3.1 Biogas production
The cumulative biogas production during the
study period is shown in Figure-1. It was observed tha
biogas production was actually slow at starting and the end
of observation. This is predicted because biogas
production rate in batch condition is directly equal to
specific growth of methanogenic bacteria [5]. During the
first 3 days of observation, there was less biogas
production and mainly due to the lag phase of microbia
growth. Whereas, in the range of 4 to 6 days 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, i
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 anaerobicdigestion of beef manure in mixed and unmixed reactors.
It is clear that cow dung is an effective feedstock for
anaerobic digestion and could significantly enhance the
cumulative biogas production. It therefore shows that
considerable amount of anaerobic bacteria in the cow dung
functions effectively to degrade the organic fraction from
cattle manure even though pH was unregulated. The
methane content of the biogas generated during the entire
operation was on average 47%. This result implies that all
the processes are most likely in balanced and stable
operation.