Biogas carbon sequestration method based on synergistic treatment of pig farm wastewater and ardealite

Abstract

The invention discloses a biogas carbon sequestration method based on synergistic treatment of pig farm wastewater and ardealite, which comprises the following steps: (1) adding phosphate and magnesium salt into biogas slurry, then adjusting the pH value to generate struvite precipitate, carrying out low-temperature pyrolysis to obtain ammonia gas, and absorbing with water to obtain ammonia water; (2) taking the ammonia water as an absorbent to synchronously capture COs in the biogas so as to form an absorption liquid containing ammonium carbonate; and (3) reacting the absorption liquid with phosphogypsum to directionally generate a calcium carbonate product. According to the invention, a biogas slurry-ardealite-biogas three-waste synergistic treatment system is innovatively constructed, and triple technologies of struvite crystallization, ammonia water dual absorption and ardealite conversion are coupled, so that the nitrogen and phosphorus recovery rate is greater than or equal to 92%, and the biogas purification rate is greater than 95%.

Description

Technical Field

[0001] The present invention belongs to the technical field of wastewater ammonium resource treatment, and particularly relates to a method for biogas carbon sequestration based on the co-treatment of pig farm wastewater and phosphogypsum. Background Art

[0002] The rapid development of large-scale pig farming has brought significant pollutant emission problems, causing a huge impact on the national water environment, and the wastewater discharge from the livestock and poultry breeding industry remains high. Therefore, there is an urgent need for a reasonable, effective and green treatment method to solve this problem. The pig farm wastewater treatment technology aiming at energy conservation, emission reduction and wastewater resource utilization is gradually becoming the mainstream. The traditional process adopts anaerobic digestion-aerobic nitrification-anaerobic denitrification method. Although this method can effectively convert organic matter into methane (CH4) and recover methane as energy to realize wastewater resource utilization, the treated biogas slurry still contains high concentrations of ammonia nitrogen and has poor biodegradability.

[0003] On the other hand, enterprises need to prepare a large amount of ammonia (NH3) for nitrogen fertilizers and chemical raw materials, but the commonly used Haber method in traditional industries has high energy consumption and large emissions. If NH4 in pig farm wastewater can be recovered, it can reduce the energy consumption of NH3 production and greenhouse gas emissions. The magnesium ammonium phosphate precipitation method is an effective solution. By adding magnesium ions (Mg) and phosphate ions (PO4), NH4 and phosphorus are converted into struvite precipitation, thereby realizing nitrogen and phosphorus recovery. This method has the advantages of simple operation, stable reaction, high denitrification rate, economic and environmental protection, etc., and has good economic and environmental benefits. + , it can reduce the energy consumption of NH3 production and greenhouse gas emissions. The magnesium ammonium phosphate precipitation method is an effective solution. By adding magnesium ions (Mg 2+ ) and phosphate ions (PO4 3- ) agents, NH4 + and phosphorus are converted into struvite precipitation, thereby realizing nitrogen and phosphorus recovery. This method has the advantages of simple operation, stable reaction, high denitrification rate, economic and environmental protection, etc., and has good economic and environmental benefits.

[0004] At the same time, biogas, as a combustible gas generated by the microbial fermentation of organic substances (such as livestock and poultry manure and food waste) under anaerobic conditions, is of great significance for promoting the adjustment of the energy structure, improving the ecological environment and enhancing the resource utilization efficiency. However, the high concentration of carbon dioxide (CO2) in biogas cannot meet the national standards, which limits its application range and further exacerbates the energy security and supply risks in China.

[0005] Phosphogypsum (CaSO4·2H2O), as a major type of industrial waste, has a huge annual output. Phosphogypsum is a by-product of wet-process phosphoric acid smelting. If not properly treated, it will cause harm to the environment and human health, and pose a serious threat to the health and balance of the global ecosystem. At present, the conventional method for treating phosphogypsum is the ammonia method of phosphogypsum. This method can effectively capture CO2 while generating building material calcium carbonate (CaCO3) and nitrogen fertilizer ammonium sulfate [(NH4)2SO4], and is an efficient carbon emission reduction technology. However, in practical applications, this method requires a large amount of ammonia water, which limits its wide application and promotion.

