Inoculum origin and waste solid content influence the biochemical methane potential of olive mill wastewater under mesophilic and thermophilic conditions

[Display omitted] •The CH4 yields of raw OMW were higher than centrifuged regardless inoculum origin.•The 74% VSS-richer raw OMW led to 21–48% higher CH4 yield than treated OMW.•The UASB reactor inoculum presented the highest yield of 121 mL CH4/g VSSadded∙d.•Inocula acclimated to OMW presented maximal substrate biodegradability. The biological valorization of olive mill wastewater (OMW) is often problematic due to the characteristically high organic load and phenolic compound content of this waste stream. The present study aimed at determining the optimal conditions (i.e. temperature, solid content) for the anaerobic digestion of OMW, striving for the maximum methane yield. Therefore, inocula originating from a wastewater treatment plant and two lab-scale bioreactors treating OMW (mesophilic Up-flow Anaerobic Sludge Blanket (UASB) reactor and thermophilic Up-flow Packed Bed Reactor (UPBR)), were tested in terms of OMW degradation potential and methane yield under mesophilic and thermophilic conditions, through Biochemical Methane Potential (BMP) assays. The methane yields were in all cases higher for the raw OMW (325–472 mL CH4/g VSadded) compared to the centrifuged OMW (219–391 mL CH4/g VSadded). Moreover, the greatest biodegradability extent (94.0–98.5%) was achieved by the mesophilic sludge originating from the UASB reactor. The maximum specific methane production rate (121 mL CH4/g VSSadded∙d) was reached by the inoculum obtained from the mesophilic UASB reactor using centrifuged OMW. These results indicate that the mesophilic temperatures are optimal for the anaerobic digestion (AD) of OMW in terms of methane productivity and biodegradation potential, while no solid removal is necessary.