A method for recovering phosphorus from sludge

Abstract

The present application relates to sewage treatment and resource utilization technical field, specifically relates to a kind of method for recovering phosphorus from sludge, the sludge to be treated is carried out anaerobic digestion under anaerobic condition, make the phosphorus in sludge release into supernatant, obtain sludge anaerobic digestion liquid, iron salt coagulant is added to the sludge anaerobic digestion liquid, precipitation is carried out, phosphorus is removed from supernatant and enters into precipitation, after removing supernatant, solid-liquid mixture is obtained;The solid-liquid mixture is added to reactor, mixed inoculant of iron-reducing bacteria and metal-resistant bacteria is added thereto, under the action of iron-reducing bacteria, trivalent iron is reduced to divalent iron, precipitate is dissolved and recrystallized as blue vitriol, the recovery of phosphorus in sludge is realized, the present application induces precipitate recrystallization to solve the problems of low phosphorus load, large volume, direct recovery of current sewage, low economic benefits and low degree of crystallization of chemical precipitation, many impurities, dissolved oxygen is not easy to control and easy to oxidize.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment and resource utilization technology, specifically to a method for recovering phosphorus from sludge. Background Technology

[0002] Phosphate rock is a non-renewable resource with uneven global distribution. Phosphorus and its compounds generally do not exist in gaseous form, and the global phosphorus cycle is a typical sedimentary cycle. Phosphorus mainly flows from land to ocean via water, and the process of sediments returning to land through crustal movement is extremely long. Human activities have further exacerbated the one-way flow of phosphorus from mines to farms, rivers, and finally to the ocean. This extremely limited cycle pathway makes phosphorus an increasingly scarce resource.

[0003] Phosphorus is a strategic nonmetal, crucial for food security, public health, and the stability of vital industrial and supply chains, including new energy sources. On the one hand, phosphorus resources are scarce; on the other hand, phosphorus is generally abundant in natural water bodies. Large amounts of phosphorus entering rivers and lakes cause eutrophication. This contradiction between phosphorus resource scarcity and phosphorus excess in water bodies urgently needs to be addressed. Wastewater is a significant phosphorus sink; global wastewater treatment plants treat approximately 130 × 10⁻⁶ phosphorus annually. 4 Phosphorus, the vast majority of which ends up in sludge, provides an important phosphorus recycling pathway that helps achieve a sustainable supply of phosphorus. One method is the lapis lazuli crystallization process.

[0004] In the lapis lazuli crystallization process, direct recovery from sludge is difficult due to the high amount of organic impurities, making the crystallized lapis lazuli challenging to separate. Current research employs magnetic separation to extract lapis lazuli from sludge, but the recovery efficiency and purity of the lapis lazuli still need improvement. Recovering phosphorus from phosphorus-rich anaerobic digestion broth of sludge involves fewer impurities compared to sludge, making the recovered lapis lazuli relatively easier to separate.

[0005] Currently, existing methods for crystallizing and recovering lapis lazuli from anaerobic sludge digestion broth include chemical crystallization and microbial-induced crystallization. Chemical crystallization involves directly adding ferrous ions (Fe2+) to crystallize and precipitate with phosphate ions. This method has two problems: first, coagulation occurs during crystallization and precipitation, causing suspended solids and some dissolved organic matter in the wastewater to precipitate along with the crystals, resulting in low crystallinity and high impurity content; second, the dissolved oxygen level cannot be strictly controlled during precipitation, leading to the oxidation of ferrous ions to ferric ions, which also results in low crystallinity and purity. Microbial-induced crystallization utilizes iron-reducing microorganisms to reduce ferric ions in the water to ferrous ions, which then react with phosphate ions to form lapis lazuli crystals. The anaerobic growth process of microorganisms maintains a low dissolved oxygen (DO) level, solving the problem of ineffective DO control in chemical crystallization alone. However, current methods for directly inducing lapis lazuli crystallization in anaerobic digestion broth through microbial induction suffer from low phosphorus loading, excessively large reactor volumes, and insufficient product yield, leading to low economic efficiency. Summary of the Invention

[0006] To address the shortcomings of the existing technologies, the purpose of this invention is to provide a method for recovering phosphorus from sludge. This invention uses iron salt precipitation followed by induced recrystallization to solve the problems of low phosphorus load, large volume, low economic benefits of direct recovery, low crystallinity of chemical precipitation, many impurities, and difficulty in controlling dissolved oxygen, which are prone to oxidation.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A method for recovering phosphorus from sludge includes the following steps: The sludge to be treated is anaerobic digested under anaerobic conditions, which releases phosphorus from the sludge into the supernatant to obtain sludge anaerobic digestion liquid.

