Sludge dewatering performance improving method based on enhanced crystallization

Abstract

The invention relates to a sludge dewatering performance improving method based on enhanced crystallization. The method for improving the dewatering performance of the sludge comprises the following steps: adding a quaternary ammonium salt flocculant into the sludge, and fully stirring and mixing to obtain a mixture; centrifugally concentrating and dehydrating the obtained mixture; transferring the concentrated and dehydrated sludge into a pressure container, and intermittently introducing high-pressure carbon dioxide in a low-temperature environment to generate a cage-shaped hydrate; and after the partial pressure of the carbon dioxide hydrate in the pressure container is stable, releasing the pressure until no gas escapes, thereby obtaining the sludge with improved dehydration performance. Compared with the prior art, the method is simple and easy to implement, flocculation and concentration treatment of the sludge can be achieved, the quaternary ammonium salt flocculating agent left in the concentrated and dehydrated sludge can effectively improve the phase equilibrium thermodynamic condition for hydrate generation, the synthesis rate of the hydrate is increased, the synthesis partial pressure of the hydrate is reduced, and the method is nontoxic and harmless to the environment.

Description

Technical Field

[0001] The present invention belongs to the technical field of sludge treatment, and in particular relates to a method for improving sludge dewatering performance based on enhanced crystallization. Background Art

[0002] Municipal sewage treatment plant sludge (hereinafter referred to as sludge) refers to the sediment formed from pollutant conversion during the sewage treatment process, as well as the microbial residues produced by the biodegradation of pollutants. my country's annual sludge production has reached 80 million tons (80% moisture content), and sludge carries 30-40% of the total pollutants in sewage treatment plant influent, including various pathogens, heavy metals, and toxic organic pollutants. The safe treatment and disposal of sludge is crucial for reducing the risk of secondary pollution from sewage treatment and improving my country's water pollution control technology.

[0003] High moisture content is one of the main factors limiting the efficiency of sludge treatment and disposal. A series of standards and specifications for sludge transportation, pyrolysis, incineration, and land use in urban sewage treatment plants all have specific technical requirements for sludge moisture content. Dehydration is a common key technical link in all sludge treatment and disposal routes. In particular, thermal treatment (incineration, pyrolysis) has become a rapidly developing direction for the final disposal of sludge in my country in recent years due to its significant benefits in volume reduction, stabilization, and energy utilization. Therefore, reducing sludge moisture content and increasing sludge calorific value in an efficient and low-cost manner are also important technical prerequisites for low-carbon, centralized, and large-scale sludge treatment and disposal in my country.

[0004] However, sludge is a highly mixed, heterogeneous, and complex system of organic and inorganic matter, exhibiting a stable colloidal flocculent state and making solid-liquid separation extremely difficult. Currently, polyacrylamide (PAM) is the most widely used flocculant in sludge dewatering and conditioning, effectively improving sludge dewatering performance. However, it has disadvantages such as high cost, poor solubility, and poor effluent quality. Furthermore, the addition of PAM inevitably introduces biotoxic acrylamide monomers, posing a long-term environmental pollution risk when sludge is used on land.

[0005] Patent CN115340285A discloses a method and system for improving sludge solid-liquid separation performance through in-situ water crystallization. The method comprises: adding sludge to a pressure-resistant container, intermittently introducing high-pressure carbon dioxide at low temperatures to allow carbon dioxide hydrate to form until the carbon dioxide partial pressure stabilizes, then releasing the pressure and stirring the sludge until no gas escapes, thereby obtaining treated sludge. However, this method suffers from a relatively slow rate of carbon dioxide hydrate formation, and achieving specific temperature and pressure conditions consumes significant energy, necessitating further improvements in the phase equilibrium thermodynamic conditions for hydrate formation. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for improving sludge dewatering performance based on enhanced crystallization in order to improve the phase equilibrium thermodynamic conditions for hydrate formation during sludge dewatering.

