Impact of advanced oxidation process (AOPs) pretreatment on enhancing sludge dewaterability: new insights through EPS protein and sludge hydrophilicity/hydrophobicity properties

Impact of advanced oxidation process (AOPs) pretreatment on enhancing sludge dewaterability: new insights through EPS protein and sludge hydrophilicity/hydrophobicity properties | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Impact of advanced oxidation process (AOPs) pretreatment on enhancing sludge dewaterability: new insights through EPS protein and sludge hydrophilicity/hydrophobicity properties Wei Zhang, Jing Zhu, Xue Yang, Xiaohu Dai This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-2997910/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Advanced oxidation processes (AOPs) are important for improving sludge dewatering efficiency. In this study, AOPs such as pretreatment with sodium persulphate and calcium peroxide were investigated as methods for improving the dewaterability of waste activated sludge (raw sludge), digested sludge (DS), and sludge digested for 3 days (DS-3). The results showed that persulphate and peroxide could effectively improve the dewaterability of sludge. The results suggested that the maximum increment of free moisture of 4.69% was achieved for the DS-3 of pretreatment with persulphate, and the mechanism investigation revealed that AOPs pretreatment could enhance sludge floc disintegration, degrade the protein (PN) fraction of extracellular polymeric substances (EPS), and accelerate the release of mechanically bound moisture. Free moisture was significantly negatively correlated with the DS PN concentration in extra-microcolony polymers ( R = 0.850, p < 0.01). Compared with RS, the organic matter content of DS and DS-3 sludge samples pretreated by AOPs was lower, viscosity was decreased, zeta potential was closer to zero, and the strength of amino, amide II and other hydrophilic functional groups decreased or disappeared. Under the same AOPs conditions, the dewateratbility of digested sludge is better than that of RS. This study reveals the mechanisms of sludge AOP conditioning and provides a theoretical basis for practical applications. pretreatment digested sludge dewaterability mechanism hydrophobicity extracellular polymeric substances Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1. Introduction Dewatering is the main method for sludge volume reduction. Anaerobic digestion (AD) is a widely used technology for sludge stabilisation and dewatering improvement (Neyens and Baeyens 2003 , Toutian et al. 2020 ). Pretreatment (thermal, chemical, etc.) has been commonly applied to enhance sludge dewaterability (Zhang et al. 2017 , Cai et al. 2018 , Yuan et al. 2019 , Li et al. 2020 ). Many studies have reported that the effect of sludge pretreatment on improving sludge dewaterability is due to sludge floc disintegration and reducing the protein (PN) content of extracellular polymeric substances (EPS), which help to release bound moisture (Wang et al. 2017 , Wu et al. 2017a , Dai et al. 2018 , Zhang et al. 2019b ). Advanced oxidation processes (AOPs) have been reported as a rapid and effective solution for enhancing sludge dewaterability (Xiao et al. 2017 , Chen et al. 2019 , Zhen et al. 2019 ). Sodium persulphate (SP) is one of the most popular oxidants utilised in various industries; it is used as a bleach for dry cleaning, as an initiator of organically synthesised monomeric polymerisation, and as microetchers for circuit boards (Zhen et al. 2019 ). It is well known that calcium peroxide (CP) is a traditional solid inorganic peroxy compound that has stable oxidation capability (Huang et al. 2019b ). Fe(II) can reduce persulphate ions and simultaneously generate strong oxydic sulphate radicals, as follows [Eq. ( 1 )]: $$F{e^{2+}}+{S_2}{O_8}^{{2 - }} \to F{e^{3+}}+S{O_4}^{{2 - }}+\cdot S{O_4}^{ - }$$ 1 Compared with hydroxyl radicals, sulphate radicals have a higher oxidation potential, which makes the oxidisation reactions with sludge easy to start (Zhen et al. 2012a ). Some studies have reported that sludge dewaterability is improved by acidification by chemical agents (Cai et al. 2018 , Wei et al. 2019 , Zhang et al. 2019a ). However, it is not clear whether the acidification stage sludge in the AD process has better dewaterability. Moreover, information on the differences in dewaterability among raw sludge (RS), digested sludge (DS), and DS in the acidification stage is lacking. In addition, parameter optimisation of the AD process and sludge dewatering efficiency improvement remain to be explored. The objective of this study was to investigate the effect of AOP (persulphate and peroxide) pretreatment on waste activated sludge (RS) and DS dewaterability. The dewatered sludge supernatant was evaluated and controlled and the mechanism of AOP pretreatment on sludge dewaterability was analysed. The results will help us to understand the effect of AOP pretreatment on sludge dewaterability and optimise the digestion process parameters for realising maximum sludge reduction. 2. Materials and Methods 2.1 Experimental Materials and Procedures The experimental materials (RS and DS) were obtained from municipal wastewater treatment plants in Shanghai (China) and stored at 4°C until use. DS was the seed sludge, RS was the feed sludge, and DS-3 samples were obtained after digestion for 3 d. Seed sludge was mixed with feed sludge at a ratio of 1:1 w/w [in terms of volatile solids (VS)]. The main characteristics of the sludge samples are listed in Table 1 . A series of experiments was conducted after pretreatment of RS, DS, and DS-3 with SP and CP, respectively. In the conditioning procedure, an appropriate amount of SP (0-300 mg/g TS) and CP (0-120 mg/g TS) were added to 100 mL sludge samples contained in a beaker under continuous stirring at 300 rpm, and the pre-oxidation treatment occurred and continued for 10 min. Ferrous sulphate (30 mg/g dry sludge) was then added, and the mixture was agitated for 10 min at 150 rpm. To investigate the suitable conditions for sludge conditioning by SP and CP, the sludge was treated at different pH levels (pH of 3, 4, 5, 6, and 7). All experiments were conducted in triplicate at room temperature in 20 ℃. 2.2 Evaluation of Sludge Dewaterability and Dewatered Filtrate (1) Sludge dewaterability index The moisture distribution was used to characterise sludge dewaterability. The moisture distribution was measured as described in our previous study (Zhang et al. 2019b ). (2) Evaluation of sludge supernatant The total organic carbon (TOC), total phosphorus (TP), total nitrogen (TN), calcium ion (Ca 2+ ), and iron ion (Fe 2+ ) contents in the supernatant were measured by standard methods after filtration through a 0.45 µm membrane. TOC was measured using a TOC analyser (TOC-L CPH, Shimadzu, Japan), TN was determined using the basic potassium persulphate process, and TP was quantified using the ammonium molybdate spectrophotometric method. Calcium and iron ions were measured using an inductively coupled plasma spectrometer (ICP-AES, Optima 2100 DV, Thermo Fisher Scientific, USA). (3) Other sludge parameters Other parameters such as total solids and VS were determined using standard methods of the Ministry of Ecology and Environment of the People’s Republic of China (HJ 761–2015, China 2015). The higher heating value was determined using a calorimeter (Kaiyuan Instrument Co., Ltd., China). The zeta potential of the supernatant was measured using a Zetasizer Nano Z (Malvern, UK). The solution was filtered through a 0.45 µm polyether sulphone membrane to remove suspended substances. The concentration was then kept constant after dilution at a pH of 7. Each sample was measured in triplicate. 2.3 EPS The EPS content at the floc level (extra-microcolony polymers; EMPS) and microcolony level (extracellular polymers; ECPS) were obtained using cation exchange resin (Frolund et al. 1996 ) and ultrasonic methods (Liu et al. 2002) respectively. EPS extraction and determination of soluble PNs, soluble polysaccharides (PS), rheological properties, and volatile fatty acid (VFA) production were conducted in accordance with the methods of our previous study (Zhang et al. 2019b ). 2.4 Apparent Characteristics of Sludge (1) Fourier transform infrared spectroscopy A mixture of 1–2 mg of freeze-dried DS samples and 100 mg of potassium bromide (IR grade) was pressed into pellets. The pellets were then ground to be homogenised and analysed using Fourier transform infrared spectroscopy (FTIR) (Nicolet 5700, Thero Electron Corporation, USA) from 400 cm − 1 to 4000 cm − 1 . The FTIR results were normalised by dividing them by the sum of the transmittance values. (2) Contact angle and surface characteristic evaluation The interfacial tension and surface free energy of the DS were estimated using a contact angle approach (JC2000A, Powereach Co., Shanghai, China). Homogeneous sludge layers were prepared and placed on slides as described by Liu et al. ( 2007), and the advancing contact angles were directly measured using the sessile drop technique with a drop of liquid water, 1-bromonaphthalene, and formamide. All contact angle values were based on the arithmetic means of at least five independent measurements. 2.5 Statistical Analysis The relationships between sludge dewaterability indicators and physical and chemical parameters were analysed by linear regression using SPSS 17.0. All batch digestion experiments in this study were conducted in triplicate, and the results were expressed as the mean ± standard deviation. One-way analysis of variance was used to evaluate the significance of the results, and statistical significance was set at p < 0.05. 