Thermal Disintegration of Waste-Activated Sludge

Abstract

The effective management of waste-activated sludge (WAS) presents a significant challenge for wastewater treatment plants (WWTPs), primarily due to the sludge’s high content of organic matter, pathogens, and hazardous substances such as heavy metals. As urban populations and industrial activities expand, the increasing volume of WAS has intensified the need for sustainable treatment solutions. Conventional approaches, such as landfilling and anaerobic digestion, are frequently ineffective and resource-intensive, particularly when dealing with the protective extracellular polymeric substances (EPS) that render WAS resistant to biodegradation. Thermal pretreatment methods have gained attention due to their ability to enhance the biodegradability of sludge, improve dewaterability, and facilitate resource recovery. These processes function by breaking down complex organic structures within the sludge, thereby increasing its accessibility for subsequent treatments such as anaerobic digestion. The integration of thermal treatment with chemical methods can further optimize the management process, resulting in higher biogas yields, reduced pathogen content, and lower environmental risks. While thermal disintegration is energy-intensive, advancements in energy recovery and process optimization have made it a more viable and environmentally friendly option. This approach offers a pathway to more sustainable and efficient sludge management practices, which align with the goals of reducing waste and complying with stricter environmental regulations.

1. Introduction

The effective management of waste-activated sludge (WAS) represents a critical concern for wastewater treatment plants (WWTPs) due to the significant environmental, operational, and financial challenges it presents. As a byproduct of biological wastewater treatment processes, WAS is distinguished by its high concentrations of organic matter, pathogens, heavy metals, and other potentially hazardous substances. The increasing volume of WAS, driven by expanding urban populations and industrial activities, serves to further exacerbate these challenges.

The improper treatment and disposal of WAS can result in the leaching of harmful contaminants into soil and water bodies, which can lead to long-term environmental degradation and public health risks. The presence of heavy metals and toxic organic compounds in WAS has the potential to contaminate groundwater and surface waters, resulting in long-term ecological damage. Furthermore, conventional disposal methods, such as landfilling or incineration, contribute to the emission of greenhouse gases, including methane, which exacerbates climate change. The emergence of concerns regarding pollutants such as microplastics, pharmaceuticals, and antibiotic-resistant genes has also led to the implementation of more stringent regulations and a reduction in public acceptance of land application of sludge as a sustainable practice [1].

From an operational point of view, the treatment and disposal of WAS represents a considerable drain on resources. Conventional methods, such as anaerobic digestion and composting, necessitate the construction of substantial infrastructure, regular maintenance, and long processing times. These processes are frequently inefficient, particularly when the sludge contains elevated levels of extracellular polymeric substances (EPS), which form a protective matrix that shields microorganisms from degradation. The gel-like matrix significantly impairs the efficiency of biological treatments, necessitating greater energy and time inputs to achieve effective results [2]. From a financial perspective, the management of WAS is a significant issue for wastewater treatment facilities. Especially for smaller WWTPs with limited financial resources, the financial costs associated with transporting, treating, and disposing of wastewater are significant [3].

The control of pathogens and the management of odors are also important in the treatment of WAS. The sludge is a rich source of microbial content, including potentially harmful bacteria, viruses, and parasites. It is important to ensure that the sludge is adequately treated before it is disposed of or reused in order to protect public health. It is not always the case that traditional treatment methods reach the necessary level of pathogen reduction. In such instances, additional steps may be required, such as thermal treatment or chemical disinfection. Furthermore, the decomposition of organic matter in sludge results in the production of foul odors, which give rise to operational difficulties and community complaints [4].

In light of these multifaceted challenges, recent research has concentrated on the development of innovative treatment technologies and the exploration of resource recovery options derived from WAS. Pretreatment methods that employ physical, chemical, and biological processes have garnered attention for their potential to enhance sludge management by improving biodegradability and dewaterability, and facilitating the recovery of valuable resources.

Physical pretreatment methods are designed to mechanically or thermally disrupt the structure of sludge flocs, thereby facilitating the solubilization of organic matter [5]. Techniques such as thermal treatment, ultrasonic disintegration [6], and high-pressure homogenization [7] are widely adopted due to their capacity to enhance sludge dewaterability and increase biogas production during subsequent anaerobic digestion. However, these methods can be energy-intensive and may require optimization depending on the characteristics of the sludge.

Among these pretreatment methods, thermal hydrolysis and hydrothermal treatment have emerged as particularly effective solutions. Thermal hydrolysis, typically conducted at temperatures in the range of 60 to 120 °C, focuses on breaking down sludge components through controlled low-temperature heating [8]. This process improves sludge solubilization and facilitates the release of bound water, enhancing downstream anaerobic digestion and biogas production. In contrast, hydrothermal treatment operates at higher temperatures (120 to 200 °C) and applies more intense thermal pressure to the sludge, resulting in a complete breakdown of organic matter [9]. This method is particularly effective in improving sludge dewaterability, pathogen reduction, and resource recovery.

In addition to physical methods, chemical pretreatment entails the use of reagents, including alkalis, acids, and oxidants, to disrupt complex organic molecules within the sludge. Alkaline pretreatment has been demonstrated to be an effective method for the breakdown of lipids and disruption of the EPS matrix, resulting in an improvement in sludge solubilization and dewaterability [10]. Acidic pretreatment has been demonstrated to enhance polysaccharide hydrolysis, thereby promoting carbon release and nutrient recovery [11]. However, chemical methods entail the risk of secondary pollution and may introduce inhibitory by-products that impact downstream biological processes.

On the other hand, biological pretreatment methods employ microbial consortia or enzymes to degrade organic matter within WAS. Approaches such as bioaugmentation and enzymatic hydrolysis target the breakdown of complex biopolymers, rendering organic compounds more accessible for treatment [12]. Biological methods are typically more energy-efficient and environmentally friendly but may be slower and less effective at treating recalcitrant materials compared to physical and chemical methods.

In addition to pretreatment strategies, advancements in nutrient-recovery technologies provide promising solutions for addressing the challenges of sludge management. Techniques such as struvite precipitation [13], phosphorus recovery [14], and the production of sludge-based fertilizers [15] are increasingly being integrated into sludge treatment strategies. These technologies not only address nutrient depletion but also reduce the environmental impact of sewage sludge disposal.

Despite the considerable potential of these innovative technologies, the high capital and operational costs, particularly for smaller WWTPs, constitute a significant barrier. Implementing advanced systems necessitates a substantial investment in equipment, infrastructure, and skilled labor, in addition to ongoing maintenance and energy expenses. Moreover, the complexity of these technologies frequently requires specialized training, which contributes to the operational costs.

The objective of this review was to evaluate the effectiveness of thermal pretreatment methods such as thermal hydrolysis and hydrothermal treatment of WAS. The review includes an in-depth analysis of these techniques aimed at improving sludge biodegradability, dewaterability, and resource recovery. It focuses on mechanisms of thermal processes, operational conditions, and potential integration into existing wastewater treatment systems. In addition, the review explores the synergistic effects of combining thermal pretreatment with chemical methods to optimize sludge management strategies. Beyond these technical aspects, the review also discusses the implications of energy consumption, cost-effectiveness, and environmental sustainability. By synthesizing the latest advances and addressing the associated challenges, this review provides a comprehensive overview of current thermal pretreatment technologies and their applicability in modern WAS management practices.

2. Data Sources and Methodology

The references and data sources used in this review were collated through a comprehensive literature search in several academic databases, including PubMed, Scopus, Web of Science, and Google Scholar. The search was conducted on peer-reviewed journal articles, conference proceedings, and authoritative reports published over the past two decades. The following keywords were employed in the search for relevant studies: “waste-activated sludge management”, “thermal disintegration”, “sludge treatment technologies”, “biodegradability enhancement”, and “sludge dewatering”. Furthermore, the most recent developments and optimization strategies in thermal disintegration were investigated by examining the most recent research articles published in high-impact journals. The review prioritized studies that provided empirical data, advanced theoretical models, or comprehensive reviews of the current state of WAS management. This systematic approach guaranteed that the review included a comprehensive and representative sample of the most current and relevant research, thus providing a robust foundation for the subsequent discussion and conclusions.

3. Mechanisms of Thermal Disintegration of WAS

The thermal disintegration of WAS represents a process employed in advanced wastewater treatment with the objective of enhancing the biodegradability of sludge and optimizing the efficacy of the wastewater treatment process. The application of controlled heat facilitates the breakdown of complex organic molecules within the sludge into simpler, more readily digestible compounds. This process facilitates subsequent treatment steps, such as anaerobic digestion, and contributes to pathogen reduction and odor control, thereby improving the safety and quality of the final sludge product. The integration of thermal disintegration into the WAS management process is a pivotal step in optimizing sludge treatment and disposal, as illustrated in Figure 1.

