Sludge dewatering conditioning method of cationic polyacrylamide coupled with deep eutectic solvent

Abstract

A sludge dewatering and conditioning method using cationic polyacrylamide coupled with a eutectic solvent, relating to the field of environmental engineering, aims to solve the problem of increased energy consumption caused by heating required in existing sludge dewatering methods using eutectic solvents. The method comprises: 1. Mixing reagents A, B, and C, heating and stirring to form a eutectic solvent; 2. Adding the eutectic solvent to the raw sludge at room temperature for preconditioning; 3. Adding cationic polyacrylamide, stirring and reacting to obtain conditioned and reconstituted sludge; 4. Pumping the conditioned and reconstituted sludge into a high-pressure dewatering device for filter pressing to obtain a dried sludge cake. This method eliminates the need for heating and is used to improve sludge dewatering capacity and reduce the final moisture content of the sludge.

Description

Technical Field

[0001] This invention relates to the field of environmental engineering, specifically to a sludge dewatering and conditioning method using cationic polyacrylamide coupled with a eutectic solvent. Background Technology

[0002] Municipal sludge is an unavoidable byproduct of wastewater treatment. Its high water content and poor dewatering properties cause problems such as difficult transportation and high costs for resource utilization, necessitating dewatering. In practical applications, flocculation conditioning technology has gained widespread acceptance and application due to its simple dosing system configuration, convenient operation and management, and good economic efficiency.

[0003] Commonly used flocculants can be broadly classified into inorganic flocculants and organic polymeric flocculants based on their properties. Inorganic flocculants mainly include iron salts and aluminum salts, but they have disadvantages such as large dosage and poor settling performance. Furthermore, the dewatered sludge contains large amounts of iron and aluminum, which is detrimental to subsequent utilization. The most commonly used organic polymeric flocculant is cationic polyacrylamide (CPAM), which efficiently promotes sludge dewatering through charge neutralization. It primarily removes free water between particles, but its treatment capacity for bound water is significantly limited; the moisture content of the sludge treated with PAM cannot be significantly reduced.

[0004] Existing methods have found that while traditional eutectic solvents can break down extracellular polymeric substances (EPS) and release bound water during sludge dewatering, they also increase the overall viscosity of the sludge, hindering dewatering. Therefore, coupling the eutectic solvent with heating reduces its viscosity and hydrogen bond strength, improving dewatering performance; alternatively, adding a large amount of inorganic flocculant on top of this coupling further improves sludge-water separation. However, using eutectic solvents directly increases energy consumption for sludge dewatering. Furthermore, sludge dewatered with inorganic flocculants contains high levels of iron, aluminum, and other metals, severely impacting subsequent resource recovery processes such as incineration, composting, or building material utilization. The large dosage of inorganic flocculant also poses a risk of secondary pollution from metal ion leaching. Therefore, while current methods achieve sludge reduction, their potential risks cannot be ignored. Summary of the Invention

[0005] The present invention aims to solve the problem that existing methods for sludge dewatering using eutectic solvents require heating, leading to increased energy consumption, and provides a sludge dewatering conditioning method using cationic polyacrylamide coupled with a eutectic solvent.

[0006] This invention provides a sludge dewatering and conditioning method using cationic polyacrylamide coupled with a eutectic solvent, comprising the following steps:

[0007] 1. Mix reagent A, reagent B, and reagent C, heat and stir to form a uniform and clear liquid, which is the eutectic solvent; reagent A is choline chloride, reagent B is carboxylic acid, and reagent C is hydrated metal chloride;

[0008] 2. Under normal temperature and no external heating conditions, add the eutectic solvent prepared in step 1 to the original sludge with a water content of 95%~99% and perform mechanical stirring and pre-conditioning for 5~30 minutes.

[0009] 3. Add cationic polyacrylamide (CPAM) to the sludge pre-conditioned in step 2, and stir to react, to obtain the conditioned and reconstituted sludge;

[0010] Fourth, the conditioned and reconstructed sludge is pumped into a high-pressure dewatering device for pressure filtration, and finally dried sludge cake with a stable moisture content of less than 40% is obtained.

