A method for improving the purity of struvite crystals in the process of landfill leachate treatment

Abstract

The invention discloses a method for improving the crystal purity of struvite in the process of treating landfill leachate, and belongs to the field of environmental engineering technology. The steps of the method for improving the crystal purity of struvite in the process of treating landfill leachate include: homogenizing the landfill leachate and performing anaerobic digestion pretreatment, injecting the gas generated during the anaerobic digestion pretreatment into the landfill leachate after the anaerobic digestion pretreatment to perform a metal passivation reaction, adjusting the pH of the landfill leachate after the metal passivation reaction to 9-11, adding a magnesium source to perform a struvite crystallization reaction, and generating struvite crystals. Through the method provided by the invention, the crystal purity of the recovered struvite can reach more than 90%.

Description

Technical Field

[0001] The invention belongs to the technical field of environmental engineering, and in particular relates to a method for improving the purity of struvite crystals in a process of treating landfill leachate. Background Art

[0002] Common methods for treating municipal solid waste include sanitary landfill, incineration, composting and comprehensive utilization. However, landfilling will occupy a large amount of land resources, and will cause secondary pollution to the atmosphere, surrounding water bodies and soil before and after landfilling; pollution control is difficult during composting and the products are difficult to find a way out. Therefore, in recent years, the treatment and disposal ratio of the two has continued to decline. In contrast, incineration has gradually gained attention due to its advantages such as heat generation, significant volume reduction, complete removal of harmful substances and small footprint. At present, waste incineration is the main method for treating municipal solid waste. However, in the process of incineration and disposal of municipal solid waste, secondary pollutants such as leachate will also be generated. According to data reported by various parties, the amount of leachate generated can reach more than 15% of the total amount of waste on average, and it is recognized as a high-risk and highly polluted difficult-to-degrade organic wastewater with high pollutant concentration and complex and changeable properties. Leachate from a waste incineration plant is a high-concentration organic wastewater leached from a storage pit before waste incineration. The water quality is generally only affected by factors such as waste composition, initial moisture content, ambient humidity, and storage pit temperature, as shown in Table 1. Typical leachate from a waste incineration plant has the following water quality characteristics: (1) high chroma and foul odor; (2) low pH value, with the leachate being weakly acidic; (3) extremely high concentration of organic pollutants, good biodegradability, and COD concentration usually exceeding 30,000 mg / L; (4) complex composition of organic pollutants, with a high content of refractory organic matter; (5) extremely high concentrations of ammonia nitrogen and total nitrogen; and (6) high salinity, with high concentrations of chloride ions, calcium ions, and magnesium ions in the leachate.

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

[0004]

[0005]

[0006] The focus of treating landfill leachate is the removal of COD, total nitrogen and total phosphorus. At present, there is a lack of research on the high-value synergistic recovery technology of nitrogen and phosphorus resources in landfill leachate while treating pollutants. The existing mainstream technologies for landfill leachate treatment include pretreatment, flocculation and sedimentation, biochemical treatment, chemical strong oxidation, MBR, ultrafiltration, nanofiltration and reverse osmosis, which combine physical and chemical treatment with biological treatment. This technology has achieved relatively good results in treating landfill leachate, and can achieve stable and standard discharge of leachate, but it has failed to effectively recycle the large amount of nitrogen and phosphorus nutrients contained in it.

