Next Article in Journal
Extra Tree Regression Algorithm for Simulation of Iceberg Draft and Subgouge Soil Characteristics
Previous Article in Journal
Multi-Model Analyses of Spatiotemporal Variations of Water Resources in Central Asia
Previous Article in Special Issue
Insight on the Ultrafast Water Treatment over NiFe-Layered Double Hydroxides via Electroactivation of Ferrate(VI): The Role of Spin State Regulation
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Water
Volume 17
Issue 16
10.3390/w17162424
water-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Ferric Tannate-Enhanced Electrochemical Conditioning Process for Improving Sludge Dewaterability

by
Yalin Yu
1,†,
Junkun Feng
1,†,
Nanwen Zhu
2 and
Dongdong Ge
1,2,*
1
School of Resources and Environmental Engineering, Jiangsu University of Technology, Changzhou 213001, China
2
Shanghai Engineering Research Center of Solid Waste Treatment and Resource Recovery, Shanghai Jiao Tong University, Shanghai 200240, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Water 2025, 17(16), 2424; https://doi.org/10.3390/w17162424 (registering DOI)
Submission received: 16 July 2025 / Revised: 8 August 2025 / Accepted: 14 August 2025 / Published: 16 August 2025
(This article belongs to the Special Issue Application of Advanced Oxidation Processes (AOPs) for Wastewater Treatment)
Browse Figures
Versions Notes

Abstract

Sludge dewatering is a key step in the overall process of sludge treatment and disposal. In this study, ferric tannate was synthesized by chemically complexing tannic acid with Fe2(SO4)3 under various conditions and then was innovatively employed to enhance electrochemical conditioning (ECC) for municipal sludge dewatering. The optimal preparation conditions of ferric tannate were determined as a tannic acid to iron ion molar ratio of 0.8:10, pH of 10, and reaction time of 2 h. Subsequently, ferric tannate-enhanced ECC was investigated under different dosages and operating parameters. The optimal conditions were identified as ferric tannate dosage of 20% total solid, voltage of 50 V, and reaction time of 30 min, under which capillary suction time, specific resistance to filtration, and water content of dewatered sludge cake decreased by 84.3%, 84.2%, and 17.6%, respectively. Results of the mechanism analysis indicated that ferric tannate effectively reduced sludge viscosity, increased zeta potential, and neutralized the negative surface charges via charge neutralization, hydrophobic interactions, and hydrogen bonding. Meanwhile, adsorption bridging promoted floc aggregation and particle growth. Compared with the ECC process alone, the addition of ferric tannate in the ferric tannate-enhanced ECC process generated more •OH, promoting the extracellular polymeric substance degradation and protein removal, thereby improving sludge hydrophobicity. Furthermore, the floc structure was reconstructed into a more compact and smooth morphology, facilitating the release of bound water during filtration. These findings provide new technical and theoretical support for the development of eco-friendly and efficient sludge conditioning and dewatering processes.

1. Introduction

With the acceleration of urbanization and rapid industrial development, the efficiency of wastewater treatment has improved, and large quantities of waste activated sludge have been generated by wastewater treatment plants [1]. Sludge dewatering is considered a key section to diminish sludge volume, lower transportation costs, and improve the efficiency of subsequent treatment and disposal. Nonetheless, the complex gel-like composition of sludge, which contains highly hydrophilic extracellular polymeric substances (EPSs) and microbial cells, severely hinders the effectiveness of conventional dewatering approaches [2]. Therefore, disrupting the EPS matrix and microbial cells to release both EPS-bound water and intracellular water has been recognized as an effective strategy to enhance sludge dewatering performance [3].
In recent years, electrochemical conditioning (ECC) has emerged as a promising pretreatment method for improving sludge dewaterability [4], as it can facilitate the release of bound water by directly and indirectly oxidizing and disrupting the EPS matrix and microbial cells. However, despite its advantages, ECC alone is limited by the relatively low generation of oxidative reactive species, resulting in incomplete EPS degradation and insufficient release of bound water. As a consequence, dewatering performance during subsequent pressure filtration remains suboptimal [5]. For instance, the maximum reduction in capillary suction time (CST) achieved by ECC alone was only 18.8% [6]. To overcome these limitations and further improve sludge dewaterability, it is essential to promote the formation of oxidative radicals during electrochemical treatment.
Adding oxidizing reagents during the ECC process or integrating ECC with other advanced oxidation processes, particularly the Fenton reaction, is a promising strategy to address this deficiency. Hu et al. introduced Ca(ClO)2 into an electrochemical system employing an iron anode. The coexistence of HOCl and Fe(II) facilitated the generation of highly oxidative radicals, including hydroxyl radical (•OH) and active chlorine, significantly reducing CST and specific resistance to filtration (SRF) by 88% and 92%, respectively [7]. Masihi and Gholikandi applied electrochemical–Fenton technology for the conditioning and dewatering of anaerobically digested sludge. Their research results demonstrated that under optimal operational conditions, SRF and time to filter were reduced by 93.8% and 75.9%, respectively [8]. Furthermore, Cai et al. employed zero-valent iron as the iron source in sludge dewatering. The addition of H2O2 and zero-valent iron in the electrochemical–Fenton pretreatment generated highly oxidative •OH, which disrupted EPS and lysed sludge cells, leading to substantial reductions in water content (Wc) and CST values [9].
In the electrochemical treatment process, hydrogen peroxide (H2O2) can be generated at the cathode via the reduction of oxygen. Therefore, activating H2O2 to produce •OH with a high non-selective oxidation potential of 2.8 V is considered an effective approach to enhance the oxidative capacity of ECC systems [10]. Tannic acid, an environmental-friendly secondary metabolite of plants, contains abundant phenolic hydroxyl and carboxyl groups capable of complexing with Fe3+ to form ferric tannate complexes [11,12]. The phenolic hydroxyl groups in a tannic acid molecule coordinate with Fe3+ to form five- or six-membered cyclic structures, in which Fe3+ in the complex can be reduced to Fe2+ and subsequently released [13]. This unique reaction can promote the continuous recycling and regeneration of Fe2+. Moreover, the phenolic hydroxyl and benzene ring structures in ferric tannate derived from the initial tannic acid can bind to proteinaceous substances through hydrogen bonding and hydrophobic interactions, respectively [14], leading to the precipitation of extracellular proteins and facilitating the water separation. Therefore, it is speculated that introducing ferric tannate into the ECC system not only can elevate the oxidative degradation of sludge but also can promote the removal of released hydrophilic organic matter, such as proteins, in external stratification of sludge [15]. This reduction in resistance to water leakage can further improve sludge dewaterability, which has not been previously reported.
In this study, to improve the sludge dewatering performance, ferric tannate was first synthesized and then combined with the ECC system to jointly regulate the sludge. The effects of ferric tannate and electrochemical parameters on sludge dewatering performance were investigated. Additionally, sludge characteristics, including viscosity, zeta potential, particle size, hydrophobicity, EPS content, fluorescent components, surface functional groups, and morphological structure, were comprehensively analyzed to elucidate the underlying mechanisms responsible for the enhanced sludge dewaterability observed in the ferric tannate-enhanced electrochemical system. The research purpose of this paper is to provide valuable theoretical and technical support for the development of ecological and efficient sludge conditioning and dewatering processes.

2. Materials and Methods

2.1. Material

2.1.1. Sludge Sample and Chemicals

Sludge was taken out from the secondary sedimentation tank of a sewage treatment plant in Changzhou city in China. The collected sludge was screened through a 20-mesh sieve (China Anping Co., Ltd., Hebei, China), with gravel and hair discarded, and then was subsequently centrifuged by the TG16-WS centrifuge (Hunan Xiangyi Laboratory Instrument Development Co., Ltd., Changsha, China) to obtain sludge with an approximately total solid (TS) content of 3%. The ratio of volatile solid (VS) to TS was nearly 0.63. The main physicochemical characteristics of the sludge are listed in Table 1. The chemicals used in this study included tannic acid (Shandong Keyuan Biochemical Company, Shandong, China), H2SO4 (Shanghai Lingfeng Chemical Reagent Co., Ltd., Shanghai, China), NaOH (Shanghai Wokai Biotechnology Co., Ltd., Shanghai, China), and Fe2(SO4)3 (Shanghai Lingfeng Chemical Reagent Co., Ltd., Shanghai, China), all of which were analytical grade.

