Peracetic acid enhanced anaerobic co-digestion of excess sludge and food waste: Performance and mechanism

Excess sludge (ES), as a byproduct of sewage biological treatment processes, represents a complex medium characterized by multiple components, media, and interactions (Wei et al. 2018). It was reported that the annual output of ES was as high as 30 million tons (Qiang et al. 2015). In addition to organic matter, sludge contains a plethora of pollutants such as heavy metals and antibiotics. Improper handling and disposal of sludge can lead to secondary environmental pollution (Cheong et al. 2022; Wu et al. 2022). Food waste (FW) is also a typical organic waste in urban areas, with an annual production of 600 million tons (Zhang et al. 2018). Currently, the main strategies for dealing with ES and FW include incineration, landfilling, composting, and anaerobic digestion (Liew et al. 2023; Zhao et al. 2023). Anaerobic digestion technology, as a sustainable biological treatment method, offers a viable approach for the decomposition of organic waste and energy recovery, presenting one of the effective methods for ecologically treating sludge (Liew et al. 2022). However, the presence of tough cell walls and membranes encapsulating sludge restricts the release and utilization of intracellular substances, thereby resulting in low anaerobic digestion efficiency. Only 30–35% of organic matter within sludge can be converted into volatile fatty acids (VFAs) through anaerobic digestion, leaving the remaining 60–65% unused and requiring further treatment and disposal (Feng et al. 2014). Anaerobic digestion of sludge alone suffers from the drawback of carbon-to-nitrogen (C/N) imbalance. Recently, co-digestion of ES with FW has garnered widespread attention, as it offers advantages such as balanced C/N ratios, synergistic disposal of organic waste, and enrichment of microbial communities (Mehariya et al. 2018; Zhao et al. 2021; Kan et al. 2021a, 2022a; Zhang et al. 2023a, 2023b).

In order to further enhance the anaerobic co-digestion of ES and FW, various pretreatment technologies, including physical, chemical, and biological methods, have been applied to improve the hydrolysis rate (Yuan et al. 2019; Kan et al. 2021b; Wang et al. 2023; Fu et al. 2024; Guan et al. 2024). Among these, the addition of chemical reagents offers the advantages of easy operation, high treatment efficiency, and good economic performance. Recently, peracetic acid (PAA) has been widely used for water disinfection, removal of difficult-to-degrade pollutants, and inactivation of pathogens due to its excellent oxidative capacity and bactericidal properties (Kiejza et al. 2021; Kan et al. 2022b). PAA has a relatively low O–OH bond dissociation energy, which means that when using PAA for water disinfection, the amount of PAA required can be significantly reduced. Ren et al. (2024) reported that PAA can enhance the release and transformation of organic matter, and alter the microbial community structure to improve the ES anaerobic digestion efficiency. Additionally, Appels et al. (2011) applied PAA to enhance organic hydrolysis to increase biogas production, with the maximum increase in biogas production reaching 21%, however, high doses of PAA led to a decrease in biogas production. PAA has also been used in the dark fermentation of sludge to produce hydrogen, where it can change the fermentation type from propionic to acetic and butyric acids (Li et al. 2022). The hydroxyl and acetate radicals produced during the PAA degradation process can significantly improve the conductivity of sludge and suppress the generation of odorous gases during the sludge dewatering process. Based on these findings, PAA can serve as an intensifier to enhance the hydrolysis of organic matter, thereby increasing the resource utilization rate of organic matter. However, studies on the use of PAA to enhance the co-digestion of ES and FW are scarce, and the underlying mechanisms are not well understood. Moreover, it is well-known that the anaerobic digestion process of organic matter involves sequential biochemical stages such as hydrolysis, acidification, and methanogenesis, and how PAA affects these processes is still unclear. Therefore, it is necessary to investigate the impact of PAA pretreatment on the anaerobic digestion of ES and FW to improve the efficient utilization rate of organic solid waste in urban areas.

Based on the aforementioned discussion, this work investigated the impact of PAA on the anaerobic co-digestion of sludge and FW, and revealed the related mechanisms. Firstly, the influence of PAA concentration on the co-digestion of ES and FW was systematically explored. Secondly, the characteristics of organic matter transformation during the anaerobic co-digestion process of ES and FW in the presence of PAA were analyzed. Thirdly, the impact of PAA on each step of the anaerobic digestion process was examined. Finally, the significance of energy recovery based on PAA enhanced anaerobic co-digestion of ES and FW is discussed. The results of the work enrich the application scope of PAA and provide a theoretical basis for the resource recovery of urban organic solid waste.

