Combined valorization of grape marc

The implementation of the concepts of the circular economy raises the qualitative and quantitative level of residue valorisation, as a higher integration of waste-derived resources in the market logics and regulations is required. In this framework, grape pomace is one of the most promising agro-industrial side-streams as it contains organic acids, sugars, and lignocellulosic materials suitable to be converted into biofuels or soil improvers, but also compounds of great interest for economic sectors such as medicine, nutraceuticals and cosmetics. The full exploitation of grape pomace, therefore, implies the adoption of a sequence of processes that allows both the recovery of intact compounds with high market value and the valorization of the exhausted biomass. The present study focused on the recovery of polyphenols from grape pomace, combined with the anaerobic digestion, composting or hydrothermal carbonization (HTC) of the exhausted biomass to convert it into value-added materials and energy. The polyphenols extraction was based on mild solvent characteristics and operating process conditions; the effects of pomace preliminary grinding and different values of the liquid-to-solid ratio were explored. The extraction yields, the concentration of total polyphenols and single compounds in the extract, and the antioxidant properties of the extracts were determined. Finally, biochemical methane potential, composting and HTC tests were performed on the exhausted grape pomace and, for comparison, also on the raw grape pomace. The results show that 76.5 g of extract per kg of dry grape pomace can be recovered, which contains 2.3% w/w of original total phenols (roughly 1.8 g total phenols per kg of dry grape pomace). Preliminary grinding had beneficial effects on the phenols extraction yield and total phenols content in the extract, and the antioxidant activity of the extracts proved to be proportional to the total phenols content. A theoretical value of biomethane recovery of about 140 NLCH4/kg volatile solids was derived from the BMP tests conducted on the exhausted grape pomace; although this figure refers to optimal process conditions, it is reasonable to assume that the production of biogas achievable on the real scale would be such as to cover at least the energy needs of the polyphenols extraction process. The composting process proved to be able to deal with the phenol presence without negative effects, being instead more influenced by the particle size of the substrate. Indeed, the total phenols content decreased during the composting treatment, probably because the polyphenols were incorporated into biosynthetic pathways leading to the formation of humic molecules. The size reduction carried out upstream of the phenol extraction resulted in faster and more complete composting. The hydrochars obtained through the thermochemical conversion of grape pomace showed a carbon content increase (+26%) and improved quality as a solid fuel (+40%), suggesting their use as soil amendment or for energy production. Further energy recovery can be achieved through biomethane production from the HTC process water (obtaining up to 137 mL of biomethane per gram of fed COD).