An integrated system and method for deep denitrification and dephosphorization of aquaculture tail water based on AOA-MBBR

Abstract

This invention relates to the field of aquaculture wastewater treatment technology and environmental protection, providing an integrated system and method for nitrogen and phosphorus removal from aquaculture wastewater based on AOA-MBBR. It primarily addresses the problem of low nitrogen removal efficiency and the need for external carbon sources due to insufficient carbon sources in aquaculture wastewater. The system sequentially connects a rotary drum microfiltration pretreatment unit, an AOA-MBBR biological treatment unit, a sedimentation tank, a vortex-type air flotation unit, and an ultraviolet disinfection unit. The AOA-MBBR biological treatment unit is divided into anaerobic, aerobic, and post-anoxic zones. A dual sludge return pipe is installed at the bottom of the sedimentation tank, connecting the anaerobic zone and the post-anoxic zone respectively. This invention uses a microfiltration unit to intercept large particles while retaining dissolved carbon sources to enter the biological system, simultaneously returning activated sludge to the post-anoxic zone. The endogenous carbon in the sludge drives deep denitrification, achieving highly efficient nitrogen and phosphorus removal without the need for external carbon sources.

Description

Technical Field

[0001] This invention relates to the field of aquaculture wastewater treatment technology and environmental protection, and provides an integrated system and method for nitrogen and phosphorus removal from aquaculture wastewater based on AOA-MBBR. Background Technology

[0002] Currently, conventional treatment equipment for aquaculture wastewater treatment, such as the integrated seawater recirculating aquaculture and wastewater treatment system disclosed in CN210261456U and the fishpond aquaculture water treatment equipment based on MBBR technology disclosed in CN212687850U, has obvious application limitations: its process links often lack scientific zoning and multi-level synergistic mechanisms for nitrogen and phosphorus removal, and relying solely on conventional biochemical or single MBBR units results in extremely limited removal capacity for total nitrogen and total phosphorus; moreover, the biofilm detached from the MBBR packing during system operation lacks a targeted and efficient separation unit, and is easily carried out with the water, leading to excessive suspended solids in the effluent.

[0003] To improve overall nitrogen removal efficiency, existing technologies attempt to introduce complex biochemical modes, such as the MBBR enhanced AOA and AAO dual-mode operation method based on circulating flow disclosed in CN114604967B. However, when applied to aquaculture wastewater, serious problems are exposed: aquaculture wastewater has special characteristics such as large particles of uneaten feed and feces, and a very low carbon-to-nitrogen ratio. Such methods lack precise front-end sludge removal and dissolved carbon source retention mechanisms, and even more so, they lack a specific design for precisely supplementing activated sludge to the post-anoxic zone to stimulate the release of endogenous carbon. As a result, the denitrification process of the system is severely hindered under low carbon water quality, and it is still unable to get rid of the heavy dependence on high-cost external carbon sources.

[0004] In summary, the fundamental flaws of existing treatment systems and processes lie in their inability to address three core pain points synergistically within a single system: first, the inability to achieve precise separation and retention of large particulate pollutants and dissolved effective carbon sources at the front end; second, the inability to establish an effective endogenous release mechanism within the system to overcome the bottleneck of deep denitrification under low-carbon conditions; and third, the lack of methods to thoroughly clarify the detached fine biofilm in the effluent. These deficiencies prevent existing technologies from meeting the actual industrial demands for low-cost, chemical-free, and high-standard discharge of aquaculture wastewater. Summary of the Invention

[0005] The purpose of this invention is to provide an integrated system and method for nitrogen and phosphorus removal in aquatic wastewater based on AOA-MBBR. Its core lies in constructing a synergistic mechanism across the entire chain: "front-end carbon retention and sludge removal, mid-stage endogenous nitrogen removal, and end-stage air flotation membrane removal." By configuring a rotary drum microfilter with a specific pore size of 70-80 micrometers, large particles of residual feed and feces are precisely intercepted, cleverly retaining valuable dissolved organic carbon sources in the water and introducing them into subsequent biological treatment units. The biological treatment section adopts an AOA-MBBR partitioned architecture and innovatively incorporates a dual-path sludge return mechanism to remove sediment... The bottom sludge is returned to the anaerobic zone and the post-anoxic zone in precise proportions. The endogenous carbon source released by the hydrolysis of the returned decaying sludge drives the deep denitrification reaction in the post-anoxic zone, completely breaking through the technical bottleneck of relying on external carbon sources under low-carbon water quality. In response to the defect of MBBR packing material being prone to detachment and debris, a unique vortex-type air flotation unit is connected in series after the sedimentation tank. Microbubble flotation is used to thoroughly remove light suspended solids and detached biofilm that are difficult to remove by conventional sedimentation, thereby achieving low-cost, drug-free, and high-standard integrated deep purification of aquaculture effluent.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] An integrated system for deep nitrogen and phosphorus removal in aquatic wastewater based on AOA-MBBR is a single unit that is connected sequentially along the water flow direction, including: a rotary drum microfiltration pretreatment unit, an AOA-MBBR biological treatment unit, a sedimentation tank, a vortex-type air flotation unit, and an ultraviolet disinfection unit.