[0006] In summary, a method for realizing carbon sequestration through the comprehensive utilization of pig farm wastewater and phosphogypsum proposed by the present invention recovers and prepares ammonia water from pig farm biogas slurry wastewater through the struvite crystallization method, and uses it for ammonia-based capture of CO2 in biogas from phosphogypsum. This method aims to realize the resource utilization of pig farm biogas slurry wastewater and phosphogypsum, while improving the quality of biogas, and has important practical significance. Summary of the Invention

[0007] In view of the above problems, the present invention provides a method for biogas carbon sequestration based on the collaborative treatment of pig farm wastewater and phosphogypsum, which realizes the comprehensive utilization of phosphogypsum and pig farm wastewater, and forms a set of efficient, low-emission, and recyclable ammonium treatment technologies applicable to pig farm wastewater.

[0008] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0009] A method for biogas carbon sequestration based on the collaborative treatment of pig farm wastewater and phosphogypsum, comprising the following steps:

[0010] S1: Add phosphate and magnesium salt to the pig farm biogas slurry wastewater containing NH4 + , and add alkali to adjust the pH to alkaline, stir and react to form struvite (MgNH4PO4) precipitation, that is, recover ammonia nitrogen in the biogas slurry wastewater through the struvite crystallization method, and then filter to obtain low-concentration ammonia nitrogen wastewater and struvite precipitation; the low-concentration ammonia nitrogen wastewater is further treated by a conventional treatment process, that is, after biochemical treatment of hydrolysis acidification and aerobic biochemistry, it is discharged up to standard;

[0011] S2: Dry the struvite precipitation obtained in step S1, the struvite decomposes by heating, generating residual magnesium hydrogen phosphate solid, and NH3 volatilizes during this process; the residual magnesium hydrogen phosphate solid is reused in the reaction of step S1;

[0012] S3: Pass the NH3 in step S2 into water in the form of aeration, so that NH3 is in full contact with water for absorption reaction to prepare an ammonia water solution;

[0013] S4: The ammonia water solution obtained in step S3 is sprayed down from the upper part of the spray packing tower, and at the same time, the biogas containing CO2 is introduced from the lower part of the spray packing tower. The ammonia water solution is in full countercurrent contact with the biogas containing CO2, and after the ammonia water solution absorbs CO2, it forms an (NH4)2CO3 water solution and is discharged from the bottom of the spray packing tower;

[0014] S5: Mix and stir the (NH4)2CO3 aqueous solution described in step S4 with phosphogypsum (CaSO4·2H2O) to convert it into an aqueous solution containing CaCO3 and (NH4)2SO4, then let it stand for crystallization to allow CaCO3 to settle fully, filter to obtain the CaCO3 precipitate for recovery, and the residual liquid rich in (NH4)2SO4 can be used as a nitrogen fertilizer.

[0015] Further, in step S1, the NH4 in the pig farm biogas slurry wastewater + concentration is > 1000 mg / L; the phosphate is sodium phosphate, and the magnesium salt is magnesium chloride. Based on the NH4 + concentration in the pig farm biogas slurry wastewater, according to the molar ratio NH4 + : PO4 3- : Mg 2+ = 1: 1.05 - 1.1: 1.05 - 1.1, determine the feeding amounts of the phosphate and the magnesium salt in the pig farm biogas slurry wastewater.

[0016] Further, in step S1, the stirring speed is 200 - 300 rpm, the base for adjusting the pH is sodium hydroxide solution, the pH range of the reaction system is adjusted to 8 - 9.5, the reaction temperature is room temperature, and the reaction time is 1.5 - 4 h.

[0017] Further, in step S2, the struvite precipitate is added to a heating tank, and after sealing the heating tank, it is heated and dried. First, the air in the heating tank is displaced with nitrogen to make the drying atmosphere nitrogen, and the struvite in the heating tank is heated to decompose and release NH3. The temperature for heating and drying is 80 - 110 °C; an NH3 on-line detector is also provided at the air outlet of the heating tank. When the detected NH3 concentration < 30 ppm, it is considered that the struvite has been completely decomposed and the heating is stopped.

