Dissolved Oxygen Auxostat

Product and nutrient based auxostats use direct measurements of chemical species that directly reflect metabolism. The types are limited because of the lack of suitable sensors. Control based on the dissolved oxygen concentration (DO) was first proposed by Ohashi (1958). Operation is based on manipulating the liquid feed rate to maintain a certain DO level and not the manipulation of the air flow rate. Even though oxygen is a nutrient, it is normally not limiting in this type of operation and the absolute value is relatively unimportant. The DO-auxostat is similar in nature to a product-based auxostat because the dilution rate does not have a direct effect on DO. The flow of fresh air is not manipulated in simple operations but contributes along with the agitation rate to an overall mass transfer coefficient (Kla), that is assumed to be constant. The governing equation for this type of auxostat is shown in the equation, that is based on an O2 mass balance.

The term in the equation that has the coefficient times the concentration difference is generally termed the oxygen transfer rate (OTR) and the term with the minus sign is the oxygen uptake rate (OUR). By manipulating the feed rate of a limiting nutrient such as sugar, the OUR can be changed thereby effecting the DO. Oxygen consumption must be growth-related for effective control.

The first control of dissolved oxygen was reported with a yeast bioprocess using on/off control of the feed rate (Hospodka, 1966). The time dependency of the DO concentration and dilution rate were not reported. Yamada, et al., (1979) used this technique for controlling the conversion of sorbitol to sorbose by Acetobacter suboxydans. They also implemented control based on the oxygen content of the bioreactor off-gas and obtained a more stable operation than with dissolved oxygen concentration. To improve the control, Konstantinov, et al.,}, (1990) used a more advanced technique where unmeasured, small, rapid disturbances were rejected through manipulation of the agitation rate, which adjusted the OTR. These disturbances were attributed to fluctuations in air flow rate and pressure. Feed rate was changed when the agitation rate moved outside a predetermined window of operation to adjust for OUR changes.