On the fit of experimental methodology for constructive alum sludge conditioning using waste polysaccharide into reuse dewatered sludge in contaminants’ immobilization

3.1.2. Morphological analyses

The surface morphology of the prepared conditioner was investigated using scanning electron microscopy (SEM), as shown in Figure 3. The micrograph at lower magnification (Figure 3a) revealed a heterogeneous structure with irregularly distributed particles embedded within a compact matrix, suggesting successful integration of the polysaccharide crosslinker with iron species. At higher magnification (Figure 3b), the particles appeared as agglomerated clusters with relatively smoother and more spherical domains, which can be attributed to the stabilizing and templating effect of the polysaccharide. Such spherical morphology is advantageous, as it generally provides higher surface area and facilitates greater contact efficiency with sludge flocs. Additionally, the presence of agglomerates also indicates strong interaction between the organic crosslinker and iron particles, which may enhance the structural stability of the conditioner. These morphological features are consistent with previous reports on bio-based polymer-iron composites, where polysaccharide matrices not only regulate particle growth but also prevent excessive aggregation, leading to more uniform and functionalized surfaces. Overall, the SEM analysis confirms that the designed polysaccharide/iron conditioner exhibits a morphology conducive to efficient sludge conditioning and dewatering applications.

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Figure 3.

SEM micrograph of the polysaccharide/iron conditioner in different magnifications. (a) at lower magnification, (b) at higher magnification

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Figure 4 displays the transmission electron microscope (TEM) micrograph of the polysaccharide/iron conditioner. The graph reveals the presence of electron-dense iron-based nanoparticles dispersed within a low-contrast organic matrix attributed to the polysaccharide crosslinker. The primary crystallites appear quasi-spherical with narrow polydispersity. These primary particles organize into ramified aggregates separated by nanoscale voids, a morphology expected for superparamagnetic iron oxides where dipole-dipole and van der Waals forces drive secondary agglomeration. The lighter continuous background indicates a conformal polysaccharide layer that acts as a soft template and capping agent, limiting anisotropic growth and favoring the observed near-spherical geometry. Such organic coordination (via hydroxyl/carboxyl groups) is consistent with chelation to Fe centers, which suppresses Ostwald ripening and sterically/electrostatically hinders uncontrolled aggregation [19,20].

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Figure 4.

TEM micrograph of polysaccharide/iron conditioner.

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From a functional standpoint, this hierarchical shape is advantageous for sludge conditioning through such nanoscale iron domains maximize accessible surface sites for adsorption/complexation with extracellular polymeric substances (EPS). Also, the polymer shell introduces hydrophilic groups that promote bridging between sludge flocs. Moreover, the inter-aggregate meso/nanopores provide diffusion pathways that facilitate water release during dewatering. The modest secondary agglomeration observed is beneficial, mechanically enhancing particle floc capture, while the preservation of small primary crystals sustains high surface reactivity [19-21]. Overall, the TEM evidence supports a polysaccharide-stabilized iron nanocomposite with controlled particle size, good dispersion within an organic network, and a multiscale porosity that together rationalize the improved conditioning/dewaterability reported elsewhere in this study.

3.1.3. FTIR analysis

Fourier transform infrared spectroscopy (FTIR) analyses were assessed to illustrate the links to the functional groups of the prepared polysaccharide/iron composite material (Figure 5a). Also, the conditioned sludge with the Fenton’s system after the oxidation reaction (Figure 5b). Additionally, the catalyst’s structure and the degradation of the dye are also attained (Figure 5c).

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Figure 5.

FTIR spectra of (a) polysaccharide/iron conditioner, (b) conditioned sludge and (c) catalyst loaded with dye.

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The FTIR analysis results can support valuable data about the changes in the functional groups of the material prior to and after sludge conditioning, as well as the conditioned material in the dye oxidation. The FTIR spectra can expose the existence or lack of specific peaks associated with the catalyst’s composition that might indicate the oxidation of the dye and the interaction between the catalyst and the dye.

Figures 5(a,b) expose the presence of a band at 569 cm⁻¹ is linked to the Fe-O stretching in the conditioner as well as the conditioned sludge. However, such a band disappeared when the conditioned sample was used for dye oxidation (Figure 5c). Also, the existence of the C-C band at 1427 cm⁻¹ in the conditioner and in the conditioned sludge, but it also disappears in the material when used as a catalyst and occupied with the dye. Further, the bands at around 3500 cm⁻¹ represent the OH group and NH, which appear in the three samples, but it appears as a weak band for the conditioned and loaded with the dye sample that was used as a catalyst (Figure 5c). Additionally, two peaks observed at wavenumbers of 2150 cm⁻¹ and 1630 cm⁻¹ are assigned for –CH- and the fundamental bonding mode of water. But it is weakly seen in the conditioned catalyst loaded with dye, and it is sharper in the conditioned and polysaccharide/iron sample.

