Methods for producing biomass energy from microalgae treated in livestock and poultry farming wastewater

Abstract

This invention belongs to the field of bioenergy technology and discloses a method for producing biomass energy from microalgae using livestock and poultry breeding wastewater. The specific steps include: 1) filtration to remove coarse suspended solids and impurities; 2) adding modified biochar to the filtered wastewater, adjusting the pH to 6-8, stirring for adsorption, retaining the suspended biochar, and obtaining a mixed solution of livestock and poultry breeding wastewater; 3) sterilizing the mixture from step 2) and cooling it to room temperature; 4) inoculating the sterilized wastewater with microalgae, nitrogen-fixing bacteria, and phosphorus-solubilizing bacteria, while simultaneously adding Fe3O4 nanoparticles; 5) continuing cultivation and harvesting microalgal biomass. This invention significantly improves microalgal biomass and lipid content through multi-component synergistic effects, achieving high removal rates of total nitrogen and total phosphorus, and realizing the integration of wastewater purification, resource recovery, and energy production, resulting in significant environmental and economic benefits.

Description

Technical Field

[0001] This invention belongs to the field of bioenergy technology, and in particular relates to a method for producing biomass energy by treating microalgae from livestock and poultry breeding wastewater. Background Technology

[0002] Livestock and poultry farming wastewater mainly includes wastewater from livestock and poultry farming as well as domestic sewage, with the livestock and poultry farming wastewater being the most critical component requiring treatment. This portion primarily consists of animal excrement, urine, feed residue, and pens flushing water, with pens flushing water and urine constituting the majority. The composition of livestock and poultry farming wastewater is extremely complex, exhibiting high COD and BOD levels, and containing a large amount of suspended solids. It is not only colored but also enriched with nitrogen, phosphorus, and various pathogens; some wastewater also contains heavy metal pollution. These characteristics can lead to a series of environmental hazards such as eutrophication and ammonia toxicity. Furthermore, animal excrement itself contains a certain amount of antibiotics, as well as nutrients such as phosphorus, copper, and zinc, while the concentration of nitrogen (especially ammonia nitrogen) is relatively high.

[0003] Compared to ordinary domestic sewage, livestock and poultry farming wastewater has three main characteristics: first, large-scale farming leads to concentrated pollutant discharge; second, pollutant concentration is high, solid-liquid mixtures are present, and treatment is difficult; and third, the investment and operating costs for treatment are high. These characteristics make livestock and poultry farming wastewater a significant threat to my country's ecological environment, especially its aquatic ecosystem. Researchers are gradually shifting their research on livestock and poultry farming wastewater treatment technologies from traditional methods to resource recovery technologies. Compared with traditional treatment technologies, resource recovery from livestock and poultry farming wastewater rich in nutrients has the following advantages: recovering raw materials such as organic matter, nitrogen, and phosphorus, reducing production costs; and producing new products such as biogas, fertilizer, and protein.

[0004] Microalgae refer to the general term for microorganisms containing chlorophyll a and capable of photosynthesis; they are a type of protist organism. As a type of photosynthetic autotrophic organism, microalgae can absorb nitrogen (to synthesize proteins and nucleic acids), phosphorus (to synthesize ATP and phospholipids), and CO2 (to synthesize carbohydrates and oils) from wastewater through photosynthesis, while simultaneously accumulating biomass (containing oils and proteins). They possess the dual functions of wastewater purification and energy production, and are considered an ideal technology to replace traditional processes.

