A method for dehydrating sludge and recycling it to produce humic acid-containing nutrient soil and its application

Abstract

The present invention provides a method for dewatering sewage sludge and recycling it to produce nutrient soil containing humic acid, and its application, aiming to solve problems in the field of resource utilization of sewage sludge. The method adds an iron-containing flocculant and high-alkali fly ash to sewage sludge and mixes them thoroughly. After the reaction, the sludge is filtered and dehydrated to obtain dewatered sludge. Subsequently, the dewatered sludge is pyrolyzed, cooled, and a pyrolysis residue is obtained. Finally, the pyrolysis residue is applied to soil in a certain proportion to produce nutrient soil containing humic acid. The present invention achieves rapid and efficient dehydration of sewage sludge by adopting a synergistic dehydration method using an iron-containing flocculant and high-alkali fly ash, while reducing the risk of heavy metals and realizing the resource utilization of high-alkali fly coal. Through pyrolysis treatment, an accelerated decomposition process is simulated, the humification degree of the sludge is increased, and thus low-carbon treatment is achieved. The resulting nutrient soil has excellent physical and chemical properties and significantly promotes plant growth.

Description

Technical Field

[0001] The present invention relates to the technical field of sewage sludge resource treatment, and in particular to a method for dehydrating sludge and recycling it to prepare humic acid-containing nutrient soil and an application thereof. Background Art

[0002] Sewage sludge refers to solid waste generated during sewage treatment. It is projected that by 2025, the production of urban sludge (moisture content >80%) in my country will exceed 100 million tons. Driven by the "Implementation Plan for the Harmless Treatment and Resource Utilization of Sludge" (National Development and Reform Commission Environmental Protection Administration

[2022] No. 1453), improving the low-carbon, harmless, and resource-based treatment of sludge, the fifth largest carbon emitter, is of great significance. Sludge is rich in nutrients such as nitrogen, phosphorus, potassium, and organic matter. Therefore, its use as a soil resource for agriculture or urban landscaping is widely considered a promising resource recovery method. However, over 40% of sludge consists of fine sand, clay, colloids, and organic matter that cannot be directly utilized by plants, making it difficult to fully utilize. Furthermore, sludge often has unfavorable factors such as high moisture content, heavy metal content, organic pollutants, and pathogens. Therefore, necessary pretreatment steps such as dehydration, composting, and other risk reduction measures are essential before sludge can be utilized as a resource.

[0003] Sludge dewatering is a key step in resource utilization, which can reduce volume and lower subsequent processing costs and risks. At present, commonly used sludge dewatering methods include adding flocculants or coagulants. Flocculants can quickly aggregate particulate matter in sludge to form larger aggregates, thereby enhancing the dewatering effect. However, this method usually requires a longer reaction time, resulting in increased processing costs, and requires precise control of the concentration and ratio of the solution to ensure the dewatering effect, increasing operation and maintenance costs. Another common method is to use coagulants. Coagulants can react with water in the sludge to form hydrates, precipitates and porous hydrophobic channels to achieve rapid dehydration, but the effect of coagulants is usually limited.

[0004] A common land use method for the various organic matter present in sludge is to use anaerobic or aerobic composting methods to decompose it into substances that are more beneficial to soil and plants, such as humic acid. However, in addition to the methane produced by the degradation of organic matter, the composting process also releases large amounts of greenhouse gases such as carbon dioxide, ammonia, and nitrogen oxides. In addition, the composting cycle is long, usually 90 to 270 days, and the humification rate is low (usually less than 20%), which may also increase the bioavailability of heavy metals. Therefore, for the treatment process after sludge dewatering, with the goal of enhancing its land use and reducing risks, it is necessary to improve the existing process. Summary of the Invention

[0005] The present invention aims to solve the above-mentioned problems in the resource utilization process of sewage sludge and proposes a method for dehydrating sludge and producing humic acid-containing nutrient soil and its application. The method of the present invention is achieved through the following technical solutions:

[0006] A method for dehydrating sludge and recycling it to produce humic acid-containing nutrient soil and its application, comprising the following steps:

[0007] 1) adding an iron-containing flocculant and high-alkali fly ash to sewage sludge and mixing them evenly, and performing filter pressing and dehydration after the reaction to obtain dehydrated sludge;

[0008] 2) pyrolyzing the dehydrated sludge obtained in step 1) and cooling it to obtain a pyrolysis residue;

[0009] 3) applying the pyrolysis residue obtained in step 2) to the soil to obtain nutrient soil containing humic acid.

[0010] Preferably, the iron-containing flocculant in step 1) is polyferric chloride or polyferric sulfate;

[0011] The high-alkali fly ash contains 5-25% by mass of alkali metals, 15-50% by mass of silicon, 1-20% by mass of calcium, and 3-5% by mass of potassium, with a bulk density of 1-2 g / cm 3 , the particle size distribution is 0.5~30μm.

[0012] Preferably, the water content of the sewage sludge in step 1) is 75-85%.

[0013] Preferably, in step 1), the dosage of the iron-containing flocculant is 0.2-1% of the dry weight of the sludge;

[0014] The addition amount of the high-alkali fly ash is 1-5% of the dry weight of the sludge.

[0015] Preferably, the uniform mixing in step 1) is performed by using a magnetic stirrer at a stirring rate of 100 to 200 rpm; the reaction time is 0.5±0.2 h; and the pressure of the filter press dehydration is 0.5 to 1 MPa.

[0016] Preferably, the moisture content of the dewatered sludge obtained in step 1) is 50-65%.

[0017] Preferably, the pyrolysis in step 2) is carried out in an air atmosphere oven, the pyrolysis temperature is 250±50° C., and the pyrolysis time is 2±0.5 h.

[0018] Preferably, in step 3), the mass ratio of pyrolysis residue to soil is 1 to 10:100.

[0019] A humic acid-containing nutrient soil prepared by the method.

[0020] The invention relates to an application of the above-mentioned humic acid-containing nutrient soil in promoting plant growth and improving soil.

[0021] Compared with the prior art, the advantages of the present invention are:

[0022] 1. The high-alkali fly ash selected in the present invention is a type of fly ash. It is a fly ash with unique properties obtained by burning coal in Zhundong, Xinjiang, the region with the largest coal reserves in my country. It has a high alkali metal content and cannot be used in traditional fly ash resource utilization fields, such as building materials, and is prone to secondary pollution. The present invention creatively uses it as a coagulant aid for sludge dewatering, providing an effective method for resource utilization of high-alkali fly ash. The pH value of high-alkali fly ash is usually high. As a sludge coagulant aid, it can precipitate and passivate heavy metal elements in the sludge and adjust the pH of the sludge to alkaline, which will cause the surface of the sludge particles to carry more negative charges, thereby promoting the reaction of cationic flocculants. Therefore, using high-alkali fly ash as a coagulant aid for sludge dewatering is a very promising resource utilization method.

[0023] 2. The present invention combines solid powdered iron-containing flocculants and high-alkali fly ash for use in the sludge dewatering process. This technical solution can not only accurately control the dosage, reduce the amount of chemicals used and operation and maintenance costs, but also solve the problems of large amounts of iron-containing flocculants and long reaction times used in traditional methods through synergistic effects with high-alkali fly ash, thereby achieving rapid and efficient sludge dewatering. This method fully utilizes the synergistic effects of iron-containing flocculants and high-alkali fly ash in the sludge dewatering process: the iron-containing flocculant can cause the sludge to agglomerate and settle, while the high-alkali fly ash can increase the alkalinity of the sludge and control the pH within the optimal flocculation pH range of the iron-containing flocculant, thereby improving its dewaterability and achieving efficient dehydration, while reducing the amount of chemicals used and realizing the resource utilization of high-alkali fly ash.

