A method for recovering nutrient elements from sludge by enhanced hydrothermal pretreatment combined with crystallization precipitation and application thereof

Abstract

The present application relates to a kind of methods for recovering nutrient elements from sludge by enhanced hydrothermal pretreatment combined with crystallization precipitation method and application, belong to the field of waste resource utilization.Utilize hydrothermal carbonization-crystallization precipitation method to carry out combined processing to urban sludge;The hydrothermal carbonization-crystallization precipitation method is first to the dewatered sludge hydrothermal carbonization treatment, control hydrothermal temperature and time, after reaction is completed, natural cooling to room temperature, solid-liquid separation;Then magnesium source and phosphorus source are added to the obtained carbonization liquid, and seed is added, and nutrient elements are recovered under alkaline conditions, and the obtained solid precipitate can be used as slow-release fertilizer.The technical route provided in the present application is short, simple operation, the recovery rate of ammonia nitrogen and phosphorus is as high as 97.06% and 99.78%, and the crystallization precipitation time is greatly shortened, organic nitrogen, phosphorus and other nutrient elements in sludge can be quickly and effectively converted into slow-release and plant-absorbed form, thereby improving the level of sludge resource utilization.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for recovering nutrients from sludge by enhanced hydrothermal pretreatment combined with crystallization precipitation, and belongs to the field of waste resource utilization. BACKGROUND

[0002] In recent years, with the increase of urban sewage and the improvement of water treatment rate, the amount of sludge produced by water treatment is also increasing. The traditional sludge treatment and disposal technology cannot meet the requirements of sustainable development, and new technologies are urgently needed to improve the level of sludge reduction, stabilization, harmlessness and resource utilization. Hydrothermal carbonization technology is an advanced technology for treating high-moisture biomass. It refers to a series of complex chemical reactions such as hydrolysis, dehydration, decarboxylation, polymerization and aromatization under certain temperature and saturation pressure, using water as solvent and reaction medium, to convert biomass into carbon products (hydrothermal carbon) and produce a large amount of liquid by-products (hydrothermal carbon liquid) and a small amount of gas. Compared with traditional high-temperature pyrolysis, hydrothermal carbonization requires lower temperature and lower energy consumption. The generated hydrothermal carbon has a high content of oxygen-containing functional groups and a high energy density, and can be used as soil additive, adsorbent and fuel, etc. Previous researches have mostly focused on hydrothermal carbon, and the research on hydrothermal carbon liquid is relatively less.

[0003] The organic matter contained in sludge is mainly protein, polysaccharide, lipid, humus and nucleic acid, among which protein and polysaccharide generally account for about 90%. After hydrothermal carbonization of sludge, the nutrient elements can be released to the maximum extent, among which 40-70% of nitrogen, 50-70% of potassium and 10-15% of phosphorus will be transferred to the hydrothermal carbon liquid. The hydrothermal carbon liquid contains rich organic and inorganic components and is a valuable potential resource. If it is not properly treated and disposed and enters natural water bodies such as lakes and reservoirs, it will cause eutrophication of the water body and cause damage to the ecological environment to a certain extent. Therefore, it is of great significance to recover nutrient elements from sludge. SUMMARY

[0004] In order to recover N, P, K and other nutrient elements in sludge, the present application provides a method for recovering nutrients from sludge by enhanced hydrothermal pretreatment combined with crystallization precipitation and application. The method of the present application can effectively transfer the nutrient elements in the sludge to the carbonization liquid at a mild reaction temperature, and then effectively recover the N, P and other nutrient elements in the sludge, which is low in cost and can turn waste into treasure. The technical problem to be solved by the present application is to make the sludge treatment method simple, transfer more nutrient elements to the carbonization liquid, at the same time, make the carbonization liquid treatment method simple, not limited by temperature, can effectively treat wastewater containing high nutrient elements, realize the resource recovery and utilization of nitrogen and phosphorus, so as to be more suitable for practical use.

[0005] The purpose of the present application and the technical problem thereof are realized by adopting the following technical scheme.