[0005] However, existing microalgae treatment technologies face three major bottlenecks: low nutrient utilization efficiency: nitrogen in livestock and poultry wastewater is mainly organic nitrogen (such as proteins and polypeptides), and microalgae lack extracellular enzymes to degrade organic nitrogen, resulting in low direct utilization; phosphorus is mainly insoluble inorganic phosphorus (such as Ca3(PO4)2, FePO4) and organic phosphorus (such as phytic acid), and microalgae can only absorb soluble phosphates (PO4). 3-This often leads to growth stagnation in the later stages of cultivation due to "nitrogen and phosphorus starvation," resulting in low biomass dry weight. Simultaneously, there is also the problem of poor stress resistance, and heavy metals (such as Cu) in the wastewater... 2+ Antibiotics (such as tetracycline, which inhibit electron transport in the microalgal photosynthetic system PSII) and antibiotics (such as tetracycline, which damage microalgal cell membrane permeability) can lead to decreased microalgal survival rates and hindered lipid synthesis (reduced lipid content). Furthermore, single-culture methods are costly; to improve microalgal growth rates, existing technologies require the addition of artificial nutrients such as sodium nitrate (nitrogen source) and potassium dihydrogen phosphate (phosphorus source), which is expensive and limits large-scale application. In summary, the severe pollution of livestock and poultry farm wastewater, the limitations of traditional treatment technologies, and the contradiction between the potential of microalgae technology and influent water requirements necessitate a low-cost production method that can efficiently purify livestock and poultry farm wastewater and generate microalgal biomass energy. Summary of the Invention

[0006] To address the aforementioned technical challenges, this invention proposes a method for treating microalgae in livestock and poultry farming wastewater to produce biomass energy. This method constructs a stable and efficient "microalgae-functional microorganism" symbiotic system, which overcomes bottlenecks in nutrient utilization and stress resistance by directionally regulating metabolic synergy, thus providing an integrated solution for environmental governance and renewable energy production.

[0007] To achieve the above objectives, the present invention provides a method for producing biomass energy by cultivating microalgae using livestock and poultry breeding wastewater, comprising the following steps:

[0008] 1) Pretreatment of original livestock and poultry breeding wastewater: filtration to remove coarse suspended solids and impurities;

[0009] 2) Modified biochar treatment: Add modified biochar to the filtered wastewater, adjust the pH to 6-8, stir to adsorb, retain the suspended biochar, and obtain a mixed liquid of livestock and poultry breeding wastewater;

[0010] 3) Sterilization treatment: Sterilize the mixture from step 2) and cool it to room temperature to obtain livestock and poultry wastewater;

[0011] 4) Inoculation and co-cultivation: Microalgae, nitrogen-fixing bacteria and phosphate-solubilizing bacteria were inoculated into the sterilized livestock and poultry wastewater, and Fe3O4 nanoparticles were added at the same time.

[0012] 5) Continue cultivation and harvest microalgae biomass.

[0013] Furthermore, the livestock and poultry breeding wastewater described in step 1) is filtered sequentially through a 1-5 mm pore size grid and a multi-layer quartz sand filtration device.

[0014] Furthermore, the filling sequence of the multi-layer quartz sand filter is coarse quartz sand, fine river sand, fine quartz sand, and coarse quartz sand.

[0015] Furthermore, the modified biochar described in step 2) is obtained by activation with 1 mol / L HCl for 2 hours, and the amount added is 5~10 g / L.

[0016] Further, in step 3), the mixture is sterilized at 120~140℃ for 25~35 minutes.

[0017] Furthermore, in step 4), the biomass ratio of the added microalgae, nitrogen-fixing bacteria and phosphate-solubilizing bacteria is 10~15:0.5~2:1~2.

[0018] Furthermore, the biomass ratio of the added microalgae, nitrogen-fixing bacteria, and phosphate-solubilizing bacteria is 10:1:1.

[0019] Furthermore, the Fe3O4 nanoparticles mentioned in step 4) have a particle size of 20~50nm, are added in an amount of 10~20mg / L, and are ultrasonically dispersed for 25~35min.

[0020] Furthermore, the nitrogen-fixing bacteria mentioned in step (4) are *Azotobacter chrysogenum*, with a bacterial concentration of 1 × 10⁻⁶. 8 CFU / mL; the phosphate-solubilizing bacteria is *Pseudomonas fluorescens*, with a phosphatase activity of 18 U / mL.

[0021] Furthermore, in step 5), the light intensity is 5000 lx, the temperature is 20~30℃, and the light-dark ratio is 12h:12h.

