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Article

Freezing Method Assists Peracetic Acid Oxidation for Promoting the Methane Production from Sludge Anaerobic Digestion

by
Zhen-Wei Liu
1,*,
Yan-Qiu Chen
2,
Zhi-Shuai Liu
1 and
Sheng-Wu Wang
1
1
Shandong Provincial Key Laboratory of Molecular Engineering, School of Chemistry and Chemical Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, China
2
Institute of Rare and Scattered Elements, College of Chemistry, Liaoning University, Shenyang 110036, China
*
Author to whom correspondence should be addressed.
Energies 2025, 18(3), 731; https://doi.org/10.3390/en18030731
Submission received: 2 January 2025 / Revised: 30 January 2025 / Accepted: 1 February 2025 / Published: 5 February 2025
(This article belongs to the Section A4: Bio-Energy)
Browse Figures
Versions Notes

Abstract

:
Peracetic acid (PAA) oxidation, which is a kind of chemical method for sludge pretreatment, has been verified to be valid for promoting sludge anaerobic digestion performance. However, the methane production is still limited at certain levels by this method, because excess PAA has negative effects on methanogens. This work selected a freezing method combined with PAA to form a composite sludge pretreatment technology for synergistically improving the biomethane production. According to the experimental data, the methane yield was largely enhanced from 166.4 ± 5.6 mL/g volatile suspended solids (VSS) in the control to 261.5 ± 7.3 mL/g VSS by the combined freezing (−10 °C) and PAA (0.08 g/g TSS) pretreatment, with a 57.2% increase rate. Kinetic analysis showed that the methane production potential, methane production rate, and hydrolysis rate were promoted, respectively, from 159.4 mL/g VSS, 17.18 mL/g VSS/d, and 0.104 d−1 to 254.9 mL/g VSS, 25.69 mL/g VSS/d, and 0.125 d−1 by the freezing + PAA pretreatment. Mechanism analysis revealed that the freezing + PAA pretreatment destroyed both extracellular polymeric substances (EPS) and microbial cells in the sludge, resulting in the increase in hydrolysis efficiency. Gene analysis showed that the hydrolytic microbes (Hyphomicrobium and norank_f_Caldilineaceae), acidogens (e.g., Petrimonas, Tissierella, and Mycobacerium) and methanogens (Methanosaeta, Methanosarcina, and Methanobacterium) were all enriched by the freezing + PAA pretreatment, with the total abundances calculated to be 10.65% and 22.07% in the control and pretreated reactors, respectively. Considering both technical and economic factors, the freezing + PAA method is feasible for sludge pretreatment.

