Effect of medium temperature-activated potassium persulfate pretreatment on Sludge Anaerobic Digestion Efficiency and Substance Transformation Processes

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Thermally activated persulfate pretreatment has been used to promote the decomposition of sludge. However, the optimal activation temperature for persulfate-enhanced anaerobic digestion remains undetermined. This study investigated the effects of medium temperature-activated potassium persulfate (K₂S₂O₈) on the decomposition of waste activated sludge (WAS) and the performance of methane production. Pretreatment was conducted at three activation temperatures of 35°C, 55°C, and 75°C, with a K₂S₂O₈ dosage of 0.43 g/g TSS for 1.5 hours. Results demonstrated that K₂S₂O₈ pretreatment significantly improved sludge hydrolysis rates and anaerobic digestion efficiencies of WAS. The soluble organic matter (SCOD) concentrations in WAS after pretreatment were 1,371.52–2,198.02 mg/L, much higher than the control(521.32mg/L), and with optimal performance observed at 75°C. The methane production potential of pretreated groups increased by 67.4%, 94.6%, and 76.5% compared to the control, respectively, with the highest yield at 55 ° C. However, K 2 S 2 O 8 pretreatment delayed methanogenesis initiation, as indicated by the Gompertz model fitting result (lag phase λ: 1.21–2.60 d in treatment groups vs. <0.1 d in the control). Although pretreatment also enhanced organic matter release during the anaerobic digestion stage, a considerable portion of dissolved organics remained unconverted to methane. Therefore, the impact mechanism of K 2 S 2 O 8 on anaerobic digestion needs further research. This study confirmed that medium temperature-activated K₂S₂O₈ pretreatment was effective for anaerobic digestion of WAS and more conducive to the stability of AD system. Waste activated sludge Anaerobic digestion Pretreatment Potassium persulfate Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1. Introduction With the growth of population and the development of society, the volume of wastewater generated by human activities continues to increase, resulting in a large amount of waste activated sludge (WAS) that needs to be properly treated. According to statistics, China's annual sludge production (80% water content) has exceeded 50 million tons, and the global sludge production will reach 103 million tons in 2025 [ 1 ]. Due to land constraints, the traditional disposal method of landfilling after dewatering is gradually being prohibited. In contrast, anaerobic digestion (AD) has been extensively studied as a sustainable alternative method for the resource and energy recovery from WAS. The microbial cell membranes and extracellular polymeric substances (EPS) in sludge generally form a dense barrier that hinders the decomposition of sludge, reducing the release and utilization of organic matter in WAS. Moreover, WAS contains a lot of organic compounds such as lignin, cellulose, and hemicellulose, which have stable structures and are difficult for microorganisms to utilize. These two factors have become the main limiting factors for WAS anaerobic digestion [ 2 ]. In response to these issues, various pretreatment technologies have been developed to promote the disintegration of sludge, including physical, chemical, biological, and combined methods[ 2 ]. Among these, advanced oxidation pretreatment, a chemical method that generates reactive oxidative radicals to promote sludge hydrolysis, has attracted significant attention due to its stable oxidation properties. Common oxidizing agents include Fenton reagents, CaO₂, and persulfates [ 3 – 5 ]. Although Fenton reagents have been widely applied in sludge treatment [ 6 ], their further application is constrained by strict pH requirements (pH 2–4) and the instability of H₂O₂ [ 7 ]. In contrast, CaO₂ and persulfate has been applied to treatment solid waste because of its strong stability and oxidation ability [ 8 ]. Persulfate-generated sulfate radicals (SO₄⁻·) exhibit higher redox potential (E₀ = 2.65–3.10 V) and longer half-life (3–4×10⁻⁵ s) than hydroxyl radicals (·OH, E₀ = 2.4–2.8 V, half-life 10⁻⁹ s), indicating stronger penetration capability through sludge cellular structures [ 9 ]. This indicates that persulfate pretreatment is more conducive to the hydrolysis of sludge and provides more substrates for anaerobic digestion. Research on persulfate applications in sludge treatment has been increasingly pursued. Liu et al. found that thermal activation of persulfate could effectively generate various free radicals (·OH and SO₄⁻·), increasing the release of short-chain fatty acids(SCFA) by 54.8%[ 10 ]. Hu et al. achieved a 53.6% enhancement in methane production using a combination of zero-valent iron foil (ZVI) and persulfate for sludge anaerobic digestion [ 6 ]. Sun et al. found that the methane production of WAS increased from 61.58 mL/g VSS to 89.24 mL/g VSS after persulfate pretreatment[ 11 ]. Persulfate has a stable structure and needs to be activated to produce SO 4 − ·. Common activation methods include thermal activation and metal activation [ 9 ]. However, metal-based methods face challenges in metal recovery and secondary pollution risks, while thermal activation demands high energy consumption. Previous studies typically employed thermal activation temperatures of 75–90°C [ 9 ]. But some studies have found that these high activation temperatures are detrimental to anaerobic digestion. Yin et al. observed that although persulfate activation at 80°C significantly increased SCFA production in sludge, methane yield was even lower than the control group due to strong inhibition of methanogens by thermal treatment[ 12 ]. Sun et al. revealed that the soluble chemical oxygen demand (SCOD) of pretreated sludge was 6.02 times higher than the control after thermally activated persulfate pretreatment at 90°C, but the increase in methane production was not significant, indicating most organic matter remained unutilized for methanogenesis[ 11 ]. At present, the optimal activation temperature for persulfate-enhanced anaerobic digestion of WAS remains undetermined. This study investigated lower activation temperatures (35–75°C) of potassium persulfate to identify the most suitable thermal activation condition for WAS anaerobic digestion. The effect of potassium persulfate on substance conversion during anaerobic digestion under this condition was also analyzed. 2. Materials and methods 2.1. Experimental materials The WAS was taken from the second sedimentation tank of Shishan Wastewater Treatment Plant in Suzhou City, China, which was retrieved and then placed at 4°C for 24 h to remove the supernatant. The inoculated sludge was taken from the anaerobic fermentation tank of Everbright Environmental Energy (Suzhou) Co. The characteristics of the raw materials were shown in Table 1 . K₂S₂O₈ was purchased from Shanghai Lingfeng Chemical Reagent Co. Table 1 Properties of WAS and inoculated sludge WAS inoculated sludge TSS 20.96±0.30g/L 0.111±0.008g/g VSS 11.62±0.30g/L 0.052±0.005g/g pH 6.80±0.07 7.60±0.16 2.2. Pretreatment test WAS was pretreated with K₂S₂O₈ using a 1000 ml serum bottle. The addition amount of K₂S₂O₈ and the reaction time of pretreatment were referred to the study of Balcioglu et al [13]. CG was the control group without K₂S₂O₈ addition, and the experimental conditions are shown in Table 2 . Samples were taken every 0.5 h from each group to analyze indicators such as SCOD, soluble protein, and soluble polysaccharides. After pre-processing, take a portion of the sample and dry it before crushing it, passed through a 100-mesh sieve, 2 mg of the sample and 200 mg of potassium bromide(KBr) were taken and subjected to Fourier transform infrared spectroscopy analysis. Table 2 Test design of thermally activated K₂S₂O₈ pretreatment Group Addition amount Temperature(℃) Time(h) CG 0 25 1.5 K1 0.43g/g TSS K₂S₂O₈ 35 1.5 K2 0.43g/g TSS K₂S₂O₈ 55 1.5 K3 0.43g/g TSS K₂S₂O₈ 75 1.5 2.3. Anaerobic digestion test The anaerobic digestion test was carried out on the pretreated sludge. It included three groups (K1, K2, K3), each of which continued from the previous pretreatment experiment. The control group CG was set for the three groups. Parallel groups were set up for each experiment group. A 500 ml (effective volume 450 ml) serum bottle was used as the anaerobic digestion reactor. WAS and the inoculated sludge were mixed in the ratio of 1:2 (VSS/VSS), then the pH of the mixture was adjusted to 7.0±0.1 with 4 mol/L HCl or NaOH. The serum bottle was sealed and purged with nitrogen for 10 min to ensure complete anaerobicity, and the anaerobic digestion was carried out in a constant-temperature water-bath shaker at 35°C for 32 days, with a rotation speed of 150 r/min. Gas volume was measured and gas composition was analyzed every 3 days. Liquid samples were taken to measure pH, VFA, NH 4 + -N, SCOD, dissolved proteins, and dissolved polysaccharides every 3 days for the first 12 d, and every five days thereafter. 