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Article

Flocculation–Electro-Osmosis-Coupled Dewatering Treatment of River-Dredged Sludge

by
Ziwei Liu
1,†,
Qing Wei
1,†,
Chunzhen Fan
1,
Shutian Li
2,* and
Suqing Wu
3,*
1
College of Life and Environmental Science, Wenzhou University, Wenzhou 325000, China
2
Department of Architecture and Art, Zhejiang College of Construction, Hangzhou 311231, China
3
National and Local Joint Engineering Research Center of Ecological Treatment Technology for Urban Water Pollution, Wenzhou University, Wenzhou 325000, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Water 2025, 17(21), 3174; https://doi.org/10.3390/w17213174
Submission received: 29 September 2025 / Revised: 30 October 2025 / Accepted: 30 October 2025 / Published: 5 November 2025
(This article belongs to the Special Issue Ecological Wastewater Treatment and Resource Utilization)
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Abstract

The presence of organic matter can alter the dewatering characteristics of river-dredged silt and affect the dewatering efficiency. This study systematically compared the dewatering effects of cationic polyacrylamide (CPAM), ferric chloride (FeCl3), and composite flocculant (CPAM + FeCl3) for sludge with different organic matter contents by using the combined flocculation–electro-osmotic dewatering technology. The results show that the presence of organic matter significantly hinders the dewatering of silt. After the combined treatment of low-, medium-, and high-organic-matter river-dredging sludge with composite flocculants and electro-osmotic treatment, the final water content was 39.53%, 45.08%, and 47.28%, respectively. Compared with the use of CPAM alone, its dewatering efficiency increased by 66.98%, 5.39%, and 13.72%, respectively. Three-dimensional fluorescence spectroscopy analysis (3D-EEM) indicates that the combined dewatering of flocculation and electro-osmosis can improve the dewatering performance of sludge by promoting the transformation of organic matter. Scanning electron microscopy (SEM) analysis shows that under the action of the composite flocculant, the sludge particles aggregate significantly, and after electro-osmosis, the structure becomes more compact and channels are formed, which further improves the sludge dewatering efficiency. This study provides a theoretical basis for the optimization of dewatering processes for dredged silt with different organic matter contents.

1. Introduction

With the acceleration of urbanization and the continuous advancement of water conservancy projects, people gradually attached greater importance to river protection and began to strengthen river conservation, restoration, and reconstruction [1,2]. However, in China, the serious problem of sludge siltation has been prevalent in a large number of rivers, lakes, and coastal rivers [3,4]. Moreover, since river sludge contains pollutants such as sludge, organic matter, microorganisms, and heavy metals, and is characterized by high water content and poor stability [5,6], its harmless, reduced-quantity, and resource-based treatment has drawn increasing attention. Some sludge treatment methods, such as landfilling, composting, incineration, and land utilization, require the moisture content (MC) of the sludge to be below 60% [7]. Therefore, efficient dewatering was the primary and essential step before sludge treatment and resource utilization.
Currently, commonly used sludge dewatering methods include flocculation, filter pressing, centrifugation, and electro-osmosis [8,9,10]. In practical applications, flocculation dewatering was widely applied due to its simple operation and relatively low cost. Zhu et al. utilized anionic polyacrylamide (APAM) and lime as flocculants and conditioning agents to conduct flocculation-sedimentation dewatering treatment on river sludge with varying organic matter contents. The research results revealed that the combined use of APAM and lime achieved higher dewatering efficiency compared to using APAM or lime alone. The turbidity of the sludge supernatant, capillary suction time (CST), and median particle size (Dx(50)) reached 2.77 NTU, 80.47 s, and 271.46 μm, respectively [11]. Liu et al. discovered through research that the combined use of inorganic flocculants and organic polymer flocculants could effectively leverage the advantages of charge neutralization and adsorption bridging. The addition of 0.8% FeCl3 and 0.08% APAM increased the drainage volume by 46.5% and 56.8%, respectively [12]. Wei et al. employed a composite flocculant of CPAM and chitosan (CS) for combined flocculation–filter pressing dewatering of sludge. The research results indicated that the dewatering effect with the composite flocculant was significantly better than that with a single flocculant. When the ratio of CPAM to CS was 1:2 and the dosage was 300 mg·L−1, the combined dewatering effect of the sludge was optimal, with the moisture content of the sludge decreasing from 239.5% to 61.5% [13].
There is still a series of membrane separation-based treatment technologies for the dewatering of sludge. In traditional processes, plate and frame filter press technology is often adopted. Cui et al.’s study shows that applying a plate and frame filter press to the sludge treated with lime can significantly improve the dewatering efficiency of the sludge, but there is a problem of pore blockage in long-term use [14]. In municipal wastewater treatment, forward osmosis (FO) can simultaneously achieve sewage purification and sludge concentration. However, due to scaling issues, the water flux decreases, and regular membrane cleaning is required [15]. To further alleviate this issue, Wang et al. combined FO with a membrane bioreactor (MBR), which can further reduce the membrane’s tendency to scale. However, salt accumulation and membrane fouling still lead to a decrease in water flux [16]. In addition, we have also summarized the currently popular sludge dewatering technologies. Their advantages and limitations are shown in Table 1.
In contrast, electric dewatering technology is regarded as one of the most effective methods to enhance the efficiency of sludge dewatering and has received extensive attention in recent years [21,22]. Its working principle is based on the double electric layer existing on the surface of colloidal particles in the sludge. By applying current to the water-containing sludge, the cations on the surface of the sludge particles move towards the cathode, while the anions move towards the anode. At the same time as the ions migrate, they drive water molecules to move towards the cathode, thereby achieving sludge dewatering. This process has low energy consumption and does not cause clogging of the filter medium [23,24]. Cai et al. employed CPAM as a skeleton builder to enhance electro-osmotic flow during pressurized vertical electro-osmotic dewatering (PVEOD). The results showed that the combination of CPAM and PVEOD achieved a sludge moisture content of 56.81%. Moreover, the addition of CPAM facilitated the disruption of EPS in PVEOD and the formation of water channels. The optimal dewatering efficiency depended on the applied CPAM dosage [25]. Liu et al. used a combination of inorganic flocculants, FeCl3 and Al2(SO4)3, along with electro-osmotic experiments. The research results indicated that flocculants had a significant impact on the electro-osmotic drainage effect of dredged sludge. The maximum drainage volume of 269.24 g was achieved when the incorporation ratio of the two flocculants was 0.1%. However, compared to pure electro-osmotic tests, as the incorporation ratio of flocculants increased, the current and electro-osmotic energy consumption also rose. Therefore, the influence of flocculant type on electro-osmotic performance needed to be considered [26].
Therefore, the selection of flocculants and the organic matter content of river-dredged sludge were identified as key factors in combined dewatering. The choice of flocculants directly influenced the flocculation effect, while the organic matter content affected the performance of electro-osmotic dewatering. Thus, optimizing the selection of flocculants became a focal point in research on combined dewatering. This study addressed the issue of efficient dewatering of river-dredged sludge. Through experimental investigations, the impacts of different flocculants on the dewatering performance of river-dredged sludge, as well as the influence of organic matter content on dewatering effects, were examined. By integrating flocculation and electro-osmotic dewatering technologies, an efficient combined dewatering process was developed to achieve effective dewatering of river-dredged sludge.