[0011] Furthermore, the molar ratio of drug A, drug B and drug C in step one is 1:(5~10):(1~5).

[0012] Furthermore, the carboxylic acid mentioned in step one is one or more of oxalic acid, citric acid, lactic acid, acetic acid, and formic acid.

[0013] Furthermore, the hydrated metal chloride mentioned in step one is one or more of aluminum chloride hexahydrate (AlCl3·6H2O), copper chloride dihydrate (CuCl2·2H2O), magnesium chloride hexahydrate (MgCl2·6H2O), and ferric chloride hexahydrate (FeCl3·6H2O).

[0014] Furthermore, the heating temperature in step one is 60~90℃.

[0015] Furthermore, in step one, the stirring speed is 100~500 rpm, and the stirring time is 1~8 hours.

[0016] Furthermore, in step three, the stirring speed is 300-400 rpm, and the stirring time is 2-10 minutes.

[0017] Furthermore, in step four, the pressure for filtration is set to 1.5~2.0 MPa, and the pressure is maintained for 60~90 minutes.

[0018] The beneficial effects of this invention are:

[0019] The eutectic solvent in this invention consists of hydrogen bond donors and acceptors. It can disrupt the stable structure of EPS and sludge particles in the sludge through hydrogen bonding, releasing internal bound water and converting it into free water. It also acts as a flocculant to improve the settling and dewatering performance of the sludge. The positively charged groups of CPAM neutralize the negative charge on the surface of the sludge particles, reducing repulsive forces. Furthermore, the long-chain polymers connect the dispersed sludge particles, forming larger flocs that facilitate rapid water escape and prevent fine particles from clogging the channels. After treatment with the eutectic solvent, the increased adsorption sites of CPAM on the sludge also improve flocculation efficiency, resulting in a denser sludge cake during the mechanical dewatering stage. Together, these components form a green and efficient "cell-wall breaking flocculation" dewatering method, significantly improving sludge dewatering capacity and reducing the final moisture content of the sludge.

[0020] CPAM primarily treats free water between particles through adsorption bridging, but its ability to treat bound water bound to EPS is extremely limited. Therefore, CPAM cannot effectively improve sludge moisture content, hindering sludge reduction. Eutectic solvents can break extracellular polymers through hydrogen bonding, releasing bound water, but most require heating, limiting their application. This invention combines CPAM and DES, overcoming CPAM's limitations and achieving deep dewatering capabilities not found in conventional applications. It also broadens the application of DES beyond heating conditions. The proposed eutectic solvent formulation effectively breaks down EPS at room temperature, facilitating bound water release. Simultaneously, CPAM's charge neutralization and adsorption bridging effects integrate the dispersed organic matter and fine particles released from DES into flocs at room temperature, enabling efficient dewatering at room temperature. This increases the practicality and feasibility of CPAM in real-world applications, providing a new approach to sludge dewatering. Subsequent experimental results revealed that the combined treatment of the two methods for sludge processing greatly improved the dewatering effect. This effect was far more than a simple superposition of their individual properties; rather, it was the synergistic effect generated by utilizing their respective advantages. Through the "wall-breaking flocculation" pathway and the resulting deep dewatering effect, the sludge dewatering performance was significantly improved. Attached Figure Description

[0021] Figure 1 The sludge dewatering effect of different treatment groups;

[0022] Figure 2 Moisture content of mud cake in different treatment groups;

[0023] Figure 3 Comparison of sludge samples from different treatment groups. Detailed Implementation

[0024] Extracellular polymers (EPS) are high-molecular-weight polymers secreted by microorganisms in activated sludge systems, mainly consisting of proteins, polysaccharides, and nucleic acids. These biopolymers form hydrogen bonds with water molecules through their abundant hydrophilic functional groups (such as hydroxyl, carboxyl, and amino groups), constructing a stable hydrated gel network structure within the sludge flocs. This gel structure not only firmly binds a large amount of bound water but also significantly increases the viscosity and filtration resistance of the sludge system. Simultaneously, the negative charge carried on its surface maintains colloidal stability through electrostatic repulsion, all contributing to a severe deterioration of sludge dewatering performance.

