3. Results
3.1. Two-stage continuous fermentation
A two-stage continuous fermentation optimized for high overall
butanol production was carried out, shown in Fig. 1. The pH in first
stage reactor was tested at 4.5 and 5.4. Second stage was operated
at pH 4.5. The dilution rate for the first reactor was kept at 0.1 h−1
and 0.04 h−1 for the second reactor. The first experiment parameters
(stage 1: pH 4.5, D = 0.1 h−1 and stage 2: pH 4.5, D = 0.04 h−1)
were chosen to investigate if the high solvents reported by Godin
and Engasser (1988) could be reproduced using a different medium.
The highest produced solvents by C. acetobutylicum in batch fermentation
before cell inhibition are around 20 g l−1. The higher pH
of 5.4 in the first stage is chosen to investigate if the results from Lai
and Traxler (1994), where they tested a pH of 6, in the first reactor
could be improved.