MATERIALS AND METHODS
The anaerobic digestion experiment was carried out under an ambient temperature of 29.6 ± 3.4 oC in a laboratory at the Department of Agricultural and Environmental Engineering, Obafemi Awolowo University, Nigeria. The materials used for this study were fresh swine and chicken manures, both collected from the University Teaching and Research Farm.
The biogas production of the SM-CM mixtures were examined at five MRs (w/w dry weight basis) 0:1 (CM only), 3:7, 1:1, 7:3 and 1:0 (SM only) in 25 L batch-type digesters adapted using rectangular plastic containers (Plate 1). Each digester had drain plug fitted at the base through which samples were collected for pH analysis. Also, a thermometer was fitted to each digester for substrate temperature measurement. Water tanks and water collector tanks were adapted using rectangular 10 and 5 L plastic containers,respectively. The digesters, water tanks and water collectors were inter-connected using rubber hoses with cork fitted tightly to prevent gas and water leakage. Each mixture was diluted to 8% TS, as recommended by Zennaki et al. (1996), agitated vigorously and screened using a 6 mm plastic mesh to remove gross solids. The digesters were loaded once during the experiment to 70% of the capacity with the substrates. The daily biogas production was measured by water displacement method. Each treatment was replicated thrice. The digesters were manually agitated once daily to ensure intimate contact between micro-organisms and the substrates. The substrates were digested for 63 days during which ambient and substrates temperatures and biogas production were measured daily, while pH was measured fortnightly.