3. Results and Discussion 3.1 Effects of AOPs and AD on Sludge Dewaterability As shown in Fig. 1 , the free moisture of RS, DS, and DS-3 increased with activated persulphate and peroxide treatments, and the mechanically bound moisture (MBM) of RS, DS, and DS-3 decreased with AOP treatments, thereby indicating that pretreatment with AOPs improved sludge dewaterability. For the SP treatments, the ratio of the free moisture (FM) content to the total moisture content for RS, DS, and DS-3 increased from 73.21%, 75.55%, and 74.44–76.92%, 78.74%, and 79.13%, respectively, thereby increasing by 3.71%, 3.19%, and 4.69%, respectively. The MBM decreased by 3.60%, 3.26%, and 4.66%, respectively. For the CP treatments, the ratio of the FM content to the total moisture content for RS, DS, and DS-3 increased from 73.21%, 75.55%, and 74.44–75.72%, 78.48%, and 74.88%, respectively, thereby increasing by 2.51%, 2.93%, and 0.44%, respectively. The MBM decreased by 3.34%, 3.25%, and 1.24%, respectively. Moreover, MBM was found to be the most strongly correlated with the PN content in EPS, zeta potential, and rheological parameters. Table S1 indicates that MBM was positively and strongly correlated with the PN-EMPS and PN-ECPS for RS ( R = 0.954** and R = 0.963**, respectively) and significantly negatively correlated with the zeta potential for RS ( R =-0.963**, p < 0.01). Moreover, a significant negative correlation was found between free moisture (FM) and MBM for RS ( R =-0.952**, p < 0.01). 3.2 Evaluation and Analysis of Dewatered Filtrate 3.2.1 Variation in TOC in Sludge Supernatant The sludge disintegration degree increased as the AOPs progressed. As shown in Fig. 2 a, the TOC concentration increased during pretreatment. The maximum TOC concentration of RS under the pretreatment condition was 420.30 mg/L, which was approximately 4.13 times higher than that of the blank. This result indicated that AOP treatment could promote sludge solubilisation and organics release (Wu et al. 2019 ). The increase in the TOC concentration could be the result of the release of intercellular substances such as EPS, which is consistent with the decrease in the EPS content of the sludge, as discussed in Section 3.3.2. An increase in the TOC concentration commonly occurs when using advanced oxidation methods for sludge conditioning (Ni et al. 2019 ). The disintegration of sludge flocs and microbial cell breakdown resulted in the release of more soluble organic matter into the sludge supernatant (Huang et al. 2019a ). It has been reported that bound water is held by sludge flocs and microorganisms and can be released when the structure or microorganisms are decomposed (Zhen et al. 2012a ). Therefore, the release of TOC could partly reflect the variation in sludge dewaterability (Zhou et al. 2015 ). This implies that the oxidisation processes facilitated the decomposition of sludge and release of bound moisture. The results agree with those reported by Wu et al. ( 2017a ). 3.2.2 Variation in TP and TN in Sludge Filtrate The TP concentrations in the supernatant of RS and DS after persulphate pretreatment decreased considerably to 80.4 mg/L and 21.4 mg/L, respectively. The concentrations of TP in RS and DS after peroxide pretreatment were 48.6 mg/L and 2.9 mg/L, respectively (Fig. 2 b). This might have resulted from ferric cations combining with PO 4 3− to form Fe-PO 4 3− during AD (Li et al. 2019 ). The nitrogen in the liquid phase after pretreatment was mainly in the form of organic nitrogen, and the TN concentrations in the RS, DS, and DS-3 were 254.94/248.04 mg/L, 1176.30/1128.90 mg/L, and 806.70/756.90 mg/L (Fig. 2 c), respectively. The TN in the liquid phase gradually increased, thereby indicating that a large amount of organic nitrogen was converted into the liquid phase. 3.2.3 Variation in Metal Cations and Conductivity The distribution of the main cations such as calcium, magnesium, iron, and aluminium in the dewatered filtrate before and after CP pretreatment is presented in Fig. 2 d. The calcium ion content was the highest, followed by that of magnesium ions. After pretreatment with CP, the calcium ion concentration was significantly increased in the sludge samples, and the calcium ion concentration in the dewatered filtrate of RS, DS, and DS-3 increased by 6.5, 3.6, and 5.6 fold compared with that in the primary samples, respectively. These results indicate that CP addition is beneficial for the removal of phosphorus compared with conventional treatment. This corresponded well with the results of the decrease in TP. This could be attributed to the release of calcium ions during AD. A large amount of calcium ions could be produced by calcium carbonate decomposition owing to the supply of H + ions from VFAs, which could easily precipitate with phosphate, as shown in Equations ( 2 ) and ( 3 ). These findings are consistent with those reported by Zhang et al. ( 2016 ). The conductivity of the dewatered filtrate of RS, DS, and DS-3 after persulphate and peroxide pretreatment was higher than that of the untreated sludge samples, but the conductivity values were lower than 10 mS/cm (Fig. 2 e). 3.3 Insights Into the Effects of AOPs and AD 3.3.1 VFA Composition and Distribution The VFA production profiles of different sludge samples are illustrated in Fig. 3 . It was observed that the VFA production of DS was lower than that of RS. Acetic acid was the most prevalent product in the RS samples, which was in accordance with the findings of Huang et al. ( 2019a ). AD contributed to a greater proportion of n -valeric acid, which accounted for 24.28% and 28.83% of the total VFAs for DS and DS-3, respectively. (Wu et al. 2017b ). reported that acetic, propionic, and iso-valeric acids were the three main products under alkaline conditions, which had a greater advantage over the improvement of acetic acid yield This may be explained by the fact that sludge hydrolysis and acidification were more efficient and sufficient in the AD process, thereby leading to the conversion of high-molecular-weight VFAs to acetic acid and transformation into methane and carbon dioxide. 3.3.2 Effect of Sludge EPS on Sludge Dewaterability The distributions of PN and PS contents in EPS are shown in Fig. 4 . During the persulphate and peroxide treatments, the PN and PS contents in EMPS and ECPS decreased. Accordingly, the trend of change in the PS content with persulphate and peroxide adjustment were roughly similar to those of the PN content in the EPS, as shown in Fig. 4 c. The initial concentrations of PN and PS of EMPS in the RS samples were 24.38 mg/g VS and 6.58 mg/g VS, respectively. The concentrations of PN and PS decreased with persulphate and peroxide treatments (Fig. 4 a, 4 c). After pretreatment with persulphate, the PN and PS contents of EMPS decreased by 57.51% and 55.47%, respectively. After pretreatment with peroxide, the PN and PS contents of EMPS decreased by 67.97% and 2.28%, respectively. PN plays a more important role than PS in influencing sludge dewaterability, especially for PN in EPS at the outer layer, including soluble EPS and loosely bound EPS (Shao et al. 2010 ). In our previous study, PN-EMPS played a key role in enhancing sludge dewaterability during AD (Zhang et al. 2019b ). These results further confirm the release of EPS owing to the disruption of sludge flocs caused by AOP pretreatment. The enhanced sludge dewaterability by AOP conditioning was due to the destruction of sludge flocs, thereby causing the degradation of EPS and the release of bound moisture. In addition, a significant positive correlation was observed between MBM and PN-EMPS and PN-ECPS for RS ( R = 0.954** and R = 0.963**, respectively) (Table S1 ). 3.3.3 Zeta Potential The zeta potential is a critical factor influencing sludge dewaterability (Zhen et al. 2012b ). All the AOP treatments for the three types of sludge significantly increased the zeta potential of the sludge flocs, and the zeta potential in the sludge supernatant approached zero (Fig. 5 ). Figure 5 shows that the zeta potentials of the RS, DS, and DS-3 flocs increased from − 20.5, -18.7, and − 20.1 mV to -5.6, -7.2, and − 1.6 mV after persulphate conditioning, respectively. These observations were consistent with those of previous reports indicating that the surface charges of sludge flocs reach nearly zero after oxidation treatments (Chen et al. 2019 ). This might have been caused by the accelerated accumulation of sludge flocs and improved sludge dewaterability. Moreover, it has been reported that the ionisation of anionic groups in EPS, such as carboxyl groups and amino groups, account for the decrease in floc surface electronegativity after oxidation treatments (Sheng et al. 2010 , Wu et al. 2017a ). Mikkelsen et al (2002) also found that floc disintegration created more exposed surfaces in the floc interior, which also increased the surface electronegativity. According to the divalent cationic bridging theory, it is believed that divalent cations can neutralise negatively charged colloids and bond with the functional groups of soluble PN and carbohydrates (Higgins and Novak. 1997). 3.3.4 Hydrophobic Properties and Functional Groups of Sludge Disintegration of sludge flocs and an increase in soluble organic matter were also observed in the persulphate and peroxide pretreatments (Fig. 6 a). The higher zeta potential in the sludge supernatant indicated that some of the hydrophilic groups in the sludge flocs were destroyed by the adsorbed hydroxyl radicals and solid-state active oxygen, with the reduction in hydrophilic groups, more bound moisture attached to organic matter could be released as free moisture (Chen et al. 2019 , Zhang et al. 2020 ). The contact angle and surface free energy values are presented in Table S4. The contact angle between the water and sludge increased after the oxidation treatment; i.e. the sludge surface became hydrophobic. The surface free energy was negative, which suggested that the sludge surface was hydrophobic, thereby indicating that sludge dewaterability was improved (Zhang et al. 2020 ). The sludge samples were further analysed by FTIR, and the corresponding spectra are shown in Fig. 6 . The peaks at 1658 cm − 1 , 1539 cm − 1 , and 1230 cm − 1 were assigned to the C = O stretching vibration of amide I, the N–H deformation vibration of amide II, and the C–N stretching vibration of amide III, respectively (Schmitt and Flemming, 1998 ). The peak at 1645 cm − 1 corresponds to the stretching vibration of the carbonyl group (C–O) in the PN amide I band. The peaks at 1614 cm − 1 and 1550 cm − 1 correspond to the C–N stretching vibration and the N–H bending vibration in the amide II band, thereby indicating the presence of PN. The peaks near 1245 cm − 1 and 1070 cm − 1 are the C–O–C and C–OH stretching vibrations, respectively, which indicate the presence of PS. Figure 6 shows that persulphate and peroxide did not affect the positions of the peaks, but did change the transmission intensity of the peaks. The peaks for RS in persulphate pretreatment and RS in peroxide pretreatment remained unchanged; however, some new peaks appeared after persulphate and peroxide conditioning, and functional groups such as –NH 2 , amide II, and –CH 2 -N + /carboxylic groups appeared. The intensity of C = O, C–N, and –OH/C–O–C increased. Moreover, the peak for DS was different from that of RS because the organic matter was degraded during the AD process. In addition, the peak shapes for DS in persulphate pretreatment and DS in peroxide pretreatment remained stable; however, some peaks disappeared after persulphate conditioning, with functional groups such as –NH 2 , amide II, and –CH 2 -N + /carboxylic groups. The intensities of C = O and C–N decreased. For DS-3, the main peak almost disappeared after the persulphate and peroxide pretreatments. This was especially true for peroxide treatment. In addition, there was no clear amide I peak for DS-3. 