A method and system for denitrification-based endogenous carbon denitrification in recirculating aquaculture

Abstract

The application provides a circulating water breeding endogenous carbon denitrification method and system with denitrification as the core, and belongs to the technical field of water treatment in circulating water breeding. The innovation is that the system core is changed from nitrification to denitrification, the independent solid-liquid separation equipment is cancelled, the biological filler in the anoxic denitrification unit is used to realize the physical interception, in-situ hydrolysis fermentation and denitrification of suspended organic matter at the same time, the organic matter in the breeding tail water is used as the endogenous carbon source to drive denitrification, and the denitrification does not depend on the addition of exogenous carbon source. The ammonia nitrogen is converted into nitrate nitrogen and / or nitrite nitrogen as the electron acceptor in the nitrification area, which is closely coupled with denitrification, and the accumulation of nitrate is avoided from the source. By detecting the water quality parameters of the effluent of the anoxic denitrification unit, the influent flow is adjusted according to the hierarchical control logic, so that the hydraulic retention time is matched with the biochemical treatment load. The system can adapt to various biochemical treatment architecture modes according to the breeding varieties and load, and the water exchange rate can be greatly reduced, and the system is especially suitable for the seawater and land breeding scene.

Description

Technical Field

[0001] This invention belongs to the field of water treatment technology for recirculating aquaculture, specifically relating to a method and system for denitrification of endogenous carbon in recirculating aquaculture with denitrification as the core. In particular, it relates to a method and system that uses an anoxic denitrification unit as the core of the system, provides electron acceptors through nitrification, drives denitrification with hydrolysis and fermentation products of suspended organic matter in the aquaculture wastewater, and manages the carbon-nitrogen ratio through detection and regulation. Background Technology

[0002] In factory-scale recirculating aquaculture systems, the current mainstream architecture is centered on nitrification: after solid-liquid separation, the aquaculture wastewater enters a nitrifying biological filter, where ammonia nitrogen is oxidized to nitrate nitrogen. The treated water is then aerated and returned to the aquaculture area. Nitrification is considered the primary treatment unit, while denitrification is considered an auxiliary unit or is completely absent. This architecture has significant drawbacks: there is no effective removal pathway for the nitrate nitrogen, the end product of nitrification, leading to its continuous accumulation in the water. Aquaculture farms must rely on large-scale water exchanges for dilution, with daily water exchange rates reaching 5% to 15% of the total water volume, resulting in water waste and water quality fluctuations. Simultaneously, nitrification continuously consumes alkalinity (7.14g of alkalinity, calculated as calcium carbonate, is consumed for every 1g of ammonia nitrogen oxidized), and the absence of denitrification prevents alkalinity compensation from functioning, causing a continuous drop in pH and necessitating frequent addition of alkali-adjusting agents.

[0003] However, a long-standing technological bias exists in this field: recirculating aquaculture systems (RAS) must be equipped with independent solid-liquid separation equipment (microfilters, drum filters, protein skimmers, etc.) as a pretreatment stage, and suspended organic matter (uneaten feed, feces, bioflocs, chitin) is considered a "pollutant" that must be removed from the water body rather than a usable "carbon resource." This bias is prevalent and long-standing in the field, with mainstream domestic and international RAS engineering design specifications, textbooks, and commercially available system products all listing solid-liquid separation equipment as standard configuration. Timmons and Ebeling, in their authoritative textbook on RAS, dedicate a chapter to "particulate matter removal," systematically discussing the core role of particulate matter separation and removal technology (Timmons MB, Ebeling J M. Recirculating Aquaculture Systems [M]. Translated by Zhu Songming et al. Hangzhou: Zhejiang University Press, 2021), further confirming this technological understanding. Based on this understanding, industry technological research and development has long focused on how to improve solid-liquid separation efficiency, without taking "retaining suspended organic matter and converting it in situ into a denitrifying carbon source under anoxic conditions" as the starting point for system design.

[0004] This bias has a reasonable basis: in traditional systems centered on nitrification, the nitrification zone is an aerobic environment, and the presence of suspended organic matter promotes the growth of heterotrophic bacteria, which compete with nitrifying bacteria for dissolved oxygen and biofilm space, thus reducing nitrification efficiency. Therefore, solid-liquid separation is indeed necessary in traditional architectures. However, after this invention changes the system core from nitrification to denitrification, the operating conditions of the anoxic denitrification unit are fundamentally different from those of the nitrification zone—suspended organic matter no longer constitutes interference, but rather is an indispensable carbon source for the denitrification reaction. The technical contribution of this invention lies in recognizing that the preconditions for the traditional bias (centered on nitrification) can be changed. By changing the system core to denitrification, suspended organic matter is transformed from a "pollutant that needs to be removed" into a "carbon resource that needs to be retained," thereby fundamentally overturning the technical perception of solid-liquid separation equipment as a necessary component. After hydrolysis, the suspended organic matter in aquaculture effluent contains abundant organic carbon, with a carbon-to-nitrogen ratio (COD:TN) far exceeding the theoretical requirement for denitrification (approximately 5:1), indicating a plentiful carbon source within the system. However, in traditional systems, after solid-liquid separation equipment removes most of the suspended organic matter, the remaining water has a carbon-to-nitrogen ratio of less than 2:1, only 1 / 3 to 1 / 4 of the theoretical requirement for denitrification. This demonstrates that the problem is not a lack of carbon source in the aquaculture effluent itself, but rather that the solid-liquid separation equipment removes a large amount of carbon as solid waste.

[0005] A few systems have added denitrification units, but the carbon source supply still relies on external sources. Traditional exogenous carbon source solutions directly add soluble organic carbon such as methanol, sodium acetate, and glucose. However, this carries the risk of insufficient addition leading to decreased nitrogen removal efficiency, or excessive addition causing excessive organic matter in the effluent. Furthermore, these solutions require independent carbon source storage, transportation, and addition equipment, increasing system complexity and operating costs. To reduce reliance on chemical reagents, some studies have attempted to use biomass materials such as agricultural straw and plant fibers as solid carbon sources. However, the release rate of these materials is unstable, making precise control of carbon source supply difficult, and problems such as increased effluent color and excessive organic matter exist. In recent years, biodegradable polymers such as polycaprolactone (PCL), polyhydroxybutyrate (PHB), and polybutylene succinate (PBS) have been developed as solid slow-release carbon sources, serving as both carriers and carbon sources. The carbon source release rate is relatively stable, but the material cost is high, and the carbon source release capacity gradually declines after long-term operation. Moreover, the dynamic matching problem between the carbon source release rate and the actual needs of denitrification remains unresolved. None of the above-mentioned external carbon source solutions utilize the organic carbon resources contained in the aquaculture wastewater itself.

