A system and method for realizing denitrification and simultaneous phosphorus recovery of anaerobic digestion liquid based on nitritation-anammox coupling magnesium-phosphorus crystallization

Abstract

This invention belongs to the field of wastewater biological treatment technology and resource recovery, and discloses a system and method for achieving simultaneous nitrogen removal and phosphorus recovery from anaerobic digestion liquid based on nitrification-anaerobic ammonium oxidation coupled with magnesium-phosphorus crystallization. The system of this invention includes: a raw water tank, a nitrification reactor, an intermediate tank, an anaerobic ammonium oxidation reactor, and a magnesium salt solution storage tank; wherein, the raw water tank, nitrification reactor, intermediate tank, and anaerobic ammonium oxidation reactor are sequentially connected by water pumps and pipelines. The anaerobic ammonium oxidation reactor includes a reflux device consisting of a reflux pipe and a reflux water pump. The magnesium salt solution storage tank is connected to the bottom of the anaerobic ammonium oxidation reactor via water pumps and pipelines. After entering the system of this invention, wastewater undergoes sequential nitrification and anaerobic ammonium oxidation coupled with magnesium salt precipitation, solving the problem of sludge easily floating and being lost with air bubbles in the anaerobic ammonium oxidation reactor, and simultaneously achieving nitrogen and phosphorus removal efficiencies of over 85% and 75% respectively in the wastewater.

Description

Technical Field

[0001] This invention belongs to the field of wastewater biological treatment technology and resource recovery, and specifically relates to a system and method for achieving simultaneous phosphorus recovery and nitrogen removal from anaerobic digestion liquid based on nitrification-anaerobic ammonia oxidation coupled with magnesium phosphorus crystallization. Background Technology

[0002] The activated sludge process is currently the most widely used wastewater treatment method, with significant advantages such as high efficiency and small footprint. However, it also generates a large amount of excess sludge during the wastewater treatment process. Anaerobic digestion of this excess sludge is a feasible and mature method for sludge reduction and resource recovery.

[0003] However, the sludge and anaerobic digestion liquid produced by anaerobic digestion contain large amounts of pollutants such as nitrogen and phosphorus, which can easily lead to eutrophication of water bodies. Furthermore, phosphorus, as a non-renewable resource, plays a crucial role in food production and industrial applications. Therefore, developing a highly efficient, low-energy-consumption, and resource-recoverable wastewater nitrogen and phosphorus removal technology is of great significance in water pollution control.

[0004] Because anaerobic digestion liquid is a typical low carbon-to-nitrogen ratio (C / N) wastewater, the carbon source required for denitrification biological denitrification is lacking, and traditional biological denitrification and phosphorus removal processes all have many problems to varying degrees, such as long process flow, high aeration energy consumption, insufficient carbon source, difficulty in meeting standards, and high construction and operation costs.

[0005] Anaerobic ammonia oxidizing bacteria use NH4+ + -N is an electron donor, NO2 - -N acts as the electron acceptor, and the reaction produces N2 and a small amount of NO3. - Anaerobic ammonia-oxidizing bacteria remove total nitrogen (TN) without requiring additional carbon sources, theoretically achieving a TN removal rate of over 85%. However, these bacteria grow and reproduce slowly, have long generation cycles, and are easily carried out of the reactor by the water flow, leading to a decrease in reactor treatment efficiency. Summary of the Invention

[0006] In order to overcome the shortcomings and deficiencies of the prior art, the primary objective of this invention is to provide a system for achieving simultaneous nitrogen removal and phosphorus recovery from anaerobic digestion liquid based on nitrification-anaerobic ammonia oxidation coupled with magnesium phosphorus crystallization.

[0007] Another objective of this invention is to provide a method for achieving simultaneous phosphorus recovery from anaerobic digestion liquid by using the above-mentioned system based on nitrification-anaerobic ammonia oxidation coupled with magnesium phosphorus crystallization.