3.3.5 Rheological properties The results of the sludge rheological properties before and after AOP conditioning are shown in Fig. 7 . The viscosity decreased after AOP treatment, i.e. sludge flowability increased as viscosity decreased. As shown in Fig. 7 , the concentrations of total organic compounds decreased, and more organic compounds, i.e. PN and PS, might have been released from the inner particles to the outer layer by AOP treatment. Moreover, EPS were transferred from particles into the bulk fluid, thereby making the flow easier and increasing the functional group and surface charge of the bulk fluid during the AD process. As shown in Tables S1–S3, the correlation between the mechanically bound moisture content and rheological properties was significant (Miryahyaei et al. 2019 ). As demonstrated in previous sections, lower viscosity was correlated with the lowest amount of MBM during the digestion period. Furthermore, viscosity was significantly correlated with PN-EMPS and PN-ECPS for RS, DS, and DS-3 ( R = 0.862**, R = 0.794*; R = 0.762*, R = 0.938**; and R = 0.612*, R = 0.812**, respectively). An exception was found for DS-3 after the treatment with peroxide; the viscosity increased, which was probably due to the increase in the sludge concentration as peroxide addition increased for DS-3. 4. Conclusion In this study, three types of sludge (RS, DS, and DS-3) were treated using persulphate and peroxide processes to study the mechanisms of sludge AOPs conditioning. The highest MBM reduction was 4.69% for DS-3 in the presence of persulphate. The mechanism revealed that AOPs pretreatment significantly accelerated sludge disintegration, which caused the degradation of EPS, promoted the degradation of the PN fraction of EPS, and induced the release of MBM. Moreover, compared with RS, the organic matter content of DS and DS-3 sludge samples pretreated by AOPs was lower, viscosity was decreased, zeta potential was closer to zero, and the strength of amino, amide II and other hydrophilic functional groups decreased or disappeared. Under the same AOPs conditions, the dewateratbility of digested sludge is better than that of RS. Declarations Acknowledges This work was financially supported by the National Natural Science Foundation of China (52100151). Ethical Approval Compliance with Ethical Standards The authors declare that they have no known potential conflicts of interest that could have appeared to influence the work reported in this paper. No human and/or animal studies were involved in this study. All authors give informed consent. The submitted work was original and have not been published elsewhere in any form or language (partially or in full) Consent to Participate No human and/or animal studies were involved in this study. All the authors agree to submit this manuscript to Environmental Science and Pollution Research. Consent to Publish This work has not been published before; No considered for publication elsewhere; Its publication has been approved by all authors. Authors Contributions All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Wei Zhang, Jing Zhu, Xue Yang and Xiaohu Dai. The first draft of the manuscript was written by Wei Zhang and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.” Funding “This work was supported by National Natural Science Foundation of China (52100151) . Author Wei Zhang has received research support from National Natural Science Foundation of China .” Competing Interests “The authors have no relevant financial or non-financial interests to disclose.” Statements and Declarations ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Availability of data and materials Other data that need to be supplemented have been uploaded as supplementary material files, and there is no other data and materials. References Cai MQ, Hu JQ, Wells G, Seo Y, Spinney R, Ho SH, Dionysiou DD, Su J, Xiao R, Wei Z (2018) Understanding Mechanisms of Synergy between Acidification and Ultrasound Treatments for Activated Sludge Dewatering: From Bench to Pilot-Scale Investigation. 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J Environ Sci 48:200–208 Zhen G, Lu X, Li Y, Zhao Y, Wang B, Song Y, Chai X, Niu D, Cao X (2012a) Novel insights into enhanced dewaterability of waste activated sludge by Fe(II)-activated persulfate oxidation. Bioresour Technol 119:7–14 Zhen G, Lu X, Wang B, Zhao Y, Chai X, Niu D, Zhao A, Li Y, Song Y, Cao X (2012b) Synergetic pretreatment of waste activated sludge by Fe(II)-activated persulfate oxidation under mild temperature for enhanced dewaterability. Bioresour Technol 124:29–36 Zhen G, Tan Y, Wu T, Wang J, Lu X, Zhao Y, Zhu X, Niu J, Xiong J (2019) Strengthened dewaterability of coke-oven plant oily sludge by altering extracellular organics using Fe(II)-activated persulfate oxidation. Sci Total Environ 688:1155–1161 Zhou X, Wang Q, Jiang G, Liu P, Yuan Z (2015) A novel conditioning process for enhancing dewaterability of waste activated sludge by combination of zero-valent iron and persulfate. Bioresour Technol 185:416–420 Tables Table 1. The characteristics of sludge samples pH TS (%) VS/TS (%) Zeta potential (mV) Water content (%) HHV (MJ/kg) RS 7.85 0.02 2.870.01 64.100.01 -20.50.85 97.130.01 18.13 DS 8.39 3.730.09 51.49 -18.731.58 96.270.10 14.38 DS-3 7.63 3.28 59.310.18 -21.100.30 96.720.01 13.55 Note: SCOD, soluble chemical oxygen demand; TS, total solid; VS, volatile solid; HHV, higher heating value. Table 2. Surface tension of digested sludge. Total interfacial tension, and total interfacial free energy between digested sludge and water (mJ/m 2 ) Zeta potential (mV) Contact angle (deg) (mJ/m 2 ) Water 1-bromonaphthalene formamide RS -20.50.85 80.10 45.44 81.90 166.48 RS-SDS -5.620.65 97.68 44.86 79.63 -45.82 RS-CaO2 -10.250.75 88.73 47.19 75.72 138.56 DS -18.731.58 75.59 46.29 72.69 621.93 DS-SDS -7.242.90 78.42 43.48 74.71 -65.03 DS-CaO2 -6.480.73 77.06 41.67 70.49 -53.65 DS-3 -21.10.30 85.19 46.80 69.64 24.33 DS-3-SDS -1.590.99 84.64 40.85 71.55 -100.59 DS-3-CaO2 -6.282.76 76.77 45.18 75.08 -9.83 Supplementary Files Supplementarymaterials.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-2997910","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":231490148,"identity":"f09e5c36-22e4-4053-94c4-733c0c432a74","order_by":0,"name":"Wei Zhang","email":"data:image/png;base64,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","orcid":"","institution":"University of Shanghai for Science and Technology","correspondingAuthor":true,"submittingAuthor":false,"prefix":"","firstName":"Wei","middleName":"","lastName":"Zhang","suffix":""},{"id":231490149,"identity":"2daf0877-8ff3-4cb6-97f3-a6cd5ec0fcfe","order_by":1,"name":"Jing Zhu","email":"","orcid":"","institution":"University of Shanghai for Science and Technology","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Jing","middleName":"","lastName":"Zhu","suffix":""},{"id":231490150,"identity":"2af18a06-db7b-4d43-a82f-83a3a0a8fc1d","order_by":2,"name":"Xue Yang","email":"","orcid":"","institution":"University of Shanghai for Science and Technology","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Xue","middleName":"","lastName":"Yang","suffix":""},{"id":231490151,"identity":"cca3d874-35ac-45e6-9912-6696f424f854","order_by":3,"name":"Xiaohu Dai","email":"","orcid":"","institution":"Tongji University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Xiaohu","middleName":"","lastName":"Dai","suffix":""}],"badges":[],"createdAt":"2023-05-30 04:28:40","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-2997910/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-2997910/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":43008949,"identity":"e6ae9f3b-f49c-4c90-b0f7-9c3860ec3e60","added_by":"auto","created_at":"2023-09-12 14:22:42","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":24863,"visible":true,"origin":"","legend":"\u003cp\u003eMoisture distribution of three kinds sludge by different approaches.\u003c/p\u003e\n\u003cp\u003eNote: FM, free moisture; MBM, mechanically bound moisture; BM, bound moisture;\u003c/p\u003e","description":"","filename":"F1.png","url":"https://assets-eu.researchsquare.com/files/rs-2997910/v1/b3fbe56cc75df885618ea33d.png"},{"id":43008950,"identity":"049cb483-fc41-43ec-b522-8c51f17ba148","added_by":"auto","created_at":"2023-09-12 14:22:42","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":59974,"visible":true,"origin":"","legend":"\u003cp\u003ea. TOC concentration of dewatered filtrate.\u003c/p\u003e\n\u003cp\u003eb. TP concentration of dewatered filtrate\u003c/p\u003e\n\u003cp\u003ec.TN concentration of dewatered filtrate\u003c/p\u003e\n\u003cp\u003ed Metals concentration of dewatered filtrate\u003c/p\u003e\n\u003cp\u003ee. Conductivity of dewatered filtrate\u003c/p\u003e\n\u003cp\u003eTOC, TP, TN, metals concentration and conductivity of dewatered filtrate\u003c/p\u003e","description":"","filename":"F2.png","url":"https://assets-eu.researchsquare.com/files/rs-2997910/v1/9e68e7d9bfc8943e83d8c7f9.png"},{"id":43007752,"identity":"f536c4ef-b951-46c0-83d3-4c555c219a7b","added_by":"auto","created_at":"2023-09-12 14:14:42","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":12813,"visible":true,"origin":"","legend":"\u003cp\u003eVFAs composition and distribution of sludge.