Method for preparing microalgae protein bait by efficiently treating livestock and poultry biogas slurry by coupling aquatic vegetables with microalgae

Abstract

The invention belongs to the technical field of bioengineering and the field of sewage treatment, and particularly relates to a method for efficiently treating livestock and poultry biogas slurry to prepare microalgae protein bait by coupling aquatic vegetables with microalgae, which comprises the following steps: carrying out primary treatment on livestock and poultry wastewater by utilizing the aquatic vegetables, and carrying out secondary treatment on the microalgae with high robustness by coupling; the pollutant content of the biogas slurry is reduced, microalgae protein bait is harvested, and pollutants such as COD content, ammonia nitrogen content and phosphorus content in the livestock and poultry biogas slurry are efficiently reduced. According to the method, the pollutant content of the livestock and poultry biogas slurry is efficiently reduced, the COD content of the treated biogas slurry is reduced by 75%-90%, the ammonia nitrogen content is reduced by 80%-92%, the phosphorus removal rate reaches 90% or above, the biomass of microalgae harvested at low cost through self-flocculation reaches 2 g / L or above, the protein content in the microalgae reaches 50%-70%, the content of essential amino acids in the microalgae is close to that of fish meal, and the content of organic matters in the microalgae is reduced by 50%-70%. The feed can be used as high-quality protein bait.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of bioengineering and wastewater treatment, and particularly relates to a method for efficiently treating livestock manure slurry by using aquatic vegetables coupled with microalgae to prepare microalgae protein bait. BACKGROUND

[0002] Livestock breeding wastewater is the most important source of water pollution. After biogas fermentation, a large amount of degradable biomass, ammonia nitrogen and phosphorus elements are still left in the slurry. If the wastewater is directly discharged into natural water bodies without deep treatment, it will cause great damage to the ecological environment and lakes and rivers. Taking pig farm wastewater as an example, after slurry fermentation, the residual COD of pig farm wastewater is as high as 700-2000 mg / L, the ammonia nitrogen content is 300-600 mg / L, and the phosphorus content is 40-60 mg / L. Direct discharge will cause water eutrophication. According to statistics of the Ministry of Agriculture, the annual production of livestock slurry in China is as high as 200 million tons, but the comprehensive utilization rate is less than 60%. Therefore, it is an inevitable trend to develop efficient methods for treating livestock slurry and comprehensively utilizing it in the future.

[0003] The treatment methods for slurry mainly include physical, chemical and biological methods. Physical or chemical methods for treating slurry often cause secondary pollution and other problems. Biological methods use microorganisms, microalgae or plants to metabolize and degrade waste in the slurry, and can also produce high-quality microalgae protein bait without secondary pollution. However, there are still some problems in the current treatment of livestock slurry:

[0004] 1. The biological toxicity of livestock slurry is strong, and a large amount of water needs to be added to dilute the slurry for conventional microalgae direct treatment, which greatly increases the equipment investment and treatment cost;

[0005] 2. Aquatic vegetables have good pollution tolerance, but the treatment effect of ammonia nitrogen is poor, and a long treatment period is needed to reach the discharge standard, which greatly reduces the treatment efficiency of the slurry;

[0006] 3. Microalgae are generally collected by centrifugal precipitation and flocculant flocculation, which has high cost and is easy to cause secondary pollution;

[0007] After searching, there is no report on using aquatic vegetables coupled with microalgae to efficiently treat livestock slurry to prepare microalgae protein bait. In addition, most of the current reported methods for treating livestock slurry are single aquatic vegetable treatment or algal-bacterial symbiosis treatment. The present application first reports a method for efficiently treating livestock slurry by using aquatic vegetables coupled with microalgae, and collects microalgae by self-flocculation to prepare high-quality protein bait. This method not only realizes efficient denitrification and dephosphorization of livestock slurry, but also reduces the cost of microalgae collection. SUMMARY

[0008] To solve the above problems, the application aims to provide a method for efficiently treating livestock and poultry biogas slurry by coupling aquatic vegetables and microalgae to prepare microalgae protein bait, which can quickly treat livestock and poultry biogas slurry, reduce the COD, ammonia nitrogen content and phosphorus content in the biogas slurry, and also prepare high-quality protein bait.