[0006] In a first aspect, the present application provides a method for recovering nutrients from sludge by a combination of hydrothermal pretreatment and crystallization precipitation, which is a combination of hydrothermal carbonization-crystallization precipitation method for treating municipal sludge; the hydrothermal carbonization-crystallization precipitation method is first to perform hydrothermal carbonization treatment on the dewatered sludge, and to perform magnesium modification and add an oxidizing agent during the hydrothermal carbonization process to obtain a carbonization liquid and modified hydrothermal carbon; then to add a magnesium source and a phosphorus source to the obtained carbonization liquid, and to add the modified hydrothermal carbon as a crystal seed, and to perform crystallization precipitation under alkaline conditions to recover nutrients.

[0007] In a preferred embodiment, the temperature for hydrothermal carbonization of the dewatered sludge is 180-260°C, the pressure is 2-10 MPa, and the hydrothermal carbonization reaction time is 0.5-1.5 h; before the hydrothermal carbonization, the dewatered sludge is pretreated by an ultrasonic method, the ultrasonic time is 5-10 min, and the ultrasonic power is 100-300 W; then the dewatered sludge is mixed with 1-5 wt% of magnesium citrate, and the dewatered sludge is subjected to hydrothermal carbonization treatment, and an oxidizing agent is added during the hydrothermal carbonization, and the amount of the oxidizing agent is 0-8% (w:v).

[0008] The addition of the oxidizing agent can strengthen the hydrothermal carbonization process, promote more N and P to transfer into the carbonization liquid, and make the subsequent obtained crystallization precipitation have a higher content of nutrients. The ultrasonic pretreatment before the hydrothermal carbonization generates hydroxyl radicals, hydrogen radicals, oxygen radicals, etc., destroys the extracellular polymeric substance and cell wall in the sludge, and decomposes organic matter, which can improve the degree of hydrothermal carbonization of the sludge, and in combination with the magnesium modification and the addition of the oxidizing agent, the modified hydrothermal carbon has a larger specific surface area and porosity, and the magnesium modification also makes the obtained carbonization liquid have a certain magnesium content, and can greatly shorten the time required for the crystallization precipitation.

[0009] Further, the initial concentration of ammonia nitrogen in the carbonization liquid is 1000-3000 mg / L, the initial concentration of phosphorus is 20-100 mg / L, and the initial concentration of magnesium ions in the carbonization liquid is 500-1000 mg / L.

[0010] In a preferred embodiment, the pH value of the system during the crystallization precipitation reaction is controlled to be 8-10; the amount of the crystal seed during the crystallization precipitation reaction is 0.1-1.0 g / L, the stirring time during the crystallization precipitation reaction is 0.2-0.4 h, and the standing time is 5-10 min; and the crystal seed is the solid product of the dewatered sludge hydrothermal carbonization reaction, i.e., the modified hydrothermal carbon. The modified hydrothermal carbon has a high specific surface area and strong adsorption capacity due to its developed pores, and can significantly improve the crystallization rate and the size of the obtained precipitation when the modified hydrothermal carbon is used as the crystal seed for the first time.

[0011] Further, the pH value of the system during the crystallization precipitation reaction is adjusted by using an alkaline solution, which is a sodium hydroxide solution with a concentration of 2-5 mol / L; and the molar ratio of magnesium, phosphorus and nitrogen during the crystallization precipitation reaction is (1-2):(1-2):1. If the magnesium source and the phosphorus source are not supplemented additionally, the precipitate cannot be generated, and the nutrient elements in the carbonization liquid cannot be recovered.

[0012] In a preferred embodiment, the method specifically comprises:

[0013] (1) The dewatered sludge is first subjected to ultrasonic pretreatment, then the sludge with a certain water content is placed in a pressure reaction kettle, 1-5 wt% of magnesium citrate is added, and an oxidizing agent is added, the hydrothermal carbonization temperature and reaction time are controlled, after the reaction is completed, it is naturally cooled to room temperature, solid-liquid separation is performed, the solid phase product is modified hydrothermal carbon, and the liquid phase product is carbonization liquid, and the carbonization liquid is further treated;

[0014] (2) The magnesium source and the phosphorus source are added to the obtained carbonization liquid, an alkaline solution is added to control the pH value, and the modified hydrothermal carbon is added as a crystal seed, after the crystallization reaction is completed, solid-liquid separation is performed to obtain a solid phase product rich in nutrient elements, which can be used as a slow-release fertilizer, and the remaining wastewater enters an anaerobic tank for further treatment.