[0022] Compared with the prior art, the present invention has the following advantages and technical effects:

[0023] (1) This invention utilizes the metabolic complementarity of microorganisms and microalgae to construct a “nitrogen-phosphorus-carbon” synergistic cycle network, thereby improving resource utilization;

[0024] (2) The modified biochar of this invention has reduced the inhibition of bacteria and algae by the adsorption of heavy metals and antibiotics. Combined with the conversion of insoluble phosphorus by phosphate-solubilizing bacteria and the supplementation and conversion of nitrogen by nitrogen-fixing bacteria, the effluent indicators meet the discharge standards, thus solving the problem of incomplete nitrogen and phosphorus removal in traditional processes.

[0025] (3) The pore structure of the modified biochar of the present invention provides a colonization carrier for bacteria and algae, forming a "biochar-bacteria-algae" composite aggregate, which increases the efficiency of metabolite transfer; Fe3O4 nanoparticles serve as electron carriers, enhancing the activity of nitrogenase and phosphatase and shortening the culture cycle.

[0026] (4) This invention utilizes livestock and poultry breeding wastewater to replace artificial nutrients, and combines the low-cost characteristics of modified biochar (prepared from agricultural straw) and Fe3O4 nanoparticles (recyclable) to reduce the unit treatment cost, while realizing a closed-loop cycle of "wastewater-biomass-energy", which is in line with the concept of circular economy. Attached Figure Description

[0027] Figure 1 This is a flowchart illustrating the process of using livestock and poultry breeding wastewater to treat microalgae and produce biomass energy in this invention. Detailed Implementation

[0028] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0029] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0030] Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this invention. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards. Experimental instruments, equipment, and reagents in the following embodiments that do not specify their sources are all commercially available materials.

[0031] Unless otherwise defined or stated, all technical and scientific terms used in this invention have the same meaning as those skilled in the art. Furthermore, any methods and materials similar to or equivalent to those described herein can be applied to the methods of this invention. It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined with each other. All instruments used in this application are commercially available.

[0032] Experimental materials:

[0033] In this embodiment of the invention, the nitrogen-fixing bacteria is *Azotobacter brownii* CICC 21685, with a bacterial concentration of 1×10⁻⁶. 8 CFU / mL; the phosphate-solubilizing bacteria were *Pseudomonas fluorescens* CICC 23252, and the microalgae were *Spirulina*.

[0034] Before inoculation, the microalgae were expanded and cultured at 25℃ and 3000 lux for 7 days until the concentration reached 1.0 g / L (dry weight). The culture medium used for expansion was BG-11 medium, with the following formula: NaNO3 added at 10 ml / L, stock solution concentration 15 g / L; K2HPO4 added at 10 ml / L, stock solution concentration 4 g / L; MgSO4·7H2O added at 10 ml / L, stock solution concentration 7.5 g / L; CaCl2·2H2O added at 10 ml / L, stock solution concentration 3.6 g / L; CitricAcid·H2O added at 10 ml / L, stock solution concentration 0.6 g / L; Ferric Ammonium The following solutions were added: Citrate at 10 ml / L, with a stock solution concentration of 0.6 g / L; Na2EDTA·2H2O at 10 ml / L, with a stock solution concentration of 0.1 g / L; Na2CO3 at 10 ml / L, with a stock solution concentration of 2 g / L; and A5 at 1 ml / L. A certain amount of water was added, and the solution was sterilized.

[0035] The preparation of A5 involves adding 2.86 g / L of H3BO3, 1.81 g / L of MnCl2·4H2O, 0.22 g / L of ZnSO4·7H2O, 0.39 g / L of Na2MoO4·2H2O, 0.079 g / L of CuSO4·5H2O, and 49.4 mg / L of Co(NO3)2·6H2O, followed by adding a certain amount of water and sterilization.

[0036] Azotobacter chroococcum was cultured in LB medium (containing 0.1% mannitol) at 30°C with shaking at 180 rpm for 24 h, with a bacterial concentration of 1×10⁻⁶. 8 CFU / mL (plate count method);

[0037] Phosphate-solubilizing bacteria: Pseudomonas fluorescens, cultured in LB medium at 30°C and 180 rpm for 20 h with shaking. Phosphatase activity was determined by the p-nitrophenyl phosphate method to ensure it reached 18 U / mL.

