Factory-pond coupled ecological breeding method and system based on nitrate cascade utilization

Abstract

The application relates to the technical field of aquaculture, and discloses a factory-pond coupling ecological breeding method and system based on nitrate gradient utilization, which directly inputs tail water without denitrification from a factory circulating water workshop into a first pond to culture microalgae and assimilate nitrate; based on the chlorophyll a concentration of the first pond, the flow rate of the second pond is controlled; in the second pond, filter-feeding animals and scraping-feeding benthic fish are cultured, physical disturbance is utilized to promote the release of sediment nutrients into liquid phase; the fluid sequentially enters subsequent ponds for multi-nutrient level breeding, and according to the ammonia nitrogen concentration feedback of the fifth pond, the reverse backflow to the second pond is controlled; and the tail water is finally purified through the sixth pond and reused. The application omits the traditional denitrification process, overcomes the technical prejudice of avoiding the breeding of scraping-feeding fish, constructs a double-parameter feedback regulation mechanism, realizes the dynamic matching of bait supply and demand, the balance of pollution load, and the synergy of pollution treatment and yield increase.

Description

Technical Field

[0001] This invention relates to the field of aquaculture technology, specifically to a factory-pond coupled ecological aquaculture method and system based on the tiered utilization of nitrates. Background Technology

[0002] Factory-style recirculating aquaculture systems (RAS) offer advantages such as high density, high yield, water conservation, and environmental friendliness, making them a crucial development direction in modern aquaculture. However, during daily operation, these systems generate large amounts of wastewater containing nutrients like nitrogen and phosphorus, with typically high nitrate concentrations. Direct discharge of this wastewater not only wastes water resources but also poses a risk of eutrophication to surrounding aquatic environments. Multitrophic level aquaculture (MLA) is an ecological farming model that systematically combines aquatic animals from different trophic levels. It utilizes the feeding and metabolic relationships between organisms to achieve multi-level utilization and internal circulation of nutrients. In recent years, combining factory-style RAS with multitrophic level pond systems, utilizing factory wastewater to cultivate feed organisms and then purifying it in series, has shown promising application potential.

[0003] However, existing coupled factory and pond aquaculture systems and methods still have many technical shortcomings. Existing high-nitrate wastewater typically requires pre-denitrification treatment before entering microalgae cultivation units. This not only increases carbon source consumption and operating costs but also fails to fully utilize the favorable conditions for microalgae growth at suitable nitrate nitrogen concentrations. There is a lack of technical solutions for directly using wastewater for microalgae cultivation. Simultaneously, to maintain water transparency and protect shellfish filter feeding, existing pond systems generally avoid introducing benthic omnivorous or scraping fish. This results in a long-term fixed state of nitrogen and phosphorus nutrients in the bottom sediment, preventing them from re-entering the water layer to participate in primary production, thus limiting the system's material cycling efficiency.

[0004] In addition, the operation of existing series systems mainly relies on fixed hydraulic retention time and water exchange rate, and has failed to establish a real-time feedback control mechanism based on water quality parameters such as chlorophyll a and ammonia nitrogen. When the stocking density or climate conditions change, the system is prone to problems such as imbalance between the supply and demand of feed organisms and mismatch between purification capacity and pollution load, making it difficult to ensure the stable operation of the overall system under complex working conditions. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a factory-pond coupled ecological aquaculture method and system based on nitrate cascade utilization. This solves the problems of high nitrate wastewater being difficult to directly utilize as a resource, low efficiency of nutrient recycling in bottom sediment, and instability in system operation due to the lack of dynamic water quality feedback control in existing factory-pond coupled aquaculture systems.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a factory-pond coupled ecological aquaculture method and system based on nitrate cascade utilization, comprising the following steps: The wastewater from the factory-style recirculating aquaculture system is fed into the first pond; The concentration of chlorophyll a in the fluid at the outlet of the first pond is monitored online. Based on the comparison between the concentration of chlorophyll a in the fluid and the set chlorophyll a concentration threshold, the flow rate of the fluid discharged from the first pond into the second pond is controlled. In the second pond, filter-feeding aquatic animals and scraping benthic fish were polycultured; The fluid discharged from the second pond is sequentially introduced into the third, fourth and fifth ponds connected in series for multi-trophic level aquaculture, and the ammonia nitrogen concentration at the outlet of the fifth pond is monitored online. When the ammonia nitrogen concentration is greater than or equal to the set ammonia nitrogen concentration reflux threshold, a portion of the fluid at the outlet of the fifth pond is reversed and refluxed back to the second pond. The non-returned fluid discharged from the fifth pond is introduced into the sixth pond, and a portion of the fluid discharged from the sixth pond is reused in the factory-style recirculating aquaculture workshop.

[0007] In one embodiment, the total volume of the first pond, the second pond, the third to fifth ponds, and the effective volume of the sixth pond are set in a fixed ratio. This fixed ratio is constrained by the reciprocal of the target chlorophyll a concentration at the outlet of the first pond, the assimilation conversion coefficient from prey organisms to filter feeders, the average gravity settling velocity of suspended particulate matter, the average daily filtration flux per unit body weight of filter feeders, and the daily drainage volume of the factory-style recirculating aquaculture system.

[0008] In one embodiment, microalgae are used in the first pond to absorb and assimilate nitrate nitrogen in the tailwater; the nitrate nitrogen concentration of the tailwater input into the first pond is controlled within the range of 5 mg / L to 50 mg / L; the volume of tailwater discharged into the first pond daily accounts for 30% to 50% of the total aquaculture water volume in the factory-style recirculating aquaculture workshop; and the hydraulic residence time of the fluid in the first pond is maintained at 18 h to 48 h by adjusting the flow rate.

[0009] Specifically, the innovation of this application lies in fully understanding the accumulation characteristics of nitrate in a factory-scale circulating water system. By directly introducing untreated effluent into the first pond, the nitrate nitrogen concentration is cleverly controlled within the range of 5 mg / L to 50 mg / L, which is easily absorbed and utilized by microalgae. This not only saves the high cost of effluent pretreatment but also provides sufficient natural food for microalgae in the subsequent ecological chain.

[0010] Furthermore, this invention sets the daily discharge of wastewater into the first pond to account for 30% to 50% of the total water volume in the workshop. This high proportion of water replacement not only ensures the nutrient supply for microalgae in the pond but also significantly reduces the load on the biofilters within the front-end recirculating aquaculture system. In conventional systems, biofilters typically occupy 20%-30% of the workshop area, while with this invention, the area of ​​biofilters within the factory workshop can be reduced to 10%-15%, freeing up more physical space for raising high-value economic fish and improving the economic efficiency per unit area of ​​the workshop.

