Method for recovering phosphorus from sludge

Abstract

The invention relates to the technical field of sewage treatment and resource utilization, in particular to a method for recovering phosphorus from sludge, which comprises the following steps: carrying out anaerobic digestion on the sludge to be treated under anaerobic conditions to release phosphorus in the sludge into supernate to obtain sludge anaerobic digestion liquid, adding an iron salt coagulant into the sludge anaerobic digestion liquid, precipitating, filtering, washing and drying to obtain the phosphorus-containing sludge. Removing phosphorus from the supernate, enabling the phosphorus to enter a precipitate, and removing the supernate to obtain a solid-liquid mixture; and adding the solid-liquid mixture into a reactor, adding a mixed bacterial agent of iron reducing bacteria and metal-resistant bacteria into the reactor, reducing ferric iron into ferrous iron under the action of the iron reducing bacteria, dissolving the precipitate, and recrystallizing to obtain the blue iron ore, thereby realizing the recovery of phosphorus in the sludge. According to the invention, ferric salt precipitates are re-induced to precipitate and recrystallize, so that the problems of low phosphorus load, large volume, low economic benefit of direct recovery, low crystallinity of chemical precipitates, more impurities, difficulty in controlling dissolved oxygen and easiness in oxidation at present are solved.

Description

Technical Field

[0001] The invention relates to the technical field of sewage treatment and resource utilization, and in particular to a method for recovering phosphorus from sludge. Background Art

[0002] Phosphate is a non-renewable resource and is unevenly distributed around the world. Phosphorus and its compounds generally do not exist in gaseous form. The global phosphorus cycle is a typical sedimentary cycle. Phosphorus mainly flows from land to the ocean by water, and it is a very long process for sediments to return to land through crustal movement. Human activities have intensified the one-way flow of phosphorus from mines to farms, rivers, and finally to the ocean. The extremely scarce circulation path has caused phosphorus to become an increasingly limited resource.

[0003] Phosphorus is a strategic non-metal that is related to the safety and stability of important industrial chains and supply chains such as food security, life and health, and new energy. On the one hand, phosphorus resources are in short supply, while on the other hand, phosphorus is generally in excess in natural water bodies. A large amount of phosphorus enters rivers and lakes, causing eutrophication of water bodies. The contradiction between phosphorus resource shortage and phosphorus excess in water bodies needs to be resolved urgently. Sewage is an important phosphorus sink. Sewage treatment plants around the world treat about 130 million tons of phosphorus per year. 4 Most of the phosphorus enters the sludge, so recovering phosphorus from sludge provides an important phosphorus recycling route, which helps to achieve a sustainable supply of phosphorus. One of the methods is the blue iron ore crystallization method.

[0004] In the cyanite crystallization method, it is difficult to separate the cyanite directly from the sludge due to the large amount of organic impurities. At present, some studies have used magnetic separation to separate cyanite from sludge, but the recovery efficiency and purity of cyanite still need to be improved. Recovering phosphorus from the anaerobic digestion liquid of phosphorus-rich sludge has fewer impurities than sludge, and the recovered cyanite is relatively easy to separate.

[0005] The existing methods for recovering blue iron ore from anaerobic digestion liquid of sludge by crystallization include chemical crystallization and microbial induced crystallization. Chemical crystallization is to directly add divalent iron ions and crystallize and precipitate with phosphate. There are two problems with this method. First, coagulation occurs during the crystallization precipitation process, which will precipitate the suspended solids and part of the soluble organic matter in the sewage, resulting in low crystallinity and high impurity content. Second, the dissolved oxygen level cannot be strictly controlled during the precipitation process, which will cause the divalent iron to be oxidized to trivalent iron, which will also lead to low crystallinity and purity. The method of microbial induced crystallization is to reduce the trivalent iron in the water to divalent iron through the reduction action of iron-reducing microorganisms, and then form blue iron ore crystals with phosphate. The anaerobic growth process of microorganisms maintains a low DO level, which solves the problem that the chemical crystallization method alone cannot effectively regulate DO. However, the current direct microbial induction of blue iron ore crystallization in anaerobic digestion liquid has the problems of low phosphorus load, too large reactor volume and too little product, resulting in low economic benefits. Summary of the invention

