Enhancing methane production from rice straw by extrusion pretreatment

Extrusion pretreatment was applied to enhance fermentative methane production from rice straw. The extrusion-pretreated rice straw (EPRS) showed significant particle size reduction, physical properties improvement and volume expansion, as compared to the milling-pretreatment rice straw (MPRS) and unpretreated rice straw (UPRS). The anaerobic digestion of EPRS, MPRS and UPRS with different inoculum-to-substrate ratios (ISRs) and solid loading rates (SLRs) was investigated. The highest specific methane production (SMP) of the EPRS was 227.3 L/g VS with an ISR of 0.4 and SLR of 50 kg/m3, which was 32.5% and 72.2% greater than that of the MPRS and UPRS at the corresponding ISR and SLR, respectively. Furthermore, the volumetric methane production (VMP) increased with increasing SLR. The highest VMP (17.0 m3/m3) of the EPRS was obtained with an SLR of 90 kg/m3, which was 1.5 times that of the MPRS. In addition, the EPRS had significantly shorter technical digestion time and higher cellulose and hemicellulose degradation efficiencies, which was due to the smaller particle size and larger specific surface area of the EPRS, thus contributing to the enhancement of methane production. The SEM analyses further demonstrated that the cellulose of the rice straw was devillicated after extrusion pretreatment, which was in agreement with the higher cellulose and hemicellulose degradation efficiencies of the EPRS.