Hydrothermal Pretreatment of Food Waste Enhances Performance of Anaerobic Co-digestion With Sludge

Hydrothermal Pretreatment of Food Waste Enhances Performance of Anaerobic Co-digestion With Sludge | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Hydrothermal Pretreatment of Food Waste Enhances Performance of Anaerobic Co-digestion With Sludge Davidraj Johnravindar, Jun Zhao, Mathikere Krishnegowda Manu, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5087495/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 05 Feb, 2025 Read the published version in Environmental Science and Pollution Research → Version 1 posted 5 You are reading this latest preprint version Abstract Food waste (FW) presents a significant opportunity for renewable energy production through anaerobic digestion (AD) when subjected to appropriate treatment. This study investigates the impact of thermal hydrolysis pretreatment (THP) on FW at varying temperature levels (90°C, 120°C, and 140°C) prior to mesophilic anaerobic co-digestion with sewage sludge (SS). Results demonstrate enhanced FW hydrolysis at 120°C, leading to a cumulative methane yield of 324.39 ± 4.5 mL/gVS add , representing a 41.75% increase over untreated FW (228.83 ± 1.13 mL/gVS add ). Shifts in microbial communities, particularly Methanosarcina , Methanobactrium , and Methanobrevibacter , support efficient methanogenesis. Co-digestion of FW pretreated at 120°C yields maximum energy production of 11.48 MJ/t, a 49.47% improvement compared to untreated processes. The economic analysis underscores the profitability of co-digestion with FW pretreated at 120°C. These findings highlight the potential for enhanced methane production and energy conversion efficiency with hydrothermally pretreated FW and SS co-digestion. Food waste Hydrothermal pretreatment Co-digestion Energy conversion efficiency Economic analysis Sewage sludge Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1. Introduction Hydrothermal pretreatment of food waste (FW) has emerged as a promising strategy to enhance the performance of anaerobic co-digestion with sludge. The escalating global population and economic activities have led to a surge in municipal solid waste (MSW) generation, posing significant environmental challenges (Chuen Chen et al. 2020 ). In densely populated cities like Hong Kong, the per capita production of FW stands at 0.30 kg/day, while sewage sludge (SS) rates exceed 0.16 kg/day (HKEPD 2019 ). Approximately 30.0% of the 11,057 tons/day of MSW generated in Hong Kong comprises FW, with SS production reaching around 1,052 tons/day (EPD 2021). Given FW's predominant presence in Hong Kong's MSW composition, the government has proposed leveraging the surplus AD capacity at existing sewage treatment plants for FW/SS co-digestion, offering superior benefits compared to mono-digestion (Mehariya et al. 2018 ). Thus, there is a pressing need to enhance the handling of FW and SS through efficient anaerobic co-digestion practices to mitigate adverse environmental and societal impacts. Anaerobic digestion is recognized as a viable method for simultaneous waste treatment and energy recovery through methane-rich biogas production (Johnravindar et al. 2022 ). FW, characterized by its high moisture content and degradability, is an ideal substrate for AD. However, the standalone application of AD for FW and SS faces challenges such as high organic loads, rapid acidification, prolonged solid retention, and the presence of inhibitory substances. Consequently, sludge from wastewater treatment plants contains significant levels of heavy metals, pathogens, and bacteria (Kaur et al. 2020 ). AD involves a series of biological processes that convert complex substrates into biogas through microbial action in the absence of oxygen, encompassing hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Hydrolysis is often the rate-limiting step for complex organic substrates due to the formation of toxic byproducts or undesirable volatile fatty acids (VFAs), while methanogenesis poses limitations for easily biodegradable substrates (Kaur et al. 2020 ). In this context, the anaerobic co-digestion of FW and SS has been shown to enhance digestion efficiency and optimize methane production (Parthiba Karthikeyan et al. 2018 ). Leveraging the alkalinity and enhanced buffering capacity of SS, coupled with the underutilized capacity of sludge digesters at wastewater treatment plants, achieves reduced waste discharge and improved resource utilization. Studies by Chakraborty et al. ( 2018 ) and Kaur et al. ( 2019 ) have demonstrated elevated concentrations of intermediate metabolites, particularly VFAs like acetic acid, butyric acid, and propionic acid, during FW/SS co-digestion with biochar addition. Acetic acid, followed by butyric acid, serves as favorable substrates for methanogenesis, while propionic acid can inhibit methane production. However, successful FW/SS co-digestion schemes have been identified for diluted FW systems with low FW loading due to reduced process performance (Wong et al. 2018 ). FW contains complex organic compounds, including animal fats, vegetable oils, and high molecular weight carbohydrates, making hydrolysis a crucial step in AD. Various pretreatment techniques have been explored to accelerate hydrolysis, produce valuable byproducts, and enhance the quality of recyclable components, such as nitrogen and phosphorus. Among these techniques, thermal hydrolysis pretreatment (THP) stands out as an effective and environmentally friendly method that does not rely on chemical additives. THP involves subjecting FW to high pressures and temperatures, leading to the hydrolysis of macromolecular materials and organic dissolution (Yin et al. 2014 ; Gahlot et al. 2022 ; Amr et al. 2022 ). This process disrupts cell membranes, promotes the dissolution of recalcitrant organic compounds, and enhances macromolecule hydrolysis under specific time and temperature conditions. Studies such as Jin et al. ( 2016 ) have shown positive effects on chemical oxygen demand (COD), lignin, cellulose, volatile solids (VS), fibers, proteins, and methane production following THP at 160°C for 20 minutes. Similarly, Ding et al. ( 2017 ) reported enhanced hydrogen and methane co-production through FW hydrothermal pretreatment at 140°C for 20 minutes. Notably, the literature review highlights the diversity of pretreatment techniques such as ultrasonic, chemical, or their combination with waste-activated sludge (WAS) and other wastes that pose challenges in comparing their efficiencies (Gao et al. 2021 ; Khanh Nguyen et al. 2021 ). While THP is commonly employed for extracting monosaccharides from agricultural waste and SS during biomass utilization, its application in pretreating FW for co-digestion to produce VFAs and methane remains underexplored. Digestate sludge, when used as a substrate, offers an opportunity to generate a carbon source without compromising biogas production. This approach also significantly reduces sludge handling volumes, leading to substantial cost savings and addressing inefficiencies in traditional THP + AD systems, particularly when wastewater sludge has a low solids content (Carranza Munoz et al., 2024). The composition of FW, varying by location and season, presents challenges due to the presence of recalcitrant components that can fluctuate significantly. Pretreatment of FW can modify its composition, potentially enhancing VFA production. Additionally, pretreatment improves AD and boosts electricity generation in fuel cells by providing simpler nutrients for microorganisms compared to untreated sludge, promoting microbial growth and resistance to inhibitors. Nonetheless, the extended detention times and limited reduction in volatile solids (VS) pose challenges in utilizing AD for biological solids (Aierzhati et al. 2019 ). THP offers a range of benefits, including improved anaerobic degradation kinetics, enhanced biodegradability, VS destruction, methane production, solubilization of macromolecular components (carbohydrates, proteins, and lipids) in FW and sludge, improved dewaterability, and higher organic loading rates in digesters (Liu et al. 2020 ). This study aims to explore the impact of thermal hydrolysis pretreatment on the characteristics of FW and its subsequent co-digestion for methane production. Additionally, the research seeks to investigate the effects of THP applied to FW on the anaerobic co-digestion process with sludge. Specifically, the study will assess how pretreatment temperature influences the characteristics of organic matter and evaluate the efficiency of biogas production based on methane potential yield. The anticipated outcomes of this research should offer new insights into the mechanisms involved and the changes in microbial communities during anaerobic co-digestion of hydrothermally pretreated FW and sludge. 2. Materials and methods 2.1. Substrates and inoculum FW was artificially prepared (using 35% bread, 25% cabbage, 25% boiled rice, and 15% boiled pork) to mimic the composition of FW with regard to the Hong Kong diet, as in our previous studies (Johnravindar et al. 2022 ). The FW was subjected to size reduction by manual chopping followed by blending to achieve a homogenous mixture suitable for co-digestion experiments. The inoculum consisted of anaerobic digestate (non-saline) sludge, while primary and secondary (biologically activated) sludge from the Shek Wu Hui wastewater treatment plant in Hong Kong served as co-substrates with FW in the co-digestion experiments. Physicochemical parameters of both the sludge and FW substrates were individually assessed before their introduction into the co-digestion systems (Table 1 ). Table 1 Characteristics of the mixed food waste and sludge used for the AD process. Parameter Unit Food waste Anaerobic digestate sludge (ADS) Primary & secondary sludge pH -- -- 7.21 ± 0.01 6.11 ± 0.02 TS % 41.00 ± 0.03 2.80 ± 0.01 3.58 ± 0.01 VS/TS % 97.35 ± 0.01 72.18 ± 0.02 76.98 ± 0.2 TOC % 42.86 ± 0.65 33.98 23.54 TKN % 3.08 ± 0.08 2.20 ± 0.07 5.29 ± 0.12 EC µS/cm -- 3.62 ± 0.003 1100 sCOD mg/L -- 1600 ± 0.025 46000 ± 0.02 Note : Dry weight bases calculated of food waste/sludge mix and inoculum addition. 2.2. Hydrothermal pretreatment of FW FW was thermally hydrolyzed and pretreated in 100 mL airtight pressure digestion vessels. Approximately 12–34 g of stimulated FW was added to the vessels (adjusting the moisture content to 80%), and reactors were operated at 90°C, 120°C, and 140°C for 60 min in an air-dry oven without adding any chemicals (Gnaoui et al., 2020 ; Liu et al., 2021 ). The reactors generated gas to maintain the headspace pressure. After 60 min of pretreatment, the reactors were cooled to ambient temperature, and the physiochemical characteristics of FW were determined. 2.3. AD process Batch AD tests were conducted using the Bioprocess AMPTS-II, an automatic biochemical methane potential system from Sweden. Co-digestion was conducted in triplicate to assess the AD performance of FW and sludge. They were linked to CO 2 taps (3 M NaOH) and gas flow meters in accordance with the manufacturer's instructions to continuously monitor methane flow rates. The mixed FW was pretreated at three different temperatures (90°C FW/WAS, 120°C FW/WAS, 140°C FW/WAS, and untreated FW control). The working volume of each reactor was 400 mL. According to Johnravindar et al. ( 2023 ), the inoculum/substrate ratio was set to 0.6, and the FW/WAS mixture ratio was 1:7 (w/w, VS basis). When necessary, lime (CaO) was used to adjust the pH of each reactor to approximately 6.5–7.0 to prevent rapid acidification of the co-digestion mixture. The pH was monitored and adjusted using a pH meter from Thermo Scientific. Each reactor was purged with N 2 for 5 min and then sealed. All reactors were operated at 35°C for 20 days with continuous stirring (80 rpm). 2.4. Analytical methods Pretreated FW was promptly utilized to assess solid and soluble indices, VFAs (acetic acid, propionic acid, butyric acid, iso-butyric acid, valeric acid, iso-valeric acid, and caproic acid) through gas chromatography (Agilent CP7675) equipped with a TG-WAXMS column (30 m × 0.32 mm) and a flame ionization detector (FID). Additionally, the analysis of soluble and total COD was performed following standard protocols (APHA, 2005 ) as detailed in the study by Johnravindar et al. ( 2021 ). Soluble protein, carbohydrates, ammoniacal nitrogen (NH 4 + -N), phosphate (PO 4 + -P), total solids (TS), and volatile solids (VS) were also analyzed (APHA, 2005 ). The AMPTS-II online data acquisition device recorded cumulative methane production statistics, which were examined and compared for various pretreatments and controls. The fermented broth was separated from the residue by centrifuging at 10,000 rpm for 10 minutes and then filtered through a filtration membrane with a pore size of 0.45 µm. Elementary analysis of the raw materials, including C, H, N, S, and O, was carried out using a Vario MACRO instrument from Germany. The final experimental data presented are the averages obtained from triplicate reactor tests. Furthermore, the dissolved organic matter (DOM) in the digestate liquid was examined using an excitation-emission matrix (EEM) spectrofluorometer, specifically the (Hitachi F-4700). The scanning protocol for the EEM spectra was performed as previously described (Johnravindar et al. 2022 ). Transform Infrared Spectrometry (FTIR) (Spectrum II, Perkin Elmer, Massachusetts, US, Chemistry) was used to characterize the functional groups in the freeze-dried HTP, highlighting changes in the chemical composition and functional groups of the samples. The filterability of the pretreated mixed FW samples at the THP temperatures was assessed using an improvised capillary suction time (CST) apparatus equipped with a single-radius test head and CST paper. This test serves as a valuable indicator of the sludge's water absorption capacity and can provide insights into its filtration characteristics (Liang et al. 2022 ). The solubilization of COD and VFAs was determined using Eq. (1): Solubilization COD = \(\:\frac{sCODt-sCOD0}{tCOD0-sCOD0}\) …………………………………………… (1). The methane amount and methane yield from the THP methods were compared to evaluate the co-digestion performance. The efficiency of the AD process for removing soluble COD (sCOD) from the digestate can be evaluated using Eq. (2) as follows: COD removal efficiency rate (%) = \(\:\frac{\:sCODinitial-sCODfinal}{sCODinitial\:}\:X\:100\) ……………. (2). where sCOD initial and sCOD final represent soluble COD (mg/L) pre- and post-digestion, respectively. 2.5. Energy conversion and economic analysis The study focused on comparing the energy conversion efficiency (ECF) of THP pretreatment temperatures in enhancing methane production from FW/WAS by analyzing energy reflections related to the mesophilic anaerobic co-digestion process and the capital cost for the pretreatments. The study primarily focused on comparing the efficiency of various temperature pretreatment methods in enhancing biogas production from FW/WAS by analyzing energy reflections related to the mesopic anaerobic co-digestion process and the capital cost of pretreatments. The supplementary materials provide the pertinent equations (3): ECF (%) \(\:\frac{\text{H}\text{e}\text{a}\text{t}\text{i}\text{n}\text{g}\:\text{v}\text{a}\text{l}\text{u}\text{e}\:\text{o}\text{f}\:\text{t}\text{h}\text{e}\:\text{m}\text{e}\text{t}\text{h}\text{a}\text{n}\text{e}\:\left(\text{k}\text{J}\right)}{\text{H}\text{e}\text{a}\text{t}\text{i}\text{n}\text{g}\:\text{v}\text{a}\text{l}\text{u}\text{e}\:\text{o}\text{f}\:\text{t}\text{h}\text{e}\:\text{s}\text{u}\text{b}\text{s}\text{t}\text{r}\text{a}\text{t}\text{e}\:\left(\text{k}\text{J}\right)}\) ……………………………………… (3) It is crucial to remember that this study examined only how energy is converted and how the processes of pretreatment and digestion benefit the economy (Chen et al. 2023 ). 2.6. Microbial population analysis The populations of methane-producing microorganisms in the digester bulk sludge/FW and sludge were analyzed. Changes in the microbial community structures in digesters using THP samples were observed on the final day of sampling, correlating with daily methane production. Samples were stored at -20°C prior to microbial community analysis. DNA was isolated from solid biomass samples of AD digestate using a Power Soil DNA Isolation Kit (Qiagen, Germany). For each sample, two independent polymerase chain reaction (PCR) tests were conducted to amplify bacterial and archaeal 16S rRNA genes using the primer pair 515-F (5′- GTGYCAGCMGCCGCGGTAA-3′) and 806-R (5′-GGACTACNVGGGTWTCTAAT-3′) with Illumina barcodes and adapters. Sequences were performed on the Illumina MiSeq platform (Major BioTech) Co., Ltd. (Shanghai, China). Raw Illumina MiSeq sequences were processed and analyzed using version 4.03 of Metagenomic Rast Server (MG-RAST) ( http://metagenomics.anl.gov/ ). The raw data were uploaded as FASTQ files after demultiplexing paired-end reads. Clustered operational taxonomic units (OTUs) were annotated at various taxonomic levels (phylum, class, order, family, and genus) using the RDP database (Johnravindar et al. 2022 ). 2.7. Statistical analysis The correlation between the two parameters was evaluated using Origin Pro® statistical software (Origin Lab, Massachusetts, USA) via one-way analysis of variance (ANOVA). The significance level for this study was set at P < 0.05 . 