[0008] This invention addresses the prominent problems of limited nitrogen removal efficiency and lack of phosphorus recovery in traditional denitrification technologies when treating low C / N, high nitrogen and phosphorus wastewater. Based on the reaction characteristics of anaerobic ammonia oxidizing bacteria, it proposes a nitrogen and phosphorus removal system and method for anaerobic digestion broth based on a highly efficient nitrogen removal reaction of nitrification-anaerobic ammonia oxidation coupled with magnesium phosphate mineralization to form crystals. With limited carbon sources, this system not only removes nitrogen and phosphorus but also recovers phosphorus resources, representing a novel anaerobic digestion broth treatment process with low-carbon nitrogen removal and simultaneous phosphorus resource recovery. Furthermore, the magnesium phosphate crystals can serve as attachment sites for microorganisms, promoting the enrichment and stable retention of anaerobic ammonia oxidizing bacteria.

[0009] The objective of this invention is achieved through the following solution:

[0010] A system for achieving simultaneous nitrogen removal and phosphorus recovery from anaerobic digestion liquid based on nitrification-anaerobic ammonium oxidation coupled with magnesium phosphorus crystallization includes: a raw water tank, a nitrification reactor, an intermediate tank, an anaerobic ammonium oxidation reactor, and a magnesium salt solution storage tank; wherein, the raw water tank, the nitrification reactor, the intermediate tank, and the anaerobic ammonium oxidation reactor are sequentially connected by water pumps and pipelines, the anaerobic ammonium oxidation reactor includes a reflux device consisting of a reflux pipe and a reflux water pump, and the magnesium salt solution storage tank is connected to the bottom of the anaerobic ammonium oxidation reactor through water pumps and pipelines.

[0011] The nitrification reactor is equipped with an aeration device and a stirring device; the anaerobic ammonia oxidation reactor and the magnesium salt solution storage tank are equipped with stirring devices; the raw water tank and the intermediate tank may be equipped with stirring devices.

[0012] The nitrification reactor is a sequencing batch reactor (SBR); the anaerobic ammonia oxidation reactor is an upflow anaerobic sludge blanket reactor (UASB).

[0013] The nitrification reactor is equipped with a stirring and aeration device; the anaerobic ammonia oxidation reactor is equipped with a reflux pipe, a three-phase separator and an overflow weir at the top; the anaerobic ammonia oxidation reactor has bottom inlet and top outlet after passing through the overflow weir.

[0014] A method for denitrifying and removing phosphorus from anaerobic digestion liquid using the above-mentioned system based on nitrite-anaerobic ammonia oxidation coupled with magnesium phosphate crystallization includes:

[0015] (1) Sludge 1 was inoculated in the nitrification reactor and sludge 2 was inoculated in the anaerobic ammonium oxidation reactor;

[0016] (2) Wastewater in the raw water tank enters the nitrification reactor to undergo nitrification reaction;

[0017] (3) The effluent from the nitrification reactor enters the anaerobic ammonia oxidation reactor for anaerobic ammonia oxidation treatment;

[0018] (4) The magnesium salt solution in the magnesium salt solution storage tank is pumped from the bottom of the anaerobic ammonia oxidation reactor into the anaerobic ammonia oxidation reactor through a water pump and pipeline to provide a magnesium source for phosphate crystallization in wastewater and achieve phosphorus removal.

[0019] (5) Wastewater at the top of the anaerobic ammonia oxidation reactor is returned to the bottom inlet of the reactor through a return pipe and a return water pump to achieve internal return of the reactor.

[0020] The sludge 1 in step (1) is sludge containing ammonia-oxidizing bacteria, preferably at least one of nitrification sludge and secondary sedimentation tank sludge; the sludge 2 is sludge containing anaerobic ammonia-oxidizing bacteria, preferably sludge from an engineered anaerobic ammonia oxidation reactor.

[0021] The wastewater in step (2) has an ammonia nitrogen concentration of 800–2000 mg / L, a total nitrogen concentration of 800–2500 mg / L, and a phosphorus concentration of 60–150 mg / L.