1. Introduction

With the acceleration of urbanization, both the number and the processing capacity of sewage treatment plants (STPs) in China showed increasing trends in recent years, and the waste-activated sludge (WAS), which served as the main byproduct of STPs, was continuously generated in daily operations. The huge amount of WAS produced from STPs has become a critical issue to be solved, because it contains a lot of organic matter and can cause serious secondary pollution of the environment without proper treatment. Recently, using WAS as the substrate to produce methane through anaerobic digestion has aroused wide concern; this is considered to be a proper method for WAS treatment because the pathway of “waste-to-resource” is constructed [1,2]. The low reaction rate, however, restricts the development of sludge anaerobic digestion technology. Among several processes (i.e., hydrolysis, acidification, and methanogenesis) contained in sludge anaerobic digestion, the hydrolysis with poor efficiency is generally considered to be the rate-limiting process [3,4], which negatively affects the efficiency of the entire anaerobic system. In other words, sludge anaerobic digestion efficiency could be largely enhanced when the hydrolysis process is accelerated, and pretreatment is found to be valid to achieve this goal.
To date, sludge pretreatment methods have mainly contained three categories, i.e., physical [5,6], chemical [7,8], and biological [9] methods. Among them, the chemical oxidation method possesses the advantages of simple operation and stable effectiveness; thus, it has received more and more attention in recent years. Peracetic acid (PAA) is a typical oxidant frequently adopted in the food and medical industries [10,11] because it has a relatively high oxidation potential of 1.81 eV and can inactivate harmful microorganisms in a wide spectrum [12]. More importantly, there is no toxic by-product generated in the PAA oxidation process. Based on these characteristics, PAA also shows great potential for application in the field of environmental engineering, and its application in sewage sludge treatment has been widely studied in the last decade [13,14]. For instance, Li et al. [15] reported that the sludge hydrolysis effect was efficiently promoted after PAA pretreatment, as the volatile suspended solid (VSS) reduction was generally enhanced with the increase in PAA content from 0 to10 mg/g of total suspended solids (TSS), and more soluble carbohydrate and protein were produced as a consequence. A similar result was observed by Wang et al. [16], in which sludge hydrolysis efficiency showed a positive correlation with PAA content from 10 to 25 mg/g TSS, and the soluble protein and carbohydrate concentrations were promoted, respectively, by 212.9% and 133.0% at the highest PAA dosage when compared with the control. Higher hydrolysis efficiency is undoubtedly favorable to the operation of the anaerobic digestion system. The research of Sun et al. [17] indicated that sludge volatile solids (VS) were decreased by 39.0%, while soluble chemical oxygen demand (SCOD) content was enhanced by 530% after PAA treatment with 30 mg/g suspended solids (SS), and the biogas yield showed a 20.0% increase in the subsequent anaerobic digestion process. Though the positive effect of PAA on the anaerobic reaction of sludge has been generally proved [7], excess PAA can bring about adverse effects on biomethane production. Appels et al. [12] found that the biogas production first increased and then decreased as the PAA dosage was enhanced from 20 to 100 g/kg dry solids (DS), and the optimum PAA concentration was 25 g/kg DS, with which the biogas yield was 21% higher than that without pretreatment. Ren et al. [18] also reported that the biogas yield was first enhanced with a PAA concentration elevated from 1 to 2 mM/g VS; it then started to reduce when the PAA dosage was further increased to 3 or 4 mM/g VS, with the highest gas production of 297.94 mL/g VS achieved at the optimal PAA dosage after 26 days of anaerobic digestion. The above analysis indicated that the effect of PAA on sludge anaerobic digestion performance was relevant to its dosage, i.e., proper dosages resulted in a positive effect, while excess dosages inhibited the anaerobic methane production. In other words, the promotion of biogas yield by PAA pretreatment was limited at certain levels, and an additional method should be adopted to help PAA oxidation further reinforce the anaerobic digestion of sludge.
Freezing is a kind of physical method, which can effectively break sludge flocs and then promote the hydrolysis efficiency. For instance, Hu et al. [19] reported that the VSS decreased while SCOD solubilization increased by 10.5% after freezing at −18 °C for 72 h, suggesting that this pretreatment method effectively promoted sludge hydrolysis efficiency. Phalakornkule et al. [20] found that the SCOD content of original sludge was only 0.10 g/L, which was largely enhanced to 0.79 g/L after being frozen at −20 °C for 24 h. She et al. [21] further found that freezing temperature and time were two crucial factors affecting the pretreatment performance, as lower temperatures (−8 °C, −16 °C, and −24 °C) and longer times (10 to 50 h) resulted in better hydrolysis efficiency of the sludge. Tang et al. [22] proposed a combined pretreatment of freezing and calcium hypochlorite and found that the SCOD concentration significantly increased from 115 to 1743 mg/L when the temperature and chemical dosage were −20 °C and 0.075 g/g VSS, respectively, resulting in a 62.1% increase in biomethane production. Hu et al. [23] combined freezing and potassium ferrate to pretreat sludge, and the results showed that the methane production was promoted by 9.3% under optimal combination when compared with solo potassium ferrate pretreatment, which means freezing effectively reinforced the efficacy of the potassium ferrate oxidization method. Thus, the freezing method could be a proper supplementary for PAA to pretreat sludge, and it is anticipated that a synergistic effect on sludge anaerobic digestion performance can be achieved; however, this has not been studied so far.
This work aims to investigate the feasibility of combining freezing and PAA for sludge pretreatment and to explore their synergistic effect on biomethane generation from anaerobic digestion and the underlying mechanisms. Firstly, the methane yields under different combinations of freezing (−5 °C, −10 °C, −15 °C, and −20 °C) and PAA (0.02, 0.04, 0.06, 0.08, and 0.1 g/g TSS) were investigated. The mechanisms for freezing coupled with PAA pretreatment that synergistically enhanced sludge methane production were then revealed from the variations in hydrolysis efficiency and reaction kinetic parameters. Finally, microbial community analysis was conducted to assess the influence of combined freezing and PAA pretreatment on some of the typical functional microbes that participated in the anaerobic digestion. The results obtained in this study enrich the theoretical system of PAA-based pretreatment technology; they also could provide an important reference for practical sludge treatment engineering.

2. Materials and Methods

2.1. Substrate and Oxidant Used

The WAS used in this study was obtained from a sewage treatment plant (28.2° N, 113.0° E) in Changsha, China, which mainly treated the domestic sewage. The inoculum was acquired from a long-term continuously operated anaerobic reactor (50 L working volume) in our laboratory, with WAS served as the solo substrate, and the temperature of this reactor was controlled to be constant at 35 °C, while the pH value was not controlled. The typical parameters of WAS and inoculum were provided in Table 1. The PAA reagent with 10% content was purchased from a chemical company in Changsha, China.