2. Materials and Methods

2.1. Experimental Materials

The low-, medium-, and high-organic-matter sludge samples used in the experiment were collected from different sections of the Wenrui Tang River in Wenzhou. A Petersen grab sampler was employed to gather surface sediment sludge from the river channel. Large stones, branches, and other foreign impurities were removed from the river-dredged sludge. Subsequently, the sludge was placed in clean storage containers and transported back to the laboratory, where it was sealed and stored in a 4 °C refrigerator for future use. The basic properties of the river-dredged sludge samples are presented in Table 2.
The experiment used cationic organic polymer flocculant polyacrylamide (CPAM) as the flocculant and inorganic flocculant ferric chloride (FeCl3) as the coagulant. Among them, CPAM was purchased from Tenglong Water Treatment Materials Co., Ltd. (Gongyi, China), with a molecular weight of 12 million. FeCl3 was acquired from Nanjing Xianglingde Environmental Protection Technology Co., Ltd. (Nanjing, China).

2.2. Experimental Procedures

First, a flocculation sedimentation column experiment was conducted on river-dredged sludge. Different types of flocculants were initially added to the river-dredged sludge. A six-paddle mixer was used to stir the mixture at 800 r/min for 5 min. Subsequently, the uniformly mixed samples were entirely poured into 500 mL graduated cylinders. After being allowed to stand for 48 h, the supernatant was extracted using a syringe, and its volume was recorded. The pH (conductivity), EC, and soluble components of both the river-dredged sludge and the supernatant were measured and analyzed. Through flocculation preliminary experiments, it was determined that the dosage ranges of CPAM and FeCl3 were 0–3 g·L−1 and 5–25 g·L−1, respectively. Therefore, in the flocculation sedimentation column experiment, the dosage gradients of CPAM were set at 0, 0.5, 1, 1.5, 2.5, and 3 g·L−1, while those of FeCl3 were set at 0, 5, 10, 15, 20, and 25 g·L−1. After the supernatant from the flocculation treatment was extracted, the remaining river-dredged sludge was stirred uniformly and then entirely poured into an electro-osmotic device (with a diameter of 12 cm and a height of 5 cm) with a voltage of 50 V, where iron and aluminum sheets were used as the anode and cathode, respectively. After the electro-osmotic dewatering process was completed, samples of the river-dredged sludge were taken and subjected to freeze-drying treatment. Subsequently, the pH, EC, and microstructure of both the sludge samples and the discharged liquid were tested.

2.3. Determination Method and Characterization

2.3.1. Determination of Indexes of Dredged Sludge in the River

The moisture content, pH, and electrical conductivity (EC) of the river-dredged sludge samples were determined using the gravimetric method (HJ 613-2011) [27], potentiometric method (HJ 962-2018) [28], and electrode method (HJ 802-2016) [29], respectively. The content of organic matter in the river-dredged sludge samples was measured by the loss-on-ignition method (HJ 761-2015) [30]. The formula for determining the moisture content in the sludge is presented as follows:
w   =   ( m 1 − m 2 ) ( m 2 − m 0 ) × 100 %
In the formula, w represents the moisture content of the sludge sample; m0 is the mass of the covered container (g); m1 is the total mass of the covered container and the sludge sample (g); and m2 is the total mass of the covered container and the dried sludge (g).
The contents of free water and bound water in the river-dredged sludge were determined by the dilatometer method. The microstructure of the river-dredged sludge during the dewatering process was characterized by using SEM (Zeiss Sigma 300, Carl Zeiss Microscopy GmbH, Jena, Germany).