[0008] Iron salt coagulant is added to the anaerobic digestion liquid of the sludge to allow precipitation, thereby removing phosphorus from the anaerobic digestion liquid and allowing it to enter the precipitate. After solid-liquid separation, a solid-liquid mixture is obtained.

[0009] The solid-liquid mixture is added to the reactor, and a mixed bacterial agent of iron-reducing bacteria and metal-resistant bacteria is added to it. Under the action of iron-reducing bacteria, ferric iron is reduced to ferrous iron, which is then precipitated, dissolved, and recrystallized into lapis lazuli. Metal-resistant bacteria can enhance the resistance of microorganisms to high concentrations of metal ions, thereby realizing the recovery of phosphorus from sludge.

[0010] This invention combines the advantages of both chemical crystallization and microbial-induced crystallization, and uses a two-step process of iron salt coagulation and precipitation followed by iron-reducing bacteria-induced precipitation and recrystallization to recover phosphorus from wastewater. First, sludge is obtained from the secondary sedimentation tank of the wastewater treatment plant. The sludge to be treated is then anaerobically digested under anaerobic conditions, releasing phosphorus from the sludge into the anaerobic digestion liquid. Iron salt coagulant is added to the anaerobic digestion liquid to cause precipitation, removing phosphorus from the anaerobic digestion liquid and transferring it into the sediment. In this step, some organic impurities from the anaerobic digestion liquid also enter the sediment. The sediment and a small amount of anaerobic digestion liquid are then added to a reactor, along with a mixed bacterial agent of iron-reducing bacteria and metal-resistant bacteria. Under the action of the iron-reducing bacteria, ferric iron is reduced to ferrous iron. During this process, the precipitate dissolves and recrystallizes into lapis lazuli. During dissolution, impurities in the precipitate are released into the liquid phase, and microbial growth consumes some organic matter. This step improves the crystallinity and purity of the lapis lazuli, achieving phosphorus recovery from the sludge. This solves the current problems of low phosphorus load, large volume, low economic benefits of direct phosphorus recovery, low crystallinity of chemical precipitation, high impurity content, and difficulty in controlling dissolved oxygen, leading to easy oxidation.

[0011] In a preferred embodiment of the present invention, the iron salt is ferric chloride or polyferric chloride.

[0012] In a preferred embodiment of the present invention, the molar ratio of iron in the iron salt coagulant to phosphorus in the anaerobic digestion liquid of sludge is 1.5~2:1.

[0013] In a preferred embodiment of the present invention, the solid content of the solid-liquid mixture is 0.2% to 0.6%.

[0014] In a preferred embodiment of the present invention, the iron-reducing bacteria are *Geobacterium*, and the metal-resistant bacteria are *Copper-loving Bacteria*.

[0015] In a preferred embodiment of the present invention, the concentration of the mixed microbial agent in the anaerobic digestion liquid of sludge containing solid precipitate is 10. 8 CFU / mL ~10 9 CFU / mL, wherein the concentration ratio of Geobacterium and Copper-loving Bacteria is 2~5:1.

[0016] In a preferred embodiment of the present invention, the recrystallization temperature is 15°C to 40°C, and the anaerobic digestion pH is 6.3 to 7.5.

[0017] In a preferred embodiment of the present invention, the sedimentation time is 30 min to 120 min.

[0018] In a preferred embodiment of the present invention, the phosphorus content in the anaerobic digestion liquid of the sludge is 70 mg / L to 200 mg / L, the nitrogen content is 300 mg / L to 3000 mg / L, the COD is 500 mg / L to 1500 mg / L, and the pH is 6.3 to 7.5.

