Method for improving struvite crystallization rate and purity based on ultrasonic pretreatment

Abstract

The invention discloses a method for improving the crystallization rate and purity of struvite based on ultrasonic pretreatment, and belongs to the technical field of environmental engineering. The method for improving the crystallization rate and purity of struvite based on ultrasonic pretreatment comprises the following steps: homogenizing landfill leachate, performing high-power ultrasonic treatment at the power of 150-250W, and then performing low-power ultrasonic treatment at the power of 20-50W to obtain ultrasonic treatment liquid; the pH value of the ultrasonic treatment liquid is adjusted to 9-10, then a magnesium source is added for struvite crystallization reaction, and struvite crystals are generated. By utilizing the ultrasonic pretreatment disclosed by the invention, SS (suspended solids) in the landfill leachate can be used as a crystal nucleus of subsequent struvite crystallization, so that the crystallization rate of the struvite is well improved. According to the method provided by the invention, the purity of the recovered struvite crystals can reach 90% or above, and the struvite crystallization reaction time is shortened to 15-25 minutes from the original 45 minutes.

Description

Technical Field

[0001] The present invention relates to the technical field of environmental engineering, and particularly to a method for improving the crystallization rate and purity of struvite based on ultrasonic pretreatment. Background Art

[0002] Incineration is one of the most commonly used methods for current domestic waste treatment. Currently, most domestic waste still relies on incineration technology for treatment. Before the waste is sent into the incinerator, it is usually stacked in a storage pit for 3 to 7 days to reduce its water content and increase the calorific value of the waste. This step will result in the generation of leachate equivalent to 5% to 28% of the total amount of waste. Research data from multiple waste incineration plants show that the leachate from such waste incineration plants contains high concentrations of organic carbon, nitrogen and other pollutants. The concentrations of chemical oxygen demand (COD), total nitrogen and ammonia nitrogen can reach 33000 - 89000 mg / L, 1300 - 3300 mg / L and 1000 - 3000 mg / L respectively. The water quality of the leachate from waste incineration plants is affected by factors such as waste composition, initial moisture content, environmental humidity, and storage pit temperature, as shown in Table 1 specifically.

[0003] Table 1 Typical water quality characteristics of the leachate from waste incineration plants

[0004] Index Range pH value 5.0-8.0 TOC (mg / L) 15000-80000 COD (mg / L) 33000-89000 <![CDATA[BOD5 (mg / L)]]> 16000-30000 <![CDATA[NH3-N (mg / L)]]> 1000-3000 <![CDATA[NO2-N (mg / L)]]> 20-400 TN (mg / L) 1300-3300 TP (mg / L) 50-150

[0005] The existing waste leachate treatment technology mainly includes steps such as pretreatment, flocculation precipitation, biochemical treatment, chemical strong oxidation, MBR, ultrafiltration, nanofiltration and reverse osmosis. It combines physical and chemical treatment and biological treatment means. This technology has achieved relatively good results in treating waste leachate, enabling the leachate to achieve stable discharge up to the standard. However, effective recycling and utilization of the large amounts of nitrogen and phosphorus nutrients contained therein have not been realized. The struvite recovery technology is an effective method for recovering nitrogen and phosphorus in the leachate. Its basic principle is to generate struvite through a chemical precipitation reaction, thereby achieving the synchronous removal and recovery of nitrogen and phosphorus. However, the leachate often contains interfering substances such as organic matter and heavy metals, which will affect the formation of struvite. For example, SS (suspended solids), the content in the leachate of general incineration plants exceeds 2000 mg / L, which has a great impact on the formation of struvite. In addition, during the crystallization process of struvite, crystal nuclei play a crucial role, and their formation and properties directly affect the crystallization rate, crystal size and morphology. Generally speaking, the removal effect of the current struvite (MAP) precipitation process still has not reached the maximum or better level, and the crystallization purity and reaction rate of struvite still need to be further improved. Exploring how to enhance the high-efficiency utilization ability of the struvite precipitation method is of great significance. Summary of the Invention