2.2. Sludge Pretreatment Procedure and Anaerobic Digestion Experiment

Test 1: This part was designed to explore the optimum parameters for the combined pretreatment of freezing and PAA, including the freezing temperatures (−5 °C, −10 °C, −15 °C, and −20 °C) and PAA concentrations (0.02, 0.04, 0.06, 0.08, and 0.1 g/g TSS). The four temperatures were adopted according to previous studies [22,24], while the PAA dosages were confirmed on the basis of our preliminary experimental results, as shown in Figure 1. There were twenty combinations considering different parameters in total; thus, twenty-one anaerobic reactors (V = 1000 mL), which were numbered as Reactors 1 to 21, were set in this part, and the pretreatment parameters for each one are shown in Table 2. Firstly, each reactor received 300 mL of WAS. Except for Reactor 1, which served as a control group and was not treated, the remanent reactors were added with different dosages of PAA according to that presented in Table 2. It should be noted that the pH was not controlled in the PAA treatment process. After being placed for 24 h to ensure a sufficient reaction between the PAA and sludge samples [18], the reactors were then operated for freezing pretreatment. The freezing temperatures for Reactors 2 to 6, Reactors 7 to 11, Reactors 12 to 16, and Reactors 17 to 21 were −5 °C, −10 °C, −15 °C, and −20 °C, respectively, and freezing time for each one was the same: 8 h. Though longer freezing time resulted in better pretreatment effect [19], the cost could be also enhanced with the increase in freezing time; thus, a moderate duration time of 8 h for the freezing pretreatment was adopted in this study to seek the balance between the effect and cost [20,22,25]. In the freezing pretreatment process, Reactor 1 was set at 4 °C to eliminate the interference from ambient temperature. Afterwards, all the reactors were thawed at 35 °C for 3 h; then, 300 mL of inoculum was added to each one. Finally, each of the reactors was aerated with nitrogen gas (N2) for 3 min to dislodge internal air; then, it was sealed and placed in a constant temperature shaker (35 °C, 180 rpm) to actuate anaerobic reaction. The period of this test lasted for 37 days, till the anaerobic reaction was completed, and the biomethane productions were periodically measured. The optimal combination of freezing and PAA was confirmed by comparing the cumulative methane yields among the different reactors.
Test 2: The test was conducted to verify whether combined freezing and PAA pretreatment synergistically promoted sludge anaerobic digestion performance and to explore the relevant mechanisms. According to results of Test 1, the optimal condition of the combined pretreatment was freezing at −10 °C coupled with PAA at a 0.08 g/g TSS dosage; thus, this test adopted the combination to assess the synergistic effect. There were four anaerobic reactors (V = 1000 mL) used in this part, named as Reactors 22 to 25, respectively. At first, each reactor accepted 400 mL of WAS. Reactor 22 served as the control group without any treatment; the other three reactors received different pretreatment methods. Among them, Reactor 23 was treated by solo freezing at −10 °C, and Reactor 24 was treated with 0.08 g/g TSS of PAA. For Reactor 25, the combination of freezing (−10 °C) and PAA (0.08 g/g TSS) was conducted. The specific procedures for both freezing and PAA pretreatments in this part were the same as that described in Test 1. When these treatment processes were accomplished, 100 mL of sludge sample was withdrawn from each reactor to analyze several hydrolysis indicators, including the contents of dissolved organics and the damage to the microbial cells and EPS. Afterwards, each reactor with 300 mL of remaining sludge received 300 mL of inoculum; it was then aerated with N2 and sealed for anaerobic reaction according to the operation used in Test 1. The whole reaction process lasted for 40 days, during which the methane yield in each reactor was periodically measured.

2.3. Kinetic Model-Based Analysis

Two typical kinetic models, the modified Gompertz model and the first-order kinetic model, were adopted to simulate the experimental methane production results in this study. The specific simulation processes were conducted using OriginPro 2022 software.

2.3.1. Modified Gompertz Model

From Equation (1), there are three kinetic parameters, Mm, Rm, and λ, included in the modified Gompertz model. Among them, Mm and Rm represent the methane production potential (mL/g VSS) and rate (mL/g VSS/d), respectively, while λ denotes lag phase time (d). By inputting anaerobic reaction time t (d) and corresponding methane yield M (mL/g VSS) into Equation (1), the three kinetic parameters can be computed.
M = M m × e x p − e x p R m × e M m λ − t + 1

2.3.2. First-Order Kinetic Model

This model contains two kinetic parameters, i.e., B0 and k, representing the methane production potential (mL/g VSS) and hydrolysis rate (d−1), respectively (Equation (2)). The two parameters can be estimated by inputting the anaerobic reaction time t (d) and the corresponding methane yield B(t) into Equation (2).
B t = B 0 ( 1 − e − k t )

