Mechanism and Parameter Optimization of Fenton’s Reagent Integrated with Surfactant Pretreatment to Improve Sludge Dewaterability

Sludge dewatering can effectively reduce the volume and mass of sludge for subsequent treatment and disposal. The work validated the potential of Fenton’s reagent combined with dodecyl dimethyl benzyl ammonium chloride (DDBAC) in improving sludge dewaterability and proposed the mechanism of joint conditioning. The composite conditioner dosage was optimized using response surface methodology. Results indicated the good conditioning capability of the composite conditioners. The optimum dosages for H2O2, Fe2+, and DDBAC were 44.6, 39.6, and 71.0 mg/g, respectively, at which a sludge cake water content of 59.67% could be achieved. Moreover, a second-order polynomial equation was developed to describe the behavior of joint conditioning. Analysis of the reaction mechanism showed that Fenton oxidation effectively decomposed extracellular polymeric substance (EPS), including loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS), into dissolved organics, such as proteins and polysaccharides. The process facilitated the conversion of the bound water into free water. Furthermore, DDBAC further released the bound water through solubilization of TB-EPS and LB-EPS after the Fenton reaction. The bound water content of the sludge conditioned with Fenton’s reagent decreased from 3.15 to 1.36 g/g and further decreased to 1.08 g/g with the addition of DDBAC. High-performance liquid chromatography analysis verified that the composite conditioning could oxidize and hydrolyze EPS into low-molecular-mass organics (e.g., formic and acetic acid), thereby facilitating the release of bound water.


Introduction
Sewage sludge, a by-product of wastewater treatment, contains a large amount of water that can be generally divided into free and bound water [1][2][3]. Free water is unrestrained by sludge solid particles and can be easily separated from the sludge by using mechanical stress, whereas the bound water is difficult to be removed using the mechanical dewatering methods because of the presence of chemical bonds between bound water and sludge flocs [4]. Thus, sludge conditioning and dewatering are essential to improve sludge dewaterability. Organic polymers can facilitate the flocculation of sludge particles by charge neutralization or adsorption bridging mechanism but do not affect the water molecule-organic matter bonding in the sludge. Moreover, the complicated degradability of residual polymer in sludge cakes cause secondary environmental pollution.
The Fenton system generates hydroxyl radicals (ÁOH) with powerful oxidizing abilities through iron-catalyzed H 2 O 2 decomposition under acidic conditions. ÁOH can decompose organic matter and rupture microbial cells [5,6]. Researchers [7][8][9][10] have concluded that Fenton's reagent can improve sludge dewaterability. Liu et al. [7] demonstrated that 84% specific resistance to filtration (SRF) reduction efficiency was achieved when 20 mg/g Fe 2+ and 125 mg/g H 2 O 2 were added to the sludge. Moreover, their studies [8][9][10] reported that the addition of Fenton's reagent increases the extracellular polymeric substance (EPS) concentration in the sludge supernatant, leading to significant effects on sludge dewatering performance.
Extracellular polymeric substance, which mainly consists of proteins and polysaccharides, is an insoluble organic matter attached to the bacterial cell surface and is closely related to the bound water [11][12][13]. According to the combined extent between organic matter and sludge flocs, EPS can be divided into slime layer EPS (S-EPS), loosely bound EPS (LB-EPS), and tightly bound EPS (TB-EPS) [14]. Researchers have reported that cationic surfactants (CTAB and DTAB) could be used to improve sludge filtration efficiency and decrease sludge cake water content substantially [15][16][17]. However, the effects of the cationic surfactant dodecyl dimethyl benzyl ammonium chloride (DDBAC) on sludge dewatering performance have been rarely investigated. Our previous studies [18,19] revealed that DDBAC can effectively release bound water from sludge because of its superior surface activity and strong adsorption/bridge capacities on sludge. Moreover, the application of Fenton's reagent integrated with DDBAC during sludge conditioning has not been investigated. Furthermore, all the above-mentioned studies focused on the relationship between S-EPS concentration and sludge dewatering performance. The effects of composite conditioners on LB-EPS and TB-EPS, and the relationship between EPS (S-EPS, LB-EPS, and TB-EPS) and bound water during joint conditioning are still unclear. In addition, optimal composite conditioner dosages have not been concluded yet.
In this study, water content of sludge cake (W C ) and capillary suction time (CST) were used as sludge dewaterability indicators. Composite conditioner dosages were optimized using single-factor experiments and response surface methodology (RSM). In addition, bound water content (W B ), EPS concentration, and organic acid concentrations in the sludge supernatant were determined to clarify the mechanism of Fenton's reagent integrated with surfactant pretreatment to improve sludge dewaterability.

