Butanol production by Clostridium acetobutylicum in a series of packed bed biofilm reactors

The continuous production of Acetone, n-Butanol and Ethanol (ABE) by immobilized cells of Clostridium acetobutylicum DSM 792 using glucose and lactose as carbon source is presented in this paper. The conversion process was successfully carried out for more than three months in 4 packed bed biofilm reactors (PBBRs) connected in series. The first PBBR of the series (fed with fresh medium) was kept under acidogenesis conditions and the three other PBBRs were kept under solventogenesis conditions. Each PBBR was a glass tube (4 cm ID, 8 cm high) with a 4 cm-bed of 3 mm-Tygon rings as carriers. The PBBR system was fed with 100g/L of lactose medium. The fermentation process was characterized in terms of metabolite production (butyric and acetic acids, acetone, butanol, and ethanol), sugar conversion and mass of biofilm. The overall dilution rate (DTOT) was varied between 0.15 h-1 and 0.9 h-1 to assess the PBBR system performance as a function of DTOT. The best PBBR system performance under optimized conditions was: butanol productivity 9.2 g/Lh, butanol concentration 10.8 g/L, acetone concentration 2.4 g/L, ethanol concentration 1.8 g/L, selectivity of butanol with respect to all solvents 72%w. To the authors’ knowledge, these butanol productivity and concentration values are the highest in the literature on lactose/(cheese whey) fermentation. An interpretation of the biofilm structure in the PBBR was put forward.