Method for rapidly recovering phosphorus from municipal sludge

Abstract

The invention discloses a method for rapidly recovering phosphorus from municipal sludge, which comprises the following steps: step 1, adding to-be-treated sludge and an iron salt catalyst into a hydrothermal reaction container, and carrying out iron catalytic hydrothermal oxidation to obtain a hydrothermal oxidation liquid phase product of the sludge; step 2, adding a sodium hydroxide solution into the obtained sludge hydrothermal oxidation liquid-phase product to quickly adjust the pH value, so that phosphorus is removed from the liquid-phase product and enters a precipitate to obtain a phosphorus-rich precipitate; and step 3, drying the obtained phosphorus-rich precipitate to obtain a final phosphorus-containing product. According to the method, iron-catalyzed hydrothermal oxidation of the sludge can be realized within a relatively low temperature range, phosphorus in the sludge is promoted to migrate and convert to a liquid phase, organic impurities in a product are greatly reduced by rapidly adjusting the pH and a precipitation process, and the time required by the whole process is short.

Description

Technical Field

[0001] This invention relates to the field of sludge treatment technology, and more particularly to a method for rapidly recovering phosphorus from municipal sludge. Background Technology

[0002] Phosphate rock is a non-renewable and highly concentrated resource, primarily circulating on Earth through sedimentary cycles. The extensive exploitation and use of phosphorus resources by humans has led to the rapid depletion of naturally occurring phosphorus resources in the form of phosphate rock. Finding alternative, renewable sources is extremely urgent, as it concerns the security and stability of the industrial and agricultural supply chain.

[0003] Currently, eutrophication of natural water bodies and the demand for municipal wastewater treatment are increasing significantly. As the core of water treatment, municipal sludge is a crucial phosphorus sink and a highly promising source of phosphorus. It is worth noting that while the phosphorus content of municipal sludge is already very high compared to water bodies, it is still far too low and its volume is enormous compared to the phosphorus products or raw materials needed for industrial and agricultural applications. Therefore, it is necessary to further enrich the phosphorus in the sludge and convert it into smaller volumes of recycled phosphorus products for further use.

[0004] Extracting phosphorus from sludge using acid leaching or traditional thermal treatment often results in insufficient phosphorus release and high energy consumption. Extracting phosphorus from the supernatant of anaerobic fermentation requires a long fermentation time, leading to a prolonged sludge storage and turnover cycle. Hydrothermal oxidation has the advantage of promoting organic matter decomposition and rapid phosphate dissolution in aqueous systems; however, conventional hydrothermal oxidation typically requires higher temperatures and pressures, necessitating high-temperature, high-pressure equipment, increasing engineering complexity and cost. Therefore, developing products that can achieve efficient phosphorus release and rapid recovery of high-phosphorus content at lower temperatures and atmospheric pressures is of great significance. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for rapidly recovering phosphorus from municipal sewage sludge, thereby solving the technical problems of low phosphorus content and large volume in municipal sewage sludge, resulting in low economic benefits from direct recovery, as well as the high cost and long processing time of traditional phosphorus recovery methods.

[0006] This invention provides a method for rapidly recovering phosphorus from municipal sewage sludge, comprising the following steps:

[0007] Step 1: Add the sludge to be treated and the iron salt catalyst together to a hydrothermal reaction vessel for iron-catalyzed hydrothermal oxidation to obtain the hydrothermal oxidation liquid phase product of the sludge;

[0008] Step 2: Add sodium hydroxide solution to the obtained sludge hydrothermal oxidation liquid phase product to quickly adjust the pH value, so that phosphorus is removed from the liquid phase product and enters the precipitate to obtain phosphorus-rich precipitate;

[0009] Step 3: Dry the obtained phosphorus-rich precipitate to obtain the final phosphorus-containing product.

[0010] Furthermore, in step 1, the iron salt catalyst is ferric chloride or ferric sulfate, and the addition ratio ranges from 2.0% to 3.0%.

[0011] Furthermore, in step 1, the oxidant added during the iron-catalyzed hydrothermal oxidation process is sodium persulfate solution with a concentration range of 0.10-0.20 mol / L.

[0012] Furthermore, in step 1, the liquid-to-solid ratio of the sludge in hydrothermal oxidation is in the range of 15:1 to 20:1, calculated on a dry basis.