Microbial EPS and different cations form a polymeric, highly hydrated, gel-like network in which different microorganisms and organic and inorganic matter are embedded, thus forming a complex three-dimensional floc structure of the activated sludge. EPS originates from the wastewater and/or are secreted and released due to cell lysis by activated sludge microorganisms. Biopolymers such as polysaccharides, proteins, lipids, humic acid-like substances, uronic acids, and nucleic acids are the main components of EPS [16,17]. Taking into account the spatial distribution of EPS in the floc of activated sludge, there can be soluble EPS (S-EPS; distributed in the aqueous phase), loosely bound EPS (LB-EPS), and tightly bound EPS (TB-EPS) [18]. LB-EPS and TB-EPS are highly porous and attached to the bacterial cell surface inside the flocs, occurring as a capsule around the bacterial cells and keeping them together [19,20]. As a result, EPS govern the physicochemical properties of activated sludge and provides functional integrity and strength of flocs, preventing the cell lysis, thus affecting the surface charge, hydrophobicity, bioflocculation ability, settle-ability, dewaterability, adsorption ability, and biodegradability of activated sludge in wastewater treatment systems, as well as the efficiency of pollutant removal from wastewater [18]. The effects of EPS on these properties are crucial because, although EPS content in WAS may vary, it may account for up to 80% of sludge biomass, thus significantly contributing to the sludge mass [20]. EPS protection and a hard structure built of glycan strands cross-linked by peptides around the microbial cells make WAS resistant to direct anaerobic digestion. That is why the degradation of volatile solids (VS) of raw (not pre-treated) sludge in conventional mesophilic anaerobic digestion reaches only 30−50% even at a long retention time of 20−30 d [21].

Thermal treatment alters the molecular structure of EPS, leading to significant changes in the macro-physical properties of sludge, such as viscosity, dewaterability, and solid–liquid separation. The specific impacts depend on the temperature range used, which distinguishes between thermal hydrolysis (low temperatures) and hydrothermal treatment (moderate to higher temperatures).

At low temperatures (30 to 120 °C), typically associated with thermal hydrolysis, the thermal energy primarily induces the solubilization of EPS components, particularly high-molecular-weight proteins and polysaccharides [22]. During this phase, hydrophilic functional groups such as hydroxyl and amino groups become exposed, increasing the electrostatic repulsion between sludge particles [23]. This results in the formation of a gel-like network that retains water within the sludge matrix, leading to higher viscosity and reduced dewaterability [24]. The release of these EPS components increases capillary suction time (CST) and specific resistance to filtration (SRF), both indicators of deteriorated dewatering performance [25]. The negative impact is more pronounced in anaerobic sludge, which exhibits a more significant drop in floc compactness and greater release of EPS, exacerbating dewatering challenges [26].

As temperatures rise to the mid-range (120 to 150 °C), characteristic of hydrothermal treatment, significant molecular transformations occur within the EPS. The hydrolysis of protein secondary structures, such as α-helices and β-sheets, leads to a transition toward more disordered forms like random coils [27]. This transformation reduces hydrophilicity, thereby facilitating the release of bound water and improving sludge dewaterability. Additionally, the onset of the Maillard reaction, involving interactions between amino acids and reducing sugars, results in the formation of stable, hydrophobic by-products such as melanoidins [28]. These reactions begin a shift from hydrophilic to hydrophobic interactions, enhancing sludge aggregation and reducing viscosity. However, the continued presence of larger EPS components, particularly those with high molecular weight, means that some water is still retained within the sludge, limiting optimal dewatering performance.

At higher temperatures (150 to 170 °C), the hydrothermal treatment process leads to the complete disintegration of the gel-like EPS structures, resulting in a significant improvement in dewaterability. The intense hydrolysis of biopolymers produces lower-molecular-weight compounds, while the Maillard reaction becomes more pronounced, generating highly hydrophobic compounds [28]. The shift toward hydrophobicity reduces the sludge’s water-holding capacity, facilitating better solid–liquid separation and lower CST and SRF values. The reduction in viscosity and the breakdown of larger colloidal aggregates into smaller particles further enhance sludge handling and dewatering efficiency.

At very-high-temperature temperatures above 170 °C, approaching the upper end of the hydrothermal treatment range, the sludge undergoes carbonization, characterized by extensive dehydration, decarboxylation, and condensation reactions [29]. These processes result in the formation of fixed carbon and stable aromatic compounds that contribute to sludge stabilization [25]. The Maillard reaction intensifies, producing nitrogen-containing heterocyclic compounds resistant to biodegradation [30]. These compounds further increase sludge hydrophobicity, leading to optimal dewatering conditions. The complete breakdown of the gel network, combined with reduced electrostatic repulsion and enhanced particle aggregation, results in a sludge matrix that is easier to process and handle.

4. Optimization Parameters for Thermal Disintegration of WAS

The optimization of the thermal disintegration of WAS necessitates the precise adjustment of parameters, including exposure time, pH levels, temperature, and pressure. These factors are crucial for enhancing the efficiency of the disintegration process, optimizing the solubilization of organic matter, and improving the efficacy of the treatment.

Thermal hydrolysis is regarded as a relatively low-energy and low-cost process for the disruption of the structures of EPS and microbial cells in WAS. This results in an increase in sludge biodegradability and the simultaneous release of phosphorus from sludge [31]. Furthermore, the greatest cost-efficiency of low-temperature WAS processing is achieved through the recovery of waste heat produced in other WWTP equipment, such as air compressors [32].

To optimize the solubilization of organic matter in WAS, Nazari et al. [3] investigated the effects of varying temperatures (40, 60, and 80 °C), exposure times (1, 3, and 5 h), and pH (4, 7, and 10). Compared to the non-thermally treated WAS, the soluble chemical oxygen demand (sCOD) concentration increased by 20%, and the volatile suspended solids (VSS) reduction increased by 44%. The optimal conditions (80 °C, 5 h, and pH 10) were employed for the thermal hydrolysis of multiple WAS samples from full-scale WWTPs, resulting in an 18.3 ± 7.5% increase in COD release and a 27.7 ± 12.3% reduction in VSS compared to the untreated WAS. The observed increase in sCOD was attributed to the release of proteins and carbohydrates into the liquid phase. However, the solubilization of carbohydrates was found to be significantly lower than proteins under the same operational conditions. This was a consequence of the preponderance of both components in the diverse structures of the cells. Carbohydrates are primarily located within the exopolymers, whereas proteins are within the cells. The degradation of carbohydrates is not effectively achieved during thermal hydrolysis. The thermal process stimulated cell lysis [19], resulting in a higher increase in proteins released from both cell lysis and EPS decomposition. At the same optimal treatment temperature (80 °C), the concentration of soluble proteins increased 12-fold compared to the non-treated WAS [33]. The predominant proteins were those of a tyrosine-like nature, with a molecular weight of less than 20 kDa. The rise in protein concentration prompted the release of total organic carbon (TOC), total organic nitrogen, and total dissolved nitrogen from soluble EPS, as well as their decomposition in tightly bound EPS.

In examining the main factors affecting thermal disintegration, it was determined that temperature plays a more pivotal role than treatment duration in the solubilization of organic matter. The highest degree of solubilization was observed during the low-temperature process, with a temperature of 90 °C [32] being the optimal condition. However, the discrepancy in solubilization effects between 90 and 75 °C led to the conclusion that 75 °C represents the threshold temperature for the effective solubilization of WAS components. With regard to the exposure duration, it was observed that the solubilization of carbohydrates and proteins increased during the initial four hours. Thereafter, the solubilization of the sludge exhibited a decline. The release of humic substances was observed to increase with increasing time above 4 h. Furthermore, the investigations into temperatures up to 210 °C confirmed that exposure time had a lesser effect on organic matter solubilization than temperature. Specifically, the solubilization did not change at 4 h compared to 30 min [34]. Nevertheless, it has been demonstrated that exposure time is a more significant factor in optimizing thermal treatments [35].

The investigation of the thermal hydrolysis of WAS at pH levels of 4, 7, and 10 revealed that the optimal pH for this process was 10 [3]. The higher solubilization of organic matter under alkaline conditions was attributable to a number of factors. Firstly, the saponification of lipids results in the release of intracellular material from the cells. Secondly, the dissociation of acidic groups in EPS occurs, resulting in the desorption of some polymers due to electrostatic forces [36]. Thirdly, the solubilization of EPS can be attributed to chemical degradation and the ionization of hydroxyl groups, which in turn result in the solubilization of organic matter [19]. The situation is distinct with regard to polysaccharides. In an alkaline environment, they remain stable. In acidic conditions, the disruption of glycosidic linkages facilitates the hydrolysis of polysaccharides into monosaccharides, which are more readily soluble [19]. It is important to note that the production of furfural and hydroxymethylfurfural may occur under strong acid conditions, which can inhibit subsequent sludge methanation [37]. As a consequence of the solubilization of organic matter (an increase in COD solubilization by 18.3 ± 7.5% and a reduction in VSS by 27.7 ± 12.3%), methane production rates increased by up to 5-fold in comparison to the untreated WAS. However, the ultimate methane yield remained unaltered [3].

