Effect of Combined Particle Size Reduction and Fe₃O₄ Additives on Biogas and Methane Yields of Arachis hypogea Shells at Mesophilic Temperature

Enzymatic hydrolysis of lignocellulose materials has been identified as the rate-limiting step during anaerobic digestion. The application of pretreatment techniques can influence the biodegradability of lignocellulose substrate. This study combined Fe₃O₄ nanoparticles, which serve as a heterogeneous catalyst during anaerobic digestion, with different particle sizes of Arachis hypogea shells. Batch anaerobic digestion was set up at mesophilic temperature for 35 days. The results showed that 20 mg/L Fe₃O₄ additives, as a single pretreatment, significantly influence biogas and methane yields with an 80.59 and 106.66% increase, respectively. The combination of 20 mg/L Fe₃O₄ with a 6 mm particle size of Arachis hypogea shells produced the highest cumulative biogas yield of 130.85 mL/gVS_added and a cumulative methane yield of 100.86 mL/gVS_added. This study shows that 20 mg/L of Fe₃O₄ additive, combined with the particle size pretreatment, improved the biogas and methane yields of Arachis hypogea shells. This result can be replicated on the industrial scale to improve the energy recovery from Arachis hypogea shells.