[0006] In view of the above-mentioned deficiencies in the prior art, the object of the present invention is to provide a method for recovering phosphorus from sludge. The present invention adopts iron salt precipitation and then induces recrystallization to solve the current problems of low phosphorus load in sewage, large volume, low economic benefit of direct recovery, low crystallinity of chemical precipitation, many impurities, and difficult to control dissolved oxygen and easy oxidation.

[0007] In order to solve the above technical problems, the present invention adopts the following technical solutions: A method for recovering phosphorus from sludge comprises the following steps: The sludge to be treated is anaerobically digested under anaerobic conditions to release phosphorus in the sludge into the supernatant to obtain sludge anaerobic digestion liquid.

[0008] An iron salt coagulant is added to the anaerobic digestion liquid of the sludge to perform precipitation, phosphorus is removed from the anaerobic digestion liquid of the sludge and enters the precipitation, and a solid-liquid mixture is obtained after solid-liquid separation.

[0009] The solid-liquid mixture is added into a reactor, and a mixed bacterial agent of iron-reducing bacteria and metal-resistant bacteria is added thereto. Under the action of the iron-reducing bacteria, the trivalent iron is reduced to divalent iron, and the precipitate is dissolved and recrystallized into blue iron ore. The metal-resistant bacteria can enhance the resistance of microorganisms to high concentrations of metal ions, thereby realizing the recovery of phosphorus in the sludge.

[0010] The invention combines the advantages of chemical crystallization and microbial induced crystallization, and recovers phosphorus from sewage in two steps of iron salt coagulation precipitation and iron-reducing bacteria induced precipitation recrystallization. First, sludge is obtained from the secondary sedimentation tank of the sewage treatment plant, and the sludge to be treated is anaerobically digested under anaerobic conditions to release phosphorus in the sludge into the anaerobic digestion liquid of the sludge. An iron salt coagulant is added to the anaerobic digestion liquid of the sludge to perform precipitation, and phosphorus is removed from the anaerobic digestion liquid of the sludge and enters the precipitate. In this step, some organic impurities in the anaerobic digestion liquid of the sludge will also enter the precipitate. Then, the precipitate and a small amount of anaerobic digestion liquid of the sludge are added to the reactor, and a mixed bacterial agent 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. In this process, the precipitate dissolves and recrystallizes into blue iron ore. During the dissolution process, impurities in the precipitate will be released into the liquid phase, and microbial growth will consume some organic matter. This step can improve the crystallinity and purity of blue iron ore, realize the recovery of phosphorus in the sludge, and solve the current problems of low phosphorus load, large volume, low economic benefit of direct recovery of sewage, low crystallinity of chemical precipitation, many impurities, and difficult to control dissolved oxygen and 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 of the genus Geobacter, and the metal-resistant bacteria are of the genus Cuprophilus.

[0015] In a preferred embodiment of the present invention, the concentration of the mixed bacterial agent in the anaerobic digestion liquid of the sludge containing solid precipitation is 10 8 CFU / mL~10 9 CFU / mL, among which the concentration ratio of Geobacter and Cuprophilus was 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 precipitation 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~200 mg / L, the nitrogen content is 300 mg / L~3000 mg / L, the COD is 500 mg / L~1500 mg / L, and the pH is 6.3~7.5.

