Manipulative Variable Selection for Stabilizing Control of a Competitive Mixed Culture

In this paper we consider the problem of controlling biomss concentrations of two microorganisms which are competing for the same limiting nutrient in a continuous, single stage bloreactor. Although it has been previously shown that feedback control can be used to stabilize this process, the choice of manipulative and controlled variables has a significant impact on how well the system can be controlled. An interaction analysis is presented which shows that the extent of interaction depends on the desired operating point (ratio of the two microorganisms), growth rate characteristics of the microbes, and the selection and pairing of manipulative and controlled variables. In addition, results of statistical screening experiments are presented which indicate the relative magnitude of manipulative variable effects on biomass concentration and yield factors for cultures of Candida utilis and Escherichia coli under ammonium ion limitation.