Method and device for enhanced phosphorus release from sludge and recovery of struvite by free ammonia

Abstract

The present application belongs to the field of sludge resource utilization and phosphorus recovery, and discloses a method and device for releasing phosphorus from excess activated sludge and recovering magnesium ammonium phosphate by free ammonia pretreatment. The method comprises the following steps: placing excess activated sludge in a pretreatment reaction system, adjusting pH and adding ammonium-containing substances to form free ammonia, and releasing phosphorus to liquid phase under the condition of free ammonia; then, obtaining phosphorus-containing liquid phase by solid-liquid separation, adding magnesium source and adjusting pH to recover phosphorus in the form of magnesium ammonium phosphate precipitation. The device comprises a sludge concentration unit, a free ammonia pretreatment unit, a pH adjustment unit, an ammonium-containing substance adding unit, a solid-liquid separation unit, a magnesium ammonium phosphate recovery unit, a magnesium source adding unit, an anaerobic digestion unit and a reflux unit. The present application can enhance the release of phosphorus from excess activated sludge and realize the recovery and utilization of liquid phase phosphorus, and has low cost and is environmentally friendly.

Description

Technical Field

[0001] This invention belongs to the field of sludge resource utilization and phosphorus recovery technology, specifically relating to a method and apparatus for enhancing phosphorus release from sludge with free ammonia and recovering magnesium ammonium phosphate. Background Technology

[0002] Phosphorus is an essential nutrient element for biological growth and metabolism, and existing phosphorus resources mainly come from non-renewable phosphate rock. When using the activated sludge process to treat municipal wastewater, phosphorus in the wastewater is taken up by polyphosphate-accumulating bacteria and enriched in the excess activated sludge in the form of polyphosphate, making the excess activated sludge a stable source, a large output, and a phosphorus-rich recovery target.

[0003] Phosphorus recovery from excess activated sludge typically requires first breaking down the sludge to release phosphorus into the liquid phase, followed by separation and recovery of the phosphorus in the liquid phase. Existing sludge pretreatment methods, such as acid treatment, alkali treatment, electrodialysis, ultrasonication, and incineration, generally suffer from problems such as high reagent consumption, high energy consumption, or heavy environmental burden; moreover, simply releasing phosphorus is insufficient to complete phosphorus resource utilization, and further separation and recovery are necessary. Therefore, it is necessary to provide a low-cost, environmentally friendly technical solution that fully releases phosphorus and enables liquid-phase phosphorus recovery. Summary of the Invention

[0004] To address the aforementioned problems, the present invention aims to provide a method and apparatus for enhancing phosphorus release from sludge and recovering magnesium ammonium phosphate using free ammonia, thereby solving the problems of insufficient phosphorus release from residual activated sludge, limited efficiency of subsequent liquid-phase phosphorus recovery, and high cost and environmental burden of related treatment methods in the prior art.

[0005] To achieve the above objectives, the present invention provides a method for enhancing phosphorus release from sludge and recovering magnesium ammonium phosphate using free ammonia, comprising the following steps: (1) Place the remaining activated sludge in the pretreatment reaction system; (2) Adjust the pH value of the pretreatment reaction system and add an ammonium-containing substance to the pretreatment reaction system to form free ammonia in the pretreatment reaction system; (3) The remaining activated sludge is pretreated under the free ammonia conditions to release phosphorus from the remaining activated sludge into the liquid phase; (4) The remaining activated sludge after pretreatment is subjected to solid-liquid separation to obtain a phosphorus-containing liquid phase; (5) Add a magnesium source to the phosphorus-containing liquid phase and adjust the pH value so that the phosphorus in the phosphorus-containing liquid phase is recovered in the form of magnesium ammonium phosphate precipitate.

[0006] Preferably, the residual activated sludge is taken from the sludge thickening tank of the sewage treatment plant and is left to stand at 4°C for 24 hours before free ammonia pretreatment; the ammonium-containing substance is ammonium chloride.