[0019] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention combines the advantages of chemical crystallization and microbial induced crystallization, and uses iron salt coagulation precipitation and iron-reducing bacteria induced precipitation recrystallization to recover phosphorus from sewage in two steps. First, sludge is obtained from the secondary sedimentation tank of the sewage treatment plant, and the sludge to be treated is anaerobically digested under anaerobic conditions to release phosphorus in the sludge into the anaerobic digestion liquid of the sludge. An iron salt coagulant is added to the anaerobic digestion liquid of the sludge to perform precipitation, and phosphorus is removed from the anaerobic digestion liquid of the sludge and enters the precipitate. In this step, some organic impurities in the anaerobic digestion liquid of the sludge will also enter the precipitate. Then, the precipitate and a small amount of anaerobic digestion liquid of the sludge are added to the reactor, and a mixed bacterial agent 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. In this process, the precipitate dissolves and recrystallizes into blue iron ore. During the dissolution process, impurities in the precipitate will be released into the liquid phase, and microbial growth will consume some organic matter. This step can improve the crystallinity and purity of blue iron ore, realize the recovery of phosphorus in the sludge, and solve the current problems of low phosphorus load, large volume, low economic benefit of direct recovery of sewage, low crystallinity of chemical precipitation, many impurities, and difficult to control dissolved oxygen and easy oxidation.

[0020] 2. The synergistic effect between various microorganisms in the present invention is manifested in: Geobacter Geobacter It is a dissimilatory iron-reducing bacterium that can convert Fe 3+ Reduction to Fe 2+ , thereby inducing the amorphous iron phosphate precipitate to dissolve and recrystallize to form cyanite. Trivalent iron is a transition metal ion rather than a typical "heavy metal", but it may still show toxicity similar to heavy metals at high concentrations. Cupriavidus It can secrete metal ion resistance substances to protect the entire microbial community from toxicity. Cupriavidus It can also secrete electroactive substances, participate in the iron reduction process, and promote the reduction of trivalent iron. The cooperation between the two can improve the crystallization efficiency of blue iron ore.

[0021] 3. The present invention first precipitates the iron salt and then induces recrystallization to solve the current problems of low phosphorus load in sewage, large volume, low economic benefits of direct recovery, low crystallinity of chemical precipitation, many impurities, and difficult to control dissolved oxygen and easy oxidation. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The present invention is a process flow chart for recovering phosphorus from sludge.

[0023] Figure 2 The XRD patterns of the precipitate at different time points under the induction of iron-reducing bacteria in Example 1 of the present invention are shown.

[0024] Figure 3 The figure is a result diagram of phosphorus recovery rate and blue iron ore recovery rate at different iron-phosphorus molar ratios of the present invention.

[0025] Figure 4 The figure is a result diagram of phosphorus recovery rate and blue iron ore recovery rate at different solid contents of the present invention.

[0026] Figure 5 This is a graph showing the phosphorus recovery rate and blue iron ore recovery rate results for different iron salt types of the present invention. DETAILED DESCRIPTION

[0027] The following is a detailed description of the technical solutions in the embodiments of the present invention in combination with the preferred embodiments and the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

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

[0029] Figure 1 The process flow chart of recovering phosphorus from sludge of the present invention comprises the following steps: obtaining sludge from the secondary sedimentation tank of a sewage treatment plant, performing anaerobically digesting the sludge to be treated to release phosphorus in the sludge into a supernatant, adding trivalent iron salt to the supernatant for precipitation, removing phosphorus from the anaerobic digestion liquid of the sludge and allowing it to enter the precipitate, obtaining an anaerobic digestion liquid containing solid precipitate, then adding iron-reducing bacteria to induce recrystallization, and then reducing trivalent iron to divalent iron under the action of the iron-reducing bacteria, dissolving the precipitate and recrystallizing it into blue iron ore, thereby realizing the recovery of phosphorus in the sludge.

