This paper describes a combined estimation/control scheme conceived to handle unmeasurable process quantities, such as viable mass in the aerobic reactor. Starting with the well known dissolved oxygen dynamic balance, an algorithm for the reconstruction of the oxygen utilization rate is derived, which in turn is used to mechanize a nonlinear state observer for the process variables. Eventually this cascaded double observer is coupled to a digital PID controller, thus forming an overall output controller based solely on dissolved oxygen measurements. Simulation analysis is used to assess the composite output controller performance and sensitivity.

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