[0022] The nitrification reactor described in step (2) is operated in a sequencing batch process, and a single cycle includes influent-reaction (aeration and stirring)-sedimentation-effluent.

[0023] The conditions for the nitrification reaction in step (2) are as follows: the concentration of sludge 1 in the nitrification reactor is 4000-6000 mg / L; the hydraulic retention time (HRT) in the nitrification reactor is 12-30 h; the effluent ratio of the nitrification reactor is 20-50%; and the aeration rate of the nitrification reactor is such that the dissolved oxygen concentration in the nitrification reactor is maintained at 0.5-3 mg / L.

[0024] The nitrification reaction described in step (2) is the conversion of ammonia nitrogen in wastewater into nitrite nitrogen under the action of ammonia-oxidizing bacteria (AOB) in sludge. By controlling the aeration rate and hydraulic retention time in the nitrification reactor, the ratio of nitrite nitrogen concentration to ammonia nitrogen concentration in the effluent after the reaction is 1.32:1 to 1:1.

[0025] The conditions for the anaerobic ammonia oxidation treatment in step (3) are as follows: the concentration of sludge 2 in the anaerobic ammonia oxidation reactor is 5000-8000 mg / L; the hydraulic retention time in the anaerobic ammonia oxidation reactor is 12-24 h; and the reaction temperature is 28-33 °C. Suitable anaerobic ammonia oxidation sludge concentration and hydraulic retention time are conducive to the anaerobic ammonia oxidation reaction.

[0026] The anaerobic ammonia oxidation treatment in step (3) involves anaerobic ammonia oxidizing bacteria using NH4+ in an anaerobic ammonia oxidation reactor. + -N is an electron donor, NO2 - -N acts as an electron acceptor, generating nitrogen gas through a biochemical reaction, thereby removing nitrogen from wastewater.

[0027] The amount of magnesium salt solution used in step (4) is such that the molar ratio of magnesium ions to phosphate entering the anaerobic ammonia oxidation reactor is 1.5 to 2:1.

[0028] The amount of magnesium salt solution used in step (4) satisfies the following: the molar ratio of magnesium ion concentration in the magnesium salt solution to phosphate concentration in the wastewater is 150-200, the flow rate of the magnesium salt solution is 1:100 to the influent flow rate of the anaerobic ammonia oxidation reactor, which is equivalent to a molar ratio of magnesium ions to phosphate entering the anaerobic ammonia oxidation reactor of 1.5-2:1.

[0029] The phosphorus removal in step (4) specifically involves the following: after the anaerobic ammonia oxidation reaction, the pH of the water environment increases, causing magnesium ions to combine with phosphate ions to form magnesium phosphorus crystal precipitates, thereby achieving the removal and recovery of phosphorus; the magnesium phosphorus crystals are mainly hydrated magnesium phosphate (Mg3(PO4)2·xH2O).

[0030] The magnesium salt mentioned in step (4) is at least one of magnesium chloride and magnesium sulfate.

[0031] The reflux ratio in step (5) is 100% to 500%, preferably 300% to 500%.

[0032] The mechanism of this invention is as follows:

[0033] Anaerobic ammonia oxidizing bacteria are autotrophic microorganisms with long generation cycles. Their growth is slow, and the anaerobic ammonia oxidation reaction generates nitrogen gas. These nitrogen gas bubbles easily adhere to the anaerobic ammonia oxidation sludge flocs, causing sludge to float. Some of the floating sludge bypasses the three-phase separator and flows out of the reactor with the water flow, leading to sludge loss and reducing the reactor's wastewater treatment performance. To address these drawbacks, this invention proposes a system and method for nitrogen and phosphorus removal from anaerobic digestion broth based on nitrification-anaerobic ammonia oxidation coupled with magnesium-phosphorus crystallization. The system consists of a nitrification reactor, an anaerobic ammonia oxidation reactor, and a magnesium salt solution supply device. The nitrification reactor converts some ammonia nitrogen in the wastewater into nitrite nitrogen, while simultaneously utilizing organic matter in the wastewater for denitrification. The anaerobic ammonia oxidation reactor uses the nitrite nitrogen produced by the nitrification reactor as an electron acceptor and ammonia nitrogen as an electron donor to generate nitrogen gas through the anaerobic ammonia oxidation reaction, thereby achieving nitrogen removal from the wastewater. Simultaneously, magnesium salt solution is added to the anaerobic ammonium oxidation reactor. Utilizing the characteristics of anaerobic ammonium oxidation—consuming hydrogen ions and increasing the system's pH—as well as the adsorption and release characteristics of ions by the sludge within the reactor, the saturation index (SI) of magnesium phosphorus compounds is increased. Furthermore, the sludge's adsorption of ions provides nucleation sites for magnesium phosphorus crystal formation, promoting crystallization. The formation of magnesium phosphorus crystals achieves the removal and recovery of phosphorus from wastewater, while also providing attachment sites for microorganisms, promoting the formation of granular sludge, improving sludge settling performance and retention capacity in the reactor, and ensuring the reactor's highly efficient nitrogen removal performance.

[0034] Anaerobic ammonia oxidation (AA) reactors contain anaerobic ammonia-oxidizing bacteria and other microorganisms that can attach and grow on the surface of magnesium phosphate crystals. Under the action of extracellular polymers and water flow shear force, flocculation occurs between the anaerobic ammonia-oxidizing microbacterial colonies, forming colony flocs. As the magnesium phosphate crystals grow and the colony flocs expand and cover the surface, granular sludge is gradually formed, with magnesium phosphate crystals as the core and microorganisms covering the outer layer. Sludge is periodically discharged, and the sludge is crushed and centrifuged. The sludge containing microorganisms is returned to the reactor, while the inorganic matter (magnesium phosphate crystals) is recovered after further treatment.

[0035] The intermediate tank allows for adjustment of the influent properties and chemical dosage based on actual conditions, such as adjusting pH and adding alkalinity.

[0036] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0037] 1) Simultaneous removal of nitrogen and phosphorus from anaerobic digestion liquid was achieved, with nitrogen and phosphorus removal rates of over 85% and 75% respectively in wastewater.

[0038] 2) It solved the problem of sludge easily floating and being lost with the bubbles in the anaerobic ammonia oxidation reactor;

[0039] 3) Anaerobic ammonia oxidation is a typical autotrophic reaction process. The nitrification reactor can consume most of the bioavailable organic matter in the wastewater, achieving simultaneous nitrification and denitrification, removing some nitrogen from the wastewater, and providing good growth conditions for subsequent anaerobic ammonia oxidizing bacteria, avoiding excessive competition with other heterotrophic bacteria.

[0040] 4) Compared with traditional precipitation methods for phosphorus recovery, such as struvite crystallization and hydroxyapatite crystallization, which require the addition of alkaline agents to adjust the pH of wastewater, the alkalinity caused by anaerobic ammonia oxidation can be reduced or even eliminated by the need for additional agents.

[0041] 5) The sludge containing magnesium and phosphorus crystals formed in the anaerobic ammonia oxidation reactor can be recycled and sold as magnesium and phosphorus composite sludge organic fertilizer; or the sludge and magnesium and phosphorus crystals can be separated and recycled and sold as fertilizer or industrial raw materials to increase revenue from the wastewater treatment process.

[0042] 6) The entire system integrates pollutant removal and resource recovery. In particular, the integrated anaerobic ammonia oxidation coupled magnesium phosphorus crystallization reactor can reduce the footprint of sewage treatment facilities and is suitable for the upgrading and renovation of current and future sewage treatment plants. Attached Figure Description

[0043] Figure 1 Here is a schematic diagram of the system of the present invention:

[0044] (1) Raw water tank, (2) Nitrification reactor, (2-1) Aerator, (2-2) Aeration head, (2-3) Stirring device, (3) Intermediate tank, (4) Anaerobic ammonia oxidation reactor, (4-1) Overflow weir, (4-2) Three-phase separator, (5) Magnesium salt solution storage tank, (6) Nitrification reactor inlet pump, (7) Anaerobic ammonia oxidation reactor inlet pump, (8) Magnesium salt solution feed pump, (9) Anaerobic ammonia oxidation reactor reflux pump, (10) Anaerobic ammonia oxidation reactor reflux pipeline. Detailed Implementation

[0045] The present invention will be further described in detail below with reference to embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto. Unless otherwise specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments used, unless otherwise specified, are all commercially available products.