[0007] The purpose of the present invention can be achieved by the following technical solutions:

[0008] The present invention provides a method for improving sludge dewatering performance based on enhanced crystallization, the method comprising the following steps:

[0009] S1: adding a quaternary ammonium salt flocculant to the sludge and stirring thoroughly to obtain a mixture;

[0010] S2: centrifugally concentrate and dehydrate the mixture obtained in S1;

[0011] S3: The concentrated and dehydrated sludge from S2 is transferred to a pressure-resistant container, and high-pressure carbon dioxide is intermittently introduced under low-temperature conditions to generate clathrate hydrates;

[0012] S4: After the partial pressure of the carbon dioxide hydrate in the pressure-resistant container is stabilized, the pressure is released until no gas escapes, thereby obtaining sludge with improved dehydration performance.

[0013] Furthermore, in step S1, the quaternary ammonium salt flocculant is selected from one or more of octadecyltrimethylammonium bromide, dioctadecyldimethylammonium chloride or behenyldimethylbenzylammonium chloride.

[0014] The quaternary ammonium salt flocculant used in the present invention is a cationic organic flocculant with a strong positive charge density and good adsorption bridging ability. The quaternary ammonium salt flocculant neutralizes the negative charge on the surface of the sludge particles through electrical neutralization, and promotes particle aggregation through adsorption bridging. It is effective for sludge with a high organic content and a large amount of colloidal substances, and can form larger and dense flocculated particles. The average particle size of the sludge flocculated particles can reach 200-500 μm. In addition, the quaternary ammonium salt flocculant partially remaining in the sludge can form hydrogen bonds through electrostatic interaction with the oxygen atoms in the cations in the quaternary ammonium salt molecules and the water molecules, so as to change the arrangement of the water molecules and make it easier to form a hydrate with a cage structure with carbon dioxide. The presence of the quaternary ammonium salt can also reduce the phase equilibrium pressure and temperature of carbon dioxide hydrate generation to expand the feasible region of hydrate generation.

[0015] Furthermore, in step S1, the dosage of the quaternary ammonium salt flocculant is 0.5-1 wt.% of the dry mass of the sludge.

[0016] Furthermore, in step S1, the stirring and mixing speed is 200-600 rpm, preferably 400 rpm.

[0017] Furthermore, in step S1, the stirring and mixing time is 2-15 minutes.

[0018] Furthermore, in step S2, the g value of the centrifugal concentration and dehydration is 2000-3000g.

[0019] Furthermore, in step S2, the time for centrifugal concentration and dehydration is 3-5 minutes.

[0020] Furthermore, in step S3, the temperature of the low-temperature environment is 0-10°C.

[0021] Furthermore, in step S3, the pressure of the carbon dioxide introduced is 1500-5000 kPa.

[0022] Furthermore, in step S4, the phase equilibrium partial pressure of the carbon dioxide hydrate is 1414-4292 kPa.

[0023] Furthermore, in step S4, the pressure-resistant container is depressurized until the gas pressure inside the pressure-resistant container is consistent with the atmospheric pressure.

[0024] Furthermore, in step S4, the initial moisture content of the sludge is not less than 98 wt.%, and the moisture content after dehydration is not higher than 93.5 wt.%.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (1) The present invention adopts a quaternary ammonium salt flocculant as a sludge dewatering conditioner to perform preliminary conditioning on the sludge, thereby achieving flocculation and concentration treatment of the sludge. After concentration and dehydration, the sludge has a lower moisture content and better dewatering performance.

[0027] (2) Compared with traditional PAM, the quaternary ammonium salt flocculant used in the present invention has a larger particle size of sludge flocculation particles, which has an excellent improvement on the conditions for hydrate formation. It also has the physical and chemical properties of being non-volatile and non-toxic to the environment. It has significant advantages in economic benefits and social and environmental benefits, and shows broad market application potential.

[0028] (3) The quaternary ammonium salt flocculant remaining in the sludge after preliminary conditioning of the present invention can act as a thermodynamic promoter to effectively improve the phase equilibrium thermodynamic conditions for the formation of carbon dioxide hydrates, and increase the driving force for the formation of hydrates by interacting with water molecules to form clathrate hydrates, thereby allowing carbon dioxide hydrates to be rapidly generated within a relatively mild temperature and pressure range. DETAILED DESCRIPTION

[0029] The present invention is described in detail below with reference to specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0030] Unless otherwise specified, the reagents, methods, instruments and equipment used in the present invention are conventional reagents, methods, instruments and equipment in the art. Unless otherwise specified, the reagents and materials used in the following examples are commercially available.

