English

• 1. Introduction

  With the growth of the world's population, the increasing amount of waste activated sludge (WAS) has become an important challenge for wastewater treatment plants (WWTPs) [1,2]. WAS contains not only high moisture content (MC), perishability, and unpleasant odors, but also a variety of toxic and hazardous substances, such as endocrine disrupting compounds, pathogens, and heavy metals [3,4]. As an important step in the WAS treatment procedure, WAS dewatering can minimize the amount of WAS, decrease the costs of transportation, storage, treatment, and disposal, and improve the efficiency of energy utilization [2].

  WAS has a non-homogeneous colloidal structure in which tiny WAS particles form a steady suspension in water and are hardly separate from aqueous phase [3]. According to the difficulty of removal, water in WAS can be categorized into three types, i.e., free water, interstitial water, and bound water [5]. Among them, bound water is difficult to remove mechanically and is a key barrier to passing WAS dewatering performance. The extracellular polymeric substances (EPS), referring to polymers with a three-dimensional gel-like structure encapsulated outside the cell wall, are the main organic components of WAS and are usually categorized into three types, i.e., soluble EPS (S-EPS), loosely bound EPS (LB-EPS), and tightly bound EPS (TB-EPS) [2,3]. The presence of large amounts of carbohydrates and proteins in EPS results in the high hydrophilicity and strong electronegativity of EPS [4], which maintains the stability of WAS floc structure and causes a large amount of water retained in WAS floc [6]. Therefore, reducing the content of EPS and enhancing the hydrophobicity of EPS are favorable for WAS dewatering.

  During the past several decades, substantial work has been paid to establish efficient pretreatment processes for improving the dewatering performance of WAS. Based on working mechanism, these pretreatment techniques can be divided into crushing, flocculation and skeleton building processes [3,4]. The floc and EPS of WAS can be broken by ultrasonic, heat and oxidation, and then the release of bound water was promoted [3]. Meantime, the charge neutralization and double electric layer compression could be realized by adding flocculant, thus promoting the increase of WAS particle size [5]. Moreover, the skeleton building processes can achieve a compressibility reduction of WAS by adding rigid inert materials as skeleton builder to ensure water channels [7]. Advanced oxidation processes (AOPs) and its combined processes in breaking process, such as Fenton process [8], ultrasonic (US)-Fenton process [9], and thermally-activated Na₂S₂O₈ process [10], have the potential to achieve the release of bound water, flocculation of WAS particles, and skeleton-building simultaneously, showing their superiority in WAS dehydration. In AOPs-based WAS dewatering, highly active species with strong oxidizing effects produced in situ by oxidants play a key role [1,11,12]. Currently, commonly used oxidants are hydrogen peroxide (H₂O₂), calcium peroxide (CaO₂), ozone (O₃), persulfates (peroxymonosulfate (HSO₅⁻, PMS) and peroxydisulfate (S₂O₈²⁻, PDS), sulfite (SO₃²⁻), potassium ferrate (K₂FeO₄, PF) and potassium permanganate (KMnO₄) [4]. The choice of different oxidants leads to the production of different highly active species, which causes different reaction mechanisms. Depending on highly active species, AOPs can be categorized into four groups, i.e., hydroxyl radical (^•OH)-based AOPs, sulfate radical (SO₄^•‒)-based AOPs, high-valent iron species-based AOPs, and high-valent manganese species (RMnS)-based AOPs. At present, the mechanisms and applications of these four types of AOPs in WAS dewatering have not been thoroughly discussed and comprehensively analyzed. Also, AOPs can degrade or convert exogenous refractory pollutants (ERPs) in WAS [13,14], which facilitates subsequent resource recovery and disposal of WAS. Meanwhile, AOPs can promote the dissolution of WAS, thus facilitating the resource recovery of WAS through anaerobic fermentation/anaerobic digestion [6,15]. However, the influence of AOPs on the subsequent harmless disposal of WAS has not been systematically analyzed.

  Bibliometrics is a useful method to reveal the evolutionary trend of a research field by analyzing the publication number, country/author distribution, literature development, and research hotspots based on a large amount of published literature to construct a network relationship of the research field, and therefore it could predict the future research direction of the research field [16]. Bibliometric analysis of researches on AOPs-based WAS dewatering could provide reliable recommendations on the development trends in the field. However, the existing WAS dewatering reviews on AOPs have not conducted a bibliometric analysis of AOPs-based WAS dewatering studies.

  Over the past few years, several excellent reviews have been published on the use of different pretreatment processes to improve WAS dewatering. For example, Xu et al. [17] and Xiao et al. [6] reviewed the application of CaO₂-based AOPs and persulfate-based AOPs for WAS dewatering, respectively, and Cao et al. [3] comprehensively reviewed some previously used WAS dewatering pretreatment processes (e.g., chemical, biological and physical treatment). However, these reviews of WAS dewatering either focused on only one type of AOPs, or on all types of pretreatment processes (i.e., physical, chemical, and biological processes). There is still no comprehensive review and detailed analysis of different AOPs-based pretreatment processes. Therefore, the challenges and opportunities of AOPs-based pretreatment processes in WAS dewatering deserve more attention.

  The scopes of this review were (1) to critically review the mechanisms and applications of various AOPs, including ^•OH-based AOPs, SO₄^•‒-based AOPs, and high-valent iron species-based AOPs, and RMnS-based AOPs, in improving WAS dewatering; (2) to conduct the bibliometric analysis of researches on AOPs for WAS dewatering from 2000 to 2023 to analyze the cooperative network, co-citation, journals, categories, and keywords of related publications; (3) to analyze the effects of AOPs on ERPs in WAS, such as recalcitrant organics, pathogenic microorganisms/viruses, and heavy metals, and propose the subsequent harmless disposal of AOPs-treated WAS. This review can not only provide guidance for the development of efficient, cost-effective, and environmentally AOPs to further improve the dewaterability of WAS, but also provide new perspectives and directions for the positive reform of the traditional WAS management plan.

  2. Overview and limitations of mechanisms for AOPs in WAS dewatering

  In AOPs systems, various physicochemical methods, e.g., thermal, US, microwave (MW), ultraviolet (UV), transition metals, carbonaceous materials, and acid/alkali, were used to activate oxidants to produce highly active species with high redox potentials [11,12,18]. Depending on highly active species that play the main role, the currently existing AOPs applied to WAS dewatering can be classified into four categories, i.e., ^•OH-based AOPs (e.g., H₂O₂, CaO₂, and O₃), SO₄^•‒-based AOPs (e.g., HSO₅⁻ and S₂O₈²⁻), high-valent iron species (Fe(Ⅳ) and Fe(Ⅴ))-based AOPs (e.g., K₂FeO₄), and RMnS-based AOPs (e.g., KMnO₄) (Fig. 1a). In addition to the above commonly used activation methods, in recent years, a series of researches have demonstrated that substances, such as surfactants, phenol, HCO₃⁻, HPO₄²⁻, and glucose, can also activate oxidants [19-23]. Interestingly, WAS, as a system with complex composition, contains a large number of the activators mentioned above. Hence, the possibility of WAS itself activating oxidants deserves further exploration (Fig. 1b).

