Fed-batch anaerobic digestion of raw and pretreated hazelnut skin over long-term operation

•Anaerobic digestion (AD) of hazelnut skin (HS) was investigated in fed-batch mode.•Higher CH4 production was obtained at lower solid retention time and higher HS load.•Maceration and organosolv pretreatment removed 82 and 97% of the HS polyphenols.•Methanol-organosolv pretreatment increased the CH4 potential of HS by 21%.•Reactor configuration enabled enriching microorganisms capable of HS degradation.