\u003c/p\u003e","description":"","filename":"F3.png","url":"https://assets-eu.researchsquare.com/files/rs-2997910/v1/658b5b3f549ae89f4d157407.png"},{"id":43007756,"identity":"2e1de018-6b00-4852-9a36-457417739a33","added_by":"auto","created_at":"2023-09-12 14:14:42","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":70724,"visible":true,"origin":"","legend":"\u003cp\u003ea. The protein content of EMPS in sludge\u003c/p\u003e\n\u003cp\u003eb. The protein content of ECPS in sludge\u003c/p\u003e\n\u003cp\u003ec. The polysaccharides content of EMPS/ECPS in sludge\u003c/p\u003e\n\u003cp\u003eThe protein and polysaccharides content of EPS.\u003c/p\u003e","description":"","filename":"F4.png","url":"https://assets-eu.researchsquare.com/files/rs-2997910/v1/d589e4f8ac76a180efb9636e.png"},{"id":43008951,"identity":"7d935942-400e-4c8c-abec-80de1a202079","added_by":"auto","created_at":"2023-09-12 14:22:42","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":13608,"visible":true,"origin":"","legend":"\u003cp\u003eZeta potential of sludge supernatant.\u003c/p\u003e","description":"","filename":"F5.png","url":"https://assets-eu.researchsquare.com/files/rs-2997910/v1/60de7b4dde8487499ccbd61b.png"},{"id":43007757,"identity":"18d6a04a-586e-4537-9a34-0411ba4c8d7b","added_by":"auto","created_at":"2023-09-12 14:14:42","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":42504,"visible":true,"origin":"","legend":"\u003cp\u003ea. FTIR of RS.\u003c/p\u003e\n\u003cp\u003eb. FTIR of DS.\u003c/p\u003e\n\u003cp\u003ec FTIR in DS-3\u003c/p\u003e\n\u003cp\u003eFTIR of sludge samples.\u003c/p\u003e","description":"","filename":"F6.png","url":"https://assets-eu.researchsquare.com/files/rs-2997910/v1/6625ea80582b8934e77b336f.png"},{"id":43007759,"identity":"be36908a-8055-4924-8275-685f2e7251a8","added_by":"auto","created_at":"2023-09-12 14:14:42","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":36097,"visible":true,"origin":"","legend":"\u003cp\u003ea viscosity for RS samples.\u003c/p\u003e\n\u003cp\u003eb. Viscosity for DS\u003c/p\u003e\n\u003cp\u003ec. Viscosity for DS-3\u003c/p\u003e\n\u003cp\u003eViscosity in sludge samples.\u003c/p\u003e","description":"","filename":"F7.png","url":"https://assets-eu.researchsquare.com/files/rs-2997910/v1/337e9eea73229435efca52f4.png"},{"id":45121029,"identity":"1ee62b77-be53-4676-a919-f1d6d95a0a0a","added_by":"auto","created_at":"2023-10-24 01:35:49","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":699703,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-2997910/v1/01006d1e-777c-4a79-9692-806f50514895.pdf"},{"id":43007761,"identity":"60ec00b7-3039-4007-8a05-faf16d84bb18","added_by":"auto","created_at":"2023-09-12 14:14:42","extension":"docx","order_by":7,"title":"","display":"","copyAsset":false,"role":"supplement","size":28664,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementarymaterials.docx","url":"https://assets-eu.researchsquare.com/files/rs-2997910/v1/ac732c81ae536dde7dabe492.docx"}],"financialInterests":"","formattedTitle":"Impact of advanced oxidation process (AOPs) pretreatment on enhancing sludge dewaterability: new insights through EPS protein and sludge hydrophilicity/hydrophobicity properties","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eDewatering is the main method for sludge volume reduction. Anaerobic digestion (AD) is a widely used technology for sludge stabilisation and dewatering improvement (Neyens and Baeyens \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2003\u003c/span\u003e, Toutian et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Pretreatment (thermal, chemical, etc.) has been commonly applied to enhance sludge dewaterability (Zhang et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2017\u003c/span\u003e, Cai et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2018\u003c/span\u003e, Yuan et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2019\u003c/span\u003e, Li et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Many studies have reported that the effect of sludge pretreatment on improving sludge dewaterability is due to sludge floc disintegration and reducing the protein (PN) content of extracellular polymeric substances (EPS), which help to release bound moisture (Wang et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2017\u003c/span\u003e, Wu et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2017a\u003c/span\u003e, Dai et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2018\u003c/span\u003e, Zhang et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2019b\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eAdvanced oxidation processes (AOPs) have been reported as a rapid and effective solution for enhancing sludge dewaterability (Xiao et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2017\u003c/span\u003e, Chen et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2019\u003c/span\u003e, Zhen et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Sodium persulphate (SP) is one of the most popular oxidants utilised in various industries; it is used as a bleach for dry cleaning, as an initiator of organically synthesised monomeric polymerisation, and as microetchers for circuit boards (Zhen et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). It is well known that calcium peroxide (CP) is a traditional solid inorganic peroxy compound that has stable oxidation capability (Huang et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2019b\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eFe(II) can reduce persulphate ions and simultaneously generate strong oxydic sulphate radicals, as follows [Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e)]:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$F{e^{2+}}+{S_2}{O_8}^{{2 - }} \\to F{e^{3+}}+S{O_4}^{{2 - }}+\\cdot S{O_4}^{ - }$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eCompared with hydroxyl radicals, sulphate radicals have a higher oxidation potential, which makes the oxidisation reactions with sludge easy to start (Zhen et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2012a\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eSome studies have reported that sludge dewaterability is improved by acidification by chemical agents (Cai et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2018\u003c/span\u003e, Wei et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2019\u003c/span\u003e, Zhang et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2019a\u003c/span\u003e). However, it is not clear whether the acidification stage sludge in the AD process has better dewaterability. Moreover, information on the differences in dewaterability among raw sludge (RS), digested sludge (DS), and DS in the acidification stage is lacking. In addition, parameter optimisation of the AD process and sludge dewatering efficiency improvement remain to be explored.\u003c/p\u003e \u003cp\u003eThe objective of this study was to investigate the effect of AOP (persulphate and peroxide) pretreatment on waste activated sludge (RS) and DS dewaterability. The dewatered sludge supernatant was evaluated and controlled and the mechanism of AOP pretreatment on sludge dewaterability was analysed. The results will help us to understand the effect of AOP pretreatment on sludge dewaterability and optimise the digestion process parameters for realising maximum sludge reduction.\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003ch2\u003e2.1 Experimental Materials and Procedures\u003c/h2\u003e\n \u003cp\u003eThe experimental materials (RS and DS) were obtained from municipal wastewater treatment plants in Shanghai (China) and stored at 4\u0026deg;C until use. DS was the seed sludge, RS was the feed sludge, and DS-3 samples were obtained after digestion for 3 d. Seed sludge was mixed with feed sludge at a ratio of 1:1 w/w [in terms of volatile solids (VS)]. The main characteristics of the sludge samples are listed in Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\n \u003cp\u003eA series of experiments was conducted after pretreatment of RS, DS, and DS-3 with SP and CP, respectively. In the conditioning procedure, an appropriate amount of SP (0-300 mg/g TS) and CP (0-120 mg/g TS) were added to 100 mL sludge samples contained in a beaker under continuous stirring at 300 rpm, and the pre-oxidation treatment occurred and continued for 10 min. Ferrous sulphate (30 mg/g dry sludge) was then added, and the mixture was agitated for 10 min at 150 rpm. To investigate the suitable conditions for sludge conditioning by SP and CP, the sludge was treated at different pH levels (pH of 3, 4, 5, 6, and 7). All experiments were conducted in triplicate at room temperature in 20 ℃.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n \u003ch2\u003e2.2 Evaluation of Sludge Dewaterability and Dewatered Filtrate\u003c/h2\u003e\u003cspan\u003e\n \u003cp\u003e(1) Sludge dewaterability index\u003c/p\u003e\n \u003c/span\u003e\n \u003cp\u003eThe moisture distribution was used to characterise sludge dewaterability. The moisture distribution was measured as described in our previous study (Zhang et al. \u003cspan class=\"CitationRef\"\u003e2019b\u003c/span\u003e).\u003c/p\u003e\u003cspan\u003e\n \u003cp\u003e(2) Evaluation of sludge supernatant\u003c/p\u003e\n \u003c/span\u003e\n \u003cp\u003eThe total organic carbon (TOC), total phosphorus (TP), total nitrogen (TN), calcium ion (Ca\u003csup\u003e2+\u003c/sup\u003e), and iron ion (Fe\u003csup\u003e2+\u003c/sup\u003e) contents in the supernatant were measured by standard methods after filtration through a 0.45 \u0026micro;m membrane.\u003c/p\u003e\n \u003cp\u003eTOC was measured using a TOC analyser (TOC-L CPH, Shimadzu, Japan), TN was determined using the basic potassium persulphate process, and TP was quantified using the ammonium molybdate spectrophotometric method. Calcium and iron ions were measured using an inductively coupled plasma spectrometer (ICP-AES, Optima 2100 DV, Thermo Fisher Scientific, USA).