[0031] The present invention provides a method for improving sludge dewatering performance based on enhanced crystallization, the method comprising the following steps:

[0032] S1: adding a quaternary ammonium salt flocculant to the sludge and stirring thoroughly to obtain a mixture;

[0033] S2: centrifugally concentrate and dehydrate the mixture obtained in S1;

[0034] S3: The concentrated and dehydrated sludge from S2 is transferred to a pressure-resistant container, and high-pressure carbon dioxide is intermittently introduced under low-temperature conditions to generate clathrate hydrates;

[0035] S4: After the partial pressure of the carbon dioxide hydrate in the pressure-resistant container is stabilized, the pressure is released until no gas escapes, thereby obtaining sludge with improved dehydration performance.

[0036] In some specific embodiments, in step S1, the quaternary ammonium salt flocculant is selected from one or more of octadecyltrimethylammonium bromide, dioctadecyldimethylammonium chloride, or behenyldimethylbenzylammonium chloride.

[0037] In some specific embodiments, in step S1, the dosage of the quaternary ammonium salt flocculant is 0.5-1 wt.% of the dry mass of the sludge.

[0038] In some specific embodiments, in step S1, the stirring and mixing speed is 200-600 rpm, preferably 400 rpm.

[0039] In some specific embodiments, in step S1, the stirring and mixing time is 2-15 minutes.

[0040] In some specific embodiments, in step S2, the g value of the centrifugal concentration and dehydration is 2000-3000g.

[0041] In some specific embodiments, in step S2, the time for centrifugal concentration and dehydration is 3-5 minutes.

[0042] In some specific embodiments, in step S3, the temperature of the low-temperature environment is 0-10°C.

[0043] In some specific embodiments, in step S3, the pressure of the carbon dioxide introduced is 1500-5000 kPa.

[0044] In some specific embodiments, in step S4, the phase equilibrium partial pressure of the carbon dioxide hydrate is 1414-4292 kPa.

[0045] In some specific embodiments, in step S4, the pressure-resistant container is depressurized until the gas pressure in the pressure-resistant container is consistent with the atmospheric pressure.

[0046] In some specific embodiments, in step S4, the initial moisture content of the sludge is not less than 98 wt.%, and the moisture content after dehydration is not higher than 93.5 wt.%.

[0047] The above embodiments can be implemented individually or in any combination of two or more. The above embodiments will be described in more detail below with reference to specific examples.

[0048] The following examples and comparative examples were all carried out at room temperature. The sludge used was taken from a municipal sewage treatment plant in Shanghai. The added quaternary ammonium surfactant was first dissolved in water to form a viscous quaternary ammonium surfactant stock solution with a concentration of 0.05 g / mL.

[0049] Example 1:

[0050] In this embodiment, a quaternary ammonium surfactant (octadecyl trimethyl ammonium bromide) is used as a quaternary ammonium flocculant. The specific conditions and steps are as follows:

[0051] (1) The initial moisture content of the sludge was 99 wt.%, and the dosage of octadecyltrimethylammonium bromide was 0.5 wt.% of the dry mass of the sludge. The mixture was stirred at a speed of 200 rpm for 15 min.

[0052] (2) Under the centrifugal force of 2000g, the sludge moisture content decreased from 99wt.% to 92.6wt.% after centrifugation for 5min;

[0053] (3) The concentrated and dehydrated sludge is transferred to a pressure-resistant container, and all the water in the sludge is converted into clathrate hydrates under the conditions of 5°C and an initial carbon dioxide pressure of 5000 kPa;

[0054] (4) When the carbon dioxide partial pressure in the pressure vessel is stable, the equilibrium partial pressure of the hydrate can be reduced to 1980 kPa, and the pressure vessel is operated to release the pressure until no gas escapes, thereby obtaining sludge with improved dehydration performance.

[0055] Example 2:

[0056] In this embodiment, a quaternary ammonium salt surfactant (dioctadecyl dimethyl ammonium chloride) is used as a quaternary ammonium salt flocculant. The specific conditions and steps are as follows:

[0057] (1) The initial moisture content of the sludge was 99 wt.%, and the dosage of dioctadecyl dimethyl ammonium chloride was 1 wt.% of the dry mass of the sludge. The mixture was stirred at a speed of 600 rpm for 2 min.