Test materials
The sludge used in this study was collected from secondary sedimentation tank of Xiaohongmen municipal wastewater treatment plant in Beijing, China. The samples were thickened by gravity to increase the concentration and stored in polypropylene containers at 4˚C before use.
All experiments were completed in 72 h. The main characteristics of the thickened sludge are listed in Table 1.
Sulfuric acid (H 2 SO 4 , 4 M) was used to adjust the initial pH of sludge to 4 before adding Fenton's reagent. Fenton's reagent was prepared by mixing FeSO 4 Á7H 2 O and H 2 O 2 (30 wt%). The cationic surfactant was DDBAC with a chemical formula of C 21 H 38 NCl and relative molecular mass of 340.00. All the chemicals used were of analytical grade, except for the bovine serum albumin of biochemical grade, and were purchased from Sinopharm Chemical Reagent Company, China.

Experimental design
Single factor experiments. The effects of H 2 O 2 (from 10 to 100 mg/g) on the W C and CST were investigated with 20 and 40 mg/g of Fe 2+ and DDBAC, respectively. Similarly, the effects of Fe 2+ (from 10 to 100 mg/g) on the W C and CST were studied with 40 and 40 mg/g of H 2 O 2 and DDBAC, respectively. The effects of DDBAC (from 0 to 100 mg/g) on the W C and CST were evaluated with 40 and 40 mg/g of H 2 O 2 and Fe 2+ .
RSM design. A Box-Behnken design [20] was chosen to evaluate the combined effects of H 2 O 2 , Fe 2+ , and DDBAC during conditioning. The range and levels of the three variables were defined according to the single-factor experiments and are presented in Table 2. Moreover, W C was examined as the response value. Seventeen runs were required for a complete set of the experimental design, as shown in Table 3, and the experimental results were analyzed using Design-Expert 8.0. The response variable (Y) was connected with the set of the independent variables using an empirical second-order polynomial model. The generalized model is in the following form [20]: where Y is the predicted response (W C , %); β 0 , β i , β ii , and β ij are defined as the model regression coefficients; and X i and X j are the independent variables.

Sludge conditioning and dewatering
Approximately 300 mL of the thickened sludge samples were poured into a 500 mL beaker and conditioned according to the following procedures: addition of 4 M H 2 SO 4 ! rapid mixing at 200 rpm for 1 min ! addition of Fe 2+ solution ! rapid mixing at 200 rpm for 1 min ! addition of H 2 O 2 ! slow mixing at 100 rpm for 120 min ! addition of 4 M NaOH to neutralize the pH of sludge samples to approximately 7 (terminating the Fenton reaction) ! addition of DDBAC ! slow mixing at 100 rpm for 30 min. After conditioning, 50 mL conditioned sludge sample was filtered and dewatered in a 150 mm standard Buchner funnel with a quantitative filter paper at a constant vacuum pressure of −0.055 MPa until no filtrate came out.

Sludge dewaterability
Sludge dewaterability was evaluated by determining W C and CST. W C was calculated using the equation: where W 1 is the sludge cake weight (g), and W 2 is the sludge cake weight after drying to a constant weight at 105˚C (g). CST was measured using a standard CST apparatus (304 M, Triton). The pH value of the sludge was determined using a digital pH-meter (FE20, METTLER-TOLEDO).