The disparate characteristics of primary sludge and WAS led to the observation of contrasting outcomes in their solubilization of organic matter. The primary sludge is typically composed of a higher proportion of readily biodegradable fats and carbohydrates, whereas the activated sludge is characterized by the presence of complex carbohydrates, proteins, and long-chain hydrocarbons. The elevated increase in sCOD and soluble protein fractions during thermal WAS treatment can be attributed to the enhanced efficiency in the release of tightly bound cellular components [3]. The increase in sCOD was not correlated with the rise in VSS reduction, likely due to the generation of a greater number of colloidal particles during thermal treatment. Consequently, a higher reduction in VSS was observed for the primary sludge.

The leakage of organic matter and phosphorus from WAS during thermal hydrolysis was compared for two different sludges: phosphorus-accumulating fluid sludge (PFS) and phosphorus-accumulating granular sludge (PGS) [38]. The solubilization of COD was significantly higher for the PGS than for the PFS. The concentrations of released sCOD, proteins, and carbohydrates were 403 ± 9, 82.7 ± 1.6, and 42.0 ± 3.2 mg/g VSS, respectively (from PGS), and 329 ± 26, 63.0 ± 0.9, and 39.1 ± 1.3 mg/g VSS, respectively (from PFS). A similar trend was observed with regard to phosphorus release. The higher concentrations of organic matter released from the PGS were attributed to three factors: the higher content of EPS, the higher proportion of phosphorus in the EPS, and the higher abundance of microorganisms (alive and dead) in the granular sludge compared to the flocculent sludge. It is also important to note that the extent of sludge solubilization at elevated temperatures was not dependent on the initial sludge concentration [39]. The EPS content was found to be 2.6 times higher in granular sludge than in flocculent sludge [38]. Cell lysis and subsequent EPS decomposition resulted in the release of organic matter. The characteristics of the granular sludge prompted the generation of butyric acid and hydrogen.

In the case of hydrothermal treatment, its ability to disintegrate WAS results in enhanced WAS solubilization and biodegradability under anaerobic conditions. For instance, a study conducted at 60 °C reported a biodegradability of 20%, while a study at 170 °C reported an enhanced biodegradability of 88% compared to the untreated WAS [31]. It was established that at elevated temperatures (172 °C), the most efficacious sludge solubilization (release of low-molecular-weight proteins, polysaccharides, and neutrals) and the optimal rate of methane production were attained [40]. Consequently, the anaerobic digester effluent exhibited a higher concentration of dissolved organic matter, predominantly comprising humic substances and low-molecular-weight proteins and neutrals. Most of these compounds were either non-biodegradable or exhibited only slow biodegradation, displaying characteristics associated with aromatic and steroid-like compounds. Additionally, high-molecular-weight compounds were identified, including some inhibitors or recalcitrant compounds (flavonoids, benzenoids, pyridines, and their derivatives). The presence of these compounds may be attributed to the hydrothermal treatment of WAS, and further purification of the anaerobic digestion supernatant is recommended. Furthermore, a minimal increase in polysaccharide content was observed, which was attributed to the reaction of released sugars with amino compounds at elevated temperatures. The Maillard reaction can occur at temperatures exceeding 150 °C, which results in the formation of recalcitrant compounds, such as melanoidins, that are resistant to degradation and can impede the degradation of other organic compounds. The detrimental effects of the Maillard reaction were also substantiated in examining the six distinct WAS at temperatures ranging from 60 to 210 °C [34]. An increase in temperature to 190 °C resulted in enhanced solubilization of organic matter, while higher temperatures had the opposite effect, reducing biodegradability.

The effect of hydrothermal treatment of WAS at 160 and 180 °C on sludge humification was investigated [41]. At lower temperatures, the WAS exhibited a total extractable carbon and humic acid carbon content of 7.3 and 8.5% higher, respectively, compared to the untreated WAS. At elevated temperatures, the WAS exhibited a reduction in carbon concentration, with a decrease of 1.4 and 7.0% for these compounds. The findings demonstrate that at 160 °C, the process of humification was enhanced, resulting in an increased availability of polysaccharides and more efficient metabolism of intracellular polyphenols. The inhibition of humification at 180 °C was attributed to the excessive production of melanoidin, which lowered the availability of polysaccharides and inhibited polyphenol synthesis. Polyphenols play a pivotal role in the synthesis of condensation enzymes, which are essential to producing humic acids.

The impact of the hydrothermal treatment of WAS at temperatures of 150 and 250 °C and pressures of 0.2–4.0 MPa was monitored and regulated regarding the alterations in TOC concentrations in both the solid and liquid phases [39]. Following a two-hour exposure, the TOC concentration in the sludge was observed to decrease by 15–18% (of the initial value) at 150 °C and 30% at 250 °C. At lower temperatures, transfer of TOC from the solid to the liquid fraction was observed to reach 25% after one hour, whereas at higher temperatures, this figure reached 50% after 15 min. These effects were attributed to the thermal disintegration of WAS particles, which resulted in the release of dissolved organic compounds into the liquid phase and the simultaneous decomposition of the insoluble solid phase into carbon dioxide. Both mechanisms were possible due to the integrated action of temperature and pressure.

5. Combination of Thermal Treatments with Chemical Treatments

The combination of thermal and chemical treatments has been demonstrated to enhance the disintegration of WAS [33]. Furthermore, these synergistic effects can facilitate the solubilization of organic matter, enhance biogas production, and increase process efficiency [42]. Chemical treatments that are complementary to thermal processes include the addition of hydrogen peroxide, alkaline and acidic conditions, and the use of various catalysts such as Fenton’s reagent, calcium peroxide, sodium percarbonate, and persulfates. Moreover, the surfactants were subjected to a thermal pretreatment of WAS with the purpose of improving disintegration. Each of these chemical agents contributes a distinct mechanism, such as oxidative degradation, saponification, or enhanced solubilization. When combined with thermal processes, these mechanisms significantly improve the breakdown of complex organic matter and enhance treatment outcomes.

5.1. Hydrogen Peroxide

The incorporation of hydrogen peroxide (H₂O₂) into the thermal disintegration of WAS represents a synergistic approach that enhances the efficiency of the process [43]. Hydrogen peroxide is a potent oxidizing agent that can significantly enhance the breakdown of complex organic molecules when combined with heat treatments [44]. The combination of these two processes harnesses the oxidative power of H₂O₂ to enhance the solubilization of organic matter, reduce the viscosity of the sludge, and improve the biodegradability of WAS. Ultimately, this leads to an increase in methane production during anaerobic digestion [45].

H₂O₂ exerts its primary effect through the formation of hydroxyl radicals (•OH), which are among the most highly reactive species in chemical oxidation processes [46]. The introduction of H₂O₂ into WAS under thermal conditions results in its decomposition, leading to the formation of •OH [47]. These radicals exhibit a high degree of reactivity towards complex organic molecules present in the sludge, including proteins, lipids, and polysaccharides. They effectively break down these molecules into simpler, more biodegradable compounds. The oxidative degradation process is complementary to the thermal disintegration process, which results in the destruction of cell structures and EPS, as well as the release of intracellular materials.

One of the principal advantages of incorporating H₂O₂ is the substantial enhancement in organic matter solubilization [48]. It has been demonstrated that the application of hydrogen peroxide at 70 °C, with a dosage of 5–30 mg H₂O₂/g TS (total solids), results in an increase in the concentration of sCOD, proteins, and carbohydrates [49]. The combination of temperature and hydrogen peroxide resulted in the mineralization of humic substances (as evidenced by a reduction in TS, VS, and COD), as well as the solubilization of proteins and carbohydrates. This process led to a shift from tightly bound to loosely bound and soluble fractions. The integration of these methods resulted in a reduction in WAS viscosity. This decrease was attributed to the increasing reaction rate of H₂O₂, which was stimulated by temperature, and a reduction in shear stress as a result of the exposure of WAS to 70 °C. The reduction in sludge viscosity may have enhanced mixing efficiency, potentially reducing the extent of dead zones in the fermenter and enhancing substrate mass transfer, which could elevate biogas and methane production rates. In addition to the previously mentioned factors that reduce the viscosity of WAS, the potential advantages of introducing Fenton’s reagent or cationic polymers and reducing particle size have been observed.

The combination of thermal hydrolysis and the addition of H₂O₂ has enhanced WAS’s dewaterability. The breakdown of the EPS and reduction in viscosity reduce the sludge’s resistance to water release. This is reflected in a reduction in CST and SRF, which are both indicative of enhanced dewatering performance. The enhanced dewatering capacity reduces the volume of sludge that must be disposed of, thereby lowering transportation and disposal costs and reducing the environmental impact.