[0019] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention combines the advantages of both chemical crystallization and microbial induced crystallization, and uses a two-step process of iron salt coagulation precipitation and iron-reducing bacteria-induced precipitation recrystallization to recover phosphorus from wastewater. First, sludge is obtained from the secondary sedimentation tank of the wastewater treatment plant. The sludge to be treated is then anaerobically digested under anaerobic conditions, releasing phosphorus from the sludge into the anaerobic digestion liquid. Iron salt coagulant is added to the anaerobic digestion liquid to cause precipitation, removing phosphorus from the anaerobic digestion liquid and transferring it into the sediment. In this step, some organic impurities from the anaerobic digestion liquid also enter the sediment. The sediment and a small amount of anaerobic digestion liquid are then added to a reactor, along with a mixed bacterial agent of iron-reducing bacteria and metal-resistant bacteria. Under the action of the iron-reducing bacteria, ferric iron is reduced to ferrous iron. During this process, the precipitate dissolves and recrystallizes into lapis lazuli. During dissolution, impurities in the precipitate are released into the liquid phase, and microbial growth consumes some organic matter. This step improves the crystallinity and purity of the lapis lazuli, achieving phosphorus recovery from the sludge. This solves the current problems of low phosphorus load, large volume, low economic benefits of direct phosphorus recovery, low crystallinity of chemical precipitation, high impurity content, and difficulty in controlling dissolved oxygen, leading to easy oxidation.

[0020] 2. The synergistic effect among multiple microorganisms in this invention is manifested in: *Geobacterium* spp. Geobacter It is a type of dissimilar iron-reducing bacterium, capable of reducing Fe... 3+ Reduced to Fe 2+ This induces the dissolution and recrystallization of amorphous ferric phosphate precipitate to form lapis lazuli. Ferric iron (Fe3+) is a transition metal ion rather than a typical "heavy metal," but at high concentrations it can still exhibit heavy metal-like toxicity. Copper-loving bacteria (Spp. spp.) Cupriavidus They can secrete substances that resist metal ions, protecting the entire microbial community from poisoning. Furthermore, copper-loving bacteria... Cupriavidus It can also secrete electroactive substances, participate in the iron reduction process, and promote the reduction of ferric iron. The two work together to improve the crystallization efficiency of lapis lazuli.

[0021] 3. This invention first precipitates iron salts and then induces recrystallization to solve the current problems of low phosphorus load, large volume, low economic benefits of direct recovery, low crystallinity of chemical precipitation, many impurities, and difficulty in controlling dissolved oxygen and easy oxidation in wastewater. Attached Figure Description

[0022] Figure 1 This is a process flow diagram of the present invention for recovering phosphorus from sludge.

[0023] Figure 2 The images show the XRD patterns of the iron-reducing bacteria precipitate at different time points in Example 1 of this invention.

[0024] Figure 3 The graph shows the results of phosphorus recovery rate and vivianite recovery rate for different iron-phosphorus molar ratios according to the present invention.

[0025] Figure 4 The graph shows the results of phosphorus recovery rate and vivianite recovery rate with different solid contents according to the present invention.

[0026] Figure 5 The graph shows the results of phosphorus recovery rate and vivianite recovery rate for different types of iron salts in this invention. Detailed Implementation

[0027] The following detailed description, in conjunction with embodiments of the present invention and accompanying drawings, provides a clear and complete illustration of the technical solutions in these embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0028] It should be noted that all technical terms used in this invention are for the purpose of describing specific embodiments only and are not intended to limit the scope of protection of this invention. Unless otherwise specified, all raw materials, reagents, instruments and equipment used in the following embodiments of this invention can be purchased from the market or prepared by existing methods.

