Granular sludge is a preferable inoculum for the biochemical methane potential assay for two complex substrates.

•The effect of inoculum selection on BMP was explored for manure and sewage sludge.•BMPs were tested with inocula obtained from four different anaerobic digesters.•Granular sludge yielded higher and had faster kinetics than suspended biomass.•Acclimation of inocula to substrate has a little effect on degradation rates.•Co-inoculation did not perform better than single inoculation.