[0006] In view of the problems of low high-value collaborative recovery efficiency of nitrogen and phosphorus nutrient elements and low product purity in the mainstream treatment process of landfill leachate in existing waste incineration plants, the present invention proposes a method for improving the crystallization rate and purity of struvite based on ultrasonic pretreatment. After homogenizing the landfill leachate, it is subjected to high-power ultrasonic treatment, then low-power ultrasonic treatment, and then the pH of the landfill leachate pretreated by ultrasonic waves is adjusted to 9-10, and a magnesium source is added for struvite crystallization reaction to generate struvite crystals.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] One of the technical solutions of the present invention: provides a method for improving the crystallization rate and purity of struvite based on ultrasonic pretreatment, including the following steps:

[0009] After homogenizing the landfill leachate, it is subjected to high-power ultrasonic treatment at a power of 150-250W, and then low-power ultrasonic treatment at a power of 20-50W to obtain an ultrasonic-treated solution;

[0010] Adjust the pH value of the ultrasonic-treated solution to 9-10, and then add a magnesium source for struvite crystallization reaction to generate struvite crystals.

[0011] Preferably, the initial concentration of ammonia nitrogen in the landfill leachate is 1000-3000mg / L, and the initial concentration of total phosphorus is 50-150mg / L.

[0012] Preferably, the time of the high-power ultrasonic treatment is 10-15min.

[0013] Preferably, the time of the low-power ultrasonic treatment is 5-10min.

[0014] Preferably, when adjusting the pH value of the ultrasonic-treated solution, a sodium carbonate-sodium bicarbonate buffer solution is used.

[0015] Preferably, the sodium carbonate and sodium bicarbonate buffer solution is a 0.1mol / L sodium carbonate and sodium bicarbonate buffer solution, and the preparation method is:

[0016] (1) Preparation of 0.1mol / L sodium carbonate solution: Gradually dissolve 10.6g of sodium carbonate powder in 100mL of pure water;

[0017] (2) Preparation of 0.1mol / L sodium bicarbonate solution: Dissolve 8.4g of sodium bicarbonate powder in 100mL of pure water;

[0018] (3) Take 50 mL of the sodium carbonate solution from step (1) in a beaker, then slowly add the sodium bicarbonate solution from step (2), and monitor it using a pH meter. Adjust the pH value to the desired target range by adding the sodium bicarbonate solution; store the prepared buffer solution at room temperature or in a refrigerated environment for subsequent use.

[0019] Preferably, the magnesium source is selected from one or more of magnesium chloride, magnesium sulfate, and magnesium carbonate.

[0020] Preferably, the molar ratio of magnesium element in the magnesium source to the total phosphorus contained in the ultrasonic treatment solution is (0.8 - 1.2):(1.0 - 1.5).

[0021] More preferably, the molar ratio of magnesium element in the magnesium source to the total phosphorus contained in the ultrasonic treatment solution is 1.1:1.5.

[0022] Preferably, the time for the struvite crystallization reaction is 15 - 25 min.

[0023] The second technical solution of the present invention: Provide an application of the above method for improving the crystallization rate and purity of struvite based on ultrasonic pretreatment in the high - value recovery of nitrogen and phosphorus from landfill leachate.

[0024] The technical principle of the present invention is as follows:

[0025] The present invention first uses high - power ultrasonic waves to pretreat the leachate. On the one hand, it directly degrades some small - particle suspended solids, reducing their impact on the purity of subsequent struvite crystallization as impurities; on the other hand, ultrasonic treatment can break up larger - particle suspended solids with a diameter of more than 100 microns, turning them into small - particle solids and changing their surface properties. The SS gradually changes from a plate - like laminated structure to a porous structure with honeycomb - like voids. The pore sites provided thereon can lower the energy barrier for struvite nucleation, regulate the crystal growth direction, stabilize the initial crystal nuclei, and promote the improvement of crystal formation rate and product purity; then, use low - power ultrasonic waves to treat the leachate to avoid the agglomeration of the small - particle suspended solids generated previously, and make the shape and particles of the subsequent - generated struvite crystallization more uniform. At the same time, the ultrasonic effect can also enhance the vibration intensity of the surface functional groups of SS. After ultrasonic treatment, the vibration peaks of amino groups (-NH2) and nitro groups (-NO2) on the SS surface are significantly enhanced. The lone pair electrons of the amino nitrogen in the amino group (-NH2) make it easy to participate in nucleophilic substitution or addition reactions, having a protonation electrostatic adsorption effect, which can combine with protons to form ammonium salts and can also adsorb PO4 3-Anionic groups such as these can promote the crystallization nucleation of struvite; the nitro group (-NO2) is a strong electron-withdrawing group, and through the inductive effect and conjugate effect, it can significantly change the charge distribution on the surface of molecules or materials. The nitro-modified suspended solids can be adsorbed on the surface of the crystal nucleus through electrostatic interaction and can serve as nucleation sites to regulate the crystal growth direction and promote oriented crystallization. In the present invention, ultrasonic pretreatment is used to make SS the heterogeneous crystal nucleus for struvite crystallization, accelerating the struvite crystallization nucleation rate. In the present invention, one or more of magnesium chloride, magnesium sulfate, and magnesium carbonate are added externally as the magnesium source for the struvite crystallization reaction, and at the same time, the pH value of the reaction system is adjusted to 9-10, which is more conducive to the formation of struvite crystallization.

[0026] The beneficial technical effects of the present invention are as follows:

[0027] By using the ultrasonic pretreatment of the present invention, SS (suspended solids) in the landfill leachate can be used as the crystal nucleus for subsequent struvite crystallization, thus significantly improving the formation rate of struvite. Through the method provided by the present invention, the reaction time for the recovered struvite crystallization can be shortened from the original 45 minutes without ultrasonic pretreatment to 15-25 minutes.

[0028] Compared with conventional struvite recovery technologies (such as the study on the recovery of ammonia nitrogen in food waste biogas slurry by the combination of struvite crystallization method and biological coagulation slag removal method, Yang Dekun; the study on the recovery of nitrogen and phosphorus by the struvite crystallization method using seawater as the magnesium source, Zhang Ping, etc.), through ultrasonic pretreatment, the present invention can effectively improve the crystallization purity of subsequent struvite, increasing the purity from the conventional 80% to over 90%, which has good application value. Brief Description of the Drawings

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0030] Figure 1 It is the treatment flow chart of the landfill leachate in each embodiment of the present invention. Detailed Description of the Embodiments

[0031] Now, various exemplary embodiments of the present invention will be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, characteristics, and implementation schemes of the present invention. It should be understood that the terms described in the present invention are only for describing specific implementation modes and are not used to limit the present invention.

[0032] In addition, for the numerical ranges in the present invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Intermediate values within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, are also included in the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0033] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Although the present invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. It should be noted that those aspects not described in detail in the present invention are all conventional operating means in the art and are not the focus of the present invention.

[0034] Regarding the use of "comprising", "including", "having", "containing", etc. in the present invention, they are all open-ended terms, meaning including but not limited to.

[0035] All raw materials used in the following examples and comparative examples of the present invention are commercially available products.

[0036] The preparation method of the sodium carbonate-sodium bicarbonate buffer solution with a concentration of 0.1 mol / L in the examples and comparative examples of the present invention is as follows:

[0037] (1) Preparation of 0.1 mol / L sodium carbonate solution: Gradually dissolve 10.6 g of sodium carbonate powder in 100 mL of pure water;

[0038] (2) Preparation of 0.1 mol / L sodium bicarbonate solution: Dissolve 8.4 g of sodium bicarbonate powder in 100 mL of pure water;

[0039] (3) Take 50 mL of the sodium carbonate solution from step (1) in a beaker, then slowly add the sodium bicarbonate solution from step (2), and monitor with a pH meter. Adjust the pH value to the required target range by adding the sodium bicarbonate solution; Store the prepared buffer solution at room temperature or in a refrigerated environment for subsequent use.

[0040] Figure 1 This is the treatment flow chart of the landfill leachate in each example of the present invention.

[0041] The treatment of the landfill leachate mainstream treatment system mentioned in the examples and comparative examples of the present invention refers to performing two-stage AO + MBR system + nanofiltration treatment in sequence.