Furthermore, the oxidative degradation of organic material by H₂O₂ has a beneficial impact on biogas production during anaerobic digestion. The addition of H₂O₂ facilitates the conversion of complex organic compounds into simpler, more biodegradable forms, thereby increasing the availability of substrates for microbial degradation and resulting in higher methane yields. This increase in biogas production enhances the energy efficiency and sustainability of the wastewater treatment process, as the biogas can be harnessed as a renewable energy source, thereby providing an alternative means of powering the treatment plants or meeting other energy demands.

5.2. Calcium Peroxide

Combining calcium peroxide (CaO₂) with thermal hydrolysis for the management of WAS improves sludge disintegration and solubilization of organic matter and enhances treatment efficiency. CaO₂ is an oxidizing agent that releases H₂O₂ and oxygen (O₂) when in contact with water [50].

CaO₂ is a strong oxidizing agent that decomposes in the presence of water to release H₂O₂ and O₂. The reactions are as follows (Equations (1) and (2)):

CaO₂ + 2H₂O → Ca(OH)₂ + H₂O₂

(1)

2CaO₂ + 2H₂O → 2Ca(OH)₂ + O₂

(2)

The H₂O₂ further decomposes to form •OH, which are highly reactive species capable of breaking down complex organic molecules.

The decomposition of calcium peroxide results in the formation of calcium hydroxide (Ca(OH)₂), which raises the pH of the sludge [51]. The resulting alkaline environment further contributes to sludge disintegration by promoting the saponification of lipids [52]. Saponification breaks down lipids into glycerol and fatty acids, making them more soluble and easier to degrade. Alkaline conditions also facilitate the breakdown of proteins and polysaccharides through hydrolysis, enhancing the solubilization of organic matter.

The combined action of thermal energy and CaO₂ leads to the disintegration of flocs into smaller particles [53], converting tightly bound water into free water, which is easier to remove during mechanical dewatering processes [54]. The oxidation weakens the bonds holding the sludge particles together, while the heat reduces the viscosity of the sludge. Improved dewaterability reduces the volume of sludge needing disposal. Moreover, it effectively inactivates pathogenic microorganisms, making the treated sludge safer for disposal or reuse. This pathogen control is particularly advantageous in meeting regulatory requirements for sludge disposal and potential agricultural applications.

The oxidative breakdown of organic matter results in the production of simpler, more readily biodegradable substrates, such as short-chain fatty acids (SCFAs). Sun et al. [55] at the conditions—70 °C combined with 0.2 g CaO₂/g VS of WAS—achieved the highest SCFA production of 3529 mg COD/L, which was 2.1 times higher than the thermal-only treatment and 1.4 times greater than the untreated control. Similarly, Li et al. [56] found that a CaO₂ dose of 0.2 g/g VSS under mesophilic conditions (35 °C) achieved an SCFA production of 284 mg COD/g VSS, which was 3.9 times higher than the control, with acetic acid accounting for 60.2% of the total SCFAs produced. These results were driven by the combination of CaO₂’s oxidative effects and enhanced microbial activity.

5.3. Sodium Percarbonate

Sodium percarbonate (SPC, Na₂CO₃·1.5H₂O₂) is a solid, crystalline compound that releases H₂O₂ when dissolved in water. The decomposition of SPC into H₂O₂ and sodium carbonate provides a source of reactive oxygen species (ROS), primarily •OH, which are powerful oxidants. These radicals play a critical role in the oxidative breakdown of organic matter in WAS.

The decomposition reaction is as follows (Equation (3)):

Na₂CO₃ ⋅ 1.5H₂O₂ → Na₂CO₃ + 1.5H₂O₂

(3)

The combined use of SPC and thermal hydrolysis has proven to be highly effective in disintegrating sludge flocs and enhancing the extraction of valuable biopolymers like alginate-like extracellular polymers (ALE) from WAS. In the study, Liu et al. [57] demonstrated that the heating-sodium percarbonate method resulted in a 30.11% increase in ALE yield compared to the conventional heating-sodium carbonate method. The oxidative properties of SPC played a critical role in breaking down EPS, leading to enhanced solubilization of organic matter and improved polymer recovery. Moreover, the molecular analysis indicated that the oxidative radicals produced by SPC contribute to structural modifications in the extracted ALE, which could be beneficial for various industrial applications.

5.4. Persulfates

Persulfates, specifically peroxydisulfate (PDS) and peroxymonosulfate (PMS), have gained significant attention in recent years as potent oxidants for improving WAS treatment [58]. Persulfates are strong oxidizing agents that can generate sulfate radicals (SO₄•⁻) under activation conditions, including heat [59], ultraviolet light [60], and chemical catalysts [61]. The sulfate radicals produced are highly reactive, possessing a redox potential of 2.65–3.10 V [62], which is comparable to or even higher than •OH. Compared to conventional advanced oxidation processes (AOPs), SO₄•⁻ are more selective, have a longer half-life, and are effective over a wider pH range (pH 2–8) [63]. These characteristics make PDS and PMS especially suitable for complex and variable sludge matrices like WAS. PDS (S₂O₈²⁻) and PMS (HSO₅⁻) each have distinct activation mechanisms and reactivity profiles. PDS activation primarily involves breaking the O-O bond to generate SO₄•⁻ and additional reactive oxygen species such as hydroxyl radicals and superoxide anions (O₂•⁻) [64]. PMS, on the other hand, tends to produce both sulfate and hydroxyl radicals, along with singlet oxygen (¹O₂), which further contribute to oxidative degradation processes [65].

Heat is one of the most straightforward and effective activation techniques, as it directly breaks the O-O bonds in persulfate molecules, facilitating the rapid generation of reactive radicals [66]. When combined with thermal hydrolysis (e.g., 70–90 °C), these radicals initiate several beneficial reactions in WAS treatment. In WAS, the sludge particles are held together by EPS, which consist of proteins, polysaccharides, and lipids that form a complex, stable matrix. The radicals generated from PDS and PMS, particularly SO₄•⁻, target the functional groups within EPS, breaking down these macromolecules and destabilizing the floc structure. This disintegration process releases bound water, converting it into free water that is easier to remove during dewatering. As a result, sludge dewaterability improves, leading to reduced sludge volume and enhanced mechanical dewatering efficiency. Lee et al. [67] investigated the performance of thermally activated (50 and 80 °C) persulfates. Thermally activated PDS at 80 °C was identified as particularly effective in improving the standardized capillary suction time (SCST), a key indicator of filterability. This improvement in SCST was attributed to the non-selective oxidation provided by sulfate radicals, which aggressively target and break down both cell structures and EPS components, reducing the viscosity of the sludge.

The oxidative power of SO₄•⁻ is particularly effective at converting complex and recalcitrant organic compounds within the sludge into simpler, more biodegradable substrates, such as SCFAs [68]. For instance, under heat-activated conditions, PMS and PDS pretreatment can significantly increase SCFA production, which is crucial for downstream processes like anaerobic digestion. Wu and Song [69] investigated the thermal activation of PDS at 55 °C for enhancing SCFA production during WAS anaerobic digestion. Their results showed that the maximum SCFA yield reached 3183.51 mg COD/L with a PDS dosage of 0.3 mM/g TSS, which was 2.1 times higher than the control. Similarly, Liu et al. [59] explored a coupling strategy involving heat-activated persulfate (PS) pretreatment combined with sulfate-reducing bacteria (SRB) triggering, demonstrating that this approach enhanced SCFA production during WAS anaerobic digestion. The study achieved a peak SCFA yield of 431.89 mg COD/gVSS, with acetic acid constituting 57.8% of the total SCFAs produced. The synergy between heat-activated PS and SRB was key, as it promoted more efficient WAS acidification, with the process driven by the decomposition of WAS by multiple free radicals (SO₄•⁻, •OH) generated through heat activation. Additionally, the enrichment of sulfate-reducing bacteria under persulfate conditions can alter the metabolic pathways, reducing hydrogen partial pressure and allowing for more stable methane production. The resulting improved methane yields and reduced digestion time enhance the energy-recovery potential of WAS treatment processes.

The heat-activated conditions with PMS and PDS pretreatment enhance the degradation of non-biodegradable organics, such as humic acids and lignin, further promoting the availability of substrates for microbial digestion and enhancing sludge treatment efficiency. Yang et al. [70] demonstrated that heat-activated PMS significantly reduced humic acid-like and fulvic acid-like substances in waste-activated sludge. The powerful SO₄•⁻ and •OH generated during heat activation broke down these recalcitrant organics, enhancing the biodegradability of the released substrates and leading to increased SCFA production during anaerobic digestion.

5.5. Alkaline and Acidic Conditions

The pH of the WAS can be adjusted during thermal disintegration, which has the potential to significantly impact the solubilization of organic matter and the efficiency of the treatment process [10]. Both alkaline and acidic conditions offer distinctive benefits and challenges that influence the degradation of complex organic molecules, pathogen reduction, and nutrient release.

