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 compared with digester PPW-FR-1, the daily CH4 production for
digester PPW-FR-2 and PPW-FR-3 with higher TS and VS contents
were somehow inhibited, which generated a peak daily CH4 yield
of only 7.7 and 5.9 L/kg VSfed, respectively. Similar results were
reported by Li et al. (2011) and Yao et al. (2013) in that CH4 production
was decreased with increasing TS and VS contents with
corn stover and poplar residue as feedstock. This could be
explained by the hydrolysis or acidogenesis step being inhibited
by a high organic loading, or toxicity of free ammonia from the
degradation of protein