A high-efficiency purification system and method for aquaculture tail water based on immobilization of tetraselmis tetrahele

Abstract

The present application relates to the field of aquaculture pollution control and tail water treatment engineering technology, and discloses a kind of based on immobilized four-tailed Scenedesmus dimorphus high-efficiency purification system and method of aquaculture tail water, including sequentially connected primary sedimentation tank, algal ball reaction tank, collection tank and circulation control system, the connecting pipeline between the primary sedimentation tank, algal ball reaction tank is equipped with water pump;The algal ball reaction tank inside adds immobilized four-tailed Scenedesmus dimorphus algal ball, bottom is equipped with aeration device, top is equipped with illumination module;The collection tank is equipped with filter.This scheme is modularized sequentially linked by "primary sedimentation tank-algal ball reaction tank-collection tank-circulation control system", matched with accurate supporting components, and builds up a hierarchical treatment, collaborative purification, integrated system of cycle control, and gives consideration to tail water purification efficiency, stability and resource utilization, solves the pain points of traditional system purification not thorough, algal body is easy to lose, working condition control rigid.

Description

Technical Field

[0001] This invention relates to the field of aquaculture pollution control and wastewater treatment engineering technology, specifically to a highly efficient aquaculture wastewater purification system and method based on the immobilization of *Scenedesmus tetracoralis*. Background Technology

[0002] Aquaculture wastewater is rich in pollutants such as ammonia nitrogen, total phosphorus, and heavy metals. Direct discharge without efficient treatment can easily lead to eutrophication, disrupt the aquatic ecological balance, and hinder the green and sustainable development of the aquaculture industry. Meanwhile, with increasingly stringent environmental regulations and ever-improving standards for aquaculture wastewater discharge, developing efficient and stable wastewater purification technologies can not only reduce environmental pollution risks but also help the aquaculture industry improve quality and efficiency. This is of significant practical importance for protecting the aquatic ecological environment and promoting the transformation and upgrading of the aquaculture industry.

[0003] In existing technologies, there are various methods to improve the efficiency and effectiveness of aquaculture wastewater purification, with the core focusing on physical, chemical, and biological purification pathways. Among these, immobilized microalgae technology has become the mainstream approach due to its good purification effect and environmental friendliness. Firstly, physical and chemical methods are used to initially remove suspended pollutants and some heavy metals from the wastewater through sedimentation, filtration, and adsorption, laying the foundation for subsequent purification. Secondly, biological purification methods utilize the photosynthesis and metabolism of microalgae to absorb and transform nutrient pollutants such as ammonia nitrogen and total phosphorus, while immobilization technology (using sodium alginate-chitosan as a common carrier) retains the microalgae, reducing microalgae loss and improving purification stability. Thirdly, optimizing the purification system design involves combining multi-stage treatment units and adjusting reaction environment parameters to enhance pollutant removal efficiency.

[0004] However, existing wastewater purification technologies, especially immobilized microalgae technology, still have the following technical problems: (1) The design of immobilized carrier parameters in existing technologies is unreasonable. The ratio and concentration of sodium alginate and chitosan lack precise optimization. If the ratio is too high, it will reduce the permeability of the algae and hinder the exchange of nutrients. If the ratio is too low, the mechanical strength will be insufficient, leading to the loss of microalgae. Improper concentration will also affect the activity of microalgae or cause the algae to swell and break. (2) The purification efficiency of existing technologies is unstable and low. Affected by the performance of the carrier, the ammonia nitrogen removal rate of existing systems is only maintained at 60%-70%, which is difficult to efficiently degrade various pollutants in the wastewater. (3) The operation of existing technology systems is easily affected by the fluctuation of aquaculture wastewater quality, which makes it difficult to meet the wastewater purification needs of large-scale aquaculture. In summary, there is an urgent need for an immobilized microalgae purification system and method that optimizes carrier parameters and improves purification stability and efficiency. Summary of the Invention

[0005] The present invention aims to provide a highly efficient purification system and method for aquaculture wastewater based on immobilized Scenedesmus tetraculus, in order to solve the technical problems of low purification efficiency and unstable purification effect of existing immobilized microalgae technology in purifying wastewater.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a high-efficiency purification system for aquaculture wastewater based on immobilized *Scenedesmus tetracauda*, comprising a primary sedimentation tank, an algae ball reaction tank, a collection tank, and a circulation control system connected in sequence; a water pump is installed on the connecting pipe between the primary sedimentation tank and the algae ball reaction tank; immobilized *Scenedesmus tetracauda* algae balls are added inside the algae ball reaction tank, an aeration device is installed at the bottom, and a light module is installed at the top; a filter is installed in the collection tank.