2.3.2. Determination of Water Sample Index

A pH meter (PHC3003-3C, Hach Company, Loveland, CO, USA) and an electrical conductivity meter (DDS-11a, Shanghai Leici Instrument Co., Ltd., Shanghai, China) were employed to directly measure the pH and EC values of the water samples.
An ultra-fast three-dimensional fluorescence spectrometer (Aqualog, HORIBA Scientific, Kyoto, Japan) was used to determine the active components in the flocculation supernatant and electro-osmotic filtrate. The main performance parameters of the instrument were as follows: the light source was a 150 W xenon lamp, the PMT voltage was set at 700 V, and the signal-to-noise ratio was greater than 110. The following parameters were configured for the test: the excitation wavelength was set in the range of Ex = 245–500 nm, the emission wavelength was fixed at Em = 244.24–826.05 nm, the scanning speed was 2400 nm/min, the integration time was 0.5 s, the increment for both excitation and emission was 5 nm, and the CCD gain was set to medium. The three-dimensional matrix formed was processed using the parallel factor analysis method, and the active components in the supernatant water samples were ultimately obtained.

3. Results

3.1. Influence of Single Flocculant on Dewatering Effect of River-Dredged Sludge with Different Organic Matter Content

3.1.1. Influence of CPAM on the Dewatering Effect of River-Dredged Sludge with Different Organic Matter Contents

The impacts of CPAM on the flocculation and dewatering effects of low-, medium-, and high-organic-matter river-dredged sludge are illustrated in Figure 1.
The results shown in Figure 1 indicate that within the dosage range of 0–3 g·L−1 of CPAM, as the dosage increased, the moisture content of the flocculated low-, medium-, and high-organic-matter river-dredged sludge (208.7~228.55%) exhibited minimal differences compared to the low-, medium-, and high-organic-matter control groups (188.09%, 214.76%, and 190.18%). As the organic matter content of the original sludge increased, the optimal dosages of CPAM were found to be 0.5, 1, and 2.5 g·L−1, respectively. After the combined treatment of flocculation and electro-osmosis, the moisture contents were reduced to 119.71%, 47.65%, and 54.8%, respectively.

3.1.2. Effect of FeCl3 on the Dewatering Effect of Dredging Sludge in Rivers with Different Organic Matter Contents

The impacts of FeCl3 on the flocculation and dewatering effects of low-, medium-, and high-organic-matter river-dredged sludge are shown in Figure 2.
The results presented in Figure 2 indicate that within the dosage range of 0–25 g·L−1 of FeCl3, as the dosage increased, the moisture content of the coagulated low-, medium-, and high-organic-matter river-dredged sludge (222.84~233.64%) exhibited relatively small differences compared to the low-, medium-, and high-organic-matter control groups (188.09%, 214.76%, and 190.18%). As the organic matter content of the original sludge increased, the optimal dosage of FeCl3 was consistently found to be 5 g·L−1. However, notable variations in moisture content were observed among the three types of sludge, with values of 125.33%, 70.2%, and 71.44%, respectively. This suggests that the cohesive electro-osmosis effect of FeCl3 on low-organic-matter river-dredged sludge was weaker than that on medium- and high-organic-matter sludge. This could be attributed to the fact that suspended particles and colloids in river-dredged sludge typically carry negative charges, and the positively charged iron ions generated from the hydrolysis of FeCl3 can neutralize these negative charges, reducing the electrical repulsion between particles and facilitating their aggregation into larger flocs. Low-organic-matter river-dredged sludge contains fewer EPS (extracellular polymeric substance) aggregates, resulting in a weaker charge neutralization effect with FeCl3 compared to medium- and high-organic-matter sludge. Consequently, after treatment with the same dosage of FeCl3, the moisture content of the river-dredged sludge remained relatively high.

3.2. Effect of Composite Flocculant on the Dewatering Effect of Dredging Sludge in Rivers with Different Organic Matter Contents

The dewatering effects of low-, medium-, and high-organic-matter river-dredged sludge after the application of composite flocculants are illustrated in Figure 3.
Notable differences in dewatering performance were observed among the river-dredged sludge samples after the addition of the composite flocculant composed of CPAM and FeCl3, followed by flocculation and electro-osmosis treatments. The moisture contents of the flocculated low-, medium-, and high-organic-matter river-dredged sludge were recorded as 71%, 73.44%, and 125.17%, respectively. After the combined treatment of flocculation and electro-osmosis, the lowest moisture contents achieved were 39.53%, 45.08%, and 47.28%, respectively. As the organic matter content of the original sludge increased, the optimal moisture contents of the low-, medium-, and high-organic-matter river-dredged sludge after the combined flocculation–electro-osmosis treatment were found to be 39.53%, 45.08%, and 47.28%, respectively. Compared to the dewatering effects achieved by individual flocculant treatments combined with electro-osmosis, the composite flocculant improved the dewatering efficiency by 66.98%, 5.39%, and 13.72% for low-, medium-, and high-organic-matter sludge, respectively. This indicates that the composite flocculant exhibited significantly superior dewatering performance compared to individual flocculants, particularly for low-organic-matter river-dredged sludge. Furthermore, when comparing the optimal dosages of the composite flocculant for different organic-matter-content river-dredged sludge, it was found that, with the same dosage of FeCl3, medium-organic-matter river-dredged sludge required a higher dosage of CPAM compared to low-organic-matter sludge. In contrast, high-organic-matter river-dredged sludge demanded higher dosages of both FeCl3 and CPAM compared to the other two types of sludge.