3.2.1. Comparison between conditioners

For comparison purposes, the values of the CST and SRF efficiency (%) were compared for different conditioners, including solo systems of polysaccharide and hydrogen peroxide, and the dual conditioner of polysaccharide/iron/H₂O₂ as a Fenton’s system. Figure 6 compares the effects of different conditioning systems in terms of CST and SRF reduction.

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Figure 6.

Effects of various systems compared with the modified-Fenton.

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The dual conditioner demonstrated a markedly superior performance as exhibited in Figure 6 for both CST and SRF reduction. Thus, the dual system is achieving 70% CST reduction and 62% SRF reduction, whereas the solo systems exhibited considerably lower efficiencies. This outcome highlights the synergistic role of the modified Fenton’s reagent in enhancing sludge dewatering. Similar findings have been reported in previous studies, where dual systems integrating natural polymers with Fenton oxidation consistently outperformed single-component conditioners in improving floc structure and water release efficiency [47-49]. The enhanced performance can be attributed to hydroxyl radicals (∙OH) generated during the Fenton reaction, which effectively degrade EPS and disrupt sludge floc matrices, thereby reducing hydrophilicity and releasing interstitial water. Such oxidative modification not only facilitates charge neutralization and bridging by polysaccharides but also creates a more porous and compact floc structure, consistent with earlier observations on sludge treated with advanced Fenton-based processes [47-49]. The significant improvement of the combined system over solo treatments, therefore, verifies the feasibility and practical advantages of employing a dual-conditioning approach for sludge dewatering applications.

Hence, modified Fenton’s reaction improves the Al-S dewatering. Overall, the results in this study are consistent with prior research demonstrating that the oxidative action of hydroxyl radicals not only facilitates the EPS degradation but also enhances the charge neutralization and bridging effects of polysaccharides, thereby producing sludge flocs with improved hydrophobicity and water release characteristics. This comparative evidence highlights the practical advantage of employing modified Fenton’s reagent with polysaccharides as a cost-effective and robust approach for sludge dewatering.

Similar findings have been reported in previous studies. For instance, Li et al. [61] demonstrated that dual-conditioning with Fenton’s reagent and a cationic biopolymer improved sludge CST by 67% compared with single treatments. Ruan et al. [62] found that EPS degradation via hydroxyl radicals significantly reduced SRF values (up to 60%), confirming the role of radical-driven breakdown of extracellular substances in enhancing water release. Xia et al. [63] also reported that the integration of natural polysaccharides with iron-based Fenton systems improved dewatering by creating more porous and compact flocs. More recently, Chen et al. [64] observed that dual conditioning achieved nearly 70% CST reduction under near-neutral pH, in agreement with the current findings.

In this regard, the application of waste-derived polysaccharides as conditioning agents offers a significant sustainability advantage compared to traditional synthetic conditioners. Polysaccharides sourced from agricultural and seafood waste (e.g., chitosan from crustacean shells or cellulose-based derivatives) are renewable, biodegradable, and non-toxic, minimizing long-term environmental burdens associated with residual chemicals in sludge cakes [5]. In contrast, conventional conditioners often leave behind recalcitrant and potentially harmful residues that complicate sludge disposal and reuse. By valorizing waste polysaccharides, this approach supports circular economy principles through the dual benefits of waste reduction and the generation of value-added products. Moreover, polysaccharide-based conditioners reduce reliance on petrochemical-derived polymers, aligning with global sustainability targets and green chemistry practices. These advantages make them particularly promising for full-scale wastewater treatment applications where cost-effectiveness, environmental safety, and regulatory compliance are critical.

3.2.2. Conditioning time of Al-S dewaterability

Figure 7 conjointly illustrates the influence of conditioning time on Al-S dewaterability using the modified Fenton system (polysaccharide/Fe/H₂O₂). In general, lower CST and SRF values correspond to improved filtration performance. The results show that conditioning time significantly affected dewatering efficiency, with both CST and SRF reduction reaching maximum values of 70% and 62%, respectively, at 2 min of flocculation. Shorter or longer times resulted in decreased performance, indicating that an optimal conditioning period exists for effective floc formation. This could be associated with the size of the floc formation is related to the flocculation time. These findings agree with Mo et al. [2], who observed that floc size and strength are strongly dependent on flocculation duration, and that over-extended conditioning can destabilize flocs and reduce filterability.

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Figure 7.

Effects of reaction time using modified-Fenton’s oxidation conditioner on CST and SRF reduction efficiency (Operating parameters: H₂O₂ =300 mg L^-1; polysaccharide = 40 mg L^-1; pH = 6.0).