[0008] The rotary drum microfiltration pretreatment unit includes a filter screen, a liquid level sensor, and an automatic backwashing assembly, with the filter screen pore size configured to be 70 micrometers to 80 micrometers.

[0009] The AOA-MBBR biological treatment unit is internally divided into an anaerobic zone, an aerobic zone, and a post-anoxic zone by partitions; MBBR suspended packing is added to both the aerobic zone and the post-anoxic zone; a microporous aeration device is provided at the bottom of the aerobic zone; and a flow mixing device is provided in the post-anoxic zone.

[0010] The sedimentation tank is equipped with a dual-path sludge return mechanism at its bottom. The dual-path sludge return mechanism includes an independently adjustable first sludge return pipe and a second sludge return pipe. The first sludge return pipe is connected to the anaerobic zone, and the second sludge return pipe is connected to the post-anoxic zone.

[0011] The inlet end of the vortex-type air flotation unit is connected to the supernatant outlet of the sedimentation tank.

[0012] The ultraviolet disinfection unit is located at the outlet end of the vortex-type air flotation unit.

[0013] Preferably, the hydraulic retention time of each zone in the AOA-MBBR biological treatment unit is configured as follows: the hydraulic retention time of the anaerobic zone is 1.0-2.0h, the hydraulic retention time of the aerobic zone is 2.5-3.5h, and the hydraulic retention time of the post-anoxic zone is 5.0-7.0h.

[0014] The hydraulic retention time of the post-anoxic zone is greater than the sum of the hydraulic retention times of the aerobic zone and the anaerobic zone.

[0015] Preferably, the volumetric filling rate of the MBBR suspended packing material added in both the aerobic zone and the post-anoxic zone is 30%-40%.

[0016] Both the outlet of the aerobic zone and the outlet of the post-anoxic zone are equipped with packing interception nets, the pore size of which is smaller than the particle size of the MBBR suspended packing.

[0017] Preferably, the dual-path sludge return mechanism includes a sludge discharge main pipe disposed at the bottom of the sedimentation tank, a sludge return pump, and a diversion valve connected to the sludge discharge main pipe;

[0018] The return ratio of the first sludge return pipe and the second sludge return pipe is independently controlled between 60% and 90%.

[0019] Preferably, a flow guide baffle is provided between the anaerobic zone, the aerobic zone and the post-anoxic zone, and the water flow is pushed in an up-and-down folding manner between the zones through the bottom opening or the top overflow weir of the flow guide baffle.

[0020] The anaerobic zone is an oxygen-free, unstirred, or intermittently stirred environment, used for the uptake of dissolved organic carbon sources in the water flow and the anaerobic phosphorus release by polyphosphate-accumulating bacteria.

[0021] Preferably, the sedimentation tank is an inclined tube sedimentation tank, with an inverted conical sludge hopper at the bottom to enrich dead cells and heavy activated sludge, and a triangular overflow weir at the top to uniformly collect the supernatant and allow it to flow by gravity to the vortex-type air flotation unit.

[0022] Preferably, the vortex-type air flotation unit includes an air filling section, an air flotation separation section, and a scum collection tank.

[0023] The bottom of the air-filling section is equipped with a vortex aerator, which uses the high-speed rotation of the impeller to generate a slight negative pressure in the water and draw in air to break it into microbubbles; the top of the air flotation separation section is equipped with a chain-type skimmer.

[0024] Preferably, the ultraviolet disinfection unit is a pipeline ultraviolet disinfection device, which has multiple ultraviolet germicidal lamps with quartz sleeves arranged in parallel along the water flow direction inside; the pipeline ultraviolet disinfection device also has a built-in ultrasonic self-cleaning device.

[0025] A method for deep nitrogen and phosphorus removal treatment of aquatic tailwater based on AOA-MBBR, comprising the following steps performed sequentially:

[0026] S1. The aquatic wastewater to be treated is introduced into the pretreatment unit of the rotary drum microfiltration machine, and the water is physically filtered using a filter screen with a pore size of 70 to 80 micrometers.

[0027] S2. The filtered water flows sequentially through the anaerobic zone, aerobic zone, and post-anoxic zone of the AOA-MBBR biological treatment unit. In the anaerobic zone, polyphosphate-accumulating bacteria (PACs) absorb dissolved organic carbon sources in the water and carry out anaerobic phosphorus release reactions. In the aerobic zone, the MBBR suspended packing material in the zone is fluidized by aeration, and nitrification is carried out using the biofilm on the packing material to convert ammonia nitrogen into nitrate nitrogen, while PACs carry out aerobic phosphorus uptake reactions. In the post-anoxic zone, the MBBR suspended packing material in the zone is prevented from settling by stirring, and denitrification is carried out under conditions without external carbon sources.

