Alkaline-thermal and mild ultrasonic pretreatments for improving biomethane yields: Impact on structural properties of chestnut shells

Abstract

The high fiber content of chestnut shells (CS) makes biomethane production inefficient when using the anaerobic digestion (AD) method. Therefore, this study has evaluated for the first time the combined effect of alkaline-thermal (AT) and mild ultrasonic (US) pretreatments prior to the AD of CS. The cumulative biomethane yields (CMY) of untreated and pretreated (4% NaOH + 100 °C thermal + US) CS were 79.10 ± 7.9 and 266.9 ± 9.3 mL CH4/g VS, respectively. The effects of the pretreatment of lignocellulosic biomass at different intensities were interpreted according to amounts of lignocellulosic solubilization, soluble chemical oxygen demand/total chemical oxygen demand, and scanning electron microscope images. Solubilization of the lignin from CS with pretreatments varied between 44.5 and 65.6% w/w. The use of AT pretreatments, together with US pretreatments, successfully increased the solubility of lignocellulosic compounds compared to the first case, whilst CMYs showed a positive correlation with lignocellulosic solubilization amounts. After pretreatments, total volatile fatty acid (VFA) concentrations were measured in the anaerobic process, from which the highest VFA was measured to be 1555 ± 108 mg HAc L-1, well below the appropriate value for AD (≤8000 mg HAc L-1). After pretreatments, the CMYs were successfully simulated by the modified Gompertz and Logistic model (R2 = 0.942–0.984). This study presents a promising strategy in the sense that the biomethane yield of CS containing large amounts of fiber can be significantly increased by combined AT and mild US pretreatment methods.