VFAs were quantified 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 (Chen et al., 2008).