  Figure 1

  

  Figure 1.  Overview of mechanisms for AOPs in WAS dewatering (a) and limitations of mechanisms for AOPs in WAS dewatering (b).

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  Through oxidation, these highly active species can effectively destroy the spatial structure of WAS floc and EPS, promoting the conversion of LB-EPS and TB-EPS into S-EPS as well as the dissolution of hydrophilic substances (e.g., proteins and polysaccharides) in EPS, reducing the ability of EPS to bind water, thereby facilitating the conversion of interstitial water within WAS floc and water bound to EPS into free water (Fig. 1a) [15]. Some researches have shown that the highly active species can disrupt the interactions of hydrogen bonds and the interactions of S-S bonds in proteins, thereby altering the secondary structure of proteins, exposing the hydrophobic sites in WAS, and improving the hydrophobicity of WAS [24,25]. Meanwhile, these highly active species also increase the permeability of cell membranes and disrupt the homogeneity of cell walls, leading to the lysis of microbial cells, which in turn releases water and organic substances from WAS microbial cells [26]. In addition, the destruction of the spatial structure of WAS floc and EPS will cause the changes in rheological properties of WAS (such as, the decrease of viscosity and network structure strength), thus improving the fluidity and dewaterability of WAS [27]. Although the release of bound water and intracellular water and the improvement of fluidity are favorable for WAS dewatering, the increase of tiny particles will block the channels in the filter cake and filter cloth, which will worsen the filtration performance of WAS and is unfavorable to WAS dewatering [2,6,28]. These conflicting conclusions deserve further exploration in future researches of dewatering mechanisms (Fig. 1b).

  Furthermore, some metal ions (e.g., Fe³⁺, Ca²⁺, and Mn²⁺) generated after the oxidation–reduction reaction can promote the conversion of more interfacial water into free water and facilitate the flocculation of WAS by compressing the double electricity layer of WAS particles and neutralizing the negative charges of WAS particles (Fig. 1a) [11,17,24,29]. However, opposite conclusions were obtained from the researches of Liu et al. [30] (oxidation is the key) and Liang et al. [31] (flocculation is the key) as to which of the two roles of oxidation and flocculation is the key factor in improving the dewatering performance of WAS (Fig. 1b). Therefore, more work needs to be put into exploring this conflicting result. In addition, the sequence in which these two roles occur deserves further exploration.

  3. Applications of AOPs-based WAS dewatering processes

  3.1 Hydroxyl radical-based AOPs for WAS dewatering

  ^•OH-based AOPs are the most researched methods among all kinds of AOPs, in which the redox potential of ^•OH is 2.80 V [2]. In general, H₂O₂, CaO₂, and O₃ are the most commonly used oxidants in ^•OH-based AOPs.

  3.1.1   Hydrogen peroxide

  Among the many H₂O₂-based AOPs, the most common is the Fenton process, which relies on the chemical reaction between Fe²⁺ and H₂O₂ (Eqs. 1-3) [12].

  $ \begin{equation} \mathrm{Fe}^{2+}+\mathrm{H}_2 \mathrm{O}_2 \rightarrow \mathrm{Fe}^{3+}+{ }^{\bullet} \mathrm{OH}+\mathrm{OH}^{-} \end{equation} $

  (1)

  $ \begin{equation} \mathrm{Fe}^{3+}+\mathrm{H}_2 \mathrm{O}_2 \rightarrow \mathrm{Fe}^{2+}+\mathrm{HO}_2^{\bullet}+\mathrm{H}^{+} \end{equation} $

  (2)

  $ \begin{equation} \mathrm{HO}_2^{\bullet} \leftrightarrow \mathrm{O}_2^{\bullet-}+\mathrm{H}^{-} \end{equation} $

  (3)

  The dewaterability of sludge was significantly improved at pH 3 with 6 g/L of Fe²⁺ and 3 g/L of H₂O₂ [8]. However, the narrow effective range of pH, wastage of H₂O₂, the unstable and difficult storage of Fe²⁺, and the formation of iron-containing sludge all contribute to the high cost of the Fenton process [5]. In order to reduce costs and maintain efficient dewatering of WAS, various combination processes and Fenton-like processes have been developed (Fig. 2a). The typical combined processes include Fenton + skeleton builders (e.g., lime and red mud) [32,33], Fenton + surfactants (e.g., dodecyl dimethyl benzyl ammonium chloride) [34], and Fenton + US [9]. In addition to the combined processes, Fenton-like processes that catalyze H₂O₂ with Fe⁰ [15,35], Fe-rich biochar [25], or Fe³⁺ [36] have also received much attention. Compared to the classical Fenton process, the Fe⁰ + H₂O₂ process can provide a cost savings of 64% [35]. However, H₂O₂, a regulated hazardous chemical, is unstable, explosive, and corrosive in nature, which makes storage and transportation difficult and limits the large-scale application of H₂O₂-based AOPs [37]. In situ generation of H₂O₂ or finding substitutes may be viable approaches.

  Figure 2

  

  Figure 2.  (a) The combined processes of ^•OH-based AOPs. (b) The activation methods and mechanisms of SO₄^•‒-based AOPs. (c) The oxidation processes possibly involved in K₂FeO₄ and the dewatering mechanisms of an innovative electro-chemical system for the activation of K₂FeO₄ using granular activated carbon as the fluidized electrode. (d) The mechanisms of RMnS and radical generation in the KMnO₄ + bisulfite/sulfite process.

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  3.1.2   Calcium peroxide

  CaO₂ is a stable, economical, environmentally friendly, and widely used solid inorganic peroxide compound that reacts with water to form H₂O₂, O₂, and Ca(OH)₂ (Eqs. 4 and 5), where H₂O₂ can further generated into various free radicals, i.e., ^•OH, hydroperoxyl radical (HO₂^•), and superoxide radical (O₂^•−) (Eqs. 6-8) [17].