3. Results and discussion 3.1. Effect of thermal hydrolysis pretreatment on physiochemical characteristics of FW FW was pretreated for 60 min at 90°C, 120°C, and 140°C using thermal hydrolysis methods. The characteristics of the pretreated FW are listed in Table 2 . The pretreated solid exhibited a darker brown color compared to untreated FW, with the color darkening as the pretreatment temperature increased. The TS content of the pretreated FW remained nearly constant below 140°C. The VS concentrations of all THP-pretreated and raw substrates are shown in Table 2 . According to this table, the VS concentration of all THP samples was lower than that of untreated FW. However, the VS decreased at temperatures of 90°C, 120°C, and 140°C, and untreated FW with 97.41 ± 0.01%, 97.65 ± 0.01%, 97.61 ± 0.05%, and 97.8 ± 0.04% of the VS, respectively, lost at 140°C. This indicates that VS, primarily hemicellulose and cellulose, is broken down into smaller molecular organics at high temperatures, including monosaccharides, furans, and organic acids (Yin et al. 2014 ; Xu et al. 2022 ). The concentration of soluble COD in FW treated with varying THP at 90°C, 120°C, and 140°C reached 41.43 ± 0.81, 67.6 ± 1.7, and 72.0 ± 0.2 g of COD/L, respectively. In comparison, the untreated FW had a soluble COD concentration of only 37.83 ± 0.04 g of COD/L. After THP, the soluble COD in the treated FW was higher than that in the untreated FW, indicating that THP facilitates the solubilization of solid organics in FW. The increased sCOD indicates that THP effectively breaks down the organic compounds in FW, making them more readily available for further digestion. Figure 1 a shows the variations in sCOD and its composition under different pretreatments. As the thermal hydrolysis temperature increased from 90°C to 120°C and 140°C, the COD solubilization yield increased to 16.22 ± 1.06%, 61.19 ± 1.19%, and 87.94 ± 2.89%, respectively. This phenomenon occurs because high temperatures accelerate the breakdown of chemical bonds, such as the breakdown of proteins and polysaccharides contained in FW, releasing organic compounds into the liquid phase (Yin et al. 2014 ). The concentrations of total VFAs (tVFAs) at 90°C, 120°C, and 140°C of FW pretreated with thermal hydrolysis increased to 2.42 ± 0.03, 2.53 ± 0.62, and 3.43 ± 0.05 g/L, respectively, approximately 1.16-, 1.20-, and 1.64-fold greater than that of the control, respectively (Fig. 1 a). The amino acid and monosaccharide ingredients considerably converted the total VFAs (tVFAs), including acetic, propionic, butyric, isobutyric, isovaleric, valeric, and caproic acids, which were formed during acidification (Fig. 1 c). Diverse compositions of VFA can result in varying methane yields, with acetic acid identified as the primary contributor to methane generation according to Zhang et al. ( 2019 ). Acetic acid exhibited the highest concentration among the VFA components in all groups, followed by lactic, propionic, ethanol, and butyric acids. Specifically, the acetic acid content among the VFAs subjected to THP at 90°C, 120°C, and 140°C was measured at 40%, 44.6%, and 43.1%, respectively. Acetic acid, the dominant substrate of methanogen metabolism, could be crucial in selecting pretreatment methods during AD. Raising the THP temperature significantly increased the total and soluble carbohydrate contents. Figure 1 b shows that the maximum content levels of soluble carbohydrates were 45.31 ± 3.3, 71.18 ± 6.12, 81.80 ± 0.8, and 35.74 g/L observed at 90°C, 120°C, 140°C, and untreated FW, respectively. The increase in soluble carbohydrates observed at temperatures ranging from 90°C to 140°C during THP can be attributed to the hydrolysis of large-molecular-weight carbohydrate polymers present in the FW. The polymers include starch, cellulose, and hemicellulose. During THP, the high temperature and pressure conditions break down these complex carbohydrates into smaller molecular weight compounds such as oligosaccharides and monosaccharides (including glucose, fructose, and xylose) (Li et al. 2014 ). This results in the release of soluble sugars from solid carbohydrates into the FW, increasing in soluble carbohydrates. These degraded sugars can be converted into short-chain VFAs, with acetic acid being a common product. As a result, the total carbohydrate content may decrease due to the conversion of certain soluble sugars into VFAs (Li et al. 2014 ). As THP pretreatment temperatures rose from 90°C to 140°C, the soluble protein content in treated FW increased by 29.52–144.63% compared to untreated FW (Table 2 ). As the temperature of the THP increased, nitrogen present in the FW and solid matter dissolved into the water, leading to a rise in the concentration of NH 4 + -N. The NH 4 + -N concentration before THP was measured at 142.9 ± 0.98 mg/L, surging by around 83% to 261.8 ± 0.1 mg/L at a THP temperature of 140°C. However, the increases in NH 4 + -N and PO 4 3 -P contents ranged from 4.8–83.20% and 0.499–24.39%, respectively. The application of THP showed a significant increase in protein solubilization. This indicates that THP effectively promoted the breakdown of proteins into soluble forms. This result is consistent with observations from a previous study (Ding et al. 2017 ), which also reported a similar pattern of increased protein solubilization and limited protein degradation following THP. Table 2 Effect of the various hydrothermal pre-treatment temperatures on the characteristics of the food waste. Parameter Unit 90℃ FW 120℃ FW 140℃ FW Untreated (control) pH -- 4.57 ± 0.02 4.51 ± 0.16 4.29 ± 0.01 4.5 ± 0.01 EC uS/cm 3.40 ± 0.01 3.39 ± 0.06 3.40 ± 0.0 3.37 ± 0.01 TS % 20.16 ± 0.21 20.91 ± 0.6 21.9 ± 0.2 20.96 ± 0.11 VS/TS % 97.41 ± 0.01 97.65 ± 0.01 97.6 ± 0.05 97.8 ± 0.04 Total soluble protein mg/L 278.7 ± 1.31 336.7 ± 0.3 526.4 ± 4.5 215.2 ± 1.6 tCOD g/L 48.7 ± 0.14 72.63 ± 2.3 104 ± 3.2 41.1 ± 0.14 NH 4 + -N mg/L 149.9 ± 0.9 189.8 ± 0.3 261.8 ± 0.0 142.9 ± 0.9 PO 4 3− -P mg/L 120.7 ± 0.02 131.63 ± 0.05 149.4 ± 0.3 120.1 ± 0.1 C % (wt%, on dry basis) 50.58 ± 0.2 50.68 ± 0.7 52.6 ± 0.6 48.11 ± 0.06 H 6.14 ± 0.04 4.2 ± 0.06 4.1 ± 0.056 7.17 ± 0.05 O 37.39 38.04 38.03 38.46 N 3.11 ± 0.03 3.8 ± 0.05 2.67 ± 0.04 3.78 ± 0.04 S 0.17 ± 0.014 0.23 ± 0.01 0.21 ± 0.14 0.16 ± 0.02 Changes in the surface functional groups of FW subjected to various pretreatment temperatures were examined using FT-IR analysis within the range of 4000 to 400 cm − 1 to assess the impact of pretreatment on the functional groups and chemical structure of both untreated and pretreated FW (Fig. 1 d). A peak observed in the range of 3000 to 3600 cm − 1 indicated the presence of N-H groups from amide I of FW and the − OH group from internal water. Additionally, the bonds within the 3000–2800 cm − 1 range are linked to aliphatic C-H stretching. The intensity becomes stronger after THP, indicating an increase in the content of aliphatic and aromatic compounds resulting from the decomposition of proteins and carbohydrates (Zhang et al. 2019 ). The observed increase in the peak with the severity of pretreatment can be attributed to the solubilization of complex insoluble organics into simpler soluble organics. However, beyond 90°C, the significant increase in − OH stretching vibrations diminishes. Peaks corresponding to C = O and C-N stretching vibrations of amides I and II of the protein were detected at wavelengths 1655 and 1545 cm − 1 . As protein degradation progressed during pretreatment, wider peaks of amino acids or smaller fragments containing a carboxyl group and NH 3 were observed with increasing temperature and duration. The optimal THP temperature for organic dissolution from FW was identified as 120°C. It is important to note that at high pretreatment temperatures, some of the dissolved organics may be transformed into insoluble substances like hydrochar or even CO 2 and H 2 O (Yin et al. 2014 ). 3.2. DOM Figure 2 shows the excitation-emission matrix (3D-EEM) fluorescence spectra of dissolved organic matter (DOM) found in the effluent resulting from the THP of FW. All spectra indicated the presence of various fluorophores, each distinguished by its excitation/emission (Ex/Em) wavelength pair. The fluorescence regional integration approach, as detailed by Chen et al. ( 2003 ), was utilized to evaluate five Ex/Em regions. This method aids in comprehending the properties of the EEM spectra of the DOM samples, thereby improving the interpretability of the data. In the initial extracts, three peaks were observed in the EEM fluorescence spectra. Peak A, associated with aromatic protein fluorescence, was detected at Ex/Em 230/330 nm, as described by Baker and Curry ( 2004 ). Notably, for the 90°C-pretreated FW, Peak A appeared distinct from the others, located at an Ex/Em ratio of 225/300 nm and was attributed to tyrosine fluorescence. Tryptophan and protein-like compounds fluoresced in peak B at an Ex/Em ratio of 280/320 nm (Chen et al. 2003 ). Peak C, with an Ex/Em ratio of 260/450 nm, contained a compound resembling fulvic acid (Baker et al. 2004 ). According to Johnravindar et al. ( 2022 ), the visible fluorescence of humic-like compounds is responsible for Peak C at about 390/450 nm. DOM in FW comprises recalcitrant fulvic and humic acids, as well as soluble microbial products like proteins and polysaccharides. The control group exhibited the lowest fluorescence intensity compared to all the pretreated groups, indicating that the pretreatment effectively enhanced the release of organic matter to varying extents. Compared to other THP temperatures, the fluorescence spectrum at 140°C shows an increase in humic acid-like materials (peak D). This indicates that THP greatly enhances the hydrolysis of hard biodegradable DOM in FW. The FW pretreated at 120°C for 60 minutes exhibited the best performance for methane production, utilizing biodegradable DOM rapidly and completely. 3.3. Effect of hydrothermal pretreatment of FW on soluble organic matter degradation in co-digestion of SS THP involves subjecting organic matter to high temperatures in water. This treatment can significantly affect the degradation of soluble organic matter during digestion. During the initial six days, the concentrations of sCOD, soluble carbohydrates, soluble proteins, and NH 4 + -N in all treatments exhibited rapid increases (Fig. 3 ), primarily attributed to the solubilization and hydrolysis of macromolecular substances (Liang et al. 2022 ). The pH reduction in the anaerobic co-digestion system was primarily influenced by the action of acidogenic microorganisms. The pH change trend for each treatment was essentially the same, as shown in Fig. 3 a; it first declined, then increased, before eventually stabilizing. In the hydrothermally-pretreated 90°C FW/WAS, 120°C FW/WAS, and 140°C FW/WAS groups, the concentrations of sCOD, soluble carbohydrates, soluble proteins, and NH 4 + -N were significantly higher than those in the control group (p < 0.05), indicating rapid solubilization of substrates and effective release of carbohydrates and proteins in the THP (Wang et al. 2014 ). The variation in NH 4 + -N concentration is shown in Fig. 3 b. The soluble protein concentration increased after four days of fermentation. Anaerobic co-digestion tended to increase NH 4 + -N concentration. The average NH 4 + -N concentration increased as the THP temperature increased: it stood at 87.39 ± 1.02, 89.68 ± 1.02, 85.08 ± 0.40, and 84.69 ± 0.11 g/gVS add for the 90°C, 120°C, 140°C, and control groups, respectively. The NH 4 + -N concentration in the 120°C FW/WAS escalated rapidly due to protein decomposition, as reported by Johnravindar et al. ( 2021 ), and this rise was associated with the increase in VFAs. Moreover, it was indicated that the 140°C treatment inhibited microbial activity, reducing the levels of microbial hydrolytic enzymes like proteases and peptidases, as suggested by Yin et al. ( 2014 ). As shown in Fig. 3 c, the soluble COD of the FW model compounds post-THP was higher than that of the control sample. Following THP at 140°C, the sCOD of the products reached 1680 ± 0.8 mg/gVS, increasing by 1%. The results showed that THP promoted the release of dissolved organic matter from the substrates, and a higher temperature was associated with higher sCOD content. Figure 3 c indicates that when untreated food waste (FW) was subjected to thermal hydrolysis at 90°C, 120°C, and 140°C, the sCOD exhibited a general decreasing trend, with reductions of 23.19%, 44.89%, and 25.39% respectively. The observed trend suggests a dynamic interplay between two opposing processes; the dissolution of organic matter into the liquid phase, which increases sCOD, and the thermal degradation of these dissolved compounds, which reduces sCOD. At higher temperatures or longer durations, degradation might outpace dissolution, leading to a net decrease in sCOD. Thus, the final sCOD content reflects the balance between these two competing processes. This indicated that within the initial six-day period, fermentation was primarily governed by the hydrolysis and acidogenesis stages. The sCOD concentration followed the sequence 140°C > untreated FW > 120°C > 90°C. This pattern aligns with the initial sCOD level observed post-hydrothermal pretreatment (Table 2 ). After six days, a decline in sCOD levels was observed in each reactor. This observation implies that THP pretreatment facilitates the breakdown of soluble organic compounds during the anaerobic co-digestion process. The sCOD removal efficiencies at 120°C and FW/WAS were higher than those of the controls with different THPs. This result was consistent with that of Wang et al. ( 2014 ), where the final soluble carbohydrate concentration at 140°C was higher than that in other reactors. High temperature and pressure during THP pretreatment can break down complex organic compounds into simpler and more soluble forms. This can increase the availability of organic matter for microbial degradation during co-digestion. This phenomenon could be attributed to the generation of potentially toxic and recalcitrant byproducts resulting from the THP of FW at elevated temperatures, particularly at 140°C for 60 minutes, potentially exerting additional stress on microorganisms (Matsakas et al. 2014 ). Some organic compounds present in feedstocks can inhibit microbial activity during co-digestion. As an illustration, the Maillard reaction could occur between proteins and carbohydrates within the raw materials under high-temperature conditions (Li et al. 2014 ). Figure 3 d-e illustrates that over the six-day period, the concentrations of soluble proteins and polysaccharides in all experimental groups gradually declined to their minimum levels, suggesting that methanogenic archaea and fermentative bacteria utilized various substrates during acidogenesis and methanogenesis processes, respectively (Kaur et al. 2020 ). Following anaerobic co-digestion, the elimination rates of soluble proteins in the control and treatments at 90°C, 120°C, and 140°C were 55.04%, 57.45%, 65.31%, and 56.01%, respectively. Similarly, the removal rates of soluble polysaccharides in these treatments were 93.53%, 96.35%, 97.09%, and 95.99%, respectively. These results demonstrate that THP pretreatment facilitated the decomposition of high-molecular-weight organic matter ( p < 0.05 ). Furthermore, the 120°C treatment attained the highest removal rate of soluble organics compared to the 90°C and 140°C treatments ( p < 0.05 ). 3.4. Exploring the VFA Dynamics in THP during food waste/sludge co-digestion Figure 4 a illustrates the total volatile fatty acid (tVFA) production during anaerobic co-digestion. The amount of tVFA produced was significantly higher in thermally hydrolyzed food waste (FW) compared to non-pretreated FW. The concentrations of tVFA at 90°C, 120°C, and 140°C during hydrothermal pretreatment of FW/WAS increased to 3.41 ± 0.89, 4.05 ± 0.093, and 5.01 ± 0.140 g/L, respectively, which is approximately 1.75- fold greater than that of the control (Fig. 1 b). Notably, VFA production from the co-digestion of FW pretreated at 120°C was 13.28% higher than that of the control. VFA production did not increase in the 140°C reactors, despite the presence of more soluble materials. As previously indicated, the synthesis of toxic compounds, such as melanoidin (Liu et al. 2012 ), under hydrothermal conditions at 140°C may have inhibited the growth of acid-forming bacteria and the biodegradation of FW. Following pretreatment at 140°C, the composition of FW changed significantly. Within this temperature range, the formation of biodegradable substances increased while the presence of toxic materials decreased. Acetate and butyrate collectively accounted for approximately 70% of the total VFAs across all reactors. Acetic acid, in particular, played a crucial role as an intermediate in methanogenesis, facilitating methane production, as indicated by Johnravindar et al. ( 2022 ). The highest concentration of acetic acid was observed in the 120°C FW/WAS treatment (0.59 ± 0.02 g/gVS), surpassing concentrations in other THP conditions (Fig. 4 a-c). The VFA concentrations during anaerobic co-digestion with or without 90°C FW/WAS, 120°C FW/WAS, and 140°C FW/WAS were significantly different (Fig. 4 ). The concentrations of VFAs, including acetic, butyric, propionic, valeric, isobutyric, isovaleric, and caproic acids, were quantified in each treatment to elucidate the influence of THP on VFA degradation. Acetic acid, a key precursor in methanogenesis responsible for methane production, was highlighted for its pivotal role (Johnravindar et al. 2021 ). In this study, the maximum acetic acid concentration was observed at THP 120°C (0.59 ± 0.02 g/gVS), which was higher than that of the untreated FW/WAS (0.54 ± 0.01 g/gVS). Consequently, a high acetic acid yield was obtained at 120°C. The VFA yield exhibited a rapid increase within the initial eight days across all experimental groups, followed by a gradual decline during the subsequent co-digestion phase. This trend indicates that the VFAs were primarily accumulated through the hydrolysis of proteins and polysaccharides, subsequently serving as substrates for the ensuing methanogenesis process (Liang et al. 2022 ). However, at the end of the co-digestion process, the VFAs content in the control, 90°C FW/WAS, 120°C FW/WAS, and 140°C FW/WAS THP treatments stood at 0.72 ± 0.05, 0.23 ± 0.02, 0.15 ± 0.007, and 0.40 ± 0.009 mg/VS add , respectively, indicating that THP further promoted the rapid decomposition of VFAs and thus accelerated methane production (Johnravindar et al. 2022 ). Additionally, the VFA composition revealed that acetic acid was the predominant component in all groups during the early phases, while propionic acid exhibited a higher proportion throughout the fermentation process. These findings support the notion that methanogenesis was enhanced in the initial stages and subsequently suppressed in the later stages of the process (Kaur et al. 2020 ). 