2.4. Analytical methods SCOD, NH 4 + -N, TSS and VSS were determined according to Methods of Analysis for Examination of Water and Wastewater [14]. Measurement of pH value using pH meter (PHS-2F, Shanghai). Proteins and polysaccharides were determined using the Folin-phenol reagent method and the phenol-sulfuric acid colorimetric method [15, 16]. Biogas components were determined using a gas chromatograph (INSEA GC112A, China) equipped with a thermal conductivity detector. Biogas was collected in a gas bag and the volume was measured with a glass syringe (100 mL) at room temperature (about 25°C) and atmospheric pressure (about 101 kpa). The determination of VFAs adopts colorimetric method: first add acidic ethylene glycol for heating, cool down, then add hydroxylamine reagent, and then add acidic ferric chloride, after color development, measure its absorbance. All experiments were in triplicate, and the data were arithmetic mean. A kinetic model, the modified Gompertz model, was used to calculate the lag period of methane production and the maximum daily methane production under different test conditions [17, 18]. The modified Gompertz model is shown in Equation 1: where M is the cumulative methane production(mL/g VSS) on day t of digestion; M 0 is the simulated maximum methane production potential(mL/g VSS); R max is the simulated maximum daily methane production rate(mL/g VSS/d); λ is the simulated methane production lag(d); and e is the natural logarithm with the value of 2.718.The parameters(M 0 , R max , λ) in this equation were estimated using the Origin2017 nonlinear fit for estimation. Changes in chemical composition and functional groups of WAS during pretreatment were determined using Fourier transform infrared spectroscopy (FT-IR, Nicolet Is50). Refer to the method of huang et al., infrared (IR) spectra were recorded on a Bruker TENSOR II spectrometer using an attenuated total reflection (ATR) device. About 2 mg grinded powders were put on the crystal device and the contact was obtained by applying a strength of about 150 N on the sample. Each spectrum was subjected to 16 scans with the resolution of 4 cm −1 from 4000 to 400 cm −1 region[8]. 3. Results and discussion 3.1. Effect of pretreatment on sludge decomposition 3.1.1. Release of dissolved organic matter in sludge The concentration of SCOD in sludge supernatant can indicate the degree of sludge disintegration. Results showed that the addition of K₂S₂O₈ significantly increased the SCOD concentration in treatment groups. As shown in Fig. 1 , the concentration of SCOD in CG group remained nearly unchanged within 1.5 h, while those in K1-K3 groups all exhibited an upward trend, reaching the final values of 1372 mg/L, 2004 mg/L, and 2198 mg/L, respectively. After K₂S₂O₈ pretreatment, the SCOD concentration of K1-K3 group increased by 163.1%, 284.5%, and 321.6%, respectively, compared to the control. The highest sludge disintegration and organic matter release were achieved at the activation temperature of 75°C. Previous studies suggested that higher activation temperatures of persulfate were conducive to the cell lysis and the release of intercellular biopolymeric substances. Sun et al. activated K₂S₂O₈ at 90 ℃, and the maximum SCOD concentration of the pretreated sludge reached 590mg/L, compared to 84 mg/L in the control group[ 11 ]. Soluble proteins and polysaccharides are important components of sludge SCOD. WAS contains large amount (50–70%) of organics, mainly including protein, carbohydrate [ 9 ]. These organic compounds will be decomposed as the sludge disintegrates, releasing soluble proteins and polysaccharides, which are the main substrates in the hydrolysis and methane production processes of sludge. Similar to the change of SCOD, the concentrations of soluble proteins and polysaccharides in the control group remained nearly unchanged, while those in the treated groups increased rapidly in 1.5h. In the K groups, soluble proteins concentrations rose most rapidly within the first 0.5 h, with slower increases observed between 0.5h and 1.5 h. Activation temperature obviously influenced the release of protein: the higher the activation temperature, the faster the protein release, and the higher the concentration of soluble proteins after K₂S₂O₈ pretreatment. However, the impact of activation temperature on soluble polysaccharide concentrations was less distinct. The concentration of soluble polysaccharides in K2 and K3 after pretreatment was similar, and both higher than that in K1. After 1.5 h, all indicators of K3 group were the highest, with the content of SCOD, soluble proteins, and polysaccharides reaching 2198.02 mg/L, 375.75 mg/L, and 451.07 mg/L, respectively. 3.1.2. Effect of pretreatment on functional groups Figure 2 presents the Fourier Transform Infrared Spectroscopy (FT-IR) analysis results of pretreated samples. The absorption peaks at 3631 − 3184 cm⁻¹ correspond to the stretching vibrations of hydroxyl (O-H) and amino (N-H) groups. O-H groups typically exist in water, alcohols, and phenols, while N-H groups are commonly found in amino acids, peptides, proteins, and other nitrogen-containing organic compounds [ 19 ]. These peaks primarily indicate protein hydrolysis in the anaerobic digestion system, reflecting the hydrolysis and acidogenesis efficiency of sludge. Compared with the CG group, the K1-K3 groups exhibited higher peak intensities in this region. This suggests that although the activity of both acidogenic and methanogenic bacteria decreased after pretreatment, the K1-K2 groups demonstrated higher overall acidogenesis efficiency due to the release of more substrates through pretreatment [ 19 ]. The peak at 1402 cm⁻¹ represents the bending vibration of -CH₂- groups, which are present in aliphatic compounds (including VFAs) [ 20 ]. Both C1 and K1 groups showed stronger peak intensities at this position compared to CG, indicating that K₂S₂O₈ pretreatment promoted VFAs production. The peak at 1115 cm⁻¹ corresponds to in-plane C-H bending vibrations, while the peak at 613 cm⁻¹ is attributed to out-of-plane C-H bending vibrations [ 21 ]. As sludge organic matter typically contains various C-H functional groups, the significantly lower peak intensities at these two positions in the K1-K3 groups compared to CG suggest that K₂S₂O₈ pretreatment enhanced organic matter degradation. 3.2. Effect of pretreatment on methane production in anaerobic digestion of sludge 3.2.1. Methane production and methane production rate Methane production is a key indicator for evaluating anaerobic digestion processes. As shown in Fig. 3 (a) , pretreatment significantly enhanced the anaerobic digestion of sludge. The methane production of the treated groups over 32 days was greatly higher than that of the control. The accumulated methane yield of K1, K2, and K3 reached 1.65, 1.92, and 1.74 times that of the control, respectively. The K2 group (with the K₂S₂O₈ activation temperature of 55°C) achieved the highest accumulated methane yield (183.2 ml/g VSS), exceeding K1 and K3 by 16.3% and 10.3%, respectively. After pretreatment at the activation temperature of 75°C, the K3 group exhibited superior sludge hydrolysis with the highest SCOD concentration. The methane production of K3 group was the highest in the first 12 days of the AD reaction, but lower than that of K2 group after 12 days due to a decrease in methane production rate in the later stage. Sun et al. reported that thermal activation at 90°C increased SCOD release of sludge by 602% compared to controls, surpassing the organic matter release effect observed in this study. However, the final methane yield only increased by 44.9% [ 11 ], significantly lower than the enhancement achieved here. These results demonstrated that medium temperature-activated potassium persulfate pretreatment was feasible and effective for improving WAS anaerobic digestion. While its solubilization efficiency was inferior to high-temperature conditions, it achieved superior enhancement of methane production. Figure 3 (b) shows the variation in methane production rates in different groups. The methane production in K1 and K2 groups started slowly, with daily methane yields of 14.2 ml/g VSS and 19.3 ml/g VSS on day 3, respectively, much lower than those of CG and K3 groups (30.7 ml/g VSS and 39.1 ml/g VSS, respectively). Between days 3–6, the daily methane production of K groups increased substantially, with peak values of K1-K3 reaching 58.5, 52.8, and 63.6 ml/g VSS respectively, maintaining high levels (≥ 33.5 ml/g VSS) until day 9. Compared to the control, K groups exhibited obvious methane production peaks and their methane production concentrated within the first 15 days. The initial methane production rates revealed the dual effects of pretreatment on anaerobic digestion processes. On the one hand, the initial methane production rates in treated groups correlated with post-pretreatment substrate concentrations, and higher pretreatment temperatures released more organic matter and resulted in higher initial methane production rates. On the other hand, only K3 showed a higher initial production rate than the control despite all pretreated groups having elevated organic matter concentrations. This suggested potential adverse effects of pretreatment on digestion initiation, possibly due to the inhibition of methanogens by residual radicals. Some studies confirmed that highly reactive chemicals could suppress methanogen activity, leading to reduced methane output [ 22 ]. Despite the K3 group having higher initial performance, the K2 group (activated at 55°C) achieved the best cumulative methane production, therefore 55°C was the optimal activation temperature for pretreatment. This conclusion contrasts with previous assessments based solely on sludge disintegration efficiency during pretreatment, indicating that the optimization of persulfate pretreatment should not rely exclusively on hydrolysis performance but must consider overall digestion outcomes. 3.2.2. Kinetic analysis of anaerobic digestion To further analyze the methane production efficiency of each group, the experimental data were fitted using the modified Gompertz model. The kinetic parameters of anaerobic digestion under different pretreatment conditions obtained from the simulation are shown in Table 3 , where M 0 (maximum methane production potential), R max (maximum daily methane production rate), and λ (methane production lag phase) represent the simulated parameters. Table 3 Kinetics of methane production based on modified Gompertz model parameter M 0 (mL/g VSS) R max (mL/g VSS/d) λ(d) R 2 CG 94.47 ± 0.31 7.10 ± 0.13 < 0.1 0.99930 K1 155.54 ± 1.00 20.13 ± 0.79 2.60 ± 0.16 0.99780 K2 182.54 ± 1.73 17.89 ± 0.84 1.91 ± 0.25 0.99623 K3 164.13 ± 0.95 22.95 ± 0.95 1.21 ± 0.16 0.99679 According to the simulation results in Table 3 , the K groups achieved significantly higher M 0 and R max values compared to CG, indicating that pretreatment effectively enhanced methane production. The K2 group with optimal performance showed an M 0 value 1.93 times that of the CG group. The CG group showed a short methane production lag phase (λ < 0.1 d), suggesting high initial activity of methanogens in the inoculated sludge. In contrast, the pretreated groups displayed λ values ranging from 1.21 d to 2.60 d, indicating a relatively delayed methane production initiation. This delay might be attributed to the influence of methanogens by residual free radicals from K 2 S 2 O 8 . Additionally, K 2 S 2 O 8 pretreatment could promote the growing of sulfate-reducing bacteria (SRBs), which compete with methanogens for substrates, thereby further reducing methanogenic activity [ 9 ]. Despite the delayed initiation, the total methane yield in the pretreated groups remained higher than the control. A similar phenomenon of methane production lag was observed by Yin et al. under the thermal activation temperature of 80°C, where methanogenesis started after 120 h without pH adjustment [ 12 ]. Notably, Tan et al. found that Fe²⁺-activated persulfate shortened the lag time of methane production [ 23 ]. Both activation methods and operational conditions were shown to influence methanogenesis dynamics. 