[0058] (2) Under the centrifugal force of 3000g, the sludge moisture content decreased from 99wt.% to 90.2wt.% after centrifugation for 5min;

[0059] (3) The concentrated and dehydrated sludge is transferred to a pressure-resistant container, and all the water in the sludge is converted into clathrate hydrates at 9°C and an initial carbon dioxide pressure of 5000 kPa;

[0060] (4) When the carbon dioxide partial pressure in the pressure vessel is stable, the equilibrium partial pressure of the hydrate can be reduced to 3570 kPa, and the pressure vessel is operated to release the pressure until no gas escapes, thereby obtaining sludge with improved dehydration performance.

[0061] Example 3:

[0062] In this embodiment, a quaternary ammonium surfactant (behenyl dimethyl benzyl ammonium chloride) is used as a quaternary ammonium flocculant. The specific conditions and steps are as follows:

[0063] (1) The initial moisture content of the sludge was 99 wt.%, and the dosage of behenyl dimethyl benzyl ammonium chloride was 0.75 wt.% of the dry mass of the sludge. The mixture was stirred at 400 rpm for 3 min.

[0064] (2) Centrifugation for 3 min at a centrifugal force of 2500 g reduced the sludge moisture content from 99 wt.% to 91.6 wt.%;

[0065] (3) The concentrated and dehydrated sludge is transferred to a pressure-resistant container, and all the water in the sludge is converted into clathrate hydrates under the conditions of 1°C and an initial carbon dioxide pressure of 1500 kPa;

[0066] (4) When the carbon dioxide partial pressure in the pressure vessel is stable, the equilibrium partial pressure of the hydrate can be reduced to 1342 kPa, and the pressure vessel is operated to release the pressure until no gas escapes, thereby obtaining sludge with improved dehydration performance.

[0067] Example 4:

[0068] In this embodiment, a quaternary ammonium surfactant (octadecyl trimethyl ammonium bromide) is used as a quaternary ammonium flocculant. The specific conditions and steps are as follows:

[0069] (1) The initial moisture content of the sludge was 99 wt.%, and the dosage of octadecyltrimethylammonium bromide was 0.5 wt.% of the dry mass of the sludge. The mixture was stirred at 400 rpm for 3 min.

[0070] (2) Centrifugation at a centrifugal force of 3000 g for 5 min reduced the sludge moisture content from 99 wt.% to 93.2 wt.%;

[0071] (3) The concentrated and dehydrated sludge is transferred to a pressure-resistant container, and all the water in the sludge is converted into clathrate hydrates at 3°C and an initial carbon dioxide pressure of 4000 kPa;

[0072] (4) When the carbon dioxide partial pressure in the pressure vessel is stable, the equilibrium partial pressure of the hydrate can be reduced to 1536 kPa, and the pressure vessel is operated to release the pressure until no gas escapes, thereby obtaining sludge with improved dehydration performance.

[0073] Comparative Example:

[0074] In this comparative example, no quaternary ammonium salt flocculant was added. The specific conditions and steps are as follows:

[0075] (1) The initial moisture content of the sludge was 99 wt.%, and no quaternary ammonium salt flocculant was added;

[0076] (2) Under the centrifugal force of 2000g, the sludge moisture content decreased from 99wt.% to 94.7wt.% after centrifugation for 5min;

[0077] (3) The concentrated and dehydrated sludge is transferred to a pressure-resistant container, and all the water in the sludge is converted into clathrate hydrates under the conditions of 5°C and an initial carbon dioxide pressure of 5000 kPa;

[0078] (4) When the carbon dioxide partial pressure in the pressure vessel is stable, the equilibrium partial pressure of the hydrate can be reduced to 2464 kPa, which is significantly higher than that of Example 1 in which a quaternary ammonium salt flocculant is added at the same operating temperature.

[0079] The capillary suction time (CST) of the sludge obtained from Examples 1-4 and the comparative example was measured using a capillary suction time meter. The capillary suction time (CST) is an important indicator for evaluating sludge dewatering performance; a higher CST value indicates poorer dewatering performance. Specific test results are shown in Table 1.