[0011] In one embodiment, the set chlorophyll a concentration threshold includes a first threshold preset to 25 μg / L to 35 μg / L and a second threshold preset to 15 μg / L to 25 μg / L; when the measured chlorophyll a concentration of the fluid is greater than the first threshold, the opening of the connecting pipe from the first pond to the second pond is increased to discharge fluid into the second pond; when the measured chlorophyll a concentration of the fluid is less than the second threshold, the connecting pipe is closed to retain the fluid.

[0012] Specifically, the advantage of this application lies in the fact that, compared to the low efficiency of previous microalgae treatment of ammonia nitrogen / nitrite, this system directly utilizes nitrate, resulting in higher absorption efficiency. Meanwhile, traditional methods often aim to completely deplete nitrogen at the source, leading to algal blooms or algal collapses in subsequent ponds due to a lack of stable nitrogen sources. This invention, through dynamic control of the chlorophyll a concentration threshold, can stably introduce healthy microalgae solution with appropriate amounts of nitrate (N fertilizer) into subsequent ponds, ensuring that beneficial microalgae always occupy advantageous ecological niches in the subsequent ponds, thus solving the technical problem of easily uncontrolled algal community in the system.

[0013] In one embodiment, the physical disturbance of the bottom sediment of the second pond by the scraping benthic fish promotes the conversion of sedimentary particles into dissolved nutrients, which are then released into the liquid phase. The scraping benthic fish is *Siniperca scaberra*, and the stocking density of *Siniperca scaberra* is limited to 0.04 fish / m³. 2 Up to 0.06 tails / m 2 The filter-feeding aquatic animals comprised a density of 0.1 individuals / m³. 2 Up to 0.3 tails / m 2 The density of the silver carp is 0.05 fish / m³. 2 Up to 0.15 tails / m 2 Bighead carp and a density of 1 fish / m³ 2 Up to 3 tails / m 2 The triangular sail mussel.

[0014] Specifically, this invention precisely controls the stocking density of scraping benthic fish (Gnaphalium gracilis) in the second pond. Its deeper objective is to promote the release of nutrients from the bottom sediment into the liquid phase through moderate physical disturbance, thereby preventing anaerobic deterioration at the bottom of the pond and ensuring its health. At the same time, the released nutrients, combined with the continuously flowing highly active microalgae from the first pond, enable beneficial microalgae to quickly and continuously occupy the ecological niche of the pond in advance, forming an absolute advantage in nutrient competition, thereby inhibiting and preventing the outbreak of harmful algae such as cyanobacteria, and maintaining the long-term stability of the aquatic ecosystem.

[0015] In one embodiment, carnivorous fish are stocked in the third pond, shrimp are stocked in the fourth pond, and omnivorous fish are stocked in the fifth pond; the scraping benthic fish are polycultured in the third, fourth, and fifth ponds at a ratio of 3% to 8% of the total number of individuals of the main cultured species.

[0016] Specifically, this invention incorporates high-value-added species such as carnivorous fish and shrimp into the multi-trophic level conversion units of the third to fifth ponds. This design not only makes full use of surplus feed and space, improving the system's space utilization efficiency, but also significantly enhances the overall economic benefits of the plant-pond coupling system, truly achieving a win-win situation for both ecological pollution control and economic revenue generation.

[0017] After the fluid passes through the third, fourth, and fifth ponds for biological feeding and absorption, the nitrate load is significantly reduced. Actual measurements show that by the time the water reaches the outlet of the fifth pond, the nitrate concentration has dropped dramatically to below 0.5 mg / L, greatly easing the pressure on the final sixth pond for deep purification.

[0018] In one embodiment, the preset ammonia nitrogen concentration reflux threshold is 1.5 mg / L to 2.5 mg / L, and the preset ammonia nitrogen concentration stop reflux threshold is 0.05 mg / L to 0.2 mg / L; when the ammonia nitrogen concentration reaches the preset ammonia nitrogen concentration reflux threshold, 20% to 40% of the volume of fluid from the outlet of the fifth pond is extracted, impurities are intercepted by a 50 μm precision sand filter, and then the fluid is pumped back to the second pond; when the ammonia nitrogen concentration is lower than the ammonia nitrogen concentration stop reflux threshold, the reflux operation is stopped.

[0019] In one embodiment, the bottom of the first pond is progressively deepened along the direction of water flow, wherein the water depth at the inlet is set to 0.6m to 0.8m and the water depth at the outlet is set to 0.8m to 1.2m; the top of the first pond is enclosed and covered with a greenhouse film.

[0020] In one embodiment, the sixth pond is planted with submerged plants for intercepting and purifying the fluid, and the submerged plants occupy 30% of the total surface area of ​​the sixth pond in the vertical projection plane; the hydraulic residence time of the fluid in the sixth pond is controlled to be 24h to 72h; the volume of fluid pumped back to the factory-style recirculating aquaculture workshop accounts for 10% to 30% of the total volume of fluid discharged into the sixth pond.

[0021] The second aspect of the present invention provides a factory-pond coupled ecological aquaculture system based on nitrate cascade utilization, which is applied to the factory-pond coupled ecological aquaculture method based on nitrate cascade utilization described in the first aspect, including a factory-scale recirculating aquaculture workshop, and a first pond, a second pond, a third pond, a fourth pond, a fifth pond and a sixth pond connected in series. The connecting pipeline between the first pond and the second pond is equipped with an online chlorophyll a sensor and an electric regulating valve; the outlet of the fifth pond is equipped with an ammonia nitrogen sensor. The fifth pond is connected to the second pond by a return pipeline for reverse fluid transport, and the return pipeline is connected in series with a return pump and a sand filter. The sixth pond is equipped with a first reuse branch and a second discharge branch at its output end. The first reuse branch is connected to the factory-style recirculating aquaculture workshop via a return water pump. It also includes a programmable logic controller, which is electrically connected to the online chlorophyll a sensor, the electric regulating valve, the ammonia nitrogen sensor and the reflux pump.

[0022] Compared with existing technologies, this invention has the following technical advantages and innovative principles: This invention achieves the tiered utilization and in-situ assimilation of nutrients such as nitrates by constructing a physical and ecological coupling model of a factory-style workshop and multi-stage ponds. In the first pond, microalgae absorb and assimilate nitrate nitrogen from aquaculture wastewater through photosynthesis, converting inorganic nutrients into microalgal biomass. The system uses chlorophyll a concentration as a feedforward control parameter to regulate the input rate of microalgal biomass to the second pond, ensuring a stable natural food source for filter-feeding aquatic animals.