[0028] S3. The sludge-water mixture after biological treatment enters the sedimentation tank for solid-liquid separation. The settled activated sludge is divided into two independent return streams. The first stream of sludge is returned to the anaerobic zone to maintain the sludge concentration and phosphorus release environment of the front-end biological system. The second stream of sludge is returned to the post-anoxic zone to replenish the post-anoxic zone with decaying activated sludge. Through endogenous respiration and cell lysis of the sludge, endogenous carbon sources are released, driving the deep denitrification reaction in the post-anoxic zone. The supernatant of the sedimentation tank overflows to the subsequent unit.

[0029] S4. The supernatant from the sedimentation tank enters the air flotation unit, where microbubbles are generated to lift the lighter suspended solids and MBBR detached biofilm remaining in the supernatant to the water surface and scrape them off, thus completing the deep clarification of the water body by air flotation.

[0030] S5. After deep clarification by air flotation, the effluent enters the ultraviolet disinfection unit for radiation sterilization treatment before being discharged in compliance with standards.

[0031] The beneficial effects of this invention are:

[0032] This invention innovatively employs a dual-path sludge recirculation mechanism to address the low carbon-to-nitrogen ratio of aquaculture effluent. By precisely recirculating a portion of the activated sludge from the bottom of the sedimentation tank to the post-anoxic zone, not only is the microbial biomass replenished, but more importantly, the "endogenous carbon" released from the hydrolysis of dying cells and the endogenous respiration of the sludge serves as an electron donor, driving deep denitrification in the post-anoxic zone. This design completely eliminates the heavy reliance on purchased carbon sources such as methanol and sodium acetate in traditional denitrification processes, achieving highly efficient total nitrogen removal while significantly reducing the long-term operating costs of the system.

[0033] This invention employs a precisely configured rotary drum microfilter in the pretreatment stage. This unit can intercept and remove large suspended particles such as undigested feed and feces from the water, significantly reducing the organic load and physical clogging risk of the subsequent biological system from the source. More ingeniously, the pore size allows dissolved small-molecule organic carbon sources in the water to penetrate smoothly and enter the anaerobic zone, providing an extremely valuable high-quality raw water carbon source for the anaerobic phosphorus release by polyphosphate-accumulating bacteria, achieving a synergistic effect between physical interception and front-end biological phosphorus removal.

[0034] This invention addresses a common industry-wide problem in MBBR (Metal-Oxide-Brain Reactor) suspended packing materials: the formation of fine, water-like biofilms during operation, which traditional gravity sedimentation tanks struggle to effectively separate. The invention incorporates a vortex-type air flotation unit connected in series after the sedimentation tank. Through a tiered solid-liquid separation mechanism of "sedimentation to remove heavy sludge + air flotation to remove light biofilms," the vortex-type microbubbles precisely capture and scrape away light sludge fragments and colloids in the water. This not only ensures extremely low suspended solids and high water clarity in the final effluent but also completely eliminates light transmission obstacles for the final ultraviolet disinfection unit, maximizing sterilization effectiveness and ensuring consistently high-standard effluent compliance. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the integrated system and method for deep nitrogen and phosphorus removal in aquatic tailwater based on AOA-MBBR according to the present invention. Detailed Implementation

[0036] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0037] Example: The "aquaculture wastewater" involved in this invention mainly refers to the wastewater discharged from high-density intensive aquaculture. Typical characteristics of this type of wastewater are: high concentration of large particulate suspended solids, but extremely low dissolved biochemical oxygen demand (BOD) and chemical oxygen demand (COD), i.e., an imbalanced carbon-to-nitrogen ratio. Traditional biological denitrification processes are severely hampered by the lack of electron donors in this type of water.

[0038] Example 1: Structural composition and hardware configuration of an integrated system for deep nitrogen and phosphorus removal in aquatic tailwater based on AOA-MBBR;

[0039] like Figure 1As shown, this embodiment provides an integrated system for deep nitrogen and phosphorus removal from aquatic wastewater based on AOA-MBBR. The system is a modular, integrated unit prefabricated using Q235 carbon steel or stainless steel with anti-corrosion welding, primarily targeting aquaculture bases with a treatment capacity of 100-5000 m³ / d. Along the water flow direction, the system is sequentially connected via flanges, pipes, and a flow guide channel, comprising: a rotary drum microfiltration pretreatment unit, an AOA-MBBR biological treatment unit, a sedimentation tank, a vortex-type air flotation unit, and an ultraviolet disinfection unit.