[0024] 3. The present invention creates ideal conditions for pyrolysis catalysis during the subsequent pyrolysis step. The combination of nano-iron oxide and high-alkali fly ash catalysts in an alkaline environment allows for the efficient conversion of organic matter in the sludge into humic acid, mimicking an accelerated decomposition process. This reduces organic matter loss, increases carbon sequestration, and achieves low-carbon resource utilization for sewage sludge. Furthermore, the alkaline environment created by the high-alkali fly ash and the pyrolysis process effectively reduce the bioavailability of heavy metals in the sludge and high-alkali fly ash, and kills various microorganisms.

[0025] 4. The iron-containing flocculant and high-alkali fly ash used in the present invention also provide abundant elements such as iron, calcium, sodium and potassium required for plant growth. By mixing with the soil matrix, the acidification of the soil matrix is ​​improved, and nutrient soil rich in humic acid is obtained, which can significantly promote plant growth. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions of the prior art, the following briefly introduces the drawings required for use. It should be noted that the following drawings only describe some embodiments of the present invention, and that a person of ordinary skill in the art can obtain other relevant drawings based on these drawings without inventive effort.

[0027] Figure 1 Flowchart of the method of the present invention.

[0028] Figure 2 This is the sewage sludge after pyrolysis and catalysis in Example 1.

[0029] Figure 3 The figures are growth trends of pakchoy grown in the nutrient soil described in Examples 1 to 3 within three weeks and pakchoy grown in the nutrient soil described in Comparative Examples 1 to 3 within three weeks. DETAILED DESCRIPTION

[0030] In the present invention, there is no particular limitation on the sewage sludge used.

[0031] In the present invention, unless otherwise specified, all raw materials used are commercially available products well known to those skilled in the art.

[0032] Example 1:

[0033] The physical and chemical properties of the sewage sludge used in this example are shown in Table 1.

[0034] Table 1 Physicochemical properties of the sewage sludge used in Example 1 (all indicators except moisture content are tested under dry weight)

[0035] index Numerical Moisture content (%) 81.24 TN (g / kg) 40.52 TP (g / kg) 20.89 TK(g / kg) 19.78 TOC (g / kg) 494.82 Pb (mg / kg) 104.93 Cd (mg / kg) 4.67 As (mg / kg) 29.16 Cu (mg / kg) 172.61 Cr (mg / kg) 84.32

[0036] 1) 250 g of sewage sludge was placed in a 1 L beaker, 0.469 g of polyferric chloride (1% of the dry weight of the sludge) and 2.345 g of high-alkali fly ash (5% of the dry weight of the sludge) were added, and the mixture was mixed uniformly at a stirring rate of 200 rpm using a magnetic stirrer. After reacting for 0.7 h, the mixture was filtered under a pressure of 1 MPa to obtain dehydrated sludge.

[0037] 2) The dehydrated sludge obtained in step 1) is placed in an air atmosphere oven at 280° C. for pyrolysis for 2.5 h, and then cooled to obtain a pyrolysis residue.

[0038] The pyrolysis residue obtained in step 2) is as follows Figure 2 .

[0039] 3) applying the pyrolysis residue obtained in step 2) to the soil at a ratio of 10:100 to obtain nutrient soil containing humic acid.

[0040] The high-alkali fly ash in step 1) refers to the fine ash collected from the flue gas of the Xinjiang Zhundong coal-fired boiler through the dust removal equipment, wherein the mass percentage of alkali metals is 12.19%, the mass percentage of silicon is 38.51%, the mass percentage of calcium is 11.48%, the mass percentage of potassium is 4.8%, and the bulk density is 1.82g / cm 3 , particle size distribution is 0.5-30μm.

[0041] The basic physical and chemical properties of the soil in step 3) are shown in Table 2.