[0030] Example 1 A method for recovering phosphorus from sludge comprises the following steps: (1) The sludge to be treated is subjected to anaerobically digestion under anaerobic conditions for 20 days to release the phosphorus in the sludge into the supernatant. After the digestion, the supernatant is poured out to obtain the sludge anaerobic digestion liquid. The sludge anaerobic digestion liquid contains: 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 5 L of the anaerobic digestion liquid of the sludge in step (1). According to the molar ratio of iron to phosphorus of 1.5:1, add a 0.0846 mol / L FeCl3 solution to the anaerobic digestion liquid while stirring. The stirring speed should not be too fast, otherwise it is not easy to form flocs. After the addition is completed, stop stirring to obtain yellow flocs. Let it settle for 30 minutes to remove phosphorus from the anaerobic digestion liquid and enter the precipitate to obtain anaerobic digestion liquid containing solid precipitates.

[0032] (3) Add the anaerobic digestion liquid containing solid precipitation into a 500 mL anaerobic bottle at a corresponding solid content of 0.6%. Connect the long tube below the liquid surface to the N2 gas cylinder and aerate for 30 minutes. Take 5 mL of the mixed bacterial solution with a syringe and inject it 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 9 CFU / mL, and place the anaerobic bottle in a 30°C shaker.

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

[0034] Example 2 A method for recovering phosphorus from sludge comprises the following steps: (1) The sludge to be treated is subjected to anaerobically digestion under anaerobic conditions for 20 days to release the phosphorus in the sludge into the supernatant. After the digestion, the supernatant is poured out to obtain the sludge anaerobic digestion liquid. The sludge anaerobic digestion liquid contains: 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 5 L of the anaerobic digestion liquid of the sludge in step (1). According to the iron-phosphorus molar ratio of 2:1, add a 0.0846 mol / L FeCl3 solution into it while stirring. The stirring speed should not be too fast, otherwise it is not easy to form flocs. After the addition is completed, stop stirring to obtain yellow flocs. Let it settle for 30 minutes to remove phosphorus from the anaerobic digestion liquid and enter the sediment to obtain anaerobic digestion liquid containing solid sediment.

[0036] (3) Add the anaerobic digestion liquid containing solid precipitation into a 500 mL anaerobic bottle at a corresponding solid content of 0.6%. Connect the long tube below the liquid surface to the N2 gas cylinder and aerate for 30 minutes. Take 5 mL of the mixed bacterial solution with a syringe and inject it 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 9 CFU / mL, and place the anaerobic bottle in a 30°C shaker.

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

[0038] Example 3 A method for recovering phosphorus from sludge comprises the following steps: (1) The sludge to be treated is subjected to anaerobically digestion under anaerobic conditions for 20 days to release the phosphorus in the sludge into the supernatant. After the digestion, the supernatant is poured out to obtain the sludge anaerobic digestion liquid. The sludge anaerobic digestion liquid contains: 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 5 L of the anaerobic digestion liquid of the sludge in step (1). According to the molar ratio of iron to phosphorus of 1.5:1, add a 0.0846 mol / L FeCl3 solution to the anaerobic digestion liquid while stirring. The stirring speed should not be too fast, otherwise it is not easy to form flocs. After the addition is completed, stop stirring to obtain yellow flocs. Let it settle for 30 minutes to remove phosphorus from the anaerobic digestion liquid and enter the precipitate to obtain anaerobic digestion liquid containing solid precipitates.

[0040] (3) Add the anaerobic digestion liquid containing solid precipitation into a 500 mL anaerobic bottle at a corresponding solid content of 0.2%. Connect the long tube below the liquid surface to the N2 gas cylinder and aerate for 30 minutes. Take 5 mL of the mixed bacterial solution with a syringe and inject it 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 9 CFU / mL, and place the anaerobic bottle in a 30°C shaker.

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

[0042] Example 4 A method for recovering phosphorus from sludge comprises the following steps: (1) The sludge to be treated is subjected to anaerobically digestion under anaerobic conditions for 20 days to release the phosphorus in the sludge into the supernatant. After the digestion, the supernatant is poured out to obtain the sludge anaerobic digestion liquid. The sludge anaerobic digestion liquid contains: 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 5 L of the anaerobic digestion liquid of the sludge in step (1). According to the molar ratio of iron to phosphorus of 1.5:1, add a 0.0846 mol / L FeCl3 solution to the anaerobic digestion liquid while stirring. The stirring speed should not be too fast, otherwise it is not easy to form flocs. After the addition is completed, stop stirring to obtain yellow flocs. Let it settle for 30 minutes to remove phosphorus from the anaerobic digestion liquid and enter the precipitate to obtain anaerobic digestion liquid containing solid precipitates.

