The inhibition of
methane production is still not entirely eliminated, as the percentage
of methane gas was ca. 30% of the total production of biogas. This
may be attributed to ongoing production of ammonia that kept rising
high above the adapted threshold. Accumulation of higher amounts
of VFA (9.4, 1.3, 1.7 g kg− 1 CM at the end of the 7th, 8th, and 9th
batches respectively) may also be a cause.