3.3. Analysis of the Properties of Filtrate

3.3.1. Electrical Conductivity of Filtrate

The EC (electrical conductivity) of river-dredged sludge and supernatant at different stages under various CPAM dosages is shown in Figure 4. The results indicated the following:
When no CPAM was added (blank group), the EC of low-organic-matter river-dredged sludge after flocculation and electro-osmosis was 0.357 and 0.564 ms/cm, respectively, while that of the supernatant was 0.381 and 0.779 ms/cm, respectively. For medium-organic-matter river-dredged sludge, the EC after flocculation and electro-osmosis was 0.44 and 0.852 ms/cm, respectively, and that of the supernatant was 0.535 and 1.695 ms/cm, respectively. For high-organic-matter river-dredged sludge, the EC after flocculation and electro-osmosis was 0.724 and 1.069 ms/cm, respectively, and that of the supernatant was 0.45 and 0.831 ms/cm, respectively.
After CPAM was added, the EC of both low- and medium-organic-matter river-dredged sludge and their discharged liquids after flocculation and electro-osmosis increased significantly, and this increase was observed to rise with the increase in CPAM dosage. For high-organic-matter river-dredged sludge, after CPAM was added, the EC of its discharged liquids at various stages exhibited changes similar to those of medium-organic-matter river-dredged sludge, while the EC of its sludge cake showed slight variations. Compared with the blank group, the EC of the discharged liquids at all stages increased after CPAM was added.
Figure 5 illustrates the EC of river-dredged sludge and supernatant at different stages under various FeCl3 dosages. The results indicated the following: The EC of both dredged sludge and its discharged liquids after flocculation and electro-osmosis showed an upward trend as the FeCl3 dosage increased. It was noteworthy that when the FeCl3 dosage exceeded 15 g·L−1, no electro-osmotic fluid was generated during electro-osmosis in the experimental groups. This could be attributed to the fact that the addition of ferric chloride increased the ionic concentration in the solution. As the dewatering process progressed, some ions might adsorb onto the surface of sludge particles or undergo precipitation, thereby reducing the concentration of free ions in the solution and leading to a decrease in EC. Moreover, during the collection of electro-osmotic fluid, the electro-osmotic reactions in some experimental groups ended at different times, so the collected electro-osmotic fluid was not collected at the same moment. At the initial stage of electro-osmotic dewatering, after ferric chloride was added to the sludge, it dissociated into Fe3+ and Cl− ions, increasing the ionic concentration in the solution and thus causing the EC to rise. As the electro-osmotic dewatering process continued, the moisture in the river-dredged sludge was gradually removed, and the relative ionic concentration increased, which might further elevate the EC. However, if an excessive amount of ferric chloride was used, it could lead to pore blockage in the river-dredged sludge, hindering ion migration and thus slowing down the upward trend of EC. At the later stage of electro-osmotic dewatering, with a significant reduction in moisture in the river-dredged sludge, ion migration was restricted. Simultaneously, some ions might undergo precipitation or adsorb onto the surface of river-dredged sludge particles, resulting in a decrease in the ionic concentration in the solution and consequently a drop in EC.
3.3.2. pH Changes During the Coagulation Process
Figure 6 shows the pH changes in low-, medium-, and high-organic-matter content sludge before and after electro-osmosis at different FeCl3 dosages. As shown in the figure, with the continuous increase of FeCl3 dosage, the pH value of the sludge decreases nonlinearly, indicating that the acidification reaction of FeCl3 and the hydrolysis reaction of FeCl3 produce additional H+, leading to a decrease in the pH of the sludge. Unlike alkaline environments, under these conditions, Fe3+ hydrolyzes and forms multi-core cations that can effectively neutralize negatively charged sludge colloidal particles, further improving the coagulation efficiency [31]. Taking high-organic-matter sediment as an example, the initial pH value of the sediment was 7.03. After adding 5 g·L−1 FeCl3, the pH value of the sediment decreased to 6.38. When FeCl3 was further added to 15 g·L−1, the pH value of the sediment was 4.25, a decrease of 2.13 compared to the addition of 5 g·L−1. Then, when FeCl3 was further added to 25 g·L−1, the pH value of the sediment was 3.26, a decrease of only 0.99 compared to 15 g·L−1. This indicates that the addition of FeCl3 will cause acidification reactions in the sludge, but the sludge itself has a certain buffering capacity for the degree of acidification, especially when the pH value of the sludge decreases gently at high doses of FeCl3.
Further comparison of the pH changes in the three types of sludge reveals that sludge with different organic matter contents also has different buffering capacities for acidification. The reason may be due to the influence of the chemical composition and buffering capacity of dredged sludge. Low organic matter sludge usually contains a higher proportion of inorganic mineral components, such as carbonates, which have strong pH buffering capacity [32,33]. In high-organic-matter sludge, carboxyl and phenolic hydroxyl groups are present, providing negatively charged sites that can directly adsorb H + and metal cations, neutralizing acid input [34].

