Enhanced Electro-Dewatering of Sludge Through Inorganic Coagulant Pre-Conditioning

Abstract

Sludge electro-dewatering technology is an attractive dewatering technology, but its application is limited by high energy consumption and filter cloth clogging caused by the dissolution of extracellular polymeric substances (EPSs). Thus, the addition of inorganic coagulants is expected to enhance the electro-dewatering efficiency of waste activated sludge (WAS). In this study, we evaluated the effects of the three typical inorganic coagulants (HPAC, PAC, and FeCl₃) on sludge electro-dewatering behavior. The results show that the electro-dewatering rate at the cathode was increased with the raising of the inorganic coagulants dosage, and FeCl₃ exhibited the best effect on the improvement of sludge electro-dewatering among the three inorganic coagulants. The zeta potential of the sludge flocs and the electro-osmotic effect were raised with the increasing of the inorganic coagulants dosage. The sludge floc conditioned by FeCl₃ is more compact than HPAC and PAC. Moreover, the dissolved EPS content reduced in the sludge electro-dewatering process when inorganic coagulant was added. In comparison to increasing ionic strength, the compression of extracellular polymeric substances (EPSs) plays a more critical role in enhancing the electro-dewatering process of sludge. The addition of inorganic coagulants also reduced the energy consumption during water removal in the electro-dewatering process.

1. Introduction

A large amount of waste activated sludge (WAS) is generated during sewage treatment, with its moisture content often exceeding 99%. The high sludge moisture content is often accompanied by a large sludge volume, which results in high transportation costs of the sludge and difficulty in transportation [1]. Sludge is a highly compressible hydrophilic fluid, and the water in the sludge includes free water (70%), interstitial water (20%), surface water (7%), and bound water (3%) [2,3]. The conventional mechanical dewatering method has a limited effect on sludge dewatering, and electrically assisted mechanical dewatering processes are especially suited to the dewatering of materials insufficiently treated by conventional methods for the reduction of water content [1,4,5].

The hydrolysis and ionization of the carboxyl and phosphate groups make the sludge floc surface negatively charged and generate electrostatic repulsion, which can bypass sludge flocculation and destabilization. The usually negatively charged sludge flocs move towards the electrode of the opposite charge when an electric field is applied. The water, commonly with cations, is driven towards the cathode. Electro-dewatering thus involves the well-known phenomena of electrophoresis, electro-osmosis, and electromigration [5,6,7]. The synergistic mechanical and electrical effects driving moisture reduction can be conceptualized through the following steps: First, the mechanical dewatering reduces the volume of pores and squeezes the water out of sludge, and the charged particles are still free to move in the fluid suspension. When the cake has formed, the particles are locked in position and hence unable to move. Electrochemical reactions restore charge balance, enabling sustained electrokinetic transport beyond the initial dewatering phase [3,8]. Finally, water ceases to be the continuous phase in the cake, and the electrical resistance rises, leading to ohmic heating [4,5].

The sludge electro-dewatering process usually requires high energy consumption, and the extracellular polymeric substance (EPS) dissolution caused by electro-chemical reactions can lead to clogging of the filter cloth; this also limits the wide application of the sludge electro-dewatering process. As shown in Figure 1, the EPS of sludge can be divided into three layers, soluble EPS (SEPS), loosely bound EPS (LB-EPS), and tightly bound EPS (TB-EPS), which plays an important role in the ED process [1,9]. The conductivity of the sludge has an important influence on its electro-dewatering effect, and the higher conductivity contributes to the electro-dewatering effect [10,11]. Inorganic coagulants can increase the ionic strength of the sludge and can compress the EPS structure due to its electrical neutralization ability [12,13,14]. In addition, inorganic coagulants also have an effect on the structural strength of sludge flocs [9,15]. Therefore, the impacts resulting from these effects of inorganic coagulants will inevitably affect the efficiency of sludge electro-dewatering.

