Experimental Study of Biodegradation of Ethanol and Toluene Vapors

Appendix C-2. Fed-batch Operation
Batch growth experiments performed at very high substrate concentrations often
result in very poor bacterial biomass yield because of substrate inhibition, oxygen
insufficiency, and the generation of toxic metabolic byproducts. Fed-batch processes are
often employed in fermentation to minimize substrate inhibition, or to avoid oxygen
insufficiency and possible overheating (resulting from high biomass growth).
In this study, a working medium volume of 1.5 L was added to the bioreactor for
fed-batch operation. The inoculum was prepared in shake flasks using the same
chemical as the growth substrate (See Section 3.2.3 for preparation procedure) and then
transferred into the bioreactor at time zero. The substrate, ethanol, was continuously
introduced from a contaminated air stream. The ethanol gas concentrations both in the
inlet and outlet gas streams were measured and in the liquid phase as well.
An ethanol inlet concentration of 25.0 mg/L was introduced into the bioreactor
at an air flowrate of 0.4 L/min with a working volume of 1.5 L. Figure C-2 shows a
fed-batch operation conducted at 450 RPM and 25.0°C. Initially, ethanol metabolism
was low and the ethanol concentration in the liquid increased similar to an ethanol mass
transfer alone (shown by the straight line in Figure C-2). After about three hours,
biomass growth for the next five hours prevented further build-up of ethanol in the
liquid phase.
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