2.1.2. Preparation of Ferric Tannate

According to previous studies [16,17], ferric tannate was prepared by mixing a Fe2(SO4)3 solution (1 M ferric ion) with a tannic acid solution (0.1 M) at ambient temperature (20 °C). Briefly, the two solutions were mixed at varying molar ratios (tannic acid to ferric ions ranging from 0.2:10 to 1:10) and stirred at 400 rpm for 60 min to form a homogeneous solution containing ferric tannate complexes. The mixtures were then centrifuged at 5000 rpm for 20 min, and the resulting precipitate was dried by the LGJ-60A vacuum freeze dryer (Shanghai Hefan Instrument Co., Ltd., Shanghai, China) at −55 °C for 72 h to obtain powdered ferric tannate particles.
In addition, ferric tannate was also prepared under different pH conditions. Specifically, the synthesis was performed by adjusting the pH of the mixed solution (prepared as described above) to values of 2, 4, 6, 8, 10, 12, and 14 using either sulfuric acid (H2SO4, 2 M) or alkali (NaOH, 2 M). After pH adjustment, the solutions were similarly stirred at 400 rpm (20 °C) for 60 min, followed by identical centrifugation and freeze-drying procedures as described above. The schematic illustration of the synthesis of ferric tannate and its application in electrochemically enhanced sludge dewatering are shown in Figure S1.

2.2. Experimental Procedure

The ECC reactor consisted of a cubic glass container with dimensions of 10 cm × 10 cm × 10 cm. During each sludge conditioning reaction, 400 mL of sludge with a TS content of 3% was first mixed with ferric tannate at a stirring speed of 300 rpm for 1 min and then transferred into the reactor. Titanium alloy mesh electrode plates were used as both the anode and cathode, with an inter-electrode distance set to 2 cm [18], and were further connected to a regulated MP6020D power supply (Shanghai Jinghua Technology Instrument Co., Ltd., Shanghai, China).
Various experimental conditions were comprehensively explored, including ferric tannate dosage (0%, 10%, 20%, 30%, and 40% TS), applied voltages (0, 10, 20, 30, 40, and 50 V), and reaction times (0, 10, 20, 30, 40, 50, and 60 min) in order to optimize the operational parameters for enhancing sludge dewaterability (Figure S2). Dewatering performance was evaluated by measuring the CST, SRF, and Wc of dewatered sludge cake.
To reveal the underlying mechanisms responsible for the improved sludge dewaterability achieved by ferric tannate-enhanced ECC, sludge samples from four different treatments were collected: raw sludge, ECC treated sludge, sludge after ferric tannate conditioning, and sludge after ferric tannate-enhanced ECC treatment. The samples were comparatively analyzed with respect to particle size distribution, zeta potential, viscosity, EPS content, fluorescent components, hydrophobicity, surface functional groups, and surface morphology characteristics. Furthermore, the contents of the key chemical components, including Fe2+, H2O2, and •OH in ferric tannate-enhanced ECC system, were comprehensively monitored. All experiments were conducted in triplicate to minimize experimental errors.

2.3. Analytical Methods

The CST values of the sludge samples were determined using 304B CST equipment (Triton, London, UK). Standard methods were used to determine the SRF and Wc of dewatered sludge cake [19]. The particle size distribution of the sludge was analyzed using a BT-9300s particle size analyzer (Bettersize, Dandong, China). A rotational viscosimeter (NDJ-8S, Shanghai Nirun Co., Ltd., Shanghai, China) was used to measure the viscosity of sludge. The zeta potential was determined using a DelsaTM Nano instrument (Beckman Coulter, California, USA). The pH values were monitored with a digital pH meter (pHs-25, Shanghai Instrument & Electrical Co., Ltd., Shanghai, China). The contact angles were measured using a DSA100 contact angle goniometer (KRüSS, Hamburg, Germany) [20]. The triple-layered EPS samples, including slime EPS (S-EPS), loosely bound EPS (LB-EPS), and tightly bound EPS (TB-EPS), were extracted from various sludge stratifications, following the method described by Li et al. [21]. Protein was determined through a modified Lowry method with bovine serum albumin as the standard [22]. Polysaccharide content was measured using the anthrone-sulfuric acid method with glucose as the standard [23]. The fluorescent compounds in the EPS were analyzed by a Hitachi F-7000 three-dimensional excitation emission matrix (3D-EEM) fluorescence spectroscopy (Hitachi, Tokyo, Japan) [24]. For morphology characterization, the raw and conditioned sludge samples were centrifuged at 7000 rpm for 10 min to remove the supernatant, and the sediments were freeze-dried at −55 °C for 72 h to obtain dried sludge residues. The surface morphology of the samples was observed by a ZEISS Sigma 500 scanning electron microscope (SEM, Zeiss, Oberkochen, Germany). The functional groups of the sludge samples were further analyzed by a Perkin Elmer Fourier transform infrared (FTIR) Spectrum 2 Instrument (NicoletIR200, Thermo Fidher Scientific, Massachusetts, USA). Fe2+ concentration in the sludge supernatant was determined using the o-phenanthroline photometric method [25]. H2O2 concentration in the solution was spectrophotometrically determined with potassium titanium oxalate [26]. The content of •OH was measured by a previous method [27].

3. Results and Discussion

3.1. Physicochemical Characteristics of the Prepared Ferric Tannate

To optimize the preparation conditions of ferric tannate complexes, the effects of the tannic acid-to-ferric ion molar ratio and pH on the yield were systematically investigated. As illustrated in Figure 1a, the yield of the tannic acid–ferric complex increased with the tannic acid-to-ferric molar ratio and reached its maximum at 0.8:10. Beyond this ratio, the yield decreased, possibly due to excess tannic acid leading to steric hindrance or competitive binding effects that inhibit efficient complex formation. Figure 1b shows that the yield significantly increased with rising pH, but the growth slowed after pH of 10. Although the maximum yield occurred at pH of 12, this condition required substantially higher alkalinity. Thus, pH of 10 was relatively a more practical and efficient choice. According to Fu et al. [28], the formation of the tannic acid–Fe(III) complex was favored under alkaline conditions (e.g., pH of 9), while under acidic conditions, the protonation of phenolic hydroxyl groups inhibits complexation. Furthermore, compared with a previous study [29], this study achieved higher complexation result with a lower ferric dosage, possibly due to differences in alkalinity and the nature of the anions present.
The FTIR spectra of both ferric tannate and tannic acid are presented in Figure 2. The prepared ferric tannate exhibited characteristic peaks at 1405 cm−1 and 1480 cm−1, corresponding to the C–C bonds of phenolic groups [30]. Additional vibration peaks at 1645 cm−1, 1244 cm−1, and 1776 cm−1 are attributed to C=C, C–O, and C=O bonds of tannic acid, respectively [31]. The vibration peak at 815 cm−1 is the external bending vibration of the C–H plane of the aromatic ring of tannic acid, and the vibration peak at 3471 cm−1 is the stretching vibration of the phenolic hydroxyl group (O-H) of tannic acid [32,33]. Notably, a distinct Fe–O stretching vibration was observed at 651 cm−1 [34]. These spectral features confirmed the successful complexation of ferric tannate complexes through coordination between tannic acid molecules and ferric ions via Fe–O bonding.
SEM images of ferric tannate are shown in Figure 3. The ferric tannate prepared without pH adjustment displayed relatively dispersed particles and limited complexation between tannic acid and ferric ions, likely due to insufficient electrostatic interactions and competitive protonation of tannic acid functional groups (e.g., phenolic hydroxyls), which hindered effective complexation (Figure 3a). In contrast, the ferric tannate prepared under the optimal conditions exhibited particles aggregated into cluster-like structures (Figure 3b). This aggregation may be due to the enhanced electrostatic interaction between ion types in the suspension and an increase in pH, thereby reducing proton competition, promoting the deprotonation of tannic acid [35], and strengthening its affinity for Fe3+ via catechol–Fe3+ coordination, which is conducive to the dispersion of tannic acid molecules and thus promoting complexation.