[0006] Regarding the utilization of endogenous carbon, van Rijn et al. from the Hebrew University of Jerusalem, in their review of denitrification in recirculating aquaculture systems, pointed out that the field "lacks a unified concept for the design and operation of denitrifying biofilters in recirculating aquaculture systems" (van Rijn J, Tal Y, Schreier H J. Denitrification in recirculating systems: Theory and applications[J]. Aquacultural Engineering, 2006, 34: 364-376). This review describes various technical schemes for denitrification driven by endogenous carbon, including schemes utilizing endogenous organic carbon from activated sludge, and schemes that release volatile fatty acids from an anaerobic digester and then introduce them into a fluidized bed reactor for denitrification. However, all of these schemes separate the retention / hydrolysis fermentation of suspended organic matter from denitrification in independent treatment units, resulting in a mismatch between the conversion rate and the time required for denitrification during the transfer of carbon sources between units. Furthermore, none of the schemes described in this review involved adjusting the influent flow rate by detecting the water quality parameters of the denitrification unit effluent to achieve a match between carbon source conversion and consumption. Until 2019, some domestic schemes attempted to use uneaten feed and feces in aquaculture tailwater as endogenous carbon sources for denitrification, but still retained sedimentation chambers for solid-liquid separation, and supplemented with biomass materials such as rice husks and straw as supplementary carbon sources (CN 110002666 A, Nitrification-Denitrification Circulating Water Treatment Method, South China Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, 2019). This further indicates that from the aforementioned early research until 2019, the field has consistently failed to break through the technical understanding that "solid-liquid separation equipment must be set up." Some suspended organic matter is removed by sedimentation, failing to maximize the retention and utilization of endogenous carbon, and exogenous biomass materials are also required as carbon sources.

[0007] In summary, existing exogenous and endogenous carbon source solutions fail to integrate the physical retention of suspended organic matter, in-situ hydrolysis and fermentation, and denitrification into a single treatment unit, nor do they form a complete technical solution applicable to engineering. It is particularly important to note that none of the above solutions address the supply-demand balance between carbon source conversion rate and denitrification requirements. After being retained, suspended organic matter in aquaculture recirculating water undergoes a gradual hydrolysis process under anoxic conditions, transitioning from particulate to dissolved state to short-chain organic acids. The carbon source supply exhibits slow-release characteristics, and its release rate is influenced by multiple factors such as the amount of retained organic matter, water temperature, and hydraulic retention time, rather than being a constant value. This results in a dynamic imbalance with the demands of denitrification, and existing technologies lack detection and control schemes for this unique slow-release carbon source supply mode. Summary of the Invention

[0008] The technical problem this invention aims to solve is that traditional recirculating aquaculture systems (RAS) rely on nitrification as their core process. After suspended organic matter is removed by solid-liquid separation equipment, the carbon source is lost, and the denitrification process is either missing or dependent on external carbon sources. This leads to the continuous accumulation of nitrates, inability to compensate for alkalinity, and reliance on large-scale water exchanges and off-site solid waste disposal. This invention shifts the core of the system from nitrification to denitrification. Denitrification is the core treatment process and the final outlet of the entire system, and nitrification is redefined as an electron acceptor supply unit. Using biological packing material within the anoxic denitrification unit, suspended organic matter is physically retained, hydrolyzed and fermented in situ, and denitrified nitrogen removal is achieved within the same unit. Endogenous carbon drives denitrification, fundamentally solving the aforementioned systemic problems.

[0009] The fundamental approach of this invention to solve the carbon source shortage problem is not "increasing carbon source supply," but rather "preventing carbon source loss." This is achieved through a three-pronged approach: system architecture selection, carbon source retention and conversion, and monitoring and regulation. System architecture selection chooses a suitable biochemical treatment architecture based on aquaculture density and scenario requirements, determining the macroscopic supply pattern of carbon sources and electron acceptors. Carbon source retention and conversion utilizes biological packing material within the anoxic denitrification unit to intercept suspended organic matter and hydrolyze and ferment it in situ into dissolved organic carbon, serving as an endogenous carbon source for denitrification and ensuring the supply capacity of endogenous carbon. Monitoring and regulation involves detecting water quality parameters reflecting the operating status and adjusting the influent flow rate to match the hydraulic retention time with the biochemical treatment load, maintaining a continuous balance between carbon source conversion and consumption. These three levels form an organic whole, jointly serving the carbon-to-nitrogen ratio balance required for endogenous carbon-driven denitrification.

[0010] To address the aforementioned technical problems, this invention provides a method for denitrification of endogenous carbon in recirculating aquaculture systems, centered on denitrification, comprising the following steps: Utilizing an aerobic nitrification zone to convert ammonia nitrogen in the recirculating aquaculture water into nitrate nitrogen and / or nitrite nitrogen, which serve as electron acceptors for the denitrification reaction; Introducing the recirculating aquaculture water into an anoxic denitrification unit, utilizing the biological packing material within the anoxic denitrification unit to achieve physical retention of suspended organic matter, in-situ hydrolysis and fermentation of the retained organic matter, and denitrification using dissolved organic carbon generated from hydrolysis and fermentation as a carbon source, thereby reducing and removing nitrate nitrogen and / or nitrite nitrogen from the recirculating aquaculture water; Detecting at least one water quality parameter reflecting the denitrification operation status of the anoxic denitrification unit, and adjusting the influent flow rate into the anoxic denitrification unit based on the detected water quality parameter values ​​to maintain the anoxic denitrification unit in a controlled anoxic state.

[0011] Furthermore, the carbon source for the denitrification is entirely provided by the dissolved organic carbon produced by the in-situ hydrolysis and fermentation of the suspended organic matter, and the method does not depend on an external carbon source.

[0012] Furthermore, the aquaculture recirculating water body does not undergo a treatment step of separating and removing suspended organic matter from the water body before entering the anoxic denitrification unit. The suspended organic matter enters the anoxic denitrification unit with the aquaculture recirculating water body and is intercepted by the biological packing material.

[0013] Furthermore, the water quality parameters are selected from at least one of nitrite concentration, dissolved oxygen concentration, and redox potential.

[0014] Furthermore, the water quality parameters include nitrite concentration, dissolved oxygen concentration, and oxidation-reduction potential. When adjusting the influent flow rate based on the detected water quality parameter values, the following sequential steps are executed: When the nitrite concentration in the effluent of the anoxic denitrification unit exceeds a preset safety threshold, the influent flow rate is reduced; provided that the nitrite concentration does not exceed the preset safety threshold, the influent flow rate is increased when the dissolved oxygen concentration is below 0.2 mg / L and decreased when it is above 0.6 mg / L; provided that both the nitrite concentration and dissolved oxygen concentration meet the requirements, the influent flow rate is reduced when the oxidation-reduction potential is above the upper limit of the preset range and increased when it is below the lower limit of the preset range.

[0015] Furthermore, the controlled anoxic state refers to a dissolved oxygen concentration of 0.2–0.6 mg / L and a redox potential of -80 mV to +60 mV within the anoxic denitrification unit. Under these conditions, the metabolic processes of the retained suspended organic matter are dominated by hydrolysis-acidification and denitrification, inhibiting excessive carbon source consumption by aerobic respiration and the diversion of carbon sources by anaerobic side reactions such as methanogenesis and sulfate reduction. Dissolved oxygen is controlled within the range of 0.2–0.6 mg / L to ensure that a thin aerobic zone is retained on the outer layer of the biofilm to consume influent dissolved oxygen, while avoiding excessive residual dissolved oxygen leading to large-scale carbon source consumption by aerobic heterotrophic processes.