[0007] The principles and advantages of this scheme are: This purification system employs a modular approach, sequentially connecting a primary sedimentation tank, an algae ball reaction tank, a collection tank, and a circulation control system. Coupled with precisely matched components, it constructs an integrated system of graded treatment, collaborative purification, and cyclical regulation, balancing effluent purification efficiency, stability, and resource utilization. This addresses the pain points of traditional systems, such as incomplete purification, easy algae loss, and rigid operational control. Specifically, the modules connect sequentially to form a closed-loop process. The primary sedimentation tank pre-treats and removes large particulate impurities, reducing the subsequent purification load. In the algae ball reaction tank, immobilized algae balls, bottom aeration devices, and top lighting modules work together to provide an optimal environment for Scenedesmus metabolism, enhancing pollutant removal. The collection tank, equipped with a filter, separates algae from effluent, facilitating algae recovery. The circulation control system, with its pumps, flexibly adjusts the effluent flow direction and residence time to adapt to different pollution concentrations. Missing or improperly matched components (such as insufficient aeration device power, uneven lighting module intensity, or filter precision deviation) can lead to insufficient dissolved oxygen, decreased algae activity, incomplete algae recovery, or poor effluent circulation, all of which weaken the system's purification efficiency and compromise process stability.

[0008] Preferably, as an improvement, the aeration device is a microporous aerator evenly distributed at the bottom of the pool. The microporous aerator is connected to an aeration pump, through which air is introduced to maintain a dissolved oxygen concentration of 5~8 mg / L in the water.

[0009] Preferably, as an improvement, an overflow pipe is connected between the algae ball reactor and the collection tank, and a return pipe is connected between the collection tank and the algae ball reactor.

[0010] Preferably, as an improvement, this solution also provides a highly efficient purification method for aquaculture wastewater based on the immobilization of *Scenedesmus tetrapoda*, which is carried out based on the above system and includes the following steps: Step 1: Preparation of immobilized Scenedesmus tetracaudus algal spheres: Step 2, System Installation: Place the immobilized *Scenedesmus tetratail* algal balls into the algal ball reaction tank and install the above system; Step 3: Purifying Aquaculture Wastewater: The aquaculture wastewater first enters the primary sedimentation tank to remove large particulate impurities. The supernatant then enters the algae ball reaction tank to fully contact the immobilized algae balls. Under aeration and light conditions, the wastewater is purified. Part of the purified wastewater overflows into the collection tank. After recovering the proliferated algae, the circulation control system uses a water pump to return part of the purified wastewater to the algae ball reaction tank, adjusting the hydraulic retention time to complete the purification of the aquaculture wastewater.

[0011] Beneficial effects: This solution relies on the immobilization technology of Scenedesmus tetraculus and the coordinated operation of the system. Through the standardized process of "immobilization preparation - system deployment - graded purification cycle", it achieves efficient purification of aquaculture wastewater and recovery of algal resources, taking into account the purification effect, operational stability and resource utilization rate, and solving the pain points of traditional purification methods such as algal loss, low purification efficiency and difficulty in achieving wastewater standards.

[0012] 1. This scheme effectively constructs a stable carrier for immobilized *Scenedesmus tetracaudus* algal pellets, laying a core foundation for efficient purification and algal recovery. Specifically, immobilization prevents *Scenedesmus tetracaudus* from being lost with the wastewater during purification, maintains a stable algal concentration in the reaction system, and ensures continuous purification efficiency. The algal pellet structure provides a suitable microenvironment for *Scenedesmus tetracaudus* growth, promotes algal proliferation and enhances metabolic activity, thereby strengthening the absorption and transformation capacity of pollutants such as nitrogen and phosphorus in the wastewater. The immobilized form facilitates subsequent algal recovery, enabling the reuse of algal resources and avoiding secondary pollution.

[0013] 2. This solution achieves precise compatibility between the immobilized algae balls and the purification system through system installation, ensuring a smooth and efficient purification process. Specifically, the algae balls are placed into the algae ball reaction tank in a directional manner to ensure uniform distribution and create conditions for sufficient contact with the effluent. The overall system installation is standardized, enabling the algae ball reaction, aeration, and lighting modules to operate in synergy, ensuring orderly connection between each purification stage and avoiding impacts on purification efficiency due to equipment compatibility issues.

[0014] 3. This solution achieves efficient and in-depth purification of effluent through graded treatment, synergistic purification, and cyclic regulation, enhancing process flexibility and stability. Specifically, primary sedimentation pretreatment removes large particulate impurities, reducing the subsequent algal purification load and preventing impurities from covering the algal surface and affecting metabolic activity; aeration and light conditions work synergistically to promote the photosynthesis and metabolism of Scenedesmus, efficiently degrading and absorbing pollutants in the effluent, thus improving the purification rate and effect; the combination of effluent overflow collection, algal recovery, and partial recirculation not only achieves compliant effluent discharge and algal resource utilization, but also allows for adjustment of hydraulic retention time through recirculation, adapting to the purification needs of effluent with different pollution concentrations and ensuring stable and compliant purification results.

[0015] Preferably, as an improvement, in step one, the preparation of immobilized *Scenedesmus tetracaudus* algal spheres includes the following steps: S1. Algal culture: The Scenedesmus tetracauda was inoculated into BG11 medium and cultured until the logarithmic growth phase to obtain the Scenedesmus tetracauda algal solution. S2. Preparation of composite carrier: Mix 2-4 wt% sodium alginate solution and 0.5-1 wt% chitosan solution in a mass ratio of 5-10:1 to obtain composite carrier; S3. Formation of algal balls: Mix the algal solution of *Scenedesmus tetratail* with the composite carrier at a volume ratio of 1:2~4. Use a syringe to drop the mixture into a 2~4% CaCl2 solution to form gel balls with a diameter of 3~5 mm. After standing and solidifying for 2~4 h, rinse three times with deionized water and transfer to BG11 medium for pre-culture for 24~48 h to obtain immobilized algal balls.

