Pretreatment Of Sewage Sludge Research Articles

Evaluation of the Feasibility and Utilizability of Pretreated Sewage Sludge in Cement Kiln Co-Processing

The treatment and resource utilization of sludge from municipal sewage treatment plants is an important environmental issue. Cement kiln co-processing offers a promising solution, but challenges remain, particularly regarding sludge properties and feasibility in kiln systems. This study analyzes the characteristics of three pretreated sludges: mechanically dewatered sludge, deeply dewatered sludge, and lime-dried sludge. Using techniques such as thermogravimetric analysis (TGA) and X-ray diffraction (XRD), this study investigates their calorific values and raw material utilizability in co-processing. As the sludge moisture content decreases from interstitial to bound water, energy consumption per ton of evaporated water rises, particularly below 30%. At 10 °C/min heating, energy consumption for mechanically dewatered sludge at 80%, 30%, and 10% moisture was 3573, 8220, and 34,751 kJ/kg, respectively; for deeply dewatered sludge at 60%, 30%, and 10%, the values were 4398, 7550, and 11,504 kJ/kg. Keeping moisture content above 30% before kiln entry reduces energy use and enhances calorific value. Sludge utilizability as a raw material depends on its pretreatment. The ash composition of deeply and mechanically dewatered sludge resembles iron-rich raw materials, while lime-dried sludge aligns more with limestone. The utilizable ash content was 23.3%, 8.1%, and 46.3%, respectively, with lime-dried sludge showing the highest potential. This study provides insights into sludge properties and their co-processing potential in cement kilns, offering scientific and technical support for practical applications.

Application of machine learning in ultrasonic pretreatment of sewage sludge: Prediction and optimization

In this research, typical industrial scenarios were analyzed optimized by machine learning algorithms, which fills the gap of massive data and industrial requirements in ultrasonic sludge treatment. Principal component analysis showed that the ultrasonic density and ultrasonic time were positively correlated with soluble chemical oxygen demand (SCOD), total nitrogen (TN), and total phosphorus (TP). Within five machine learning models, the best model for SCOD prediction was XG-boost (R2 = 0.855), while RF was the best for TN and TP (R2 = 0.974 and 0.957, respectively). In addition, SHAP indicated that the importance feature for SCOD, TN, and TP was ultrasonic time, and sludge concentration, respectively. Finally, the typical industrial scenario of ultrasonic pretreatment of sludge was analyzed. In the secondary sludge, treatment volume at 0.6 L, the pH at 7.0, and the ultrasonic time at 20 min was best to improve the SCOD. In the ultrasonic pretreatment primary sludge, treatment volume of 0.3 L, pH of 7.0, and ultrasonic time of 15 min was best to improve the SCOD. Furthermore, the ultrasonic power at 700 W and ultrasonic time at 20 min were best to improve the C/N and C/P in the secondary sludge. In the primary sludge, the ultrasonic power at 600 W, and the ultrasonic time at 15 min were best to improve C/N and C/P. This study lays a foundation for the practical application of ultrasonic pretreatment of sludge and provides basic information for typical industrial scenarios.

Synergistic effect of biological pre-treatment on co digestion of rice straw and sewage sludge: Process optimization and microbial interactions

The increasing demand for renewable energy sources has prompted a search for innovative and efficient approaches for biogas production. This study investigates the potential of achieving sustainable biogas production through a synergistic combination of biological pre-treatment and co-digestion of rice straw (RS) and sewage sludge (SS). The goal is to enhance the overall methane yield, mitigate substrate limitations, and optimize the biogas production process. In this study, rice straw and sewage sludge were anaerobically co-digested in ratios of 100:0, 70:30, 50:50, 30:70, and 30:70. It was observed that the co-digestion ratio of 70:30 is optimal to achieve maximum methane yield of 0.3 L CH4/(g VS added). Biological pre-treatment with five different organisms Sphingobium sp., Paenibacillus sp., Microbacterium sp., Pseudomonas sp., and Stenotrophomonas sp. was also examined for RS and co-digested with SS at the optimized ratio to improve the degradability of RS. This pre-treatment strategy is anticipated to enhance the accessibility of microorganisms to substrates and accelerate the rates of hydrolysis. The biological pre-treatment resulted in a 20–23% improvement in methane yield compared to untreated substrates. Metagenomic studies revealed the dominance of Bacteroidetes, Firmicutes, and Proteobacteria, which are capable of utilizing lignocellulosic biomass and ultimately converting it to methane. The acetoclastic methanogensis is the major methane generating pathway which is supported by the complete dominance of genus Methanosaeta.