3.5. Methane production Figure 5 illustrates the daily and cumulative methane production observed throughout the co-digestion process. As depicted in Fig. 5 a, the early phases of anaerobic co-digestion displayed a notable increase in methane production from thermally pretreated FW, attributed to the enhanced hydrolysis of complex organic compounds within the FW. Initially, methane production in the co-digestion system exhibited growth, irrespective of the substrate being FW. However, there was variation in the peak values of daily methane production across different groups. The highest daily methane production occurred on the second and ninth days, recorded at 24.83 ± 1.6 mL/gVS.d (120°C FW/WAS), while the lowest was 19.82 ± 1.8 mL/gVS.d (control). These results indicate that THP treatment has the potential to enhance daily methane production in the co-digestion of FW and sludge. The most significant increase in methane production was observed with the 120°C FW/WAS THP-treated FW/sludge. This is because FW positively affects the hydrolysis of organic matter, resulting in rapid hydrolysis acidification and a relatively low pH of the slurry, which hinders the growth of methanogenic archaea. Daily methane production in these treatments increased significantly with the gradual consumption of the accumulated acids. The higher temperature of 140°C during pretreatment led to a significant decline in daily methane yield on the ninth day, likely due to the formation of undesirable substances that hindered the activities of methanogens (Ariunbaatar et al. 2014 ). The specific cumulative methane yields of each THP and untreated FW are shown in Fig. 5 b. All pretreated FW samples achieved higher volumetric methane production than the control. The highest volumetric methane production was obtained for FW treated at 120°C. Furthermore, the impact of THP pretreatment of FW was particularly pronounced in terms of methane production. This effect primarily stemmed from the enhanced solubilization of FW and the liberation of substantial quantities of soluble organic material, as detailed in Table 2 . The THP process has the potential to augment the solubilization and anaerobic biodegradability of the FW/WAS mixture. Additionally, the AD of FW is thought to disrupt the physical structure of the organic solids. Consequently, methane production rates can be elevated through anaerobic co-digestion (Park et al. 2020 ). The cumulative methane production (Fig. 5 b) showed a threshold value for an increase in methane production. Indeed, it increased with the THP temperature until 120°C, from 324.3 ± 4.5 mL CH 4 /gVSadd for the untreated FW/WAS to 228.8 ± 1.13 mL CH4/gVSadd for the FW/WAS pretreated at 120°C. The increase was statistically significant at p < 0.05. Thus, among the cumulative methane productions, the optimum THP temperature was found to be 120℃. Moreover, at 140°C, significant solubilization of particulate organic matter occurred, but the FW biodegradability was inhibited (324.3 ± 4.5 and 267.4 ± 1.46 mL/gVS add ). It was determined that, at 140°C, carbohydrates in the soluble phase react with other components to form products that are slowly or hardly biodegradable (Li et al. 2014 ; Park et al. 2020 ). Kim et al. ( 2015 ) suggested that a changed chemical structure due to the "burn sugar" reaction and Maillard reactions occurs at high pretreatment temperatures, as evidenced by the FT-IR spectrum (Fig. 1 d). The brown color of the soluble phase of the FW at 140°C confirmed the presence of new compounds such as Amadori compounds and melanoidins, with final pH values recorded at 7.2–7.4. The Gompertz model was applied to the methane production results to estimate the co-digestion parameters. The parameters were estimated from the best fit to the experimental data (Fig. 5 and Appendix Table S1 ). This advantageous phenomenon is attributed to a notable decrease in lignin content under these conditions, leading to a heightened hemicellulose concentration. Hemicellulose is more readily hydrolyzed into sugar monomers such as glucose, mannose, and galactose compared to cellulose. These monomeric substrates are more easily biodegraded via methanogenesis, thereby facilitating increased methane production, as highlighted by Parra-Orobio et al. ( 2020 ). The rise in VFA concentrations in the initial stages of the AD process is conducive to methanogen growth, consequently boosting methane production and shortening the lag phase ( λ ). This suggests that excessive temperatures can lead to the generation of harmful compounds within the system, disrupting the normal functioning of the digestion process, aligning with the findings of Wang et al. ( 2018 ). 3.6. Carbon Balance Analysis THP pretreatment can significantly affect the carbon balance during the co-digestion process. Carbon balance refers to the distribution and transformation of carbon-containing compounds during co-digestion. During anaerobic co-digestion, the carbon in the volatile solids (VS) is converted to VFAs, soluble carbohydrates, soluble proteins, lactate, alcohol, methane, and other products (Table 3 ). The carbon balance calculation was based on the maximum VFA production. Appendix S1 shows that most of the THPs stood at 30.9%, 40.07%, and 23.8% when subjected to temperatures of 90°C, 120°C, and 140°C, respectively. This result indicates that pretreatments could promote VFA production. However, the contents of the residual VS post-anaerobic co-digestion remained over 50% across all three treatments. Residual VS consists of proteins, lipids, and sugars. The carbon balance of the fermentation process was evaluated using the experimental data to assess carbon conversion efficiency. The carbon content of dry VS, VFAs, proteins, and carbohydrates was quantified. Other carbon-containing compounds, such as CO 2 , CH 4, and other organic substances in soluble chemical oxygen demand (sCOD), were collectively categorized as "others." Following the co-digestion process, approximately 58% of the carbon present in the VS was converted. Notably, the carbon content of the VFAs was most prominent at 140°C, accounting for 40.3% of the total, with a lower proportion of other carbon compounds. Table 3 Overall performance of the co-digestion systems at various hydrothermal pre-treatment. Parameters Pre-treatment Condition Untreated Control co-digestion 90℃ FW + WAS 120℃ FW + WAS 140℃ FW + WAS VS reduction (%) 55.224 ± 0.6 57.98 ± 0.5 53.205 ± 0.1 47.78 ± 0.74 Cumulative CH 4 production (mL/gVS added ) 307.13 ± 3.8 324.39 ± 4.5 267.43 ± 1.4 228.83 ± 1.13 sCOD max (mg/gVS) 1537 ± 3.4 1557.7 ± 9.2 1680 ± 8.10 1660.6 ± 4.1 Increase rate of methane yield compared to no pre-treatment AD process (%) 34.21 41.75 16.86 -- 3.7. Microbial Community Composition of Various HTP Pretreatments during Anaerobic Co-Digestion The alpha diversity indices, including ACE, Chao1, operational taxonomic units (OTUs), Shannon, and Simpson (Appendix Table S2), indicated that the ecological roles of the bacterial community during the methanogenesis stages were strengthened at 120°C and 90°C. This is supported by the significantly higher alpha values of THP at these temperatures compared to the control (Zhou et al. 2024 ). Figure 6 a illustrates the similarity in bacterial phyla composition within the community structure, although notable differences in their relative abundances exist. The dominant bacterial phyla observed were Firmicutes, Bacteroidetes, Halobacterota, Euryarchaeota, Actinobacteria, and Synergistota, collectively accounting for more than 92.53% of the bacterial content across all samples. Specifically, the control group exhibited relative abundances of 45.074% for Firmicutes, 26.40% for Halobacterota, 17.13% for Bacteroidetes, and 2.68% for Euryarchaeota. However, upon the addition of THP FW, there was a significant increase in the relative abundance of Firmicutes from 48.13–71.43%. Similarly, the relative abundances of Halobacterota, Bacteroidetes, and Euryarchaeota increased from 7.35–13.85%, 9.35–22%, and 1.77–2.41%, respectively. Notably, the relative abundance of Firmicutes decreased from 48.13–45.72% at 140°C in FW/WAS. The presence of methanogens in Halobacterota and Euryarchaeota significantly increased following the inoculation of HTP. Bacteroidetes, known as acid-forming bacteria, exhibited the ability to convert various substances into butyric, acetic, isovaleric acids, hydrogen, and carbon dioxide. An increase in the abundance of Bacteroidetes was observed in the 90°C FW/WAS (22.18%), 140°C FW/WAS (19.75%), and 120°C FW/WAS (17.13%). The findings from the bacterial community analysis align with the VFA and sCOD results, indicating a correlation between VFA generation and Bacteroidetes. Acetogenic bacteria play a crucial role in methane production during AD by converting organic acids into acetate and hydrogen. Hydrolytic bacteria, such as Clostridium, Bacteroides, and Cellulomonas, are responsible for the initial breakdown of complex organic compounds into simpler molecules through enzyme secretion. The increase during THP 120°C FW/WAS resulted in an enhanced number of hydrolyzing bacteria and acid-producing microorganisms, leading to faster acidification and hydrolysis. Bacteroidetes, known as proteolytic bacteria, degrade proteins into VFAs and ammonium nitrogen (NH 4 + -N) (Liu et al. 2021 ). The high soluble protein concentration in the 120°C FW/WAS sample resulted in a significantly higher relative abundance. Further investigations of bacterial communities were performed at a general level (Fig. 6 b). The major genera detected in the THP FW/WAS digestate samples showed increased relative abundance. Petrimonas (27.46–38.38%), Proteiniphilum (16.8–25.04%), Bacteroidetes_vadin HA17 (0.97–2.35%), Macellibacteroides (2.22–2.69%), and unclassified genera (12.03–24.34%) are proteolytic bacteria capable of metabolizing carbohydrates to produce VFAs. In the THP for FW/WAS at 120°C, the hydrogen-producing bacteria Petrimonas were enriched (38.38%), potentially improving carbon recovery from WAS. Proteiniphilum , belonging to the phylum Bacteroidetes, exhibited significantly higher relative abundance. Notably, Proteiniphilum was present at a considerable abundance of 35.65% in the control (untreated FW). This species accelerates propionate degradation by utilizing pyruvate, an intermediate in the propionate degradation pathway (Jiang et al. 2022 ). Bacteroides predominated in the THP-pretreated digestate (1.68%) and control systems (0.6%). The dominance of methanogenic archaea characterizes the archaeal microbial communities in anaerobic co-digestion systems, specifically those involving FW/WAS and THP pretreatment. Methanogens play a pivotal role in methane production (Fig. 6 c). The specific composition of these archaeal communities exhibits variability, contingent upon factors such as the feedstock, operating conditions, and pretreatment methodology employed (Jiang et al. 2022 ). It is noteworthy that methanogens, being obligate anaerobic microorganisms, exhibit greater vulnerability compared to hydrolytic or acidogenic bacteria. Among the various genera of methanogens, Methanobrevibacter , Methanosarcina , Methanobacterium , Methanocorpusculum , and Methanosaeta assume particular significance in the context of co-digestion. In an investigation involving FW/sludge samples from anaerobic co-digestion reactors, the relative abundance of Methanosarcina in the digestate samples exhibited an increase with higher THP temperatures, whereas the abundance of Methanosaeta displayed a decrease. This shift in abundance can be attributed to the availability of substrates. Specifically, Methanosarcina and Methanosaeta were seen to compete for acetate and free ammonia, respectively. Further, the abundance of hydrogen, carbon dioxide, and formate-utilizing Methanobrevibacter was observed to be low during the treatments (ranging from 0.39–2.67%), whereas it was relatively high in the raw sludge inoculum (41.18%). On day 0, Methanosarcina dominated, accounting for 16.42%, 41.15%, 30.25%, 21.02%, and 15.53% of the inoculum in 90°C FW/WAS, 120°C FW/WAS, 140°C FW/WAS, and control, respectively. During AD, the abundance of Methanosarcina increased significantly, reaching final abundances of 164.97%, 94.78%, and 35.41%, respectively. Methanosarcina exhibited significant growth during the active methane-generation phase across all treatments. Competitive growth occurred between Methanosarcina and Methanosaeta , resulting in a decrease in the latter and an increase in the former. The control group consisted of Methanocorpusculum (60.8%), Methanosarcina (15.5%), Methanofollis (10.53%), and Methanobactrium (5.65%). With regard to 90°C FW/WAS, 120°C FW/WAS, and 140°C FW/WAS, Methanosarcina was the most dominant methanogen, ranging from 21.02–41.15%, followed by Methanobacterium (15.80–38.48%). Methanosarcina , unlike other methanogens, has a rapid growth rate and is more tolerant of abrupt pH shifts of about 0.8–1.0 units (Johnravindar et al. 2022 ). The relative abundance of Methanosarcina in digestate samples increases with rising THP temperature, possibly due to the availability of specific substrates. Conversely, Methanosaeta experiences a decrease in relative abundance. Methanobrevibacter , utilizing hydrogen, carbon dioxide, and formate, exhibits a significantly higher prevalence across THP conditions compared to the control (Shao et al. 2024). 3.9. Comparison of energy yield with comparable studies This study calculated the overall energy production using the methane yield and its calorific value of 11.48 MJ/KgVS. The energy conversion efficiency (ECE) of methane varied from 16.97–49.47%. High methane yields corresponded to a THP for 120°C FW/WAS. Evidently, the overall ECEs were significantly impacted by the thermal hydrolysis temperature. The enhanced ECE of the treated FW increased to 49.47% when the hydrothermal temperature was elevated to 120°C, surpassing that of untreated FW. However, a negative correlation was observed between the overall ECEs and hydrothermal temperatures greater than 140°C. The overall ECE of treated FW dropped to 16% when the temperature reached 140°C. Additionally, energy consumption primarily comprised mechanical crushing throughout the entire cycle, THP, and operation (heating). Mechanical crushing and thermal hydrolysis accounted for the maximum energy usage. The ECE for methane production stood at 324.39 mL/gVS, which constituted 41.75% of the overall ECE of 49.47%. However, experimental results obtained from various studies have shown significant variations due to the considerable differences between the sources and components of food waste and waste lipids. Furthermore, the ECE can reflect the relative degree of substrate conversion. In this study, through hydrothermal pretreatment and co-digestion using methane, the overall energy conversion efficiency was increased to 41.75%, which improved methane yield and shortened the time duration for the same. In comparison, the influence of hydrothermal retention time on overall ECEs was deemed to be moderately significant. Based on the collective findings of the study, the optimal operational parameters for co-digestion of FW/WAS were identified to be 120°C and 60 min. Table 4 shows that the extra energy produced by applying pretreatment methods ranges around 11.48 MJ/KgVS FW. Table 4 Comparison of cumulative methane production and energy yield from pre-treated food waste. FW origin Pre-treatment Digestion type Energy yield (MJ/kgVS) *Increase in energy (ECF) yield (%) References Food waste (waste process facility in Canada) Ultrasonic and acid pretreatment (sonicated at 79 kJ/gTS and pH at 3.0 with 1 N HCl for 24 h) One-stage batch dark H 2 fermentation 1.3 18.0 Elbeshbishy et al. ( 2011 ) Food waste: sewage sludge, ratio 2:1 (TS based) Thermophilic conditions (56℃) Batch anaerobic digestion 7.1 -- Sosnowski et al. ( 2003 ) Food waste (institute cafeteria in Korea) Alkali pretreatment (pH at 12.0 with 6 N KOH for 6 h) Single-stage batch dark H 2 fermentation 1.8 -- Jang et al. ( 2015 ) Food waste (University canteen in Singapore) Enzymatic pretreatment using fungal mash (0.2 g/gTS-FW) Single-stage batch anaerobic digestion 29.2 33.9 Yin et al. ( 2016 ) Synthetic food waste (based on the average composition of FW in some European countries) Thermal pretreatment (80℃, 90 mins) Ozonation pretreatment (0.068 gO3/gTS) Single-stage batch anaerobic digestion 23.2 52.0 Ariunbaatar et al. ( 2014 ) Pelagic Sargassum/FW co-digestion Hydrothermal pre-treatment 140℃ and 30 min Single-stage batch AD 21.57 51.07 Thompson et al. ( 2021 ) Strawberry extrudate and Sewage sludge Hydrothermal 120°C, 15 min Single-stage batch anaerobic co-digestion -- 31 Serrano et al. ( 2023 ) Food waste Thermal pretreatment (100℃, 30mins) Single stage continuously studies AD (different OLR) 13.75 23.68 Gnaouia, et al., 2020 Food waste: sludge (stimulated FW) Hydrothermal pre-treatment 120℃ and 60 min Single-stage batch anaerobic co-digestion 11.48 Control-7.68 49.47 This study Sludge dewatering affects digestate yield and transportation costs. Dewaterability increased in reactors at 90°C, 120°C, and 140°C, as well as in the control in Appendix S2. The initial capillary suction time (CST) of the raw FW-SS mixture was 420.3 seconds. After HTP treatment, the CST of the FW/WAS blend demonstrated a notable reduction to 365.6, 325.76, and 225.7 seconds at 90°C, 120°C, and 140°C, respectively. This reduction in CST can be attributed to the degradation of cell walls in the WAS and the subsequent alteration of the physical structure of FW, resulting in increased difficulty in dewatering (Zhang et al. 2019 ). Cao et al. ( 2023 ) found that dewaterability is linked to the degradation of organic matter. However, a reduction in sludge concentration and viscosity decreases dewaterability. The sludge network structure is disrupted, and extracellular polymeric substances (EPS) are dissolved through THP, thereby releasing bound water and improving dewaterability. THP effectively enhances the dewaterability of digestate post-co-digestion using FW and WAS. 4. Conclusions The study's conclusions highlight the effectiveness of THP in enhancing the co-digestion performance of FW. The findings indicate that the solubilization of sCOD, VFAs, and organic components such as carbohydrates and proteins increased with rising temperatures, achieving peak values of 87.94%, 64.45%, 64.54%, and 34.64% at treatment temperatures of 90°C, 120°C, and 140°C for 60 minutes. Notably, THP conducted at 120°C for 60 minutes proved highly efficient in improving the solubilization of organic constituents, resulting in a substantial rise in methane generation during the co-digestion process of FW and sludge. Furthermore, the microbial community analysis revealed that THP at 120°C for the FW/WAS mixture notably elevated the proportions of acetogenic bacteria and methanogenic organisms. The study also observed that the highest energy conversion factor (ECF) output was obtained for THP at 120°C for FW/WAS, recording an output of 11.48 MJ per ton of fed VS, representing a 49.47% increase compared to the control condition. These results collectively demonstrate the potential of THP as a viable pretreatment method for enhancing the efficiency and performance of FW co-digestion processes, with significant improvements observed in methane generation and energy output. Further research could explore the applicability of these conditions to others organic waste types including agricultural residues or mixed municipal waste, to assess their methane production potential and scalability. Future studies should evaluate the long- term stability and performance of anaerobic co-digestion systems. Declarations Authors Contributions Davidraj Johnravindar: Investigation, Methodology, Writing – original draft. Manu Mathikere Krishnegowda: Investigation, Jun Zhao: Conceptualization, Data curation, Funding acquisition, Jonathan Wong Woon Chung: Project administration, Supervision, Writing – review & editing. Acknowledgment/Funding: This work was supported by the Environment and Conservation Fund, Hong Kong, (Grant No ECF Project 46/2020, 09/2021). Data availability: Availability of data and materials will be available upon request. Ethical approval : Not applicable. Consent to participate : Not applicable. Consent for publication: Not applicable. Competing interests: The authors declare no competing interests. References Aierzhati A, Stablein MJ, Wu NE, Kuo CT, Si B, Kang X, Zhang Y (2019). Experimental and model enhancement of food waste hydrothermal liquefaction with combined effects of biochemical composition and reaction conditions. Bioresour Technol. 284: 139-147. Amr I, Farokh laqa K, Elsayed E, George N (2022). 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Effect of thermal pretreatment on the physical and chemical properties of municipal biomass waste. Waste Manage. 