Enhancing the efficiency of high solid anaerobic digestion of empty fruit bunches under thermophilic conditions by particle size reduction and co-digestion with palm oil mill effluent

This study investigates the impact of thermophilic high solid anaerobic digestion (HS-AD) on biogas production from empty fruit bunches (EFB), focusing on the effects of total solids (TS) loading (5–40 %), particle size reduction (0.5, 3.25, and 6 cm), and co-digestion with palm oil mill effluent (POME) (10–30 % VS basis). The HS-AD at a 15–20 % TS loading has a methane yield of 103.4–105.3 mL CH₄/g-VS with 24.6–25.1 % biodegradability. Particle size reduction to 0.5 cm enhanced methane yield by 54–61 % and improved hydrolysis rates by 45 % compared to the untreated EFB (6 cm) at a 15–20 % TS loading. Co-digestion of EFB with POME at a ratio of 31:1 based on VS basis led to a synergistic effect of 17.77 mL CH₄/g-VS, increasing methane yield by 24–46.5 % and improving process stability, as evidenced by a 22.8–38.1 % reduction in volatile fatty acids (VFAs) accumulation. Microbial community analysis showed a 2-fold increase in the relative abundance of hydrogenotrophic methanogens (Methanothermobacter sp. and Methanoculleus sp.) during co-digestion, while the abundance of key cellulolytic bacteria (Clostridium sp. and Fibrobacter sp.) increased by 1.5-fold. The optimized HS-AD process achieved a maximum methane yield of 287.77 mL CH₄/g-VS and a biodegradability of 61.2 % under thermophilic conditions, with a 20 % POME co-digestion addition (31:1 VS ratio) and 0.5 cm particle size. These findings demonstrate the potential of thermophilic HS-AD for the sustainable management of EFB and highlight the importance of process optimization and co-digestion strategies for enhanced biogas production from EFB.