The ES was sourced from the secondary sedimentation tank of a wastewater treatment plant. The retrieved ES was filtered to remove impurities and was kept on standby for use, with the main characteristics of the experimental ES presented in Table 1.

The FW utilized in the experiments was collected from a university student cafeteria. After collection, the FW was manually screened to remove non-degradable impurities such as chopsticks, plastic bags, and animal bones. It was then pulverized into a paste-like substance using a high-speed blender (Joyoung brand), with the main characteristics of the FW also displayed in Table 1.

The inoculum was derived from a continuous-flow anaerobic digester in a laboratory, which is primarily used for sludge anaerobic digestion experiments. Upon retrieval, the inoculum was allowed to settle for 24 h to remove the supernatant before being prepared for use, with the main properties of the inoculum showcased in Table 1.

PAA was purchased from a chemical reagent company in Shanghai and stored in a brown bottle in the laboratory upon arrival. The main characteristics of PAA are as follows: Boiling point: 105 °C, relative density (water = 1): 1.15 (at 20 °C), saturated vapor pressure: 2.67 kPa (at 25 °C), flash point: 41 °C. A schematic diagram of the anaerobic SBR device used in this work is presented in the Supplementary material.

This experiment was conducted in five sets of identical reactors, with each set comprising three identical reactors. Initially, each reactor was fed with 3.0 L of inoculum and 3.0 L of digestion substrate (the ES and FW were mixed at a mass ratio of 1:1 calculated based on total suspended solids (TSS) quality). Secondly, different amounts of PAA were introduced into each group, controlling the mass concentration to be 0, 3, 6, 9, and 12%, respectively (calculated based on the TSS of the digestion substrate). Thirdly, after the addition of materials to each reactor was complete, 2.0 M hydrochloric acid or 3.0 M sodium hydroxide was manually added to adjust the initial pH to 7.0, with no further intervention during the reaction process. Finally, high purity (over 99.99%) nitrogen gas was introduced into each group for 1.0 min to purge the oxygen from the reactors, ensuring a strictly anaerobic environment. The reactors were then quickly sealed with rubber stoppers and transferred to a constant temperature shaker for mesophilic anaerobic digestion (35 °C). Each reactor was equipped with a 5.0 L gas collection bag above it, and the biogas production was measured daily using the water displacement method. The characteristics of the digestion liquid and the digestion substrate were regularly determined during the anaerobic digestion process to assess the impact of PAA on the ES and FW anaerobic co-digestion process.

This batch experiment was conducted to investigate the impact of PAA exposure on every step of the ES and FW co-digestion process. Initially, three sets of reactors were established, each containing five fermenters of the same volume (500 mL), defined as G-I, G-II, and G-III. These sets were used to assess the effects of PAA on hydrolysis, acidification, and methanization processes. All groups were inoculated with 200 mL of inoculum and 300 mL of synthetic wastewater. PAA was then added to the reactors in varying doses to control its concentrations at 0, 3, 6, 9, and 12%. G-I's synthetic wastewater contained 2.0 g/L of bovine serum albumin (BSA) and 3.0 g/L of dextran. G-II's synthetic wastewater contained 2.0 g/L of L-alanine and 3.0 g/L of glucose. G-III's synthetic wastewater contained 2.0 g/L of sodium acetate. The impact of PAA on each anaerobic step was determined by measuring the degradation of the simulated compound in each group.

The samples retrieved from the reactors were immediately passed through a 0.45-μm acetate cellulose membrane to obtain the digestion liquid for the analysis of water quality characteristics. The determination of chemical oxygen demand (COD), ammonium nitrogen (NH₄⁺-N), soluble orthophosphate (SOP), TSS, and volatile suspended solids (VSS) was carried out using international standard methods (APHA 1985). Methane and VFAs were measured using gas chromatography with a chromatograph model Agilent 7890A, equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD), respectively. The calculation of methane yield is based on the amount of methane produced per unit reduction of organic matter. Specifically, it is the ratio of the cumulative methane production on a particular day to the reduction in VSS. The conditions and steps for the determination of VFA are as follows: the chromatographic column type is FFAP - MM0071, with dimensions of 30 m × 0.25 mm × 0.25 μm. The temperatures of the injection port, column oven, and detector were set at 100, 80, and 100 °C, respectively. Argon was used as the carrier gas at a flow rate of 44 mL/min, with a split ratio of 1:20 (Cheng et al. 2021; Kan et al. 2023; Li et al. 2023). Soluble protein (SPN) and soluble polysaccharide (SPS) were determined by the Folin phenol method and the anthrone colorimetric method, respectively, and BSA and glucose were used as standard substrates, respectively.