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The maximum reduction in both CST and SRF was observed at a flocculation time of 2 min, beyond which only negligible improvements in dewatering performance were achieved. Although the underlying mechanism remains uncertain, it is likely associated with the structural evolution of flocs, where enlargement improves filterability by creating more porous networks that facilitate water release. This observation is consistent with previous studies indicating that floc size and structure are critical determinants of sludge dewaterability.

The integration of natural polysaccharide-based polymers, iron salts, and hydrogen peroxide in the modified Fenton system further enhanced sludge conditioning efficiency. The synergistic action of these components not only disrupted EPS but also promoted the formation of larger, more compact flocs with superior drainage characteristics. These findings highlight the significant potential of the modified Fenton process as a viable alternative to conventional oxidation and chemical conditioning methods.

The experimental results demonstrate that the polysaccharide/iron-based modified Fenton system achieved significant improvements in sludge dewaterability, with CST and SRF reductions of up to 70% and 62%, respectively. Such enhancements are highly relevant for real-world waterworks operations, where dewatering efficiency directly reduces sludge handling, transport, and disposal costs. Moreover, the use of environmentally benign polysaccharide-based conditioners mitigates concerns about long-term toxicity, offering a sustainable alternative to synthetic polymers. Therefore, these results confirm that the modified Fenton system is not only effective but also time-efficient, requiring only a brief conditioning period to achieve optimal results. This efficiency, coupled with the system’s reliance on low-cost and environmentally benign reagents, underscores its promise as a practical alternative to classical acidic Fenton processes. Beyond its technical efficiency, the modified Fenton approach also offers economic and environmental benefits. By valorizing polysaccharide-rich waste materials and reducing dependence on synthetic chemical conditioners, this method contributes to circular economy strategies and sustainable sludge management practices. Moreover, its lower chemical demand and environmentally benign nature strengthen its superiority over classical oxidation systems [33,50-52]. Comparable trends have been documented in earlier studies. Li et al. [61] reported that optimal flocculation time was critical in dual Fenton–polymer conditioning, where excessive mixing reduced sludge dewaterability by breaking fragile flocs. Ruan et al. [62] confirmed that hydroxyl radical-driven EPS degradation improves water release, but its efficiency depends on ensuring sufficient, but not prolonged, contact time for radical-sludge interaction. Similarly, Xia et al. [63] found that modified Fenton-polysaccharide systems achieved maximum CST reduction within 2-3 min, after which performance declined. Faye et al. [64] further demonstrated that near-neutral Fenton-biopolymer systems provided rapid floc formation and stabilization within a short conditioning window, reducing chemical consumption while sustaining high dewaterability.

3.2.3. Catalyst effect on Al-S dewaterability

According to the experimental results, Fenton’s parameters play a decisive role in Al-S conditioning. A series of experiments was conducted to evaluate the influence of catalyst dosage and identify the optimal operating conditions. As shown in Figure 8, the catalyst dosage was gradually increased from 10 to 100 mg L^-1, and the corresponding CST and SRF reduction efficiencies were monitored. The findings confirmed the critical role of the catalyst in promoting hydroxyl radical (·OH) generation, which directly improves sludge filterability. Specifically, CST reduction increased markedly from 32% to 70% as the catalyst dosage was elevated to 40 mg L^-1. However, further increases beyond this level led to a decline in performance, with CST reduction decreasing to 42%. A similar trend was observed for SRF, where reduction peaked at 62% at 40 mg L^-1 and subsequently dropped to 27% at higher dosages.

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Figure 8.

CST reduction efficiency at various catalyst doses of modified-Fenton’s reagent (Operating parameters: pH 6.0; reaction time is 2 min; catalyst concentration 40 mg L^-1).

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Such a trend can be explained by the dual role of the catalyst in the modified Fenton system. At optimal concentrations, the catalyst enhances the decomposition of H₂O₂ and maximizes ·OH radical production, thereby improving coagulation, flocculation, and EPS disruption. Conversely, excessive catalyst concentrations may promote scavenging reactions in which ·OH radicals are consumed or inhibited, ultimately reducing their availability for effective oxidation and floc formation. Such behavior has been consistently reported in the literature, where an optimal catalyst window was demonstrated for achieving the highest dewatering efficiency. For example, Li et al. [61] and Ruan et al. [62] both observed that increasing catalyst dosages beyond the optimum led to radical quenching and performance deterioration. Similarly, Xia et al. [63] reported that polysaccharide-assisted Fenton oxidation showed maximum dewatering improvement at moderate catalyst levels, with excessive loading resulting in sludge destabilization and decreased filterability.

These findings suggest that the efficiency of the modified Fenton system is strongly dependent on maintaining the catalyst dosage within a narrow optimal range. From both an economic and environmental perspective, this highlights the importance of dose optimization: insufficient catalyst addition fails to provide adequate radical production, while excessive dosing not only reduces efficiency but also increases chemical demand and residual metal content in the sludge. Therefore, selecting the appropriate catalyst concentration is vital for balancing performance, sustainability, and cost-effectiveness in Al-S conditioning [47-49,53-56].

