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BACKGROUNDOlive mill wastewater (OMW) represents an environmental problem due to its high organic load and relevant concentration of phenolic compounds (PCs). OMW treatment and disposal represents a relevant challenge and cost for olive mills and multi‐utilities in charge of waste management in Mediterranean countries. The goal of this study was to develop an anaerobic co‐digestion (co‐AD) process of OMW and sewage sludge (SwS) from municipal wastewater treatment.RESULTSDifferent volumetric OMW:SwS ratios up to 100% OMW were fed in continuous 1.7‐L bioreactors. The reactors fed with raw OMW (rOMW) performed better than those fed with OMW dephenolized by adsorption (dOMW). At a 23‐day hydraulic retention time, the best performances were obtained in the reactor fed with 25% rOMW, with a 105% increase in methane yield in comparison to the 100% SwS test. At a 40‐day hydraulic retention time, the reactor fed with 40% rOMW attained a 268 NLCH4/kgvolatile solids methane yield. The conversion of phenolic compoundsreached 70% when the hydraulic retention time was increased from 23 to 40 days. A cost–benefit analysis indicated that both rOMW co‐AD in existing digesters and phenolic compounds recovery from OMW followed by co‐AD of dOMW can lead to relevant additional revenues for the multi‐utilities in charge of wastewater management.CONCLUSIONThis work proves that, using the existing network of SwS anaerobic digesters, it is feasible to co‐digest the entire OMW production in regions characterized by intense olive oil production, thus attaining a relevant increase in methane production yield (a 144% increase in comparison to 100% SwS). © 2020 Society of Chemical Industry (SCI)

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