To ensure the dependability of the results, each test was repeated three times, and the error bars were calculated from the standard deviations of these repetitions. A p-value less than 0.05 was set as the threshold for statistical significance, which helps to identify the differences between the sample groups.

Figure 1(b) further illustrates the effect of PAA on the daily methane production during the co-digestion process of ES and FW. It can be observed that the daily methane production exhibits two peaks over time, approximately at 7 and 15 days. The first peak in daily gas production is primarily associated with the ample substrate, allowing methanogenic archaea to efficiently consume the substrate. The second peak emerges as the methanogens gradually adapt to the metabolic environment. PAA effectively enhances the daily methane production during both peaks. For instance, in the presence of PAA, the highest daily methane production increased to a range of 22.6 to 33.8 mL/g VSS, which is higher than the 18.6 mL/g VSS observed in the control group without PAA. Similar results were found at other time intervals. These findings indicate that PAA can significantly promote the efficiency of daily methane production.

PAA possesses certain oxidative properties, which effectively facilitate the solubilization of refractory organic matter within ES and FW, accelerating the hydrolysis rate and providing a material basis for methanogenic microorganisms. By enhancing the solubility of organic substrates, PAA improves the bioavailability for microbial degradation, leading to an increased rate of methane production (Ao et al. 2021; Guo et al. 2024). This suggests that PAA not only influences the immediate metabolic processes but also contributes to the overall efficiency of the anaerobic digestion system by improving the breakdown of complex organic compounds into simpler forms that can be metabolized by the methanogens, ultimately resulting in higher methane yields.

To clarify the reasons behind the enhancement of methane production by PAA, this study also evaluated the impact of PAA on each step of the anaerobic co-digestion process. During the hydrolysis stage, BSA and dextran are degraded by hydrolytic bacteria into small molecular amino acids and glucose. As shown in Table 2, PAA increased the degradation of BSA and dextran. When the PAA concentration was 9%, the degradation efficiencies of BSA and dextran were 73.2 and 85.6%, respectively, significantly higher than the 62.2 and 72.3% in the control group. This indicates that PAA improved the hydrolysis process of organic matter, which is consistent with the results in Figure 2 where the SCOD levels in the PAA-treated groups were higher than those in the control group.

During the acidification process, various amino acids and glucose are further degraded into small molecular carboxylic acids such as acetic acid. As depicted in Figure 5(b), PAA also enhanced the degradation of L-alanine and dextran, especially in the 9% group, where the degradation efficiencies of L-alanine and dextran were 65.5 and 59.6%, respectively, again higher than the 52.3 and 49.6% in the control group. This also suggests that PAA improved the acidification metabolic process of organic matter. It should be noted that in the 12% PAA group, the degradation efficiency of the simulated substrate in the acidification process was lower than that in the group with PAA of 3–9%, but higher than that in the control group, which was related to the strong oxidizing properties of PAA, which in turn inhibited the activity of acidified microorganisms.

The promotion of hydrolysis and acidification of organic matter by PAA is related to its oxidizing properties. PAA increases the contact points between organic matter and key enzymes, enlarges the reaction area, and enhances the degradation of organic matter. The oxidizing nature of PAA not only aids in breaking down complex organic compounds into simpler forms that are more accessible for microbial attack but also potentially alters the microbial community structure and function to favor methanogenic processes (Ren et al. 2024).

Regarding the methanogenesis process, it was found that PAA severely inhibits the methanization process, with a trend that the higher the concentration of PAA, the more pronounced the inhibition of methanization. In the group treated with 12% PAA, the degradation efficiency of acetate dropped to 42.6%, which is only 56.6% of that in the control group, indicating that PAA significantly inhibits the consumption of acetate by methanogenic archaea to produce methane. One might question why, despite the inhibition of methanogenesis by PAA, the group treated with 9% PAA showed the highest methane production. This paradox can be explained by considering the dual role of PAA in the anaerobic digestion process. While high concentrations of PAA can inhibit methanogenesis, PAA also promotes the hydrolysis and acidification processes, which provide ample substrates for methanogenesis.

