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).
3.2. Gas production
The quantity of biogas produced from Poultry droppings, Cow
dung and Lemon grass over a period of 30 days SRT is shown in
Fig. 2. Biogas production was observed on the first day for reactor
B (Cow dung), on the second day for reactor C (Poultry droppings)
while production in rector A (Lemon grass) started on the third day
of loading the digesters and these increased gradually until the
maximum values were recorded on the 20th, 23rd and 16th day
respectively. Apart from the 22nd and 28th day when sudden increase
was observed, biogas production dropped progressively
after the day 16 for reactor A. In reactor B, production dropped progressively
after the 20th day except on the 22nd and 26th day
when sudden increase was observed while the production in reactor
C decreased progressively after the 23rd day with a little increase
on day 30. It was observed that the digester temperature
fluctuated between 28 C and 36.7 C while the pH of the medium
changed progressively from acidic to slightly alkaline fluctuating
optimally between 6.5 and 7.8 except for reactor C (Poultry droppings)
that recorded very alkaline pH (8.85) on the 9th day and
was maintained above 8.0 till the last day of the study (Fig. 3). This
could be attributed to the nature of feed materials used and agrees
with earlier submission of Ojolo et al. (2007) and Ahmadu et al.
(2009) that the organic matter content of the poultry wastes is a
factor that affects the digestion environment as well as the microbial
habitat. Also, the observed pH falls within the acceptable range
for anaerobic digestion (Abubakar and Ismail, 2012).