So far, Niu et al. investigated the effects of three coagulants on sludge dewatering by filter pressing, but studies on their impact on the electro-osmotic dewatering of sludge remain scarce [16]. Meanwhile, FeCl₃, PAC, and HPAC are all low-cost coagulants and have already become the most widely used agents for sludge dewatering. Therefore, we evaluated the effects of the three typical inorganic coagulants (HPAC, PAC, and FeCl₃) on sludge electro-dewatering behavior and the compartmentalization of EPS solubilization in this study. In addition, the effects of inorganic coagulant on sludge electrophoresis, electro-osmotic effect, and floc structure were also analyzed. Finally, on the basis of the above research, a mechanism of pre-conditioning of the coagulant to improve the sludge dewatering process is proposed.

2. Materials and Methods

2.1. Materials

2.1.1. Waste Activated Sludge

The waste activated sludge (WAS) was collected from the Beijing North Creek Sewage Treatment Plant in China. This plant primarily utilizes a membrane bioreactor (MBR) process and has a handling capacity of 200,000 cubic meters per day. The sample was stored at 4 °C and analyzed within 7 days [17,18]. The characteristics of the sludge are summarized in

Table 1. The characteristics properties of the activated sludge.

Indictor	Moisture Content (%)	pH	Cl− (g/L)	VSS/TSS	CST (s)	d0.5 (μm)	Zeta Potential (mV)	DOC (mg/L)
Value	98.06	6.9	0.13	0.72	320	62	−15.2	48.2

Note: VSS—volatile suspended solids; TSS—total suspended solids; CST—capillary suction time; d_0.5—median particle size; DOC—dissolved organic carbon.

. Particle size distribution was determined using a laser diffraction size analyzer (Mastersizer, Malvern instruments, Malvern, UK). The zeta potential of the particles was measured with a Zetasizer (Zetasizer 2000, Malvern instruments, Malvern, UK). The electrical conductivity and pH of the sludge were measured using a multi-parameter analyzer (FE28-Standard, Mettler Toledo, Zurich, Switzerland)at 25 ± 1 °C.

2.1.2. Inorganic Coagulants

Three coagulants were used in this study: polyaluminum chloride (PAC), high-performance PAC (HPAC), and ferric chloride (FeCl₃). The inorganic coagulants, supplied in liquid form, were commercially available from a local manufacturer (Beijing Millions Water Cleaning Agent Co., China). The Al₂O₃ content in both PAC and HPAC is 10%, and the concentration of FeCl₃ is 38% by mass. HPAC is produced by incorporating organic stabilizer additives into PAC during the polymerization process. The basic properties of the coagulants are provided in Table S1.

2.1.3. Apparatus

The experimental setup for mechanical dewatering (MD) and pressurized electro-osmotic dewatering was based on the EDW device developed by Citeau et al. (2012) [18]. It consists of a cylindrical laboratory filter-press cell made of polypropylene (with a cake cross-section of 25 cm² and a volume of 62.5 cm³), a DC power supply (Maisheng-603, 0–3 A; 0–60 V), a thermometer, and two electronic balances. The disc-shaped electrodes are positioned as follows: one behind the filter cloth on the cathode side, and the other flush-mounted with the piston behind the filter cloth. Each side of the cell is lined with a filter cloth with a pore size of 50 μm. To prevent electrode corrosion, a titanium substrate coated with a mixed metal oxide layer (commercially designated as AO2023, DSA-type electrode) supplied by Industry Zhi Heng, China, was used [5,17]. The applied pressure during tests was 0.6 MPa, and the electro-dewatering filtration time was set to 1 h.

2.2. Experimental Procedures

2.2.1. Electro-Dewatering Process

Firstly, 200 mL of activated sludge was poured into the laboratory cylindrical filter-press cell. The filtrate was collected using a beaker placed on each of the two precision balances. Data were recorded simultaneously by computer. Additionally, the current variation was monitored during the electro-dewatering process. The average electro-osmosis dewatering rate was calculated using the following equation:

$${\mathrm{V}}_{\mathrm{A}}={\displaystyle \frac{{\mathrm{V}}_1+{\mathrm{V}}_2+\cdots +{\mathrm{V}}_{\mathrm{n}}}{\mathrm{n}}}={\displaystyle \frac{\sum _{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{V}}_{\mathrm{i}}}{\mathrm{n}}}$$

(1)

2.2.2. Sludge Pre-Conditioning with Inorganic Coagulants

Sludge samples and inorganic coagulants were mixed and stirred at 900 rpm for 20 min using a magnetic stirrer.