[0013] Furthermore, in step 1, the initial pH range of hydrothermal oxidation is 1~3; the reaction temperature range is 60~90℃; and the reaction time range is 60~120 min.

[0014] Furthermore, in step 1, the phosphorus content in the hydrothermal oxidation liquid phase product of the sludge ranges from 500 to 850 mg / L, the nitrogen content ranges from 45 to 100 mg / L, the COD content ranges from 650 to 1300 mg / L, and the pH ranges from 1.0 to 2.0.

[0015] Furthermore, in step 2, the rapid adjustment of pH value specifically involves adjusting the pH value over a time range of 5 to 10 minutes.

[0016] Furthermore, in step 2, the result of rapidly adjusting the pH value is within the range of 9~10.

[0017] Furthermore, in step 2, the process begins with rapid adjustment of the pH value and continues until the phosphorus-rich precipitate is formed, with a time range of 15-20 minutes.

[0018] Furthermore, in step 3, the drying temperature is 60℃ and the drying time ranges from 30 to 60 minutes.

[0019] The beneficial effects of this invention are:

[0020] This invention enables iron-catalyzed hydrothermal oxidation of sludge within a relatively low temperature range of 60-90℃, promoting the migration and transformation of phosphorus in the sludge into the liquid phase. By rapidly adjusting the pH and inducing precipitation, this invention effectively mitigates the co-precipitation of residual humic acid and phosphate ions in the liquid phase after sludge oxidation, significantly reducing organic impurities in the product. The entire process of this invention is short, with the total time from iron-catalyzed hydrothermal oxidation of sludge to the recovery of the final precipitated product controlled within 4 hours. Attached Figure Description

[0021] The features and advantages of the invention will be more clearly understood by referring to the accompanying drawings, which are schematic and should not be construed as limiting the invention in any way. In the drawings:

[0022] Figure 1 This is a flowchart illustrating a specific embodiment of the present invention;

[0023] Figure 2 This is a graph showing the release rate of municipal sludge after hydrothermal oxidation at different temperatures and with different concentrations of sodium persulfate oxidant in a specific embodiment of the present invention.

[0024] Figure 3 This is a graph showing the phosphorus release rate of municipal sludge after hydrothermal oxidation under different concentrations of sodium persulfate oxidant at different oxidation times in a specific embodiment of the present invention.

[0025] Figure 4 The graph shows the phosphorus content of the precipitate obtained at different pH adjustment endpoints in a specific embodiment of the present invention.

[0026] Figure 5 This is the XRD pattern of the final precipitate in Example 1 of this invention. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] The present invention will be further illustrated below with reference to specific embodiments. Those skilled in the art should understand that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Modifications to the present invention in various equivalent forms all fall within the scope defined by the appended claims.

[0029] like Figure 1 As shown, the present invention provides a method for rapidly recovering phosphorus from municipal sewage sludge, comprising the following steps:

[0030] Step 1: Add the sludge to be treated and the iron salt catalyst together to a hydrothermal reaction vessel for iron-catalyzed hydrothermal oxidation to obtain the hydrothermal oxidation liquid phase product of the sludge;

[0031] The iron salt catalyst is ferric chloride or ferric sulfate, with an addition ratio ranging from 2.0% to 3.0%. The liquid-to-solid ratio of the sludge in the hydrothermal oxidation process, calculated on a dry basis, ranges from 15:1 to 20:1. The oxidant added during the iron-catalyzed hydrothermal oxidation process is sodium persulfate solution, with a concentration ranging from 0.10 to 0.20 mol / L. The phosphorus content, nitrogen content, COD content, and pH range of the hydrothermal oxidation liquid phase products of the sludge are all within the range of 500–850 mg / L, 45–100 mg / L, and 650–1300 mg / L, respectively.

[0032] Step 2: Add sodium hydroxide solution to the obtained sludge hydrothermal oxidation liquid phase product to rapidly adjust the pH value, specifically as follows:

[0033] The pH adjustment time range is 5~10 min, and the pH adjustment result range is 9~10.

[0034] Phosphorus is removed from the liquid phase product and enters the precipitate to obtain a phosphorus-rich precipitate. The process starts with rapid pH adjustment and continues until the phosphorus-rich precipitate is formed. The time range of the process is 15-20 minutes.