[0040] Aquaculture wastewater first enters the pretreatment unit of a rotary drum microfiltration unit via an influent lift pump. Traditional water treatment methods often use screens or grit chambers, which cannot effectively remove non-soluble fine feed residues. This embodiment abandons conventional practices and selects a rotary drum microfiltration unit of a specific specification, the core of which lies in the precise screening that "removes sludge and retains carbon".

[0041] The surface of the microfilter drum is covered with a 316L special nylon microporous filter screen with a precisely configured pore size of 70 to 80 micrometers. Experimental results have shown that pore sizes larger than 80 micrometers lead to excessive debris entering the biological treatment tank, causing a surge in sludge load; while pore sizes smaller than 70 micrometers intercept many valuable colloidal and some large dissolved organic molecules in the water, further depriving the already scarce carbon source in the subsequent biological treatment tank. A pore size of 70 to 80 micrometers can intercept more than 95% of large-particle solid feed and feces, while allowing dissolved organic carbon sources to pass through smoothly.

[0042] The microfilter is equipped with a liquid level sensor and an automatic backwashing assembly. During normal filtration, the drum rotates at a low speed of 3-5 rpm. When the water permeability decreases due to dirt adhesion on the inner and outer surfaces of the filter screen, and the water level difference between the inside and outside of the drum reaches a set threshold, the PLC control system automatically determines that the filter screen is clogged and immediately starts the backwash water pump and the external array of high-pressure nozzles. The high-pressure water flow washes the filter screen in a fan shape from the outside to the inside, and the high-concentration solid waste washed off falls into the sludge collection tank inside the drum and is discharged to the sludge collection pool outside the system. This self-cleaning mechanism ensures a constant water head and continuous operation of the pretreatment unit.

[0043] The filtered water from the microfiltration unit flows by gravity or is pumped into the AOA-MBBR biological treatment unit via an intermediate tank. This unit is the core reaction engine of the system. Its internal space is sequentially divided into anaerobic, aerobic, and post-anoxic zones by multiple baffles. To prevent short-circuiting of the water flow, the baffles are arranged with alternating bottom openings or top overflow weirs, forcing the water flow in a "vertical and undulating, wave-like" manner within the tank. The hydraulic flow pattern is close to a combination of an ideal flow promoter and a completely mixed reactor, effectively eliminating dead zones.

[0044] This system redefines the volumetric ratio of each zone based on the characteristics of aquaculture effluent. The effective volume ratio of the anaerobic zone, aerobic zone, and post-anoxic zone is strictly designed to be 1:2:4. At the design flow rate, the hydraulic retention times (HRT) are configured as follows: anaerobic zone HRT 1.0-2.0 h, preferably 1.5 h; aerobic zone HRT 2.5-3.5 h, preferably 3 h; and post-anoxic zone HRT 5.0-7.0 h, preferably 6 h. The extra-long post-anoxic zone design provides the physical space necessary to stimulate endogenous denitrification.

[0045] Anaerobic Zone (Pre-treatment): This zone does not have aeration equipment but is equipped with a low-speed submersible mixer for intermittent slow mixing, maintaining the oxidation-reduction potential between -250mV and -150mV. Its main function is to receive raw water containing dissolved small-molecule carbon sources and the first-pass return sludge from the sedimentation tank.

[0046] Aerobic Zone (Centralized): Microporous aeration discs or perforated silicone aeration pipes are evenly distributed at the bottom of the tank, connected to an external Roots blower. Dissolved oxygen is strictly controlled at 1.0-3.0 mg / L. This zone contains modified polyethylene MBBR suspended packing material. The MBBR packing material is a porous cylinder with a diameter of approximately 15-25 mm and a density of 0.96-0.98 g / cm³. 3 Its internal effective specific surface area is as high as 500-800 m². 2 / m 3 The volumetric filling rate of the MBBR packing material in this zone was set at 30%-40%. Under the tumbling shear force of the aeration airflow, the packing material is in a three-dimensional fluidized state in the aerobic zone, colliding and rubbing against each other, which promotes the shedding of aged biofilm and the replacement of new biofilm.

[0047] Post-anoxic zone (end-stage): This zone does not have aeration equipment but is equipped with multiple submersible mixers to maintain dissolved oxygen at a micro-aerobic / anoxic state of 0.2-0.5 mg / L. MBBR suspended media with a volumetric filling rate of 30%-40% is also added to this zone. The mixers prevent the MBBR media and returned sludge from settling, ensuring sufficient contact between sludge, water, and media.

[0048] To prevent the fluidized MBBR packing material from penetrating to the next zone with the water flow, 304 stainless steel packing interceptor screens are installed at an angle at the outlet of both the aerobic zone and the post-anoxic zone. The screen aperture is designed to be 8mm-10mm, much smaller than the minimum outer diameter of the packing material, thus preventing packing material loss. The interceptor screen is installed at a 60-degree angle to the horizontal plane, and utilizes the drop in water flow from the aerobic zone to create a lateral flow velocity of 0.3-0.5m / s for self-cleaning. Combined with the tilt angle and the flushing effect of the water flow, or the purge air pipe installed below, this prevents suspended sludge from forming a biofilm and clogging the interceptor screen.

