Introduction
The increasing demand for sustainable and renewable energy sources has intensified research on bioenergy production from agricultural wastes. Anaerobic digestion is a widely applied biological process for converting organic residues into valuable gaseous fuels such as biohydrogen and biomethane. However, the efficiency of this process is often limited by the complex structure of lignocellulosic biomass and the slow hydrolysis step, which acts as a rate‑limiting factor. Therefore, various pretreatment techniques have been proposed to enhance substrate biodegradability and improve gas yields. Among physical pretreatment methods, ultrasonic pretreatment has attracted significant attention due to its ability to disrupt cell walls, reduce particle size, and enhance solubilisation of organic matter through cavitation effects. Despite numerous studies on biogas enhancement using ultrasonic pretreatment, limited research has simultaneously investigated its effect on biohydrogen production, gas composition (H₂, CH₄, CO, and H₂S), and the associated metabolic pathways for different agricultural residues. Accordingly, the main objective of this study was to evaluate the impact of ultrasonic pretreatment on the anaerobic digestion performance of selected agricultural wastes, including corn residues, potato waste, and banana waste. In addition to gas production performance, particular emphasis was placed on analysing changes in metabolic pathways and alcohol formation to better understand the mechanisms governing hydrogen and methane generation.
Materials and Methods
In this study three types of agricultural wastes, namely corn residues, potato waste, and banana waste, were used as feedstocks. The substrates were mixed with animal manure and water to provide appropriate microbial inoculation and moisture content. The prepared mixtures were mechanically stirred at 500 rpm for 10 minutes to ensure homogeneity. Ultrasonic pretreatment was applied using an ultrasonic device operating at 300 W for 5 minutes. Both pretreated and untreated samples were then subjected to anaerobic digestion under controlled conditions. Total solids (TS) and volatile solids (VS) were measured to characterise the substrates before digestion. During the anaerobic digestion process, the composition of the produced gases, including hydrogen (H₂), methane (CH₄), carbon monoxide (CO), and hydrogen sulfide (H₂S), was monitored. In addition, total alcohol concentration (ALC) was measured to assess shifts in fermentation pathways. The obtained data were analysed to compare the performance of ultrasonic pretreatment across different feedstocks and to evaluate its influence on metabolic reactions.
Results and Discussion
The results demonstrated that the effect of ultrasonic pretreatment on gas production strongly depended on the type of agricultural waste. For corn residues, ultrasonic pretreatment significantly enhanced biohydrogen production, increasing hydrogen concentration from approximately 2,585 ppm to 3,900 ppm. Methane production also showed a moderate increase, rising from about 104,000 ppm to 107,000 ppm. These improvements can be attributed to enhanced solubilisation of organic matter and improved accessibility of fermentable substrates. In contrast, potato waste exhibited decreased hydrogen and methane production following ultrasonic pretreatment. This behaviour suggests that excessive disruption of the substrate structure may have promoted alternative metabolic pathways unfavourable for gas generation. Banana waste showed a substantial percentage increase in hydrogen production after ultrasonic pretreatment, although its effect on methane production was less pronounced. Across all substrates, ultrasonic pretreatment led to an increase in carbon monoxide concentration and a noticeable reduction in hydrogen sulfide, which is considered beneficial due to the corrosive nature of H₂S. Metabolic pathway analysis revealed that ultrasonic pretreatment, particularly in banana and corn wastes, promoted pathways leading to alcohol production, such as ethanol and methanol formation. This shift explains the observed reduction or stagnation in hydrogen and methane production during later stages of digestion, as substrates were partially diverted toward solventogenic reactions.
Conclusion
The results of this study indicate that ultrasonic pretreatment can be an effective approach for improving the performance of anaerobic digestion; however, its effectiveness strongly depends on the type of substrate used. Among the investigated agricultural residues, corn waste demonstrated the most favourable response to ultrasonic pretreatment, showing improvements in both hydrogen and methane production. Banana peel also exhibited a noticeable increase in hydrogen generation after pretreatment, whereas potato waste showed a less favourable response and did not benefit significantly from the ultrasonic treatment. In addition, the variations observed in gas composition and alcohol production suggest that changes in microbial metabolic pathways play an important role in determining the outcomes of pretreatment processes. Overall, these findings suggest that ultrasonic pretreatment has considerable potential for enhancing bioenergy recovery from agricultural wastes, provided that pretreatment conditions are carefully optimised according to the characteristics of each substrate.