Method for recycling phosphorus-containing wastewater

Abstract

The present invention provides a method for recycling phosphorus-containing wastewater, which includes step 1) subjecting the phosphorus-containing wastewater to membrane concentration treatment until the organic phosphorus in the membrane concentrate is solidified to obtain a high-concentration concentrate a containing solid organic phosphorus and a permeate a; step 2) subjecting the membrane concentrate obtained in step 1) to membrane separation to obtain a solid organic phosphorus-containing concentrate b and a salt-containing permeate b, evaporating and crystallizing the salt-containing permeate b to obtain industrial salt; and step 3) filtering the solid organic phosphorus-containing concentrate b to obtain a solid organic phosphorus-containing filtrate concentrate and a clarified filtrate, and drying the solid organic phosphorus-containing filtrate concentrate to obtain solid organic phosphorus. The phosphorus-containing wastewater treatment method recovers organic phosphorus simultaneously while recovering inorganic salts, is simple and easy to control, improves resource utilization, and reduces wastewater treatment costs for enterprises.

Description

[Technical Field]

[0001] The present invention belongs to the technical field of wastewater treatment, and relates to a method for treating phosphorus-containing wastewater to recover resources. [Background technology]

[0002] In recent years, phosphorus-based flame retardants have become an important alternative to bromine-based flame retardants due to their advantages of low corrosiveness and low toxicity, and have broad market prospects. Among them, aluminum diethylbisphosphinate is the most representative of alkyl phosphinate flame retardants, with the advantages of small particle size, high whiteness, good thermal stability, and non-toxicity, and is widely used in the flame retardant treatment of thermoplastics, thermosetting plastics, textiles, and textile products.

[0003] In the production process of aluminum diethylbisphosphinate, a large amount of refractory organic wastewater is generated, which is characterized by relatively high COD concentration and high total phosphorus content, and the main component of total phosphorus is aluminum diethylbisphosphinate.Existing treatment methods mainly include oxidation method, precipitation method, extraction method, biochemical method, etc., which can neutralize total phosphorus and discharge it in accordance with the standard, and at the same time, the inorganic salts in the wastewater can be further treated and reused as resources, but organic phosphorus cannot be recovered, which results in the waste of resources (aluminum diethylbisphosphinate).

[0004] Patent publication number CN108609788A discloses a process for treating phosphorus-based flame retardant wastewater, comprising: (1) adjusting the pH of phosphorus-based flame retardant wastewater to 2-9, adding a soluble iron(III) salt, and stirring and filtering the mixture to obtain filtrate I; (2) neutralizing and flocculating filtrate I and separating and removing the precipitate to obtain filtrate II; and (3) evaporating, concentrating, and crystallizing filtrate II to desalt it. The evaporated condensate is either directly discharged or recycled, and the concentrated liquid is directly returned to step (1) for repeated treatment. The treatment process of the present invention is applicable to continuous treatment of phosphorus-based flame retardant wastewater, and employs a main cycle of chemical pretreatment-neutralization and flocculation-evaporation and condensation. It is easy to operate, inexpensive, and capable of achieving zero discharge of phosphorus-based flame retardant wastewater.

[0005] Patent publication number CN108975585A discloses a method for recycling wastewater from the production of phosphorus-containing flame retardants, comprising: (1) wet catalytic oxidation of wastewater from the production of phosphorus-containing flame retardants to obtain an oxidized solution; (2) membrane separation of the oxidized solution obtained in (1) to obtain a membrane concentrate and a membrane permeate; and (3) crystallization of the membrane concentrate obtained in (2) to obtain industrial phosphate salts by freeze-crystallization, and (4) evaporation of the membrane permeate obtained in (2) to crystallize and desalinate the phosphate salts. The method of the present invention relies primarily on wet catalytic oxidation in combination with membrane separation and crystallization processes to treat phosphorus-containing flame retardant wastewater, which is simple, effectively decomposes organic phosphorus in the wastewater, and recovers salts in the wastewater as a resource, thereby achieving the goal of zero discharge.