[0042] Table 2 Basic physical and chemical properties of the soil used in Example 1

[0043] index Numerical pH 4.92 TN (g / kg) 0.65 TP (g / kg) 0.23 TK(g / kg) 6.2 TOC (g / kg) 8.66

[0044] Example 2:

[0045] The sludge was dehydrated and resource-recycled to produce humic acid-containing nutrient soil using the technical scheme described in Example 1, with the only difference being that: in step 1), 0.0938 g of polyferric chloride (0.2% of the dry weight of the sludge) and 0.469 g of high-alkali fly ash (1% of the dry weight of the sludge) were added, mixed evenly with a magnetic stirrer at a stirring rate of 100 rpm, reacted for 0.3 h, and then filtered to obtain dehydrated sludge; in step 2), the pyrolysis conditions in the oven were pyrolysis at 220° C. for 1.5 h, and the dehydrated sludge was obtained after filtration under a pressure of 0.5 MPa; and in step 3), the obtained pyrolysis residue was applied to the soil at a ratio of 1:100.

[0046] Example 3:

[0047] The sludge was dehydrated and resource-recycled to produce humic acid-containing nutrient soil using the technical scheme described in Example 1, with the only difference being that: in step 1), 0.2814 g of polyferric sulfate (0.6% of the dry weight of the sludge) and 1.407 g of high-alkali fly ash (3% of the dry weight of the sludge) were added, mixed evenly with a magnetic stirrer at a stirring rate of 150 rpm, reacted for 0.5 h, and then filtered to obtain dehydrated sludge; in step 2), the pyrolysis condition in the oven was pyrolysis at 250° C. for 2 h; and in step 3), the obtained pyrolysis residue was applied to the soil at a ratio of 5:100.

[0048] Comparative Example 1:

[0049] The sludge is dehydrated and recycled to produce humic acid-containing nutrient soil using the technical solution described in Example 1, with the only difference being that polyferric chloride is not added in step 1).

[0050] Comparative Example 2:

[0051] The sludge is dehydrated and recycled to produce humic acid-containing nutrient soil using the technical solution described in Example 1, with the only difference being that high-alkali fly ash is not added in step 1).

[0052] Comparative Example 3:

[0053] The sludge is dehydrated and recycled to produce humic acid-containing nutrient soil using the technical solution described in Example 1, with the only difference being that polyferric chloride and high-alkali fly ash are not added in step 1).

[0054] Test Example 1:

[0055] The moisture content of the sewage sludge after the dehydration step in each embodiment and comparative example was measured, and the measurement steps were as follows:

[0056] 1) 10 g of the dehydrated sludge obtained in step 1) was placed in an oven at 80° C. for 12 h, and then weighed every 2 h until the weight did not decrease after three consecutive weighings.

[0057] 2) Calculate the moisture content of the dewatered sludge according to the formula: moisture content = (measured weight (g) / 10) × 100%.

[0058] The moisture content of the dewatered sludge of each embodiment and comparative example is shown in Table 3.

[0059] Table 3 Moisture content of dewatered sludge in each embodiment and each comparative example

[0060]

[0061] As can be seen from Table 3, the technical solution of the present invention can achieve rapid and efficient sludge dehydration, and can control the moisture content of the sludge to below 65%, which meets the subsequent treatment standards.

[0062] Test Example 2:

[0063] The pyrolysis residues in the examples and comparative examples were subjected to water-soluble and exchangeable heavy metal extraction experiments using the DTPA extraction method. The determination steps are as follows.

[0064] 1) Dissolve 1.967 g of triethylenetriaminepentaacetic acid (DTPA) in 14.92 g of triethanolamine (TEA) and a small amount of water. Then dissolve 1.47 g of CaCl2·2H2O in water. Transfer the mixture to a 1 L volumetric flask, add water to about 950 mL, and adjust the pH to 7.30 with 6 mol / L HCl solution. Finally, add water to the volume to obtain the DTPA extractant. Store in a plastic bottle for later use.