3.3.3. Three-Dimensional Fluorescence Analysis of Filtrate

The data obtained from the three-dimensional fluorescence tests on the filtrates from the flocculation and electro-osmosis stages, after subtracting the blank control values of ultrapure water, were imported into the Origin software. Subsequently, fluorescence spectra of the raw sludge, the supernatant after flocculation, and the discharged liquid from electro-osmosis for low-, medium-, and high-organic-matter river-dredged sludge were plotted. A three-dimensional fluorescence spectral analysis was then conducted, and the results are shown in Figure 7.
From Figure 7a, it can be observed that the supernatant of the raw low-organic-matter river sludge contains humic-like fluorescence peaks (λEx (nm)/λEm (nm) = 290~325/370~440) and fulvic acid fluorescence peaks (λEx (nm)/λEm (nm) = 230~310/380~460). After electro-osmosis, the fulvic acid and humic-like peaks in the electro-osmotic fluid disappear, and a tryptophan peak emerges. After CPAM was added, the fluorescence intensity of the fulvic acid peak in the supernatant after flocculation weakened, and most of them are visible-region fulvic acid peaks, while the range of the humic-like peak expanded. After electro-osmosis, the fluorescence intensity of the visible-region fulvic acid peak increased, the range of the humic-like peak decreased, and a weak tryptophan peak appeared. In medium-organic-matter river-dredged sludge, the organic matter is mainly composed of humic-like substances. Under electro-osmotic treatment, protein-like substances are released to improve the dewatering effect. In high-organic-matter river-dredged sludge, the organic matter is composed predominantly of protein-like substances. Compared with medium- and high-organic-matter sludge, the organic matter in low-organic-matter river-dredged sludge mainly consists of humic-like substances and fulvic acid. Through comparison, it is found that after CPAM was added, the organic matter in low-organic-matter river-dredged sludge can be decomposed into easily degradable substances such as humic-like substances, fulvic acid, and tryptophan. Moreover, after electro-osmosis, humic-like substances can be further converted into more easily degradable fulvic acid.
The fluorescence spectra of the combined dewatering process using CPAM in combination with a flocculant are shown in Figure 7. From Figure 7e,g,i, it can be observed that after flocculation with the composite flocculant, the organic matter in river-dredged sludge mainly consists of humic-like substances, fulvic acid, and tryptophan. As the organic matter content in the river-dredged sludge increased, the humic-like peaks became more dispersed, indicating that the humic-like substances in low-organic-matter river-dredged sludge had a relatively simple composition and were prone to adsorb onto the surface of sludge particles, forming stable complexes. Additionally, there was a more uniform type of fulvic acid. In high-organic-matter river-dredged sludge, there was a wide variety of humic-like substances, which might partially inhibit dewatering through colloidal interactions with the flocculant and were related to the pH of the water sample. Meanwhile, the fluorescence intensity of the fulvic acid peaks decreased significantly, suggesting that the fulvic acid content in high-organic-matter river-dredged sludge was lower than that in low-organic-matter sludge.

3.4. Properties and Microstructure of River-Dredged Sludge

3.4.1. Free Water/Bound Water of River-Dredged Sludge

The ratios of free water to bound water in river-dredged sludge at different stages under various CPAM dosages are presented in Figure 8a–c. It was observed that after CPAM was added, the ratios of free water to bound water in river-dredged sludge increased significantly, indicating that the addition of CPAM promoted the conversion of bound water to free water in river-dredged sludge, thereby facilitating its dewatering. In addition, as the CPAM dosage increased, the ratio of free water to bound water initially rose and then declined. When the CPAM dosages were 0.5, 1, and 2.5 g·L−1, respectively, the ratios of free water to bound water in low-, medium-, and high-organic-matter river-dredged sludge reached their peak values after flocculation–electro-osmosis, with values of 3.85, 4.12, and 3.54, respectively. However, after electro-osmotic dewatering, the ratios of free water to bound water in medium- and high-organic-matter river-dredged sludge decreased significantly, by 18.18%, 18.93%, and 11.86%, respectively. This suggested that the moisture in river-dredged sludge tended to stabilize after electro-osmosis, and the difficulty of dewatering the sludge increased. In comparison, as the organic matter content of the original sludge increased, the initial ratio of free water to bound water in river-dredged sludge gradually rose. For low-organic-matter river-dredged sludge, the change in the ratio of free water to bound water was not significant with an increase in CPAM dosage. Moreover, the flocculant had a more pronounced effect on increasing the ratio of free water to bound water in medium-organic-matter river-dredged sludge compared to high-organic-matter river-dredged sludge. This result was corroborated by the fact that a lower CPAM dosage was required during the dewatering process of low-organic-matter river-dredged sludge.