[0018] Further, the residual solid of magnesium hydrogen phosphate obtained in step S2 is reused in the ammonium recovery reaction in step S1. The specific implementation steps are as follows: The residual solid of magnesium hydrogen phosphate obtained in step S2 is reused in the ammonium recovery reaction in step S1. The specific implementation steps are as follows: The residual solid of magnesium hydrogen phosphate also contains phosphoric acid. The residual solid of magnesium hydrogen phosphate is added to the pig farm biogas slurry wastewater. Based on the NH4 + concentration in the pig farm biogas slurry wastewater, according to the molar ratio NH4 + : PO4 3- : Mg 2+= 1: 1.05 - 1.1: 1.05 - 1.1, determine the dosage of phosphoric acid reagent and magnesium chloride added to the biogas slurry wastewater in the pig farm, and the stirring speed is 300 - 400 rpm. Magnesium hydrogen phosphate solid, when recycled, is dissolved by adding phosphoric acid. The presence of phosphoric acid not only dissolves magnesium hydrogen phosphate but also replenishes phosphate ions, which is beneficial for removing ammonium ions. In addition, solid magnesium hydrogen phosphate is insoluble in water, but the solubility of magnesium ammonium phosphate is lower than that of magnesium hydrogen phosphate. Therefore, there is a reaction driving force to form a precipitate with lower solubility. To accelerate the occurrence of this reaction, the stirring speed is increased to a certain extent to increase the reaction area between magnesium hydrogen phosphate and NH4 + to better realize the recycling of magnesium hydrogen phosphate.

[0019] Further, in step S3, the temperature of the absorption reaction is room temperature, and the mass concentration of the prepared ammonia water is 3% - 5%.

[0020] Further, in step S4, the volume concentration of CO2 in the biogas is 20 - 40%, the residence time of ammonia water absorbed in the packing of the spray packing tower is controlled to be ≥ 60 s, the absorption temperature is normal temperature, the CO2 concentration of the exhaust gas at the gas outlet of the spray packing tower is ≤ 1.5%, and the mass concentration of the (NH4)2CO3 aqueous solution at the bottom of the spray packing tower is 8 - 12%.

[0021] Further, in step S5, the molar ratio of the CaSO4 component of phosphogypsum to (NH4)2CO3 in the mixed liquid is 1.3 - 1.8:1, the reaction temperature is normal temperature, the reaction time is 1 - 1.5 h, and the stirring speed of the reaction is 100 - 200 rpm. Since phosphogypsum is a by - product of the phosphoric acid preparation process, its main impurities are phosphates (such as Ca3(PO4)2, FePO4, AlPO4, etc.), but the solubility of these phosphates is relatively low. For example, calcium phosphate (Ca3(PO4)2) is insoluble in water, and iron phosphate (FePO4) and aluminum phosphate (AlPO4) are also not easily soluble. Therefore, it has no impact on the produced liquid nitrogen fertilizer, and there is no need to specially treat the impurity situation.

[0022] Further, the static crystallization time in step S5 is 1 - 3 h.

[0023] Compared with the prior art, the beneficial effects achieved by the present invention are:

[0024] 1) The present invention adopts the comprehensive utilization of pig farm wastewater and phosphogypsum to realize the carbon fixation method, which is a wastewater resource utilization technology aiming at energy conservation and emission reduction, and is a process for preparing ammonium products by struvite crystallization for pig farm wastewater quality. The present invention realizes the efficient precipitation of NH4 + in pig farm wastewater based on optimized process parameters, can achieve the recovery of struvite, and can convert its NH4 + into ammonium salt products, reducing greenhouse gas emissions caused by the preparation of NH3.

[0025] The specific mechanism is as follows: Mg 2+ +PO4 3- +NH4 + →MgNH4PO4;

[0026] MgNH4PO4→MgHPO4 + 3NH3↑;

[0027] NH3 + H2O→NH3·H2O;

[0028] 2NH3·H2O + CO2 + H2O→(NH4)2CO3 + 2H2O;

[0029] CaSO4 + (NH4)2CO3→(NH4)2SO4 + CaCO3↓.

[0030] 2) The NH3·H2O is prepared by drying the struvite precipitate (MgNH4PO4) at low temperature to generate NH3, which is fully absorbed in the form of aeration, realizing the recycling of waste ammonium resources; the CO2 comes from biogas, which is brought into full contact with ammonia water to form (NH4)2CO3 solution and enters the crystallization tank, realizing the carbon fixation of biogas; the CaSO4 is provided by phosphogypsum, realizing the reuse of phosphogypsum resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a process flow diagram of a method for biogas carbon fixation based on the coordinated treatment of pig farm wastewater and phosphogypsum according to the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0032] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be described in detail below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without any creative work shall fall within the scope protected by the present invention.