[0065] 2) 10 g of the pyrolysis residue from step 3) was passed through a 1 mm sieve and placed in a 50 mL centrifuge tube. 20 mL of DTPA extractant was added, and the mixture was shaken at 180 rpm for 2 h at room temperature. The mixture was then filtered. The heavy metal elements in the filtrate, blank solution, and standard solution were determined by ICP-OES.

[0066] The water-soluble and exchangeable heavy metal contents in the pyrolysis residues of the embodiments and comparative examples are shown in Table 4.

[0067] Table 4 Water-soluble and exchangeable heavy metal contents in the pyrolysis residues of each embodiment and each comparative example

[0068] heavy metal Pb (mg / kg) Cd (mg / kg) As (mg / kg) Cu (mg / kg) Cr (mg / kg) Example 1 0.56 - 0.14 7.76 - Example 2 1.27 0.07 0.57 14.94 0.51 Example 3 0.94 - 0.31 12.99 0.09 Comparative Example 1 7.91 0.22 1.23 22.72 2.64 Comparative Example 2 6.52 0.31 1.84 25.67 2.42 Comparative Example 3 8.49 0.44 1.56 36.52 2.91

[0069] *'-' indicates that the heavy metal concentration is below the detection limit.

[0070] As can be seen from Table 4, the technical solution of the present invention can reduce the bioavailability of heavy metal elements in sewage sludge and reduce the risks of subsequent treatment and resource utilization of sewage sludge.

[0071] Test Example 3:

[0072] The humic acid in the pyrolysis residues of each embodiment and comparative example was measured to determine the humification rate of organic matter in the sewage sludge. The measurement steps are as follows:

[0073] 1) 10 g of air-dried sewage sludge was placed in a ceramic crucible and calcined at 600°C in a muffle furnace for 2 h. After cooling, the calcined residue was weighed and the organic matter content of the sewage sludge was calculated according to the formula: organic matter content = (10-mass of the calcined residue) × 100%.

[0074] 2) Take 10 g of the pyrolysis residue from step 3) and add potassium hydroxide solution based on 30% of the mass of organic matter to extract humic acid. Place the liquid obtained after washing and filtering in an oven at 105°C to dry the resulting solid, which is potassium humate.

[0075] 3) Weigh the potassium humate obtained by drying. The humification rate of the sewage sludge is calculated as follows: Humification rate = ((mass of potassium humate - mass of added potassium hydroxide) / total organic matter content of the sewage sludge) × 100%.

[0076] 4) 10 g of the pyrolysis residue from step 3) was placed in a ceramic crucible and calcined in a muffle furnace at 600°C for 2 h. After cooling, the calcined residue was weighed and the organic matter content of the pyrolysis residue was calculated according to the formula: organic matter content = (10 - mass of calcined residue) × 100%. The carbon sequestration rate was calculated according to the formula: carbon sequestration rate = (organic matter content of residue / organic matter content of sewage sludge) × 100%.

[0077] The humification rate of organic matter and the carbon sequestration rate in the sewage sludge in each embodiment and comparative example are shown in Table 5.

[0078] Table 5 Humification rate of organic matter in sewage sludge in various examples and comparative examples

[0079]

[0080] As can be seen from Table 5, the technical solution of the present invention can significantly improve the humification rate and carbon sequestration rate of sewage sludge, optimize the subsequent treatment steps of sewage sludge, and achieve low carbonization.

[0081] Test Example 4:

[0082] The nutrient soil in each embodiment and comparative example was subjected to a pot culture experiment, and the steps were as follows:

[0083] 1) A potting experiment was carried out using the nutrient soil obtained in each embodiment and comparative example as a substrate. 5 kg of nutrient soil was added to each pot and the pot was watered thoroughly with tap water.

