Development of a biofilm bioreactor for enhanced ethanol production

Biofilms are a natural form of cell immobilization that result from microbial attachment to solid supports. Plastic-composite supports were prepared by high temperature extrusion of polypropylene and up to 25% (w/w) various agricultural materials and micronutrients into 2-3 mm chips. Pure cultures of the ethanol producers Zymomonas mobilis (ATCC 31821) or Saccharomyces cerevisiae (ATCC 24859), and mixed cultures with either ethanol-producing microorganism and the biofilm forming Streptomyces viridosporus T7A (ATCC 39115) were used. A maximum ethanol productivity of 374 g/l/h with 44% yield was obtained using soybean hull-zein-plastic composite supports with Z. mobilis and a 10% glucose feed. Productivities and yields were generally lower with Z. mobilis and S. viridosporus fermentations. With S. cerevisiae, the ethanol productivities were lower in both pure and mixed-cultures than those observed with Z. mobilis. Biofilm presence on the support was confirmed by weight gain, support clumping and Gram staining of supports. With these differences between pure- and mixed-culture fermentations, long-term performance studies of the pure-culture fermentations were further evaluated.; A packed bed reactor that approximated a trickling bed was custom-made for Z. mobilis fermentation. Soy hull-zein polypropylene composites or polypropylene supports were used. For S. cerevisiae, a plug-flow reactor was filled with soybean hull-soybean flour polypropylene composite supports. Glucose-yeast extract medium containing 10% glucose for Zymomonas and 7.5% for Saccharomyces was used. Continuous fermentations in replicates of two were carried out for 60 days at various dilution rates. Suspended culture fermentations were carried out in 2-liter B. Braun fermentors. There was no difference in the productivities obtained in fermentation between the two supports for Z. mobilis, most likely because it is a good biofilm former. Maximum productivities of 536 g/l/h and 499 g/l/h were obtained with Z. mobilis using polypropylene alone and soybean hull-zein polypropylene composite supports respectively. These productivities are the highest reported in the literature so far. With S. cerevisiae a maximum productivity of 76.1 g/l/h was obtained on the soybean hull-soybean flour plastic composite supports.; Visible biofilm formation was observed in all the reactors within two weeks of operation. Suspension culture fermentations resulted in ethanol productivities of 4.8 g/l/h and 5.2 g/l/h with yeast and bacteria respectively. A washout was observed in suspension culture fermentation when the reactor was operated at a dilution rate of 1.0 h{dollar}sp{lcub}-1{rcub}{dollar} These results suggest that, biofilm bioreactors with composite support materials can be used to improve ethanol productivity and lower the costs of fermentation.