[0038] Preparation method of modified biochar:

[0039] Corn stalks were crushed to a particle size of 1-2 cm and pyrolyzed at 600℃ for 2 hours under nitrogen protection. After cooling, the stalks were passed through a 100-mesh sieve to obtain biochar raw powder. The raw powder was added to a 1 mol / L HCl solution (solid-liquid ratio 1:10), stirred and activated at 60℃ for 2 hours, washed with distilled water until neutral, and dried at 80℃ to obtain modified biochar (specific surface area 180 m²). 2 / g, pore size 10~30nm).

[0040] Preparation and dispersion of Fe3O4 nanoparticles:

[0041] Fe3O4 nanoparticles were prepared by coprecipitation: FeCl3·6H2O and FeCl2·4H2O were dissolved in deionized water at a molar ratio of 2:1, nitrogen gas was introduced to remove oxygen, 25% ammonia water was added dropwise at 30℃ until pH=10, the reaction was stirred for 1 h, the product was collected by magnetic separation, washed with deionized water until neutral, and vacuum dried to obtain Fe3O4 nanoparticles with a particle size of 30 nm; before use, they were ultrasonically dispersed at 300 W for 30 min to avoid agglomeration.

[0042] Figure 1 This is a flowchart illustrating the process of using livestock and poultry breeding wastewater to treat microalgae and produce biomass energy in this invention.

[0043] Example 1

[0044] The original livestock and poultry wastewater is passed through a 3mm pore size screen to remove coarse suspended solids such as straw, feathers, and large pieces of manure, resulting in coarsely filtered livestock and poultry wastewater.

[0045] The coarsely filtered livestock and poultry wastewater is then filtered through a filter device filled with coarse quartz sand, fine river sand, fine quartz sand, and coarse quartz sand. After filtration, the livestock and poultry wastewater is obtained.

[0046] Add 10 g / L of modified biochar (activated with 1 mol / L HCl for 2 h) to the filtered livestock and poultry wastewater, adjust the pH to about 7, stir and adsorb for 2 h, and then retain the suspended biochar (without filtration).

[0047] Sterilize at 130℃ for 30 minutes, then allow to return to room temperature.

[0048] Microalgae (initial concentration 1.0 g / L), nitrogen-fixing bacteria, and phosphate-solubilizing bacteria were inoculated at a biomass ratio of microalgae: nitrogen-fixing bacteria: phosphate-solubilizing bacteria = 10:1:1.

[0049] During inoculation, 15 mg / L Fe3O4 nanoparticles (dispersed by ultrasound for 30 min to avoid aggregation) were added to the livestock and poultry wastewater and cultured simultaneously with bacteria and algae.

[0050] Example 2

[0051] The original livestock and poultry wastewater is passed through a 5mm pore size screen to remove coarse suspended solids such as straw, feathers, and large pieces of manure, resulting in coarsely filtered livestock and poultry wastewater.

[0052] The coarsely filtered livestock and poultry wastewater is then filtered through a filter device filled with coarse quartz sand, fine river sand, fine quartz sand, and coarse quartz sand. After filtration, the livestock and poultry wastewater is obtained.

[0053] Add 5 g / L of modified biochar (activated by 1 mol / L HCl for 2 h) to the filtered livestock and poultry wastewater, adjust the pH to about 7, stir and adsorb for 2 h, and then retain the suspended biochar (without filtration).

[0054] Sterilize at 135℃ for 30 minutes, then allow to return to room temperature.

[0055] Microalgae (initial concentration 1.0 g / L), nitrogen-fixing bacteria, and phosphate-solubilizing bacteria were inoculated at a biomass ratio of microalgae: nitrogen-fixing bacteria: phosphate-solubilizing bacteria = 5:1:1.