[0006] In the above patented technical solutions, all of them achieve the discharge of organic phosphorus-containing wastewater in compliance with the standard and recover inorganic salts as resources, but do not recover organic phosphorus, which results in the waste of organic phosphorus resources. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention provides a method for recovering organic phosphorus from phosphorus-containing wastewater to solve the problem of organic phosphorus being wasted due to the inability to recover it. This method can recover inorganic salts and organic phosphorus at the same time, and the process is simple and easy to control, improving resource utilization and reducing wastewater treatment costs for enterprises. [Means for solving the problem]

[0008] In order to achieve the object of the invention, the present invention employs the following technical solution: A method for treating phosphorus-containing wastewater for resource recovery, comprising: Step 1) of subjecting phosphorus-containing wastewater to membrane concentration treatment until the organic phosphorus in the membrane concentrate becomes solid, thereby obtaining a highly concentrated concentrate a containing solid organic phosphorus and a permeate a; Step 2) is to subject the membrane concentrate obtained in step 1 to membrane separation treatment to obtain a solid organic phosphorus-containing concentrate b and a salt-containing permeate b, and evaporate and crystallize the salt-containing permeate b to obtain industrial salt; and 3) filtering the solid organic phosphorus-containing concentrated solution b to obtain a solid organic phosphorus-containing filtrate concentrate and a clarified filtrate, and drying the solid organic phosphorus-containing filtrate concentrate to obtain solid organic phosphorus.

[0009] In the treatment of phosphorus-containing wastewater in the present invention, first, a membrane concentration process is used to precipitate the organic phosphorus, aluminum diethylbisphosphinate, in the wastewater in a solid state, and then a membrane separation process is used to separate the solid aluminum diethylbisphosphinate from the salt dissolved in the wastewater, thereby achieving the separation and recovery of organic phosphorus and salt, and the process is simple and easy to control.

[0010] Preferably, in step 1), the phosphorus-containing wastewater is further subjected to a pre-filtration treatment before being subjected to membrane concentration treatment.

[0011] Preferably, the product water obtained after the pretreatment is returned to the subsequent membrane concentration treatment step, and the concentrated water obtained after the pretreatment is returned to the upstream production process.

[0012] Preferably, the filtration pretreatment method is one or more of plate and frame filtration, hollow fiber membrane filtration, tubular membrane filtration, filter bag filtration, and filter element filtration.

[0013] Preferably, the membrane used in the membrane concentration treatment process of step 1) is a nanofiltration membrane and / or a reverse osmosis membrane.

[0014] Preferably, the system employed in the membrane concentration process is divided into at least two stages.

[0015] Preferably, the water recovery rate of the membrane concentration system is controlled to 75-90%.

[0016] Preferably, the system employed in the membrane concentration treatment process employs two or three stages, each stage being composed of three to six membrane elements connected in series.

[0017] Research conducted during the experimental process of the present invention has shown that when a single-stage membrane concentration is used in the membrane concentration process, the concentration ratio of the organophosphorus aluminum diethylbisphosphinate is insufficient, and the organophosphorus aluminum diethylbisphosphinate cannot be precipitated in a fixed state from the raw material solution. Therefore, the membrane concentration system of the present invention employs at least two stages, allowing the organophosphorus aluminum diethylbisphosphinate to be precipitated in a fixed state.

[0018] Preferably, when the membrane concentration system has two stages, the operating parameters are that in the first stage, the water recovery rate is controlled to 50-60% and the operating pressure is controlled to 50-70 bar, and in the second stage, the water recovery rate is controlled to 40-50% and the operating pressure is controlled to 65-100 bar. Preferably, when the membrane concentration system has three stages, the operating parameters are that in the first stage, the water recovery rate is controlled to 50-60% and the operating pressure is controlled to 50-70 bar, and in the second stage, the water recovery rate is controlled to 40-50% and the operating pressure is controlled to 65-100 bar, and in the third stage, the water recovery rate is controlled to 30-50% and the operating pressure is controlled to 70-100 bar.