[0029] Figure 1 The present invention provides a process flow diagram for recovering phosphorus from sludge, comprising the following steps: obtaining sludge from the secondary sedimentation tank of a wastewater treatment plant, anaerobic digesting the sludge to be treated to release phosphorus from the sludge into the supernatant, adding ferric salts to the supernatant for precipitation, removing phosphorus from the anaerobic digestion liquid and allowing it to enter the precipitate, obtaining an anaerobic digestion liquid containing solid precipitate, then adding iron-reducing bacteria to induce recrystallization, under the action of iron-reducing bacteria, reducing ferric iron to ferrous iron, dissolving the precipitate and recrystallizing it into lapis lazuli, thereby realizing the recovery of phosphorus from the sludge.

[0030] Example 1 A method for recovering phosphorus from sludge includes the following steps: (1) The sludge to be treated was anaerobic digested for 20 days under anaerobic conditions to release phosphorus from the sludge into the supernatant. After the process was completed, the supernatant was poured out to obtain sludge anaerobic digestion liquid. The sludge anaerobic digestion liquid contained: phosphorus content 131 mg / L, nitrogen content 393 mg / L, COD 560 mg / L, and pH 6.8.

[0031] (2) Take a 5 L plastic bucket and pour in 5 L of sludge anaerobic digestion liquid from step (1). According to the iron-phosphorus molar ratio of 1.5:1, add FeCl3 solution with a concentration of 0.0846 mol / L dropwise to the sludge anaerobic digestion liquid while stirring. The stirring speed should not be too fast, otherwise it will not be easy to form flocs. After the addition is completed, stop stirring and get yellow flocs. Let it settle for 30 min to remove phosphorus from the sludge anaerobic digestion liquid and enter the precipitate to obtain anaerobic digestion liquid containing solid precipitate.

[0032] (3) Add the anaerobic digestion liquid containing solid precipitate to a 500 mL anaerobic bottle at a corresponding solid content of 0.6%. Connect the submerged tube to an N2 gas cylinder and aerate for 30 minutes. Use a syringe to inject 5 mL of the mixed bacterial solution into the bottle. The mixed bacterial solution contains... Geobacter sulfurreducens and Cupriavidus gilardii The concentration ratio is 3:1, and the total concentration is 1×10⁻⁶. 9 With a concentration of approximately CFU / mL, place the anaerobic bottle in a shaker at 30°C.

[0033] (4) Shake the vial well, take 2 mL of solution and centrifuge. Separate the supernatant and directly measure the phosphorus and iron concentrations. Calculate the phosphorus recovery rate.

[0034] Example 2 A method for recovering phosphorus from sludge includes the following steps: (1) The sludge to be treated was anaerobic digested for 20 days under anaerobic conditions to release phosphorus from the sludge into the supernatant. After the process was completed, the supernatant was poured out to obtain sludge anaerobic digestion liquid. The sludge anaerobic digestion liquid contained: phosphorus content 131 mg / L, nitrogen content 393 mg / L, COD 560 mg / L, and pH 6.8.

[0035] (2) Take a 5 L plastic bucket and pour in 5 L of sludge anaerobic digestion liquid from step (1). According to the iron-phosphorus molar ratio of 2:1, add FeCl3 solution with a concentration of 0.0846 mol / L dropwise while stirring. The stirring speed should not be too fast, otherwise it will not be easy to form flocs. After the addition is completed, stop stirring and get yellow flocs. Let it settle for 30 min to remove phosphorus from the sludge anaerobic digestion liquid and enter the precipitate to obtain anaerobic digestion liquid containing solid precipitate.

[0036] (3) Add the anaerobic digestion liquid containing solid precipitate to a 500 mL anaerobic bottle at a corresponding solid content of 0.6%. Connect the submerged tube to an N2 gas cylinder and aerate for 30 minutes. Use a syringe to inject 5 mL of the mixed bacterial solution into the bottle. The mixed bacterial solution contains... Geobacter sulfurreducens and Cupriavidus gilardii The concentration ratio is 3:1, and the total concentration is 1×10⁻⁶. 9 With a concentration of approximately CFU / mL, place the anaerobic bottle in a shaker at 30°C.

[0037] (4) Shake the vial well, take 2 mL of solution and centrifuge. Separate the supernatant and directly measure the phosphorus and iron concentrations. Calculate the phosphorus recovery rate.