[0056] During inoculation, 10 mg / L Fe3O4 nanoparticles (dispersed by ultrasound for 30 min to avoid aggregation) were added to the livestock and poultry wastewater and cultured simultaneously with bacteria and algae.

[0057] Example 3

[0058] The original livestock and poultry wastewater is passed through a screen with a pore size of 1mm to remove coarse suspended solids such as straw, feathers, and large pieces of manure, resulting in coarsely filtered livestock and poultry wastewater.

[0059] The coarsely filtered livestock and poultry wastewater is then filtered through a filter device filled with coarse quartz sand, fine river sand, fine quartz sand, and coarse quartz sand. After filtration, the livestock and poultry wastewater is obtained.

[0060] Add 8 g / L of modified biochar (activated with 1 mol / L HCl for 2 h) to the filtered livestock and poultry wastewater, adjust the pH to about 7, stir and adsorb for 2 h, and then retain the suspended biochar (without filtering).

[0061] Sterilize at 130℃ for 30 minutes, then allow to return to room temperature.

[0062] Microalgae (initial concentration 1.0 g / L), nitrogen-fixing bacteria, and phosphate-solubilizing bacteria were inoculated at a biomass ratio of microalgae: nitrogen-fixing bacteria: phosphate-solubilizing bacteria = 15:1:1.

[0063] During inoculation, 15 mg / L Fe3O4 nanoparticles (dispersed by ultrasound for 30 min to avoid aggregation) were added to the livestock and poultry wastewater and cultured simultaneously with bacteria and algae.

[0064] Example 4

[0065] The original livestock and poultry wastewater is passed through a 2mm pore size screen to remove coarse suspended solids such as straw, feathers, and large pieces of manure, resulting in coarsely filtered livestock and poultry wastewater.

[0066] The coarsely filtered livestock and poultry wastewater is then filtered through a filter device filled with coarse quartz sand, fine river sand, fine quartz sand, and coarse quartz sand. After filtration, the livestock and poultry wastewater is obtained.

[0067] Add 10 g / L of modified biochar (activated with 1 mol / L HCl for 2 h) to the filtered livestock and poultry wastewater, adjust the pH to about 7, stir and adsorb for 2 h, and then retain the suspended biochar (without filtration).

[0068] Sterilize at 130℃ for 30 minutes, then allow to return to room temperature.

[0069] Microalgae (initial concentration 1.0 g / L), nitrogen-fixing bacteria, and phosphate-solubilizing bacteria were inoculated at a biomass ratio of microalgae: nitrogen-fixing bacteria: phosphate-solubilizing bacteria = 10:0.5:1.

[0070] During inoculation, 20 mg / L Fe3O4 nanoparticles (dispersed by ultrasound for 30 min to avoid aggregation) were added to the livestock and poultry wastewater and cultured simultaneously with bacteria and algae.

[0071] Example 5

[0072] The original livestock and poultry wastewater is passed through a screen with a pore size of 4mm to remove coarse suspended solids such as straw, feathers, and large pieces of manure, resulting in coarsely filtered livestock and poultry wastewater.

[0073] The coarsely filtered livestock and poultry wastewater is then filtered through a filter device filled with coarse quartz sand, fine river sand, fine quartz sand, and coarse quartz sand. After filtration, the livestock and poultry wastewater is obtained.

[0074] Add 5 g / L of modified biochar (activated by 1 mol / L HCl for 2 h) to the filtered livestock and poultry wastewater, adjust the pH to about 7, stir and adsorb for 2 h, and then retain the suspended biochar (without filtration).

[0075] Sterilize at 125°C for 30 minutes, then allow to return to room temperature.

[0076] Microalgae (initial concentration 1.0 g / L), nitrogen-fixing bacteria, and phosphate-solubilizing bacteria were inoculated at a biomass ratio of microalgae: nitrogen-fixing bacteria: phosphate-solubilizing bacteria = 10:2:1.

[0077] During inoculation, 15 mg / L Fe3O4 nanoparticles (dispersed by ultrasound for 30 min to avoid aggregation) were added to the livestock and poultry wastewater and cultured simultaneously with bacteria and algae.