[0023] In the second pond, filter-feeding aquatic animals consume microalgae, converting plant protein into animal protein and transforming primary productivity into secondary productivity. Simultaneously, scraping benthic fish are introduced, utilizing their habit of stirring up the bottom sediment to suspend particulate organic matter deposited at the bottom and promote its conversion into dissolved nutrients, which then re-enter the aquatic ecological cycle, reducing the risk of eutrophication of the bottom sediment.

[0024] The third to fifth ponds constitute a multi-trophic-level aquaculture zone, sequentially utilizing residual feed and organic matter in the water. The system incorporates an ammonia nitrogen concentration feedback mechanism at the outlet of the fifth pond. When the ammonia nitrogen concentration reaches a set threshold, a recirculation process is initiated, transferring the ammonia-rich water back to the second pond, where it re-enters the upstream ecological food web for assimilation and degradation, preventing high concentrations of ammonia nitrogen from entering subsequent purification units or causing water quality deterioration. The sixth pond utilizes submerged plants for deep purification and suspended solids retention, meeting the reuse requirements of the recirculating aquaculture system and establishing a complete water resource closed loop. This system transforms waste into biological resources, improving nitrogen utilization efficiency and reducing the overall total pollution emissions.

[0025] This invention provides a factory-pond coupled ecological aquaculture method and system based on the tiered utilization of nitrates. It has the following beneficial effects: 1. This invention directly inputs the untreated tailwater from the factory-scale recirculating aquaculture workshop into the first pond, utilizing microalgae to absorb and assimilate the nitrate nitrogen in the tailwater, eliminating the need for the traditional pre-denitrification process. This directly converts nitrogen pollutants in the water into high-value natural feed for subsequent aquaculture, reducing not only the overall energy consumption of the system but also the comprehensive feed conversion ratio, achieving synergistic effects of pollution control and increased production in aquaculture as well as nutrient removal.

[0026] 2. This invention involves polyculturing filter-feeding aquatic animals and scraping benthic fish at a specific density in a second pond. The physical disturbance of the bottom sediment by the scraping benthic fish promotes the conversion of sedimentary particles into dissolved nutrients and their release into the liquid phase. This overcomes the technical bias in the field that polyculturing scraping fish in filter-feeding ponds can easily lead to water quality deterioration. While ensuring the survival rate of filter-feeding organisms, it achieves the remineralization of sediment nutrients, thereby improving the material cycling efficiency of the system and the comprehensive yield per unit water body.

[0027] 3. This invention constructs a dual-parameter real-time feedback control mechanism based on chlorophyll a and ammonia nitrogen concentration. By monitoring the chlorophyll a concentration in the first pond, the flow rate of microalgae entering the second pond is controlled. Based on the ammonia nitrogen concentration in the fifth pond, it determines whether to initiate reverse flow to the second pond. This changes the traditional operation mode with a fixed water exchange rate, realizes the dynamic matching of natural feed supply and animal demand, and the real-time balance between system purification capacity and pollution load. This ensures the stable operation of the multi-stage series aquaculture system under different operating conditions and ultimately meets environmental emission requirements. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the system architecture of the present invention; Figure 2 This is a schematic diagram of the method flow of the present invention; Figure 3 This is a schematic diagram of the system volume allocation logic of the present invention; Figure 4 This is a schematic diagram of the feedforward control logic based on dynamic matching of chlorophyll a concentration according to the present invention. Figure 5 This is a schematic diagram of the multi-trophic level aquaculture fluid distribution and ammonia nitrogen feedback reflux logic flow of the present invention. Detailed Implementation

[0029] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] See attached document Figure 1 The present invention provides a factory-pond coupled ecological aquaculture system based on nitrate cascade utilization, which may include: a factory-scale recirculating aquaculture workshop, and a first pond, a second pond, a third pond, a fourth pond, a fifth pond and a sixth pond connected in series.

[0031] The drainage outlet of the factory-style recirculating aquaculture system is directly connected to the inlet of the first pond via a main pipeline. The bottom of the first pond is constructed of cement. The top space of the first pond is equipped with a large membrane structure covering the entire water surface. The bottom of the first pond features a progressively deeper design along the direction of water flow from the inlet to the outlet, with the water depth at the inlet set at 0.6m to 0.8m and the water depth at the outlet set at 0.8m to 1.2m. The length-to-width ratio of the first pond is set at 4 to 6:1.

[0032] A physical environment regulation component is installed inside the space covered by the greenhouse film. Temperature sensors, automatic roller shutter ventilation fans, shading nets, and LED supplemental lighting components are fixedly installed on the inner wall and within the space of the greenhouse film. The temperature sensors, automatic roller shutter ventilation fans, shading nets, and LED supplemental lighting components are electrically connected to the control unit via signal lines and control lines, respectively.

[0033] A horizontal connecting pipe is laid between the outlet of the first pond and the inlet of the second pond. Sensing and actuation devices are fixed to the outside and inside of this connecting pipe. Specifically, an online chlorophyll a sensor, an electric regulating valve, and a flow meter are sequentially installed on the connecting pipe. The probe of the online chlorophyll a sensor penetrates the pipe wall and is exposed to the fluid inside the pipe.

[0034] The outlet of the second pond is connected to the inlet of the third pond via a water supply pipe. The third, fourth, and fifth ponds are arranged in series according to the direction of water flow via overflow pipes or connecting pipes. The horizontal projection shape of the third, fourth, and fifth ponds is independently constructed as a square or an approximately square with an aspect ratio of 1 to 1.2:1. The physical vertical depth of the third, fourth, and fifth ponds is constructed to be 1.8m to 2.2m.

[0035] The bottom surfaces of the third, fourth, and fifth ponds are each covered with a network of microporous aeration pipes. A central drain outlet is located at the center of the bottom surface of each of the three ponds. Flow regulating valves are independently connected in series on each section of the inlet pipe connecting the third, fourth, and fifth ponds.

[0036] A water quality monitoring sensor array is installed at the outlet area of ​​the fifth pond. This sensor array is fixedly installed underwater and includes ammonia nitrogen, nitrite, and dissolved oxygen sensors.

[0037] The system includes a return pipeline for reverse liquid transport. The inlet of the return pipeline is connected to the outlet of the fifth pond, and the outlet is connected to the inlet of the second pond. A return pump, a sand filter, and an electric return valve are connected in series along the return pipeline. The sand filter is located downstream of the return pump's output.

[0038] The gravity overflow outlet of the fifth pond is connected to the inlet of the sixth pond via the main drainage pipe. The outflow structure of the sixth pond includes a first reuse branch and a second discharge branch. One end of the first reuse branch connects to the sixth pond, and the other end connects to the water storage inlet of the recirculating aquaculture system. The second discharge branch is connected to an external drainage ditch.

