Journal of Energy Bioscience 2024, Vol.15, No.2, 118-131 http://bioscipublisher.com/index.php/jeb 119 This systematic study aims to provide a comprehensive overview of the current advancements and challenges in enhancing the efficiency of converting agricultural waste into biomethane using anaerobic digestion technology. The specific objectives are to evaluate the latest innovations and process configurations that improve biomethane production efficiency, to assess the economic and environmental benefits of integrating anaerobic digestion with other technologies, to identify the key factors influencing the performance and stability of anaerobic digestion systems, and to highlight future research directions and potential policy implications for promoting the adoption of anaerobic digestion technology in agricultural waste management. By synthesizing findings from recent studies, this study seeks to inform researchers, policymakers, and industry stakeholders about the potential of anaerobic digestion technology to contribute to sustainable energy production and waste management. 2 Agricultural Waste as a Feedstock for Biomethane Production 2.1 Types of agricultural waste suitable for anaerobic digestion Agricultural waste encompasses a variety of materials that can be effectively utilized as feedstock for anaerobic digestion to produce biomethane. The primary types of agricultural waste include: Crop Residues: These are the remnants of crops after the harvest, such as straw, stalks, and husks. Crop residues are rich in lignocellulosic materials, which can be challenging to break down but offer significant potential for biomethane production when pretreated appropriately (Salman et al., 2017; Noor et al., 2021). Animal Manure: Manure from livestock such as cattle, swine, and poultry is a common feedstock for anaerobic digestion. Co-digestion of animal manure with other organic wastes has been shown to significantly enhance methane yields (Muscolo et al., 2017; Ma et al., 2020). Food Processing Waste: This includes waste generated from the processing of agricultural products, such as fruit and vegetable peels, pulp, and other organic residues. Food processing waste is often rich in easily degradable organic matter, making it an excellent candidate for anaerobic digestion (Bong et al., 2018; Neri et al., 2023). 2.2 Characteristics and composition of agricultural waste The efficiency of anaerobic digestion largely depends on the characteristics and composition of the agricultural waste used. Key factors include: Carbon to Nitrogen (C/N) Ratio: An optimal C/N ratio is crucial for maintaining microbial activity and enhancing methane production. For instance, co-digestion studies have shown that a C/N ratio ranging from 26 to 34 is ideal for maximizing methane yields (Ma et al., 2020; Pan et al., 2021). Volatile Solids (VS): The amount of organic matter available for microbial degradation is indicated by the volatile solids content. Higher VS content generally correlates with higher biogas production potential (Meng et al., 2015; Ma et al., 2020). Particle Size and Pretreatment: Smaller particle sizes and appropriate pretreatment methods, such as mechanical, thermal, or chemical treatments, can enhance the digestibility of lignocellulosic materials, thereby improving biomethane yields (Salman et al., 2017; Bong et al., 2018). pH and Temperature: Maintaining optimal pH (around 7.0-7.5) and temperature (mesophilic: 30℃-40℃, thermophilic: 40℃-50℃) conditions is essential for the stability and efficiency of the anaerobic digestion process (Noor et al., 2021; Neri et al., 2023). 2.3 Challenges associated with using agricultural waste as feedstock Despite the potential benefits, several challenges are associated with using agricultural waste as feedstock for biomethane production:
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