PhD Graduation

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 possibility to use municipal and industrial waste as biomass feedstock for fuel production reduces competition with food production, and potentially saves large greenhouse gasses emissions. Waste is a complex substrate with a diverse composition and a high water content. It can be homogenized by anaerobic conversion to volatile fatty acids (VFA) while retaining its energy value. Producing fuel from VFA would give the opportunity to process cheap and abundantly present biomass residues instead of sugar containing crops or vegetable oil. This thesis describes the feasibility to convert VFA to compounds with a higher energy density using mixed culture fermentations. Three approaches were theoretically and experimentally studied: hydrogenation to an alcohol with 1) hydrogen and 2) with an electrode as electron donor, and 3) chain elongation with hydrogen and ethanol. A critical issue in this study was the suppression of competitive microbial processes for substrate consumption.

Of the three VFA conversion processes, chain elongation dominated during mixed culture fermentation. It was noticed that mixed cultures converted acetate faster and to higher concentrations of medium chain fatty acids then to ethanol. Medium chain fatty acids have good fuel properties with their longer hydrocarbon tail compared to ethanol. 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 rate 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 chain elongation is performed by stable microbial populations, that were dominated by relatives of Clostridium kluyveri.

The proven mixed culture fermentation technology is promising to valorize VFA at production rates comparable to other anaerobic conversions.

Promotor: Prof.dr.ir. C.J.N. Buisman

Co-promotor: Dr.ir. H.V.M. Hamelers