A method and system for two-stage thermal hydrolysis of municipal sludge

Abstract

The invention relates to the technical field related to sludge treatment and disposal, and provides a method and system for two-stage thermal hydrolysis of municipal sludge, which can solve the technical problems of incomplete hydrolysis of municipal sludge and poor solid-liquid separation effect, improve the fertilizer efficiency of subsequent liquid fertilizer and reduce the moisture content of dry residue, and realize the resource utilization of sludge. The method is characterized by: step 1, slurrying sludge; step 2, outputting the sludge to a first hydrolysis device, adding a dilute nitric acid solution containing calcium nitrate in the first hydrolysis device, adjusting the pH value of the sludge to acidic, and performing thermal acid hydrolysis at high temperature; step 3, outputting the sludge to a second hydrolysis device, adding industrial-grade diatomaceous earth in the second hydrolysis device, adjusting the pH value of the sludge to weak alkalinity by using quicklime, and performing low-temperature thermal hydrolysis; step 4, outputting the sludge to a solid-liquid separation device, collecting filtrate through the solid-liquid separation device, recovering nutrients in the sludge, and using the dry residue as landfill covering soil to realize the resource utilization of the sludge.

Description

Technical Field

[0001] The present invention relates to the technical field related to sludge treatment and disposal, and in particular to a method and system for two-stage thermal hydrolysis of municipal sludge. Background Art

[0002] With the rapid development of the number and scale of cities in the world, the discharge of municipal sewage is also increasing sharply. A steady stream of municipal sludge is produced in the process of sewage purification. How to scientifically and effectively treat such a large amount of sludge has become a new problem in urban development. Municipal sludge not only has extremely complex composition and extremely high water content, but also contains a large amount of nutrients and pathogenic bacteria and microorganisms, heavy metals and other toxic and harmful substances, and most sludge is often accompanied by a foul odor, affecting air quality. If the sludge is not scientifically treated and randomly piled on the land, the pollutants in the sludge will penetrate into the groundwater and soil under the scouring of rainwater, directly affecting the country's water and soil security and our health. Therefore, the reduction and resource utilization of sludge has become an imminent problem facing my country and even the world.

[0003] Thermal hydrolysis of sludge is currently a commonly used technology for sludge treatment. During the thermal hydrolysis process, sludge flocs are decomposed, cells are broken, and organic matter is released, which can achieve the harmlessness of sludge, reduce sludge volume, improve dewatering performance, and reduce the content of volatile organic matter in sludge, thereby achieving sludge stabilization. However, at present, thermal hydrolysis technology faces the problems of incomplete hydrolysis, poor solid-liquid separation, and dispersed heat utilization in the promotion of industrial scale, resulting in low fertilizer efficiency of subsequent liquid fertilizers and high moisture content of dry residues, which affects subsequent resource utilization. Summary of the invention

[0004] The invention provides a method and system for two-stage thermal hydrolysis of municipal sludge, which can solve the technical problems of incomplete hydrolysis of municipal sludge and poor solid-liquid separation effect, improve the fertilizer efficiency of subsequent liquid fertilizer and reduce the moisture content of dry residue, so as to realize resource utilization of sludge.

[0005] The technical solution is as follows:

[0006] A method for two-stage thermal hydrolysis of municipal sludge, characterized in that:

[0007] Step 1, slurrying sludge;

[0008] Step 2, outputting the sludge to a first hydrolysis device, adding a dilute nitric acid solution containing calcium nitrate to the first hydrolysis device, adjusting the pH of the sludge to acidic, and performing thermal acid hydrolysis at high temperature;

[0009] Step 3: outputting the sludge to a second hydrolysis device, adding industrial-grade diatomaceous earth to the second hydrolysis device, adjusting the pH of the sludge to weak alkalinity with quicklime, and performing low-temperature thermal hydrolysis;

[0010] Step 4: Output the sludge to a solid-liquid separation device, collect the filtrate through the solid-liquid separation device, recover the nutrients in the sludge, and use the dry residue as landfill covering soil to realize resource utilization of the sludge.

[0011] Furthermore, the step 1 is specifically as follows: in a slurrying tank, water is added to dilute the municipal dewatered sludge having a moisture content of 80% to 85% to 88% to 90% and the sludge is homogenized and stirred.

