Comparative Analysis of the Effects of Five Pretreatment Methods on Morphological and Methane Yield of Groundnut Shells

Agricultural residues have been identified as potential substrates for anaerobic digestion, which indicates them as alternative energy sources. But enzymatic hydrolysis of these residues has been observed as the rate-limiting step due to their recalcitrant characteristics. Therefore, pretreatment before anaerobic digestion is required to reduce the retention time and increase the methane yield. This study investigates the influence of thermal, alkali, acid, nanoparticle additive, and combined pretreatments on the methane yield of groundnut shells. The effect of pretreatment methods on the structural arrangement was investigated with scanning electron microscopy, X-ray diffraction, and Fourier transform infrared, and methane yield was investigated after anaerobic digestion at mesophilic temperature. The morphological analysis shows that all the treatment methods alter the microstructural arrangement of groundnut shells at varying degrees, except the particle size reduction, which does not significantly influence them. Cumulative methane yields of 222.92, 214.00, 171.02, 140.99, and 261.36 ml CH₄/g VS_added were recorded for thermal, alkali, acid, nanoparticle additive, and combined pretreatment methods, which represents 112.77, 70.03, 40.18, and 159.85% improvement compared to the untreated substrate (100.58 ml CH₄/g VS_added). Combined pretreatment was found to release the highest cumulative yield, and thermal pretreatment produced the least retention period. Therefore, combined and thermal pretreatments could be a bright means to deconstruct the recalcitrant characteristics of lignocellulose feedstocks arrangement to reduce the retention period and improve the methane yield. These methods can be experimented with further at the industrial scale to establish economic viability. Graphical Abstract: [Figure not available: see fulltext.].