[0019] Preferably, in step 1), before the phosphorus-containing wastewater is subjected to membrane concentration treatment, an acid is added to adjust the pH and / or a temperature-lowering treatment is performed. Preferably, sulfuric acid is added to the phosphorus-containing wastewater to adjust the pH to 2.0 to 2.5, and the temperature of the phosphorus-containing wastewater is lowered to 20 to 25°C. In the present invention, the purity of the organic phosphorus, aluminum diethylbisphosphinate, can be improved to a certain extent by adding an acid to the flame retardant wastewater or performing a temperature-lowering treatment. Furthermore, when adding an acid to lower the pH in the present invention, sulfuric acid is preferably used, as sulfuric acid can react with carbonate in the flame retardant wastewater to produce sulfate, thereby improving the purity of the sodium sulfate recovered as a resource.

[0020] Preferably, in step 1), permeate a is introduced into a single-stage reverse osmosis concentration treatment to obtain concentrate c and permeate c, and permeate c is directly reused or discharged, or introduced into a multi-stage reverse osmosis treatment and then reused. The multi-stage reverse osmosis is not limited to two-stage reverse osmosis, three-stage reverse osmosis, four-stage reverse osmosis, or five-stage reverse osmosis, but is preferably two-stage reverse osmosis, and the concentrate after the multi-stage reverse osmosis treatment is returned to the previous stage of the single-stage reverse osmosis treatment, and the product water from the multi-stage reverse osmosis treatment is directly reused or discharged.

[0021] Preferably, the concentrate c is returned to the previous stage of the membrane concentration treatment process in step 1), or the concentrate c is sequentially treated by medium pressure reverse osmosis and high pressure reverse osmosis, or the concentrate c is sequentially treated by medium pressure reverse osmosis and disc tube reverse osmosis, the high pressure reverse osmosis is preferably spiral high pressure reverse osmosis, and the concentrate after high pressure reverse osmosis or disc tube reverse osmosis treatment is evaporated and crystallized to recover sodium sulfate, and the permeate after the two-stage reverse osmosis treatment is returned to the first stage reverse osmosis and mixed with the product water of the first stage reverse osmosis.

[0022] Preferably, in step 2), the membrane used for membrane separation is an ultrafiltration membrane and / or a loose nanofiltration membrane.

[0023] Preferably, the system employed for the membrane separation is divided into one, two or three stages, and each stage is composed of two to four membrane elements connected in series.

[0024] Preferably, the water recovery rate of the membrane separation is controlled to be 70 to 95%.

[0025] Preferably, the condensate obtained by condensing the salt-containing permeate (b) in the evaporation crystallization process is reused or directly discharged.

[0026] Preferably, the molecular weight cutoff of the ultrafiltration membrane is 2K to 100K.

[0027] Preferably, the loose nanofiltration membrane has a divalent salt permeability of ≧50%.

[0028] Preferably, in step 3), before filtering the concentrated solution b of solid organic phosphorus, a washing filtration step is further performed to obtain a concentrated solution and a permeate of the washed filtrate, which is then introduced into a subsequent filtration step, where the permeate of the washed filtrate is evaporated and crystallized to obtain industrial salt. The present invention can significantly improve the purity of the recovered organic phosphorus through the washing filtration step.

[0029] Preferably, in step 3), the filtered clarified liquid is returned to the previous production process.

[0030] Preferably, the condensate produced by condensing the solid organic phosphorus-containing filtered concentrate during the drying process is reused or directly discharged. [Effects of the Invention]

[0031] The present invention has the following beneficial effects.

[0032] First, a membrane concentration process is first used to precipitate the organophosphorus aluminum diethylbisphosphinate in the wastewater in a solid state, and then a membrane separation process is used to separate the solid organophosphorus aluminum diethylbisphosphinate from the salt dissolved in the wastewater, thereby achieving the separation and recovery of the organophosphorus and salt, and the process is simple and easy to control. Second, the organophosphorus aluminum diethylbisphosphinate and sodium sulfate recovered as resources in the present invention have higher purity. [Brief explanation of the drawings]

[0033] [Figure 1] FIG. 1 is a flow chart of the process for recovering phosphorus-containing wastewater from resources according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0034] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to explain the technical contents, the objects and effects achieved by the present invention in detail, the present invention will be described below with reference to the following embodiments.