3.3 Effect of pretreatment on material conversion in sludge anaerobic digestion process 3.3.1 Changes of soluble organic matter during anaerobic digestion Figure 4 displays the concentration variations of dissolved organic matters during the anaerobic digestion (AD) of WAS, including soluble proteins, soluble polysaccharides, and volatile fatty acid (VFA). As shown in Fig. 4 (a) , different trends of change in soluble organic matter concentrations were observed between treatment groups and the control group (CG). The CG group exhibited an overall decline in SCOD concentration, particularly after day 17, indicating the active consumption of soluble organic matter during methanogenesis. In contrast, the SCOD content of K1 and K2 groups obviously increased from day 3 to day 6, then followed by a slow increase. Notably, SCOD of K3 group greatly increased after day 17. This indicates that the production rate of soluble organic matter during anaerobic digestion was higher than its consumption rate by methanogens in these pretreated groups. At the end of the experiment, the SCOD concentration in K1 and K2 groups reached 3540.82 and 3348.50 mg/L, respectively, substantially higher than that in CG (730.9 mg/L). Both the accumulated methane production and residual SCOD concentrations in K groups were higher than the control. This phenomenon indicated that K₂S₂O₈ pretreatment also enhanced organic matter release during AD, but a significant portion of dissolved organics remained unutilized by methanogens. Lv et al. reported similar findings. In their study, SCOD of control groups maintained stable over 12 days of AD, while that of persulfate-pretreated groups exhibited continuous increases [ 24 ]. Different patterns of changes in soluble proteins are shown in Fig. 4 (b) : the content of soluble proteins in the CG group exhibited an initial increase in 0–3 days followed by continuous decrease, whereas that in K1 and K2 groups experienced significant increase in 0–3 days, then remained relatively stable, decreased after day 12, and finally slightly increased. The early-phase protein accumulation across all groups suggested its dissolution rates exceeded consumption rates. Previous studies indicated in the early stages of anaerobic digestion, the consumption of soluble carbohydrates took precedence over protein [ 25 ]. Notably, the soluble protein in the K3 group increased significantly after 17 days, which resulted in an obvious increase of SCOD in the K3 group after 17 days. The relevant research did not mention this phenomenon [ 20 , 21 ]. It might potentially link to sulfate-reducing bacteria (SRBs)-mediated H 2 S production from SO₄²⁻[ 26 ]. High H 2 S concentrations can induce microbial cell lysis, releasing intracellular proteins. The significant increase of soluble proteins in K3 in the later stage may be released by dead microorganisms. This highlights potential instability in anaerobic digester systems in high-temperature thermal activation processes. Soluble carbohydrate concentrations in all groups fluctuated throughout AD (Fig. 4 (c) ), with K1-K3 groups maintaining significantly higher levels than CG. Intriguingly, the soluble polysaccharide concentration of K3 was higher than that of K1 and K2 during days 0–9, but lower than that of K1 and K2 during days 9–27. This implied that high temperature-activated pretreatment did not substantially inhibit the utilization of soluble polysaccharides compared to mesophilic conditions. Volatile fatty acids (VFAs) content profiles (Fig. 4 (d) ) demonstrated the rapid depletion of VFAs within the first 6 days in all groups, coinciding with the peak methane production rates. This reflected the high methanogenic activity of microorganisms in the inoculum sludge, which led to the rapid conversion of VFAs into methane. After 6 days, the VFAs concentrations in all groups were at a low level. Throughout the anaerobic digestion process, the concentration of VFAs in K groups was higher than that in CG group. Throughout AD, K groups had higher initial SCOD concentrations than CG due to pretreatment-enhanced organic matter release. The concentrations of SCOD, soluble proteins, and soluble carbohydrates in K groups were obviously higher than those in the control during later stages. This suggested that K₂S₂O₈ pretreatment also promoted substantial dissolution of organic matter in AD stage, and the incomplete conversion to methane implied complex interaction mechanism requiring further investigation. 3.3.2 Changes in pH and NH 4 + -N concentration in the anaerobic digestion The pH variation trends of all groups during anaerobic digestion were similar, showing rapid initial increase followed by gradual rise, with final pH values stabilizing at 7.6–7.9 (Fig. 5 (a) ). No distinct acidogenic phase was observed in this experiment, with only slight pH decreases occurring between days 3–6 in some groups. This might be attributed to the high activity of inoculated sludge, which enabled rapid conversion and consumption of VFAs. The initial increase of pH value in all groups corresponded with VFAs reduction shown in Fig. 4 (d) . Subsequently, pH values continued rising steadily due to the microbial decomposition of organic matter and the production of alkaline substances like ammonia during anaerobic digestion [ 9 ]. Figure 5 (b) reveals that NH 4 + -N concentrations in all groups exhibited an overall upward trend throughout the digestion process. The K groups consistently showed higher NH 4 + -N levels than the CG group, indicating that more organic matter was degraded in K groups. The K₂S₂O₈ pretreatment promoted protein hydrolysis in sludge, generating more NH 4 + -N. Final NH 4 + -N concentration reached 1431.32 mg/L in CG group, compared to 1829.84 mg/L, 1594.03 mg/L, and 1529.75 mg/L in K1, K2, and K3, respectively. Overall, potassium persulfate pretreatment exerted limited influence on pH and NH 4 + -N concentration during the anaerobic digestion stage. 4. Conclusion To promote sludge decomposition and improve anaerobic digestion efficiency, potassium persulfate pretreatment of sludge was carried out in this study at a lower activation temperature of 35 ° C-75 ° C. The results indicated that the thermal activation K 2 S 2 O 8 pretreatment under medium temperature conditions was feasible and effective. Compared with the control, the content of SCOD increased by a maximum of 321.6% and the methane production potential increased by a maximum of 93.0% after pretreatment. The hydrolysis effect of sludge in the pretreatment stage is positively correlated with the activation temperature, and the content of SCOD was the highest at 75 ℃. However, the K2 group with an activation temperature of 55℃ had the highest total methane yield, which was 16.3% and 10.3% higher than the K1 and K3 groups, respectively. The analysis of the Gompertz model showed that the pretreated groups had a distinct lag-phase of methane production compared to the control. At the end of AD, the SCOD concentration in K1 and K2 groups reached 3540.82 and 3348.50 mg/L, respectively, substantially higher than that in CG (730.9 mg/L). K₂S₂O₈ pretreatment also enhanced organic matter release during AD, but a significant portion of dissolved organics remained unutilized by methanogens. Notably, the soluble protein in the K3 group increased significantly after 17 days, highlighting potential instability in anaerobic digesting systems after high-temperature thermal activation pretreatment. The mechanism by which K 2 S 2 O 8 promotes anaerobic digestion of sludge needs further investigation. Declarations Competing Interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: The authors declare that they have no competing interest. Data Availability The data supporting the findings of this study are available within the paper and its Supplementary Information files. Should any raw data files be needed in another format they are available from the corresponding author upon reasonable request. Author contributions Material preparation, data collection, and analysis were performed by Quan Chen, Ao Zheng and Jiao Wu. The manuscript was written by Quan Chen and Juan Mei. All authors read and approved the final manuscript. Funding This work was supported by National Natural Science Foun dation of China (No. 51508367). Ethics Approval Not applicable. Consent to Participate Not applicable. Consent to Publish Not applicable. 