2.4. Analytical Methods

The COD, TSS, VSS, carbohydrate, and protein contents were measured using standard methods [26]. Sludge EPS were separated step by step into soluble EPS (S-EPS), lightly bound EPS (LB-EPS), and tightly bound EPS (TB-EPS) based on the method reported in the literature [17]. The release of DNA was measured as that reported by Liang et al. [27], and the lactate dehydrogenase (LDH) concentration was detected using an LDH cytotoxicity assay kit (Beyotime Biotechnology, Haimen, China). A gas chromatograph (Shimadzu GC-2010, Japan) was adopted to detect the methane concentration in the biogas, and the operations were consistent with the literature [28]. Microbial analysis was conducted using the Illumina Miseq sequencing technique. Firstly, the DNA in the different samples was extracted using the E.Z.N.A.® soil DNA kit (Omega Bio-tek, Norcross, GA, USA). Then, the polymerase chain reaction (PCR) was performed, and 515FmodF (5′-GTGYCAGCMGCCGCGGTAA-3′) and 806RmodR (5′-GGACTACNVGGGTWTCTAAT-3′) were adopted as the primers. Finally, the amplicons were purified and then sequenced using the Illumina MiSeq PE300 platform (Illumina, San Diego, CA, USA). The raw sequencing data were clustered based on 97% similarity, from which various operational taxonomic units (OTUs) were generated. For economic analysis, the cost of freezing pretreatment and benefits from the increased methane yield and reduced disposal cost were calculated according to the literature [22,29], while the cost of the PAA agent was acquired from www.alibaba.com.

3. Results and Discussion

3.1. Determination of the Optimal Parameters of Combined Freezing and PAA Pretreatment

Table 2 exhibits the cumulative methane yield under the different pretreatment conditions of combined freezing and PAA when the anaerobic reactions were completed. Compared with the control, the methane productions from the different freezing/PAA combinations were all significantly promoted, and the specific values changed a lot with the variations in both freezing temperature and PAA dosage (Table 2). For example, the methane yield first enhanced and then reduced with the rise in PAA concentration when the freezing temperature was unchanged. Meanwhile, the variation in freezing temperature also affected the methane yield. From Table 2, when the PAA concentration was 0.02 g/g TSS, the methane yield gradually increased with the decrease in freezing temperature from −5 to −20 °C. The results were similar to those of some of the previously reported co-treatment methods, such as microwave + alkaline [30] and freezing + calcium hypochlorite [22], as the methane production was affected, respectively, by two pretreatment factors. Considering both of the two factors, the optimum parameters were freezing at −10 °C combined with 0.08 g/g TSS of PAA, from which the maximal methane yield of 259.0 mL/g VSS was achieved. In the following context, this combination was selected to conduct further analysis.

3.2. The Synergistic Effect of Combined Freezing and PAA Pretreatment on Methane Generation

Figure 2 shows the variations in methane production with time in the control, solo freezing (−10 °C), solo PAA (0.08 g/g TSS), and freezing + PAA (freezing at −10 °C combined with 0.08 g/g TSS of PAA) reactors. Similar trends were found in these reactors, as methane productions grew rapidly in the initial stage and then leveled off, and not all were obviously enhanced after 40 days; the daily methane yield during three consecutive days was <1% of the accumulative production, suggesting that the anaerobic reactions were all accomplished [31]. In the control, the cumulative methane production on the 40th day was only 166.4 mL/g VSS, which was relatively low because of the poor hydrolysis efficiency [32,33]. The data were observably enhanced to 212.7 and 230.1 mL/g VSS when treated by solo freezing or solo PAA, respectively, suggesting that the two methods were both effective for enhancing sludge anaerobic digestion performance. In the freezing + PAA pretreated reactor, the methane production was further enhanced to 261.5 mL/g VSS, which was significantly higher than the solo freezing or solo PAA reactors (the statistical p values were 0.003 and 0.006, respectively), which means that combined freezing and PAA pretreatment synergistically enhanced the biomethane production.
Kinetic analysis was then performed to reveal the effect of freezing + PAA pretreatment on some kinetic parameters relevant to methane production. Figure 3 exhibits fitted curves of the modified Gompertz model and first-order kinetic model under different conditions. It can be seen that both models captured the experimental data well, which were then confirmed by the specific fitting results shown in Table 3 and Table 4, as the R2 values for the reactors with or without pretreatment were all higher than 0.98. Also from Table 3, the three kinetic parameters of the modified Gompertz model all varied a lot after the pretreatments. For example, the Mm was only 159.4 mL/g VSS in the control group; it was enhanced to 206.8 and 221.8 mL/g VSS in the solo freezing and solo PAA pretreated reactors, respectively. The highest Mm value of 254.9 mL/g VSS was obtained under the freezing + PAA condition. The results indicated that the freezing and PAA methods both promoted the methane production potential of sludge, and their combination realized the synergistic effect. A similar trend was shown in Rm, which was enhanced from 13.15 to 17.18, 20.67, and 25.69 mL/g VSS/d by the solo freezing, solo PAA, and freezing + PAA pretreatments (Table 3), respectively, suggesting that the combined method synergistically accelerated the methane generation rate of the sludge anaerobic digestion system. The λ value, which refers to the lag phase time, was also enhanced in the treated reactors, suggesting that the starting of the anaerobic reaction was delayed after the pretreatments, especially under the freezing + PAA condition. Table 4 shows the kinetic parameters of the first-order kinetic model in different situations. B0 also represents the methane production potential, which is similar to Mm in the modified Gompertz model. The results in Table 4 show that B0 was generally enhanced after the solo freezing, solo PAA, and freezing + PAA pretreatments, which further proved that these methods positively affected the methane production potential of sludge, especially for the combined freezing and PAA pretreatments. The k value refers to the hydrolysis rate, and it was only 0.104 d−1 in the control group, suggesting that sludge hydrolysis efficiency was extremely low without any pretreatment. When treated by solo freezing or solo PAA, the k values increased to 0.108 and 0.114 d−1, respectively, and the maximum value of 0.125 d−1 was attained under the freezing + PAA condition, which means the sludge hydrolysis rate was efficiently accelerated by these methods. The previous research also found that both methane production potential and hydrolysis rate were promoted by the combined pretreatment of potassium ferrate and freezing, resulting in an increase in the practical methane yield [23]. The results of the kinetic model-based analysis above indicated that freezing coupled with PAA pretreatment largely enhanced the methane production potential, methane production rate, and hydrolysis rate of sludge, resulting in a great improvement in the practical methane yield (Figure 2).

