Journal of Energy Bioscience 2024, Vol.15, No.2, 118-131 http://bioscipublisher.com/index.php/jeb 121 3.2 Stages of anaerobic digestion The anaerobic digestion process can be divided into four main stages: Hydrolysis: In this initial stage, complex organic polymers such as carbohydrates, proteins, and fats are broken down into simpler monomers like sugars, amino acids, and fatty acids by hydrolytic enzymes (Mlaik et al., 2019). Enzymatic pretreatment has been shown to enhance this stage significantly, improving the solubilization of organic matter (Mlaik et al., 2019). Acidogenesis: The monomers produced during hydrolysis are further broken down by acidogenic bacteria into volatile fatty acids (VFAs), alcohols, hydrogen, and carbon dioxide (Li et al., 2019). Acetogenesis: In this stage, the VFAs and alcohols are converted into acetic acid, hydrogen, and carbon dioxide by acetogenic bacteria (Li et al., 2019). Methanogenesis: The final stage involves methanogenic archaea converting acetic acid, hydrogen, and carbon dioxide into methane and water. This stage is crucial for the production of biogas and can be enhanced by improving enzyme activity and electron transfer processes (Salman et al., 2017; Li et al., 2019). 3.3 Types of anaerobic digesters There are several types of anaerobic digesters, each suited to different types of feedstock and operational conditions: Batch Digesters: These operate in a fill-and-draw mode, where the digester is filled, sealed, and left to digest the material for a set period before being emptied (Li et al., 2019). Continuous Digesters: These systems continuously feed organic material into the digester and continuously remove digested material, allowing for a steady production of biogas (Li et al., 2019). Plug Flow Digesters: These are designed for high-solids content feedstocks and operate by pushing the material through the digester in a plug flow manner, ensuring that the material moves as a unit through the system (Li et al., 2019). 3.4 Parameters influencing the anaerobic digestion process Several parameters significantly influence the efficiency and stability of the anaerobic digestion process: Temperature: The process can be operated under mesophilic (30℃-40℃) or thermophilic (50℃-60℃) conditions. Thermophilic digestion generally results in higher biogas yields but requires more energy input (Li et al., 2019). pH: The optimal pH range for anaerobic digestion is between 6.8 and 7.2. Deviations from this range can inhibit microbial activity and reduce biogas production (Li et al., 2019). C/N Ratio: The carbon to nitrogen (C/N) ratio is crucial for maintaining microbial balance. An optimal C/N ratio of around 20-30:1 is recommended for efficient digestion (Li et al., 2019). Hydraulic Retention Time (HRT): This is the average time the feedstock remains in the digester. Longer HRTs generally improve biogas yields but require larger digester volumes (Li et al., 2019). Organic Loading Rate (OLR): This parameter refers to the amount of organic material fed into the digester per unit volume per day. Optimizing OLR is essential to prevent overloading and ensure stable operation (Li et al., 2019). By understanding and optimizing these parameters, the efficiency of converting agricultural waste into biomethane using anaerobic digestion technology can be significantly enhanced.
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