Exploring nanobubble technology for enhanced anaerobic digestion of thermal-hydrolysis pre-treated sewage sludge

Nanobubble technology was used to enhance anaerobic digestion (AD) of thermal-hydrolysis pre-treated sewage sludge for bioenergy recovery. The prepared air, CO2, and H2 nanobubble solutions, with concentrations of 9.88–10.2 × 107 bubbles/mL, remained stable for at least 7 days. After adding them into AD reactors, significantly higher CH4 production (37.1 %) was observed for the CO2 nanobubble treatment, followed by air (25.6 %) and H2 (14.5 %) nanobubble treatments, compared to the control group. CO2 nanobubble treatment performed the best in improving acidogenesis/acetogenesis, resulting in significantly higher volatile fatty acid generation during the initial 3–4 days. A comparison of reactors supersaturated and non-saturated with oxygen has demonstrated most of the biogas uplift observed to result from the nanobubbles rather than from initial oxygen soluble levels, demonstrating the crucial role of nanobubbles in upgrading AD. This study demonstrates, for the first time, that nanobubbles can provide additional benefits when combined with stablished sludge pre-treatment technologies.

Comprehensive hydrothermal pretreatment of municipal sewage sludge: A systematic approach

This study provides a comprehensive analysis of the potential impact of hydrothermal pretreatment (HTP) on municipal thickened waste-activated sludge (TWAS) and its integration with anaerobic digestion (AD). The research demonstrates that HTP conditions (170 °C, 3 bars for 30 min) can increase the solubilization of macromolecular organic compounds by 41%, which enhances biodegradability in semicontinuous bioreactors. This treatment also results in a 50% reduction in chemical oxygen demand (COD) and a 63% increase in the destruction of volatile solids (VS). The combination of HTP with AD significantly boosts methane yields by 51%, reaching 176 ml/g COD, and improves the digestate dewaterability, doubling the solid content in the dewatered cake. However, a higher polymer dose is required compared to conventional AD. Microbial community analysis correlates the observed performance and alterations; it indicates that HTP enhances resilience to stress conditions such as ammonia toxicity. This comprehensive study provides valuable insights into the transition from wastewater treatment plants (WWTPs) to resource recovery facilities (RRF) in line with circular economy principles.

Evaluation of the optimal sewage sludge pre-treatment technology through continuous reactor operation: Process performance and microbial community insights

This study investigated the impact of two low-temperature thermal pre-treatments on continuous anaerobic reactors’ performance, sequentially fed with sludge of different total solids content (∼3 % and ∼6 %) and subjected to progressively increasing Organic Loading Rates (OLR) from 1.0 to 2.5 g volatile solids/(LReactor⋅day). Assessing pre-treatments’ influence on influent sludge characteristics revealed enhanced organic matter hydrolysis, facilitating sludge solubilization and methanogenesis; volatile fatty acids concentration also increased, particularly in pre-treated sludge of ∼6 % total solids, indicating improved heating efficiency under increased solids content. The reactor fed with sludge pre-treated at 45 °C for 48 h and 55 °C for an extra 48 h exhibited the highest methane yield under all applied OLRs, peaking at 240 ± 3.0 mL/g volatile solids at the OLR of 2.5 g volatile solids/(LReactor⋅day). 16S rRNA gene sequencing demonstrated differences in the reactors’ microbiomes as evidence of sludge thickening and the different pre-treatments applied, which promoted the release of organic matter in diverse concentrations and compositions. Finally, the microbial analysis revealed that specific foam-related genera increased in abundance in the foam layer of reactors’ effluent bottles, dictating their association with the sludge foaming incidents that occurred inside the reactors during their operation at 2.0 g volatile solids/(LReactor⋅day).

