Global warming, steadily increasing energy demand, and limited fossil fuel reserves are growing concerns of modern society. In the past few decades, significant advances in renewable energy research have helped reduce dependence on conventional non-renewable energy sources. Biofuels are sustainable and can replace petroleum-based fuels. Biofuels can be produced through three different platforms: thermochemical, sugar, and carboxylate. Based on experimental results, this dissertation suggests process improvements in the carboxylate and sugar platform to make biofuels more economically attractive. The carboxylate platform is a robust and scalable technology that produces fuels and chemicals from biomass. It employs methane-inhibited anaerobic fermentation to produce mainly short-chain fatty acids (SCFA, e.g., acetic, propanoic, butanoic, pentanoic). Medium-chain fatty acids (MCFA, e.g., hexanoic, heptanoic, octanoic acid) are more valuable than SCFAs. By feeding ethanol to the fermentor, MCFA formation is enhanced through chain elongation. To maximize MCFA production, alcohol concentrations and temperature were optimized in the mixed-culture fermentation. Chain elongation occurs at low temperatures (<=40 0C) and does not occur at 55 0C. Using the sugar platform, enzymes are a major cost contributor in biofuel production. Conventionally, enzymatic saccharification is performed in batch. To more efficiently use enzymes, a new continuous countercurrent method is explored. Pseudo-continuous countercurrent saccharification was performed on lime-pretreated corn stover at enzyme loadings of 1 mg CTec3/g dry biomass and (1 mg CTec3 + 1 mg HTec3)/g dry biomass and the results were compared with batch. To achieve the same glucan conversion as compared to batch, countercurrent saccharification reduced enzyme loading by 1.6 and 1.4 times at 1 mg protein/g biomass and 2 mg protein/g biomass, respectively. In rapidly growing developing countries, waste disposal is a major challenge. To address this challenge, the MixAlco process was investigated as an alternative to create economic incentives for waste disposal. The MixAlco process is one example of carboxylate platform. This work focuses on fermenting municipal solid waste in batch fermentations. Using the Continuum Particle Distribution Model (CPDM), the performance of continuous countercurrent fermentation was predicted at different volatile solid loading rates (VSLR) and liquid residence times (LRT).
