P76: Conversion of biodiesel-glycerin to methane via syntrophic microorganisms

Biodiesel is an alternative liquid transportation fuel derived from biological oils, and one of the major by-products from biodiesel production is crude glycerin.  Glycerin was once considered a valuable co-product but now is considered a low-value by-product due to increased production.  In the following study, different cultures 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 SRB were grown syntrophically in different combinations with hydrogentrophic and/or acetotrophic methanogens.  Cultures 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 cultures 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, and methane production depended on the carbon source and culture composition. Cultures grown on pure glycerol and fractionated glycerin displayed a decrease of methane production as growth rate increased. The opposite was observed with growth on crude glycerin. With most cultures, the addition of an acetotrophic methanogen increased methane concentrations significantly.  This is the first study to characterize the utilization of crude glycerin from biodiesel production by syntrophic cultures of SRB and methanogenic archaea.