[0033] As Figure 1 shown, it is a process flow chart of the present invention, and the process flow of the present invention will be further described below.

[0034] The pig farm biogas slurry wastewater is taken from an ecological breeding company, and the biogas is taken from a company for the recycling of kitchen waste. The ammonia nitrogen content of the pig farm biogas slurry wastewater is 1200 mg / L; in the CO2-containing biogas, the CO2 volume concentration is about 30 ± 2%, and the rest is mainly methane.

[0035] Example 1 A method for biogas carbon fixation based on the coordinated treatment of pig farm wastewater and phosphogypsum, comprising the following steps:

[0036] S1 Struvite crystallization and separation:

[0037] Add pig farm biogas slurry wastewater into the pig farm biogas slurry wastewater treatment tank, and add sodium phosphate and magnesium chloride in sequence to make the molar ratio of each substance in the reaction system NH4 + :PO4 3- :Mg 2+ =1:1.05:1.05. Start stirring, and the stirring speed is 200 rpm. Dropwise add sodium hydroxide solution to adjust the pH range of the reaction system to 8 - 9. The reaction temperature is normal temperature, and the reaction time is 3 - 4 h. After stirring to form struvite (MgNH4PO4) precipitate, filter to obtain low-concentration ammonia nitrogen wastewater with NH4 + concentration < 80 mg / L and struvite precipitate. The low-concentration ammonia nitrogen wastewater is removed through conventional treatment processes.

[0038] S2 Struvite heating and NH3 volatilization:

[0039] Add the struvite precipitate into the heating tank. After sealing the heating tank, perform heating and drying treatment. First, displace the air in the heating tank with nitrogen to make the drying atmosphere environment nitrogen. Use the waste heat steam of the factory to heat the struvite in the heating tank to make it decompose and release NH3. The temperature of the heating and drying is 100 - 105 °C; an NH3 on-line detector is also set at the air outlet of the heating tank. When the monitored NH3 concentration < 30 ppm, it is considered that the struvite has been completely decomposed, and heating is stopped. Struvite decomposes to form magnesium hydrogen phosphate and phosphoric acid, and the NH3 volatilized during this process is input to the next process.

[0040] The residual solid magnesium hydrogen phosphate is recycled to the reaction in step S1 for recycling and recovering waste ammonium.

[0041] S3, NH3 absorption to form ammonia water:

[0042] The NH3 released from the struvite is transported to the ammonia water preparation tank through a pipeline and is in full contact with the water in the tank in the form of aeration. The temperature of the absorption reaction is room temperature, and finally, ammonia water solution with a concentration of 4% is prepared after the absorption reaction.

[0043] S4 Spray absorption reaction:

[0044] The 4% ammonia water obtained in step S3 is transported to the upper part of the spray packing tower for spraying. At the same time, the biogas containing CO2 is transported to the lower air inlet of the spray packing tower. The flow rate of the biogas containing CO2 into the spray packing tower is 10000 m 3 / d, and the flow rate of ammonia water into the spray packing tower is 100 - 110 m 3 / d, ammonia and biogas containing CO2 are in countercurrent contact reaction in the packing of a spray packing tower. The absorption temperature is normal temperature. The residence time of ammonia in the packing of the spray packing tower for absorption is controlled to be ≥60 s. Ammonia absorbs CO2 in the biogas to form an aqueous solution of (NH4)2CO3 and flows into the bottom of the spray packing tower. The mass concentration of (NH4)2CO3 in the aqueous solution at the bottom of the spray packing tower is 10-12%. After the biogas is treated with ammonia, the CO2 concentration in the tail gas is <1.5%.