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Chemical Engineering Journal 497:154707. https://doi.org/10.1016/j.cej.2024.154707 Xu Q, Liu X, Wang D, et al (2018) Free ammonia-based pretreatment enhances phosphorus release and recovery from waste activated sludge. Chemosphere 213:276–284. https://doi.org/10.1016/j.chemosphere.2018.09.048 Sedmak JJ, Grossberg SE (1977) A rapid, sensitive, and versatile assay for protein using Coomassie brilliant blue G250. Analytical Biochemistry 79:544–552. https://doi.org/10.1016/0003-2697(77)90428-6 Dai X, Hu C, Zhang D, et al (2017) Impact of a high ammonia-ammonium-pH system on methane-producing archaea and sulfate-reducing bacteria in mesophilic anaerobic digestion. Bioresource Technology 245:598–605. https://doi.org/10.1016/j.biortech.2017.08.208 Kafle GK, Kim SH (2013) Anaerobic treatment of apple waste with swine manure for biogas production: Batch and continuous operation. Applied Energy 103:61–72. https://doi.org/10.1016/j.apenergy.2012.10.018 Wang Y, Wei Y, Liu J (2009) Effect of H2O2 dosing strategy on sludge pretreatment by microwave-H2O2 advanced oxidation process. Journal of Hazardous Materials 169:680–684. https://doi.org/10.1016/j.jhazmat.2009.04.001 Wacławek S, Lutze HV, Grübel K, et al (2017) Chemistry of persulfates in water and wastewater treatment: A review. Chemical Engineering Journal 330:44–62. https://doi.org/10.1016/j.cej.2017.07.132 Zhang R, Lu X, Tan Y, et al (2021) Disordered mesoporous carbon activated peroxydisulfate pretreatment facilitates disintegration of extracellular polymeric substances and anaerobic bioconversion of waste activated sludge. Bioresource Technology 339:125547. https://doi.org/10.1016/j.biortech.2021.125547 Liu C, Wu B, Chen X (2018) Sulfate radical-based oxidation for sludge treatment: A review. Chemical Engineering Journal 335:865–875. https://doi.org/10.1016/j.cej.2017.10.162 Tan Y, Zhang R, Lu X, et al (2021) Mechanistic insights into promoted dewaterability, drying behaviors and methane-producing potential of waste activated sludge by Fe2+-activated persulfate oxidation. Journal of Environmental Management 298:113429. https://doi.org/10.1016/j.jenvman.2021.113429 Lv J, Gong L, Chen X, et al (2021) Enhancements of short-chain fatty acids production via anaerobic fermentation of waste activated sludge by the combined use of persulfate and micron-sized magnetite. Bioresource Technology 342:126051. https://doi.org/10.1016/j.biortech.2021.126051 Wang Z, Wang Y, Li X, et al (2022) Effect of calcium peroxide pretreatment on anaerobic digestion of primary and secondary sludge of A2/O process. Journal of Water Process Engineering 49:102994. https://doi.org/10.1016/j.jwpe.2022.102994 Vu HP, Nguyen LN, Wang Q, et al (2022) Hydrogen sulphide management in anaerobic digestion: A critical review on input control, process regulation, and post-treatment. Bioresource Technology 346:126634. https://doi.org/10.1016/j.biortech.2021.126634 Cite Share Download PDF Status: Published Journal Publication published 15 Jul, 2025 Read the published version in Brazilian Journal of Chemical Engineering → Version 1 posted Reviewers agreed at journal 06 May, 2025 Reviewers invited by journal 05 May, 2025 Editor invited by journal 27 Apr, 2025 Editor assigned by journal 19 Apr, 2025 First submitted to journal 16 Apr, 2025 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. 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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-6461581","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":452282568,"identity":"6e40b251-da07-42a1-a9eb-302939143e13","order_by":0,"name":"Quan Chen","email":"","orcid":"","institution":"Suzhou University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Quan","middleName":"","lastName":"Chen","suffix":""},{"id":452282569,"identity":"564623fd-37f1-4a26-9ac9-fbc4fcfe1144","order_by":1,"name":"Juan MEI","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAxklEQVRIiWNgGAWjYJCCAyCCjb2x8eEH0rTwHG42liDNLon0NgEeYhTKz0h/eOADwzY5PsmHbQwSDHZyug0EtBjcyDE4OIPhtjGbdGLbgwKGZGOzA4S0SOQwHOZhuJ3YJp3YbiDBcCBxGyEtQIc9OPyH4XZ9m+TBNgkeYrQw3EgwOMzAcDuBTYKRSC0GZ94YHOxhuG3YxpMIDGQDIvwi357++MMPhtvy8u3HHz78UGEnR1ALGDD+g1tKjPJRMApGwSgYBQQBAPOiQcNzRpZkAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0009-0007-2147-7405","institution":"Suzhou University of Science and Technology","correspondingAuthor":true,"prefix":"","firstName":"Juan","middleName":"","lastName":"MEI","suffix":""},{"id":452282570,"identity":"7010d8ac-4492-4cc7-9414-b5143ccaacbd","order_by":2,"name":"Ao Zheng","email":"","orcid":"","institution":"Suzhou University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Ao","middleName":"","lastName":"Zheng","suffix":""},{"id":452282571,"identity":"5ac1a857-aecb-44b3-808e-d87aa356be0f","order_by":3,"name":"Jiao Wu","email":"","orcid":"","institution":"Suzhou University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Jiao","middleName":"","lastName":"Wu","suffix":""}],"badges":[],"createdAt":"2025-04-16 08:56:12","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6461581/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6461581/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s43153-025-00581-0","type":"published","date":"2025-07-15T15:57:11+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":82325350,"identity":"95d617b2-e385-47b8-89c8-f298b239f22f","added_by":"auto","created_at":"2025-05-09 06:08:19","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":41191,"visible":true,"origin":"","legend":"\u003cp\u003eThe change of SCOD, soluble proteins and soluble polysaccharides concentration in pretreatment process\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6461581/v1/d173b16f32b7879c64408ad6.png"},{"id":82324172,"identity":"a32e8c06-3aa5-4203-95ae-9dc095d65b03","added_by":"auto","created_at":"2025-05-09 06:00:10","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":35025,"visible":true,"origin":"","legend":"\u003cp\u003eFTIR spectra analysis of sludge after pretreatment\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6461581/v1/74cd1c5edeb339b01da83635.png"},{"id":82325226,"identity":"3eed0a8e-d81f-45a0-ae41-35b131e7d609","added_by":"auto","created_at":"2025-05-09 06:07:15","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":27853,"visible":true,"origin":"","legend":"\u003cp\u003eThe accumulated methane yield (a) and methane production rate (b) in anaerobic digestion process\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6461581/v1/6b369ecaebaf7a1bba6898bb.png"},{"id":82324253,"identity":"e23b2b87-e44c-4f32-8225-a080adba6f5b","added_by":"auto","created_at":"2025-05-09 06:00:24","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":47367,"visible":true,"origin":"","legend":"\u003cp\u003eConcentration changes of SCOD, soluble protein and soluble polysaccharide during anaerobic digestion\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6461581/v1/9c7a290dbd4b76ba4e30231b.png"},{"id":82325231,"identity":"3def5395-bd87-4480-9c6d-471ebca9987f","added_by":"auto","created_at":"2025-05-09 06:07:21","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":28978,"visible":true,"origin":"","legend":"\u003cp\u003eChanges of pH value and NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N concentration during anaerobic digestion\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6461581/v1/5b9a77f4f995e28b089da950.png"},{"id":87219905,"identity":"b38ef107-f6c7-499b-87e7-d5d0b17573a6","added_by":"auto","created_at":"2025-07-21 16:07:38","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":971614,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6461581/v1/27b07b17-465e-4718-a553-eb1284045286.pdf"}],"financialInterests":"","formattedTitle":"Effect of medium temperature-activated potassium persulfate pretreatment on Sludge Anaerobic Digestion Efficiency and Substance Transformation Processes","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eWith the growth of population and the development of society, the volume of wastewater generated by human activities continues to increase, resulting in a large amount of waste activated sludge (WAS) that needs to be properly treated. According to statistics, China's annual sludge production (80% water content) has exceeded 50\u0026nbsp;million tons, and the global sludge production will reach 103\u0026nbsp;million tons in 2025 [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Due to land constraints, the traditional disposal method of landfilling after dewatering is gradually being prohibited. In contrast, anaerobic digestion (AD) has been extensively studied as a sustainable alternative method for the resource and energy recovery from WAS.\u003c/p\u003e \u003cp\u003eThe microbial cell membranes and extracellular polymeric substances (EPS) in sludge generally form a dense barrier that hinders the decomposition of sludge, reducing the release and utilization of organic matter in WAS. Moreover, WAS contains a lot of organic compounds such as lignin, cellulose, and hemicellulose, which have stable structures and are difficult for microorganisms to utilize. These two factors have become the main limiting factors for WAS anaerobic digestion [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. In response to these issues, various pretreatment technologies have been developed to promote the disintegration of sludge, including physical, chemical, biological, and combined methods[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Among these, advanced oxidation pretreatment, a chemical method that generates reactive oxidative radicals to promote sludge hydrolysis, has attracted significant attention due to its stable oxidation properties. Common oxidizing agents include Fenton reagents, CaO₂, and persulfates [\u003cspan additionalcitationids=\"CR4\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Although Fenton reagents have been widely applied in sludge treatment [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e], their further application is constrained by strict pH requirements (pH 2\u0026ndash;4) and the instability of H₂O₂ [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. In contrast, CaO₂ and persulfate has been applied to treatment solid waste because of its strong stability and oxidation ability [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Persulfate-generated sulfate radicals (SO₄⁻\u0026middot;) exhibit higher redox potential (E₀ = 2.65\u0026ndash;3.10 V) and longer half-life (3\u0026ndash;4\u0026times;10⁻⁵ s) than hydroxyl radicals (\u0026middot;OH, E₀ = 2.4\u0026ndash;2.8 V, half-life 10⁻⁹ s), indicating stronger penetration capability through sludge cellular structures [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. This indicates that persulfate pretreatment is more conducive to the hydrolysis of sludge and provides more substrates for anaerobic digestion. Research on persulfate applications in sludge treatment has been increasingly pursued. Liu et al. found that thermal activation of persulfate could effectively generate various free radicals (\u0026middot;OH and SO₄⁻\u0026middot;), increasing the release of short-chain fatty acids(SCFA) by 54.8%[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Hu et al. achieved a 53.6% enhancement in methane production using a combination of zero-valent iron foil (ZVI) and persulfate for sludge anaerobic digestion [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Sun et al. found that the methane production of WAS increased from 61.58 mL/g VSS to 89.24 mL/g VSS after persulfate pretreatment[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e].