3.2.4. Hydrogen peroxide effect on Al-S dewaterability

To assess the effectiveness of the hydrogen peroxide reagent in the modified-Fenton system, its concentration is changed in the system to reach the optimized dose. Figure 9 demonstrates the CST and SRF reduction rate (%), which is highly influenced by the hydrogen peroxide reagent alteration concentration. As seen in Figure 9, 300 mg L^-1 leads to 70% and 62% for CST and SRF reduction when 40 mg L^-1 of the polysaccharide is the initiator catalyst of the modified-Fenton test. While the promotion of a greater dose in the reagent concentration (400 mg L^-1) results in a reduction in both CST and SRF reduction effectiveness. This could be attributed to the increase in the peroxide conditioner doses; the Al-S effective tendency (CST) revealed a trend of initial lessening followed by improvement. Such impact is due to hydrogen peroxide triggering the hydroxyl radicals’ (·OH) production [57].

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Figure 9.

CST reduction efficiency at various hydrogen peroxide concentrations of modified-Fenton’s reagent (Operating parameters: pH 6.0; reaction time is 2 min; Hydrogen peroxide 300 mg L^-1).

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It is noteworthy that hydroxyl radicals (·OH) act as the primary driving force of the oxidation reaction, thereby exerting a pivotal influence on the overall flocculation and sludge conditioning process. The concentration of hydrogen peroxide (H₂O₂) plays a crucial role in regulating ·OH radical formation. At optimal dosages, H₂O₂ decomposition is maximized, leading to successive oxidation reactions that disrupt EPS and enhance sludge dewaterability. However, both insufficient and excessive amounts of H₂O₂ can be detrimental. Insufficient dosing results in limited radical generation, while overdosing promotes self-scavenging reactions (H₂O₂ acting as a radical producer), thus reducing the effective radical availability for oxidation and destabilizing the flocculation system.

The results of this study are consistent with previous findings on Al-S oxidation using the classical Fenton process. For instance, Faye et al. [64] demonstrated that near-neutral Fenton systems required precise optimization of H₂O₂ to achieve enhanced sludge filterability, with excess H₂O₂ leading to diminished performance due to radical quenching. Similarly, Li et al. [61] and Xia et al. [63] reported that moderate peroxide dosages yielded the most favorable balance between ·OH radical generation and flocculation improvement. Moreover, Ruan et al. [62] confirmed that inappropriate H₂O₂ levels not only impaired radical efficiency but also increased operational costs and left residual oxidants in the treated sludge, raising environmental concerns.

These findings underline the necessity of careful optimization of H₂O₂ dosage in modified Fenton systems. From a practical perspective, determining the optimal peroxide concentration not only maximizes dewatering performance but also minimizes chemical consumption, operational costs, and potential risks associated with excess oxidant residues. Thus, the outcomes of this study reinforce the broader consensus in the literature that maintaining a balance between catalyst and H₂O₂ is essential for achieving sustainable, efficient, and environmentally responsible sludge management [47-50,57].

3.2.5. pH effect on Al-S dewaterability

Al-S dewaterability performance with the change in pH value is illustrated in Figure 10. According to the data displayed in Figure 10, the modified-Fenton oxidation based on polysaccharides, the Al-S dewaterability possesses a minor enriched with the pH variation. The pH variation altered the CST reduction from 44 to 70%. However, such a result indicates that the conditioning could happen at varied pH. Changing the pH values results in a modification of the sludge particles’ surface appearances and characterization, besides its definite impact on the extracellular polysaccharide in the Al-S-based sludge. But, minor enhancement in the sludge dewaterability is attained. Also, it is noteworthy to mention that the neutral pH conditions correspond to higher CST and SRF reduction rates. This confirms the suitability and economic efficiency of the process [23]. Large Al-S flocs might form under the conditioning system of the modified Fenton’s system. This could be because the charge conflict between the sludge molecules through the neutral circumstances is bigger than that under alkaline or acidic environments.

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Figure 10.

CST reduction efficiency at various pH conditions of modified-Fenton’s reagent (Operating parameters: pH 6.0; reaction time is 2 min; Hydrogen peroxide(H²O²) 300 mg L^-1; catalyst concentration 40 mg L^-1).

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This trend can be attributed to the influence of pH on sludge surface chemistry and polymer interactions. Under alkaline conditions, the negative charge density of Al-S particles and polymeric/iron species increases, leading to stronger electrostatic repulsion and reduced flocculation efficiency. With appropriate pH adjustment, Al-S can adsorb both H⁺ and OH⁻ ions, thereby altering its surface charge characteristics and influencing self-chargeability. Additionally, pH strongly affects the stability of polysaccharide-based EPS, which in turn governs sludge filterability.