  $ \begin{equation} \mathrm{CaO}_2+2 \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Ca}(\mathrm{OH})_2+\mathrm{H}_2 \mathrm{O}_2 \end{equation} $

  (4)

  $ \begin{equation} 2 \mathrm{CaO}_2+2 \mathrm{H}_2 \mathrm{O} \rightarrow 2 \mathrm{Ca}(\mathrm{OH})_2+\mathrm{O}_2 \end{equation} $

  (5)

  $ \begin{equation} \mathrm{H}_2 \mathrm{O}_2+\mathrm{e}^{-} \rightarrow \mathrm{OH}^{-}+{ }^{\bullet} \mathrm{OH} \end{equation} $

  (6)

  $ \begin{equation} \mathrm{H}_2 \mathrm{O}_2+{ }^{\bullet} \mathrm{OH} \rightarrow \mathrm{H}_2 \mathrm{O}+\mathrm{HO}_2^{\bullet} \end{equation} $

  (7)

  $ \begin{equation} \mathrm{HO}_2^{\bullet} \rightarrow \mathrm{O}_2^{\bullet-}+\mathrm{H}^{+} \end{equation} $

  (8)

  The dewaterability of WAS showed a tendency to first rise and then decrease with the increase of CaO₂ addition [38]. When the dosage of CaO₂ was 20 mg/g total suspended solid (TSS), the dewaterability of WAS was the best with specific resistance to filtration (SRF) of 1.28 × 10¹³ m/kg [38]. However, the improvement in dewaterability of WAS by CaO₂ alone is limited and does not meet the standard of good dewatering performance (SRF < 0.4 × 10¹³ m/kg) [39]. To improve the dewaterability and environmental-economic benefits simultaneously, CaO₂ is usually utilized as a peroxidant for activation applications or in combination with other processes for pre-oxidation (Fig. 2a) [17]. The existing modified CaO₂-based AOPs could be classified into five kinds, (ⅰ) combinations of CaO₂ and activators (e.g., Fe(Ⅱ), nitrilotriacetic acid-Fe⁰, and MW) [40-42], (ⅱ) combinations of CaO₂ and skeleton builders (e.g., montmorillonite) [43], (ⅲ) combinations of CaO₂ and coagulants/flocculants (e.g., FeCl₃, polyaluminium chloride and polyacrylamide (PAM)) [38], (ⅳ) combinations of CaO₂ and other oxidants (e.g., PDS and O₃) [44,45], and (ⅴ) combinations of CaO₂ and physical conditioning (e.g., dielectric barrier discharge plasma) [46]. CaO₂-based AOPs typically have multiple effects, such as oxidation by free radicals, flocculation by Ca²⁺, and skeleton building by Ca(OH)₂ [47]. However, the contribution of each effect of CaO₂-based AOPs to the improvement of WAS dewaterability hasn't been fully elucidated. In addition, the high cost of CaO₂ (1020.0 ＄/ton) limits the practical application of CaO₂-based AOPs.

  3.1.3   Ozone

  Pretreating WAS by O₃, without the addition of toxic chemicals and the generation of toxic by-products, is considered a class of environmentally friendly AOPs [48]. It has been reported that when the ozonation process is used for WAS dewatering, various free radicals such as HO₂^•, ozone radical (O₃^•−), and ^•OH are generated, which can decompose WAS flocs [49].

  The MC of dewatered sludge cake (MC_dsc) increased from 81.93% to 91.56% in the range of 0–40 mg O₃/g total solid (TS), indicating that the dewaterability of WAS was deteriorated [49]. To improve the oxidation performance of the O₃ process, the combinations of O₃ with activators or other processes (Fig. 2a), such as polyferric sulfate (PFS) + O₃ [37], Fe⁰ + O₃ [50], bimetallic Fe/Ce loading sludge-derived biochar + O₃ [51], heat-modified drinking water sludge + O₃ [52], H₂O₂ + O₃ [53], PMS + O₃ [49], and chitosan + O₃ [54], have been investigated. The combined pretreatment of chitosan (20 mg/g TS) and O₃ (60 mg/g TS) reduced MC_dsc from 82.1% to 56.5%, which meet the level of deep dewatering (MC_dsc < 60%) [54]. With the exception of the O₃ process for WAS dewatering in a pilot-scale sludge treatment plant [55], all the remaining researches on O₃-based AOPs have been limited to the laboratory-scale. The pilot or large-scale researches are still to be investigated to assess the feasibility and stability of O₃-based AOPs in scaled-up applications.

  3.2 Sulfate radical-based AOPs for WAS dewatering

  Compared to ^•OH, SO₄^•‒ has better oxidative efficacy and versatility due to wider pH range (3–10), higher standard redox potential (2.5–3.1 eV), and longer lifetime (half-life 30–40 µs) [12]. SO₄^•‒-based AOPs are another commonly investigated method for WAS dewatering (Table S1 in Supporting information).

  In general, persulfates (PMS and PDS) are the most commonly used oxidants in SO₄^•‒-based AOPs [56]. When sludge was pretreated with 100 mg PMS/g TS at pH 5.0, SRF of sludge was 3.96 × 10¹³ m/kg, which was 22.50% lower than that of the control [57]. Another study showed that MC_dsc decreased from 95.5% to 82.1% as the dosage of PDS was increased from 0 to 1.2 mmol/g-volatile solids (VS) [13]. However, due to the good stability of persulfate, pretreatment with persulfate alone has limited improvement in WAS dewaterability. In order to obtain higher reactivity, the O—O bonds of PMS and PDS can be broken by various activation measures (e.g., thermal, US, UV, transition metals, carbonaceous materials, and alkali) to generate free radicals such as SO₄^•‒ (Fig. 2b) [6,10,56,58]. MC_dsc decreased from 82.30% to 68.24% at 1.00 mmol/g TS of PDS + 2.00 kW/L of US for 15 min [58]. It is worth noting that although persulfate-based AOPs can significantly improve the dewaterability of WAS, the high cost of PMS (~2.2 ＄/kg) and PDS (~0.74 ＄/kg) may be a key impediment to their practical application [59]. In addition, the published researches on the use of persulfate-based AOPs for WAS dewatering have been limited to laboratory scale, and there is a lack of actual WAS dewatering cases.

  Compared to PMS and PDS, sulfite is a cost-effective alternative (~0.43 ＄/kg) [59]. Activation of sulfite by transition metals (e.g., Fe, Co, Cr, Mn, and Cu) in the presence of oxygen can produce SO₄^•‒ [59,60]. Among them, Fe-based activators have received more attention due to their widespread existence in nature and relative non-toxicity (Fig. 2b) [61]. Under near-neutral conditions (pH 6.87 ± 0.26), SRF, capillary suction time (CST), and MC_dsc were reduced from (7.71 ± 0.16) × 10¹² m/kg, 131.60 ± 4.90 s, and 89.2% ± 3.7% to (1.57 ± 0.41) × 10¹² m/kg, 54.82 ± 3.38 s, and 79.7% ± 2.6%, respectively, after treatment with 1.79 mmol Fe²⁺/L-TSS, 1.43 mmol Na₂SO₃/L-TSS, and 4.76 mg PAM/g-TSS [60]. Compared to Fe²⁺, Fe⁰ can be slowly dissolved to produce Fe²⁺, thus reducing the depletion of SO₄^•‒ by excess Fe²⁺. Under the optimum operational conditions of 0.9 mmol Fe⁰/g VS, 1.2 mmol sulfite/g VS, a reaction time of 60 min, and pH 6, MC_dsc was reduced from 89.1% ± 1.9% to 69.3% ± 1.2% [61]. Currently, there are relatively few researches on sulfite-based AOPs for WAS dewatering, and the activation methods of sulfite as well as the combinations of sulfite with other processes should be further explored in the future. In addition, SO₄^•‒-based AOPs always introduce dissolved sulfur compounds, such as SO₄²⁻ and S²⁻, into the pretreated WAS, and thus potential secondary pollution should be paid extra attention when applying SO₄^•‒-based AOPs.