\u003c/p\u003e\u003cspan\u003e\n \u003cp\u003e(3) Other sludge parameters\u003c/p\u003e\n \u003c/span\u003e\n \u003cp\u003eOther parameters such as total solids and VS were determined using standard methods of the Ministry of Ecology and Environment of the People\u0026rsquo;s Republic of China (HJ 761\u0026ndash;2015, China 2015). The higher heating value was determined using a calorimeter (Kaiyuan Instrument Co., Ltd., China). The zeta potential of the supernatant was measured using a Zetasizer Nano Z (Malvern, UK). The solution was filtered through a 0.45 \u0026micro;m polyether sulphone membrane to remove suspended substances. The concentration was then kept constant after dilution at a pH of 7. Each sample was measured in triplicate.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n \u003ch2\u003e2.3 EPS\u003c/h2\u003e\n \u003cp\u003eThe EPS content at the floc level (extra-microcolony polymers; EMPS) and microcolony level (extracellular polymers; ECPS) were obtained using cation exchange resin (Frolund et al. \u003cspan class=\"CitationRef\"\u003e1996\u003c/span\u003e) and ultrasonic methods (Liu et al. 2002) respectively. EPS extraction and determination of soluble PNs, soluble polysaccharides (PS), rheological properties, and volatile fatty acid (VFA) production were conducted in accordance with the methods of our previous study (Zhang et al. \u003cspan class=\"CitationRef\"\u003e2019b\u003c/span\u003e).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n \u003ch2\u003e2.4 Apparent Characteristics of Sludge\u003c/h2\u003e\u003cspan\u003e\n \u003cp\u003e(1) Fourier transform infrared spectroscopy\u003c/p\u003e\n \u003c/span\u003e\n \u003cp\u003eA mixture of 1\u0026ndash;2 mg of freeze-dried DS samples and 100 mg of potassium bromide (IR grade) was pressed into pellets. The pellets were then ground to be homogenised and analysed using Fourier transform infrared spectroscopy (FTIR) (Nicolet 5700, Thero Electron Corporation, USA) from 400 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e to 4000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The FTIR results were normalised by dividing them by the sum of the transmittance values.\u003c/p\u003e\u003cspan\u003e\n \u003cp\u003e(2) Contact angle and surface characteristic evaluation\u003c/p\u003e\n \u003c/span\u003e\n \u003cp\u003eThe interfacial tension and surface free energy of the DS were estimated using a contact angle approach (JC2000A, Powereach Co., Shanghai, China). Homogeneous sludge layers were prepared and placed on slides as described by Liu et al. ( 2007), and the advancing contact angles were directly measured using the sessile drop technique with a drop of liquid water, 1-bromonaphthalene, and formamide. All contact angle values were based on the arithmetic means of at least five independent measurements.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\n \u003ch2\u003e2.5 Statistical Analysis\u003c/h2\u003e\n \u003cp\u003eThe relationships between sludge dewaterability indicators and physical and chemical parameters were analysed by linear regression using SPSS 17.0. All batch digestion experiments in this study were conducted in triplicate, and the results were expressed as the mean\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviation. One-way analysis of variance was used to evaluate the significance of the results, and statistical significance was set at \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\n \u003ch2\u003e3.1 Effects of AOPs and AD on Sludge Dewaterability\u003c/h2\u003e\n \u003cp\u003eAs shown in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e, the free moisture of RS, DS, and DS-3 increased with activated persulphate and peroxide treatments, and the mechanically bound moisture (MBM) of RS, DS, and DS-3 decreased with AOP treatments, thereby indicating that pretreatment with AOPs improved sludge dewaterability.\u003c/p\u003e\n \u003cp\u003eFor the SP treatments, the ratio of the free moisture (FM) content to the total moisture content for RS, DS, and DS-3 increased from 73.21%, 75.55%, and 74.44\u0026ndash;76.92%, 78.74%, and 79.13%, respectively, thereby increasing by 3.71%, 3.19%, and 4.69%, respectively. The MBM decreased by 3.60%, 3.26%, and 4.66%, respectively.\u003c/p\u003e\n \u003cp\u003eFor the CP treatments, the ratio of the FM content to the total moisture content for RS, DS, and DS-3 increased from 73.21%, 75.55%, and 74.44\u0026ndash;75.72%, 78.48%, and 74.88%, respectively, thereby increasing by 2.51%, 2.93%, and 0.44%, respectively. The MBM decreased by 3.34%, 3.25%, and 1.24%, respectively. Moreover, MBM was found to be the most strongly correlated with the PN content in EPS, zeta potential, and rheological parameters. Table \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e indicates that MBM was positively and strongly correlated with the PN-EMPS and PN-ECPS for RS (\u003cem\u003eR\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.954** and \u003cem\u003eR\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.963**, respectively) and significantly negatively correlated with the zeta potential for RS (\u003cem\u003eR\u003c/em\u003e=-0.963**, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.01). Moreover, a significant negative correlation was found between free moisture (FM) and MBM for RS (\u003cem\u003eR\u003c/em\u003e=-0.952**, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.01).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2 Evaluation and Analysis of Dewatered Filtrate\u003c/h2\u003e\n \u003cdiv id=\"Sec11\" class=\"Section3\"\u003e\n \u003ch2\u003e3.2.1 Variation in TOC in Sludge Supernatant\u003c/h2\u003e\n \u003cp\u003eThe sludge disintegration degree increased as the AOPs progressed. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea, the TOC concentration increased during pretreatment. The maximum TOC concentration of RS under the pretreatment condition was 420.30 mg/L, which was approximately 4.13 times higher than that of the blank. This result indicated that AOP treatment could promote sludge solubilisation and organics release (Wu et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e). The increase in the TOC concentration could be the result of the release of intercellular substances such as EPS, which is consistent with the decrease in the EPS content of the sludge, as discussed in Section 3.3.2. An increase in the TOC concentration commonly occurs when using advanced oxidation methods for sludge conditioning (Ni et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e\n \u003cp\u003eThe disintegration of sludge flocs and microbial cell breakdown resulted in the release of more soluble organic matter into the sludge supernatant (Huang et al. \u003cspan class=\"CitationRef\"\u003e2019a\u003c/span\u003e). It has been reported that bound water is held by sludge flocs and microorganisms and can be released when the structure or microorganisms are decomposed (Zhen et al. \u003cspan class=\"CitationRef\"\u003e2012a\u003c/span\u003e). Therefore, the release of TOC could partly reflect the variation in sludge dewaterability (Zhou et al. \u003cspan class=\"CitationRef\"\u003e2015\u003c/span\u003e). This implies that the oxidisation processes facilitated the decomposition of sludge and release of bound moisture. The results agree with those reported by Wu et al. (\u003cspan class=\"CitationRef\"\u003e2017a\u003c/span\u003e).\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e\n \u003ch2\u003e3.2.2 Variation in TP and TN in Sludge Filtrate\u003c/h2\u003e\n \u003cp\u003eThe TP concentrations in the supernatant of RS and DS after persulphate pretreatment decreased considerably to 80.4 mg/L and 21.4 mg/L, respectively. The concentrations of TP in RS and DS after peroxide pretreatment were 48.6 mg/L and 2.9 mg/L, respectively (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb). This might have resulted from ferric cations combining with PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e to form Fe-PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e during AD (Li et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e\n \u003cp\u003eThe nitrogen in the liquid phase after pretreatment was mainly in the form of organic nitrogen, and the TN concentrations in the RS, DS, and DS-3 were 254.94/248.04 mg/L, 1176.30/1128.90 mg/L, and 806.70/756.90 mg/L (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ec), respectively. The TN in the liquid phase gradually increased, thereby indicating that a large amount of organic nitrogen was converted into the liquid phase.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec13\" class=\"Section3\"\u003e\n \u003ch2\u003e3.2.3 Variation in Metal Cations and Conductivity\u003c/h2\u003e\n \u003cp\u003eThe distribution of the main cations such as calcium, magnesium, iron, and aluminium in the dewatered filtrate before and after CP pretreatment is presented in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ed. The calcium ion content was the highest, followed by that of magnesium ions. After pretreatment with CP, the calcium ion concentration was significantly increased in the sludge samples, and the calcium ion concentration in the dewatered filtrate of RS, DS, and DS-3 increased by 6.5, 3.6, and 5.6 fold compared with that in the primary samples, respectively.\u003c/p\u003e\n \u003cp\u003eThese results indicate that CP addition is beneficial for the removal of phosphorus compared with conventional treatment. This corresponded well with the results of the decrease in TP. This could be attributed to the release of calcium ions during AD. A large amount of calcium ions could be produced by calcium carbonate decomposition owing to the supply of H\u003csup\u003e+\u003c/sup\u003e ions from VFAs, which could easily precipitate with phosphate, as shown in Equations (\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e) and (\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e). These findings are consistent with those reported by Zhang et al. (\u003cspan class=\"CitationRef\"\u003e2016\u003c/span\u003e).\u003c/p\u003e\n \u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cimg src=\"data:image/png;base64,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\"\u003e\u003c/div\u003e\n \u003cp\u003eThe conductivity of the dewatered filtrate of RS, DS, and DS-3 after persulphate and peroxide pretreatment was higher than that of the untreated sludge samples, but the conductivity values were lower than 10 mS/cm (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ee).