[0007] In terms of recovering nitrogen resources from high-ammonia nitrogen wastewater, the stripping method has good development potential due to its high ammonia nitrogen recovery rate and low cost. However, at present, sulfuric acid solution is mainly used to absorb the ammonia nitrogen stripped out to form ammonium sulfate fertilizer, which has a low selling price. In particular, it is impossible to make a profit by stripping / stripping ammonia nitrogen from wastewater with lower ammonia nitrogen concentrations to make fertilizers. In addition, chemical precipitation is a technically feasible and economically reasonable treatment method for treating high-ammonia nitrogen wastewater, especially for high-concentration ammonia nitrogen. The struvite (MAP) precipitation method has received widespread attention in the field of treating high-ammonia nitrogen wastewater, and it also achieves the purpose of waste recycling and has great application prospects. However, the performance of the treatment process in different treatment systems varies, and is subject to various factors, such as temperature, pH, and interferences in the water. At the same time, using wastewater to add phosphorus to "produce" struvite deviates from the original intention of wastewater treatment and has poor benefits: when recovering nitrogen and phosphorus by the struvite crystallization method, since the molar ratio of nitrogen and phosphorus contained in general organic wastewater often exceeds 5.0, in many "large-scale" or pilot wastewater struvite recovery nitrogen and phosphorus experiments, researchers often need to add phosphorus to the wastewater, which not only leads to a low phosphorus recovery rate in wastewater, but also inevitably leads to poor economic benefits. In general, the removal effect of the current struvite (MAP) precipitation process has not yet reached the maximum or better level, and the crystal purity of struvite still needs to be further improved. It is very meaningful to explore how to further promote the decontamination and high-value utilization capabilities of struvite (MAP) precipitation. Summary of the invention

[0008] In view of the low synergistic recovery rate of high-value nitrogen and phosphorus nutrient elements and low economic benefits of wastewater treatment in the mainstream treatment process of leachate from existing garbage incineration plants, the present invention proposes a method for improving the purity of struvite crystals in the process of treating garbage leachate. The product purity of struvite crystals is improved through anaerobic digestion pretreatment and metal passivation, and the ammonia nitrogen produced in the ammonia stripping process is absorbed by phosphoric acid to make diammonium phosphate products, further improving the value of the recovered products, and finally making the nitrogen recovery rate in the garbage leachate reach more than 90%, the phosphorus recovery rate reach more than 95%, and the purity of struvite crystals reach more than 90%.

[0009] To achieve the above object, the present invention provides the following technical solutions:

[0010] One of the technical solutions of the present invention is to provide a method for improving the purity of struvite crystals in the process of treating landfill leachate, comprising the following steps:

[0011] The landfill leachate is homogenized and then subjected to anaerobic digestion pretreatment. The gas generated during the anaerobic digestion pretreatment is injected into the landfill leachate after the anaerobic digestion pretreatment to carry out a metal passivation reaction. The pH of the landfill leachate after the metal passivation reaction is adjusted to 9-11, and a magnesium source is added to carry out a struvite crystallization reaction to generate struvite crystals.

[0012] Preferably, the initial concentration of ammonia nitrogen in the landfill leachate is 1000-2000 mg / L, and the initial concentration of total phosphorus is 80-160 mg / L.

[0013] Preferably, the anaerobic digestion pretreatment time is 2-3 days.

[0014] Preferably, 2-4 mol / L sodium hydroxide solution is used to adjust the pH of the landfill leachate after the metal passivation reaction.

[0015] Preferably, the magnesium source is selected from one or more of magnesium chloride, magnesium carbonate and magnesium hydroxide.

[0016] Preferably, the molar ratio of the magnesium element in the magnesium source to the total phosphorus contained in the landfill leachate after the metal passivation reaction is (0.8-1.5):(0.8-1.5); the most preferred ratio is 1.2:1.

[0017] Preferably, the process of generating struvite crystals also includes ammonia nitrogen recovery treatment, and the specific steps include: preheating the landfill leachate after generating struvite crystals to 110-130° C., adjusting the pH to 11-12, and then introducing the leachate into an ammonia stripping device, using phosphoric acid solution to absorb the generated ammonia nitrogen to generate diammonium phosphate, and completing ammonia nitrogen recovery.

[0018] The second technical solution of the present invention is to provide an application of the above-mentioned method for improving the purity of struvite crystals in the process of landfill leachate treatment in the high-value recovery of nitrogen and phosphorus in landfill leachate.

