Umudike Journal of Engineering and Technology

This work evaluated the effects of agitation, feed-inoculum ratio (F/I) and total solid content (TSC) on the anaerobic digestion of mixed animal wastes (cattle dung, pig dung, poultry droppings) co-digested with mixed fruit wastes (of mango, orange, and pineapple) as co-substrate and contents of chicken-gizzard as inoculums for efficient and high recovery of bioenergy (biogas/biomethane). A F/I ratio of 1:1, 1:2, 1:3, 2:1 and 3:1; TSC of 2, 4, 6, 8 and 10% and agitation speed of 0 (i.e. no agitation) and 30 rpm were studied. The anaerobic digestion experiments were carried out in several 20 kg capacity batch anaerobic biodigesters operated at an agitation speed of 30 rpm and incubated at ambient temperature (28 ± 2^oC) for 10 weeks. The results showed that the cumulative biogas/biomethane yield obtained from the anaerobic digestion of mixed animal wastes co-digested with mixed fruit wastes and contents of chicken-gizzard as inoculums generally increased with increase in F/I ratio, TSC, and minimum agitation of 30 rpm, respectively. F/I ratio of 1:3 and 3:1 as well as TSC of 8% resulted in a maximum cumulative biogas/biomethane yield of 6.2 dm³/g/63.3%, 6.1 dm³/g/62.4% .and 5.8 dm³/g/60%, with an energy content (EC) of 22.6 MJ/m³, 22.1 MJ/m³ and 22.3 MJ/m³, respectively. A minimum agitation of 30 rpm exhibited a cumulative biogas/biomethane yield of 5.8 dm³/g/60% with an EC of 21.4 MJ/m³ higher than 5.2 dm³/g/55% (EC of 19.6 MJ/m³) obtained without agitation. The biogas/biomethane production data were fitted to three kinetic growth models of Logistic, Exponential Rise to Maximum and Modified Gompertz, respectively. Modified Gompertz and Exponential Rise to Maximum growth models fitted very well to the data ) and thus showed a better correlation of cumulative biomethane production than the Logistic model.