[0012] Furthermore, the step 2 is specifically as follows: the sludge slurry homogenized in step 1 is pumped into the first hydrolysis device through a plunger pump, and a dilute nitric acid solution containing calcium nitrate is added to the first hydrolysis device to adjust the pH of the sludge, the thermal hydrolysis temperature is maintained at 120-220°C, and the thermal hydrolysis reaction time is maintained at 30-120min.

[0013] Furthermore, the step 3 is specifically as follows: the sludge hydrolyzed in step 2 is transported to the second hydrolysis device through the positive pressure in the first hydrolysis device, industrial grade diatomaceous earth is added to the second hydrolysis device, and the pH value of the sludge is adjusted with quicklime, the thermal hydrolysis temperature is maintained at 80-100°C, and the thermal hydrolysis reaction time is 30-60min.

[0014] Furthermore, in step 2, the mass fraction of dilute nitric acid in the dilute nitric acid solution is 30%, and the pH value of the sludge is adjusted to 3-5; and the amount of calcium nitrate added is 10%-25% of the dry weight of the sludge.

[0015] Furthermore, in step 3, the amount of industrial-grade diatomaceous earth added is 10% to 25% of the dry weight of the sludge, and quicklime is used to adjust the sludge pH to 7 to 8.

[0016] The present invention also includes a system for two-stage thermal hydrolysis of municipal sludge based on the above method, characterized in that it includes a slurrying tank, a first hydrolysis device, a second hydrolysis device, a solid-liquid separation device, a liquid fertilizer collecting device and a dry residue collecting device which are connected in sequence through pipelines, the first reagent tank is connected to the first hydrolysis device through a pipeline, the second reagent tank is connected to the second hydrolysis device through a pipeline, and a negative pressure fan and a condenser are connected in sequence on the branch pipeline connecting the air outlet port of the second hydrolysis device and the slurrying tank for recovering heat and reusing grey water.

[0017] Furthermore, the slurrying tank is used to dilute the municipal dewatered sludge with a moisture content of 80% to 85% to 88% to 90% by adding water and to perform homogenous stirring.

[0018] Furthermore, the first hydrolysis device is used to add a dilute nitric acid solution containing calcium nitrate, adjust the pH of the sludge to acidic, and then perform high-temperature thermal acid hydrolysis on the sludge.

[0019] Furthermore, the first hydrolysis device is provided with a heating jacket, wherein the heating medium is saturated steam or heat transfer oil.

[0020] Furthermore, the second hydrolysis device is used to add industrial-grade diatomaceous earth, use quicklime to adjust the pH of the sludge to weak alkalinity, and then perform low-temperature thermal hydrolysis on the sludge.

[0021] Furthermore, the second hydrolysis device is a normal pressure container, and a cooling jacket is provided on the outer wall to ensure that the hydrolysis reaction is carried out at a suitable temperature.

[0022] Furthermore, the solid-liquid separation device is a plate and frame filter press device, a centrifugal dehydration device, or a belt filter press device.

[0023] Furthermore, the slurrying tank, the first hydrolysis device and the second hydrolysis device are all provided with a variable frequency stirring device, and the stirring speed is set to 50-75 rpm.

[0024] The beneficial effects of the method of the present invention are:

[0025] The present invention arranges a two-stage hydrolysis, firstly performs high-temperature hot hydrolysis in the first hydrolysis device, utilizes the strong oxidizing property of calcium nitrate under acidic conditions, can effectively destroy the sludge flocs under hot acidic conditions, makes the extracellular polymer fall off and dissolve in the water phase, because the sludge surface is negatively charged, the divalent calcium ions can play the role of bridging flocculation and electrical neutralization on the sludge cells, and at the same time, the calcium ions have the alkali metal chelating property, react with the protein to form a complex, and improve the dehydration performance of the sludge through hydrophobic action and hydrogen bonding, and calcium nitrate as a common agricultural fertilizer can increase the fertilizer efficiency of liquid fertilizer, and the calcium-containing dry residue can be used for the improvement of alkaline soil; then in the second hydrolysis device, the sludge flocs are effectively destroyed, and the extracellular polymers are detached and dissolved in the water phase, and the sludge surface is negatively charged, and the divalent calcium ions can play the role of bridging flocculation and electrical neutralization on the sludge cells, and at the same time, the calcium ions have the alkali metal chelating property, and react with the protein to form a complex, and improve the dehydration performance of the sludge through hydrophobic action and hydrogen bonding, and the calcium nitrate as a common agricultural fertilizer can increase the fertilizer efficiency of liquid fertilizer, and the calcium-containing dry residue can be used for the improvement of alkaline soil; then in the second hydrolysis device, the sludge flocs are effectively destroyed, and the extracellular polymers are effectively destroyed, and the extracellular polymers are effectively dissolved in the water phase, and the calcium ions ... Low-temperature thermal hydrolysis is carried out in the device, and industrial-grade diatomaceous earth contains a small amount of Al2O3, Fe2O3, CaO, MgO and other substances, which play a role of electrical neutralization in the process of neutralizing nitric acid. At the same time, the specific surface area of ​​diatomaceous earth is improved in a dilute nitric acid environment, and the diatom pores are modified. The loose and porous structure plays a role of skeleton support in the sludge filter pressing process, while increasing the permeability of water in the mud cake, and further improving the dewatering performance of the sludge; thereby solving the technical problems of incomplete hydrolysis of municipal sludge and poor solid-liquid separation effect, achieving the beneficial effects of improving the fertilizer efficiency of liquid fertilizer and reducing the moisture content of dry residue, and realizing the resource utilization of sludge. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the overall structure of the system of the present invention. DETAILED DESCRIPTION

[0027] In order to better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0028] Example 1

[0029] A system for two-stage thermal hydrolysis of municipal sludge comprises a pulping tank, a first hydrolysis device, a second hydrolysis device, a solid-liquid separation device, a liquid fertilizer collection device and a dry residue collection device which are connected in sequence through pipelines; the first reagent tank is connected to the first hydrolysis device through a pipeline, and the second reagent tank is connected to the second hydrolysis device through a pipeline; the outlet port of the second hydrolysis device is connected to a branch pipeline of the pulping tank, on which a negative pressure fan and a condenser are connected in sequence for recovering heat and reusing reclaimed water.

[0030] The slurrying tank is used to dilute the municipal dewatered sludge with a moisture content of 80% to 85% to 88% to 90% by adding water and to perform homogenous stirring.

[0031] The first hydrolysis device is provided with a heating jacket, wherein the heating medium is saturated steam or heat transfer oil. The first hydrolysis device is used to add a dilute nitric acid solution containing calcium nitrate, adjust the pH of the sludge to acidic, and then perform high-temperature thermal acid hydrolysis on the sludge.

[0032] The second hydrolysis device is a normal pressure container, and the outer wall is provided with a cooling jacket to ensure that the hydrolysis reaction is carried out at a suitable temperature. The second hydrolysis device is used to add industrial grade diatomaceous earth, use quicklime to adjust the pH of the sludge to weak alkalinity, and then perform low temperature hot hydrolysis on the sludge.

[0033] The solid-liquid separation device is a plate-and-frame filter press device, a centrifugal dehydration device, or a belt filter press device.

[0034] The slurrying tank, the first hydrolysis device and the second hydrolysis device are all provided with a variable frequency stirring device, and the stirring speed is set to 50-75 rpm.

[0035] Example 2

[0036] A method for two-stage thermal hydrolysis of municipal sludge comprises the following steps:

[0037] (1) Sludge slurrying: In a slurrying tank, the municipal dewatered sludge with a moisture content of 80% to 85% is diluted to 90% by adding water and then homogenized and stirred.

[0038] (2) The first thermal hydrolysis of sludge: the homogenized sludge slurry is pumped into the first hydrolysis device through a plunger pump. The first reagent tank is used to store dilute nitric acid and calcium nitrate, respectively. Calcium nitrate and dilute nitric acid solution are added to the first hydrolysis device through pipelines, wherein the mass fraction of dilute nitric acid is 30%. The pH value of the sludge is adjusted to 3, and the amount of calcium nitrate added is 25% of the dry weight of the sludge. The thermal hydrolysis temperature is maintained at 120°C, and the thermal hydrolysis reaction time is maintained at 120 min.

