Dialysate was sterilised and temperature controlled to 35 °C. In this set-up, circulation
through the dialysis cycle had a residence time of 35 and 43 s in 2- and 300-l scale, respectively. To avoid oxygen depletion due to fast growth and high cell densities in the external loop, cells were supplied with oxygen via dialysate in the membrane module. Therefore, dialysate was aerated with pure oxygen at low aeration rates of 0.2 or 0.05 vvm in 2- and 300-l scale, respectively, to keep the concentration of dissolved oxygen in the dialysate reservoir 80% oxygen saturation. Aeration in the module is feasible because of the high ratio of membrane area to volume of cell suspension (1.3·104 m2 m−3) inside the module. An oxygen transfer rate of 2.5 g l−1 h−1 can be realised in the module. This was calculated according to Ma¨rkl (1989) from measured permeability of glucose in the module. Compared to oxygen transfer in the fermenter of 25 g l−1h−1, this is rather low, but seems to be enough to prevent organisms from metabolic stress.