32 (2): 249-255. Mehariya S, Patel AK, Obulisamy PK, Punniyakotti E, Wong JWC (2018). Co-digestion of food waste and sewage sludge for methane production: Current status and perspective. Bioresour Technol. 265: 519-531. Matsakas L, Kekos D, Loizidou M, Christakopoulos P (2014). Utilization of household food waste for the production of ethanol at high dry material content. Biotechnol Biofuels: 7; 4. https://doi.org/10.1186/1754-6834-7-4. Park S, Han SK, Song E, Kim H, Kim M, Lee W (2020). Effect of hydrothermal pre-treatment on physical properties and co-digestion from food waste and sewage sludge mixture. Waste Manag Res. 38(5): 546-553. Parthiba Karthikeyan O, Trably E, Mehariya S, Bernet N, Wong JWC, Carrere H (2018). Pretreatment of food waste for methane and hydrogen recovery: A review. Bioresour Technol. 249: 1025-1039. doi: 10.1016/j.biortech.2017.09.105. Parra-Orobio BA, Donoso-Bravo A, Torres-Lozada P (2020). Energy balance and carbon dioxide emissions comparison through modified anaerobic digestion model No 1 for single-stage and two-stage anaerobic digestion of food waste. Biomass and Bioenergy. 142: 105814. doi:10.1016/j.biombioe.2020.10581 Serrano A, Russo E, Chaves-Quesada B, Cubero-Cardoso J, Trujillo-Reyes Á, Esposito G, Xu X, Fermoso FG (2023). Accumulation of Volatile Fatty Acids from Hydrothermally Treated Strawberry Extrudate through Anaerobic Fermentation at Different pH Values. Agronomy. 13: 120. https://doi.org/10.3390/ agronomy13010120. Sosnowski P, Wieczorek A, Ledakowicz S (2003). Anaerobic co-digestion of sewage sludge and organic fraction of municipal solid wastes . J Environ Manage. 7(3): 0-616 . doi:10.1016/s1093-0191(02)00049-7. Thompson TM, Young BR, Baroutian, S (2021). Enhancing biogas production from caribbean pelagic Sargassum utilising hydrothermal pretreatment and anaerobic co-digestion with food waste. Chemosphere. 275: 130035. doi: 10.1016/j.chemosphere.2021.130035. Wang D, Shen F, Yang G, Zhang Y, Deng S, Zhang J, Zeng Y, Luo T, Mei Z (2018). Can hydrothermal pretreatment improve anaerobic digestion for biogas from lignocellulosic biomass? Bioresour Technol. 249: 117-124. Wang K, Yin J, Shen DS, Li N (2014). Anaerobic digestion of food waste for volatile fatty acids (VFAs) production with different types of inoculum: effect of pH. Bioresour Technol. 161: 395-401. Wong JWC, Kaur G, Mehariya S, Karthikeyan OPK, Chen G (2018). Food waste treatment by anaerobic co-digestion with saline sludge and its implications for energy recovery in Hong Kong. Bioresour Technol. 268: 824- 828. Xu Q, Luo L, Li D, Johnravindar D, Varjani S, Wong JWC, Zhao J (2022). Hydrochar prepared from digestate improves anaerobic co-digestion of food waste and sewage sludge: Performance, mechanisms, and implication. Bioresour Technol. 362: 127765. Yin J, Wang K, Yang Y, Shen D, Wang M, Mo H (2014). Improving production of volatile fatty acids from food waste fermentation by hydrothermal pretreatment. Bioresour Technol. 171: 323-9. Yin Y, Liu YJ, Meng SJ, Kiran EU, Liu Y (2016). Enzymatic pretreatment of activated sludge, food waste and their mixture for enhanced bioenergy recovery and waste volume reduction via anaerobic digestion. Appl Energy. 179: 1131-1137. doi:10.1016/j.apenergy.2016.07.083. Zhang D, Jiang H, Chang J, Sun J, Tu W, Wang H (2019). Effect of thermal hydrolysis pretreatment on volatile fatty acids production in sludge acidification and subsequent polyhydroxyalkanoates production. Bioresour Technol. 279: 92-100. https://doi.org/10.1016/j.biortech.2019.01.077. Zhou P, Li D, Zhang C, Ping Q, Wang L, Li Y (2024). Comparison of different sewage sludge pretreatment technologies for improving sludge solubilization and anaerobic digestion efficiency: A comprehensive review. Sci Total Environ. 15: 921, 171175. doi: 10.1016/j.scitotenv.2024.171175. Supplementary Files Supplementarymaterials.docx Cite Share Download PDF Status: Published Journal Publication published 05 Feb, 2025 Read the published version in Environmental Science and Pollution Research → Version 1 posted Editorial decision: Accept 12 Jan, 2025 Reviewers agreed at journal 20 Dec, 2024 Reviewers invited by journal 20 Dec, 2024 Editor invited by journal 18 Dec, 2024 First submitted to journal 16 Dec, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5087495","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":392859525,"identity":"4884ed01-b482-4bb3-add5-5b3434659098","order_by":0,"name":"Davidraj Johnravindar","email":"","orcid":"","institution":"Hong Kong Baptist University","correspondingAuthor":false,"prefix":"","firstName":"Davidraj","middleName":"","lastName":"Johnravindar","suffix":""},{"id":392859526,"identity":"8d4be965-d973-4832-a2f4-0c8b3e9160f8","order_by":1,"name":"Jun Zhao","email":"","orcid":"","institution":"Hong Kong Baptist University","correspondingAuthor":false,"prefix":"","firstName":"Jun","middleName":"","lastName":"Zhao","suffix":""},{"id":392859527,"identity":"37ba7ec5-c406-43bc-9e41-8bea6a62074b","order_by":2,"name":"Mathikere Krishnegowda Manu","email":"","orcid":"","institution":"Hong Kong Baptist University","correspondingAuthor":false,"prefix":"","firstName":"Mathikere","middleName":"Krishnegowda","lastName":"Manu","suffix":""},{"id":392859528,"identity":"f6940df9-3225-45f7-af72-53c50e14e29f","order_by":3,"name":"Jonathan Woon Chung Wong","email":"data:image/png;base64,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","orcid":"","institution":"Dongguan University of Technology","correspondingAuthor":true,"prefix":"","firstName":"Jonathan","middleName":"Woon Chung","lastName":"Wong","suffix":""}],"badges":[],"createdAt":"2024-09-14 06:59:08","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5087495/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5087495/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s11356-025-35944-0","type":"published","date":"2025-02-05T15:57:20+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":72168343,"identity":"f72e8e22-9210-4b08-a068-977913772f2c","added_by":"auto","created_at":"2024-12-23 10:43:24","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":298474,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of the various hydrothermal pre-treatment temperatures on the characteristics of food waste, including a) Total soluble carbohydrate, b) COD solubilization percentage, c) tVFA production profile, and d) THP pretreated FW FTIR spectra.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5087495/v1/e66b3ebfcba1e3435fb8ddb4.png"},{"id":72168853,"identity":"32840973-f4d2-45a2-9ffe-0fd019cc831b","added_by":"auto","created_at":"2024-12-23 10:51:24","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":174002,"visible":true,"origin":"","legend":"\u003cp\u003eThe fluorescence EEM of the DOM fractions from the food waste THP under different temperatures. a) 90℃ FW, b) 120℃ FW, c) 140℃ FW, and d) Control (Untreated).\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5087495/v1/fdc01ca5072db45abc10a054.png"},{"id":72168333,"identity":"59587e82-963b-4e52-9577-8b6db420fa43","added_by":"auto","created_at":"2024-12-23 10:43:23","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":124098,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of\u003cstrong\u003e \u003c/strong\u003ea)\u003cstrong\u003e \u003c/strong\u003epH, b) NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N, c) sCOD, d) total soluble carbohydrates, and e) total soluble protein change during food waste co-digestion under different hydrothermal pre-treatment conditions and control.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5087495/v1/9e1438ae8ee531fa89be0d79.png"},{"id":72168335,"identity":"f7ca1c96-8099-480c-82b3-83171499bb53","added_by":"auto","created_at":"2024-12-23 10:43:24","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":390460,"visible":true,"origin":"","legend":"\u003cp\u003eInfluence of different hydrothermal pre-treatment on total VFA kinetics during food waste/sludge co-digestion and its comparison with control, a) 90℃, b) 120℃, c) 140℃, and d) Control (untreated FW).\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5087495/v1/115aae98d17caa248ee55502.png"},{"id":72170478,"identity":"7d196a1f-35f4-4239-a047-d4e289145bc4","added_by":"auto","created_at":"2024-12-23 10:59:26","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":75972,"visible":true,"origin":"","legend":"\u003cp\u003ea) Daily and b) cumulative methane production in the anaerobic digestions operated at different hydrothermal pre-treatments.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-5087495/v1/567a6ebaba99cd5187a0c08f.png"},{"id":72168339,"identity":"b98a9007-aa1f-4bd3-b1e1-f3cff66689c4","added_by":"auto","created_at":"2024-12-23 10:43:24","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":212799,"visible":true,"origin":"","legend":"\u003cp\u003eMicrobial community structure in different food waste HTP anaerobic co-digestion system along with sewage sludge addition: Phylum level (a); Bacteria at the genus level (b); and Archaea at the genus level (c).\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-5087495/v1/5efd75e6c2c3f8d30339cb48.png"},{"id":75930385,"identity":"f8cb7cac-de2c-47d0-9f42-098a3c0e30a5","added_by":"auto","created_at":"2025-02-10 16:10:52","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2296591,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5087495/v1/25daddac-bd34-4d6d-849f-7a3e444ade80.pdf"},{"id":72168401,"identity":"87ee3354-6b28-43cb-a5e2-87b5e1c9b567","added_by":"auto","created_at":"2024-12-23 10:43:30","extension":"docx","order_by":7,"title":"","display":"","copyAsset":false,"role":"supplement","size":491217,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementarymaterials.docx","url":"https://assets-eu.researchsquare.com/files/rs-5087495/v1/e9344b1c53807d4ceb321932.docx"}],"financialInterests":"","formattedTitle":"Hydrothermal Pretreatment of Food Waste Enhances Performance of Anaerobic Co-digestion With Sludge","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eHydrothermal pretreatment of food waste (FW) has emerged as a promising strategy to enhance the performance of anaerobic co-digestion with sludge. The escalating global population and economic activities have led to a surge in municipal solid waste (MSW) generation, posing significant environmental challenges (Chuen Chen et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). In densely populated cities like Hong Kong, the per capita production of FW stands at 0.30 kg/day, while sewage sludge (SS) rates exceed 0.16 kg/day (HKEPD \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Approximately 30.0% of the 11,057 tons/day of MSW generated in Hong Kong comprises FW, with SS production reaching around 1,052 tons/day (EPD 2021). Given FW's predominant presence in Hong Kong's MSW composition, the government has proposed leveraging the surplus AD capacity at existing sewage treatment plants for FW/SS co-digestion, offering superior benefits compared to mono-digestion (Mehariya et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Thus, there is a pressing need to enhance the handling of FW and SS through efficient anaerobic co-digestion practices to mitigate adverse environmental and societal impacts. Anaerobic digestion is recognized as a viable method for simultaneous waste treatment and energy recovery through methane-rich biogas production (Johnravindar et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). FW, characterized by its high moisture content and degradability, is an ideal substrate for AD. However, the standalone application of AD for FW and SS faces challenges such as high organic loads, rapid acidification, prolonged solid retention, and the presence of inhibitory substances. Consequently, sludge from wastewater treatment plants contains significant levels of heavy metals, pathogens, and bacteria (Kaur et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). AD involves a series of biological processes that convert complex substrates into biogas through microbial action in the absence of oxygen, encompassing hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Hydrolysis is often the rate-limiting step for complex organic substrates due to the formation of toxic byproducts or undesirable volatile fatty acids (VFAs), while methanogenesis poses limitations for easily biodegradable substrates (Kaur et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). In this context, the anaerobic co-digestion of FW and SS has been shown to enhance digestion efficiency and optimize methane production (Parthiba Karthikeyan et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Leveraging the alkalinity and enhanced buffering capacity of SS, coupled with the underutilized capacity of sludge digesters at wastewater treatment plants, achieves reduced waste discharge and improved resource utilization. Studies by Chakraborty et al. (\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) and Kaur et al. (\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) have demonstrated elevated concentrations of intermediate metabolites, particularly VFAs like acetic acid, butyric acid, and propionic acid, during FW/SS co-digestion with biochar addition. Acetic acid, followed by butyric acid, serves as favorable substrates for methanogenesis, while propionic acid can inhibit methane production. However, successful FW/SS co-digestion schemes have been identified for diluted FW systems with low FW loading due to reduced process performance (Wong et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). FW contains complex organic compounds, including animal fats, vegetable oils, and high molecular weight carbohydrates, making hydrolysis a crucial step in AD. Various pretreatment techniques have been explored to accelerate hydrolysis, produce valuable byproducts, and enhance the quality of recyclable components, such as nitrogen and phosphorus. Among these techniques, thermal hydrolysis pretreatment (THP) stands out as an effective and environmentally friendly method that does not rely on chemical additives. THP involves subjecting FW to high pressures and temperatures, leading to the hydrolysis of macromolecular materials and organic dissolution (Yin et al. \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Gahlot et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Amr et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). This process disrupts cell membranes, promotes the dissolution of recalcitrant organic compounds, and enhances macromolecule hydrolysis under specific time and temperature conditions. Studies such as Jin et al. (\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) have shown positive effects on chemical oxygen demand (COD), lignin, cellulose, volatile solids (VS), fibers, proteins, and methane production following THP at 160\u0026deg;C for 20 minutes. Similarly, Ding et al. (\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2017\u003c/span\u003e) reported enhanced hydrogen and methane co-production through FW hydrothermal pretreatment at 140\u0026deg;C for 20 minutes. Notably, the literature review highlights the diversity of pretreatment techniques such as ultrasonic, chemical, or their combination with waste-activated sludge (WAS) and other wastes that pose challenges in comparing their efficiencies (Gao et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Khanh Nguyen et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eWhile THP is commonly employed for extracting monosaccharides from agricultural waste and SS during biomass utilization, its application in pretreating FW for co-digestion to produce VFAs and methane remains underexplored. Digestate sludge, when used as a substrate, offers an opportunity to generate a carbon source without compromising biogas production. This approach also significantly reduces sludge handling volumes, leading to substantial cost savings and addressing inefficiencies in traditional THP\u0026thinsp;+\u0026thinsp;AD systems, particularly when wastewater sludge has a low solids content (Carranza Munoz et al., 2024). The composition of FW, varying by location and season, presents challenges due to the presence of recalcitrant components that can fluctuate significantly. Pretreatment of FW can modify its composition, potentially enhancing VFA production. Additionally, pretreatment improves AD and boosts electricity generation in fuel cells by providing simpler nutrients for microorganisms compared to untreated sludge, promoting microbial growth and resistance to inhibitors. Nonetheless, the extended detention times and limited reduction in volatile solids (VS) pose challenges in utilizing AD for biological solids (Aierzhati et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). THP offers a range of benefits, including improved anaerobic degradation kinetics, enhanced biodegradability, VS destruction, methane production, solubilization of macromolecular components (carbohydrates, proteins, and lipids) in FW and sludge, improved dewaterability, and higher organic loading rates in digesters (Liu et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThis study aims to explore the impact of thermal hydrolysis pretreatment on the characteristics of FW and its subsequent co-digestion for methane production. Additionally, the research seeks to investigate the effects of THP applied to FW on the anaerobic co-digestion process with sludge. Specifically, the study will assess how pretreatment temperature influences the characteristics of organic matter and evaluate the efficiency of biogas production based on methane potential yield. The anticipated outcomes of this research should offer new insights into the mechanisms involved and the changes in microbial communities during anaerobic co-digestion of hydrothermally pretreated FW and sludge.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003ch2\u003e2.1. Substrates and inoculum\u003c/h2\u003e\n \u003cp\u003eFW was artificially prepared (using 35% bread, 25% cabbage, 25% boiled rice, and 15% boiled pork) to mimic the composition of FW with regard to the Hong Kong diet, as in our previous studies (Johnravindar et al. \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e). The FW was subjected to size reduction by manual chopping followed by blending to achieve a homogenous mixture suitable for co-digestion experiments. The inoculum consisted of anaerobic digestate (non-saline) sludge, while primary and secondary (biologically activated) sludge from the Shek Wu Hui wastewater treatment plant in Hong Kong served as co-substrates with FW in the co-digestion experiments. Physicochemical parameters of both the sludge and FW substrates were individually assessed before their introduction into the co-digestion systems (Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable id=\"Tab1\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eCharacteristics of the mixed food waste and sludge used for the AD process.