In the evening, when the cooking test was concluded (about
7 pm local time), the gas holder was completely emptied. The values
given by the calibration were written down in order to obtain
the daily production by subtracting this value from the one of the
day before. It was assumed that other impurities apart from carbon
dioxide were negligible, thus, the difference in volume of gas produced
before and after scrubbing were used to estimate the methane
content.
2.5. Statistical analysis
Analysis was carried out using IBM SPSS software for Windows
version 20.0. The values obtained were confirmed using one-way
ANOVA at 0.05 level of significance.
3. Results and Discussion
3.1. The characteristics of the substrates used
The characteristics of the substrates (Poultry droppings, Cow
dung and Lemon grass), used for this study are as shown in Table 1.
Among these substrates, Cow dung was the densest followed by
Poultry dropping while Lemon grass has the lowest in terms of total
solid content. Also, great variation was found in the volatile solids
content of the three substrates; while Cow dung and Poultry
dropping have values close to each other, Lemon grass recorded
very low value. Nitrogen was highest in Poultry dropping with a
value of 72.2 ± 2.78 and lowest in Lemon grass with 12.0 ± 4.61
as value. Value recorded for phosphorus was highest in Poultry
dropping (5.10 ± 0.01) and lowest in Cow dung with value of
(3.50 ± 1.03). Calcium value was highest in Lemon grass with
51.22 ± 8.43 while it was lowest in Cow dung with 32.1 ± 2.13 as
the value. Sodium was highest in Poultry dropping (4.70 ± 0.02)
as it may have been included in the poultry feed during compounding
and lowest in Lemon grass (2.09 ± 0.13). For potassium, magnesium,
iron and zinc, highest values were recorded in Lemon grass
(31.4 ± 4.09; 5.21 ± 1.62; 0.99 ± 0.18 and 1.00 ± 0.08) because they
are all elements needed by the green plant in different quantities
and for different functions while their lowest values were found
in Cow dung (20.61 ± 2.13; 2.22 ± 0.02; 0.41 ± 0.01 and
0.02 ± 0.03) respectively possibly because most of them have
undergone modifications/reduction via digestion in the animal’s
alimentary canal prior to excretion. Also, for copper, lead and cadmium,
highest values were recorded in Poultry dropping
(92.6 ± 7.41; 36.21 ± 3.81 and 13.62 ± 1.80) probably due to the
presence of these metals in the poultry feed as different materials
are incorporated into such feeds during production. They were
however lowest in Lemon grass (67.4 ± 9.90; 27.7 ± 5.00 and
8.2 ± 2.06) respectively. For aluminium however, Lemon grass recorded
the highest value of 1.03 ± 0.09 while the lowest value
(0.62 ± 0.12) was recorded for Cow dung. E. coli and Enterobacteriaceae
counts were both highest in Poultry dropping and Cow dung
respectively (11.2 105 ± 3.23 and 1.21 104 ± 0.11) and lowest
in Lemon grass (3.2 105 ± 1.23 and 1.02 103 ± 0.01).