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2.3 Two-stage anaerobic digestion process

Two-stage anaerobic fermentation process is often used for sequential hydrogen and
methane production. The process contains two phases in two separate reactors comprehensive (i)
the production of VFAs and hydrogen by acidogens and hydrogen producers, by the results of
the hydrolysis and acidogenesis, occur in the first phase and (ii) the second phase is
methanogenesis, in which the hydogenogenic effluent of the first stage is transferred to and used
as a substrate to produce methane by methanogens (Cooney et al., 2007). Two-stage
fermentation of hydrogen and methane is more stable and effective than the one-stage process
since each stage can be optimized separately, leading to a greater overall reaction rate and biogas
yield (Mataalvarez et al., 1993; Park et al., 2010; Liu et al., 2006).

Fig. 2.4 below shows the anaerobic process is categorized into two-stage based on the
products of hydrogen and methane. Hydrogen released from the first stage is called dark
hydrogen fermentation, while the soluble end products generated by this stage are fed into the
second stage, a methane phase for further anaerobic methane production by sequential
acetogenesis and methanogenesis steps (Liu et al., 2006).

Fig. 2.4 Flow diagram of two-stage anaerobic process (Adapted from Kongjan, 2010).