[0042] Example 1

[0043] A method for improving the crystallization rate and purity of struvite based on ultrasonic pretreatment, the steps are as follows:

[0044] Select the leachate generated by a waste incineration plant in Shanghai. Its COD is 42,600 mg / L, ammonia nitrogen is 2,000 mg / L, and total phosphorus is 150 mg / L. Introduce the leachate into the regulation tank. A submersible mixer is installed at the bottom of the regulation tank, and the stirring rate is 50 rpm to continuously stir the leachate. Uniformly introduce the leachate in the regulation tank into the ultrasonic pretreatment device, and conduct high-power ultrasonic treatment at a power of 250 W for 12.5 min. Then adjust the ultrasonic power to 25 W and conduct low-power ultrasonic treatment for 10 min. Introduce the effluent after ultrasonic treatment into the struvite crystallization reaction tank, use 0.1 mol / L sodium carbonate-sodium bicarbonate buffer solution to adjust the pH value of the system to 9.5, and then by adding magnesium chloride, make the molar ratio of magnesium to phosphorus 1.1:1.5, and conduct crystallization reaction for 15 min to generate struvite crystals.

[0045] After detection, through the treatment of this process, the recovery rate of phosphorus in the leachate can reach 96.8%, the reaction rate of struvite crystallization is increased by 62.5% compared with that without ultrasonic pretreatment (Comparative Example 4), and the purity of the generated struvite crystals is as high as 92.47%.

[0046] Example 2

[0047] A method for improving the crystallization rate and purity of struvite based on ultrasonic pretreatment, the steps are as follows:

[0048] Select the leachate generated by a waste incineration plant in Shanghai. Its COD is 53,300 mg / L, ammonia nitrogen is 3,000 mg / L, and total phosphorus is 80 mg / L. Introduce the leachate into the regulation tank. A submersible mixer is installed at the bottom of the regulation tank, and the stirring rate is 50 rpm to continuously stir the leachate. Uniformly introduce the leachate in the regulation tank into the ultrasonic pretreatment device, and conduct high-power ultrasonic treatment at a power of 250 W for 15 min. Then adjust the ultrasonic power to 50 W and conduct low-power ultrasonic treatment for 5 min. Introduce the effluent after ultrasonic treatment into the struvite crystallization reaction tank, use 0.1 mol / L sodium carbonate-sodium bicarbonate buffer solution to adjust the pH value of the system to 10, and then by adding magnesium sulfate, make the molar ratio of magnesium to phosphorus 1.2:1.0, and conduct crystallization reaction for 20 min to generate struvite crystals.

[0049] After detection, through the treatment of this process, the recovery rate of phosphorus in the leachate can reach 91.36%, the reaction rate of struvite crystallization is increased by 53.49% compared with that without ultrasonic pretreatment (Comparative Example 5), and the purity of the generated struvite crystals is as high as 90.8%.

[0050] Example 3

[0051] A method for improving the crystallization rate and purity of struvite based on ultrasonic pretreatment, the steps are as follows:

[0052] Select the leachate generated by a waste incineration plant in Shanghai, with a COD of 48100 mg / L, ammonia nitrogen of 2000 mg / L, and total phosphorus of 50 mg / L. Introduce the leachate into the regulation tank. A submersible mixer is installed at the bottom of the regulation tank, with a stirring rate of 50 rpm, and continuously stir the leachate; evenly introduce the leachate in the regulation tank into the ultrasonic pretreatment device, and perform high-power ultrasonic treatment for 110 min at a power of 200 W, then adjust the ultrasonic power to 20 W and perform low-power ultrasonic treatment for 7.5 min; introduce the effluent after ultrasonic treatment into the struvite crystallization reaction tank, use 0.1 mol / L sodium carbonate-sodium bicarbonate buffer solution to adjust the pH value of the system to 9, and then by adding magnesium carbonate, make the molar ratio of magnesium to phosphorus 0.8:1.0, and crystallize for 25 min to generate struvite crystals.

[0053] After testing, through this process, the recovery rate of phosphorus in the leachate can reach 89.9%, the reaction rate of struvite crystallization is increased by 44.44% compared with no ultrasonic pretreatment (Comparative Example 3), and the purity of the generated struvite crystals is as high as 90.3%.