[0016] Beneficial Effects: This solution utilizes a standardized process of "logarithmic algal culture - precise formulation of composite carrier - controllable pre-culture" to prepare immobilized *Scenedesmus tetracauda* algal balls with stable structure and high algal activity. This provides a core carrier for efficient wastewater purification. Precise matching of parameters is crucial; deviations will degrade the algal ball's performance and weaken its purification efficiency. Through long-term experiments, the inventors discovered that when the mass ratio of sodium alginate to chitosan is <5:1 (i.e., too little sodium alginate), the algal ball's mechanical strength is insufficient, leading to a breakage rate >30%. Conversely, when the mass ratio is >10:1 (i.e., too much sodium alginate), poor algal ball permeability reduces the ammonia nitrogen removal rate to below 70%. If the sodium alginate concentration is too low, the resulting algal balls easily swell, resulting in a microalgae loss rate greater than 25%. Conversely, if the sodium alginate concentration is too high, the algal balls become too hard, causing the total phosphorus removal rate to drop below 65%. If the chitosan concentration is too low, the continuous use period will be less than 10 days due to the poor anti-swelling properties of the algal balls; if the chitosan concentration is too high, the ammonia nitrogen removal rate will decrease by 15-20% due to the inhibition of microalgal activity. If the volume ratio of algal solution to carrier is <1:2 (i.e., too little carrier), the activity will decrease due to competitive inhibition caused by excessively dense microalgae; if the volume ratio of algal solution to carrier is >1:4 (i.e., too much carrier), the purification efficiency will decrease by 20-30% due to insufficient microalgae density.

[0017] Preferably, as an improvement, in step two, the amount of the immobilized *Scenedesmus tetratail* algae balls added is 10-15% of the effective volume of the algae ball reaction tank.

[0018] Preferably, as an improvement, in step three, the light intensity is 2000~4000 lux, and the daily light duration is 12~16h; the aeration intensity is controlled to have dissolved oxygen at 5~8mg / L.

[0019] Beneficial Effects: This solution precisely controls the light intensity (2000-4000 lux) and daily light duration (12-16 hours), while simultaneously maintaining dissolved oxygen at 5-8 mg / L. These two factors synergistically enhance the metabolism of *Scenedesmus tetrapoda*, maximizing effluent purification efficiency while balancing algal activity and process stability. Parameter deviations significantly weaken the purification effect. Through long-term experiments, the inventors discovered that if the light intensity is below 2000 lux and the daily light duration is less than 12 hours, insufficient photosynthesis in *Scenedesmus tetrapoda* leads to decreased metabolic activity, weakened absorption and conversion of nitrogen and phosphorus pollutants, and a slower purification rate. Conversely, if the light intensity exceeds 4000 lux and the light duration exceeds 16 hours, it easily triggers algal photoinhibition, even damaging cell structure and causing algal death, leading to secondary pollution. When dissolved oxygen is below 5 mg / L, the lack of oxygen in the reaction tank inhibits the aerobic metabolism and proliferation of Scenedesmus, and may also breed anaerobic bacteria, affecting the purification effect. If dissolved oxygen is above 8 mg / L, excessive oxygen will inhibit the activity of key enzymes in the photosynthesis of Scenedesmus, reduce the efficiency of pollutant removal, increase aeration energy consumption, and increase process costs, all of which undermine the stability and economy of effluent purification.

[0020] Preferably, as an improvement, in step three, the hydraulic residence time regulated by the circulation control system is 8~24h, the flow rate is 0.1~0.3m / s, and the temperature is 15~30℃.

[0021] Beneficial Effects: This solution precisely regulates the hydraulic retention time (8-24h), flow velocity (0.1-0.3m / s), and temperature (15-30℃) through a circulation control system, coordinating with light and aeration parameters to provide an optimal environment for the metabolism of *Scenedesmus tetragonus*, ensuring thorough effluent purification and process stability. Deviation of any parameter will weaken the purification efficiency. Through long-term experiments, the inventors discovered that when the hydraulic retention time is less than 8h, the contact between the effluent and the algae is insufficient, pollutants are not completely absorbed and transformed, and the treatment effect is substandard; while if it is longer than 24h, the treatment efficiency is low, increasing process energy consumption and operating costs. When the flow velocity is below 0.1m / s, the effluent is slowly renewed, and localized pollutant accumulation inhibits algal activity; while if it is above 0.3m / s, excessive water flow impact can easily lead to algal breakage and loss, while also shortening the contact time. When the temperature is below 15℃, the metabolic rate of Scenedesmus drops sharply, and its purification capacity is greatly weakened; while if the temperature is above 30℃, the algae are easily killed by high temperature stress, causing secondary pollution, both of which destroy the efficiency and stability of the effluent purification.

[0022] Preferably, as an improvement, in step three, the continuous use period of the immobilized *Scenedesmus tetratail* algal balls is 15-20 days, and they can be reused after being soaked in BG11 medium for 24 hours during regeneration.