  3.3 High-valent iron species-based AOPs for WAS dewatering

  K₂FeO₄ is a strong oxidant with an unusual + 6 oxidation state of iron that can effectively oxidize organic compounds [62]. The oxidation processes involved in K₂FeO₄ may be as follows (Fig. 2c): (ⅰ) Fe(Ⅵ) forms Fe(Ⅴ) and free radicals (X^•) by 1-e⁻ transfer, (ⅱ) Fe(Ⅴ) forms Fe(Ⅳ) and X^• by 1-e⁻ transfer, (ⅲ) Fe(Ⅴ) forms Fe(Ⅲ) and oxygenation products (X(O)) via oxygen-atom transfer (OAT), (ⅳ) Fe(Ⅵ) forms Fe(Ⅳ) and dimers (X₂) via 1-e⁻ transfer, (ⅴ) Fe(Ⅵ) forms Fe(Ⅳ) and X(O) via OAT, (vi) Fe(Ⅳ) forms Fe(Ⅲ) and X^• via 1-e⁻ transfer, and (vii) Fe(Ⅳ) forms Fe(Ⅱ) and X(O) via OAT [63]. After a series of oxidation–reduction reactions, Fe(Ⅵ) is eventually reduced to Fe(Ⅲ)/Fe(Ⅱ), which is an excellent flocculant [4]. Among them, Fe(Ⅳ) and Fe(Ⅴ) are high-valent iron species with 2–6 orders of magnitude higher reactivity than Fe(Ⅵ) [64]. Recently, K₂FeO₄-based AOPs have attracted more and more attention in WAS dewatering due to their oxidation and flocculation.

  Under 500 mg/L of K₂FeO₄, SRF of WAS was reduced by 85.5% from 18.6 × 10¹² m/kg in the control group to 2.7 × 10¹² m/kg [65]. The standard potentials of K₂FeO₄ were 2.20 and 0.70 V under acidic and alkaline conditions, respectively [24]. The initial SRF of 1.05 × 10¹³ m/kg was reduced to 0.315 × 10¹³ m/kg and 0.960 × 10¹³ m/kg after treatment with acid (pH 2) + K₂FeO₄ (0.1 g/g TSS) and alkali (pH 11) + K₂FeO₄ (0.1 g/g TSS), respectively [66]. Notably, despite the higher oxidizing property of K₂FeO₄ under acidic conditions, its stability is better under alkaline conditions. MC_dsc was 83% at 700 mg Fe(Ⅵ)/L of alkaline ferrate containing Fe(Ⅵ) and KOH, which was 6.2% lower than that at 700 mg Fe(Ⅵ)/L of K₂FeO₄ alone (89.2%) [67]. Although pH adjustment can improve the oxidation performance of K₂FeO₄, both acidic and alkaline conditions aren't favorable for subsequent treatment and disposal of WAS.

  In order to expand the suitable pH range of K₂FeO₄ for improving its stability and oxidizing property at neutral pH, the combinations of K₂FeO₄ with other conditions for WAS dewatering were widely explored, such as WAS biochar [68], US [69], and electrolysis system [70]. Among them, an innovative electrochemical system for the activation of K₂FeO₄ using granular activated carbon (GAC) as the fluidized electrode is an attractive process (Fig. 2c) [70]. Under the condition of 0.5 g K₂FeO₄/g TS + 3.0 g GAC/g TS with the voltage of 16 V and the pretreatment time of 15 min, MC_dsc was found to be 8.2% and 5.8% lower than that of the control and K₂FeO₄-treated WAS, respectively [70]. The high commercial price of K₂FeO₄ (132.28 ＄/kg) has been a key barrier to its application, but a study has successfully reduced the cost of K₂FeO₄ to about 11.02 ＄/kg using an electrochemical preparation method [71], which has made it possible to use K₂FeO₄-based AOPs in large-scale WAS dewatering.

  3.4 High-valent manganese species-based AOPs for WAS dewatering

  KMnO₄ is a kind of green oxidant with the advantages of high oxidizing power, stability during storage and transportation, and safety and convenience in dosing [11,72]. KMnO₄ is applicable to a wider pH range, with different pH conditions corresponding to different redox potentials, of which the highest redox potential is +1.70 V under acidic-neutral conditions [73]. During pretreatment of WAS with KMnO₄-based AOPs, a variety of RMnS with highly reactive activity, including soluble Mn(Ⅲ), Mn(Ⅴ), and Mn(Ⅵ), are produced [11].

  With the increase in the dosage of KMnO₄ from 0 to 20 g/kg dry solids (DS), MC_dsc decreased from 83.38% to 76.15% [74]. Recently, KMnO₄ was often used in conjunction with other conditioning processes, such as KMnO₄ + Fe(Ⅲ) + PDS [75], KMnO₄ + PMS [76], KMnO₄ + thermal-acid-wash (TA) + Fe⁰ [15], KMnO₄ + US [77], KMnO₄ + FeCl₃/PAM/a cationic starch-based flocculant (St-WH) [78,79], KMnO₄ + Fe(Ⅱ) [80], KMnO₄ + acidification + St-WH [81], and KMnO₄ + bisulfite/sulfite [29,82], to improve the dewatering effectiveness. In particular, the KMnO₄ + bisulfite/sulfite process has received much attention due to its ability not only to form RMnS and radicals (e.g., SO₃^•‒, SO₄^•‒, SO₅^•‒, and ^•OH) but also to stabilize Mn(Ⅲ) under neutral conditions (Fig. 2d) [11]. MC_dsc was reduced from 84.24% to 72.57% at 0.05 mol/L of KMnO₄ + 0.25 mol/L of NaHSO₃ with a voltage of 30 V for 90 min [29]. It is worth noting that the KMnO₄ + bisulfite/sulfite process usually caused deterioration of WAS filtration performance [82], and combining it with flocculants/coagulants or skeleton builders may help to achieve better dewatering performance of WAS. However, compared to K₂FeO₄, KMnO₄ needs to be further investigated for WAS dewatering.

