A model to describe the oxygen transfer in an airlift-loop reactor (ALR) is developed in order to build an optimal digital control algorithm for the dissolved oxygen concentration. Thereafter the model is discretized in place and time resulting in a set of linear difference equations. These equations are used to implement an optimal linear discrete control algorithm. The discrete controller has as an advantage a wider application and can be used with more measuring and control points in comparison with a PID controller. Moreover, the optimal control parameters can be predicted and do not have to be revealed empirically as in the case of a PID controller. The resulting optimal discrete controller has been tested in a pilot plant ALR (0.165 m3) and compared with a PID controller. In all cases tested the discrete controller acted comparably with the PID controller. © 1992.