From the profile of pH changes in the first two weeks of
digestion, it was observed that blending of the cassava
peels with these animal wastes stabilized the waste for
gas production (Figure 3). This could be as a result of its
high fiber and carbon contents (Table 1). Swine in this
part of the country are normally fed with spent grains occasionally,
which may contain a lot of fiber. The presence
of hydrocyanic acid in the cassava peels may have brought
about the de-lignification of the fibrous plant structure
of the spent grain observed in the swine waste making
nutrients available for the methanogens during the digestion
period (Mathewson, 1980). The CP: PD combination
had a cumulative gas yield close to that of CP: SD (Table
2) but faster onset of gas flammability. This may be as a
result of its low total solids (TS) of the undigested single
poultry waste (Table 1). Adequate physicochemical properties
are known to favor biogas production. A higher TS
level for poultry droppings implies high ammonia content
of the slurry. Shivaraj and Seenayya (1994) reported that
digesters fed with 8% TS of poultry waste gave better
biogas yield than the higher TS levels. Again, earlier work
carried out by Waksman and Hutchings (1936), pointed
out the significance of organic sources of nitrogen in the
decomposition of lignin in plant materials. They asserted
that lignin-decomposing microbes prefer organic proteinnitrogen
to inorganic forms. Tinsley and Nowkawski
(1959) also submitted that, application of poultry droppings
as fluid slurry to Brewer’s spent grain brought an
abundant and vigorous micro-flora immediately into contact
with feedstock substrate. They further explained that
as uric acid is decomposed, ammonia is produced which
diffuse rapidly so that the cellulose-decomposing organisms
were well supplied with nitrogen from an early
stage. This organic source of nitrogen as biogas production
catalyst was also highlighted in the report of Ezeonu
et al. (2002) in the biomethanation of Brewery spent grain
(BSG) with chicken droppings and Cow rumen liquor. The
system with BSG / droppings ratio of4:1 had the highest
gas yield when compared with the other ratios of 5:1 and