[0039] The system is equipped with a programmable logic controller (PLC). This PLC establishes bidirectional communication connections with the online chlorophyll a sensor, electric regulating valve, and flow meter on the aforementioned connecting pipelines. The PLC also establishes bidirectional communication connections with the water quality monitoring sensor group at the end of the fifth pond, the return pump on the return pipeline, and the electric return valve. This allows the PLC to receive digital signals from the sensors and output electrical drive signals to the electric valves and pumps.

[0040] See attached document Figure 3 In constructing the system of this invention, to match the high-frequency drainage of the factory workshop with the biological metabolic rhythm of the ponds, the effective volumes of the first to sixth ponds are set and fixed according to a specific mathematical relationship. The physical ratio of the volumes of the first pond, the second pond, the total volume of the main aquaculture area, and the sixth pond satisfies the following formula constraint: ; In the formula, , , , , , These represent the effective volumes of the first, second, third, fourth, fifth, and sixth ponds, respectively, in cubic meters (m³). 3 ; The first configuration coefficient corresponds to the specific growth rate of microalgae under environmental conditions with nitrate nitrogen concentrations of 5 mg / L to 50 mg / L, and the value is set to 0.15 to 0.25; The target chlorophyll a concentration at the outlet of the first pond is set in μg / L. The second configuration coefficient represents the assimilation conversion coefficient from prey organisms to filter feeders, and its value is set to 0.10 to 0.20. The average daily output mass of food organisms generated in the first pond to the downstream end, expressed in kg / d; The daily average filtration flux per unit body weight of filter-feeding organisms such as silver carp, bighead carp, and triangular sail mussels placed in the second pond is expressed in m³. 3 / (kg·d); The third configuration coefficient represents the factor that hinders and compensates for the impact of outdoor wind and wave disturbances on gravitational settlement, with a value set between 3.5 and 4.8. This refers to the daily drainage volume of the front-end recirculating aquaculture system, expressed in m³. 3 / d; The design water surface area of ​​the sixth pond is in m². 2 ; The average gravity settling velocity of suspended particles with a diameter greater than 50 μm in the sixth pond at the end of the system is given in m / d.

[0041] First pond volume The configuration benchmark is affected by the target chlorophyll a concentration. The mathematical reciprocal constraint applies. Under the physical premise of a constant system influent flow rate, when the operator needs to obtain a high concentration of algal solution output, the required proportion of the first pond volume decreases according to law, thereby reducing the water dilution factor.

[0042] Second pond volume The physical space setting is based on the computational logic of upstream and downstream material balance. The average daily output mass in the numerator of the formula... This determines the upper limit of the required storage space, and the filtration flux in the denominator. This determines the safe flow velocity of water through the second pond. Configuration coefficient. The introduction of this method aligns the physical residence time of the suspended feed transported from the first pond in the second pond with the intestinal filtration and emptying cycle of the organisms, thus setting a lower limit for the hydraulic residence time.

[0043] The combined volume of the third, fourth, and fifth ponds With the sixth pond volume The distribution between them is jointly determined by the input load at the front end and the limit of the surface load at the end. Daily drainage volume of the front-end workshop. The increase requires a linear proportional increase in the volume of the main aquaculture area. (Formula denominator term) This constitutes the theoretical surface loading flux of the settling tank. The disturbance compensation factor... Integrating this into the proportion term, the volume of the main aquaculture area is used to slow down the overall transport rate of water flow, so that the fluid velocity entering the sixth pond at the end is reduced to the average gravitational settling velocity of suspended particulate matter. Within the defined threshold for settling retention velocity.

[0044] See attached document Figure 2 This invention provides a factory-pond coupled ecological aquaculture method based on nitrate cascade utilization. The effluent from the factory-scale recirculating aquaculture system (RAS) is continuously or intermittently transported to the inlet of a first pond via a main drainage pipeline. The daily volume of effluent entering the first pond is limited to 30% to 50% of the total volume of the aquaculture water in the RAS. The nitrate nitrogen concentration in the effluent flowing into the first pond is between 5 mg / L and 50 mg / L. During its physical transport path from the RAS to the first pond, the effluent undergoes no denitrification treatment, and the system pipeline does not include anaerobic denitrification filters or dosing devices for adding organic carbon sources.

[0045] The nitrate-containing fluid entering the first pond serves directly as a nutrient substrate for microalgal cells. When microalgal cells are exposed to nitrate nitrogen concentrations of 15 mg / L to 30 mg / L, the catalytic activity of nitrate transport proteins and nitrate reductases within the cells reaches physiological limits. Microalgae directly transport nitrate into the cytoplasm via transmembrane uptake, where, driven by adenosine triphosphate (ATP) produced during photosynthesis, inorganic nitrate is reduced and assimilated into amino acids and protein molecules. This cellular-scale nitrogen assimilation process converts dissolved nitrogen in the tailwater into particulate microalgal biomass.

[0046] The outflow rate of the first pond is controlled and regulated to maintain the hydraulic retention time of the liquid within the pond between 18 and 48 hours. This hydraulic retention time parameter of 18 to 48 hours is synchronized with the cell division and doubling cycle of microalgal cells during the logarithmic growth phase. By controlling the daily water exchange volume parameter of 30% to 50%, the first pond can continuously receive the newly generated nitrate load discharged from the upstream workshop and simultaneously discharge mature microalgae and zooplankton downstream, thereby limiting the in-situ algal community from entering the decline phase caused by nutrient depletion.

[0047] The gradual physical structure at the bottom of the first pond causes its water depth to change along the direction of water flow. In the inlet area, the vertical depth of the water is controlled within the range of 0.6m to 0.8m. This water depth of 0.6m to 0.8m reduces the scattering and absorption cross-section of suspended particles in the water on incident photons, resulting in photosynthetically active radiation transmitted to the bottom of the pond that is higher than the light compensation point parameter of the microalgae, supporting the initial concentration of microalgal cells to perform photosynthetic carbon fixation.

[0048] In the outlet area of ​​the first pond, the vertical depth of the water increases from 0.8m to 1.2m with the transition of topography. The increased longitudinal water volume provides underwater distribution space for zooplankton such as cladocerans and copepods that feed on microalgae. The greenhouse film covering structure forms a closed cavity above the first pond, preventing external rainfall from directly mixing into the pond water and maintaining the physical boundary stability of the salinity, temperature, and pH parameters of the fluid inside the first pond.

