A Novel Bioreactor‐Cell Precipitator Combination for High‐Cell Density, High‐Flow Fermentations

A novel bioreactor design has been investigated for carrying out in a single unit a fermentation and cell recycle process simultaneously. The reactor consisted of a typical fermentor and a side‐arm attached at a certain fixed angle to the fermentor. Due to the enhanced sedimentation in the inclined side‐arm, the cells precipitate quickly and flow back into the fermentor. This allowed an essentially cell‐free fermentation broth to be withdrawn at high rates through the sidearm while maintaining a high cell density culture in the fermentor. Continuous flow fermentation runs of S. cerevisiae demonstrated these features and showed that many fold increases in the steady‐state cell density could be achieved using high flowrates, which in an ordinary chemostat would lead to washout. Ethanol productivities were high and can further be increased several‐fold through a straightforward scale‐up of the precipitator sidearm to sustain even larger flow rates. Discussed also are various other reactor features such as the enhanced resistance to contamination, possible reduction of the plasmid instability problems of recombinant microorganisms, potential applications with flocculent strains, steady‐state coexistence of competing populations in mixed culture, and the possibility of carrying out fermentations with stationary cultures under continuous flow conditions.