The oxygen uptake rate (or respiration) is a crucial variable in the operation of an activated sludge plant. A method has been developed and tried in full scale to estimate the respirat ion while controlling the dissolved oxygen. The strategy allows a simultaneous tracking of the respiration rate and the oxygen transfer rate. This has not been possible to do in earlier methods. The control strategy is dual in the sense that the control signal, the air flow rate, is used both to control the DO concentration and to excite the DO dynamics sufficiently to allow the parameters to be estimated. The excitation is introduced automatically by a relay feedback resulting in an oscillation of the DO around its set point. Tuning of the relay gain makes then a nice compromise between fast tracking and accurate control. The dual controller and estimator has been tested during a limited series of experiments in full scale at the Malmö Sewage works in Sweden.

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