The acidogenesis stage is a complex phase involving acid-forming fermentation, hydrogen production and an acetogenic (acetic acid-forming) step. Once complex organics are hydrolyzed, acidogenic (acid-forming) bacteria convert sugars, amino acids and fatty acids to smaller organic acids, hydrogen and carbon dioxide. The products formed vary with the types of bacteria as well as with environmental conditions. The community of bacteria responsible for acid production may include facultative anaerobic bacteria, strict anaerobic bacteria or both (e.g. Bacteroides, Bifidobacterium, Clostridium, Lactobacillus, Streptococcus). Hydrogen is produced by the acidogenic bacteria including hydrogen-producing aceto-genic bacteria. Acetogenic bacteria such as Syntrobacter wolini and Syntrophomonas wolfei convert fatty acids (e.g. propionic acid and butyric acid) and alcohol into acetate, hydrogen and carbon dioxide, which are used in methanogenesis. These microorganisms are related and can tolerate a wide range of environmental conditions (Novaes, 1986; Parkin and Owen, 1986).

Hydrogen has been shown to play a key role in regulating organic acids production and consumption. Under relatively high H2 pressure, acetic acid formation is reduced and the substrate is converted into propionic acid, butyric acid and ethanol, rather than methane. Thus, to maintain efficient anaerobic treatment of sludge to produce methane, the hydrogen levels must be maintained at least below 10-4 atom level. The syntrophic association between acetogenic bacteria and CO2-reducing methanogenic bacteria can ensure maintenance of low hydrogen concentration (Harper and Pohland, 1986). Because of its key role, hydrogen is considered a performance indicator of anaerobic methane fermentation. Acetogenic bacteria grow much faster than methanogenic bacteria. The former group has a growth rate (mmax) of approximately 1/h, whereas the latter is around 0.04/h (Stronach et al., 1986).

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