[0009] The technical scheme of the application is as follows:

[0010] The method for efficiently treating livestock and poultry biogas slurry by coupling aquatic vegetables and microalgae to prepare microalgae protein bait comprises the following steps:

[0011] (1) Pretreatment of livestock and poultry biogas slurry: the livestock and poultry biogas slurry is first deposited in a primary sedimentation tank, and then filtered through a grid to remove most of the solid particles;

[0012] (2) Primary treatment and utilization of livestock and poultry biogas slurry: the treated biogas slurry is transported to a primary treatment tank in which aquatic vegetables are planted on floating beds, and the biogas slurry is treated by using a sequencing batch process, and after being retained for 15-25 days, the biogas slurry is discharged to a secondary treatment tank after adjusting the pH in a regulating tank;

[0013] (3) Secondary treatment and utilization of livestock and poultry biogas slurry: the biogas slurry treated by the primary treatment is subjected to secondary treatment in a microalgae raceway tank in which algae with strong pollution-eating and pollution-tolerant abilities are inoculated, and the secondary treatment is ended when the biomass of the microalgae and the removal rate of the pollutants reach the standard, and the microalgae are collected to prepare microalgae protein bait.

[0014] Preferably, in step (1), the initial livestock and poultry biogas slurry has a COD content of 600-2200 mg / L, an ammonia nitrogen content of 300-800 mg / L and a phosphorus content of 30-100 mg / L; preferably, in step (1), the livestock and poultry biogas slurry is deposited in the primary sedimentation tank for 12-24 h, and then filtered through a grid with a mesh size of 300-500 to remove most of the solid particles.

[0015] Preferably, in step (2), the aquatic vegetables are selected from one or two of water celery, water bamboo and water lettuce; more preferably, the aquatic vegetables are water celery and water bamboo commonly seen in Jiangsu.

[0016] The sequencing batch process refers to stopping the inflow of the biogas slurry after the biogas slurry reaches a depth of 30-60 cm in the primary treatment tank, reducing the COD content by more than 60%, and reducing the ammonia nitrogen content and the phosphorus content by more than 70% to start the next stage of secondary treatment, so as to avoid the inhibition of the subsequent microalgae treatment caused by the high content of pollutants.

[0017] Preferably, in step (2), the water celery and water bamboo are planted on the floating beds in a column planting mode, and the column number ratio of the water bamboo to the water celery is in the range of 1:1 to 3:1, and the unit coverage rate is in the range of 80% to 90%. More preferably, the column number ratio of the water bamboo to the water celery is 2:1.

[0018] Preferably, in step (2), the biogas slurry after the first-stage treatment is subjected to pH adjustment in the conditioning tank, and 8-10% (w / w) Ca(OH)2 is used to adjust the pH to 7.0-7.5.

[0019] Preferably, in step (3), one or more of Chlamydomonas reinhardtii, Chlorella pyrenoidosa and Tetradesmus obliquus are selected, which are all purchased from the National Freshwater Algae Culture Collection. The algae are inoculated into the raceway pond after being expanded.

[0020] Preferably, in step (3), Chlamydomonas reinhardtii, Chlorella pyrenoidosa and Tetradesmus obliquus are respectively cultured to a biomass of 2-4 g / L. More preferably, Chlamydomonas reinhardtii, Chlorella pyrenoidosa and Tetradesmus obliquus are respectively cultured to a biomass of 2-4 g / L, and are inoculated at a volume ratio of 1:1:2-1:1:5, with a total inoculation amount of 1%-5% (v / v). Most preferably, Chlamydomonas reinhardtii, Chlorella pyrenoidosa and Tetradesmus obliquus are inoculated at a ratio of 1:1:3.