[0016] This invention also provides an endogenous carbon removal system for recirculating aquaculture systems with denitrification as its core, comprising: an aerobic nitrification zone for converting ammonia nitrogen in the recirculating aquaculture water into nitrate nitrogen and / or nitrite nitrogen, serving as an electron acceptor for the denitrification reaction; an anoxic denitrification unit for receiving the recirculating aquaculture water and utilizing biological packing material within the unit to physically retain suspended organic matter in the recirculating aquaculture water, perform in-situ hydrolysis and fermentation of the retained organic matter, and denitrify nitrogen removal using dissolved organic carbon generated by hydrolysis and fermentation as a carbon source, thereby reducing and removing nitrate nitrogen and / or nitrite nitrogen in the recirculating aquaculture water; a water quality monitoring device for acquiring at least one of the water quality parameters reflecting the denitrification operation status of the anoxic denitrification unit; and a flow control device for adjusting the influent flow rate into the anoxic denitrification unit according to the water quality parameter values ​​acquired by the water quality monitoring device, so that the anoxic denitrification unit maintains a controlled anoxic state.

[0017] Furthermore, the system does not have an independent external carbon source addition device; the carbon source required for denitrification is entirely provided by the dissolved organic carbon produced by the in-situ hydrolysis and fermentation of the suspended organic matter.

[0018] Furthermore, the system does not include a separate solid-liquid separation device to separate and remove suspended organic matter from the aquaculture recirculating water. The suspended organic matter in the aquaculture recirculating water enters the anoxic denitrification unit along with the aquaculture recirculating water and is intercepted by the biological packing material.

[0019] Furthermore, the system adopts at least one of the following biochemical treatment architecture modes: nitrification-denitrification series mode, in which the aerobic nitrification zone and the anoxic denitrification unit are connected in series along the water flow direction, and the effluent is returned to the aquaculture zone; denitrification-nitrification series mode, in which the anoxic denitrification unit and the aerobic nitrification zone are connected in series along the water flow direction, and the effluent is returned to the aquaculture zone; parallel split mode, in which the anoxic denitrification unit and the aerobic nitrification zone form independent loops with the aquaculture zone, or the two effluents are mixed and then returned to the aquaculture zone; and a series-parallel hybrid mode, a hybrid hydraulic architecture formed by combining the above series mode and parallel mode.

[0020] Furthermore, the biological filler is a carrier with a three-dimensional porous spatial structure, with a specific surface area greater than 240 m² / m³, a porosity greater than 70%, and a filling rate of 60%~95%. The biological filler is made of at least one of polyolefin materials, polyurethane materials, and silicone materials, and has elastic deformation characteristics, which are used to generate elastic deformation under the action of water flow to alleviate pore blockage and promote the shedding of aging biofilm.

[0021] Furthermore, within the anoxic denitrification unit, a front-section packing zone and a rear-section packing zone are sequentially arranged along the water flow direction. The porosity of the front-section packing zone is higher than that of the rear-section packing zone, and the specific surface area of ​​the rear-section packing zone is higher than that of the front-section packing zone. In a preferred embodiment, the front-section packing zone has a filling rate of 80%~95%, a specific surface area of ​​240~300 m² / m³, and a porosity of 85%~95%; the rear-section packing zone has a filling rate of 60%~80%, a specific surface area of ​​800~1200 m² / m³, and a porosity of 75%~90%.

[0022] Furthermore, the water quality testing device includes at least one of an online sensor and / or a portable testing tool installed at the outlet of the anoxic denitrification unit; the flow control device is at least one of a variable frequency pump and / or a regulating valve, which is controlled by an automatic controller or an operator according to the detection value of the water quality testing tool.

[0023] Furthermore, the flow control device performs at least one of the following control actions based on the detected water quality parameter values: when the detected nitrite concentration exceeds a preset safety threshold, the influent flow rate is reduced; when the detected dissolved oxygen concentration is below 0.2 mg / L, the influent flow rate is increased, and when it is above 0.6 mg / L, the influent flow rate is reduced; when the detected oxidation-reduction potential is above the upper limit of a preset range, the influent flow rate is reduced, and when it is below the lower limit of a preset range, the influent flow rate is increased.

[0024] Working principle of the anoxic denitrification unit: The anoxic denitrification unit is the core processing unit of this invention. In existing recirculating aquaculture systems, the physical retention of suspended organic matter, the conversion of organic carbon, and denitrification are all completed by separate equipment or units: solid-liquid separation equipment is responsible for retaining suspended organic matter, external anaerobic fermenters are responsible for carbon source conversion, and the anoxic tank is responsible for denitrification. This separation results in the retention of organic matter being removed as solid waste, and the carbon source conversion rate is difficult to match with the denitrification requirements. This invention integrates the three functions of physical retention, in-situ hydrolysis fermentation, and denitrification into an anoxic denitrification unit, achieving the following synergistic effects that cannot be achieved when each function is operated separately:

[0025] (1) Slow-release effect of carbon source. After suspended organic matter is intercepted by biological packing, it undergoes a gradual hydrolysis process under anaerobic conditions, from particulate matter to dissolved matter to short-chain organic acids, rather than being released immediately. This slow-release characteristic makes the carbon source supply rate relatively stable, avoiding the direct transmission of fluctuations in influent organic matter concentration to drastic changes in carbon source concentration. When the three functions are completed by independent equipment, the intercepted organic matter is directly removed (solid-liquid separation equipment) or concentratedly hydrolyzed in an independent fermenter (anaerobic fermenter), and the carbon source release is pulsed, which is difficult to match the continuous demand of denitrification. This invention achieves the coupling of interception and in-situ hydrolysis within the same packing space, so that the carbon source release and denitrification consumption are closely connected in space and naturally coupled in time, fundamentally solving the problem of mismatch between carbon source supply and denitrification demand in traditional systems.

[0026] (2) Functional self-organization driven by dissolved oxygen gradient. Under controlled anoxic conditions, a dissolved oxygen gradient naturally forms within the packing bed along the water flow direction. The dissolved oxygen concentration is higher at the inlet, where aerobic or facultative bacteria are active and rapidly consume dissolved oxygen; the dissolved oxygen concentration gradually decreases along the water flow direction, and the activity of hydrolytic fermentation bacteria and denitrifying bacteria gradually increases; until the dissolved oxygen concentration at the outlet drops to a controlled level of 0.2~0.6 mg / L. At the same time, there is also a concentration gradient from the surface to the carrier within the biofilm. The combined effect of these two gradients causes aerobic or facultative bacteria, hydrolytic fermentation bacteria, and denitrifying bacteria to form a functional distribution in different spatial locations. This functional distribution is the result of the natural succession of the microbial community under dissolved oxygen gradient and carbon-nitrogen coexistence conditions, and does not depend on specific inoculation of bacteria or artificial spatial separation. When the three functions are performed by independent equipment, the environmental conditions within each equipment are singular and fixed, making it impossible to form this type of gradient-driven functional distribution and synergistic microbial community.

[0027] (3) Negative feedback self-stabilizing effect. Denitrification consumes carbon source and nitrate / nitrite, maintaining an anaerobic environment; the anaerobic environment inhibits excessive consumption of carbon source by aerobic respiration, protecting the carbon source supply; the continuous supply of carbon source sustains the denitrification reaction. The three form a negative feedback self-stabilizing cycle in the same space. When the three functions are performed by independent devices, the devices are connected by pipelines, and the feedback signal transmission is delayed and attenuated, so the system stability depends on external control. This invention enables the system to have inherent self-stabilizing ability through functional coupling, reducing the dependence on external control.