At extreme acidic or alkaline pH values, EPS undergoes hydrolysis or structural decomposition, causing polysaccharide fragments and other organic components to leach into the supernatant. This destabilizes the floc structure, reduces sludge compactness, and weakens dewatering performance. In contrast, near-neutral pH conditions promote the formation of larger, more compact flocs by maintaining EPS integrity and optimizing the interaction between polysaccharides, iron species, and sludge particles. Consequently, sludge dewatering is more efficient under mild pH conditions, eliminating the need for extensive pre- or post-treatment adjustments.

These findings are consistent with reports in the literature. Ruan et al. [62] and Xia et al. [63] demonstrated that neutral to slightly acidic pH conditions (pH 5-7) favored optimal hydroxyl radical generation in Fenton-based conditioning and promoted stable floc formation. Similarly, Ma et al. [50] highlighted that near-neutral Fenton-biopolymer systems not only improved dewatering but also minimized chemical input compared to highly acidic treatments. Furthermore, Liu et al. [47] noted that extreme acidic environments accelerated EPS solubilization, leading to organic release into the liquid phase and reduced filterability. Collectively, these studies reinforce that maintaining a near-neutral pH provides a balance between chemical reactivity, EPS stability, and floc structural integrity, ensuring both efficiency and sustainability in sludge conditioning processes.

3.2.6. Temperature effect on Fenton’s conditioning

Usually, the Fenton system, as an oxidizing reaction of pollutants, is carried on at ordinary room temperature. But increasing the temperature should be evaluated since it might influence the dewatering performance. According to such a concept, the temperature of the sludge is ranged the conditioning test is performed in the temperature range of room temperature to 60°C. The data displayed in Figure 11 demonstrated that the conditioning trend designates the influence of elevating temperature in the modified Fenton, which attains a reversible effect on the dewaterability effectiveness.

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Figure 11.

Temperature effect on Al-S dewaterability by Fenton reaction.

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The effect of temperature on Al-S conditioning was evaluated as displayed in Figure 11 and found to negatively influence dewatering performance. Specifically, CST reduction declined from 70% to 33%, while SRF reduction decreased from 62% to 22% as the temperature increased from room temperature to 60°C. These results indicate that the dominant factor governing Al-S dewatering efficiency is the modified Fenton system itself, rather than thermal enhancement. Indeed, the elevated temperature exerted an adverse effect on conditioning performance, which can be attributed to the accelerated decomposition of hydrogen peroxide at higher temperatures. This rapid consumption reduces the effective availability of ·OH radicals, thereby diminishing the oxidative and flocculating capacity of the system. For this reason, maintaining room temperature conditions is more favorable for achieving optimal sludge filterability.

Interestingly, this trend contrasts with some previous reports on waste-activated sludge (WAS). For instance, Mustranta and Viikari [37] observed that elevated temperatures enhanced WAS dewatering due to improved solubilization of organic matter and greater EPS disruption. However, the differing behavior between Al-S and WAS may arise from their distinct physicochemical compositions: while WAS contains high levels of organic EPS that benefit from thermal disruption, Al-S is predominantly inorganic with limited EPS content. In Al-S systems, the excessive decomposition of H₂O₂ at higher temperatures outweighs any minor thermal benefits, leading to overall performance deterioration.

Similar observations were also reported by Faye et al. [64], who emphasized that moderate thermal conditions were beneficial only up to a point, beyond which oxidant consumption became excessive. In agreement, Xia et al. [63] highlighted that maintaining ambient temperatures provided the most sustainable balance between radical generation and sludge stability in Fenton-assisted conditioning. These findings underscore that, unlike WAS, Al-S conditioning is best performed at room temperature to preserve oxidant efficiency, minimize chemical demand, and ensure stable dewatering performance.

3.2.7. Sludge surface micromorphology:

SEM images prior to and after conditioning of the sludge cake were investigated. SEM analysis was employed to examine the surface features of raw and conditioned Al-S cakes (Figures 12a, b). The raw Al-S (Figure 12a) exhibited relatively loose floc structures with open voids and irregular particles, indicating a weak network that is less favorable for efficient water release. In contrast, the conditioned Al-S cake (Figure 12b) presented a more compact morphology with smoother and denser surfaces. This densification is attributed to the action of the modified Fenton-polysaccharide conditioner, which promotes flocculation and restructuring of the sludge matrix through redox-induced aggregation and polymer bridging effects.

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Figure 12.

SEM images of (a) raw and (b) Fconditioned Al-S cake sludge with Fenton conditioner.

