Biogas production from lignocellulosic materials

Abstract: Lignocellulosic materials such as agricultural plant residues are widely available in large amounts and can be used for production of biogas without the risk of competition for arable land. However, the intricate structure of lignocellulose, a major component of the plant cell wall, limits microbial degradation and consequently results in low degradation rate and low biogas yield. The aim of this thesis was to investigate microbial communities engaged in the degradation of lignocellulose and search for correlations with process operating parameters and degradation efficiency. Such information could be used in a long-term perspective for management of biogas processes towards more efficient degradation. The bacterial and archaeal communities as well as functional gene communities of glycoside hydrolase family 5 (cel5) and 48 (cel48), representing potential cellulose-degrading bacteria, were investigated in laboratory-scale and industrial-scale digesters; and their incidence was related to process parameters. The laboratory-scale digesters were operated with manure, alone or in co-digestion with wheat straw (mechanically chopped or treated with steam explosion), at different temperatures. The results demonstrated that all digesters had similar overall process performance, e.g. degree of degradation and biogas yield, irrespective of straw addition or changes in temperature. However, the microbial communities, including potential cellulose-degrading bacteria, changed in response to the changes in operation, e.g. addition of straw, pre-treatment and operating temperature. In a survey of degradation efficiency of cellulose and wheat straw in 10 industrial-scale biogas plants in Sweden, free ammonia level was identified as a potential factor affecting degradation efficiency as well as the species richness and taxonomic composition of bacterial communities, including the cel5 community. This thesis presents novel information about microbial communities in biogas processes degrading lignocellulosic materials and their response to operating parameters.