\u003c/p\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\n \u003ch2\u003e3.3 Insights Into the Effects of AOPs and AD\u003c/h2\u003e\n \u003cdiv id=\"Sec15\" class=\"Section3\"\u003e\n \u003ch2\u003e3.3.1 VFA Composition and Distribution\u003c/h2\u003e\n \u003cp\u003eThe VFA production profiles of different sludge samples are illustrated in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e. It was observed that the VFA production of DS was lower than that of RS. Acetic acid was the most prevalent product in the RS samples, which was in accordance with the findings of Huang et al. (\u003cspan class=\"CitationRef\"\u003e2019a\u003c/span\u003e). AD contributed to a greater proportion of \u003cem\u003en\u003c/em\u003e-valeric acid, which accounted for 24.28% and 28.83% of the total VFAs for DS and DS-3, respectively. (Wu et al. \u003cspan class=\"CitationRef\"\u003e2017b\u003c/span\u003e). reported that acetic, propionic, and iso-valeric acids were the three main products under alkaline conditions, which had a greater advantage over the improvement of acetic acid yield This may be explained by the fact that sludge hydrolysis and acidification were more efficient and sufficient in the AD process, thereby leading to the conversion of high-molecular-weight VFAs to acetic acid and transformation into methane and carbon dioxide.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec16\" class=\"Section3\"\u003e\n \u003ch2\u003e3.3.2 Effect of Sludge EPS on Sludge Dewaterability\u003c/h2\u003e\n \u003cp\u003eThe distributions of PN and PS contents in EPS are shown in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e. During the persulphate and peroxide treatments, the PN and PS contents in EMPS and ECPS decreased. Accordingly, the trend of change in the PS content with persulphate and peroxide adjustment were roughly similar to those of the PN content in the EPS, as shown in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ec.\u003c/p\u003e\n \u003cp\u003eThe initial concentrations of PN and PS of EMPS in the RS samples were 24.38 mg/g VS and 6.58 mg/g VS, respectively. The concentrations of PN and PS decreased with persulphate and peroxide treatments (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea, \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ec). After pretreatment with persulphate, the PN and PS contents of EMPS decreased by 57.51% and 55.47%, respectively. After pretreatment with peroxide, the PN and PS contents of EMPS decreased by 67.97% and 2.28%, respectively.\u003c/p\u003e\n \u003cp\u003ePN plays a more important role than PS in influencing sludge dewaterability, especially for PN in EPS at the outer layer, including soluble EPS and loosely bound EPS (Shao et al. \u003cspan class=\"CitationRef\"\u003e2010\u003c/span\u003e). In our previous study, PN-EMPS played a key role in enhancing sludge dewaterability during AD (Zhang et al. \u003cspan class=\"CitationRef\"\u003e2019b\u003c/span\u003e). These results further confirm the release of EPS owing to the disruption of sludge flocs caused by AOP pretreatment. The enhanced sludge dewaterability by AOP conditioning was due to the destruction of sludge flocs, thereby causing the degradation of EPS and the release of bound moisture. In addition, a significant positive correlation was observed between MBM and PN-EMPS and PN-ECPS for RS (\u003cem\u003eR\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.954** and \u003cem\u003eR\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.963**, respectively) (Table \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e).\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec17\" class=\"Section3\"\u003e\n \u003ch2\u003e3.3.3 Zeta Potential\u003c/h2\u003e\n \u003cp\u003eThe zeta potential is a critical factor influencing sludge dewaterability (Zhen et al. \u003cspan class=\"CitationRef\"\u003e2012b\u003c/span\u003e). All the AOP treatments for the three types of sludge significantly increased the zeta potential of the sludge flocs, and the zeta potential in the sludge supernatant approached zero (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e shows that the zeta potentials of the RS, DS, and DS-3 flocs increased from \u0026minus;\u0026thinsp;20.5, -18.7, and \u0026minus;\u0026thinsp;20.1 mV to -5.6, -7.2, and \u0026minus;\u0026thinsp;1.6 mV after persulphate conditioning, respectively. These observations were consistent with those of previous reports indicating that the surface charges of sludge flocs reach nearly zero after oxidation treatments (Chen et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e\n \u003cp\u003eThis might have been caused by the accelerated accumulation of sludge flocs and improved sludge dewaterability. Moreover, it has been reported that the ionisation of anionic groups in EPS, such as carboxyl groups and amino groups, account for the decrease in floc surface electronegativity after oxidation treatments (Sheng et al. \u003cspan class=\"CitationRef\"\u003e2010\u003c/span\u003e, Wu et al. \u003cspan class=\"CitationRef\"\u003e2017a\u003c/span\u003e). Mikkelsen et al (2002) also found that floc disintegration created more exposed surfaces in the floc interior, which also increased the surface electronegativity.\u003c/p\u003e\n \u003cp\u003eAccording to the divalent cationic bridging theory, it is believed that divalent cations can neutralise negatively charged colloids and bond with the functional groups of soluble PN and carbohydrates (Higgins and Novak. 1997).\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec18\" class=\"Section3\"\u003e\n \u003ch2\u003e3.3.4 Hydrophobic Properties and Functional Groups of Sludge\u003c/h2\u003e\n \u003cp\u003eDisintegration of sludge flocs and an increase in soluble organic matter were also observed in the persulphate and peroxide pretreatments (Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003ea). The higher zeta potential in the sludge supernatant indicated that some of the hydrophilic groups in the sludge flocs were destroyed by the adsorbed hydroxyl radicals and solid-state active oxygen, with the reduction in hydrophilic groups, more bound moisture attached to organic matter could be released as free moisture (Chen et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e, Zhang et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e\n \u003cp\u003eThe contact angle and surface free energy values are presented in Table S4. The contact angle between the water and sludge increased after the oxidation treatment; i.e. the sludge surface became hydrophobic. The surface free energy was negative, which suggested that the sludge surface was hydrophobic, thereby indicating that sludge dewaterability was improved (Zhang et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e\n \u003cp\u003eThe sludge samples were further analysed by FTIR, and the corresponding spectra are shown in Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e.\u003c/p\u003e\n \u003cp\u003eThe peaks at 1658 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 1539 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and 1230 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e were assigned to the C\u0026thinsp;=\u0026thinsp;O stretching vibration of amide I, the N\u0026ndash;H deformation vibration of amide II, and the C\u0026ndash;N stretching vibration of amide III, respectively (Schmitt and Flemming, \u003cspan class=\"CitationRef\"\u003e1998\u003c/span\u003e). The peak at 1645 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e corresponds to the stretching vibration of the carbonyl group (C\u0026ndash;O) in the PN amide I band. The peaks at 1614 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1550 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003ecorrespond to the C\u0026ndash;N stretching vibration and the N\u0026ndash;H bending vibration in the amide II band, thereby indicating the presence of PN. The peaks near 1245 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1070 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are the C\u0026ndash;O\u0026ndash;C and C\u0026ndash;OH stretching vibrations, respectively, which indicate the presence of PS.\u003c/p\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e shows that persulphate and peroxide did not affect the positions of the peaks, but did change the transmission intensity of the peaks. The peaks for RS in persulphate pretreatment and RS in peroxide pretreatment remained unchanged; however, some new peaks appeared after persulphate and peroxide conditioning, and functional groups such as \u0026ndash;NH\u003csub\u003e2\u003c/sub\u003e, amide II, and \u0026ndash;CH\u003csub\u003e2\u003c/sub\u003e-N\u003csup\u003e+\u003c/sup\u003e/carboxylic groups appeared. The intensity of C\u0026thinsp;=\u0026thinsp;O, C\u0026ndash;N, and \u0026ndash;OH/C\u0026ndash;O\u0026ndash;C increased.\u003c/p\u003e\n \u003cp\u003eMoreover, the peak for DS was different from that of RS because the organic matter was degraded during the AD process. In addition, the peak shapes for DS in persulphate pretreatment and DS in peroxide pretreatment remained stable; however, some peaks disappeared after persulphate conditioning, with functional groups such as \u0026ndash;NH\u003csub\u003e2\u003c/sub\u003e, amide II, and \u0026ndash;CH\u003csub\u003e2\u003c/sub\u003e-N\u003csup\u003e+\u003c/sup\u003e/carboxylic groups. The intensities of C\u0026thinsp;=\u0026thinsp;O and C\u0026ndash;N decreased.\u003c/p\u003e\n \u003cp\u003eFor DS-3, the main peak almost disappeared after the persulphate and peroxide pretreatments. This was especially true for peroxide treatment. In addition, there was no clear amide I peak for DS-3.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec19\" class=\"Section3\"\u003e\n \u003ch2\u003e3.3.5 Rheological properties\u003c/h2\u003e\n \u003cp\u003eThe results of the sludge rheological properties before and after AOP conditioning are shown in Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e. The viscosity decreased after AOP treatment, i.e. sludge flowability increased as viscosity decreased. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e, the concentrations of total organic compounds decreased, and more organic compounds, i.e. PN and PS, might have been released from the inner particles to the outer layer by AOP treatment. Moreover, EPS were transferred from particles into the bulk fluid, thereby making the flow easier and increasing the functional group and surface charge of the bulk fluid during the AD process. As shown in Tables S1\u0026ndash;S3, the correlation between the mechanically bound moisture content and rheological properties was significant (Miryahyaei et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e). As demonstrated in previous sections, lower viscosity was correlated with the lowest amount of MBM during the digestion period. Furthermore, viscosity was significantly correlated with PN-EMPS and PN-ECPS for RS, DS, and DS-3 (\u003cem\u003eR\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.862**, \u003cem\u003eR\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.794*; \u003cem\u003eR\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.762*, \u003cem\u003eR\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.938**; and \u003cem\u003eR\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.612*, \u003cem\u003eR\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.812**, respectively). An exception was found for DS-3 after the treatment with peroxide; the viscosity increased, which was probably due to the increase in the sludge concentration as peroxide addition increased for DS-3.\u003c/p\u003e\n \u003c/div\u003e\n\u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eIn this study, three types of sludge (RS, DS, and DS-3) were treated using persulphate and peroxide processes to study the mechanisms of sludge AOPs conditioning. The highest MBM reduction was 4.69% for DS-3 in the presence of persulphate. The mechanism revealed that AOPs pretreatment significantly accelerated sludge disintegration, which caused the degradation of EPS, promoted the degradation of the PN fraction of EPS, and induced the release of MBM. Moreover, compared with RS, the organic matter content of DS and DS-3 sludge samples pretreated by AOPs was lower, viscosity was decreased, zeta potential was closer to zero, and the strength of amino, amide II and other hydrophilic functional groups decreased or disappeared. Under the same AOPs conditions, the dewateratbility of digested sludge is better than that of RS.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledges\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was financially supported by the National Natural Science Foundation of China (52100151).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical Approval\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eCompliance with Ethical Standards\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known\u0026nbsp;potential conflicts of interest\u0026nbsp;that could have appeared to influence the work reported in this paper. No human and/or animal studies were involved in this study. All authors give informed consent. The submitted work was original and have not been published elsewhere in any form or language (partially or in full)\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to Participate\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNo human and/or animal studies were involved in this study. \u0026nbsp;All the authors agree to submit this manuscript to Environmental Science and Pollution Research.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to Publish\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work has not been published before; No considered for publication elsewhere; Its publication has been approved by all authors.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors Contributions\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eAll authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Wei Zhang, Jing Zhu, Xue Yang and Xiaohu Dai. The first draft of the manuscript was written by Wei Zhang and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.\u0026rdquo;\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003e\u0026nbsp;\u0026ldquo;This work was supported by\u0026nbsp;\u003c/em\u003eNational Natural Science Foundation of China (52100151)\u003cem\u003e. Author Wei Zhang has received research support from\u0026nbsp;\u003c/em\u003eNational Natural Science Foundation of China\u003cem\u003e.\u0026rdquo;\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003e\u0026ldquo;The authors have no relevant financial or non-financial interests to disclose.\u0026rdquo;\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eStatements and Declarations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eOther data that need to be supplemented have been uploaded as supplementary material files, and there is no other data and materials.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eCai MQ, Hu JQ, Wells G, Seo Y, Spinney R, Ho SH, Dionysiou DD, Su J, Xiao R, Wei Z (2018) Understanding Mechanisms of Synergy between Acidification and Ultrasound Treatments for Activated Sludge Dewatering: From Bench to Pilot-Scale Investigation. 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J Environ Sci 48:200\u0026ndash;208\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhen G, Lu X, Li Y, Zhao Y, Wang B, Song Y, Chai X, Niu D, Cao X (2012a) Novel insights into enhanced dewaterability of waste activated sludge by Fe(II)-activated persulfate oxidation. Bioresour Technol 119:7\u0026ndash;14\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhen G, Lu X, Wang B, Zhao Y, Chai X, Niu D, Zhao A, Li Y, Song Y, Cao X (2012b) Synergetic pretreatment of waste activated sludge by Fe(II)-activated persulfate oxidation under mild temperature for enhanced dewaterability. Bioresour Technol 124:29\u0026ndash;36\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhen G, Tan Y, Wu T, Wang J, Lu X, Zhao Y, Zhu X, Niu J, Xiong J (2019) Strengthened dewaterability of coke-oven plant oily sludge by altering extracellular organics using Fe(II)-activated persulfate oxidation. Sci Total Environ 688:1155\u0026ndash;1161\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhou X, Wang Q, Jiang G, Liu P, Yuan Z (2015) A novel conditioning process for enhancing dewaterability of waste activated sludge by combination of zero-valent iron and persulfate. Bioresour Technol 185:416\u0026ndash;420\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"},{"header":"Tables","content":"\u003cp\u003eTable 1. The characteristics of sludge samples\u003c/p\u003e\n\u003cdiv\u003e\n \u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" width=\"633\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd width=\"8.860759493670885%\" valign=\"top\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"14.240506329113924%\" valign=\"top\"\u003e\n \u003cp\u003epH\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"14.082278481012658%\" valign=\"top\"\u003e\n \u003cp\u003eTS\u003c/p\u003e\n \u003cp\u003e(%)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.031645569620252%\" valign=\"top\"\u003e\n \u003cp\u003eVS/TS\u003c/p\u003e\n \u003cp\u003e(%)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.455696202531644%\" valign=\"top\"\u003e\n \u003cp\u003eZeta potential (mV)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"20.88607594936709%\" valign=\"top\"\u003e\n \u003cp\u003eWater content\u003c/p\u003e\n \u003cp\u003e(%)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"10.443037974683545%\" valign=\"top\"\u003e\n \u003cp\u003eHHV (MJ/kg)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"8.860759493670885%\" valign=\"top\"\u003e\n \u003cp\u003eRS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"14.240506329113924%\" valign=\"top\"\u003e\n \u003cp\u003e7.85\u003cimg width=\"9\" src=\"data:image/png;base64,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\" alt=\"image\"\u003e0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"14.082278481012658%\" valign=\"top\"\u003e\n \u003cp\u003e2.870.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.031645569620252%\" valign=\"top\"\u003e\n \u003cp\u003e64.100.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.455696202531644%\" valign=\"top\"\u003e\n \u003cp\u003e-20.50.85\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"20.88607594936709%\" valign=\"top\"\u003e\n \u003cp\u003e97.130.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"10.443037974683545%\" valign=\"top\"\u003e\n \u003cp\u003e18.13\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"8.860759493670885%\" valign=\"top\"\u003e\n \u003cp\u003eDS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"14.240506329113924%\" valign=\"top\"\u003e\n \u003cp\u003e8.39\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"14.082278481012658%\" valign=\"top\"\u003e\n \u003cp\u003e3.730.09\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.031645569620252%\" valign=\"top\"\u003e\n \u003cp\u003e51.49\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.455696202531644%\" valign=\"top\"\u003e\n \u003cp\u003e-18.731.58\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"20.88607594936709%\" valign=\"top\"\u003e\n \u003cp\u003e96.270.10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"10.443037974683545%\" valign=\"top\"\u003e\n \u003cp\u003e14.38\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"8.860759493670885%\" valign=\"top\"\u003e\n \u003cp\u003eDS-3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"14.240506329113924%\" valign=\"top\"\u003e\n \u003cp\u003e7.63\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"14.082278481012658%\" valign=\"top\"\u003e\n \u003cp\u003e3.28\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.031645569620252%\" valign=\"top\"\u003e\n \u003cp\u003e59.310.18\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.455696202531644%\" valign=\"top\"\u003e\n \u003cp\u003e-21.100.30\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"20.88607594936709%\" valign=\"top\"\u003e\n \u003cp\u003e96.720.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"10.443037974683545%\" valign=\"top\"\u003e\n \u003cp\u003e13.55\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eNote: SCOD, soluble chemical oxygen demand; TS, total solid; VS, volatile solid; HHV, higher heating value.