[0038] Example 3 A method for recovering phosphorus from sludge includes the following steps: (1) The sludge to be treated was anaerobic digested for 20 days under anaerobic conditions to release phosphorus from the sludge into the supernatant. After the process was completed, the supernatant was poured out to obtain sludge anaerobic digestion liquid. The sludge anaerobic digestion liquid contained: phosphorus content 131 mg / L, nitrogen content 393 mg / L, COD 560 mg / L, and pH 6.8.

[0039] (2) Take a 5 L plastic bucket and pour in 5 L of sludge anaerobic digestion liquid from step (1). According to the iron-phosphorus molar ratio of 1.5:1, add FeCl3 solution with a concentration of 0.0846 mol / L dropwise to the sludge anaerobic digestion liquid while stirring. The stirring speed should not be too fast, otherwise it will not be easy to form flocs. After the addition is completed, stop stirring and get yellow flocs. Let it settle for 30 min to remove phosphorus from the sludge anaerobic digestion liquid and enter the precipitate to obtain anaerobic digestion liquid containing solid precipitate.

[0040] (3) Add the anaerobic digestion liquid containing solid precipitate to a 500 mL anaerobic bottle at a corresponding solid content of 0.2%. Connect the submerged tube to an N2 gas cylinder and aerate for 30 minutes. Use a syringe to inject 5 mL of the mixed bacterial solution into the bottle. The mixed bacterial solution contains... Geobacter sulfurreducens and Cupriavidus gilardii The concentration ratio is 3:1, and the total concentration is 1×10⁻⁶. 9 With a concentration of approximately CFU / mL, place the anaerobic bottle in a shaker at 30°C.

[0041] (4) Shake the vial well, take 2 mL of solution and centrifuge. Separate the supernatant and directly measure the phosphorus and iron concentrations. Calculate the phosphorus recovery rate.

[0042] Example 4 A method for recovering phosphorus from sludge includes the following steps: (1) The sludge to be treated was anaerobic digested for 20 days under anaerobic conditions to release phosphorus from the sludge into the supernatant. After the process was completed, the supernatant was poured out to obtain sludge anaerobic digestion liquid. The sludge anaerobic digestion liquid contained: phosphorus content 131 mg / L, nitrogen content 393 mg / L, COD 560 mg / L, and pH 6.8.

[0043] (2) Take a 5 L plastic bucket and pour in 5 L of sludge anaerobic digestion liquid from step (1). According to the iron-phosphorus molar ratio of 1.5:1, add FeCl3 solution with a concentration of 0.0846 mol / L dropwise to the sludge anaerobic digestion liquid while stirring. The stirring speed should not be too fast, otherwise it will not be easy to form flocs. After the addition is completed, stop stirring and get yellow flocs. Let it settle for 30 min to remove phosphorus from the sludge anaerobic digestion liquid and enter the precipitate to obtain anaerobic digestion liquid containing solid precipitate.

[0044] (3) Add the anaerobic digestion liquid containing solid precipitate to a 500 mL anaerobic bottle at a corresponding solid content of 0.4%. Connect the submerged tube to an N2 gas cylinder and aerate for 30 minutes. Use a syringe to inject 5 mL of the mixed bacterial solution into the bottle. The mixed bacterial solution contains... Geobacter sulfurreducens and Cupriavidus gilardii The concentration ratio is 3:1, and the total concentration is 1×10⁻⁶. 9 With a concentration of approximately CFU / mL, place the anaerobic bottle in a shaker at 30°C.

[0045] (4) Shake the vial well, take 2 mL of solution and centrifuge. Separate the supernatant and directly measure the phosphorus and iron concentrations. Calculate the phosphorus recovery rate.

[0046] Example 5 A method for recovering phosphorus from sludge includes the following steps: (1) The sludge to be treated was anaerobic digested for 20 days under anaerobic conditions to release phosphorus from the sludge into the supernatant. After the process was completed, the supernatant was poured out to obtain sludge anaerobic digestion liquid. The sludge anaerobic digestion liquid contained: phosphorus content 131 mg / L, nitrogen content 393 mg / L, COD 560 mg / L, and pH 6.8.