3.4.2. Microstructure of Dredged Sludge in the River

The structural diagram of low-organic-matter river-dredged sludge subjected to flocculation–electro-osmosis treatment is shown in Figure 9. From Figure 9a, it can be observed that the raw low-organic-matter river sludge, after flocculation, formed irregular clumps with a smooth surface. These clumps were stacked in a sheet-like manner and arranged relatively loosely, while small pore structures were also present. After electro-osmosis, the surface of the raw sludge became smoother, and the particle arrangement was more orderly compared to Figure 9a. The number of small pore structures decreased, but the proportion of micropores increased, which might be attributed to the rapid drainage of water during local dewatering in electro-osmosis. In Figure 9c, after the addition of CPAM flocculant, under the combined effects of adsorption bridging by CPAM and charge neutralization by FeCl3, a branched network floc structure was formed. This structure became denser and more conducive to dewatering. After electro-osmosis, the orderliness of particle arrangement was enhanced, and distinct pore channels were generated. Additionally, a smooth cemented surface was clearly visible, and the structure became more compact (Figure 9d). It can thus be concluded that after CPAM was added, its long-chain structure could adsorb sludge particles to form a “particle-polymer-particle” network structure, generating large and dense flocs and improving the efficiency of solid–liquid separation [35,36].
The structural diagrams of river-dredged sludge subjected to flocculation–electro-osmosis dewatering using a composite flocculant are shown in Figure 9. From Figure 9e,g,i, it can be observed that as the organic matter content increased, the particle arrangement in the river-dredged sludge flocculated by the composite flocculant gradually became more orderly. The size of the formed flocs increased with the rise in organic matter content, accompanied by the aggregation of tiny particles into micro-flocs. In low-organic-matter river-dredged sludge, the particles were more concentrated in distribution, and the surface area increased. For medium-organic-matter sludge, the microaggregates formed by the compression of the electric double layer due to FeCl3 were more pronounced, along with a large number of small pore structures. In high-organic-matter sludge, the flocs were larger, and the structure was slightly looser compared to the former two, making it more prone to water retention. This might also explain why the dewatering effect of high-organic-matter river-dredged sludge was weaker than that of the other two. From Figure 9f,h,j, it can be seen that after electro-osmosis, the overall floc structure became denser, accompanied by the formation of pore channels. As the organic matter content increased, the volume of the flocs became larger, and the particles were arranged more tightly, resulting in stronger structural stability of the river-dredged sludge.

4. Discussion

The effective disposal of dredged bottom sludge is an urgent issue that needs to be addressed at present, and efficient dewatering serves as a key step in the reuse and disposal of sludge. The addition of flocculants has led to the recombination of ions and compounds within river-dredged sludge, promoting the release of some bound ions in the sludge [37]. Among these ions, there are some stable and non-biodegradable organic substances that can fix certain ions, such as metal ions, through complexation [38]. These strong complexation interactions are difficult to disrupt by flocculants, and the fixed ions are also hard to release. As a result, the improvement effect of flocculant addition on low-organic-matter river-dredged sludge and its supernatant is significantly greater than that on high-organic-matter river-dredged sludge. Meanwhile, river sludge with different organic matter contents in rivers exhibits different characteristics during the combined flocculation–electro-osmosis dewatering process. As the organic matter content increases, the presence of extracellular polymeric substances (EPSs) hinders the conversion of bound water to free water in river-dredged sludge, resulting in the stabilization of moisture [39]. This leads to a decrease in dewatering capacity. The increase in organic matter content causes an increase in the negative charge density on the surface of river sludge particles in rivers, forming a thicker electric double layer. Under the action of an electric field, the compression and slippage of the electric double layer become difficult, leading to a weakened electro-osmotic effect and relatively lower dewatering efficiency. Moreover, the fluorescence peak of humic acid or fulvic acid (humic-like substances) (λEx (nm)/λEm (nm) = 330–350/320–380) narrows under acidic conditions and expands under alkaline conditions due to the dissociation of carboxyl groups. Their presence further affects the dewatering performance of river-dredged sludge [38,40]. Secondly, electrochemical reactions can convert and precipitate some free ions [41,42], resulting in a significant decrease in the EC of medium-organic-matter river-dredged sludge and its discharged liquid after electro-osmosis.
In this study, the flocculation–electro-osmotic coupling technology was adopted to establish flocculant gradient experiments for sludge with different organic matter contents, revealing the influence of different organic matter contents on dewatering performance and overcoming the limitations of the traditional dewatering methods, as described in Table 1. The results show that the composite flocculant composed of CPAM and FeCl3 significantly improves the dewatering efficiency of low-, medium-, and high-organic-matter sludge, increasing it by 66.98%, 5.39%, and 13.72%, respectively, compared with the single flocculant. The minimum water content after flocculation and electro-osmosis was 39.53%, 45.08%, and 47.28%, respectively, which was 15–20% lower than that after flocculation and pressure filtration alone [13]. The moisture content of the sludge treated by traditional physical and chemical regulators is far superior (60–70%) [43], and it is also lower than that treated by piezoelectric technology (85.2–63.9%) [44]. This is attributed to the synergistic effect of flocculants and electro-osmosis. Among them, CPAM mainly causes sludge particles to aggregate and form aggregates through bridging, and in addition, CPAM also reduces the electrostatic repulsion between sludge particles through electro-neutralization, making the aggregates denser [25,45,46]. FeCl3 mainly serves as a coagulant, and its high valence positive charge generated by hydrolysis effectively neutralizes the negative charge on the surface of sludge particles. At the same time, it destroys the TB-EPS and LB-EPS structures, degrades hydrophilic organic matter, and releases bound water [47,48]. The combined effect of the two makes the floc structure of the silt more compact and stable, and forms drainage channels within the silt, as shown in the SEM image. On this basis, the application of electro-osmosis technology will be more conducive to the movement of sludge water molecules towards the cathode, further improving the dewatering efficiency. This coupling technology consumes less energy and occupies less land compared with the thermal drying technology [49]. In addition, compared with the membrane-based sludge dewatering technology that uses forward osmosis (FO) and a membrane bioreactor (MBR), the coupling of flocculation and electro-osmosis has obvious advantages. Electro-osmosis technology utilizes the electric field generated by current as the driving force for separation and the sludge itself as the solid–liquid separation membrane, effectively avoiding problems such as clogging, contamination, and decreased water flux in membrane processes. The experiment uncovered the influence mechanism of multi-factor coupling on the dewatering performance of river sludge. It provided a theoretical basis for optimizing dewatering process parameters in practical engineering applications and effectively promoted the development of dewatering technology for river sludge towards a resource-saving and environmentally friendly direction, contributing to the achievement of the green governance goal of high low-efficiency carbon sustainability.