[0028] (4) Natural carbon-nitrogen coupling during startup. During the system startup phase, the particulate organic matter retained by the packing has entered the hydrolysis and acidification stage. The production of volatile fatty acids and the enrichment of denitrifying bacteria occur synchronously in time, and the denitrification function can be started without the need for an external carbon source. In traditional systems, the solid-liquid separation equipment begins to remove suspended organic matter when the system is started, resulting in a continuous loss of carbon source from the beginning of system establishment; external anaerobic digesters require an independent startup acclimatization period, and the carbon source supply and denitrification demand are difficult to match during the startup phase. This invention achieves retention and in-situ hydrolysis on the same packing, so that the carbon source supply is naturally established from the beginning of system startup, without the need for additional startup procedures or external carbon source input.

[0029] The aforementioned effect is not a simple superposition of the three functions, but rather a dynamic synergistic effect resulting from functional coupling. When solid-liquid separation equipment, anaerobic digesters, and anoxic denitrification tanks are set up separately, each function is physically separated. Carbon source transfer between equipment involves losses, delays, and concentration fluctuations, and the overall system efficiency is limited by the degree of matching between each component. This invention integrates the three functions into the same biological packing material, eliminating spatial distance and time delays between functions. This allows carbon source retention, conversion, and consumption to be completed continuously within the same space. System efficiency is no longer limited by the matching between equipment, but is determined by the biochemical processes within the packing bed itself. This synergistic effect endows the anoxic denitrification unit with inherent self-stabilizing capabilities, but these capabilities have boundaries. When influent load fluctuations exceed the self-stabilizing range, it is necessary to detect water quality parameters reflecting the operating status and adjust the influent flow rate to actively maintain dissolved oxygen concentration and redox potential within the designed operating range, i.e., maintain the aforementioned "controlled anoxic state," to ensure continuous matching between carbon source conversion and denitrification consumption.

[0030] Detection and Control Strategy: This invention employs nitrite concentration, dissolved oxygen concentration, and redox potential as detection parameters, with different parameters playing different control roles. A tiered control strategy is adopted, based on the functional roles of safety outcome → process conditions → efficiency optimization. During the steady-state operation phase after the system's microbial community is established, the influent flow rate is adjusted progressively according to the three parameters' functional roles and priorities to achieve a continuous balance between carbon source conversion and consumption. The three control levels can be used independently or in combination depending on the aquaculture scenario. For detailed control procedures and parameter selection, please refer to the specific implementation method.

[0031] System Architecture Patterns. This invention supports multiple biochemical processing architecture patterns; four representative patterns are listed below (see [link to relevant documentation]). Figure 1 ): (1) Nitrification-denitrification series mode, the aquaculture recirculating water flows through the aerobic nitrification zone and the anoxic denitrification unit in sequence and then flows back to the aquaculture zone. It is suitable for scenarios where the supply of electron acceptors needs to be prioritized. (2) Denitrification-nitrification series mode: the aquaculture recirculating water flows through the anoxic denitrification unit and the aerobic nitrification zone in sequence and then flows back to the aquaculture area. It is suitable for aquaculture scenarios with abundant carbon sources and low load. (3) Parallel flow mode: the aquaculture recirculating water is introduced into the anoxic denitrification unit and the aerobic nitrification zone respectively and then returned to the aquaculture zone, or the two outflows are mixed and then returned to the aquaculture zone. The two lines operate independently, which is suitable for high-density aquaculture scenarios and can avoid the interference of organic matter on the nitrification zone. (4) Series-parallel hybrid mode, which is a hybrid hydraulic architecture formed by combining the series mode and the parallel mode mentioned above. In a preferred embodiment, this mode includes a main road that operates continuously and a branch road that discharges intermittently with pulses. This mode is suitable for aquaculture species such as shrimp that have molting habits. The pulse discharge can simultaneously solve the engineering problems of discharging molting waste and preventing the aquaculture species from escaping. Beneficial effects

[0032] 1. Overcome industry technical biases and eliminate independent solid-liquid separation equipment. Break down the industry technical bias that recirculating aquaculture systems must be equipped with solid-liquid separation equipment. Suspended organic matter is intercepted by biological packing and converted in situ into denitrification carbon source, eliminating the generation and disposal of solid waste at the source. At the same time, it eliminates the equipment investment, operation and maintenance costs associated with independent solid-liquid separation equipment.

[0033] 2. Denitrification using endogenous carbon sources, independent of external carbon sources. The in-situ hydrolysis and fermentation products of suspended organic matter in the aquaculture recirculating water serve as the carbon source for denitrification, eliminating the need for external carbon sources. This avoids the risks of insufficient addition leading to decreased denitrification efficiency or excessive addition causing excessive organic matter levels in the effluent. The "slow-release" hydrolysis characteristics of the biological packing material ensure a stable carbon source supply rate, eliminating the need for external carbon sources during the start-up phase.

[0034] 3. Nitrification and denitrification coupling significantly reduces water exchange rate. Nitrification products are reduced in situ to nitrogen gas by denitrification, preventing nitrate accumulation at the source; denitrification alkalinity production partially compensates for nitrification alkalinity consumption, which helps maintain pH stability. In freshwater scenarios, the daily water exchange rate can be reduced to 1 / 5 to 1 / 3 of that of traditional systems; in marine aquaculture scenarios, near-zero water exchange operation can be achieved.

[0035] 4. The system has a compact structure and is easy to operate and maintain. Physical interception, in-situ hydrolysis fermentation, and denitrification are integrated into the same biological packing material, eliminating the need for an external carbon source storage and transportation system. This reduces the number of processing units, simplifies piping, and decreases the floor space required. Water quality monitoring and flow control can be performed automatically or manually, adapting to farms of different sizes and investment levels.

[0036] 5. Water-saving, energy-efficient, and near-zero solid waste; environmentally friendly. Significantly reduced water exchange rate conserves water resources, decreases wastewater discharge and reduces energy consumption for temperature regulation during water replenishment, with particularly noticeable energy savings in temperature-controlled aquaculture scenarios. Suspended organic matter is converted into a carbon source within the system instead of being discharged as solid waste, eliminating the need for external solid waste transportation and disposal at the source.

[0037] 6. Marine aquaculture offers significant advantages and is suitable for land-based fish and shrimp farming. Marine fish and shrimp have a natural tolerance to the accumulation of cations generated by alkaline agents, allowing for near-zero water exchange operations and greatly reducing dependence on natural seawater. High-protein marine fish and shrimp environments often have high nitrogen loads; this invention addresses the denitrification problem at its source by preserving and utilizing endogenous carbon sources. These two advantages synergistically significantly improve the economic feasibility of land-based fish and shrimp farming. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of four biochemical treatment architecture modes of the system of the present invention, wherein mode 1 is a nitrification-denitrification series mode, mode 2 is a denitrification-nitrification series mode, mode 3 is a split parallel mode, and mode 4 is a series-parallel mixed mode. Figure 2 This is a flowchart of the detection, regulation, and control process for the combined use of three parameters in this invention; Figure 3 This is a schematic diagram of the structure and hydraulic process of the experimental simulation system in Embodiment 1 of the present invention.

[0039] Attached labels: 1. Aquaculture area; 2. Aerobic nitrification zone; 3. Anaerobic denitrification unit; 4. Variable frequency pump; 5. Monitoring point; 6. Aeration and degassing device; 7. Fluidized bed biofilm reactor; 8. Aeration disc; 9. Automatic water replenisher; 10. Three-dimensional elastic packing zone; 11. Bio-rope packing zone; 12. Rain shower pipe; 13. Coarse-pore cotton fiber; 14. Axial flow fan; 15. Pall rings; 16. Heating rod; 17. Bacterial filter media. Detailed Implementation

[0040] The following five embodiments illustrate the specific implementation of the present invention.