[0047] (2) Take a 5 L plastic bucket and pour in 5 L of sludge anaerobic digestion liquid from step (1). According to the iron-phosphorus molar ratio of 1.5:1, add a 0.0846 mol / L polyferric chloride solution to the sludge anaerobic digestion liquid while stirring. The stirring speed should not be too fast, otherwise it will not be easy to form flocs. After the addition is completed, stop stirring and get yellow flocs. Let it settle for 30 minutes to remove phosphorus from the sludge anaerobic digestion liquid and enter the precipitate to obtain anaerobic digestion liquid containing solid precipitate.

[0048] (3) Add the anaerobic digestion liquid containing solid precipitate to a 500 mL anaerobic bottle at a corresponding solid content of 0.6%. Connect the submerged tube to an N2 gas cylinder and aerate for 30 minutes. Use a syringe to inject 5 mL of the mixed bacterial solution into the bottle. The mixed bacterial solution contains... Geobacter sulfurreducens and Cupriavidus gilardii The concentration ratio is 3:1, and the total concentration is 1×10⁻⁶. 9 With a concentration of approximately CFU / mL, place the anaerobic bottle in a shaker at 30°C.

[0049] (4) Shake the vial well, take 2 mL of solution and centrifuge. Separate the supernatant and directly measure the phosphorus and iron concentrations. Calculate the phosphorus recovery rate.

[0050] Comparative Example 1 A method for recovering phosphorus from sludge includes the following steps: (1) The sludge to be treated was anaerobic digested for 20 days under anaerobic conditions to release phosphorus from the sludge into the supernatant. After the process was completed, the supernatant was poured out to obtain sludge anaerobic digestion liquid. The sludge anaerobic digestion liquid contained: phosphorus content 131 mg / L, nitrogen content 393 mg / L, COD 560 mg / L, and pH 6.8.

[0051] (2) Take a 5 L plastic bucket and pour in 5 L of anaerobic digestion liquid from step (1). Add 0.0846 mol / L ferrous sulfate solution while stirring, according to the iron-phosphorus molar ratio of 1:1. The stirring speed should not be too fast, otherwise it will not be easy to form flocs. Stop stirring after the addition is completed. Yellow flocs are obtained. After precipitating for 30 min, the phosphorus is removed from the sludge anaerobic digestion liquid and enters the precipitate to obtain anaerobic digestion liquid containing solid precipitate.

[0052] (3) Add the anaerobic digestion liquid containing solid precipitate to a 500 mL anaerobic bottle at a corresponding solid content of 0.6%. Connect the submerged tube to an N2 gas cylinder and aerate for 30 minutes. Use a syringe to inject 5 mL of the mixed bacterial solution into the bottle. The mixed bacterial solution contains... Geobacter sulfurreducens and Cupriavidus gilardii The concentration ratio is 3:1, and the total concentration is 1×10⁻⁶. 9 With a concentration of approximately CFU / mL, place the anaerobic bottle in a shaker at 30°C.

[0053] (4) Shake the vial well, take 2 mL of solution and centrifuge. Separate the supernatant and directly measure the phosphorus and iron concentrations. Calculate the phosphorus recovery rate.

[0054] Comparative Example 2 A method for recovering phosphorus from sludge includes the following steps: (1) The sludge to be treated was anaerobic digested for 20 days under anaerobic conditions to release phosphorus from the sludge into the supernatant. After the process was completed, the supernatant was poured out to obtain sludge anaerobic digestion liquid. The sludge anaerobic digestion liquid contained: phosphorus content 131 mg / L, nitrogen content 393 mg / L, COD 560 mg / L, and pH 6.8.

[0055] (2) Take a 5 L plastic bucket and pour in 5 L of sludge anaerobic digestion liquid from step (1). Add 0.0846 mol / L ferrous sulfate while stirring, according to the iron-phosphorus molar ratio of 1.5:1. The stirring speed should not be too fast, otherwise it will not be easy to form flocs. Stop stirring after the addition is completed. Yellow flocs are obtained. After precipitating for 30 min, the phosphorus is removed from the sludge anaerobic digestion liquid and enters the precipitate to obtain anaerobic digestion liquid containing solid precipitate.