[0035] Example 1

[0036] The water quality indicators for wastewater from a factory producing phosphorus-containing flame retardants are: COD content 2500 mg / L, total phosphorus content 800 mg / L, salt content 30,000 mg / L (mainly sodium sulfate), pH value 3.8, and temperature 55°C.

[0037] FIG. 1 is a flowchart of the phosphorus-containing wastewater recycling treatment process according to the present invention. First, the wastewater is cooled to 25°C using a heat exchanger, and then pre-treated by plate and frame filtration to remove solids. The concentrated water obtained by plate and frame filtration is returned to the previous production process, and the product water is introduced into the subsequent membrane concentration system for further treatment.

[0038] 1) Membrane concentration treatment The water produced by plate and frame filtration is introduced into a membrane concentration system. The membrane concentration system is composed of three membrane concentration units connected in series. The recovery rate of the water system is 85%. Each membrane concentration unit is composed of four nanofiltration membrane elements connected in series. The operating parameters set for each membrane concentration unit are: In the first stage, the water recovery rate is controlled at 50% and the operating pressure is controlled at 55 bar. In the second stage, the water recovery rate is controlled at 50% and the operating pressure is controlled at 70 bar. In the third stage, the water recovery rate is controlled to 40% and the operating pressure is controlled to 75 bar. The system is maintained at the preset operating parameters until the organic phosphorus in the membrane concentrate precipitates in a fixed state and becomes suspended in the concentrate. The concentrate from the membrane concentrate system is introduced into a membrane separation system for subsequent separation. The permeate from the membrane concentrate system is filtered through a single-stage reverse osmosis membrane to obtain a single-stage reverse osmosis permeate and a single-stage reverse osmosis concentrate. The single-stage reverse osmosis concentrate is returned to the upstream stage of the membrane concentration process in step 1. The single-stage reverse osmosis permeate is introduced into a two-stage reverse osmosis process. The two-stage reverse osmosis concentrate is returned to the upstream stage of the single-stage reverse osmosis process. The two-stage reverse osmosis permeate is directly reused or discharged. The COD of the two-stage reverse osmosis permeate is detected to be less than 30 mg / L and conductivity less than 50 μs / cm, meeting the water requirements of the production process.

[0039] After being treated by the nanofiltration membrane concentration system, the indicators of the concentrate water of the membrane concentration system (i.e., the concentrate water of the third stage) are COD content 15,250 mg / L, total phosphorus content 5,107 mg / L, and salt content 166,000 mg / L. The indicators of the product water of the membrane concentration system (a mixture of the product waters of the first, second, and third stages) are COD content 250 mg / L, total phosphorus content 40 mg / L, and salt content 6,000 mg / L.

[0040] 2) Membrane separation treatment The concentrated water from the membrane concentration system is introduced into the membrane separation system to obtain the membrane separation system product water and membrane separation system concentrated water. The membrane separation system is a single-stage system consisting of three ultrafiltration membrane elements connected in series. The molecular weight cutoff of the ultrafiltration membrane is 2K. The water system has a recovery rate controlled at 90% and an operating pressure controlled at 3 bar.

[0041] The concentrated water from the membrane separation system was washed and filtered. The washing and filtration was carried out three times in total. In the first washing, water was added to the concentrated water from the membrane concentration system in a 1:1 ratio to control the system recovery rate to 50%. The second and third washing and filtration were carried out by repeating the above process.

[0042] After the above washing and filtration process, a wash filtrate concentrate and a wash filtrate permeate are obtained. The wash filtrate concentrate is introduced into the subsequent filtration process, and the product water of the membrane separation system and the wash filtrate permeate are mixed and subjected to evaporation and crystallization to obtain industrial-grade sodium sulfate. The sodium sulfate yield and purity tested are 81.5% and 98.9%, respectively.