[0041] Example 1: Experimental simulation system, specifically as follows Figure 3 As shown Simulation conditions: A 500 L high-density adult sea bass farming system was simulated, with a farming density of 50 kg / m³, a feed protein content of 48% (crude fat >6%, crude fiber <6%, crude ash <16%, moisture <12%), a daily feeding rate of 1% (based on the 500 L system, the corresponding feed intake is 250 g / day), feeding twice a day, and an operating water temperature of 24~32 ℃. The system did not have a real farming area (1), and the farming metabolic environment was simulated by artificial feed preparation.

[0042] Simulation Formula Explanation: In fish-free simulation experiments, urea needs to be added to ensure the carbon-to-nitrogen ratio of the fish-free system matches that of the fish-containing system, accurately simulating the metabolic characteristics of the fish-containing system. Taking a 48% protein feed as an example, based on the approximate nutrient composition of the feed and the carbon loss ratio of fish respiration metabolism, adding 5.4g of urea to every 44.3g of feed can accurately simulate 100g of real-world feeding in a fish-containing system. Therefore, in this example, 125g of simulated feed (111.5g of feed and 13.5g of urea) is added daily, corresponding to a simulated real-world feeding of 250g / day.

[0043] System architecture and water flow path: The system adopts a main-path series nitrification-denitrification mode. The water flows through the aerobic nitrification zone (2), the anoxic denitrification unit (3), the variable frequency pump (4), and the oxygenation and deaeration device (6) in sequence before returning to the aerobic nitrification zone (2). The aquaculture simulation feed is directly fed into the aerobic nitrification zone (2) and then enters the anoxic denitrification unit (3) with the water flow. It is then intercepted in the three-dimensional elastic packing zone (10) and the biological rope packing zone (11) in sequence. Under controlled anoxic conditions, it undergoes in-situ hydrolysis and acidification to produce dissolved organic carbon, which serves as the endogenous carbon source for the denitrification reaction. The system does not have an independent suspended organic matter separation and removal device.

[0044] System Structure: The aerobic nitrification zone (2) has an effective volume of 60 L. A fluidized bed biofilm reactor (7) is installed inside as the nitrification zone. K5 type rolling packing is used with a filling rate of 45%. An aeration disc (8) is installed at the bottom. The packing is fluidized by a 5 W adjustable air pump. An automatic water replenisher (9) is installed to replenish evaporated water. The anoxic denitrification unit (3) has an effective volume of 80 L. A three-dimensional elastic packing zone (10) and a biological rope packing zone (11) are set up in sequence along the water flow direction. The front three-dimensional elastic packing zone (10) has a filling rate of 95%, a specific surface area of ​​280 m² / m³, a porosity of 90%, and is made of polypropylene elastic filament material. The rear biological rope packing zone (11) has a filling rate of 75%, a specific surface area of ​​1100 m² / m³, a porosity of 80%, and is made of polypropylene rope winding material. The variable frequency pump (4) is adjustable in 100 levels, with a rated flow rate of 100~1000 L / h. It is installed at the end of the anoxic denitrification unit (3) and pumps water to the oxygenation and deaeration device (6) with a head of about 55 cm. The oxygenation and deaeration device (6) adopts a three-stage drip device with a total volume of 45 L. The first layer is equipped with a rain shower pipe (12) and coarse-pore cotton (13) for uniform water distribution. The second layer is equipped with an axial flow fan (14) and Pall rings (15) to enhance gas-liquid exchange for oxygenation and removal of supersaturated gases. The third layer is equipped with a 500 W heater (16) to maintain water temperature and a filter media (17) containing bacteria taken from a stable aquarium system to accelerate the establishment of the biochemical system. The system also supplements alkalinity with sodium bicarbonate to maintain pH stability.

[0045] Detection device: Detection point (5) is set at the outlet of the anoxic denitrification unit (3), and oxidation-reduction potential, pH and temperature sensors are installed to provide online data; dissolved oxygen is detected at the detection point (5) at regular intervals using a high-precision handheld dissolved oxygen meter; ammonia nitrogen, nitrate, nitrite, hydrogen sulfide and alkalinity are detected by taking water through the detection point (5) and manually using test paper and reagent kit.

[0046] Initial conditions and acclimatization: Standard tap water was used for initial water filling of the system, and bacterial filter media (17) taken from a stably operating aquarium system was placed in the system as a source of bacteria. Commercial bacterial strains were also added to assist in the establishment of the biochemical system. The initial feeding amount during the start-up phase was 10% of the daily feeding amount, increasing by 5% each day, and reaching the standard feeding amount after about 3 weeks. During the acclimatization period, nitrifying bacteria gradually accumulated in the aerobic nitrification zone (2), and the packing material in the anoxic denitrification unit (3) intercepted suspended organic matter and gradually formed a biofilm with hydrolysis, acidification, and denitrification functions. If the effluent water quality did not meet the standards, feeding was suspended, and the cause was analyzed based on the test results and the system was adjusted accordingly. After 5 consecutive days of steady-state compliance, the system entered the formal operation phase. In this embodiment, the time from the first feeding to entering the formal operation phase was about 7 weeks.

[0047] Control strategy: In this embodiment, three parameters are used for joint control: nitrite concentration, dissolved oxygen concentration and redox potential. The flow rate is adjusted by manually regulating the variable frequency pump (4) speed. The specific control process is as follows: Figure 2 As shown. The three parameters are executed in a progressive order of priority: safety outcome → process conditions → efficiency optimization. The functional roles of each parameter are consistent with those described in the technical solution section.

[0048] The specific control parameters in this embodiment are as follows: the safety threshold for nitrite concentration is set according to the safety limit of the cultured object. In this embodiment, freshwater bass culture is taken as an example, the safety threshold is set to 0.3 mg / L, the dissolved oxygen concentration is maintained at 0.2~0.6 mg / L, and the preset range of oxidation-reduction potential is -80 mV to +60 mV.

[0049] Typical operating conditions and adjustment experience in actual operation: Occasional increase in nitrite concentration usually occurs when the amount of feed increases or the denitrification activity temporarily decreases due to insufficient hydrolyzed carbon source. In this case, the flow rate can be reduced first to restore it; Abnormal dissolved oxygen usually originates from fluctuations in the upstream oxygenation link. The aeration intensity of the aerobic nitrification zone (2) should be checked to see if it is too high; The diurnal fluctuation of oxidation-reduction potential is mainly affected by the feeding rhythm - after feeding, the amount of suspended organic matter entering increases, the carbon source release rate temporarily increases, and the oxidation-reduction potential temporarily decreases. This is a normal fluctuation and does not require frequent adjustment.

[0050] Operational Results: The system operated for 30 days without an external carbon source or suspended organic matter separation and removal equipment. Simulated aquaculture feed (125 g) was added twice daily. During the formal operation period, the effluent quality monitoring results of the denitrification unit were as follows: ammonia nitrogen <0.5 mg / L, nitrite nitrogen <1.0 mg / L, nitrate nitrogen <10.0 mg / L, hydrogen sulfide undetectable, effluent pH 7.2–7.8, oxidation-reduction potential range: -80 mV to +60 mV, dissolved oxygen range: 0.2–0.6 mg / L. These data indicate that during operation, only evaporation water and sodium bicarbonate were needed to maintain alkalinity balance, and the ammonia nitrogen and organic matter converted from the simulated feed were effectively converted.