\u003c/p\u003e \u003cp\u003ePersulfate has a stable structure and needs to be activated to produce SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026middot;. Common activation methods include thermal activation and metal activation [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. However, metal-based methods face challenges in metal recovery and secondary pollution risks, while thermal activation demands high energy consumption. Previous studies typically employed thermal activation temperatures of 75\u0026ndash;90\u0026deg;C [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. But some studies have found that these high activation temperatures are detrimental to anaerobic digestion. Yin et al. observed that although persulfate activation at 80\u0026deg;C significantly increased SCFA production in sludge, methane yield was even lower than the control group due to strong inhibition of methanogens by thermal treatment[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Sun et al. revealed that the soluble chemical oxygen demand (SCOD) of pretreated sludge was 6.02 times higher than the control after thermally activated persulfate pretreatment at 90\u0026deg;C, but the increase in methane production was not significant, indicating most organic matter remained unutilized for methanogenesis[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. At present, the optimal activation temperature for persulfate-enhanced anaerobic digestion of WAS remains undetermined. This study investigated lower activation temperatures (35\u0026ndash;75\u0026deg;C) of potassium persulfate to identify the most suitable thermal activation condition for WAS anaerobic digestion. The effect of potassium persulfate on substance conversion during anaerobic digestion under this condition was also analyzed.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cp\u003e\u003cstrong\u003e2.1. Experimental materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe WAS was taken from the second sedimentation tank of Shishan Wastewater Treatment Plant in Suzhou City, China, which was retrieved and then placed at 4\u0026deg;C for 24 h to remove the supernatant. The inoculated sludge was taken from the anaerobic fermentation tank of Everbright Environmental Energy (Suzhou) Co. The characteristics of the raw materials were shown in \u003cstrong\u003eTable 1\u003c/strong\u003e. K₂S₂O₈ was purchased from Shanghai Lingfeng Chemical Reagent Co.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 1\u003c/strong\u003e Properties of WAS and inoculated sludge\u0026nbsp;\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 189px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 189px;\"\u003e\n \u003cp\u003eWAS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 189px;\"\u003e\n \u003cp\u003einoculated sludge\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 189px;\"\u003e\n \u003cp\u003eTSS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 189px;\"\u003e\n \u003cp\u003e20.96\u0026plusmn;0.30g/L\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 189px;\"\u003e\n \u003cp\u003e0.111\u0026plusmn;0.008g/g\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 189px;\"\u003e\n \u003cp\u003eVSS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 189px;\"\u003e\n \u003cp\u003e11.62\u0026plusmn;0.30g/L\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 189px;\"\u003e\n \u003cp\u003e0.052\u0026plusmn;0.005g/g\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 189px;\"\u003e\n \u003cp\u003epH\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 189px;\"\u003e\n \u003cp\u003e6.80\u0026plusmn;0.07\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 189px;\"\u003e\n \u003cp\u003e7.60\u0026plusmn;0.16\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e\u003cstrong\u003e2.2. \u0026nbsp;Pretreatment test\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWAS was pretreated with K₂S₂O₈ using a 1000 ml serum bottle. The addition amount of K₂S₂O₈ and the reaction time of pretreatment were referred to the study of Balcioglu et al\u0026nbsp;[13]. CG was the control group without K₂S₂O₈ addition, and the experimental conditions are shown in \u003cstrong\u003eTable 2\u003c/strong\u003e. Samples were taken every 0.5 h from each group to analyze indicators such as SCOD, soluble protein, and soluble polysaccharides. After pre-processing, take a portion of the sample and dry it before crushing it, passed through a 100-mesh sieve,\u0026nbsp;2 mg of the sample and 200 mg of potassium bromide(KBr) were taken and subjected to Fourier transform infrared spectroscopy analysis.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 2\u003c/strong\u003e Test design of thermally activated\u0026nbsp;K₂S₂O₈\u0026nbsp;pretreatment\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 132px;\"\u003e\n \u003cp\u003eGroup\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 158px;\"\u003e\n \u003cp\u003eAddition amount\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 173px;\"\u003e\n \u003cp\u003eTemperature(℃)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 91px;\"\u003e\n \u003cp\u003eTime(h)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 132px;\"\u003e\n \u003cp\u003eCG\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 158px;\"\u003e\n \u003cp\u003e0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 173px;\"\u003e\n \u003cp\u003e25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 91px;\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 132px;\"\u003e\n \u003cp\u003eK1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 158px;\"\u003e\n \u003cp\u003e0.43g/g TSS K₂S₂O₈\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 173px;\"\u003e\n \u003cp\u003e35\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 91px;\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 132px;\"\u003e\n \u003cp\u003eK2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 158px;\"\u003e\n \u003cp\u003e0.43g/g TSS K₂S₂O₈\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 173px;\"\u003e\n \u003cp\u003e55\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 91px;\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 132px;\"\u003e\n \u003cp\u003eK3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 158px;\"\u003e\n \u003cp\u003e0.43g/g TSS K₂S₂O₈\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 173px;\"\u003e\n \u003cp\u003e75\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 91px;\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e\u003cstrong\u003e2.3. Anaerobic digestion test\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe anaerobic digestion test was carried out on the pretreated sludge. It included three groups (K1, K2, K3), each of which continued from the previous pretreatment experiment. The control group CG was set for the three groups. Parallel groups were set up for each experiment group. A 500 ml (effective volume 450 ml) serum bottle was used as the anaerobic digestion reactor. WAS and the inoculated sludge were mixed in the ratio of 1:2 (VSS/VSS), then the pH of the mixture was adjusted to 7.0\u0026plusmn;0.1 with 4 mol/L HCl or NaOH. The serum bottle was sealed and purged with nitrogen for 10 min to ensure complete anaerobicity, and the anaerobic digestion was carried out in a constant-temperature water-bath shaker at 35\u0026deg;C for 32 days, with a rotation speed of 150 r/min. Gas volume was measured and gas composition was analyzed every 3 days. Liquid samples were taken to measure pH, VFA, NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N, SCOD, dissolved proteins, and dissolved polysaccharides every 3 days for the first 12 d, and every five days thereafter.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.4. Analytical methods\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSCOD, NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N, TSS and VSS were determined according to\u0026nbsp;Methods of Analysis for Examination of Water and Wastewater\u003csup\u003e\u0026nbsp;\u003c/sup\u003e[14]. Measurement of pH value using pH meter (PHS-2F, Shanghai). Proteins and polysaccharides were determined using the Folin-phenol\u0026nbsp;reagent method and the phenol-sulfuric acid colorimetric method\u003csup\u003e\u0026nbsp;\u003c/sup\u003e[15, 16]. Biogas components were determined using a gas chromatograph (INSEA GC112A, China) equipped with a thermal conductivity detector. Biogas was collected in a gas bag and the volume was measured with a glass syringe (100 mL) at room temperature (about 25\u0026deg;C) and atmospheric pressure (about 101 kpa). The determination of VFAs adopts colorimetric method: first add acidic ethylene glycol for heating, cool down, then add hydroxylamine reagent, and then add acidic ferric chloride, after color development, measure its absorbance. All experiments were in triplicate, and the data were arithmetic mean.