3.3. Effect of Combined Freezing and PAA Pretreatment on Sludge Hydrolysis Efficiency

EPS grab most of the organic matter in sludge, which should be dissolved through hydrolysis before being used by functional microbes for methane generation [34]. Figure 4a shows the COD concentrations of EPS under different conditions, including TB-EPS and LB-EPS, both of which reduced after the pretreatments. For example, the TB-EPS decreased from 2894 to 2075 and 1162 mg/L by the solo freezing and solo PAA pretreatments, respectively, which further decreased to 537 mg/L under the co-treatment condition. The variation in LB-EPS showed a similar trend, with the COD contents detected to be 544, 489, 277, and 205 mg/L in the control, solo freezing, solo PAA, and freezing + PAA reactors, respectively. Based on these results, the sludge EPS structure was effectively destroyed by the combined freezing and PAA pretreatment, as both LB-EPS and TB-EPS were decomposed after the treatment process. There are also parts of the organic matter that exist in microbial cells, which can be utilized when cell membrane/wall is damaged. DNA and LDH are two intracellular substances that are commonly used to reflect the destruction of microbial cells [35,36], and their relative contents under different conditions are provided in Figure 4b. Compared with the control group, the relative contents of LDH in the solo freezing and solo PAA pretreated reactors were 186.4% and 255.2%, respectively, which were then enhanced to 303.7% by the freezing + PAA pretreatment, suggesting that many microbial cells were damaged under the co-pretreatment condition. For the DNA content, similar results were observed, as it was generally higher in the pretreated reactors than the control, especially under the freezing + PAA condition, which further confirmed our deduction regarding microbial cell destruction by the co-treatment of freezing and PAA. The previous study found that the main reason for the freezing method destructing the EPS and microbial cells was the existence of physical forces during ice front formation [19], while the effect of PAA on EPS and cell destruction was mainly based on chemical oxidation [37]. According to the above analysis, freezing coupled with PAA pretreatment effectively destroyed the EPS and microbial cells, which could be beneficial to hydrolysis, and more dissolved organics are expected to be produced.
Figure 5a depicts the SCOD concentrations of the sludge samples from the different reactors. A relatively low SCOD content of 109 mg/L was detected in the control group, which was generally enhanced to 1284, 4517, and 5994 mg/L by the solo freezing, solo PAA, and freezing + PAA pretreatments, respectively, indicating that the freezing and PAA pretreatments both enhanced sludge hydrolysis efficiency, and their combination realized the synergistic effect. Protein and carbohydrate are the main biodegradable organics in sludge, and their concentrations were also detected (Figure 5b). It was observed that the protein content was first enhanced from 36.7 to 348.3 and 1426.6 mg/L by the solo freezing and solo PAA pretreatments, respectively, then it further increased to 1812.5 mg/L under the freezing + PAA condition. Similar results appeared in the carbohydrate concentration, indicating that more biodegradable substances are generated through high-efficiency hydrolysis by freezing coupled with PAA treatment. The increased concentration of soluble organic matters, especially for the biodegradable substances, from hydrolysis by the combined pretreatment was undoubtedly beneficial to the subsequent fermentation and methane generation process, which served as an important reason for the enhanced methane production (Figure 2).