[0051] Endogenous carbon source supply: Dissolved organic carbon comes from suspended organic matter trapped by the biological packing material in the anoxic denitrification unit (3) after feed input. It is produced by in-situ hydrolysis and acidification under controlled anoxic conditions and does not depend on external carbon source. During the start-up phase, the particulate organic matter trapped by the packing material gradually enters the hydrolysis and acidification phase. The supply of volatile fatty acids and the demand for denitrification are naturally coupled in time, and no external carbon source is required during the start-up period.

[0052] Example 2: Denitrification-nitrification series mode for low-density freshwater ornamental fish farming Breeding scenario: Ornamental koi, with a breeding water volume of about 10 m³, a density of 10~15 kg / m³, a feed protein content of 30%~36%, and a water temperature of 18~30℃.

[0053] System configuration: Denitrification-nitrification series mode is selected (see...) Figure 1 Mode 2). The aquaculture recirculating water is first pumped by the main circulation pump to the anoxic denitrification unit (3) (first intercepting carbon source, first denitrification), then flows through a three-stage series trickling nitrification filter box, and finally flows back to the aquaculture area (1). The nitrification zone (2) uses a multi-stage trickling filter box, which relies on gravity trickling to fully mix water and air and naturally oxygenate. The construction and control of the anoxic denitrification unit (3) are described in Example 1. In this example, the unit volume is adjusted to 1 m³. The system uses biological packing material of the denitrification unit to intercept suspended organic matter instead of traditional physical filtration. Denitrification does not rely on any external carbon source. In this example, only nitrite is periodically detected. The variable frequency pump (4) is controlled independently based on the safety results of nitrite concentration.

[0054] Design Basis: This embodiment is based on the engineering design of the endogenous carbon-driven denitrification principle verified in Example 1.

[0055] Based on carbon-nitrogen balance analysis, the carbon-nitrogen ratio of aquaculture metabolites in this aquaculture scenario is far higher than the theoretical carbon-nitrogen ratio requirement for denitrification (approximately 5:1), indicating abundant endogenous carbon. In low-density, low-load scenarios, only safety-level control of nitrite concentration is needed to ensure effluent nitrite nitrogen levels meet standards, pH remains stable, and the system structure is simple. Compared to traditional systems: Traditional denitrification-free systems, under the same conditions, continuously accumulate nitrate nitrogen in the effluent, requiring a daily water exchange rate of 5%–15%. This invention can significantly reduce the water exchange rate and eliminates the need for an external carbon source.

[0056] Example 3: Nitrification-denitrification series mode (short-cut nitrification) for medium-density, high-protein marine aquaculture Aquaculture scenario: Pearl grouper, seawater recirculating aquaculture, aquaculture water volume of about 35 m³, density of 20~35 kg / m³, feed protein of 48%~53%, water temperature of 24~28℃.

[0057] System configuration: Nitrification-denitrification series mode is selected (see...) Figure 1 (Mode 1). The aquaculture recirculating water is first introduced into the nitrification zone (2) of a single-stage moving bed biofilm reactor. The fluidization intensity is controlled to inhibit nitrite-oxidizing bacteria and heterotrophic bacteria, so that nitrite nitrogen becomes the main product (short-cut nitrification saves about 40% of carbon source consumption compared with full-process nitrification). The nitrified effluent is introduced into the anoxic denitrification unit (3). After degassing by oxygen cone aeration, the denitrification effluent is returned to the aquaculture zone (1). Nano-oxygenation tubes are installed in the aquaculture zone (1). A detection point (5) is set at the outlet of the anoxic denitrification unit (3). A dissolved oxygen detection sensor and a nitrite detection device are installed. The control is based on the safety result control of nitrite concentration and the process condition control of dissolved oxygen concentration. The construction and control of the anoxic denitrification unit (3) are described in Example 1. In this example, the unit volume is adjusted to 5 m³, and the dissolved oxygen control range is 0.2~0.6 mg / L.

[0058] Design Basis: This embodiment is based on the engineering design of the endogenous carbon-driven denitrification principle verified in Example 1.

[0059] According to the carbon-nitrogen balance analysis, the carbon-nitrogen ratio of the aquaculture metabolites in this aquaculture scenario is higher than the theoretical carbon-nitrogen ratio requirement for denitrification (approximately 5:1). The endogenous carbon is sufficient, but the nitrogen load is high and the carbon source is relatively tight in the high-protein scenario. Therefore, the short-cut nitrification pathway (ammonium nitrogen is converted into nitrite nitrogen) is selected, which saves about 40% of the carbon source consumption compared with full nitrification, further ensuring that the endogenous carbon meets the denitrification requirements, without the need for external carbon sources. According to the verification results of Example 1 and the carbon-nitrogen balance analysis, the system can achieve the standards for total nitrogen and nitrite nitrogen in the effluent, and the pH is stable. Compared with the traditional system: the traditional system has a large carbon source requirement for full nitrification under high-protein conditions, and the insufficient endogenous carbon leads to low denitrification efficiency. This invention saves carbon source consumption through the short-cut nitrification pathway and retains endogenous carbon in conjunction with the anoxic denitrification unit (3), thus solving the above problems in a coordinated manner.

[0060] Example 4: High-density freshwater California bass farming in a parallel, split-flow mode. Aquaculture scenario: California bass, freshwater recirculating aquaculture system, aquaculture water volume of about 100 m³, density of 45~55 kg / m³, feed protein of 42%~48%, water temperature of 24~28℃.

[0061] System configuration: A split-parallel architecture is selected (see...) Figure 1 Mode 3). The underflow concentrate is introduced into the anoxic denitrification unit (3). The supernatant is introduced into the full-process nitrification zone (2) of the multi-stage series moving bed biofilm reactor to ensure that ammonium nitrogen is completely converted into nitrate nitrogen. The two effluents are mixed and transported to the degassing tower through the hydraulic drive unit. After removing the supersaturated gas, it is returned to the aquaculture area (1). Liquid oxygen ceramic aeration plates are installed in the aquaculture area (1). The outlet of the anoxic denitrification unit (3) is equipped with a detection point (5) and online sensors for oxidation-reduction potential, dissolved oxygen and nitrite are installed. The online control is based on the safety result control of nitrite concentration, the process condition control of dissolved oxygen concentration and the efficiency optimization control of oxidation-reduction potential. The construction and control of the anoxic denitrification unit (3) are described in Example 1. In this example, the unit volume is adjusted to 15 m³, the dissolved oxygen control range is 0.2~0.6 mg / L, and the oxidation-reduction potential is -80 mV to +60 mV.

[0062] Design Basis: This embodiment is based on the engineering design of the endogenous carbon-driven denitrification principle verified in Example 1.