[0007] Preferably, in the free ammonia pretreatment step, the pH value of the pretreatment reaction system is 9.0±0.05, the temperature is 25±0.5℃, the treatment time is 24h, the stirring speed is 150rpm, and the initial concentration of free ammonia is 20.0~300.0 mg NH3-N / L.

[0008] Preferably, the magnesium source is magnesium sulfate; in the magnesium ammonium phosphate precipitation and recovery step, the pH value of the phosphorus-containing liquid phase is 9.5±0.05, the Mg / P molar ratio is 1.5, the recovery step is carried out at 25±0.5℃, and the stirring speed is 250 rpm.

[0009] Preferably, the concentration of the free ammonia is calculated using the following formula: , , In the formula, This refers to the concentration of free ammonia. The total concentration of ammonia nitrogen in the system. The temperature of the system, The ionization constant of ammonia monohydrate, is the ion product constant of water.

[0010] The present invention also provides an apparatus for enhancing phosphorus release from sludge and recovering magnesium ammonium phosphate using free ammonia, comprising: The sludge thickening unit is used to thicken the excess activated sludge. A free ammonia pretreatment unit is connected to the sludge thickening unit and is used to pretreat the remaining activated sludge after thickening with free ammonia. A pH adjustment unit is connected to the free ammonia pretreatment unit and is used to adjust the pH value of the system within the free ammonia pretreatment unit. An ammonium-containing substance addition unit is connected to the free ammonia pretreatment unit and is used to add an ammonium-containing substance into the free ammonia pretreatment unit to form free ammonia within the free ammonia pretreatment unit. A solid-liquid separation unit, connected to the free ammonia pretreatment unit, is used to perform solid-liquid separation on the remaining activated sludge after free ammonia pretreatment to obtain a phosphorus-containing liquid phase and a sludge phase. A magnesium ammonium phosphate recovery unit is connected to the solid-liquid separation unit and is used to contain the phosphorus-containing liquid phase and perform magnesium ammonium phosphate precipitation and recovery. A magnesium source dosing unit is connected to the magnesium ammonium phosphate recovery unit and is used to add a magnesium source into the magnesium ammonium phosphate recovery unit; An anaerobic digestion unit, connected to the solid-liquid separation unit, is used to perform anaerobic digestion on the sludge phase; A reflux unit, connected to the anaerobic digestion unit and the free ammonia pretreatment unit, is used to reflux the anaerobic digestion liquid rich in ammonium nitrogen back to the free ammonia pretreatment unit; The pH adjustment unit is also used to adjust the pH value of the system within the magnesium ammonium phosphate recovery unit.

[0011] Compared with the prior art, the present invention has at least the following beneficial effects: (1) Pretreatment of the remaining activated sludge by free ammonia can promote sludge disintegration and enhance the release of phosphorus into the liquid phase. Compared with acid treatment and other methods, the consumption of reagents and environmental burden are lower. (2) Based on enhanced phosphorus release, further recover liquid phosphorus through magnesium ammonium phosphate precipitation to achieve phosphorus separation and resource recovery; (3) The sources of ammonium-containing substances used are wide-ranging, and the ammonium nitrogen rich in the anaerobic digestion liquid of sewage treatment plants can also be used as a source, which has a basis for engineering application; (4) The device organically combines sludge pretreatment, liquid phase phosphorus recovery and sludge anaerobic digestion, and returns the ammonium-rich digestion liquid to the pretreatment unit through the reflux unit, which is conducive to reducing the amount of external reagents and promoting the resource utilization of sludge. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of the structure of the device of the present invention.

[0013] In the diagram: 1-Sludge thickening unit; 2-Free ammonia pretreatment unit; 3-pH adjustment unit; 4-Ammonium-containing substance addition unit; 5-Solid-liquid separation unit; 6-Ammonium magnesium phosphate recovery unit; 7-Magnesium source addition unit; 8-Anaerobic digestion unit; 9-Reflux unit. Detailed Implementation

[0014] The present invention will be further described below with reference to specific embodiments, but this does not limit the scope of protection of the present invention.