3.4. Influence of Combined Freezing and PAA Pretreatment on Microbial Community

Anaerobic digestion is driven by various functional microbes, such as hydrolytic microbes, acidogens, and methanogens; thus, the microbial community structure can directly affect biomethane production [38,39]. To disclose the microscopic mechanisms for enhanced methane yield by the freezing coupled with PAA pretreatment, microbial community analysis was conducted. The Venn diagram in Figure 6 exhibits the connection between the control and freezing + PAA treated reactors in microbial OTUs. It was observed that the total OTUs were 2154 and 1827 in the control and freezing + PAA treated reactors, respectively, suggesting that microbial diversity was reduced by the combined freezing and PAA pretreatment. According to the results in Figure 4a, many microbial cells were damaged by the combined pretreatment, resulting in the decrease in microbial diversity. The dead microbes could serve as digestion substrates for survival microbes, especially for the functional microbes that participate in methane production. Also from Figure 6, there were 1689 of the same OTUs shared by two reactors, while a total of 603 unique OTUs were generated, demonstrating that the microbial community structure changed a lot after the freezing coupled with PAA pretreatment.
Figure 7 further compares the genus level microbial components between the control and freezing + PAA treated reactors. There were some functional microbes involved in the anaerobic reactions detected in both reactors, such as Hyphomicrobium, norank_f_Caldilineaceae, Tissierella, Mycobacerium, norank_f_Bacteroidetes_vadinHA17, Longilinea, Petrimonas, norank_f__Anaerolineaceae, Sedimentibacter, Acetobacteroides, Romboutsia, Methanosarcina, Methanosaeta, and Methanobacterium. Among them, Hyphomicrobium and norank_f_Caldilineaceae both participate in the hydrolysis process [40], and their total abundances were 1.87% and 2.71% in the control and freezing + PAA treated reactors, respectively, suggesting that hydrolytic microbes were enriched by the co-treatment. The abundances of Tissierella and Petrimonas, two acidogenic genera that can degrade organic substances to generate acetic acid and hydrogen [41], were, respectively, 1.24% and 0.41% in the control, which were correspondingly enhanced to 1.92% and 1.14% in the freezing + PAA pretreated reactor. Mycobacerium is relevant to both the hydrolysis and acidification processes [42], and its abundances were 1.06% and 1.33% in the control and freezing + PAA treated reactors, respectively. The genus norank_f_Bacteroidetes_vadinHA17, which belongs to the phylum Bacteroidetes, was found to be responsible for lactic acid and propionic acid generation [43], and its abundance increased from 0.78% to 2.11% after the freezing + PAA pretreatment. All of Longilinea, norank_f__Anaerolineaceae, and Romboutsia mainly utilize carbohydrates to generate hydrogen as well as VFAs [44,45], and their total abundances were 2.86% and 5.33% in the control and freezing + PAA treated reactors, respectively. Sedimentibacter, a hydrogen and VFA producer that mainly utilizes pyruvate and amino acids as substrates [46], was enriched from 0.98% to 3.13% after the freezing + PAA pretreatment. The genus Acetobacteroides, a hydrogen producer that is widely detected in the anaerobic environment [47], was enriched from 0.76% to 1.23% when treated by freezing + PAA. Methanosaeta, Methanosarcina, and Methanobacterium are three typical methanogens that are widely found in anaerobic reactors, among which the first two genera belong to the acetoclastic category, while Methanobacterium is a kind of hydrogenotrophic methanogen [23,48]. From Figure 7, the total abundance of these three methanogenic genera was only 0.69% in the control, which was mainly attributable to their characteristic of low growth velocity [49]. When treated by freezing + PAA, their abundance was largely enhanced to 3.17%, which means the acetoclastic and hydrogenotrophic methanogens were both enriched by the combined freezing and PAA pretreatment. In Figure 7, the most abundant functional species was Sedimentibacter under the freezing + PAA condition, with the abundance detected to be 3.13%. The result was different from that of the previous literature [22], in which norank_f__Anaerolineaceae possessed the highest abundance under the freezing + calcium hypochlorite condition. According to above analysis, the functional microbes involved in different biochemical processes were all enriched under the freezing + PAA condition, with the total abundances calculated to be 10.65% and 22.07% in the control and freezing + PAA treated reactors, respectively, which was a crucial factor for increased methane yield by the freezing coupled with PAA pretreatment (Figure 2).