The addition of agents such as sodium hydroxide (NaOH) or lime (CaO) is typically employed to achieve alkaline conditions, which are conducive to a high-pH environment that facilitates the thermal disintegration of WAS [71]. A number of reactions are observed when the pH is increased to values between 9 and 12. One of the most significant effects of alkaline conditions is the saponification of lipids, which results in the breakdown of fats into glycerol and fatty acids [72]. This increases the solubility of compounds, thereby facilitating microbial degradation during subsequent anaerobic digestion. Ruffino et al. [73] investigated the efficacy of alkali (NaOH and Ca(OH)₂) and hybrid thermo-alkaline pre-treatments at low temperatures (70–90 °C) for enhancing the anaerobic digestion of WAS. The study demonstrated that NaOH was more effective than Ca(OH)₂ in promoting sludge digestion and COD dissolution and that significant results could be achieved even at lower doses. The application of a thermo-alkaline pretreatment at 70 °C with NaOH resulted in a 26.8% enhancement in biogas yield in comparison to the untreated samples.

The elevated pH environment also disrupts the EPS that encapsulate the microbial cells in the sludge [74]. The EPS consists of biopolymers, including polysaccharides, proteins, and nucleic acids, which ensure the structural integrity of the flocs. In alkaline conditions, the dissociation of acidic groups within the EPS results in their degradation and desorption, thereby promoting the release of intracellular materials [75]. The enhanced solubilization of organic matter is evidenced by increased concentrations of sCOD and a reduction in VSS, which results in a more biodegradable sludge. Furthermore, alkaline conditions facilitate the release of ammonia from proteins, thereby enhancing the recovery of nutrients and the efficiency of the anaerobic digestion process [76]. The resulting WAS is also more hygienic, as the high pH is inhospitable to many pathogens, which helps to reduce the number of pathogens present. Nevertheless, the management of elevated pH levels necessitates precise control to prevent complications such as scale formation or mineral precipitation, which can complicate the treatment process and increase maintenance requirements.

The addition of acids, such as sulfuric acid (H₂SO₄) or hydrochloric acid (HCl) is typically employed to achieve acidic conditions, whereby the pH of the sludge is reduced to values between 4 and 6 [77]. The hydrolysis of polysaccharides is promoted in acidic conditions, whereby complex carbohydrates are broken down into simpler sugars. This process enhances the solubility of the carbohydrates, rendering them more accessible for microbial degradation. Acid hydrolysis is an effective method for breaking the glycosidic bonds in polysaccharides, facilitating their conversion into monosaccharides and other low-molecular-weight compounds [78]. One of the principal benefits of acidic conditions is the enhanced solubilization of metals and phosphorus from sewage sludge [11]. A reduction in pH can result in the transfer of these elements into the liquid phase, thereby facilitating their recovery and recycling. This aspect is of particular benefit with regard to the recovery of nutrients and contributes to more sustainable waste management practices. Nevertheless, the use of acidic conditions also presents a challenge. The formation of inhibitory compounds, such as furfural and hydroxymethylfurfural, is a potential issue when highly acidic conditions are present, particularly at elevated temperatures [79]. Such inhibition can result in the disruption of subsequent biological processes, such as anaerobic digestion, thereby reducing the efficiency of the treatment process. It is essential to maintain a balance in order to optimize the positive effects of acid hydrolysis while minimizing the formation of inhibitory compounds. Furthermore, the corrosive nature of acids necessitates the utilization of robust materials for the construction and maintenance of the treatment infrastructure, which consequently increases the cost [80]. The handling and storage of acids also present a significant safety risk, necessitating the implementation of rigorous safety protocols and comprehensive staff training.

In certain instances, the combination of alkaline and acidic treatments can yield synergistic benefits. Initial alkaline treatment can disrupt the EPS matrix and facilitate the release of intracellular materials, which can then be further enhanced by an acidic treatment to facilitate the solubilization of polysaccharides and metals [10]. This sequential approach can optimize organic solubilization and nutrient recovery while minimizing the drawbacks associated with each pH condition.

5.6. Potassium Ferrate

Potassium ferrate (K₂FeO₄, PF) has been demonstrated to be an efficacious and environmentally benign chemical oxidizing agent in the treatment of WAS. Its use is predicated on its high redox potential, which renders it highly efficacious in oxidizing a diverse array of organic and inorganic compounds present in sludge. The appeal of PF lies not only in its strong oxidative capabilities but also in its ability to simultaneously act as a coagulant due to the iron (III) ion (Fe³⁺) formed as a by-product of its reduction. This dual functionality facilitates enhanced sludge dewatering and pollutant removal [81]. Furthermore, PF enhances anaerobic sludge fermentation by facilitating the solubilization and hydrolysis of organic matter, which in turn leads to increased production of volatile fatty acids (VFAs) and methane. Additionally, it has been demonstrated to enhance biogas yields by selectively stimulating the microbial activity involved in fermentation [82]. Furthermore, PF is effective in the breakdown of recalcitrant pollutants, including endocrine-disrupting compounds (EDCs) and persistent organic pollutants (POPs), thereby reducing the environmental impact of sewage sludge. Additionally, the iron ions are generated to assist in the immobilization of heavy metals and the mitigation of associated environmental risks [83].

The research conducted by Guo et al. [84] underscores the synergistic impact of integrating PF with thermal hydrolysis on methane generation during anaerobic digestion of WAS. The study demonstrated that the utilization of 0.05 g/g PF in conjunction with thermal hydrolysis at 80 °C for 30 min led to an increase in methane yield, from 170.66 to 232.73 mL/g VSS. This improvement was attributed to enhanced sludge digestion, elevated biodegradability, and an augmented microbial community, which exhibited an increase in active microorganisms from 11.87% to 20.45%. Similarly, Ma et al. [85] investigated the potential of combining PF with low-temperature thermal hydrolysis to improve WAS treatment. The co-pretreatment resulted in an enhancement in sludge degradation, organic matter disintegration, and anaerobic digestion. The oxidative properties of PF, when combined with thermal hydrolysis at 75 °C and a PF dosage of 0.25 g/g TSS (total suspended solids), effectively degraded EPS and cell walls, resulting in increased sCOD and total phosphorus release. This resulted in an increased production of SCFAs, which are essential for methane production. Wang et al. [86] observed that the combination of PF with thermal conditions (55 °C + 0.1 g PF/g TSS) resulted in an increase in the production of SCFAs, reaching 4068.4 mg COD/L, with acetic acid reaching 1766.1 mg COD/L. The application of heat during PF pretreatment has been demonstrated to enhance sludge digestion by facilitating the degradation of EPS and cell walls, which in turn improves the release of organic matter and the biodegradability of the sludge. Furthermore, it elevated the bioactivity of pivotal enzymes and augmented the population of SCFA-producing bacteria while concurrently curbing the proliferation of SCFA-consuming bacteria.

5.7. Sodium Dodecylbenzene Sulfonate

Combining sodium dodecylbenzene sulfonate (SDBS) with thermal pretreatment of WAS improves disintegration, SCFAs production, and dewaterability. SDBS is an anionic surfactant that can disrupt the structure of EPS, which are responsible for the stability and resistance of sludge flocs. In the study by Wan et al. [87], the combined pretreatment of 0.01 g SDBS/g TS of WAS with thermal hydrolysis at 70 °C for 60 min achieved an SCFA yield of 320 mg COD/g VS, significantly higher than the yields from individual treatments. The synergy between SDBS and heat accelerates sludge disintegration and solubilization, promoting faster hydrolysis and acidogenesis. This improvement is supported by increased solubilization of EPS and enhanced microbial community shifts favoring fermentative bacteria like Firmicutes. Moreover, Wu et al. [88] found that SDBS significantly improved hydrogen production during the dark fermentation of WAS by increasing organic solubilization and inhibiting hydrogen-consuming pathways. The addition of 30 mg/g TSS SDBS increased hydrogen yield from 2.47 to 10.73 mL/g VSS. The surfactant disrupted sludge floc structure, facilitating enzyme access to substrates and optimizing the fermentation process towards butyric acid production. Additionally, the study noted that SDBS altered microbial communities by enhancing the abundance of hydrogen producers like Synergistaceae, while simultaneously suppressing methanogens, which are responsible for hydrogen consumption. In a related study by Shi et al. [89], surfactant-assisted thermal hydrolysis with SDS at a dosage of 0.06 g/g VSS achieved a VFA concentration of 2168 mg COD/L. The increase in VFAs was attributed to the effective disruption of EPS and the breakdown of complex organics, leading to improved substrate availability. Additionally, the dewaterability of the sludge was greatly enhanced, with the CST value decreasing from 205 to 50 s/g TSS, further supporting the benefits of surfactant-assisted thermal hydrolysis in WAS treatment. These findings underscore the potential of surfactant-assisted thermal hydrolysis as a highly effective approach for enhancing sludge management and resource recovery, particularly in optimizing VFA production for downstream processes.