[0015] According to the method for recovering nutrient elements from sludge by using the combined method of enhanced hydrothermal pretreatment and crystallization precipitation, more specifically, the process method is as follows: the dewatered sludge is first subjected to ultrasonic pretreatment, then the sludge with a certain water content is placed in a pressure reaction kettle, 1-5 wt% of magnesium citrate is added, a certain amount of hydrogen peroxide is added, the hydrothermal carbonization temperature, reaction time and pressure are controlled, after the reaction is completed, it is naturally cooled to room temperature, solid-liquid separation is performed, and the liquid phase product (carbonization liquid) is collected for further crystallization treatment. The magnesium source and the phosphorus source are added to the obtained carbonization liquid, an alkaline solution is added to control the pH value of the system, the solid phase product (modified hydrothermal carbon) in the early stage is added as a crystal seed, after the crystallization reaction is completed, solid-liquid separation is performed to obtain a solid phase product rich in nutrient elements, which can be used as a slow-release fertilizer, and the remaining wastewater enters an anaerobic tank for further treatment.

[0016] For the first time, the modified hydrothermal carbon is added as a crystal seed, so that the precipitation speed is faster, and the stability of the precipitate is better. When the modified hydrothermal carbon is prepared, magnesium citrate is added, and the carbonization liquid and the modified hydrothermal carbon both contain Mg, and the microstructure of the hydrothermal carbon changes. It is found that using the modified hydrothermal carbon as a crystal seed and then adding the carbonization liquid can significantly improve the crystallization precipitation speed, and the corresponding crystallization precipitate rich in nutrient substances can be obtained only by standing for 5-10 min.

[0017] The foregoing method for recovering nutrient elements from sludge by using the combined method of enhanced hydrothermal pretreatment and crystallization precipitation, the magnesium source includes but is not limited to magnesium chloride and magnesium sulfate.

[0018] The method for recovering nutrient elements from sludge by the enhanced hydrothermal pretreatment combined with crystallization precipitation method as described above, wherein the phosphorus source includes, but is not limited to, disodium hydrogen phosphate and sodium dihydrogen phosphate.

[0019] The method for recovering nutrient elements from sludge by the enhanced hydrothermal pretreatment combined with crystallization precipitation method as described above, wherein the crystallization precipitate is a good slow-release fertilizer.

[0020] In a second aspect of the present application, the crystallization precipitate prepared by the method for recovering nutrient elements from sludge by the enhanced hydrothermal pretreatment combined with crystallization precipitation method as described above is disclosed. The crystallization precipitate is rich in nutrient elements.

[0021] In a third aspect of the present application, a slow-release fertilizer comprising the crystallization precipitate as described above is provided.

[0022] In a fourth aspect of the present application, a mixed fertilizer comprising the slow-release fertilizer as described above is disclosed. The slow-release fertilizer can be used in combination with conventional chemical fertilizers, organic fertilizers, etc. to form a mixed fertilizer.

[0023] In a fifth aspect of the present application, the use of the method for recovering nutrient elements from sludge by the enhanced hydrothermal pretreatment combined with crystallization precipitation method as described above in the preparation of a slow-release fertilizer, the resource utilization of sludge or the resource utilization of carbonization liquid is provided. The method of the present application can quickly and effectively convert organic nitrogen and phosphorus in sludge into a slow-release and plant-absorbable form, thereby improving the resource utilization level of sludge.

[0024] Compared with the prior art, the present application has the following beneficial effects:

[0025] The method for recovering nutrient elements from sludge (dewatered sludge from a municipal wastewater treatment plant) by the enhanced hydrothermal pretreatment combined with crystallization precipitation method as described above adopts a method combining hydrothermal carbonization technology with crystallization precipitation, which can transfer nutrient elements in sludge to carbonization liquid at a relatively mild reaction temperature, thereby reducing energy consumption. The crystallization precipitation of carbonization liquid is not limited by temperature, and the design and operation are very simple. The method can effectively recover nutrient elements in sludge, and the recovery rates of ammonia nitrogen and phosphorus are as high as 97.06% and 99.78%, respectively. Moreover, the crystallization precipitation time is greatly shortened, and the generated crystallization precipitate is a good slow-release fertilizer, thereby realizing the reduction and resource utilization of sludge. BRIEF DESCRIPTION OF DRAWINGS