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"5\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eParameter\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eUnit\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eFood waste\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eAnaerobic digestate sludge (ADS)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003ePrimary \u0026amp; secondary sludge\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003epH\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e--\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e--\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e7.21\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e6.11\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003eTS\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e%\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e41.00\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.80\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.58\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003eVS/TS\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e%\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e97.35\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e72.18\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e76.98\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003eTOC\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e%\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e42.86\u0026thinsp;\u0026plusmn;\u0026thinsp;0.65\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e33.98\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e23.54\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003eTKN\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e%\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.08\u0026thinsp;\u0026plusmn;\u0026thinsp;0.08\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.20\u0026thinsp;\u0026plusmn;\u0026thinsp;0.07\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e5.29\u0026thinsp;\u0026plusmn;\u0026thinsp;0.12\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003eEC\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026micro;S/cm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e--\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.62\u0026thinsp;\u0026plusmn;\u0026thinsp;0.003\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1100\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003esCOD\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003emg/L\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e--\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1600\u0026thinsp;\u0026plusmn;\u0026thinsp;0.025\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e46000\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cp\u003e\u003cstrong\u003eNote\u003c/strong\u003e: Dry weight bases calculated of food waste/sludge mix and inoculum addition.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n \u003ch2\u003e2.2. Hydrothermal pretreatment of FW\u003c/h2\u003e\n \u003cp\u003eFW was thermally hydrolyzed and pretreated in 100 mL airtight pressure digestion vessels. Approximately 12\u0026ndash;34 g of stimulated FW was added to the vessels (adjusting the moisture content to 80%), and reactors were operated at 90\u0026deg;C, 120\u0026deg;C, and 140\u0026deg;C for 60 min in an air-dry oven without adding any chemicals (Gnaoui et al., \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e; Liu et al., \u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e). The reactors generated gas to maintain the headspace pressure. After 60 min of pretreatment, the reactors were cooled to ambient temperature, and the physiochemical characteristics of FW were determined.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n \u003ch2\u003e2.3. AD process\u003c/h2\u003e\n \u003cp\u003eBatch AD tests were conducted using the Bioprocess AMPTS-II, an automatic biochemical methane potential system from Sweden. Co-digestion was conducted in triplicate to assess the AD performance of FW and sludge. They were linked to CO\u003csub\u003e2\u003c/sub\u003e taps (3 M NaOH) and gas flow meters in accordance with the manufacturer\u0026apos;s instructions to continuously monitor methane flow rates. The mixed FW was pretreated at three different temperatures (90\u0026deg;C FW/WAS, 120\u0026deg;C FW/WAS, 140\u0026deg;C FW/WAS, and untreated FW control). The working volume of each reactor was 400 mL. According to Johnravindar et al. (\u003cspan class=\"CitationRef\"\u003e2023\u003c/span\u003e), the inoculum/substrate ratio was set to 0.6, and the FW/WAS mixture ratio was 1:7 (w/w, VS basis). When necessary, lime (CaO) was used to adjust the pH of each reactor to approximately 6.5\u0026ndash;7.0 to prevent rapid acidification of the co-digestion mixture. The pH was monitored and adjusted using a pH meter from Thermo Scientific. Each reactor was purged with N\u003csub\u003e2\u003c/sub\u003e for 5 min and then sealed. All reactors were operated at 35\u0026deg;C for 20 days with continuous stirring (80 rpm).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n \u003ch2\u003e2.4. Analytical methods\u003c/h2\u003e\n \u003cp\u003ePretreated FW was promptly utilized to assess solid and soluble indices, VFAs (acetic acid, propionic acid, butyric acid, iso-butyric acid, valeric acid, iso-valeric acid, and caproic acid) through gas chromatography (Agilent CP7675) equipped with a TG-WAXMS column (30 m \u0026times; 0.32 mm) and a flame ionization detector (FID). Additionally, the analysis of soluble and total COD was performed following standard protocols (APHA, \u003cspan class=\"CitationRef\"\u003e2005\u003c/span\u003e) as detailed in the study by Johnravindar et al. (\u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e). Soluble protein, carbohydrates, ammoniacal nitrogen (NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N), phosphate (PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-P), total solids (TS), and volatile solids (VS) were also analyzed (APHA, \u003cspan class=\"CitationRef\"\u003e2005\u003c/span\u003e). The AMPTS-II online data acquisition device recorded cumulative methane production statistics, which were examined and compared for various pretreatments and controls. The fermented broth was separated from the residue by centrifuging at 10,000 rpm for 10 minutes and then filtered through a filtration membrane with a pore size of 0.45 \u0026micro;m. Elementary analysis of the raw materials, including C, H, N, S, and O, was carried out using a Vario MACRO instrument from Germany. The final experimental data presented are the averages obtained from triplicate reactor tests. Furthermore, the dissolved organic matter (DOM) in the digestate liquid was examined using an excitation-emission matrix (EEM) spectrofluorometer, specifically the (Hitachi F-4700). The scanning protocol for the EEM spectra was performed as previously described (Johnravindar et al. \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e). Transform Infrared Spectrometry (FTIR) (Spectrum II, Perkin Elmer, Massachusetts, US, Chemistry) was used to characterize the functional groups in the freeze-dried HTP, highlighting changes in the chemical composition and functional groups of the samples. The filterability of the pretreated mixed FW samples at the THP temperatures was assessed using an improvised capillary suction time (CST) apparatus equipped with a single-radius test head and CST paper. This test serves as a valuable indicator of the sludge\u0026apos;s water absorption capacity and can provide insights into its filtration characteristics (Liang et al. \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e\n \u003cp\u003eThe solubilization of COD and VFAs was determined using Eq.\u0026nbsp;(1):\u003c/p\u003e\n \u003cp\u003eSolubilization COD = \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\frac{sCODt-sCOD0}{tCOD0-sCOD0}\\)\u003c/span\u003e\u003c/span\u003e\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip; (1).\u003c/p\u003e\n \u003cp\u003eThe methane amount and methane yield from the THP methods were compared to evaluate the co-digestion performance. The efficiency of the AD process for removing soluble COD (sCOD) from the digestate can be evaluated using Eq.\u0026nbsp;(2) as follows:\u003c/p\u003e\n \u003cp\u003eCOD removal efficiency rate (%) = \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\frac{\\:sCODinitial-sCODfinal}{sCODinitial\\:}\\:X\\:100\\)\u003c/span\u003e\u003c/span\u003e\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;. (2).\u003c/p\u003e\n \u003cp\u003ewhere sCOD\u003csub\u003einitial\u003c/sub\u003e and sCOD\u003csub\u003efinal\u003c/sub\u003e represent soluble COD (mg/L) pre- and post-digestion, respectively.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\n \u003ch2\u003e2.5. Energy conversion and economic analysis\u003c/h2\u003e\n \u003cp\u003eThe study focused on comparing the energy conversion efficiency (ECF) of THP pretreatment temperatures in enhancing methane production from FW/WAS by analyzing energy reflections related to the mesophilic anaerobic co-digestion process and the capital cost for the pretreatments. The study primarily focused on comparing the efficiency of various temperature pretreatment methods in enhancing biogas production from FW/WAS by analyzing energy reflections related to the mesopic anaerobic co-digestion process and the capital cost of pretreatments. The supplementary materials provide the pertinent equations (3):\u003c/p\u003e\n \u003cp\u003eECF (%) \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\frac{\\text{H}\\text{e}\\text{a}\\text{t}\\text{i}\\text{n}\\text{g}\\:\\text{v}\\text{a}\\text{l}\\text{u}\\text{e}\\:\\text{o}\\text{f}\\:\\text{t}\\text{h}\\text{e}\\:\\text{m}\\text{e}\\text{t}\\text{h}\\text{a}\\text{n}\\text{e}\\:\\left(\\text{k}\\text{J}\\right)}{\\text{H}\\text{e}\\text{a}\\text{t}\\text{i}\\text{n}\\text{g}\\:\\text{v}\\text{a}\\text{l}\\text{u}\\text{e}\\:\\text{o}\\text{f}\\:\\text{t}\\text{h}\\text{e}\\:\\text{s}\\text{u}\\text{b}\\text{s}\\text{t}\\text{r}\\text{a}\\text{t}\\text{e}\\:\\left(\\text{k}\\text{J}\\right)}\\)\u003c/span\u003e\u003c/span\u003e\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip; (3)\u003c/p\u003e\n \u003cp\u003eIt is crucial to remember that this study examined only how energy is converted and how the processes of pretreatment and digestion benefit the economy (Chen et al. \u003cspan class=\"CitationRef\"\u003e2023\u003c/span\u003e).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n \u003ch2\u003e2.6. Microbial population analysis\u003c/h2\u003e\n \u003cp\u003eThe populations of methane-producing microorganisms in the digester bulk sludge/FW and sludge were analyzed. Changes in the microbial community structures in digesters using THP samples were observed on the final day of sampling, correlating with daily methane production. Samples were stored at -20\u0026deg;C prior to microbial community analysis. DNA was isolated from solid biomass samples of AD digestate using a Power Soil DNA Isolation Kit (Qiagen, Germany). For each sample, two independent polymerase chain reaction (PCR) tests were conducted to amplify bacterial and archaeal 16S rRNA genes using the primer pair 515-F (5\u0026prime;- GTGYCAGCMGCCGCGGTAA-3\u0026prime;) and 806-R (5\u0026prime;-GGACTACNVGGGTWTCTAAT-3\u0026prime;) with Illumina barcodes and adapters. Sequences were performed on the Illumina MiSeq platform (Major BioTech) Co., Ltd. (Shanghai, China). Raw Illumina MiSeq sequences were processed and analyzed using version 4.03 of Metagenomic Rast Server (MG-RAST) (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttp://metagenomics.anl.gov/\u003c/span\u003e\u003c/span\u003e). The raw data were uploaded as FASTQ files after demultiplexing paired-end reads. Clustered operational taxonomic units (OTUs) were annotated at various taxonomic levels (phylum, class, order, family, and genus) using the RDP database (Johnravindar et al. \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\n \u003ch2\u003e2.7. Statistical analysis\u003c/h2\u003e\n \u003cp\u003eThe correlation between the two parameters was evaluated using Origin Pro\u0026reg; statistical software (Origin Lab, Massachusetts, USA) via one-way analysis of variance (ANOVA). The significance level for this study was set at \u003cem\u003eP\u0026thinsp;\u0026lt;\u0026thinsp;0.05\u003c/em\u003e.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\n \u003ch2\u003e3.1. Effect of thermal hydrolysis pretreatment on physiochemical characteristics of FW\u003c/h2\u003e\n \u003cp\u003eFW was pretreated for 60 min at 90\u0026deg;C, 120\u0026deg;C, and 140\u0026deg;C using thermal hydrolysis methods. The characteristics of the pretreated FW are listed in Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e. The pretreated solid exhibited a darker brown color compared to untreated FW, with the color darkening as the pretreatment temperature increased. The TS content of the pretreated FW remained nearly constant below 140\u0026deg;C. The VS concentrations of all THP-pretreated and raw substrates are shown in Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e. According to this table, the VS concentration of all THP samples was lower than that of untreated FW. However, the VS decreased at temperatures of 90\u0026deg;C, 120\u0026deg;C, and 140\u0026deg;C, and untreated FW with 97.41\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01%, 97.65\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01%, 97.61\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05%, and 97.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04% of the VS, respectively, lost at 140\u0026deg;C. This indicates that VS, primarily hemicellulose and cellulose, is broken down into smaller molecular organics at high temperatures, including monosaccharides, furans, and organic acids (Yin et al. \u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e; Xu et al. \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e). The concentration of soluble COD in FW treated with varying THP at 90\u0026deg;C, 120\u0026deg;C, and 140\u0026deg;C reached 41.43\u0026thinsp;\u0026plusmn;\u0026thinsp;0.81, 67.6\u0026thinsp;\u0026plusmn;\u0026thinsp;1.7, and 72.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2 g of COD/L, respectively. In comparison, the untreated FW had a soluble COD concentration of only 37.83\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04 g of COD/L. After THP, the soluble COD in the treated FW was higher than that in the untreated FW, indicating that THP facilitates the solubilization of solid organics in FW. The increased sCOD indicates that THP effectively breaks down the organic compounds in FW, making them more readily available for further digestion. Figure \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ea shows the variations in sCOD and its composition under different pretreatments. As the thermal hydrolysis temperature increased from 90\u0026deg;C to 120\u0026deg;C and 140\u0026deg;C, the COD solubilization yield increased to 16.22\u0026thinsp;\u0026plusmn;\u0026thinsp;1.06%, 61.19\u0026thinsp;\u0026plusmn;\u0026thinsp;1.19%, and 87.94\u0026thinsp;\u0026plusmn;\u0026thinsp;2.89%, respectively. This phenomenon occurs because high temperatures accelerate the breakdown of chemical bonds, such as the breakdown of proteins and polysaccharides contained in FW, releasing organic compounds into the liquid phase (Yin et al. \u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e). The concentrations of total VFAs (tVFAs) at 90\u0026deg;C, 120\u0026deg;C, and 140\u0026deg;C of FW pretreated with thermal hydrolysis increased to 2.42\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03, 2.53\u0026thinsp;\u0026plusmn;\u0026thinsp;0.62, and 3.43\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05 g/L, respectively, approximately 1.16-, 1.20-, and 1.64-fold greater than that of the control, respectively (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ea). The amino acid and monosaccharide ingredients considerably converted the total VFAs (tVFAs), including acetic, propionic, butyric, isobutyric, isovaleric, valeric, and caproic acids, which were formed during acidification (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ec). Diverse compositions of VFA can result in varying methane yields, with acetic acid identified as the primary contributor to methane generation according to Zhang et al. (\u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e). Acetic acid exhibited the highest concentration among the VFA components in all groups, followed by lactic, propionic, ethanol, and butyric acids. Specifically, the acetic acid content among the VFAs subjected to THP at 90\u0026deg;C, 120\u0026deg;C, and 140\u0026deg;C was measured at 40%, 44.6%, and 43.1%, respectively. Acetic acid, the dominant substrate of methanogen metabolism, could be crucial in selecting pretreatment methods during AD. Raising the THP temperature significantly increased the total and soluble carbohydrate contents. Figure \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eb shows that the maximum content levels of soluble carbohydrates were 45.31\u0026thinsp;\u0026plusmn;\u0026thinsp;3.3, 71.18\u0026thinsp;\u0026plusmn;\u0026thinsp;6.12, 81.80\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8, and 35.74 g/L observed at 90\u0026deg;C, 120\u0026deg;C, 140\u0026deg;C, and untreated FW, respectively. The increase in soluble carbohydrates observed at temperatures ranging from 90\u0026deg;C to 140\u0026deg;C during THP can be attributed to the hydrolysis of large-molecular-weight carbohydrate polymers present in the FW. The polymers include starch, cellulose, and hemicellulose. During THP, the high temperature and pressure conditions break down these complex carbohydrates into smaller molecular weight compounds such as oligosaccharides and monosaccharides (including glucose, fructose, and xylose) (Li et al. \u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e). This results in the release of soluble sugars from solid carbohydrates into the FW, increasing in soluble carbohydrates. These degraded sugars can be converted into short-chain VFAs, with acetic acid being a common product. As a result, the total carbohydrate content may decrease due to the conversion of certain soluble sugars into VFAs (Li et al. \u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e). As THP pretreatment temperatures rose from 90\u0026deg;C to 140\u0026deg;C, the soluble protein content in treated FW increased by 29.52\u0026ndash;144.63% compared to untreated FW (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e). As the temperature of the THP increased, nitrogen present in the FW and solid matter dissolved into the water, leading to a rise in the concentration of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N. The NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N concentration before THP was measured at 142.9\u0026thinsp;\u0026plusmn;\u0026thinsp;0.98 mg/L, surging by around 83% to 261.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 mg/L at a THP temperature of 140\u0026deg;C. However, the increases in NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N and PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u003c/sup\u003e-P contents ranged from 4.8\u0026ndash;83.20% and 0.499\u0026ndash;24.39%, respectively. The application of THP showed a significant increase in protein solubilization. This indicates that THP effectively promoted the breakdown of proteins into soluble forms. This result is consistent with observations from a previous study (Ding et al. \u003cspan class=\"CitationRef\"\u003e2017\u003c/span\u003e), which also reported a similar pattern of increased protein solubilization and limited protein degradation following THP.