[0045] Step S4: The aqueous solution of (NH4)2CO3 at the bottom of the spray packing tower is transported to a crystallization tank, and then 87% phosphogypsum (the mass fraction of the effective component CaSO4·2H2O in the phosphogypsum is 87%) is added to the crystallization tank. The molar ratio of the CaSO4 component of the phosphogypsum to (NH4)2CO3 in the mixed liquid is controlled to be 1.5:1. Stirring is started at a stirring speed of 100 rpm, the reaction temperature is normal temperature, and the reaction time is 1.5 h. (NH4)2CO3 reacts with the CaSO4 component of the phosphogypsum to form CaCO3 precipitate. Then, stirring is stopped and left standing for more than 1 h to complete the crystallization and sedimentation process. The liquid nitrogen fertilizer containing the aqueous solution of (NH4)2SO4 is filtered out, and then the remaining CaCO3 precipitate is taken out. The concentration of (NH4)2SO4 in the liquid nitrogen fertilizer is 13-15%. The purity of the remaining CaCO3 precipitate can reach 82% after drying. The liquid nitrogen fertilizer rich in (NH4)2SO4 can be used as nitrogen fertilizer.

[0046] The present invention creatively adopts the concept of "treating waste with waste", and jointly applies two solid wastes, waste ammonium salts and phosphogypsum, to the resource treatment of CO2 in biogas. Through this innovative method, the present invention successfully generates (NH4)2SO4 products, which can be used as high-quality nitrogen fertilizers for flower cultivation; at the same time, the generated calcium carbonate can be used as building materials, and the purified biogas can be used for power generation.

[0047] Example 2:

[0048] The residual magnesium hydrogen phosphate obtained in step S2 of Example 1 is reused in the recovery of waste ammonium reaction in step S1 of Example 1. The specific implementation steps are as follows: The residual magnesium hydrogen phosphate solid also contains phosphoric acid. The residual magnesium hydrogen phosphate solid is dissolved by adding phosphoric acid, and the mixed solution is re-added to the biogas slurry wastewater for reuse, and a certain amount of magnesium chloride is supplemented to the NH4 in the pig farm biogas slurry wastewater + concentration meter to make the molar ratio of each substance in the reaction system NH4 + :PO4 3- :Mg 2+= 1:1.05:1.05, start stirring at a stirring speed of 400 rpm. Dropwise add sodium hydroxide solution to adjust the pH range of the reaction system to 8 - 9. The reaction temperature is at room temperature. After fully stirring the reaction to form struvite (MgNH4PO4) precipitate, filter to obtain low-concentration ammonia nitrogen wastewater with an NH4 + concentration < 80 mg / L and struvite precipitate.

[0049] Example 2 continues to heat and decompose the obtained struvite precipitate in the manner of step S2 of Example 1, and the residual solid magnesium hydrogen phosphate after decomposition is repeatedly reused in the recovery of ammonium from pig farm biogas slurry wastewater. According to the above experimental process, after the residual solid magnesium hydrogen phosphate is repeatedly applied in the reaction 5 times, the recovery rate of ammonia nitrogen in the pig farm biogas slurry wastewater is still > 92%.

[0050] As described above, the above are only specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention can easily think of changes or substitutions, which should all be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

1. A biogas carbon fixation method based on the coordinated treatment of pig farm wastewater and phosphogypsum, characterized in that: The following steps are involved: S1: In the presence of NH4 + Phosphate and magnesium salt are added to the biogas wastewater from the pig farm, and alkali is added to adjust the pH to alkaline, and the reaction is stirred to form struvite MgNH4PO4 precipitation, that is, the ammonia nitrogen in the biogas wastewater is recovered by the struvite crystallization method, and then filtered to obtain low-concentration ammonia nitrogen wastewater and struvite precipitation; S2: drying the struvite precipitate obtained in step S1, whereby the struvite is thermally decomposed to generate residual solid magnesium hydrogen phosphate, and NH3 is volatilized in the process; wherein the residual solid magnesium hydrogen phosphate is reused in the reaction of step S1; S3: introducing the NH3 described in step S2 into water in the form of aeration, so that the NH3 and water are fully in contact with each other for absorption reaction, thereby preparing an ammonia solution; S4: the ammonia solution obtained in step S3 is sprayed downward from the upper part of the spray packed tower, and the CO2-containing biogas is introduced from the lower part of the spray packed tower. The ammonia solution and the CO2-containing biogas are fully contacted in countercurrent, and the ammonia solution absorbs CO2 to form a (NH4)2CO3 aqueous solution, which is discharged from the bottom of the spray packed tower; S5: The (NH4)2CO3 aqueous solution described in step S4 is mixed and stirred with phosphogypsum (CaSO4·2H2O) to react and convert into an aqueous solution containing CaCO3 and (NH4)2SO4, and then allowed to stand and crystallize to allow the CaCO3 to fully settle. The CaCO3 precipitate is obtained by filtration and recovered. The residual liquid is rich in (NH4)2SO4 and can be used as a nitrogen fertilizer.