[0078] Comparative Example 1

[0079] The original livestock and poultry wastewater is passed through a 3mm pore size screen to remove coarse suspended solids such as straw, feathers, and large pieces of manure, resulting in coarsely filtered livestock and poultry wastewater.

[0080] The coarsely filtered livestock and poultry wastewater is then filtered through a filter device filled with coarse quartz sand, fine river sand, fine quartz sand, and coarse quartz sand. After filtration, the livestock and poultry wastewater is obtained.

[0081] Add 10 g / L of modified biochar (activated with 1 mol / L HCl for 2 h) to the filtered livestock and poultry wastewater, adjust the pH to about 7, stir and adsorb for 2 h, and then retain the suspended biochar (without filtration).

[0082] Sterilize at 130℃ for 30 minutes, then allow to return to room temperature.

[0083] Microalgae (initial concentration 1.0 g / L), nitrogen-fixing bacteria, and phosphate-solubilizing bacteria were inoculated at a biomass ratio of microalgae: nitrogen-fixing bacteria: phosphate-solubilizing bacteria = 10:2:0.

[0084] During inoculation, 15 mg / L Fe3O4 nanoparticles (dispersed by ultrasound for 30 min to avoid aggregation) were added to the livestock and poultry wastewater and cultured simultaneously with bacteria and algae.

[0085] They were cultured for 15 days under conditions of 5000 lx light intensity, 25℃ temperature, and a light-dark ratio of 12h:12h.

[0086] Comparative Example 2

[0087] The original livestock and poultry wastewater is passed through a 3mm pore size screen to remove coarse suspended solids such as straw, feathers, and large pieces of manure, resulting in coarsely filtered livestock and poultry wastewater.

[0088] The coarsely filtered livestock and poultry wastewater is then filtered through a filter device filled with coarse quartz sand, fine river sand, fine quartz sand, and coarse quartz sand. After filtration, the livestock and poultry wastewater is obtained.

[0089] Add 10 g / L of modified biochar (activated with 1 mol / L HCl for 2 h) to the filtered livestock and poultry wastewater, adjust the pH to about 7, stir and adsorb for 2 h, and then retain the suspended biochar (without filtration).

[0090] Sterilize at 130℃ for 30 minutes, then allow to return to room temperature.

[0091] Microalgae (initial concentration 1.0 g / L), nitrogen-fixing bacteria, and phosphate-solubilizing bacteria were inoculated at a biomass ratio of microalgae: nitrogen-fixing bacteria: phosphate-solubilizing bacteria = 10:0:2.

[0092] During inoculation, 15 mg / L Fe3O4 nanoparticles (dispersed by ultrasound for 30 min to avoid aggregation) were added to the livestock and poultry wastewater and cultured simultaneously with bacteria and algae.

[0093] Practical applications:

[0094] Detection indicators and methods:

[0095] Microalgae dry weight: centrifuge at 10,000 rpm for 10 min, and dry at 80℃ to constant weight;

[0096] Total nitrogen / total phosphorus: alkaline potassium persulfate digestion-ultraviolet spectrophotometry / ammonium molybdate spectrophotometry;

[0097] Oil content: Soxhlet extraction method (n-hexane as solvent).

[0098] Blank control group:

[0099] The original livestock and poultry wastewater is passed through a 3mm pore size screen to remove coarse suspended solids such as straw, feathers, and large pieces of manure, resulting in coarsely filtered livestock and poultry wastewater.

[0100] The coarsely filtered livestock and poultry wastewater is then filtered through a filter device filled with coarse quartz sand, fine river sand, fine quartz sand, and coarse quartz sand. After filtration, the livestock and poultry wastewater is obtained.

[0101] Sterilize the filtered livestock and poultry wastewater at 130℃ for 30 minutes, and then allow it to return to room temperature.

[0102] Microalgae (initial concentration 1.0 g / L), nitrogen-fixing bacteria, and phosphate-solubilizing bacteria were inoculated and cultured at a biomass ratio of microalgae: nitrogen-fixing bacteria: phosphate-solubilizing bacteria = 10:0:0.