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable id=\"Tab2\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eEffect of the various hydrothermal pre-treatment temperatures on the characteristics of the food waste.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"6\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eParameter\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eUnit\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e90℃ FW\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e120℃ FW\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e140℃ FW\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eUntreated (control)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003epH\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e--\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4.57\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4.51\u0026thinsp;\u0026plusmn;\u0026thinsp;0.16\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4.29\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eEC\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003euS/cm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.40\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.39\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.40\u0026thinsp;\u0026plusmn;\u0026thinsp;0.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.37\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eTS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e%\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e20.16\u0026thinsp;\u0026plusmn;\u0026thinsp;0.21\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e20.91\u0026thinsp;\u0026plusmn;\u0026thinsp;0.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e21.9\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e20.96\u0026thinsp;\u0026plusmn;\u0026thinsp;0.11\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eVS/TS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e%\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e97.41\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e97.65\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e97.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e97.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eTotal soluble protein\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003emg/L\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e278.7\u0026thinsp;\u0026plusmn;\u0026thinsp;1.31\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e336.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e526.4\u0026thinsp;\u0026plusmn;\u0026thinsp;4.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e215.2\u0026thinsp;\u0026plusmn;\u0026thinsp;1.6\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003etCOD\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eg/L\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e48.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.14\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e72.63\u0026thinsp;\u0026plusmn;\u0026thinsp;2.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e104\u0026thinsp;\u0026plusmn;\u0026thinsp;3.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e41.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.14\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003emg/L\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e149.9\u0026thinsp;\u0026plusmn;\u0026thinsp;0.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e189.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e261.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e142.9\u0026thinsp;\u0026plusmn;\u0026thinsp;0.9\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ePO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e-P\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003emg/L\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e120.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e131.63\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e149.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e120.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eC\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" rowspan=\"5\"\u003e\n \u003cp\u003e% (wt%, on dry basis)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e50.58\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e50.68\u0026thinsp;\u0026plusmn;\u0026thinsp;0.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e52.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e48.11\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eH\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e6.14\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.056\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e7.17\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eO\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e37.39\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e38.04\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e38.03\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e38.46\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eN\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.11\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.67\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.78\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.17\u0026thinsp;\u0026plusmn;\u0026thinsp;0.014\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.23\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.21\u0026thinsp;\u0026plusmn;\u0026thinsp;0.14\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.16\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cp\u003eChanges in the surface functional groups of FW subjected to various pretreatment temperatures were examined using FT-IR analysis within the range of 4000 to 400 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e to assess the impact of pretreatment on the functional groups and chemical structure of both untreated and pretreated FW (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ed). A peak observed in the range of 3000 to 3600 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e indicated the presence of N-H groups from amide I of FW and the \u003csup\u003e\u0026minus;\u003c/sup\u003eOH group from internal water. Additionally, the bonds within the 3000\u0026ndash;2800 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e range are linked to aliphatic C-H stretching. The intensity becomes stronger after THP, indicating an increase in the content of aliphatic and aromatic compounds resulting from the decomposition of proteins and carbohydrates (Zhang et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e). The observed increase in the peak with the severity of pretreatment can be attributed to the solubilization of complex insoluble organics into simpler soluble organics. However, beyond 90\u0026deg;C, the significant increase in \u003csup\u003e\u0026minus;\u003c/sup\u003eOH stretching vibrations diminishes. Peaks corresponding to C\u0026thinsp;=\u0026thinsp;O and C-N stretching vibrations of amides I and II of the protein were detected at wavelengths 1655 and 1545 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. As protein degradation progressed during pretreatment, wider peaks of amino acids or smaller fragments containing a carboxyl group and NH\u003csub\u003e3\u003c/sub\u003e were observed with increasing temperature and duration. The optimal THP temperature for organic dissolution from FW was identified as 120\u0026deg;C. It is important to note that at high pretreatment temperatures, some of the dissolved organics may be transformed into insoluble substances like hydrochar or even CO\u003csub\u003e2\u003c/sub\u003e and H\u003csub\u003e2\u003c/sub\u003eO (Yin et al. \u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2. DOM\u003c/h2\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e shows the excitation-emission matrix (3D-EEM) fluorescence spectra of dissolved organic matter (DOM) found in the effluent resulting from the THP of FW. All spectra indicated the presence of various fluorophores, each distinguished by its excitation/emission (Ex/Em) wavelength pair. The fluorescence regional integration approach, as detailed by Chen et al. (\u003cspan class=\"CitationRef\"\u003e2003\u003c/span\u003e), was utilized to evaluate five Ex/Em regions. This method aids in comprehending the properties of the EEM spectra of the DOM samples, thereby improving the interpretability of the data. In the initial extracts, three peaks were observed in the EEM fluorescence spectra. Peak A, associated with aromatic protein fluorescence, was detected at Ex/Em 230/330 nm, as described by Baker and Curry (\u003cspan class=\"CitationRef\"\u003e2004\u003c/span\u003e). Notably, for the 90\u0026deg;C-pretreated FW, Peak A appeared distinct from the others, located at an Ex/Em ratio of 225/300 nm and was attributed to tyrosine fluorescence. Tryptophan and protein-like compounds fluoresced in peak B at an Ex/Em ratio of 280/320 nm (Chen et al. \u003cspan class=\"CitationRef\"\u003e2003\u003c/span\u003e). Peak C, with an Ex/Em ratio of 260/450 nm, contained a compound resembling fulvic acid (Baker et al. \u003cspan class=\"CitationRef\"\u003e2004\u003c/span\u003e). According to Johnravindar et al. (\u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e), the visible fluorescence of humic-like compounds is responsible for Peak C at about 390/450 nm. DOM in FW comprises recalcitrant fulvic and humic acids, as well as soluble microbial products like proteins and polysaccharides. The control group exhibited the lowest fluorescence intensity compared to all the pretreated groups, indicating that the pretreatment effectively enhanced the release of organic matter to varying extents. Compared to other THP temperatures, the fluorescence spectrum at 140\u0026deg;C shows an increase in humic acid-like materials (peak D). This indicates that THP greatly enhances the hydrolysis of hard biodegradable DOM in FW. The FW pretreated at 120\u0026deg;C for 60 minutes exhibited the best performance for methane production, utilizing biodegradable DOM rapidly and completely.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\n \u003ch2\u003e3.3. Effect of hydrothermal pretreatment of FW on soluble organic matter degradation in co-digestion of SS\u003c/h2\u003e\n \u003cp\u003eTHP involves subjecting organic matter to high temperatures in water. This treatment can significantly affect the degradation of soluble organic matter during digestion. During the initial six days, the concentrations of sCOD, soluble carbohydrates, soluble proteins, and NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N in all treatments exhibited rapid increases (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e), primarily attributed to the solubilization and hydrolysis of macromolecular substances (Liang et al. \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e). The pH reduction in the anaerobic co-digestion system was primarily influenced by the action of acidogenic microorganisms. The pH change trend for each treatment was essentially the same, as shown in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea; it first declined, then increased, before eventually stabilizing. In the hydrothermally-pretreated 90\u0026deg;C FW/WAS, 120\u0026deg;C FW/WAS, and 140\u0026deg;C FW/WAS groups, the concentrations of sCOD, soluble carbohydrates, soluble proteins, and NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N were significantly higher than those in the control group (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05), indicating rapid solubilization of substrates and effective release of carbohydrates and proteins in the THP (Wang et al. \u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e). The variation in NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N concentration is shown in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb. The soluble protein concentration increased after four days of fermentation. Anaerobic co-digestion tended to increase NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N concentration. The average NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N concentration increased as the THP temperature increased: it stood at 87.39\u0026thinsp;\u0026plusmn;\u0026thinsp;1.02, 89.68\u0026thinsp;\u0026plusmn;\u0026thinsp;1.02, 85.08\u0026thinsp;\u0026plusmn;\u0026thinsp;0.40, and 84.69\u0026thinsp;\u0026plusmn;\u0026thinsp;0.11 g/gVS\u003csub\u003eadd\u003c/sub\u003e for the 90\u0026deg;C, 120\u0026deg;C, 140\u0026deg;C, and control groups, respectively. The NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N concentration in the 120\u0026deg;C FW/WAS escalated rapidly due to protein decomposition, as reported by Johnravindar et al. (\u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e), and this rise was associated with the increase in VFAs. Moreover, it was indicated that the 140\u0026deg;C treatment inhibited microbial activity, reducing the levels of microbial hydrolytic enzymes like proteases and peptidases, as suggested by Yin et al. (\u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e). As shown in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec, the soluble COD of the FW model compounds post-THP was higher than that of the control sample. Following THP at 140\u0026deg;C, the sCOD of the products reached 1680\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8 mg/gVS, increasing by 1%. The results showed that THP promoted the release of dissolved organic matter from the substrates, and a higher temperature was associated with higher sCOD content. Figure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec indicates that when untreated food waste (FW) was subjected to thermal hydrolysis at 90\u0026deg;C, 120\u0026deg;C, and 140\u0026deg;C, the sCOD exhibited a general decreasing trend, with reductions of 23.19%, 44.89%, and 25.39% respectively. The observed trend suggests a dynamic interplay between two opposing processes; the dissolution of organic matter into the liquid phase, which increases sCOD, and the thermal degradation of these dissolved compounds, which reduces sCOD. At higher temperatures or longer durations, degradation might outpace dissolution, leading to a net decrease in sCOD. Thus, the final sCOD content reflects the balance between these two competing processes.\u003c/p\u003e\n \u003cp\u003eThis indicated that within the initial six-day period, fermentation was primarily governed by the hydrolysis and acidogenesis stages. The sCOD concentration followed the sequence 140\u0026deg;C\u0026thinsp;\u0026gt;\u0026thinsp;untreated FW\u0026thinsp;\u0026gt;\u0026thinsp;120\u0026deg;C\u0026thinsp;\u0026gt;\u0026thinsp;90\u0026deg;C. This pattern aligns with the initial sCOD level observed post-hydrothermal pretreatment (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e). After six days, a decline in sCOD levels was observed in each reactor. This observation implies that THP pretreatment facilitates the breakdown of soluble organic compounds during the anaerobic co-digestion process. The sCOD removal efficiencies at 120\u0026deg;C and FW/WAS were higher than those of the controls with different THPs. This result was consistent with that of Wang et al. (\u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e), where the final soluble carbohydrate concentration at 140\u0026deg;C was higher than that in other reactors. High temperature and pressure during THP pretreatment can break down complex organic compounds into simpler and more soluble forms. This can increase the availability of organic matter for microbial degradation during co-digestion. This phenomenon could be attributed to the generation of potentially toxic and recalcitrant byproducts resulting from the THP of FW at elevated temperatures, particularly at 140\u0026deg;C for 60 minutes, potentially exerting additional stress on microorganisms (Matsakas et al. \u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e). Some organic compounds present in feedstocks can inhibit microbial activity during co-digestion. As an illustration, the Maillard reaction could occur between proteins and carbohydrates within the raw materials under high-temperature conditions (Li et al. \u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e). Figure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed-e illustrates that over the six-day period, the concentrations of soluble proteins and polysaccharides in all experimental groups gradually declined to their minimum levels, suggesting that methanogenic archaea and fermentative bacteria utilized various substrates during acidogenesis and methanogenesis processes, respectively (Kaur et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e). Following anaerobic co-digestion, the elimination rates of soluble proteins in the control and treatments at 90\u0026deg;C, 120\u0026deg;C, and 140\u0026deg;C were 55.04%, 57.45%, 65.31%, and 56.01%, respectively. Similarly, the removal rates of soluble polysaccharides in these treatments were 93.53%, 96.35%, 97.09%, and 95.99%, respectively. These results demonstrate that THP pretreatment facilitated the decomposition of high-molecular-weight organic matter (\u003cem\u003ep\u0026thinsp;\u0026lt;\u0026thinsp;0.05\u003c/em\u003e). Furthermore, the 120\u0026deg;C treatment attained the highest removal rate of soluble organics compared to the 90\u0026deg;C and 140\u0026deg;C treatments (\u003cem\u003ep\u0026thinsp;\u0026lt;\u0026thinsp;0.05\u003c/em\u003e).