Conditioning mechanism investigation
As shown in Table 4, a set of experiments with different conditioning preparations were conducted to elucidate the mechanism of the composite conditioner pretreatment to improve sludge dewaterability. The sludge before and after conditioning was analyzed in terms of EPS concentration, W B and organic acid concentrations in the sludge supernatant.
EPS extraction and analysis. EPS samples were extracted according to our previous studies [15]. All the S-EPS, LB-EPS, and TB-EPS extractions were analyzed for proteins (PN) and polysaccharides (PS). The PN concentration was determined based on the method of Lowry et al. [21] using bovine serum albumin as the standard; whereas the PS concentration was measured according to the method of Riesz et al. [22] using glucose as the standard.
Bound water content measurement. The W B was measured using a differential scanning calorimetry (DSC) analyzer (404 F3 Pegasus, NETZSCH) equipped with a liquid nitrogen cooling system. The sludge cake sample was first cooled to -20˚C, assuming that all free water was frozen at this temperature, and then warmed to 20˚C at the rate of 2˚C/min [23]. The DSC curve showed an endothermic peak during the free water transition. Thus, the free water can be calculated using the area of the endothermic curve below the baseline. W B was determined by the difference between total and free water in the sludge cake: where W B is the bound water content (g/g, is the matter content in per gram of dry sludge, based on dry solids, similarly hereinafter), W T is the total water content (g/g), ΔH is the endothermic curve area of the sample per gram of dry solid (J/g), and ΔH 0 is the standard fusion heat of ice at 334.7 J/g. HPLC study. Organic matter hydrolysis in the sludge supernatant was analyzed using HPLC (LC-20AD, Shimadzu Corp., Japan). HPLC was applied to determine the concentration of small molecular organic acids (i.e., formic, acetic, and propionic acids).
HPLC was performed under the following conditions: the mobile phase was 7% CH 3 OH-0.20 mol/L KH 2 PO 4 (pH = 4.0) buffer solution (v/v) filtered with a 0.45 μm membrane and then degassed by ultrasonication for 10 min. The flow rate was 0.5 mL/min, the column temperature was 40 ± 1˚C, the measurement wavelength was 215 nm, the injection volume was 20 μL, and the high peaks were quantified using the external standard method.

Results and Discussion
Single factor experiments to determine the optimum range of three factors Fig 1 shows that the effects of the composite conditioner on sludge dewaterability. The W C and CST decreased to 63.36% and 31.8 s (Fig 1a), respectively, when the H 2 O 2 dosage was increased to 40 mg/g (mg/g, being the H 2 O 2 dosage per gram of dry sludge, similarly hereinafter). This was likely due to the fact that ÁOH concentration increased as H 2 O 2 dosage increased, thereby enhancing the capacity of ÁOH in oxidizing the sludge [24]. However, H 2 O 2 could react with ÁOH and form hydroperoxyl radicals (HO 2 Á) whose oxidizing capacity is inferior to that of ÁOH [24,25]. Thus, the decreasing trends of W C and CST slowed down as H 2 O 2 dosage further increased. The optimum H 2 O 2 dosage was thought to be approximately 40 mg/g. The finding was in agreement with that reported by Zhou et al. [5], who observed that adding more H 2 O 2 cannot further improve the sludge dewaterability. Similarly, the effects of Fe 2+ on the sludge dewaterability are shown in Fig 1b. The W C and CST of the sludge decreased as Fe 2+ dosage increased (>10 mg/g) and the lowest W C (62.50%) and CST (27.3 s) were obtained at 40 mg/g of Fe 2+ . This is mainly because Fe 2+ was a catalyst in the Fenton reaction system and could generate ÁOH after reacting with H 2 O 2 . Thus, ÁOH could be increased with the increase of Fe 2+ dosage. The ÁOH oxidized sludge flocs and microorganisms, which boosted the sludge dewatering performance by releasing the bound water into the liquid phase [26]. However, with a further increase in Fe 2+ dosage, the W C and CST rapidly increased. The excess Fe 2+ (>40 mg/g) could consume the ÁOH, thereby producing the non-oxydic OH − [6]. The concentration of oxydic ÁOH decreased compared with that at 40 mg/g of Fe 2+ dosage, thereby decreasing the treatment efficiency.
The effects of DDBAC on sludge dewatering performance, as depicted in Fig 1c, were obvious and similar in effect to that of H 2 O 2 (Fig 1a). The W C and CST decreased as DDBAC dosage increased ( 60 mg/g). After that, sludge dewatering performance was almost unchanged. The results coincided with those in our previous studies [18,19]. Thus, the optimum DDBAC dosage is approximately 60 mg/g.