[0039] (3) Second thermal hydrolysis of sludge. The sludge after the first thermal hydrolysis is transported to the second hydrolysis device by its own positive pressure. Industrial grade diatomaceous earth is added to the second hydrolysis device. The amount of diatomaceous earth added accounts for 10% of the dry weight of the sludge. The pH is adjusted to about 7 with quicklime. The thermal hydrolysis temperature is maintained at 90°C and the thermal hydrolysis reaction time is 60 min.

[0040] (4) The sludge after the second thermal hydrolysis is pumped into a diaphragm plate and frame filter press through a plunger pump for solid-liquid separation. The feed pressure is 1.0 MPa, the feed time is 40 min, the pressing pressure is 1.0 MPa, and the pressing time is 60 min. After the filtrate passes through the filtering device, the water insoluble matter in the filtrate is removed. The obtained filtrate passes through the MVR evaporation concentration device and is concentrated 20 times at an evaporation temperature of 95°C. The obtained concentrated liquid is used as liquid fertilizer and the filter cake is used as landfill covering soil.

[0041] The main implementation principle of the system and method is as follows: first, high-temperature thermal hydrolysis is carried out in the first hydrolysis device. Calcium nitrate has strong oxidizing properties under acidic conditions and can effectively destroy sludge flocs under hot acid conditions, causing extracellular polymers to fall off and dissolve in the water phase. Since the sludge surface is negatively charged, divalent calcium ions can bridge flocculation and charge neutralization for sludge cells. At the same time, calcium ions have alkali metal chelating properties and react with proteins to form a complex reaction. Through hydrophobic action and hydrogen bonding, the dehydration performance of sludge is improved. In addition, calcium nitrate, as a common agricultural fertilizer, can increase The fertilizer effect of liquid fertilizer is added, and the calcium-containing dry residue can be used for the improvement of alkaline soil; then low-temperature hot hydrolysis is carried out in the second hydrolysis device, and industrial-grade diatomaceous earth contains a small amount of Al2O3, Fe2O3, CaO, MgO and other substances, which play a role of electrical neutralization in the process of neutralizing nitric acid. At the same time, the specific surface area of ​​diatomaceous earth is improved in a dilute nitric acid environment, and the diatom pores are modified. The loose and porous structure plays a role of skeleton support in the sludge filtration process, while increasing the permeability of water in the mud cake, further improving the dewatering performance of the sludge.

[0042] The experimental data are as follows: the sludge hydrolyzate in the second hydrolysis device and the dry residue after filtration were detected, wherein the capillary water absorption time (CST) was 20.4s, the moisture content of the filter cake was stabilized below 40%, and the dehydration performance was greatly improved, showing that it was very easy to dehydrate; the composition of the obtained liquid fertilizer was detected, and the free amino acid was ≥100g / L, wherein the free amino acid was mainly alanine, glutamic acid and glycine; the organic matter was ≥200g / L, and the humic acid was ≥50g / L. Applying it to the soil can increase the soil organic matter, improve the quality of cultivated land, and improve the root environment of crops; humic acid promotes the growth and development of crops, and stimulates the activity of crop roots, which is conducive to promoting the absorption and utilization of nutrients by crops. Therefore, the present invention solves the technical problems of incomplete hydrolysis of municipal sludge and poor solid-liquid separation effect, and achieves the beneficial effects of improving the fertilizer efficiency of liquid fertilizer and reducing the moisture content of dry residue, and realizing the resource utilization of sludge.

[0043] The above is a further detailed description of the present invention in combination with specific implementation contents. It is only a preferred embodiment, and it cannot be determined that the specific implementation of the present invention is limited to the above description. For ordinary technicians in this field, simple parameter replacements can be made without departing from the basic concept of the present invention, which can be regarded as within the scope of protection of the present invention.