[0026] The accompanying drawings are included to provide a further understanding of the present application, and constitute a part of the specification, illustrate the present application and are used to explain the present application, and do not constitute a limitation on the present application; in the drawings:

[0027] Figure 1 FTIR graphs of crystallization precipitation products under different reaction conditions;

[0028] Figure 2 XRD patterns of the crystalline precipitates obtained under different reaction conditions. DETAILED DESCRIPTION

[0029] The experimental methods used in the following examples are conventional methods unless otherwise specified. The materials, reagents, instruments, etc. used in the following examples are commercially available unless otherwise specified.

[0030] Example 1

[0031] The dewatered sludge from a municipal wastewater treatment plant was used as raw material. The dewatered sludge with a water content of 80% was subjected to ultrasonic treatment (ultrasonic time of 5 min, ultrasonic power of 200 w) and then placed in a pressure reactor. 1 wt% of magnesium citrate (i.e., the mass of magnesium citrate accounted for 1% of the total mass of the mixture of magnesium citrate solution and dewatered sludge) was added. The hydrothermal carbonization reaction temperature was controlled at 180°C, 220°C, and 260°C, respectively. The pressure was 5 MPa, the residence time was 1 h, and the oxidant dosage was 4% and 8% H2O2 (w:v, g / ml), respectively. After the reaction was completed, the system was naturally cooled to room temperature. Solid-liquid separation was performed, and the water quality indicators of the liquid phase products obtained under different reaction conditions, such as pH value, ammonia nitrogen, and TP, and magnesium concentration, were determined. The results are shown in Table 1.

[0032] In combination with ultrasonic treatment, magnesium modification, and the addition of oxidants, the hydrothermal reaction time in Example 1 is shorter than the usual duration of existing hydrothermal carbonization reactions. Ultrasonic treatment, magnesium modification, and the addition of oxidants also help to improve the reaction rate and product stability during subsequent crystalline precipitation.

[0033] Table 1 Water quality indicators of carbonization liquid obtained under different reaction conditions

[0034]

[0035] After the dewatered sludge from a municipal wastewater treatment plant was pretreated by hydrothermal carbonization, the pH value of the carbonization liquid obtained under different reaction conditions was weakly alkaline, the ammonia nitrogen concentration was relatively high, and the TP concentration was relatively low.

[0036] The carbonization liquid obtained under the reaction conditions of hydrothermal temperature of 220°C, residence time of 1 h, and oxidant dosage of 4% H2O2 in Table 1 was used as the experimental water quality for crystalline precipitation. The pH value was 7.80, the ammonia nitrogen concentration was 2600 mg / L, the TP concentration was 75 mg / L, and the Mg 2+ concentration was 700 mg / L.

[0037] Examples 2-5 were all carried out at room temperature.

[0038] Example 2

[0039] Take 25 mL of sludge carbonization liquid (i.e. the carbonization liquid obtained by hydrothermal carbonization experiment under the reaction conditions of hydrothermal temperature 220 ℃, residence time 1 h and oxidant dosage 4% H2O2 in Table 1, the same below), calculate Mg:P:N according to the magnesium concentration, ammonia nitrogen concentration and TP concentration in Example 1, do not add N source, add magnesium chloride and disodium hydrogen phosphate, so that the molar ratio of Mg:P:N in the final system is 2:2:1, add modified hydrothermal carbon and make the concentration of modified hydrothermal carbon in the crystallization precipitation system be 0.5 g / L, the crystallization precipitation system is a mixed solution composed of the aforementioned sludge carbonization liquid, magnesium chloride, disodium hydrogen phosphate and modified hydrothermal carbon, adjust the pH value of the solution to 9.0 with 2.5 mol / L NaOH solution, stir at room temperature with a magnetic stirrer at a speed of 180 rpm for 0.3 h, after the reaction is completed, stand for 5 min, filter with a 0.45 um filter membrane, and then take the filtrate for determination. In addition, take part of the solid sample, analyze the characteristic functional groups and crystal morphology in the solid with a Fourier infrared spectrometer and an X-ray diffractometer.