[0025] Eutectic solvents (DES), as a green conditioner, exhibit unique advantages in improving sludge dewatering performance. The cationic components in DES (such as quaternary ammonium salts) effectively neutralize the negative charge on the surface of sludge particles, significantly increasing the zeta potential, weakening electrostatic repulsion between particles, and promoting floc aggregation and sedimentation. More importantly, DES components can disrupt the conformation of protein and polysaccharide molecules in EPS through competitive hydrogen bonding, breaking down their hydrated gel network structure and converting bound water into free water. Furthermore, some acidic DES can further weaken the hydration capacity of EPS by lowering the system pH and promoting the protonation of hydrophilic functional groups. This multi-synergistic mechanism of DES makes it significantly effective in disrupting the water-holding structure of sludge and improving solid-liquid separation performance, while also creating favorable conditions for subsequent resource recovery.

[0026] Existing methods require coupling the eutectic solvent with heating, or adding large amounts of inorganic flocculants on top of this coupling. Therefore, using eutectic solvents for sludge dewatering directly increases energy consumption. Furthermore, the sludge dewatered with inorganic flocculants contains high levels of iron, aluminum, and other metallic elements, severely impacting subsequent resource recovery processes such as incineration, composting, or utilization in building materials. Additionally, the large dosage of inorganic flocculants poses a risk of secondary pollution from metal ion leaching. Therefore, while current methods achieve sludge reduction, their potential risks cannot be ignored.

[0027] Based on the above description, this invention provides a method for improving sludge dewatering performance by coupling CPAM with a eutectic solvent. This method utilizes the destructive effect of the eutectic solvent on extracellular polymers under heating-free conditions, and couples it with CPAM to treat the sludge. Through a two-stage conditioning process, the sludge dewatering effect is improved, and the moisture content is reduced. Compared to existing methods using eutectic solvents for sludge dewatering, this method can be carried out at room temperature, reducing energy consumption and making it more feasible in practical applications. Furthermore, neither CPAM nor the eutectic solvent introduces large amounts of metal elements into the sludge, thus offering better prospects for resource utilization of the dewatered sludge. This aligns with the development direction of sludge treatment—reduction, harmlessness, and resource recovery—and achieves significant progress in dewatering efficiency, sludge cake moisture content, and environmental friendliness.

[0028] Specific Implementation Method 1: This implementation method for sludge dewatering and conditioning using cationic polyacrylamide coupled with a eutectic solvent is characterized by the following steps:

[0029] 1. Mix reagent A, reagent B, and reagent C, heat and stir to form a uniform and clear liquid, which is the eutectic solvent; reagent A is choline chloride, reagent B is carboxylic acid, and reagent C is hydrated metal chloride;

[0030] 2. Under normal temperature and no external heating conditions, add the eutectic solvent prepared in step 1 to the original sludge with a water content of 95%~99% and perform mechanical stirring and pre-conditioning for 5~30 minutes.

[0031] 3. Add polyacrylamide to the sludge that has been pre-conditioned in step 2, and stir to react, to obtain the conditioned and reconstituted sludge.

[0032] Fourth, the conditioned and reconstructed sludge is pumped into a high-pressure dewatering device for pressure filtration, and finally dried sludge cake with a stable moisture content of less than 40% is obtained.

[0033] Specific Implementation Method Two: In this implementation method, the molar ratio of drug A, drug B, and drug C in step one is 1:(5~10):(1~5). Other steps and parameters are the same as in Specific Implementation Method One.

[0034] Specific Implementation Method 3: The carboxylic acid mentioned in step one of this implementation method is one or more of oxalic acid, citric acid, lactic acid, acetic acid, and formic acid. Other steps and parameters are the same as in Specific Implementation Method 1 or 2.

[0035] Specific Implementation Method Four: The hydrated metal chloride mentioned in step one of this implementation method is one or more of aluminum chloride hexahydrate, copper chloride dihydrate, magnesium chloride hexahydrate, and ferric chloride hexahydrate. Other steps and parameters are the same as in any of Specific Implementation Methods One to Three.

[0036] Specific Implementation Method 5: The heating temperature described in step one of this implementation method is 60~90℃. Other steps and parameters are the same as in any of Specific Implementation Methods 1 to 4.

