Using anaerobic digestion (AD) for biogas production has several environmental advantages. AD is divided into wet digestion (total solid 15%) and solid-state digestion (total solid 15%). Solid-state anaerobic digestion (SSAD) is preferred to wet digestion, especially for lignocellulosic feedstocks due to their low moisture content. However, digestion of lignocelluloses is limited due to their recalcitrance structure and low hydrolysis rate. Therefore, using a pretreatment method seems necessary to improve lignocelluloses digestibility. The purpose of this dissertation was improvement of SSAD performance using pretreatment methods. The effects of six pretreatment methods, i.e., phosphoric acid, sulfuric acid, sodium hydroxide, ammonia solution, sodium carbonate, and organosolv, on SSAD of lignocelluloses were studied. After that, three methods, i.e., phosphoric acid, organosolv, and sodium carbonate, were chosen for more investigation. Also, the idea of using rice straw digestate to produce nanosilica were proposed and investigated. The effect of organosolv pretreatment on methane yield from SSAD of lignocellulosic substrate was studied. The pretreatment was carried out using ethanol as an organic solvent together with sulfuric acid as a catalyst at 150 and 180 °C for 30 and 60 min. Statistical analysis showed that the pretreatment temperature was the most important factor affecting methane production. After pretreating lignocelluloses at the optimum temperature, the highest methane yields were about 153, 94, and 71 liters per kg of carbohydrate in rice straw, elmwood and pinewood, respectively, which showed 32, 73, and 84% enhancement in comparison with the untreated ones. A reverse relation between methane yield and lignin content was observed. The kinetic data of all untreated and pretreated samples were fitted on first-order kinetic model. Phosphoric acid pretreatment (COSLIF) has been repeatedly shown to be a cost-effective and promising process to modify the structure of different lignocelluloses. Improvement of solid-state biogas production from lignocelluloses using concentrated phosphoric acid pretreatment was studied. Feed to inoculum (F/I) ratio, which plays a major role in SSAD, was set to 3, 4, and 5. After the pretreatment, 24 to 39% higher methane yield from pine was achieved. Considerable reduction in hemicellulose and lignin content, reduction in cellulose crystallinity, and increase in accessible surface area were the changes observed after the pretreatment. Mild sodium carbonate pretreatment as a modern and promising method were also studied. The results showed that sodium carbonate pretreatment markedly enhanced the sugar yields from enzymatic hydrolysis of the lignocellulosic biomass. The glucose yields from enzymatic hydrolysis were increased up to 62 to 96% (2 to 4 fold higher than that of untreated feedstocks) and the xylose yields were improved up to 69-77% (5 to 20 fold more than that of untreated feedstocks). The most important effect of the pretreatment was lignin removal, while the amounts of glucan and xylan degradation were low. Using ultrasonic with sodium carbonate, could only increase xylose yield. Finally, the rice straw residue from SSAD was applied for silica preparation. SSAD was used instead of direct burning of rice straw which is the common methods for silica production. By converting organic fraction of rice straw into biogas while generating solid digestate rich in silica, the economy of the process can be improved. The methane yield obtained from dry digestion was 119 L/kg volatile solid of the substrate. Silica content in the digestate ash was 21% more than that in the ash of raw rice straw. Nano particles with diameter between 15 to 30 nm were obtained. Keywords Biogas, Lignocellulosic feedstocks, Pretreatment, Digestate, Silica, Solid-state anaerobic digestion.