Microbial conversion of biodiesel by-products to biofuel
O'Shea, Kelly Frances
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Biodiesel is an alternative liquid transportation fuel derived from biological oils. It is a renewable form of transportation fuel that can be easily integrated into society's current infrastructure. Biodiesel is cleaner burning than petroleum, emitting less carbon pollution and harmful toxins (i.e. sulfur, benzene). One of the major by-products from biodiesel production is crude glycerin. With the increased production of biodiesel, glycerin production will continue to increase. Glycerin was once considered a valuable co-product but now is considered a low-value by-product. In the following study, different co/tricultures of sulfate reducing bacteria (SRB) and methanogens were grown with crude glycerin as a means to convert the waste product into a renewable energy source, methane. The SRBs, Desulfovibrio vietnamensis and Desulfovibrio alcoholovorans 6133, were grown syntrophically, in different co/triculture combinations, with Methanococcus maripaludis, Methanoculleus marisnigri, and Methanosarcina acetivorans. Co/tricultures were investigated for the ability to produce methane via the utilization of pure glycerol, fractionated glycerin, and crude glycerin as carbon and energy sources. In order to gain insight into cellular physiology, glycerol, acetate, free fatty acid, and methane concentrations were measured throughout growth. The co/tricultures grew fastest on pure glycerol and experienced a lag phase in growth on fractionated glycerin and longer lag phases when transferred to crude glycerin. However, methane yields were similar on all three carbon sources. Methane production depended on the carbon source and culture composition. Co/tricultures growing on pure glycerol and fractionated glycerin displayed a decrease of methane production as growth rate increased. The opposite was seen with growth on crude glycerin. With most cultures, the addition of M. acetivorans increased methane concentrations significantly. M. acetivorans displayed the capability of utilizing the by-product, acetate, from SRB oxidation of glycerol and the methanol layer from fractionated and crude glycerin. M. acetivorans appeared to interfere with the coculturing of D. vietnamensis and M. marisnigri based on decreased methane production. Cocultures with M. maripaludis grew poorly and produced little methane when grown on the supernatant of M. acetivorans. This is the first study to characterize the utilization of crude glycerin from biodiesel production by syntrophic cultures of SRB and methanogenic archaea.