The digester PPW-1 and PPW-FR-1 with identical TS and VS
contents shared similar trends of daily CH4 production (Fig. 1B).
A small peak was observed at the beginning of digestion in both
PPW-1 and PPW-FR-1, which was due to the quick conversion of
available VFAs into biogas, as shown in Fig. 2. After 4–6 days incubation,
the CH4 production increased dramatically and reached the
highest daily yield of 16.8 and 20.8 L/kg VSfed on day 10 and 8,
respectively, in digester PPW-1 and PPW-FR-1. However, as municipal organic solid (Dreschke et al., 2015). Coats et al.
(2012) studied the biogas production from dairy manure with a
two-stage AD process, and found that the additional prefermentation
in the first stage with short solid retention time of
4 days increased CH4 content and enhanced the biogas production
as compared to single stage AD process. It is postulated that the
pre-fermentation process can loosen (or disrupt) the recalcitrant
components in the biomass, which make them susceptible to
microbial attack and assimilation by AD. Insufficient digestion
effects were obtained in digester PPW-FR-2 and PPW-FR-3, which
had cumulative CH4 yields of 150 and 118 L/kg VSfed, respectively.
It should be noted that the CH4 contents in these two digesters
reached a similar level with the other two digesters (PPW-1 and
PPW-FR-1).