[0015] Example 1:

[0016] This embodiment provides a device for enhancing phosphorus release and recovering magnesium ammonium phosphate from excess activated sludge through free ammonia pretreatment. The device is arranged sequentially along the material flow direction as follows: a sludge thickening unit 1, a free ammonia pretreatment unit 2, a solid-liquid separation unit 5, a magnesium ammonium phosphate recovery unit 6, and an anaerobic digestion unit 8. The effluent end of the anaerobic digestion unit 8 is connected to the free ammonia pretreatment unit 2 via a reflux unit 9. The free ammonia pretreatment unit 2 and the magnesium ammonium phosphate recovery unit 6 are equipped with the same pH adjustment unit 3; the free ammonia pretreatment unit 2 is also connected to an ammonium-containing substance addition unit 4, and the magnesium ammonium phosphate recovery unit 6 is also connected to a magnesium source addition unit 7.

[0017] The sludge thickening unit 1 is a sludge thickening tank used to receive and thicken the excess activated sludge generated by the wastewater treatment system before sending it to the free ammonia pretreatment unit 2. The free ammonia pretreatment unit 2 is a reaction tank equipped with a stirring device and a temperature control device. The top of the tank has an ammonium chloride solution inlet and a pH adjustment solution inlet. The tank contains a first stirring unit and a first temperature control unit. The solid-liquid separation unit 5 is a sedimentation tank. Its inlet is connected to the outlet of the free ammonia pretreatment unit 2. The upper outlet is used to discharge the phosphorus-containing liquid phase, and the lower sludge outlet is used to discharge the sludge phase.

[0018] The magnesium ammonium phosphate recovery unit 6 uses a reaction tank equipped with a stirring device and a temperature control device. Its inlet is connected to the upper outlet of the solid-liquid separation unit 5. The top of the tank has a magnesium source inlet and a pH adjustment solution inlet. A second stirring unit and a second temperature control unit are installed inside the tank. The anaerobic digestion unit 8 uses an anaerobic digester. Its inlet is connected to the sludge discharge port of the solid-liquid separation unit 5 to receive the pretreated sludge phase. The reflux unit 9 uses a reflux pump and reflux pipeline. One end is connected to the outlet of the anaerobic digestion unit 8, and the other end is connected to the free ammonia pretreatment unit 2, used to return the ammonium-rich anaerobic digestate to the free ammonia pretreatment unit 2.

[0019] In operation, the remaining activated sludge first enters the sludge thickening unit 1 for thickening, and the thickened sludge is then sent to the free ammonia pretreatment unit 2. In the free ammonia pretreatment unit 2, ammonium chloride solution is added through the ammonium-containing substance addition unit 4, and the pH of the system is adjusted through the pH adjustment unit 3 to create a free ammonia environment in the reaction tank. Simultaneously, the stirring and temperature conditions are controlled by the first stirring unit and the first temperature control unit to promote the release of phosphorus into the liquid phase. The pretreated material enters the solid-liquid separation unit 5. The separated phosphorus-containing liquid phase enters the magnesium ammonium phosphate recovery unit 6, while the sludge phase enters the anaerobic digestion unit 8.

[0020] After the phosphorus-containing liquid phase enters the magnesium ammonium phosphate recovery unit 6, magnesium sulfate solution is added through the magnesium source addition unit 7. The pH value, stirring, and temperature conditions of the system are adjusted by the pH adjustment unit 3, the second stirring unit, and the second temperature control unit, so that phosphorus precipitates out in the form of magnesium ammonium phosphate. The digestate produced by the anaerobic digestion unit 8 is returned to the free ammonia pretreatment unit 2 through the reflux unit 9.