3.2. Effects of Process Parameters on Sludge Dewaterability

Effective sludge dewatering is crucial for volume reduction and subsequent treatment/disposal processes, with CST serving as a fundamental performance index [36,37]. In this novel ferric tannate-enhanced ECC system, ferric tannate dosage, applied voltage, and reaction time were considered key operation parameters for optimizing sludge dewatering efficiency.
As illustrated in Figure 4a, when the sludge underwent electrochemical treatment without the addition of ferric tannate, its dewaterability deteriorated. The addition of ferric tannate significantly improved the dewatering performance of ECC treatment. When the ferric tannate dosage increased, the CST value of the sludge progressively decreased. At a reaction time of 30 min, the CST value declined from 309 s to 52 s (an 83.17% reduction) with a ferric tannate dosage of 10%, and further dropped to 22 s (a 92.88% reduction) with a dosage of 20%. These results demonstrated that ferric tannate effectively enhanced sludge dewaterability. However, increasing the dosage beyond 20% yielded only marginal improvements, with additional CST reductions of 0.88% and 1.88% observed at dosages of 30% and 40%, respectively. Therefore, considering both dewatering performance and cost-effectiveness, the optimal ferric tannate dosage was determined to be 20%.
With ferric tannate dosage fixed at 20% TS, the dewaterability showed voltage-dependent behavior (Figure 4b). Under low-voltage conditions (0–20 V), the CST values of the sludge initially decreased and then increased, with CST values exceeding that of the raw sludge, indicating a deterioration in dewaterability. In contrast, at higher voltages (30–50 V), the CST exhibited a continuous downward trend. The most significant improvement was observed at 50 V, where the CST decreased from 197 s to 24.3 s, representing an 87.66% reduction. Based on these experimental trends and practical considerations, the optimal voltage was selected as 50 V.
Combining the results from both sets of experiments, the optimal parameters for ferric tannate-enhanced sludge dewatering were identified as follows: ferric tannate dosage of 20%, reaction time of 30 min, and applied voltage of 50 V. Under these optimized conditions, the CST of the sludge was reduced by over 90%.
SRF is used to quantify filtration resistance [38]. Figure 5a presents the SRF values under different treatment conditions. The electrochemical treatment alone increased the SRF from 34.04 × 1012 m/kg (raw sludge) to 40.96 × 1012 m/kg, suggesting an adverse effect on dewaterability. By comparison, significant SRF reductions were achieved in both the ferric tannate treatment group (28.77 × 1012 m/kg) and the ferric tannate-enhanced ECC group (5.74 × 1012 m/kg). These results clearly indicated that while standalone electrochemical treatment modestly compromised sludge dewaterability, the incorporation of ferric tannate substantially enhanced the dewatering performance, with the combined treatment showing particularly remarkable improvement.
The Wc of the dewatered sludge cake is another critical indicator, reflecting dewatering effectiveness. As shown in Figure 5b, compared with the original sludge, the moisture content of the filter cake decreased from 94.1% to 93.7% in the electrochemical treatment group, to 73.0% in the ferric tannate group, and to 64.1% in the ferric tannate-enhanced ECC group.
Based on the observed changes in CST values, filter cake moisture content, and SRF, electrochemical treatment alone led to a decline in sludge dewatering performance [39]. However, the addition of ferric tannate markedly improved the dewatering performance of sludge. Notably, the combined application of ferric tannate and electrochemical treatment significantly enhanced the dewatering performance of sludge compared with either treatment alone.

3.3. Effects of Ferric Tannate-Enhanced Electrochemical Process on Sludge Properties

3.3.1. Viscosity

Viscosity is an important indicator reflecting the content of organic colloids, the water-holding capacity of sludge flocs, and the connection of biopolymers between sludge particles [40]. In general, higher sludge viscosity implies greater difficulty in dewatering and increased resistance during the flow and compression processes. As shown in Figure 6, the viscosity of the raw sludge was 409 mPa·s. After ECC, ferric tannate treatment, and the ferric tannate-enhanced ECC treatment, sludge viscosity decreased to 310 mPa·s, 190 mPa·s, and 142 mPa·s, respectively. These results further indicated that the combination of ferric tannate and ECC effectively improved the sludge dewatering performance.

3.3.2. Zeta Potential

The stability of the colloid dispersion system is described by the zeta potential, which also governs the coagulation behavior of sludge colloid particles. Commonly, sludge with lower absolute zeta potential values exhibits enhanced settling properties of sludge [41]. As shown in Figure 7, following ferric tannate-enhanced ECC treatment, the zeta potential of the sludge markedly increased from −12.12 mV to −1.96 mV. The initial electronegativity of sludge flocs was primarily due to the presence of large amounts of negatively charged biomass, such as proteins and polysaccharides, whose surfaces carried numerous anionic functional groups including carboxyl, hydroxyl, and phosphate groups. The phenolic hydroxyl groups in tannic acid can complex with Fe3+, forming positively charged ferric tannate complexes that neutralize the negative charges on the surface of sludge colloidal particles. It should be noted that electrochemical oxidation can also degrade hydrophilic organic matter [42]. However, the resulting degradation products may carry both positive and negative charges, and thus relying solely on electrochemical treatment may lead to a decrease in the zeta potential. In contrast, the ferric tannate-enhanced ECC can effectively neutralize and oxidize these negative charged components, gradually reducing the overall electronegativity of sludge. Consequently, this process promotes the destabilization and subsequent settling of fine organic fragments within the liquid phase. The combined treatment thus significantly reduced the hydrophilicity of the conditioned sludge and pronouncedly improved its dewaterability.

3.3.3. Particle Size

Particle size is another key factor regulating sludge dewaterability, as differences in specific surface areas result in distinct charge distributions and sludge microstructures [43]. As shown in Figure 8, the particle sizes of the sludge samples dramatically varied after the different treatments, indicating that ferric tannate and ECC can effectively aggregate smaller sludge flocs into larger ones. This aggregation can be primarily attributed to the charge neutralization effect of ferric tannate, which promotes the agglomeration of fine particles via van der Waals forces [44]. Additionally, the polyphenolic structure of ferric tannate enables the adsorption of multiple colloidal particles through hydrogen bonding or hydrophobic interactions, forming a network-like architecture that directly increases the floc size [45]. Furthermore, electrochemical oxidation degrades EPS, thereby reducing steric hindrance and facilitating the densification of sludge flocs [46]. In addition, the size of sludge particles plays a significant role in the compaction process. Under equivalent moisture content, larger particles required less force to form a mud cake [47], which was consistent with the variation trends in sludge Wc discussed earlier.

3.3.4. Contact Angle

The contact angle serves as a valuable parameter for evaluating sludge surface hydrophobicity, with larger angles generally suggesting stronger hydrophobic characteristics [43]. As presented in Figure 9a, the contact angle of raw sludge was 80°, indicating that the sludge was dominantly hydrophilic. Figure 9b,c show that the contact angles increased to 100° and 121° for sludge samples treated by ECC and ferric tannate, respectively, demonstrating that both treatments enhanced the hydrophobicity of the sludge surface. Notably, the ferric tannate-enhanced ECC treatment achieved the maximum contact angle of 131° (Figure 9d), indicating a substantial transformation from hydrophilic to hydrophobic surface properties. These results confirmed that the synergistic oxidation effects of ferric tannate and ECC process effectively modified the conformation and surface properties of sludge particle, enhancing hydrophobicity and thereby reducing the affinity between water molecules and the sludge matrix [47].

