Anaerobic bioreactors can be classified into two-phase anaerobic digestion (TPAD) and two-stage anaerobic digestion (TSAD). The TPAD system refers to the development of unique biomasses in separate reactors. The first phase is referred to as 'acid fermentation' and is designed to produce primarily VFAs, while the second phase is referred to as 'methane fermentation' because in this phase the VFAs are converted to mainly methane and carbon dioxide. Due to short SRT in the first phase, acidogens are predominant, while both acidogens and methanogens are found in the longer SRT of the second phase. The TSAD system refers to two consecutive reactors in which a common microbial consortium is recycled between the second-stage reactor and the first-stage reactor. The same microbes are thus exposed to different environmental conditions, because the first reactor tends toward acidification of the waste and the second is more methane formation. Thus, the same microbes are exposed to diverse substrate and metabolic intermediate concentrations in a TSAD system. Staging can be accomplished in both suspended growth and attached growth systems (Stronach et al., 1986; Azbar and Speece, 2001).
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