\u003c/p\u003e\n\u003cp\u003eA kinetic model, the modified Gompertz model, was used to calculate the lag period of methane production and the maximum daily methane production under different test conditions\u003csup\u003e\u0026nbsp;\u003c/sup\u003e[17, 18]. The modified Gompertz model is shown in Equation 1:\u003c/p\u003e\n\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\"\u003e\u003c/p\u003e\n\u003cp\u003ewhere M is the cumulative methane production(mL/g VSS) on day t of digestion; M\u003csub\u003e0\u003c/sub\u003e is the simulated maximum methane production potential(mL/g VSS); R\u003csub\u003emax\u003c/sub\u003e is the simulated maximum daily methane production rate(mL/g VSS/d); \u0026lambda; is the simulated methane production lag(d); and e is the natural logarithm with the value of 2.718.The parameters(M\u003csub\u003e0\u003c/sub\u003e, R\u003csub\u003emax\u003c/sub\u003e, \u0026lambda;) in this equation were estimated using the Origin2017 nonlinear fit for estimation.\u003c/p\u003e\n\u003cp\u003eChanges in chemical composition and functional groups of WAS during pretreatment were determined using Fourier transform infrared spectroscopy (FT-IR, Nicolet Is50).\u0026nbsp;Refer to the method of huang et al., infrared (IR) spectra were recorded on a Bruker TENSOR II spectrometer using an attenuated total reflection (ATR) device. About 2 mg grinded powders were put on the crystal device and the contact was obtained by applying a strength of about 150 N on the sample. Each spectrum was subjected to 16 scans with the resolution of 4 cm\u003csup\u003e\u0026minus;1\u003c/sup\u003e from 4000 to 400 cm\u003csup\u003e\u0026minus;1\u003c/sup\u003e region[8].\u003c/p\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Effect of pretreatment on sludge decomposition\u003c/h2\u003e \u003cdiv id=\"Sec9\" class=\"Section3\"\u003e \u003ch2\u003e3.1.1. Release of dissolved organic matter in sludge\u003c/h2\u003e \u003cp\u003eThe concentration of SCOD in sludge supernatant can indicate the degree of sludge disintegration. Results showed that the addition of K₂S₂O₈ significantly increased the SCOD concentration in treatment groups. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, the concentration of SCOD in CG group remained nearly unchanged within 1.5 h, while those in K1-K3 groups all exhibited an upward trend, reaching the final values of 1372 mg/L, 2004 mg/L, and 2198 mg/L, respectively. After K₂S₂O₈ pretreatment, the SCOD concentration of K1-K3 group increased by 163.1%, 284.5%, and 321.6%, respectively, compared to the control. The highest sludge disintegration and organic matter release were achieved at the activation temperature of 75\u0026deg;C. Previous studies suggested that higher activation temperatures of persulfate were conducive to the cell lysis and the release of intercellular biopolymeric substances. Sun et al. activated K₂S₂O₈ at 90 ℃, and the maximum SCOD concentration of the pretreated sludge reached 590mg/L, compared to 84 mg/L in the control group[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eSoluble proteins and polysaccharides are important components of sludge SCOD. WAS contains large amount (50\u0026ndash;70%) of organics, mainly including protein, carbohydrate [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. These organic compounds will be decomposed as the sludge disintegrates, releasing soluble proteins and polysaccharides, which are the main substrates in the hydrolysis and methane production processes of sludge. Similar to the change of SCOD, the concentrations of soluble proteins and polysaccharides in the control group remained nearly unchanged, while those in the treated groups increased rapidly in 1.5h. In the K groups, soluble proteins concentrations rose most rapidly within the first 0.5 h, with slower increases observed between 0.5h and 1.5 h. Activation temperature obviously influenced the release of protein: the higher the activation temperature, the faster the protein release, and the higher the concentration of soluble proteins after K₂S₂O₈ pretreatment. However, the impact of activation temperature on soluble polysaccharide concentrations was less distinct. The concentration of soluble polysaccharides in K2 and K3 after pretreatment was similar, and both higher than that in K1. After 1.5 h, all indicators of K3 group were the highest, with the content of SCOD, soluble proteins, and polysaccharides reaching 2198.02 mg/L, 375.75 mg/L, and 451.07 mg/L, respectively.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section3\"\u003e \u003ch2\u003e3.1.2. Effect of pretreatment on functional groups\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e presents the Fourier Transform Infrared Spectroscopy (FT-IR) analysis results of pretreated samples. The absorption peaks at 3631\u0026thinsp;\u0026minus;\u0026thinsp;3184 cm⁻\u0026sup1; correspond to the stretching vibrations of hydroxyl (O-H) and amino (N-H) groups. O-H groups typically exist in water, alcohols, and phenols, while N-H groups are commonly found in amino acids, peptides, proteins, and other nitrogen-containing organic compounds [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. These peaks primarily indicate protein hydrolysis in the anaerobic digestion system, reflecting the hydrolysis and acidogenesis efficiency of sludge. Compared with the CG group, the K1-K3 groups exhibited higher peak intensities in this region. This suggests that although the activity of both acidogenic and methanogenic bacteria decreased after pretreatment, the K1-K2 groups demonstrated higher overall acidogenesis efficiency due to the release of more substrates through pretreatment [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe peak at 1402 cm⁻\u0026sup1; represents the bending vibration of -CH₂- groups, which are present in aliphatic compounds (including VFAs) [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Both C1 and K1 groups showed stronger peak intensities at this position compared to CG, indicating that K₂S₂O₈ pretreatment promoted VFAs production. The peak at 1115 cm⁻\u0026sup1; corresponds to in-plane C-H bending vibrations, while the peak at 613 cm⁻\u0026sup1; is attributed to out-of-plane C-H bending vibrations [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. As sludge organic matter typically contains various C-H functional groups, the significantly lower peak intensities at these two positions in the K1-K3 groups compared to CG suggest that K₂S₂O₈ pretreatment enhanced organic matter degradation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Effect of pretreatment on methane production in anaerobic digestion of sludge\u003c/h2\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003e3.2.1. Methane production and methane production rate\u003c/h2\u003e \u003cp\u003eMethane production is a key indicator for evaluating anaerobic digestion processes. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e\u003cb\u003e(a)\u003c/b\u003e, pretreatment significantly enhanced the anaerobic digestion of sludge. The methane production of the treated groups over 32 days was greatly higher than that of the control. The accumulated methane yield of K1, K2, and K3 reached 1.65, 1.92, and 1.74 times that of the control, respectively. The K2 group (with the K₂S₂O₈ activation temperature of 55\u0026deg;C) achieved the highest accumulated methane yield (183.2 ml/g VSS), exceeding K1 and K3 by 16.3% and 10.3%, respectively. After pretreatment at the activation temperature of 75\u0026deg;C, the K3 group exhibited superior sludge hydrolysis with the highest SCOD concentration. The methane production of K3 group was the highest in the first 12 days of the AD reaction, but lower than that of K2 group after 12 days due to a decrease in methane production rate in the later stage. Sun et al. reported that thermal activation at 90\u0026deg;C increased SCOD release of sludge by 602% compared to controls, surpassing the organic matter release effect observed in this study. However, the final methane yield only increased by 44.9% [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e], significantly lower than the enhancement achieved here. These results demonstrated that medium temperature-activated potassium persulfate pretreatment was feasible and effective for improving WAS anaerobic digestion. While its solubilization efficiency was inferior to high-temperature conditions, it achieved superior enhancement of methane production.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e \u003cb\u003e(b)\u003c/b\u003e shows the variation in methane production rates in different groups. The methane production in K1 and K2 groups started slowly, with daily methane yields of 14.2 ml/g VSS and 19.3 ml/g VSS on day 3, respectively, much lower than those of CG and K3 groups (30.7 ml/g VSS and 39.1 ml/g VSS, respectively). Between days 3\u0026ndash;6, the daily methane production of K groups increased substantially, with peak values of K1-K3 reaching 58.5, 52.8, and 63.6 ml/g VSS respectively, maintaining high levels (\u0026ge;\u0026thinsp;33.5 ml/g VSS) until day 9. Compared to the control, K groups exhibited obvious methane production peaks and their methane production concentrated within the first 15 days.\u003c/p\u003e \u003cp\u003eThe initial methane production rates revealed the dual effects of pretreatment on anaerobic digestion processes. On the one hand, the initial methane production rates in treated groups correlated with post-pretreatment substrate concentrations, and higher pretreatment temperatures released more organic matter and resulted in higher initial methane production rates. On the other hand, only K3 showed a higher initial production rate than the control despite all pretreated groups having elevated organic matter concentrations. This suggested potential adverse effects of pretreatment on digestion initiation, possibly due to the inhibition of methanogens by residual radicals. Some studies confirmed that highly reactive chemicals could suppress methanogen activity, leading to reduced methane output [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eDespite the K3 group having higher initial performance, the K2 group (activated at 55\u0026deg;C) achieved the best cumulative methane production, therefore 55\u0026deg;C was the optimal activation temperature for pretreatment. This conclusion contrasts with previous assessments based solely on sludge disintegration efficiency during pretreatment, indicating that the optimization of persulfate pretreatment should not rely exclusively on hydrolysis performance but must consider overall digestion outcomes.