[0043] 3) Filtration The concentrate after cleaning and filtering through the membrane separation system is subjected to plate and frame filtration, and the clarified filtered liquid is returned to the previous production process. The filtered concentrate is then dried to obtain a solid organophosphorus product. The condensate collected during the drying process is reused or directly discharged. As a result of the detection, the obtained organophosphorus product has a yield of 62.0% and a purity of 92.1%.

[0044] Example 2 The difference between Example 2 and Example 1 is that the first stage reverse osmosis concentrate is not returned to the previous stage of the membrane concentration treatment process in step 1). Instead, The first-stage reverse osmosis concentrate is introduced into a medium-pressure reverse osmosis treatment to obtain a medium-pressure reverse osmosis permeate and a concentrate, and then the medium-pressure reverse osmosis concentrate is introduced into a high-pressure spiral reverse osmosis treatment to obtain a high-pressure spiral reverse osmosis concentrate and a permeate. The high-pressure spiral reverse osmosis concentrate is evaporated and crystallized to recover sodium sulfate, and the product water of the medium-pressure reverse osmosis and high-pressure spiral reverse osmosis treatment is mixed with the first-stage reverse osmosis permeate, which is then introduced into the subsequent first-stage reverse osmosis treatment process.

[0045] After calculations were performed for the membrane concentration process, the concentrate was evaporated and crystallized after being treated with single-stage reverse osmosis, medium-pressure reverse osmosis, and high-pressure spiral reverse osmosis. The final detection results showed that the yield of sodium sulfate was 88.6% and the purity was 98.5%, and the yield of organic phosphorus products was 52.1% and the purity was 91.1%.

[0046] Example 3 The difference between Example 3 and Example 2 is that the high-pressure spiral reverse osmosis system is replaced with a disc tube reverse osmosis system.

[0047] After calculations are made for the membrane concentration process steps, the concentrate after being treated with single-stage reverse osmosis, medium-pressure reverse osmosis and high-pressure spiral reverse osmosis is evaporated and crystallized.

[0048] Sodium sulfate has a yield of 88.3% and a purity of 98.7%, and organophosphorus products have a yield of 53.2% and a purity of 90.6%.

[0049] Example 4 The difference between Example 4 and Example 1 is that the temperature of the wastewater is lowered to 20°C by a heat exchanger.

[0050] As a result of the detection, the obtained sodium sulfate had a yield of 81.6% and a purity of 98.6%, and the organic phosphorus product had a yield of 65.1% and a purity of 92.4%.

[0051] Example 5 The difference between Example 5 and Example 1 is that the temperature of the wastewater is not lowered by a heat exchanger before treatment.

[0052] As a result of the detection, the obtained sodium sulfate had a yield of 77.3% and a purity of 98.2%, and the organic phosphorus product had a yield of 50.2% and a purity of 82.0%.

[0053] Example 6 Example 6 differs from Example 1 in that the step of lowering the temperature of the wastewater using a heat exchanger is replaced with a step of adjusting the pH value of the wastewater to 2 with sulfuric acid.

[0054] As a result of the detection, the obtained sodium sulfate had a yield of 81.5% and a purity of 98.9%, and the organic phosphorus product had a yield of 62.0% and a purity of 92.1%.

[0055] Example 7 Example 7 differs from Example 1 in that the step of lowering the temperature of the wastewater using a heat exchanger is replaced with a step of adjusting the pH value of the wastewater to 2.5 with sulfuric acid.

[0056] As a result of the detection, the obtained sodium sulfate has a yield of 81.3% and a purity of 98.7%, and the organic phosphorus product has a yield of 57.3% and a purity of 91.8%.

[0057] Example 8 The difference between Example 8 and Example 1 is that the wastewater from the production of phosphorus-containing flame retardants is directly introduced into the membrane concentration system for treatment without undergoing a pretreatment step using plate and frame filtration.