[0046] Unless otherwise specified, all reagents used in the examples are commercially available.

[0047] The system for simultaneous denitrification and phosphorus recovery of anaerobic digestion liquid based on nitrification-anaerobic ammonia oxidation coupled with magnesium phosphorus crystallization includes (1) raw water tank, (2) nitrification reactor, (2-1) aerator, (2-2) aeration head, (2-3) stirring device, (3) intermediate tank, (4) anaerobic ammonia oxidation reactor, (4-1) overflow weir, (4-2) three-phase separator, (5) magnesium salt solution storage tank, (6) nitrification reactor inlet pump, (7) anaerobic ammonia oxidation reactor inlet pump, (8) magnesium salt solution feed pump, (9) anaerobic ammonia oxidation reactor reflux pump, and (10) anaerobic ammonia oxidation reactor reflux pipeline.

[0048] The nitrification reactor (2) contains an aerator (2-1), an aeration head (2-2), and a stirring device (2-3). The anaerobic ammonia oxidation reactor (4) contains an overflow weir (4-1) and a three-phase separator (4-2). The raw water tank (1) is used to store the anaerobic digestion liquid, and the intermediate tank (3) is used to store the effluent from the nitrification reactor. The magnesium salt solution storage tank (5) is used to store the magnesium salt solution. The raw water tank (1) is connected to the nitrification reactor via a pipeline equipped with a nitrification reactor inlet pump (6). The outlet of the nitrification reactor (2) is connected to the intermediate tank (3) via a pipeline. The intermediate tank (3) is connected to the bottom inlet of the anaerobic ammonia oxidation reactor (4) via a pipeline equipped with an anaerobic ammonia oxidation reactor inlet pump (7). The magnesium salt solution storage tank (5) is connected to the bottom inlet of the anaerobic ammonia oxidation reactor (4) via a magnesium salt solution dosing pump (8). The anaerobic ammonia oxidation reactor reflux pipeline (10) connects the top outlet and bottom inlet of the reactor. The anaerobic ammonia oxidation reactor reflux pipeline (10) is equipped with an anaerobic ammonia oxidation reactor reflux pump (9).

[0049] Sludge 1 was inoculated into the nitrification reactor, and sludge 2 was inoculated into the anaerobic ammonia oxidation reactor. During wastewater treatment, wastewater from the raw water tank (1) entered the nitrification reactor (2) through the nitrification reactor inlet pump (6). The nitrification reactor operated in a sequencing batch reactor (SBR) manner, with each cycle consisting of influent-reaction (aeration and stirring)-sedimentation-effluent. The average concentration of sludge 1 in the nitrification reactor was 4000–6000 mg / L, the effluent ratio of the nitrification reactor was 20–50%, the hydraulic retention time in the nitrification reactor was 12–30 h, and the aeration rate of the nitrification reactor was such that the dissolved oxygen concentration in the nitrification reactor was maintained at 0.5–3 mg / L. Under the action of ammonia-oxidizing bacteria (AOB) in the sludge, ammonia nitrogen in the wastewater was partially converted into nitrite nitrogen. By controlling the aeration rate and hydraulic retention time in the reactor, the ratio of nitrite nitrogen concentration to ammonia nitrogen concentration in the effluent after the reaction was controlled between 1.32:1 and 1:1.

