Development of a biofilm bioreactor for enhanced ethanol production

Abstract

Biofilms are a natural form of cell immobilization that result from microbial attachment to solid supports. Biofilm reactors with polypropylene composites containing up to 25% (w/w) of various agricultural materials (com hulls, cellulose, oat hulls, soybean hulls or starch) and micronutrients (soy bean flour or zein) were used for ethanol production. Plastic composite supports were prepared by a high temperature extrusion of polypropylene and agricultural material into 2-3 mm chips. Pure-cultures of Zymomonas mobilis (ATCC 31821) ox Saccharomyces cerevisiae (ATCC 24859) and mixed-cultures with either of the ethanol producing microorganism and the biofilm forming Streptomyces viridosporus T7A (ATCC 39115) were evaluated. Pureand mixed-culture combinations were evaluated in continuous fermentation in glucose-yeast extract medium in a bioreactor with a 20 ml working volume, 50 ml plastic composite supports and dilution rates of 0.18 to 10.5 h'. An ethanol productivity of 374 g/l/h with a 44% yield was obtained on ' Graduate student and Associate Professor respectively, Department of Food Science and Human Nutrition and Center for Crops Utilization Research, Iowa State University. ' Author for correspondence. 60 soybean hull-zein-polypropylene composite supports using Z mobilis in pure-culture with 10% glucose feed. The ethanol productivity obtained is the highest reported to date. With mixed-culture fermentations employing Z mobilis and S. viridosporus, an ethanol productivity of 147.5 g/l/h was obtained on com starch-soybean flour composite support. With pure-culture fermentation with S. cerevisiae, maximimi productivity of 40 g/l/h with a 47% yield was obtained on soybean hull-soybean flour plastic composite support. Mixed-culture fermentation using S. cerevisiae and S. viridosporus resulted in ethanol productivity of 190 g/I/h with a 35% yield, when oat hull-polypropylene composite supports were used. The maximum productivities obtained in continuously stirred reactors without supports were 124 g/l/h and 5.4 g/l/h with Z mobilis and S. cerevisiae, respectively. These productivities are significantly lower than the productivities obtained vdth biofilm reactors using composite supports. Percent yields were generally lower with mixed-culture fermentations than those observed with pureculture fermentations. Biofilm formation on the chips was detected by the change in weight and Gram staining of the support material at the end of the fermentation. Ethanol production rate and concentrations were consistently higher in biofilm reactors than in suspension cultures. Biofilm reactors with pure culture of Z mobilis had higher ethanol productivity, whereas, biofilm reactors with mixed-cultures of S. cerevisiae and S. viridosporus resulted in higher productivities. Fermentations with S. cerevisiae resulted in lower productivities than fermentations with Z mobilis. 6