[0021] Preferably, the operating conditions for the free ammonia pretreatment unit 2 are: system pH value of 9.0±0.05, temperature of 25±0.5℃, stirring speed of 150 rpm, pretreatment time of 24 h, and initial free ammonia concentration of 20.0~300.0 mg NH3-N / L. The operating conditions for the magnesium ammonium phosphate recovery unit 6 are: system pH value of 9.5±0.05, temperature of 25±0.5℃, stirring speed of 250 rpm, and Mg / P molar ratio of 1.5.

[0022] The following examples and comparative examples were all performed on the device described in Example 1.

[0023] Example 2:

[0024] The residual activated sludge from the sludge thickening tank of a municipal wastewater treatment plant operating on an anaerobic-anoxic-aerobic process was taken as the treatment target. Its basic physicochemical characteristics were: pH 6.85±0.05, TSS (total suspended solids) 25.31±0.41 g / L, TP (total phosphorus) 12.9±1.5 mg-P / g-TS, initial NH4+-N 22.8±0.8 mg / L, initial liquid phase PO43--P 8.2±0.3 mg / L, and sludge age approximately 20 days. These parameters were used to calculate the free ammonia concentration and phosphorus release efficiency. The free ammonia concentration was calculated using the following formula: , , In the formula, This refers to the concentration of free ammonia. The total concentration of ammonia nitrogen in the system. The temperature of the system, The ionization constant of ammonia monohydrate, is the ion product constant of water.

[0025] The remaining activated sludge after sampling was left to stand at 4℃ for 24 hours before use.

[0026] The remaining activated sludge after settling was sent to the free ammonia pretreatment unit 2, and 4.0 M ammonium chloride stock solution was added. The pH value of the system was adjusted to 9.0±0.05 by the pH adjustment unit 3, so that the initial concentration of free ammonia in the system reached 225.7 mgNH3-N / L. The temperature was controlled at 25±0.5℃ by the first temperature control unit, and the system was stirred at 150 rpm by the first stirring unit. The pretreatment lasted for 24 hours, so that the phosphorus in the sludge was released into the liquid phase.

[0027] After pretreatment, the material is introduced into solid-liquid separation unit 5 for separation. The phosphorus-containing liquid phase is collected and introduced into magnesium ammonium phosphate recovery unit 6. Magnesium sulfate is added as a magnesium source, and the pH value is adjusted to 9.5±0.05 by pH adjustment unit 3 to control the Mg / P molar ratio at 1.5. The temperature is controlled at 25±0.5℃ by the second temperature control unit, and the mixture is stirred at 250 rpm by the second stirring unit to precipitate phosphorus in the phosphorus-containing liquid phase as magnesium ammonium phosphate. After the reaction, the precipitate is filtered, collected using a 0.45μm filter membrane, washed with ultrapure water, and dried at room temperature for 36 hours to obtain the magnesium ammonium phosphate product.

[0028] Example 3:

[0029] This embodiment is basically the same as Embodiment 2, except that: after adding 4.0 M ammonium chloride stock solution, the pH value of the pretreatment system is adjusted to 9.0±0.05 by pH adjustment unit 3, so that the initial concentration of free ammonia in the system reaches 117.0 mgNH3-N / L; the other pretreatment conditions, solid-liquid separation and magnesium ammonium phosphate recovery steps are the same as in Embodiment 2.

[0030] Example 4:

[0031] This embodiment is basically the same as Example 2, except that: after adding 4.0 M ammonium chloride stock solution, the pH value of the pretreatment system is adjusted to 9.0±0.05 so that the initial concentration of free ammonia in the system reaches 62.6 mg NH3-N / L; the other conditions are the same as in Example 2.

[0032] Example 5:

[0033] This embodiment is basically the same as Example 2, except that: after adding 4.0 M ammonium chloride stock solution, the pH value of the pretreatment system is adjusted to 9.0±0.05 so that the initial concentration of free ammonia in the system reaches 26.4 mg NH3-N / L; the other conditions are the same as in Example 2.