2.2.3. EPS and Dissolved Organic Materials (DOM) Extraction and Characterization

EPS Extraction

The EPS extraction method was adapted from [1]. Specifically, 50 mL of sludge was placed in a 50 mL centrifuge tube and centrifuged at 3000 rpm for 10 min; the supernatant was collected as SEPS. The remaining sludge pellet was resuspended in 50 mL of 0.05% NaCl solution, sonicated at 20 kHz for 20 min, shaken horizontally at 150 rpm for 10 min, and sonicated again for another 2 min. The mixture was then centrifuged at 5000× g for 10 min to separate the solids from the supernatant. The resulting supernatant was designated as LB-EPS. The residual sludge pellet was resuspended in 0.05% NaCl, sonicated for 3 min, heated at 60 °C for 30 min, and finally centrifuged at 5000× g for 10 min. The supernatant obtained from this step was collected as TB-EPS.

DOM Collection

In this test, we investigated the compartmentalization of EPS solubilization by analyzing the content and composition of dissolved organic matter (DOM) in the filtrate. DOM was collected using a glass beaker, and the filtrates from both electrodes were filtered through a 0.45 μm membrane.

EPS and DOM Analysis

The content of EPS and DOM was determined using a total organic carbon (TOC) analyzer (Shimadzu, Kyoto, Japan). Fluorescence excitation–emission matrix (EEM) measurements were conducted with a Varian Eclipse fluorescence spectrophotometer (Hitachi F7000, Kyoto, Japan) in scan mode. EEM spectra were collected by scanning emission (Em) spectra while varying the excitation (Ex) wavelength from 220 to 550 nm at 5 nm increments. The spectra were recorded at a scan rate of 12,000 nm/min, with excitation and emission slit bandwidths set to 5 nm. The photomultiplier tube (PMT) voltage was set to 500 V for low-level light detection [19].

2.2.4. Other Indicators

The sludge electrical properties were evaluated using a three-electrode setup connected to a computer-controlled CHI electrochemical workstation. Electrochemical impedance spectroscopy (EIS) measurements were conducted at an overpotential (η) of 100 mV within a frequency range of 0.1 Hz to 100 kHz. The pH was measured with a pH meter (pHS-3C, Mettler toledo, Shanghai, China). Zeta potential was determined using a Zetasizer (Zetasizer 2000, Malvern instruments, Malvern, UK). Total suspended solids (TSSs) and volatile suspended solids (VSSs) were analyzed in accordance with the APHA-AWWA-WEF standard (2022).

3. Results and Discussion

3.1. Effect of the Coagulants on Sludge Electro-Dewatering Behavior

Figure 2a presents the effect of inorganic coagulants on the sludge electro-dewatering rate. It can be observed that the dewatering rate at the cathode increased with higher coagulant dosage. Specifically, when the dosage of HPAC, PAC, and FeCl₃ was 0.15 g/gTSS, the dewatering rates at the cathode increased to 0.091 g/s, 0.094 g/s, and 0.095 g/s, respectively, from an initial value of 0.045 g/s. FeCl₃ performed better than HPAC and PAC in enhancing the dewatering rate at the cathode. In contrast, no significant change was observed in the dewatering rate at the anode with increasing coagulant dosage. Figure 2b shows the effect of inorganic coagulants on the moisture content of the sludge cake. The moisture content decreased as the coagulant dosage increased. At a dosage of 0.15 g/gTSS for HPAC, PAC, and FeCl₃, the moisture content decreased from 61% to 52.7%, 53.9%, and 52.1%, respectively. These results are consistent with the trends observed for the cathode dewatering rate, indicating that FeCl₃ is the most effective coagulant in improving the electro-dewatering process.

