Evaluation of biogas yield and kinetics from the anaerobic co-digestion of cow dung and horse dung: a strategy for sustainable management of livestock manure

In this study, investigation was done to determine the optimum combination of cow dung (CD) and horse dung (HD) for enhanced biogas production and plant stability. Anaerobic co-digestion of CD and HD at varying percentage combination was carried out in five (5) identical 25 L cylindrical digesters (A–E) for a retention period of 37 days, at an average ambient temperature of 33 °C. Using the Microsoft excel solver function, 2010 version, the modified Gompertz model was applied to predict the relevant kinetic variables of the digestion process. Result obtained shows that digester D with 25% CD and 75% HD produced the highest daily biogas, followed by C (50% CD and 50% HD), B (75% CD–25% HD), A (100% CD) and E (100% HD). Digester D also had maximum biogas production potential (A) of 13.8 L/gVS, maximum biogas production rate (µ) of 0.69 L/gVS/day and shortest lag phase (λ) of 5.20 days. Digester E with 100% HD, though had a short lag phase of 5.72 days, had the least total biogas yield of 5.1 L/gVS. The closeness of the coefficients of determination (R²) to 1 reflects a good fit, between experimental and simulated data. The study found that increase in the amount of cow dung beyond 25% led to decrease in biogas yield. It has also shown that biogas production from CD and HD is feasible and can serve as way of removing CD and HD from the environment while serving as a source of bioenergy. Further study on best ways of pre-treating the substrates for greater biogas yield is recommended.