  3.5 Intrinsic limitations and prospects

  AOPs have shown excellent performance in WAS dewatering, but intrinsic limitations, such as high price, material residuals, and unclear mechanism analysis, still exist. Generally, the dewaterability of WAS increases with the increasing dosages of activators and/or oxidants, but beyond a certain range, excessive activators and/or oxidants rather lead to meaningless consumption and excessive decomposition of EPS, which prevents WAS from achieving good dewaterability, or even deteriorate the dewatering performance of WAS [83]. In addition, excessive activators and/or oxidants can lead to inappropriate residues, which affect the resource utilization and disposal of WAS. However, most of the current researches on the dosage optimization of AOPs-based WAS dewatering processes have been conducted at the laboratory or pilot scale. In addition, the physicochemical properties of WAS are always affected by various factors (e.g., season and influent effluent composition) and aren't constant in practical engineering [3]. In order to realize continuous optimal control, the application of online monitoring system and real-time intelligent decision-making system is essential in practical engineering.

  Compared with flocculation methods commonly used in practical engineering nowadays, AOPs usually have higher costs, which is a key obstacle to the engineering application of AOPs-based WAS dewatering processes. The economic analysis of AOPs-based WAS dewatering processes was summarized in Table 1 [15,37,44,60,68,83-85]. Different AOPs correspond to greatly different WAS dewatering efficiencies (8.46%−49.51%) and costs (23.63–102.0 ＄/t DS). Among them, the combined processes of AOPs (e.g., TA + Fe⁰ + KMnO₄) usually have lower costs and maintain higher dewatering efficiencies. This provides a promising solution for the practical engineering application of AOPs-based WAS dewatering processes. It is worth noting that it has been mentioned that the substances (e.g., surfactants, phenol, HCO₃⁻, HPO₄²⁻, and glucose) contained in WAS can also activate oxidants, which has the potential to facilitate zero addition of activators. Therefore, more researches can be conducted in this direction in the future. In addition, most economic analyses of AOPs-based WAS dewatering processes have been done for laboratory-scale optimal conditions and have focused only on the purchase cost of chemicals and energy costs. In future researches, economic analysis of AOPs-based sludge dewatering processes should be carried out on a full-flow and whole life-cycle range at optimal operating conditions for the practical engineering scale.

  Table 1

  

  Table 1.  The economic analysis of typical AOPs for WAS dewatering.

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  Condition methods	Optimum dosages(t/t DS)	MCdsc(%)			Total costa (＄/t DS)	Ref.
  		Initial	End	Reduction
  Fenton	H2SO4: 0.016;	96.80	48.87	49.51	71.1	[84]
  	H2O2: 0.141;
  	FeSO4: 0.366.
  CaO2	0.1	88	78	11.36	102.0	[44]
  PFS + O3	PFS: 0.04;	78.70	59.79	24.03	88.80	[37]
  	O3: 0.06.
  Pyrite + PMS	H2SO4: 0.025;	92.60	70.14	24.25	79.9	[85]
  P	yrite: 0.076;
  	PMS: 0.076.
  Fe2+ + PDS	Fe2+: 0.307;	95.52	87.44	8.46	53.75	[83]
  	PDS: 0.237.
  Fe2+ + Na2SO3 + PAM	Fe2+: 096;	89.2	79.7	10.65	49.6	[60]
  	Na2SO3: 0.173;
  	PAM: 0.005.
  K2FeO4	K2FeO4: 0.017.	78.8	68.3	13.32	23.63	[68]
  TA + Fe0 + KMnO4	TA: 45 ℃, 15 min;	89.48	60.08	32.86	54.37	[15]
  	Fe0: 0.14;
  	KMnO4: 0.06.
  a The price of chemicals comes from: http://www.alibaba.com/

  4. Bibliometric analysis of AOPs-based WAS dewatering researches

  In order to provide a more comprehensive and thorough review and to propose future research directions of AOPs-based WAS dewatering researches, bibliometrics were used to analyze the country/author distribution, co-citations, categories, and keywords of AOPs-based WAS dewatering researches.

  4.1 Data collection and methods

  The Science Citation Index Expanded of Web of Science Core Collection databases was used for the bibliometric analysis of researches related to the AOPs-based WAS dewatering (Fig. 3). The search strategy is as follows: TS = (sludge) AND TS = (dewatering OR dewater OR dehydrate OR dewaterability OR "dewatering performance") AND ALL = (oxidation). 682 English-language articles associated with AOPs-based WAS dewatering were found in the database during 2000–2023 (searched on 10 December 2023). Subsequently, the obtained articles were roughly screened to exclude non-conforming articles, and finally 438 articles were selected for bibliometric analysis. The literature information was visualized using CiteSpace software (version 6.1.R6) and VOSviewer software (version 1.6.20.0) (Text S1 in Supporting information).

  Figure 3

  

  Figure 3.  Technical flow graph for bibliometric analysis of AOPs-based WAS dewatering research.

  DownLoad: Full-Size Img PowerPoint

  4.2 Cooperative network analysis for countries and authors relating to AOPs for WAS dewatering

  The number of publications published on AOPs-based WAS dewatering grew faster than before after 2016 in different countries worldwide, with China, Australia, USA, Canada, and Poland as the leading countries (Fig. S1 in Supporting information). The network of cooperation relations between countries showed that the earliest study was conducted in Poland, with most countries starting to perform the researches on AOPs-based WAS dewatering in 2006 (Fig. S2a in Supporting information). So far, the vast majority of publications in this research field have been published by Chinese researchers, indicating that using AOPs to promote WAS dewatering has become a hot topic in China [12].

  The collaboration network of authors showed that the top 10 authors with the most publications were all Chinese, and Dongbo Wang ranked first with 17 publications in the research field of AOPs-based WAS dewatering (Fig. S2b and Table S2 in Supporting information). In addition, the analysis of co-cited authors showed that E. Neyens laid the foundation for the research field of AOPs-based WAS dewatering with 197 citations (Table S2). Most of the authors collaborated less and only a few collaborated more closely with mostly in the same organization. In the future, researchers from different countries and institutions should strengthen cooperation to promote the development of the research field of AOPs-based WAS dewatering.

  4.3 Co-citation and categories analysis relating to AOPs for WAS dewatering

  The documents published in various journals in the past have provided important theoretical support and references for researchers in the research field of AOPs-based WAS dewatering. Most of the top 10 co-citated journals belonged to the environmental field while also exhibiting values in engineering applications and management (Fig. 4a and Table S3 in Supporting information). Among them, Water Research was the most co-cited journal with 429 citations. The significant publications that attracted enormous focus and interest could be found according to citation frequency (Table S4 in Supporting information). The top 10 publications were associated with WAS dewatering and the majority employed AOPs. In the Web of Science Core Collection databases, the researches of AOPs-based WAS dewatering were divided into 19 categories. The 10 categories with the highest frequency of simultaneous occurrences and the corresponding centrality values demonstrated that the study of AOPs-based WAS dewatering is a multidisciplinary research field (Figs. 4b and c). "Environmental sciences" was the most frequent category, indicating that the employment of AOPs as a pretreatment process for WAS dewatering was ultimately aimed at achieving environmental protection. "Engineering, chemical" had the highest centrality value and was the key node in the category co-occurrence mapping, which indicated the importance and influence of engineering applications and chemical principles in the researches of AOPs-based WAS dewatering.