[0023] Beneficial effects: Based on the regulation of light, aeration and circulation parameters, this scheme precisely controls the continuous use cycle of immobilized Scenedesmus tetra-tailed algal balls to 15-20 days. Regeneration is achieved by soaking in BG11 medium for 24 hours and then reusing. This scheme balances the sustainability of purification, algal activity and process economy. Parameter deviations will degrade the performance of the algal balls and the purification effect.

[0024] The principles and advantages of this solution are as follows: 1. High purification efficiency: Through immobilization technology, Scenedesmus tetracoralis can be stably colonized in the system, and the removal rate of ammonia nitrogen and total phosphorus can reach more than 80%, which is much higher than that of free algae (60%-70%) and traditional constructed wetlands (30%-60%).

[0025] 2. High stability: The immobilized algae balls can be used continuously for more than 15 days and are less affected by temperature and light fluctuations (e.g., removal rate fluctuation <10% within the range of 10-35℃), solving the problem of free algae being "easy to lose and having unstable efficiency".

[0026] 3. Resource Utilization and Value Added: The recovered algae have a protein content of 40%-50%, which can be used directly as an aquatic feed additive; or after drying and composting, they can be made into organic fertilizer. Each ton of algae can generate economic benefits of 200-500 yuan, significantly improving the economic efficiency of the system.

[0027] 4. Clear parameters: The optimal range of key parameters is determined through experimental verification, which facilitates industrial scale-up and standardized production.

[0028] 5. Easy to operate: The system has a compact structure and occupies only 1 / 5 to 1 / 3 of the area of ​​traditional wetlands. It requires no complicated maintenance and is suitable for large-scale promotion and application. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of an efficient aquaculture wastewater purification system based on immobilized Scenedesmus tetrapoda in an embodiment of the present invention.

[0030] Figure 2 This refers to the morphological changes of the immobilized *Scenedesmus tetracauda* algal spheres during operation in Example 1 of this invention (including the morphology of the algal spheres at 0d and 18d of operation, with a scale bar of 500μm).

[0031] Figure 3 This refers to the morphological changes of the immobilized *Scenedesmus tetracauda* algal spheres during operation in Example 2 of this invention (including the morphology of the algal spheres at 0d and 15d of operation, with a scale bar of 500μm).

[0032] Figure 4 This refers to the morphological changes of the immobilized *Scenedesmus tetracauda* algal spheres during operation in Example 3 of the present invention (including the morphology of the algal spheres at 0d and 15d of operation, with a scale bar of 500μm).

[0033] Figure 5 This is a comparative example of the morphological changes of the immobilized *Scenedesmus tetracauda* algal spheres during operation (including the morphology of the algal spheres at 0d and 10d of operation, with a scale bar of 500μm).

[0034] Figure 6 This is a comparative example of the morphological changes of the immobilized *Scenedesmus tetracauda* algal spheres during operation (including the morphology of the algal spheres at 0d and 5d of operation, with a scale bar of 500μm).

[0035] Figure 7 This is a comparative example of the morphological changes of the immobilized *Scenedesmus tetracauda* algal spheres during operation (including the morphology of the algal spheres at 0d and 5d of operation, with a scale bar of 500μm).

[0036] The reference numerals in the accompanying drawings include: primary sedimentation tank 1, inlet pipe 11, inclined plate 12, connecting water pipe 13, water pump 14, algae ball reaction tank 2, algae ball 21, LED light source module 22, microporous aerator 23, overflow pipe 24, collection tank 3, filter 31, and return pipe 4. Detailed Implementation

[0037] The present invention will be further described in detail below with reference to embodiments, but the implementation of the present invention is not limited thereto. Unless otherwise specified, the technical means used in the following embodiments and experimental examples are conventional means well known to those skilled in the art, and the materials and reagents used can all be obtained commercially.

[0038] Overview of the Plan This solution provides a highly efficient aquaculture wastewater purification system based on the immobilization of *Scenedesmus tetrapoda*, such as... Figure 1 As shown, it includes a primary sedimentation tank, an algae ball reaction tank, and a collection tank connected in sequence, as well as a supporting circulation control system. A water pump is installed on the connecting pipe between the primary sedimentation tank and the algae ball reaction tank. The primary sedimentation tank is used to remove large particulate impurities and some suspended solids in the effluent. The algae ball reaction tank is filled with immobilized Siegesbeckia tetrandra algae balls, and has an aeration device at the bottom and a light module at the top. The collection tank is equipped with a filter.

[0039] The core improvement of this scheme lies in the optimization of the preparation parameters for immobilized Scenedesmus tetracauda algal spheres. The system specifically includes the following components: (1) System composition Primary sedimentation tank: An inclined plate sedimentation tank is adopted with a hydraulic retention time of 1~2 hours. Particle impurities with a diameter >50μm (such as uneaten feed and feces) in the effluent are removed by gravity sedimentation, reducing the load on subsequent treatment units.

[0040] Algae ball reaction tank: Rectangular structure, effective volume designed according to treatment capacity (based on hydraulic load of 0.5~1.0m³). 3 / (m 2•d) Calculation), add immobilized Scenedesmus tetra-tailed algal balls inside, the amount of which is 10~15% of the effective volume of the reaction tank.

[0041] Aeration device: It consists of an LED light source module and microporous aerators. The LED light source is arranged at the top of the reaction tank to provide continuous or intermittent illumination; the microporous aerators are evenly distributed at the bottom of the tank and air is introduced through the aeration pump to maintain the dissolved oxygen concentration in the water at 5~8 mg / L.