6. Effects on Sludge Properties

6.1. Improvement of Sludge Dewaterability

The thermal processing of WAS was originally employed with the objective of enhancing the dewaterability of the sludge and facilitating its handling and ultimate disposal. This necessitates a reduction in the extracellular protein water affinity and the prevention of the formation of stable colloidal aggregates. The transformation of proteins can be achieved by disrupting cellular membranes and other encapsulating structures, thereby releasing intracellular substances [90]. For instance, pilot-scale thermal processing of WAS demonstrated that a 20 min exposure of sludge to a temperature of 65 °C and a pressure of 6 bar resulted in increased removal of VS from 26 to 42%, indicating enhanced dewaterability [91].

The impact of thermal processing on the WAS dewaterability potential is predominantly influenced by a range of parameters, including CST, SRF, the composition of cake dry matter and bound water, and shear sensitivity. An additional indicator of treatment efficiency is the characterization of dissolved organic matter (DOM), which provides insight into the properties of the sludge, including hydrophobicity, settleability, degradability, and dewaterability [92]. The most significant parameter that affects the dewaterability of sludge is the fraction of DOM below 20,000 Dalton, which comprises proteins of low molecular weight.

The processing of WAS to increase its dewaterability considers two mechanisms: disruption of EPS polysaccharides and proteins, which serves to decrease the water retention properties of the EPS and facilitate flocculation, thereby reducing the number of fine flocs [19]. The degradation of EPS to enhance sludge dewaterability was conducted in a combination of thermal processing and the utilization of natural deep eutectic solvents (NADES), which contained betaine and oxalic acid as harmless and biodegradable catalysts for efficient dehydration [93]. In a comparative study, Liu et al. [94] examined the effects of three NADES-based pretreatment methods on WAS: NADES treatment alone, NADES with 80 °C thermal hydrolysis, and NADES with 180 °C hydrothermal treatment. The highest dewatering ability was observed for NADES combined with hydrothermal treatment at 180 °C, indicating that higher temperatures facilitate the degradation of EPS and the release of bound water. Moreover, all three NADES-based pretreatments resulted in an enhancement in the production of VFAs from the sludge. The highest yield of VFAs was obtained with the combination of NADES and hydrothermal treatment at 180 °C, corresponding to an increase of 133% compared to the control. Similarly, Liu et al. [93] demonstrated that NADES-based pretreatments, particularly at 180 °C, significantly enhanced the dewaterability of sludge while also promoting the production of 5-methylfurfural. These changes resulted in improved sludge properties, including a reduction in capillary drawdown time and lower bound water content. Additionally, enhanced resource recovery was observed, with a yield of 114.67 mg/L of 5-methylfurfural. It is noteworthy that 5-methylfurfural is a versatile platform chemical with a wide range of potential applications. Under optimized conditions, a production rate of 11.54 kg of 5-methylfurfural per tonne of volatile solids (VS) was achieved.

6.2. Effect on the Composition of Supernatant

WAS has been found to retain approximately 50% of the carbon within its biomass, which equates to about 250–300 mg C/g TS. It also contains various amounts of nitrogen, phosphorus, sulfur, and potassium. The thermal processing of WAS results in the release of intracellular substances, which presents a challenge in the form of residual ammonia. This is the consequence of protein solubilization during thermal processing, resulting in the release of nitrogen to the soluble phase [22]. Furthermore, the release of sulfur to the aquatic phase can undergo transformation into ferrous sulfide or colloidal sulfur [3].

An increase in processing temperature from 100 to 180 °C during WAS treatment led to a 3-fold increase in the degree of hydrolysis and a corresponding tripling of total nitrogen concentrations, including its ammoniacal form. However, the phosphate concentration remained constant [95]. In similar conditions, nitrogen levels increased by 32%, with ammonium accounting for 5% of this increase [96]. The release of nitrogen from sludge was observed to range from 0.5% to 3.5% when the sludge was exposed to temperatures between 40 and 70 °C for durations of one to three hours [97]. In wastewater treatment plants, where thermal sludge is processed by anaerobic digestion, the resulting supernatant contains highly concentrated ammonium streams (800–1000 mg/L) and an unfavorable C/N ratio. This may contribute 15–20% to the influent nitrogen load if it is to be treated in the main technological line [98].

6.3. Pathogen Reduction

Pathogen reduction is a crucial element of thermal disintegration of WAS, enhancing the safety and quality of the treated sludge. Pathogens, including bacteria, viruses, and parasites, are commonly found in untreated sludge and pose a significant health risk if not adequately treated [99]. Thermal disintegration is an effective method for mitigating the risks associated with pathogenic microorganisms, as it kills these organisms through exposure to high temperatures [100]. At temperatures exceeding 100 °C, thermal energy disrupts the cellular structures of pathogens, leading to their inactivation and death [101]. This process, frequently designated as thermal disinfection, is especially efficacious in that it eradicates even the most resilient microorganisms [102]. High temperatures induce a denaturation of the proteins and nucleic acids within the cells of the pathogens, rendering them nonviable and incapable of further multiplication [103]. This markedly diminishes the microbial load of the sludge and enhances its safety during handling and disposal.

The hydrothermal treatments, particularly those exceeding 160 °C, are highly effective in reducing pathogens [4]. At these temperatures, the combined effects of thermal energy and pressure result in the rapid and thorough inactivation of various pathogens. Studies have shown that sludges at temperatures of 170–190 °C can be almost completely cleared of pathogens for a short duration (typically 30 min to 1 h) [104,105]. The efficacy of high-temperature thermal disintegration makes it a preferred method for facilities seeking to produce Class A sewage sludge, which is characterized by the absence of detectable pathogens and can be used for land application without restrictions.

The capacity to diminish pathogens considerably influences the management of sludge. The production of pathogen-free sludge facilitates the handling, transportation and processing of sludge, thereby reducing the operational challenges and health risks associated with sludge treatment [1]. Those engaged in the handling and processing of sludge are less likely to be exposed to harmful pathogens, thereby enhancing workplace safety [106]. Furthermore, the reduction in pathogens allows for the reuse of treated sludge in agriculture, whereby the resulting soil conditioner promotes plant growth and soil health.

Although thermal disintegration is an effective method, the combination with chemical treatments can result in a further reduction in pathogens. The addition of alkaline agents, such as lime, can elevate the pH of the sludge, thereby creating an unfavorable environment for pathogens [107]. Similarly, the combination of oxidizing agents, such as H₂O₂, with thermal treatment has been demonstrated to enhance the inactivation of microorganisms [108]. Such combinations can be particularly beneficial in optimizing pathogen reduction while simultaneously achieving other treatment objectives, such as enhanced solubility and dewaterability.

6.4. Odor Removal

Thermal disintegration of WAS provides a significant advantage in odor elimination, addressing a particularly challenging aspect of sludge management [109]. The presence of odorous compounds in sludge not only complicates handling and processing but also poses substantial environmental and community concerns, particularly in urban and suburban areas [110]. Thermal disintegration effectively mitigates these challenges by breaking down and removing the volatile organic compounds (VOCs) responsible for malodors [111].

The primary source of odors in sewage sludge is the anaerobic decomposition of organic matter, which results in the formation of malodorous compounds such as hydrogen sulfide, ammonia, volatile fatty acids and mercaptans [112]. During thermal disintegration, the sludge is subjected to high temperatures, which cause the odorous compounds to volatilize and undergo degradation. The thermal process not only results in the destruction of existing VOCs but also prevents the formation of new odorous compounds by stabilizing the organic substances present in the sludge [113].

The efficacy of odor elimination through thermal disintegration is inextricably linked to the temperature at which the process is conducted. Thermal hydrolysis has been demonstrated to significantly reduce odors by both volatilizing and degrading some lighter VOCs [114]. Nevertheless, hydrothermal treatment is considerably more efficacious, as it ensures the complete thermal degradation of a broader range of odorants [22].

Although thermal disintegration is an effective method for odor elimination, its performance can be further enhanced through the integration of chemical treatments. The addition of alkaline agents can neutralize acidic odors, while oxidizing agents, such as H₂O₂, promote the oxidation and removal of odor-causing compounds. These chemical treatments can be tailored to target specific malodorous substances, providing a versatile and adaptive approach to odor management.

6.5. The Effect of Thermal Treatment on Resource Recovery

Thermal treatment plays a key role in improving the efficiency of anaerobic digestion and anaerobic fermentation, both of which are critical for resource recovery from WAS. As the global demand for sustainable waste management solutions increases, the integration of thermal pre-treatment methods with anaerobic digestion and anaerobic fermentation processes has gained attention for its ability to improve the yield and quality of valuable end products such as biogas, VFAs, and nutrients such as phosphorus.