\u003c/p\u003e\n\u003cp\u003eTable 2. Surface tension of digested sludge. Total interfacial tension, and total interfacial free energy between digested sludge and water (mJ/m\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\n\u003cdiv\u003e\n \u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" width=\"737\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd width=\"12.754409769335142%\" rowspan=\"2\" valign=\"top\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"14.111261872455902%\" rowspan=\"2\" valign=\"top\"\u003e\n \u003cp\u003eZeta potential (mV)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"56.44504748982361%\" colspan=\"3\" valign=\"top\"\u003e\n \u003cp\u003eContact angle (deg)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.689280868385346%\" rowspan=\"2\" valign=\"top\"\u003e\n \u003cp\u003e\u0026nbsp;(mJ/m\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"27.403846153846153%\" valign=\"top\"\u003e\n \u003cp\u003eWater\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"38.46153846153846%\" valign=\"top\"\u003e\n \u003cp\u003e1-bromonaphthalene\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"34.13461538461539%\" valign=\"top\"\u003e\n \u003cp\u003eformamide\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"12.754409769335142%\" valign=\"top\"\u003e\n \u003cp\u003eRS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"14.111261872455902%\" valign=\"top\"\u003e\n \u003cp\u003e-20.50.85\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.468113975576662%\" valign=\"top\"\u003e\n \u003cp\u003e80.10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"21.709633649932158%\" valign=\"top\"\u003e\n \u003cp\u003e45.44\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"19.26729986431479%\" valign=\"top\"\u003e\n \u003cp\u003e81.90\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.689280868385346%\" valign=\"top\"\u003e\n \u003cp\u003e166.48\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"12.754409769335142%\" valign=\"top\"\u003e\n \u003cp\u003eRS-SDS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"14.111261872455902%\" valign=\"top\"\u003e\n \u003cp\u003e-5.620.65\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.468113975576662%\" valign=\"top\"\u003e\n \u003cp\u003e97.68\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"21.709633649932158%\" valign=\"top\"\u003e\n \u003cp\u003e44.86\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"19.26729986431479%\" valign=\"top\"\u003e\n \u003cp\u003e79.63\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.689280868385346%\" valign=\"top\"\u003e\n \u003cp\u003e-45.82\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"12.754409769335142%\" valign=\"top\"\u003e\n \u003cp\u003eRS-CaO2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"14.111261872455902%\" valign=\"top\"\u003e\n \u003cp\u003e-10.250.75\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.468113975576662%\" valign=\"top\"\u003e\n \u003cp\u003e88.73\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"21.709633649932158%\" valign=\"top\"\u003e\n \u003cp\u003e47.19\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"19.26729986431479%\" valign=\"top\"\u003e\n \u003cp\u003e75.72\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.689280868385346%\" valign=\"top\"\u003e\n \u003cp\u003e138.56\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"12.754409769335142%\" valign=\"top\"\u003e\n \u003cp\u003eDS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"14.111261872455902%\" valign=\"top\"\u003e\n \u003cp\u003e-18.731.58\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.468113975576662%\" valign=\"top\"\u003e\n \u003cp\u003e75.59\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"21.709633649932158%\" valign=\"top\"\u003e\n \u003cp\u003e46.29\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"19.26729986431479%\" valign=\"top\"\u003e\n \u003cp\u003e72.69\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.689280868385346%\" valign=\"top\"\u003e\n \u003cp\u003e621.93\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"12.754409769335142%\" valign=\"top\"\u003e\n \u003cp\u003eDS-SDS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"14.111261872455902%\" valign=\"top\"\u003e\n \u003cp\u003e-7.242.90\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.468113975576662%\" valign=\"top\"\u003e\n \u003cp\u003e78.42\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"21.709633649932158%\" valign=\"top\"\u003e\n \u003cp\u003e43.48\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"19.26729986431479%\" valign=\"top\"\u003e\n \u003cp\u003e74.71\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.689280868385346%\" valign=\"top\"\u003e\n \u003cp\u003e-65.03\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"12.754409769335142%\" valign=\"top\"\u003e\n \u003cp\u003eDS-CaO2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"14.111261872455902%\" valign=\"top\"\u003e\n \u003cp\u003e-6.480.73\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.468113975576662%\" valign=\"top\"\u003e\n \u003cp\u003e77.06\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"21.709633649932158%\" valign=\"top\"\u003e\n \u003cp\u003e41.67\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"19.26729986431479%\" valign=\"top\"\u003e\n \u003cp\u003e70.49\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.689280868385346%\" valign=\"top\"\u003e\n \u003cp\u003e-53.65\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"12.754409769335142%\" valign=\"top\"\u003e\n \u003cp\u003eDS-3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"14.111261872455902%\" valign=\"top\"\u003e\n \u003cp\u003e-21.10.30\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.468113975576662%\" valign=\"top\"\u003e\n \u003cp\u003e85.19\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"21.709633649932158%\" valign=\"top\"\u003e\n \u003cp\u003e46.80\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"19.26729986431479%\" valign=\"top\"\u003e\n \u003cp\u003e69.64\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.689280868385346%\" valign=\"top\"\u003e\n \u003cp\u003e24.33\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"12.754409769335142%\" valign=\"top\"\u003e\n \u003cp\u003eDS-3-SDS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"14.111261872455902%\" valign=\"top\"\u003e\n \u003cp\u003e-1.590.99\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.468113975576662%\" valign=\"top\"\u003e\n \u003cp\u003e84.64\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"21.709633649932158%\" valign=\"top\"\u003e\n \u003cp\u003e40.85\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"19.26729986431479%\" valign=\"top\"\u003e\n \u003cp\u003e71.55\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.689280868385346%\" valign=\"top\"\u003e\n \u003cp\u003e-100.59\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"12.754409769335142%\" valign=\"top\"\u003e\n \u003cp\u003eDS-3-CaO2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"14.111261872455902%\" valign=\"top\"\u003e\n \u003cp\u003e-6.282.76\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.468113975576662%\" valign=\"top\"\u003e\n \u003cp\u003e76.77\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"21.709633649932158%\" valign=\"top\"\u003e\n \u003cp\u003e45.18\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"19.26729986431479%\" valign=\"top\"\u003e\n \u003cp\u003e75.08\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.689280868385346%\" valign=\"top\"\u003e\n \u003cp\u003e-9.83\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"pretreatment, digested sludge, dewaterability, mechanism, hydrophobicity, extracellular polymeric substances","lastPublishedDoi":"10.21203/rs.3.rs-2997910/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-2997910/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAdvanced oxidation processes (AOPs) are important for improving sludge dewatering efficiency. In this study, AOPs such as pretreatment with sodium persulphate and calcium peroxide were investigated as methods for improving the dewaterability of waste activated sludge (raw sludge), digested sludge (DS), and sludge digested for 3 days (DS-3). The results showed that persulphate and peroxide could effectively improve the dewaterability of sludge. The results suggested that the maximum increment of free moisture of 4.69% was achieved for the DS-3 of pretreatment with persulphate, and the mechanism investigation revealed that AOPs pretreatment could enhance sludge floc disintegration, degrade the protein (PN) fraction of extracellular polymeric substances (EPS), and accelerate the release of mechanically bound moisture. Free moisture was significantly negatively correlated with the DS PN concentration in extra-microcolony polymers (\u003cem\u003eR\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.850, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.01). Compared with RS, the organic matter content of DS and DS-3 sludge samples pretreated by AOPs was lower, viscosity was decreased, zeta potential was closer to zero, and the strength of amino, amide II and other hydrophilic functional groups decreased or disappeared. Under the same AOPs conditions, the dewateratbility of digested sludge is better than that of RS. This study reveals the mechanisms of sludge AOP conditioning and provides a theoretical basis for practical applications.\u003c/p\u003e","manuscriptTitle":"Impact of advanced oxidation process (AOPs) pretreatment on enhancing sludge dewaterability: new insights through EPS protein and sludge hydrophilicity/hydrophobicity properties","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2023-09-12 14:14:37","doi":"10.21203/rs.3.rs-2997910/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"d12a3135-175b-4053-b227-19c5089e40e7","owner":[],"postedDate":"September 12th, 2023","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2023-10-24T01:27:41+00:00","versionOfRecord":[],"versionCreatedAt":"2023-09-12 14:14:37","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-2997910","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-2997910","identity":"rs-2997910","version":["v1"]},"buildId":"rHA-KDH7Qsr4HCuvH75dn","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}