3.3.5. EPS Contents

EPS, comprising primarily proteins and polysaccharides, comprises plenty of hydrophilic high-molecular-weight compounds distributed in the extracellular space, accounting for approximately 50–80% of the total organic components in sludge [48]. Notably, it is mainly composed of proteins and polysaccharides. As shown in Figure 10a, the protein content in the S-EPS layer of the electrochemical treatment group was higher than that of raw sludge, likely due to electrochemical disruption of microbial cells and the subsequent release of S-EPS protein. In contrast, in the ferric tannate-enhanced ECC group, the S-EPS protein content markedly decreased from 48.48 mg/L to 23.26 mg/L. Furthermore, compared with the raw sludge, the protein contents in the LB-EPS layer and the TB-EPS layer decreased by 60.9% and 90.7%, respectively. Figure 10b showed elevated polysaccharide content in the S-EPS layer after the ferric tannate-enhanced ECC treatment. This might be attributed to the neutralization of negatively charged functional groups on the sludge surface by positively charged groups, leading to compression of the EPS matrix and the release of polysaccharides. However, it is currently widely accepted that extracellular protein, rather than extracellular polysaccharide, is the primary contributor affecting sludge dewatering performance [49,50]. To present the comparison of detailed data, the relevant results are shown in Table S1.

3.3.6. Fluorescence Components

The 3D-EEM fluorescence technology has been extensively employed to monitor organic substances due to its advantages of rapid detection, high sensitivity, and strong selectivity. Studies have pointed out that EPS contains lots of fluorescent substances including tyrosine- and tryptophan-contained proteins, which are rich in hydrophilic functional groups. These compounds can combine with water molecules, block dewatering channels, and thereby deteriorate sludge dewaterability [51]. Therefore, it is necessary to further compare the three-dimensional fluorescence spectra of different EPS layers in sludge samples subjected to various treatments. As shown in Figure 11a1,a2,a3, for the raw sludge, three distinct peaks were observed: peak A (Ex/Em = 220 nm/350 nm), peak B (Ex/Em = 280 nm/358 nm), and peak C (Ex/Em = 275 nm/330 nm), which corresponded to aromatic protein substances II, tryptophan protein substances, and tyrosine protein substances, respectively [52]. Noteworthily, all the identified peaks were related to protein-like substances. This is consistent with previous reports [53,54].
As shown in Figure 11b1,b2,b3, after the ferric tannate-enhanced ECC treatment, the fluorescence intensities of the peaks in different EPS layers were significantly reduced, indicating a substantial decrease in the content of the relevant fluorescent components. This effect might be attributed to the adsorption of hydrophilic EPS components by ferric tannate via electrostatic interaction, hydrogen bonding, and hydrophobic associations, resulting in the formation of insoluble complexes. Meanwhile, a Fenton-like reaction can occur in which ferric tannate constantly promoted the recycling and regeneration of Fe2+, which subsequently reacted with hydrogen peroxide produced at the cathode to generate reactive oxygen species (ROS). These ROS oxidatively destroyed and degraded the aromatic ring structures in the fluorescent components of sludge EPS, breaking them down into small molecule substances or mineralizing them into CO2 and H2O, thereby enhancing hydrophobicity. In addition, under the applied electric field, the negatively charged protein-like substances containing hydrophilic fluorescent groups migrated toward the anode [55], where some were further adsorbed onto or oxidized at the electrode surface. This process contributed to the further reduction in the fluorescent intensity of peaks A and B and weakened the water affinity of EPS.

3.3.7. Functional Groups

The organic functional groups on the sludge surface have a direct impact on its dewaterability [56]. To further investigate the effects of different conditioning processes on sludge surface chemistry, FTIR was employed to analyze the hydrophilic and hydrophobic functional groups of the treated samples [43]. As presented in Figure 12, the sludge samples exhibited characteristic absorption peaks at 3422 cm−1, 2925 cm−1, 1656 cm−1, 1410 cm−1, and 1023 cm−1. Specifically, the peak at 3422 cm−1 corresponded to the stretching vibration of O–H groups, while the peak at 2925 cm−1 was triggered by the symmetric and asymmetric stretching vibrations of CH2 in lipids. The peak at 1656 cm−1 represented the typical stretching vibration of C=O bonds, indicating the presence of C–N in the protein amide I band [57]. The peak at 1023 cm−1 was associated with the tensile vibration of polysaccharide C–O bonds. These observations clearly demonstrated that the primary organic matters on sludge surface were mainly composed of hydrocarbons, proteins, and polysaccharides.
In addition, it was evident that the protein-related absorption peaks weakened after ferric tannate-enhanced ECC treatment, indicating that the integrated treatment effectively reduced the hydrophilic groups in the sludge matrix. Moreover, the addition of ferric tannate led to the gradual appearance of characteristic peaks corresponding to polyphenolic structures, confirming the incorporation of ferric tannate components. Notably, the positions of typical peaks for proteins and polysaccharides remained unchanged, suggesting that their chemical structures were largely preserved. Meanwhile, with the addition of ferric tannate, the intensities of these characteristic peaks also intensified, indicating the accumulation of related biomolecular substances on the surface of sludge flocs. This trend further suggested that higher dosages of ferric tannate promoted the precipitation of biological organic matters, contributing to improved sludge aggregation and dewatering behavior.

3.3.8. Morphological Structure

The microstructural differences among the sludge samples were observed using SEM. As shown in Figure 13, the raw sludge mainly consisted of large, irregularly shaped flocs, corresponding to poor dewatering and compression performance. In contrast, after ferric tannate-enhanced ECC treatment, the sludge flocs exhibited obvious aggregation, characterized by smoother, flatter surfaces and a more orderly arrangement. This improvement might be due to the positively charged complexes formed by the phenolic hydroxyl groups in tannic acid and Fe3+, which exerted the effective electro-neutralization reaction with the negative charges of sludge colloids, thereby accelerating the discharge of bound water in sludge flocs.

3.3.9. Chemical Components

Tannic acid (TA), as a polyphenolic compound (PPC), is recognized as a powerful complexant and metal chelator. Fe3+ readily reacts with PPC (viz. benzene-1,2-diols and -1,2,3-triols) and facilitates the formation of extremely stable complexes between Fe3+ and the 1,2-dihydroxy function groups of PPC ligands. Under neutral pH conditions, ferric tannate can continuously promote the recycling and regeneration of Fe2+, as illustrated in Equation (1) [16,58]. Simultaneously, in the electrochemical system, H2O2 was generated through the oxygen reduction reaction (ORR) at the cathode. The produced H2O2 subsequently reacted with Fe2+ to produce hydroxyl radicals, as shown in Equations (2) and (3).
Fe3+ + 1/2phenol → Fe2+ + 1/2(O-quinone) + H+
O2 + 2H+ + 2e− → H2O2
Fe2+ + H2O2 + H+ → Fe3+ + •OH + H2O
Hence, the ferric tannate complex can boost the Fenton-like reaction, since the Fe3+/Fe2+ redox cycle was able to promote the generation of •OH by reacting with the H2O2 produced by cathode in the ferric tannate-enhanced ECC system. These ROS can oxidatively degrade the highly hydrophilic substances and protein-like fluorescent components within the extracellular stratification, thereby extremely beneficial for the release of bound water linked with organic biopolymeric molecules.
Considering the changes in sludge properties and the reducing property of TA, it is reasonable to infer that TA may accelerate the conversion of Fe3+ to Fe2+, generate H2O2, and enhance oxidation capacity by free radicals in Fe2+/H2O2 for the decomposition of sludge substrates [59]. To better reveal the role of TA, the concentrations of Fe2+, H2O2, and •OH were monitored, which are illustrated in Figure 14. It was found that added Fe2+ was rapidly consumed by 68.0% within 2.5 min and continually decreased, which was attributed to its hydrolysis via Equation (4) and the quick reaction with O2 via Equation (5) but very few regenerations of Fe2+ [60]. The Fe2+ concentration rose by the reducibility of TA, and TA was oxidized to quinine (Q) via Equation (6) [61]. TA also can chelate ferrous and ferric iron for higher solubility, further benefiting the regenerations of free radicals via Equations (7) and (8). Noteworthily, Fe2+ concentration fluctuated over time, indicating a dynamic balance with the changes in concentrations of Fe2+, H2O2, and •OH. On the other hand, it was reported that Q was capable of generating semiquinone radical (SQ•) via Equation (9) [62]. SQ• is a key substance in the reaction and can degrade organic pollutants through electron transfer or free radical chain reactions in electrochemical processes [63,64]. Therefore, sludge flocs and EPS were constantly being destroyed, releasing more bound water to improve the sludge dewatering performance.
Fe2+ + H2O → Fe(OH)2 + H+
4 Fe2+ + O2 + 4H+ → 4 Fe3+ + 2H2O
Fe3+ + TA → Fe2+ + Q
Fe2+ + S2O82− → Fe3+ + SO4•− + SO42−
SO4•− + H2O → •OH + SO42− + H+
Q + H2O →SQ• + H+