5. Conclusions

This study systematically discussed the characteristics and mechanisms of the flocculation–electro-osmotic coupling technology for different organic sludges. The results show that the use of a combined flocculant of CPAM and FeCl3 can significantly improve the dewatering efficiency, and its effect is superior to that of a single flocculant treatment. The organic matter content of sludge has a significant impact on the dewatering effect of sludge. After electro-osmosis, the final moisture content of low-, medium-, and high-organic-matter sludge was reduced to 39.53%, 45.08%, and 47.28%, respectively, and the dewatering efficiency was increased by up to 66.98%, 5.39%, and 13.72% compared with that of a single flocculant. In terms of mechanism, the flocculation–electro-osmotic coupling technology can not only promote the aggregation of sludge particles to form a more compact structure but also create dewatering channels. At the same time, by changing the organic matter composition of the sludge and improving the water molecule binding state, it can promote the deep dewatering of the sludge. This technology avoids the shortcomings of sludge dewatering technologies based on membrane processes and thermal drying, providing a reliable technical path for the resource utilization treatment of organic dredged sludge.

Author Contributions

Conceptualization, Z.L., Q.W., and C.F.; methodology, Z.L., Q.W., and C.F.; software, Z.L., Q.W., and C.F.; validation, Z.L., Q.W., and C.F.; formal analysis, Z.L., Q.W., and C.F.; investigation, Z.L., Q.W., and C.F.; resources, Z.L., Q.W., and C.F.; data curation, Z.L., Q.W., and C.F.; writing—original draft preparation, Z.L., Q.W., and C.F.; writing—review and editing, S.L. and S.W.; visualization, Z.L., Q.W., and C.F.; supervision, S.L. and S.W.; project administration, Z.L., Q.W. and C.F.; funding acquisition, S.L. and S.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Wenzhou Ecological Park Research Project (grant numbers SY2022ZD-1002-02 and SY2022ZD-1002-07).

Data Availability Statement

The data presented in this study are available on request from the corresponding authors. The data are not publicly available due to the funder’s restrictions.