Thermal treatment at temperatures between 60 and 150 °C has been demonstrated to effectively degrade complex organic matter in waste-activated sludge (WAS), thereby enhancing substrate bioavailability for microbial processes during anaerobic digestion and fermentation. The disruption of extracellular polymeric substances (EPS) and the hydrolysis of recalcitrant compounds promote the conversion of organic matter into simpler, more readily digestible forms, including soluble proteins, carbohydrates, and lipids. This enhanced solubilization directly contributes to augmented biogas production during anaerobic digestion [115]. The application of thermal treatment can increase the biogas yield by up to 30%, primarily by enhancing the methane production potential of the sludge [116]. Additionally, thermal treatment reduces the retention time required in anaerobic digesters, by increasing the efficiency and throughput of the process.

In the context of anaerobic fermentation, thermal pretreatment has proven to be an effective strategy for enhancing the production of SCFAs, which are intermediates in various biorefinery applications. The improved solubilization and hydrolysis achieved through thermal treatment led to increased SCFA yields, which can be recovered and utilized as precursors for bio-based chemicals, bioplastics, and bioenergy. For instance, thermal hydrolysis can selectively enhance the production of acetic acid and butyric acid, both of which are valuable for downstream processes [117]. This selective production is driven by the altered microbial community dynamics induced by thermal pretreatment, which favors fermentative pathways over those involved in methane production.

The combination of thermal treatment with anaerobic processes offers additional advantages in terms of resource recovery. The thermal disintegration of sludge facilitates the release of nutrients such as nitrogen and phosphorus, which can be recovered through processes such as struvite precipitation or phosphorus crystallization [13]. The recovered nutrients can then be repurposed as fertilizers, thereby contributing to the attainment of the circular economy goals. Moreover, thermal pretreatment improves the dewaterability of digested sludge, reducing the final sludge volume and lowering disposal costs while simultaneously enhancing the quality of biosolids for land application.

To sum up, the thermal disintegration of WAS significantly enhances sludge treatment by breaking down complex organic molecules into simpler, more biodegradable compounds (Figure 2). This process improves biodegradability, increasing methane production during anaerobic digestion and enhanced dewaterability, making sludge easier to handle and reducing disposal costs. High temperatures kill pathogens and eliminate odors, contributing to safer and more acceptable sludge treatment. Additionally, thermal disintegration promotes the release of soluble organic matter and nutrients, facilitating nutrient recovery and reuse. The process also reduces sludge volume and recalcitrant compounds’ formation while enabling energy recovery. This aligns with environmental sustainability goals by reducing greenhouse gas emissions and improving treatment efficiency.

7. Full-Scale Implementation and Technological Readiness

Thermal disintegration of WAS has reached a high level of technological maturity, and several applications have been successfully implemented worldwide. These technologies help to increase the efficiency of sludge treatment processes and support sustainable waste management practices. The technology readiness level (TRL) of thermal disintegration methods is generally high, indicating their maturity and widespread application in commercial and industrial settings [118]. This was based on the full-scale implementation of the main thermal disintegration technologies and their TRLs, including CambiTHP™, BioThelys^TM, Exelys^TM, Turbotec^TM, Lysotherm^TM, and BioRefinex^©.

CambiTHP™ is one of the world’s most widely used thermal disintegration technologies. CambiTHP™ works at temperatures between 150 and 165 °C and a pressure of 8–9 bar [119]. It breaks down complex organic molecules into simpler compounds to increase biodegradability and improve dewaterability. This technology has already been used in over 70 plants worldwide, including the installation at the Blue Plains Advanced Wastewater Treatment Plant in Washington (USA) [119]. With a TRL of 9, CambiTHP™ is fully mature and operationally reliable, making it a cornerstone of modern sludge treatment practice [120].

Similarly, BioThelys^TM is a well-known thermal hydrolysis technology that operates at around 165 °C and 9 bar pressure [35]. It focuses on breaking down the sludge into more biodegradable components, allowing for improved methane production during anaerobic digestion. BioThelys^TM has been successfully used in several WWTPs, such as WWTP in Saumur (France) [121]. The installations underline the technology’s ability to efficiently process large volumes of sludge. With a TRL of 9, BioThelys^TM is valued for its robustness and ability to increase biogas production. This reflects its widespread application and proven effectiveness in full-scale operations.

Exelys^TM, developed by Veolia (Saint-Maurice Cedex, France) is an advanced thermal hydrolysis process that combines thermal and chemical hydrolysis to maximize sludge dissolution and treatment efficiency [122]. The Exelys^TM process is a continuous version of the BioThelys^TM process, which operates at high temperatures of around 160 °C and a pressure of 6–9 bar. The technology has already been used in the Marquette-lez-Lille WWTP in France [123]. Plants benefit from improved biogas production and reduced sludge production. Exelys^TM, like CambiTHP™ and Biothelys^®, has achieved TRL 9, indicating full-scale implementation and operational readiness, with significant improvements in sludge treatment performance and energy efficiency.

Turbotec^TM is designed to improve the digestibility of sewage sludge by operating at temperatures between 140 and 160 °C. The process involves rapid heating and cooling cycles that promote the degradation of organic matter and improve dewaterability. Turbotec^TM has already been used in various facilities, including the WWTPs in Venlo and Apeldoorn in the Netherlands [124], where it has shown significant improvements in sludge treatment and biogas production. With a TRL of 9, Turbotec^TM is recognized for its efficiency in thermal hydrolysis applications and its successful use on a large scale.

Lysotherm^TM is another thermal hydrolysis process that works at temperatures between 140 and 160 °C and a pressure of 6–8 bar. It improves the solubilization of organic matter in the sludge, improving biodegradability and increasing biogas yields. Lysotherm^TM has already been used in several European WWTPs, including Austria [125]. These plants report improved sludge dewaterability and increased biogas production. Lysotherm^TM, like other leading technologies, has reached TRL 9, indicating that it is ready for large-scale application and has established itself on the market.

BioRefinex^© offers a unique approach that combines thermal hydrolysis with biorefinery principles to maximize the recovery of resources from sewage sludge. BioRefinex^© operates at temperatures of 150–170 °C and focuses on converting sewage sludge into valuable bioproducts, including biogas and organic fertilizers. This technology has already been successfully used in the Lethbridge biogas plant in Canada, where it converts agricultural and municipal waste into biogas and high-quality organic fertilizers [126]. With a TRL of 9, Biorefinex^© proves its maturity and successful integration into large-scale operations and is praised for its innovative approach to resource recovery and sustainability in sludge treatment.

Table 1. Comparison of thermal disintegration technologies.

Technology	Temperature (°C)	Pressure (bar)	Energy Consumption	Performance Outcomes
CambiTHP™	150–165	8–9	High	High biogas yield, improved dewaterability
BioThelys™	165	9	High	Enhanced biodegradability, increased methane production
Exelys™	160	6–9	Moderate	Maximized sludge dissolution, efficient biogas production
Turbotec™	140–160	NA	Moderate	Improved dewaterability, increased biogas yield
Lysotherm™	140–160	6–8	Moderate	Enhanced solubilization, higher biogas yields
Biorefinex©	150–170	NA	Moderate	Resource recovery, production of biogas and fertilizers

NA—not available.

provides a comparative overview of various thermal disintegration technologies used for WAS treatment. Each technology operates at different temperature and pressure ranges, affecting energy consumption and performance outcomes. CambiTHP™ and BioThelys™ are characterized by higher energy consumption but deliver high biogas yields and improved biodegradability. Technologies like Exelys™, Turbotec™, Lysotherm™, and Biorefinex^© operate at moderate energy consumption levels, maximizing sludge dissolution, enhancing solubilization, and efficient resource recovery. These technologies demonstrate varied capabilities in improving dewaterability and increasing biogas production, contributing significantly to sustainable waste management practices.

These technologies are TRL 9, reflecting their operational maturity and widespread use in the industry. By improving the biodegradability of sewage sludge, dewaterability, and biogas production, these technologies contribute significantly to sustainable waste management and environmental protection. The successful use of these systems in numerous WWTPs worldwide underlines their crucial role in the further development of sludge treatment processes.

8. Economic and Environmental Implications

The economic viability and environmental impact of the thermal disintegration of WAS are pivotal considerations in determining the potential for the widespread application of this technology in WWTPs [127]. This technology offers advantages in terms of cost savings, energy efficiency, and environmental impact [49]. However, it also presents certain challenges and drawbacks that must be addressed in order to optimize its performance and feasibility.

The implementation of thermal disintegration requires a substantial initial capital investment. The primary costs are attributed to the construction of high-temperature reactors, pressure vessels, heat exchangers, and auxiliary systems necessary for the thermal hydrolysis process. Additionally, the incorporation of advanced monitoring and control systems is critical for maintaining optimal operational parameters, further increasing the financial burden. Although operating costs are generally lower than the initial capital expenses, they still include significant expenditures related to energy consumption, maintenance, and labor [128]. Although the initial investment may be considerable, the long-term operational savings, including reduced sludge disposal costs and enhanced biogas production, can offset these expenses [129]. The successful deployment of thermal disintegration technologies, such as Cambi-THP™ and Biothelys™, has been observed to result in significant cost savings over time, attributed to enhanced process efficiency and a reduction in the volume of sludge requiring disposal.