Claims

1. A method for two-stage thermal hydrolysis of municipal sludge, characterized in that: Step 1, slurrying sludge; Step 2, outputting the sludge to a first hydrolysis device, adding a dilute nitric acid solution containing calcium nitrate to the first hydrolysis device, adjusting the pH of the sludge to acidic, and performing thermal acid hydrolysis at high temperature; Step 3, outputting the sludge to a second hydrolysis device, adding industrial-grade diatomaceous earth to the second hydrolysis device, adjusting the pH of the sludge to weak alkalinity with quicklime, and performing low-temperature thermal hydrolysis; Step 4: Output the sludge to the solid-liquid separation device, collect the filtrate through the solid-liquid separation device, and use the dry residue as landfill covering soil.

2. A method for two-stage thermal hydrolysis of municipal sludge according to claim 1, characterized in that: The step 1 specifically comprises: adding water to dilute the municipal dewatered sludge having a moisture content of 80% to 85% to 88% to 90% in a slurrying tank and homogenizing and stirring the sludge.

3. A method for two-stage thermal hydrolysis of municipal sludge according to claim 1, characterized in that: The step 2 specifically comprises: pumping the homogenized sludge slurry in step 1 into the first hydrolysis device through a plunger pump, adding a dilute nitric acid solution containing calcium nitrate into the first hydrolysis device to adjust the pH value of the sludge, maintaining the thermal hydrolysis temperature at 120-220° C., and maintaining the thermal hydrolysis reaction time at 30-120 min; the mass fraction of dilute nitric acid in the dilute nitric acid solution is 30%, and the pH value of the sludge is adjusted to 3-5; and the amount of calcium nitrate added is 10%-25% of the dry weight of the sludge.

4. A method for two-stage thermal hydrolysis of municipal sludge according to claim 1, characterized in that: The step 3 is specifically as follows: the sludge hydrolyzed in step 2 is transported to the second hydrolysis device through the positive pressure of the first hydrolysis device, industrial-grade diatomaceous earth is added to the second hydrolysis device, and the pH value of the sludge is adjusted with quicklime, the thermal hydrolysis temperature is maintained at 80-100° C., and the thermal hydrolysis reaction time is 30-60 min; the amount of the industrial-grade diatomaceous earth added accounts for 10%-25% of the dry weight of the sludge, and the quicklime is used to adjust the sludge pH value to 7-8.

5. A system based on the method for two-stage thermal hydrolysis of municipal sludge according to any one of claims 1 to 4, characterized in that: The system includes a pulping tank, a first hydrolysis device, a second hydrolysis device, a solid-liquid separation device, a liquid fertilizer collecting device and a dry residue collecting device which are sequentially connected through pipelines; the first reagent tank is connected to the first hydrolysis device through a pipeline, the second reagent tank is connected to the second hydrolysis device through a pipeline, and a negative pressure fan and a condenser are sequentially connected on a branch pipeline connecting an outlet port of the second hydrolysis device and the pulping tank.

6. A system for two-stage thermal hydrolysis of municipal sludge according to claim 5, characterized in that: The slurrying tank is used to dilute the municipal dewatered sludge with a moisture content of 80% to 85% to 88% to 90% by adding water and to perform homogenous stirring.

7. A system for two-stage thermal hydrolysis of municipal sludge according to claim 5, characterized in that: The first hydrolysis device is provided with a heating jacket, wherein the heating medium is saturated steam or heat transfer oil; the first hydrolysis device is used to add a dilute nitric acid solution containing calcium nitrate, adjust the pH of the sludge to acidic, and then perform high-temperature thermal acid hydrolysis on the sludge.

8. A system for two-stage thermal hydrolysis of municipal sludge according to claim 5, characterized in that: The second hydrolysis device is a normal pressure container with a cooling jacket on its outer wall; the second hydrolysis device is used to add industrial grade diatomaceous earth, adjust the pH value of the sludge to weak alkalinity with quicklime, and then perform low-temperature thermal hydrolysis on the sludge.

9. A system for two-stage thermal hydrolysis of municipal sludge according to claim 5, characterized in that: The solid-liquid separation device is a plate-and-frame filter press device, a centrifugal dehydration device, or a belt filter press device.

10. A system for two-stage thermal hydrolysis of municipal sludge according to claim 5, characterized in that: The slurrying tank, the first hydrolysis device and the second hydrolysis device are all provided with a variable frequency stirring device, and the stirring speed is set to 50-75 rpm.

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