Optimization of the composite conditioner using RSM
The three-dimensional surface and contour plots of the response W C , obtained using Design-Expert 8.0, visualized the predicted model. Fig 2a-2c display the relationship between two interacting variables with the response when the third variable was kept at its zero level. In Fig  2a, an obvious decrease of W C was observed as H 2 O 2 dosage increased. W C was almost unchanged when the H 2 O 2 dosage was at higher value, which agreed with the single factor experimental results (Fig 1a). Similarly, the W C decreased as Fe 2+ dosage increased within a certain limit. After that, the increasing Fe 2+ severely deteriorated sludge dewatering performance, as described above (Fig 1b). The 3D surface and the corresponding contour plotted in Fig 2b illustrate Table 5 presents the results of the experiments based on the Box-Behnken design. After data fitting, a second-order polynomial equation was obtained as: The statistical test of the model was performed using Fisher's statistical method for analysis of variance (ANOVA). The result of ANOVA for W C , as shown in Table 6, implied a good agreement between the experimental and predicted data because the regression coefficient R 2 reached 0.9585. The Model F-value was 17.97 and the value of "Prob > F = 0.0005" was less than 0.05, indicating significance of the model. Moreover, the total determination coefficient R 2 value of 0.9052 demonstrated that only 9% of the total variation could not be explained by the model. Therefore, the mathematical model (Eq (4)) was able to describe the effects of Fenton's reagent combined with DDBAC on sludge dewaterability and predict the W C during sludge conditioning. From Eq (4), the minimal coded values of independent variables were determined using the Mathematical software and response surface analysis as follows: X 1 = 0.23, X 2 = − 0.12 and X 3 = 0.55. Accordingly, H 2 O 2 , Fe 2+ , and DDBAC dosages were 44.6, 39.6, and 71.0 mg/g, at which the predicted value of the W C was estimated to be 59.67%.
Considering these optimal conditions, three additional experiments were performed to validate the accuracy and practicability of the established polynomial equation. The replicated experiments yielded an average W C of 59.54% ± 0.37%, which showed high agreement with the predicted value. Therefore, the quadratic model optimized the joint conditioning processes effectively.  using the composite conditioner were lower than those of the sludge conditioned using Fenton's reagent alone or DDBAC alone, suggesting that the effect of joint conditioning was better than individual usage. EPS changes in sludge. As shown in Fig 4, the variations of PN and PS concentrations in S-EPS, LB-EPS, and TB-EPS were very obvious during conditioning. For the raw sludge, the PN and PS were mainly distributed in TB-EPS, accounting for 85.72% of total PN and 85.37% of total PS. The PN and PS in LB-EPS and S-EPS were less distributed. These findings agree with those in the previous literature [27], which demonstrated that more than 80% of EPS existed in the form of TB-EPS. For sludge conditioned using Fenton's reagent, the PN concentration in TB-EPS decreased from 3028.17 to 773.18 mg/L, whereas the PN concentration in LB-EPS increased from 143.16 to 639.83 mg/L, and those in S-EPS increased from 363.31 to 838.86 mg/L (Fig 4a). Furthermore, the PS variation trend was similar to that of PN (Fig 4b).

