The theoretical methane production was 516 L kg1-VS
added for swine wastes (Moller et al., 2004). However, in
this study the methane production was 356 L kg1-VS
added at an OLR of 3.6 kg-VS m3 day1 and 321 L kg1
-VS added at an OLR of 4.9 kg-VS m3 day1. These
results were in accordance with those reported in the previous
work (Moller et al., 2004). The methane content of the
biogas was relatively high because of high carbon dioxide
dissolution into bicarbonate form, which enhanced the buffering
capacity of the methane fermentation reactor. In
addition, the methane content in the biogas was higher,
accounting for 50–80% in a two-phase system compared
to 40–60% in a single-phase system (Yu et al., 2002). In
the whole period of Stage 1, the pH in the methane fermentation
reactor increased significantly from 7.2 to 8.2,
regardless of the pH in the source material