Strategies to enhance the biomethane potential of hemp biomass

Among agricultural residues, lignocellulosic materials (LMs) are highly attractive substrates for anaerobic digestion (AD), given their high availability, low cost and no direct competition with food and feed production. Hemp (Cannabis sativa L.) is a multipurpose crop, and its cultivation has boosted again in the last years. Nevertheless, due to contrasting legislation, in some European Countries the harvesting and manufacturing of plant components (e.g. leaves and inflorescences) has stopped, resulting in a massive amount of lignocellulosic biomass to be diversely disposed. This research explores the valorization of hemp biomass (whole stalk, bast fiber, decorticated hurds, and a mixture of leaves and inflorescences) by AD, as a first major step for the establishment of a wider, future biorefinery platform. Both physical (grinding) and chemical (acid and alkali) pretreatments have been investigated to break down the lignocellulosic matrix of different hemp biomass components. Their biomethane yield has been then evaluated through batch biochemical methane potential (BMP) tests performed in 100 mL serum bottles under controlled mesophilic (37°C) conditions for 45 days. The highest cumulative biomethane production (422 mL CH4/g VS) was obtained with the raw bast fiber, while the BMP of the raw whole stalk and decorticated hurds reached 271 and 240 mL CH4/g VS, respectively. The alkali pretreatment with NaOH increased the BMP of the hurds by 15% obtaining a final biomethane yield of 277 mL CH4/g VS. The mixture of leaves and inflorescences resulted in the lowest BMP values, with the NaOH-pretreated material reaching approximately 150 mL CH4/g VS.