[0044] (3) Add the anaerobic digestion liquid containing solid precipitation into a 500 mL anaerobic bottle at a corresponding solid content of 0.4%. Connect the long tube below the liquid surface to the N2 gas cylinder and aerate for 30 minutes. Take 5 mL of the mixed bacterial solution with a syringe and inject it 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 9 CFU / mL, and place the anaerobic bottle in a 30°C shaker.

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

[0046] Example 5 A method for recovering phosphorus from sludge comprises the following steps: (1) The sludge to be treated is subjected to anaerobically digestion under anaerobic conditions for 20 days to release the phosphorus in the sludge into the supernatant. After the digestion, the supernatant is poured out to obtain the sludge anaerobic digestion liquid. The sludge anaerobic digestion liquid contains: 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 5 L of the anaerobic digestion liquid of the sludge in step (1). According to the iron-phosphorus molar ratio of 1.5:1, add a 0.0846 mol / L polyferric chloride solution to the anaerobic digestion liquid while stirring. The stirring speed should not be too fast, otherwise it is not easy to form flocs. After the addition is completed, stop stirring to obtain yellow flocs. Let it settle for 30 minutes to remove phosphorus from the anaerobic digestion liquid and enter the precipitate to obtain anaerobic digestion liquid containing solid precipitates.

[0048] (3) Add the anaerobic digestion liquid containing solid precipitation into a 500 mL anaerobic bottle at a corresponding solid content of 0.6%. Connect the long tube below the liquid surface to the N2 gas cylinder and aerate for 30 minutes. Take 5 mL of the mixed bacterial solution with a syringe and inject it 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 9 CFU / mL, and place the anaerobic bottle in a 30°C shaker.

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

[0050] Comparative Example 1 A method for recovering phosphorus from sludge comprises the following steps: (1) The sludge to be treated is subjected to anaerobically digestion under anaerobic conditions for 20 days to release the phosphorus in the sludge into the supernatant. After the digestion, the supernatant is poured out to obtain the sludge anaerobic digestion liquid. The sludge anaerobic digestion liquid contains: 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 5 L of the anaerobic digestion liquid in step (1) into it. According to the iron-phosphorus molar ratio of 1:1, add a 0.0846 mol / L ferrous sulfate solution into it while stirring. The stirring speed should not be too fast, otherwise it is not easy to form flocs. After the addition is completed, stop stirring to obtain yellow flocs. Settle for 30 minutes to remove phosphorus from the sludge anaerobic digestion liquid and enter the sediment to obtain anaerobic digestion liquid containing solid sediment.

[0052] (3) Add the anaerobic digestion liquid containing solid precipitation into a 500 mL anaerobic bottle at a corresponding solid content of 0.6%. Connect the long tube below the liquid surface to the N2 gas cylinder and aerate for 30 minutes. Take 5 mL of the mixed bacterial solution with a syringe and inject it 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 9 CFU / mL, and place the anaerobic bottle in a 30°C shaker.

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

[0054] Comparative Example 2 A method for recovering phosphorus from sludge comprises the following steps: (1) The sludge to be treated is subjected to anaerobically digestion under anaerobic conditions for 20 days to release the phosphorus in the sludge into the supernatant. After the digestion, the supernatant is poured out to obtain the sludge anaerobic digestion liquid. The sludge anaerobic digestion liquid contains: 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 5 L of the anaerobic digestion liquid of the sludge in step (1). According to the iron-phosphorus molar ratio of 1.5:1, add ferrous sulfate with a concentration of 0.0846 mol / L while stirring. The stirring speed should not be too fast, otherwise it is not easy to form flocs. After the addition is completed, stop stirring to obtain yellow flocs. Settle for 30 minutes to remove phosphorus from the anaerobic digestion liquid of the sludge and enter the sediment to obtain anaerobic digestion liquid containing solid sediment.