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\n \u003ch2\u003e3.4. Exploring the VFA Dynamics in THP during food waste/sludge co-digestion\u003c/h2\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea illustrates the total volatile fatty acid (tVFA) production during anaerobic co-digestion. The amount of tVFA produced was significantly higher in thermally hydrolyzed food waste (FW) compared to non-pretreated FW. The concentrations of tVFA at 90\u0026deg;C, 120\u0026deg;C, and 140\u0026deg;C during hydrothermal pretreatment of FW/WAS increased to 3.41\u0026thinsp;\u0026plusmn;\u0026thinsp;0.89, 4.05\u0026thinsp;\u0026plusmn;\u0026thinsp;0.093, and 5.01\u0026thinsp;\u0026plusmn;\u0026thinsp;0.140 g/L, respectively, which is approximately 1.75- fold greater than that of the control (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eb). Notably, VFA production from the co-digestion of FW pretreated at 120\u0026deg;C was 13.28% higher than that of the control. VFA production did not increase in the 140\u0026deg;C reactors, despite the presence of more soluble materials. As previously indicated, the synthesis of toxic compounds, such as melanoidin (Liu et al. \u003cspan class=\"CitationRef\"\u003e2012\u003c/span\u003e), under hydrothermal conditions at 140\u0026deg;C may have inhibited the growth of acid-forming bacteria and the biodegradation of FW. Following pretreatment at 140\u0026deg;C, the composition of FW changed significantly. Within this temperature range, the formation of biodegradable substances increased while the presence of toxic materials decreased. Acetate and butyrate collectively accounted for approximately 70% of the total VFAs across all reactors. Acetic acid, in particular, played a crucial role as an intermediate in methanogenesis, facilitating methane production, as indicated by Johnravindar et al. (\u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e). The highest concentration of acetic acid was observed in the 120\u0026deg;C FW/WAS treatment (0.59\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02 g/gVS), surpassing concentrations in other THP conditions (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea-c). The VFA concentrations during anaerobic co-digestion with or without 90\u0026deg;C FW/WAS, 120\u0026deg;C FW/WAS, and 140\u0026deg;C FW/WAS were significantly different (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e). The concentrations of VFAs, including acetic, butyric, propionic, valeric, isobutyric, isovaleric, and caproic acids, were quantified in each treatment to elucidate the influence of THP on VFA degradation. Acetic acid, a key precursor in methanogenesis responsible for methane production, was highlighted for its pivotal role (Johnravindar et al. \u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e). In this study, the maximum acetic acid concentration was observed at THP 120\u0026deg;C (0.59\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02 g/gVS), which was higher than that of the untreated FW/WAS (0.54\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01 g/gVS). Consequently, a high acetic acid yield was obtained at 120\u0026deg;C. The VFA yield exhibited a rapid increase within the initial eight days across all experimental groups, followed by a gradual decline during the subsequent co-digestion phase. This trend indicates that the VFAs were primarily accumulated through the hydrolysis of proteins and polysaccharides, subsequently serving as substrates for the ensuing methanogenesis process (Liang et al. \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e). However, at the end of the co-digestion process, the VFAs content in the control, 90\u0026deg;C FW/WAS, 120\u0026deg;C FW/WAS, and 140\u0026deg;C FW/WAS THP treatments stood at 0.72\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05, 0.23\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02, 0.15\u0026thinsp;\u0026plusmn;\u0026thinsp;0.007, and 0.40\u0026thinsp;\u0026plusmn;\u0026thinsp;0.009 mg/VS\u003csub\u003eadd\u003c/sub\u003e, respectively, indicating that THP further promoted the rapid decomposition of VFAs and thus accelerated methane production (Johnravindar et al. \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e). Additionally, the VFA composition revealed that acetic acid was the predominant component in all groups during the early phases, while propionic acid exhibited a higher proportion throughout the fermentation process. These findings support the notion that methanogenesis was enhanced in the initial stages and subsequently suppressed in the later stages of the process (Kaur et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\n \u003ch2\u003e3.5. Methane production\u003c/h2\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e illustrates the daily and cumulative methane production observed throughout the co-digestion process. As depicted in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ea, the early phases of anaerobic co-digestion displayed a notable increase in methane production from thermally pretreated FW, attributed to the enhanced hydrolysis of complex organic compounds within the FW. Initially, methane production in the co-digestion system exhibited growth, irrespective of the substrate being FW. However, there was variation in the peak values of daily methane production across different groups. The highest daily methane production occurred on the second and ninth days, recorded at 24.83\u0026thinsp;\u0026plusmn;\u0026thinsp;1.6 mL/gVS.d (120\u0026deg;C FW/WAS), while the lowest was 19.82\u0026thinsp;\u0026plusmn;\u0026thinsp;1.8 mL/gVS.d (control). These results indicate that THP treatment has the potential to enhance daily methane production in the co-digestion of FW and sludge. The most significant increase in methane production was observed with the 120\u0026deg;C FW/WAS THP-treated FW/sludge. This is because FW positively affects the hydrolysis of organic matter, resulting in rapid hydrolysis acidification and a relatively low pH of the slurry, which hinders the growth of methanogenic archaea. Daily methane production in these treatments increased significantly with the gradual consumption of the accumulated acids. The higher temperature of 140\u0026deg;C during pretreatment led to a significant decline in daily methane yield on the ninth day, likely due to the formation of undesirable substances that hindered the activities of methanogens (Ariunbaatar et al. \u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e).\u003c/p\u003e\n \u003cp\u003eThe specific cumulative methane yields of each THP and untreated FW are shown in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003eb. All pretreated FW samples achieved higher volumetric methane production than the control. The highest volumetric methane production was obtained for FW treated at 120\u0026deg;C. Furthermore, the impact of THP pretreatment of FW was particularly pronounced in terms of methane production. This effect primarily stemmed from the enhanced solubilization of FW and the liberation of substantial quantities of soluble organic material, as detailed in Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e. The THP process has the potential to augment the solubilization and anaerobic biodegradability of the FW/WAS mixture. Additionally, the AD of FW is thought to disrupt the physical structure of the organic solids. Consequently, methane production rates can be elevated through anaerobic co-digestion (Park et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e). The cumulative methane production (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003eb) showed a threshold value for an increase in methane production. Indeed, it increased with the THP temperature until 120\u0026deg;C, from 324.3\u0026thinsp;\u0026plusmn;\u0026thinsp;4.5 mL CH\u003csub\u003e4\u003c/sub\u003e/gVSadd for the untreated FW/WAS to 228.8\u0026thinsp;\u0026plusmn;\u0026thinsp;1.13 mL CH4/gVSadd for the FW/WAS pretreated at 120\u0026deg;C. The increase was statistically significant at p\u0026thinsp;\u0026lt;\u0026thinsp;0.05. Thus, among the cumulative methane productions, the optimum THP temperature was found to be 120℃. Moreover, at 140\u0026deg;C, significant solubilization of particulate organic matter occurred, but the FW biodegradability was inhibited (324.3\u0026thinsp;\u0026plusmn;\u0026thinsp;4.5 and 267.4\u0026thinsp;\u0026plusmn;\u0026thinsp;1.46 mL/gVS\u003csub\u003eadd\u003c/sub\u003e). It was determined that, at 140\u0026deg;C, carbohydrates in the soluble phase react with other components to form products that are slowly or hardly biodegradable (Li et al. \u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e; Park et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e). Kim et al. (\u003cspan class=\"CitationRef\"\u003e2015\u003c/span\u003e) suggested that a changed chemical structure due to the \u0026quot;burn sugar\u0026quot; reaction and Maillard reactions occurs at high pretreatment temperatures, as evidenced by the FT-IR spectrum (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ed). The brown color of the soluble phase of the FW at 140\u0026deg;C confirmed the presence of new compounds such as Amadori compounds and melanoidins, with final pH values recorded at 7.2\u0026ndash;7.4. The Gompertz model was applied to the methane production results to estimate the co-digestion parameters. The parameters were estimated from the best fit to the experimental data (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e and Appendix Table \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e). This advantageous phenomenon is attributed to a notable decrease in lignin content under these conditions, leading to a heightened hemicellulose concentration. Hemicellulose is more readily hydrolyzed into sugar monomers such as glucose, mannose, and galactose compared to cellulose. These monomeric substrates are more easily biodegraded via methanogenesis, thereby facilitating increased methane production, as highlighted by Parra-Orobio et al. (\u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e). The rise in VFA concentrations in the initial stages of the AD process is conducive to methanogen growth, consequently boosting methane production and shortening the lag phase (\u003cem\u003e\u0026lambda;\u003c/em\u003e). This suggests that excessive temperatures can lead to the generation of harmful compounds within the system, disrupting the normal functioning of the digestion process, aligning with the findings of Wang et al. (\u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\n \u003ch2\u003e3.6. Carbon Balance Analysis\u003c/h2\u003e\n \u003cp\u003eTHP pretreatment can significantly affect the carbon balance during the co-digestion process. Carbon balance refers to the distribution and transformation of carbon-containing compounds during co-digestion. During anaerobic co-digestion, the carbon in the volatile solids (VS) is converted to VFAs, soluble carbohydrates, soluble proteins, lactate, alcohol, methane, and other products (Table \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e). The carbon balance calculation was based on the maximum VFA production. Appendix S1 shows that most of the THPs stood at 30.9%, 40.07%, and 23.8% when subjected to temperatures of 90\u0026deg;C, 120\u0026deg;C, and 140\u0026deg;C, respectively. This result indicates that pretreatments could promote VFA production. However, the contents of the residual VS post-anaerobic co-digestion remained over 50% across all three treatments. Residual VS consists of proteins, lipids, and sugars. The carbon balance of the fermentation process was evaluated using the experimental data to assess carbon conversion efficiency. The carbon content of dry VS, VFAs, proteins, and carbohydrates was quantified. Other carbon-containing compounds, such as CO\u003csub\u003e2\u003c/sub\u003e, CH\u003csub\u003e4,\u003c/sub\u003e and other organic substances in soluble chemical oxygen demand (sCOD), were collectively categorized as \u0026quot;others.\u0026quot; Following the co-digestion process, approximately 58% of the carbon present in the VS was converted. Notably, the carbon content of the VFAs was most prominent at 140\u0026deg;C, accounting for 40.3% of the total, with a lower proportion of other carbon compounds.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable id=\"Tab3\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eOverall performance of the co-digestion systems at various hydrothermal pre-treatment.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"5\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003eParameters\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colspan=\"3\"\u003e\n \u003cp\u003ePre-treatment Condition\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003eUntreated\u003c/p\u003e\n \u003cp\u003eControl co-digestion\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e90℃ FW\u0026thinsp;+\u0026thinsp;WAS\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e120℃ FW\u0026thinsp;+\u0026thinsp;WAS\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e140℃ FW\u0026thinsp;+\u0026thinsp;WAS\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eVS reduction (%)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e55.224\u0026thinsp;\u0026plusmn;\u0026thinsp;0.6\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e57.98\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e53.205\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e47.78\u0026thinsp;\u0026plusmn;\u0026thinsp;0.74\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003eCumulative CH\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e4\u003c/strong\u003e\u003c/sub\u003e \u003cstrong\u003eproduction (mL/gVS\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003eadded\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e307.13\u0026thinsp;\u0026plusmn;\u0026thinsp;3.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e324.39\u0026thinsp;\u0026plusmn;\u0026thinsp;4.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e267.43\u0026thinsp;\u0026plusmn;\u0026thinsp;1.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e228.83\u0026thinsp;\u0026plusmn;\u0026thinsp;1.13\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003esCOD\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003emax\u003c/strong\u003e\u003c/sub\u003e \u003cstrong\u003e(mg/gVS)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1537\u0026thinsp;\u0026plusmn;\u0026thinsp;3.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1557.7\u0026thinsp;\u0026plusmn;\u0026thinsp;9.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1680\u0026thinsp;\u0026plusmn;\u0026thinsp;8.10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1660.6\u0026thinsp;\u0026plusmn;\u0026thinsp;4.1\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003eIncrease rate of methane yield compared to no\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003epre-treatment AD process (%)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e34.21\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e41.75\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e16.86\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e--\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec17\" class=\"Section2\"\u003e\n \u003ch2\u003e3.7. Microbial Community Composition of Various HTP Pretreatments during Anaerobic Co-Digestion\u003c/h2\u003e\n \u003cp\u003eThe alpha diversity indices, including ACE, Chao1, operational taxonomic units (OTUs), Shannon, and Simpson (Appendix Table S2), indicated that the ecological roles of the bacterial community during the methanogenesis stages were strengthened at 120\u0026deg;C and 90\u0026deg;C. This is supported by the significantly higher alpha values of THP at these temperatures compared to the control (Zhou et al. \u003cspan class=\"CitationRef\"\u003e2024\u003c/span\u003e). Figure \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003ea illustrates the similarity in bacterial phyla composition within the community structure, although notable differences in their relative abundances exist. The dominant bacterial phyla observed were Firmicutes, Bacteroidetes, Halobacterota, Euryarchaeota, Actinobacteria, and Synergistota, collectively accounting for more than 92.53% of the bacterial content across all samples. Specifically, the control group exhibited relative abundances of 45.074% for Firmicutes, 26.40% for Halobacterota, 17.13% for Bacteroidetes, and 2.68% for Euryarchaeota. However, upon the addition of THP FW, there was a significant increase in the relative abundance of Firmicutes from 48.13\u0026ndash;71.43%. Similarly, the relative abundances of Halobacterota, Bacteroidetes, and Euryarchaeota increased from 7.35\u0026ndash;13.85%, 9.35\u0026ndash;22%, and 1.77\u0026ndash;2.41%, respectively. Notably, the relative abundance of Firmicutes decreased from 48.13\u0026ndash;45.72% at 140\u0026deg;C in FW/WAS. The presence of methanogens in Halobacterota and Euryarchaeota significantly increased following the inoculation of HTP. Bacteroidetes, known as acid-forming bacteria, exhibited the ability to convert various substances into butyric, acetic, isovaleric acids, hydrogen, and carbon dioxide. An increase in the abundance of Bacteroidetes was observed in the 90\u0026deg;C FW/WAS (22.18%), 140\u0026deg;C FW/WAS (19.75%), and 120\u0026deg;C FW/WAS (17.13%). The findings from the bacterial community analysis align with the VFA and sCOD results, indicating a correlation between VFA generation and Bacteroidetes. Acetogenic bacteria play a crucial role in methane production during AD by converting organic acids into acetate and hydrogen. Hydrolytic bacteria, such as Clostridium, Bacteroides, and Cellulomonas, are responsible for the initial breakdown of complex organic compounds into simpler molecules through enzyme secretion. The increase during THP 120\u0026deg;C FW/WAS resulted in an enhanced number of hydrolyzing bacteria and acid-producing microorganisms, leading to faster acidification and hydrolysis. Bacteroidetes, known as proteolytic bacteria, degrade proteins into VFAs and ammonium nitrogen (NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N) (Liu et al. \u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e). The high soluble protein concentration in the 120\u0026deg;C FW/WAS sample resulted in a significantly higher relative abundance.\u003c/p\u003e\n \u003cp\u003eFurther investigations of bacterial communities were performed at a general level (Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003eb). The major genera detected in the THP FW/WAS digestate samples showed increased relative abundance. \u003cem\u003ePetrimonas\u003c/em\u003e (27.46\u0026ndash;38.38%), \u003cem\u003eProteiniphilum\u003c/em\u003e (16.8\u0026ndash;25.04%), \u003cem\u003eBacteroidetes_vadin\u003c/em\u003e HA17 (0.97\u0026ndash;2.35%), \u003cem\u003eMacellibacteroides\u003c/em\u003e (2.22\u0026ndash;2.69%), and unclassified genera (12.03\u0026ndash;24.34%) are proteolytic bacteria capable of metabolizing carbohydrates to produce VFAs. In the THP for FW/WAS at 120\u0026deg;C, the hydrogen-producing bacteria \u003cem\u003ePetrimonas\u003c/em\u003e were enriched (38.38%), potentially improving carbon recovery from WAS. \u003cem\u003eProteiniphilum\u003c/em\u003e, belonging to the phylum Bacteroidetes, exhibited significantly higher relative abundance. Notably, \u003cem\u003eProteiniphilum\u003c/em\u003e was present at a considerable abundance of 35.65% in the control (untreated FW). This species accelerates propionate degradation by utilizing pyruvate, an intermediate in the propionate degradation pathway (Jiang et al. \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e). Bacteroides predominated in the THP-pretreated digestate (1.68%) and control systems (0.6%).