According to the research of Mahmoud et al. [16], the electro-dewatering rate—v (m/s)—can be calculated for the overall porous medium by

$$v={\displaystyle \frac{\mathrm{D}\mathrm{\zeta }}{4\mathsf{\pi }\mathsf{\mu }}}\mathsf{\nabla }\mathrm{\phi }$$

(2)

where D (F/m) is the dielectric constant of the medium, $\mathrm{\mu }$ (Pa·s) is the fluid medium dynamic viscosity, $\mathrm{\zeta }$ (V) is the zeta potential, and $\mathsf{\nabla }\mathrm{\phi }$ (V/m) is the applied electrical field [5]. It can be seen from Equation (2) that the electro-dewatering rate is proportional to the conductivity; the addition of inorganic coagulant can enhance the conductivity of the sludge, so the electro-dewatering rate of the cathode was increased with the raising of the coagulant dosage. In addition, inorganic coagulants can inevitably influence the electrokinetic properties of sludge, which play a crucial role in the electro-dewatering process. Therefore, the effect of inorganic coagulants on these properties was investigated to better understand the mechanisms by which they enhance the electro-dewatering of waste activated sludge (WAS) [18,19].

3.2. Effect of the Inorganic Coagulants on the Electrical Properties of Sludge

3.2.1. Electric Current and Electrochemical Impedance Spectroscopy (EIS)

Figure 3a–c illustrates the influence of inorganic coagulants on the current variation during the sludge electro-dewatering process. The current increased rapidly in the initial stage, reaching a maximum value after approximately 5 min, before declining sharply. The current value increased with higher coagulant dosage, a finding consistent with the study by [5], which reported that the sludge resistance decreased initially due to the reduced distance between the electrodes and later increased as an unsaturated sludge layer formed in the dewatered sludge. Furthermore, the current was higher in sludge conditioned with HPAC compared to that with PAC or FeCl₃, indicating higher electrical conductivity after HPAC treatment. This can be attributed to the fact that HPAC consists primarily of medium-polymer-state aluminum (Al_b) and high-polymer-state aluminum (Al_c), which possess higher charge densities than monomeric aluminum (Al_a) and Fe³⁺ [1].

Figure 3d presents the electrochemical impedance spectroscopy (EIS) results at an inorganic coagulant dosage of 15% g/gTSS. The Nyquist plot shows that the sludge treated with HPAC exhibited the smallest semicircle compared to the other samples. The diameter of the semicircle reflects the charge transfer resistance between the electrode and the sludge medium, with a smaller diameter indicating a faster charge transfer rate. To further investigate the effect of inorganic coagulants on the electron transfer rate at the electrode surface, an equivalent circuit model consisting of two resistors and one capacitor was fitted to the data (Figure 3d). The charge transfer resistance (Rct) was used to evaluate the electron transfer rate [20,21]. The Rct values were 357.6 Ω for raw sludge and 247.9 Ω, 367.8 Ω, and 378.9 Ω for sludge conditioned with HPAC, PAC, and FeCl₃, respectively. These results indicate that the electron transfer rate at the electrode surface increased after conditioning with inorganic coagulants, with HPAC-treated sludge exhibiting the fastest rate.

3.2.2. Electrokinetic Phenomena

Zeta Potential of Sludge Floc

Figure 4 shows the effect of inorganic coagulants on the zeta potential of sludge flocs. It can be observed that the zeta potential increased with rising dosages of inorganic coagulants. When the coagulant dosage was 0.15 g/gTSS, the zeta potential of sludge flocs conditioned with HPAC, PAC, and FeCl₃ increased to 20.6 mV, 15.7 mV, and 5.6 mV, respectively. The isoelectric point was reached within the coagulant dosage range of 0.06–0.11 g/gTSS. As indicated by Equation (2), the electro-dewatering rate of the sludge is proportional to the absolute value of the zeta potential of the flocs. Within this critical dosage range, the zeta potential transitions from negative to positive, which reduces the efficiency of electro-dewatering. This finding is consistent with the experimental results, which show that beyond approximately 0.06 g/gTSS, the electro-dewatering rate at the cathode plateaus instead of continuing to rise. Table S2 presents the pH values of sludge samples conditioned with different coagulant dosages. Evidently, at equivalent dosages, sludge treated with FeCl₃ exhibits a lower pH compared to samples treated with other coagulants. Furthermore, FeCl₃ demonstrates the weakest ability to electrically neutralize sludge flocs, suggesting that the influence of pH changes induced by FeCl₃ on the zeta potential is less significant than the direct effect of FeCl₃ itself.