[0021] Further, the Chlamydomonas reinhardtii is cultured using TAP medium (0.2‰-0.4‰ NH4Cl, 0.2‰-0.4‰ anhydrous calcium chloride, 0.1‰-0.12‰ MgSO4·7H2O, 2‰-3‰ Tris, 0.15‰-0.2‰ K2HPO4, 0.04‰-0.06‰ KH2PO4, 1-1.5 mL / L concentrated hydrochloric acid, 1-1.5 mL / L glacial acetic acid, 2‰-2.5‰ anhydrous sodium acetate), at a temperature of 20-24℃, and a light intensity of 10000-20000 Lux;

[0022] The Chlorella pyrenoidosa and Tetradesmus obliquus are cultured using commercial BG-11 medium, and the temperature is controlled at 22-28℃, and the light intensity is controlled at 1000-4000 Lux.

[0023] Further, the Chlamydomonas reinhardtii needs to be inoculated into the raceway pond after being subjected to secondary expansion. The first-stage seed culture of Chlamydomonas reinhardtii is cultured in a 100-500 mL triangular flask added with TAP medium for 1-4 days to the logarithmic growth phase. The secondary seed culture of Chlamydomonas reinhardtii is carried out in TAP medium additionally added with 5%-10% biogas slurry after the first-stage treatment, and is placed in a column photobioreactor, with a ventilation rate of 10-40 L / min, and is cultured for 2-8 days to obtain the secondary seed liquid;

[0024] Further, the Chlorella pyrenoidosa or Scenedesmus obliquus needs to be inoculated into a raceway pond after secondary expansion, the primary seed culture is cultured in a 100-500 mL triangular flask added with BG-11 culture medium for 3-8 days to the logarithmic growth phase; the secondary seed culture is carried out in the BG-11 culture medium additionally added with 5%-20% of the first digestate, and is placed in a column type photobioreactor, with a ventilation rate of 10-40 L / min, and is cultured for 6-20 days to obtain the secondary seed liquid;

[0025] Preferably, in step (3), the inoculated microalgae are subjected to secondary digestate treatment in the raceway pond by using a stainless steel flat paddle, and the paddle rotation speed is 15-20 r / min.

[0026] Preferably, in step (3), the first digestate is subjected to circulation treatment in the raceway pond, and can be discharged and the microalgae can be collected until meeting the discharge standard; the liquid depth of the digestate in the microalgae raceway pond is 15-30 cm, the digestate stays in the microalgae raceway pond for 10-15 days, and can be discharged until the ammonia nitrogen content is ≤80 mg / L, the COD content is ≤400 mg / L, and the phosphorus content is ≤8 mg / L (referring to the Pollutant Discharge Standard for Livestock and Poultry Breeding Industry GB 18596-2001).

[0027] Preferably, in step (3), the collection of the microalgae needs to add 8%-10% (w / w) Ca(OH)2 to adjust the pH to 7.0-7.2, and the supernatant is collected after the microalgae are completely flocculated after being static for 6-12 h.

[0028] In step (3), the microalgae are collected by self-flocculation; it is determined that the prepared microalgae protein bait contains rich amino acid components, and the protein content is as high as 50%-70% (w / w), which can be used as high-quality bait for aquaculture.

[0029] The present application can harvest microalgae biomass of more than 2 g / L by self-flocculation at low cost, the protein content in the microalgae is as high as 50%-70%, and the essential amino acid content is close to that of fish meal, which can be used as high-quality protein bait.

[0030] The present application has the following beneficial effects compared with the prior art:

[0031] (1) The present application uses aquatic vegetables to carry out primary digestate treatment on the pretreated livestock and poultry digestate, which can effectively reduce the biological toxicity of the digestate to the subsequent microalgae treatment, and avoid the need to add water to dilute the digestate in the secondary digestate stage; the aquatic vegetable coupled microalgae treatment of the livestock and poultry digestate is rapid and efficient, and the purified digestate meets the national discharge standard.

