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Stage I - Hydrolysis and Fermentation Large, insoluble organic molecules are broken down by extra-cellular enzymes through hydrolysis. High amounts of cellulose, or other material that is hydrolyzed slowly, can limit the performance of the entire system (Poulsen, 2003). Short-chain fatty acids (e.g. acetic acid, formic acid) are formed by the enzymes while by-products undergo fermentation to produce alcohols, C02, and H2. Stage II - Acidogenesis Any short-chain fatty acids longer than acetic acid are oxidized by the acidogenic bacteria to produce formic acid, acetic acid, and H2. Research is currently being conducted on means of promoting the H2 producing feature of acidogenic bacteria for use as an alternative fuel source (Valdez-Vazquez et al, 2004). Stage III - Methanogenesis Methanogenic bacteria convert the acetic acid and H2 to methane and carbon dioxide. Of the three primary groups of bacteria, the methanogenic are the most unstable (Price and Cheremisinoff, 1981). Figure 1 indicates two more groups (Fermentation Group IV, Oxidation Group V) as being part of the anaerobic process. Group IV can either produce acetic acid or break it down if the concentration of acetate is too high. Group V oxidizes acetic acid to H2 and CO2. Neither group is central to the basic understanding of anaerobic digestion. Figure 2 summarizes the main processes in a simplified flowchart. Figure 2 Carbon flow in anaerobic environments with active methanogens (Ahring, 2003) It is also important to note that these processes must be in balance with each other. For example, if the methanogenic bacteria do not use the hydrogen produced by the acidogenic bacteria almost immediately, the higher hydrogen concentration will increase the production of alcohols and fatty acids, causing the system to collapse (Poulsen, 2003). 3PDF Image | Electricity and Heat Production Using Biogas from the Anaerobic Digestion of Livestock Manure - Literature Review
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