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Furthermore, the compacted and less porous appearance of the conditioned flocs suggests that EPS and released intracellular matter were redistributed and redeposited over the surface, partially clogging pores and binding particles together. Such surface modifications corroborate the improved dewaterability observed experimentally, since stronger floc matrices resist collapse under vacuum filtration while promoting water drainage through interparticle channels. Moreover, the presence of spherical and plate-like features in Figure 12(b) supports the hypothesis that iron-aluminum hydroxo-complexes formed during conditioning act as cementing agents, bridging adjacent colloids into stable aggregates. These findings are consistent with literature reports on chemically conditioned sludge [20], where surface reorganization and pore blocking were observed as critical microstructural changes leading to enhanced sludge dewatering.

3.2.8. Box-Behnken factorial design optimization

A numerical design approach based on RSM was employed to optimize multiple operational parameters, including hydrogen peroxide dosage, polysaccharide/iron concentration, and pH conditions. These factors were chosen because they represent the most critical operational variables influencing both the oxidative efficiency of the modified Fenton system and the flocculation/dewatering performance of Al-S.

Polysaccharide dosage (X₁) was considered essential, as this natural polyelectrolyte governs floc formation, charge neutralization, and bridging mechanisms, which are fundamental for improving CST and SRF. Optimizing its level is necessary to ensure effective sludge conditioning while maintaining economic feasibility. Additionally, hydrogen peroxide concentration (X₂) was chosen as another key variable, since H₂O₂ acts as the source of hydroxyl radicals in the Fenton reaction, thereby determining the extent of extracellular polymeric substance degradation. However, excessive dosing may result in radical scavenging and elevated operational costs, highlighting the need for balance. Also, the third selected factor is the pH (X₃) that exerts a decisive influence on both iron speciation and the decomposition efficiency of H₂O₂. Near-neutral conditions are of practical importance, as they reduce the requirement for extensive chemical adjustment compared to classical acidic Fenton systems, thereby lowering costs.

In this regard, the Box-Behnken design was therefore selected as an efficient statistical tool to systematically evaluate the individual effects, quadratic effects, and interactions of these three parameters with a reduced number of experimental runs with statistical assessment. Such design provides an efficient framework for optimization with a reduced number of experimental runs while ensuring highest dewaterability performance. Initially, the pre-RSM stage is to reveal an appropriate approximation between the SRF (%) response and the set of the independent parameters the levels have been detailed in Table 2.

The polynomial second-order model for the three independent parameters is correlated according to Eq. (1).

where x₁, x₂ and x₃ are the coded independent variables, as tabulated in Table 2.

The modified Fenton system is a multi-variable process. Polysaccharide-based catalyst concentration, H₂O₂ concentrations, and pH value are essential to be optimized to maximize the response (using SAS software, SAS (1990 Version). The range of the multi-parameter levels is located according to the above-mentioned study. The Box-Behnken design was selected, and the design matrix has been tabulated in Table 3. For this, 15 runs of combined experiments using various selected operating ranges were experimentally conducted, and thereby the response values are evaluated and compared with the predicted values.

The statistical evaluation of the Box-Behnken design was carried out using analysis of variance (ANOVA), as summarized in Table 4. The model demonstrated a strong agreement between the experimental results and predicted values, confirming its adequacy for sludge conditioning optimization. The overall regression was statistically significant (Probability, p < 0.047), with a high coefficient of determination (R² 0.92). Such values are widely regarded as indicators of reliable predictive performance in process optimization studies. In general, models with p-values < 0.05 and R² > 0.80 are considered both statistically valid and practically robust [52]. Thus, the present model can be considered statistically dependable, with the observed R² (0.92) highlighting its predictive strength.

Among the linear factors, polysaccharide dosage (X₁) and pH (X₃) exerted statistically significant influences (p < 0.05), underlining their dominant roles in enhancing sludge filterability. The quadratic effect of polysaccharide dosage (X₁²) was highly significant (p = 0.005), revealing a strong non-linear relationship in which excessive dosage negatively impacts dewaterability. This phenomenon is consistent with reports of floc over-stabilization and destabilization in polymer-conditioned sludge [52]. In contrast, hydrogen peroxide dosage (X₂) and interaction effects were not significant within the tested ranges. Collectively, these findings confirm that precise adjustment of polysaccharide dosage and maintenance of near-neutral pH are the most critical parameters for maximizing CST and SRF reductions. The close alignment between predicted and observed values further supports the robustness of the model and its utility as a predictive tool for sustainable sludge conditioning.

Graphical representation is also applied for the excess model illustration. 3-D surface and its corresponding 2-D contour graphical illustrations operated by Matlab software version 7.11.0.584 are carried out between each two independent variables. According to Figure 13, the SRF response (%, reduction) is improved by the increase in both polysaccharide-based catalyst and hydrogen peroxide (Figures 13a-d) as seen in the surface and contour plot. But, the suggestive SRF advancement is declined with the further increase in both the reagents. Thus, there is a certain limit to their addition. Additionally, the SRF reduction (as seen in Figures 13e and f) with low values of pH is favorable near neutral pH, which corresponds to the natural pH of Al-S. It is noteworthy to mention that optimizing polysaccharide-based catalyst and hydrogen peroxide concentrations, besides controlling the pH value, is crucial to the conditioning process. Further Mathematical software (V 5.2. Wolfram research Inc.) was applied to locate the accurate optimized values which correspond to to 42 and 270 polysaccharide-based catalysts and hydrogen peroxide, respectively, at pH 6.5.