[0034] Comparative Example 1: This comparative example is basically the same as Example 2, except that: no ammonium chloride stock solution is added during the free ammonia pretreatment stage, and the pH value of the pretreatment system is not adjusted; the other conditions are the same as in Example 2.

[0035] Comparative Example 2: This comparative example is basically the same as Example 2, except that: no ammonium chloride stock solution is added during the free ammonia pretreatment stage, but the pH value of the pretreatment system is adjusted to 9.0±0.05 by pH adjustment unit 3; the other conditions are the same as in Example 2.

[0036] The treatment conditions and phosphorus release results of each embodiment and comparative example are shown in Table 1.

[0037] Table 1. Comparison of processing conditions and results for different embodiments and comparative examples.

[0038] As shown in Table 1, compared with Comparative Examples 1 and 2, the concentration of PO43--P in the liquid phase and the phosphorus release efficiency of Examples 2-5 were significantly improved after pretreatment with free ammonia, indicating that free ammonia pretreatment is beneficial to promoting the release of phosphorus from the remaining activated sludge into the liquid phase. Furthermore, with the increase of the concentration of ammonium-containing substances and the increase of the initial free ammonia concentration, both the concentration of PO43--P in the liquid phase and the phosphorus release efficiency showed an upward trend. The phosphorus release effect of Comparative Example 2 (only pH was increased, without the addition of ammonium-containing substances) was higher than that of Comparative Example 1, but still lower than that of Examples 2-5 under the same conditions after free ammonia pretreatment, indicating that free ammonia pretreatment has a more significant effect on enhancing phosphorus release than simply increasing pH.

[0039] After obtaining the phosphorus-containing liquid phase, adding a magnesium source and adjusting the pH value can cause the phosphorus in the liquid phase to precipitate out as magnesium ammonium phosphate, thus realizing the recovery of phosphorus from the liquid phase. This demonstrates that the present invention not only enhances the release of phosphorus from excess activated sludge into the liquid phase but also further realizes the recovery and utilization of phosphorus resources.

[0040] The above description of the embodiments is provided to facilitate understanding and use of the present invention by those skilled in the art. Without departing from the spirit and technical solutions of the present invention, those skilled in the art can make several modifications or equivalent substitutions to the present invention. All simple improvements and modifications that do not depart from the content of the technical solutions of the present invention should fall within the protection scope of the present invention.

Claims

1. A method for enhancing phosphorus release from sludge and recovering magnesium ammonium phosphate using free ammonia, characterized in that, Includes the following steps: The remaining activated sludge is placed in the pretreatment reaction system; the pH value of the pretreatment reaction system is adjusted, and an ammonium-containing substance is added to the pretreatment reaction system to form free ammonia in the pretreatment reaction system; The remaining activated sludge is pretreated under the free ammonia conditions to release phosphorus from the remaining activated sludge into the liquid phase; The remaining activated sludge after pretreatment is subjected to solid-liquid separation to obtain a phosphorus-containing liquid phase. A magnesium source is added to the phosphorus-containing liquid phase and the pH value is adjusted so that the phosphorus in the phosphorus-containing liquid phase is recovered in the form of magnesium ammonium phosphate precipitate.

2. The method according to claim 1, characterized in that, The remaining activated sludge was taken from the sludge thickening tank of the wastewater treatment plant and was left to stand at 4°C for 24 hours before being pretreated with free ammonia; the ammonium-containing substance was ammonium chloride.

3. The method according to claim 1, characterized in that, In the free ammonia pretreatment step, the pH value of the pretreatment reaction system is 9.0±0.05, the temperature is 25±0.5℃, the treatment time is 24h, the stirring speed is 150rpm, and the initial concentration of free ammonia is 20.0~300.0 mg NH3-N / L.

4. The method according to claim 1, characterized in that, In the magnesium ammonium phosphate precipitation recovery step, the magnesium source is magnesium sulfate, the pH value of the phosphorus-containing liquid phase is adjusted to 9.5±0.05, the Mg / P molar ratio is 1.5, and the recovery step is carried out at 25±0.5℃ with a stirring speed of 250rpm.