Efficient removal of heavy metals from sewage sludge using a low-cost protic ionic liquid: Investigation of mechanism and ecological risk

Land utilization for the disposal of sewage sludge (SS) is an effective strategy, yet the existence of heavy metals (HMs) in the SS represents critical constraints to its implementation on a larger scale. By the principles of simple synthesis steps and low synthesis cost, a low-cost double cationic protic ionic liquid was produced in this study for HMs removal from SS. The influence of reaction time, protic ionic liquid to SS mass ratio, and reaction temperature on HM removal were thoroughly investigated. This result indicated that the integration of protic ionic liquid and heat treatment was efficacious in the efficient elimination of HMs. Under 60°C, 15 min, and protic ionic liquid: SS of 0.5, the removal efficiency of Zn, Fe, Ni, and Cd exceeded 80%, and Cr achieved a removal efficiency of up to 93%. The BCR sequential extraction procedure and calculation of the potential ecological risk index revealed a significant reduction in the ecological risk associated with the treated SS, transitioning from a high to a low-risk level. The alterations in elemental and chemical bonding within the solid phase of SS and changes in polysaccharide and fluorescence properties within the liquid phase of SS were investigated using redundancy analysis, two-dimensional correlation analysis, and parallel factor analysis. These findings suggest that dehydration, deamination, and decarboxylation reactions may be responsible for removing HMs within SS. These findings have a positive impact on further elucidating the effects of protic ionic liquid in SS pretreatment, thereby facilitating the efficient utilization of municipal SS resources.

Methane Production and Nutrient Recovery After Applying Microwave Technology in Sewage Sludge Pretreatment

Methane Production and Nutrient Recovery After Applying Microwave Technology in Sewage Sludge Pretreatment

Synergistic application of alum sludge and sequential extraction for phosphorus recovery from sewage sludge char

Phosphorus (P) recovery from sewage sludge (SS) (or its ash/char) is challenging due to the presence of heavy metals (HMs) in SS. This study proposed the integration of SS, alum sludge (AS), sequential wet extraction, and pyrolysis for P recovery without simultaneous HM extraction. SS + AS mixture (P:Al molar ratio = 3:1) was first acid pretreated to obtain amended SS, then pyrolyzed, and finally, P was extracted from the char using an alkali. Pyrolysis was conducted at low (400 °C), medium (600 °C), and high (900 °C) temperatures. The highest alkaline P recovery efficiency was 88 % from the amended SS char, while 32 % from the unamended SS char (both prepared at 400 °C). Overall, the highest P recovery in the amended process was 48 % higher than the unamended process despite 11 % P loss during acidic pretreatment. Standards measurements and testing (SMT) protocol and solid-state NMR (31P and 27Al) analyses revealed that acidic pretreatment of SS with AS induced apatite phosphate (AP) to non-apatite inorganic phosphate (NAIP) conversion in SS. As NAIP is more soluble in alkali than AP and HMs, higher alkaline P recovery was achieved for amended SS char than unamended SS char, without HM contamination. However, higher pyrolytic temperatures impaired P recovery due to the immobilization of P in the char carbon matrix and the formation of recalcitrant P phases. These findings were corroborated by complementary characterization techniques such as XPS, XRD, and FTIR. Some major HMs in SS and AS were concentrated in char, except Zn, which partially volatilized above 600 °C. This study demonstrated the applicability of AS for extracting P from SS, offering a potential pathway for the synergistic management of sludge waste streams.

Maximizing Bio-Hydrogen and Energy Yields Obtained in a Self-Fermented Anaerobic Bioreactor by Screening of Different Sewage Sludge Pretreatment Methods

Egypt faces significant challenges in managing its sewage sludge generated in large quantities from wastewater treatment plants. This study investigates the feasibility of utilizing sewage sludge as a renewable resource for hydrogen production through anaerobic digestion at the 100 L bioreactor level. Hydrogen is considered a promising alternative energy source due to its high energy content and environmental benefits. To optimize the microbial degradation process and maximize hydrogen production from sewage sludge, a specialized pretreatment is necessary. Various pretreatment methods have been applied to the sewage sludge, individually and in combination, to study the bio-hydrogen production from sewage sludge. The four methods of treatment were studied in batch assays as a pilot scale. Thermal pretreatment of sewage sludge significantly increases bio-hydrogen production yield compared to other sewage sludge pretreatment methods, producing the highest H2 yield (6.48 LH2/g VS). In general, the hydrogen yield of any type of pretreated inoculum was significantly higher than the untreated inoculum. At the same time, alkaline pretreatment improved the hydrogen yield (1.04 LH2/g VS) more than acid pretreatment (0.74 LH2/g VS), while the hydrogen yield for the combination of pretreatments (shock alkali pretreatment) was higher than both (1.73 LH2/g VS), On the other hand, untreated sewage sludge (control) had almost no hydrogen yield (0.03 LH2/g VS). The self-fermented anaerobic bioreactor improved sewage sludge utilization, increased bioenergy yields, and seems to be promising for treating complex wastes at this scale.