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Figure 13.

SRF reduction (%) (a) 3-D surface coded polysaccharide based catalyst and H²O²; (b) 2-D contour plots of polysaccharide based catalyst and H₂O₂; (c) 3-D surface coded polysaccharide based catalyst and pH value; (d) 2-D contour plots of polysaccharide based catalyst and pH value and H₂O₂;(e) 3-D surface coded H₂O₂ and pH value; (f) 2-D contour plots of coded H₂O₂ and pH value.

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Additionally, to confirm and verify such a predicted model, the model verification, three extra replicates of the experiment are conducted using the above-mentioned optimum values. The results of such tests verified the optimal values that achieved a maximal SRF deduction rate that could be reached to 63% which clearly proved the efficacy of the proposed predicted model.

3.2.9. Mechanism of modified Fenton-assisted sludge dewatering:

Schematic representation of the modified Fenton mechanism for sludge dewatering. Sludge flocs, encased by an EPS layer, retain bound and interstitial water that hinders dewatering. The combined action of hydrogen peroxide (H₂O₂), iron catalyst (Fe²⁺/Fe³⁺), and polysaccharide biopolymer generates hydroxyl radicals (·OH), which disrupt the EPS matrix and open pores. This process promotes the formation of larger, denser flocs with reduced water retention as seen in Figure 14. The schematic illustrates the mechanism of sludge dewatering improvement through a modified Fenton system incorporating polysaccharides and iron catalysts. The proposed mechanism highlights the central role of hydroxyl radicals (·OH) in disrupting the EPS matrix. EPS, primarily composed of polysaccharides and proteins, forms a protective hydrogel-like barrier around sludge flocs that strongly retains bound water within pores and interstitial water between flocs. This structural arrangement contributes to the poor filterability of Al-S. The sludge flocs, shown as irregular aggregates, are surrounded by an EPS layer represented by a dashed red halo, which normally entraps both bound water within the pores and interstitial water between the flocs, reducing dewaterability.

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Figure 14.

Proposed mechanism of modified Fenton-assisted sludge dewatering.

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The addition of hydrogen peroxide (H₂O₂), iron catalyst (Fe²⁺/Fe³⁺) derived from magnetite, and a polysaccharide biopolymer initiates Fenton-like reactions that generate hydroxyl radicals (·OH). Upon the addition of H₂O₂ in the presence of Fe²⁺/Fe³⁺ catalysts derived from magnetite, classical Fenton reactions are initiated, generating highly reactive ·OH species. These radicals attack and disrupt the EPS matrix, leading to structural loosening and pore opening. The inclusion of a polysaccharide biopolymer enhances the process by improving floc bridging and providing an eco-friendly alternative to synthetic polymers. Also, the polysaccharide component acts as a natural polyelectrolyte, providing charge neutralization and inter-particle bridging effects. This enhances floc formation, resulting in larger, denser, and more compact aggregates that exhibit lower compressibility and improved filterability. The synergistic effect of the oxidative breakdown of EPS and the structural reinforcement by polysaccharides leads to more efficient water release and improved dewatering performance. The generated ·OH radicals attack the EPS matrix, breaking down complex macromolecules, loosening the gel structure, and facilitating the release of trapped water. This disruption leads to floc restructuring, characterized by the formation of larger, denser aggregates with higher porosity and better permeability.

Furthermore, iron salts not only catalyze radical generation but also enhance floc stability through the formation of Fe(OH)₃ precipitates, which act as binding agents within the sludge matrix. These combined mechanisms explain the significant improvements observed in CST and SRF, confirming that the dual Fenton–polysaccharide system provides both chemical and structural enhancements compared to traditional conditioning methods.

As a result, both CST and SRF are significantly reduced, confirming improved dewaterability. The mechanism also reflects a synergistic effect, where chemical oxidation (via ·OH) and biopolymer conditioning act simultaneously to degrade EPS and strengthen floc formation. This dual action contrasts with conventional single-conditioning strategies, which often require higher chemical dosages or result in incomplete water release.

These observations are consistent with earlier studies [60-64], which demonstrated that oxidative damage to EPS is a key pathway for enhancing sludge filterability. However, the integration of polysaccharide biopolymers in the modified Fenton system offers an additional sustainable dimension by reducing reliance on synthetic flocculants and lowering the environmental footprint of sludge treatment. Thus, the use of waste-derived polysaccharides as natural conditioners not only valorizes residual biomass but also reduces dependence on synthetic polymers and harsh chemical agents traditionally applied in sludge treatment. The integration of polysaccharides with the Fenton system lowers the required dosage of chemical oxidants, thereby reducing the overall chemical footprint and associated environmental risks. This approach contributes to circular economy practices, minimizes secondary pollution, and aligns with global strategies for sustainable sludge management. These points have been incorporated into the revised discussion section, with appropriate references to highlight both the environmental impact and the reduction of chemical inputs.