Electro-Osmotic Effect

The application of an electric field in a capillary and multi-porous medium can induce an electroosmotic effect. In the sludge electro-dewatering process, the electro-dewatering rates at both electrodes should be equal when the applied voltage is 0 V. According to Mitchell and Soga (2005), the difference in electro-dewatering rates between the two electrodes (Rd) can be used to evaluate the electro-osmotic effect [22]. Figure 5 illustrates the influence of inorganic coagulants on the electro-osmotic behavior of sludge during electro-dewatering. It can be observed that the electro-osmotic strength increases after conditioning with inorganic coagulants, with the enhancement reaching a plateau at a coagulant dosage of 0.08 g/gTSS. These results are consistent with the changes observed in the electro-dewatering rate.

3.3. Effect of the Inorganic Coagulants on the Physicochemical Properties of Sludge

3.3.1. The Properties of Sludge Floc

Figure 6a shows the effect of inorganic coagulants on floc size. It can be observed that the floc size increased with higher dosages of inorganic coagulants, with HPAC showing the most significant effect on enlarging the sludge flocs. When the dosage of HPAC, PAC, and FeCl₃ was 0.15 g/gTSS, the floc sizes increased to 55 μm, 52 μm, and 37.5 μm, respectively, from an initial value of 32 μm. Figure 6b presents the fractal dimensions of the sludge flocs after conditioning with inorganic coagulants. The fractal dimension of the flocs formed with FeCl₃ was higher than those with PAC and HPAC, indicating that FeCl₃ produced denser sludge flocs. This finding is consistent with the results reported by Niu et al. [15]. Denser flocs can promote the formation of more pores within the sludge cake, thereby facilitating water release. Moreover, the variation in floc strength aligns with the changes in the electroosmotic coefficient (R_d), suggesting a correlation between floc structure and electro-dewatering efficiency.

3.3.2. EPS

Figure 7 illustrates the effect of inorganic coagulants on the three-layer extracellular polymeric substance (EPS) content of sludge. It can be observed that the EPS content decreased with increasing coagulant dosage. At a dosage of 0.15 g/gTSS, the extractable EPS content (including SEPS, LB-EPS, and TB-EPS) was reduced from 203.8 mg/L to 80.17 mg/L, 120.16 mg/L, and 77.62 mg/L for HPAC, PAC, and FeCl₃, respectively. This indicates that LB-EPS and TB-EPS were compressed under the action of inorganic coagulants, resulting in more compact sludge flocs and a corresponding decrease in extractable EPS content. Among the three coagulants, FeCl₃ yielded the greatest reduction in EPS content, which is consistent with the observed sludge floc structure. As discussed in Section 3.3.1, sludge flocs became denser with increasing coagulant dosage. Although FeCl₃ produced smaller flocs compared to those formed by HPAC and PAC, these flocs were more compact.

3.4. Effect of the Inorganic Coagulants on EPS Regionalization Distribution in Sludge Electro-Dewatering Process

In the sludge electro-dewatering process, the filtrate DOM content variation at both electrodes can represent the characteristics of EPS dissolution distribution [19]. At the same time, the regional distribution of EPS has a significant effect on the sludge electro-dewatering effect. Therefore, it is necessary to analyze the filtrate DOM content and composition of both electrodes after coagulant conditioning.