[0056] (3) Pour the anaerobic digestion liquid containing solid precipitation into a 500 mL anaerobic bottle at a corresponding solid content of 0.6%. Connect the long tube below the liquid surface to the N2 gas cylinder and aerate for 30 minutes. Take 5 mL of the mixed bacterial solution with a syringe and inject it 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 9 CFU / mL, and place the anaerobic bottle in a 30°C shaker.

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

[0058] The calculation formulas for phosphorus recovery and blue iron ore recovery are as follows: C0(P) is the phosphorus concentration in the anaerobic digestion solution, V0 is the volume of the anaerobic digestion solution, C(P) is the phosphorus concentration of 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 The XRD patterns of the precipitate at different time points under the induction of iron-reducing bacteria in Example 1 of the present invention are shown in FIG. Figure 2 It can be seen that the intensity of the characteristic diffraction peak belonging to violaceous iron ore increases with time. The results show that the precipitate gradually recrystallizes into violaceous iron ore under the action of iron-reducing bacteria.

[0060] Figure 3The results of the phosphorus recovery rate and blue iron ore recovery rate under different iron-phosphorus molar ratios of Examples 1 to 2 of the present invention and Comparative Example 1 are shown. According to the change in phosphorus concentration during the entire experiment, it can be observed that when the iron-phosphorus ratio is 1.5:1, the phosphorus recovery rate and blue iron ore recovery rate are both the highest, and the recovery effect is the best. When the iron-phosphorus ratio is 1:1, not all the phosphorus in the sludge digestion liquid is precipitated, so the phosphorus recovery rate is low. When the iron-phosphorus ratio is 2:1, the excessive iron content will lead to an increase in the proportion of hydrated iron oxide in the precipitate. Therefore, although the phosphorus recovery rate is higher, the proportion of blue iron ore in the precipitate decreases, and the blue iron ore recovery rate decreases.

[0061] Figure 4 The results of the phosphorus recovery rate and the blue iron ore recovery rate at different solid contents in Example 1 and Example 3 to Example 4 of the present invention show 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 when the solid content is 0.6%, the phosphorus recovery rate and the blue iron ore recovery rate are the highest, and the recovery effect is the best. This is because as the solid content increases, the iron and phosphorus loads in the entire system increase, and the crystallization effect is better.

[0062] Figure 5 The phosphorus recovery rate and blue iron ore recovery rate results of different iron salt types in Example 1, Example 5 and Comparative Example 2 of the present invention show that the recovery rate varies with the type of iron salt added in the flocculation precipitation step. When FeCl3 is used, the phosphorus recovery rate is the highest, and when polyferric chloride (PFC) is used, the blue iron ore recovery rate is the highest, but the difference between FeCl3 and PFC is not large. The effect of using FeSO4 is the worst, because divalent iron cannot be used by iron-reducing bacteria, so it cannot induce precipitation and recrystallization, and the blue iron ore generated is directly generated by chemical precipitation without secondary mineralization of microorganisms.

[0063] In summary, the present invention recovers phosphorus from sludge in two steps: iron salt coagulation precipitation and iron-reducing bacteria-induced precipitation recrystallization. First, sludge is obtained from the secondary sedimentation tank of the sewage treatment plant, and the sludge to be treated is anaerobically digested under anaerobic conditions to release phosphorus in the sludge into the anaerobic digestion liquid of the sludge. An iron salt coagulant is added to the anaerobic digestion liquid of the sludge to perform precipitation, and phosphorus is removed from the anaerobic digestion liquid of the sludge and enters the precipitate. In this step, some organic impurities in the anaerobic digestion liquid of the sludge will also enter the precipitate. Then, the precipitate and a small amount of anaerobic digestion liquid of the sludge are added to the reactor, and a mixed bacterial agent 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. In this process, the precipitate dissolves and recrystallizes into blue iron ore. During the dissolution process, impurities in the precipitate will be released into the liquid phase, and microbial growth will consume some organic matter. This step can improve the crystallinity and purity of blue iron ore, realize the recovery of phosphorus in the sludge, and solve the current problems of low phosphorus load, large volume, low economic benefit of direct recovery of sewage, low crystallinity of chemical precipitation, many impurities, and difficult to control dissolved oxygen and easy oxidation.

