AbstractA reactor with size‐selective properties was found to allow a mixed culture of Escherichia coli and Saccharomyces cerevisiae to coexist under continuous culture conditions. The reactor consisted of a fermentor with an attached inclined side‐arm to recycle the cells by enhanced sedimentation. The larger size yeast population was retained preferentially and recycled at high efficiencies while the smaller yet faster growing bacteria were removed through the side‐arm. Stability analysis indicated that the coexistence of this system could be stable only if the net yeast removal rate through the side‐arm as a function of the reactor yeast concentration was concave up. This would occur if growth continued in the side‐arm or if settling efficiency decreased at higher cell densities. Another experimental system was devised to measure this net removal rate function. A negative removal rate (i.e., a net addition of yeast to the fermentor) was observed at low biomass indicating growth in the settler and explaining the stability of the coexistence ste
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