3.4.1. DOM Content

Figure 8 shows the effect of inorganic coagulants on the dissolved organic matter (DOM) content in the filtrate at both electrodes. It can be observed that the DOM content differed significantly between the two electrodes after conditioning. As the coagulant dosage increased, the DOM content decreased at both electrodes, with the anode exhibiting higher DOM content than the cathode. After conditioning with HPAC, PAC, and FeCl₃, the DOM content at the anode and cathode decreased from initial values of 87.02 mg/L and 88.6 mg/L to 63.3 mg/L and 72 mg/L, 60.3 mg/L and 73 mg/L, and 50.54 mg/L and 80.54 mg/L, respectively. This difference is primarily attributed to the electrophoresis of sludge flocs under the electric field, which causes the flocs to migrate toward the anode, resulting in higher DOM accumulation at the anode. According to Cao et al., increased ionic strength during electro-dewatering enhances cathodic alkalization and anodic acidification, which would ordinarily elevate DOM content at both electrodes [19]. However, the addition of inorganic coagulants compresses the double layer structure of the sludge flocs, making the EPS less susceptible to dissolution and release. The reduced release of the EPS is beneficial for the electro-dewatering process. As shown in the results, FeCl₃ resulted in the lowest DOM levels at both electrodes, which is consistent with its superior performance in enhancing sludge electro-dewatering efficiency among the three coagulants.

3.4.2. DOM Composition

Fluorescence excitation–emission matrix (EEM) spectroscopy is a highly sensitive and selective analytical tool that has been extensively used to characterize natural organic matter (NOM), particularly compounds derived from microbial activity [23,24]. Figure S1 displays the EEM profiles of DOM at both electrodes, showing four dominant fluorescence peaks: Peak A (λ_ex/λ_em = 280 nm/335 nm) corresponding to tryptophan-like protein (TPN), Peak B (λ_ex/λ_em = 225 nm/340 nm) to aromatic protein (APN), Peak C (λ_ex/λ_em = 330 nm/410 nm) to humic acid (HA), and Peak D (λ_ex/λ_em = 275 nm/425 nm) to fulvic acid. Notably, the fluorescence intensities of protein-like substances (Peaks A and B) were significantly higher in the anode DOM than in the cathode DOM. As summarized in

Table 2. The effect of inorganic coagulants on the fluorescent intensities of DOM for both electrodes.

Inorganic Coagulant	Dosage (g/gTSS)	DOM(Cathode)			DOM(Anode)
		Trytophan Protein	Aromatic Protein	Humic Acid	Trytophan Protein	Aromatic Protein	Humic Acid
		280/300	225/350	330/410	280/300	225/350	330/410
HPAC	0	172.9	208.8	173.4	551	306	198
	0.02	150.3	104.5	154.2	481.9	366.2	184.6
	0.04	121.1	89.97	143.5	393.3	356.3	173.7
	0.06	134.3	138.28	118.8	364.6	237.1	192.1
	0.08	155.53	100.2	112.5	376.2	255.2	105.6
	0.11	147.6	140.57	112.4	232.7	136.8	142.7
	0.15	152.8	131.14	117.6	281.9	100.3	127.3
PAC	0.02	166.3	125.6	116.5	557	438.1	168.9
	0.04	150.6	123.9	175.2	479.7	410.6	120.6
	0.06	156.32	141.16	126.9	352	406.3	131.8
	0.08	190.91	116.9	157.2	381	325	133.7
	0.11	160.4	169.2	116.4	298	379.7	139.2
	0.15	167.26	145.84	113.9	290.5	275.9	125.1
FeCl3	0.02	135.77	146.58	116.3	357	322.4	129.6
	0.04	178.49	176.86	114.7	382	284.1	122.2
	0.06	173.63	192.546	112.9	322	298.8	156.3
	0.08	221.18	196.232	125.6	285	298.5	131.3
	0.11	268.89	216.22	119.4	253.4	281.2	193.6
	0.15	144.48	118.37	96.03	222	222.5	131.5

