Biogas production from anaerobic digestion of manure at different operative conditions

Anaerobic digestion is an established technology to simultaneously produce biogas and treat different kinds of wastes. This work deals with water buffalo manure digested under mesophilic (37°C) and thermophilic (55°C) conditions with initial pH varying from 6.0 to 8.7. The digestion is investigated following the evolution of the biogas composition in time, in terms of the volume fraction of CH4, CO2 and their ratio. The growths of the biogas products are interpolated with a Gompertz equation and in all the investigated cases the methane productivity is above 63% with a CH4/CO2 ratio rather high and equal to about 2. In mesophilic conditions, an initial pH of 7.0 must be preferred to optimize the methane yield and to minimize the time required to attain it. The thermophilic conditions resulted very promising since they almost halved the time required to reach the final yield. The speed up of the process at 55°C, in our case, resulted to be due to the increase of the digestion kinetic more than to the selection of a new metabolic pathway.