Application of renewable and environmental friendly types of energy is obligatory for the near future. Biogas production is one of the routes for the future market of energy. Biogas is produced under anaerobic conditions by conversion of organic materials to methane (CH4) gas, which is a flammable gas. Lignocellulosic material as an agriculture residue has a great potential for biogas production. Rice straw is one of the lignocellulosic materials with a huge amount of world wide annual production. Recalcitrant structure of rice straw, which is mainly due to high crystalline cellulose and protection by lignin and hemicelluloses, causes hardships in the way of microorganisms accessibility to rice straw carbohydrates. Pretreatment is a way to enhance biogas production by destroying the complex structure of the straw as well as lignin removal. In the present work, the main target is to improve biogas production by alkali pretreatment of rice straw in a pressurized reactor with 0.25 and 0.5 M sodium carbonate at 90, 110, and 130?C and for 1, 2 and 3 hours. All treated and untreated rice straws were digested under mesophilic conditions (37?C) for 47 days. The best results obtained by pretreatment with 0.5 M sodium carbonate at 110?C for 2 hours and resulted in production of 292 ml CH4/g VS while untreated rice straw produced 130 ml CH4/g VS. The results also showed that by increasing the concentrations from 0.25 M to 0.5 M, biogas production would significantly be improved while time interval increasing showed less impact. NREL, SEM and FTIR analyses confirmed straw structure destruction, lignin removal, and cellulose crystalline reduction. Furthermore, biogas production by using anaerobic membrane bioreactor (An-MBR) was investigated. Biogas production yield in the An-MBR for about 55 days and at 35?C was 141 ml CH4/g VS while a traditional bioreactor with the same conditions produced 111 ml CH4/g VS. Keywords An-MBR, biogas, pretreatment, rice straw, sodium carbonate
