Enhancing ethanol and methane production from rice straw by pretreatment with liquid waste from biogas plant

Effluent of biogas digestate, a waste stream with serious environmental problems, was used to pretreat rice straw in order to improve biofuel production. To investigate the effect of components presented in this waste stream and compare the results, water was also applied at the same conditions for the pretreatment. The straw was pretreated at 130, 160, and 190 °C for 30 and 60 min and subjected to enzymatic hydrolysis, simultaneous saccharification and fermentation (SSF), liquid anaerobic digestion (L-AD), and dry anaerobic digestion (D-AD). The highest improvements in hydrolysis and ethanol yield were 100 and 125%, achieved from the straw pretreated with effluent of biogas digestate at 190 °C for 60 min. The best methane yield was obtained through L-AD and D-AD of straw pretreated at 190 °C and 30 min with effluent of biogas digestate, resulting in 24 and 26% enhancements in produced methane. However, treatment with water had no significant effect on methane yield. Compositional and FTIR analyses indicated hemicellulose omission through the treatment under severe conditions. Furthermore, SEM images showed major enhancement in straw porosity by the pretreatment.