Liquid biofuel production from volatile fatty acids

The production of renewable fuels and chemicals reduces the dependency on fossil fuels and limits the increase of CO2 concentration in the atmosphere only if a sustainable feedstock and an energy efficient process are used. The thesis assesses the possibility to use municipal and industrial waste as biomass feedstock to have little of no competition with food production, and to save greenhouse gasses emissions. Waste is a complex substrate with a diverse composition and high water content. It can be homogenized without losing its initial energy value by anaerobic conversion to volatile fatty acids. Using VFA gives the opportunity to process cheap and abundantly present biomass residues to a fuel and chemical instead of sugar containing crops or vegetable oil. This thesis describes the feasibility to convert VFA to compounds with a higher energy content using mixed culture fermentations by eliminating of oxygen and/or increasing the carbon and hydrogen content. At high hydrogen pressure, protons and electrons release via the reduction of organic products such as VFA becomes thermodynamically more attractive. Three VFA reduction reactions were studied: hydrogenation to an alcohol with 1) hydrogen and 2) an electrode as electron donor, and 3) by chain elongation with hydrogen and ethanol. Based on concentration, production rate and efficiency, elongation of acetate with hydrogen and/or ethanol was the best technique to convert VFA into a fuel. In a continuous flow CSTR, 10.5 g L-1 caproic acid and 0.48 g L-1 caprylic acid were produced with ethanol and/or hydrogen at a specific MCFA production activity of 2.9 g caproate and 0.09 g caprylate per gram VSS d-1. The products were selectively removed by calcium precipitation and solvent extraction with ethyl hexanoate and petroleum ether. Microbial characterization revealed that the microbial populations were stable and dominated by relatives of Clostridium kluyveri. VFA could also be reduced to alcohols. Acetic, propionic and butyric acids were biohydrogenated with hydrogen and acetic acid also with an electrode. Observed alcohol concentrations were 0.62 g L-1 ethanol, 0.49 g L-1 propanol and 0.27 g L-1 n-butanol. Methanogenesis was successfully inhibited after thermal pre-treatment incubated at pH 6, while acetate reduction was enhanced. In the second study, ethanol (0.084 g L-1) was produced at the cathodic compartment of a bioelectrochemical system, in which the electron transport was mediated by methyl viologen. The ethanol production activity at the cathode was only of very short term, since the mediator irreversibly reacted at the surface of the cathode. Of the two VFA conversion processes, biohydrogenation and chain elongation, chain elongation was a more dominant process that consumes ethanol with acetate to medium chain fatty acids. With this technology, wet organic waste can be converted to biofuels carbon and energy efficient. The technology is promising due to the good fuel and separation properties of medium chain fatty acids, and the possibility to produce them at high concentrations and specific production rates comparable to other anaerobic conversions.