3.4. Mechanism Exploration

According to the above results, the prepared ferric tannate effectively promoted the diminishing of sludge viscosity and the elevation of the zeta potential of sludge particle. These findings reflected that the formed Fe3+–TA complex can compensate for the anionic nature of TA to a certain extent and efficaciously eliminate negatively charged colloidal particles in extracellular space through charge neutralization, hydrophobic interaction, and hydrogen bonding to form insoluble complexes. Then, with adsorption bridging and net capture effects of ferric tannate, the tiny sludge flocs gathered, and particle size apparently increased.
Moreover, during the ferric tannate-enhanced ECC process, the Fenton-like reaction occurred, continuously generating ROS. These ROS directly and indirectly oxidized and destroyed sludge cells and EPS, dramatically enhancing the removal of extracellular proteins and protein-like substances, thereby further reducing the hydrophilic functional groups and strengthening sludge hydrophobicity [65]. Simultaneously, the structure of sludge flocs underwent collapse and reconstruction, ultimately forming smooth, flat surface and orderly morphology, which was better to facilitate the compression of agglomerated matrixes and the release of bound water during the subsequent pressure filtration. The main chemical mechanism of ECC for the enhanced sludge dewaterability is depicted in Figure 15.

4. Conclusions

This study systematically investigated the effects of a novel ferric tannate-enhanced ECC process on sludge dewatering and preliminarily explored the underlying mechanisms. The prepared ferric tannate proved to be an eco-friendly and effective sludge conditioner, which, combined with ECC, showed substantial capacity in improving sludge dewaterability. Under optimal conditions (ferric tannate dosage of 20% TS, voltage of 50 V, and reaction time of 30 min), reductions of 84.3% in CST, 84.2% in SRF, and 17.6% in Wc of dewatered sludge were achieved. The enhanced sludge dewaterability was primarily attributed to the ability of ferric tannate to neutralize surface charges, promote floc growth, and increase sludge hydrophobicity. When combined with ECC, it triggered ROS production, leading to EPS disruption and protein removal. Ultimately, the resulting compact and smooth floc structure can further improve sludge filterability by facilitating the release of bound water during subsequent pressure filtration. Overall, these findings offer new insights into the development of eco-friendly and efficient strategies for sludge conditioning and dewatering.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w17162424/s1, Figure S1: Schematic illustration of the synthesis of ferric tannate and its application in electrochemically enhanced sludge dewatering; Figure S2: Digital images of the experimental setup for ferric tannate-enhanced electrochemical sludge treatment; Table S1: Comparison of EPS contents of various sludge treatment methods in this work.

Author Contributions

Conceptualization, N.Z. and D.G.; methodology, N.Z. and D.G.; validation, Y.Y. and J.F.; formal analysis, Y.Y. and N.Z.; investigation, Y.Y., J.F. and N.Z.; data curation, J.F.; writing—original draft preparation, Y.Y. and J.F.; writing—review and editing, Y.Y., N.Z. and D.G.; visualization, Y.Y. and N.Z.; supervision, N.Z. and D.G.; funding acquisition, D.G. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (22306078), the Natural Science Foundation of Jiangsu Province (BK20230714), the Natural Science Research of Jiangsu Higher Education Institutions of China (23KJB610006), the project of the Leading Innovative Talent in Changzhou city (CQ20230078), and the Postgraduate Practice Innovation Program of Jiangsu University of Technology (XSJCX24_ 90).

Data Availability Statement

The data presented in this study are available from the corresponding author upon reasonable request.

Acknowledgments

The authors express their gratitude to the editors and reviewers for their insightful comments and suggestions.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
EPS Extracellular polymeric substances
ECCElectrochemical conditioning
CSTCapillary suction time
SRFSpecific resistance to filtration
WcWater content
•OHHydroxyl radical
H2O2Hydrogen peroxide
TSTotal solid
VSVolatile solid
S-EPSSlime EPS
LB-EPSLoosely bound EPS
TB-EPSTightly bound EPS
3D-EEMThree-dimensional excitation emission matrix
SEMScanning electron microscope
FTIRFourier transform infrared
ROSReactive oxygen species
TATannic acid
PPCPolyphenolic compound
ORROxygen reduction reaction
QQuinine
SQ•Semiquinone radical