[0040] Result analysis: the ammonia nitrogen concentration of the filtrate is 87.00±2.8 mg / L, the ammonia nitrogen recovery rate is 96.25%; the TP concentration is 337.21±6.3 mg / L, and the phosphorus recovery rate is 98.62%. The structural characteristics of the obtained crystalline precipitate are shown in Figure 1 and Figure 2 .

[0041] Example 3:

[0042] Take 25 mL of sludge carbonization liquid, calculate Mg:P:N according to the magnesium concentration, ammonia nitrogen concentration and TP concentration in Example 1, do not add N source, add magnesium sulfate and sodium dihydrogen phosphate, so that the molar ratio of Mg:P:N in the final system is 2:1.5:1, add modified hydrothermal carbon and make the concentration of modified hydrothermal carbon in the crystallization precipitation system be 0.5 g / L, the crystallization precipitation system is a mixed solution composed of the aforementioned sludge carbonization liquid, magnesium sulfate, sodium dihydrogen phosphate and modified hydrothermal carbon, adjust the pH value of the solution to 8.5 with 2.5 mol / L NaOH solution, stir at room temperature with a magnetic stirrer at a speed of 180 rpm for 0.3 h, after the reaction is completed, stand for 5 min, filter with a 0.45 um filter membrane, and then take the filtrate for determination. In addition, take part of the solid sample, analyze the characteristic functional groups and crystal morphology in the solid with a Fourier infrared spectrometer and an X-ray diffractometer.

[0043] Result analysis: the ammonia nitrogen concentration of the filtrate is 69.58±1.6 mg / L, the ammonia nitrogen recovery rate is 97.06%; the TP concentration is 40.55 mg / L, and the phosphorus recovery rate is 99.78%. The structural characteristics of the obtained crystalline precipitate are shown in Figure 1 and Figure 2 .

[0044] Example 4:

[0045] Take 25 mL of sludge carbonization liquid, according to the magnesium concentration, ammonia nitrogen concentration, TP concentration in example 1 to calculate Mg:P:N, without additional N source, add magnesium chloride and disodium hydrogen phosphate, so that the molar ratio of Mg:P:N in the final system is 1:2:1, add modified hydrothermal carbon and make the concentration of modified hydrothermal carbon in the crystallization precipitation system 0.5 g / L, the crystallization precipitation system is a mixed solution composed of the aforementioned sludge carbonization liquid, magnesium chloride, disodium hydrogen phosphate and modified hydrothermal carbon, adjust the pH value of the solution to 9.0 with 2.5 mol / L NaOH solution, stir at room temperature with a magnetic stirrer at a speed of 180 rpm for 0.3 h, after the reaction is completed, stand for 5 min, filter with a 0.45 um filter membrane, and then take the filtrate for determination. In addition, take part of the solid sample, analyze the characteristic functional groups and crystal morphology in the solid with a Fourier infrared spectrometer and an X-ray diffractometer.

[0046] Result analysis: the ammonia nitrogen concentration of the filtrate is 200.64±9.2 mg / L, the ammonia nitrogen recovery rate is 91.51%; the TP concentration is 2382.77±20.8 mg / L, and the phosphorus recovery rate is 90.45%. The structural characteristics of the obtained crystalline precipitate are shown in Figure 1 and Figure 2 The obtained crystalline precipitate under this reaction condition is struvite crystal.

[0047] Example 5:

[0048] Take 25 mL of sludge carbonization liquid, according to the magnesium concentration, ammonia nitrogen concentration, TP concentration in example 1 to calculate Mg:P:N, without additional N source, add magnesium sulfate and sodium dihydrogen phosphate, so that the molar ratio of Mg:P:N in the final system is 1:1:1, add modified hydrothermal carbon and make the concentration of modified hydrothermal carbon in the crystallization precipitation system 0.5 g / L, the crystallization precipitation system is a mixed solution composed of the aforementioned sludge carbonization liquid, magnesium sulfate, sodium dihydrogen phosphate and modified hydrothermal carbon, adjust the pH value of the solution to 9.0 with 2.5 mol / L NaOH solution, stir at room temperature with a magnetic stirrer at a speed of 180 rpm for 0.3 h, after the reaction is completed, stand for 5 min, filter with a 0.45 um filter membrane, and then take the filtrate for determination. In addition, take part of the solid sample, analyze the characteristic functional groups and crystal morphology in the solid with a Fourier infrared spectrometer and an X-ray diffractometer.