[0103] The inoculated livestock and poultry wastewater from the control group, Examples 1, 2, 3, 4, 5, Comparative Example 1, and Comparative Example 2 were cultured for 15 days under the conditions of 5000 lx light intensity, 25℃ temperature, and a light-dark ratio of 12h:12h. The concentrations of ammonium nitrogen, total nitrogen, and phosphorus phosphate after culture were measured to be 476 mg / L, 518 mg / L, and 31 mg / L, respectively. The removal rate of nitrogen and phosphorus in the culture medium by microalgae was greater than 90%. The control group (single microalgae) had a total nitrogen removal rate of only 75% and a total phosphorus removal rate of 70% due to the lack of synergistic effects between bacteria and algae and the assistance of functional materials.

[0104] Microalgal biomass and lipid content were measured. Specific results are shown in Table 1.

[0105] Table 1 Microalgal biomass and lipid content

[0106]

[0107] As shown in Table 1, microalgae cultured after treatment of livestock and poultry farm manure exhibit high biomass and high lipid content when mixed with nitrogen-fixing bacteria and phosphate-solubilizing bacteria in a specific ratio. Moreover, this is related to the biomass ratio among microalgae, nitrogen-fixing bacteria, and phosphate-solubilizing bacteria. The synergistic effect is optimal when the biomass ratio of microalgae: nitrogen-fixing bacteria: phosphate-solubilizing bacteria reaches 10:1:1. The higher the biomass of the cultured microalgae, the higher the proportion of liquid products, and the higher the lipid content of the cultured microalgae.

[0108] In summary, this study proposes a method for producing biomass energy from microalgae using livestock and poultry farming wastewater. The interaction between nitrogen-fixing bacteria, phosphate-solubilizing bacteria, and microalgae is a comprehensive result of "metabolic complementarity + functional synergy + environmental adaptation." Nitrogen-fixing bacteria address "nitrogen limitation," phosphate-solubilizing bacteria address "phosphorus limitation," and microalgae provide "carbon and oxygen sources." Specifically, nitrogen-fixing bacteria can convert atmospheric N2 into ammonia nitrogen, supplementing the nitrogen deficiency in the later stages of wastewater treatment (especially since nitrogen consumption is rapid in the later stages of microalgae growth). Simultaneously, their metabolic products (vitamin B...)... 12 Growth hormones can promote microalgae growth; microalgae release O2 through photosynthesis, providing conditions for aerobic respiration of nitrogen-fixing bacteria. Furthermore, phosphorus in livestock and poultry wastewater is mostly in the form of organic phosphorus (such as phytic acid) or insoluble inorganic phosphorus (such as calcium phosphate), which is difficult for microalgae to absorb directly. Phosphate-solubilizing bacteria can secrete phosphatases to convert it into soluble phosphate (PO42-). 3- The wastewater is used by microalgae, which in turn provide carbon sources such as organic acids for phosphate-solubilizing bacteria. Thus, the three form a closed-loop nutrition system through material exchange, construct a resilient microenvironment through biofilm, and ultimately achieve efficient purification of livestock and poultry farming wastewater and efficient production of microalgal biomass energy. This mechanism provides core theoretical support for the synergy between "waste treatment" and energy production.