[0019] The recycling principle of the present invention is as follows:

[0020] First, the biogas produced by anaerobic digestion is used to recycle part of the organic bioenergy in the landfill leachate, and part of the suspended solid particles are degraded during the anaerobic digestion process to prevent their influence on the purity of the struvite crystallization; at the same time, the biogas produced by anaerobic digestion is introduced into the landfill leachate after anaerobic digestion, and the carbon dioxide in the biogas is used to react with the calcium ions in the landfill leachate to achieve metal passivation, and further improve the purity of the subsequent struvite crystallization. The present invention uses exogenous magnesium chloride as a magnesium source for the struvite crystallization reaction, and at the same time uses sodium hydroxide to adjust the pH value of the reaction system to 9-11 to generate struvite crystals as an additive for organic fertilizer; the leachate after the struvite crystallization reaction is introduced into an ammonia stripping reaction device, and the generated ammonia nitrogen is absorbed by a phosphoric acid solution to generate a diammonium phosphate product with high economic value. Traditional ammonia nitrogen recovery uses sulfuric acid solution for absorption to generate a low-value ammonium sulfate product. The leachate after the ammonia stripping reaction is treated by the current mainstream landfill leachate treatment process, and the tail water meets the discharge standards.

[0021] The beneficial technical effects of the present invention are as follows:

[0022] (1) Obvious resource utilization effect: The present invention can recover energy gas (biogas) to the greatest extent, and further improve the purity of biogas by metal passivation treatment.

[0023] (2) Phosphorus can be efficiently recovered: MAP is used for phosphorus removal, and phosphorus can be recovered in the form of magnesium ammonium phosphate (struvite) and added to the produced organic fertilizer as a slow-release fertilizer additive.

[0024] (3) MAP has a high purity: The use of anaerobic digestion and metal ion passivation treatment can effectively improve the purity of subsequent struvite crystallization, raising the purity from the conventional 80% (research on the recovery of ammonia nitrogen from food waste sludge by struvite crystallization combined with biological coagulation and slag removal, Yang Dekun; research on the recovery of nitrogen and phosphorus by struvite crystallization using seawater as magnesium source, Zhang Ping, etc.) to more than 90%.

[0025] (4) Nitrogen can be efficiently recovered: The ammonia stripping method is used to recover the nitrogen element in the leachate. Traditionally, sulfuric acid solution is used to absorb the generated ammonia nitrogen to generate an ammonium sulfate product, but the product economic efficiency is not high. The present invention uses phosphoric acid solution for absorption to generate a diammonium phosphate product, which has a higher economic value.

[0026] (5) The process of combining struvite crystallization and ammonia stripping is adopted. The various treatment steps work synergistically to improve the recovery efficiency of ammonia nitrogen and total phosphorus, which can reach 90% and 95% or more respectively. At the same time, after being treated by this process and entering the mainstream treatment process of "two-stage AO+MBR system+nanofiltration" of landfill leachate (without the need to pass through the reverse osmosis membrane treatment device, which greatly reduces the treatment cost), the effluent can stably meet the tailwater discharge standard specified in the "Pollution Control Standards for Municipal Waste Landfills" (GB16889-2008). BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Flow chart of the treatment of landfill leachate in each embodiment of the present invention. DETAILED DESCRIPTION

[0028] Now, various exemplary embodiments of the present invention are described in detail, and this detailed description should not be considered as a limitation of the present invention, but should be understood as a more detailed description of certain aspects, characteristics and embodiments of the present invention. It should be understood that the terms described in the present invention are only for describing specific embodiments and are not used to limit the present invention.

[0029] In addition, for the numerical range in the present invention, it is understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. The intermediate value in any stated value or stated range, and each smaller range between any other stated value or intermediate value in the range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0030] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the invention pertains. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention.

[0031] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0032] The process flow chart of the landfill leachate treatment in each embodiment of the present invention is shown in Figure 1 .

[0033] The mainstream treatment system mentioned in the embodiments and comparative examples of the present invention refers to a two-stage AO+MBR system+nanofiltration treatment in sequence.