[0035] Step 3: Dry the obtained phosphorus-rich precipitate at a temperature of 60℃ for a time of 30-60 minutes to obtain the final phosphorus-containing product.

[0036] Example 1:

[0037] Sludge pretreatment and batching process:

[0038] Dewatered sludge from a municipal wastewater treatment plant was used as raw material. Its moisture content was recorded, and the dry weight of the sludge was calculated as m (g) on ​​a dry basis. Iron salt catalyst was added at 2.5% of the dry weight of the sludge. The preferred iron salt catalyst is ferric chloride or ferric sulfate. In this embodiment, ferric chloride is used as an example. Deionized water was added to make the liquid-solid ratio (on a dry basis) 18:1 (mL:g). After mixing, the initial pH was adjusted to 2.0 with acid (hydrochloric acid or sulfuric acid).

[0039] Iron-catalyzed hydrothermal oxidation process for phosphorus release:

[0040] Add sodium persulfate (Na2S2O8) solution as an oxidant to the above mixture to make the oxidant concentration in the system 0.15 mol / L. Transfer the mixture to a hydrothermal reaction vessel and react at 80°C for 90 min while stirring / rotating to mix; after the reaction is completed, cool to room temperature.

[0041] Solid-liquid separation process for obtaining hydrothermal oxidation liquid phase products:

[0042] The slurry after the reaction is subjected to solid-liquid separation. The separation method can be centrifugation or filtration. The supernatant / filtrate is collected as the hydrothermal oxidation liquid phase product of the sludge. Its volume is recorded, and indicators such as total phosphorus concentration and pH can be detected.

[0043] Rapid pH-adjusted precipitation for phosphorus recovery (single-stage precipitation):

[0044] The hydrothermal oxidation liquid phase product of the above sludge was placed under stirring conditions, and a 1 mol / L sodium hydroxide solution was rapidly added to quickly adjust the pH from acidic to 9 within 2 minutes. Stirring was continued and the mixture was allowed to stand, so that the total sedimentation time (including the rapid pH adjustment time and the standing time) was 17 minutes. Solid-liquid separation was then performed to obtain the precipitate.

[0045] The process of precipitation, washing, and drying to obtain the final product:

[0046] The precipitate was washed 1–3 times with deionized water, followed by solid-liquid separation. The resulting precipitate was dried at 60°C for 45 min, cooled, and then ground to obtain a phosphorus-rich precipitate (a mixture of various phosphates). Different precipitates could be characterized by XRD to obtain... Figure 5 The phase information corresponding to different precipitation times is shown.

[0047] Example 2:

[0048] Based on Example 1, the amount of iron salt catalyst added was kept at 2.5% of the dry weight of sludge, the liquid-to-solid ratio (dry weight) was kept at 18:1 (mL:g), and the initial pH was kept constant at 2.0. The reaction temperature and sodium persulfate (Na2S2O8) concentration in the iron catalytic hydrothermal oxidation stage were changed, and the reaction temperature was set at 60℃, 90℃, and 120℃, and the Na2S2O8 concentration was set at 0.10 mol / L, 0.15 mol / L, and 0.20 mol / L, respectively. The reaction time was fixed at 60 min.

[0049] After the reaction, solid-liquid separation was performed as in Example 1. The total phosphorus concentration in the bile (supernatant / filtrate) was measured, and the phosphorus release rate was calculated based on the proportion of total phosphorus migrating from the solid phase to the liquid phase. The phosphorus release rate results obtained under different temperatures and different Na2S2O8 concentrations were plotted as follows: Figure 2 This is used to characterize the effect of "temperature-oxidant concentration" on the phosphorus release process.

[0050] Example 3:

[0051] Based on the "sludge pretreatment and batching process" in Example 1, the amount of iron salt catalyst added was kept at 2.5% of the dry weight of the sludge, the liquid-to-solid ratio (dry weight) was kept at 18:1 (mL:g), and the initial pH was kept constant at 2.0. The reaction temperature was kept constant at 90℃. The reaction time and Na2S2O8 concentration of the iron catalytic hydrothermal oxidation stage were changed. The reaction time was set to 60 min, 90 min, and 120 min, and the Na2S2O8 concentration was set to 0.10 mol / L, 0.15 mol / L, 0.20 mol / L, and 0.25 mol / L.