[0109] In addition, 5-10 g / L of modified biochar with a porous structure (specific surface area 150-200 m²) is added to the wastewater during the pretreatment stage. 2 / g), can adsorb heavy metals (Cu) in wastewater. 2+ The biochar exhibits an adsorption capacity of 0.3–0.5 mmol / g, adsorbing antibiotics (with a tetracycline adsorption rate exceeding 90%) and some large organic molecules, reducing their toxicity to microalgae and functional bacteria. Simultaneously, the functional groups (carboxyl and hydroxyl groups) on the biochar surface can buffer pH fluctuations in wastewater (maintaining a stable range of 7.0–8.0), providing a suitable microenvironment for the symbiotic system. The porous structure of biochar provides attachment sites for nitrogen-fixing and phosphate-solubilizing bacteria, forming a "biochar-bacteria-algae" complex aggregate, increasing the contact area between bacteria and microalgae, promoting direct metabolite transfer, and reducing nutrient loss. Furthermore, residual potassium, calcium, and other minerals in the biochar can be slowly released, replenishing the trace elements needed for microalgae growth and alleviating growth stagnation caused by nutrient imbalances in wastewater. During inoculation, Fe3O4 nanoparticles are added to livestock and poultry wastewater. Due to their excellent conductivity, Fe3O4 acts as an "electron carrier" in bacterial and algal metabolism, accelerating the electron transfer efficiency of nitrogenase in nitrogen-fixing bacteria (which requires Fe as a cofactor), thus enhancing nitrogen-fixing activity. Simultaneously, it promotes the catalytic reaction of phosphate-solubilizing bacteria's phosphatase, accelerating the conversion of insoluble phosphorus. Furthermore, Fe3O4 nanoparticles can be absorbed by microalgal cells (through endocytosis), serving as an iron source for chlorophyll synthesis and enhancing the light absorption efficiency of the photosynthetic system, promoting carbohydrate synthesis and providing more carbon sources for the bacterial community. Moreover, nano-Fe3O4 can remove reactive oxygen species (ROS) in wastewater through "redox regulation," reducing oxidative damage to microalgal cell membranes caused by heavy metals and antibiotics.

[0110] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for producing biomass energy using microalgae cultivated with livestock and poultry breeding wastewater, characterized in that, The method comprises the following steps: 1) livestock and poultry breeding wastewater pretreatment: filtration, removal of coarse suspended solids and impurities; 2) modified biochar treatment: adding modified biochar to the filtered wastewater, adjusting the pH to 6-8, stirring and adsorbing, retaining suspended biochar, and obtaining a livestock and poultry breeding wastewater mixture; the modified biochar is prepared from agricultural straw, activated by 1 mol / L HCl for 2 h; 3) sterilization treatment: sterilizing the mixture of step 2), cooling to room temperature, and obtaining livestock and poultry wastewater; 4) inoculation and synergistic culture: inoculating microalgae, nitrogen-fixing bacteria and phosphorus-solubilizing bacteria into the sterilized livestock and poultry wastewater, and adding Fe3O4 nanoparticles; the biomass ratio of microalgae, nitrogen-fixing bacteria and phosphorus-solubilizing bacteria is 10-15:0.5-2:1-2; the nitrogen-fixing bacteria are Azotobacter chroococcum, and the phosphorus-solubilizing bacteria are Pseudomonas fluorescens; 5) continue to culture and harvest microalgae biomass.

2. The method of claim 1, wherein, The livestock and poultry breeding wastewater in step 1) is sequentially filtered by a 1-5 mm grid filter and a quartz sand multi-layer filter device.

3. The method of claim 2, wherein, The filling order of the quartz sand multi-layer filter device is coarse quartz sand, fine river sand, fine quartz sand and coarse quartz sand.

4. The method of claim 1, wherein, The amount of modified biochar added in step 2) is 5-10 g / L.

5. The method of claim 1, wherein, In step 3), the mixture is sterilized at 120-140°C for 25-35 minutes.

6. The method of claim 1, wherein, In step 4), the biomass ratio of microalgae, nitrogen-fixing bacteria and phosphorus-solubilizing bacteria is 10:1:

1.

7. The method of claim 1, wherein, In step 4), the particle size of the Fe3O4 nanoparticles is 20-50 nm, and the amount added is 10-20 mg / L, and the nanoparticles are dispersed by ultrasonic for 25-35 min.

8. The method of claim 1, wherein, The concentration of the Rhizobium rhizomosarum in Step 4) is 1 x 10 8 CFU / mL; and the phosphatase activity of the Pseudomonas fluorescens is 18 U / mL.

9. The method of claim 1, wherein, In step 5), during the continuous culture, the light intensity is 5000 lx, the temperature is 20-30°C, and the light-dark ratio is 12h:12h.

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