[0042] Collection tank: Connected to the algae ball reaction tank via an overflow pipe. The collection tank is equipped with a filter. For reference, in this scheme, the filter consists of an inclined bottom plate (5° slope) set at the bottom of the tank to facilitate algae sedimentation and recovery.

[0043] The circulation control system includes a variable frequency water pump, a temperature sensor, and a PLC controller. Based on the concentration of pollutants in the influent, the system dynamically adjusts the hydraulic retention time of the effluent in the system to 8-24 hours, controls the flow rate to 0.1-0.3 m / s, and maintains the temperature at the optimal range of 15-30℃ for the growth of Scenedesmus tetrapoda.

[0044] (2) Immobilized Scenedesmus tetracaudus ( Scenedesmus quadricauda Algal ball preparation Specifically, the steps are as follows; Step 1: Algal Culture: *Scenedesmus tetraculus* was inoculated into BG11 medium and cultured at 25°C and 3000 lux light intensity until the logarithmic growth phase (cell concentration reached 1×10⁻⁶). 7 ~5×10 7 (each cell / mL) to obtain a four-tailed Scenedesmus algal solution.

[0045] Step 2, Preparation of composite carrier: Dissolve sodium alginate (analytical grade) in deionized water, heat and stir until completely dissolved to prepare a sodium alginate solution with a mass concentration of 2-4%; separately dissolve chitosan (degree of deacetylation ≥90%) in 1% acetic acid solution to prepare a chitosan solution with a mass concentration of 0.5-1%; mix sodium alginate and chitosan at a mass ratio of 5-10:1 to obtain the composite carrier.

[0046] Step 3, Algal Sphere Formation: Mix the logarithmic growth phase of *Scenedesmus tetraculus* slurry with the composite carrier at a volume ratio of 1:2~4. Use a syringe to drop the mixture into a 2~4% CaCl2 solution to form gel spheres with a diameter of 3~5 mm. After standing and solidifying for 2~4 h, rinse three times with deionized water and transfer to BG11 medium for pre-culture for 24~48 h to obtain immobilized algal spheres.

[0047] The inventors determined the optimal range for the following key parameters through single-variable experiments. The experimental data are shown in Table 1: Table 1. Optimization of key parameters and results

[0048] (3) System operation process The aquaculture wastewater first enters the primary sedimentation tank, where large particulate impurities are removed after 1-2 hours of sedimentation. The supernatant then enters the algae ball reaction tank, where it comes into full contact with the immobilized algae balls. Under the conditions of 3000 lux light and 5-8 mg / L dissolved oxygen provided by the aeration device, *Scenedesmus tetrapoda* absorbs nutrients such as ammonia nitrogen and total phosphorus through photosynthesis. Part of the treated wastewater overflows into the collection tank, where the algae that have proliferated are recovered (through gravity settling or filter interception). The circulation control system uses a water pump to return a portion of the wastewater to the algae ball reaction tank, adjusting the hydraulic retention time to 8-24 hours to ensure treatment effectiveness. After 15 days of use, the immobilized algae balls can be regenerated by soaking in BG11 medium for 24 hours, and the removal rate after regeneration remains above 85% of the initial level.

[0049] Example 1: Treatment of effluent by immobilized Scenedesmus tetra-tailed algae under optimized parameters 1. Composite carrier configuration: Accurately weigh 30g of sodium alginate and slowly add it to 1000ml of deionized water. Stir continuously for 2 hours in a 50℃ water bath until completely dissolved to obtain a 3% sodium alginate solution. Separately weigh 8g of chitosan and dissolve it in a 1% acetic acid solution. Stir until completely dissolved and then bring the volume to 1000ml to obtain a 0.8% chitosan solution. Mix the two solutions at a mass ratio of 7:1 (i.e., mix 700g of sodium alginate solution with 100g of chitosan solution) and stir with a magnetic stirrer at 300rpm for 30 minutes to ensure uniform mixing.

[0050] 2. Preparation of algal balls: Collect *Scenedesmus tetracauda* cells in the logarithmic growth phase, centrifuge at 4000 rpm for 10 min, collect the cell pellet, wash three times with BG11 medium, resuspend in BG11 medium, and adjust the cell concentration to 2 × 10⁻⁶ cells / mL. 7 The algal solution of *Scenedesmus tetraculus* was obtained by measuring cells / mL. This algal solution was mixed with the aforementioned composite carrier at a volume ratio of 1:3, and stirred with a glass rod at 150 rpm for 15 min to ensure uniform dispersion of algal cells in the carrier. Using a 10 ml syringe (1 mm needle diameter), the mixture was slowly dripped into a 3% calcium chloride solution to form small spheres approximately 4 mm in diameter. The spheres were allowed to stand in the calcium chloride solution for 3 h to allow for complete solidification, followed by rinsing three times with deionized water, each time using five times the volume of the spheres, to remove residual calcium chloride. The rinsed algal spheres were then transferred to BG11 medium and pre-cultured at 25°C and 3000 lux for 36 h.

