > MgC > MgPR > MgPT > CK, and the cumulative methanogenesis was MgPR > MgPT > MgC > CK > MgPC. Because the porous structure of MgPR provides abundant microbial attachment sites, promotes biofilm formation and accelerates substrate mass transfer efficiency, and the iron, potassium, and magnesium it contains synergistically promote the activity of anaerobic microorganisms, the cumulative methanogenesis reached a maximum of 40.09 ml / gVS. Figure 2The removal rate of nitrogen and phosphorus in MgPR was 19.1% higher than that of the control group. Simultaneously, the magnesium and phosphorus components in MgPR reacted with ammonia nitrogen to form struvite precipitate, achieving a removal rate of 537.5 mg / L of ammonia nitrogen, which was 3.77 times higher than that of MgC. In summary, magnesium-phosphorus modified hydrothermal carbon, through a coupling effect of "pore adsorption-metal synergy-chemical precipitation," achieves in-situ removal of nitrogen and phosphorus in the anaerobic stage while simultaneously increasing methane production.

[0026] Example 3: Removal of nitrogen and phosphorus by adding magnesium-phosphorus modified hydrothermal charcoal to biogas slurry

[0027] According to a solid-liquid ratio of 20 g / L, MgPC, MgPT, and MgPR obtained in Example 1 were added to the biogas slurry obtained in Example 2, and samples were taken for analysis after adsorption at room temperature for 6 hours. Figure 3 As shown in (a), the nitrogen and phosphorus removal rates were MgPR > MgPT > MgPC, respectively. All were higher than the removal rates of nitrogen and phosphorus from biogas slurry by commercial coconut shell activated carbon (16.89% and 17.22%, respectively). MgPR showed the best simultaneous nitrogen and phosphorus removal capacity from biogas slurry, at 93.72% and 62.41%, respectively. The specific surface area of ​​MgPR was also the highest, measured at 58.021 m². 2 / g, followed by MgPT at 49.628m 2 / g and MgPC 27.332m 2 / g (Table 1). High specific surface area MgPR has a significant advantage in nitrogen and phosphorus removal. Compared with unmodified coconut shell hydrothermal char (HC), unmodified tobacco stalk hydrothermal char (HT), and unmodified biogas residue hydrothermal char (HR), magnesium-phosphorus modified hydrothermal char shows a significant increase in infrared spectrum of 10¹⁶ cm⁻¹. -1 (PO key) and 563cm -1 (Mg-O bond) proves the successful loading of magnesium phosphorus ( Figure 3 b) and after adsorbing nitrogen and phosphorus, at 1431 cm -1 New peaks appeared at 765 cm⁻¹ for both NH₃ bonds and PO₄⁻¹, with MgPR exhibiting the highest peak intensity, indicating its optimal nitrogen and phosphorus adsorption capacity. The selected MgPR was subjected to scanning electron microscopy (SEM), and the surface morphology after adsorption is shown below. Figure 3 As shown in (c), the platy structure on the carbon surface becomes more uniform and regularly distributed, and the surface roughness is significantly reduced, indicating that struvite precursors are directionally deposited on the material surface and self-assembled into ordered lamellae during nitrogen and phosphorus adsorption. The formation of struvite crystals was also confirmed by XRD characterization. Figure 3 d).

[0028] Table 1

[0029]

[0030] Example 4: Enhancing Nitrogen and Phosphorus Removal by Inoculating Microalgae with Carbon Quantum Dots in Biogas Slurry

[0031] Chlorella was inoculated into the treated biogas slurry as described in Example 3, and carbon quantum dots produced in conjunction with it were added at a ratio of 10 mg / L. The mixture was then subjected to a light intensity of 50 μmol / s at 25°C. 2 Under a 24-hour photoperiod and 5% carbon dioxide atmosphere for 7 days, the addition of carbon quantum dots positively promoted microalgae growth. Compared with the control group without carbon quantum dots, the growth rates of 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; and photosynthetic efficiency increased by 27.74%, 20.36%, and 18.59%, respectively. Simultaneously, the ammonia nitrogen removal rates in the biogas slurry reached 55.72%, 53.66%, and 50.12%, respectively, representing increases of 16.18%, 15.20%, and 14.33% compared to the control group without carbon quantum dots. The concentrations of ammonia nitrogen and phosphorus in the biogas slurry met the "Emission Standards for Pollutants from Livestock and Poultry Farming".