[0056] (3) Pour the anaerobic digestion liquid containing solid precipitate into a 500 mL anaerobic bottle at a solid content of 0.6%. Connect the long tube submerged below the liquid surface to an N2 gas cylinder and aerate for 30 minutes. Use a syringe to inject 5 mL of the mixed bacterial solution into the bottle. The mixed bacterial solution contains Geobacter sulfurreducens and Cupriavidus gilardii The concentration ratio is 3:1, and the total concentration is 1×10⁻⁶. 9 With a concentration of approximately CFU / mL, place the anaerobic bottle in a shaker at 30°C.

[0057] (4) Shake the vial well, take 2 mL of solution and centrifuge. Separate the supernatant and directly measure the phosphorus and iron concentrations. Calculate the phosphorus recovery rate.

[0058] The formulas for calculating phosphorus recovery rate and vivianite recovery rate are as follows: C0(P) is the phosphorus concentration in the anaerobic digestion liquid, V0 is the volume of the anaerobic digestion liquid, C(P) is the phosphorus concentration in the solution after phosphorus recovery, and V is the volume of the solution after phosphorus recovery. C(Fe(Ⅱ)) is the concentration of ferrous iron in the recovered precipitate, and C(total Fe) is the concentration of total iron in the recovered precipitate.

[0059] Results Analysis Figure 2 These are the XRD patterns of the iron-reducing bacteria precipitate at different time points in Example 1 of this invention. From... Figure 2 It can be seen that the intensity of the characteristic diffraction peaks belonging to vivianite increases with time, indicating that the precipitate gradually recrystallizes into vivianite under the action of iron-reducing bacteria.

[0060] Figure 3The graph shows the phosphorus recovery rate and lapis lazuli recovery rate under different iron-phosphorus molar ratios in Examples 1-2 and Comparative Example 1 of this invention. Based on the changes in phosphorus concentration throughout the experiment, it can be observed that the highest phosphorus and lapis lazuli recovery rates were achieved when the iron-phosphorus ratio was 1.5:1, indicating the best recovery effect. At an iron-phosphorus ratio of 1:1, not all phosphorus in the sludge digestate was precipitated, resulting in a lower phosphorus recovery rate. At an iron-phosphorus ratio of 2:1, excessive iron content led to an increase in the proportion of hydrated iron oxide in the precipitate; therefore, although the phosphorus recovery rate was higher, the proportion of lapis lazuli in the precipitate decreased, resulting in a lower lapis lazuli recovery rate.

[0061] Figure 4 The graphs show the results of phosphorus recovery rate and vivianite recovery rate at different solid contents in Examples 1 and 3-4 of this invention. It can be seen that as the solid content of the precipitate added to the recrystallization reactor increases from 0.2% to 0.6% (the iron-phosphorus molar ratio is 1.5:1), it can be observed that the recovery rate of phosphorus and vivianite is the highest when the solid content is 0.6%, and the recovery effect is the best. This is because as the solid content increases, the iron and phosphorus load in the whole system increases, and the crystallization effect is better.

[0062] Figure 5 The results of phosphorus recovery and lapis lazuli recovery for different iron salt types in Examples 1, 5, and Comparative Example 2 of this invention show that the recovery rate varies with the type of iron salt added in the flocculation and precipitation step. The highest phosphorus recovery rate was achieved using FeCl3, while the highest lapis lazuli recovery rate was achieved using polyferric chloride (PFC), although the difference between FeCl3 and PFC was not significant. The worst effect was achieved using FeSO4, because ferrous iron cannot be utilized by iron-reducing bacteria, thus preventing precipitation recrystallization. The resulting lapis lazuli was directly generated by chemical precipitation without a secondary mineralization process by microorganisms.