References

  1. Hernández-Chover, V.; Bellver-Domingo, Á.; Hernández-Sancho, F. Efficiency of wastewater treatment facilities: The influence of scale economies. J. Environ. Manag. 2018, 228, 77–84. [Google Scholar] [CrossRef]
  2. He, S.; Feng, L.; Zhao, W.; Li, J.; Zhao, Q.; Wei, L. Composition and molecular structure analysis of hydrophilic/hydrophobic extracellular polymeric substances (EPS) with impacts on sludge dewaterability. Chem. Eng. J. 2023, 462, 142234. [Google Scholar] [CrossRef]
  3. Mądzielewska, W.I.; Jachimowicz, P.; Otieno, J.O.; Cydzik-Kwiatkowska, A. Impact of Tire-Derived Microplastics on Microbiological Activity of Aerobic Granular Sludge. Int. J. Mol. Sci. 2025, 26, 4136. [Google Scholar] [CrossRef]
  4. Feng, J.; Yu, Y.; Huang, S.; Zhu, N.; Mojiri, A.; Ge, D. Tannic acid as a green chemical for the removal of various heavy metals: A critical review of recent developments. J. Environ. Manag. 2025, 375, 124390. [Google Scholar] [CrossRef]
  5. Yang, M.; Hasaer, B.; Bai, Y.; Liu, R.; Hu, C.; Qu, J. Using activated peroxymonosulfate by electrochemically generated FeII for conditioning and dewatering of anaerobically digested sludge. Chem. Eng. J. 2020, 391, 123603. [Google Scholar] [CrossRef]
  6. Yuan, H.; Zhu, N.; Song, L. Conditioning of sewage sludge with electrolysis: Effectiveness and optimizing study to improve dewaterability. Bioresour. Technol. 2010, 101, 4285–4290. [Google Scholar] [CrossRef] [PubMed]
  7. Hu, S.; Zhao, W.; Hu, J.; Liu, B.; Wang, D.; Zhu, Q.; Yang, J.; Hou, H. Integration of electrochemical and calcium hypochlorite oxidation for simultaneous sludge deep dewatering, stabilization and phosphorus fixation. Sci. Total Environ. 2021, 750, 141408. [Google Scholar] [CrossRef] [PubMed]
  8. Masihi, H.; Gholikandi, G.B. Employing Electrochemical-Fenton process for conditioning and dewatering of anaerobically digested sludge: A novel approach. Water Res. 2018, 144, 373–382. [Google Scholar] [CrossRef]
  9. Cai, M.; Wang, Q.; Wells, G.; Dionysiou, D.D.; Song, Z.; Jin, M.; Hu, J.; Ho, S.-H.; Xiao, R.; Wei, Z. Improving dewaterability and filterability of waste activated sludge by electrochemical Fenton pretreatment. Chem. Eng. J. 2019, 362, 525–536. [Google Scholar] [CrossRef]
  10. Feng, C.; Zhang, H.; Guo, J.; Yu, S.-Y.; Luo, M.; Zhang, J.; Ren, Y.; Liu, Y.; Zhou, P.; He, C.-S. Boosted H2O2 utilization and selective hydroxyl radical generation for water decontamination: Synergistic roles of dual active sites in H2O2 activation. Water Res. 2024, 267, 122453. [Google Scholar] [CrossRef]
  11. Fu, M.; Jiang, X.-h.; Wang, M.; Fan, Z.-n.; Gu, Y.-l.; Zou, R.-h.; Zhao, L.-l.; Liu, L. Catalase catalyzed tannic acid-Fe3+ network coating: A theranostic strategy for intestinal barrier restoration. Int. J. Biol. Macromol. 2024, 274, 133304. [Google Scholar] [CrossRef]
  12. Qin, Z.; Huang, Y.; Xiao, S.; Zhang, H.; Lu, Y.; Xu, K. Preparation and characterization of high mechanical strength chitosan/oxidized tannic acid composite film with Schiff Base and hydrogen bond crosslinking. Int. J. Mol. Sci. 2022, 23, 9284. [Google Scholar] [CrossRef] [PubMed]
  13. Zhao, X.-L.; Tian, P.-X.; Li, Y.-D.; Zeng, J.-B. Biobased covalent adaptable networks: Towards better sustainability of thermosets. Green Chem. 2022, 24, 4363–4387. [Google Scholar] [CrossRef]
  14. Wang, N.; Liu, B.; Han, X.; Wang, L.; Yang, F.; Wang, T.; Yu, D. Tannic acid/non-covalent interaction-mediated modification of hemp seed proteins: Focused on binding mechanisms, oil-water interface behavior, and rheological properties. Food Hydrocoll. 2024, 157, 110483. [Google Scholar] [CrossRef]
  15. Zuliani, L.; Frison, N.; Jelic, A.; Fatone, F.; Bolzonella, D.; Ballottari, M. Microalgae cultivation on anaerobic digestate of municipal wastewater, sewage sludge and agro-waste. Int. J. Mol. Sci. 2016, 17, 1692. [Google Scholar] [CrossRef]
  16. Zhang, L.; Guan, Q.; Jiang, J.; Khan, M.S. Tannin complexation with metal ions and its implication on human health, environment and industry: An overview. Int. J. Biol. Macromol. 2023, 253, 127485. [Google Scholar] [CrossRef]
  17. Nguyen-Thi, K.-S.; Fareed, H.; Le-Thi, A.-D.; Lee, Y.; Kang, S.; Kim, I.S. Leveraging tannic acid-ferric iron (TA/Fe3+) complexation in polyamide selective layer for synthesizing robust thin film composite reverse osmosis (TFC-RO) hollow fiber membranes for saline water treatment. Sep. Purif. Technol. 2024, 348, 127669. [Google Scholar] [CrossRef]
  18. Castro, J.; López-Maldonado, J.; Cárdenas, J.; Orozco, G.; Bustos, E.; Rivera, F. Design of an electrochemical flow reactor prototype to the electro-oxidation of amoxicillin in aqueous media using modified electrodes with transition metal oxides. J. Environ. Chem. Eng. 2022, 10, 107165. [Google Scholar] [CrossRef]
  19. Liu, J.; Yang, Q.; Wang, D.; Li, X.; Zhong, Y.; Li, X.; Deng, Y.; Wang, L.; Yi, K.; Zeng, G. Enhanced dewaterability of waste activated sludge by Fe (II)-activated peroxymonosulfate oxidation. Bioresour. Technol. 2016, 206, 134–140. [Google Scholar] [CrossRef]
  20. Zemfira, T.; Milanovskiy, E. The contact angle of wetting of the solid phase of soil before and after chemical modification. Eurasian J. Soil Sci. 2015, 4, 191–197. [Google Scholar] [CrossRef]
  21. Li, H.; Song, L.; Han, B.; Song, H.; Bai, R.; Li, H.; Wang, Q.; Lin, Z.; Hu, W. Highly efficient enhancement of municipal sludge dewaterability using persulfate activation with nZVI/HA. Water Sci. Technol. 2019, 79, 1309–1315. [Google Scholar] [CrossRef]
  22. Yang, X.; Zhang, L.; Li, S.; Zhang, H.; Zhang, S.; Wan, Y.; Yu, H. Fast start-up of partial nitrification for high-ammonia wastewater treatment using zeolite with in-situ bioregeneration. J. Water Process Eng. 2024, 59, 105077. [Google Scholar] [CrossRef]
  23. Zhang, W.-H.; Wu, J.; Weng, L.; Zhang, H.; Zhang, J.; Wu, A. An improved phenol-sulfuric acid method for the determination of carbohydrates in the presence of persulfate. Carbohydr. Polym. 2020, 227, 115332. [Google Scholar] [CrossRef]
  24. Wang, S.; Liu, Z.; Yang, M.; Zhou, Y.; Yang, M.; Long, M.; Fang, F.; Guo, J. The differences in characteristics of extracellular polymeric substances of flocs and anammox granules impacted aggregation. Bioprocess. Biosyst. Eng. 2021, 44, 1711–1720. [Google Scholar] [CrossRef] [PubMed]
  25. Divjak, B.; Franko, M.; Novič, M. Determination of iron in complex matrices by ion chromatography with UV–Vis, thermal lens and amperometric detection using post-column reagents. J. Chromatogr. A 1998, 829, 167–174. [Google Scholar] [CrossRef]
  26. Ye, C.; Liu, P.; Ma, Z.; Xue, C.; Zhang, C.; Zhang, Y.; Liu, J.; Liu, C.; Sun, X.; Mu, Y. High H2O2 concentrations observed during haze periods during the winter in Beijing: Importance of H2O2 oxidation in sulfate formation. Environ. Sci. Technol. Lett. 2018, 5, 757–763. [Google Scholar] [CrossRef]
  27. Wang, G.; Ge, D.; Bai, L.; Dong, Y.; Bian, C.; Xu, J.; Zhu, N.; Yuan, H. Insight into the roles of electrolysis-activated persulfate oxidation in the waste activated sludge dewaterability: Effects and mechanism. J. Environ. Manag. 2021, 297, 113342. [Google Scholar] [CrossRef]
  28. Fu, Z.; Chen, R. Study of complexes of tannic acid with Fe (III) and Fe (II). J. Anal. Methods Chem. 2019, 2019, 3894571. [Google Scholar] [CrossRef] [PubMed]