[0058] As a result of the detection, the obtained sodium sulfate has a yield of 80.9% and a purity of 98.3%, and the organic phosphorus product has a yield of 59.8% and a purity of 91.6%.

[0059] Example 9 The difference between Example 9 and Example 1 is that washing filtration is not performed in the membrane separation step.

[0060] Membrane Separation Treatment The concentrated water from the membrane concentration system is introduced into the membrane separation system to obtain the membrane separation system product water and membrane separation system concentrated water. The membrane separation system is a single-stage system consisting of three ultrafiltration membrane elements connected in series. The molecular weight cutoff of the ultrafiltration membrane is 2K. The water system has a recovery rate controlled at 90% and an operating pressure controlled at 3 bar.

[0061] The concentrate from the membrane separation system is introduced into a subsequent filtration process, and the product water from the membrane separation system is subjected to evaporation crystallization to obtain industrial grade sodium sulfate.

[0062] As a result of the detection, sodium sulfate has a yield of 81.5% and a purity of 98.6%.

[0063] Filtration The concentrate from the membrane separation system is subjected to plate and frame filtration, and the clarified filtrate is returned to the upstream production process. The filtrate concentrate is then dried to obtain a solid organophosphorus product. The condensate collected during the drying process is reused or directly discharged.

[0064] As a result of the detection, the obtained organophosphorus product has a yield of 70.4% and a purity of 55.6%.

[0065] Example 10 The differences between Example 10 and Example 1 are as follows: The membrane concentration step uses a two-stage membrane concentration system, the recovery rate of the water system is 75%, and each stage of the membrane concentration unit is composed of four nanofiltration membrane elements connected in series. The operating parameters set for each stage of the membrane concentration unit are:

[0066] In the first stage, the water recovery rate is controlled at 50% and the operating pressure is controlled at 55 bar. In the second stage, the water recovery rate is 50% and the operating pressure is controlled at 70 bar. As a result of the detection, the obtained sodium sulfate had a yield of 83.5% and a purity of 98.3%, and the organic phosphorus product had a yield of 40.2% and a purity of 88.1%.

[0067] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that a single-stage membrane concentration system is used in the membrane concentration step. The operating parameters are that the water recovery rate is 50% and the operating pressure is controlled to 55 bar.

[0068] As a result of the detection, the yield of the obtained sodium sulfate was 88.4%, the purity was 95.7%, and the yield of the organic phosphorus product was zero.

[0069] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that the membrane concentration step was replaced with concentration by electrodialysis.

[0070] As a result of the detection, the yield of the obtained sodium sulfate was 81.5%, the purity was 98.6%, and the yield of the organic phosphorus product was zero.

[0071] The above has disclosed embodiments of the present invention, but the invention is not limited to the applications recited in the specification and embodiments, which are fully applicable to various fields suitable for the present invention, and additional modifications will be readily apparent to those skilled in the art. Therefore, the present invention is not limited to the specific details and examples described herein, without departing from the general concept defined by the scope of the claims and the scope of equivalents.

Claims

1. A method for treating phosphorus-containing wastewater for recycling, comprising: Step 1) subjecting phosphorus-containing wastewater to membrane concentration treatment until the organic phosphorus in the membrane concentrate is in a solid state to obtain a highly concentrated concentrate a containing solid organic phosphorus and a permeate a; Step 2) is to subject the membrane concentrate obtained in step 1) to membrane separation treatment to obtain a solid organic phosphorus-containing concentrate b and a salt-containing permeate b, and evaporate and crystallize the salt-containing permeate b to obtain industrial salt; and (3) filtering the solid organic phosphorus-containing concentrated solution (b) to obtain a solid organic phosphorus-containing filtrate concentrate and a clarified filtrate, and drying the solid organic phosphorus-containing filtrate concentrate to obtain solid organic phosphorus.