[0032] (2) The present application utilizes the self-flocculation characteristics of microalgae, and realizes low-cost recovery of microalgae cells by adjusting pH value, and the prepared microalgae protein content is as high as 50%-70% (w / w), which belongs to a high-quality protein bait.

[0033] (3) The present application realizes efficient reduction of pollutant content of livestock and poultry biogas liquid, and the COD content of the treated biogas liquid is reduced by 75%-90%, the ammonia nitrogen content is reduced by 80%-92%, and the phosphorus removal rate is more than 90%. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 Process flow chart of high-efficiency treatment of livestock and poultry biogas liquid by aquatic vegetables coupled with microalgae;

[0035] Figure 2 Growth conditions of three kinds of microalgae under different biogas liquid treatment conditions.

[0036] DETAILED EMBODIMENT

[0037] The present application can be better understood according to the following examples. However, it is easy for those skilled in the art to understand that the content described in the examples is only used to illustrate the present application, and should not and will not limit the present application described in detail in the claims.

[0038] Culture medium used in the present application

[0039] TAP culture medium (calculated by mass): 0.2 ‰ NH4Cl, 0.2 ‰ anhydrous calcium chloride, 0.1 ‰ MgSO4·7H2O, 2 ‰ Tris, 0.15 ‰ K2HPO4, 0.04 ‰ KH2PO4, 1 mL / L concentrated hydrochloric acid, 1 mL / L glacial acetic acid, 2 ‰ anhydrous sodium acetate);

[0040] The BG-11 culture medium is purchased from Qingdao Haibo Biotechnology Co., Ltd., product specification 250 g, product code HB8793.

[0041] Detection method used in the present application:

[0042] The COD is determined by potassium dichromate oxidation method, the ammonia nitrogen content is determined by Nash reagent method, the phosphorus content is determined by ammonium molybdate spectrophotometry, the microalgae growth condition is determined by determining the cell dry weight (DCW), and the protein content is determined by Coomassie brilliant blue method.

[0043] The above culture medium and related reagents are purchased from local suppliers.

[0044] Example 1: Primary consumption treatment of biogas liquid by aquatic vegetables

[0045] The separated sewage from the cow dung after the sedimentation in the sedimentation well is introduced into the sewage anaerobic treatment system for anaerobic methane fermentation for 20-30 days. When the pollutant content of the livestock and poultry slurry meets the requirements (COD content is 600-2200 mg / L, ammonia nitrogen content is 300-800 mg / L, and phosphorus content is 30-100 mg / L), the livestock and poultry slurry is discharged into the primary sedimentation tank for sedimentation for 24 hours, and then is filtered through the grid with a pore size of 300 mesh and flows into the primary treatment tank. It is determined that the initial slurry has a COD of 830 mg / L, an ammonia nitrogen content of 541 mg / L, and a phosphorus content of 52 mg / L. The area of the primary treatment tank is 100 m 2 In the small primary treatment tank, the water vegetables are transplanted, the liquid level of the added slurry is controlled at 40 cm, the coverage rate of the water vegetables is controlled at 80%, and the floating bed is arranged in the column planting mode. The water shield seedlings are transplanted into the small primary treatment tank when the growth reaches 3-5 tillers, and the water celery seedlings can be directly used for planting. The column number ratio of the water shield seedlings to the water celery seedlings is between 1:1 and 3:1. After 20 days of treatment, the COD, ammonia nitrogen content and phosphorus content in the slurry are determined.

[0046] As shown in Table 1, the single water shield treatment has a higher ammonia nitrogen and phosphorus removal rate, but has a poor COD removal efficiency. The single water celery treatment has a lower ammonia nitrogen and phosphorus removal rate. When the water shield and water celery are planted in a ratio of 2:1, the COD, ammonia nitrogen and phosphorus removal effect is the best.