3.2.10. Comparative data with literature

The dewatering performance achieved in this study through the evaluation of both CST reduction of 70% and SRF reduction of 62% at 40 mg L^-1 of polysaccharide-Fe catalyst, 300 mg L^-1 H₂O₂, pH 6.0, and only 2 min conditioning is comparable to values reported in conventional Fenton-based studies. Additionally, previous investigations have consistently shown that Fenton and Fenton-assisted conditioning markedly improve sludge dewaterability. However, such outcomes are generally obtained under strongly acidic conditions (pH 3-4), higher reagent dosages, and extended conditioning times reported by Mo et al., 2015 [2]; Liang et al., 2015 [23]; Zhen et al., 2014 [14]; Tony et al., 2009 [65]; Ding et al., 2018 [53]; Fontmorin and Sillanpää, 2017 [54]; Ferrentino et al., 2020 [56]. In contrast, the near-neutral operation demonstrated here substantially reduces chemical consumption and minimizes the need for post-neutralization, while sustaining robust CST and SRF reductions. Importantly, the dual role of the waste-derived polysaccharide-iron conditioner introduces a waste-to-resource pathway: the conditioned sludge residue retained catalytic activity, functioning as a hetero-Fenton catalyst capable of achieving up to 98% MG removal. This added functionality extends the process beyond conventional sludge conditioning toward integrated pollutant abatement.

Relative to the displayed studies in Table 5, typical CST/SRF reductions are about 45–65% at pH 3-4 over 15-30 min, whereas the current system offers shorter residence time with a reduced chemical, including lower oxidant dose and no bulk post-neutralization. Also, the current advantage is a waste-to-resource valorization via a waste-derived polysaccharide-iron dual conditioner. Notably, the conditioned residue retained hetero-Fenton catalytic activity (dye removal), extending process value beyond dewatering toward pollutant abatement, an attribute seldom demonstrated in dewatering-focused works. These distinctions position the proposed modified Fenton route as a low-acid, low-dose, rapid alternative to classical Fenton conditioning while maintaining robust dewaterability gains.

While the modified Fenton-assisted sludge conditioning process demonstrated significant improvements in dewaterability, certain limitations should be acknowledged. Scaling up from laboratory to full-scale operations may face challenges, particularly with respect to reagent dosing control, mixing efficiency, and sludge heterogeneity, all of which can influence treatment consistency. This includes the accumulation of residual iron salts or the release of reactive oxygen species. Moreover, although the incorporation of waste-derived polysaccharides reduces reliance on synthetic polymers, the continued use of hydrogen peroxide and iron salts may introduce secondary environmental concerns, such as residual iron accumulation or the generation of excess reactive oxygen species. These considerations highlight the need for careful process optimization and environmental management to minimize unintended impacts.

3.2.11. Future perspectives and valorization pathways

Future research should therefore focus on pilot-scale implementation, long-term stability assessments, and life-cycle evaluations to fully verify the environmental and economic feasibility of this approach. Positioning the method against alternative emerging strategies, such as enzymatic conditioning, nanomaterial-assisted flocculation, or others that might also help to clarify its role within sustainable sludge management. Ultimately, while challenges remain, the ability to operate at near-neutral pH using waste-derived polysaccharides offers a promising pathway toward eco-friendly and resource-efficient conditioning practices.

Although the modified Fenton-biopolymer conditioning process clearly enhances sludge dewaterability, the potential of the final conditioned residue should also be considered within a circular economy framework. In addition to functioning as a hetero-Fenton catalyst for pollutant removal, the sludge residue enriched with organic fractions, iron species, and waste-derived polysaccharides may serve as a valuable feedstock for higher-value applications.

Recent advances demonstrate that waste-derived biopolymers can be exploited in the development of reinforced bioplastics for food packaging [66] and in Maillard-reaction-driven glycation of polymeric films to improve barrier and functional properties [67]. Likewise, zein, a maize-derived protein, has been reviewed as a promising candidate for tissue engineering and nanotechnology applications [68]. These examples highlight the feasibility of transforming organic- and mineral-rich residues into biopolymeric composites, nanostructured films, or functional materials for packaging, biomedical, or nanotechnological sectors.

Such valorization would extend the benefits of the modified Fenton approach beyond wastewater remediation, linking sludge management with the production of sustainable materials. This dual functionality significantly enhances the circular economy potential of the process by transforming a problematic waste stream into a renewable resource with added industrial value.