Co-hydrothermal carbonization of pretreated sludge and polyethylene terephthalate for the preparation of low-nitrogen clean solid fuels.

In this work, polyethylene terephthalate (PET) and sewage sludge (SS) were co-hydrothermally carbonized to produce low-nitrogen solid fuels. To minimize the effect of nitrogen, this work introduces a co-hydrothermal carbonization method involving alkali (A), ultrasonic cell disruptor (UCC), and sodium dodecyl sulfate (SDS) for both individual and combined pretreatment of SS and PET. Comparative analysis of the products shows that the combined pretreatment with sodium dodecyl sulfate (SDS) and alkali (A) effectively disrupts the SS cell structure, leading to the loosening of stable extracellular polymeric substances (EPS). This condition is conducive to the release and hydrolysis of proteins during hydrothermal carbonization. Moreover, under conditions where PET serves both as an acid producer and a carbon source, and through parameter optimization at a temperature of 240 °C, reaction time of 2 h, PET addition of 20 wt%, and water addition of 0.6 g cm-3, a high-quality, low-nitrogen clean solid fuel was produced (N: 0.51 wt%, C: 19.10 wt%).

Recent Developments in the Application of Ultrasonication in Pre-Treatment of Municipal Sewage Sludge

Recent Developments in the Application of Ultrasonication in Pre-Treatment of Municipal Sewage Sludge

Conversion of Materials and Energy in Anaerobic Digestion of Sewage Sludge with High-Pressure Homogenization Pretreatment

High pressure homogenization (HPH) pretreatment can improve sludge anaerobic digestion; however, the relationship among the material, energy conversion, and gas production efficiency was unclear under different operating conditions in sludge anaerobic digestion by HPH pretreatment. In this study, the performance of HPH pretreatment before sludge anaerobic digestion was investigated, and the relationship among the material, energy conversion, and gas production efficiency was explored. HPH pretreatment induced organic solubilization, and a maximum soluble chemical oxygen demand (SCOD)/total chemical oxygen demand (TCOD) of about 30% was achieved. Results showed that HPH pretreatment significantly improved the biogas production of sludge anaerobic digestion; the maximum increase in CH4 yield was 57%; and the anaerobic digestion period was shortened by about 10 days. The ratio of CH4 yield increment to volatile dissolved solids (VDS) increment was 0.21 mL/mg. The CH4 yield increment of 1 L/g volatile solid (VS) required a specific energy of 0.10 MJ/kg total solid (TS) by increasing the pressure with one cycle and 0.72 MJ/kg TS by increasing the cycle at 60 MPa. The minimum additive energy consumption of HPH pretreatment was 125 J/mL CH4 yield increment at 20 MPa with one cycle. Considering CH4 yield improvement and energy conservation, HPH pretreatment should maintain a pressure of no more than 60 MPa in one cycle. This study provides a theoretical reference for the practical application of HPH pretreatment in anaerobic digestion. HPH holds promise as a potential strategy for sewage sludge pretreatment to produce CH4 in anaerobic digestion.

Sewage sludge pretreatment: current status and future prospects.

Sewage sludge is regarded by wastewater treatment plants as problematic, from a financial and managerial point of view. Thus, a variety of disposal routes are used, but the most popular is methane fermentation. The proportion of macromolecular compounds in sewage sludges varies, and substrates treated in methane fermentation provide different amounts of biogas with various quality and quantity. Depending on the equipment and financial capabilities for methane fermentation, different methods of sewage sludge pretreatment are available. This review presents the challenges associated with the recalcitrant structure of sewage sludge and the presence of process inhibitors. We also examined the diverse methods of sewage sludge pretreatment that increase methane yield. Moreover, in the field of biological sewage sludge treatment, three future study propositions are proposed: improved pretreatment of sewage sludge using biological methods, assess the changes in microbial consortia caused with pretreatment methods, and verification of microbial impact on biomass degradation.

