ABSTRACT
Biogas production is one of the oldest and most widely used conversion processes for most wastewater treatment plants (Turovskiy and Mathai, 2006) and including animal manure (NAS, 1976). The technical term for the process is anaerobic digestion. It is a biochemical conversion process carried out by various microorganisms in which the biomass organic material is converted into organic acids, which in turn are converted into methane gas with carbon dioxide as the other by-product. The process takes place under very strict anaerobic conditions (no oxygen). The entire process takes place in four clearly dened process, as shown in Figure 7.1, and include the following four distinct events:
1. Hydrolysis-converting insoluble complex biomass into soluble com pounds
2. Acidogenesis-converting higher organics into simpler organics
3. Acetogenesis-converting higher organics and organics acids into acetic acid
4. Methanogenesis-converting organic acid and hydrogen into methane
7.1.1 Hydrolysis
Hydrolysis is the process of converting insoluble complex biomass into soluble compounds by the action of extracellular enzymes of hydrolytic bacteria. Thus, these bacteria are the rst set of microorganisms that breaks down the biomass into simpler compounds. Carbohydrates, fats, and proteins are converted into sugars, fatty acids, and amino acids. The bulk of microorganisms are fermentative in nature (i.e., organisms that digest cellulose, hemicellulose, starch, proteins, and sugars, converting them into some other forms such as fatty acids and alcohols). In most anaerobic digesters, this stage can be a rate-limiting step if the digester is not properly inoculated with fermentative microbes. Thus, in most newly operated anaerobic digesters, the rst few days of operation could be challenging if the microbial population in the reactor is too small for this initial step. At the end of this step, the complex organics are converted into simpler organics, mostly volatile fatty acids.