Acknowledgments

The authors express their sincere gratitude for the work of the editor and the anonymous reviewers.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. CPAM treatment for low-organic-matter (a), medium-organic-matter (b), and high-organic-matter (c) dehydration moisture content.
Figure 1. CPAM treatment for low-organic-matter (a), medium-organic-matter (b), and high-organic-matter (c) dehydration moisture content.
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Figure 2. FeCl3 treatment for low-organic-matter (a), medium-organic-matter (b), and high-organic-matter (c) dehydration moisture content.
Figure 2. FeCl3 treatment for low-organic-matter (a), medium-organic-matter (b), and high-organic-matter (c) dehydration moisture content.
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Figure 3. Dehydration moisture content of the river-dredged sludge treated with composite flocculants, categorized by (a) low organic matter, (b) medium organic matter, and (c) high organic matter.
Figure 3. Dehydration moisture content of the river-dredged sludge treated with composite flocculants, categorized by (a) low organic matter, (b) medium organic matter, and (c) high organic matter.
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Figure 4. Electrical conductivity of the river-dredged sludge (a–c) and supernatant (d–f) at various stages, with different dosages of cationic polyacrylamide (CPAM), categorized by (a,d) low organic matter, (b,e) medium organic matter, and (c,f) high organic matter.
Figure 4. Electrical conductivity of the river-dredged sludge (a–c) and supernatant (d–f) at various stages, with different dosages of cationic polyacrylamide (CPAM), categorized by (a,d) low organic matter, (b,e) medium organic matter, and (c,f) high organic matter.
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Figure 5. Electrical conductivity of the river-dredged sludge (a–c) and supernatant (d–f) at various stages, with different dosages of FeCl3, categorized by (a,d) low organic matter, (b,e) medium organic matter, and (c,f) high organic matter.
Figure 5. Electrical conductivity of the river-dredged sludge (a–c) and supernatant (d–f) at various stages, with different dosages of FeCl3, categorized by (a,d) low organic matter, (b,e) medium organic matter, and (c,f) high organic matter.
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Figure 6. FeCl3 treatment for low-organic-matter (a), medium-organic-matter (b), and high-organic-matter (c) pH values.
Figure 6. FeCl3 treatment for low-organic-matter (a), medium-organic-matter (b), and high-organic-matter (c) pH values.
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Figure 7. 3D-EEM analysis of supernatant of low-organic raw sludge (a), electro-osmotic fluid of low-organic raw sludge (b), supernatant after optimal flocculation with CPAM dosage (c), electro-osmotic fluid after optimal flocculation with CPAM dosage (d), supernatant after optimal flocculation with low-organic composite flocculant (e), electro-osmotic fluid after optimal flocculation with low-organic composite flocculant (f), supernatant after optimal flocculation with medium-organic composite flocculant (g), electro-osmotic fluid after optimal flocculation with medium-organic composite flocculant (h), supernatant after optimal flocculation with high-organic composite flocculant (i), and electro-osmotic fluid after optimal flocculation with high-organic composite flocculant (j).
Figure 7. 3D-EEM analysis of supernatant of low-organic raw sludge (a), electro-osmotic fluid of low-organic raw sludge (b), supernatant after optimal flocculation with CPAM dosage (c), electro-osmotic fluid after optimal flocculation with CPAM dosage (d), supernatant after optimal flocculation with low-organic composite flocculant (e), electro-osmotic fluid after optimal flocculation with low-organic composite flocculant (f), supernatant after optimal flocculation with medium-organic composite flocculant (g), electro-osmotic fluid after optimal flocculation with medium-organic composite flocculant (h), supernatant after optimal flocculation with high-organic composite flocculant (i), and electro-osmotic fluid after optimal flocculation with high-organic composite flocculant (j).
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Figure 8. Free water and bound water content of the river-dredged sludge at various stages, categorized by (a,d) low organic matter, (b,e) medium organic matter, and (c,f) high organic matter, under different dosages of cationic polyacrylamide (CPAM) (a–c) and combinations of flocculants (d–f).
Figure 8. Free water and bound water content of the river-dredged sludge at various stages, categorized by (a,d) low organic matter, (b,e) medium organic matter, and (c,f) high organic matter, under different dosages of cationic polyacrylamide (CPAM) (a–c) and combinations of flocculants (d–f).
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Figure 9. SEM characterization: (a) after flocculation of low-organic raw river-dredged sludge, (b) after electro-osmosis of low-organic raw river-dredged sludge, (c) after optimal flocculation with CPAM dosage, (d) after optimal electro-osmosis with CPAM dosage, (e) after optimal flocculation with low-organic composite flocculant, (f) after optimal electro-osmosis with low-organic composite flocculant, (g) after optimal flocculation with medium-organic composite flocculant, (h) after optimal electro-osmosis with medium-organic composite flocculant, (i) after optimal flocculation with high-organic composite flocculant, (j) after optimal electro-osmosis with high-organic composite flocculant.
Figure 9. SEM characterization: (a) after flocculation of low-organic raw river-dredged sludge, (b) after electro-osmosis of low-organic raw river-dredged sludge, (c) after optimal flocculation with CPAM dosage, (d) after optimal electro-osmosis with CPAM dosage, (e) after optimal flocculation with low-organic composite flocculant, (f) after optimal electro-osmosis with low-organic composite flocculant, (g) after optimal flocculation with medium-organic composite flocculant, (h) after optimal electro-osmosis with medium-organic composite flocculant, (i) after optimal flocculation with high-organic composite flocculant, (j) after optimal electro-osmosis with high-organic composite flocculant.
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Table 1. Summary of Sludge Dewatering Technology.
Table 1. Summary of Sludge Dewatering Technology.
Technical PrincipleTechnical NameAdvantagesDisadvantagesSources
Mechanical dehydrationFilter pressHigh solid recovery rate and filtrate clarification.The processing capacity decreases as the filtration time increases.[17]
Horizontal screw centrifugeIt can operate continuously and has a low moisture content.Low recovery rate (especially when the feed is high).
Thermal energy dryingThermal dryingHighly efficient, stable, and widely applicable.High energy consumption and heavy environmental burden.[18]
Solar dryingClean and environmentally friendly, with low cost.It occupies a large amount of land and is affected by the climate, thus having limitations.
Flocculation technologyPolyferric chloride (PAFC) flocculationHighly efficient and rapid dehydration, economical and practical.An optimal dosage exists. Excessive dosage may hinder the final compression settlement.[19]
Flocculation + mechanicalGeotextile filtration coupled with chitosan and polyacrylamideHigh filtration efficiency and improved effluent quality.The dehydration cycle is long.[20]
Table 2. The basic properties of river-dredged sludge.
Table 2. The basic properties of river-dredged sludge.
Sample NameMoisture Content (%)Organic Matter Content (%)pHElectrical Conductivity (μs/cm)
Low organic sludge222.744.86.02357
Medium organic sludge226.926.656.88440
High organic sludge225.6812.97.03724
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Liu, Z.; Wei, Q.; Fan, C.; Li, S.; Wu, S. Flocculation–Electro-Osmosis-Coupled Dewatering Treatment of River-Dredged Sludge. Water 2025, 17, 3174. https://doi.org/10.3390/w17213174

AMA Style

Liu Z, Wei Q, Fan C, Li S, Wu S. Flocculation–Electro-Osmosis-Coupled Dewatering Treatment of River-Dredged Sludge. Water. 2025; 17(21):3174. https://doi.org/10.3390/w17213174

Chicago/Turabian Style

Liu, Ziwei, Qing Wei, Chunzhen Fan, Shutian Li, and Suqing Wu. 2025. "Flocculation–Electro-Osmosis-Coupled Dewatering Treatment of River-Dredged Sludge" Water 17, no. 21: 3174. https://doi.org/10.3390/w17213174

APA Style

Liu, Z., Wei, Q., Fan, C., Li, S., & Wu, S. (2025). Flocculation–Electro-Osmosis-Coupled Dewatering Treatment of River-Dredged Sludge. Water, 17(21), 3174. https://doi.org/10.3390/w17213174

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