[0049] See attached document Figure 4 On the connecting pipe between the outlet of the first pond and the inlet of the second pond, an online chlorophyll a sensor emits excitation light of a fixed wavelength into the fluid within the pipe. This light detects the endogenous fluorescence signal emitted by suspended microalgae in the fluid and converts it into a digital electrical signal characterizing the chlorophyll a concentration in the water. A programmable logic controller (PLC) receives this digital electrical signal at a set sampling period, compares it with first and second threshold data recorded in its internal registers, and outputs a current command signal to the actuator of the electric regulating valve based on the calculation result.

[0050] The first threshold parameter set in the programmable logic controller (PLC) is located within the range of 25 μg / L to 35 μg / L. When the controller's comparison logic determines that the chlorophyll a concentration received online is greater than the set first threshold, the controller outputs an opening signal or an increase in the opening area signal to the electric regulating valve via a hard-wired connection, driving the flow rate through the connecting pipe to increase. The concentration boundary of 25 μg / L to 35 μg / L corresponds to the physical critical point at which the density of the microalgal population in the first pond generates a self-shading effect of the population photons. The physical action of opening or increasing the water outlet channel discharges the high-concentration algal solution into the second pond behind, forcibly reducing the particulate matter density per unit volume of water in the first pond. This density reduction changes the light transmittance distribution within the water column of the first pond, causing the photosynthetically active radiation intensity obtained by the deeper water layers to be higher than the light compensation point value, maintaining the photosynthetic oxygen production rate of the deeper algal cells.

[0051] The second threshold parameter set in the programmable logic controller (PLC) is within the range of 15 μg / L to 25 μg / L. When the controller's comparison logic determines that the chlorophyll a concentration received online is lower than the set second threshold, the controller outputs a signal to the electric regulating valve to reduce the opening area until the pipeline channel is closed. The concentration boundary of 15 μg / L to 25 μg / L corresponds to the extreme value of basal mother cell density required to maintain the logarithmic exponential growth of the microalgal population in the first pond. Closing or reducing the opening of the outlet valve blocks the leakage of the liquid phase from the first pond. The fluid retention state causes the remaining microalgal mother cells in the first pond to continue absorbing the nitrate nitrogen source continuously input from the workshop inlet, performing the transverse cell division cycle, until the chlorophyll a concentration data accumulated in the water exceeds the first threshold again in subsequent sampling cycles, triggering a new valve opening response.

[0052] Individual organisms targeting different water niches were introduced into the second pond, which receives water from the first pond. Filter-feeding fish were deployed in the upper and middle fluid layers of the second pond, with a specific stocking density of 0.1–0.3 silver carp per m³. 2 Bighead carp: 0.05–0.15 fish / m 2 Filter-feeding mollusks were deployed on the bottom surface of the second pond, with the stocking density of the triangular sail mussel set at 1–3 individuals / m². 2 In the same pond bottom area, the scraping fish *Gnaphalium affine* was simultaneously released, with the stocking density of this species limited to 0.04–0.06 individuals / m². 2 Within the spatially constrained interval.

[0053] In existing conventional physical configuration schemes, to avoid secondary suspension of particles settled by gravity, benthic fish that disturb the bottom sediment are usually not deployed in the same confined water body where filter-feeding shellfish are present. This system, by setting a specific distribution density for the *Siniperca scabra*, utilizes its unique lower jaw structure to perform scraping action on the surface attachments of the bottom sediment. The *Siniperca scabra*'s lower jaw makes physical contact with the pond bottom sediment interface, generating surface shear force that forces organic debris and sedimentary algal cells attached to the bottom layer to detach from the solid substrate and re-enter the lower fluid phase. This physical scraping process disrupts the static boundary layer constraint at the bottom sediment-water interface, allowing particulate nitrogen and phosphorus sealed within the bottom sediment pores to be converted into dissolved nutrients that enter the overlying liquid phase through fluid diffusion. These nutrients are then transported upwards with water convection for transmembrane ion absorption during photosynthesis by surface plankton.

[0054] 0.04~0.06 tails / m 2 The stocking density parameter constitutes the critical condition for maintaining the biophysical synergy. When the stocking density of *Siniperca scabra* is less than 0.03 individuals / m², the stocking density becomes critical. 2 At this time, the drag force generated by individual fish swimming and feeding cannot completely cover the total projected area of ​​the bottom surface of the second pond. Under this condition, the pore water flow in the bottom sediment of the undisturbed area stagnates, and the mineralized products of organic debris are trapped inside the solid sediment, cutting off the material transport path for bottom nutrients to be released back into the liquid water column.

[0055] When the stocking density of *Siniperca scabra* is higher than 0.08 individuals / m², 2 At this time, the cumulative frequency of mechanical contact between the fish and the bottom surface exceeded the critical value of the physical cohesion between the bottom sediment particles. The intense, continuous bottom-touching caused a large number of inorganic sediment particles smaller than 50 μm and undecomposed excrement to escape gravity and be entrained into the water column on a large scale, resulting in a sharp increase in the concentration of non-nutritive suspended particulate matter in the fluid. When this fluid flowed through the inlet tube of the triangular sail mussel on the same plane, the inflow flux of inorganic suspended particles exceeded the physical rejection and cleaning limit of the cilia on the gill filaments of the shellfish. A large number of inorganic sediment particles gradually became physically clogged in the tiny pores of the gills, cutting off the diffusion channels of dissolved oxygen and the filtering path of edible microalgae, causing respiratory failure and physical death in the bivalve individuals. (0.04–0.06 fish / m³) 2 The density parameter setting ensures that the amount of bottom mud resuspended due to scraping is kept within the physical flux limit that the physiological rejection mechanism of filter-feeding shellfish can withstand.

[0056] See attached document Figure 5The fluid discharged from the second pond is sequentially introduced into the third, fourth, and fifth ponds according to the gravity gradient. Along this fluid transfer path, the three ponds house different predatory species. The third pond contains carnivorous fish, the fourth pond contains shrimp, and the fifth pond contains omnivorous fish. Within the third, fourth, and fifth ponds, scraping benthic fish are introduced at a ratio of 3% to 8% of the total number of individuals of the main cultured species. These benthic fish physically contact the pond bottom with their snouts, ingesting the settled solid excrement and unconsumed pelleted feed from the upper cultured species. This jaw-feeding action disrupts the physical stratification of solid particles on the bottom sediment surface, reducing the vertical thickness of the solid sediment and limiting the reaction flux of dissolved ammonia nitrogen and nitrite released from organic matter under anaerobic conditions through bacterial fermentation.