  29. Zhang, R.; Li, L.; Liu, J. Synthesis and characterization of ferric tannate as a novel porous adsorptive-catalyst for nitrogen removal from wastewater. RSC Adv. 2015, 5, 40785–40791. [Google Scholar] [CrossRef]
  30. Morimune-Moriya, S.; Nagata, Y. Tough polyimide nanocomposites with high thermal resistance at extremely low filler content. Polym. Compos. 2025. [Google Scholar] [CrossRef]
  31. Tomma, J.; Abbas, S.; Al-Dujaili, A. New Quinolin-2-one, Indazole, and Benzisoxazole Derivatives Derived from Chalcones: Synthesis, Characterization, and Biological Activity. Russ. J. Org. Chem. 2023, 59, 1027–1032. [Google Scholar] [CrossRef]
  32. Hamimed, M.Y.; Belfilali, I.; Guenfoud, F.; Roisnel, T. Efficient heavy metal removal from water using a novel thiosemicarbazone-based ligand. J. Coord. Chem. 2025, 78, 1284–1300. [Google Scholar] [CrossRef]
  33. Januário, E.F.D.; Vidovix, T.B.; Araújo, L.A.d.; Bergamasco Beltran, L.; Bergamasco, R.; Vieira, A.M.S. Investigation of Citrus reticulata peels as an efficient and low-cost adsorbent for the removal of safranin orange dye. Environ. Technol. 2022, 43, 4315–4329. [Google Scholar] [CrossRef]
  34. Drescher, D.; Büchner, T.; McNaughton, D.; Kneipp, J. SERS reveals the specific interaction of silver and gold nanoparticles with hemoglobin and red blood cell components. Phys. Chem. Chem. Phys. 2013, 15, 5364–5373. [Google Scholar] [CrossRef] [PubMed]
  35. Zhang, Y.; Yang, F.; Qin, S.; Huang, J.; Yang, X.; Wang, W.; Li, Y.; Wu, C.; Shao, L. Deprotonated tannic acid regulating pyrrole polymerization to enhance nanofiltration performance for molecular separations under both aqueous and organic solvent environments. Sep. Purif. Technol. 2022, 298, 121647. [Google Scholar] [CrossRef]
  36. Scholz, M. Review of recent trends in capillary suction time (CST) dewaterability testing research. Ind. Eng. Chem. Res. 2005, 44, 8157–8163. [Google Scholar] [CrossRef]
  37. Cobbledick, J.; Nguyen, A.; Latulippe, D.R. Demonstration of FBRM as process analytical technology tool for dewatering processes via CST correlation. Water Res. 2014, 58, 132–140. [Google Scholar] [CrossRef] [PubMed]
  38. Liu, Z.; Luo, F.; He, L.; Wang, S.; Wu, Y.; Chen, Z. Physical conditioning methods for sludge deep dewatering: A critical review. J. Environ. Manag. 2024, 360, 121207. [Google Scholar] [CrossRef] [PubMed]
  39. Mu’azu, N.D.; Zubair, M.; Jarrah, N.; Alagha, O.; Al-Harthi, M.A.; Essa, M.H. Sewage sludge ZnCl2-activated carbon intercalated MgFe–LDH nanocomposites: Insight of the sorption mechanism of improved removal of phenol from water. Int. J. Mol. Sci. 2020, 21, 1563. [Google Scholar] [CrossRef]
  40. Tan, X.; Zeng, S.; Chen, Z.; Lv, M.; Tang, X.; He, X.; Chen, Y.; Wan, Y.; Zhang, J. Synergistic effect of organic matter-floc size-bound water and multifactorial quantitative model of optimal reagent demand in sewage sludge conditioning process prior to dewatering. Water Res. 2024, 251, 121108. [Google Scholar] [CrossRef]
  41. Jiang, Y.; Gao, F.; Zhang, N.; Li, J.; Xu, M.; Jiang, Y. Dehydration performance of municipal sludge and its dewatering conditioning methods: A review. Ind. Eng. Chem. Res. 2023, 62, 11337–11357. [Google Scholar] [CrossRef]
  42. Lu, W.; Chen, N.; Feng, C.; Deng, Y.; Zhang, J.; Chen, F. Treatment of polluted river sediment by electrochemical oxidation: Changes of hydrophilicity and acute cytotoxicity of dissolved organic matter. Chemosphere 2020, 243, 125283. [Google Scholar] [CrossRef] [PubMed]
  43. Zhang, W.; Dai, X.; Dong, B.; Dai, L. New insights into the effect of sludge proteins on the hydrophilic/hydrophobic properties that improve sludge dewaterability during anaerobic digestion. Water Res. 2020, 173, 115503. [Google Scholar] [CrossRef]
  44. Yang, J.; Zhang, Z.; Hao, Z.; Liu, X.; Tian, X.; Wang, D. Molecular mechanisms of interaction between protease and humic acid of waste activated sludge. J. Environ. Manag. 2025, 389, 126229. [Google Scholar] [CrossRef]
  45. Chen, Y.; Wei, Q.; Chen, Y.; Feng, A.; Zhang, W. Enhancement of hydrogen bonds between proteins and polyphenols through magnetic field treatment: Structure, interfacial properties, and emulsifying properties. Food Res. Int. 2024, 192, 114822. [Google Scholar] [CrossRef]
  46. Dong, Y.; Yuan, H.; Ge, D.; Zhu, N. A novel conditioning approach for amelioration of sludge dewaterability using activated carbon strengthening electrochemical oxidation and realized mechanism. Water Res. 2022, 220, 118704. [Google Scholar] [CrossRef]
  47. Liu, X.; Zhai, Y.; Xu, Z.; Liu, L.; Ren, W.; Xie, Y.; Li, C.; Zhu, Y.; Xu, M. Unraveling the impacts of Cu+-based treatments on sludge dewaterability: The overlooked role of Cu3+. Chem. Eng. J. 2023, 457, 141106. [Google Scholar] [CrossRef]
  48. Zhou, D.; Liang, M.; Xia, Y.; Li, C.; Huang, M.; Peng, S.; Huang, Y. Reduction mechanisms of V5+ by vanadium-reducing bacteria in aqueous environments: Role of different molecular weight fractionated extracellular polymeric substances. Sci. Total Environ. 2022, 852, 158394. [Google Scholar] [CrossRef]
  49. Yuan, H.-p.; Cheng, X.-b.; Chen, S.-p.; Zhu, N.-w.; Zhou, Z.-y. New sludge pretreatment method to improve dewaterability of waste activated sludge. Bioresour. Technol. 2011, 102, 5659–5664. [Google Scholar] [CrossRef]
  50. Yu, G.-H.; He, P.-J.; Shao, L.-M. Novel insights into sludge dewaterability by fluorescence excitation–emission matrix combined with parallel factor analysis. Water Res. 2010, 44, 797–806. [Google Scholar] [CrossRef] [PubMed]
  51. Li, Y.; Pan, L.; Zhu, Y.; Yu, Y.; Wang, D.; Yang, G.; Yuan, X.; Liu, X.; Li, H.; Zhang, J. How does zero valent iron activating peroxydisulfate improve the dewatering of anaerobically digested sludge? Water Res. 2019, 163, 114912. [Google Scholar] [CrossRef] [PubMed]
  52. Chen, W.; Westerhoff, P.; Leenheer, J.A.; Booksh, K. Fluorescence excitation− emission matrix regional integration to quantify spectra for dissolved organic matter. Environ. Sci. Technol. 2003, 37, 5701–5710. [Google Scholar] [CrossRef] [PubMed]
  53. Lv, H.; Liu, D.; Zhang, Y.; Yuan, D.; Wang, F.; Yang, J.; Wu, X.; Zhang, W.; Dai, X. Effects of temperature variation on wastewater sludge electro-dewatering. J. Cleaner Prod. 2019, 214, 873–880. [Google Scholar] [CrossRef]
  54. Xiao, K.; Seow, W.Y.; Chen, Y.; Lu, D.; Jiang, X.; Zhou, Y. Effects of thermal-Fe (II) activated oxone treatment on sludge dewaterability. Chem. Eng. J. 2017, 322, 463–471. [Google Scholar] [CrossRef]
  55. Song, G.; Zhao, S.; Wang, J.; Zhao, K.; Zhao, J.; Liang, H.; Liu, R.; Li, Y.-Y.; Hu, C.; Qu, J. Enzyme-enhanced acidogenic fermentation of waste activated sludge: Insights from sludge structure, interfaces, and functional microflora. Water Res. 2024, 249, 120889. [Google Scholar] [CrossRef]
  56. Yu, W.; Wen, Q.; Yang, J.; Xiao, K.; Zhu, Y.; Tao, S.; Lv, Y.; Liang, S.; Fan, W.; Zhu, S. Unraveling oxidation behaviors for intracellular and extracellular from different oxidants (HOCl vs. H2O2) catalyzed by ferrous iron in waste activated sludge dewatering. Water Res. 2019, 148, 60–69. [Google Scholar] [CrossRef]
  57. Yusuf, M.O. Bond characterization in cementitious material binders using Fourier-transform infrared spectroscopy. Appl. Sci. 2023, 13, 3353. [Google Scholar] [CrossRef]
  58. Powell, H.K.J.; Taylor, M.C. Interactions of iron (II) and iron (III) with gallic acid and its homologues: A potentiometric and spectrophotometric study. Aust. J. Chem. 1982, 35, 739–756. [Google Scholar] [CrossRef]