[0032] As can be seen from the above, the efficient nitrogen and phosphorus pollutant removal method for biogas projects of the present invention, by adding magnesium-phosphorus modified hydrothermal carbon in the anaerobic fermentation stage of biogas projects to remove nitrogen and phosphorus in situ, shows that the nitrogen and phosphorus removal efficiency is MgPC > MgPR > MgPT. Specifically, MgPC removes 570 mg / L and 4.725 mg / L of nitrogen and phosphorus, respectively; MgPR removes 537.5 mg / L and 3.15 mg / L of nitrogen and phosphorus, respectively; and MgPT removes 525 mg / L and 1.575 mg / L of nitrogen and phosphorus, respectively. In the biogas slurry purification stage, magnesium-phosphorus modified hydrothermal carbon is added first to remove high concentrations of nitrogen and phosphorus. The nitrogen and phosphorus removal efficiency is MgPR > MgPT > MgPC. Specifically, MgPR removes 93.72% and 62.41% of nitrogen and phosphorus, respectively; MgPT removes 88.06% and 75.78% of nitrogen and phosphorus, respectively; and MgPC removes 69.61% and 42.35% of nitrogen and phosphorus, respectively. Then, microalgae are inoculated and coupled with carbon quantum dots to enhance nitrogen and phosphorus removal. The removal effect is MgPR > MgPC > MgPT, achieving the effect of multi-stage and tiered pollution reduction in biogas projects. Since the biogas slurry purification stage removes high concentrations of nitrogen and phosphorus, the overall nitrogen and phosphorus removal effect of the whole process is MgPR > MgPT > MgPC.

Claims

1. A method for efficient removal of nitrogen and phosphorus pollutants in biogas projects, characterized in that, The method includes the following steps: 1) Mix agricultural and livestock waste with 0.2-3M Mg(OH)2-KH2PO4 solution at a solid-liquid ratio of 1g:(10-20)ml, and perform a hydrothermal reaction at 180℃-280℃ for 0.5-8h. After the reaction, separate the solid and liquid phases, and dry the solid phase to obtain magnesium-phosphorus modified hydrothermal carbon. Dialyze the liquid phase through a 1000Da dialysis bag and freeze-dry to obtain carbon quantum dot powder. The magnesium-phosphorus molar ratio in the Mg(OH)2-KH2PO4 solution is 0.8-1.

2. The agricultural and livestock waste is selected from coconut shell powder, tobacco straw powder, and biogas residue. 2) During the anaerobic fermentation stage of livestock and poultry manure raw material inoculation sludge, magnesium-phosphorus modified hydrothermal carbon obtained in step 1) is added to remove nitrogen and phosphorus in situ. The addition ratio of magnesium-phosphorus modified hydrothermal carbon is 10-30% of the mass of volatile components of the raw material. 3) Add the magnesium-phosphorus modified hydrothermal carbon obtained in step 1) to the biogas slurry after fermentation in step 2) at a solid-liquid ratio of 1~30g / L to adsorb and remove nitrogen and phosphorus; 4) Microalgae are inoculated into the biogas slurry obtained in step 3), and carbon quantum dots are added at a ratio of 1-10 mg / L. Carbon dioxide is introduced for cultivation under light, achieving efficient nitrogen and phosphorus removal in a cascade manner during biogas engineering. When coconut shells are used as substrates, 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 ℃ for 0.5 h. When tobacco stalks are used as substrates, 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 ℃ for 0.5 h. When biogas residue is used as substrates, 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 ℃ for 1 h. h; A "one-pot method" is used to prepare magnesium-phosphorus modified hydrothermal char and co-produce carbon quantum dots. This fully utilizes the "pore adsorption-metal synergy-chemical precipitation" function of magnesium-phosphorus modified hydrothermal char for nitrogen and phosphorus, as well as the function of carbon quantum dots to enhance microalgal photosynthesis and further utilize low-concentration nitrogen and phosphorus. Magnesium-phosphorus modified hydrothermal char 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 biogas engineering, magnesium-phosphorus modified hydrothermal char is added to remove nitrogen and phosphorus in situ. In the biogas slurry purification stage, magnesium-phosphorus modified hydrothermal char is first added to remove high-concentration nitrogen and phosphorus, and then microalgae are inoculated and coupled with carbon quantum dots to enhance nitrogen and phosphorus removal.

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

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