[0057] At the point where the fluid exits the outlet interface of the fifth pond, a fixed ammonia nitrogen sensor electrochemically collects the total concentration of molecular ammonia and ionic ammonium within the flowing water column. The sensor's output transmits a digital signal to the input port of a programmable logic controller (PLC) via a transmission cable. The controller's internal register has a preset third threshold for ammonia nitrogen concentration of 1.5–2.5 mg / L. When the online measured ammonia nitrogen concentration reaches or exceeds this third threshold, the controller outputs an execution level, activating the reflux pump connected between the fifth and second ponds and opening the electric reflux valve.

[0058] The total ammonia nitrogen concentration parameter of 1.5–2.5 mg / L corresponds to the critical triggering condition in which free ammonia molecules penetrate the gill epithelial cell membrane of fish and shrimp through gradient diffusion and cause physical edema damage to the tissues under specific pH and temperature conditions. When the feed load at the front end of the aquaculture increases, causing the ammonia nitrogen concentration in the fluid to exceed this critical point, the return pump extracts 20%–40% of the volume of ammonia-rich fluid from the outlet of the fifth pond. This fluid passes through a sand filter with a filtration precision of 50 μm in the return pipeline, where large particles of solid impurities are physically intercepted, and then it is pressurized and sprayed back to the inlet of the second pond. The returned liquid physically mixes with the fresh liquid phase output from the first pond inside the second pond. The large volume of liquid phase carried by the second pond acts as a buffer medium, producing a spatial volume dilution effect on the high-concentration ammonia nitrogen water, and the free ammonia carried in the fluid is absorbed and consumed again through the transmembrane transport mechanism of suspended microalgae and attached microbial communities.

[0059] The programmable logic controller (PLC) internal register synchronously presets a fourth threshold for ammonia nitrogen concentration as 0.5–1.0 mg / L. During the reflux operation, the ammonia nitrogen sensor continuously performs online electrochemical sampling periodically. When the system detects that the ammonia nitrogen concentration in the outflowing fluid has decreased below the fourth threshold, the controller cuts off the power supply to the reflux pump motor and simultaneously controls the actuator of the electric reflux valve to rotate to the closed angle. The concentration range of 0.5–1.0 mg / L indicates that the water quality inside the system has moved away from the state of rapid toxic accumulation. Closing the reflux pipeline valve interrupts the forced reverse hydraulic transport process of the water, and the fluid inside the fifth pond resumes its forward flow along the gravity pipeline network to enter the next treatment stage.

[0060] The fluid that did not enter the return branch at the outlet of the fifth pond flows into the inlet of the sixth pond along the main water supply pipeline connected in series, using its gravitational potential energy.

[0061] Large submerged plants were pre-fixed and planted within the bottom sedimentary matrix of the sixth pond. The area occupied by the stems and leaves of these submerged plants in the vertical projection plane was pre-set to account for 30% of the total surface area of ​​the sixth pond. Filter-feeding bivalves were placed on the bottom surface of the sixth pond and at the attachment points of some of the hard stems of the submerged plants. The spatial distribution density of bivalves in this pond was limited to 1–3 individuals / m². 2 .

[0062] As the fluid entering the sixth pond moves towards the terminal outlet, it penetrates the three-dimensional physical barrier formed by the submerged plant community. The microscopic textures on the surface of the plant stems and leaves generate contact friction on the water flow, causing a decrease in velocity and loss of kinetic energy in the local flow field. Due to the reduced drag from the water flow, the tiny suspended particles carried within the fluid have a greater settling velocity in the direction of gravity vector than their horizontal drift velocity as they move forward with the water flow, thus separating from the liquid phase column and settling to the bottom of the pond.

[0063] In the simultaneous fluid reaction, the microbial biofilm community attached to the surface of submerged plants and the plant's own epidermal cells, through the ion channel structures on their cell membranes, perform passive diffusion and active transport absorption of low concentrations of nitrate molecules and free phosphate ions in the flowing fluid under a concentration gradient. 1–3 plants / m² are arranged in the same flow field. 2 Filter-feeding bivalve mollusks draw in water through the rhythmic contraction of their inlet and outlet ducts, using the cilia on their gills to physically intercept tiny suspended impurities that fail to settle in the fluid, as well as algal cell remnants that break down due to the decrease in nutrients.

[0064] By adjusting the opening of the inlet valve and the outlet weir, the hydraulic residence time parameter of the fluid in the sixth pond is physically anchored within a time span of 24–72 hours. This 24–72 hour timeframe corresponds to the gravity settling cycle of suspended particles in the presence of physical resistance from aquatic plants, and the physical transport time for low-concentration nutrient molecules to cross the water boundary layer and reach the plant cell absorption sites. The fluid flowing through this residence time to the final discharge outlet exhibits phase characteristics showing a suspended solids concentration of less than 80 mg / L and a nitrate nitrogen concentration of less than 5 mg / L.

[0065] In the section downstream of the discharge outlet of the sixth pond, the pipeline is split into two parallel water flow branches via a physical tee joint. The first branch of the tee joint connects to the inlet of the return water pump. The return water pump motor generates negative pressure, drawing 10%–30% of the total volume of fluid flowing into the discharge outlet into the main return water pipe. This main return water pipe extends in the opposite direction and connects to the inlet storage container of the factory-style recirculating aquaculture system at the front end of the system. This pumped-in fluid mixes with the source water in the storage container, directly compensating for the fluid mass loss caused by surface evaporation and physical wastewater discharge operations in the aquaculture system. The second branch of the tee joint receives the remaining liquid volume that has not been pumped out and discharges it into the external receiving water area through an open gravity flow channel.

[0066] Specific application examples: To verify the above-mentioned technical solution based on the principle of system volume optimization and various control mechanisms, this embodiment provides an engineering application example with complete physical landing parameters.

[0067] This embodiment is based on a largemouth bass factory-style recirculating aquaculture system (RAS) facility. The total aquaculture water volume of the RAS facility at this facility is 250 m³. 3 The system is set to a daily water exchange rate of 40%, which is the daily drainage volume of the workshop. Fixed at 100m 3 / d. The measured initial nitrate nitrogen concentration in the effluent discharged from the workshop was 35 mg / L.

[0068] The system allocates pond volumes at each level based on a physical configuration formula. A first configuration coefficient is set. The target chlorophyll a concentration is 0.2. 30 μg / L; set the second configuration coefficient. The output mass of the first pond is 0.15. The filtration flux is 4 kg / d. 6m 3 / (kg·d); Set the third configuration coefficient The gravitational settling velocity of suspended matter is 4.2. The surface area of ​​the sixth pond is 1.5 m / d. 1200m 2 After calculation and physical topographical adjustments, the effective volume of the first pond was finally constructed. 100m 3 The effective volume of the second pond 1500m 3 The effective volumes of the third, fourth, and fifth ponds are 1000 m³ and 1000 m³, respectively. 3 1000m 3 1000m 3 (That is, the total volume of the main breeding area is 3000m³) 3 The effective volume of the sixth pond 1800m 3 .

