BACKGROUND: Biodiesel is an alternative to fossil fuels and can be used directly in internal combustion engines when mixed with diesel. The economic feasibility of biodiesel production necessitates the valorisation of glycerol, which is produced in large quantities (equal to 10% of the biodiesel produced). Anaerobic digestion is applicable to a variety of organic residues yielding biogas rich inmethane. In order to estimate the net potential of glycerol to yield methane, pure glycerol was selected to avoid any effect from the impurities in crude glycerol. RESULTS: The anaerobic digestion of pure glycerol was studied in two types of bioreactors: a continuous stirred tank reactor (CSTR) and a baffled reactor (periodic anaerobic baffled reactor, PABR). Both reactors were operated in mesophilic conditions (35 degrees C) at various organic loading rates. The maximum glycerol loading achieved in a CSTR was 0.25 g COD L-1 d(-1), yielding 0.074 +/- 0.009 L CH4 L-1 d(-1). On the other hand, PABR allowed glycerol degradation at a loading of 3 g COD L-1 d(-1) yielding 0.993 +/- 0.102 L CH4 L-1 d(-1). CONCLUSION: PABR was proved to be more efficient since it was subjected to a 10-fold higher organic loading rate than CSTR. Moreover, its performance was much higher in terms of COD removal andmethane productivity. (C) 2013 Society of Chemical Industry