[0050] The effluent from the nitrification reactor enters the intermediate tank (3), and the wastewater in the intermediate tank is then fed into the anammox reactor (4) by the anammox influent pump (7). The sludge concentration in the anammox reactor is 5000–8000 mg / L; the hydraulic retention time in the anammox reactor is 12–24 h. In the anammox reactor, anammox bacteria utilize NH4+ as fuel. + -N is an electron donor, NO2 - -N acts as an electron acceptor, generating nitrogen gas through a biochemical reaction, thus removing nitrogen from wastewater. After the wastewater passes through a three-phase separator (4-2) to separate sludge, water, and gas, it is discharged from the top overflow weir (4-1). The anaerobic ammonia oxidation reactor achieves internal reflux through a reflux pipeline (10), with a reflux ratio of 100% to 500%.

[0051] The magnesium salt solution in the magnesium salt solution storage tank (5) enters the anaerobic ammonia oxidation reactor (4) through the magnesium salt solution feed pump (8). The molar ratio of magnesium ion concentration in the magnesium salt solution to phosphate concentration in the wastewater is 150-200, and the flow rate of the magnesium salt solution to the influent flow rate of the anaerobic ammonia oxidation reactor is 1:100, which is equivalent to a molar ratio of magnesium ions to phosphate entering the anaerobic ammonia oxidation reactor of 1.5-2:1. The anaerobic ammonia oxidation reaction will increase the pH of the surrounding water environment. Under suitable pH conditions, magnesium ions and phosphate ions combine to form magnesium phosphate crystals, thereby achieving the removal and recovery of phosphorus. The magnesium phosphate crystals are mainly hydrated magnesium phosphate (Mg3(PO4)2·xH2O).

[0052] Optionally, magnesium salt solutions such as magnesium chloride or magnesium sulfate can be selected.

[0053] Example

[0054] The water quality characteristics of the simulated anaerobic digestion liquid are shown in the table below:

[0055]

[0056] Experimental conditions:

[0057] Sludge 1 originates from: sludge from the secondary sedimentation tank of the wastewater treatment plant of Guangzhou Water Purification Co., Ltd.

[0058] Sludge 2 originates from the anaerobic ammonia oxidation sludge (red bacteria) in the anaerobic ammonia oxidation engineering device of Foshan Huaer Ammonium Biotechnology Co., Ltd.

[0059] In the nitrification reactor, the sludge concentration of 1 is 5500 mg / L, the effluent ratio is 50%, and the HTR is 24 h. The aeration rate of the nitrification reactor is adjusted to maintain the dissolved oxygen concentration in the reactor at 1–1.5 mg / L. The ratio of nitrite nitrogen concentration to ammonia nitrogen concentration in the effluent after the reaction is 1.1–1.3. In the anammox reactor, the sludge concentration of 2 is 7500 mg / L, the HRT is 19 h, the reflux ratio is 500%, and the temperature is 32 ± 1 °C. The concentration of magnesium chloride solution in the magnesium salt solution storage tank is 400 mM, and the ratio of magnesium salt solution flow rate to anammox reactor influent flow rate is 1:100, that is, the Mg / P molar ratio in the anammox reactor is 1.6:1.

[0060] Experimental results show that the average concentrations of ammonia nitrogen, total nitrogen, and phosphorus in the effluent from the system are 4.5, 132.8, and 18 mg / L, respectively. The system can achieve a total nitrogen removal rate of about 87% and a phosphorus removal rate of about 77%.

