Dual setpoint auxostat
Excerpts from the Ph.D. thesis of Peter Gostomski
INTRODUCTION
Continuous bioprocessing permits the observation of microorganisms
at steady state. Traditionally chemostats are used, however, the advantages of auxostats in which a concentration in the bioreactor is held constant by varying the nutrient feed rate have been recognized for sometime (Agrawal and Lim, 1984, Fraleigh, et al., 1989). An auxostat is generally more stable than the open-loop chemostat at dilution rates near the maximum growth rate
of the microorganism. Instabilities arise because of fluctuations in pumping rate and reactor volume, but in an auxostat the dilution rate becomes a dependent variable and is a function of the setpoint of the controlled variable.
Chemostat operation with toxic nutrients or chemicals is impossible at
inhibiting concentrations, but auxostat stability
is achieved through manipulation of the pumping rate of fresh medium using
feedback control strategies to maintain a desired concentration.
The pH-auxostat is a robust type of auxostat that controls nutrient
concentration indirectly. The pH change is often an excellent indication of growth and meets the requirements as a growth-dependent parameter as defined by Frederickson et al., (1970). As growth perturbs the pH, adding fresh medium restores the pH setpoint. The pH-auxostat was simultaneously reported by different research groups and was probably first operated in the 1960's (e.g., Bungay, 1972, Watson, 1972) and a detailed development was presented by Martin and Hempfling (1976). An extensive study of buffering capacity by Rice and Hempfling (1985) showed that the specific growth rate could be varied by an order of magnitude by changing the buffering capacity.
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