[0047] Table 1. Primary treatment of slurry by water vegetables

[0048]

[0049]

[0050] Example 2: Selection of liquid level of primary treatment of slurry

[0051] This example is carried out in a small primary treatment tank with an area of 100 m 2 In the small primary treatment tank, the water vegetables (water shield: water celery = 2:1) are transplanted, the liquid level of the added slurry is controlled at 30-60 cm, the coverage rate of the water vegetables is controlled at 80%, and the floating bed is arranged in the column planting mode. The water shield seedlings are transplanted into the small primary treatment tank when the growth reaches 3-5 tillers, and the water celery seedlings can be directly used for planting. After 20-30 days of treatment, the COD, ammonia nitrogen content and phosphorus content in the slurry are determined.

[0052] From the data of Table 2, it can be seen that the liquid level of 30-40 cm can achieve the goal of primary treatment for 20 days, and will not affect the subsequent secondary treatment of microalgae. Further increasing the liquid level depth is difficult to effectively reduce the COD, ammonia nitrogen and phosphorus content in the livestock manure slurry due to the limitation of the treatment capacity of aquatic vegetables. Further extending the primary treatment time, the removal efficiency of COD, ammonia nitrogen and phosphorus content is difficult to further improve substantially, and the input cost will also increase. Therefore, the subsequent process selects the liquid level depth of 40 cm, and the primary treatment time of 20 days.

[0053] Table 2. Liquid level depth of primary treatment of biogas slurry

[0054]

[0055] Example 3: Growth of three kinds of microalgae in biogas slurry after primary treatment

[0056] The culture method of the microalgae in this example is as follows:

[0057] Culture of Chlamydomonas reinhardtii: Chlamydomonas reinhardtii was inoculated into a 150 mL shake flask containing 40 mL TAP medium, and cultured at 24°C with a light intensity of 10,000 Lux for 3-4 days to the logarithmic growth phase to obtain a seed solution;

[0058] Chlorella pyrenoidosa: Chlorella pyrenoidosa was inoculated into a 150 mL shake flask containing 40 mL BG-11 medium, and cultured at 24°C with a light intensity of 2,500 Lux for 8-10 days to the logarithmic growth phase to obtain a seed solution;

[0059] Scenedesmus obliquus: Scenedesmus obliquus was inoculated into a 150 mL shake flask containing 40 mL BG-11 medium, and cultured at 24°C with a light intensity of 3,500 Lux for 3-4 days to the logarithmic growth phase to obtain a seed solution;

[0060] The treatment method of the biogas slurry in this example was carried out according to the best conditions in Example 1. After primary treatment, the biogas slurry was adjusted to a pH value of 7.5 by 10% (w / w) Ca(OH)2 in the adjustment tank. Three kinds of microalgae were inoculated into a 500 mL shake flask containing 100 mL of the above treated biogas slurry, and the inoculation amount was controlled at 5% (v / v). The temperature was controlled at 24°C, and the light intensity was adjusted according to the requirements of the three kinds of microalgae. The biogas slurry without primary treatment (adjusted to a pH value of 7.5 by 10% (w / w) Ca(OH)2) was used as a control group. After 15 days of culture, the microalgae were collected by filtration, and the biomass of the three kinds of microalgae in the biogas slurry was determined. Figure 2 It can be seen from the results that the three kinds of microalgae are difficult to grow in the biogas slurry without primary treatment. After primary treatment, the microalgae toxicity of the biogas slurry is significantly reduced, and the three kinds of microalgae can grow therein, and the growth condition of Scenedesmus obliquus is the best.