  Figure 4

  

  Figure 4.  (a) The visualization mapping of journals co-citation network, (b) frequency and centrality of categories in AOPs-based WAS dewatering research field, and (c) co-occurrence network of categories in AOPs-based WAS dewatering research field. In graph (c), larger node indicated higher frequency of category, and the color closer to gray in the central of node indicated earlier occurrence of the category.

  DownLoad: Full-Size Img PowerPoint

  4.4 Keywords analysis relating to AOPs for WAS dewatering

  4.4.1   Co-occurrence analysis for keywords related to AOPs-based WAS dewatering

  To gain an in-depth understanding of the research field of AOPs-based WAS dewatering, the keyword co-occurrence analysis was conducted by CiteSpace software and VOSviewer software, and the timeline graph and trend graph of keywords were obtained (Fig. 5 and Fig. S3 in Supporting information).

  Figure 5

  

  Figure 5.  Keywords of research field in AOPs for WAS dewatering: (a) Co-occurrence network by CiteSpace software, (b) co-occurrence network by VOSviewer software, and (c) timeline graph based on CiteSpace software. Note: larger node indicated higher frequency of keyword, and the color closer to red in the central of node indicated earlier occurrence of the keyword.

  DownLoad: Full-Size Img PowerPoint

  CiteSpace software and VOSviewer software captured 20 and 33 high-frequency keywords, respectively (Figs. 5a and b and Table S5 in Supporting information). The keywords "Fenton technology" and "persulfate" obtained from CiteSpace software, and the keywords "fenton", "calcium peroxide", "ozonation", "hydrogen peroxide", "peroxymonosulfate", "peroxydisulfate", and "persulfate" obtained from VOSviewer software all indicated that ^•OH-based AOPs and SO₄^•‒-based AOPs were the more concerned AOPs in the research field of WAS dewatering. Among them, the keywords "Fenton technology" and "fenton" appeared most frequently, indicating that the Fenton process has received the most attention in the past 23 years. In addition, the Fenton process was used in WAS dewatering research in 2000 and is still of interest to date, whereas persulfate-based AOPs were used in WAS dewatering research only after 2012 (Fig. 5c). The high frequency of keywords "zero valent iron" and "biochar" demonstrated that stable, easily storable, recyclable and environmentally friendly activators have attracted more attention from researchers [31,86]. Among them, the keyword "zero valent iron" first appeared in 2016 and has received a lot of attention since 2019 (Fig. 5c). The keywords "hydrothermal treatment" and "flocculation" obtained from CiteSpace software and the keywords "hydrothermal treatment", "coagulation", "thermal hydrolysis", and "flocculation" obtained from VOSviewer software all demonstrated that AOPs were frequently combined with a variety of physicochemical processes in the research field of WAS dewatering [79,87,88]. In addition, the flocculation process has been combined with AOPs since 2004, while the hydrothermal treatment has only been combined with AOPs since 2011, indicating that the physicochemical processes associated with AOPs are being enriched year by year (Fig. S3).

  4.4.2   Clustering analysis for keywords related to AOPs-based WAS dewatering

  Keywords clustering analysis was conducted by CiteSpace software to explore the differences and connections between keywords (Fig. S4 and Table S6 in Supporting information). Cluster #0 was marked as "dewaterability" and included 107 keywords, which was the biggest cluster with a silhouette value of 0.758. The top 5 keywords with the highest frequency in the "dewaterability" cluster were "EPS", "dewaterability", "physicochemical property", "bound water" and "organic matter". Cluster #0, Cluster #8 (marked as "particle size distribution"), and Cluster #9 (marked as "moisture distribution") have the same characteristics and all reflect the influencing factors of WAS dewaterability, indicating that the influencing factors of WAS dewaterability have been the research focus in the field of WAS dewatering. Cluster #1 was marked as "waste activated sludge" and included 93 keywords with a silhouette value of 0.699. The top 5 keywords with the highest frequency in the "waste activated sludge" cluster were "waste activated sludge", "sewage sludge", "sludge degradation", "AOPs" and "pretreatment". This cluster reflected that AOPs were commonly used as pretreatment technologies for WAS to promote the degradation of WAS and release of bound water and intracellular water, thus improving WAS dewatering performance. In addition, Cluster #1, Cluster #3 (marked as "waste water treatment"), Cluster #4 (marked as "anaerobic digestion sludge"), and Cluster #7 (marked as "sewage sludge conditioning"), displayed the application of AOPs as pretreatment for the dewatering of various types sludge. Cluster #2 (marked as "peroxymonosulfate"), Cluster #5 (marked as "sludge solubilization"), and Cluster #6 (marked as "thermal hydrolysis"), demonstrated the AOPs and its combined processes that have been used for WAS dewatering. Cluster #2 included 90 keywords with a silhouette value of 0.729. In Cluster #2, the top 5 keywords with the highest frequency were "Fenton technology", "zero valent iron", "persulfate", "activated method" and "hydroxyl radical". This cluster reflected that ^•OH-based AOPs and SO₄^•‒-based AOPs were the major AOPs explored in WAS dewatering. In addition, the Fe⁰-based activation methods can increase the utilization efficiency of activators and reduce the cost of activators, so Fe⁰-based activation methods need to be further explored in future researches. Cluster #5 included 53 keywords with a silhouette value of 0.897, and reflected the effects of CaO₂-based AOPs on WAS dewatering. Cluster #6 included 53 keywords with a silhouette value of 0.849. This cluster reflected that AOPs is often combined with thermal hydrolysis and so on to reduce the oxidant dose and further improve the dewaterability of WAS. Therefore, the combined processes of AOPs with other physicochemical processes should be considered more in future researches.