[0084] 2) After the nutrient soil in the pot is properly dry, evenly sprinkle 30 pakchoy seeds (Jingfeng No. 1) into the pot and control the watering amount to 100 mL per day.

[0085] After three weeks, the growth of plants in the potted plants using the nutrient soil obtained in each embodiment and comparative example as the matrix is ​​as follows: Figure 2 shown.

[0086] Depend on Figure 2 The growth of the Chinese cabbage shown in the figure indicates that, compared with simple soil and direct application of dried sludge, the humic acid-containing nutrient soil provided by the technical solution of the present invention can significantly promote the growth of Chinese cabbage.

[0087] After harvest, the soil in each group was sampled and analyzed, and the results are shown in Table 6.

[0088] Table 6 Physicochemical properties of the nutrient soil potted plants obtained in each embodiment and each comparative example after three weeks

[0089] Group pH TN (g / kg) TP (g / kg) TK(g / kg) TOC (g / kg) Example 1 6.94 4.29 1.91 7.1 49.14 Example 2 6.32 1.72 0.51 6.5 12.27 Example 3 6.91 2.91 1.32 6.8 28.93 Comparative Example 1 7.01 3.91 1.42 6.4 32.94 Comparative Example 2 4.94 2.98 1.27 6.0 30.46 Comparative Example 3 5.02 3.14 1.31 5.9 23.19

[0090] The physical and chemical properties of the nutrient soil after harvest shown in Table 6 indicate that the nutrient soil obtained by the technical solution of the present invention can significantly alleviate the acidification of the matrix soil, while improving soil fertility, improving soil structure, and reducing nutrient loss.

[0091] Although the above embodiment provides a detailed description of the present invention, it is only a part of the embodiments of the present invention, not all of the embodiments. People can also obtain other embodiments based on this embodiment without creativity, and these embodiments all fall within the scope of protection of the present invention.

Claims

1. A method for dehydrating sludge and recycling it to produce humic acid-containing nutrient soil, characterized in that: The following steps are involved: 1) Adding the iron-containing flocculant and high-alkali fly ash to the sewage sludge and mixing them evenly, and then performing filter pressing and dehydration after the reaction to obtain dehydrated sludge; 2) pyrolyzing the dewatered sludge obtained in step 1) and cooling it to obtain a pyrolysis residue; 3) applying the pyrolysis residue obtained in step 2) to the soil to obtain nutrient soil containing humic acid; Step 1) The high alkaline fly ash contains 12.19% by mass of alkali metals, 38.51% by mass of silicon, 11.48% by mass of calcium, and 4.8% by mass of potassium, with a bulk density of 1.82 g / cm 3 , particle size distribution is 0.5~30μm; The iron-containing flocculant in step 1) is polyferric chloride or polyferric sulfate; In step 1), the dosage of the iron-containing flocculant is 0.2-1% of the dry weight of the sludge; the dosage of the high-alkali fly ash is 1-5% of the dry weight of the sludge; The pyrolysis in step 2) is carried out in an air atmosphere oven at a temperature of 250±50°C and a pyrolysis time of 2±0.5h; The method can significantly improve the humification rate and carbon sequestration rate of sewage sludge.

2. The method according to claim 1, characterized in that The water content of the sewage sludge in step 1) is 75-85%.

3. The method according to claim 1, characterized in that The uniform mixing in step 1) is performed by using a magnetic stirrer at a stirring rate of 100-200 rpm; the reaction time is 0.5±0.2 h; and the pressure of the filter press dehydration is 0.5-1 MPa.

4. The method according to claim 1, wherein The moisture content of the dewatered sludge obtained in step 1) is 50-65%.

5. The method according to claim 1, wherein In step 3), the mass ratio of pyrolysis residue to soil is 1-10:

100.

6. A humic acid-containing nutrient soil obtained by the method according to any one of claims 1 to 5.

7. Use of the humic acid-containing nutrient soil according to claim 6 in promoting plant growth and improving soil.

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