5. The method according to claim 1, characterized in that, The magnesium ammonium phosphate precipitate was filtered through a 0.45 μm filter membrane, washed with ultrapure water, and dried at room temperature.

6. An apparatus for enhancing phosphorus release from sludge with free ammonia and recovering magnesium ammonium phosphate, characterized in that, include: The sludge thickening unit (1) is used to thicken the remaining activated sludge; A free ammonia pretreatment unit (2) is connected to the sludge thickening unit (1) and is used to pretreat the remaining activated sludge after thickening with free ammonia; a pH adjustment unit (3) is connected to the free ammonia pretreatment unit (2) and is used to adjust the pH value of the system in the free ammonia pretreatment unit (2); an ammonium-containing substance addition unit (4) is connected to the free ammonia pretreatment unit (2) and is used to add ammonium-containing substances into the free ammonia pretreatment unit (2) to form free ammonia in the free ammonia pretreatment unit (2); A solid-liquid separation unit (5) is connected to the free ammonia pretreatment unit (2) and is used to perform solid-liquid separation on the remaining activated sludge after free ammonia pretreatment to obtain a phosphorus-containing liquid phase and a sludge phase; a magnesium ammonium phosphate recovery unit (6) is connected to the solid-liquid separation unit (5) and is used to contain the phosphorus-containing liquid phase and perform magnesium ammonium phosphate precipitation recovery; a magnesium source addition unit (7) is connected to the magnesium ammonium phosphate recovery unit (6) and is used to add a magnesium source into the magnesium ammonium phosphate recovery unit (6); an anaerobic digestion unit (8) is connected to the solid-liquid separation unit (5) and is used to perform anaerobic digestion on the sludge phase; a reflux unit (9) is connected to the anaerobic digestion unit (8) and the free ammonia pretreatment unit (2) and is used to reflux the anaerobic digestion liquid rich in ammonium nitrogen back to the free ammonia pretreatment unit (2); wherein, the pH adjustment unit (3) is also used to adjust the pH value of the system in the magnesium ammonium phosphate recovery unit (6) so that the phosphorus in the phosphorus-containing liquid phase is recovered in the form of magnesium ammonium phosphate precipitation.

7. The apparatus according to claim 6, characterized in that, The sludge thickening unit (1) is a sludge thickening tank; the solid-liquid separation unit (5) is a sedimentation separation unit; and the ammonium-containing substance addition unit (4) is used to add ammonium chloride solution.

8. The apparatus according to claim 6, characterized in that, The device further includes a first temperature control unit and a first stirring unit; the pH adjustment unit (3) is used to adjust the pH value of the system in the free ammonia pretreatment unit (2) to 9.0±0.05, the first temperature control unit is used to maintain the temperature of the system in the free ammonia pretreatment unit (2) at 25±0.5℃, the first stirring unit is used to stir the material in the free ammonia pretreatment unit (2) at a speed of 150rpm, and the free ammonia pretreatment unit (2) is used to pretreat the remaining activated sludge for 24h, and the initial concentration of free ammonia in it is 20.0~300.0 mg NH3-N / L.

9. The apparatus according to claim 6, characterized in that, The magnesium source addition unit (7) is used to add magnesium sulfate into the magnesium ammonium phosphate recovery unit (6); the pH adjustment unit (3) is used to adjust the pH value of the system in the magnesium ammonium phosphate recovery unit (6) to 9.5±0.05, and the magnesium source addition unit (7) and the magnesium ammonium phosphate recovery unit (6) work together to make the Mg / P molar ratio 1.

5.

10. The apparatus according to claim 6, characterized in that, The device further includes a second stirring unit and a second temperature control unit. The second stirring unit is used to stir the material in the magnesium ammonium phosphate recovery unit (6) at a speed of 250 rpm. The second temperature control unit is used to maintain the system temperature in the magnesium ammonium phosphate recovery unit (6) at 25 ± 0.5 °C.

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