3.2. System stability
The reactor was operated effectively under
steady state conditions for 87 days at a dilution
rate of 1.0 h1. Beyond this period, slight decline
in productivity was observed due to a decrease in
cell viability. The cell viability and hence productivity
could be restored and maintained by sparging
1 1 h1 air intermittently. The reactor was
operated for 127 days and was shut down at this
point. The present system is superior in terms of
operational stability to most of the systems reported
in the literature except the report of Nagashima
et al. (1984) (Table 2) who used calcium
aiginate beads. However, the ethanol productivity
(20 g 11 h1) reported by these workers is lower
in comparison to the present study
Table 3 shows a comparison of the data obtained
with free cells Nigam (1999), and immobilized
cells. The ethanol productivity was about
11.5 times higher with immobilized cells.