[0037] Specific Implementation Method Six: In step one of this implementation method, the stirring speed is 100~500 rpm, and the stirring time is 1~8 hours. Other steps and parameters are the same as in any of Specific Implementation Methods One to Five.

[0038] Specific Implementation Method Seven: In step three of this implementation method, the stirring speed is 300-400 rpm, and the stirring time is 2-10 minutes. Other steps and parameters are the same as in any of Specific Implementation Methods One to Six.

[0039] Specific Implementation Method Eight: In step four of this implementation method, the pressure is set to 1.5~2.0 MPa, and the pressure is maintained for 60~90 minutes. Other steps and parameters are the same as in any of Specific Implementation Methods One to Seven.

[0040] The embodiments of the present invention will be described in detail below. The following embodiments are implemented based on the technical solution of the present invention, and detailed implementation schemes and specific operation processes are given. However, the protection scope of the present invention is not limited to the following embodiments.

[0041] Example 1:

[0042] This embodiment describes a method for improving sludge dewatering performance using cationic polyacrylamide coupled with a eutectic solvent, comprising the following steps:

[0043] I. Preparation of Eutectic Solvent (DES)

[0044] Mix reagents A, B, and C, heat and stir at 70°C at 300 rpm for 4 hours to form a homogeneous and clear liquid, which is the eutectic solvent; reagent A is choline chloride (ChCl), reagent B is oxalic acid, and reagent C is aluminum chloride hexahydrate; the molar ratio of reagents A, B, and C is 1:10:1.

[0045] II. DES Preprocessing

[0046] Under normal temperature and no external heating conditions, the eutectic solvent prepared in step one is added to the original sludge with a water content of 99% and mechanically stirred for 30 minutes for pre-conditioning.

[0047] III. PAM Deep Conditioning

[0048] Cationic polyacrylamide (CPAM) was added to the sludge pre-conditioned in step two, and the mixture was stirred at a speed of 300 rpm for 5 minutes to obtain the conditioned and reconstituted sludge.

[0049] IV. Mechanical filtration for deep dehydration

[0050] The conditioned and reconstituted sludge is pumped into a high-pressure dewatering device, the pressure is set to 2.0 MPa, and the pressure is maintained for 60 minutes to obtain dried sludge cake with a stable moisture content of less than 40%.

[0051] Capillary suction time (CST) and sludge specific resistance (SRF) are important indicators for evaluating sludge dewatering performance. Four groups were set up for comparative experiments: raw sludge (RS), eutectic solvent alone (DES), cationic polyacrylamide alone (CPAM), and cationic polyacrylamide coupled with eutectic solvent (CPAM+DES). The sludge dewatering effects under different treatment conditions are as follows: Figure 1As shown. From Figure 1 As can be seen, the CST of sludge decreased after CPAM treatment alone and after CPAM coupled with DES treatment. The coupled treatment showed the most significant improvement in dewatering performance, with the CST of the CPAM+DES group decreasing by 84.7%, 85.6%, and 60.9% compared to the original sludge, DES-treated sludge alone, and CPAM-treated sludge alone. The SRF of the CPAM+DES group also decreased significantly, decreasing by 67.9%, 68.8%, and 35.8% compared to the original sludge, DES-treated sludge alone, and CPAM-treated sludge alone, respectively. It is noteworthy that when DES treated the sludge alone, both CST and SRF increased compared to the original sludge. This is attributed to the strong destructive effect of DES on sludge particles and EPS, breaking down the original sludge particles into finer particles, clogging the original pores and water channels, resulting in an increase in CST and SRF instead of a decrease. Compared to traditional flocculation conditioning, CPAM coupled with DES treatment has better results and significantly improves sludge dewatering performance.

[0052] Figure 2 The study demonstrates the moisture content of sludge cakes after different treatments under laboratory conditions. The moisture content of the sludge cake after PAM treatment is close to that of the original sludge. The moisture content of the sludge cake after DES treatment decreased from 88% to 85.8%, proving that DES can indeed disrupt the stable structure of EPS and sludge particles in the sludge, releasing bound water and converting it into free water, thereby reducing the sludge moisture content. This is consistent with previous results. Furthermore, the moisture content of the sludge cake after the coupled treatment decreased from 88% to 83.7%. This indicates that CPAM coupled with DES treatment can significantly improve sludge dewatering, reduce sludge moisture content, and is beneficial for sludge volume reduction.