[0069] The horizontal physical boundaries of the first pond are defined as 25m long and 5m wide (length-to-width ratio 5:1). The bottom is constructed with C20 grade concrete. The inlet water depth is set at 0.7m, and the outlet water depth at 1.0m. (Workshop 100m) 3 The effluent from the first pond is pumped directly into the first pond through a 110mm diameter PVC main pipe without passing through a denitrification reactor. The hydraulic retention time of the water in the first pond is physically controlled to be 24 hours.

[0070] The online chlorophyll a sensor installed on the outlet pipe of the first pond is set to detect every 10 minutes. The programmable logic controller (PLC) has a first threshold parameter of 30 μg / L and a second threshold parameter of 20 μg / L. When the input electrical signal indicates a chlorophyll a concentration ≥ 30 μg / L, the controller outputs a 24V DC voltage to fully open the electric regulating valve; when the input electrical signal indicates a chlorophyll a concentration ≤ 20 μg / L, the controller drives the electric regulating valve to physically close.

[0071] The projected area of ​​the second pond is 1000m². 2 The specific physical quantity parameters of the filter-feeding aquatic animals released into the pond are: 200 silver carp (corresponding to a density of 0.2 individuals / m³). 2 ), 100 silver carp individuals (corresponding to a density of 0.1 individuals / m³) 2 ), 2000 benthic triangular sail mussels (corresponding to a density of 2 mussels / m²) 2 At the same time, 50 individuals of the fine-scaled gudgeon were introduced, and their spatial distribution density was strictly controlled at 0.05 individuals / m². 2 .

[0072] The third, fourth, and fifth ponds were all constructed as square structures with horizontal dimensions of 22m × 22m, and a physical vertical water depth of 2.0m. The third pond was stocked with carnivorous mandarin fish, the fourth with Litopenaeus vannamei, and the fifth with omnivorous yellow catfish. In each of the three ponds, individuals of the fine-scaled gudgeon were introduced at a ratio of 5% of the total number of individuals of the main cultured species to engage in benthic feeding.

[0073] The ammonia nitrogen sensor installed at the outlet of the fifth pond is set to sample at a frequency of 5 minutes per sampling. The third threshold parameter programmed into the programmable logic controller is 2.0 mg / L, and the fourth threshold parameter is 0.8 mg / L. When the detected ammonia nitrogen concentration is ≥2.0 mg / L, the return pump is powered on, transferring 30% of the fluid volume from the outlet of the fifth pond through a 75 mm diameter pipe to a 50 μm quartz sand filter. The filtered fluid is then returned to the inlet of the second pond via a physical pipe network. When the detected ammonia nitrogen concentration is ≤0.8 mg / L, the return pump is powered off, and the return operation stops.

[0074] The fluid entering the sixth pond flows through the bottom area. This area is planted with *Vallisneria natans* and *Hydrilla verticillata* (submerged plants), whose stems and leaves cover a physical projection area of ​​1200 m² of the total pond bottom area. 2 30%. Triangular sail mussels were placed on the bottom, with a density parameter of 2 mussels / m³. 2 The hydraulic retention time of the sixth pond is set to 48 hours by adjusting the height of the weir plate at the drain outlet. A return water pump is activated at the drain outlet to pump 20% of the total outflowing liquid volume back through a polyethylene pipe to the raw water tank in the factory-style recirculating aquaculture system, while the remaining 80% of the liquid volume is discharged into the external drainage channel.

[0075] Based on the engineering application system architecture described in Part 3 above, a 90-day continuous load operation record was executed on the system to extract specific technical indicators.

[0076] During the 90-day operating cycle, the total volume of physical water entering through the main drainage pipeline of the factory workshop is 9000 m³. 3 The online monitoring values ​​of nitrate nitrogen concentration in the input fluid ranged from 28.5 mg / L to 42.1 mg / L. After the system reached mass transfer and biological metabolic equilibrium, laboratory chemical analysis of 60 consecutive fluid samples collected from the discharge outlet at the end of the sixth pond showed that the average detected value of nitrate nitrogen concentration in the outflowing fluid was 3.8 mg / L, and the maximum detected value was 4.7 mg / L, both of which were limited to below the threshold concentration of 5.0 mg / L. The average detected value of suspended particulate matter (SS) concentration in the fluid was 42 mg / L, and the maximum detected value was 68 mg / L.

[0077] During the dynamic feedback monitoring of ammonia nitrogen concentration, the ammonia nitrogen sensor at the outlet of the fifth pond recorded 14 events exceeding the third threshold of 2.0 mg / L within 90 days. The triggering events all corresponded to a physical delay period of 6 to 8 hours after the centralized feeding operation of the front-end aquaculture unit. The controller closed the control loop according to logic, instructing the return pump to start. After the return system performed physical suction and reverse transport at a set flow rate of 30%, the total concentration of free ammonia and ionic ammonium at the fifth pond showed a linear decreasing trend over a time span of 4 to 7 hours, eventually decaying to below the fourth threshold of 0.8 mg / L, triggering the return pump to shut down. Throughout the closed-loop control period, the total ammonia nitrogen concentration in the terminal output fluid did not show a sustained unidirectional increase or accumulation.

[0078] The physical statistics of material flow and mass conversion data show that a total of 365 kg of suspended microalgae biomass (dry weight) was transferred from the first pond to the second pond. The total mass of industrially produced pelleted feed input into ponds two through five over 90 days was 1850 kg. At the system's terminal, the total aquatic organisms produced in ponds two through five—including silver carp, bighead carp, razor clams, mandarin fish, Litopenaeus vannamei, and yellow catfish—were physically harvested and weighed, resulting in a net increase in animal mass of 1540 kg. The overall feed conversion ratio (comprehensive feed conversion ratio) was determined to be 1.20.

[0079] Data from the system's power consumption monitoring instruments showed that, over the entire 90-day operation period, the total active power consumption for maintaining water flow transport and control between the six-stage pond system (including 24 / 7 power supply for the chlorophyll a and ammonia nitrogen sensors, power consumption for the physical displacement of electric regulating valves and return valves, and power consumption for the motor rotation of the return pump and some water reuse pumps) was 1245 kWh. Considering the net gain of 1540 kg of animals produced in the pond section, the auxiliary power consumption per unit of output mass during the tailwater physical transport and mineralization process in this downstream system was 0.81 kWh / kg.

