In this stage, the MSFUG was fed at an OLR of 2.9 kg-
VS m3 day1 in the first 14 days, and later the OLR was
increased to 5.0 kg-VS m3 day1 at the HRT of 9 days
for 39 days. To investigate the maximum capacity of the
methane fermentation system, the OLR was increased to
9.3 kg-VS m3 day1 at an HRT of 6 days. The biogas production
increased stably to 865 L kg1-VS at 5.0 kg-
VS m3 day1. However, the biogas production decreased
dramatically to 481 L kg1-VS after a 6-day feeding period
at 9.3 kg-VS m3 day1. The experiment was then stopped
since it was considered that the fermentation would fail if
the experiment continued. Subsequently, the OLR was
decreased to 5.3 kg-VS m3 day1 to restore the performance
of the fermentation system, with 67% of VS removal
and 73% of COD removal (Table 2). The methane content
in the biogas was 62%, lower than in Stages 1 and 2, while
the biogas production in this stage was larger than that in
Stages 1 and 2. At 5.0 kg-VS m3 day1 the removal of VS
was 75% and that of COD was 74% (Table 2). The pH in
the methane fermentation reactor increased consistently
to approximately 9; then it decreased until day 200 and
increased again in this stage, as a result of the increase in
the ammonia concentration.