Hydrothermal and thermal-acid pretreatments of wheat straw: Methane yield, recalcitrant formation, process inhibition, kinetic modeling

Bio-methanation of agro-residue could fulfill the rising energy demands and curb the environmental pollution. Nevertheless, hydrolysis is the rate limiting step in anaerobic digestion (AD) of agro-waste (wheat straw, WS) due to its recalcitrant lignocellulosic composition. Hydrothermal (100–175 °C, 30–120 min) and thermal-acid (100–175 °C, 0.5–2% H2SO4 v/v) pretreatments of WS were performed to assess the pretreatment effects on WS solubilization, recalcitrant formation, lignocellulosic composition and improvement in methane yield. The 60-min hydrothermal pretreatments had an optimum chemical oxygen demand (COD) solubilization. The hydrothermal pretreatment degraded predominantly the hemicellulose by 53.4%. The furan derivatives, i.e., furfural and 5-hydroxyl-methyl-furfural (5-HMF) were formed during hydrothermal pretreatment of WS at 175 °C. The furfural and 5-HMF were generated at all studied thermal-acid pretreatment conditions owing to hemicellulose solubilization. The anaerobic co-digestion (AcoD) of hydrothermally and thermal-acid pretreated WS was performed with food waste and cow manure in a batch assay. The hydrothermally pretreated WS showed 4–14% higher methane production, while the thermal-acid pretreated WS had 29–44% less methane production than untreated WS. High concentrations of furfural, 5-HMF, total volatile fatty acids (tVFA), and NH4–N affected the methane production in digesters treating thermal-acid pretreated WS. The kinetic analysis of the assays revealed that methane production was affected by furfural, 5-HMF, temperature and acid dosing. Therefore, the calculated values through modified Gompertz and logistic models deviated from the experimental values.