[0063] According to the carbon-nitrogen balance analysis, the carbon-nitrogen ratio of the aquaculture metabolites in this aquaculture scenario is higher than the theoretical carbon-nitrogen ratio requirement for denitrification (approximately 5:1), indicating sufficient endogenous carbon. However, the absolute nitrogen load is high in high-density scenarios. Therefore, a split-flow parallel mode is selected. The bottom flow concentrate enriches suspended organic matter and introduces it into the anoxic denitrification unit (3) to further ensure the carbon source supply. The supernatant is introduced into the nitrification zone (2) to avoid interference from organic matter. Based on the verification results of Example 1 and the carbon-nitrogen balance analysis, this system can achieve the standards for total nitrogen and nitrite nitrogen in the effluent, and the pH is stable, without the need for an external carbon source. Compared with the traditional system: the traditional system requires a large amount of water exchange at high densities. The split-flow parallel mode of this invention allows the denitrification zone (3) and the nitrification zone (2) to operate independently, with sufficient hydraulic retention time for each, and without the need for an external carbon source.

[0064] Example 5: Series-parallel hybrid mode for medium-density, low-salinity Litopenaeus vannamei farming Aquaculture scenario: Litopenaeus vannamei, low salinity seawater recirculating aquaculture (salinity 2~5‰), aquaculture water volume of about 50 m³, density of 10~15 kg / m³, feed protein of 38%~42%, water temperature of 24~30℃.

[0065] System configuration: A hybrid serial-parallel architecture is selected (see...) Figure 1 Mode 4). This architecture consists of a continuously operating main circuit and an intermittent pulse discharge branch circuit, which converge at the anoxic denitrification unit. The main circuit is a continuous nitrification-denitrification series loop. The supernatant from the aquaculture area is continuously pumped to the aerobic nitrification zone (2) by the main circulation pump. The nitrified effluent is introduced into the anoxic denitrification unit (3). After aeration and degassing, the denitrified effluent is returned to the aquaculture area (1) to ensure continuous removal of ammonia nitrogen and stable water quality. The branch circuit is a bottom liquid pulse discharge loop. A normally closed discharge port is set at the bottom of the aquaculture area, equipped with a spherical movable switch. It is opened 4-6 times a day, with each discharge lasting about 1-2 minutes. During discharge, the bottom liquid carries the shrimp shells, feces and uneaten feed accumulated near the discharge port and is quickly discharged. The shrimp are startled and quickly move away from the discharge port to avoid escape. The discharge port is then closed. The bottom liquid concentrate is directly introduced into the anoxic denitrification unit (3). The suspended organic matter in it is intercepted by the biological packing and hydrolyzed and fermented in situ, serving as an endogenous carbon source for denitrification. The main pipeline continuously supplies nitrate nitrogen, while the branch pipeline intermittently pulses to supply concentrated carbon source, and the two are coupled within the anoxic denitrification unit. A detection point (5) is set at the outlet of the anoxic denitrification unit (3), and online sensors for nitrite and dissolved oxygen are installed. Control is combined based on the safety control of nitrite concentration and the process condition control of dissolved oxygen concentration. The construction and control of the anoxic denitrification unit (3) are described in Example 1. In this example, the unit volume is adjusted to 6 m³, and the dissolved oxygen control range is 0.2~0.6 mg / L.

[0066] Design Basis: This embodiment is based on the engineering design of the endogenous carbon-driven denitrification principle verified in Example 1.

[0067] According to carbon-nitrogen balance analysis, the carbon-nitrogen ratio of aquaculture metabolites in this aquaculture scenario is higher than the theoretical carbon-nitrogen ratio requirement for denitrification (approximately 5:1), indicating an abundance of endogenous carbon. Based on the verification results of Example 1 and carbon-nitrogen balance analysis, this system can achieve compliance with standards for total nitrogen and nitrite nitrogen in the effluent, maintain stable pH, and eliminate the need for an external carbon source. Summary of Implementation Examples

[0068] The five embodiments cover the entire chain from experimental verification to engineering applications, with the common logic that all designs serve the management of the carbon-to-nitrogen ratio for endogenous carbon-driven denitrification.

[0069] Example 1 is an experimental verification system that confirms the feasibility of endogenous carbon-driven denitrification. The system operated for 30 days under closed conditions without an external carbon source, suspended organic matter separation and removal equipment, and the ammonia nitrogen and organic matter converted from the simulated feed were effectively converted. The system could operate safely under zero discharge conditions. Examples 2-5 are engineering design schemes based on the technical principles and carbon-nitrogen balance analysis verified in Example 1, demonstrating the application of this invention under different stocking densities, different feed protein contents, and different cultured species. In the above examples, the feed protein content covered the range of 30% to 53%, and the cultured species included fish and shrimp, verifying the applicability of this invention to endogenous carbon-driven denitrification under different cultured species and protein content scenarios. The biological packing interception-in-situ hydrolysis-dennitrification denitrification mechanism of this invention is applicable to the conventional recirculating aquaculture temperature range (18-32℃), and the specific control parameters can be appropriately adjusted according to the actual water temperature and cultured species.

[0070] Example 5 further extends the application of this invention to the field of shrimp farming. Current mainstream shrimp farming methods achieve in-situ conversion of ammonia nitrogen by maintaining high concentrations of suspended microbial flocs in the farming pond. However, this method has inherent limitations: flocs lead to turbid water quality in the farming pond, causing shrimp to be under constant stress; continuous high-intensity aeration is required to maintain floc suspension, resulting in high energy consumption; ammonia nitrogen is converted into bacterial protein and remains in the water rather than being truly removed from the system, requiring water exchange and removal when flocs accumulate to a certain extent; and the risk of disease increases under high organic load conditions. This invention achieves this by moving the interception of suspended organic matter and denitrification to an independent anoxic denitrification unit, resulting in cleaner water quality and less shrimp stress in the farming area; denitrification reduces nitrate nitrogen to nitrogen gas, truly removing it from the system; and the denitrification unit is an anoxic environment, eliminating the need for high-intensity aeration. The series-parallel hybrid architecture pulse discharge bottom liquid design simultaneously solves the engineering problems of shrimp shell discharge and escape prevention. The suspended organic matter in the concentrated bottom liquid is introduced into the anoxic denitrification unit as an endogenous carbon source, achieving multiple benefits.

[0071] Regarding the differences between freshwater and seawater scenarios: Data from a 30-day closed-loop operation in Example 1 shows that after supplementing alkalinity with sodium bicarbonate, the system pH can be maintained within a stable range of 7.2-7.8. However, in a freshwater closed-loop system, the accumulation of sodium ions from continuous sodium bicarbonate addition leads to an increase in total dissolved solids. Freshwater fish have low tolerance to changes in sodium ion concentration, ultimately requiring water exchange to remove excess salt. Therefore, while this invention can significantly reduce the water exchange rate in freshwater systems, it is difficult to achieve truly near-zero water exchange. This limitation stems from the low tolerance of freshwater fish to fluctuations in ion concentration, rather than a problem with the denitrification mechanism of this invention itself. In seawater systems, seawater fish and shrimp have a natural tolerance to cations such as sodium, calcium, and magnesium. The accumulation of ions from supplementing alkalinity has little impact on seawater fish and shrimp. Denitrification alkali production combined with alkalinity-regulating agents can maintain pH stability for a long time, enabling near-zero water exchange operation. Therefore, this invention has a more prominent technical advantage in marine aquaculture scenarios, especially in land-based fish and shrimp farming scenarios.