[0061] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for denitrification and phosphorus removal from anaerobic digestion liquid based on nitrite-anaerobic ammonia oxidation coupled with magnesium phosphate crystallization, characterized in that, The method is implemented using a system comprising: a raw water tank, a nitrification reactor, an intermediate tank, an anaerobic ammonia oxidation reactor, and a magnesium salt solution storage tank; wherein the raw water tank, the nitrification reactor, the intermediate tank, and the anaerobic ammonia oxidation reactor are sequentially connected by water pumps and pipelines, the anaerobic ammonia oxidation reactor includes a reflux device consisting of a reflux pipe and a reflux water pump, and the magnesium salt solution storage tank is connected to the bottom of the anaerobic ammonia oxidation reactor by water pumps and pipelines; The method includes the following steps: (1) Sludge 1 was inoculated in the nitrification reactor and sludge 2 was inoculated in the anammox reactor; (2) Wastewater in the raw water tank enters the nitrification reactor for nitrification reaction; (3) The effluent from the nitrification reactor enters the anaerobic ammonia oxidation reactor for anaerobic ammonia oxidation treatment; (4) The magnesium salt solution in the magnesium salt solution storage tank is pumped from the bottom of the anaerobic ammonia oxidation reactor into the anaerobic ammonia oxidation reactor through a water pump and pipeline to provide a magnesium source for phosphate crystallization in wastewater and achieve phosphorus removal; (5) Wastewater from the top of the anaerobic ammonia oxidation reactor is returned to the bottom inlet of the reactor through a return pipe and a return water pump to achieve internal reflux within the reactor; In step (1), sludge 1 is sludge containing ammonia-oxidizing bacteria; sludge 2 is sludge containing anaerobic ammonia-oxidizing bacteria; The ammonia nitrogen concentration of the wastewater in step (2) is 800~2000 mg / L; the total nitrogen concentration is 800~2500 mg / L; and the phosphorus concentration is 60~150 mg / L. The conditions for the anaerobic ammonia oxidation treatment in step (3) are: the concentration of sludge 2 in the anaerobic ammonia oxidation reactor is 5000~8000 mg / L; the hydraulic retention time in the anaerobic ammonia oxidation reactor is 12~24 h; The amount of magnesium salt solution used in step (4) is such that the molar ratio of magnesium ions to phosphate entering the anaerobic ammonia oxidation reactor is 1.5 to 2:

1.

2. The method for denitrification and phosphorus removal from anaerobic digestion liquid based on nitrite-anaerobic ammonia oxidation coupled with magnesium phosphate crystallization according to claim 1, characterized in that: The nitrification reactor is a sequencing batch reactor; the anaerobic ammonia oxidation reactor is an upflow anaerobic sludge bed reactor. The nitrification reactor is equipped with a stirring and aeration device; the anaerobic ammonia oxidation reactor is equipped with a reflux pipe, a three-phase separator and an overflow weir at the top; the anaerobic ammonia oxidation reactor has bottom inlet and top outlet after passing through the overflow weir.

3. The method for denitrification and phosphorus removal from anaerobic digestion liquid based on nitrite-anaerobic ammonia oxidation coupled with magnesium phosphate crystallization according to claim 1, characterized in that: The conditions for the nitrification reaction in step (2) are as follows: the concentration of sludge 1 in the nitrification reactor is 4000~6000 mg / L; the hydraulic retention time in the nitrification reactor is 12~30 h; the effluent ratio of the nitrification reactor is 20~50%; and the aeration rate of the nitrification reactor is such that the dissolved oxygen concentration in the nitrification reactor is maintained at 0.5~3 mg / L.

4. The method for denitrification and phosphorus removal from anaerobic digestion liquid based on nitrite-anaerobic ammonia oxidation coupled with magnesium phosphate crystallization according to claim 1, characterized in that: Adjust the aeration rate and hydraulic retention time in the nitrification reactor so that the ratio of nitrite nitrogen concentration to ammonia nitrogen concentration in the effluent after the reaction is 1.32:1 to 1:

1.

5. The method for denitrification and phosphorus removal from anaerobic digestion liquid based on nitrite-anaerobic ammonia oxidation coupled with magnesium phosphate crystallization according to claim 1, characterized in that: The conditions for the anaerobic ammonia oxidation treatment in step (3) are: the reaction temperature is 28~33℃.

6. The method for denitrification and phosphorus removal from anaerobic digestion liquid based on nitrite-anaerobic ammonia oxidation coupled with magnesium phosphate crystallization according to claim 1, characterized in that: The magnesium salt mentioned in step (4) is at least one of magnesium chloride and magnesium sulfate.

7. The method for denitrification and phosphorus removal from anaerobic digestion liquid based on nitrite-anaerobic ammonia oxidation coupled with magnesium phosphate crystallization according to claim 1, characterized in that: The reflux ratio in step (5) is 100%~500%.

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