[0051] 3. Treatment of aquaculture wastewater: 10L of aquaculture wastewater was collected. The wastewater was tested and found to have an ammonia nitrogen concentration of 30mg / L, a total phosphorus concentration of 6mg / L, and a copper ion concentration of 5mg / L. The prepared immobilized *Scenedesmus tetracauda* algal balls (total mass 1000g) were added to the wastewater and placed in a constant temperature and light incubator. The incubator was set at 25℃, a light intensity of 3000 lux, and a pH of 7.5. Intermittent aeration was used (10min aeration, 5min rest), and the hydraulic retention time was controlled to be 12h.

[0052] 4. Test Results: After treatment, wastewater samples were filtered through a 0.45 μm filter membrane. The ammonia nitrogen concentration was determined using Nessler's reagent spectrophotometry, yielding a result of 2.4 mg / L, with a removal rate of 92%. The total phosphorus concentration was determined using ammonium molybdate spectrophotometry, yielding a result of 0.66 mg / L, with a removal rate of 89%. The copper ion concentration was determined using atomic absorption spectrophotometry, yielding a result of 0.8 mg / L, with a removal rate of 84%. The algal pellets were observed to be 95% intact, with no obvious damage or swelling. At day 0 of operation, the pores in the algal pellet cross-section were uniformly distributed, with a pore size of 30–50 μm and a pore connectivity rate ≥85%. Algal cells were uniformly embedded in the carrier framework, with no aggregation or leakage. After day 18 of operation, the pore size remained at 25–45 μm, with a connectivity rate ≥80%, no obvious blockage, intact algal cells, chloroplast fluorescence intensity retention ≥90%, and no carrier degradation or damage. Continuous use tests were conducted on the algal pellets. Under the same conditions, after 18 days of continuous treatment, the ammonia nitrogen removal rate remained above 85%, and the total phosphorus removal rate remained above 80%. The state of the prepared algal pellets (after 0 days of operation) and after 18 days of operation is shown below. Figure 2 As shown.

[0053] Example 2: Treatment of effluent by immobilized Scenedesmus tetrapoda under low temperature conditions This embodiment is basically the same as Embodiment 1, with the following differences: 1. Composite carrier configuration: 3.5% sodium alginate solution and 0.8% chitosan solution are mixed at a mass ratio of 7:1 (high concentration sodium alginate is suitable for low temperature environment).

[0054] 2. Preparation of algal balls: Same as in Example 1.

[0055] 3. Treatment conditions: Ammonia nitrogen 30mg / L, total phosphorus 6mg / L, copper ion 5mg / L in effluent, temperature 10℃, light intensity 3500 lux (enhanced light intensity to compensate for the effects of low temperature), hydraulic retention 16h.

[0056] 4. Results: Ammonia nitrogen removal rate was 80%, total phosphorus removal rate was 76%, copper ion removal rate was 75%, and algal ball integrity rate was 92%. At day 0 of operation, the pore size of the algal ball cross-section was 25-40 μm, and the pore connectivity was approximately 82%, indicating a flexible and strong carrier structure. After 15 days of operation, the pore size remained at 20-35 μm, the connectivity was approximately 78%, the algal cell morphology was intact, and there was no significant damage to photosynthetic-related structures. Even after 15 days of continuous use, the removal rate remained above 75%. The states of the prepared algal balls (at day 0 and after 15 days of operation) are shown below. Figure 3 As shown.

[0057] Example 3: Treatment of effluent by immobilized Scenedesmus tetrapoda under high ammonia nitrogen concentration This embodiment is basically the same as Embodiment 1, with the following differences: 1. Composite carrier configuration: 3.2% sodium alginate solution and 0.8% chitosan solution are mixed at a mass ratio of 8:1 (high concentration sodium alginate is suitable for high ammonia nitrogen wastewater).

[0058] 2. Algal ball preparation: *Scenedesmus tetracauda* algal solution (3×10⁻⁶) 7 The algae (each cell / mL) was mixed with the composite carrier at a volume ratio of 1:2 and added dropwise to a 3.5% CaCl2 solution to form 5 mm algal balls. The mixture was then pre-cultured in BG11 medium for 40 h.

[0059] 3. Treatment conditions: Ammonia nitrogen 50mg / L, total phosphorus 10mg / L, zinc ion 8mg / L in effluent, temperature 28℃, light intensity 3800 lux, hydraulic retention 20h (the retention time may be extended according to the control system adjustment standard).

[0060] 4. Results: Ammonia nitrogen removal rate was 86% (reduced to 7 mg / L), total phosphorus removal rate was 82% (reduced to 1.8 mg / L), zinc ion removal rate was 80% (reduced to 1.6 mg / L), and algal ball integrity rate was 93%. At day 0 of operation, the pore size of the algal ball cross-section was 35–55 μm, the pore connectivity was approximately 83%, and the algal cell density was high and uniformly distributed. After day 15 of operation, the carrier showed no swelling or collapse, the pores were not completely blocked by metabolic products, and the algal cell integrity was ≥92%. Even after 15 days of continuous use, the removal rate remained above 80%. The states of the prepared algal balls (day 0 of operation) and algal balls after 15 days of operation are shown below. Figure 4 As shown.