  59. Qu, J.; Li, C.; Xu, J.; Liu, S.; Wang, Q.; Tang, Z.; Sun, H.; Liang, J.; Ma, P.a.; Yang, P. Natural Tannin and Upconversion Photons Co-Potentiate Fe Fenton Anticancer Therapy. Adv. Funct. Mater. 2025, 2503641. [Google Scholar] [CrossRef]
  60. Song, Y.; Yang, L.; Wang, H.; Sun, X.; Bai, S.; Wang, N.; Liang, J.; Zhou, L. The coupling reaction of Fe2+ bio-oxidation and resulting Fe3+ hydrolysis drastically improve the formation of iron hydroxysulfate minerals in AMD. Environ. Technol. 2021, 42, 2325–2334. [Google Scholar] [CrossRef] [PubMed]
  61. Bolobajev, J.; Trapido, M.; Goi, A. Interaction of tannic acid with ferric iron to assist 2, 4, 6-trichlorophenol catalytic decomposition and reuse of ferric sludge as a source of iron catalyst in Fenton-based treatment. Appl. Catal. B 2016, 187, 75–82. [Google Scholar] [CrossRef]
  62. Taguchi, K.; Fujii, S.; Yamano, S.; Cho, A.K.; Kamisuki, S.; Nakai, Y.; Sugawara, F.; Froines, J.R.; Kumagai, Y. An approach to evaluate two-electron reduction of 9, 10-phenanthraquinone and redox activity of the hydroquinone associated with oxidative stress. Free Radical Biol. Med. 2007, 43, 789–799. [Google Scholar] [CrossRef]
  63. Yang, J.; Zhang, Y.; Lu, Y.; Wang, L.; Lu, F.; Zhong, D. Ultrafast dynamics of nonequilibrium short-range electron transfer in semiquinone flavodoxin. J. Phys. Chem. Lett. 2022, 13, 3202–3208. [Google Scholar] [CrossRef] [PubMed]
  64. Ge, D.; Dong, Y.; Zhang, W.; Yuan, H.; Zhu, N. A novel Fe2+/persulfate/tannic acid process with strengthened efficacy on enhancing waste activated sludge dewaterability and mechanism insight. Sci. Total Environ. 2020, 733, 139146. [Google Scholar] [CrossRef] [PubMed]
  65. de Freitas, V.; Mateus, N. Protein/polyphenol interactions: Past and present contributions. Mechanisms of astringency perception. Curr. Org. Chem. 2012, 16, 724–746. [Google Scholar] [CrossRef]
Water 17 02424 g001
Figure 1. Effects of (a) tannic acid-to-ferric ion molar ratio and (b) pH values on the yield of ferric tannate complexes.
Figure 1. Effects of (a) tannic acid-to-ferric ion molar ratio and (b) pH values on the yield of ferric tannate complexes.
Water 17 02424 g001
Water 17 02424 g002
Figure 2. FTIR spectra of tannic acid and ferric tannate.
Figure 2. FTIR spectra of tannic acid and ferric tannate.
Water 17 02424 g002
Water 17 02424 g003
Figure 3. SEM (×10,000) images of (a) ferric tannate without pH adjustment and (b) ferric tannate prepared under the optimal conditions.
Figure 3. SEM (×10,000) images of (a) ferric tannate without pH adjustment and (b) ferric tannate prepared under the optimal conditions.
Water 17 02424 g003
Water 17 02424 g004
Figure 4. (a) CST diagrams of conditioned sludge with different ferric tannate dosages at 50 V voltage and (b) CST plots of sludge conditioned with different voltages at a dosage of 20%TS ferric tannate.
Figure 4. (a) CST diagrams of conditioned sludge with different ferric tannate dosages at 50 V voltage and (b) CST plots of sludge conditioned with different voltages at a dosage of 20%TS ferric tannate.
Water 17 02424 g004
Water 17 02424 g005
Figure 5. The ferric tannate-enhanced ECC process on (a) SRF and (b) Wc of dewatered sludge cake.
Figure 5. The ferric tannate-enhanced ECC process on (a) SRF and (b) Wc of dewatered sludge cake.
Water 17 02424 g005
Water 17 02424 g006
Figure 6. Effects of various treatment processes on the viscosity.
Figure 6. Effects of various treatment processes on the viscosity.
Water 17 02424 g006
Water 17 02424 g007
Figure 7. Effects of various treatment processes on the zeta potential.
Figure 7. Effects of various treatment processes on the zeta potential.
Water 17 02424 g007
Water 17 02424 g008
Figure 8. Effects of various treatment processes on the particle size of sludge.
Figure 8. Effects of various treatment processes on the particle size of sludge.
Water 17 02424 g008
Water 17 02424 g009
Figure 9. The contact angles of the (a) raw sludge, (b) ECC treated sludge, (c) sludge after ferric tannate conditioning, and (d) sludge after ferric tannate-enhanced ECC treatment.
Figure 9. The contact angles of the (a) raw sludge, (b) ECC treated sludge, (c) sludge after ferric tannate conditioning, and (d) sludge after ferric tannate-enhanced ECC treatment.
Water 17 02424 g009
Water 17 02424 g010
Figure 10. Effects of various treatment processes on the (a) protein and (b) polysaccharide in EPS.
Figure 10. Effects of various treatment processes on the (a) protein and (b) polysaccharide in EPS.
Water 17 02424 g010
Water 17 02424 g011
Figure 11. Fluorescence spectral characteristics of (a1) S-EPS, (a2) LB-EPS, and (a3) TB-EPS of raw sludge and (b1) S-EPS, (b2) LB-EPS, and (b3) TB-EPS of sludge after ferric tannate-enhanced ECC treatment.
Figure 11. Fluorescence spectral characteristics of (a1) S-EPS, (a2) LB-EPS, and (a3) TB-EPS of raw sludge and (b1) S-EPS, (b2) LB-EPS, and (b3) TB-EPS of sludge after ferric tannate-enhanced ECC treatment.
Water 17 02424 g011
Water 17 02424 g012
Figure 12. FTIR spectra of raw sludge, ECC treated sludge, sludge after ferric tannate conditioning, and sludge after ferric tannate-enhanced ECC treatment.
Figure 12. FTIR spectra of raw sludge, ECC treated sludge, sludge after ferric tannate conditioning, and sludge after ferric tannate-enhanced ECC treatment.
Water 17 02424 g012
Water 17 02424 g013
Figure 13. SEM (×10,000) images of (a) raw sludge and (b) sludge after ferric tannate-enhanced ECC treatment.
Figure 13. SEM (×10,000) images of (a) raw sludge and (b) sludge after ferric tannate-enhanced ECC treatment.
Water 17 02424 g013
Water 17 02424 g014
Figure 14. The contents of (a) Fe2+, (b) H2O2, and (c) •OH in ferric tannate-enhanced ECC system.
Figure 14. The contents of (a) Fe2+, (b) H2O2, and (c) •OH in ferric tannate-enhanced ECC system.
Water 17 02424 g014
Water 17 02424 g015
Figure 15. The main chemical mechanism of ECC for the enhanced sludge dewaterability.
Figure 15. The main chemical mechanism of ECC for the enhanced sludge dewaterability.
Water 17 02424 g015
Table 1. The main characteristics of raw sludge.
Table 1. The main characteristics of raw sludge.
SampleTS (g/L)VS (g/L)CST (S)SRF (m/kg)pHWc of Dewatered Sludge Cake (%)
Raw sludge32.0 ± 0.420.1 ± 0.4183 ± 3.534.6 ± 0.26.98 ± 0.0394.6 ± 0.2
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Yu, Y.; Feng, J.; Zhu, N.; Ge, D. Ferric Tannate-Enhanced Electrochemical Conditioning Process for Improving Sludge Dewaterability. Water 2025, 17, 2424. https://doi.org/10.3390/w17162424

AMA Style

Yu Y, Feng J, Zhu N, Ge D. Ferric Tannate-Enhanced Electrochemical Conditioning Process for Improving Sludge Dewaterability. Water. 2025; 17(16):2424. https://doi.org/10.3390/w17162424

Chicago/Turabian Style

Yu, Yalin, Junkun Feng, Nanwen Zhu, and Dongdong Ge. 2025. "Ferric Tannate-Enhanced Electrochemical Conditioning Process for Improving Sludge Dewaterability" Water 17, no. 16: 2424. https://doi.org/10.3390/w17162424

APA Style

Yu, Y., Feng, J., Zhu, N., & Ge, D. (2025). Ferric Tannate-Enhanced Electrochemical Conditioning Process for Improving Sludge Dewaterability. Water, 17(16), 2424. https://doi.org/10.3390/w17162424

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Article metric data becomes available approximately 24 hours after publication online.
Back to TopTop