[0049] Result analysis: the ammonia nitrogen concentration of the filtrate is 164.50±7.9 mg / L, the ammonia nitrogen recovery rate is 93.80%; the TP concentration is 206.77±7.3 mg / L, and the phosphorus recovery rate is 98.52%. The structural characteristics of the obtained crystalline precipitate are shown in Figure 1 and Figure 2 The obtained crystalline precipitate under this reaction condition is struvite crystal.

[0050] The above description of the examples is merely exemplary in nature and is in no way intended to limit the application, specifically as the application is described herein in sufficient detail to provide those of ordinary skill in the art with a complete understanding of the application. It should be understood that various modifications can be made to the application without departing from the true spirit and scope of the application. It is not intended that the application be limited as described above. The scope of the application is defined by the claims set forth below.

Claims

1. A method for recovering nutrients from sludge by a combination of intensified hydrothermal pretreatment and crystallization precipitation, characterized in that, The method is combined treatment of municipal sludge by hydrothermal carbonization-crystallization precipitation method; The hydrothermal carbonization-crystallization precipitation method is that the dewatered sludge is treated by hydrothermal carbonization, and magnesium modification is simultaneously carried out in the hydrothermal carbonization process to obtain carbonization liquid and modified hydrothermal carbon; then magnesium source and phosphorus source are added to the obtained carbonization liquid, and the modified hydrothermal carbon is added as crystal seeds, and the crystallization precipitation is carried out under alkaline conditions to recover nutrient elements; The method specifically comprises: (1) first, the dewatered sludge is pretreated by ultrasonic wave, then the sludge with a certain water content is placed in a pressure reaction kettle, 1-5wt% of magnesium citrate is added, the hydrothermal carbonization temperature and reaction time are controlled, an oxidizing agent is added, and after the reaction is completed, the system is naturally cooled to room temperature, solid-liquid separation is carried out, the solid-phase product is modified hydrothermal carbon, and the liquid-phase product is carbonization liquid, and the carbonization liquid is further treated; (2) magnesium source and phosphorus source are added to the obtained carbonization liquid, an alkaline solution is added to control the pH value, the modified hydrothermal carbon is added as crystal seeds, the molar ratio of magnesium, phosphorus and nitrogen in the crystallization precipitation reaction is (1-2):(1-2):1, after the crystallization reaction is completed, solid-liquid separation is carried out to obtain a solid-phase product rich in nutrient elements, which is used as slow-release fertilizer, and the remaining wastewater enters an anaerobic tank for further treatment; The pH value of the system in the crystallization precipitation reaction is controlled at 8-10; the crystal seed dosage in the crystallization precipitation reaction is 0.1-1.0g / L, and the stirring time in the crystallization precipitation reaction is 0.2-0.4h, and then the system is statically placed for 5-10min.

2. The method of recovering nutrients from sludge according to claim 1, wherein, The temperature of the dewatered sludge hydrothermal carbonization is 180-260℃, the pressure is 2-10MPa, and the hydrothermal carbonization reaction time is 0.5-1.5h; before the hydrothermal carbonization, the dewatered sludge is pretreated by ultrasonic wave, the ultrasonic time is 5-10min, and the ultrasonic power is 100-300w; then the dewatered sludge is mixed with 1-5wt% of magnesium citrate, and then the dewatered sludge is treated by hydrothermal carbonization, and an oxidizing agent is added in the hydrothermal carbonization, and the oxidizing agent dosage is 0-8% g / ml.

3. The method of recovering nutrients from sludge according to claim 1, wherein, The initial ammonia nitrogen concentration of the carbonization liquid is 1000-3000mg / L, the initial phosphorus concentration is 20-100mg / L, and the initial magnesium ion concentration in the carbonization liquid is 500-1000mg / L.

4. The method of recovering nutrients from sludge according to claim 1, wherein, The alkaline solution is used to adjust the pH value of the system in the crystallization precipitation reaction, and the alkaline solution is a sodium hydroxide solution with a concentration of 2-5mol / L; The molar ratio of magnesium, phosphorus and nitrogen in the crystallization precipitation reaction is (1-2):(1-2):1.

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