[0061] Example 4: The bioremediation of livestock manure by aquatic vegetable coupled with microalgae

[0062] The treatment method of the biogas slurry in this example was carried out according to the best conditions in Example 1. After the first-stage bioremediation, the biogas slurry was adjusted to pH 7.5 in the conditioning tank by adding 10% (w / w) Ca(OH)2. Three microalgae were expanded to a biomass of 2 g / L in the second-stage bioreactor, respectively. Then, the microalgae were inoculated into the biogas slurry after the first-stage bioremediation at a ratio of 5% (v / v). The second-stage bioremediation was carried out in a raceway pond at a paddle speed of 15-20 r / min. After 20 days of cyclic treatment, the COD content, ammonia nitrogen content, and phosphorus content in the biogas slurry were determined. The pH value was adjusted to 7.0 by adding 10% (w / w) Ca(OH)2 to promote the flocculation of microalgae cells. The microalgae cells were collected after 12 h of standing in the flocculation tank. All the collected microalgae cells were filtered and used as the total amount of microalgae. The flocculation efficiency of microalgae was calculated (Table 3). In addition, the types and contents of amino acids in the collected microalgae cells were analyzed, and the results are shown in Table 4.

[0063] As shown in Table 3, Scenedesmus obliquus had a good flocculation effect and could promote the flocculation effect when it was cultured with Chlamydomonas reinhardtii and Chlorella pyrenoidosa. However, the removal effect of ammonia nitrogen and phosphorus was general, and it needed to be used in combination with Chlamydomonas reinhardtii and Chlorella pyrenoidosa. The flocculation effect of Chlamydomonas reinhardtii or Chlorella pyrenoidosa alone was poor. By comparison, it was found that when the three microalgae were inoculated at a ratio of Chlamydomonas reinhardtii: Chlorella pyrenoidosa: Scenedesmus obliquus = 1:1:3, the COD, ammonia nitrogen, and phosphorus contents were 49.0 mg / L, 34.1 mg / L, and 2.5 mg / L, respectively, which could meet the discharge standard (GB 18596-2001). The collected microalgae cells were 2.6 g / L, and the flocculation recovery rate of microalgae reached 88.3%, which could realize the low-cost recovery of microalgae cells.

[0064] As shown in Table 4, after the evaluation of the collected microalgae cells inoculated at a ratio of Chlamydomonas reinhardtii: Chlorella pyrenoidosa: Scenedesmus obliquus = 1:1:3, it was found that the protein content of the microalgae cells was as high as 62.4%. By detecting the contents of ten essential amino acids (methionine, lysine, tryptophan, histidine, leucine, isoleucine, phenylalanine, threonine, valine, and arginine) commonly found in aquatic animals in the microalgae feed, and comparing them with the commercially available fish meal (crude protein content 65%), it was found that the protein feed prepared from the three microalgae at a ratio of Chlamydomonas reinhardtii: Chlorella pyrenoidosa: Scenedesmus obliquus = 1:1:3 had higher contents of all amino acids except for arginine, histidine, and lysine than the fish meal. This indicated that the microalgae protein feed prepared by the efficient treatment of livestock manure by aquatic vegetable coupled with microalgae had high application value and could be used to replace fish meal as the protein feed for aquatic animal breeding.

[0065] Table 3. The assimilation of livestock biogas slurry by aquatic vegetable coupled microalgae

[0066]

[0067] Note: The removal rates of COD, ammonia nitrogen and phosphorus were calculated based on the initial concentration in the biogas slurry.

[0068] Table 4. Types and contents of amino acids in microalgae feed

[0069]

[0070]

Claims

1. A method for efficiently treating livestock biogas slurry by coupling aquatic vegetables and microalgae to produce microalgal protein bait, characterized in that, It comprises the following steps: (1) Pretreatment of livestock and poultry biogas slurry: the livestock and poultry biogas slurry is first deposited in a primary sedimentation tank, and then filtered through a grid to remove most of the solid particles; (2) Primary treatment and utilization of livestock and poultry biogas slurry: the treated biogas slurry is transported to a primary treatment tank in which aquatic vegetables are planted, and a sequencing batch process is used to treat the biogas slurry, which is discharged to a secondary treatment tank after being adjusted in pH for 15-25 days; (3) Secondary treatment and utilization of livestock and poultry biogas slurry: the biogas slurry treated in the primary treatment tank is subjected to secondary treatment in a microalgae raceway tank inoculated with algae strains with strong pollution-eating and pollution-tolerant abilities, and the secondary treatment is ended when the biomass of the microalgae and the removal rate of pollutants reach the standard, and the microalgae are collected to prepare microalgae protein bait.

