To prevent organic acid inhibition in Stage 4, swine
urine was added to the garbage slurry at an OLR of
2.3 kg-VS m3 day1 (HRT 20 days) and 2.2 kg-
VS m3 day1 (HRT 11 days) in this stage. The pHs in
the acidification reactor and the methane fermentation
reactor increased with the same tendency. The pH in the
methane fermentation reactor exceeded 7.5 from day 310,
indicating that the fermentation was stable. The specific
biogas production values were 676 L kg1-VS added at
2.3 kg-VS m3 day1 (HRT 20 days) and 551 L kg1-VS
added at 2.2 kg-VS m3 day1 (HRT 11 days), while the
methane content was 62%. Similarly, Sterling et al. (2001)
reported that the addition of urea led to nearly immediate
increases in pH and alkalinity during anaerobic digestion
of dairy cattle manure. Excess ammonia probably contributed
to the increased alkalinity in two ways. First, ammonia
increased the bicarbonate concentration in the digesters
by forming an ammonium salt with bicarbonate taken
from dissolved CO2 (Georgacakis et al., 1982):