[0080] Table 1 Sludge dewatering performance test results

[0081]

[0082]

[0083] As shown in Table 1, the present invention significantly improves sludge dewatering performance by coupling clathrate hydrate formation with a quaternary ammonium flocculant. The sludge moisture content can be reduced from 99% to approximately 90%, and the sludge capillary absorption time can be reduced from 32.3s to 21.8s, significantly improving dewatering performance. In contrast, the method without the addition of a quaternary ammonium flocculant only reduces the sludge capillary absorption time from 32.3s to 28.3s, resulting in only a slight improvement in dewatering performance, far inferior to the dewatering performance improvement method with the addition of a quaternary ammonium flocculant.

[0084] The present invention utilizes a quaternary ammonium salt flocculant coupled with clathrate hydrate generation to improve sludge dewatering performance. The treatment method is simple and easy to implement. Not only can the quaternary ammonium salt flocculant be used as a sludge dewatering conditioner to achieve flocculation and concentration treatment of the sludge, but the quaternary ammonium salt flocculant remaining in the sludge after concentration and dehydration can effectively improve the phase equilibrium thermodynamic conditions for hydrate formation, increase the hydrate synthesis rate, and reduce the hydrate synthesis partial pressure. The invention is non-toxic and harmless to the environment.

[0085] The above description of the embodiments is intended to facilitate understanding and use of the invention by those skilled in the art. It will be apparent that those skilled in the art can readily make various modifications to these embodiments and apply the general principles described herein to other embodiments without requiring inventive effort. Therefore, the present invention is not limited to the above-described embodiments. Improvements and modifications made by those skilled in the art based on the disclosure of the present invention, without departing from the scope of the present invention, should be within the scope of protection of the present invention.

Claims

1. A method for improving sludge dewatering performance based on enhanced crystallization, characterized in that: The method for improving sludge dewatering performance comprises the following steps: S1: adding a quaternary ammonium salt flocculant to the sludge and stirring thoroughly to obtain a mixture; S2: centrifugally concentrate and dehydrate the mixture obtained in S1; S3: The concentrated and dehydrated sludge from S2 is transferred to a pressure-resistant container, and high-pressure carbon dioxide is intermittently introduced under low-temperature conditions to generate clathrate hydrates; S4: After the partial pressure of the carbon dioxide hydrate in the pressure-resistant container is stabilized, the pressure is released until no gas escapes, thereby obtaining sludge with improved dehydration performance.

2. The method for improving sludge dewatering performance based on enhanced crystallization according to claim 1, characterized in that: In step S1, the quaternary ammonium salt flocculant is selected from one or more of octadecyltrimethylammonium bromide, dioctadecyldimethylammonium chloride or behenyldimethylbenzylammonium chloride.

3. The method for improving sludge dewatering performance based on enhanced crystallization according to claim 1, characterized in that: In step S1, the dosage of the quaternary ammonium salt flocculant is 0.5-1 wt.% of the dry mass of the sludge.

4. The method for improving sludge dewatering performance based on enhanced crystallization according to claim 1, characterized in that: In step S1, the stirring and mixing speed is 200-600 rpm, and the time is 2-15 minutes.

5. The method for improving sludge dewatering performance based on enhanced crystallization according to claim 1, characterized in that: In step S2, the centrifugal concentration and dehydration is carried out at a g value of 2000-3000 g and for a time of 3-5 minutes.

6. The method for improving sludge dewatering performance based on enhanced crystallization according to claim 1, characterized in that: In step S3, the temperature of the low-temperature environment is 0-10°C.

7. The method for improving sludge dewatering performance based on enhanced crystallization according to claim 1, characterized in that: In step S3, the pressure of the carbon dioxide introduced is 1500-5000 kPa.

8. The method for improving sludge dewatering performance based on enhanced crystallization according to claim 1, characterized in that: In step S4, the phase equilibrium partial pressure of the carbon dioxide hydrate is 1414-4292 kPa.

9. The method for improving sludge dewatering performance based on enhanced crystallization according to claim 1, characterized in that: In step S4, the pressure-resistant container is depressurized until the gas pressure in the pressure-resistant container is consistent with the atmospheric pressure.

10. The method for improving sludge dewatering performance based on enhanced crystallization according to claim 1, characterized in that: In step S4, the initial moisture content of the sludge is not less than 98 wt.%, and the moisture content after dehydration is not higher than 93.5 wt.%.

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