[0053] Comparison of sludge samples under different treatment conditions is shown in the following figures. Figure 3 As shown. By Figure 3The specific effects of the RS, CPAM, and CPAM+DES experiments can be observed. In the RS group, the raw sludge was untreated. No obvious sludge-water separation was observed; the sludge particles were uniformly dispersed in the liquid phase, with neither a separate supernatant layer nor a significant bottom sediment layer. The entire system was in a highly dispersed suspended state, indicating strong electrostatic repulsion between sludge particles, making spontaneous aggregation and sedimentation difficult, resulting in poor sludge dewatering performance. In the CPAM group, significant sludge-water separation was observed. Sludge particles aggregated into flocs and settled to the bottom under the action of CPAM, achieving preliminary sludge-water separation and improving sludge dewatering. The CPAM+DES group showed the most significant sludge-water separation. Under the combined action of DES and CPAM, larger flocs were formed, and more fine particles were effectively flocculated and settled, resulting in a clearer supernatant. This indicates that the synergistic effect of the two agents made the flocculation and sedimentation of sludge particles more thorough and complete. The combined experimental results of the three groups show that the RS group did not achieve sludge-water separation, the CPAM group achieved preliminary separation, and the CPAM+DES group had the best effect, indicating that the coupling of CPAM and DES has a better effect on improving sludge dewatering performance than other treatment methods.

Claims

1. A method for sludge dewatering and conditioning using cationic polyacrylamide coupled with a eutectic solvent, characterized in that, The method includes the following steps:

1. Mix reagent A, reagent B, and reagent C, heat and stir to form a uniform and clear liquid, which is the eutectic solvent; reagent A is choline chloride, reagent B is carboxylic acid, and reagent C is hydrated metal chloride; 2. Under normal temperature and no external heating conditions, add the eutectic solvent prepared in step 1 to the original sludge with a water content of 95%~99% and perform mechanical stirring and pre-conditioning for 5~30 minutes.

3. Add polyacrylamide to the sludge that has been pre-conditioned in step 2, and stir to react, to obtain the conditioned and reconstituted sludge. Fourth, the conditioned and reconstructed sludge is pumped into a high-pressure dewatering device for pressure filtration, and finally dried sludge cake with a stable moisture content of less than 40% is obtained.

2. The sludge dewatering and conditioning method using cationic polyacrylamide coupled with a eutectic solvent according to claim 1, characterized in that, The molar ratio of drug A, drug B and drug C in step one is 1:(5~10):(1~5).

3. The sludge dewatering and conditioning method using cationic polyacrylamide coupled with a eutectic solvent according to claim 2, characterized in that, The carboxylic acid mentioned in step one is one or more of oxalic acid, citric acid, lactic acid, acetic acid, and formic acid.

4. The sludge dewatering and conditioning method using cationic polyacrylamide coupled with a eutectic solvent according to claim 3, characterized in that, The hydrated metal chloride mentioned in step one is one or more of aluminum chloride hexahydrate, copper chloride dihydrate, magnesium chloride hexahydrate, and ferric chloride hexahydrate.

5. The sludge dewatering and conditioning method using cationic polyacrylamide coupled with a eutectic solvent according to claim 4, characterized in that, The heating temperature in step one is 60~90℃.

6. The sludge dewatering and conditioning method using cationic polyacrylamide coupled with a eutectic solvent according to claim 5, characterized in that, In step one, the stirring speed is 100~500 rpm and the stirring time is 1~8 hours.

7. The sludge dewatering and conditioning method using cationic polyacrylamide coupled with a eutectic solvent according to claim 6, characterized in that, In step three, the stirring speed is 300-400 rpm, and the stirring time is 2-10 minutes.

8. The sludge dewatering and conditioning method using cationic polyacrylamide coupled with a eutectic solvent according to claim 7, characterized in that, In step four, the pressure is set to 1.5~2.0 MPa, and the pressure is maintained for 60~90 minutes.

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