The amount of total biogas produced, ammonia concentration, and
pH values at the end of each batch were not of equal values, and this is
attributed to different culture times (Fig. 1).
Uric acid, which is the principal nitrogenous compound in CM,
significantly contributes to the accumulation of ammonia (21). As
shown in Fig. 1, production of ammonia increased during anaerobic
fermentation from 2.1 g-N kg− 1 CM at time zero to 5.4 g-N kg− 1 CM at
the end of the first batch. Production of ammonia kept increasing
with subsequent batches, until it reached its peak at the end of the
7th batch (at 308 d) of 14 g-N kg− 1 CM, after which it decreased to
8 g-N kg− 1 CM for the last two batches. This accumulation of
ammonia inhibited production of methane and continued for a very
long period of time for about 254 d. Since the anaerobic consortia in
the seeded sludge, which used as inoculum, was not previously
adapted to fresh CM, production of methane would be already
inhibited by high concentration of ammonia. Furthermore, at the end
of each batch, after production of biogas stopped, fresh CM was
added, thus introducing more organic nitrogen. The resultant
production of more ammonia would in turn inhibit methanogens