[0034] Example 1

[0035] The landfill leachate produced by a waste incineration plant in Shanghai was selected, with a COD of 46800 mg / L, a total nitrogen of 2250 mg / L, ammonia nitrogen of 1500 mg / L, and a total phosphorus of 120 mg / L. The leachate was introduced into a regulating tank, and a submersible agitator was installed at the bottom of the regulating tank with a stirring rate of 50 rpm to continuously stir the leachate; the leachate in the regulating tank was uniformly introduced into an anaerobic digestion pretreatment tank with a hydraulic retention time of 3.0 d; the biogas produced by the reaction in the anaerobic digestion pretreatment tank was introduced into a metal passivation reaction tank, so that the carbon dioxide in the biogas reacted with the calcium ions in the leachate to form carbonate precipitation; the precipitated water of the metal passivation reaction tank was introduced into a struvite crystallization reaction tank, and the pH of the system was adjusted to 10.5 with a 4 mol / L sodium hydroxide solution, and the molar ratio of magnesium to phosphorus was made to be 1.2:1.0 by adding magnesium chloride, and struvite crystals were generated by reaction. The effluent is preheated to 120°C and the pH is adjusted to 11.5 using 4 mol / L sodium hydroxide solution. Then, it is introduced into the ammonia stripping device for reaction. The ammonia nitrogen produced is absorbed by phosphoric acid solution to finally generate diammonium phosphate products. The remaining wastewater from ammonia stripping enters the mainstream leachate treatment system for treatment. The COD in the effluent is 92-98 mg / L, the total nitrogen concentration is 32-37 mg / L, and other effluent indicators can also meet the direct discharge standards for tail water specified in the "Standards for Pollution Control of Municipal Waste Incineration" (GB18485-2014). In this process, the nitrogen recovery rate in the leachate can reach 93.5%, the phosphorus recovery rate can reach 98.7%, and the generated struvite crystal purity can reach 94.5%.

[0036] Example 2

[0037] The landfill leachate produced by a waste incineration plant in Shanghai was selected, with a COD of 44320 mg / L, a total nitrogen of 1800 mg / L, an ammonia nitrogen of 1000 mg / L, and a total phosphorus of 80 mg / L. The leachate was introduced into a regulating tank, and a submersible agitator was installed at the bottom of the regulating tank with a stirring rate of 50 rpm to continuously stir the leachate; the leachate in the regulating tank was uniformly introduced into an anaerobic digestion pretreatment tank with a hydraulic retention time of 2.5 d; the biogas produced by the reaction in the anaerobic digestion pretreatment tank was introduced into a metal passivation reaction tank, so that the carbon dioxide in the biogas reacted with the calcium ions in the leachate to form carbonate precipitates; the precipitated water of the metal passivation reaction tank was introduced into a struvite crystallization reaction tank, and the pH of the system was adjusted to 11 with a 3 mol / L sodium hydroxide solution, and the molar ratio of magnesium to phosphorus was made to be 1.2:0.8 by adding magnesium carbonate, and struvite crystals were formed by reaction. The effluent is preheated to 110°C and the pH is adjusted to 11 with 3 mol / L sodium hydroxide solution. Then, the ammonia stripping device is introduced for reaction. The ammonia nitrogen is absorbed by phosphoric acid solution to finally generate diammonium phosphate product. The remaining wastewater from ammonia stripping enters the mainstream leachate treatment system for treatment. The COD in the effluent is 88-95 mg / L, the total nitrogen concentration is 28-35 mg / L, and other effluent indicators can also meet the direct discharge standards for tail water specified in the "Standards for Pollution Control of Municipal Waste Incineration" (GB18485-2014). In this process, the nitrogen recovery rate in the leachate can reach 92.3%, the phosphorus recovery rate can reach 96.8%, and the generated struvite crystal purity is 92.1%.