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section3\"\u003e \u003ch2\u003e\u003cb\u003e3.2.2. Kinetic analysis of anaerobic digestion\u003c/b\u003e\u003c/h2\u003e \u003cp\u003eTo further analyze the methane production efficiency of each group, the experimental data were fitted using the modified Gompertz model. The kinetic parameters of anaerobic digestion under different pretreatment conditions obtained from the simulation are shown in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, where M\u003csub\u003e0\u003c/sub\u003e (maximum methane production potential), R\u003csub\u003emax\u003c/sub\u003e (maximum daily methane production rate), and λ (methane production lag phase) represent the simulated parameters.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eKinetics of methane production based on modified Gompertz model\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eparameter\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eM\u003csub\u003e0\u003c/sub\u003e(mL/g VSS)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eR\u003csub\u003emax\u003c/sub\u003e(mL/g VSS/d)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eλ(d)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCG\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e94.47\u0026thinsp;\u0026plusmn;\u0026thinsp;0.31\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e7.10\u0026thinsp;\u0026plusmn;\u0026thinsp;0.13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.99930\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eK1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e155.54\u0026thinsp;\u0026plusmn;\u0026thinsp;1.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e20.13\u0026thinsp;\u0026plusmn;\u0026thinsp;0.79\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.60\u0026thinsp;\u0026plusmn;\u0026thinsp;0.16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.99780\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eK2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e182.54\u0026thinsp;\u0026plusmn;\u0026thinsp;1.73\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e17.89\u0026thinsp;\u0026plusmn;\u0026thinsp;0.84\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.91\u0026thinsp;\u0026plusmn;\u0026thinsp;0.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.99623\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eK3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e164.13\u0026thinsp;\u0026plusmn;\u0026thinsp;0.95\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e22.95\u0026thinsp;\u0026plusmn;\u0026thinsp;0.95\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.21\u0026thinsp;\u0026plusmn;\u0026thinsp;0.16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.99679\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eAccording to the simulation results in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, the K groups achieved significantly higher M\u003csub\u003e0\u003c/sub\u003e and R\u003csub\u003emax\u003c/sub\u003e values compared to CG, indicating that pretreatment effectively enhanced methane production. The K2 group with optimal performance showed an M\u003csub\u003e0\u003c/sub\u003e value 1.93 times that of the CG group. The CG group showed a short methane production lag phase (λ\u0026thinsp;\u0026lt;\u0026thinsp;0.1 d), suggesting high initial activity of methanogens in the inoculated sludge. In contrast, the pretreated groups displayed λ values ranging from 1.21 d to 2.60 d, indicating a relatively delayed methane production initiation. This delay might be attributed to the influence of methanogens by residual free radicals from K\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003e. Additionally, K\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003e pretreatment could promote the growing of sulfate-reducing bacteria (SRBs), which compete with methanogens for substrates, thereby further reducing methanogenic activity [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Despite the delayed initiation, the total methane yield in the pretreated groups remained higher than the control. A similar phenomenon of methane production lag was observed by Yin et al. under the thermal activation temperature of 80\u0026deg;C, where methanogenesis started after 120 h without pH adjustment [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Notably, Tan et al. found that Fe\u0026sup2;⁺-activated persulfate shortened the lag time of methane production [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Both activation methods and operational conditions were shown to influence methanogenesis dynamics.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Effect of pretreatment on material conversion in sludge anaerobic digestion process\u003c/h2\u003e \u003cdiv id=\"Sec15\" class=\"Section3\"\u003e \u003ch2\u003e3.3.1 Changes of soluble organic matter during anaerobic digestion\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e displays the concentration variations of dissolved organic matters during the anaerobic digestion (AD) of WAS, including soluble proteins, soluble polysaccharides, and volatile fatty acid (VFA). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e \u003cb\u003e(a)\u003c/b\u003e, different trends of change in soluble organic matter concentrations were observed between treatment groups and the control group (CG). The CG group exhibited an overall decline in SCOD concentration, particularly after day 17, indicating the active consumption of soluble organic matter during methanogenesis. In contrast, the SCOD content of K1 and K2 groups obviously increased from day 3 to day 6, then followed by a slow increase. Notably, SCOD of K3 group greatly increased after day 17. This indicates that the production rate of soluble organic matter during anaerobic digestion was higher than its consumption rate by methanogens in these pretreated groups. At the end of the experiment, the SCOD concentration in K1 and K2 groups reached 3540.82 and 3348.50 mg/L, respectively, substantially higher than that in CG (730.9 mg/L). Both the accumulated methane production and residual SCOD concentrations in K groups were higher than the control. This phenomenon indicated that K₂S₂O₈ pretreatment also enhanced organic matter release during AD, but a significant portion of dissolved organics remained unutilized by methanogens. Lv et al. reported similar findings. In their study, SCOD of control groups maintained stable over 12 days of AD, while that of persulfate-pretreated groups exhibited continuous increases [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eDifferent patterns of changes in soluble proteins are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e \u003cb\u003e(b)\u003c/b\u003e: the content of soluble proteins in the CG group exhibited an initial increase in 0\u0026ndash;3 days followed by continuous decrease, whereas that in K1 and K2 groups experienced significant increase in 0\u0026ndash;3 days, then remained relatively stable, decreased after day 12, and finally slightly increased. The early-phase protein accumulation across all groups suggested its dissolution rates exceeded consumption rates. Previous studies indicated in the early stages of anaerobic digestion, the consumption of soluble carbohydrates took precedence over protein [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. Notably, the soluble protein in the K3 group increased significantly after 17 days, which resulted in an obvious increase of SCOD in the K3 group after 17 days. The relevant research did not mention this phenomenon [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. It might potentially link to sulfate-reducing bacteria (SRBs)-mediated H\u003csub\u003e2\u003c/sub\u003eS production from SO₄\u0026sup2;⁻[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. High H\u003csub\u003e2\u003c/sub\u003eS concentrations can induce microbial cell lysis, releasing intracellular proteins. The significant increase of soluble proteins in K3 in the later stage may be released by dead microorganisms. This highlights potential instability in anaerobic digester systems in high-temperature thermal activation processes.\u003c/p\u003e \u003cp\u003eSoluble carbohydrate concentrations in all groups fluctuated throughout AD (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e \u003cb\u003e(c)\u003c/b\u003e), with K1-K3 groups maintaining significantly higher levels than CG. Intriguingly, the soluble polysaccharide concentration of K3 was higher than that of K1 and K2 during days 0\u0026ndash;9, but lower than that of K1 and K2 during days 9\u0026ndash;27. This implied that high temperature-activated pretreatment did not substantially inhibit the utilization of soluble polysaccharides compared to mesophilic conditions. Volatile fatty acids (VFAs) content profiles (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e \u003cb\u003e(d)\u003c/b\u003e) demonstrated the rapid depletion of VFAs within the first 6 days in all groups, coinciding with the peak methane production rates. This reflected the high methanogenic activity of microorganisms in the inoculum sludge, which led to the rapid conversion of VFAs into methane. After 6 days, the VFAs concentrations in all groups were at a low level. Throughout the anaerobic digestion process, the concentration of VFAs in K groups was higher than that in CG group.