Two pipes were inserted from the bottom of the larger pot. One is from
digester and the other from the manometer. The smaller pot
was placed over these two pipes inside the larger pot upside
down. Water was poured into the larger pot until water was
reached the top of the pipe. When water level touches the
bottom of the smaller pot, the air inside the smaller pot was
removed through the pipe with which the manometer was
connected. The gas pressure is measured by a U-tube water
manometer. Before the production of gases, the volume of
gas storage chamber and gas pipe lines are occupied by air.
When gas is produced in the digester it pushes the air into
the smaller pot of the gas measuring arrangement and the
pot is uplifted. Initially this air is taken outside the pot and
after sometime biogas was collected and measured. The
volume of the gas was measured by the formula V=pr2h (r
is the radius of the cylindrical pot and h is the height of the
pot from the water surface). Throughout the experiment
the maximum gas pressure recorded by the manometer was
only about 4 cm of water which is only 0.48% of the
atmospheric pressure. For this reason the effect of pressure
was neglected in the volume calculation.
3. RESULTS AND DISCUSSION
3.1. Cow Dung as Fermentable Material
Cow dung, coming from a rumen animal is known to
contain the native microbial flora that aids in faster biogas
production. It has also been reported severally that cow
dung is a very good starter for poor producing feedstock’s
[23-25]. Figure 4 shows the gas production from cow dung.
The average temperature of the digester was about 33.5ºC.
It shows that the hydraulic retention time for cow dung is
40 days and gas production starts at the 5th day. Maximum
gas is produced at the 26th day which is 0.0263 m3. Figure 5
shows the gas production in volume percent in four slots of
hydraulic retention time. In first slot from 0-10 days about
10% gas is produced. From 11-20 days gas production
increases to about 35%. In the third slot from 21-30 days
the maximum gas is produced which is about 46%. Finally,
from 31-40 days gas production reduces to about 9% and
gas production ceases after the 40 day.
This is predicted because biogas production rate in batch
condition is directly equal to specific growth of
methanogenic bacteria [26]. It was reported by Chen Ye, et
al [27] that high concentration of ammonia nitrogen is toxic
to anaerobes, which will decrease the efficiency of the
digestion and upset the process. This yield seems
particularly similar to that reported by [27] during the
anaerobic digestion 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.