High production of acetone-butanol-ethanol with high cell density culture by cell-recycling and bleeding

A continuous acetone-butanol-ethanol (ABE) production system with high cell density obtained by cell-recycling of Clostridium saccharoperbutylacetonicum N1-4 has been studied. In conventional continuous culture of ABE without cell-recycling, the cell concentration was below 5.2 g l^-1 and the maximum ABE productivity was only 1.85 g l^-1 h^-1 at a dilution rate of 0.20 h^-1. To obtain a high cell density at a faster rate, we concentrated the solventogenic cells of the broth 10 times by membrane filtration and were able to obtain approximately 20 g l^-1 of active cells after only 12 h of cultivation. Continuous culture with cell-recycling was then started, and the cell concentration increased gradually through cultivation to a value greater than 100 g l^-1. The maximum ABE productivity of 11.0 g l^-1 h^-1 was obtained at a dilution rate of 0.85 h^-1. However, a cell concentration greater than 100 g l^-1 resulted in heavy bubbling and broth outflow, which made it impossible to carry out continuous culture. Therefore, to maintain a stable cell concentration, cell-bleeding was performed together with cell-recycling. At dilution rates of 0.11 h^-1 and above for cell-bleeding, continuous culture with cell-recycling could be operated for more than 200 h without strain degeneration and the overall volumetric ABE productivity of 7.55 g l ^-1 h^-1 was achieved at an ABE concentration of 8.58 g l^-1.