[0049] The biochemically treated mud-water mixture, containing some fine biofilm fragments detached from the MBBR packing material, flows by gravity over the overflow weir into the sedimentation tank.

[0050] This embodiment employs a honeycomb inclined tube sedimentation tank structure. The lower part of the tank features an inverted conical sludge hopper with an inward inclination angle of 60 degrees. This steep design facilitates the rapid accumulation and compaction of dead cells and heavy activated sludge, which lack viscosity due to endogenous respiration, at the bottom. The upper part of the sedimentation tank is equipped with a peripheral or star-shaped radial triangular toothed overflow weir to uniformly collect clarified water with extremely low surface load.

[0051] Traditional AAO processes typically have only one sludge return pipeline from the sedimentation tank to the anaerobic zone. This invention innovatively designs a dual-path regulating network. A large-diameter main sludge discharge pipe is connected to the bottom of the inverted conical sludge hopper in the sedimentation tank, and a non-clogging sludge pump is connected in series on this pipeline. A tee and an electromagnetic diversion regulating valve are installed at the outlet of the sludge return pump, dividing the sludge flow into two independently controlled pipelines:

[0052] The first sludge return pipe connects to the anaerobic zone at the front end and is equipped with an electromagnetic flow meter. The return ratio—the ratio of return flow rate to system influent flow rate—is adjustable between 60% and 90%.

[0053] The second sludge return pipe connects to the post-anoxic zone in the middle and later stages, and is also equipped with an electromagnetic flow meter. The return ratio is also adjustable between 60% and 90%.

[0054] In addition, an excess sludge discharge pipe is also led out from the main sludge discharge pipe to periodically discharge excess sludge that has grown. Due to the large amount of internal respiration consumption in this system, the actual amount of sludge discharged is very small, thus achieving sludge reduction.

[0055] Addressing the pain points of the MBBR process, where suspended packing material constantly collides with each other, resulting in extremely fine detached biofilm with tiny air bubbles adsorbed on its surface, causing its specific gravity to be close to or even slightly less than 1.0, it cannot be completely settled in conventional gravity sedimentation tanks, often leading to excessive suspended solids (SS) in the effluent. This invention creatively connects a vortex-type air flotation unit in series after the effluent end of the sedimentation tank.

[0056] The supernatant from the sedimentation tank flows by gravity into the aeration section of the air flotation unit. A high-speed vortex aerator with a rotation speed of up to 2900 r / min is installed at the bottom of the aeration section. The impeller rotates at high speed underwater, generating a very strong localized negative pressure at the center of the impeller, drawing air from above the liquid surface into the water through the air intake pipe. The air is forcefully sheared and pulverized by the high-speed rotating impeller and guide vanes, forming extremely fine bubbles with a diameter of 30-50 micrometers.

[0057] Microbubble water mixes with the supernatant entering the flotation separation section. These microbubbles with weak charges have a strong adsorption capacity, like countless tiny "bubble balloons". They quickly attach to the surface of the residual light sludge, colloidal substances and detached MBBR aged biofilm in the supernatant, greatly reducing the overall specific gravity of the flocs and forcing them to float to the surface of the flotation tank quickly, forming a thick layer of scum.

[0058] The top of the flotation tank is equipped with a chain-driven flapper skimmer that runs slowly against the water flow, scraping the scum on the surface into a scum collection trough at one end, which is then discharged into the sludge tank. The extremely clear water at the bottom of the flotation unit flows out through the collection pipe below.

[0059] After deep clarification via air flotation, the effluent has extremely low suspended solids (SS) and a light transmittance (T%) > 95%, at which point it enters the pipeline-type ultraviolet (UV) sterilizer. The equipment employs a cavity design, with multiple low-pressure, high-intensity UV germicidal lamps, each with a high-purity quartz sleeve, arranged parallel to the water flow direction. The wavelength is 253.7 nm, ensuring that the water receives a UV radiation dose greater than 30 mJ / cm² at this point. 2 It instantly penetrates the cell membranes of residual bacteria and viruses in the water, destroying their DNA and RNA structures for thorough sterilization. The sterilizer has a built-in ultrasonic self-cleaning device that activates periodically to remove trace amounts of calcium and magnesium scale and colloids from the surface of the quartz tube, maintaining long-lasting light transmission.