[0064] At present, chemical phosphorus removal technology is widely used in sewage treatment. The present invention improves and transforms the existing technology to expand the application scenarios, has high connectivity, strong adaptability, and reduces transformation and operation costs; the microbial induced recrystallization process is expected to recover blue iron ore with higher purity at low cost, thereby increasing the added value of products; in view of the problem that the blue iron ore crystallization method has low purity and difficulty in separating the phosphorus recovery products from sewage, inspired by the microbial induced mineralization process in nature, a phosphorus recovery system coupled with chemical precipitation and microbial secondary mineralization is constructed, and the blue iron ore purification and recovery is achieved through the synergy of iron-phosphorus precipitation recrystallization induced by iron-reducing bacteria and organic carbon metabolism.

[0065] It should be noted that when the present invention involves a numerical range, it should be understood that the two 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 those in the embodiments, in order to avoid redundancy, the present invention describes a preferred embodiment. Although the preferred embodiments of the present invention have been described, those skilled in the art may make additional changes and modifications to these embodiments once they know the basic creative concept. Therefore, the attached claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0066] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A method for recovering phosphorus from sludge, characterized in that: The following steps are involved: Anaerobic digestion of the sludge to be treated is carried out under anaerobic conditions to release phosphorus in the sludge into the supernatant to obtain anaerobic digestion liquid of the sludge; Adding an iron salt coagulant to the anaerobic digestion liquid of the sludge to perform precipitation, removing phosphorus from the anaerobic digestion liquid of the sludge and allowing it to enter the precipitate, and obtaining a solid-liquid mixture after solid-liquid separation; The solid-liquid mixture is added into a reactor, and a mixed bacterial agent of iron-reducing bacteria and metal-resistant bacteria is added thereto. Under the action of the iron-reducing bacteria, the trivalent iron is reduced to divalent iron, and the precipitation is dissolved and recrystallized into blue iron ore, thereby realizing the recovery of phosphorus in the sludge.

2. The method for recovering phosphorus from sludge according to claim 1, characterized in that: The iron salt coagulant is ferric chloride or polyferric chloride.

3. The method for recovering phosphorus from sludge according to claim 1, characterized in that: The molar ratio of iron in the iron salt coagulant to phosphorus in the anaerobic digestion liquid of sludge is 1.5~2:

1.

4. 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%~0.6%.

5. The method for recovering phosphorus from sludge according to claim 1, characterized in that: The iron-reducing bacteria belonged to the genus Geobacter, and the metal-resistant bacteria belonged to the genus Cuprophilus.

6. The method for recovering phosphorus from sludge according to claim 5, characterized in that: The concentration of mixed bacterial agent in anaerobic digestion liquid of sludge containing solid precipitation is 10 8 CFU / mL~10 9 CFU / mL, among which the concentration ratio of Geobacter and Cuprophilus was 2-5:

1.

7. The method for recovering phosphorus from sludge according to claim 1, characterized in that: The recrystallization temperature is 15℃~40℃, and the anaerobic digestion pH is 6.3~7.

5.

8. The method for recovering phosphorus from sludge according to claim 1, characterized in that: The precipitation time is 30min~120min.

9. 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 70mg / L~200mg / L, the nitrogen content is 300mg / L~3000mg / L, the COD is 500mg / L~1500mg / L, and the pH is 6.3~7.5.

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