Claims

1. A factory-pond coupled ecological aquaculture method based on nitrate cascade utilization, characterized in that, Includes the following steps: The wastewater from the factory-style recirculating aquaculture system is fed into the first pond; The concentration of chlorophyll a in the fluid at the outlet of the first pond is monitored online. Based on the comparison between the concentration of chlorophyll a in the fluid and the set chlorophyll a concentration threshold, the flow rate of the fluid discharged from the first pond into the second pond is controlled. In the second pond, filter-feeding aquatic animals and scraping benthic fish were polycultured; The fluid discharged from the second pond is sequentially introduced into the third, fourth and fifth ponds connected in series for multi-trophic level aquaculture, and the ammonia nitrogen concentration at the outlet of the fifth pond is monitored online. When the ammonia nitrogen concentration is greater than or equal to the set ammonia nitrogen concentration reflux threshold, a portion of the fluid at the outlet of the fifth pond is reversed and refluxed back to the second pond. The non-returned fluid discharged from the fifth pond is introduced into the sixth pond, and a portion of the fluid discharged from the sixth pond is reused in the factory-style recirculating aquaculture workshop.

2. The factory-pond coupled ecological aquaculture method based on nitrate cascade utilization according to claim 1, characterized in that, The total volume of the first pond, the second pond, the third to fifth ponds, and the effective volume of the sixth pond are set in a fixed ratio. This fixed ratio is constrained by the reciprocal of the target chlorophyll a concentration at the outlet of the first pond, the assimilation conversion coefficient from prey organisms to filter feeders, the average gravity settling velocity of suspended particulate matter, the average daily filtration flux per unit body weight of filter feeders, and the daily drainage volume of the factory-style recirculating aquaculture system.

3. The factory-pond coupled ecological aquaculture method based on nitrate cascade utilization according to claim 1, characterized in that, In the first pond, microalgae are used to absorb and assimilate nitrate nitrogen in the tailwater; The nitrate nitrogen concentration in the effluent from the first pond is controlled within the range of 5 mg / L to 50 mg / L; The daily volume of the wastewater discharged into the first pond accounts for 30% to 50% of the total aquaculture water volume in the factory-style recirculating aquaculture workshop; The hydraulic residence time of the fluid in the first pond is maintained between 18 and 48 hours by adjusting the flow rate.

4. The factory-pond coupled ecological aquaculture method based on nitrate cascade utilization according to claim 1, characterized in that, The set chlorophyll a concentration thresholds include a first threshold preset to 25 μg / L to 35 μg / L and a second threshold preset to 15 μg / L to 25 μg / L; When the measured concentration of chlorophyll a in the fluid is greater than the first threshold, the opening of the connecting pipe from the first pond to the second pond is increased to discharge fluid into the second pond. When the measured concentration of chlorophyll a in the fluid is less than the second threshold, the connecting pipe is closed to retain the fluid.

5. The factory-pond coupled ecological aquaculture method based on nitrate cascade utilization according to claim 1, characterized in that, The physical disturbance of the bottom mud of the second pond by the scraping benthic fish promotes the conversion of sedimentary particles into dissolved nutrients and their release into the liquid phase. The scraping benthic fish is *Siniperca scaberra*, and the stocking density of *Siniperca scaberra* is limited to 0.04 fish / m². 2 Up to 0.06 tails / m 2 ; The filter-feeding aquatic animals include those with a density of 0.1 individuals / m³. 2 Up to 0.3 tails / m 2 The density of the silver carp is 0.05 fish / m³. 2 Up to 0.15 tails / m 2 Bighead carp and a density of 1 fish / m³ 2 Up to 3 tails / m 2 The triangular sail mussel.

6. The factory-pond coupled ecological aquaculture method based on nitrate cascade utilization according to claim 1, characterized in that, The third pond is stocked with carnivorous fish, the fourth pond with shrimp, and the fifth pond with omnivorous fish. In the third, fourth, and fifth ponds, the scraping benthic fish are polycultured at a ratio of 3% to 8% of the total number of individuals of the main cultured species.

7. The factory-pond coupled ecological aquaculture method based on nitrate cascade utilization according to claim 1, characterized in that, The preset ammonia nitrogen concentration reflux threshold is 1.5 mg / L to 2.5 mg / L, and the preset ammonia nitrogen concentration stop reflux threshold is 0.05 mg / L to 0.2 mg / L; When the ammonia nitrogen concentration reaches the set ammonia nitrogen concentration reflux threshold, 20% to 40% of the volume of fluid from the outlet of the fifth pond is extracted, impurities are intercepted by a 50μm precision sand filter, and then pumped back to the second pond. When the ammonia nitrogen concentration is lower than the ammonia nitrogen concentration stop reflux threshold, the reflux operation is stopped.

8. The factory-pond coupled ecological aquaculture method based on nitrate cascade utilization according to claim 1, characterized in that, The bottom of the first pond is progressively deeper along the direction of water flow, with the water depth at the inlet set to 0.6m to 0.8m and the water depth at the outlet set to 0.8m to 1.2m; the top of the first pond is enclosed and covered with a greenhouse film.

9. The factory-pond coupled ecological aquaculture method based on nitrate cascade utilization according to claim 1, characterized in that, The sixth pond is planted with submerged plants to intercept and purify the fluid, and the submerged plants occupy 30% of the total surface area of ​​the sixth pond in the vertical projection plane; The hydraulic residence time of the fluid in the sixth pond is controlled to be between 24 hours and 72 hours. The volume of fluid pumped back to the factory-style recirculating aquaculture workshop accounts for 10% to 30% of the total volume of fluid discharged into the sixth pond.

10. A factory-pond coupled ecological aquaculture system based on nitrate cascade utilization, characterized in that, The method for factory-pond coupled ecological aquaculture based on nitrate cascade utilization as described in any one of claims 1-9 includes a factory-scale recirculating aquaculture workshop and a first pond, a second pond, a third pond, a fourth pond, a fifth pond, and a sixth pond connected in series. The connecting pipeline between the first pond and the second pond is equipped with an online chlorophyll a sensor and an electric regulating valve; The outlet of the fifth pond is equipped with an ammonia nitrogen sensor; The fifth pond is connected to the second pond by a return pipeline for reverse fluid transport, and the return pipeline is connected in series with a return pump and a sand filter. The sixth pond is equipped with a first reuse branch and a second discharge branch at its output end. The first reuse branch is connected to the factory-style recirculating aquaculture workshop via a return water pump. It also includes a programmable logic controller, which is electrically connected to the online chlorophyll a sensor, the electric regulating valve, the ammonia nitrogen sensor and the reflux pump.

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