Fe/Mn modified biochar as electrode particles in electrochemical system for efficient anaerobic sludge digestion

Electrochemical pretreatment (EPT) of sewage sludge demonstrates an aggressive effect on extracellular polymeric substance destruction and sludge solubilization. While the low efficiency of methane production does not match the initial dissolution of the sludge after EPT. Here we proposed a novel method, using Fe-Mn-modified biochar (FMBC) strengthening electrochemical pretreatment, aiming at improving the methanogenic efficiency. The strengthening mechanism of FMBC was studied according to the physicochemical parameters of sludge at different concentrations and the change of microbial community. Experimentally, with the addition of 2 g/L FMBC, the methane production increased by 7.96% and 10.06% compared with electrochemical pretreatment alone and the control group. The sludge organic matter solubilization varied from 344.6 mg/L in E0 to 383.4 mg/L in E2 after pretreatment. Simultaneously, analysis of microbial communities showed that the addition of 2 g/L FMBC obviously increased the relative abundance of Methanofastidiosum and Methanosaeta from 5.5% and 20.9% in CK to 16% and 28.7% in E2, respectively. The addition of FMBC also intensified the acetoclastic methanogenesis and methylotrophic methanogenesis. This study provides new ideas for the enhancement of electrochemical pretreatment of sludge.

Oxygen limitation in aerobic polyhydroxyalkanoates production from sewage sludge anaerobic fermentation liquids under low and medium organic loading rate

The present study describes the microbial production of polyhydroxyalkanoates (PHA) from thermally pre-treated sewage sludge at pilot scale level, investigating for the first time the effect of the organic loading rate (OLR) under oxygen limitation on biomass storage properties and kinetics. Polymer characteristics have been also evaluated. The selection/enrichment of PHA-storing biomass was successfully achieved in a Sequencing Batch Reactor (SBR) under short hydraulic retention time (HRT; 2 days). Low OLR (2.05 g COD/L d) was ideal for the selection of an efficient PHA-producing consortium cultivated under limited oxygen availability. In the fed-batch accumulation conducted under high DO regime, such biomass was characterized by 51% of PHA content on cell dry weight, with a related storage yield (YP/Sbatch) of 0.61 CODPHA/CODS. On the contrary, medium OLR (4.56 g COD/L d) was not technically feasible to sustain the required consortium's selection under low DO regime. The PHA produced by biomass cultivated under low DO regime was characterized higher thermal stability and crystalline domain compared to PHA traditionally produced under high DO regime. The mass balance assessment highlighted a global yield of 51 g PHA/kg VS (volatile solids of thickened sludge), which was 9% lower than yield obtained under high DO regime, in the face of a realistic reduction of the energy cost of the process.

Co-production of biohydrogen and biomethane utilizing halophytic biomass Atriplexcrassifolia by two-stage anaerobic fermentation process.

The excessive use of fossil has resulted in the drastic exhaustion of natural energy sources, leading to environmental challenges and energy crises. Owing to rising energy demand there is a dire need to shift towards renewable energies from lignocellulosic biomass. The present study assessed the co-production of biohydrogen (H2) and biomethane (CH4) by utilizing a less explored halophyte Atriplexcrassifolia. Various reaction parameters were evaluated for their effect on biohydrogen and biomethane production in batch experiments. One parameter at a time experimental strategy was chosen for production optimization. Hydrogen and methane yields along with their production rates were assessed at different incubation times, temperatures, pH, substrate concentrations, and inoculum sizes in acidogenesis and methanogenesis stages, respectively. In the first stage, maximum cumulative hydrogen production of 66 ± 0.02mL, with hydrogen yield of 13.2 ± 0.03mL/g, and hydrogen production rate (HPR) of 1.37 ± 0.05mL/h was attained when the reaction mixture (5g Atriplexcrassifolia and 10mL pretreated sewage sludge) was processed at 37°C and pH 5.5 after 48h of incubation. While in the second stage, maximum cumulative methane production, i.e., 343 ± 0.12mL, methane yield (MY) of 8.5 ± 0.07mL/mL, and methane production rate (MPR) of 0.8 ± 0.05mL/h was achieved after 18days of incubation of reaction mixture (40mL of hydrogenic slurry with 80mL inoculum) at 45°C and pH 8. Furthermore, a 51% and 24% rise in biohydrogen and biomethane production respectively were recorded when the gases were produced at these optimized reaction conditions. The results ensure halophyte Atriplexcrassifolia as an imperative renewable energy resource and proposed that effective optimization of the process further increased the coproduction of biohydrogen and biomethane.