[0054] Comparative Example 1

[0055] The difference from Example 1 is only that the high-power ultrasonic treatment is omitted, only the low-power ultrasonic treatment is performed, and the time of the low-power ultrasonic treatment is modified to 117.5 min. In this case, the struvite crystallization reaction time is 18 min.

[0056] After testing, through this process, the recovery rate of phosphorus in the leachate is 94.3%, the reaction rate of struvite crystallization is increased by 55.0% compared with no ultrasonic pretreatment (Comparative Example 4), and the purity of the generated struvite crystals is 90.34%.

[0057] Comparative Example 2

[0058] The difference from Example 2 is only that the low-power ultrasonic treatment is omitted, only the high-power ultrasonic treatment is performed, and the time of the high-power ultrasonic treatment is modified to 117.5 min. In this case, the struvite crystallization reaction time is 22 min.

[0059] After testing, through this process, the recovery rate of phosphorus in the leachate is 90.60%, the reaction rate of struvite crystallization is increased by 48.84% compared with no ultrasonic pretreatment (Comparative Example 5), and the purity of the generated struvite crystals is 89.16%.

[0060] Comparative Example 3

[0061] The difference from Example 3 is only that the low-power ultrasonic treatment and the high-power ultrasonic treatment are omitted, and the leachate in the regulating tank is directly introduced into the struvite crystallization reaction tank, and the reaction time for struvite crystallization is 45 min.

[0062] After detection, after treatment by this process, the recovery rate of phosphorus in the landfill leachate is only 86.42%, and the purity of the generated struvite crystals is 81.54%.

[0063] Comparative Example 4

[0064] The difference from Example 1 is only that the low-power ultrasonic treatment and the high-power ultrasonic treatment are omitted, and the leachate in the regulating tank is directly introduced into the struvite crystallization reaction tank, and the reaction time for struvite crystallization is 40 min.

[0065] After detection, after treatment by this process, the recovery rate of phosphorus in the landfill leachate is only 88.31%, and the purity of the generated struvite crystals is 84.47%.

[0066] Comparative Example 5

[0067] The difference from Example 2 is only that the low-power ultrasonic treatment and the high-power ultrasonic treatment are omitted, and the leachate in the regulating tank is directly introduced into the struvite crystallization reaction tank, and the reaction time for struvite crystallization is 43 min.

[0068] After detection, after treatment by this process, the recovery rate of phosphorus in the landfill leachate is only 87.92%, and the purity of the generated struvite crystals is 83.29%.

[0069] The above-described embodiments are only descriptions of the preferred modes of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for improving the crystallization rate and purity of struvite based on ultrasonic pretreatment, characterized in that, It includes the following steps: After homogenizing the landfill leachate, perform high-power ultrasonic treatment at a power of 150 - 250 W, and then perform low-power ultrasonic treatment at a power of 20 - 50 W to obtain an ultrasonic-treated solution; Adjust the pH value of the ultrasonic-treated solution to 9 - 10, and then add a magnesium source for struvite crystallization reaction to generate struvite crystals.

2. The method according to claim 1, wherein The initial concentration of ammonia nitrogen in the landfill leachate is 1000 - 3000 mg / L, and the initial concentration of total phosphorus is 50 - 150 mg / L.

3. The method according to claim 1, wherein The time of the high-power ultrasonic treatment is 10 - 15 min.

4. The method according to claim 1, wherein The time of the low-power ultrasonic treatment is 5 - 10 min.

5. The method according to claim 1, characterized in that When adjusting the pH value of the ultrasonic-treated solution, use a sodium carbonate and sodium bicarbonate buffer solution.

6. The method according to claim 1, wherein The magnesium source is selected from one or more of magnesium chloride, magnesium sulfate, and magnesium carbonate.

7. The method according to claim 1, characterized in that The molar ratio of magnesium element in the magnesium source to the total phosphorus contained in the ultrasonic-treated solution is (0.8 - 1.2):(1.0 - 1.5).

8. The method according to claim 1, wherein The time of the struvite crystallization reaction is 15 - 25 min.

9. Application of the method for improving the crystallization rate and purity of struvite based on ultrasonic pretreatment according to any one of claims 1 - 8 in the high-value recovery of nitrogen and phosphorus from landfill leachate.

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