A Two-stage CSTR Cascade for Studying the Effect of Inhibitory and Toxic Substances in Bioprocesses

In some biotechnological processes like wastewater treatment and biotransformation, substances are involved which are inhibitory or even toxic to the microorganisms. Their presence changes the cell physiology or even acts lethal on the cells so that the process breaks down completely. For studying such processes, a two-stage continuous-flow stirred tank reactor (CSTR) cascade was developed where the toxic substance is only supplied to the second reactor. Mathematical modeling of the system showed that identical steady-state conditions can be established in both bioreactors of the two-stage CSTR cascade when the dilution rate of the second reactor is twice as high as the dilution rate in the first reactor, provided that both reactors are fed with the same culture medium and possess an identical working volume. The theoretically derived concept was verified by cultivating Saccharomyces cerevisiae CBS 8066 under glucose-limited aerobic conditions. Independently of the dilution rates established (D-1 in the range of 0.26 to 0.38 h(-1) and D-2 = 2.D-1), the steady-state values of the biomass, glucose and ethanol concentration were almost identical in both reactors. Moreover, the dynamic behavior after each stepwise change of the dilution rates was also identical in both reactors, which was detected by dissolved-oxygen measurements. Finally, the system was applied to the whole-cell biotransformation of ethyl 2-chloro-3-oxo-butanoate as an example.