[0061] Comparative Example 1: The mass ratio of sodium alginate to chitosan exceeded the preferred range. This comparative example is basically the same as Example 1, with the following differences: 1. Composite carrier configuration: Prepare a 3% sodium alginate solution and a 0.8% chitosan solution according to the method in Example 1. Mix the two solutions at a mass ratio of 3:1 (i.e., mix 300g of sodium alginate solution with 100g of chitosan solution, i.e., the amount of sodium alginate used is relatively small) and stir evenly.

[0062] 2. The remaining steps are the same as in Example 1: During the preparation of the algal balls, the concentration of the *Scenedesmus tetraculus* algal solution, the volume ratio to the composite carrier, the concentration of the added calcium chloride solution, the diameter of the formed algal balls, and the pre-culture conditions were all the same as in Example 1. When treating aquaculture wastewater, the initial concentration of the wastewater, the amount of algal balls added, the temperature, the light intensity, the pH value, the aeration method, and the hydraulic retention time were also consistent with those in Example 1.

[0063] 3. Test Results: After treatment, the ammonia nitrogen concentration in the wastewater was 7.5 mg / L, with a removal rate of 75%; the total phosphorus concentration was 1.92 mg / L, with a removal rate of 68%; and the copper ion concentration was 1.8 mg / L, with a removal rate of 64%. At day 0 of operation, the algal pellets showed insufficient mechanical strength, with a breakage rate of 35%, and significant algal cell leakage. After 10 days of continuous use, due to extensive pellet breakage, the wastewater could no longer be stably treated, and the ammonia nitrogen removal rate dropped below 50%, and the total phosphorus removal rate dropped below 45%. The states of the prepared algal pellets (at day 0 and after 10 days of operation) are shown below. Figure 5 As shown.

[0064] Comparative Example 2: Sodium alginate concentration exceeds the preferred range This comparative example is basically the same as Example 1, with the following differences: 1. Composite carrier configuration: Accurately weigh 50g of sodium alginate and add it to 1000ml of deionized water. Stir in a 50℃ water bath for 2 hours to obtain a 5% sodium alginate solution. Prepare a 0.8% chitosan solution according to the method in Example 1. Mix the 5% sodium alginate solution and the 0.8% chitosan solution at a mass ratio of 7:1 (i.e., mix 700g of sodium alginate solution with 100g of chitosan solution, indicating that the amount of sodium alginate used is slightly higher) and stir until homogeneous.

[0065] 2. The remaining steps are the same as in Example 1: During the preparation of the algal balls, the concentration of the *Scenedesmus tetraculus* algal solution, the volume ratio of the algal balls to the composite carrier, the concentration of the calcium chloride solution, the diameter of the algal balls, and the pre-culture conditions were all the same as in Example 1. The conditions for treating the wastewater, such as the initial concentration of the wastewater, the amount of algal balls added, the temperature, the light intensity, the pH value, the aeration method, and the hydraulic retention time, were also consistent with those in Example 1.

[0066] 3. Test Results: After treatment, the ammonia nitrogen concentration in the wastewater was 10.5 mg / L, with a removal rate of 65%; the total phosphorus concentration was 2.46 mg / L, with a removal rate of 59%; and the copper ion concentration was 2.1 mg / L, with a removal rate of 58%. At day 0 of operation, the algal pellets were observed to be excessively hard. Microscopic examination of the cross-section revealed an overly dense structure with pore sizes <15 μm and pore connectivity <40%, indicating impaired material exchange among algal cells. After day 5, the algal cell chloroplasts shrank, fluorescence intensity decreased by approximately 30%, overall microalgal activity decreased by 30%, and the photosynthetic rate significantly decreased. The states of the prepared algal pellets (day 0 and day 5) are shown below. Figure 6 As shown.

[0067] Comparative Example 3: Chitosan concentration exceeded the range This comparative example is basically the same as Example 1, with the following differences: 1. Composite carrier preparation: Accurately weigh 30g of sodium alginate and add it to 1000ml of deionized water. Stir in a 50℃ water bath for 2 hours to obtain a 3% sodium alginate solution. Separately weigh 12g of chitosan and dissolve it in 1% acetic acid solution to prepare 1000ml of a 1.2% chitosan solution (0.5%-1% higher than the preferred range). Mix the 3% sodium alginate solution and the 1.2% chitosan solution at a mass ratio of 7:1 (i.e., mix 700g of sodium alginate solution with 100g of chitosan solution, indicating excessive chitosan). Stir until homogeneous.

[0068] 2. The remaining steps are the same as in Example 1: During the preparation of the algal balls, the concentration of the *Scenedesmus tetraculus* algal solution, the volume ratio to the composite carrier, the concentration of the calcium chloride solution, the diameter of the algal balls, and the pre-culture conditions are all the same as in Example 1. The conditions for treating the wastewater, such as the initial concentration of the wastewater, the amount of algal balls added, temperature, light, pH value, aeration method, and hydraulic retention time, are also consistent with those in Example 1.

[0069] 3. Test Results: After treatment, the ammonia nitrogen concentration in the wastewater was 12.6 mg / L, with a removal rate of 58%; the total phosphorus concentration was 2.88 mg / L, with a removal rate of 52%; and the copper ion concentration was 2.1 mg / L, with a removal rate of 58%. Observation of the algal balls revealed a rough surface and a relatively hard texture. At day 0, chitosan excessively filled the pores, with pore size <10 μm and connectivity <30%, indicating excessive carrier rigidity. After day 5, some algal cells deformed and ruptured, showing significant autolysis. Chloroplast fluorescence intensity decreased by approximately 40%, algal cell activity decreased by 40%, and the photosynthetic rate decreased by 35%. The states of the prepared algal balls (day 0 and day 5) are shown below. Figure 7 As shown.