  4.4.3   Citation burst analysis for keywords related to AOPs-based WAS dewatering

  The citation burst identifies keywords that proliferate in a brief time, which reveals research trends and hotspots [16]. In the research field of AOPs-based WAS dewatering, the 20 strongest citation burst keywords were obtained and categorized into 6 groups (Table 2). The keywords in the first group were associated with AOPs, such as "Fenton technology", "peroxidation", "Fe(Ⅱ) activated persulfate oxidation", "hydrogen peroxide", "persulfate", "iron", and "hydroxyl radical". This indicated that AOPs used for WAS dewatering researches have been continuously updated during the last 23 years, with a gradual development from the initial Fenton process to the later persulfate-based process and the current ^•OH-based AOPs. The keywords in the second group were associated with dewatering mechanism, including "bound water", "3D-EEM", "EPS extraction method", and "protein secondary structure". This suggested that the analysis of WAS dewatering mechanisms was progressively shifting from the floc and cellular level to the molecular level. The keywords in the third group were "hydrolysis", "alkaline conditioning", "skeleton builder", "coagulation", and "polyacrylamide", which were physicochemical processes frequently combined with AOPs. This indicated that in the research field of AOPs-based WAS dewatering, researchers were initially more concerned with the destruction of WAS flocs while later more concerned with the reconstruction of WAS flocs. The keywords in the fourth group were "capillary suction time" and "specific resistance to filtration (SRF)", which were the evaluation indexes of WAS dewaterability. The fifth group, "anaerobic digestion sludge", demonstrated that digestate dewatering after WAS anaerobic digestion has also received attention. The sixth group, "heavy metal", related to post-treatment of dewatered WAS, indicating that research on WAS dewatering based on AOPs has begun to focus on the impact of AOPs on the subsequent treatment process of WAS, rather than merely focusing on WAS dewatering itself. Furthermore, the citation burst analysis of keywords showed that "iron", "protein secondary structure", and "hydroxyl radical" are the current research hotspots in the field of AOPs-based WAS dewatering and may be the future research hotspots as well.

  Table 2

  

  Table 2.  The strongest citation bursts keywords.

  DownLoad: CSV

  The results of co-occurrence analysis, cluster analysis, and citation burst analysis of keywords indicated that the research on WAS dewatering based on AOPs is delving into ^•OH-based AOPs and SO₄^•‒-based AOPs, and more abundant modes of oxidant activation (e.g., Fe⁰) and process combinations (e.g., oxidants + skeleton builder and oxidants + coagulation/flocculation) are being explored from the perspective of green and sustainable development in order to further improve the dewatering performance of WAS and reduce the cost at the same time. In addition, the molecular level exploration of substances (e.g., proteins of EPS) in WAS may remain a focus of future research as analytical tools continue to be improved.

  5. Application of AOPs-based WAS dewatering processes for WAS subsequent harmless disposal

  5.1 Removal of exogenous refractory pollutants

  During the wastewater treatment process, a large number of ERPs (e.g., recalcitrant organics, pathogenic microorganisms/viruses, and heavy metals) are eventually enriched in WAS due to adsorption [17]. During AOPs-based WAS dewatering, the structure of recalcitrant organics [14,89], the activity of pathogenic microorganisms [1], and the content and toxicity of heavy metals [13,90], can be effectively destroyed and decreased by electron transfer and formation of free radicals, thus reducing the potential risk of WAS [6,18]. The treatment of sludge with 3 g/L of PDS for 5 min at 55 ℃ resulted in complete degradation of toluene, along with a 28.0% reduction of CST [89]. At 2.0 g Fe⁰/g TSS and 0.5 g PDS/g TSS, the dewaterability of sludge was significantly improved, meanwhile the contents of 4-nonylphenols, estrone, and bisphenol A were reduced by 40.7%, 43.5%, and 42.1% respectively, and the fecal coliform population in the dewatered sludge cake was significantly reduced to meet the US standards for class A biosolids [1]. After treating WAS with pH 2 and 1% of CaO₂, MC_dsc was reduced from 82.5% to 66.5%, and the removal rates of Pb, Zn, Cu, and Cd in WAS reached 53.0%, 58.3%, 64.8%, and 2.1%, respectively [90]. However, the degradation/conversion pathways and the removal mechanisms of ERPs during AOPs-based WAS dewatering have not been thoroughly elucidated, e.g., (ⅰ) the generation of toxic by-products, (ⅱ) whether highly active species directly degrade the ERPs bound to EPS or break the binding bond between the ERPs and EPS. In addition, in terms of pathogenic microorganisms/viruses, only the inactivation of fecal coliform during WAS dewatering based on AOPs has been investigated, while other pathogenic microorganisms (e.g., Klebsiella pneumoniae and E. coli) and/or viruses (e.g., HCoV-HKU1, poliovirus 1, and SARS-CoV-2) have not yet been mentioned.

  5.2 Resource utilization

  WAS contains a large amount of organic matter and nutrients, which can be recovered as a resource through anaerobic fermentation for the production of short-chain fatty acids (SCFAs) and anaerobic digestion for the production of methane [2,6]. During AOPs-based WAS dewatering, the EPS structure and microbial cells in WAS can be destroyed by oxidants and their highly active species, thereby facilitating the disintegration of WAS and the release of organic matter [26]. Under CaO₂ (0.1 g/g volatile suspended solids (VSS)) + MW (480 W, 2 min) treatment, SRF and CST of WAS were reduced by 43.31% and 47.27%, respectively, and at the same time the optimum SCFAs yield of 227.8 mg COD/g VSS was reached on day 4, which was 363.95% more than the optimum SCFAs yield of the control on day 6 [91]. Under 500 mg/L of K₂FeO₄, SRF of WAS was reduced by 85%, and the cumulative biogas production at day 35 was 44% higher than that of the control [65]. However, only a few AOPs-based WAS dewatering researches (only 12 articles as of December 2023) also focused on WAS anaerobic fermentation/anaerobic digestion. In future researches, all three (i.e., dewatering, ERPs, and anaerobic fermentation/anaerobic digestion) should be taken into account simultaneously, and their intrinsic linkages should be explored in order to provide more rationale for the selection of AOPs-based WAS dewatering processes.

  5.3 An integrated plan for the harmless disposal of AOPs-treated WAS

  Normally, 20%−30% of nitrogen (N) and phosphorus (P) in wastewater is present in the dewatered liquid, which makes it to be a nutrient-rich resource [12]. After treatment by AOPs-based WAS dewatering processes, a large number of flocs and microbial cells in WAS were decomposed, leading to a further increase (> 100%) in the N and P content of dewatered liquid [92]. Producing struvite (magnesium ammonium phosphate, MgNH₄PO₄·6H₂O) as a fertilizer is an effective method to recover N and P from dewatered liquids [93]. In addition to N and P, the dewatered liquid also contains a large amount of C, which can be used as a supplementary carbon source for wastewater treatment processes and anaerobic fermentation/digestion tanks.

  Currently, treatment and disposal of dewatered WAS is available through incineration, landfill, land application, and functional products [5,94]. AOPs-based WAS dewatering processes can result in dewatered WAS with MC less than 60%, which can accommodate WAS for self-sustained incineration [2]. Meantime, the calorific value of dewatered WAS can be further increased by AOPs using pyrolyzed biochar as an activator. AOPs-based WAS dewatering processes can promote the degradation of ERPs in WAS, which is beneficial for the land application of dewatered WAS. Converting dewatered WAS into functional products (e.g., building materials, biochar, and biorefinery products) is a sustainable disposal method [12,25,95].

