Comparison of anaerobic digestion capability of rice straw with different pretreatment methods

Abstract: China is a large agricultural country with the most abundant straw resources in the world, and about 700 million tons of crop straw is produced annually. However, most of the stalks are burned in harvest season, leading to severe air pollution and resource waste. Methane production through anaerobic digestion (AD) using agricultural straw is an important way to resolve the energy shortage in rural China. However, the AD technology is limited by low conversion efficiency due to the tight association among lignin, cellulose and hemicellulose. Therefore, pretreatments are necessary to facilitate biogas production by overcoming hydrolysis limitations. In the present study, we applied 4 pre-treatment methods i.e. sodium hydroxide (NaOH), urea, fungus and biogas slurry, to evaluate biogasification performance of rice straw via AD in laboratory scale. The pre-treatment temperature was 25℃, fermentation temperature was 35℃ and the total solids (TS) concentration was about 5%. Untreated rice straw substrate resulted into specific yields of methane and biogas of 149.1 and 261.1 L/kg, respectively. We found that AD with NaOH and biological treatment for rice straw increased biogas yield by 25%-44% and 19.4%-48.0%, respectively. Rice straw pretreated with urea of low concentration could not improve the methane yield (P=0.499), but could significantly shorten the fermentation period. Applying biogas slurry pre-treatment was found to be optimal, whose total biogas production was higher by 27.9%, whose methane yield was higher by 21.2%, and whose technical digestion time was shortened by 17.4%, compared to the untreated one. It was shown that weightlessness rate was proportional to the methane production, and methane production rate was negatively correlated to the content of cellulose and hemicelluloses after AD. Although 1.5% NaOH pretreatment could obtain the highest gas production rate, chemical oxygen demand (COD) of high concentration in biogas slurry after AD may increase the difficulty of subsequent treatment. The pretreatment costs were calculated to analyze which methods were fit for popularization. The variable costs included the fees for both raw materials and pretreatment reagents. We conclude that selecting the best pretreatment of straw in biogas project, not only needs to consider the gas production rate, subsequent processing, variable costs and other issues should also be taken into account. The pretreatment method with biogas slurry would be a more conducive and environmental friendly method.