Fermentations with external dialysis modules were carried out in a 2- and 300-l reactor, both from Bioengineering (Switzerland) with initial working volume of 1.3 and 100 l, respectively. For fermentations in 2-l scale, one module was used, resulting in a ratio of membrane area to initial fermentation volume of Amembrane/Vfermenter=1000 m2 m−3. In fermentations in 300-l scale, four modules were installed in parallel yielding a ratio of membrane area to initial fermentation volume of Amembrane/Vfermenter=52 m2 m−3 (see Fig. 1).