[0038] Example 3

[0039] The landfill leachate produced by a waste incineration plant in Shanghai was selected, with a COD of 48540 mg / L, a total nitrogen of 2400 mg / L, an ammonia nitrogen of 2000 mg / L, and a total phosphorus of 160 mg / L. The leachate was introduced into a regulating tank, and a submersible agitator was installed at the bottom of the regulating tank with a stirring rate of 50 rpm to continuously stir the leachate; the leachate in the regulating tank was uniformly introduced into an anaerobic digestion pretreatment tank with a hydraulic retention time of 2.0 d; the biogas produced by the reaction in the anaerobic digestion pretreatment tank was introduced into a metal passivation reaction tank, so that the carbon dioxide in the biogas reacted with the calcium ions in the leachate to form carbonate precipitation; the precipitated water of the metal passivation reaction tank was introduced into a struvite crystallization reaction tank, and the pH of the system was adjusted to 11 with a 2 mol / L sodium hydroxide solution, and the molar ratio of magnesium to phosphorus was made to be 0.8:1.2 by adding magnesium hydroxide, and struvite crystals were formed by reaction. The effluent is preheated to 130°C and the pH is adjusted to 12 using a 2 mol / L sodium hydroxide solution. The effluent is then introduced into an ammonia stripping device for reaction. The ammonia nitrogen produced is absorbed by a phosphoric acid solution to eventually generate a diammonium phosphate product. The remaining wastewater from ammonia stripping enters the mainstream leachate treatment system for treatment. The COD in the effluent is 84-92 mg / L, the total nitrogen concentration is 25-32 mg / L, and other effluent indicators can also meet the direct tailwater discharge standards specified in the "Standards for Pollution Control of Municipal Waste Incineration" (GB18485-2014). In this process, the nitrogen recovery rate in the leachate can reach 90.4%, the phosphorus recovery rate can reach 95.3%, and the generated struvite crystal purity is 91.6%.

[0040] Comparative Example 1

[0041] Compared with Example 1, the only difference is that the biogas generated during the anaerobic digestion pretreatment is not introduced into the metal passivation reaction tank. In this process, the recovery rate of nitrogen in the landfill leachate is 88.4%, the recovery rate of phosphorus is 94.3%, and the purity of the generated struvite crystals is 87.6%.

[0042] Comparative Example 2

[0043] Compared with Example 2, the only difference is that no anaerobic digestion pretreatment is provided and metal passivation reaction treatment is lacking. In this process, the recovery rate of nitrogen in the landfill leachate is 84.4%, the recovery rate of phosphorus is 92.6%, and the purity of the generated struvite crystals is 81.6%.

[0044] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for improving the purity of struvite crystals in the process of treating landfill leachate, characterized in that: It consists of the following steps: The landfill leachate is homogenized and then subjected to anaerobic digestion pretreatment, the gas generated during the anaerobic digestion pretreatment is injected into the landfill leachate after the anaerobic digestion pretreatment to perform a metal passivation reaction, the pH of the landfill leachate after the metal passivation reaction is adjusted to 9-11, and a magnesium source is added to perform a struvite crystallization reaction to generate struvite crystals; The initial concentration of ammonia nitrogen in the landfill leachate is 1000-2000 mg / L, and the initial concentration of total phosphorus is 80-160 mg / L; The anaerobic digestion pretreatment time is 2-3 days; The molar ratio of the magnesium element in the magnesium source to the total phosphorus contained in the landfill leachate after the metal passivation reaction is (0.8-1.5):(0.8-1.5); After the formation of struvite crystals, ammonia nitrogen recovery treatment is also included. The specific steps include: preheating the landfill leachate after the formation of struvite crystals to 110-130°C, adjusting the pH to 11-12, and then introducing it into an ammonia stripping device, using phosphoric acid solution to absorb the generated ammonia nitrogen to generate diammonium phosphate, and completing ammonia nitrogen recovery.

2. The method for improving the purity of struvite crystals in the process of treating landfill leachate according to claim 1, characterized in that: 2-4 mol / L sodium hydroxide solution is used to adjust the pH of the landfill leachate after the metal passivation reaction.

3. The method for improving the purity of struvite crystals in the process of treating landfill leachate according to claim 1, characterized in that: The magnesium source is selected from one or more of magnesium chloride, magnesium carbonate and magnesium hydroxide.

4. Application of the method for improving the purity of struvite crystals in the process of treating landfill leachate according to any one of claims 1 to 3 in high-value recovery of nitrogen and phosphorus in landfill leachate.

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