Biogas production from lignocellulosic agricultural residues : microbial approaches for enhanced efficiency

Abstract: Methane, produced through microbial anaerobic digestion of various organic materials, is seen as a promising sustainable bioenergy source with the potential to reduce the current dependence on fossil fuels. Among organic materials, lignocellulosic materials, especially agriculture residues, are highly interesting due to high abundance and potential for methane production. However, low nutrient content and highly recalcitrant structure often limit process efficiency. This thesis presents the results of in-depth studies conducted in order to obtain new information about lignocellulose-degrading bacteria in biogas processes and to identify ways to enable more efficient biogas production. Different biogas processes were investigated in terms of their overall microbial community (bacteria and archaea) and potential lignocellulose degraders. The results showed that the biogas processes differed with regard to overall microbial community and chemical composition, but also composition of the cellulose-degrading bacterial community. These differences significantly influenced the degradation efficiency of both cellulose and wheat straw in batch digestion systems and also performance during start-up of semi-continuous stirred tank reactor (CSTR) processes. A positive correlation was found between lignocellulose degradation efficiency and relative abundance of Clostridium cellulolyticum. Ammonia level in the inoculum was identified as the most significant factor potentially affecting microbial community structure and methane production from lignocellulosic materials. Microbial and chemical composition of the original inoculum sources also influenced long-term degradation of lignocellulose in CSTR and appeared to influence residual methane potential. Different molecular methods for microbial community analysis were explored, with the aim of building an appropriate pipeline for in-depth studies of lignocellulose degraders in anaerobic reactors. This thesis provides novel information about the microbial communities involved in degradation of lignocellulosic materials and possible connections to process parameters. This information could potentially enable biogas production to be steered towards a more efficient and controllable process for degradation and biogas production from agriculture residues and plant-based materials.