BMP tests were used in order to evaluate the anaerobic digestibility of ensiled sorghum forage. Being a
batch test, the BMP assay is strongly dependent on initial conditions especially on the nature of the
anaerobic sludge inoculum. The inoculum selection may have a relevant impact on both biomethanization
rate an final BMP value after a selected operative digestion time. These aspects can make BMP values
difficult to compare. To explain the origin of the observed differences on methane production, BMP tests
were run on sorghum forage using four inocula (urban, agricultural, mixture of agricultural and urban,
granular sludge) and metabolic and enzymatic activities were measured during the course of BMP tests.
Results indicate that lower differences were observed in terms of BMP values with a slight higher value
when agricultural sludge was used as inoculum, possibly due to the adaptation of the bacterial consortium to
similar agricultural wastes. Significant differences can be observed among different inocula, in terms of
biomethanization rate. The fastest biomethanization occurred when using the urban sludge while the slowest
one was obtained from the agricultural sludge, in agreement with the observed hydrolytic activities.
Differences on the hydrolytic and enzymatic activities are also discussed