\u003c/p\u003e \u003cp\u003eThroughout AD, K groups had higher initial SCOD concentrations than CG due to pretreatment-enhanced organic matter release. The concentrations of SCOD, soluble proteins, and soluble carbohydrates in K groups were obviously higher than those in the control during later stages. This suggested that K₂S₂O₈ pretreatment also promoted substantial dissolution of organic matter in AD stage, and the incomplete conversion to methane implied complex interaction mechanism requiring further investigation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section3\"\u003e \u003ch2\u003e\u003cb\u003e3.3.2 Changes in pH and\u003c/b\u003e NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N \u003cb\u003econcentration in the anaerobic digestion\u003c/b\u003e\u003c/h2\u003e \u003cp\u003eThe pH variation trends of all groups during anaerobic digestion were similar, showing rapid initial increase followed by gradual rise, with final pH values stabilizing at 7.6\u0026ndash;7.9 (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e\u003cb\u003e(a)\u003c/b\u003e). No distinct acidogenic phase was observed in this experiment, with only slight pH decreases occurring between days 3\u0026ndash;6 in some groups. This might be attributed to the high activity of inoculated sludge, which enabled rapid conversion and consumption of VFAs. The initial increase of pH value in all groups corresponded with VFAs reduction shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e\u003cb\u003e(d)\u003c/b\u003e. Subsequently, pH values continued rising steadily due to the microbial decomposition of organic matter and the production of alkaline substances like ammonia during anaerobic digestion [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e \u003cb\u003e(b)\u003c/b\u003e reveals that NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N concentrations in all groups exhibited an overall upward trend throughout the digestion process. The K groups consistently showed higher NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N levels than the CG group, indicating that more organic matter was degraded in K groups. The K₂S₂O₈ pretreatment promoted protein hydrolysis in sludge, generating more NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N. Final NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N concentration reached 1431.32 mg/L in CG group, compared to 1829.84 mg/L, 1594.03 mg/L, and 1529.75 mg/L in K1, K2, and K3, respectively. Overall, potassium persulfate pretreatment exerted limited influence on pH and NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N concentration during the anaerobic digestion stage.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eTo promote sludge decomposition and improve anaerobic digestion efficiency, potassium persulfate pretreatment of sludge was carried out in this study at a lower activation temperature of 35 \u0026deg; C-75 \u0026deg; C. The results indicated that the thermal activation K\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003e pretreatment under medium temperature conditions was feasible and effective. Compared with the control, the content of SCOD increased by a maximum of 321.6% and the methane production potential increased by a maximum of 93.0% after pretreatment. The hydrolysis effect of sludge in the pretreatment stage is positively correlated with the activation temperature, and the content of SCOD was the highest at 75 ℃. However, the K2 group with an activation temperature of 55℃ had the highest total methane yield, which was 16.3% and 10.3% higher than the K1 and K3 groups, respectively.\u003c/p\u003e \u003cp\u003eThe analysis of the Gompertz model showed that the pretreated groups had a distinct lag-phase of methane production compared to the control. At the end of AD, the SCOD concentration in K1 and K2 groups reached 3540.82 and 3348.50 mg/L, respectively, substantially higher than that in CG (730.9 mg/L). K₂S₂O₈ pretreatment also enhanced organic matter release during AD, but a significant portion of dissolved organics remained unutilized by methanogens. Notably, the soluble protein in the K3 group increased significantly after 17 days, highlighting potential instability in anaerobic digesting systems after high-temperature thermal activation pretreatment. The mechanism by which K\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003e promotes anaerobic digestion of sludge needs further investigation.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp; The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;The authors declare that they have no competing interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u0026nbsp; The data supporting the findings of this study are available within the paper and its Supplementary Information files. Should any raw data files be needed in another format they are available from the corresponding author upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u0026nbsp; Material preparation, data collection, and analysis were performed by Quan Chen, Ao Zheng and Jiao Wu. The manuscript was written by Quan Chen and Juan Mei. All authors read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u0026nbsp; This work was supported by National Natural Science Foun dation of China (No. 51508367).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics Approval\u0026nbsp;\u003c/strong\u003e Not applicable.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to Participate\u003c/strong\u003e\u0026nbsp; Not applicable.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to Publish\u003c/strong\u003e\u0026nbsp; Not applicable.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eXu Y, Geng H, Chen R, et al (2021) Enhancing methanogenic fermentation of waste activated sludge via isoelectric-point pretreatment: Insights from interfacial thermodynamics, electron transfer and microbial community. 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Bioresource Technology 346:126634. https://doi.org/10.1016/j.biortech.2021.126634\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":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"brazilian-journal-of-chemical-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"bjce","sideBox":"Learn more about [Brazilian Journal of Chemical Engineering](http://link.springer.com/journal/43153)","snPcode":"43153","submissionUrl":"https://www.editorialmanager.com/bjce/default2.aspx","title":"Brazilian Journal of Chemical Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Waste activated sludge, Anaerobic digestion, Pretreatment, Potassium persulfate","lastPublishedDoi":"10.21203/rs.3.rs-6461581/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6461581/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSlow hydrolysis is a major limiting factor in sludge anaerobic digestion. Thermally activated persulfate pretreatment has been used to promote the decomposition of sludge. However, the optimal activation temperature for persulfate-enhanced anaerobic digestion remains undetermined. This study investigated the effects of medium temperature-activated potassium persulfate (K₂S₂O₈) on the decomposition of waste activated sludge (WAS) and the performance of methane production. Pretreatment was conducted at three activation temperatures of 35°C, 55°C, and 75°C, with a K₂S₂O₈ dosage of 0.43 g/g TSS for 1.5 hours. Results demonstrated that K₂S₂O₈ pretreatment significantly improved sludge hydrolysis rates and anaerobic digestion efficiencies of WAS. The soluble organic matter (SCOD) concentrations in WAS after pretreatment were 1,371.52–2,198.02 mg/L, much higher than the control(521.32mg/L), and with optimal performance observed at 75°C. The methane production potential of pretreated groups increased by 67.4%, 94.6%, and 76.5% compared to the control, respectively, with the highest yield at 55 ° C. However, K\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e8 \u003c/sub\u003epretreatment delayed methanogenesis initiation, as indicated by the Gompertz model fitting result (lag phase λ: 1.21–2.60 d in treatment groups vs. \u0026lt;0.1 d in the control). Although pretreatment also enhanced organic matter release during the anaerobic digestion stage, a considerable portion of dissolved organics remained unconverted to methane. Therefore, the impact mechanism of K\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003e on anaerobic digestion needs further research. This study confirmed that medium temperature-activated K₂S₂O₈ pretreatment was effective for anaerobic digestion of WAS and more conducive to the stability of AD system.\u003c/p\u003e","manuscriptTitle":"Effect of medium temperature-activated potassium persulfate pretreatment on Sludge Anaerobic Digestion Efficiency and Substance Transformation Processes","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-05-09 05:58:50","doi":"10.21203/rs.3.rs-6461581/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2025-05-06T11:51:52+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-05-05T20:56:05+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Brazilian Journal of Chemical Engineering","date":"2025-04-28T02:26:28+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-04-19T11:43:40+00:00","index":"","fulltext":""},{"type":"submitted","content":"Brazilian Journal of Chemical Engineering","date":"2025-04-16T04:55:11+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"brazilian-journal-of-chemical-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"bjce","sideBox":"Learn more about [Brazilian Journal of Chemical Engineering](http://link.springer.com/journal/43153)","snPcode":"43153","submissionUrl":"https://www.editorialmanager.com/bjce/default2.aspx","title":"Brazilian Journal of Chemical Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"85f0d9a2-b15a-4781-9862-5208fb07d445","owner":[],"postedDate":"May 9th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-07-21T16:06:09+00:00","versionOfRecord":{"articleIdentity":"rs-6461581","link":"https://doi.org/10.1007/s43153-025-00581-0","journal":{"identity":"brazilian-journal-of-chemical-engineering","isVorOnly":false,"title":"Brazilian Journal of Chemical Engineering"},"publishedOn":"2025-07-15 15:57:11","publishedOnDateReadable":"July 15th, 2025"},"versionCreatedAt":"2025-05-09 05:58:50","video":"","vorDoi":"10.1007/s43153-025-00581-0","vorDoiUrl":"https://doi.org/10.1007/s43153-025-00581-0","workflowStages":[]},"version":"v1","identity":"rs-6461581","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6461581","identity":"rs-6461581","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}