3.5. Economic Analysis

Economic rationality is an important factor in assessing the potential of a new technology, especially when it is considered for application in practical engineering. Therefore, the economic analysis of the different pretreatments proposed in this work was conducted by calculating the costs and benefits, respectively. The results in Table 5 showed that all the solo freezing, solo PAA, and freezing + PAA methods realized net benefits when compared with the control group, as the benefit from increased methane yield and saved dewatering, transportation, and landfill costs surpassed the relevant pretreatment cost, which means these methods were all economically viable. It should be noted that the freezing + PAA method achieved the highest net benefit among these three methods, though the pretreatment cost was also the highest, which further confirmed the synergistic effect of the combined freezing and PAA pretreatment on the sludge anaerobic digestion performance.

4. Conclusions

The effect of the freezing coupled with PAA pretreatment on sludge anaerobic digestion performance was studied in this work. The optimal parameters for the co-treatment were freezing at −10 °C combined with 0.08 g/g TSS of PAA, from which the maximum methane production of 261.5 mL/g VSS was realized and was, respectively, 57.2%, 22.9% and 13.7% higher than the control, solo freezing, and solo PAA conditions. The mechanism analysis showed that the freezing + PAA pretreatment destroyed both EPS and microbial cells, leading to a significant improvement in hydrolysis efficiency. Microbial community analysis revealed that the functional microbes involved in anaerobic digestion were generally enriched by the freezing + PAA pretreatment. The freezing + PAA method proposed in this study has advantages in terms of both technology and economy, which could be particularly suitable for applications in cold regions where natural freezing is easily achieved.

Author Contributions

Conceptualization, Z.-W.L. and Y.-Q.C.; data curation, Z.-W.L.; formal analysis, Z.-W.L.; funding acquisition, Z.-W.L.; investigation, Z.-W.L.; methodology, Z.-W.L.; project administration, Z.-W.L.; resources, Z.-W.L.; software, Y.-Q.C.; supervision, Z.-W.L.; validation, Z.-S.L.; visualization, S.-W.W.; writing—original draft, Z.-W.L.; writing—review and editing, Z.-S.L. and S.-W.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Grant Number 22178184), the Shandong Provincial Natural Science Foundation of China (Grant Number ZR2021MB007), and the Science, Education and Industry Integration Innovation Pilot Project from Qilu University of Technology (Shandong Academy of Sciences) (Grant Number 2024ZDZX12).

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

Correction Statement

This article has been republished with a minor correction to the title. This change does not affect the scientific content of the article.