  Based on the discussion above, an integrated plan for the harmless disposal of AOPs-treated WAS was proposed (Fig. 6). The plan used AOPs as the pretreatment process for WAS, which could simultaneously realize the removal of ERPs from WAS and the improvement of WAS dewatering performance. It can realize the resourceful and harmless treatment and disposal of WAS, meanwhile reducing the external emission of WWTPs, which provides an effective and competitive plan for the reform of traditional WAS management plan.

  Figure 6

  

  Figure 6.  An integrated path to achieve harmless disposal of AOPs-treated WAS.

  DownLoad: Full-Size Img PowerPoint

  6. Conclusions

  AOPs have been shown to be effective in improving WAS dewatering. This review summarized the AOPs-based WAS dewatering researches since the 21^st century, including the mechanism and application of AOPs in WAS dewatering as well as the bibliometric analysis of AOPs-based WAS dewatering researches, and proposed the AOPs-based integrated treatment and disposal process of WAS.

  (1) Oxidants can be activated by various physicochemical methods to generate highly active species and metal ions, which in turn improves the dewaterability of WAS through the double action of oxidation and flocculation. However, so far, the activation mechanisms of AOPs in WAS are not clear, and the possibility of WAS itself activating oxidants is not clarified. Meanwhile, flocculation and oxidation as the main mechanisms of AOPs to improve WAS dewatering, which one is the key and the occurrence order of the two are still inconclusive.

  (2) The combined processes of AOPs and other physicochemical processes usually result in a better dewatering performance of WAS, which is accompanied by a wider application range of pH and lower chemical dosage as well as cost. However, in terms of practical engineering applications, AOPs-based WAS dewatering processes still have intrinsic limitations, such as unstable chemicals, high prices, and substance residues. Furthermore, full-scale, full-process, and full-life-cycle based process optimization and economic analyses still need to be discovered.

  (3) In the bibliometric analysis of AOPs, ^•OH-based AOPs and SO₄^•‒-based AOPs are being intensively investigated, and richer activation methods and combinatorial processes deserve to be further explored in the future, meanwhile the molecular level exploration of substances in WAS may remain the focus of future researches.

  (4) AOPs usually also have the effects of promoting pollutants degradation and resource recovery. However, the degradation/conversion pathways and removal mechanisms of ERPs during AOPs-based WAS dewatering have yet to be thoroughly elucidated so far. In addition, all three (i.e., dewatering, ERPs, and anaerobic fermentation/anaerobic digestion) should be taken into account simultaneously and their intrinsic linkages should be explored. Finally, an integrated plan for the harmless disposal of WAS was constructed based on AOPs.

  Declaration of competing interest

  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.

  CRediT authorship contribution statement

  Xinwen Li: Writing – review & editing, Writing – original draft, Methodology, Conceptualization. Lili Li: Writing – review & editing, Supervision, Funding acquisition, Conceptualization. Junqiu Jiang: Writing – review & editing, Supervision, Funding acquisition, Conceptualization. Wangyang Mei: Methodology. Zhaoxia Wang: Methodology. Qingwei Gao: Methodology, Conceptualization. Huimin Zhou: Methodology, Conceptualization. Liangliang Wei: Methodology, Conceptualization. Qingliang Zhao: Methodology, Conceptualization.

  Acknowledgments

  This research was funded under the auspices of the National Key Research and Development Program of China (No. 2023YFC3207404-01), the Postdoctoral Fellowship Program of CPSF (No. GZC20233450), and the Heilongjiang Province Postdoctoral Science Foundation (No. LBHZ23154).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.110847.

  
  
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• 

  Figure 1  Overview of mechanisms for AOPs in WAS dewatering (a) and limitations of mechanisms for AOPs in WAS dewatering (b).

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  Figure 2  (a) The combined processes of ^•OH-based AOPs. (b) The activation methods and mechanisms of SO₄^•‒-based AOPs. (c) The oxidation processes possibly involved in K₂FeO₄ and the dewatering mechanisms of an innovative electro-chemical system for the activation of K₂FeO₄ using granular activated carbon as the fluidized electrode. (d) The mechanisms of RMnS and radical generation in the KMnO₄ + bisulfite/sulfite process.

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  Figure 3  Technical flow graph for bibliometric analysis of AOPs-based WAS dewatering research.

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  Figure 4  (a) The visualization mapping of journals co-citation network, (b) frequency and centrality of categories in AOPs-based WAS dewatering research field, and (c) co-occurrence network of categories in AOPs-based WAS dewatering research field. In graph (c), larger node indicated higher frequency of category, and the color closer to gray in the central of node indicated earlier occurrence of the category.

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  Figure 5  Keywords of research field in AOPs for WAS dewatering: (a) Co-occurrence network by CiteSpace software, (b) co-occurrence network by VOSviewer software, and (c) timeline graph based on CiteSpace software. Note: larger node indicated higher frequency of keyword, and the color closer to red in the central of node indicated earlier occurrence of the keyword.

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  Figure 6  An integrated path to achieve harmless disposal of AOPs-treated WAS.

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  Table 1.  The economic analysis of typical AOPs for WAS dewatering.

  Condition methods	Optimum dosages(t/t DS)	MCdsc(%)			Total costa (＄/t DS)	Ref.
  		Initial	End	Reduction
  Fenton	H2SO4: 0.016;	96.80	48.87	49.51	71.1	[84]
  	H2O2: 0.141;
  	FeSO4: 0.366.
  CaO2	0.1	88	78	11.36	102.0	[44]
  PFS + O3	PFS: 0.04;	78.70	59.79	24.03	88.80	[37]
  	O3: 0.06.
  Pyrite + PMS	H2SO4: 0.025;	92.60	70.14	24.25	79.9	[85]
  P	yrite: 0.076;
  	PMS: 0.076.
  Fe2+ + PDS	Fe2+: 0.307;	95.52	87.44	8.46	53.75	[83]
  	PDS: 0.237.
  Fe2+ + Na2SO3 + PAM	Fe2+: 096;	89.2	79.7	10.65	49.6	[60]
  	Na2SO3: 0.173;
  	PAM: 0.005.
  K2FeO4	K2FeO4: 0.017.	78.8	68.3	13.32	23.63	[68]
  TA + Fe0 + KMnO4	TA: 45 ℃, 15 min;	89.48	60.08	32.86	54.37	[15]
  	Fe0: 0.14;
  	KMnO4: 0.06.
  a The price of chemicals comes from: http://www.alibaba.com/

  下载: 导出CSV

  Table 2.  The strongest citation bursts keywords.

  下载: 导出CSV
•