[0060] The entire integrated system is equipped with an industrial-grade PLC control cabinet and an HMI touch screen, achieving fully automated unattended operation. The system has multiple built-in closed-loop control loops: for example, adjusting the frequency converter frequency of the Roots blower in real time through the DO meter in the aerobic zone; precisely maintaining the set dual-path sludge return ratio through flow meters and electric regulating valves on each return pipe; and automatically triggering pretreatment backwashing based on the liquid level difference of the microfilter.

[0061] Example 2: A method for deep nitrogen and phosphorus removal in aquatic tailwater based on an AOA-MBBR system;

[0062] Based on the hardware system of Example 1, this invention also provides a method for treating wastewater from low-carbon aquaculture. The innovation of this method lies not only in the superposition of physical processes, but also in the deep coupling of biochemical mechanisms. The specific treatment steps and underlying biological mechanisms are as follows:

[0063] Step S1: Microfiltration pretreatment "removing residue and retaining carbon";

[0064] The aquaculture wastewater enters a 74-micron rotary drum microfilter. A large amount of uneaten feed and fish feces particles are physically intercepted, removing over 60% of suspended solids (SS) and most of the particulate COD from the raw water. This prevents these solid substances from consuming large amounts of dissolved oxygen and transforming into inert sludge in the biological treatment tank. Crucially, the already scarce dissolved organic matter in the raw water can successfully penetrate the microfilter, serving as valuable "carbon source seeds" and entering the biological treatment stage with the filtered water.

[0065] Step S2: Anaerobic phosphorus release and carbon source retention;

[0066] The pretreated water enters the anaerobic zone and mixes with the first batch of returned sludge from the sedimentation tank. In this absolutely anaerobic environment, polyphosphate-accumulating bacteria absorb large amounts of the small-molecule organic carbon source retained in step S1, converting it into intracellular energy storage substances such as polyhydroxy fatty acid esters. Simultaneously, they decompose polyphosphates within their cells to release energy and release orthophosphates into the water. This step converts the effective carbon source in the raw water into a solid phase, preventing the carbon source from being unnecessarily consumed as energy by ordinary heterotrophic bacteria in the subsequent aerobic zone.

[0067] Step S3: Aerobic nitrification and superphosphate uptake (MBBR enhancement);

[0068] The anaerobic effluent enters the aerobic zone. Under sufficient dissolved oxygen conditions:

[0069] Polyphosphate-accumulating bacteria suspended in water use the PHA stored in their cells as an energy source to excessively absorb orthophosphate in the water to synthesize intracellular polyphosphate. The amount of phosphorus absorbed is much greater than the amount of phosphorus released by anaerobic digestion, thus achieving the removal of phosphorus from the water.

[0070] The 30% MBBR packing material used in this area plays a decisive role. Because the aquatic effluent, after microfiltration and anaerobic treatment, has extremely low BOD content and is of poor quality, it provides an ideal growth environment for slow-growing, autotrophic nitrifying bacteria with long growth cycles. These bacteria extensively attach to the pores inside the MBBR packing material, which has a large specific surface area, forming a highly active biofilm, unaffected by hydraulic retention time. They rapidly and completely oxidize ammonia nitrogen in the effluent into nitrate nitrogen.

[0071] Step S4: Endogenous denitrification in the post-anoxic zone;

[0072] The effluent from the aerobic zone contains a large amount of nitrate nitrogen, but the carbon source has been depleted, and it enters the post-anoxic zone with a long HRT. At this point, the denitrification reaction faces the desperate situation of "no carbon available". Traditional processes must artificially add carbon sources such as methanol at this point.

[0073] The system activates the second sludge return pipe, pumping 60%-90% of the sludge-water from the bottom of the sedimentation tank directly into the post-anoxic zone. After undergoing the initial circulation, most of the returned activated sludge is already in the endogenous respiration phase or even in its decay phase.

[0074] Under the long-term micro-oxygen and hypoxic agitation in the post-anoxic zone, on the one hand, a "latent growth" phenomenon occurs - the cell walls of dying microorganisms rupture, releasing their internal contents such as proteins, polysaccharides, and nucleic acids into the aqueous phase, where they are hydrolyzed and converted into usable organic carbon; on the other hand, the surviving microorganisms in the system use the intracellular carbon sources they have pre-stored in their bodies for endogenous respiration.

[0075] The abundant denitrifying bacteria in the post-anoxic zone, including the denitrifying biofilm attached to the MBBR packing, immediately release endogenous carbon as electron donors, reducing nitrate nitrogen from the aerobic zone into nitrogen gas that escapes to the water surface. The ultra-long 6-hour retention time and the dual-path directional reflux of decaying sludge perfectly support the completion of the entire endogenous denitrification process, achieving deep denitrification with absolutely "zero external carbon source".