\u003c/p\u003e\n \u003cp\u003eThe dominance of methanogenic archaea characterizes the archaeal microbial communities in anaerobic co-digestion systems, specifically those involving FW/WAS and THP pretreatment. Methanogens play a pivotal role in methane production (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003ec). The specific composition of these archaeal communities exhibits variability, contingent upon factors such as the feedstock, operating conditions, and pretreatment methodology employed (Jiang et al. \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e). It is noteworthy that methanogens, being obligate anaerobic microorganisms, exhibit greater vulnerability compared to hydrolytic or acidogenic bacteria. Among the various genera of methanogens, \u003cem\u003eMethanobrevibacter\u003c/em\u003e, \u003cem\u003eMethanosarcina\u003c/em\u003e, \u003cem\u003eMethanobacterium\u003c/em\u003e, \u003cem\u003eMethanocorpusculum\u003c/em\u003e, and \u003cem\u003eMethanosaeta\u003c/em\u003e assume particular significance in the context of co-digestion. In an investigation involving FW/sludge samples from anaerobic co-digestion reactors, the relative abundance of \u003cem\u003eMethanosarcina\u003c/em\u003e in the digestate samples exhibited an increase with higher THP temperatures, whereas the abundance of \u003cem\u003eMethanosaeta\u003c/em\u003e displayed a decrease. This shift in abundance can be attributed to the availability of substrates. Specifically, \u003cem\u003eMethanosarcina\u003c/em\u003e and \u003cem\u003eMethanosaeta\u003c/em\u003e were seen to compete for acetate and free ammonia, respectively. Further, the abundance of hydrogen, carbon dioxide, and formate-utilizing \u003cem\u003eMethanobrevibacter\u003c/em\u003e was observed to be low during the treatments (ranging from 0.39\u0026ndash;2.67%), whereas it was relatively high in the raw sludge inoculum (41.18%). On day 0, \u003cem\u003eMethanosarcina\u003c/em\u003e dominated, accounting for 16.42%, 41.15%, 30.25%, 21.02%, and 15.53% of the inoculum in 90\u0026deg;C FW/WAS, 120\u0026deg;C FW/WAS, 140\u0026deg;C FW/WAS, and control, respectively. During AD, the abundance of \u003cem\u003eMethanosarcina\u003c/em\u003e increased significantly, reaching final abundances of 164.97%, 94.78%, and 35.41%, respectively. \u003cem\u003eMethanosarcina\u003c/em\u003e exhibited significant growth during the active methane-generation phase across all treatments. Competitive growth occurred between \u003cem\u003eMethanosarcina\u003c/em\u003e and \u003cem\u003eMethanosaeta\u003c/em\u003e, resulting in a decrease in the latter and an increase in the former. The control group consisted of \u003cem\u003eMethanocorpusculum\u003c/em\u003e (60.8%), \u003cem\u003eMethanosarcina\u003c/em\u003e (15.5%), \u003cem\u003eMethanofollis\u003c/em\u003e (10.53%), and \u003cem\u003eMethanobactrium\u003c/em\u003e (5.65%). With regard to 90\u0026deg;C FW/WAS, 120\u0026deg;C FW/WAS, and 140\u0026deg;C FW/WAS, \u003cem\u003eMethanosarcina\u003c/em\u003e was the most dominant methanogen, ranging from 21.02\u0026ndash;41.15%, followed by \u003cem\u003eMethanobacterium\u003c/em\u003e (15.80\u0026ndash;38.48%). \u003cem\u003eMethanosarcina\u003c/em\u003e, unlike other methanogens, has a rapid growth rate and is more tolerant of abrupt pH shifts of about 0.8\u0026ndash;1.0 units (Johnravindar et al. \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e). The relative abundance of \u003cem\u003eMethanosarcina\u003c/em\u003e in digestate samples increases with rising THP temperature, possibly due to the availability of specific substrates. Conversely, \u003cem\u003eMethanosaeta\u003c/em\u003e experiences a decrease in relative abundance. \u003cem\u003eMethanobrevibacter\u003c/em\u003e, utilizing hydrogen, carbon dioxide, and formate, exhibits a significantly higher prevalence across THP conditions compared to the control (Shao et al. 2024).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec18\" class=\"Section2\"\u003e\n \u003ch2\u003e3.9. Comparison of energy yield with comparable studies\u003c/h2\u003e\n \u003cp\u003eThis study calculated the overall energy production using the methane yield and its calorific value of 11.48 MJ/KgVS. The energy conversion efficiency (ECE) of methane varied from 16.97\u0026ndash;49.47%. High methane yields corresponded to a THP for 120\u0026deg;C FW/WAS. Evidently, the overall ECEs were significantly impacted by the thermal hydrolysis temperature. The enhanced ECE of the treated FW increased to 49.47% when the hydrothermal temperature was elevated to 120\u0026deg;C, surpassing that of untreated FW. However, a negative correlation was observed between the overall ECEs and hydrothermal temperatures greater than 140\u0026deg;C. The overall ECE of treated FW dropped to 16% when the temperature reached 140\u0026deg;C. Additionally, energy consumption primarily comprised mechanical crushing throughout the entire cycle, THP, and operation (heating). Mechanical crushing and thermal hydrolysis accounted for the maximum energy usage.\u003c/p\u003e\n \u003cp\u003eThe ECE for methane production stood at 324.39 mL/gVS, which constituted 41.75% of the overall ECE of 49.47%. However, experimental results obtained from various studies have shown significant variations due to the considerable differences between the sources and components of food waste and waste lipids. Furthermore, the ECE can reflect the relative degree of substrate conversion. In this study, through hydrothermal pretreatment and co-digestion using methane, the overall energy conversion efficiency was increased to 41.75%, which improved methane yield and shortened the time duration for the same. In comparison, the influence of hydrothermal retention time on overall ECEs was deemed to be moderately significant. Based on the collective findings of the study, the optimal operational parameters for co-digestion of FW/WAS were identified to be 120\u0026deg;C and 60 min. Table \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e shows that the extra energy produced by applying pretreatment methods ranges around 11.48 MJ/KgVS FW.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable id=\"Tab4\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eComparison of cumulative methane production and energy yield from pre-treated food waste.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"6\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eFW origin\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003ePre-treatment\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eDigestion type\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eEnergy yield (MJ/kgVS)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e*Increase in energy (ECF) yield (%)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eReferences\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eFood waste (waste process facility in Canada)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eUltrasonic and acid pretreatment (sonicated at 79 kJ/gTS and pH at 3.0 with 1 N HCl for 24 h)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eOne-stage batch dark H\u003csub\u003e2\u003c/sub\u003e fermentation\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e18.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eElbeshbishy et al. (\u003cspan class=\"CitationRef\"\u003e2011\u003c/span\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eFood waste: sewage sludge, ratio 2:1 (TS based)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eThermophilic conditions (56℃)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eBatch anaerobic digestion\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e7.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e--\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSosnowski et al. (\u003cspan class=\"CitationRef\"\u003e2003\u003c/span\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eFood waste (institute cafeteria in Korea)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eAlkali pretreatment (pH at 12.0 with 6 N KOH for 6 h)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSingle-stage batch dark H\u003csub\u003e2\u003c/sub\u003e fermentation\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e--\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eJang et al. (\u003cspan class=\"CitationRef\"\u003e2015\u003c/span\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eFood waste (University canteen in Singapore)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eEnzymatic pretreatment using fungal mash (0.2 g/gTS-FW)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSingle-stage batch anaerobic digestion\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e29.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e33.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eYin et al. (\u003cspan class=\"CitationRef\"\u003e2016\u003c/span\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSynthetic food waste (based on the average composition of FW in some European countries)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eThermal pretreatment (80℃, 90 mins)\u003c/p\u003e\n \u003cp\u003eOzonation pretreatment\u003c/p\u003e\n \u003cp\u003e(0.068 gO3/gTS)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSingle-stage batch anaerobic digestion\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e23.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e52.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eAriunbaatar et al. (\u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ePelagic\u0026nbsp;Sargassum/FW co-digestion\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eHydrothermal pre-treatment 140℃ and 30 min\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSingle-stage batch AD\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e21.57\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e51.07\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eThompson et al. (\u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eStrawberry extrudate and Sewage sludge\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eHydrothermal 120\u0026deg;C, 15 min\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSingle-stage batch anaerobic co-digestion\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e--\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e31\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSerrano et al. (\u003cspan class=\"CitationRef\"\u003e2023\u003c/span\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eFood waste\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eThermal pretreatment (100℃, 30mins)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSingle stage continuously studies AD (different OLR)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e13.75\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e23.68\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eGnaouia, et al., 2020\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eFood waste: sludge (stimulated FW)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eHydrothermal pre-treatment 120℃ and 60 min\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSingle-stage batch anaerobic co-digestion\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e11.48\u003c/p\u003e\n \u003cp\u003eControl-7.68\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e49.47\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eThis study\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cp\u003eSludge dewatering affects digestate yield and transportation costs. Dewaterability increased in reactors at 90\u0026deg;C, 120\u0026deg;C, and 140\u0026deg;C, as well as in the control in Appendix S2. The initial capillary suction time (CST) of the raw FW-SS mixture was 420.3 seconds. After HTP treatment, the CST of the FW/WAS blend demonstrated a notable reduction to 365.6, 325.76, and 225.7 seconds at 90\u0026deg;C, 120\u0026deg;C, and 140\u0026deg;C, respectively. This reduction in CST can be attributed to the degradation of cell walls in the WAS and the subsequent alteration of the physical structure of FW, resulting in increased difficulty in dewatering (Zhang et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e). Cao et al. (\u003cspan class=\"CitationRef\"\u003e2023\u003c/span\u003e) found that dewaterability is linked to the degradation of organic matter. However, a reduction in sludge concentration and viscosity decreases dewaterability. The sludge network structure is disrupted, and extracellular polymeric substances (EPS) are dissolved through THP, thereby releasing bound water and improving dewaterability. THP effectively enhances the dewaterability of digestate post-co-digestion using FW and WAS.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eThe study's conclusions highlight the effectiveness of THP in enhancing the co-digestion performance of FW. The findings indicate that the solubilization of sCOD, VFAs, and organic components such as carbohydrates and proteins increased with rising temperatures, achieving peak values of 87.94%, 64.45%, 64.54%, and 34.64% at treatment temperatures of 90\u0026deg;C, 120\u0026deg;C, and 140\u0026deg;C for 60 minutes. Notably, THP conducted at 120\u0026deg;C for 60 minutes proved highly efficient in improving the solubilization of organic constituents, resulting in a substantial rise in methane generation during the co-digestion process of FW and sludge. Furthermore, the microbial community analysis revealed that THP at 120\u0026deg;C for the FW/WAS mixture notably elevated the proportions of acetogenic bacteria and methanogenic organisms. The study also observed that the highest energy conversion factor (ECF) output was obtained for THP at 120\u0026deg;C for FW/WAS, recording an output of 11.48 MJ per ton of fed VS, representing a 49.47% increase compared to the control condition. These results collectively demonstrate the potential of THP as a viable pretreatment method for enhancing the efficiency and performance of FW co-digestion processes, with significant improvements observed in methane generation and energy output. Further research could explore the applicability of these conditions to others organic waste types including agricultural residues or mixed municipal waste, to assess their methane production potential and scalability. Future studies should evaluate the long- term stability and performance of anaerobic co-digestion systems.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAuthors Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eDavidraj Johnravindar: Investigation, Methodology, Writing – original draft. Manu Mathikere Krishnegowda: Investigation, Jun Zhao: Conceptualization, Data curation, Funding acquisition, Jonathan Wong\u0026nbsp;Woon Chung: Project administration, Supervision, Writing – review \u0026amp; editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgment/Funding:\u0026nbsp;\u003c/strong\u003eThis work was supported by the Environment and Conservation Fund, Hong Kong, (Grant No ECF Project 46/2020, 09/2021).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability:\u0026nbsp;\u003c/strong\u003eAvailability of data and materials will be available upon request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical approval\u003c/strong\u003e: Not applicable.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to participate\u003c/strong\u003e: Not applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication:\u003c/strong\u003e Not applicable.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests:\u003c/strong\u003e The authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eAierzhati A, Stablein MJ, Wu NE, Kuo CT, Si B, Kang X, Zhang Y (2019). 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Sci Total Environ. 15: 921, 171175. doi: 10.1016/j.scitotenv.2024.171175.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"environmental-science-and-pollution-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"espr","sideBox":"Learn more about [Environmental Science and Pollution Research](https://www.springer.com/journal/11356)","snPcode":"11356","submissionUrl":"https://submission.nature.com/new-submission/11356/3","title":"Environmental Science and Pollution Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Food waste, Hydrothermal pretreatment, Co-digestion, Energy conversion efficiency, Economic analysis Sewage sludge","lastPublishedDoi":"10.21203/rs.3.rs-5087495/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5087495/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eFood waste (FW) presents a significant opportunity for renewable energy production through anaerobic digestion (AD) when subjected to appropriate treatment. This study investigates the impact of thermal hydrolysis pretreatment (THP) on FW at varying temperature levels (90°C, 120°C, and 140°C) prior to mesophilic anaerobic co-digestion with sewage sludge (SS). Results demonstrate enhanced FW hydrolysis at 120°C, leading to a cumulative methane yield of 324.39 ± 4.5 mL/gVS\u003csub\u003eadd\u003c/sub\u003e, representing a 41.75% increase over untreated FW (228.83 ± 1.13 mL/gVS\u003csub\u003eadd\u003c/sub\u003e). Shifts in microbial communities, particularly \u003cem\u003eMethanosarcina\u003c/em\u003e, \u003cem\u003eMethanobactrium\u003c/em\u003e, and \u003cem\u003eMethanobrevibacter\u003c/em\u003e, support efficient methanogenesis. Co-digestion of FW pretreated at 120°C yields maximum energy production of 11.48 MJ/t, a 49.47% improvement compared to untreated processes. The economic analysis underscores the profitability of co-digestion with FW pretreated at 120°C. These findings highlight the potential for enhanced methane production and energy conversion efficiency with hydrothermally pretreated FW and SS co-digestion.\u003c/p\u003e","manuscriptTitle":"Hydrothermal Pretreatment of Food Waste Enhances Performance of Anaerobic Co-digestion With Sludge","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-12-23 10:43:18","doi":"10.21203/rs.3.rs-5087495/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Accept","date":"2025-01-12T09:57:17+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2024-12-20T16:19:22+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-12-20T13:10:17+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Environmental Science and Pollution Research","date":"2024-12-19T04:26:12+00:00","index":"","fulltext":""},{"type":"submitted","content":"Environmental Science and Pollution Research","date":"2024-12-17T04:43:53+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"environmental-science-and-pollution-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"espr","sideBox":"Learn more about [Environmental Science and Pollution Research](https://www.springer.com/journal/11356)","snPcode":"11356","submissionUrl":"https://submission.nature.com/new-submission/11356/3","title":"Environmental Science and Pollution Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"fe31631a-c8f4-47b8-9b36-93453c4d0981","owner":[],"postedDate":"December 23rd, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-02-10T16:01:14+00:00","versionOfRecord":{"articleIdentity":"rs-5087495","link":"https://doi.org/10.1007/s11356-025-35944-0","journal":{"identity":"environmental-science-and-pollution-research","isVorOnly":false,"title":"Environmental Science and Pollution Research"},"publishedOn":"2025-02-05 15:57:20","publishedOnDateReadable":"February 5th, 2025"},"versionCreatedAt":"2024-12-23 10:43:18","video":"","vorDoi":"10.1007/s11356-025-35944-0","vorDoiUrl":"https://doi.org/10.1007/s11356-025-35944-0","workflowStages":[]},"version":"v1","identity":"rs-5087495","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5087495","identity":"rs-5087495","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}