[0070] Through the comparison of the above embodiments and comparative examples (the comparison results are detailed in Table 2), it can be clearly seen that the immobilized algal cells prepared by the present invention have significant advantages in treating aquaculture wastewater. They can efficiently remove pollutants such as heavy metal ions, ammonia nitrogen, and total organic carbon from the wastewater, providing an excellent method for treating aquaculture wastewater.

[0071] Table 2 Comparison of results from Examples 1-2 and Comparative Examples 1-3

[0072] The above descriptions are merely embodiments of the present invention, and common knowledge such as specific technical solutions and / or characteristics are not described in detail here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the technical solutions of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A high-efficiency purification system for aquaculture wastewater based on immobilized Scenedesmus tetrapoda, characterized in that: The system includes a primary sedimentation tank, an algae ball reaction tank, a collection tank, and a circulation control system connected in sequence. A water pump is installed on the connecting pipe between the primary sedimentation tank and the algae ball reaction tank. Immobilized Scenedesmus tetra-tailed algae balls are added inside the algae ball reaction tank, and an aeration device is installed at the bottom and a light module is installed at the top. The collection tank is equipped with a filter.

2. The high-efficiency purification system for aquaculture wastewater based on immobilized *Scenedesmus tetrapoda* as described in claim 1, characterized in that: The aeration device is a microporous aerator evenly distributed at the bottom of the pool. The microporous aerator is connected to an aeration pump, through which air is introduced to maintain the dissolved oxygen concentration in the water at 5~8 mg / L.

3. The high-efficiency purification system for aquaculture wastewater based on immobilized *Scenedesmus tetrapoda* as described in claim 2, characterized in that: An overflow pipe connects the algae ball reactor to the collection tank, and a return pipe connects the collection tank to the algae ball reactor.

4. A highly efficient method for purifying aquaculture wastewater based on the immobilization of *Scenedesmus tetraculus*, characterized in that: The system based on any one of claims 1 to 3 is completed, comprising the following steps: Step 1: Preparation of immobilized Scenedesmus tetracaudus algal spheres: Step 2, System Installation: Place the immobilized *Scenedesmus tetratail* algal balls into the algal ball reaction tank and install the above system; Step 3: Purifying Aquaculture Wastewater: The aquaculture wastewater first enters the primary sedimentation tank to remove large particulate impurities. The supernatant then enters the algae ball reaction tank to fully contact the immobilized algae balls. Under aeration and light conditions, the wastewater is purified. Part of the purified wastewater overflows into the collection tank. After recovering the proliferated algae, the circulation control system uses a water pump to return part of the purified wastewater to the algae ball reaction tank, adjusting the hydraulic retention time to complete the purification of the aquaculture wastewater.

5. The method for efficient purification of aquaculture wastewater based on immobilized Scenedesmus tetrapoda according to claim 4, characterized in that: In step one, the preparation of immobilized Scenedesmus tetratail algal spheres includes the following steps: S1. Algal culture: The Scenedesmus tetracauda was inoculated into BG11 medium and cultured until the logarithmic growth phase to obtain the Scenedesmus tetracauda algal solution. S2. Preparation of composite carrier: Mix 2-4 wt% sodium alginate solution and 0.5-1 wt% chitosan solution in a mass ratio of 5-10:1 to obtain composite carrier; S3. Formation of algal balls: Mix the algal solution of *Scenedesmus tetratail* with the composite carrier at a volume ratio of 1:2~4. Use a syringe to drop the mixture into a 2~4% CaCl2 solution to form gel balls with a diameter of 3~5 mm. After standing and solidifying for 2~4 h, rinse three times with deionized water and transfer to BG11 medium for pre-culture for 24~48 h to obtain immobilized algal balls.

6. The method for efficient purification of aquaculture wastewater based on the immobilization of *Scenedesmus tetrapoda* according to claim 4, characterized in that: In step two, the amount of the immobilized *Scenedesmus tetratail* algae balls added is 10-15% of the effective volume of the algae ball reaction tank.

7. The method for efficient purification of aquaculture wastewater based on the immobilization of *Scenedesmus tetrapoda* according to claim 4, characterized in that: In step three, the light intensity is 2000~4000 lux, and the daily light duration is 12~16h; the aeration intensity is controlled to have dissolved oxygen at 5~8mg / L.

8. The method for efficient purification of aquaculture wastewater based on immobilized *Scenedesmus tetrapoda* according to claim 4, characterized in that: In step three, the hydraulic residence time controlled by the circulation control system is 8~24h, the flow rate is 0.1~0.3m / s, and the temperature is 15~30℃.

9. A method for efficient purification of aquaculture wastewater based on immobilized *Scenedesmus tetrapoda* according to any one of claims 5 to 8, characterized in that: In step three, the continuous use cycle of the immobilized Scenedesmus tetra-tailed algal balls is 15-20 days. During regeneration, they can be reused after being soaked in BG11 medium for 24 hours.

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