2. In the step 1), the phosphorus-containing wastewater is subjected to a pre-filtration treatment before being subjected to membrane concentration treatment; The product water obtained after the pretreatment is returned to the subsequent membrane concentration treatment step, and the concentrated water obtained after the pretreatment is returned to the previous production process, 2. The method for treating phosphorus-containing wastewater for recycling as claimed in claim 1, wherein the filtration pretreatment method is one or more of plate and frame filtration, hollow fiber membrane filtration, tubular membrane filtration, filter bag filtration, and filter element filtration.

3. The membrane used in the membrane concentration treatment process of step 1) is a nanofiltration membrane and / or a reverse osmosis membrane; The system employed in the membrane concentration treatment process is divided into at least two stages, 2. The method for treating phosphorus-containing wastewater for recycling as a resource according to claim 1, wherein the water recovery rate of the membrane concentration system is controlled to 75 to 90%.

4. The system used in the membrane concentration treatment process employs two or three stages, each stage being composed of three to six membrane elements connected in series; When the membrane concentration system has two stages, the operating parameters are as follows: in the first stage, the water recovery rate is controlled at 50-60% and the operating pressure is controlled at 50-70 bar; in the second stage, the water recovery rate is controlled at 40-50% and the operating pressure is controlled at 65-100 bar; 4. The method for treating phosphorus-containing wastewater for recycling according to claim 3, wherein when the membrane concentration system has three stages, the operating parameters are as follows: in the first stage, the water recovery rate is controlled to 50-60% and the operating pressure is controlled to 50-70 bar; in the second stage, the water recovery rate is controlled to 40-50% and the operating pressure is controlled to 65-100 bar; and in the third stage, the water recovery rate is controlled to 30-50% and the operating pressure is controlled to 70-100 bar.

5. In the step 1), before the phosphorus-containing wastewater is subjected to membrane concentration treatment, an acid is added to adjust the pH value and / or a temperature is lowered; 2. The method for recovering phosphorus-containing wastewater as claimed in claim 1, wherein sulfuric acid is added to the phosphorus-containing wastewater to adjust the pH value to 2.0 to 2.5, and the temperature of the phosphorus-containing wastewater is lowered to 20 to 25°C.

6. In step 1), the permeate a is introduced into a single-stage reverse osmosis concentration treatment to obtain a concentrate c and a permeate c, and the permeate c is directly reused or introduced into a multi-stage reverse osmosis treatment and then reused; 2. The method for recovering phosphorus-containing wastewater as claimed in claim 1, wherein the concentrated solution c is returned to the upstream stage of the membrane concentration treatment step of step 1), or the concentrated solution c is successively treated by medium pressure reverse osmosis and then high pressure reverse osmosis, or the concentrated solution c is successively treated by medium pressure reverse osmosis and then disk reverse osmosis.

7. In the step 2), the membrane used for membrane separation is an ultrafiltration membrane and / or a loose nanofiltration membrane; The system employed for the membrane separation is divided into one, two or three stages, each stage being composed of two to four membrane elements connected in series, and the water recovery rate of the membrane separation is controlled to 70 to 95%; 2. The method for recycling phosphorus-containing wastewater according to claim 1, wherein the condensate obtained by condensing the salt-containing permeate (b) in the evaporation and crystallization process is reused or directly discharged.

8. The molecular weight cutoff of the ultrafiltration membrane is 2K to 100K, 8. The method for recovering phosphorus-containing wastewater as claimed in claim 7, wherein the loose nanofiltration membrane has a divalent salt permeability of ≥ 50%.

9. 2. The method for recovering phosphorus-containing wastewater as claimed in claim 1, wherein in step 3), before the concentrated solution b of solid organic phosphorus is filtered, a washing filtration step is further performed to obtain a concentrated solution of the washed filtrate and a permeate of the washed filtrate, the concentrated solution of the washed filtrate is introduced into a subsequent filtration process, and the permeate of the washed filtrate is evaporated and crystallized to obtain industrial salt.

10. In step 3), the filtered clarified liquid is returned to the previous production process, 2. The method for recycling phosphorus-containing wastewater according to claim 1, wherein the condensate produced by condensation in the drying process of the solid organic phosphorus-containing filtered concentrate is reused or directly discharged.

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