[0052] After the reaction, solid-liquid separation was performed as in Example 1. The total phosphorus concentration in the bile was measured and the phosphorus release rate was calculated. The phosphorus release rate results obtained under different oxidation times and different Na2S2O8 concentrations were plotted as follows: Figure 3 This is used to characterize the effect of "reaction time-oxidant concentration" on the phosphorus release process.

[0053] Example 4:

[0054] The sludge hydrothermal oxidation liquid phase product was prepared according to Example 1. The reaction temperature of 90℃, the reaction time of 60min, and the Na2S2O8 concentration of 0.20 mol / L can be the same as in Example 1. Then, a rapid pH adjustment precipitation step was performed.

[0055] During the rapid pH-adjusted precipitation process, the sodium hydroxide solution concentration was kept consistent with the total precipitation time (15–20 min), and only the pH adjustment endpoint was changed, with the pH being rapidly adjusted to 9.0. After precipitation, solid-liquid separation was performed to obtain a phosphorus-rich precipitate. The phosphorus-rich precipitate was washed, dried, and ground, and the phosphorus content of the precipitate was determined (e.g., on a dry basis). The phosphorus content results corresponding to different pH adjustment endpoints were plotted. Figure 4 This is used to characterize the effect of pH endpoint on product enrichment.

[0056] Although embodiments of the invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the invention, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A method for rapidly recovering phosphorus from municipal sewage sludge, characterized in that, Includes the following steps: Step 1: Add the sludge to be treated and the iron salt catalyst together to a hydrothermal reaction vessel for iron-catalyzed hydrothermal oxidation to obtain the hydrothermal oxidation liquid phase product of the sludge; Step 2: Add sodium hydroxide solution to the obtained sludge hydrothermal oxidation liquid phase product to quickly adjust the pH value, so that phosphorus is removed from the liquid phase product and enters the precipitate to obtain phosphorus-rich precipitate; Step 3: Dry the obtained phosphorus-rich precipitate to obtain the final phosphorus-containing product.

2. The method for rapidly recovering phosphorus from municipal sewage sludge as described in claim 1, characterized in that, In step 1, the iron salt catalyst is ferric chloride or ferric sulfate, and the addition ratio ranges from 2.0% to 3.0%.

3. The method for rapidly recovering phosphorus from municipal sewage sludge as described in claim 1, characterized in that, In step 1, the oxidant added during the iron-catalyzed hydrothermal oxidation process is sodium persulfate solution with a concentration range of 0.10-0.20 mol / L.

4. The method for rapid phosphorus recovery from municipal sewage sludge as described in any one of claims 1-3, characterized in that, In step 1, the liquid-to-solid ratio of the sludge in hydrothermal oxidation is 15:1 to 20:1, calculated on a dry basis.

5. The method for rapidly recovering phosphorus from municipal sewage sludge as described in claim 1, characterized in that, In step 1, the initial pH range of hydrothermal oxidation is 1~3; the reaction temperature range is 60~90℃; and the reaction time range is 60~120 min.

6. The method for rapid phosphorus recovery from municipal sewage sludge as described in claim 1, characterized in that, In step 1, the phosphorus content in the hydrothermal oxidation liquid phase product of the sludge ranges from 500 to 850 mg / L, the nitrogen content ranges from 45 to 100 mg / L, the COD content ranges from 650 to 1300 mg / L, and the pH ranges from 1.0 to 2.

0.

7. The method for rapidly recovering phosphorus from municipal sewage sludge as described in claim 1, characterized in that, In step 2, the rapid adjustment of pH value specifically involves adjusting the pH value within a time range of 5 to 10 minutes.

8. The method for rapid phosphorus recovery from municipal sludge as described in claim 1 or 7, characterized in that, In step 2, the result of rapid pH adjustment is within the range of 9~10.

9. The method for rapid phosphorus recovery from municipal sludge as described in claim 1 or 7, characterized in that, In step 2, the process begins with rapid adjustment of the pH value and continues until the phosphorus-rich precipitate is formed, with a time range of 15-20 minutes.

10. The method for rapidly recovering phosphorus from municipal sewage sludge as described in claim 1, characterized in that, In step 3, the drying temperature is 60℃ and the drying time ranges from 30 to 60 minutes.

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