The formation of methane, which is the ultimate product of anaerobic treatment, occurs by two major routes. Formic acid, acetic acid, methanol and hydrogen can be used as energy sources by the various methanogens. The primary route is the fermentation of the major product of the acid-forming phase, acetic acid, to methane and carbon dioxide. Bacteria that utilize acetic acid are acetoclastic bacteria (acetate splitting bacteria). The overall reaction is
The acetoclastic group comprises two main genera: Methanosarcina and Methanothrix. During the thermophilic digestion of lignocellulose waste, Methanosarcina is the dominant acetoclastic bacterium encountered in the bioreactor. About two-thirds of methane gas is derived from acetate conversion by acetoclastic methanogens.
Some methanogens use hydrogen to reduce carbon dioxide to methane (hydrogenophilic methanogens) according to the following overall reaction (Novaes, 1986; Morgan et al, 1991):
There is a synergistic relationship between the hydrogen producers and the hydrogen scavengers. Subtle changes in hydrogen conditions can change the end products of the acid-forming phase. Alcohol degradation is also inhibited by high hydrogen levels (Gujer and Zehnder, 1983).
The methane-forming bacteria are more fastidious in their environmental requirements than acid-forming bacteria. Their metabolic rates are lower than those of acid-forming bacteria and methane production is generally the rate-limiting step in anaerobic digestion (Speece, 1996).
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