Enzymatic and metabolic activities of four anaerobic sludges and their impact on methane production from ensiled sorghum forage

•Xylanase was the prevailing enzymatic activity for all inocula.•Xylanase activity exhibited its maximum activity earlier than esoglucanase.•The inoculum type influenced methane production rates but not methane potentials.•Correlations between enzymatic activities and methane production rates were found. Biochemical methane potential (BMP) tests were run on ensiled sorghum forage using four inocula (urban, agricultural, mixture of agricultural and urban, granular) and differences on their metabolic and enzymatic activities were also discussed. Results indicate that no significant differences were observed in terms of BMP values (258±14NmLCH4g−1VS) with a slightly higher value when agricultural sludge was used as inoculum. Significant differences can be observed among different inocula, in terms of methane production rate. In particular the fastest biomethanization occurred when using the urban sludge (hydrolytic kinetic constant kh=0.146d−1) while the slowest one was obtained from the agricultural sludge (kh=0.049d−1). Interestingly, positive correlations between the overall enzymatic activities and methane production rates were observed for all sludges, showing that a high enzymatic activity may favour the hydrolysis of complex substrate and accelerate the methanization process of sorghum.