2. The method for preparing microalgal protein bait by coupling hydroponic vegetables and microalgae to efficiently treat livestock and poultry biogas slurry according to claim 1, characterized in that, In step (2), the aquatic vegetables are selected from one or both of water celery and wild rice shoots.

3. The method for preparing microalgal protein bait by coupling aquatic vegetables and microalgae to efficiently treat livestock and poultry biogas slurry according to claim 1, characterized in that, The sequencing batch process is that after the biogas slurry is transported to the primary treatment tank, the biogas slurry is stopped flowing in when the depth of the biogas slurry reaches 30-60 cm, and the next stage of secondary treatment is started when the degradation of COD and ammonia nitrogen content reaches the standard.

4. The method for preparing microalgal protein bait by coupling hydroponic vegetables and microalgae to efficiently treat livestock and poultry biogas slurry according to claim 1, characterized in that, The algae strains are selected from one or more of Chlamydomonas reinhardtii, Chlorella pyrenoidosa and Scenedesmus obliquus, and the algae strains are inoculated into the raceway tank after being expanded.

5. The method for preparing microalgal protein bait by coupling aquatic vegetables and microalgae to efficiently treat livestock and poultry biogas slurry according to claim 4, characterized in that, The Chlamydomonas reinhardtii is cultured using TAP medium (0.2‰-0.4‰ NH4Cl, 0.2‰-0.4‰ anhydrous calcium chloride, 0.1‰-0.12‰ MgSO4·7H2O, 2‰-3‰ Tris, 0.15‰-0.2‰ K2HPO4, 0.04‰-0.06‰ KH2PO4, 1-1.5 mL / L concentrated hydrochloric acid, 1-1.5 mL / L glacial acetic acid, 2‰-2.5‰ anhydrous sodium acetate), and the culture is carried out at 20-24℃ with light intensity of 10,000-20,000 Lux; The Chlorella pyrenoidosa and Scenedesmus obliquus are cultured using commercial BG-11 medium, and the culture is carried out at a temperature of 22-28℃ with light intensity of 1,000-4,000 Lux.

6. According to claim 4, the expansion of Chlamydomonas reinhardtii, Chlorella pyrenoidosa or Scenedesmus obliquus comprises primary seed culture and secondary seed culture; The primary seed culture is carried out in a 100-500 mL triangular flask with inoculation amount of 1%-5% (v / v), and the culture is carried out to the logarithmic growth phase of the microalgae; The secondary seed culture is carried out in a column type photobioreactor with inoculation amount of 1%-5% (v / v) and aeration rate of 10-40 L / min, and the culture is carried out to the biomass of the microalgae reaching 2-4 g / L.

7. The method according to claim 4, wherein, The Chlamydomonas reinhardtii, Chlorella pyrenoidosa and Scenedesmus obliquus are respectively cultured to the biomass reaching 2-4 g / L, and are inoculated in a volume ratio of 1:1:2-1:1:5, with total inoculation amount of 1%-5% (v / v).

8. The method according to claim 1, wherein the method is characterized in that, In step (3), the liquid depth of the biogas slurry in the microalgae raceway tank is 15-30 cm, and the biogas slurry is retained in the microalgae raceway tank for 10-15 days until the ammonia nitrogen content, COD content and phosphorus content are reduced to the discharge standard.

9. The method according to claim 1, wherein the method is characterized in that, In step (3), a flat paddle is arranged in the microalgae raceway tank, and the stirring rate of the flat paddle is 15-20 r / min.

10. The method according to claim 1, wherein the method is characterized in that, In step (3), the microalgae are collected by self-flocculation; the prepared microalgae protein bait contains rich amino acid components, and the protein content is as high as 50%-70% (w / w), which can be used as high-quality bait for aquaculture.

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