Coupling acid pretreatment and dosing of Ni and Se enhances the biomethane potential of hazelnut skin

This study offers new insights and promising perspectives in view of the energy valorization of hazelnut skin (HS) as a potential feedstock for renewable biomethane production. The process originally comprised the coupling of chemical pretreatments and individual dosing of cobalt (Co), nickel (Ni) and selenium (Se) as enhancement strategies of the biochemical methane potential (BMP) of HS. The anaerobic digestion (AD) of the untreated HS resulted in a biomethane production of 299 ± 3 mL CH4·g VS−1. This was not positively nor negatively influenced by the addition of the three trace elements (TEs). Performing a 1% H2SO4 pretreatment on HS, a 104% increase of the biomethane yield (i.e. 612 ± 20 mL CH4·g VS−1) was obtained. Under the same pretreatment conditions, a peak of 66.2 mg HAc g VS−1 of total volatile fatty acids (VFAs) was achieved. The BMP of the H2SO4-pretreated HS was further improved by 12 and 16% through the addition of 90 μg Ni·g TS−1 and 270 μg Se·g TS−1 of HS, respectively. The application of a 1.6% NaOH pretreatment on HS led to a 41% higher BMP (i.e. 423 ± 24 mL CH4·g VS−1) compared to that achieved with the untreated material. However, not statistically significant effects were observed when dosing the TEs. In conclusion, a positive synergy between chemical pretreatments and TE supplementation was only observed with the H2SO4- pretreated HS. This was due to the higher VFA production and the more effective VFA conversion to methane in the presence of Ni and Se.