Anaerobic digestion is a promising biochemical technology to produce renewable biofuel in the form of biogas, as well as reducing the solid organic wastes. This study explored strategies to enhance the biofuel production through anaerobic digestion of different biomass wastes. Multiple methods including feedstock pretreatment, hybrid process with thermochemical pathway, operation condition optimization, co-digestion, adding additive and metabolic engineering of the microorganism were used to enhance the anaerobic digestion process.

This dissertation comprises four distinct topics organized by chapters based on journal manuscripts: 1) The hybrid process of anaerobic digestion and fast pyrolysis was studied in which the aqueous phase in bio-oil was used as feedstock for anaerobic digestion. Through pretreatment, operation optimization and directed engineering of the anaerobic microorganism, the digestibility of the aqueous phase was significantly improved with higher methane yield; 2) Hybrid process of anaerobic digestion and fast pyrolysis was fulfilled by using biochar as additive in anaerobic digestion. The physiochemical properties of two types of biochar were compared and their function in anaerobic digestion was evaluated. The main properties contributed to the enhanced biomethane yield in the biochar were identified; 3) The proper type of biochar was selected as an effective additive in enhancing anaerobic co-digestion of municipal sludge and food waste. Biochar with high alkalinity relieved the acidification of the digestion process and also had impacts on the microbial structures; 4) Extraction of hydroxycinnamic acids was developed as a pretreatment in anaerobic digestion of corn stover, not only improving the efficiency of the digestion process but also the economics of the whole biorefinery process by extracting high value chemicals. In summary, the results demonstrated in this work provide abetter understanding of the anaerobic digestion and provides many strategies to further develop a more effective and efficient process to produce low-cost, clean and sustainable biofuel.