[0076] Step S5: Step-by-step solid-liquid separation and disinfection;

[0077] After undergoing biochemical reactions, the sludge and water undergo gravity sedimentation in a settling tank, completing the separation and recycling of most of the activated sludge. The supernatant containing detached fine biofilm enters the air flotation unit, where it undergoes tiered air flotation for deep clarification under the action of microbubbles, removing extremely small suspended solids. Finally, the clear and transparent water enters the ultraviolet disinfection unit for spectral penetration and residue-free sterilization, completing the purification of aquatic effluent to meet standards.

[0078] Example 3: Actual water quality engineering verification operation test and data comparison;

[0079] To fully demonstrate the technical effects of the above-described specific embodiments of the present invention, an integrated system of the present invention with a treatment capacity of 100 m3 / d was established based on the actual wastewater from a high-density factory-style recirculating aquaculture base and operated continuously for 60 days.

[0080] Without the addition of chemical reagents and carbon sources, the influent pollutants showed significant fluctuations, with average values ​​as follows: Chemical Oxygen Demand (COD) 100 mg / L; Ammonia Nitrogen 10 mg / L; Total Nitrogen 15 mg / L; Total Phosphorus 3.5 mg / L; and Suspended Solids 200 mg / L. This is typical effluent with extremely low C / N ratio and high suspended solids, with a COD / TN ratio of less than 7, and an effective C / N ratio of less than 3 after deducting particulate COD.

[0081] Microfiltration pretreatment: 74-micron filter.

[0082] AOA-MBBR biochemical section: anaerobic zone HRT 1.5h, aerobic zone HRT 3h, anoxic zone HRT 6h; MBBR packing fill rate in both aerobic and anoxic zones is 30%.

[0083] Dual-path sludge return: The return ratio from the sedimentation tank to the anaerobic zone is set at 75%, and the return ratio from the sedimentation tank to the anoxic zone is set at 75%.

[0084] The average data from continuous testing during the stable operation period of the system is shown in Table 1 below (average data from day 30 to day 60).

[0085] Table 1 Comparison of influent and effluent water quality and removal rate

[0086]

[0087] As can be seen from the data in Table 1:

[0088] Under extremely low carbon source conditions, total nitrogen decreased from 15 mg / L to 2.1 mg / L, far below conventional emission standards. This fully demonstrates that the "post-processed long HRT anoxic zone + 75% second-stage decaying sludge recirculation" effectively stimulates sludge cell lysis and endogenous denitrification, completely breaking through the deadlock of traditional AAO processes that rely on chemical carbon supplementation. With influent suspended solids as high as 200 mg / L, the final effluent concentration was only 5 mg / L. This proves that the combined physical barrier of front-end microfiltration intercepting large particles of residual feed and back-end air flotation capturing detached biofilm successfully compensates for the inherent effluent turbidity defect of the MBBR process, providing excellent optical transmission conditions for terminal UV sterilization.

[0089] The innovative physical structure of this invention, "microfiltration pretreatment + dual-path reflux network + air flotation synergistic clarification," and the innovative biochemical mechanism, "AOA temporal spatial design + endogenous denitrification activation + MBBR composite packing," create an extremely close and inseparable synergistic effect. The system has a small footprint, is easy to operate and maintain, and, without requiring the purchase or addition of any external carbon sources or phosphorus removal chemicals, the effluent quality meets the Class I standard of SC / T 9101 "Requirements for Freshwater Pond Aquaculture Water Discharge" and the Class IV standard of GB 3838 "Surface Water Environmental Quality Standard."

[0090] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0091] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. An integrated system for deep nitrogen and phosphorus removal in aquatic tailwater based on AOA-MBBR, characterized in that, The system is an integrated unit, which is connected in sequence along the water flow direction: a rotary drum microfiltration pretreatment unit, an AOA-MBBR biological treatment unit, a sedimentation tank, a vortex-type air flotation unit, and an ultraviolet disinfection unit. The rotary drum microfiltration pretreatment unit includes a filter screen, a liquid level sensor, and an automatic backwashing assembly, with the filter screen pore size configured to be 70 micrometers to 80 micrometers. The AOA-MBBR biological treatment unit is internally divided into an anaerobic zone, an aerobic zone, and a post-anoxic zone by partitions; MBBR suspended packing is added to both the aerobic zone and the post-anoxic zone; a microporous aeration device is provided at the bottom of the aerobic zone; and a flow mixing device is provided in the post-anoxic zone. The sedimentation tank is equipped with a dual-path sludge return mechanism at its bottom. The dual-path sludge return mechanism includes an independently adjustable first sludge return pipe and a second sludge return pipe. The first sludge return pipe is connected to the anaerobic zone, and the second sludge return pipe is connected to the post-anoxic zone. The inlet end of the vortex-type air flotation unit is connected to the supernatant outlet of the sedimentation tank. The ultraviolet disinfection unit is located at the outlet end of the vortex-type air flotation unit.

