of
observation; biogas production increases substantially due
to exponential growth of methanogens. Highest biogas
production rate of 3.4 L was measured on day 6. On the
commencement of semi-continuous digestion, biogas
production was observed to decrease considerably and,
this is probably due to unregulated pH region employed,
which concurrently leads to increase in concentration of
ammonia nitrogen that might be assumed to inhibit the
process. It was reported by Chen Ye, et al [6] that high
concentration of ammonia nitrogen is toxic to anaerobes,
which will decrease the efficiency of the digestion and
upset the process. Besides, the fluctuations in the daily
biogas production found during the semi-continuous, it
could also be attributed to the varying input of VS load. At
the end of the observation, cumulative biogas yield of 0.15
L kg-1
VSadded was achieved. This yield seems particularly
similar to that reported by [7] during the anaerobic