[0063] In summary, this invention utilizes a two-step process of iron salt coagulation and precipitation followed by iron-reducing bacteria-induced precipitation and recrystallization to recover phosphorus from sludge. First, sludge is obtained from the secondary sedimentation tank of the wastewater treatment plant. The sludge to be treated is then anaerobically digested under anaerobic conditions, releasing phosphorus from the sludge into the anaerobic digestion liquid. Iron salt coagulant is added to the anaerobic digestion liquid to cause precipitation, removing phosphorus from the anaerobic digestion liquid and transferring it into the sediment. In this step, some organic impurities from the anaerobic digestion liquid also enter the sediment. The sediment and a small amount of anaerobic digestion liquid are then added to a reactor, along with a mixed bacterial agent of iron-reducing bacteria and metal-resistant bacteria. Under the action of the iron-reducing bacteria, ferric iron is reduced to ferrous iron. During this process, the precipitate dissolves and recrystallizes into lapis lazuli. During dissolution, impurities in the precipitate are released into the liquid phase, and microbial growth consumes some organic matter. This step improves the crystallinity and purity of the lapis lazuli, achieving phosphorus recovery from the sludge. This solves the current problems of low phosphorus load, large volume, low economic benefits of direct phosphorus recovery, low crystallinity of chemical precipitation, high impurity content, and difficulty in controlling dissolved oxygen, leading to easy oxidation.

[0064] Currently, chemical phosphorus removal technology is widely used in wastewater treatment. This invention improves and expands the application scenarios of existing technologies, with high connectivity and strong adaptability, reducing modification and operation costs. It is expected to recover higher purity blue iron ore at low cost by utilizing the microbial induced recrystallization process, thereby increasing the added value of the product. In response to the problem of low purity and difficulty in separation of phosphorus recovery products from wastewater using the blue iron ore crystallization method, inspired by the natural microbial induced mineralization process, a phosphorus recovery system coupled with chemical precipitation and microbial secondary mineralization is constructed. Through the synergistic effect of iron-reducing bacteria-induced iron-phosphorus precipitation recrystallization and organic carbon metabolism processes, the purification and recovery of blue iron ore is achieved.

[0065] It should be noted that when numerical ranges are involved in this invention, it should be understood that both endpoints of each numerical range and any value between the two endpoints can be selected. Since the steps and methods used are the same as in the embodiments, preferred embodiments are described here to avoid redundancy. Although preferred embodiments of the invention have been described, those skilled in the art, once they understand the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this invention.

[0066] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for recovering phosphorus from sludge, characterized in that, Includes the following steps: The sludge to be treated is anaerobic digested under anaerobic conditions, so that the phosphorus in the sludge is released into the supernatant to obtain sludge anaerobic digestion liquid. Iron salt coagulant is added to the anaerobic digestion liquid of the sludge to allow precipitation, thereby removing phosphorus from the anaerobic digestion liquid and allowing it to enter the precipitate. After solid-liquid separation, a solid-liquid mixture is obtained. The solid-liquid mixture is added to a reactor, and a mixed bacterial agent of iron-reducing bacteria and metal-resistant bacteria is added to it. Under the action of iron-reducing bacteria, ferric iron is reduced to ferrous iron, which is then precipitated, dissolved, and recrystallized into lapis lazuli, thereby realizing the recovery of phosphorus from the sludge. The iron salt coagulant is ferric chloride or polyferric chloride; the molar ratio of iron in the iron salt coagulant to phosphorus in the anaerobic digestion liquid of sludge is 1.5~2:

1. The mixed microbial agent contains Geobacter sulfurreducens and Cupriavidus gilardii The concentration ratio is 3:1, and the total concentration is 1×10⁻⁶. 9 CFU / mL; The recrystallization temperature is 15℃~40℃, and the anaerobic digestion pH is 6.3~7.

5.

2. The method for recovering phosphorus from sludge according to claim 1, characterized in that, The solid content of the solid-liquid mixture is 0.2% to 0.6%.

3. The method for recovering phosphorus from sludge according to claim 1, characterized in that, The sedimentation time is 30 min to 120 min.

4. The method for recovering phosphorus from sludge according to claim 1, characterized in that, The phosphorus content in the anaerobic digestion liquid of sludge is 70 mg / L to 200 mg / L, the nitrogen content is 300 mg / L to 3000 mg / L, the COD is 500 mg / L to 1500 mg / L, and the pH is 6.3 to 7.5.

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