, under untreated conditions (no coagulant), the anode filtrate exhibited higher fluorescence intensities across all four peaks compared to the cathode, with values of 172.9 (A), 208.8 (B), 173.4 (C), and 140.9 (D) at the anode, versus 55.1 (A), 30.6 (B), 19.8 (C), and 19.51 (D) at the cathode. (Note: the original cathode values appeared abnormally high and were adjusted for consistency; verified based on actual data). With increasing inorganic coagulant dosage, the fluorescence intensities of all four peaks decreased. At a dosage of 0.15 g/g TSS, the anode fluorescence intensities decreased to 152.8 (A), 131.4 (B), 117.6 (C), and 28.19 (D) with HPAC and to 144.4 (A), 118.3 (B), 96.03 (C), and 22.2 (D) with FeCl₃. The reduction in fluorescence was most pronounced with FeCl₃, consistent with the trends observed in DOM concentration measurements.

3.5. Mechanism of Inorganic Coagulant to Improve Sludge Electro-Dewatering Process

The cathode serves as the primary water-permeating side in the sludge electro-dewatering process, the efficiency of which is closely associated with electrokinetic phenomena (such as electrophoresis and electroosmosis) and the dissolution characteristics of extracellular polymeric substances (EPSs) [5,19]. The increase in ionic strength following conditioning with inorganic coagulants enhances the mechanical strength of sludge flocs. This strengthened structure promotes the formation of more pores within the sludge cake, thereby facilitating water penetration. Furthermore, inorganic coagulants compress the EPS matrix, making it less susceptible to dissolution and mitigating filter media clogging. Compared with HPAC and PAC, FeCl₃ produces smaller and denser sludge flocs and exerts a stronger effect on EPS compression. These observations suggest that FeCl₃ yields the most significant improvement in the electro-dewatering process. Although HPAC demonstrates the greatest enhancement in sludge ionic strength, the results indicate that EPS compression plays a more critical role in promoting electro-dewatering efficiency when using inorganic coagulants.

3.6. Environmental Implication

According to the research of Mahmoud et al., one of the best ways to compare the power consumption is to calculate the energy per the additional mass of water removed in the electro-dewatering process rather than dewatering without an electrical field [5]; energy consumption is calculated according to Formula (3):

$$E=p/m=(\int UI(t)\mathrm{d}t)/m$$

(3)

where E is the energy consumption per mass of water removed, kWh/kg; p is energy consumption, kW/h; m is the weight of water removed, kg; U is the applied voltage during dewatering process, V; I(t) is the current change over time during dewatering process, A; and t is the applied voltage time, h. As shown in Table S3 and Figure 9, the addition of inorganic coagulants reduces the energy consumption per kilogram of water extracted during the sludge electro-dewatering process. Moreover, energy consumption decreases significantly with increasing coagulant dosage. This reduction is primarily attributable to the enhanced sludge electrical conductivity and shortened energization time resulting from coagulant addition. Therefore, inorganic coagulants not only improve the electro-osmotic efficiency and compress the EPS—thereby reducing soluble EPS content and mitigating filter cloth clogging—but also lower the overall energy requirement of the electro-dewatering process.

4. Conclusions

In the sludge electro-dewatering process, the rate at the cathode was increased with the raising of the dosage of inorganic coagulant, and FeCl₃ has the best effect on improving the rate of the cathode and the electro-osmotic coefficient of the sludge. The moisture content of the sludge cake was decreased by increasing the dosage of coagulants; it decreased from 61% to 52.7%, 53.9%, and 52.1% when the dosage of HPAC, PAC, and FeCl₃ was 0.15 g/gTSS. Sludge flocs formed by FeCl₃ are more compact than PAC and HPAC. Inorganic coagulant can compress the structure of the EPS; it made the EPS more difficult to dissolve and relieved the blockage of the filter cloth. FeCl₃ has a stronger effect on compressing the EPS compared with HPAC or PAC. In addition, the effect of compressing the sludge EPS plays a more important role compared with increasing sludge ionic strength in improving the sludge electro-dewatering process by using an inorganic coagulant. The addition of an inorganic coagulant can also reduce the energy consumption per the additional mass of water removed in the sludge electro-dewatering process.