Influence of sodium hypochlorite/ultrasonic pretreatment on sewage sludge and subsequent hydrothermal liquefaction: Study on reaction mechanism and properties of bio-oil

Subcritical hydrothermal liquefaction (HTL) is an efficient approach for bio-oil production from wet sewage sludge (SS). Before HTL, pretreatment of wet SS by sodium hypochlorite (NaClO) or combination of NaClO and ultrasonic treatment was implemented with different NaClO dosage or ultrasonic temperature for the first time. The results demonstrated that TOC and content of protein in EPS of SS increased after two pretreatment methods. Meanwhile, the bio-oil yield was improved and the maximum of 39.4 wt% was obtained from the bio-oil under combined pretreatment (9.39 mg NaClO/g dried SS and ultrasounds at 80 °C) when HTL was performed at 300 °C for 15 min. Additionally, the maximum energy recovery reached 58.0% with the highest carbon recovery (61.9%) and the highest hydrogen recovery (39.0%) in bio-oils obtained from HTL of combined pretreated SS. GC-MS and NMR analysis showed that the chemical compositions of bio-oil were not affected significantly by pretreatment methods but relative percentage of each compound was changed. The esters formation was favored after combined pretreatment and increased to 16.3%, while the contents of acids and N-containing compounds decreased by 8.2% and 4.0%, respectively. TGA of bio-oils revealed that both pretreatments could promote the formation of light oil components such as diesel. Finally, the main possible reaction pathways for HTL of SS have been elucidated. Overall, this research provided a theoretical foundation for utilization of wet SS from sewage treatment plant directly for bioenergy production.

Development of a Primary Sewage Sludge Pretreatment Strategy Using a Combined Alkaline–Ultrasound Pretreatment for Enhancing Microbial Electrolysis Cell Performance

Ultrasound and combined alkaline–ultrasound pretreatment (AUP) strategies were examined for primary sewage sludge (SS) disintegration and were utilized to evaluate the degree of solubilization (DS). Further, the pretreated primary SS was operated in microbial electrolysis cells (MECs) to maximize methane production and thereby improve the reactor performance. The highest DS of 67.2% of primary SS was recorded with the AUP. MEC reactors operated with the AUP showed the highest methane production (240 ± 6.4 mL g VSin−1). VS (61.1%) and COD (72.2%) removal in the MEC ALK-US showed the best organic matter removal efficiency. In the modified Gompertz analysis, the substrate with the highest degree of solubilization (AUP) had the shortest lag phase (0.2 ± 0.1 d). This implies that forced hydrolysis via pretreatment could enhance biodegradability, thereby making it easy for microorganisms to consume and leading to improved MEC performances. Microbial analysis implicitly demonstrated that pretreatment expedited the growth of hydrolytic bacteria (Bacteroidetes and Firmicutes), and a syntrophic interaction with electroactive microorganisms (Smithella) and hydrogenotrophic methanogens (Methanoculleus) was enriched in the MECs with AUP sludge. This suggests that the AUP strategy could be useful to enhance anaerobic digestion performance and provide a new perspective on treating primary SS in an economical way.

Thermo-kinetic analysis of pyrolysis of thermally pre-treated sewage sludge from the food industry

Due to the specific characteristics of sewage sludge from the food industry, including its high fat content, its treatment is quite complex. Therefore, in this study, the effect of the pre-treatment processes torrefaction (T) and hydrothermal carbonization (HTC) on the pyrolysis of industrial sewage sludge (SS) from the vegetable oil industry was investigated by thermogravimetric analysis. Kinetic and thermodynamic analysis was performed using the Kissinger-Akahira-Sunose (KAS), Flynn-Wall-Ozawa (FWO), and Friedman (FRI) iso-conversional kinetic models. In addition, the influence of water replacement by whey in the hydrothermal carbonization process was investigated on the subsequent pyrolysis kinetics.The activation energy (Eα) values for pyrolysis of industrial sewage sludge ranged from 49 to 372 kJ/mol. Pre-treatment (torrefaction, hydrothermal carbonization) of sewage sludge increases the activation energy significantly: the Eα values for torrefied (T-SS) and hydrothermally treated (HTC-SS) samples ranged from 177 to 689 kJ/mol and from 161 to 486 kJ/mol, respectively. The variations in activation energy and the generally lower activation energies for the HTC-SSW sample (158–445 kJ/mol) indicate that the use of whey in the HTC process affects the hydrochar properties and subsequent pyrolysis kinetics significantly. According to the results, the pre-treatment of the samples is reflected in better thermochemical properties and stability of the treated samples, as well as in the thermodynamic parameters of pyrolysis, since the pre-treated samples (especially the torrefied sample, T-SS) exhibited higher entropies and enthalpies and lower Gibbs free energies.