2. A biogas carbon fixation method based on the coordinated treatment of pig farm wastewater and phosphogypsum as claimed in claim 1, characterized in that: In step S1, the NH4 + The concentration is >1000mg / L; the phosphate is sodium phosphate, the magnesium salt is magnesium chloride, and the NH4 + Concentration meter, according to the molar ratio of NH4 + :PO4 3- :Mg 2+ =1: 1.05~1.1:1.05~1.1, determine the dosage of phosphate and magnesium salt in pig farm biogas wastewater.

3. The biogas carbon fixation method based on the coordinated treatment of pig farm wastewater and phosphogypsum as claimed in claim 1, characterized in that: In step S1, the stirring speed is 200-300 rpm, the base for adjusting the pH is sodium hydroxide solution, the pH range of the reaction system is adjusted to 8-9.5, the reaction temperature is room temperature, and the reaction time is 3-4 hours.

4. The method for biogas carbon fixation based on the coordinated treatment of pig farm wastewater and phosphogypsum as claimed in claim 1, characterized in that: In step S1, ammonia nitrogen in biogas slurry wastewater is recovered by struvite crystallization method, so that NH4 + Concentration <80mg / L.

5. The method for biogas carbon fixation based on the coordinated treatment of pig farm wastewater and phosphogypsum as claimed in claim 1, characterized in that: In step S2, the struvite precipitate is added to a heating tank, which is sealed and then heated and dried. First, the air in the heating tank is replaced with nitrogen to make the drying atmosphere nitrogen. The struvite in the heating tank is heated to decompose and release NH3. The heating and drying temperature is 80-110°C. An NH3 online detector is also provided on the air outlet of the heating tank. When the concentration of released NH3 is monitored to be less than 30ppm, it is considered that the struvite has been completely decomposed and the heating is stopped.

6. The method for biogas carbon fixation based on the coordinated treatment of piggery wastewater and phosphogypsum as claimed in claim 1, characterized in that: The residual solid of magnesium hydrogen phosphate obtained in step S2 is reused in the waste ammonium recovery reaction of step S1. The specific implementation steps are: the magnesium hydrogen phosphate solid is dissolved by adding phosphoric acid, the mixed solution is re-added to the biogas wastewater for reuse, and magnesium chloride is added to the NH4 + Concentration meter, according to the molar ratio of NH4 + :PO4 3- :Mg 2+ =1: 1.05~1.1: 1.05~1.1, determine the dosage of phosphoric acid agent and magnesium chloride to be added to the pig farm sludge wastewater, and the stirring speed is 300-400rpm.

7. The method for biogas carbon fixation based on the coordinated treatment of piggery wastewater and phosphogypsum as claimed in claim 1, characterized in that: In step S3, the temperature of the absorption reaction is room temperature, and the mass concentration of the prepared ammonia water is 3% to 5%.

8. The biogas carbon fixation method based on the coordinated treatment of piggery wastewater and phosphogypsum as claimed in claim 1, characterized in that: In step S4, the volume concentration of CO2 in the biogas is 20-40%, the residence time of ammonia water absorbed in the packing of the spray packing tower is controlled to be ≥60s, the absorption temperature is room temperature, the CO2 concentration of the exhaust gas at the gas outlet of the spray packing tower is ≤1.5%, and the mass concentration of the (NH4)2CO3 aqueous solution at the bottom of the spray packing tower is 8-12%.

9. The biogas carbon fixation method based on the coordinated treatment of piggery wastewater and phosphogypsum as claimed in claim 1, characterized in that: In step S5, the molar ratio of the CaSO4 component of phosphogypsum to the (NH4)2CO3 in the mixed liquid is 1.3-1.8:1, the reaction temperature is room temperature, the reaction time is 1-1.5h, and the stirring speed of the reaction is 100-200rpm; the standing crystallization time in step S5 is 1-3h.

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