In essence, the enhancement of hydrolysis and acidification by PAA leads to increased availability of VFA, particularly acetate, which is the primary substrate for acetoclastic methanogens. Even though PAA at higher concentrations can inhibit the methanogenic process, the overall effect at optimal concentrations (such as 9% in this case) may still result in higher methane production due to the increased supply of substrates. This suggests that the balance between the stimulation of earlier stages of anaerobic digestion (hydrolysis and acidification) and the potential inhibition of the methanogenic stage is crucial for optimizing methane production in the presence of PAA.

Therefore, the key to maximizing methane yield in systems employing PAA may lie in carefully controlling the concentration of PAA to ensure that the benefits of enhanced hydrolysis and acidification outweigh the inhibitory effects on methanogenesis. This finding underscores the complexity of the interactions between PAA and the microbial processes in anaerobic digestion and highlights the importance of optimizing PAA dosage for effective biogas enhancement.

Given the strong oxidizing nature of PAA, it has been utilized to enhance the oxidation and removal of refractory organic substances in wastewater, as well as in the anaerobic fermentation of waste activated sludge to produce VFA and hydrogen (Liu et al. 2020; Ren et al. 2024). In addition to these advantages, the current work further applies PAA in the anaerobic co-digestion of ES and FW to obtain the energy substrate methane, and the related mechanisms have been systematically revealed. This provides a theoretical basis for the development of treatment technologies for urban organic solid waste based on PAA processing. As far as we know, this is the first application of PAA in the ES and FW anaerobic co-digestion, and the mechanisms involved have been unveiled. This work not only provides an alternative technology for the treatment of urban organic solid waste but also further expands the application field of PAA. The application of PAA in this work showcases its versatility and potential as an environmentally friendly and efficient agent for enhancing anaerobic digestion processes. By improving the conversion of organic matter into valuable end products such as methane, PAA contributes to the circular economy and sustainable waste management strategies. The findings of this study are significant as they pave the way for the integration of PAA into existing and new waste-to-energy systems, potentially leading to more efficient energy recovery from organic waste streams and contributing to the reduction of greenhouse gas emissions.

The application of PAA to enhance the anaerobic co-digestion of ES and FW for methane production is a green and environmentally friendly strategy, as the degradation products of PAA are carbon dioxide and water, which are less harmful to the environment. PAA is capable of generating a higher amount of oxidative free radicals, which are essential precursors for the oxidation of organic matter in the sludge and FW, leading to improved solubilization and biogas production. Furthermore, various materials, including transition metals and activated carbon, can enhance the activity of PAA, thereby increasing its oxidation efficiency and accelerating the breakdown of sludge. These materials can act as catalysts, promoting the generation of more reactive oxygen species (ROS) and other free radicals that are more effective in degrading complex organic compounds. In future research, it would be beneficial to explore the development of suitable catalysts to strengthen the oxidative properties of PAA. By doing so, the efficiency of the anaerobic co-digestion process could be further improved, leading to increased methane yields and more effective treatment of organic waste. The use of catalysts could also potentially reduce the required amount of PAA, thereby lowering the operational costs and minimizing the environmental impact of the process.

Overall, the integration of PAA with appropriate catalysts presents a promising avenue for the advancement of anaerobic co-digestion technology, contributing to the sustainable management of organic waste and the production of renewable energy in the form of biogas.

This work reports a new strategy for enhancing methane production from the anaerobic co-digestion of ES and FW using PAA, and reveals the related mechanisms. The optimal dose of PAA is 9%, with the maximum methane yield being 416 mL/g. The methane production in the PAA-treated group is higher than that of the control group, and the fermentation time is also shortened to varying degrees. PAA promotes the release of organic matter within the co-digestion substrate and increases the concentrations of SPN and SPS. PAA facilitates the production and conversion of VFA, preventing excessive acidification, thereby increasing methane yield. Additionally, PAA promotes substrate degradation in hydrolysis and acidification process, but reduces substrate consumption in the methanogenic process. PAA is proven to be a green and efficient strategy for promoting methane production from the co-digestion of ES and FW.

This work was financially supported by the project of the Natural Science Foundation of Henan Province (No. 152208318).

X.Y.: Writing – original draft. J.F.: Writing – review and editing. Y.X.: Writing – review and editing, Project administration.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.