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Figure 1. Cumulative methane production from sludge with different PAA dosages. Error bars represent standard deviations of triplicate tests.
Figure 1. Cumulative methane production from sludge with different PAA dosages. Error bars represent standard deviations of triplicate tests.
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Figure 2. Variations in cumulative methane production with time under different conditions. Error bars represent standard deviations of triplicate tests.
Figure 2. Variations in cumulative methane production with time under different conditions. Error bars represent standard deviations of triplicate tests.
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Figure 3. The fitting curves of methane production data by the modified Gompertz model (a) and first-order kinetic model (b).
Figure 3. The fitting curves of methane production data by the modified Gompertz model (a) and first-order kinetic model (b).
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Figure 4. The COD contents of EPS (a) and relative release of LDH and DNA (b) under different conditions. Error bars represent standard deviations of triplicate tests.
Figure 4. The COD contents of EPS (a) and relative release of LDH and DNA (b) under different conditions. Error bars represent standard deviations of triplicate tests.
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Figure 5. The SCOD (a), soluble protein, and carbohydrate (b) contents in sludge samples with or without pretreatment. Error bars represent standard deviations of triplicate tests.
Figure 5. The SCOD (a), soluble protein, and carbohydrate (b) contents in sludge samples with or without pretreatment. Error bars represent standard deviations of triplicate tests.
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Figure 6. The relationship of microbial OTUs between control and freezing + PAA pretreated reactors.
Figure 6. The relationship of microbial OTUs between control and freezing + PAA pretreated reactors.
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Figure 7. Heat map of the microbial abundance at genus level in control and freezing + PAA pretreated reactors.
Figure 7. Heat map of the microbial abundance at genus level in control and freezing + PAA pretreated reactors.
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Table 1. The primary characteristics of raw WAS and inoculum used in this study.
Table 1. The primary characteristics of raw WAS and inoculum used in this study.
ParametersMeasurement MethodsWASInoculum
pHpH meter6.8 ± 0.17.7 ± 0.1
TSS (total suspended solids) (mg/L)Weight method21,063 ± 18518,640 ± 154
VSS (volatile suspended solids) (mg/L)Weight method13,796 ± 1617882 ± 105
TCOD (total chemical oxygen demand) (mg/L)Dichromate titration20,849 ± 13311,074 ± 78
SCOD (soluble chemical oxygen demand) (mg/L)Dichromate titration87 ± 4255 ± 6
Table 2. The variations in methane production by combined freezing and PAA pretreatment.
Table 2. The variations in methane production by combined freezing and PAA pretreatment.
ReactorsPAA PretreatmentFreezing PretreatmentMethane Yield (mL/g VSS)
Dosage (g/g TSS)Time (h)Temperature (°C)Time (h)
102448166.3 ± 3.8
20.0224−58197.3 ± 4.0
30.0424−58214.2 ± 4.3
40.0624−58229.3 ± 4.3
50.0824−58237.8 ± 5.1
60.124−58212.3 ± 4.4
70.0224−108205.7 ± 4.2
80.0424−108220.4 ± 4.5
90.0624−108241.6 ± 5.4
100.0824−108259.0 ± 5.7
110.124−108207.2 ± 4.1
120.0224−158211.7 ± 4.6
130.0424−158228.5 ± 4.8
140.0624−158247.5 ± 5.3
150.0824−158251.3 ± 5.5
160.124−158205.6 ± 4.5
170.0224−208214.7 ± 4.2
180.0424−208233.4 ± 4.7
190.0624−208251.7 ± 5.0
200.0824−208238.8 ± 5.2
210.124−208202.5 ± 4.0
Table 3. Kinetic parameters of the modified Gompertz model under different pretreatment conditions.
Table 3. Kinetic parameters of the modified Gompertz model under different pretreatment conditions.
Pretreatment ConditionsKinetic Model Parameters
Mm (mL/g VSS)Rm (mL/g VSS/d)λ (d)R2
Control159.4 ± 1.613.15 ± 0.530.23 ± 0.240.9940
Freezing206.8 ± 1.617.18 ± 0.520.47 ± 0.180.9967
PAA221.8 ± 1.820.67 ± 0.690.62 ± 0.180.9961
Freezing + PAA254.9 ± 1.425.69 ± 0.640.67 ± 0.120.9979
Table 4. Kinetic parameters of first-order kinetic model under different pretreatment conditions.
Table 4. Kinetic parameters of first-order kinetic model under different pretreatment conditions.
Pretreatment ConditionsB0 (mL/g VSS)k (d−1)R2
Control169.8 ± 2.40.104 ± 0.0050.9931
Freezing221.3 ± 4.00.108 ± 0.0040.9896
PAA236.0 ± 4.60.114 ± 0.0060.9860
Freezing + PAA269.0 ± 5.30.125 ± 0.0070.9835
Table 5. Economic assessment for different pretreatment processes compared to the control a.
Table 5. Economic assessment for different pretreatment processes compared to the control a.
Pretreatment ConditionsCost b (USD)Increase in Methane Production (USD)Dewatering, Transportation, and Landfill CostsNet Saving Compared to the Control d (USD)
Amount of Solid c (ton)Decrease in Cost (USD)
Freezing31.648.220.5958.2434.82
PAA50.7611.320.5666.6427.20
Freezing + PAA82.4016.890.43103.0437.53
a All results are shown for per ton solid treatment compared to the control. b Energy input cost for freezing pretreatment and chemical cost for PAA pretreatment. c Amount of solid after pretreatment and anaerobic digestion. d Net saving compared to control = increase in methane (USD) + reduction in dewatering, transportation, and landfill costs (USD)—pretreatment cost (USD).
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Liu, Z.-W.; Chen, Y.-Q.; Liu, Z.-S.; Wang, S.-W. Freezing Method Assists Peracetic Acid Oxidation for Promoting the Methane Production from Sludge Anaerobic Digestion. Energies 2025, 18, 731. https://doi.org/10.3390/en18030731

AMA Style

Liu Z-W, Chen Y-Q, Liu Z-S, Wang S-W. Freezing Method Assists Peracetic Acid Oxidation for Promoting the Methane Production from Sludge Anaerobic Digestion. Energies. 2025; 18(3):731. https://doi.org/10.3390/en18030731

Chicago/Turabian Style

Liu, Zhen-Wei, Yan-Qiu Chen, Zhi-Shuai Liu, and Sheng-Wu Wang. 2025. "Freezing Method Assists Peracetic Acid Oxidation for Promoting the Methane Production from Sludge Anaerobic Digestion" Energies 18, no. 3: 731. https://doi.org/10.3390/en18030731

APA Style

Liu, Z.-W., Chen, Y.-Q., Liu, Z.-S., & Wang, S.-W. (2025). Freezing Method Assists Peracetic Acid Oxidation for Promoting the Methane Production from Sludge Anaerobic Digestion. Energies, 18(3), 731. https://doi.org/10.3390/en18030731

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