Thermal disintegration is an energy-intensive process that requires substantial heat input to achieve the temperatures necessary for effective sludge treatment [130]. The primary purpose of this energy input is to raise the sludge temperature and maintain pressure for the required duration. However, many modern systems are designed with energy-recovery capabilities, significantly improving energy efficiency [131]. For instance, waste heat generated during the process can be captured and reused to preheat incoming sludge or produce steam for other applications [132]. This energy recovery reduces net energy consumption and subsequently lowers operating costs. Technologies such as Exelys^TM and Turbotec^TM have integrated energy-recovery systems that facilitate the optimal utilization of waste heat, thereby enhancing the sustainability and cost-effectiveness of the process.

Thermal disintegration presents a number of environmental advantages, rendering it an appealing option for WWTPs. One of the most significant advantages is the reduction in greenhouse gas emissions [127]. The conversion of organic matter in the sludge into biogas, which can be utilized as a renewable energy source, serves to diminish reliance on fossil fuels. It results in a reduction in carbon dioxide emissions [133]. Furthermore, the process results in a reduction in sludge volume, thereby reducing the necessity for landfill space [134]. The removal of sludge from landfills serves to mitigate the risk of groundwater contamination and curtail methane emissions, which are typically associated with landfill decomposition. Additionally, the enhanced dewaterability of sludge subjected to thermal disintegration results in a higher dry solids content, which reduces transportation costs and further diminishes the carbon footprint of sludge treatment [19].

Despite the numerous advantages offered by thermal disintegration, this process faces challenges that can impact its economic and environmental viability. The implementation of thermal disintegration technologies, including CambiTHP™, BioThelys™, Exelys™, Turbotec™, Lysotherm™, and Biorefinex^©, necessitates a substantial capital investment. These costs encompass the construction of high-temperature reactors, pressure vessels, heat exchangers, and auxiliary systems that are necessary for the process. For example, the installation of the CambiTHP™ system at the Blue Plains Advanced Wastewater Treatment Plant in Washington, D.C., necessitated an investment of tens of millions of dollars [135]. Moreover, these systems necessitate the deployment of sophisticated monitoring and control technologies to ensure optimal performance, thereby further escalating the initial capital cost [3].

Once operational, thermal disintegration systems incur significant operating costs, largely driven by energy consumption. The process requires substantial energy input to heat the sludge to the necessary temperatures (150 to 170 °C) and maintain the required pressure, which constitutes a major portion of ongoing operational expenses. Additionally, maintenance and labor costs further contribute to the financial burden due to the complexity of the equipment and the need for skilled personnel to ensure continuous, efficient operation [3].

Nevertheless, the elevated energy expenses can be alleviated through the implementation of energy recovery systems, a pivotal component of contemporary thermal disintegration technologies. Systems such as Exelys™ and Turbotec™ are equipped with mechanisms that capture waste heat and reuse it within the process, thereby significantly improving energy efficiency and reducing net operating costs [45]. The recovered energy can be employed to preheat the incoming sludge or generate steam, thereby effectively offsetting a portion of the energy required for the process and enhancing the sustainability of the operation [22].

In addition to reducing energy costs, thermal disintegration technologies offer considerable potential for cost savings and revenue generation. By reducing the quantity of sludge that must be discarded, these technologies result in diminished transportation and disposal costs. Some facilities have reported a reduction in these expenses by 50% [36]. Additionally, the enhanced biodegradability of sludge subjected to thermal disintegration facilitates augmented biogas generation during anaerobic digestion. The biogas can be captured and utilized as a renewable energy source, thereby providing a valuable source of revenue and further reducing operating costs. An increase in biogas production of up to 50% has been observed in some plants, which has significantly contributed to the financial viability of these technologies [34].

Despite the significant initial capital investment, the long-term economic benefits of thermal disintegration technologies are substantial. Reductions in sludge disposal costs, decreased energy consumption through energy recovery, and increased biogas production often result in a favorable return on investment (ROI). Many facilities have reported payback periods ranging from five to ten years, depending on the scale of operations and the specific technology employed [136]. Additionally, these technologies align with broader sustainability objectives by reducing greenhouse gas emissions and ensuring compliance with environmental regulations, which can help avoid penalties and enhance the facility’s reputation [45].

Nevertheless, the economic viability of thermal disintegration is not without risks, particularly for smaller WWTPs. The high capital and operating costs can be a significant barrier for smaller plants, making it challenging to justify such an investment without external financial support, such as government grants or innovative financing options [29]. Moreover, the formation of recalcitrant compounds, such as melanoidins, during high-temperature processing can impede subsequent biological processes, including anaerobic digestion. This necessitates additional treatment steps, increasing both complexity and cost [137].

In conclusion, while thermal disintegration technologies involve significant initial and operational costs, their long-term benefits—such as lower sludge disposal expenses, increased biogas production, and improved energy efficiency—make them a financially viable option for large wastewater treatment plants (WWTPs). For smaller facilities, the economic feasibility of adopting thermal disintegration technologies may require careful evaluation of financing options and the implementation of solutions tailored to the plant’s scale. Overall, the adoption of thermal disintegration aligns with sustainable waste management objectives and offers both environmental and economic advantages.

9. Future Directions and Research Needs

The thermal disintegration of WAS represents a promising technology that has already demonstrated significant benefits in the treatment and management of sludge. Nevertheless, further research and technological advancements are required to enhance the efficiency, integration, scalability, and compliance of these processes with environmental and regulatory standards [119]. Future research in this area should concentrate on the improvement of existing techniques, the development of innovative approaches, and the expansion of the adoption of these technologies in order to meet the growing demand for effective WAS management.

A crucial area for future research is the optimization of thermal disintegration efficiency. It is recommended that efforts be directed toward the optimization of operational parameters, including temperature, pressure, and residence time, with the objective of maximizing organic solubilization while minimizing energy consumption [138]. Moreover, the investigation of alternative heating techniques, such as microwave or ultrasonic treatment, may result in the development of more energy-efficient solutions. Further advances in catalyst development, including the creation of novel chemical or biological catalysts, could enhance the degradation of complex organic molecules, thereby improving process efficiency and reducing costs.

Another crucial area for investigation is the integration of thermal disintegration with other waste-treatment technologies. The combination of thermal treatment with anaerobic digestion has the potential to enhance biogas production and optimize the efficacy of the treatment process [139]. Moreover, the combination of thermal disintegration with AOPs [140] or membrane filtration [141] could result in more comprehensive treatment solutions that effectively remove recalcitrant compounds and reduce the environmental impact of sludge disposal. The investigation of hybrid systems that integrate these technologies in a seamless manner may result in the development of more robust and versatile treatment processes capable of addressing a diverse range of waste management challenges.

Addressing environmental and regulatory challenges is crucial for the sustainable adoption of thermal disintegration. Developing advanced monitoring and control systems to detect and mitigate by-products can help facilities comply with stringent environmental regulations [142]. Additionally, further research should explore the potential for utilizing treated sludge in beneficial applications, such as soil amendments or bioenergy production. This approach can convert waste into valuable resources, thereby supporting the principles of a circular economy.

10. Conclusions

Waste-activated sludge management continues to be a critical challenge for wastewater treatment plants due to increasing sludge volumes and more stringent environmental regulations. Traditional disposal methods such as landfilling and incineration are becoming less viable, necessitating advanced treatment technologies. Among these, thermal pretreatment techniques such as thermal hydrolysis and hydrothermal treatment have shown promising results in increasing sludge biodegradability, improving dewaterability, and facilitating resource recovery.

Thermal hydrolysis effectively breaks down complex organic compounds, making them more accessible for subsequent anaerobic digestion, which can increase biogas production and reduce sludge volumes. Hydrothermal treatment at higher temperatures provides superior solid–liquid separation and pathogen reduction. When combined with chemical methods, these processes can further optimize sludge management by improving solubilization and treatment efficiency.

Despite concerns about energy costs and by-product formation, advances in energy recovery and process optimization have made thermal pretreatment more sustainable and cost-effective. Optimizing parameters such as temperature, pressure, and residence time can reduce energy consumption, while integrating anaerobic digestion increases biogas production, helping to offset costs. Scalable systems also make thermal pretreatment more accessible and economical for facilities of all sizes, supporting regulatory and sustainability goals. Therefore, future research should prioritize optimization of process parameters, integration of hybrid systems, and overcoming challenges related to energy efficiency and scalability. As the industry moves towards circular economy principles, thermal pre-treatment methods will play an increasingly important role in achieving sustainable sludge management.