[0072] The architecture selection of the above embodiments follows the technical logic of the system architecture selection described above in this invention, that is, selecting series, parallel or series-parallel mixed mode according to the breeding density, and selecting full-process nitrification or short-process nitrification path according to the protein content of the feed. Specific parameters can be adjusted according to actual breeding conditions. As long as the core technical features described in the claims are met, they are all within the scope of protection. The denitrification unit (3) of this invention has modular characteristics. The breeding area (1) may contain one or more breeding units, the aerobic nitrification area (2) may contain one or more nitrification treatment units, and the anoxic denitrification unit (3) may contain one or more denitrification reactors. As long as the water connection relationship conforms to the architecture configuration mode described in this invention, they are all within the scope of protection. Water quality detection can be carried out using online sensors or portable detection tools, and regulation can be carried out using automatic control or manual regulation, adapting to farms of different scales and investment levels. The above description is only a preferred specific embodiment of this invention, but the scope of protection of this invention is not limited thereto. Those skilled in the art, within the technical scope disclosed in this invention, shall make equivalent substitutions or changes according to the technical solution and inventive concept of this invention, which shall be covered within the scope of protection of this invention.

Claims

1. A method for removing endogenous carbon nitrogen in recirculating aquaculture systems, with denitrification as its core, characterized in that, Includes the following steps: The aerobic nitrification zone is used to convert ammonia nitrogen in the aquaculture recirculating water into nitrate nitrogen and / or nitrite nitrogen, which serve as electron acceptors for the denitrification reaction. The aquaculture recirculating water is introduced into the anoxic denitrification unit. The biological packing material in the anoxic denitrification unit is used to physically intercept suspended organic matter, perform in-situ hydrolysis and fermentation of the intercepted organic matter, and denitrify nitrogen removal using dissolved organic carbon produced by hydrolysis and fermentation as a carbon source, thereby reducing and removing nitrate nitrogen and / or nitrite nitrogen. At least one of the water quality parameters reflecting the denitrification and nitrogen removal operation status of the anoxic denitrification unit is detected, and the influent flow rate into the anoxic denitrification unit is adjusted according to the detected water quality parameter value so that the anoxic denitrification unit maintains a controlled anoxic state.

2. The method according to claim 1, characterized in that, The carbon source for denitrification is entirely provided by dissolved organic carbon produced by in-situ hydrolysis and fermentation of the suspended organic matter, and the method does not depend on an external carbon source.

3. The method according to claim 1, characterized in that, The aquaculture recirculating water does not undergo a treatment step to separate and remove suspended organic matter from the water before entering the anoxic denitrification unit. The suspended organic matter enters the anoxic denitrification unit with the aquaculture recirculating water and is intercepted by the biological packing material.

4. The method according to claim 1, characterized in that, The water quality parameters are selected from at least one of nitrite concentration, dissolved oxygen concentration, and redox potential.

5. The method according to claim 4, characterized in that, The water quality parameters include nitrite concentration, dissolved oxygen concentration, and oxidation-reduction potential; when adjusting the influent flow rate based on the detected water quality parameter values, the following steps are performed sequentially: When the nitrite concentration exceeds the preset safety threshold, reduce the influent flow rate; Provided that the nitrite concentration does not exceed the preset safety threshold, increase the influent flow rate when the dissolved oxygen concentration is below 0.2 mg / L and decrease the influent flow rate when it is above 0.6 mg / L. Provided that both nitrite concentration and dissolved oxygen concentration meet the requirements, the influent flow rate is reduced when the oxidation-reduction potential is higher than the upper limit of the preset range, and increased when it is lower than the lower limit of the preset range.

6. The method according to claim 1, characterized in that, The controlled hypoxia state refers to a dissolved oxygen concentration of 0.2~0.6 mg / L and an oxidation-reduction potential of -80 mV~+60 mV within the hypoxic denitrification unit.

7. A recirculating aquaculture system for removing endogenous carbon and nitrogen from water, with denitrification as its core, characterized in that, include: The aerobic nitrification zone is used to convert ammonia nitrogen in the aquaculture recirculating water into nitrate nitrogen and / or nitrite nitrogen, serving as an electron acceptor for the denitrification reaction. The anoxic denitrification unit is used to receive aquaculture recirculating water. The biological packing material in the anoxic denitrification unit is used to physically intercept the suspended organic matter, perform in-situ hydrolysis and fermentation of the intercepted organic matter, and perform denitrification and nitrogen removal using dissolved organic carbon produced by hydrolysis and fermentation as a carbon source, thereby reducing and removing nitrate nitrogen and / or nitrite nitrogen. Water quality testing device, used to acquire at least one of the water quality parameters reflecting the denitrification and nitrogen removal operation status of the anoxic denitrification unit; The flow control device is used to adjust the influent flow rate into the anoxic denitrification unit according to the water quality parameter values ​​obtained by the water quality detection device, so that the anoxic denitrification unit maintains a controlled anoxic state.

8. The system according to claim 7, characterized in that, The system does not have an independent external carbon source addition device; the carbon source required for denitrification is entirely provided by the dissolved organic carbon produced by the in-situ hydrolysis and fermentation of the suspended organic matter.

9. The system according to claim 7, characterized in that, The system does not have a separate solid-liquid separation device to separate and remove suspended organic matter from the aquaculture recirculating water. The suspended organic matter enters the anoxic denitrification unit with the aquaculture recirculating water and is intercepted by the biological packing material.

10. The system according to claim 7, characterized in that, The system employs at least one of the following biochemical processing architecture patterns: In the nitrification-denitrification series mode, the aerobic nitrification zone and the anoxic denitrification unit are connected in series along the water flow direction, and the effluent is returned to the aquaculture area. In the denitrification-nitrification series mode, the anoxic denitrification unit and the aerobic nitrification zone are connected in series along the water flow direction, and the effluent is returned to the aquaculture area. In the parallel flow mode, the anoxic denitrification unit and the aerobic nitrification zone form independent loops with the aquaculture zone, or the two effluents are mixed and then returned to the aquaculture zone. The series-parallel hybrid mode is a hybrid hydraulic architecture formed by combining the series mode and the parallel mode mentioned above.

11. The system according to claim 7, characterized in that, The biological filler is a carrier with a three-dimensional porous spatial structure, with a specific surface area greater than 240 m² / m³, a porosity greater than 70%, and a filling rate of 60%~95%. The biological filler is made of at least one of polyolefin materials, polyurethane materials, and silicone materials, and has elastic deformation characteristics. It is used to generate elastic deformation under the action of water flow to alleviate pore blockage and promote the shedding of aging biofilm.

12. The system according to claim 7, characterized in that, The anoxic denitrification unit is provided with a front packing zone and a rear packing zone in sequence along the water flow direction. The porosity of the front packing zone is higher than that of the rear packing zone, and the specific surface area of ​​the rear packing zone is higher than that of the front packing zone.

13. The system according to claim 7, characterized in that, The water quality testing device includes at least one of an online sensor and / or a portable testing tool installed at the outlet of the anoxic denitrification unit; the flow control device is at least one of a variable frequency pump and / or a regulating valve.

14. The system according to claim 7, characterized in that, The flow control device performs at least one of the following control actions based on the detected water quality parameter values: When the nitrite concentration is detected to exceed the preset safety threshold, reduce the influent flow rate; Increase the influent flow rate when the dissolved oxygen concentration is below 0.2 mg / L, and decrease the influent flow rate when it is above 0.6 mg / L. When the detected oxidation-reduction potential is higher than the upper limit of the preset range, the influent flow rate is reduced; when it is lower than the lower limit of the preset range, the influent flow rate is increased.

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