Results of mechanism
These results indicated that Fenton oxidation could effectively degrade EPS into dissolved organics that could be released into the filtrate [28][29][30].
After Fenton reaction, adding DDBAC further conditioned the sludge. The PN and PS concentrations in TB-EPS and S-EPS decreased, whereas the PN and PS concentrations in LB-EPS were almost unchanged. This trend may be because DDBAC weakened the surface tension of sludge and promoted the solubility of the non-dissolvable EPS. Therefore, a portion of insoluble EPS (TB-EPS and LB-EPS) was turned into soluble EPS (S-EPS) and was released into the liquid phase. However, the S-EPS hydrolysis was stronger than the solubilization of TB-EPS and LB-EPS under the action of DDBAC because most EPS had been peeled off, thereby decreasing S-EPS concentration.
Bound water content. Bound water is deeply embedded in sludge flocs due to intermolecular forces and cannot be removed mechanically, and this influences sludge dewaterability. Colin and Gazbar [31] reported that W B could be directly used to measure the difficulty of mechanical dewatering. To be specific, a high W B will cause mechanical dewatering difficulty, and vice versa. Fig 5 shows W B and sludge cake composition of the raw and conditioned sludge. The W B of sludge conditioned using Fenton's reagent decreased from 3.15 to 1.36 g/g and further decreased to 1.08 g/g after the sludge was jointly conditioned using Fenton's reagent and DDBAC. Approximately 68% of the bound water was transformed into free water. The results were in good agreement with the sludge dewaterability, as depicted in Fig 3, which demonstrated that the high W B in sludge causes poor sludge dewaterability. These findings are consistent with the conclusion of Kwon et al. [32] that stated a negative correlation between W B and sludge dewatering performance.
The sludge cake composition showed that approximately 45% of the bound water in the conditioned sludge still remained, which could be because the water combined with sludge flocs through strong chemical bond and could not be released into the liquid phase by composite conditioner conditioning. The proportion of free water in the sludge cake had no significant change and remained within 11%-13% during conditioning, indicating that the free water removal rate depended on mechanical dehydrating units and had no correlation with sludge conditioning.
Organic acids concentration changes in the sludge supernatant. Fig 6 shows that the three organic acid concentrations in the sludge supernatant, especially formic and acetic acid, markedly increased after conditioning. For the sludge conditioned using Fenton's reagent, the formic and acetic acid concentrations increased to 535.72 and 575.42 mg/L, respectively. This finding further indicated that EPS was effectively degraded because of strong oxidation reactions and released into the liquid phase, which was conducive to the release of the bound water. After the Fenton reaction, the formic and acetic acid concentrations further increased with the addition of DDBAC. The results verified that the PN and PS in S-EPS were resolved into low-molecular-mass organics due to DDBAC hydrolysis, which was similar to the findings reported by Hong et al. [18,19] and Huang et al. [33].

Conclusions
Fenton's reagent conditioning integrated with DDBAC significantly improved sludge dewatering performance when the sludge pH value was 4. The optimum dosages, determined by RSM, of the composite conditioner in this study were 44.6 mg/g H 2 O 2 , 39.6 mg/g Fe 2+ , and 71.0 mg/ g DDBAC, at which 59.67% W C was achieved. Three additional experiments at optimal conditions yielded an average W C of 59.54% ± 0.37%, which showed high agreement with the predicted value.
The mechanism investigations showed that the composite conditioner could effectively decompose EPS into dissolved organics, resulting in the conversion of the bound water into free water. The W B of the sludge conditioned using Fenton's reagent decreased from 3.15 to 1.36 g/g and further decreased to 1.08 g/g after the sludge was jointly conditioned using Fenton's reagent and DDBAC.