2. The integrated system for deep nitrogen and phosphorus removal in aquatic tailwater based on AOA-MBBR according to claim 1, characterized in that, The hydraulic retention time configuration for each zone within the AOA-MBBR biological treatment unit is as follows: 1.0-2.0 h for the anaerobic zone, 2.5-3.5 h for the aerobic zone, and 5.0-7.0 h for the post-anoxic zone. The hydraulic retention time of the post-anoxic zone is greater than the sum of the hydraulic retention times of the aerobic zone and the anaerobic zone.

3. The integrated system for deep nitrogen and phosphorus removal in aquatic tailwater based on AOA-MBBR according to claim 1, characterized in that, The volumetric filling rate of the MBBR suspended packing material added in both the aerobic zone and the post-anoxic zone is 30%-40%. Both the outlet of the aerobic zone and the outlet of the post-anoxic zone are equipped with packing interception nets, the pore size of which is smaller than the particle size of the MBBR suspended packing.

4. The integrated system for deep nitrogen and phosphorus removal in aquatic tailwater based on AOA-MBBR according to claim 1, characterized in that, The dual-path sludge return mechanism includes a sludge discharge main pipe installed at the bottom of the sedimentation tank, a sludge return pump, and a diversion valve connected to the sludge discharge main pipe. The return ratio of the first sludge return pipe and the second sludge return pipe is independently controlled between 60% and 90%.

5. The integrated system for deep nitrogen and phosphorus removal in aquatic tailwater based on AOA-MBBR according to claim 1, characterized in that, A flow guide baffle is provided between the anaerobic zone, the aerobic zone and the post-anoxic zone. The water flow is pushed in an up-and-down zigzag manner between the zones through the bottom opening or the top overflow weir of the flow guide baffle. The anaerobic zone is an oxygen-free, unstirred, or intermittently stirred environment, used for the uptake of dissolved organic carbon sources in the water flow and the anaerobic phosphorus release by polyphosphate-accumulating bacteria.

6. The integrated system for deep nitrogen and phosphorus removal in aquatic tailwater based on AOA-MBBR according to claim 1, characterized in that, The sedimentation tank is an inclined tube sedimentation tank, with an inverted conical sludge hopper at the bottom to enrich dead cells and heavy activated sludge, and a triangular overflow weir at the top to uniformly collect the supernatant and allow it to flow by gravity to the vortex-type air flotation unit.

7. The integrated system for deep nitrogen and phosphorus removal in aquatic tailwater based on AOA-MBBR according to claim 1, characterized in that, The vortex-type air flotation unit includes an air filling section, an air flotation separation section, and a scum collection tank. The bottom of the air-filling section is equipped with a vortex aerator; the top of the air flotation separation section is equipped with a chain-type skimmer.

8. The integrated system for deep nitrogen and phosphorus removal in aquatic tailwater based on AOA-MBBR according to claim 1, characterized in that, The ultraviolet disinfection unit is a pipeline ultraviolet disinfection device, with multiple ultraviolet germicidal lamps with quartz sleeves arranged in parallel along the water flow direction inside.

9. A method for deep nitrogen and phosphorus removal treatment of aquatic tailwater based on AOA-MBBR, characterized in that, The method includes the following steps performed sequentially: S1. The aquatic wastewater to be treated is introduced into the pretreatment unit of the rotary drum microfiltration machine, and the water is physically filtered using a filter screen with a pore size of 70 to 80 micrometers. S2. The filtered water flows sequentially through the anaerobic zone, aerobic zone, and post-anoxic zone of the AOA-MBBR biological treatment unit. In the anaerobic zone, polyphosphate-accumulating bacteria (PACs) absorb dissolved organic carbon sources in the water and carry out anaerobic phosphorus release reactions. In the aerobic zone, the MBBR suspended packing material in the zone is fluidized by aeration, and nitrification is carried out using the biofilm on the packing material to convert ammonia nitrogen into nitrate nitrogen, while PACs carry out aerobic phosphorus uptake reactions. In the post-anoxic zone, the MBBR suspended packing material in the zone is prevented from settling by stirring, and denitrification is carried out under conditions without external carbon sources. S3. The sludge-water mixture after biochemical treatment enters the sedimentation tank for solid-liquid separation. The settled activated sludge is divided into two independent return streams. The first sludge stream is returned to the anaerobic zone to maintain the sludge concentration and phosphorus release environment of the front-end biological system. The second sludge is returned to the post-anoxic zone to replenish the post-anoxic zone with decaying activated sludge. The sludge releases endogenous carbon sources through endogenous respiration and cell lysis, driving the deep denitrification reaction in the post-anoxic zone. The supernatant from the sedimentation tank overflows to the subsequent unit. S4. The supernatant from the sedimentation tank enters the air flotation unit to complete the deep clarification by air flotation. S5. After deep clarification by air flotation, the effluent enters the ultraviolet disinfection unit for radiation sterilization treatment before being discharged in compliance with standards.

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